CHAPTER 1: Integrated Forest Biorefineries: Current State and Development Potential

The motivation for development and use of biofuels is currently driven by several important factors: 1) diminishing reserves of readily recoverable oil; 2) increasing demand and prices of petroleum-derived fuel; 3) concerns over increasing greenhouse gas emissions and global climate change; 4) growing food needs; and 5) desire for energy independence and security. The total energy world consumption in 2005 was 488 EJ with U.S. consumption of 22% of the world total, which is expected to surpass 650 EJ by 2025 and grow approximately 40% over the next 25 years. Future oil supply is not unlimited or assured as currently available petroleum fuel reserves are estimated to become nearly depleted within 40 years. Crude oil prices have risen from less than $20/barrel in the 1990s to nearly $100/barrel in 2007 with a current annual volatility of crude oil prices exceeding 30%.¹  Government-controlled national oil companies and organizations, many in countries that are unstable or prone to conflict, command and control more than 75% of the world's known oil reserves and global oil production. The U.S. imports 10 million barrels of oil per day of the existing oil reserves of 1.3 trillion barrels.

In 2004, fossil fuels accounted for 86% of the U.S. total energy consumption, with an additional 8% from nuclear power and only 6% from renewable sources, including 3% of biomass-derived biofuels. Biomass, however, is the single renewable resource on earth, reproduced at 60 billion tons per year (as organically-bound carbon), that has the potential to supplant the use of liquid transportation fuels and help create a more stable energy future. In general, around 30% of the world's primary energy is derived from biomass with around 430 g carbon produced per square meter of land per year. The U.S. and other regions of the world have abundant biomass resources which are much more evenly distributed and accessible throughout the planet than the oil reserves. In their “billion ton vision”, the U.S. Department of Energy (DOE) reported that nearly 1.3 billion dry tons of biomass could become available to produce biofuels and displace more than 30% of the nation's consumption of liquid transportation fuels.²  However, the biomass share of the U.S. energy supply in 2004 was less than 3% of the total, compared to 40% and 23%, derived from petroleum and coal, respectively. Although biomass ranks well below petroleum, natural gas, and coal and is about one-half of nuclear, it surpasses hydroelectric and other renewable sources, and in 2009, the share of biomass in the total U.S. energy consumption exceeded 4% for the first time.

While ethanol production from corn and sugarcane (first generation biofuels) is a well-established process, cellulosic ethanol (second generation biofuels) is yet to be commercialized. The DOE Roadmap envisages large-scale production of second generation biofuels to become a nation-wide reality beyond 2020.³  The cellulosic biomass base is composed of a wide variety of forestry and agricultural resources that include forest thinning, wood mill residues, logging residues, paper waste, tree trimmings, grass clippings, energy crops such as switchgrass and miscanthus, sugarcane waste (bagasse), wheat straw, rice straw, corn stover and corn cobs.

According to the “billion ton vision” of DOE, two-thirds of the biomass resources in the U.S. represent agricultural waste, whereas about a third is forest-based. Forests cover 30% of the earth (about 3.9 billion hectares) and play a major role in preservation of biodiversity, soil conservation, and prevention of climate change serving as a major carbon dioxide sink.⁴  The forest resources are sustainable and provide long-term economic benefits to more than 1.6 billion people with a market of forest products estimated at $327 billion per year. Wood is used to produce heat for domestic and industrial purposes, and due to its strength properties, it is the primary construction material in more than 145 countries around the world. Wood is also used as a chemical feedstock to produce charcoal, tar, pitch, pulp fibers, paper, etc. Therefore, the wood-based value chain includes wood products, paper products, energy, and wood-derived chemicals. Due to the wood's increasing use and demand, approximately 130000 km² of forest are lost as deforestation every year. In addition, large areas of forest lands are littered with an unnatural accumulation of stunted, overcrowded trees and woody debris. Decades of fire suppression have disrupted the natural fire cycle of U.S. forests. Fires on these overstocked stands are more intense and harder to control than forest fires in previous decades, and they often result in catastrophic crown fires that kill large areas of forestlands. An estimated 8.4 billion dry tons of material needs to be removed from the national forests to reduce the risk of fire hazard, insect infestation, and disease. This vast source of biomass is available for production of wood products, chemicals, and energy. The biomass-derived energy use is projected to grow 35% from 2010 to 2024.

To combat the above processes, sustainable forest management practices need to include activities such as forest carbon credits, afforestation, reforestation, and the active support of government, forest companies and landowners. Government policies, carbon credits and carbon markets can be used as a tool to offset greenhouse gas (GHG) emissions. “Carbon credit” is a generic term for any tradable certificate or permit representing the right to emit one ton of carbon dioxide or the mass of another greenhouse gas with a carbon dioxide equivalent to one ton of carbon dioxide. Since GHG mitigation projects generate credits, this approach can be used to finance carbon reduction schemes between trading partners and around the world though carbon markets.⁵  Plant-derived biofuels as a carbon-neutral technology have to achieve at least 60% lower emissions than petroleum fuel based on lifecycle studies that include all emissions resulting from making the fuel from the field to the tank. Meeting these goals will require significant and rapid advances in biomass feedstock and conversion technologies; availability of large volumes of sustainable biomass feedstock; demonstration and deployment of large scale, integrated biofuels production facilities; and development of an adequate biofuels infrastructure.

The use of non-food cellulosic biomass to produce biofuels presents a solution to the growing food vs. fuel debate - the dilemma regarding the risk of diverting farmland or crops for biofuels production in detriment of the food supply on a global scale, thereby having potential adverse impacts on food price, land use change, carbon and energy balance. For example, the share of corn destined to ethanol production in the U.S. reached 25% in 2007 and according to a recent study for the International Centre for Trade and Sustainable Development, the market-driven expansion of ethanol in the U.S. increased corn prices by 21% in 2009, in comparison with what prices would have been, had ethanol production been frozen at the 2004 level.⁶  However, the United Nation's Organization for Economic Cooperation and Development (OECD) released a report in 2008 that provided estimates according to which up to 12% of the global coarse grain production and 14% of global vegetable oil production could be used for biofuels without having any significant impact on food prices.⁷ 

The Energy Independence and Security Act (EISA) of 2007 established a life-cycle GHG standard of 20% emission reduction for corn-based ethanol, based on the 2005 emission level, which would discourage the use of coal for process heat and limit further expansion of corn-based ethanol plants. The life-cycle analysis (LCA) studies mandated by EISA, dealing with data quality, allocation, system boundaries and sensitivity analysis, can profoundly shape the conclusions of biofuels production and land use change analysis. Using GHG and energy as functional units, the biofuel LCA accounts for the inputs and outputs of biofuel production, characterized in terms of energy requirements and yields, economic costs and benefits, and environmental costs and values. Most LCA results for lignocellulosic crops conclude that biofuels can supplement energy demands and mitigate GHG emissions to the atmosphere⁸  because fermentation-derived ethanol is already part of the global carbon cycle. Also, blending oxygenates such as ethanol and methyl tertiary butyl ether (MTBE) are well recognized for causing reduced carbon monoxide levels by improving the overall combustion of the fuel. However, global equilibrium economic models have estimated that indirect land-use change associated with an increased use of corn-based ethanol could potentially double GHG emissions associated with that fuel pathway in the next 30 years.⁹  This model is based on projections which suggest that biofuels would cause farmers to convert forests and grasslands to new agricultural lands that would otherwise be conserved. Measuring what really changes as a result of bioenergy policies and crops compared to what is expected to occur in their absence is an important challenge that must be addressed to improve land-use accounting. To protect ecosystems and improve livelihoods through more sustainable land-use practices, the forces that actually drive deforestation should be better understood. From this perspective, provided environmental preservation concerns are met, deforestation could be minimized, if much underutilized land is used for biomass production.

EISA contains a number of provisions to increase energy efficiency and the availability and use of renewable energy in the U.S. One key provision of EISA is the setting of a revised Renewable Fuels Standard (RFS). The revised RFS mandates the use of 36 billion gallons per year (BGY) of renewable fuels by 2022. The revised RFS has specific fuel allocations for 2022 that include use of: 16 BGY of cellulosic biofuels; 4 BGY of advanced biofuels; 1 BGY of biomass-based biodiesel; and 15 BGY of conventional biofuels (e.g., corn starch-based ethanol). This potential resource is more than sufficient to provide feedstock to produce the required 20 BGY of cellulosic biofuels by 2022 – the year in which the revised RFS mandates the use of 36 BGY of renewable fuels. By 2030 the target is to replace 30% of the transportation fuel supply with biofuels, equal to 60 billion gallons of ethanol, which would require the use of approximately 0.75 billion tons of biomass.

In 2009, the U.S. produced 10.94 billion gallons of ethanol, and together with Brazil, both countries accounted for nearly 90% of the world's production. In 2010, the ethanol production in the U.S. reached 13.30 billion gallons which exceeded the RFS mandate for the previous year – 2010, whereas in 2011, 13.95 billion gallons of ethanol were produced. In U.S. alone, the number of ethanol biorefineries increased from 110 (as of January 2007) to 209 (as of January 2012). For the same period, the ethanol production capacity increased by more than 60% – from 5.5 BGY to 14.9 BGY (http://www.ethanolrfa.org/pages/statistics/). The recent increase in ethanol production was driven by a combination of high crude oil prices, RFS for domestic renewable fuel consumption, tax credits for ethanol blenders, and large net exports in 2010 and 2011.

However, ethanol production in the U.S. is still mainly corn-based, therefore, breakthrough technologies are needed to make cellulosic ethanol cost-competitive with corn-based ethanol. Although significant progress has been recently made towards commercialization of cellulosic ethanol, there are still economic, social and environmental challenges that need to be addressed. These include significant and rapid advances in biomass feedstock and conversion technologies; availability of large volumes of sustainable biomass feedstock; demonstration and deployment of large scale, integrated biofuels production facilities; and development of an adequate biofuels infrastructure. A minimum profitable ethanol selling price of $2.50/gallon can compete on an energy-adjusted basis with gasoline derived from oil costing $75–$80/barrel. At the lower oil prices ($45–$50/barrel), cellulosic technology may not be as competitive and could require policy supports and regulatory mandates to drive the market. The biofuels and bioproducts strategies need to be based on a thorough assessment of opportunities and costs associated with the upward pressure on food prices, intensified competition for land and water, and deforestation. As the feedstock costs comprise more than 20% of the production costs, it has now been widely recognized that biomass waste such as agricultural and forest waste can provide a cost-effective alternative to improve the economic viability of bioethanol production.¹⁰  Despite technology advancements and declining processing costs for biofuels production, the profit margins for ethanol plants have been shrinking due to increasing feedstock costs and soaring prices of agricultural commodities. Costs and subsidies for biofuels are partly compensated by the expected economic, environmental and social benefits including increased energy security and reduced dependence on imported fossil-based fuels; diversification of energy and chemicals supply and markets; reduction of GHG emissions to mitigate climate change; job creation opportunities in rural areas; and overall improvement of quality of human health and life.

The supply and demand forces of market fundamentals have contributed to volatility in oil prices in recent years, and by transitioning toward higher energy efficiency and additional domestic sources of renewable fuels, such as biofuels, there is high potential to reduce U.S. market uncertainty and increase energy security. The depleting oil reserves and the use of traditional fuel with the associated logistics issues can be offset by deploying biorefineries that integrate various conversion technologies to derive energy and chemicals from locally available biomass resources. Furthermore, co-products such as corn gluten feed and meal, corn oil, glycerin, natural plastics, fibers, cosmetics, liquid detergents and other bioproducts, will increase with biofuel production and improve profitability. Currently, however, of the 100 million metric tons of chemicals produced annually in the U.S., only about 10% are biobased.¹¹ 

A biorefinery is a facility that integrates biomass conversion processes and equipment for sustainable processing of biomass into a spectrum of value-added bio-based products (food, feed, chemicals, materials) and bioenergy (biofuels, power and/or heat). The biorefinery is analogous to today's petroleum refinery, which generates multiple fuels and products from petroleum. However, in contrast to the petroleum-based products, the biorefinery products are non-toxic, biodegradable, reusable and recyclable. The biorefinery takes advantage of the various components in biomass and their intermediates, therefore maximizing the value derived from the biomass feedstock. It employs a multidisciplinary approach that integrates physical and mechanical methods, chemical and biological conversion, catalysis and biocatalysis to obtain high-efficiency, low-cost, and low-energy consumption. Biorefineries are continuously evolving as new advancements in research of biomass feedstock, related processes and products become available for sustainable energy production – a challenge and an opportunity that we currently face in our endeavors to transition to a biobased economy and society.

An Integrated Forest Biorefinery (IFBR) is a biorefinery that can process forest-based biomass such as wood and forestry residues to bioenergy and bioproducts including cellulosic fibers for pulp and paper production (Figure 1.1). As lignocellulose consists of four major components – cellulose, hemicellulose, lignin and extractives (Table 1.1) – the IFBR has four production platforms that can be used in an integrated manner for production of biofuels and high-value bioproducts. A unique feature of the IFBR is that the cellulose platform is predominantly dedicated to production of pulp and paper rather than cellulosic ethanol. The existing prototype of the future IFBR are the pulp and paper mills, in particular the chemical pulp mills. The pulp and paper industry has the world's largest non-food biomass collection system that provides a primary source of cellulosic feedstocks. The U.S. paper and forest products industry made a commitment to increase the development of biomass fuels with the strategic goal of evolving existing pulp and paper mills into forest biorefineries that export substantial amounts of renewable, sustainable energy and chemical products while continuing to meet the growing demand for traditional pulp, paper and wood products. The pulp and paper mills are most suited for biorefinery large-scale developments as they are located near the forest and agricultural residuals and have existing infrastructure to transport the raw materials and finished products. In the U.S. alone, mills collect and utilize over 120 million dry tons of wood per year as a raw material and produce power from biomass of which nearly 60% is derived from wood residuals and spent liquors. Furthermore, pulp and paper mills also have a highly trained workforce capable of operating energy and biorefinery systems. The U.S. pulp and paper industry is the world's largest manufacturer of forest products¹²  that employs nearly 1.3 million people with a payroll of over $50 billion per year. In 2010, the U.S. produced 76 million tons of paper and paperboard and nearly 50 million tons of wood pulp. However, the U.S. pulp and paper and other fiber processing industries need to create additional revenues and diversify their products and markets to remain competitive. This would enhance the profitability of these facilities thereby providing a higher degree of technological and market flexibility and economic independence. There are a number of reasons that necessitate the conversion of pulp and paper mills into IFBRs: 1) unstable, fluctuating oil prices and uncertainties about oil reserves; 2) strong, increasing off-shore competition; 3) global warming and increasing GHG emissions; 4) global movement toward and incentives for green fuels and chemicals.

