- 1.1 Introduction
- 1.1.1 Definitions
- 1.1.2 Value of Major Biorefinery Products
- 1.1.3 Obtaining Pure Bio-Fractions
- 1.2 From Historical Milestones to Modern Operations
- 1.3 Modern Competitive Technologies
- 1.3.1 Sugar and Starch Biorefineries (SSB)
- 1.3.2 Lignocellulosic Biorefinery (LCB)
- 1.3.3 Oil and Fats Biorefinery (OFB)
- 1.4 Early Industrial Biorefining Examples
- 1.4.1 Europe
- 1.4.2 USA
- 1.4.3 Brazil
- 1.4.4 China
- 1.4.5 USSR/Russian Wood Hydrolysis Plants
- 1.5 Future Technologies for Biorefining: Catalysis and Ionic Liquids
- 1.5.1 Recalcitrance Reduction
- 1.5.2 Fractionation of Biomass
- 1.5.3 Thorough Chemical Modification of Biomass
- 1.5.4 Enhanced Analytics
- 1.6 Conclusions
- References
CHAPTER 1: The Biorefinery and Green Chemistry
-
Published:29 Sep 2015
-
Special Collection: 2015 ebook collection , 2011-2015 industrial and pharmaceutical chemistry subject collectionSeries: Green Chemistry Series
J. Mikkola, E. Sklavounos, A. W. T. King, and P. Virtanen, in Ionic Liquids in the Biorefinery Concept: Challenges and Perspectives, ed. R. Bogel-Lukasik, The Royal Society of Chemistry, 2015, pp. 1-37.
Download citation file:
If the living standards of western societies are to be maintained while those of the developing world change at their current tempo, then, because of the depletion of fossil resources and concerns about the environment, the concept of biorefining, following the principles of green chemistry as well as sustainability, will surely become more and more important in the future. This chapter introduces the concept of what biorefineries are and discusses their sustainability, taking into account green chemistry and engineering aspects. As well as a brief history, today's situation in the biorefining industry is covered with several examples. Also, the directions for the future biorefineries are considered.
1.1 Introduction
1.1.1 Definitions
A biorefinery is a facility where different low-value renewable biomass materials are the feedstock to the processes where they are transformed, in multiple steps including fractionations, separations and conversions, to several higher-value bio-based products. Examples of these products can include fibres, food, feed, fine chemicals, transportation fuels and heat. A biorefinery can be formed by a single unit or can combine several facilities targeted for a single purpose that further process products as well as by-products or wastes of combined facilities. In biorefining one can find similarities to oil refining, with the exception that in oil refining the raw material comes from fossil resources. According to the International Energy Agency ‘Biorefineries will contribute significantly to the sustainable and efficient use of biomass resources, by providing a variety of products to different markets and sectors. They also have the potential to reduce conflicts and competition over land and feedstock, but it is necessary to measure and compare the benefits of biorefineries with other possible solutions to define the most sustainable option.’1 Although it is possible to produce the same products in a biorefinery as in an oil refinery, this is not the target, which instead is to produce products which can replace the products from oil refining.
In the development of biorefinery processes, as well as any industrial processes, it is crucial for the future of the Earth that the new processes follow the principals of sustainable development and green chemistry. It is good to remind what these terms really mean.
The term ‘sustainable development’ was famously used by the Brundtland Commission in its report to the United Nations. In the report the term ‘sustainable development’ was defined as, ‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs.’2 In other words, it can be said that we have every right to utilize resources that the Earth provides to us for our needs as long as we make sure that future generations have the same possibility. The United Nations Millennium Declaration identified principles and treaties on sustainable development, including economic development, social development and environmental protection.3
‘Green Chemistry’ is a term which is often applied when chemistry and chemical processes are defined as environmentally benign. Paul Anastas and John Werner developed and introduced widely accepted 12 principles of Green Chemistry. The following list briefly presents the principles which, if followed, would make chemical processes or products greener.4
Prevention: it is better to prevent waste than to treat or clean up waste after it has been created.
Atom economy: synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
Less hazardous chemical syntheses: where ever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
Designing safer chemicals: chemical products should be designed to affect their desired function while minimizing their toxicity.
Safer solvents and auxiliaries: the use of auxiliary substances should be made unnecessary wherever possible and harmless when used.
Design for energy efficiency: energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
Use of renewable feedstock: a raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
Reduce derivatives: unnecessary derivatization should be minimized or avoided if possible, because such steps require additional reagent and can generate waste.
Catalysis: catalytic reagents are superior to stoichiometric reagents.
Design for degradation: chemical products should be designed so that at the end of their function they break down into harmless degradation products and do not remain in the environment.
Real-time analysis for pollution prevention: analytical methodologies need to further develop to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
Inherently safer chemistry for accident prevention: substances and the form of a substance used in chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.
Based on these Green Chemistry principles, Paul Anastas and Julie Zimmermann have also developed 12 principles of Green Engineering which should be kept in mind when developing new processes. The principles are briefly listed here, but more detailed information can be found from their publication.5
Inherent rather than circumstantial.
Prevention instead of treatment.
Design for separation.
Maximize efficiency.
Output-pulled versus input-pushed.
Converse complexity.
Durability rather than immortality.
Meet need, minimize excess.
Minimize material diversity.
Integrate material and energy flows.
Design for commercial ‘afterlife’.
Renewable rather than depleting.
1.1.2 Value of Major Biorefinery Products
1.1.2.1 Fibre Versus Chemicals
A major question concerning the definition of a ‘biorefinery’ comes when you consider existing fibre-lines and pulping technology. Is the modern kraft or sulfite mill to be considered as a biorefinery? If so, modern kraft mills can already be considered as ‘green biorefineries’.
The more modern mills use relatively harmless chemicals.
Most of the materials are recycled so the net consumption of sulfur or caustic is very low.
Importantly, the process is aqueous-based.
With current totally chlorine free (TCF) bleaching stages, the production of chlorinated compounds in waste waters can be eliminated.
Release of sulfides can also be minimized.
However, developing a green-field site with the most modern technology is a billion euro investment. The main product from kraft pulping is fibrous pulp. Bleached European softwood kraft pulp has a current value of ∼0.8 € per kg. The price has been known to fluctuate within an approximate range of ±0.5 € per kg. Softwood kraft, in particular pine and spruce, are most valued due to the long fibre length. Dissolving pulp, produced through the sulfite pulping process or pre-hydrolysis kraft pulping, is even more valuable but is typically used for chemical production, e.g. cellulose acetate, carboxymethyl cellulose (CMC) and other esters or ethers. Therefore, this nicely fits the definition of a biorefinery. The fibrous properties of kraft pulp are the basis of its value, with high-quality graphical papers being the largest market for this pulp.
However, over the last 10 years demand for graphical papers has decreased, in particular in North America and Europe (i.e. the ‘saturated’ markets) and this lowers the value of pulp. This is due to competition mainly from China and Brazil and the use of graphical paper in general has decreased. This makes the use of fibrous biomass for production of energy and chemicals more attractive, despite the lower cost structure. In addition, unstable oil prices, global warming and eventually international commitment by governments to increase biofuels share in the transportation sector (EU 10% by 2020) has resulted in strong demand for bioethanol from biomass.
This is also reflected in the growing market price of bioethanol over the last 10 years, i.e. from about 0.3 € per kg to about 0.6 € per kg.6 This is uncertain to continue in the long term, as crude oil prices are currently low (at the time of writing), which will decrease bioethanol demand, and will also decrease demand from applications other than biofuels, i.e. as platform chemical for synthesis of green chemicals (diethyl ether, ethylene). Ethanol is a high volume bioproduct; however, its market value is considered low-to-average versus specialty chemicals from biomass, importantly in comparison to chemical pulp.
