On the other hand, the appeal of the reduction of greenhouse gas (GHG) emission has been the hottest topic in every recent United Nations Climate Change Conference. In 1997, the Kyoto Protocol was signed by most of the industrialized countries with the aim of reducing the global GHG emission. After the Kyoto Protocol's first commitment period expired on 2012, 37 countries, including 28 members of the European Union, agreed to a second commitment period of GHG emission reduction in Doha. Although the two largest GHG emission countries did not participate in the Kyoto Protocol, they both set their own CO₂ emission targets (UNFCCC, 2015). According to the latest report of the Intergovernmental Panel on Climate Change (IPCC, 2014), the GHG concentration in the atmosphere could reach from 750 to 1300-ppm CO₂ equivalents. As a consequence, the global average surface temperature could increase by 3.7–4.8°C. If we would like to control the temperature change within 3°C in 2100 compared to that of preindustrial levels, the GHG concentration in the atmosphere should be controlled to lower than 650-ppm CO₂ equivalents. This means a change of GHG emission should at least not exceed 24% of the 2010 emission level (IPCC, 2014). Since 78% of GHG emissions in recent decades came from fossil fuel combustion and industrial processes, the development of a low-carbon economic system to replace the fossil fuel–based system is urgent.

Along with several other renewable technologies, biofuel has made and will continuously make a significant contribution to meet targets on the usage of renewable energy resources and the reduction of GHG emission. Besides the above-mentioned major reasons, the advantages of development and application of biofuels also include: improving national energy security, utilizing existing transportation system, utilizing existing fuel distribution system, and facilitating rural development.

Currently, in the first generation of bioethanol, food crops such as corn, sugar cane, and wheat are used for the production of energy. These are starch- or sucrose-rich feedstocks that are readily fermented by microorganisms. However, these crops are also used for food and feed production, resulting in competition. At present, commercial production of the first-generation biomass utilizes readily-available sugars from these food plants for the fermentation process of biofuel production.

However, the second generation of bioethanol uses lignocellulosic raw materials as the main substrate, which has a more complex composition as compared to the first-generation feedstocks. Lignocellulosic feedstocks are high in cellulose, hemicellulose, and lignin. Second-generation feedstocks avoid competition with food and feed products. Examples are waste streams from food- or feed-crops such as wheat straw or corn stover, also municipal or industrial waste streams, or energy crops that grow on marginal lands that are unsuitable for regular agriculture. To use the preferred second-generation feedstocks, further advances in technological development are needed to unlock the more hidden sugars in the crop residues or woody plant materials. Significant research efforts and investment have been spent to improve the technology in order to enable the commercial use of the second-generation feedstocks.

Different generations of biofuels also differ in other characteristics. While the food part of the food crops is made of easily digestible sugars, the sugars captured in lignocellulosic compositions of the second-generation feedstocks are more difficult to utilize. So why do we want to use these more challenging second-generation feedstocks? This is to reduce competition with food, arable land, and water. Using residues can help to avoid land-use changes, and energy crops can be genetically engineered to reduce water usage. It can also bring in extra income for farmers. In the future, water-based feedstocks such as algae may become as important as the third-generation feedstocks. The third-generation feedstock is used for processes where CO₂ is utilized as one of the substrates. A common example would be photosynthetic algae that use sunlight and CO₂ to produce useful organic molecules. These third-generation systems would completely eliminate the need for agricultural land.

This book aims to provide an overview of the latest progresses in various technologies for biofuel production. The special emphasis has been focused on the advanced generation of biofuels, which produce biofuels from nonfood materials. We keep the same the classification method, dividing different technologies into three main sections: chemical, biological, and thermochemical conversions.