For the implementation of the IFBR concept, new and more advanced bioprocessing and conversion technologies, that are both cost efficient and environmentally friendly, are needed to bring about the expected economic, environmental and social benefits. This transformation and technology upgrade would only be possible through development of sound economic studies in partnership with government and industry. The IFBR research needs are grouped in three focus areas:¹³ 

1. Sustainable Forest Productivity: Development of fast-growing biomass plantations designed to produce economic, high-quality feedstocks for bioenergy and bioproducts;

2. Extracting Value Prior to Pulping: Extraction of hemicellulose from wood prior to pulping to produce commercially viable chemical and liquid fuel products;

3. New Value Streams from Residuals and Spent Pulping Liquors: Use of thermo-chemical conversion technologies for production of fuels and chemicals and physico-chemical extraction processes to recover high-value materials from residuals and spent liquors.

As indicated earlier, the cellulose platform in the IFBR is reserved mainly for production of cellulosic fibers for pulp and paper, which is the core business of the pulp and paper industry. This is in contrast to a lignocellulosic feedstock biorefinery such as ethanol biorefinery where cellulose is hydrolyzed and fermented to ethanol and biochemicals. In the IFBR scenario, cellulose from pulp and paper mill waste such as paper mill sludge, that is not used in paper production because of inferior quality, can be converted to ethanol and other biofuels.¹⁴  In the U.S., the pulp and paper industry generates about 5 million tons of paper sludge per year which has low value as a waste product but is a promising feedstock for ethanol production with more than 15% return on investment that requires no pretreatment as it is already delignified. In the past decade, ethanol production from cellulosic biomass has been extensively researched.¹⁵  There are ongoing efforts to reduce production costs of the entire biomass to ethanol process: from feedstock production, harvesting and transportation to enzymatic hydrolysis, ethanol fermentation and product recovery. Process and cost improvements are focused on feedstock pretreatment efficiency, enzyme costs for cellulose hydrolysis and strain improvement for ethanol fermentation.¹⁶  Simultaneous saccharification and fermentation (SSF) and consolidated bioprocessing (CBP) are well recognized strategies for integration of process steps with outstanding potential for cost reductions of up to 50%.¹⁷ 

The wood delignification (pulping) methods are well established.¹⁸  Chemical (cellulose) pulp can be manufactured from wood using mechanical, semi-chemical or fully chemical methods (kraft and sulfite processes). Over the past 60 years the kraft process has proven as the most versatile and economical pulping process which is now the predominant method for production of cellulose pulp. For example, in 2000, 131 million tons of cellulose pulp were produced of which 117 million tons was kraft pulp (89% of total). In this chapter, the cellulose platform for pulp and paper production will not be discussed as this technology is well established and commercialized. However, emphasis has been given on the hemicellulose, lignin and extractives platforms for bioenergy and biochemicals production that relate to the IFBR focus areas of “Extracting Value Prior to Pulping” and “New Value Streams from Residuals and Spent Pulping Liquors”.

Hemicellulose, the second most abundant polysaccharide after cellulose, are amorphous heterogeneous polymers comprising 15–35% of lignocellulosic biomass with a degree of polymerization (DP) of 80–200.¹⁹  Hemicellulose forms an interface in the cell wall matrix with binding properties mediated by covalent and noncovalent interactions with lignin, cellulose and other polysaccharides.²⁰  The close association between the biopolymers in plant biomass is realized via chemical bonds, predominantly between lignin and hemicelluloses, in lignin–carbohydrate complexes (LCCs) that include benzyl-ether, benzyl-ester and phenyl-glycoside types of linkages. The composition and structure of hemicellulose (heteropolymer) are more complicated than that of cellulose (homopolymer) and can vary quantitatively and qualitatively in various woody species.²¹  Due to the lower DP, the chemical and thermal stability of hemicelluloses is lower and their alkali solubility - higher than that of cellulose. The building blocks of hemicellulose (polyoses) include pentoses (D-xylose and L-arabinose) and hexoses (D-glucose, D-galactose and D-mannose). Sugar acids (acetic, 4-O-methyl glucuronic acid, ferulic/coumaric acids) make up the remainder of the hemicellulose structure. Xylans and glucomannans are the two predominant types of hemicellulose in hardwoods and softwoods, respectively, and their composition and proportion varies by species. Typically, softwoods have more mannose and galactose and less xylose and acetyl groups than hardwood. The hardwood xylans as complex heteropolysaccharides, comprising β-1,4-linked D-xylopyranose units, are highly substituted (Figure 1.2). The xylopyranose unit of the xylan main chain can be substituted at the C2 and/or C3 positions with acetic acid (at both C2 and C3 position in hardwoods), 4-O-methylglucuronic acid (at C2 position in both hardwoods and softwoods), and arabinose (at C3 position in softwoods). Arabinose may be further esterified by phenolic acids which crosslink xylan and lignin in LCCs in the cell wall matrix. The uronic acid groups in hardwood xylans are not evenly distributed, with one uronic group for every ten xylose units. In softwoods, every eight xylose residues are substituted with arabinose by α-1,3-glycosidic linkages whereas the ratio of xylose to glucuronic acid is 4:1.¹⁸  The galactoglucomannans can be classified into two fractions with different galactose contents – galactose-poor fraction and galactose-rich fraction with a corresponding galactose/glucose/mannose ratio of 0.1/1/3, and 1/1/3, respectively, and acetyl content of 6% in both fractions. The softwood xylan is a linear polymer of D-xylopyranose units slightly branched with 1-2 side chains of arabinofuranose and glucoronic acid per molecule. The degree of substitution of hardwood xylan with acetyl groups can vary from 8% to 17% corresponding to 3.5-7 acetyl groups per 10 xylose units, and on average every second xylose unit is acetylated.

In the kraft process, hemicellulose is partially depolymerized, debranched and solubilized in the cooking liquor.²²  The two important reactions of carbohydrate degradation during the kraft cooking are the “peeling” reaction and the ß-elimination of 4-O-methyl-D-glucuronic acid. In the peeling reaction, a stepwise depolymerization of the carbohydrate occurs at the reducing end sites of the polymer chain. The reaction generates a monosaccharide that undergoes a benzilic acid rearrangement to form an isosaccharinic acid. The reaction also forms a new reducing end on the remainder of the polymer, which can undergo further peeling reactions. The peeling reaction continues in carbohydrates until the introduction of a carboxyl group at the reducing end which protects the carbohydrate against further peeling. The carbohydrate material lost in the peeling reaction is converted to various hydroxyl acids that consume alkali and reduce the effective concentration of the pulping liquor. Subsequent losses of hemicellulose occur during the heating period of the kraft cook thereby about 40% of xylan is lost and glucuronic acid is converted to hexenuronic acid by ß-elimination of methanol. By the end of the cook, 60–70% of the glucuronosyl and 10% of the arabinosyl substituents in softwood xylan are removed. Due to a pH drop in the pulping liquor caused by debranched acetyl residues towards the end of pulping, part of dissolved xylan, lignin and lignin-xylan complexes are reprecipitated back onto the fiber surface. The extent of this readsorption depends on the alkaline cooking conditions and wood species, however, the reprecipitated xylan has a low molecular weight without side-chain groups and a high degree of crystallinity.²³  For instance, half the xylan content of pine kraft pulp is estimated as relocated xylan whereas up to 14% of birchwood xylan can be reprecipitated during kraft pulping. During acid sulfite pulping, redeposition of xylans onto the fiber surface has not been observed. The possible reasons for this would be that the harsh cooking conditions and presence of acid-resistant residual acetyl and 4-O-methylglucuronic acid groups act as barriers against the adsorption and intercrystallization of xylan onto the cellulose micromolecules. Significant amounts of xylans are hydrolyzed and solubilized in the sulfite pulping process. For instance, in sulfite cooking of birch only 45% of the original xylan remains in pulp after 20min and its original DP of 200 is reduced to less than 100. The bonds between the pentose units (arabinose and xylose) are hydrolyzed much more rapidly than the glycopyranosidic bonds. However, the glucuronic acid-xylose and xylose-acetic acid linkages are relatively more resistant to the acid hydrolysis conditions and little cellulose is lost in the sulfite cook.²⁴  The degradation products of the hemicellulose acid hydrolysis appear in the cooking liquor in the following approximate order: arabinose>galactose>xylose>mannose>glucose>acetic acid>glucuronic acid. In the above order, glucose is derived mostly from the glucomannan rather than cellulose polymer. Thus, the residual xylan in sulfite pulps is less accessible since it is localized mainly in the secondary cell walls, although the xylan distribution across the cell wall is more uniform than in kraft pulps.

Kraft pulping is a low-yield process (yield is typically less than 50%) since half the hemicellulose (or up to 15% of the wood weight) and almost all of the lignin components of the wood are dissolved into the spent pulping liquor. This “black liquor” is processed downstream in a recovery boiler which burns the organics in the black liquor to produce process steam and recovers the inorganic cooking chemicals for re-use. A typical kraft pulp mill can process about 600000 metric tons of wood per year of which about 15% (90000 metric tons) is released into the spent liquors as degraded hemicellulose. Because lignin has a relatively high heating value (26.9MJ/kg), it is cost-effective to recoup the heating value by combustion. The hemicelluloses, however, have a low heating value (13.6 MJ/kg) and remain underutilized through the incineration process as hemicellulose contributes about a quarter of the total energy recovered in the recovery furnace. Therefore, extracting the hemicellulose from wood chips prior to pulping could be used to produce higher value chemicals and polymers and enhance the profitability and competitiveness of the paper mills. Selective removal of the hemicelluloses prior to pulping can be accomplished without degrading the wood fibers. Hemicellulose extraction of up to 10% of wood weight has been shown to have no impact on the pulping process and strength properties of the resulting pulps. In fact, kraft mills may benefit from decreased cooking times and increased pulp throughput of up to 20%, especially for mills with limited capacity of their recovery boilers. Different process options for hemicellulose extraction before pulping have been described.^25–27  One proposed method of hemicellulose removal uses green liquor as a pretreatment chemical.^27,28  Green liquor is an alkaline aqueous solution generated in the pulping recovery process which is comprised of sodium hydroxide, sodium carbonate and sodium sulfide. Under alkaline extraction conditions, xylan in wood chips is dissolved in oligomeric form while glucomannans are degraded by the peeling reaction,²¹  therefore this method is applicable to hardwoods. Extraction of hardwood chips using alkaline chemicals to extract up to 10% of hemicellulose results in a final liquor that is near-neutral pH, preserving the pulp yield and paper quality.²⁹  According to another hemicellulose extraction method, known as “hot water extraction”, wood chips are treated in absence of mineral acids or bases at 160–170°C with water only, replacing alternative costly pre-treatment methods.³⁰  Extraction using hot water generates acidic conditions (pH of 3.5) due to release of acetyl groups from wood hemicellulose. Under these conditions, hemicellulose is autohydrolyzed to generate a wood hydrolyzate, containing monosugars and acetic acid, that is then subjected to multiple separation and fractionation steps to produce commercial chemicals and fermentation products. The extracted woody biomass has a higher energy content and contains fewer easily degradable components which allows for its efficient processing and conversion to pulp and paper, wood pellets, fiberboard and nanocellulose.^31,32  Other methods for hemicellulose extraction, that have been researched with a varying degree of success, include microwave-assisted extraction, use of supercritical carbon dioxide, ionic liquids, near critical water, ammonium hydroxide, etc. Based on the “billion ton vision” of DOE, nearly 400 million tons of hemicellulose are available in the U.S. for bioprocessing to fuels and chemicals. In addition, every year approximately 15 million tons of hemicellulose are produced by the pulp and paper industry alone, and according to preliminary results, this can yield in excess of 2 billion gallons of ethanol and 600 million gallons of acetic acid with a net cash flow of $3.3 billion.

