The lowest-cost biofuels are expected to continue to be ethanol produced from sugar cane and biodiesel produced from recycled cooking oil and waste grease. Beyond these two least-cost options, the costs for producing next-generation biofuels are expected to be in a range that should make them generally competitive with first- generation technologies. The ability of next-generation technologies to use abundant cellulosic feedstock that do not rely on food crops offers the promise of dramatically expanding the amount of biofuels that could be produced for transportation needs in the future.
When considering the potential sources of biomass energy, a distinction can be made between biomass that is specifically cultivated for energy purposes (i.e. energy crops grown on existing agricultural or marginal lands), and primary, secondary, and tertiary residues and wastes:
Primary residues are produced during production of food crops and forest products. They include straw, corn stover (stalks), or wood thinnings from commercial forestry. Such biomass streams are typically available “in the field” and must be collected to be available for further use.
Secondary residues are generated during processing of biomass for production of food products or biomass materials. They include nutshells, sugar cane bagasse (the residue from cane crushing), and sawdust, and are typically available at food and beverage industries, saw and paper mills, etc.
Tertiary residues become available after a biomass-derived commodity has already been used. A diversity of waste streams is part of this category, from the organic fraction of municipal solid waste (MSW) to waste and demolition wood, sludges, etc.
Table 3 on the following page provides an overview of the potential contribution of each of these biomass types to the global energy supply by the year 2050. (For a more detailed discussion of the potential ethanol yields from MSW, see also Sidebar 1.)
Bioenergy’s potential is enormous. Studies suggest that biomass could potentially supply anywhere from 0 EJ to more than 1,000 EJ of energy by the year 2050. In the most optimistic scenarios, bioenergy could provide for more than two times the current global energy demand, without competing with food production, forest protection efforts, and biodiversity. In the least favorable scenarios, however, bioenergy could supply only a fraction of current energy use by 2050, perhaps even less than it provides today.