Current bioenergy resources consist of residues from forestry and agriculture, various organic waste streams and dedicated biomass production from pasture land, wood plantations and sugar cane (Figure 2.2). At pre­sent, the main biomass feedstocks for electricity and heat generation are forestry and agricultural residues and municipal waste in cogeneration and cofiring power plants. In the longer term, lignocellulosic crops could provide bioenergy resources for second-generation bio­fuels, which are considered more sustainable, provide land use opportunities and will reduce the competition with food crops (http://www. ga. gov. au/image_cache/ GA16706.pdf).

Major feedstock sources for future biofuel production are likely to be high biomass producing plant species such as poplar, pine, switchgrass, sorghum maize, Miscanthus, hemp, Jatropha, willow and cassava. With
growing interest in the utilization of plant biomass for the production of ethanol and other biofuels, the use of plant species as biofuel feedstocks has become a focal point in research. Due to concerns about diverting grain and seed from human food and livestock feed to biofuel feedstock production, emphasis has shifted to the use of lignocellulose-derived biofuel production, and research is now directed at improving not only lignocellulosic yield but also quality traits in these species (Banerjee, 2011; Mueller et al., 2011; Tyner, 2010).

A long-term opportunity exists to produce fuels from nonedible lignocellulosic biomass from plants (Heather and Somerville, 2012). Sugarcane, energy cane, elephant grass, switchgrass, and Miscanthus have intrinsically higher light, water and nitrogen use efficiency and are fast-growing biomass/crops for bioenergy work pro­gram. Work on perennial grasses such as switchgrass (Panicum spp.), prairie cordgrass (Spartina spp.), big bluestem (Andropogon spp.), little bluestem (Schizachy — rium spp.) and others could produce significant biomass in a variety of biomass throughout the northern plains and southeastern grasslands in the United States (Gonzalez-Hernandez et al., 2009). Woody biomass can be harvested sustainably for lumber and paper and may, therefore, provide biofuel feedstock for some re­gions (Malmsheimer et al., 2011). Table 2.5 summarizes the countrywise contribution of current biofuel yield from different feedstocks.

As mentioned previously, biomass energy can come from numerous sources and produce several types of fuels. Ethanol is typically produced from biomass

Wood industry

residues 5%

Recovered

wood 6%

Municipal solid

waste and landfill gas

high in carbohydrates (sugar, starch and cellulose) dur­ing a fermentation process. Recent developments in fermentation processes now allow almost any plant type to be used to produce ethanol. The most prom­ising natural oils, such as rapeseed oil, have been used to produce biodiesel, which performs much like petroleum-derived diesel fuel. Apart from agricultural, forestry and other by-products, the main source of lignocellulosic biomass for second-generation biofuels is likely to be from "dedicated biomass feedstocks", such as certain perennial grass and forest tree species. Genomics, genetic modifications and other biotechnol­ogies are all being investigated as tools to produce plants with desirable characteristics for second — generation biofuel production, for example, plants that produce less lignin (a compound that cannot be fermented into liquid biofuel), plants that produce en­zymes themselves for cellulose and/or lignin degrada­tion, or plants that produce increased cellulose or overall biomass yields. Grass, leaves, agri crops, agri­crop residues and currently available nonfood plant biomass are the dominant source of lignocellulosic materials (Carpita, 2012; Ambavaram et al., 2011;

Abramson et al., 2010; Davison et al., 2006; Nguyen et al., 1999, 2000).

Bioenergy resources used in current biofuels develop­ment programs, potential future resources and the related bioenergy outputs are summarized in Table 2.6. Bioenergy resources are difficult to estimate due to their multiple and competing uses. Production statistics exist for current commodities such as grain, sugar, pulp wood and saw logs; however, these commodities are currently largely committed to food, animal feed and materials markets. Potential feedstocks for the future include modified strains of existing crops, new tree crops and algae. There are many factors to be taken into ac­count for each bioenergy resource, such as moisture con­tent, resource location and distribution, and type of conversion process that is most suitable. Different sour­ces of biomass require very different production systems and therefore a variety of sustainability issues can arise. These range from very positive benefits (e. g. use of waste material, or growing woody biomass on degraded agricultural land) through to large-scale diversion of high-input agricultural food crops for bio­fuels (O’Connell et al., 2009).