Due to their complex structure, the complete breakdown of naturally occurring branched hemicelluloses requires the concerted action of several enzymes with different functions. These are classified in two groups, hydrolases and esterases, based on the nature of linkages that they can cleave. The glycosyl hydrolases are involved in the enzymatic hydrolysis of the glycosidic bonds of hemicellulose. Of major importance are the endo-β-1,4-xylanases or 1,4-β-D-xylan xylanohydrolases (3.2.1.8) that can randomly hydrolyze internal xylosidic linkages on the backbone of xylan polysaccharide. The main products formed from xylan hydrolysis by xylanase are xylobiose, xylotriose and substituted xylooligosaccharides depending on the mode of action of the particular enzyme (Table 1.2). The xylooligomers liberated by xylanase are converted to xylose by 1,4-ß-D-xylosidase (EC 3.2.1.37). The so-called accessory enzymes such as acetyl xylan esterases, phenolic acid esterases, arabinofuranosidases and glucuronidases cleave side groups from the xylan backbone. All xylanolytic enzymes act synergistically in xylan hydrolysis. Xylanases can be classified structurally into two major groups, family 10 and family 11. Family 10 enzymes have a relatively high molecular weight whereas family 11 xylanases are relatively low molecular weight with low or high pI values. The release of reducing sugars from xylan however has not been shown to correlate to the family belonging of enzyme. The enzyme-substrate interaction is dependent on substrate specificity and kinetic properties of enzyme and can be influenced by pH, presence of xylan binding domain and ionic strength of protein and xylan molecule. Since xylan is negatively charged due to the presence of glucuronic acid side-chain groups, the efficiency of binding of enzyme to xylan is affected by the pH of reaction and pI of protein. For instance, if pH is below the pI value, the enzyme can be completely bound to the polysaccharide. The xylanolytic enzyme system of a variety of microorganisms have been extensively investigated and several exhaustive reviews have appeared.^33–35 

The optimization of fermentation techniques and isolation of more efficient microbial strains has led to a significant increase in the production rates of xylanase. Fungal systems are excellent xylanase producers, but often co-secrete cellulases which can adversely affect pulp quality. One way of overcoming this is by using suitable separation methods to purify xylanases from contaminating cellulase activity. This approach, however, is expensive and impractical. By applying appropriate screening methods and selection of growth conditions, it is possible to isolate naturally occurring microorganisms which produce totally cellulase-free xylanases or contain negligible cellulase activity. Alternatively, genetically engineered organisms could be used to produce exclusively xylanase. Most xylanases studied are active in slightly acidic conditions (pH 4-6) and temperatures between 40 and 60°C. The current trend is, however, to produce enzymes with improved thermostability and activity in alkaline conditions to fit operations at harsh industrial conditions. ß-Xylanases are produced by many microorganisms on xylan-rich substrates.³⁵ 

Currently, most commercial enzymes are mainly produced in a conventional submerged fermentation process, which is an inherently expensive operation best suited for high value antibiotics and other pharmaceutical products. Solid substrate fermentation is an economically viable alternative for enzyme production which offers numerous advantages over the submerged fermentation systems as many enzymes and other biochemicals can be produced by solid state fermentation at a fraction of the cost for submerged fermentation.³⁶  The solid state fermentation allows the direct use of in-situ enzymes (i.e. xylanase for pulp pretreatment and bleaching) without their prior downstream processing. The substrate (i.e. paper pulp which contains xylan), which is initially used as a carbon source for enzyme production, subsequently becomes the target substrate of enzyme (xylanase) action. This approach could certainly improve the economics and enhance the efficiency of the biobleaching technology due to the operational simplicity of solid state fermentation, high volumetric productivity and concentration of enzyme and production of substrate-specific enzymes in a water-restricted environment.³⁷  Advantages include high concentration of the product and simple fermentation equipment as well as low effluent generation and low requirements for aeration and agitation during enzyme production.³⁸  Due to the considerably lower production costs, the in-situ xylanase has been shown to be more cost-efficient when compared to commercial liquid products.

Spent sulfite liquor (SSL) is derived from the delignification of wood chips in an aqueous solution of acid bisulphites with an excess of SO₂, resulting in the solubilisation of lignin and leaving the wood cellulose largely undegraded.³⁹  The solids of the resultant black liquor contain 50 to 65% lignosulphonates, 15 to 22% sugars and 2 to 5% volatile acids such as acetic acid. The sugars found in the SSL include xylose, mannose, galactose, arabinose and glucose with xylose concentrations of 70–85% of the total sugars (Table 1.3). Following concentration, SSL becomes a concentrated waste with a high BOD and COD (>1000 g/L) levels and needs treatment prior to disposal. The utilization and recovery of the valuable organics in this effluent would, therefore, be more desirable than its simple discharge. The microbial utilisation of SSL has been studied for production of various metabolites such as lactic acid, single-cell yeast protein³⁹  and ethanol.⁴⁰  Recently, the use of this inexpensive carbon source as inducer of xylanase activity has also been demonstrated.⁴¹  Potential advantages include reduced xylanase production costs and development of effluent-free technology that impact positively on the environment. However, the xylose present in SSL is difficult to ferment due to the presence of inhibitory compounds such as acetic acid (>10 g/L) and polyphenols (>10 g/L), therethore strain improvement through genetic engineering and microbial adaptation on SSL have been employed.⁴²  Currently, Tembec Inc., Temiscaming, Quebec, an acid bisulfite dissolving pulp manufacturer, produces 14 million liters per year of industrial alcohol by fermenting hexose sugars in SSL.⁴³ 

Xylan-degrading enzymes, and in particular xylanases, have a great potential in industrial processes such as saccharification of lignocellulosic biomass to fermentable sugars for production of biofuels and biochemicals, bread making, clarification of beer and juices, enzymatic retting of flax, surface softening and smoothing of jute-cotton blended fabrics.⁴⁴  Nevertheless, the most important application of these enzymes to date is their use in the pulp and paper industry. Xylanases have been reported to enhance inter-fiber bonding through fibrillation without reducing pulp viscosity. Xylanase-treated pulps have shown improved beatability and brightness stability. When applied together with cellulases, xylanases can improve the drainage rates of recycled fibers and can facilitate the release of toners from office waste and the following flotation and washing steps. The xylanase production on large scale constitutes approximately 50% of the total enzyme market and the demand for xylanases grows about 25% per year, with a major application in bleaching of paper pulps.

The use of xylanases at pulp and paper mills to facilitate bleaching (biobleaching) and improve fiber properties is one the most important large-scale biotechnological applications of recent years.^45,46  The enzymatic improvement in pulp bleachability depends on a number of factors such as the wood source, pulping and bleaching processes as well as properties and substrate specificity of the enzyme. Factors such as inhibitory effect of residual pulping and bleaching chemicals in pulp as well as degradation end products on xylanase efficiency, presence of xylan-lignin and xylan-cellulose bonds may as well impact on extent of xylan hydrolysis and pulp bleachability. Restrictions in the enzymatic removal of xylan from pulp have been assigned to retarded accessibility and chemical modification of residual hemicellulose. Accessibility problems arise from the fact that chemical pulping and bleaching apparently remove the more accessible portion of xylan from the cell walls, leaving the remaining part in locations, that are less accessible to xylanase. Xylanases should contain no or very low cellulase activity as cellulases prove detrimental to yield and strength properties of pulp. The bleaching efficiency of xylanase is measured either as the reduction in the amount of chemicals used for bleaching of pulp or the brightness gain induced by the enzyme. As the biobleaching effect is dependent on the amount of enzyme used, the enzyme production costs should be kept as low as possible to ensure a cost-effective bleaching process. The major benefits from the enzyme bleaching are: 1) Reduced bleaching costs; 2) Reduced chemical consumption; 3) Increased pulp throughput; and 4) Reduced pollution.⁴⁷  A few hypotheses exist to explain the phenomenon of xylanase-aided bleaching of pulp, although the exact mechanism is not completely understood. It should be noted that the proposed mechanisms for biobleaching are not mutually exclusive and more than one model can be involved depending on pulp type, on one side, and substrate specificity of xylanase to a specific xylan type in pulp, on another.⁴⁸ 

The initial model proposed suggested that xylanases attack and hydrolyze mainly xylan redeposited on the fiber surface thereby enabling the bleaching chemicals a better and smoother access to residual lignin.⁴⁹  During kraft pulping, pulp xylan is first solubilized and later on part of it is redeposited back onto the pulp fibres. Xylanase acts on these reprecipitated xylans by partially hydrolysing them to facilitate extraction of lignin during pulp bleaching (Figure 1.3). The second hypothesis suggests that xylanases can partly hydrolyze xylan that is involved in lignin-xylan complexes thereby reducing the size of these complexes and improving their mobility and extractability from the cell walls. Indirect evidence does exist that lignin-carbohydrate bonds are formed during biosynthesis and aging of wood as well as during kraft pulping and that xylose is released as the main sugar component of isolated lignin-carbohydrate complexes. The biobleaching effect appeared to be accompanied by a decrease in the DP of xylan and a slight reduction in xylan content. It has also been reported that xylan-chromophore associations can be generated during alkaline pulping which contribute to pulp color and brightness reversion of pulps. A direct brightening effect has been observed following xylanase pretreatment of pulp. This could be due to a direct removal of lignin fragments involved in lignin-xylan complexes and/or removal of xylan derived chromophore structures. This hypothesis is supported by the recent findings that during the kraft cook the methylglucuronic acid of xylan can be modified to hexenuronic acid giving rise to double bond chromophore-type formations. Xylanases may also be able to disrupt, to an extent, the physical interlinking between xylan and cellulose within the fiber matrix thereby improving the fiber swelling and generating macropores to facilitate lignin removal. The biobleaching effect observed with some hardwood sulfite pulps may also be caused by improved pulp porosity. This suggestion is based on the fact that in acid sulfite pulps, in contrast to kraft pulps, xylan is not reprecipitated on the fiber surface but is largely entrapped across the fiber the cell walls.

Another enzyme of industrial importance, that can be produced on hemicellulose-based substrates such as xylan and xylose, is glucose (xylose) isomerase (EC 5.3.1.5). This enzyme is used industrially to convert glucose to fructose in the manufacture of high-fructose corn syrups, HFCS.⁵⁰  HFCS is produced by milling corn to produce corn starch which is first treated with alpha-amylase to produce shorter chains oligosaccharides and then with glucoamylase to produce glucose. Finally, xylose isomerase (also known as glucose isomerase) converts glucose to a mixture of about 42% fructose and 50–52% glucose (HFCS-42) with some other sugars mixed in. This 42–43% fructose-glucose mixture is then subjected to a liquid chromatography step, where the fructose is enriched to about 90% and then back-blended with 42% fructose to achieve a 55% fructose final product (HFCS-55). While the relatively inexpensive alpha-amylase and glucoamylase enzymes are added directly to the slurry and used only once, the more costly xylose isomerase is packed into columns and used repeatedly until it loses its activity. Thus, production of HFCS using xylose isomerase is the major application of immobilized enzyme technology.⁵¹  The most widely used varieties of high-fructose corn syrup are HFCS-55 (mostly used in soft drinks) and HFCS-42 (used in many foods and baked goods). In the U.S., HFCS is among the sweeteners that have primarily replaced sucrose. Factors for this include governmental production quotas of domestic sugar, subsidies of U.S. corn, and an import tariff on foreign sugar, all of which combine to raise the price of sucrose to levels above those of the rest of the world, making HFCS less costly for many sweetener applications. Pure fructose is the sweetest of all naturally occurring carbohydrates and 1.73 times as sweet as sucrose with 44% less calories than sucrose.⁵²  Fructose has the lowest glycemic index (GI of 23) of all the natural sugars and may be used in moderation by diabetics. In comparison, ordinary table sugar (sucrose) has a GI of 65 and honey has a GI of 55. Per relative sweetness, HFCS-55 (containing 55% fructose and 45% glucose) is comparable to sucrose. Sweetness is measured against sucrose as a reference with a sweetness index of 1.0. However, compared to sucrose, HFCS is less expensive, has better solubility and stability in solution, easier transportation and use than sucrose. HFCS represents at least 40% of all sweeteners added to foods, beverages and soft drinks. From 1970 to 1999 the HFCS production in the U.S. increased 10-fold and currently, together with sucrose, dominates the industrial sugar market in the U.S.⁵³  The average American consumed approximately 17.1 kg of HFCS in 2008 versus 21.2 kg of sucrose. In Japan, HFCS consumption accounts for one quarter of total sweetener consumption. The world market for HFCS was 5 million tons in 2004. In 2010, HFCS accounted for 37% of the caloric sweetener market in the U.S. At wholesale, HFCS is most often priced at $0.05-0.20 per pound lower than refined cane and beet sugar. However, the increase in the HFCS consumption has coincided with the increase in incidence of obesity, diabetes, and other cardiovascular diseases and metabolic syndromes. There are also major concerns about the mercury contamination of HFCS during production and its toxicity to honey bees with possible contribution to colony collapse disorder of honey bees.⁵¹ 

Hemicelluloses such as xylan need to have a certain degree of purity before they can be utilized in any process of industrial importance. Their isolation however is restricted as they form hydrogen bonds with cellulose, covalent bonds with lignin (mainly ether linkages), acetic and hydroxycinnamic acids (ester linkages). Xylan source and recovery process (extraction) directly impact the physical and chemical properties of the recovered polysaccharide and determine its applicability. Xylans can be extracted from lignocellulosic materials or partially delignified pulps. Xylan fractionation from lignified materials yields polysaccharides with major proportions of lignin, whereas higher purity xylans are obtained when isolated from pulps, especially bleached pulps.⁵⁴  However, the properties of xylans have not been fully characterized, defined and exploited. Although annual plants have been proven a rich source of xylan, because of the difficulties in extraction and purification of xylans and hemicellulose in general, an efficient isolation process has never been realized.⁵⁵  Furthermore, the great variety of xylan structures makes their individual use difficult as a better understanding of their physico-chemical and functional properties is needed. Xylans from hardwoods have been isolated using a combination of alkali and steam,⁵⁶  aqueous ammonia⁵⁷  and steam explosion.⁵⁸  Barium hydroxide was used as a selective extraction chemical for fractionation of arabinoxylans.⁵⁹  Xylan extractability is related to their interaction with other cell wall constituents such as lignin. Table 1.4 summarizes some of the most important current and potential applications of xylan in the pulp and paper, pharmaceutical, chemical, food and fermentation industries.⁶⁰ 