1.1.2.2 Bulk Chemicals
The production of bulk chemicals (>1000 tonnes per year) from biomass remains rather limited as the majority of organic chemicals and polymers are still derived from fossil-based feedstocks, predominantly oil and gas.7 Hence, most of the bulk chemicals originating from biomass do not show any dramatic increase in market value. However, a steady increase in demand is reported for lactic acid at 10% annual growth rate.7 Lactic acid can be converted, e.g. to polylactic acid (PLA). PLA is mainly used in production of sustainable biopolymers for use in the packaging industry (thin films) but is also found in applications elsewhere. According to reports, European demand for PLA is currently 25000 tonnes per year and could reach 650000 tonnes per year in 2025.7 Furans derived from biomass, such as furfural derived from pentoses and 5-hydroxymethylfurfural (HMF) derived from hexoses, are one major platform feedstock of interest. Furfural has already been produced on an industrial scale for almost 100 years. The first industrial process for its production was by the Quaker Oat company where it was discovered that furfural could be obtained by sulfuric acid-catalysed dehydration of their oat hull stockpiles.8 A multitude of applications ensued. Nowadays, China produces the majority of the capacity, much of it from grass-based waste material. Most of these producers are dedicated towards furfural production as the main product. There are many estimates on the production of furfural, but they typically range between 300000 and 800000 tonnes per year and several smaller producers produce furfural as a secondary product. For example, Lenzing AG produces furfural on a 5000 tonnes per year scale during their pulping of beech wood.9 The main product for this process is their cellulosic pulp destined for textile production and only about 1% of the dry mass of the wood is converted to furfural. The market price of furfural ranges from roughly between 0.5 and 1.5 € per kg and, not surprisingly, the prices are lowest in China. Hydroxymethyl furfural (HMF) by contrast, is not yet produced industrially due to the difficulty in accessing hexoses, difficulties in isolation as well as the instability of it at the process conditions. Most hexoses are bound up in softwood, which is much more recalcitrant than the abundant pentoses in grass species. Advanced techniques and enabling technologies, such as ionic liquids, are now required to allow us to access and selectively convert these saccharides whereby applications of both furans are likely to be wide ranging. Sequential catalytic dehydration, hydrolysis, hydrogenation and hydrogenolysis steps can be applied to convert them into a wide range of commodity chemicals, into potential biofuels and solvents.
One example of biofuel production is the Sylvan process (Figure 1.1).10 This process involves the hydrogenation of furfural to 2-methylfuran, dimerization or trimerization of 2-methylfuran and hydrogenation/hydrogenolysis to the fully saturated alkane.11
The sylvan process: production of biodiesel from pentose dehydration, furfural hydrogenolysis, 2-methylfuran (sylvan) trimerization and final hydrogenation.11
The sylvan process: production of biodiesel from pentose dehydration, furfural hydrogenolysis, 2-methylfuran (sylvan) trimerization and final hydrogenation.11
This product can be used as a high-quality paraffinic diesel, but the cost is rather high and so it will likely only find immediate access for high-end engines. In general, the cost of furfural is still too high compared to crude oil to have utility as a fuel precursor. Thus, large process improvements need to be made to access fuel markets. Solvents, however, can have a high price. It has been suggested that 2-methyltetrahydrofuran (2-MTHF), accessible from hydrogenation of furfural, has potential to replace THF in certain applications and is a potential biofuel itself. gamma-Valerolactone (GVL) is also now being studied intensively as a media for the conversion of polymeric pentoses and hexoses into oligomers and monomers.12 Thus, the media can be derived from the biomass feedstock. GVL can itself be also be converted into liquid transportation fuels (Figure 1.2).13
Biofuel production by acid-catalysed biomass degradation in GVL:H2O. GVL itself can be derived in the reaction sequence by fully hydrolysing xylan, dehydrating xylose to furfural, hydrolysing furfural to levulinic acid, cyclizing and then hydrogenating levulinic acid to GVL.11,12
Biofuel production by acid-catalysed biomass degradation in GVL:H2O. GVL itself can be derived in the reaction sequence by fully hydrolysing xylan, dehydrating xylose to furfural, hydrolysing furfural to levulinic acid, cyclizing and then hydrogenating levulinic acid to GVL.11,12
This media unfortunately cannot access bulk hexoses bound up in softwood at lower temperatures. Increased temperatures do allow for almost complete solubilization of hexoses but lower temperature treatments do allow for fractionation of birch sawdust, resulting in high-purity cellulosic pulps.14
1.1.2.3 Specialty Chemicals
Specialty chemicals from biomass are sold at relatively high prices (>10 € per kg) due to their limited production (<1000 tonnes per year), ‘green credentials’, unique quality and properties. Demand for specialty chemicals by the industry (pharmaceutical, cosmetics, food sector) is growing stronger and so is their market value. Specialty chemicals fit well to specific, so-called ‘niche’, markets.
An example is Borregaard’s vanillin produced by oxidation of sulfite lignin. The company has established a profitable business from vanillin as it is the exclusive producer and supplier of wood vanillin to the food industry. Vanillin derived from petrochemical sources is sold at much lower market prices.
1.1.3 Obtaining Pure Bio-Fractions
One of the major challenges in industrial biorefining is related to the need of fractionation and purification of the heterogeneous biomass streams to be processed; in fact, a majority of industrially and techno-economically feasible routes to chemicals, fuels and bio(composite) materials depend on the availability of pure, non-contaminated raw material fractions, whether those are carbohydrates, their polymers, fats and oils, various fragrances, nutraceuticals, extractives from lignocellulose or lignin. Further, the existence of various ash-elements (inorganic minerals) further complicates the task.
Throughout the years many processes were developed to facilitate the fractionation/separation targets: among others, the early acidic sulfite pulping, the alkaline sulfate pulping, mechanical pulping, various organosolv processes. More recently, ionic liquid or deep eutectic facilitated processes for biomass pre-treatments, fractionations, cellulose as well as hemicellulose separations and manipulations have all shown their potential in providing fractionated biomass components for further use. Indeed, if performing a simple SciFinder® search for the key words ‘ionic liquid & biorefinery’, this search string gives the very first paper as being published in 2008 with already 10 papers by the year 2014. From the early technologies based on (expensive) alkyl-imidazolium systems, the field has seen the rise of new concepts based on switchable ILs, strong organic bases and acid gases, the application of distillable ILs, as well as the use of bio-based, low-cost ILs, mixtures of ‘cooperative’ ionic liquids and deep eutectic solvents (DES), also in conjunction to radiofrequency and microwave heating or acoustic cavitation.15–17 The systems are not easily understood but the hydrogen bond basicity of the ionic liquid–water mixtures apparently relates to cellulose dissolution, lignin depolymerization and even to sugar yields obtained.18–38
In addition, liquid CO2 (sub- or super-critical), gas-expanded liquids or simply water under relatively harsh conditions (often near-critical) offer other alternative processing possibilities.39,40 Nevertheless, it is estimated that around 60–80% of the processing costs are still related to the separation steps.41
1.2 From Historical Milestones to Modern Operations
Early biorefining has its roots deep in the Nordic forests where many small-scale production facilities for tar, a sticky crude oil-like product with a complex chemical composition, were erected. Wood tar was particularly sought after by the rapidly expanding maritime industries of the 18th century to impregnate the (wooden) vessel hulls, manila ropes, etc. against rotting. Especially Finland and Sweden capitalized on this lucrative export product produced via a kind of slow pyrolysis of pine stumps and fatwood. At the same time, charcoal, ideally suited as a fuel for steam engines, was getting more attractive. Both tar and charcoal were initially produced in primitive tar pits and charcoal kilns. Still, in the early days, a third industrially important product was potash (soluble potassium salts). Potash was produced from birch (Betula pendula) ash and used in the production of glass and soap. It was exported in large amounts until sometime in the mid-19th century when the lack of birch trees saw to the downfall of this industry. In fact, timber and sawn wood products took over as more important export products only sometime in the middle of 19th century.15
As indicated, the saw-mill industries grew during the first half of the 19th century, soon giving way and allowing the growth of the pulping industry. In the beginning, the pulp mills were a way to increase the value of the saw-mill operations by taking advantage of the waste (saw dust) and under-dimensioned wood fractions.42 After the invention of the paper machine, the pulping industry grew in relative importance, a trend lasting until the late 20th century. In general, the paper and pulp industry has during the recent years been facing harsh competition and declining profitability. The underlying reasons are obvious: declining home markets in the western developed economies (shrinking consumption of paper); overcapacity in various paper qualities; emergence and development of pulping in Latin America and Asia (particularly from eucalyptus); lack of vision for the industry’s future directions; and resistance to change. The once so lucrative business in refining coniferous trees to high-quality paper was until recently considered the most feasible and rational use of Nordic softwood. However, signs of changing attitudes are around: besides sawmills as well as paper and pulp combines, many other new or transformed industries use increasing amounts of lignocellulose from wood.
Here, a short overview is compiled describing the changes and trends occurring today in Fennoscandia. This area was once a ‘paper-belt’, in future perhaps again it will be known as a ‘bio-belt’ for its biorefineries – as well as other areas of the world where sustainable and biodegradable chemicals, transportation fuels, and materials are co-produced along with carbon dioxide neutral heat and electric energy.
