Biofuels account for the major proportion of bioenergy production worldwide, with most of the fuels being derived through biochemical processes. For this reason, this review will focus in the main on current prac­tices used in the production of the main biofuels. The major producers of bioethanol are Brazil and the United States, both of which account for about 89% of world pro­duction (World Development Report, 2008; Lichts, 2010), while the European Union is the world’s largest producer of biodiesel (OECD-FAO, 2009). The United States has been the world’s largest producer of ethanol fuel since 2005 and the world’s largest exporter since 2010. In 2011, the United States produced 52.6 billion liters (13.9 billion US liquid gallons) of ethanol, while Brazil pro­duced 21.1 billion liters (5.57 billion US liquid gallons), representing 24.9% of the world’s total ethanol used as fuel (Renewable Fuels Association, 2012).

Fuel ethanol production is considerably more modest in the European Union, where France, Germany and Spain are the largest producers of bioethanol producing 950, 581 and 346 million liters, respectively, in 2008 (European Bioethanol Fuel Association, 2009). Countries such as Poland, Hungary and Slovakia have also increased their bioethanol output producing 200, 150 and 94 million liters of bioethanol, respectively (European Bioethanol Fuel Association, 2009). Sweden is the leading country in Europe in terms of the use of ethanol as fuel, the impetus for which is driven by government policy. Although most of the ethanol is imported, Swedish gas stations are required by an act of parliament to offer at least one alternative fuel. Furthermore, reductions in biofuel prices to the consumer have also encouraged biofuel consumption. Government incentives for biofuel replacement of gasoline are now being implemented in other countries worldwide, motivated by ever-increasing oil costs, depleting fossil fuel resources, GHG emission targets and the need for greater diversification to support agri­cultural and rural development (Mussato et al., 2010).

The major feedstock for bioethanol in Brazil is sugar­cane including bagasse, while corn grain/maize is the main feedstock used for bioethanol production in the United States. As mentioned earlier, bioethanol can be produced from any sugar or starch crop in first — generation processes, but other potential resources for bioethanol include sugar beet, cassava, maize, oil palm, rapeseed, soybean, corn stover, grass, leaves, agri­crop residues and various locally available nonfood plant biomass like Jatropha, Miscanthus, willow, hemp and switchgrass. Table 2.1 summarizes the major crops/biomass currently (ranked in order of impor­tance) in use for biofuel and bioenergy production in different countries. Shapouri (1995), Shapouri et al. (2002) concluded that the energy content of bioethanol was higher than the energy required to produce it, although other researchers would argue as to the economic viability of bioethanol in the absence of an accompanying high-value biorefinery process.

Production of ethanol from lignocellulosic biomass is a complex process where the biomass often requires pre­treatment to render the holocellulose more accessible to a mixture of enzymes, which are utilized to saccharify or hyrolyze the complex polysaccharides to fermentable sugars. Pretreatment processes can be expensive, toxic and corrosive and may require a subsequent costly detox­ification step (Agbor et al., 2011; Zhang and Lynd, 2004; Sun and Cheng, 2002). In addition, preparation of fermentable sugars and the inhibitory effect of lignin and carbohydrate-derived compounds, formed during pretreatment of the lignocelluloses, are the major bottle­necks in bioconversion processes (Viikari et al., 2007). However, since biomass energy is derived from renew­able resources, its production can still be advantageous if proper management technologies are utilized in biomass harvesting, pretreatment and processing, and if biomass feedstocks are produced sustainably. Plant

2. BIOENERGY RESEARCH: AN OVERVIEW ON TECHNOLOGICAL DEVELOPMENTS AND BIORESOURCES

biomass to energy or chemicals can be economical only if all of the components in the biomass are converted into fuel, chemicals or other value-added components in a true biorefinery approach (FitzPatrick et al., 2010; Cherubini, 2010; Percival Zhang, 2008; Kamm and Kamm, 2004).