For the economic production of ethanol from lignocellulosics, the fermentation of both hexose and pentose sugars is an economic necessity. Saccharomyces cerevisiae is used universally for industrial ethanol production because of its ability to produce high concentrations of ethanol with high inherent ethanol tolerance. However, native S. cerevisiae cannot ferment xylose, hence, engineering S. cerevisiae for xylose utilization has focused on adapting xylose metabolic pathway from xylose-utilizing yeasts. This can be achieved by introducing pentose-utilizing capability into efficient ethanol producers such as Saccaromyces and Zymomonas.⁶¹  Genes encoding for xylose reductase, xylose isomerase (xi), xylulokinase (xk), transaldolase (tal) and transketolase (tkl) are inserted to enable the pentose-phosphate pathway through which xylose can enter the glycolysis pathway of glucose fermentation to ethanol (Figure 1.4). Employing this strategy for Z. mobilis, 85% of the theoretical ethanol yield on xylose was attained.⁶²  In attempts to generate S. cerevisiae strains that are able to ferment D-xylose, the XYL1 and XYL2 genes of Pichia stipitis coding for the xylose reductase (xr) and xylose dehydrogenase (xdh), respectively, were introduced into S. cerevisiae by means of genetic engineering.⁶³  Although P. stipitis ferments pentose and hexose sugars (xylose, glucose, mannose, galactose and celliobiose) and produces ethanol at faster rates and yields than other pentose-fermenting yeasts, it is not as ethanol and inhibitor tolerant as the traditional ethanol producing yeasts.⁶⁴  The maximum ethanol concentration achieved with P. stipitis is 6.5% as compared to 18% with S. cerevisiae. The ethanol production rate of P. stipitis on glucose is also lower than that of S. cerevisiae. Nearly all reported xylose isomerase-based pathways in S. cerevisiae suffer from poor ethanol productivity, low xylose consumption rates and poor cell growth compared with the oxidoreductase pathway. To increase the xylose isomerase activity before expressing the mutant enzyme in S. cerevisiae, directed evolution can be used. This approach improved the aerobic growth rate by 61-fold and both ethanol production and xylose consumption rates in S. cerevisiae by nearly 8-fold. Moreover, the mutant enzyme, which had 77% higher activity than the native enzyme, enabled ethanol production under oxygen-limited fermentation conditions.⁶⁵ 

It is well known that bacterial fermentations of xylose to ethanol are associated with low ethanol yields, slow fermentation rates, byproduct formation (acids) which requires additional product separation, contamination problems due to the neutral pH requirements for bacterial growth, bacterial sensitivity to inhibitors, and intolerance to high ethanol concentrations.⁶⁶  To overcome these problems, the carbon flow in bacterial species is diverted from native fermentation products to ethanol by introducing pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adh). PDC catalyzes the nonoxidative decarboxylation of pyruvate to produce acetaldehyde and carbon dioxide, whereas ADH catalyzes the reduction of acetaldehyde to ethanol during fermentation. Several Gram-negative bacteria such as Escherichia coli, Z. mobilis, Klebsiella oxytoca, Klebsiella planticola and Enterobacter cloacae have been engineered for ethanol production.^66–70  On mixed sugars (xylose and glucose), a recombinant E. coli produced 103–106% of the theoretical yield of ethanol.⁷¹  Recombinant Gram-negative E. coli KO11⁷²  and Gram-positive Clostridium cellulolyticum⁷³  were constructed to produce ethanol from acid hydrolysates of hemicellulose and lignocellulosic biomass, respectively. The ethanol production by the recombinant C. thermocellum increased 53%. It should however be noted that the most common problems with recombinant microorganisms that still need to be resolved are their instability, slower production rates and reduced robustness compared to the wild strains.⁶³ 

Ethanol fermentation can also be accomplished with existing industrial xylose isomerase (xi) products that convert xylose to xylulose (see Figure 1.4), which is a fermentable sugar by S. cerevisiae. However, there is a mismatch between the optimum pH and temperature of xylose isomerase (pH 7.5, 55°C) and those for ethanol fermentation (pH 5, 30°C), which makes this approach economically unviable. The major challenges in bioethanol production are the ethanol and inhibitor tolerance of fermenting microorganisms, fermentation rates and low sugar concentrations. There is a need to develop stable and robust ethanologenic microorganisms capable of tolerating high ethanol and inhibitor concentrations that can convert high levels of sugar concentrations with a solids content of 20% or higher. The economics of bioethanol production would be significantly improved if these microorganisms could produce cellulose and xylan degrading enzymes and ferment mixed sugars. The ultimate goal is to develop a consolidated bioprocessing (CBP) technology that integrates enzyme production, cellulosic biomass hydrolysis and fermentation of mixed sugars in a single step.⁷⁴  The key objectives of this goal are to: 1) perform metabolic engineering of native cellulolytic organisms such as C. thermocellum and increase the product yield and titer,^75,76  or alternatively clone the cellulolytic abilities that are lacking in native highly efficient ethanologens such as S. cerevisiae; 2) improve the microbial tolerance to substrate and product inhibition.^77,78  The functional expression of cellobiohydrolase (CBH) in S. cerevisiae increased 20-120-fold.⁷⁹  Despite these advances of genetic engineering in yeasts, their cellulose-degrading abilities are still much lower than those of C. thermocellum and T. reesei.⁸⁰ 

The U.S. DOE has identified twelve building block chemicals that can be produced from sugars via biological or chemical conversions.⁸¹  The twelve building blocks can be subsequently converted to a number of high-value bio-based chemicals or materials. Building block chemicals, as considered for this analysis, are molecules with multiple functional groups that possess the potential to be transformed into new families of useful molecules (Table 1.5). The synthesis for each of the top building blocks and their derivatives was examined as a two-part pathway: 1) transformation of sugars to the building blocks; and 2) conversion of the building blocks to secondary chemicals or families of derivatives. A second-tier group of building blocks was also identified as viable candidates. These include gluconic acid, lactic acid, malonic acid, propionic acid, the triacids (citric and aconitic); xylonic acid, acetoin, furfural, levoglucosan, lysine, serine and threonine. Most of the organic acids identified as building block chemicals from the first- and second-tier group can be produced from lignocellulosic biomass including biomass waste – that offers a less expensive alternative in replacement of their petrochemical counterparts, provides a sustainable way of waste disposal and generates income to the industrial and rural sectors.⁸² 

In the last decade, microbially-produced organic acids^83,84  find increased use in the food industry and as raw materials for manufacture of biodegradable polymers.⁸⁵  For instance, lactic acid, with a current price of $1.5/kg for a 88% purity food-grade product, has the potential of becoming a very large volume, commodity-chemical intermediate produced from renewable carbohydrates for use as feedstocks for biodegradable polymers (polylactic acid, PLA), oxygenated chemicals, environmentally friendly “green” solvents (ethyl lactate), plant growth regulators, and specialty chemical intermediates (acrylic acid). Currently, lactic acid is the most promising starting material for chemical synthesis and transformations due to the presence of the adjacent and highly reactive carboxylic and hydroxyl groups. Lactic acid in food products usually serves either as a pH regulator or as a flavoring agent and is used in a wide range of food applications such as yougurt, bakery products, beverages, meat products, confectionery, dairy products, salads, dressings, ready meals, etc.⁸⁴  Organic acids find application in food preservation because of their effects on bacteria.⁸⁶  The non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria such as E. coli, Salmonella and Campylobacter species that are pH-sensitive and cannot tolerate a wide internal and external pH gradient. Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids dissociate and the cations lower the bacteria internal pH, leading to situations that impair or stop the growth of bacteria. Furthermore, the anions of the dissociated organic acids accumulate within the bacteria and disrupt their metabolic functions leading to osmotic pressure increase that is incompatible with the bacterial survival. The world production of lactic acid in 2009 was 258000 tons with a projected growth of 329000 tons by year 2015 and 367000 tons – by 2017. Primary growth catalysts include sustained demand from end-use industries, heightened R&D activity and emergence of new applications. Lactic acid consumption in chemical applications, which include PLA polymer and new “green solvents”, such as ethyl lactate, is expected to expand 19 % per year.⁸⁷  Lactic acid based biodegradable polymers is a niche area rapidly gaining ground, and is poised to become one of the most promising end-use applications for lactic acid. In addition, growing environmental concerns arising from use of plastics will further clear the path for biopolymers in the long run. The recent announcements of plant expansions and building of new development-scale plants for production of lactic acid and/or polymer intermediates by major U.S. companies such as Cargill, Chronopol, A.E. Staley, and Archer Daniels Midland (ADM) attest to this potential. Major international manufacturers of fermentative lactic acid include Purac (Netherlands), Galactic (Belgium), and several Chinese companies. In late 1997, Cargill joined forces with Dow Chemical and established a Cargill-Dow PLA polymer venture, NatureWorks LLC, which exists today as a stand-alone company. NatureWorks LLC has constructed a major lactic acid facility in Blair, Nebraska, which began operations in 2002 with a capacity of 180000 metric tons of lactic acid per year.⁸⁸ 

Levulinic acid can be produced from renewable resources including hemicellulose by the Biofine process through a two-step, acid-catalyzed, high-temperature reactions.⁸⁹  The process involves acid hydrolysis of the biomass polysaccharides to monosugars at 210–220°C in the first step, followed by their degradation to levulinic acid and tars (Biofine char) at 190–200°C – in the second step. The by-products that are formed during the process are formic acid (from the cellulose fraction of biomass) and furfural – from the hemicellulose fraction. If the two fractions are separated, as in the case of IFBR, the cellulose fraction is used for pulp and paper production whereas the hemicellulose fraction can be converted into furfural. As xylose and other 5-carbon sugars in the hemicellulose fraction are recalcitrant to ethanol fermentation, their utilization for production of furfural could be a viable alternative. Furfural can then be used as a commodity chemical or converted to levulinic acid. The carbon-rich Biofine char is potentially an ideal feedstock for a gasification reactor which allows it to be converted to a high energy “syngas” that can either be used as source of other chemicals via a Fischer Tropsch conversion process or burned as fuel gas in a boiler or gas turbine for energy (energy content of 12000 BTU per pound or 25000 KJ/kg). A Biofine plant with a capacity of 300 tons per day could produce 13000 tons of furfural per year from lignocellulosic biomass containing 25% hemicellulose by mass. This represents 2.9% of the global furfural consumption in 2004. Using only heat and pressure in a carefully controlled chemical environment, the Biofine conversion avoids the many challenges facing other biomass conversion technologies. The Biofine process enables the use of a broader range of lignocellulosic feedstock including low-value forest residues, whole tree chips, agricultural residues, food wastes, recycled paper, sewage, paper mill sludge and municipal solid waste. Levulinic acid at 77% of the maximum theoretical yield was produced on paper mill sludge that contained 57 wt% cellulose and 8 wt% hemicellulose.⁹⁰  The feedstock flexibility is one of the greatest strengths of this process in the marketplace of biomass conversion technologies. By far the largest potential market for levulinic acid is in the production of oxygenated fuels for both transportation (gasoline and diesel) and energy generation (heating oil and gas turbine fuels). Levulinic acid can be converted into the oxygenated gasoline fuel additive methyltetrahydrofuran (MTHF). MTHF can also be produced directly from the pentose fraction in hemicellulose via furfural.⁹¹  MTHF has several attractive properties as a gasoline fuel additive:⁹²  1) can be mixed with gasoline in amounts of up 30% with no adverse impact on engine performance; 2) has good anti-knock properties (motor octane value of 80); 2) has energy density of 90% of that of gasoline; 3) has relatively low volatility (R.V.P=3 psi), approximately that of gasoline; 4) can serve as co-solvent for ethanol in gasoline mixtures (MTHF significantly reduces the vapor pressure of ethanol when co-blended in gasoline). The Biofine process has been commercialized and is currently one of the most advanced lignocellulose-processing technology available. Table 1.6 summarizes the most prominent applications of organic acids that can be derived from hemicellulose.

Furfural represents a renewable building block chemical which is currently regaining attention as a biobased alternative for the production of industrial and household chemicals¹⁰⁰  – from antacids and fertilizers to plastics and paints (Figure 1.5). It is the first and most important product derived from hemicellulose on an industrial scale. About 10% of the mass of xylan-rich plant residues such as agriwaste and forest waste can be recovered as furfural.¹⁰¹  Furfural is usually obtained through dehydration of pentoses, particularly xylose, or hemicelluloses, at high temperature (200–250°C) or in the presence of mineral acids as catalysts. The applications of furfural are grouped as follows:

1. A sustainable substitute for petroleum-based building blocks used in production of fine chemicals and plastics.¹⁰²  Furfural is used as a feedstock for production of furfuryl alcohol, furan, methylfuran, tetrahydrofuran and furoic acid. The vast majority of furfural (more than 60%) is converted into furfuryl alcohol, a well-established industrial commodity, which has found growing applications as a source of a variety of materials with notable recent progress. Furoic acid can be obtained via oxidation¹⁰³  and furan – via palladium catalyzed vapor phase decarbonylation.¹⁰⁴  Tetrahydrofurfuryl alcohol is a widely used precursor for specialty chemicals and as a binder in catalyst for the new pebble bed reactors. Another furfural derivative, tetrahydro-2-furanmethanol, is being developed as a solvent for cleaning electronic components, as chemical coupling agent in organic syntheses and for making vinyl resin, dyes and rubber.¹⁰⁵ 

2. Intermediate in the conversion of biomass to alkane-based liquid fuels (biomass-to-liquid, BTL). Polyoses and/or pentosans are first converted to furfural by acid-catalyzed dehydration, followed by aldol condensation and hydrogenation over solid base catalysts to obtain a C7-C15 fraction of hydrocarbon fuel.¹⁰⁶  Currently, the typical process for BTL production is gasification following by Fischer–Tropsch process, a technology which is feasible economically only on large scale.

