Chapter 1: Origin and Historical Perspectives of Green Gasoline

In the recent past, the utilization of bioenergy as a renewable energy source has been emphasized in the context of energy independence and sustainable development.¹  The extensive use of fossil fuels has led to diminishing oil reserves and increased greenhouse gases in the atmosphere, which has encouraged researchers to develop environmentally friendly techniques and products for green and sustainable bio-based energy.²  Biomass such as plant and animal waste-derived green gasoline is considered a renewable green fuel that is environmentally acceptable because of the regenerative and eco-friendly nature of the feedstocks that are used to make green gasoline.³  Green gasoline is produced quickly in comparison with conventional fuels, such as oil, the formation of which requires lengthy geological processes. Vegetable oil, agricultural waste, algae and leftovers from breweries are just a few of the many sources from which biofuels may be produced. Depletion of fossil fuels, sustainability and environmental and economic concerns associated with convention gasoline can be ameliorated using green gasoline as a promising energy source that is reliable and less polluting towards the environment.⁴  The use of green gasoline will not only assist energy independence but will also significantly reduce greenhouse gas emissions.

Global environmental issues have attracted the attention of scientists and researchers and increasing emphasis has been directed towards renewable energy replacing fossil fuels on different global platforms such as the Kyoto Protocol 1992 and Glasgow Climate Summit 2021.⁵  Subsequently, via their legislation, most nations of the world have tried to lessen their reliance on fossil fuels by blending biodiesel or bioethanol with conventional fuel. Although not all biofuels are commercially viable, their variety reflects the wide range of opportunities available. Because of its low cost and efficient combustion, bio-based energy has become one of the most popular sources of energy. Overall, green gasoline production and utilization will have a positive impact on the economy in areas where raw feed materials are available to produce and utilize green gasoline locally, leading to huge savings on foreign fossil fuel imports. Apart from economic benefits, the use of green gasoline also produces health benefits due to less pollution, lower emissions of hazardous gases and improved air quality by reducing carbon dioxide levels in the environment. Green gasoline has been considered a safer alternative to conventional fossil fuels as compared with other available renewable energy options. Furthermore, owing to the range of feed materials and technology that can be used to produce green gasoline, it may be possible to reduce energy crises and energy politics, promoting peace in the world. It is estimated that bioenergy derived from a variety of plant biomass and other feedstocks can fulfil the current global energy demand, which is 3–4 times less than the stored energy of annually produced terrestrial plant biomass.⁶  Reduced emissions and reliance on fossil fuels from green gasoline made from biomass might help to slow the effects of climate change. Therefore, it has been predicted that the growth of biomass resources will be crucial to sustainable development. Various biomass feed materials have been examined for green gasoline production and classified into first-generation biofuel made from food crops, second-generation biofuel made from agricultural waste, third-generation biofuel made from algal biomass and fourth-generation biofuel made from genetically modified plants.⁷ 

Fuel from vegetable oil was conceived in 1890 by the French inventor Rudolph Diesel, who used vegetable oil to power an engine. Aiming at improved efficiency, the engine involved, which works on the principle of compression ignition, was further modified, developed and named after Diesel.⁸  The diesel engine was tested with vegetable oil on 10th August, 1893, and this historical event is now observed annually on 10th August as “World Biofuel Day”.

It is important to understand that green gasoline is not the same fuel as biodiesel. Green gasoline is really a transport fuel made from biomass that can be used in spark-ignition engines. Green gasoline meets the American Society for Testing and Materials standard specification ASTM D4814 in the USA and EN 228 in Europe, whereas biodiesel meets ASTM D6751.⁹ 

The utilization of ethanol as a motor fuel began in 1897 when Nikolas Otto invented the internal combustion engine that used ethanol as a fuel. The early development of green gasoline production was started in 1920 when Franz Fischer and Hans Tropsch developed a catalyst, reactor and process in order to convert biomass into liquid transportation fuel. The gasification process that they used is popularly known as the Fischer–Tropsch synthesis process, in which wood pellets were used as feed biomass to be converted into liquid fuel. Although the process developed to obtain liquid fuel from biomass was not economically viable at that time, currently green gasoline is attracting attention again as an alternative fuel.¹⁰  Green gasoline started to be established as an alternative fuel after the 1970s oil crises, when many nations adopted policies and began programmes on green gasoline production and feed materials. Many countries promoted the application of green gasoline for sustainable development using their local convenient feed materials.

Green gasoline consists of hydrocarbons processed from biomass sources using a variety of chemical, thermal and biological processes to produce fuel consisting of a mixture of chemical compounds very similar to the components of gasoline derived from petroleum. Researchers have explored various approaches, such as hydrotreatment, pyrolysis, gasification and other thermochemical methods, to obtain gasoline from different plant materials using a variety of microbes, enzymes and catalysts. Engineered catalyst materials have been formulated by researchers to obtain fuels from various plant-based source materials. The production of green gasoline is categorized as first, second, third and fourth generation depending on the type of feed biomass material, as shown in Figure 1.1.