In particular, managed forests possess a vast resource and potential as sources of biomaterials, chemicals and energy. Also, one of the potential tools enabling increased utilization of woody biomass is presented by careful selective breeding as well as cultivation of new generations of trees with improved genome. Nevertheless, particularly Europe is notoriously frightened by the GMO plants – and not for no reason. For instance, the Nordic boreal forests and rest of the sensitive nature took a long time to evolve, adapt and thrive; if we were to risk that balance overnight (like is already happening due to unpredictable weather chances evidenced by us living in the area), something irreversible might happen. Let us alone consider the currently popular eucalyptus plantations (originally native to Australia and transplanted and gene engineered around the more temperate climate zones of the world) which might end up as deserts in a few decades since the species is well equipped to siphon the aquifers with its deep roots. Indeed, even the imminent threat of climate change might give rise to some unprecedented complications since the domestic fauna has throughout millennia slowly adapted to the current climatic conditions. If the climate changes dramatically and rapidly, the current species are perhaps not any more the optimal ones as a source of raw materials to the industry. Nevertheless, chemical pulping has a long history in Nordic countries but it is difficult to find original literature from earlier times written in any other language than Swedish (such as ‘Ur de stora skogarnas historia’ by Bertil Boëthius, 1917, or ‘Kungliga Vetenskapsakademiens Historia’, 1967) which sheds light to the debates of 18th century about forest and forestry.43
Later on, we saw the rise of sawmills and chemical pulp mills based on the so-called sulfite pulping. The acidic sulphite mills, nowadays an increasingly rare approach compared to the dominant alkaline sulfate or ‘kraft’ mills, are characterized by their high flexibility in terms of fine-tuning the cooking procedure (in batches, contrary to the modern kraft mills that operate continuous cookers), although one was not historically able to solve some critical issues related to this process.44 Pine (Scots pine) was not a suitable raw material, thus limited to spruce (Norway spruce) in the Nordic area. Moreover, the environmental concerns related to the process resulted in serious problems with the air quality (sulfur dioxide), as evidenced by my parents who on a holiday trip through northern Sweden in the year of 1963 entered the town of Örnsköldsvik and still remember it as ‘that place where blue-green smoke that made you cough loomed around and caused a throbbing headache’ – as well as in the local fjord stretching to the sea: the pollution from the mill (the BOD-load and release of cooking chemicals) rendered the nearby waters of the Gulf of Bothnia nearly lifeless for a long period of time. How a complete change is possible as evidenced by the sustainable operations of the today mill (Aditya Birla Domsjö, see later).
The work horse of today’s pulping industry is a kraft pulp mill. The yield is only about 50% because most of the hemicelluloses and almost all the lignin dissolve in the aqueous pulping stream, called black liquor. Typically, black liquor is combusted for steam and electricity generation in the recovery boiler and, at the same time, the dissolved inorganic cooking chemicals are recovered in the reductive environment of the boiler and recycled for pulping. Since the heating value of the carbohydrates is rather low (much less than that of lignin), combustion of dissolved hemicelluloses does not constitute an optimal economical use of this resource. In the older sulfite process, hemicelluloses were frequently recovered and fermented to lignocellulosic bioethanol. Already in 1908, ‘diplomingenjör’ Gösta Ekström filed a Swedish patent application concerning recovery of sulfite alcohol from a sulfite process, although a rival, ‘civilingenjör’ Hugo Wallin had filed a competing patent application in 1907. The fight between the two gentlemen and other interest groups went on for years until, in 1917, both of the patents were declared not valid! Finally, in 1919, both patents were again declared valid after vigorous negotiations.
More recently, a boom in bioenergy, partially enforced by the new European Union regulations and political incentives, has given birth to numerous enterprises producing wood pellets (combusted both at small family homes and centrally for district heating for heat). The local energy companies in many towns have built biomass-combusting power stations that supply the grid with electricity, that supply the towns with district heating and, sometimes, even supply nearby industries with steam.
Another example of an industry in transition is presented by the Chemrec company45 who in collaboration with academia, research institutes,46 the Smurfit Kappa pulp mill47 and Volvo Trucks48 developed ‘From Wood-to-Wheel’ black liquor gasification technologies for methanol and Bio-DME (bio-dimethyl ether) to be utilized as green, renewable diesel fuel for trucks.
At this site, the Piteå Science Park,49 another company, Sunpine,50 in collaboration with a classical petroleum refining company, Preem AB,51 has launched its crude tall diesel process integrating the tall oil production from the adjacent pulp mills to the petroleum-based transportation fuels.
In other parts of Fennoscandia, the Borregaard company52 – in its original location in Sarpsborg, Norway, advertising itself as ‘the world’s most advanced biorefinery’ – is a representative of another ‘sulfite mill that didn’t die’ since the focus is on biomaterials, biochemicals and bioethanol, instead of pulp for the paper industry.
In Finland, Varkaus, a demonstration-scale unit was erected and operated for a long period of time for black liquor gasification by NSE Biofuels,53 producing diesel-type waxes ready to be supplied into the existing refining processes in an oil refinery. Unfortunately, this operation is now discontinued, presumably due to the very large investment costs required to construct full-scale industrial operations.
Chempolis Oy54 is a small, entrepreneurship-generated company located in a Northern Finnish town of Oulu and primarily marketing a biorefining concept based on chemical pulping taking advantage of formic acid. Nevertheless, the company has also a demonstration-scale and R&D facility in Oulu.
The company ST155 has developed concepts utilizing waste materials as the source of bioethanol which is blended with regular gasoline, for example to E85 – a blend with 85% ethanol and 15% gasoline. The production plants are modular and fully automated, thus enabling utilization of local, municipal waste fractions.
A lesser known player is the company Sybimar Oy,56 who builds and markets technology (besides selling renewable oils) for utilizing various waste fractions, such as fish processing industry wastes to bio-based oils usable either as heating oil or as a raw material for further refining.
In terms of early chemicals production, the utilization of wood hemicelluloses via fermentation to ethyl alcohol upon sulfite pulping saw its rise and fall before and after the 2nd World War: in particular at that time on the site of the ‘Domsjö’ pulp mill in Sweden today,57 a whole range of chemical production was based on ethanol chemistry. At best, dozens of chemicals (well over 50 of today’s bulk chemicals) were produced to compensate for oil that was not available.44
Today, other examples of obtainable specialty products produced from side-streams of pulping operations are sodium lignosulfonate (e.g. a dispersant), xylose (the raw material of xylitol,58 a sugar alcohol and alternative sweetener with nutraceutical properties) and vanillin (a flavouring agent).52 Further, turpentines and tall oil have since long been utilized not only as a source of energy (combustion to energy) but also as components in varnishes, paints and other chemicals,59,60 including more recent uses as raw materials for renewable diesel synthesis.61
Around the same time as the Swedish and Finnish biorefining efforts were starting, in Norway, the Borregaard company also introduced sulfite-based ethyl alcohol production and has since then introduced many other refined products from lignocellulose.
In Switzerland, Attizholts closed its mill in 2008. Georgia-Pacific in the USA closed in 2001 whereas in Canada Tembec is still in operation. Further, several Russian mills used to operate until recently but the current status is unclear.62
Biogas – or biomethane – is rapidly gaining momentum as yet another, partial, solution for the oil-deficient future of ours. Besides that, it can be used as a direct energy source (combustion with subsequent co-production of electricity and heat). It is entirely feasible to use it as a transportation fuel, both in gasoline and diesel engines after gas purification and appropriate modifications to the fuel supply systems of the vehicles. Perhaps the biggest challenge is the purification: various technologies aiming at competitive, small-scale economics are under development. Typically, the technologies are based on physical or chemical (or combined) binding of CO2, the major by-product from anaerobic digestion of waste. Indeed, various lignocellulosic streams as well as municipal (e.g. communal waste water treatment plants) or industrial waste (e.g. dairy industry) can be used as a feed. The most challenging gas sources are, perhaps, the multiple waste dumps established throughout the years that can contain an astonishing mix of dumped industrial wastes and degradation products of chemicals long ago banned in developed countries. In these gases, besides carbon dioxide, many other toxic and dangerous components detrimental to health and internal combustion engines are found.
In the Jyväskylä area of Finland, a small, entrepreneur-driven facility, Metener Oy,63 is operational and selling vehicle-grade biogas derived from cattle manure. In addition, co-production of electricity and heat takes place on site. The company also sells biogas digesters and gas purification equipment. However, this is just one example. In particular throughout the European continent many biogas plants are operated.