3. Solvent in the refining of lubricating oils, diesel fuels and vegetable oils. Furfural has been widely utilized as an extractant due to its broad solubility in ethanol, ether and water, for separation of saturated from unsaturated compounds and selective extraction of aromatics from hydrocarbon oils in petroleum refining.^107–109 

4. Fungicide/Pesticide/Nematicide: Furfural is a new pesticidal active ingredient intended for use as a fumigant to control root-infesting plant parasitic nematodes and fungal plant diseases in greenhouse soil used for growing ornamentals and other non-food commodities.^110,111  The technical formulation contains 99.7% furfural and is for use in formulating end-use products. The end-use product contains 90% furfural in a liquid formulation.¹¹²  Furfuryl alcohol can be further hydrogenated to tetrahydrofurfuryl alcohol (THFA). THFA is used as a nonhazardous solvent in agricultural formulations and as an adjuvant to help herbicides penetrate the leaf structure. Plant parasitic nematodes cause an estimated annual loss to agriculture of $35 billion worldwide. Furfural derivatives have been used as fungicides and herbicides.¹¹³ 

China is the biggest supplier of furfural and accounts for around half of the global capacity. The world production of furfural in 2005 was about 250000 t/year. Currently, the well-established acid-based technologies produce some 300000 tons of furfural per year at a stable price of $1000/t ($1/kg). Provided the production cost of furfural can be further reduced, new application opportunities would rise, such as for drug and specialty chemical manufacture, replacement of phenol in foundry resins, and specialized polymers.¹⁰⁵ 

Xylitol is a five-carbon naturally occurring sugar alcohol found in the fibers of fruits, vegetables and beech wood.¹¹⁴  However, because of the small amounts, the quantitative extraction of xylitol from natural sources is economically unfeasible. Xylitol is currently produced chemically on a large scale by hydrogenation of xylose, which converts the sugar aldehyde into a primary alcohol.¹¹⁵  Hydrogenation is carried out at high pressures (up to 50atm), high temperature (80–140°C) using expensive catalysts (Nickel Raney) and expensive purification processes.¹¹⁶  The xylitol yields are low – on average 50–60% from xylan. Xylitol price is about $7/kg¹¹⁷  which is comparatively higher than that of natural sweeteners.

The drawbacks of the chemical process can be overcome by using a biological route of xylitol production that is carried out by microorganisms at low temperature (30–35°C). The microbial conversion employs naturally fermenting yeasts (Candida) such as C. tropicalis and C. guillermondi that yield 65–90% of the theoretical maximum from xylan. Alternatively, recombinant strains containing a xylose reductase gene (i.e. recombinant S. cerevisiae) can be used with a very high production yield of 95% from the theoretical maximum. The microbially produced xylitol requires less purification than the chemical process.¹¹⁸  Hemicellulosic hydrolyzates from hardwoods and agricultural residues are used as feedstock for xylitol production. Hemicellulose hydrolyzates from aspen,¹¹⁹  poplar,¹²⁰  eucalyptus,¹²¹  rice straw,¹²²  barley bran¹²³  and corn cobs¹²⁴  have been studied for xylitol production. Due to the presence of inhibitory compounds, pretreatment of these hydrolyzates is usually required to detoxify them prior to microbial conversion.¹²⁵  Various approaches are being considered to remove fermentation inhibitors or minimize their formation, such as neutralization,¹²¹  use of activated charcoal,¹²⁶  overliming,¹²⁷  ion exchange resins,¹²⁸  solvent extraction,¹²⁹  intracellular acidification,¹³⁰  yeast strain variation,¹³¹  laccase¹³²  recombinant strains¹³³  and adaptation of the microbial strains.¹²⁴  Although the biotechnological production of xylitol has made significant progress over the past decade, further process and product optimization is necessary to make this technology compete with the chemical production method on large scale. Milestone on the way of commercialization include better understanding of xylose metabolism into xylitol (Figure 1.4), product inhibition, air supply, lag phase of xylitol formation, etc. New and more efficient methods for xylitol production using genetic engineering, immobilized cells, mixed cultures and enzymatic biocatalysis are currently being researched.¹³⁴ 

The primary interest in xylitol is based on its properties as an alternative sweetener. It is as sweet as sucrose, twice as sweet as sorbitol, and nearly three times sweeter than mannitol. Xylitol is a non-fermentable sugar alcohol with dental health benefits in caries prevention, showing superior performance to other polyols (polyalcohols). Xylitol inhibits the microbial deterioration of tooth enamel as it is not utilized by the acid-forming oral bacteria.¹³⁵  Due to its anti-cariogenic and anti-plaque action, xylitol is used around the world as a sweetener in chewing gums, pastilles, and oral hygiene products such as toothpaste, fluoride tablets and mouthwashes. Its plaque-reducing effect is manifested by attracting and starving harmful micro-organisms because cariogenic bacteria prefer fermentable six-carbon sugars as opposed to the nonfermentable xylitol.¹³⁶  More than 10% of its use is in sugar-free chewing gums which have a world market of more than $12 million per annum.

Possessing approximately 40% less food energy, xylitol is a low-calorie alternative to table sugar. Absorbed more slowly than sugar, it does not contribute to high blood sugar levels or the resulting hyperglycemia caused by insufficient insulin response. Because xylitol does not depend on insulin to enter the glycogenolytic metabolic pathways, it is used for treatment of diabetes. Its glycemic index is approximately ten-fold lower than that of sucrose and more than two times lower than fructose. This characteristic has also proven beneficial for people suffering from metabolic syndrome, a common disorder that includes insulin resistance, hypertension, hypercholesterolemia, and an increased risk for blood clots.

Xylitol also has potential as a treatment for osteoporosis – it prevents weakening of bones and improves bone density.¹³⁷  Studies have shown that the xylitol-containing chewing gum can help prevent upper air and ear infections.¹³⁸  When bacteria enter the body, they adhere to the tissues using a variety of sugar complexes. The open nature of xylitol and its ability to form many different sugar-like structures appears to interfere with the ability of many bacteria to adhere which was attributed to the increased effectiveness of endogenous (naturally present in the body) antimicrobial factors.¹³⁹  Xylitol is also one of the building block chemicals that can be used in production of ethylene glycol, propylene glycol, lactic acid, xylaric acid, and for synthesis of unsaturated polyester resins, antifreeze, etc. (Figure 1.6).

A simplified diagram of IFBR based on generation of multiple products from hemicellulose is presented in Figure 1.7. The flowchart illustrates the enormous potential that the IFBR hemicellulose platform has to produce an array of biofuels and high-value products in an integrated, cost-efficient and environmentally-friendly way.¹⁴⁰  There are however technological challenges that need to be overcome to make these processes economically viable.¹⁴¹  These challenges are related to optimization of process conditions to maximize the biorefinery-derived value such as: 1) improvements in extraction efficiency of hemicellulose for minimal sugar degradation while preserving the pulp and paper properties; and 2) improvements in pentose fermentation and tolerance of microbial producers to inhibitors present in the hemicellulose extracts and hydrolyzates. The schematic in Figure 1.7 only depicts some of the possible scenarios of processes and products for the IFBR development. It would be impractical and probably impossible to present all pathways of the hemicellulose platform. Moreover, the IFBR development is a dynamic, technology- and market-driven process of continuous improvements and adjustments. The strive for more cost-effective manufacturing technologies, robust supply chains, new markets, commercial opportunities and economic benefits will determine the best products and direct the necessary changes of the transformation process to a IFBR for a given mill. Recent studies of the economic and commercial potential of IFBR have suggested that the longer-term competitive advantage of implementing IFBR is more likely related to the supply chain and manufacturing flexibility of pulp and paper mills than technology.¹⁴²  Therefore, mills would need to first establish a viable market for a specific product before investing in the implementation of a particular process technology, meaning that product design and marketing should precede process design.

After cellulose, lignin is the second most abundant organic polymer on earth. Softwoods generally contain more lignin (25–35%) than hardwoods (20–25%). Lignins are complex, three-dimentional biopolymers consisting of phenylpropanoid units containing both aromatic and aliphatic groups.¹⁴³  The phenilpropane units (C9 or C6C3), known as monolignols or lignin precursors, are linked together through C-C and C-O-C bonds and have different amounts of methoxy groups (Me). The dominant bond is the β-O-4 linkage. Three types of monolignols have been identified: p-coumaroyl alcohol, confineryl alcohol and synapyl alcohol (Figure 1.8). Hardwood lignins are synthesized from mainly sinapyl and coniferyl precursors in proportions from 1:1 to 1:3. Softwood lignins contain up to 95% coniferyl units with small quantities of p-coumarolyl monolignols (up to 5%).^18,144,145  The lignin macromolecule also contains a variety of functional groups that have an impact on its reactivity such as methoxyl groups, phenolic hydroxyl groups, and few terminal aldehyde groups. Only a small proportion of the phenolics hydroxyl groups are free since most are occupied in linkages to neighboring phenylpropane linkages. Carbonyl and alcoholic hydroxyl groups are incorporated into the lignin structure during enzymatic dehydrogenation. Lignin is more concentrated in the middle lamella and primary cell wall. Lignins surround the cellulose-hemicellulose matrix to provide stiffness to the cell walls and glue the cells together. Lignin as a hydrophobic polymer serves as a barrier against water penetration and is resistant toward degradation by most microorganisms except white-rot fungi and some bacteria.¹⁴⁶ 

In the pulp and paper industry, lignin is separated from other components of wood (delignification) in a process called chemical pulping using two principal methods – alkaline (kraft, soda) and acidic (sulfite) pulping processes. The removal of lignin allows individual fibers to be freed from the wood matrix with mild mechanical treatment. Pulping must be able to remove lignin from fibers through chemical degradation, while minimizing damage to the cellulosic portion of the fibers to maintain strength. The kraft pulping process accounts for 98% of chemical pulp production in the U. S. and 92% of chemical pulp production in the world. Kraft pulping is an alkaline process that uses sodium hydroxide and sodium sulfide (white liquor) at 160–180°C and pH above 12. The hydroxide and hydrosulfide anions react with lignin during the kraft cook. The alkaline attack causes fragmentation of native lignin into lower molecular weight segments. These lignin fragments are reacted with the thiol groups generated by the sodium sulfide to form thiolignins which are soluble in strong alkali and can be precipitated upon acidification. The lignin solubility in the cooking liquor is increased by cleavage of the linkages holding the phenylpropane units together, thereby generating free phenolic hydroxyl groups.¹⁸  The most prevalent degradation reactions occurring during kraft pulping include the cleavage of α-aryl ether and β-aryl ether bonds which increases the amount of phenolics hydroxyl groups. The formation of quinine and catechol structure and carboxyl groups is increased due to the oxidative conditions during delignification. The carbon-carbon linkages, however, are more stable and tend to survive the kraft pulping process. In addition to the native alkali-stable LCCs surviving the cook, alkali-stable LCCs may be formed during the cook. The lignin fragment of the native LCCs is believed to be linked exclusively with hemicelluloses, while the lignin fragment of the LCCs that are formed during pulping may be more frequently linked to cellulose.¹⁴⁷  The alkali-stable linkages between lignin and carbohydrates in the LCCs survive the kraft cook and have been suggested to be present in kraft pulps. Overall, about 85–95% of the lignin originally present in wood is dissolved in the cooking liquor.¹⁴⁸  The black liquor is the waste liquor that is released after the kraft pulping process is completed. It contains most of the cooking chemicals and the dissolved wood substances including lignin, organic acids (acetic acid, formic acid, saccharinic acid), hemicellulose (xylan) and other compounds. In the sulfite process, a mixture of sulfonic acid and a bisulfite salt is used to degrade and solubilize lignin in the form of lignosulfonates. The water-soluble lignosulfonates are released with the waste pulping liquor (sulfite spent liquor) after the sulfite cook. The lignosulfonates are normally mixtures, with wide molecular weight profiles, which contain 70–75% lignin and up to 30% impurities such as carbohydrates, ash, and other inorganic materials. Incorporation of the sulfur into biorefinery products could be a serious environmental problem and its removal would add to the overall production costs. In soda pulping, the oldest chemical pulping method, delignification occurs by the action of sodium hydroxide which causes fragmentation of native lignin and its dissolution in the cooking liquor. Compared to the kraft and sulfite lignin, the soda lignin is sulfur-free and chemically less modified. In the Organosolv pulping, a mixture of organic solvents with or without water is used as the cooking liquor. The Organosolv lignin is produced by a selective solubilisation in the solvent mixture. The Organosolv process makes it possible to obtain a lignin product with a higher homogeneity which is less modified compared to other lignins. Overall, the chemical pulping solubilizes 40–50% of the wood dry weight – mainly lignin (on average, 90% of original lignin), up to 50% hemicellulose, up to 20% cellulose and most of the extractives.¹⁴⁹ 

It has been estimated that the current lignin production in the existing pulp and paper industry is between 50 and 60 million tons per year of which 98% is burned as a low cost fuel in the chemical recovery boiler and only 2% is used commercially. The commercial lignin is mainly lignosulphonates originating from sulfite pulping (about 1 million tons/year) and less than 100000 tons/year of kraft lignin.¹⁵⁰  In addition, a major fraction of the kraft lignin is converted into a water-soluble sulfonated lignin that competes with the lignosulfonates in some applications. As a result, the existing lignin products are based on the low-value lignosulphonates used for dispersing and binding applications, and the lignin markets are stagnated at $300 million and very low growth rates.¹⁵¹  One of the major reasons for the lignin market stagnation is the heterogeneous nature of lignin products having non-uniform and non-standard quality and properties. For example, the molecular weight of kraft lignin extracted from the black liquor can range from 200 to 200000 grams per mole. The lignosulfonates have a very high polydispersity of 4.2 to 7 and degree of sulfonization of up to 0.5 per phenylpropanoid unit which corresponds to up to 8% sulfur content.¹⁵²  In comparison, kraft lignin has a polydispersity of 2.5 to 3.5 and contains 1–3% sulfur. The heterogeneity of lignin is a result of the biomass source, pulping method and recovery process used that impact on the lignin composition, size, properties and reactivity. Lignins possess unique chemical reactivity because of their heterogeity, presence of different functional groups and limited accessibility of reactive site at o- and p-positions, which makes it difficult to control a specific reaction in a desirable direction.¹⁵³  Impurities of organic and inorganic nature in lignins and difficulties in recovering lignins from product streams also contribute to the challenges associated with lignin markets.¹⁵⁴  A recent report from DOE estimates that 0.225 billion tons of lignin (biorefinery lignin) can be produced from processing 750 million tons of biomass feedstock for biofuels production.¹⁵⁵  According to this report, opportunities for commercial applications of lignin can be grouped into: power, fuel and syngas products; macromolecules; and low molecular weight aromatic, phenolic and/or miscellaneous monomer compounds. About 75% of the commercial uses of lignin as dispersants, emulsifiers, binders, and sequestrants are based on its polymer and polyelectrolyte properties. The large-scale challenge for application of industrial lignins is to find methods for their utilization which will bring profits high enough to justify the development and commercialization of lignin technologies. Some opportunities for lignin utilization in an IFBR are shown in Figure 1.9.