Using dietary biomass rich in sugar and starch, such as maize, molasses and sugarcane juice, first-generation bioethanol is produced. The biomass used in the production of bioethanol is further categorized as being made of cellulose-based compounds, sugars and starches. Sugars present in sugarcane juice or molasses is fermented directly to produce bioethanol. Using enzymes, starch-rich food crops such as corn, potatoes and cassava are hydrolysed to fermentable sugars that can then be turned into fuel. Similarly, cellulose-rich materials such as agricultural waste and wood are turned into sugars using cellulolytic enzymes or acids. These feed biomass materials based on food crops compromise food security and are even unable to satisfy the rising fuel demands. The United Nations Food and Agriculture Organization (FAO) has taken serious note of the food security issues arising from food-based biofuels, including ethanol, and have calling its development a “crime against humanity”. Furthermore, it may have a negative impact on biodiversity, leading to deforestation.⁸  On the other hand, numerous types of biomass are abundantly available, containing lignocellulose in the majority, which are a promising feed option for ethanol production. The circumstances encouraged the growing interest that has developed towards second-generation biofuels produced from non-edible materials containing lignocellulose such as agricultural residues, waste solid biomass and energy crops.

Agriculture-intensive countries such as India produce a wide variety of crops resulting in the generation of huge amounts of waste agricultural residues that have no economical utility and are either left in the fields or are burned. Utilizing these waste agricultural residues as feed materials to produce green gasoline may help in managing energy crises and also solid agricultural waste management in an environmentally friendly manner. Furthermore, the utilization of agricultural crop residues for green gasoline production reduces the dependence on forest wood biomass and reduces deforestation. Moreover, the short harvest period of crops allows continuous biomass availability for energy production. In India, agricultural crops such as rice, wheat and sugarcane are produced in the majority of agricultural areas and the residues of these crops contain a majority of lignocellulosic biomass. The lignocellulosic agricultural residues contain cellulose lignin, hemicellulose and extractives as major components. It is estimated that lignocellulosic biomass can produce 442 billion litres of bioethanol annually; if total agricultural leftovers and lost crops are also taken into account, this amount can increase to 491 billion litres or almost 16 times the current world production.¹¹  Another feedstock for second-generation green gasoline is non-edible oil, which has been considered for efficient green gasoline production. A variety of non-edible oils have been tested as green gasoline feedstock, such as karanja (Pongamia pinnata) oil, castor (Ricinus communis L.) oil, neem (Azadirachta indica) oil, sea mango (Cerbera odollam), mahua (Madhuca indica) oil, yellow oleander (Thevetia peruviana Schum.) and Jatropha curcas L., to name just a few.¹²  The oils of these non-edible seeds contain free fatty acids (FFAs), used as a feed for biofuels.

The third generation of green gasoline is produced from wet biomass, including microalgae, which has attracted significant attention from researchers. One of the greatest problems in the development of CO₂ utilization methods, technologies and processes for growing microalgae is microalgae biodiesel production (MDP).⁷  Four main procedures are used in the production: cultivating microalgae, harvesting biomass, extracting oil and producing biodiesel.

The quality of the biomass had a major impact on its conversion into biofuels with good yields. Biomass must first be screened and carried through the treatment process before being used to make biofuels. The pretreatment of biomass produced some chemical alterations that aided in optimizing the various techniques used in the conversion of biofuels to bioethanol. An early stage of biomass-based biorefining is the pretreatment process.

The available biomass can be pretreated using a variety of techniques, including thermal, mechanical and chemical methods and their combinations. These pretreatment techniques are not environmentally friendly and, in some ways, they harm our ecosystem. Scientists have worked to devise a more environmentally friendly method of pretreatment of the biomass in order to overcome this type of situation.

Lignin and cellulose make up the bulk of the biomass and their saccharification is necessary for the production of bioethanol. With this in mind, researchers started to use various kinds of microbes for the pretreatment of biomass. Recent emphasis has been devoted to microorganisms and environmentally benign methods of microbial pretreatment to improve the enzymatic saccharification of lignocellulosic biomass. This process uses microorganisms, mostly actinomycetes, bacteria and white and soft-rot fungi, which break down lignin, the most resistant polymer in biomass. A low-cost biological technology that integrates lignin removal, cellulose hydrolysis and fermentation to transform lignocellulose to alcohol would result from biological delignification when combined with solid-state culture technology to produce and use low-cost cellulose on specific lignocellulosic materials.