In the first few introductory chapters, details on policies, socioeconomic, and environmental implications of the implementation of biofuels (chapter: Multiple objectives policies for biofuels production: environmental, socioeconomic and regulatory issues), life-cycle assessment (LCA) (chapter: Life cycle sustainability assessment of biofuels), techno-economic assessment (chapter: Techno-economic studies of biofuels), environmental concern (chapter: Multiple objectives policies for biofuels production: environmental, socio-economic and regulatory issues), and the different biofuel feedstocks (chapter: Feedstocks and challenges to biofuel development) will be presented. The rest of the book is aimed to give a detailed and balanced overview on key technologies and processes for the production of various type of biofuels, including but not limited to, bioethanol, biodiesel, biohydrogen, biogas from anaerobic digestion, biosyngas from gasification, and bio-oil from pyrolysis.

Biodiesel is a mixture of long-chain fatty acid methyl ester (FAME) that is produced from biomaterials through transesterification of triacylglycerol (TAG, ie, plant oil and animal fats) with methanol. In chemistry, the biodiesel synthesis could be expressed by the reaction as shown in Fig. 1.2. In principle, any triacylglycerol could be used for biodiesel production. In fact, the first generation of biodiesel was mainly produced from edible plant oil, such as soybean, rapeseed, and palm oil. The low price of plant oil before 2008 and high diesel price in the European countries allowed enough profit for the biodiesel production. For example, during 2004–2005, the rapeseed oil price was less than €600/ton, while the diesel price was €1.11/L. However, in 2008, the plant oil price increased sharply, and the margin of biodiesel production from plant oil was low. During the soaring of raw material cost, together with the concern of food shortage, production of biodiesel from nonedible oil, eg, waste cooking oil, grease, Jatropha oil, and microalgae attracted increasing interest. It was reported that in the United Kingdom between April 2014 and 2015, over 50% of the biodiesel would be derived from waste cooking oil (Biofuel statistics, UK government, 2014).

In terms of biodiesel conversion processes, chemical conversion using alkali and acid-based catalysts is still the most favorite approach. Various investigations have been carried out to develop novel catalysts and/or novel processes for efficient conversion of TAG to FAME. This part was reviewed in the chapter “Production of biodiesel via catalytic upgrading & refining of sustainable oleageneous feedstocks.” The chapter “Biochemical catalytic production of biodiesel” introduced a promising alternative way of biodiesel production via enzyme-catalyzed processes. Recently, microalgae has been demonstrated to have the potential for biodiesel production. Significant progress has been made since the concept was first introduced. The character of microalgae oil showed its great potential of dominating biodiesel production. Since the publication of the first edition, further intensive investigation on this field has been carried out. The advantages and limitations of microalgae oil production are discussed in the chapters “Production of fuels from microbial oil using oleaginous microorganisms” and “Utilization of biofuels in diesel engines.”

Besides bioethanol (chapter: Biochemical production of bioalcohols), anaerobic digestion of organic waste materials for the biogas production progressed rapidly worldwide in the past 5 years. For example, the annual biogas production in China increased dramatically from 10.5 billion m³ to 248 billion m³ from 2007 to 2010 (Deng et al., 2014; Wellinger, 2011). The chapters “Production of biogas via anaerobic digestion,” “Biological and fermentative production of hydrogen,” and “Biological and fermentative conversion of syngas” reviewed biogas, biohydrogen, and fermentative conversion of syngas (synthesis gas), respectively.

Biopyrolysis is a typical thermochemical process, which converts biomass into biosyngas, bio-oil, and biochar at elevated temperature with limited supply of air (oxygen). Bio-oil is the target product of biopyrolysis. A wood chip–derived bio-oil normally has a density of around 1.2 kg/L with an energy density of around 18.0 MJ/kg, which is around 2–4 times higher than those of the wood chips. However, the complex composition of the pyrolysis oil, the high water content, and the high acidity prevent wide applications of bio-oil. Various investigations have been carried out with the hope of upgrading bio-oils into a replacement of fossil transportation fuels, including optimizing operation conditions, pretreating biomass before pyrolysis, introducing catalysts, designing novel reactors, and many others, as reviewed in the chapters “Catalytic fast pyrolysis for improved liquid quality,” “Production of biofuels via hydrothermal conversion,” and “Production of biofuels via bio-oil upgrading and refining.”