The textile industry of today is facing enormous challenges since cultivation of the classical raw material, cotton, is associated with a huge water demand and extensive use of pesticides. Moreover, today most of the cotton farming takes place in third-world countries with dubious labour ethics and lack of water.64,65 Clothing is needed throughout all human societies; however, recently more people are raising the question whether we already have passed ‘peak cotton’ (analogous to ‘peak oil’) since the analysis of data shows that the market for the specialty cellulose is booming and the worldwide supply of cotton has fallen. Dissolving cellulose refers to high-quality cellulose with a very low hemicellulose content that is classically processed further by various spinning technologies to form fibres that can be used for textile weaving.66
In today’s world, many biorefinery operations have been commenced throughout the world. These include Neste Oils NextBtl® renewable diesel and jet fuels,67 the two operating production lines in Finland and two renewable diesel plants operating in Rotterdam (the Netherlands) and in Singapore. There are also the Brazilian and US efforts towards bioethanol particularly, albeit heavy criticism has been directed towards the use of corn as an ethanol source that merely results in increased greenhouse gas emissions.68–70 More actors are constantly moving into this environment and one of the latest newcomers is the small BioEndev company71 commercializing torrefaction technology. Other examples are represented by the Roquette operations in Lestrem, France, albeit the mill suffers from the same ethical dilemma as many operations in the American continent – the use of edible crops for the production of fuels and chemicals.72 Nevertheless, a recent US opening introduced a cellulosic biorefinery in Hugoton, Kansas, based on the use of corn stover residues advertised to exceed the capacity of the GranBio facility in Alagoas, Brazil, operating on bagasse and straw wastes.73 In Europe, many companies and countries have operations either aiming at demonstrating or as commercial production in line with the biorefinery philosophy. Examples are the Portucel Soporcel Group or the Respol Group in Portugal that is actively pursuing further development in the area,74,75 as well as the Austrian focus on mainly grassland and agricultural residues,76 the German very holistic view77 or the many Chinese efforts. The reader should turn to the many published reviews, papers and reports to obtain more detailed information and by no means do these examples cover all past and ongoing efforts towards a more sustainable future of tomorrow.
1.3 Modern Competitive Technologies
Biorefineries can be classified into three broad categories on the basis of biomass chemistry: sugar and starch, lignocellulose and triglyceride biorefineries.
1.3.1 Sugar and Starch Biorefineries (SSB)
This type of biorefinery can utilize a wide range of sucrose-containing feedstocks (sugar beet, sugar cane, etc.) and starchy biomass (corn, wheat, barley, maize, etc.) to produce bioethanol. Bioethanol is a promising biofuel when mixed with gasoline but is also a useful platform chemical for synthesis of diethyl ether, ethylene, etc. At present, commercial ethanol production is predominantly based on edible sugar and starchy biomass; for instance, sugar cane in Brazil, corn grains in USA and wheat and sugar beets in European Union countries. The use of edible feedstocks and large areas of arable land for biorefining operations is strongly debated by many as it competes with food production.
The main steps involved in the production of ethanol from sugar and starchy feedstocks (SSF) are shown in Figure 1.3.
Pre-treatment; needed only for lignocellulosic (sucrose containing) feedstocks. Pre-treatment by different methods removes most of the lignin and hemicellulose, increases porosity and disrupts the crystalline structure of cellulose.
Enzymatic hydrolysis of lignocellulosic and starchy biomass (after milling) by cellulases–hemicellulases and amylases, respectively.
Fermentation of the hexose-rich hydrolysate from starchy biomass by naturally occurring yeasts (usually Baker’s yeast). The hydrolysate obtained from lignocellulosic feedstocks is more complex as it contains both pentose (xylose and arabinose) and hexose sugars (glucose, galactose and mannose). The fermentation of pentose sugars is challenging and costly as only a few yeast strains are available for fermentation to ethanol.78
Sugar and starchy biorefinery (SSB): process scheme for the production of ethanol from (a) sugar cane (b) dry ground corn.
Sugar and starchy biorefinery (SSB): process scheme for the production of ethanol from (a) sugar cane (b) dry ground corn.
1.3.1.1 Current SSB Biorefinery Examples
A typical sugar and starch-based biorefinery is the ‘Les Sohettes’ complex located in Pomacle, France.79 The biorefinery comprises of a sugar beet processing unit, a wheat refinery and a sugar plant, an ethanol distillery (Cristanol), a research centre (ARD), a demo-plant for second-generation ethanol (Futurol), a straw-based paper production pilot unit (CIMV), and a succinic acid pilot plant (BioAmber). Succinic acid is a useful platform chemical for the production of polyurethanes, coatings, adhesives, sealants and personal care ingredients.
This facility is a good example of integration in the product networks as two major crops are being used as feedstocks: sugar beets and wheat. The combination of both crops allows year-long biorefining operations as the harvesting period of sugar beet is rather short (typically a few months in a year) and this would render sugar beet only production of ethanol and other bioproducts uneconomical.
Cargill Inc. has been operating a corn biorefinery since 2002 (Nebraska, USA).80 This integrated biorefinery processes corn to produce corn oil, sugar, ethanol, lactic acid and polylactic acid (Natureworks LLC). Polylactic acid is a natural biopolymer that can be used to produce biodegradable films and plastic products.
DuPont Tate & Lyle BioProducts is producing 63000 tonnes per year of 1,3-propanediol (1,3-PDO) from corn in their Loudon plant in Tennessee, USA (2006).81 In fact, 1,3-PDO is a key building block for producing polypropylene terephthalate which is used as biopolymer film.
1.3.2 Lignocellulosic Biorefinery (LCB)
The lignocellulosic biorefinery uses naturally dry biomass such as cellulosic biomass (wood energy crops) and agriculture waste to produce biofuels and other bioproducts. Woody energy crops are fast growing hardwood trees that are harvested within 5–8 years of planting. These crops include poplar, willow, silver maple, eastern cottonwood, black walnut, sweetgum, etc. and they are traditionally used for manufacture of paper and pulp. Agricultural wastes include sugar cane bagasse, corn stover (stalks, leaves, husks and cobs), wheat straw, rice straw, rice hulls, nut hulls, barley straw, olive stones, etc. Unused sawdust, bark, branches, and leaves/needles that are produced during processing of wood for bioproducts or pulp are also included in this category of wastes. Animal manure and municipal and industrial wastes are another potential source of biomass for LCB.82
In an LCB the raw biomass is first cleaned, pre-treated to improve accessibility of sugars for subsequent processing and then broken down into its primary constituents (cellulose, hemicellulose and lignin) through biochemical (enzymatic hydrolysis, Figure 1.4a) or chemical (acid hydrolysis, Figure 1.4b) routes. The cellulose and hemicellulose are converted to monomeric sugars. The glucose obtained from hydrolysis of cellulose is further converted to valuable products such as ethanol, acetic acid, acetone, butanol, succinic acid, etc. via fermentation. The fermentation step is typically performed by yeast. Another alternative is to use bacteria to produce biobutanol and other chemicals via ABE fermentation. This approach is more advantageous if the hemicellulose hydrolysate is fed simultaneously to fermentation. However, a requirement is that the hemicellulose hydrolysate is free of fermentation inhibitors. Soudham et al.83 and Sklavounos84 have reported efficient methods to detoxify acidic hydrolysates from wood for fermentation. Except alcohols, the LCB produces furfural from xylose. Lignin is typically used as an adhesive or binder and as a fuel for direct combustion.82
Lignocellulosic biorefinery (LCB): basic schematic process flow diagram for the production of ethanol and butanol via (a) the biochemical route and (b) the chemical route.
Lignocellulosic biorefinery (LCB): basic schematic process flow diagram for the production of ethanol and butanol via (a) the biochemical route and (b) the chemical route.
The major advantage of LCB is that it does not use edible feedstocks and therefore it eliminates the need of sacrificing arable lands. However, commercialization of the chemical and biochemical LCB is currently limited due to technological constraints and high processing costs (pre-treatment, detoxification and enzymatic hydrolysis steps).