Biomass pretreatment for biomass to green gasoline conversion, enzymatic hydrolysis, fermentation and product recovery are some of the processes in the bioconversion process. A cost-efficient and sustainable method for increasing gasoline output depends on the appropriate combination of the various stages. Diverse biomass sources can be used to produce renewable hydrocarbon biofuels.¹¹  Examples are lipids (such as animal fats, vegetable oils, algae and greases), in addition to cellulosic material (such as woody biomass, crop residues and dedicated energy crops).^4,13  Commercial facilities where production is taking place mostly concentrate on producing renewable diesel, while greater production of SAF (sustainable aviation fuel) is anticipated with many plants now under development.¹⁴  Often, renewable biodiesel and SAF are produced at the same site. Green gasoline can be produced through biomass gasification, which converts biomass into syngas. The syngas is then converted into liquid hydrocarbons through Fischer–Tropsch synthesis. Another method is hydrothermal liquefaction, where wet biomass is transformed into biocrude oil. Pyrolysis converts biomass into bio-oil, biochar, and syngas, which can be refined into green gasoline. Renewable hydrocarbon synthesis directly synthesizes liquid hydrocarbons from renewable sources like biomass or algae using catalytic processes.¹⁴ 

Solid biomass, such as wood chips, is converted into green gasoline using a methanol–dimethyl ether synthesis from the syngas created by the feed gasification process. The process begins with the gasification of wood chips within a gasifier (at temperatures ranging from 800 to 1000 °C and pressures ranging from 145 to 185 psia) (TIGAS project). Gas (mixture of carbon monoxide, hydrogen, methane, carbon dioxide, etc.) is the main output of the gasifier; after entering the tar reformer unit, which operates at 750 °C, 85% CH₄, 99.5% naphthalene and 99% benzene are formed.¹⁵ 

The primary decomposition of biomass starts between 122 and 202 °C. This primary stage is named the pre-pyrolysis stage. During this process, the chemical constituents of the biomass undergo some internal bond reorganization (bond breaking and formation), resulting in the formation of free radicals, peroxides and carbonyl and carboxyl groups and elimination of water molecules. Following the pre-pyrolysis stage, the solid constituents start to degrade and this stage mainly belongs to the pyrolysis process. Therefore, based on the applied environment, rate of heating and retention time of the precursor, pyrolysis is categorized into the following sub-types: conventional or slow pyrolysis, fast pyrolysis, flash pyrolysis, hydro-pyrolysis and hydrous pyrolysis. We can now consider these different categories of pyrolysis individually (Table 1.1).

1. Conventional Pyrolysis or Slow Pyrolysis

   This category is based on the rate of heating of biomass. During this process, the retention time of the biomass substrate at the assigned temperature of the pyrolytic reaction is lower than the heating time of the biomass substrate.⁴  In other words, the heating rate of the biomass substrate is very slow. The slow heating rate of the biomass allows the formation of solid, liquid and gaseous products in suitable proportions.

2. Fast Pyrolysis

   This category is also related to the heating rate of the biomass. During this process, the retention time of the biomass substrate at the assigned temperature of the pyrolytic reaction is higher than the initial heating time of the biomass substrate.^17,27  In other words, the heating rate of the biomass substrate is very fast. Fast pyrolysis of the biomass is preferred over slow pyrolysis if obtaining a liquid or gaseous product as the end product is the main goal. The maximum yield of fast pyrolysis is 17% by weight. Fast pyrolysis requires a fine particle size of the biomass substrate, a high temperature and a short contact time.

3. Flash Pyrolysis

   During flash pyrolysis, both the heating rate and the operating temperature are kept very high compared with fast pyrolysis. The retention time of the biomass substrate is shorter than in fast pyrolysis. This type of pyrolysis is also referred as an ultra-fast pyrolytic process owing to the very fast cooling of the gaseous product, i.e. the retention time of the gaseous product is less than 1 s, with very fast removal of char from the pyrolytic chamber. The fraction of liquid in the product can reach up to ∼ 60–75% by weight because of the rapid cooling of the gaseous product.¹⁸ 

4. Hydro-pyrolysis and Hydrous Pyrolysis

   In hydro-pyrolysis, pyrolysis of the biomass substrate is carried out in the presence of a hydrogen atmosphere. As a liquid product with a high content of hydrocarbons is obtained from the pyrolysis of the biomass, this method has high potential. On the other hand, hydrous pyrolysis is carried out in the presence of water.¹⁹ 

Thermal energy and chemical processes are combined step by step to accomplish gasification. Therefore, it can be thought of as a thermochemical process that converts solid or liquid biomass into gaseous products.⁴  During this process, the biomass feedstock is transformed into synthesis gas, also known as syngas, which is essentially a mixture of tar, carbon monoxide, carbon dioxide, hydrogen, hydrogen sulfide, methane and various other species in trace amounts. The raw material characteristics, gasifier characteristics, temperature, pressure and catalyst type all affect the relative fractions of these syngas constituents.