Further increase to the reaction temperature in the biopyrolysis process would lead the thermochemical reactions to shift toward biosyngas production. Such a process is then termed gasification. The main components of biosyngas are CO and H₂. The ratio of CO and H₂ is depended on the type of substrate and gasification conditions, eg, whether steam is used in the gasification process. The resultant biosyngas could be burnt directly for energy generation, or to be used for the synthesis of biofuels via the Fischer-Tropsch process or a newly-emerged gas fermentation process. These contents were reviewed in the chapters “Production of biosyngas and bio-hydrogen via gasification,” “Production of bioalcohols via gasification,” “Production of biofuels via Fischer-Tropsch synthesis: biomass-to-liquids,” and “Production of biofuels via bio-oil upgrading & refining”. Besides these, the chapter “Chemical routes for the conversion of cellulosic platform molecules into high-energy density biofuels” discussed alternative approaches that could be used for the high-energy-density biofuels production via chemical conversion routes.

Actually, the biorefinery concept has already been applied in the first generation of biofuels. Along with bioethanol production, Distiller's Dried Grains with Solubles (DDGS) is generated. DDGS is sold as an animal feed, and is an important income stream for a bioethanol company. Even more, companies consider themselves to be animal feed–producing or commodity food–producing companies—the biofuel production is just to utilize the low nutritional parts of the biomass or the organic waste to generate another product. Similarly, glycerol is the principle by-product of biodiesel, which is produced from transesterification of TAG with a glycerol to biodiesel mass ratio of 1:10. At the earlier stage in a biodiesel business model, glycerol is normally refined to pure glycerol and is sold as an income stream. However, due to the soaring of biodiesel production, the glycerol market was quickly saturated. As a consequence, the glycerol price dropped significantly. Therefore, various research has been carried out to convert glycerol, or crude glycerol into other value-added products, such as 1,3-propanediol, succinic acid, PHB, and biogas via anaerobic digestion (Koutinas et al., 2014).

Biofuel production actually plays a major role in the economics of biorefineries. The chapter “Biofuel production from food wastes” reviewed the topic of biofuel-driven biorefineries, and the chapters “Biochar in thermal and thermochemical biorefineries—production of biochar as a coproduct,” “Algae for biofuels: an emerging feedstock,” and “Utilization of biofuels in diesel engines” focused on the biofuels and other value-added production formation from the following interesting raw biomass: food waste, lignocellulose, and algae. Last but not least, engine tests are of utmost importance to test the feasibility of biofuels implementation and are still on-going activities. Chapter “Utilization of biofuels in diesel engines” summarized some experimental results on the implementation of biofuels in engine tests.

In the past, most biofuel companies received government subsidies and tax reduction. These policies stimulated the rapid growth of bioenergy industry. Nowadays, some governments have started to withdraw this kind of support to biofuel producers. On one hand, the profit of biofuel production dropped significantly, and some companies had to shut down or reduce their activities. On the other hand, this change enables only the highly efficient, highly competitive technologies to survive, and therefore increases the competitiveness of the whole biofuel industry in a nonprotective energy market.

The next 5–10 years will be a crucial period for the development of bioenergy technologies. Most attention has been put in the industrialization progress of lignocellulosic bioethanol production. Various life-cycle assessments have demonstrated that utilizing lignocellulosic biomass for bioethanol fermentation would lead to a significant reduction of GHG. Identification of effective pretreatment methods, production of low-cost cellulolytic enzymes and enhanced fermentation yield and productivity using the hydrolysate from biomass into bioethanol are the most feasible fields to advance the technology.

