by David R. Shonnard, PhD, Michigan Technological University
Q: What biofuels are in development other than corn-based ethanol and biodiesel blends that have good potential? — Gordon M., York, ME
A: Feedstocks for biofuels with the greatest potential to displace imported petroleum are in the form of cellulosic biomass, including residues from agriculture and forest operations, urban woody waste, and energy crops such as hybrid poplar, hybrid willow and switchgrass. Therefore, to answer this question, we need to look at biofuels derived from woody biomass.
Ethanol can be produced from woody biomass, as it can from corn. However, there are several important differences between cellulosic ethanol and corn-starch ethanol. The sugars derived from woody biomass are a mixture of 5- and 6-carbon sugars, and as a result, fermentation organisms and processing conditions are more challenging than for corn ethanol.
Research continues to improve the enzymes and microorganisms needed to boost sugar and ethanol yields from woody biomass and to reduce costs of production. A fraction of the woody biomass known as lignin, which is not converted to sugars, is dried and burned to produce heat and power for the conversion process. This eliminates the need for fossil energy resources for the conversion steps to ethanol. As a result, the life-cycle reduction of fossil energy and greenhouse gas from cellulosic ethanol compared to petroleum gasoline on an equivalent energy basis is much improved compared to fossil gasoline AND corn ethanol.
Thus far I have not discussed biofuels other than ethanol. Woody biomass can also be converted to a true hydrocarbon fuel (no oxygen in the fuel) that is nearly identical at a molecular level to petroleum liquid fuels, using thermochemical conversion steps: gasification and pyrolysis. Gasification converts the solid biomass into a synthesis gas comprised mostly of CO and H2. Pyrolysis converts solid biomass into an oily substance. In either case, these components are catalytically converted to hydrocarbon fuels called synthesis gasoline and synthesis diesel (syngas and syndiesel).
By adjusting reaction conditions, other fuels like jet fuel can also be produced. The obvious advantage of producing these synfuels is that their compatibility with existing vehicular transportation infrastructure is almost perfect, requiring very little if any modification. Studies have shown that the life-cycle, fossil energy, and greenhouse gas reduction of woody biomass-based synfuels compared to petroleum gasoline on an equivalent energy basis also is much improved compared to fossil gasoline AND corn ethanol.
Given the availability of woody biomass on a sustainable annual basis in the U.S. (1 billion tons per year without competing with food production), these cellulosic biofuels could displace approximately 50% of current gasoline consumption (equivalent to 20% of total petroleum use or 1/3 of imported petroleum).
Biofuels are not a total solution to the issues of energy security or greenhouse gas reduction, but significant benefits are expected in the near future, at the same time that technology continues to advance in other sustainable energy technologies (solar power, hydrogen fuels, carbon capture and sequestration, etc.).
Source: Renewable Energy World