As already discussed, gasification is a process that combines thermal and chemical reactions. It involves several intermediate steps before it produces gasoline from biomass. The biomass must first be dried, followed by pyrolysis, oxidation and reduction reactions. The feedstock for biomass is dried on a temperature scale up to 120 °C and char starts to gasify at 350 °C. The required heat must be obtained either externally or intrinsically to complete the second step of pyrolysis.

The entire gasification process can be divided into primary, secondary and tertiary stages based on various chemical reactions and their respective temperature ranges. Water, carbon dioxide and low molecular weight monomeric oxygenated vapour and liquid species are all present in the primary stage range, which occurs below 500 °C. The most significant result of the primary stage is that no chemical reaction occurs between these chemical species. As the temperature increases further, we proceed to the second stage, which produces solid tars, gaseous olefins, water vapour, carbon dioxide, hydrogen and other by-products of chemical reactions. The temperature at this stage is between 500 and 850 °C. In the tertiary stage, which occurs between 850 and 1000 °C, liquid tar and polynuclear aromatic hydrocarbons are produced.

The hydrothermal process is a thermochemical process, just like gasification. The process is carried out with the aid of hot, pressurized water, which aids in dissolving the polymeric structure of the biochemical species and converting them to liquid form. Hence this method operates at a moderate temperature and a high pressure. The result is a direct transformation of the biomass feedstock into liquid form. Hence the hydrothermal process is denoted a direct liquefaction process. An operating temperature range of 250–375 °C is sufficient to enable the pyrolysis of the biochemical species. Water must be present in a liquid state during this process to ensure that the operating pressure is kept high, between 4 and 22 MPa. This combination of pressure and temperature ranges is actually identical with supercritical water conditions. According to the operating conditions, the entire hydrothermal process can also be divided into three sub-stages, much like gasification. The first stage, also referred to as the hydrothermal carbonization process, can extend to temperatures of up to 250 °C and the main output from this stage is hydrochar. Hydrochar has qualities that are comparable to those of low-rank coal. The second stage is known as the hydrothermal liquefaction stage because it produces liquid fuel (biocrude) and has a temperature range of 250–374 °C. Above 374 °C, syngas is produced as the gasification process begins to dominate the liquefaction process. The third phase is therefore referred to as the hydrothermal gasification process.²⁰ 

For the manufacture of renewable hydrocarbon biofuels, many technological avenues are being investigated, as follows:

1. Traditional Hydrotreating

   This technique is currently utilized in commercial plants. If used in oil refineries, hydrotreating entails reacting the feed (lipids) with hydrogen at high pressures and temperatures in the presence of a catalyst.²¹ 

2. Biochemical Upgrading of Sugars

   With the addition of microorganisms that convert carbohydrates to hydrocarbons, this method utilizes a biochemical deconstruction procedure similar to that used in biofuel production.²² 

3. Catalytic Conversion of Sugars

   This route converts a stream of carbohydrates into hydrocarbon fuels through a sequence of catalytic reactions. The aim of this route is to reduce the oxygen content in the feedstock and increase the number of carbon bonds in intermediates so that the molecular weight of the final hydrocarbon product can be increased.²³ 

Petroleum and its by-products are currently the main energy source in numerous economies. With growing concerns about the environment and the unpredictability of petroleum feedstock prices, it is essential to identify sustainable replacements for petroleum-based fuels that may be utilized without needing major adjustments to the present fuel distribution and utilization infrastructure due to consequences connected with the production of these fuels. Fast pyrolysis is the preferred main method for processing lignocellulosic biomass to biofuel owing to its ease of use and scalability.²³  However, the bio-oil produced by the fast pyrolysis of biomass has a high oxygen content and other properties that prevent it from being used in combustion engines. Therefore, further catalytic cracking must be performed on the bio-oil to make it suitable for use as a transport fuel. Numerous studies have evaluated the practical and commercial potential of employing rapid pyrolysis to produce sustainable hydrocarbon fuels.¹⁶  The projected total expenditure required for these studies to generate 2000 dry tonnes (2 MT) of hydrocarbon fuel every day from biomass feedstock ranges from $287 million to $700 million. The sensitivity analysis of these studies shows that the capital cost, fuel output and feedstock cost all significantly affect the minimum selling price of gasoline.^24,25  One of the possible biomass feedstock resources for producing biofuel is forestry waste. The ash percentage is a crucial component in biomass conversion because it has catalytic impacts that can alter depolymerization processes and the distribution of the yields of biomass conversion products.^26,27  The need for fuels made from biomass (biofuels) is continuing to rise as a result of concerns about climate change and government initiatives to reduce the emissions of greenhouse gases by using fossil fuels. In comparison with fossil fuels, the ability of biofuels to reduce greenhouse gas emissions to combat climate change is questionable, especially when modifications to land use are taken into consideration in life cycle assessments.^24,26  However, because it is a by-product of the forestry sector, gasoline made from wood waste (forest or mill) can lower carbon discharges without igniting a debate over shifts in land usage.²³ 