Alternatively, lignocellulosic raw materials could be used for biofuel and biochemical production via thermochemical processes, such as fast pyrolysis, catalytic pyrolysis, and gasification. Fast pyrolysis actually has been commercialized, and the main product is bio-oil that can be readily stored, transported, and used for the production of liquid fuels and various chemicals (Bridgwater, 2012). Bio-oils have been successfully tested as fuels in engines, turbines, and boilers, and upgraded to high-quality hydrocarbon fuels (Czernik and Bridgwater, 2004). However, upgrading of the bio-oils to a quality of transport liquid fuel still faces several technical challenges due to the very complex compositions, and the process is not currently economically feasible. Catalytic pyrolysis refers to the pyrolysis process of biomass using various catalysts with aims of elimination and substitution of oxygen and oxygen-containing functionalities, in addition to increasing the hydrogen to carbon ratio of the final products (Dickerson and Soria, 2013). However, robust and highly-selective catalysts have to be further developed, and the cost of the process has to be reduced for commercial application. Gasification is one of the most promising technologies to produce gas fuels from lignocellulosic biomass. It is a thermochemical partial-oxidation process converting carbonaceous substances such as biomass into gas in the presence of a gasifying agent such as air, steam, oxygen, CO₂, or a mixture of these. Syngas (synthesis gas) is the main product generated by biomass gasification, which consists mainly of H₂, CO, CO₂, N₂, small particles of char, ashes, tars, and oils. However, in most markets, biomass gasification has yet to become consolidated as a mature technology to compete with other methods of energy conversion (Ruiz et al., 2013).

Utilizing various waste materials and by-products for biofuel and biochemical production not only reduced the burden of waste treatment but also provided an alternative way to generate green fuels and chemicals. Such waste feedstocks include various organic wastewater and residues from food processing plants, pulp mills, sugar mills, ethanol or biodiesel plants, and other biorefinery plants. The main components of the waste materials including starch, sugars, glycerol, etc. could be used as carbon sources of various microorganisms for producing bioethanol, biochemicals, and biodiesel feedstocks such as microbial lipids. Sugarcane molasses is a by-product of sugar processing, and has been successfully used for bioethanol production (Dasgupta et al., 2014). Organic effluents from different plants could be well-converted to intracellular lipid by oleaginous microorganisms, which can be used as a feedstock for biodiesel production (Marjakangas et al., 2015; Sun et al., 2015). The by-product glycerol from biodiesel production has many applications for producing chemicals and intermediates. A promising way to utilize this glycerol is to produce 1,3-propanediol, a monomer for producing polytrimethylene terephthalate (PTT). Actually, biological conversion of biodiesel by-product glycerol to 1,3-propanediol has been successfully industrialized in China (Liu et al., 2010). However, the efficiency of conversion of various waste materials to fuels and chemical needs yet to be enhanced. The impurities and inhibitors present in the waste might exert inhibition to the enzymes and microorganisms during the biological conversion. The economic feasibility of the processes still needs comprehensive evaluations.

Marine biomass, including microalgae and macroalgae (seaweeds), would still be one of the centers of bioenergy research. The potential of microalgae for the biodiesel production has been well-recognized. However, the high energy input in microalgae cultivation and microalgae processing limited its application in biofuel production. One of the most important challenges for autotrophic microalgae cultivation is the low growth rate, biomass density, and oil content. Another challenge refers to the high energy consumption for oil extraction because most microalgae has rigid cell wall structure and it is usually energy-intensive to disrupt the wall and release the intracellular oils. Therefore, currently the production of algal oil is primarily confined to high-value specialty oils with nutritional value such as polyunsaturated fatty acid, rather than commodity oils for biofuels (Hu et al., 2008). Modification of the microalgae by genetic engineering might improve the efficacy of CO₂ to oil conversion and increase biomass density. However, more work should be done to extract the intracellular oils in a lower cost and increase the economic competiveness of the microalgae oil-based biofuel system.

The significant progress of the bioenergy industry encourages further exploration on low-carbon technologies for the production of advanced-generation biofuels (and biochemicals) from low-value waste biomass. Collective efforts from various aspects surrounding bioenergy technologies, including politicians, economists, environmentalists, scientists, and engineers, are needed to come up with alternatives, policies, and choices to advance the key technologies for a more sustainable future.