1.3.2.1 Current LCB Biorefinery Examples (Chemical and Biochemical Processing)
Borregaard’s integrated biorefinery in Sarpsborg, Norway, is a special case of a pulp mill that has gradually evolved to an LCB. In this biorefinery spruce chips are treated with acidic calcium bisulfite cooking liquor. Hemicellulose is hydrolysed to monomeric sugars during the cooking process. After concentration of the spent sulfite liquor (SSL), the sugars are fermented and ethanol is distilled off in several steps. The Borregaard integrated biorefinery is particularly successful as it can produce cellulosic ethanol but also a whole range of bioproducts.85 The latter include vanillin from lignin and lignosulfonates. Vanillin is a high value flavouring agent with numerous uses in the food industry whereas lignosulfonates are used in many special applications, i.e. as dispersants, in paints, oil drilling agents, etc. The company is currently running a demonstration plant that showcases its latest concept – called the ‘BALI® biorefinery’.85,86 This plant can handle sugar cane bagasse, straw, wood, energy crops and other lignocellulosics to produce ethanol and lignin specialty chemicals.
GranBio recently announced (2014) start-up of its 2nd generation lignocellulosic ethanol biorefinery in Alagoas, Brazil.73 The facility is one of the largest ethanol biorefineries in the world. This biorefinery uses straw and bagasse as feedstocks. GranBio’s facility employs feedstock pre-treatment technology from Beta Renewables, enzymatic hydrolysis and fermentation by yeast.
Abengoa’s biorefinery in Kansas, USA, also began operation in 2014. The biorefinery is built with a planned capacity of up to 75000 tonnes per year ethanol. Abengoa’s facility utilizes agricultural crop residues (such as stalks and leaves) that do not compete with food or feed grain.87
A recent (2012) development in Europe is the Beta Renewables biorefinery in Crescentino, Italy. This biorefinery uses straw and energy crops (giant reed) to produce about 60000 tonnes per year ethanol.88
CIMV built its first pilot plant biorefinery in Pomacle, France in 2007. The facility processes 50–70 kg h−1 of hardwood, straw and bagasse.89 The feedstocks are processed by the organosolv (ethanol–water) method. The produced pulp is then enzymatically hydrolysed and released sugars are fed to fermentation. The plant produces lignin (Biolignin™), C5 sugar syrup, paper grade cellulose pulp, bioethanol and chemicals.90 The company is planning to commercialize its technology in 2015.
Finnish energy company St1 is planning start-up of a new bioethanol plant in Kajaani, Finland (capacity of 10 million L per year) in 2015. The plant will produce bioethanol transportation fuel from sawdust, which comes as a side-product of the sawmill industry. The company opened its first ethanol plant in 2007 and currently has seven plants in Finland producing ethanol from biowaste and food industry residues using its Etanolix® technology.91
An alternative approach to chemical and biochemical processing of lignocellulosic biomass are the thermochemical conversion processes of gasification and fast pyrolysis.
1.3.2.2 Gasification
Gasification of biomass generates synthesis gas (syngas), heat and electricity. In gasification, the biomass is converted to a combustible gas mixture of H2, CO, CO2, CH4, N2 (for gasification with air) and traces of higher hydrocarbons in the temperatures range of 800–900 °C.92,93 The gasification is a combination of pyrolysis and partial oxidation. The heat required for endothermic pyrolysis is generated by partial oxidation of biomass using air (most common) or oxygen. However, the former technology suffers from drawback of low heating value (4–7 MJ m−3) of resulting synthesis gas that limits its application for boiler, engine and turbine operation only.94 Biomass gasification by oxygen has potential to produce syngas with an improved heating value (10–18 MJ m−3); however, the economics favour use of hydrocarbons, i.e. natural gas and inexpensive coal, as feedstock.92
A key drawback of syngas from biomass gasification is its composition, as it is rich in tars and methane. Tars affect gasification efficiency and foul processing equipment whereas the presence of methane makes syngas unsuitable for Fischer–Tropsch (FTS) synthesis. It is, however, technically possible to overcome these barriers, i.e. by operating the gasifier at a slightly lower temperature with use of suitable catalysts. Today there are very few commercial biomass gasifiers operating without government support or subsidies. Most of them are used for power generation only.82 Downstream catalyst poisoning is also a major issue, requiring multi-step gas purification stages if the syngas is to be used as synthesis gas, i.e. for FTS, and not for energy production.
1.3.2.3 Biomass to Liquid Technology
Biomass to liquid (BTL) is the technology that allows the production of synthetic fuels (gasoline, diesel, heating oil, jet fuel, methanol, dimethyl ether, ethanol, etc.) from biomass-derived syngas using FTS. The low temperature FTS (200–250 °C) is generally used for the production of jet fuel and diesel whereas the high temperature FTS (300–350 °C) is used to produce gasoline range hydrocarbons.82 However, with the exception of producing methanol, dimethyl ether and synthetic natural gas, the BTL technology suffers from poor selectivity to fuel products.95 Moreover, FTS require syngas with a specific H2/CO ratio, which requires adjustment of biomass-derived syngas composition using the water gas shift reaction. As mentioned earlier, impurities in the gas need to be removed prior to FTS, to avoid catalyst poisoning. These impurities are present in the biomass feedstock and can poison catalysts rapidly even at the ppm level. Other drawbacks include high capital investment costs as the scale of gasification complexes is generally very large, by economic necessity.
1.3.2.4 Current LCB-BTL Biorefinery Examples
It is only a few large companies that have managed to overcome the previously described techno-economic barriers. All of them use natural gas (Sasol, Shell Pearl GTL) or coal (ExonMobil, Linc Energy) as the feedstock for FTS to fuels. A recent effort by NSE biofuels, as already mentioned (a joint venture between Neste Oil and Stora Enso) to build a commercial biorefinery in Porvoo or Imatra, Finland, that would produce Fischer–Tropsch diesel from biomass feedstock (planned capacity of 100000 tonnes per year) was abandoned due to the massive investment costs needed. Application for funding under EC’s NER 300 programme was discontinued in 2012.96
Chemrec AB, Sweden, inaugurated the world’s first dimethyl ether (DME) plant in Piteå in 2010.45 The plant produces DME from syngas formed after gasification of black liquor (spent cooking liquor of kraft pulping containing dissolved hemicelluloses and lignin) followed by Fischer–Tropsch synthesis. BioDME is a synthetic 2nd generation biofuel which can be mixed with diesel.
1.3.2.5 Fast Pyrolysis
Fast pyrolysis has better potential over BTL due to its simplicity, lower equipment requirements and hence lower capital investment costs.97 Fast pyrolysis with high heating rate (500 °C s−1) is performed in the absence of oxygen to produce ‘bio-oils’ (highly acidic ‘oils’ with a significant water content) in high yields. Bio-oils are a mixture of more than 300 chemical compounds with considerable variation in physical and chemical composition depending on the type of biomass. They are a potential feedstock for producing chemicals. However, separation of chemical compounds from these mixtures is challenging by fractional distillation or extraction. There are though methods for upgrading bio-oils to get specific types of chemicals in high concentrations (aqueous phase dehydration/hydrogenation or APD/H).98 Bio-oils could also be used as transportation fuels. However, high water and oxygen contents, immiscibility with petroleum fuels, low heating value, poor storage stability and high corrosiveness currently prohibit their use in engines for vehicles. Upgrading of bio-oils for use as fuels is possible, i.e. by steam reforming, hydrodeoxygenation (HDO) and zeolite upgrading. Bio-oils are currently used only for specific applications, i.e. in firing boilers, running turbines and heavy duty diesel engines.82
It is reported that fast pyrolysis is going to be the leading thermochemical biomass processing technology due to its favourable credentials as one can see from recent technological advancement in this area.
1.3.2.6 Current LCB-Pyrolysis Biorefinery Examples
Envergent Technologies (a joint venture between UOP LLC and Ensyn Corp.) started operation of a pyrolysis biorefinery in Les Plains, Illinois, USA, in 2008.99 The facility converts forest and agricultural waste to bio-oils using RTP® (Rapid Thermal Process); a fast thermal process whereby biomass is heated rapidly (for 2 s) in a fluidized bed reactor. Produced bio-oils can then be used for generation of electricity and for the production of process heat. Development is underway to upgrade RTP bio-oils into green gasoline, green diesel and green jet fuel.