The world’s energy supply may not be able to fulfil rising needs or sustainable environmental goals as the global population rises and fossil fuel energy supplies eventually decrease. As a result, there is a growing need for many nations to guarantee future energy security and a sustainable environment. Energy security is described by the International Energy Agency (IEA) as “the ongoing availability of energy sources at an acceptable price”.²⁸  In order to ensure an energy supply that is in line with their economic growth and sustainable environment goals, several nations have undertaken efforts to build a low-carbon economy and green sectors as long-term ambitions.^28–30  However, the amount of greenhouse gas emissions in the Earth’s atmosphere is increasing as a result of the widespread use of energy sources and the depletion of oil reserves, which has prompted the creation of eco-friendly products and practices for green, sustainable, bio-based energy. As a result of recent advances, including the production of green gasoline, biorefinery facilities have even taken centre stage in some areas. The world’s civilization and economy are currently undergoing a transition from fossil fuels to biofuels in line with the Sustainable Development Goals. Biofuels are considered a component of renewable energy sources. This transition is motivated by factors such as the increased availability of alternative fuel options and the persistent rise in greenhouse gas concentrations in the Earth's atmosphere, contributing to the need for sustainable energy solutions.³¹ 

An attempted plan with a promising strategy for replacing fossil fuels has been developed in energy sectors, together with developing technology and co-processing methods that are now on the market. Utilizing renewable energy sources is one strategy that several nations across the world are adopting to minimize their dependence on fossil fuels. The accessibility of reasonably priced and environmentally friendly biomass and the financial viability of potential projects are two key issues when scaling up a study-scale project to the commercial scale. The first element influences a factory’s attempt to be as large as feasible in order to reduce the cost of producing green fuel. Additionally, the price and location, which are two important factors in raw materials costs, are limited with regard to the availability of biomass raw materials.³²  Since the US Department of Energy began their Integrated Refineries Program, concerted efforts have been made to improve collaboration with industries in order to progress, construct, launch and legalize an integrated biorefinery industry at the design, demo, pilot and commercial scales across the USA through numerous technical advances in the production of green gasoline from biomass.³³  Through numerous research studies, regulations and application of cutting-edge technology, other countries are beginning to use these green fuel production methods, including Brazil, Indonesia, India and others. One example is a demonstration unit of the integrated biorefinery provided by UPM Company that can manufacture raw gasoline products with a capacity of 22.5 barrels per day (bpd) from 21.6 bone-dry tonnes of wood debris. Haldor Topsøe discovered an innovative process, which was used in that programme called the Topsoe Improved Gasoline Synthesis (TIGAS).³⁴  Syngas can be converted by TIGAS into gasoline with an octane rating of 88. The process developed makes use of the fluidized-bed steam–oxygen gasification technique developed by Andritz AG together with an advanced tar catalytic system that can create pure syngas from wood waste.¹⁰  Commercialization considerations involved a number of issues, includes leakage that occurred in the TIGAS in the testing of the pilot plant and the continuation of the manufacturing process after the deadline of 5 days had passed. Saxena and Viswanadham conducted a laboratory-scale experiment and succeeded in converting jatropha oil from the Jatropha curcas plant to green gasoline using a zeolite catalyst. Four distinct types of zeolite catalysts, each with unique properties but all capable of producing a RON (research octane number) of 95, were used.¹²  This work provided a different method for making green gasoline.¹²  Virent Energy System Inc. developed a green gasoline method using a variety of raw sources, including non-food plant waste and agricultural waste. Utilizing a bioforming method that combines hydrolysis and aqueous phase reforming processes, Virent produced green gasoline in the laboratory under the direction of James Dumesic with support from the National Science Foundation. In contrast, UOP carried out a number of pilot- and laboratory-scale studies in 2014.³⁵  UOP successfully demonstrated the transformation of pyrolysis oil into fuel for transportation on a laboratory scale. Additionally, utilizing an equilibrium fluid catalytic cracking catalyst, experiments were carried out at a higher level by infusing 3% of bio-oil from Ensyn Corporation with vacuum gas oil (VGO).³⁵  Research conducted by Mota et al. in 2017 utilizing unrefined palm oil as raw material and thermal catalytic cracking in a 143 L reactor with Na₂CO₃ as the catalyst constituted a further investigation of green gasoline conducted at the pilot scale.³⁶  In a laboratory study, the organic liquid product that was obtained in the cracking process revealed favourable qualities of the biofuel in accordance with Brazilian National Agency standards. However, even though the generated hydrocarbon molecules fitted into the gasoline category, the performance and composition of the green gasoline still reflected the presence of oxygen.³¹  In 2018, Numaligarh Refinery Limited (NRL) established Assam Bio Refinery Private Limited (ABRPL), which was the first biorefinery in India. This biorefinery was intended to produce bioethanol along with other chemicals and electricity from bamboo biomass. ABRPL is a joint venture company that has three partners, with major shares held by NRL in India (50%), Fortum 3 BV in The Netherlands (28%) and Chempolis Oy in Finland (22%). NRL has the capacity to produce 49 000 tonnes of ethanol per annum that can be used for petrol and diesel blending to cater for the northeastern and Indian markets. The feedstock demand for bamboo for the production of bioethanol is about 500 000 tons per annum. Such efforts would support bamboo farming and increase bamboo cultivation in the region.³⁷ 