Kraft lignin precipitates upon acidification of the black liquor using carbon dioxide or acid available at the mill. Apart from providing lignin for various applications, its precipitation from the black liquor would alleviate the load on recovery-limited boilers which would in turn increase the pulp production capacity. To date, most of the kraft lignin applications are low-tonnage or pilot scale products. Kraft lignins have been used to produce binders, resins including ion-exchange resins, carriers for fertilizers and pesticides, thermoplastic polymers, dyes and pigment chemicals, mineral technology, asphalt, lead storage batteries, phenolic resins, activated carbon and carbon fibers.¹⁵⁶  Thermoplastic blends of lignin and lignin-derivatives in synthetic polymers were shown to be subject to property engineering via lignin content and lignin modification.¹⁵⁷  Both unmodified or chemical modified lignin have been used as a filler or in blends with other natural and/or synthetic polymers^158,159  Phase-compatabilizing lignin modifications are revealed for the incorporation of lignin into such thermoplastic polymers as polyolefins, polyacrylics, PVC and others.¹⁶⁰  The first prototype printed circuit board for the electronics industry was formulated by IBM with a 50% lignin-containing epoxy resin.¹⁵⁷  As kraft lignins are insoluble in water, advanced modifications are needed to make products such as asphalt emulsifiers.¹⁵⁰  The wet strength of kraftliner has been reported to increase by laccase-aided grafting of lignin model compounds.^161,162  An increased wet strength in kraftliner could therefore be facilitated by use of black liquor lignin derivatives with a high free phenolic content. The use of kraft lignin derivatives in this application could become a large scale business as 25–30% of the world paper production is corrugated board material including testliner and kraftliner.¹² 

Another potentially large market for lignin is the low cost production of carbon nanofibers for use in the automobile and light truck industry. As currently more than half the vehicle weight is due to its metal construction, by lowering the overall weight, the use of the lighter weight carbon fiber composites could dramatically decrease the vehicle fuel consumption. Furthermore, carbon fibers are known for their superior mechanical properties as measured by their tensile index. For example, the strongest carbon fiber is about five times stronger than steel. Carbon fibers, due to their strength and light weight, are currently used in space technology and production of sports equipment such as tennis rackets. Today, more than 90% of the carbon fibers are made from petroleum-derived materials: polyacrylonitrile and petroleum pitch. However, to permit economic use of carbon fiber composites in commercial production vehicles, fiber production will need to be substantially increased and fiber price decreased from the current $25/kg to $7/kg or less. To achieve this cost objective, high-volume, renewable or recycled raw materials such as lignin are particularly attractive because the cost of these materials is inherently both low and insensitive to changes in petroleum price. Sufficient fiber to provide 10 to 100 kg for each of the 13 million cars and light trucks produced annually in the U.S. will require an increase of 5 to 50-fold in worldwide carbon fiber production. The volume of lignin produced and burned by the domestic pulp and paper industry is about 1000 times greater than the worldwide carbon fiber production of about 28000 tons per year.¹⁶³  It has been estimated that 20% of the lignin potentially available in the U.S. could produce enough carbon fiber to displace all the steel used in domestic passenger vehicles. Kraft lignin, organosolv lignin and steam exploded lignin have successfully been used to produce carbon fiber.^164,165  The technical challenges for low cost carbon fiber production from lignin, as for other lignin applications, include lignin impurities and lignin molecular weight polydispersity. A large commercial worldwide market for carbon fibers currently exists at $125 million with a projection of $3.6 billion in 2014.

A potential advent for lignin as a low cost additive would be its use in polyurethane formulations to displace petroleum-derived compounds²⁷  thereby improving the thermal and mechanical properties of kraft lignin urethanes.¹⁶⁶  However, sulfur-free lignins such as Organosolv or soda lignin should be used to avoid any environmental problems caused by the release of sulfur-containing gases. The global market for polyurethanes was estimated at 13.65 million tons in 2010 with a revenue of $33 billion and is expected to reach close to 18 million tons and $55 billion by 2016. Besides its use in polyurethanes and polyesters, technical lignins are also of interest in phenolic and epoxy resins where lignin, due to its phenol-like structure, can replace phonolic compounds in the synthesis of phenol-formaldehyde (PF) resins.^167,168  The PF resins were the first completely synthetic commercial polymer. Kraft lignin can be used to displace up to 70% of the phenol required for PF resins.

The lignosulfonate characteristics are different from those of kraft lignins. Due to the presence of sulfonic, carboxylic and hydroxyl groups, lignosulfonates possess unique colloidal properties which allow their application as dispersing agents for oil well drilling products, detergents, stabilizers, binders, surfactants, adhesives, cement additives, battery expanders, for use in dyestuffs, pesticides particleboards and animal feeds. Their usage as dust binding liquor on gravel roads was one of the first large products based on lignosulphonates. Molding resins take advantage of the their water adsorption, dispersing, adhesion and lubricating properties. Methods of separating and modifying lignosulphonates have been developed and a wide range of applications such as concrete additives, feed binders and surface modification additive in lead acid batteries have been found.¹⁵⁰  Lignosulfonates have been extensively studied for their adhesive properties with poor reproducibility of the bonding effects, due to variable properties of lignin from various sources.¹⁶⁷  The lignosulfonates were shown to reduce the amount of water to produce a more uniform concrete product with improved durability, density and strength. Due to their dispersing properties, they reduce the amount of water needed in the manufacture of bricks and ceramic products. The lignosulfonate market is currently dominated by Borregaard LignoTech with a capacity of 500000 tons per year. Tembec of Canada sells about 75000 tons/year on 50% dry weigh basis of liquid and powdered sulfite spent liquor and products.

The sorption properties of lignins open up new avenues for their utilization in medicinal products.¹⁵⁷  It is known that dietary fiber, mainly composed of cellulose and lignin is resistant to hydrolysis by the digestive enzymes of humans and animals. Research indicates that lignin, a major dietary insoluble fiber source, may alter the fate and metabolism of soluble fibers.^169,170  Digestibility of dietary fiber and crude protein in animals was inhibited with cellulose and lignin being the major determinants for changes in digestibility.¹⁷¹  Lignin was found to have a strong negative influence on fiber digestion and was undigested in both the small and large bowel of humans.¹⁷²  Lignosulfonates are used in animal feeds as a pellet binder and to increase viscosity of molasses for easier transportation. Kraft lignins and lignosulfonates possess certain antibiotic activity due to the presence of phenolic and carboxyl groups. Lignin extracts from corn stover residues generated during ethanol production were shown to exhibit antimicrobial activities against pathogenic bacteria and yeast which could be an application in antimicrobial packaging.¹⁷³  Lignin acted as an antioxidant against oxidative agents and had a DNA-protective effect in mice cells.¹⁷⁴  Organosolv lignins from hybrid poplar with more phenolic and less aliphatic hydroxyl groups, low molecular weight and narrow polydispersity were reported to have high antioxidant activity.¹⁷⁵  Research is underway to demonstrate the use of lignin nanotubes as carriers of cancer-fighting drugs (http://news.ufl.edu/2012/03/29/nanotech/).

Combustion, gasification, pyrolysis or liquefaction can degrade lignin to a different extent, depending on the severity of the process conditions, from partial depolymerization to low molecular weight lignin fractions (pyrolysis at 400–650°C in absence of air) to fully oxidized end products of lignin – carbon dioxide and water (combustion at up to 2000°C in excess of air). These processes convert lignin to power, liquid fuel and syngas products. The choice will depend on the process economics, lignin availability (in the form of waste cooking liquor, forest waste, etc.) and the desired end product.^176–180  As lignin has a heating value of nearly 27MJ/kg or 17000 Btu/lb and contributes as much as 40% of the energy content of lignocellulosic biomass, lignin combustion from the black liquor is widely practiced today in paper mills to produce process heat, power, steam and to recover pulping chemicals. Black liquor is normally concentrated via multiple effect evaporators to 40–50% solids and then burned for its heating value (12000 to 13000 Btu/dry lb). However, the value of lignin realized through heat and power only reflects the price of fossil fuel. It has been estimated that a 30 tons per day lignin-to-fuel plant would require an installed capital cost of $10 million and would have a payback period of 3–4 years. Kraft pulp mills (more than 90% of the world production) have increasingly experiencing bottleneck problems in their recovery boiler as the installed capacity becomes too small after increase of the fiber lines capacity. Only in the U.S., black liquor is produced at about 80 million tons a day as dry solids which represents 40% of the global production rate per day. Debottlenecking of recovery boilers by partial lignin precipitation or gasification of the black liquor are new strategies already contemplated by some pulp mills. Compared to a conventional recovery boiler, a black liquor gasifier can increase the total energy efficiency of a chemical pulp mill and produce a synthesis gas for production of fuels and chemicals.

Gasification of lignin (black liquor) is carried out at 750–900°C to convert it to a gaseous fuel (syngas) through partial oxidation (1.5–1.8 kg air per kg lignin vs 6–7 kg air/kg lignin in case of combustion). In case of black liquor gasification, concentration of black liquor to 70–80% solids precedes its gasification (Figure 1.10). In addition to syngas, a mixture of carbon monoxide and hydrogen, the gas stream contains water vapors, carbon dioxide, nitrogen, ammonium, hydrogen sulfide, hydrogen chloride, and methane.¹⁸¹  The presence of contaminants in syngas can cause various problems such as GHG emissions (nitrogen and chlorine compounds), corrosion (sulfur and chlorine compounds, alkali metals), deactivation of catalyst (sulfur and chlorine compounds), water and air pollution and clogging of equipment (tars and particulates). Cyclones, barrier filters, electrostatic precipitators, venture scrubbers and catalytic cracking are used for gas cleanup.¹⁷⁶  Thereafter, syngas can be used for heat and power applications, production of hydrogen employing water-gas-shift technology,¹⁸²  methanol, ethanol and Fischer-Tropsch (FT) hydrocarbons (wax, diesel, gasoline and naphta).¹⁸³  The power applications include the use of syngas in spark ignition gas engines, in a gas turbine to produce electricity, or in a boiler to produce heat by combustion.¹⁸⁴  Hydrogen can be used for production of chemicals and fertilizers, in fuel cells for electricity generation, for refinery hydrotreating and as a transportation fuel. Methanol is a starting chemical for production of chemicals such as formaldehyde, methyl acetate, acetic acid, ethylene propylene and fuels such as dimethyl ether (DME). Methanol and ethanol can be converted to acrylic acid and ethylene, respectively, and from there – to a variety of synthetic products including polymers, adhesives, surfactants, paints, etc. (see Figure 1.9). Technologies to produce DME and FT chemicals are well established. In the FT, the carbon monoxide in the syngas adsorbs on the catalyst surface which induces the hydrocarbon chain reaction.¹⁸⁵  This reaction is a sequence of hydrogenations and carbon monoxide additions occurring repeatedly until the chain growth ends by desorbing the hydrocarbon product from the catalyst surface. The product composition is influenced by the nature and reactivity of the catalyst, the hydrogen to carbon monoxide ratio and operating conditions (temperature of 260°C and pressure of 400 to 2600 psi). The technical needs for FT synthesis include economical purification of syngas streams and catalyst and process improvements to reduce unwanted products such as methane and higher molecular weight products such as waxes.^186,187  Catalyst and process improvements are still needed to scale up the thermochemical conversion of syngas to ethanol and other alcohols. It has been estimated that syngas production accounts for 50% to 75% of the total cost of the end product such as hydrogen, alcohols, DME etc. To reduce costs, the biomass-to-fuels process should be optimized in order to obtain the highest yield, least cost configuration.¹⁸⁸  The gasification economics of different lignin sources could differ, however, it has been assumed that one ton of lignin can yield 55 gallons of ethanol and 19 gallons of 3 to 5 carbon mixed alcohols.¹⁵⁵  It is expected that costs will decrease as gasification technology matures and with increasing scale of production.¹⁸⁹ 

Pyrolysis of lignocellulosic biomass produces three fractions: liquid (bio-oil), gaseous (hydrogen, methane, carbon dioxide, carbon monoxide, etc) and solid (bio-char). Typically, fast pyrolysis (650°C for less than 5 s) results in 60–70% bio-oil, 15–25% bio-char and 10–20% gases.

Bio-oil are multicomponent mixtures derived from depolymerization and fragmentation reactions that, depending on the biomass source and pyrolysis method, can be composed of more than 300 organic compounds that belong to acids, aldehydes, ketones, alcohols, esters, anhydrosugars, furans, phenols, guaiacols, syringols, nitrogen compounds, hemicellulose-, cellulose- and lignin-derived oligomers.¹⁹⁰  Pyrolysis has the advantages of: 1) reduced size of biomass (for easier transportation) with increased energy density of bio-oil (21 MJ/kg of bio-oil derived from wood) and gases as potential biofuels; 2) lower process temperature in comparison to combustion and gasification while limiting gas pollutants such as dioxins; 3) process simplicity despite its chemistry complexity; 4) economically feasible technology for small-scale application (50–100 tons/day) of portable units distributed close to the biomass source with potential for job creation in rural areas. Bio-oil can be used in three ways: 1) directly for combustion in boilers, diesel engines and gas turbines for CHP generation; 2) upgraded via gasification and hydroprocessing to FT chemicals, fuels and power; or 3) as a source of valuable chemical compounds (Figure 1.11).