KiOR (a joint venture between Bioecon and Khosla Ventures) has developed a catalytic cracking technology to convert biomass into renewable crude oil that is processed into gasoline, diesel and fuel oil blends. The company built the first commercial scale cellulosic fuel facility in Columbus, MS, USA, which started production in 2012.100
1.3.2.7 The ‘Green Biorefinery’
A special type of lignocellulose-based biorefinery is the ‘green biorefinery’. In the context of biorefineries, the term ‘green’, as used here, does not have any reference to ‘green chemistry’ but rather the general colour of the ripe feedstock. This biorefinery utilizes naturally ‘wet’ herbaceous biomass such as switch grass, Miscanthus, wheatgrass, reed canary grass, alfalfa hay, etc. and can produce a wide range of products, i.e. biogas, electricity, novel biomaterials (fibre-reinforced plastics, insulation materials) and fertilizers by employing mechanical means to fractionate biomass into a protein-rich liquid (green juice) and a solid fraction (press cake). The green juice is treated by biotechnological methods (fermentation) towards the production of lactic acid, amino acids, ethanol and proteins. The press cake can be used for the production of green feed pellets, as raw material for the production of chemicals (i.e. levulinic acid) and for conversion to syngas and hydrocarbons (synthetic fuels).76
Biowert in southern Hessen, Germany, has operated an industrial grass biorefinery to produce high-quality cellulose fibres for many applications since 2005.101
1.3.2.8 The Tall Oil Biorefinery
A tall oil biorefinery uses crude tall oil (CTO), which is a residue of the kraft pulping process. CTO is derived from rosin and fatty acids, which occur naturally in wood used for pulping. These acids are converted into corresponding sodium salts by the caustic conditions in kraft cooking. These salts are suspended in the spent black liquor from the kraft cooking and are referred to as ‘soap’. The quantity of tall oil soap varies according to wood species, geographical location, season of the year and wood storage practices. For example, the typical CTO yields in Finland are 40–50 kg per tonne of softwood pulp and about 20 kg per tonne of hardwood pulp. Pine and in particular Scots pine (Pinus sylvestris) affords the highest yields of CTO. Pine is also highly prized for the long length of the kraft fibre resulting from pulping. CTO consists of around 30 to 50% fatty acids, 15 to 35% rosin acids and 30 to 50% pitch, a bio-liquid that is used for energy generation. These fractions are separated by distillation over wide pressure ranges and they are marketed as wood-based chemicals for use in many applications, such as paper sizing agents, dispersants and surfactants.
1.3.2.9 Current Tall Oil Biorefinery Examples
Forchem Tall Oil biorefinery59 is located in Rauma, Finland, and its annual distillation capacity is approximately 175000 tonnes per year of CTO from pine. The biorefinery produces Tall Oil Fatty Acid for use as raw material for many chemical reactions and intermediates. It is also produces Tall Oil Rosin which is used as raw material in adhesives and tackifyers (glues). Distilled Tall Oil, a complex mixture of mainly fatty acids and rosin acids (more than 10% rosin acids), is produced for use in many chemical reactions and blended products. The biorefinery also produces Tall Oil Pitch which is sold as low-sulfur content biofuel to be used in communal and industrial boilers. Forchem’s new owner since 2013 is the Portuguese Respol Group which is one of the leading rosin upgraders in Europe.60
Arizona Chemical is the largest producer of pine chemicals in the world.102 The company has been operating a Tall Oil biorefinery in Sandarne, Sweden, since the early 1930s. Today the biorefinery is used to refine and upgrade CTO and Crude Sulfate Turpentine. The latter is also a by-product of the pulp and paper industry. The company sells natural pine-based products to customers in many diverse markets including adhesives, roads and construction, tires and rubber, lubricants, fuel additives, and mining. In 2007, Arizona Chemical was sold by International Paper to the private equity company Rhône Capital to enable the company’s further growth. In 2010, Arizona Chemical was acquired by American Securities, LLC, a US-based private equity firm.
As already mentioned, Sunpine has operated a Tall Oil Biorefinery in Piteå, Sweden, since 2010. The facility has a capacity of up to 10000 m3 per year crude tall diesel. The raw materials used are CTO, acid vegetable oils and methanol. The latter is used for esterification of CTO to biodiesel. Distillation of the crude fraction gives crude biodiesel (which is purified downstream), rosin acids and bio-oil.103
1.3.3 Oil and Fats Biorefinery (OFB)
This biorefinery utilizes vegetable oils and animal fats to produce biodiesel with comparable properties to petrodiesel. The production of biodiesel – as Fatty Acid Methyl Ester (FAME) – is based on transesterification of triglycerides with methanol. The reaction is commonly catalysed by acid, alkali or enzymes, depending on the free fatty acids (FFA) content of the feedstock.
The process of making biodiesel produces glycerol as by-product (∼10 wt.% of biodiesel).82 The glycerol is either etherified with alcohols or esterified with acetic acid to produce ether/esters for applications as fuel additives. Alternatively, glycerol can be converted to value-added chemical intermediates such as 1,2-PDO or 1,3-PDO by reduction and acrolein by dehydration or syngas by steam reforming.82 This versatile feedstock can also be converted to many other chemicals such as epichlorohydrin.
Currently more than 95% of biodiesel is produced from edible oils such as rapeseed and sunflower oil in Europe, soybean oil in USA and palm oil in tropical countries.104 This is a major issue, as the excessive use of edible oils necessitates that large fractions of arable land – that would otherwise be used for food production – is needed to satisfy the increasing demand for biodiesel. The use of low-cost non-edible feedstocks is thus necessary for economic and sustainable production of biodiesel. In addition, many farmers in tropical Asian countries are deforesting virgin forest to allow for the planting of vegetable oil-producing cash crops, such as oil palm (Elaeis guineensis). Therefore the long- and short-term development of vegetable oil sources is hard to justify, from a green perspective.
1.3.3.1 Current OFB Biorefinery Examples
UPM has recently announced (2014) the commercial start-up of its Tall Oil biorefinery in Lappeenranta, Finland. This biorefinery uses lower grade CTO fractions to produce biodiesel at a capacity of 100000 tonnes per year. It is estimated that the produced volumes of biodiesel will cover approximately 25% of Finland’s biofuel target. UPM’s biodiesel (UPM BioVerno™) is produced by hydrotreatment of CTO to modify its chemical structure. Then a fractionation unit removes hydrogen sulfide and incondensable gases. The remaining liquid fraction is distilled to separate biodiesel.
Likewise, Neste Oil, Finland, operates triglyceride- (or oil-) based industrial biorefineries in Finland (Porvoo), Holland (Rotterdam) and Singapore that produce biodiesel (called NExTBTL®) from vegetable oils and fats. The Singapore and Rotterdam refineries are two of the largest triglyceride biorefineries in the world, with similar capacities of about 800000 tonnes per year biodiesel, each. Renewable diesel is produced by hydrogenation of vegetable oils (HVO). Currently about 50% of used feedstock is palm oil.105 In 2011 the company had to face fierce protests by Greenpeace regarding the use of palm oil as a feedstock for its biorefining operations.106 Neste Oil (among others) was buying palm oil from the IOI Group; a Malaysian company allegedly responsible for illegal deforestation.107 Since 2013 Neste Oil claims that 100% of the crude palm oil used for its biorefining operations is certified to be sustainably produced.108
Research at Neste Oil is currently directed at microbial lipid production by using lignocellulosic biomass as the feedstock. Specifically, a process is being developed that involves hydrolysis of biomass into sugar-rich hydrolysates, which are then used by oleaginous microorganisms as the carbon and energy sources to produce lipids. However, the costs of microbial lipids are currently prohibitively high for commercialization (simultaneous saccharification and enhanced lipid production, SSELP, process).109 Also research is currently underway on using microbial oil and algae oil to produce renewable diesel (see below). A dedicated pilot plant has been built at Neste Oil’s Porvoo refinery to study the opportunities offered by microbial oil and the company is involved in a number of international research projects working on algae oil, in Australia and elsewhere.108
BioMCN (Farmsum, Netherlands) uses crude glycerine (residue from biodiesel plants) which is purified, evaporated and cracked to obtain syngas.110 Syngas is used to synthesize methanol. Methanol is an extremely versatile product, either as a fuel in its own right or as a feedstock for other biofuels. It can be used as a chemical building block for a range of future-oriented products, including MTBE, DME, hydrogen and synthetic biofuels (synthetic hydrocarbons).82
1.3.3.2 Algal Biorefinery
A key bottleneck for successful realization of biodiesel is the requirement for large areas of arable for cultivation of oil crops. A promising prospect is the algal biorefinery, which can utilize microalgae as source of lipids for the production of biodiesel. The microalgae grow rapidly (commonly double their biomass within 24 h), they give a high productivity of oils per hectare, have high oil content (up to 80 wt.% of dry biomass with 20–50 wt.% oils being common) and can grow in a variety of aquatic environments.111,112 Microalgae fix solar energy in the form of biomass and oxygen using CO2 and inexpensive growth medium containing water and inorganics. Microalgae do not require arable land and need much less water compared to energy crops. They can be cultivated in fresh water, sea water, lakes, rivers and even waste water from municipal, agricultural and industrial activities. Cultivation on a large scale is performed either in open ponds or enclosed tubular photobioreactors. The latter configuration offers many benefits including higher volumetric productivity.82 There is a lot of academic research on microalgae currently ongoing. For example, research in SLU Umeå, Sweden, is focused on algal cultivation in waste water and flue gases and transformation to bioethanol and biodiesel.113 Laboratory tests showed that microalgae (phytoplankton) grew well on a combination of nutrients from untreated sewage and CO2 dioxide from a thermal (bio) power plant. The algae absorbed CO2 from flue gases and removed up to 90% of the nitrogen and phosphorus in the sewage. They are also able to fix heavy metals. The selected microalgae were found to have a substantially lower ash concentration than marine algae (seaweed), for example, which is important if dried algae are used directly as biofuel. Research is ongoing and there are plans to use yeast, which is capable of converting the carbohydrates in the biomass into ethanol.