The US government considers that producing and using biofuels have less of an impact on the environment than using fuels based on fossil fuels. When the use of biofuels eliminates the need to import petroleum, there may also be significant benefits to the nation’s economy and security.³⁸  Government initiatives that encourage and/or mandate the use of biofuels, including California’s Low Carbon Fuel Standard and the US Renewable Fuel Standard, specify the kinds of green gasoline that can be produced and the methods or low-carbon production pathways that must be used for those biofuels in order to be eligible for use under the initiatives. Despite the fact that green gasoline is environmentally friendly, the environment is nonetheless impacted by its production and consumption.³⁸  If spilled, pure ethanol and biodiesel decompose into harmless chemicals that are non-toxic and biodegradable. Denaturants in gasoline ethanol, however, render it unfit for human consumption. Biofuels must be carried carefully since they are combustible, much like petroleum-based fuels. Pure biofuels often emit less sulfur dioxide, particulates and air toxicants when burned compared with their equivalents generated from fossil fuels. Additionally, compared with fuels without biofuels, biofuel–petroleum mixtures often have fewer emissions. In comparison with petroleum diesel, the combustion of biodiesel may produce more nitrogen oxides. Although ethanol and ethanol–gasoline blends burn more smoothly and have better octane ratings than regular gasoline, they also produce greater emissions from fuel tanks and transmission systems. These emissions from evaporation help to create dangerous smog and ground-level ozone. Before blending with ethanol, gasoline needs additional processing to decrease evaporative emissions.³⁸ 

The greenhouse gas carbon dioxide (CO₂) is produced whenever green gasoline is burned. However, in accordance with an international convention, CO₂ emissions from burning biofuels are not included in national inventories of greenhouse gas emissions since producing the biomass feedstocks needed to make biofuels might offset the CO₂ emissions generated when burning the biofuels. Depending on how the green gasoline is produced and whether farmland cultivation-related emissions are taken into account in the calculations, the use of green gasoline has an impact on net CO₂ emissions. Increasing the use of plants for fuel is a contentious issue because some people think that the resources necessary to cultivate biofuel crops – land, fertilizer and energy – should rather be utilized to generate food. Large areas of vegetation and forests have already been cleared in many parts of the world to produce soybeans and oil palms for the manufacture of biodiesel. A heat source is necessary for the production of ethanol, renewable diesel and renewable aviation fuel, and the majority of these biofuels are currently made using fossil fuels.³⁸  This is connected with the ethanol synthesis energy balance and also comparison with the fossil fuel premium motor spirit (FPMS) emission pattern. Input feedstock and by-products are both factors in the energy balance.³⁹  Owing to its vegetable basis, bioethanol has a probability of minimizing the release of harmful gases. Because it is oxygenated, it aids in full combustion, lowering emissions by up to 30%. Utilizing this substantial use of lignocellulosic biomass might result in transportation cost savings, which significantly improve the energy balance. Lignocellulose-based ethanol production is superior to that of first-generation biofuels because it uses inexpensive biomass and an ecologically benign manufacturing technique. Additionally, biodiesel offers the benefit of lubricating and lowering the temperature of internal combustion engines. As the blend concentration in the fuel mixture rises, the overall NO _x emissions are not much impacted.^38,39 