Rapid heating of fast pyrolysis reactor and rapid cooling of pyrolysis vapors are two important parameters that can maximize the bio-oil yield. Bio-char can be utilized as solid fuel (18 MJ/kg), for production of activated carbon and other chemicals, for carbon sequestration, bioremediation and for improving soil functions such as soil erosion, water and nutrients retention, etc.^191,192  The gaseous fraction containing syngas and other low molecular hydrocarbons can be used to provide heat to the pyrolysis unit or other process streams in the IFBR. Biochar production is maximized by flash pyrolysis that involves heating of biomass under moderate to high pressure in a retort with yields of 60% biochar and 40% volatiles (bio-oil and syngas) whereas slow pyrolysis typically yields up to 35% biochar.¹⁹³  A number of pyrolysis reactors have been developed for the bio-oil production, including bubbling fluidized bed, entrained bed, circulating fluidized bed, rotating cone, screw pyrolysis reactor, vacuum pyrolysis reactor, etc. BTG (www.btgworld.com) and Dynamotive (www.dynamotive.com) have already established demonstration plants for biomass fast pyrolysis, suggesting that the fast pyrolysis is near commercial. However, there are still some technical hurdles that need to be overcome before large-scale implementation of pyrolysis. Challenges with the use of bio-oil are related to its high acidity that can cause corrosion problems, high viscosity that makes it difficult to transport in pipes; high inorganic and suspended char content that can cause erosion and equipment plugging; high water content that leads to low homogeneity; high oxygen content that can reduce stability and heat value; and thermal instability leading to decomposition into less useful products.¹⁹⁴  Pyrolysis research including bio-oil production and characterization, reactor design for pyrolysis, pyrolysis parameters, reactions and kinetic mechanisms, etc. have been recently reviewed in several publications.^{193,195–199}  Current technology developments focus on stabilization of pyrolysis oil, catalyst improvements and bio-oil compatibility with existing petroleum infrastructure and technology.

Fast pyrolysis of lignin²⁰⁰  begins with its thermal softening at around 200°C and yields bio-oil liquid products that contain a high molecular weight fraction (pyrolytic lignin), monomeric phenolic compounds and low degradation compounds²⁰¹  such as methanol, hydroxyacetaldehyde and acetic acid. The pyrolytic lignins account for 13.5–27.7 wt% of crude bio-oils²⁰²  with average molecular weight of 650–1300 g/mol²⁰³  and typically characterized by dimeric and monomeric biphenyl, phenyl coumaran, diphenyl ethers, stilbene and resinol structures.²⁰⁴  Therefore, pyrolysis may be useful as a technology for the controlled molecular weight reduction of lignin that can offer some unique possibilities for conversion to useful aromatics. The lignin-derived phenolic compounds can be recovered from bio-oils²⁰⁵  and used to displace phenol in production of PF resins.²⁰⁶  Work at the National Renewable Energy Laboratory (NREL) has focused on a solvent-aided recovery of phenolic stream from bio-oil.²⁰⁷  To increase the formation of monomeric phenolic compounds in fast pyrolysis, biomass can be impregnated with alkali^208,209  or use catalytic cracking of pyrolytic lignin.²¹⁰  The major obstacle in the extraction of valuable products from bio-oil is their low concentrations that renders current recovery technically difficult and economically unattractive.

The first industrial use of lignin was in production of vanillin which today is based on nitrobenzene oxidation of lignosulfonates under alkaline conditions.²¹¹  Vanillin is a phenolics aldehyde used in the food industry as a flavoring agent.²¹²  The ice cream and chocolate industries together comprise 75% of the market for vanillin with smaller amounts being used in confections and baked goods. The global market for vanillin is estimated to be between 15–16000 tons worth of $180 million per year. Vanillin and related phenols can also be produced by microbial degradation of lignin.²¹³  White-rot fungi are believed to degrade wood lignin by excreting extracellular oxidative enzymes – laccases and peroxidases.^214,215  Due to size limitation and redox potential incompatibility of these enzymes, lignin oxidation is mediated by small molecular compounds, mediators, that are capable of penetrating the cell walls to access lignin.²¹⁶  Recent research suggests that bacteria, in particular soil- and, often aromatic-degrading bacterial species, are also capable of breaking down lignin.²¹⁷  Sphingobium sp. SYK-6²¹⁸  can metabolize lignin ß-aryl ether to vanillin Degradation of Kraft lignin by Bacillus sp. and Aneurinibacillus aneurinilyticus produced ferulic acid – a compound used as an anti-oxidant food additive.²¹⁹  Thermotoga fusca and Streptomyces viridosporus metabolized lignin to a water-soluble intermediate polyphenolic polymeric lignin (APPL) that can be precipitated by acidification^220,221  Phenolics have been extensively studied for their antimicrobial, anti-oxidative, and anticorrosive properties.²²²  Investigations revealed that phenolic fractions from kraft and sulpfite spent liquors can provide a source of antioxidants with very strong radical scavenging activity.²²³  Phenols have some pharmacological properties that could suppress the activation and expression of the HIV-1 gene.²²⁴ 

Wood extractives, also called extraneous compounds or secondary metabolites, are low molecular weight, nonpolymeric chemical compounds that are not considered essential to the cell wall structure and can be extracted from wood with various neutral solvents such as organic solvents or water. The wood extractives vary significantly within the same tree family, genera, species, from heartwood to sapwood and from season to season. Their content is normally less than 5% and may be concentrated in higher quantities in bark, tree branches, roots and wounded wood. They impact color, odor, taste, decay resistance to wood and may be toxic to fungi, bacteria and termites. For example, heartwood extractives retard wood decay, rosin is exuded by conifers to protect wounded tissues and as protection from insects, bark compounds minimize animal invasions, etc. Some of these phytochemicals may be also toxic to the producing plant²²⁵  and may cause some undesirable effects such as brightness reversion in pulps, pitch problems in papermaking, inhibition of concrete and glue setting, etc. It should be noted that the extract composition is a function of the solvent used. Furthermore, the volatile components in the wood resin cannot be measured as they will evaporate during the solvent recovery and extract dessication. No single solvent can extract all extractives present in wood. For the pulp and paper industry, the most important class of extractives is the wood resins - lipophilic compounds that are soluble in solvents such as hexane, acetone, ethanol, benzene, dichloromethane and diethylether. Due to environmental and health concerns for the use of aromatic and chlorinated solvents, determination of extractives in wood and pulp is currently standardized using acetone as a solvent. Acetone extracts more wood substance as dry weight compared to dichloromethane which was previously used as a standard solvent in the pulp and paper industry. More recently, supercritical fluid extraction with carbon dioxide was used.^226,227  The advantages of using this method include reduced use of organic solvents, higher extraction yield in comparison to acetone and dichloromethane, and shorter analysis time. Wood resins may contain fats and fatty acids, steryl esters and sterols, terpenoids and waxes. Chemically they are grouped into: terpenes, phenolic and aliphatic compounds.²²⁸ 

Terpenes are cyclyc compounds composed of isoprene units (C₅) such as monoterpenes (2 isoprene units, C₁₀), sesquiterpenes (3 isoprene units, C₁₅), diterpenes (4 isoprene units, C₂₀), triterpenes (six isoprene units, C₃₀) and tetraterpenes (eight isoprene units, C₄₀). Terpenes are contained in high quantities in the resin ducts of softwoods such as pine. Turpentine consists of volitile oils such as α- and ß-pinene (monoterpenes) used in household pine oil cleaners with pleasant aroma. The softwood monoterpenes and sesquiterpenes provide the typical pine forest odor and are collected in the turpentine fraction from the digester. Resin acids such as abietic and pimaric acid are oxygenated diterpenes that are used in rozin sizing to control the water absorption in paper products. Resin acids can be allergenic and toxic. Together with the fatty acids, they are separated from the black liquor as tall oil soap. Oxygenated terpenes such as resin acids are also called terpenoids. Some terpenoids may be allergenic and should be washed from pulps intended for use in hygienic products. Of commercial significance is the triterpenoid betulin, C₃₀H₅₀O₂, contained in the bark, pulp and pitch deposits of European birch (Betula alba). Although betulin is strictly a bark component (25–35%) that gives the typical white look of birch trees, it is introduced in the pulp and paper mill through the residual bark of incompletely debarked birchwood chips. Betulinic acid has been explored as a potential treatment for skin cancer for more than five years. Pure betulin, its derivatives and other extractives from birch bark are also tested for their effectiveness in treating HIV and respiratory syncytial virus (RSV), viruses that can cause severe cold-like symptoms and pneumonia. Betulin and some of its derivatives have shown strong antifeedant properties towards many important pest species. Phytol, acyclic diterpene (C₂₀) is found in leaves of woods and plants and comprises more than 30% of the chlorophyll molecule. Extracts from stevoside, a terpene glycoside, have been used as sweeteners²²⁹  as stevoside is more than 300 times sweeter than sucrose.²³⁰  The red and yellow plant pigments contain carotenoids – tetraterpenes (C₄₀) used in food coloring, cosmetics (lotions, powders, lipsticks) and vitamin synthesis.²³¹  Rubber, a polymeric isoprene containing up to 6000 isoprene units, is used for its elastic properties.²³²  The global natural rubber production is likely to rise 7.8% to 11.8 million tons in 2012 against 10.9 million tons in 2011, while consumption may touch 11.7 million tons in 2012. Sterols contain a tetracyclic ring and relate to triterpenes with sitosterol being the main wood sterol commonly occurring in extractives from conifers including tall oil. Sterol glycosides can be used for heart treatment as they have a strong effect on the heart muscle. Sitosterol is structurally close to cholesterol – the main sterol in humans. Recent dietary recommendations emphasize the possibility of lowering LDL cholesterol levels through consumption of products enriched with plant sterols and stanols as these are not synthesized in humans.²³³ 

Phenolic compounds contain one or more aromatic rings with a varying amount of hydroxyl groups. They are mainly found in the bark of many wood sciences and are common in heartwood. For example, suberin is a typical component of bark and can be extracted from the cork tissue of birch bark. The suberin structure is not completely determined but resembles that of lignin. The potential applications of suberinic acid are for production of coatings, surfactants and lubricants. Phenolic compounds are removed from wood during pulping and are present in the spent liquors. Some phenolics like dihydroquercetin can interfere with sulfite pulping. Resistance to decay in Scots pine is due to pinosylvins²³⁴  – a dimeric phenol belonging to the class of stilbenes. Stilbenes are commonly found in the heartwood of pine species and can be hydroxylated, methylated or glycosylated.

Flavonoids, tannins and lignans are common classes of phenolics compounds. Over 4000 different flavonoids have been isolated. Their main function is to provide resistance of trees to insects and microbial degradation.²³⁵  Polyflavonoids are used to convert animal hides to leather. Some flavonoids such as quercetin, chatechin and xanthohumol are potent antioxidant with potential health benefits including reduced risk of cancer, heart disease, asthma, and stroke. Flavonoids act as antioxidants by neutralizing oxidizing free radicals, including the superoxide and hydroxyl radicals, which are formed in the human body by the reduction of oxygen and may cause cancer and coronary heart disease and accelerate human aging.^236,237  The flavonoid compounds catechin, epicatechin, and quercetin were identified and quantified in spruce, pine, and fir species with white spruce bark containing the most abundant source of catechin (3600±100 ppm). Naringenin is a flavonoid that was isolated from the bark of Choerospondias axillarisis and is considered to have a bioactive effect on human health as antioxidant, free radical scavenger, anti-inflammatory, carbohydrate metabolism promoter and immune system modulator. Certain flavonoids have antihistamine, antimicrobial, memory- and even mood-enhancing properties. Procyanidin, extracted from Pinus radiata bark, is a flavonoid-based antioxidant that has been approved for human nutrition.²³⁸  Another flavonoid, anthocyanin, a water-soluble vacuolar pigment and a powerful antioxidant that occurs in all tissues of higher plants, can protect eyes from cataracts.

Tannins have molecular weights ranging from 500 to over 3000 (gallic acid esters) and up to 20000 (proanthocyanidins) and are considered to have antifeedant properties. They can bind to and precipitate proteins, amino acids and alkaloids. Wood tannins from oak are used in tanning animal hides into leather. Tannins have been separated in two classes: hydrolyzable and condensed tannins. The hydrolyzable tannins are further divided into gallotannins and ellagitannins, with gallic acid and ellagic acid, respectively, as essential components. The hydrolyzable tannins are mixtures of simple phenols and glucose esters of gallic and digallic acids.²³⁹  They have been used to displace up to 50% of phenol in the manufacture of PF resins.²⁴⁰  Ellagic acid has antitumor activity and is used as a sedative and tranquilizer.²⁴¹  Bark contains 10–12% tannins that are used as pharmaceuticals (anti-diarrheal agents), corrosion inhibitors and adhesives. More than 90% of the commercial production of tannins is for condensed tannins that are used in a number of applications including PF resins, tyre cord adhesives, foundry core binders and wood preservatives.²⁴² 

Lignans are one of the major classes of phytoestrogens, derived from phenylalanine via dimerization of substituted cinnamic alcohols (monolignols) to a dibenzylbutane structure. Lignans act as antioxidants and display a range of biological activities including enzyme inhibition, fish toxicisty, growth inhibition, insect antifeedant properties.²⁴³  Lignans are capable of binding to estrogen receptors and interfering with the cancer-promoting effects of estrogen on breast tissue. For example, podophyllotoxin is currently studied for its possible effects on breast, prostate and colon cancer. Hydroxymatairesinol was detected as the major lignan constituent in knots of Norwegian spruce with approximate content of 5.5wt%.²⁴⁴  Hydroxymatairesinol has anti-cancer properties used in treatments of breast, colon and prostate cancer, cardiovascular diseases and as a dietary supplement. Recent research has uncovered the naturally occurring existence of hydroxymatairesinol as the dominant lignan in wheat, triticale, barley, corn, amaranth, millet and oat bran.