Despite having enormous potential, the commercial scale construction of an algal biorefinery has not been realized yet due to the high costs of oil production.114 The collection of microalgae from diluted biomass streams, the dewatering and the extraction of oils are energy intensive and therefore expensive processes. The oil extraction step requires addition of organic solvents (e.g. hexane or chloroform) and very harsh conditions to allow disruption of the thick cell wall that covers the microalgae. Adoption of an integrated biorefinery approach where a multitude of bioproducts (biofuels, platform chemicals, biogas, fertilizers, animal feed) are produced at a large scale could improve economics of the algal biorefinery and lead to its successful commercialization in the near future.
1.4 Early Industrial Biorefining Examples
1.4.1 Europe
The concept of biomass biorefining is not new, as it has been around for many years in the form of paper mills. Two of the oldest paper mills in Europe that are nowadays operating as biorefineries are the Borregaard biorefinery115 in Sarpsborg, Norway, and the Domsjö biorefinery116 in Örnskoldsvik, Sweden.
1.4.1.1 The Borregaard Biorefinery
Production of pulp and paper in Sarpsborg started in 1889, based on the acid sulfite process. In 1938 hemicellulose monosugars (mainly mannose) started to be fermented to ethanol by yeast. Ethanol was intended for use as a solvent and disinfectant. In 1967 when a process line for lignin and vanillin was installed, chemical products were produced from all components of wood (spruce). A wide range of biochemicals were produced during the period 1960–1980. Production of most of them was soon discontinued as they could not compete economically with their petroleum-derived counterparts. Nowadays the company is focused on producing ethanol and specialty cellulose for the manufacture of cellulose derivatives, such as cellulose ethers and esters, acetate cellulose and micro-crystalline cellulose. Borregaard is also one of the leading manufacturers of lignosulfonates and an exclusive producer of vanillin from wood.
1.4.1.2 The Domsjö Biorefinery
The original sulfite mill started operating in 1903 by the then Mo and Domsjö AB in Örnskoldsvik. Manufacturing of viscose pulp began in 1930 and a chemicals plant was developed on the site in the 1940s. Dömsjö is one of the first industrial biorefinery complexes where innovative and environmentally sound measures were implemented in both its processes and waste treatment. It was one of the first facilities in the world to bleach cellulose to the highest degree of brightness in a process totally free from the use of chlorine or chlorine dioxide. Nowadays the Domsjö biorefinery produces dissolving cellulose (255000 tonnes per year), lignin as sodium lignosulfonate (120000 tonnes per year) and bioethanol (14000 tonnes per year). Other products include carbonic acid, biogas and energy. The bioenergy generated by the process is used internally and makes Domsjö virtually independent of fossil energy sources. The raw material consists largely of spruce and pine. Domsjö was acquired by the Indian Aditya Birla Group in April 2011, and is now a member of its pulp and fibre business.57
1.4.2 USA
The United States pioneered the corn industry. In 1844, the Wm. Colgate & Company wheat starch plant in Jersey City, NJ, became the first dedicated corn starch plant in world.117 However, the multi-product corn wet mills appeared in their current form much later – in the 1970s – prompted by the development of commercial technology for the production of high fructose corn syrup for use in the soft drinks industry.118 Early development of other types of industrial biorefining in the US was rather limited; the only lignocellulosic biorefinery in the United States was operated by Georgia Pacific in Bellingham, Washington. The biorefinery produced ethanol from cellulosic feedstock using the sulfite process. This plant operated from 1976 to 2001.119
1.4.3 Brazil
In November 1975, Brazil initiated a program (Pro Alcool) to start the production of ethanol from sugar cane and increase the use of ethanol as a substitute for gasoline. The program was supported by the national petroleum company Petrobras and started partly because of the quadrupling of world oil prices in 1973, and also as a means of assisting the sugar industry in times of low international sugar prices. The program promoted the installation of many ‘mini-ethanol plants’ capable of producing 20000 L per day ethanol from sugar cane and other plants such as cassava. Sucrose from sugar cane or hydrolysed starch from cassava were subjected to fermentation by yeast.120 Bagasse (fibrous residue of sugar cane) was used as boiler fuel and for electricity production to make the ethanol plants energy self-sufficient.
1.4.4 China
China has a long history of biomass biorefining; for instance the ancient Chinese developed soy processing to extract protein for bean curd, to ferment sugars into wine, to produce soybean oil and to make fertilizers for crops (from waste materials).121 Historical examples of biorefining at an industrial scale are the hydrolysis and ABE batch fermentation plants of the period 1960–2000. These plants aimed mostly at producing acetone and ethanol solvents. The raw materials used were corn, cassava, potato and sweet potato. Strong competition from the petrochemical industry (cheap solvents) and the high costs for product purification led to the gradual demise of these plants at the turn of the last century.122
1.4.5 USSR/Russian Wood Hydrolysis Plants
Since the early 1930s, the focus in USSR was on production of ethanol, single cell protein (SCP) yeast and furfural/xylitol. Hydrolysis of softwood/hardwood feedstock was typically performed with weak sulfuric acid (130–150 °C) in one or two steps. Production of ethanol was performed by yeast. A unique facility was the Dokshukino plant, which started operation in 1960. The plant utilized sugars from starch for ABE fermentation by Clostridia bacteria. It was a prime example of continuous fermentation efficiency as it yielded a 20% productivity increase versus batch fermentation and saved 65 kg of starch per tonne of solvents produced.123,124 It was the only industrial continuous ABE fermentation plant in the world.
At least seven more ABE plants were constructed; their design was based on the experience acquired from the Dokshukino facility. These ABE plants remained in operation until the late 1980s.125 All of them were closed down after USSR lost economic power but also because cheaper petrochemical processes became available. A short review on USSR hydrolysis plants by Frölander85 revealed that over the period of 1935–1985, 18 ethanol, 16 SCP yeast and 15 furfural/xylitol plants were in operation. It was also concluded that none of them was profitable without state subsidies because they lacked the integrated approach (multitude of products from a single feedstock). Today there is only one sulfite ethanol plant that still in operation in Russia (Kirov).
1.5 Future Technologies for Biorefining: Catalysis and Ionic Liquids
Clearly the bioprocessing industry has already adopted or is adopting many of the 12 principals of Green Chemistry. However, economics are still the major deciding factor when it comes to running processes. Legislation often only changes when competitive technologies become available that give market control to a limited number of actors. The major developments in green bioprocessing will be facilitated to a large degree by application of the 9th principal: application of catalysts is superior to the use of non-stoichiometric reagents. The Pimentel report ‘Opportunities in Chemistry’126 in 1985 estimated that 20% of the US gross national product was produced through the use of catalytic processes. Long experience in the understanding and development of catalytic processes has now been gained through petrochemical industrial development. This must now be combined with the existing bioprocessing infrastructure to afford selective reactions to high-value products.
An example of this kind of catalytic process applied in future biorefinery can be upgrading of biomass extractives to fine chemicals over heterogeneous catalysts.127,128
Ionic liquids are also seen as an enabling technology. Different biomass components have selective solubility in different solvents. For example, hot water is known to solubilize native (acetylated) hemicelluloses. Dipolar aprotic solvents can also solubilize both lignin and hemicelluloses. However, basic ionic liquids are known to dissolve all three components (Figure 1.5).129–131
Potential for homogenization of biomass using basic ionic liquids.