Renewable gasoline can be derived from various biomass sources, including lipids (such as vegetable oils, animal fats, greases, and algae) and cellulosic materials (like crop residues, woody biomass, and energy crops). Researchers are exploring multiple methods to produce renewable gasoline: traditional hydrotreating, biological sugar upgrading, catalytic conversion of sugars, gasification, pyrolysis, and hydrothermal processing. Each method involves different processes like high-temperature reactions, biochemical deconstruction, catalytic conversions, and thermal decomposition to generate hydrocarbon fuels from biomass feedstocks.⁴⁰  Traditional hydrotreating is a well-established method employed in petroleum refineries. It involves subjecting the feedstock (lipids) to high temperatures and pressures, along with the presence of a catalyst and hydrogen, to induce a chemical reaction. This technology is currently utilized in commercial plants for the production of hydrocarbon fuels.^40,41  Biological sugar upgrading involves a biochemical conversion process akin to cellulosic ethanol production. However, it incorporates the use of specific organisms to convert sugars into hydrocarbons. This additional step improves the conversion process, facilitating the production of hydrocarbon fuels from sugar feedstock.^40,42,43  The catalytic conversion of sugars involves a sequential series of reactions that efficiently converts a carbohydrate stream into hydrocarbon fuels. This pathway enables the transformation of carbohydrates into valuable fuels with high efficiency.⁴⁴  Gasification is a thermal conversion process that converts biomass into syngas through a series of chemical reactions. This syngas can then be catalytically transformed into valuable hydrocarbon fuels, enabling the efficient production of fuels from biomass resources. Pyrolysis is a process that chemically decomposes organic materials at high temperatures in the absence of oxygen. The outcome is a liquid pyrolysis oil, which can be independently processed or combined with crude oil in a standard petroleum refinery to produce hydrocarbon fuels.²⁷  Hydrothermal processing employs elevated pressure and moderate temperatures to initiate the chemical decomposition of biomass or wet waste materials. The result is the production of an oil, which can be catalytically upgraded into hydrocarbon fuels.^46–54  The facility is designed to utilize 2000 dry metric tons per day of hybrid poplar wood chips for the production of 76 million gallons per year of gasoline and diesel. The process involves several steps: feed drying and size reduction, fast pyrolysis to obtain a highly oxygenated liquid product, hydrotreating of the pyrolysis oil to achieve a stable hydrocarbon oil with less than 2% oxygen, hydrocracking of the heavier portion of the stable hydrocarbon oil, distillation of the hydrotreated and hydrocracked oil into gasoline and diesel fuel blendstocks, and hydrogen production to support the hydrotreater reactors.^47–52  Efforts are being made by the Idaho National Laboratory (INL) to streamline feedstock logistics, eliminating the need for drying and size reduction at the plant, which would further reduce production costs. The estimated capital cost for an independent plant is $303 million (as of 2007), with a minimum selling price for gasoline and diesel fuels of $2.04 per gallon ($1.34 per gallon ethanol equivalent basis) to achieve a 10% return on investment (ROI).⁴⁷  According to the Energy Information Agency (2007), the average refiner prices for gasoline and diesel in 2007 were $2.18 and $2.20 per gallon, respectively. Considering this, a fuel product price of $2.04 per gallon presents an attractive incentive for the development of motor fuels derived from biomass.⁴⁸ 

Any organic substance or waste originating from plants or animals has biomass. Examples include wood waste, animal manure, municipal solid agro-industrial waste and a variety of other items such as domestic garbage and waste from land management activities. Because lignocellulosic components are present and with the high abundance of this resource globally, biomass has enormous potential for conversion into value-added chemicals and also heat and power unlike other renewable energy sources, (solar and wind) that only produce heat and power. Additionally, biomass is essentially a potential renewable source of carbon fixation that may be used to create bio-based chemicals and fuels to replace traditional hydrocarbon-based consumer products and transportation fuels. After proper upgrading and refining, bio-oil, a very valuable by-product of biomass pyrolysis, may be utilized in a variety of downstream applications by dividing the complicated blend of bio-oil molecules into discrete fine chemicals.

Bioenergy refers to renewable energy derived from organic materials that have absorbed sunlight and stored it in the form of chemical energy. Bioenergy is dependent on the source of feedstock, the technology used and the end products formed via biomass to energy conversion. Bioenergy is a significant renewable energy source, and constitutes about 13–14% of the total energy consumption in the world.^53,54  The global energy consumption from oil and oil-based products is about 40%, whereas coal contributes 20% and gas 20%. The combined global energy consumption in 2017 from fossil fuels was about 80%. Renewable energy contributes about 17.7% of the global energy consumption among renewable energy sources; bioenergy is the largest and about 70% of energy comes from biomass.^53,54  Energy conversion technologies from renewable sources have made considerable progress in electricity production. In 2017, about 25% of the electricity generated globally was from renewable sources and electricity from biomass-based sources produced 596 TW h.

Biofuels that are obtained from biomass are biodiesel, bioethanol and hydrogenated vegetable oil (HVO). The market status and global country/region-wise biofuel production are given in Table 1.2. From 2013 to 2018 in India, ethanol production was 1.3 billion litres and constituted 1.2% of global output. The projected ethanol production India from 2019 to 2024 is 2.2 billion litres.⁵⁴ 

When using green gasoline as a fuel or energy carrier, the following issues need to be considered and properly solved. A wide range of strategies are being investigated experimentally or are in use. Adopting a clean and emission-free energy cycle is seen as a significant advance in this area. Following the global energy crisis, the concept of employing green energy as a form of transportation was significantly enhanced. Green energy is being produced in large quantities using a variety of techniques. Depending on its availability, green energy may be produced using a wide range of techniques, some of which are clean and “green,” or derived from a variety of substances and materials. Green energy can be created everywhere on Earth, which is essential. Some of the benefits of this material as an energy carrier are exceptional energy per unit of production, favourable storage and transportation options, enhanced safety features and decreased hazardous emissions.

