For biofuels to reach their full potential in meeting future transportation needs, it is critical to develop and deploy economically competitive technologies that can convert abundant cellulosic biomass resources into liquid. Development efforts to date have demonstrated that it is possible to produce a variety of liquid fuels from cellulosic biomass for use in existing vehicles. As of mid-2006, however, the costs of producing liquid fuels from cellulosic biomass were not competitive with either petroleum-derived fuels or more conventional biofuels. Various government and industry-sponsored efforts are under way to lower the costs of making liquid fuel from cellulosic biomass by improving the conversion technologies.
Figure 6 below highlights four primary pathways for bioenergy production: combustion, gasification, pyrolysis, and hydrolysis. This report focuses primarily on gasification (a thermochemical pathway) and hydrolysis (a biochemical pathway). Both pathways can provide a variety of products in addition to producing liquid fuels for transportation uses.
Figure 6. Lignocellulose Processing Pathways
Power generation and/or heat transmission
Power and/or heat
Sugars & lignin
Fuels, Power, Chemicals, Materials, and/or heat (many combinations possible)
Source: Darmouth College and NRDC
The hydrolysis pathway relies on advanced enzymes that can catalyze cellulose and lignocellulose into sugars and then ethanol. The gasification (and specifically, the Fischer-Tropsch synthesis) pathway uses high temperatures, controlled levels of oxygen, and chemical catalysts to convert biomass into liquid fuels, including synthetic diesel and di-methyl ether (DME).
The gasification pathway is also called the biomass-to-liquid (BTL) pathway, and generally requires a larger-sized facility and a larger capital investment. In general, improvements in this area appear to be occurring more slowly than the advances in biotechnology that are propelling the hydrolysis pathway. However, the BTL pathway can also process lignin, which comprises about one-third of plant solid matter, and can thus achieve higher liquid yields, displacing more petroleum. Accordingly, one detailed analysis of different conversion pathways concluded that a combination of the hydrolysis and BTL pathways was the most economical and energetically