Aliphatic compounds of wood extractives include fatty acids, fatty alcohols and hydrocarbons (alkanes). The fatty acids are mainly present as esters with glycerol. Triglycerides are saponified during kraft pulping to produce soaps such as sodium soaps (liquid) and potassium soaps (solid). The dominating fatty acids are the unsaturated C₁₈ fatty acid – oleic, linoleic and linolenic acids that constitute 75–85% of the total amount of fatty acids. Alkanes can also accumulate in wood tissues – n-heptane is a component of turpentine from Pinus sabiniana.²⁴⁵  The content of nitrogen compounds in wood is less than 0.1% and is due to presence of amino acids and proteins involved in cell wall biosynthesis²⁴⁶  and alkaloids.

Alkaloids are a group of naturally occurring chemical compounds that contain basic nitrogen atoms and are found in higher concentrations in bark, roots and leaves. Alkaloids can be purified from crude extracts by acid-base extraction and have pharmacological effects, can act as poisons and are used as medications, as recreational drugs, or in entheogenic rituals. Cocaine, caffeine and nicotine are classified as alkaloids.²⁴⁷  Reserpine is an alkaloid first isolated from Rauwolfia species of South American evergreen trees and shrubs. It is used as an antihypertensive drug and to treat disorders including schizophrenia.²⁴⁸  Another alkaloid isolated from the bark of the South American plant Chondrodendron tomentosum, tubocurarine, is used adjunctively in anesthesia to provide skeletal muscle relaxation during surgery or mechanical ventilation. Quinine, an antimalarial alkaloid, has been isolated from the tropical rain forest – the most dense and biogenetically diverse forest areas of the world that is largely unexplored and is currently an untapped source of valuable, biologically active natural products including pharmaceuticals and neutraceuticals. Some of these products still have superior attributes over synthetically derived drugs and/or are more economically extracted from its natural sources.²⁴⁹  Taxol, originally isolated from the bark of Pacific yew, Taxus brevifolia,²⁵⁰  is the most effective antitumor agent developed in the past three decades. It has been used for effective treatment of a variety of cancers including refractory ovarian cancer, breast cancer, non-small cell lung cancer, AIDS related Kaposi's sarcoma, head and neck carcinoma and other cancer types.²⁵¹  In attempts to make taxol less expensive and more widely available via industrial fermentation, recent research has focused on taxol-producing endophytic fungi.²⁵² 

With respect to their influence on the pulping process, extractives are classified in two groups: saponifiables such as fatty and resin acids, glycerides and some steryl esters that form soluble soaps with alkali; and unsaponifiables (also called neutrals) such as waxes, fatty alcohols, sterols, terpene alcohols, etc. that do not form soaps and can cause pitch problems. Fatty acids (30–60%), resin acids (40–60%) and unsaponifiables (5–10%) constitute the tall oil fraction of black liquor (Figure 1.12) which is formed during kraft pulping by saponification of softwood extractives-based fats and waxes to sodium salts of fatty and resin acids.²⁵³  They contain a polar hydrophilic (carboxylic) end and a nonpolar, hydrophobic (hydrocarbon) end and associate to form a micellar colloid called micelle. The electric charge of the polar carboxyl groups stabilizes the micelle and prevents their agglomeration. The neutrals (unsaponifiables) such as sterols and waxes can be solubilised within the hydrocarbon nonpolar interior of the micelle. However, as the cationic strength (sodium cations) increase, the negative charge on the outer surface of the micelle decrease and is eventually neutralized which causes aggregation and precipitation of the micelles as raw rosin soap from the black liquor.²⁵⁴  The raw rosin soap is removed from the black liquor during its evaporation (from 15% solids to about 25-30%) by skimming the surface with an average yield of 30–50 kg/ ton pulp produced from highly resinous softwood specie like southern pines. Thereafter the soap is allowed to settle to release any retrained black liquor and acidified with sulphuric acid to produce the crude tall oil. The crude tall oil is normally fractionated by vacuum distillation to several commercial fractions and sold for soaps, rosin size, adhesives, rubbers, inks and other products. Globally, about 2 million tons/year of tall oil are refined whereas in the U.S. alone more than 700 million liters of tall oil and turpentine could be produced. The unsaponifiable fraction contains mainly sterols such as sitosterol and sitostanol which, upon further isolation and purification, can be used as cholesterol-lowering food additives.²³³  The resin acids can be utilized together with the fatty acids in biodiesel production, or used for production of cetane enhancers by catalytic hydrogenation and cracking (Figure 1.12).²⁵⁵  Cetane enhancers are compounds added to diesel fuel oil to raise the fuel's measured cetane level. The addition of cetane enhancers to diesel fuel is one recognized retrofit technology used to reduce diesel engine emissions. In a IFBR scenario, the hydrogen and methanol required in the production of cetane enhancers and biodiesel, respectively, could be generated and supplied from the lignin platform via the syngas route as discussed earlier.

Most of the wood extractives are removed during the kraft cooking process. However, some sterols and alcohols, formed from steryl esters and wastes, have low solubility in the cooking liquor and may be carried out entrapped in the micelles with the brownstock pulp. Sulfite pulps normally contain more extractives than kraft pulps as fatty and resin acids are insoluble in the acid cooking. These extractives can agglomerate to form particles, often referred to as pitch, which may deposit on the equipment causing operating difficulties, or may appear in the finished pulp and paper as contaminating particles. Pitch deposits may cause spots, specks, streaks, breakage or holes in the paper that altogether result in significant downtime for cleaning in the papermaking process. Extractives can lower the surface energy of paper leading to decreased wettability with a negative impact on products such as tissue paper. Extractives can also decrease the bonding of toner particles in printing papers, or decrease the bonding between individual fibers in the paper thereby reducing the paper strength.²²⁸  Oxidation of wood extractives, especially unsaturated fatty acids and their esters, can result in formation of volatile, off-flavor compounds that, if used as food wrapping paper, can affect the food odor or taste.²⁵⁶  Extractives can react with the bleaching chemicals to form oxidized and modified extractives that can impact brightness stability and cause brightness reversion.²⁵⁷  For example, oxidized lignans have colored structures, whereas chlorinated extractives liberate hydrochloric acid that can cause formation of colored compounds from polysaccharides.^258,259  To avoid this problem, removal of extractives prior to bleaching becomes necessary. An alkali pretreatment on pine bisulfite pulp prior to hydrogen peroxide bleaching was shown to remove 86% of extractives.²⁶⁰  Hot water treatment²⁶¹  as well as pressing and washing²⁶²  have been proposed as effective methods for dissolution of the lipophilic extractives. Anionic surfactants (naphthalene sulphonate or lignosulphonate) and non-anionic surfactants (fatty alcohol ethoxylate or alkyl phenol ethoxylate) have been reported to reduce 30 % of the dichloromethane extractives in peroxide bleaching of CTMP aspen pulp.²⁶³  Pitch in mechanical pulps acts as polyanions to cause anionic trash. Traditional methods to control pitch problems include wood seasoning before pulping or adsorption and dispersion of pitch particles with chemicals such as talc during the pulping and papermaking process. Methods for pitch control using resin-degrading fungi and lipase enzymes have also been developed.^264,265 

The main technological challenges that need to be overcome to maximize the fiber value in a future IFBR are related to process improvements in extraction of hemicellulose for minimal sugar degradation and preservation of pulp and paper properties; fermentation of mixed sugars and tolerance of microbial producers to inhibitors and ethanol; pyrolysis and gasification efficiency. Process integration in a IFBR would reduce the number of process steps and reuse the process streams in a waste-free and closed cycle operation mode thereby reducing the overall energy demands. Waste heat could be utilized to integrate other manufacturing opportunities whereas additional energy requirements could be met by combustion (or another thermal process such as gasification or pyrolysis) of waste biomass. Process integration tools can be employed to identify products that can be economically produced by a pulp and paper mill. It is anticipated that, in addition to pulp and paper products, the future IFBR will extract/generate significant amounts of hemicellulose and lignin, respectively, and create new market opportunities for production of chemicals, polymers, new wood composites, liquid fuels, ethanol, etc. Hemicellulose pre-extraction from wood chips prior to pulping, or lignin precipitation from black liquor provide opportunities for higher paper production by 20% (hemicellulose extraction), or 15% (lignin precipitation). These new biorefinery technologies could be used as a recovery debottlenecking tool at existing mills.²⁶⁶  Additional benefits from hemicellulose extraction include reduced alkali consumption and black liquor load at increased delignification rate. Debottlenecking through black liquor gasification is another technological opportunity that is expected to reach commercial readiness by 2015 and is currently actively being pursued by the industry seeking federal and state grants, loans and investors funding. Black liquor gasification would allow replacing aged, low-efficient and high-maintenance recovery boilers, that are currently still in use at pulp mills, offering the potential for overall cost reductions, more efficient energy recovery, emissions reduction, improved safety, separation of pulping chemicals to maximize pulp yield through modified pulping technologies at internal rate of return on incremental capital investment of 14–18% assuming $50/bbl of oil.^267,268  To supplement the bioenergy and bioproducts derived from hemicellulose and lignin, the use of bark and foliage extracts from wood would enable the IFBR to enter the markets for functional food additives, neutraceuticals and pharmaceuticals that have shown a steady 15% growth per year over the past decade. Extractives usually comprise a minor proportion of wood biomass, however, for a large scale IFBR operation, they can be a potential source of high-value co-products.²⁶⁹  A simplified flow diagram of the IFBR with the four production platforms is displayed in Figure 1.13.

The key to a successful, convergent value chain is the identification of potential mill-specific products and processes that can be implemented and integrated at the IFBR. The following factors will have to be considered when identifying the IFBR technology pathway and specific product mix: 1) mill-specific factors – location, existing technology, wood species, production levels; 2) market analysis; 3) product design analysis – price competitiveness, supply chain analysis, life-cycle assessment; 4) process design analysis of emerging, cost-effective technologies; 5) process flexibility; 6) investment risk; 7) speed of new product to market. It should be noted that the flexibility in product diversification and risk minimization would decrease with increase in the IFBR scale of production. The distinction between main products and by-products will have a major influence on the functioning and organization of the forest products markets.²⁷⁰  Depending on the reliance of forest resources for product and revenue diversification and on the demand for new forest products on the market, the competition for and prices of forest-based biomass and materials may increase. Following the development of a technology strategy, pulp and paper mills need to formulate their business strategy: 1) formulate their competitive and marketing strategy and long-term objectives of a phased approach for incremental implementation of IFBR; 2) identify and secure supply chains and marketplace; 3) form strategic alliances with companies in other sectors with established marketplace; 4) invest in innovation and R&D to verify and optimize the products and technology choice and explore alternative platforms and concepts. The techno-economic and commercial risks of the technology and business strategy of the IFBR must be identified and mitigated.²⁷¹  To accomplish the IFBR implementation plan, government support will be of vital importance. The IFBR economic and operational feasibility may be evaluated using a demonstration or pilot plant at an existing pulp mill, or a small kraft mill that has been closed. The IFBR options could add another 30–50% of profitable revenue with 25–40% return on investment for the pulp and paper sector.

Biotechnology will continue to play a major role in the future R&D to provide cost efficient and environmentally friendly solutions to increased forest productivity and product diversification. The forest biotechnology research is expected to revolutionize advancements in tree genetic engineering for improved resistance to insects, pathogens and environmental stressors, leading to development of fast-growing biomass plantations designed to produce economic, high-quality material for building, construction, pulp, paper, bioenergy and bioproducts. For instance, Weyerhaeuser envisions a yearly production of over 100 million seedlings of Douglas-fir and loblolly pine developed through the company's biotechnology program.²⁷²  A number of microbial and enzymatic processes have been already developed and some of them commercialized in the pulp and paper industry. Enzymatic and microbial processes include biopulping, biobleaching, biorefining, bioremediation, enzymatic grafting of paper, enzymatic delignification, pitch control, enzymatic control of fines and drainage properties, enzymatic control of tissue softness, enzymatic deinking, enzymatic depithing, etc.^273–276 

Additional jobs, tax benefits and air emissions reductions may be generated to address the societal need for utilizing renewable resources rather than fossil fuels in the production of commodity products, liquid fuels and electricity. The socio-economic challenges need to address: 1) the complex systems of policies and regulations in different countries and make them more compatible; 2) the environmental impact of biomass removal; 3) the pressure from environmental groups on policy makers; and 4) the unstable commodity and fuel prices. Barriers, that can impact on IFBR development projects, include regulatory uncertainties, opposition from local communities due to environmental or other local concerns, difficulties securing permits for new technologies or power purchasing agreements, negative effect from new company failures, etc. Policy integration and policy interventions should be carefully considered prior to introduction to avoid undesirable effects on markets and prices of raw materials and end products. The successful implementation of the IFBR will likely require the establishment of strategic collaborations and partnerships with government and industry experts. Different technological, value-chain, commercial and financial partnerships and strategic alliances, depending on the business model and strategy of the IFBR, will have to be created in order to enable risk mitigation and value creation for a successful transformation into a IFBR.²⁷⁷  New ways for integration and cooperation with other industries such as the forest, agricultural and chemical sectors must be identified. The IFBRs are a pivotal milestone in our efforts to implement sustainable, low footprint, environmentally beneficial technologies that meet the global demands for energy and bioproducts through the intelligent use of our renewable resources toward a bio-based economy.

Financial support by the Center for Bioprocessing Research and Development (CBRD) at the South Dakota School of Mines & Technology (SDSM&T), the South Dakota Board of Reagents (SD BOR), the South Dakota Governor's Office for Economic Development (SD GOED), and the U.S. Air Force Research Laboratory (AFRL) is gratefully acknowledged. I am thankful to D. Christopher and K. M. Christopher for the excellent editorial assistance with the manuscript.