• Low energy density per unit volume.

• Infrastructures at the national size of emerging nations are necessary for renewable energy sources to qualify as fossil fuel alternatives.

• High prices for transportation.

• Utilizing the excess energy lost in phantom loads in large-scale grid systems is very profitable for producing green gasoline.

• Encouraging technological awareness and knowledge.

  The main obstacle in waste management technology is the relative lack of information about the best techniques for collecting and separating organic waste from municipal solid garbage. Therefore, education on waste characteristics and source segregation in biogas plants is necessary. Project designers must take part in training programmes for human resources to reduce operating expenses and boost the technical personnel for biogas upgrading technologies. The criteria of the Bureau of Indian Standards (BIS) for the utilization of biogas in engines and other vehicles in India have lately been set by regulations that must be applied in relation to the quality criteria according to the use of biogas. According to the BIS, biogas used as a fuel for vehicles in India should have the following composition: CH₄, moisture, H₂S and CO₂ + N₂ + O₂. For the application of upgrading biogas technology, changes are needed to national and international biogas standards.⁵⁵ 

• Innovative biological upgrade techniques.

  In contrast to chemical absorption, organic acids resist gaseous pollutants such as hydrogen sulfide. In biological upgrading, there are two metabolic routes: the direct approach through hydrogenotrophic methanogen conversion of CO₂ to methane and the indirect process in which homoacetogenic bacteria convert CO₂ first to acetate and subsequently to CH₄.^55,56 

• Global adoption and promotion of assisting regulations.

  Key government initiatives and financial incentives are necessary for the development and expansion of biogas plants in the commercial sector. As in the UK and Germany, financial encouragements and continued safety may strengthen risk-free environments throughout digestion. In the UK, the Climate Change Levy exempts renewable energy from paying taxes. The cleaning and upgrading of biogas are successful in areas with the right legislation and government backing. Finland’s Renewable Energy Development Act exempts biomethane production taxes, and South Africa grants incentives for bioenergy production. In India, the National Biogas and Manure Management Programme provides small-scale production of biogas from alternative sources for the provision of power, along with enhanced sanitation laws. The Malaysian government backed legislation for handling trash from palm oil production. The National Green Tribunal in India issued an instruction to use crop wastes for recycling or the creation of biogas rather than burning them outside.^46,56,57 

• Promotion of small-scale upgrade facilities that are financially viable.

  The equipment required for the biogas upgrade process makes it more expensive for small-scale plants. For a developing nation such as India, the high investment costs of sensors and other control systems provide a challenge. Therefore, one of the sustainable methods is the use of a single upgrading unit for several biogas units. Control systems should be straightforward and have a CH₄ content of at least 95% if they are to save shipping costs and operational costs. Small-scale plants should use effective materials and appropriate, sustainable operating procedures because the economics of upgrading plants depends on their size.

• Process improvement for upgrading biogas.

  Process variables including temperature, flow rate and pressure drop should be improved in the water scrubbing technique to provide a better design, less water usage, lower biogas compression costs, lower operating costs and greater CH₄ recovery. Indirect cooling systems, energy-efficient compressors and heat exchangers are key points in cryogenic research and development.

• Biogas slurry-derived fertilizer.

  Digested slurry might take the place of synthetic fertilizers during the anaerobic fermentation process. Slurry, which is free from bacteria and pathogens, makes excellent fertilizer for plants and agricultural land. The digested slurry contains a variety of micro/macronutrients that the plants can utilize as biofertilizer. The ammoniacal nitrogen content of the slurry may be utilized directly by plants in the soil. The obstacle to the public promotion of this technology is the deficiency of use of this organic fertilizer compared with inorganic fertilizer.^55,57 

Significant benefits of green gasoline include reduced emissions, flexibility, compatibility and greater energy security. Research and development are ongoing, with a focus on developing sustainable bio-based energy through the use of a range of feedstocks, processes and green gasoline products. The major emphasis in green gasoline production is on the development of the catalyst, reactor and process facilities depending on the feedstock. This chapter has summarized the evolution of biomass conversion processes, the technoeconomic elements of green gasoline and the commercial, environmental and present state of technologies for producing green gasoline.

ABRPL

Assam Bio-Refinery Private Limited

ASEAN

Association of Southeast Asian Nations

ASTM

American Society for Testing and Materials

FAO

Food and Agriculture Organization

FFA

Free fatty acid

HVO

Hydrogenated vegetable oil

IEA

International Energy Agency

MDP

Microalgae biodiesel production

PSA

Pressure swing adsorption

SAF

Sustainable aviation fuel

TIGAS

Topsoe Improved Gasoline Synthesis

VGO

Vacuum gas oil

The authors are grateful to the Council of Scientific & Industrial Research, Ministry of Science and Technology, Government of India.