This therefore allows for the potential for homogeneous processing of biomass, in addition to selective extraction of different components. This opens the door to a wide range of possibilities that were not accessible before.
1.5.1 Recalcitrance Reduction
Recalcitrance reduction is where ionic liquids are used to pre-treat biomass to enhance downstream biofuel and chemical production. Bioethanol production through recalcitrance reduction, polysaccharide enzymatic hydrolysis and fermentation is of major interest. The ionic liquid pre-treatment typically allows for much higher hydrolysis rates and yields, based on the saccharide content of the feedstock. By applying an ionic liquid treatment to lignocellulosic feedstocks the crystallinity of cellulose can be significantly reduced or converted into the more digestible cellulose II. Lignin can also be removed at the same time, which prevents enzyme inhibition and allows enzymes or other catalysts to attack the saccharide portions of the biomass. Aside from the high cost of enzymes, the major drawbacks in this process are related to the high cost of the ionic liquid and poor resistance of the downstream enzymes or microorganisms to residual ionic liquid. New strains of microorganism are now being found that can tolerate high doses of ionic liquid.132 A recent proposal for a one-pot saccharification and enzymatic hydrolysis was also recently made (Figure 1.6).133
One-pot process for ionic liquid-aided biomass recalcitrance reduction and enzymatic saccharification.133
One-pot process for ionic liquid-aided biomass recalcitrance reduction and enzymatic saccharification.133
Recycling the ionic liquid to a very high degree is implicit in all these processes. There are many methods that are under development however, including the use of switchable ionic liquids (SILs),30–34,38,134,135 distillable ionic liquids (DILs),136–138 phase-separable ionic liquids (PSILs),139,140 chromatography141 and auxiliaries142 for purification of ionic liquid or isolation of oligomers, post biomass treatment. In the case of SILs and DILs the ionic liquids are protic and are derived from the combination of superbases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or 1,1,3,3-tetramethylguanidine (TMG) with either acid gases (in the case of SILs) or organoacids (in the case of DILs). Therefore, recycling of the components can be performed by vaporization of the neutral starting species (Figure 1.7). These are accessible by increasing the temperature of the mixtures to between 100 and 180 °C.
Mechanism of distillation for the switchable ionic liquid (SIL) DBU–CO2–Glycerol (top)38 and the distillable ionic liquid [DBNH][CO2Et] (bottom, adapted from ref. 138 from Wiley-VCH).
Major advances are being made, however, to reduce the cost of the ionic liquids designed for biomass pre-treatment and generally improve performance.143 Competing recalcitrance reduction technologies are predominantly aqueous acid digestion and ammonia fibre explosion/expansion (AFEX).
1.5.2 Fractionation of Biomass
Fractionation of biomass affords pure or tuneable polymeric fractions. The opportunity to selectively extract components or for complete dissolution and selective precipitation of different components offers the potential for highly tuneable fractions from biomass. Some studies have suggested that fraction purity also depends very much on molecular weight and the degree of linkage between lignin and carbohydrates.144,145 However there have been some recent success stories with, in particular, SILs30–34,38,134,135 and pre-treatments being applied to improve the efficiency of separation of components in hard and softwoods.146 The growing body of work on SIL fractionation in particular offers very high efficiency in separation of lignin from polysaccharide and the production of fibrous pulps. This is in part due to the fact that cellulose is not soluble in the SILs but lignin can be selectively extracted. A typical process scheme for the highly optimized short-time high-temperature (STHT) SIL fractionation is given in Figure 1.8.
Typical simplified process scheme for fractionation of wood chips with switchable ionic liquids (SILs).
Typical simplified process scheme for fractionation of wood chips with switchable ionic liquids (SILs).
In this case, and under the optimum conditions, the SIL was DBU–SO2–monoethanolamine (DBU–SO2–MEA). Wood chips were used (birch or spruce). The water composition in the SIL was 37 wt.%. The tolerance of the process to water is rather important due to the fact that dried chips will not be accessible on an industrial scale, due to the energy required in removal of water. Water contents can be as high as 50 wt.% for industrial chips. Interestingly, the resulting pulps are actually fibrous (Figure 1.9).
Fibrous pulps from birch (left) and spruce (right), resulting from DBU–SO2–MEA fractionation under optimum conditions (adapted from ref. 32 with permission from Wiley-VCH).
Fibrous pulps from birch (left) and spruce (right), resulting from DBU–SO2–MEA fractionation under optimum conditions (adapted from ref. 32 with permission from Wiley-VCH).
Much of the success of the different fractionation trials is, however, highly dependent on the biomass sources and chemical pre-treatments applied. Particle size is also of major concern. Finely divided sawdust is not a practical feedstock to use due to the high cost of production. Wet wood chips are much more realistic to use, which is where most homogeneous ionic liquid processes will fail, as they cannot dissolve chips, without significant degradation. However, some ionic liquids are known to fibrillate wood chips, potentially avoiding costs associated with biomass pulverization.31,147,148 Hypothetically, homogeneous processing of wood chips to produce polymers is possible but not achievable in practice, without the development of selective degradation treatments that do not degrade the constituent polymers. The main candidate linkages for selective degradation are suggested to be lignin-carbohydrate complexes130 but the existence of these entities, as covalent linkages, in native wood has not been rigorously established. At least, maintaining the fibrous properties of the resulting pulps is not undesirable as they may enter more traditional fibre-based value chains. Simpler fractionation concepts are also appearing, such as the extraction of kraft pulp using ionic liquid–water compositions. This allows for quite efficient removal of hemicelluloses, thus affording a higher purity cellulose pulp which may be useful for chemicals or textiles production. This process has been termed as the IONCELL-P process.149
1.5.3 Thorough Chemical Modification of Biomass
Thorough chemical modification of biomass to produce high bio-content polymeric materials is facilitated by homogeneous dissolution in ionic liquid. Ionic liquids allow for a wide range of chemistries on wood. The most common of these studied is the acylation of wood.130,150 Ionic liquids can allow for complete reaction of all hydroxyl groups in wood or wood biopolymers using typical acylating reagents. These include anhydrides, acid chlorides, isocyanates, etc. More sustainable methods are also under development, such as alkylcarbonate formation151 or even acylation by transesterification.152 Etherification is also possible.37
1.5.4 Enhanced Analytics
Enhanced analytics are possible to improve our understanding and analysis of lignocellulosics. As ionic liquids have exquisite biomass dissolving capabilities this has allowed for improved analytics where the native structure of the biomass is largely preserved. Ionic liquids are finding application as media for biomass derivatization for chromatography,153 as mobile phases for chromatography154 and as media for high-resolution solution-state NMR.139,155,156 In the other hand, the analysis of ionic liquid-treated lignocellulosics can become cumbersome, since traces of ionic liquids can cause several problems in traditional chromatographic methods.157,158 However, these problems can be overcome by developing methods like capillary electrophoresis.159
Further applications of ionic liquids in biomass processing are appearing but the major challenges that need to be overcome are:
recycling the ionic liquids
reduction in the cost of ionic liquids
reduction in toxicity of ionic liquids160
use of cost-effective and selective pre-treatments to preserve molecular weights upon ionic liquid fractionation and
develop ionic liquid-resistant enzymes and microorganisms for saccharification.
1.6 Conclusions
It is evident that the field of biorefining is experiencing not only a ‘renaissance’ but also rapid growth in terms of the volume of the research and development. The constant aim towards more green and sustainable processes and products is motivated by the impeding threat of global warming as well as increased awareness about the danger contained into ever increasing chemical loads on biosphere and humankind. To facilitate this, we need to re-think not only the today’s extensive use of fossil resources but also what kinds of synthetic molecules are released to our habitat. Future integrated biorefineries need to evolve in order to give solutions in assisting our quest towards these aspirations. In general, neoteric solvents or solvent–catalyst systems will most likely be a part of the solution but only if the principles and the spirit of ‘Green Chemistry’ are respected. Therefore, appropriate choice of concepts and chemistries, efficient recycling and separation concepts, re-designed benign chemical derivatives as building blocks for our commodities and daily consumables are the trends of today. At the same time, we need to change the way how the global industry, consumer markets, business and societies perceive our consumption: reversing the clock from today’s throw away and designed to fail culture to seeing durable, well-designed and long-lasting commodities of the past – only this time with clearly improved energy-efficiency.