Though wind and solar energy are important alternative energy sources, it will take second generation biofuels technologies to reduce U.S. dependence on petroleum. While debate rages over the impact of first generation biofuels (corn ethanol) on food and feed prices, experts believe that advanced biofuels, created from non-food resources, will avoid direct competition with food and feed supplies. They may, however, compete for land use.
Controversial ethanol (C2H5OH) is produced by fermentation of starches from corn grain--an important feed and food commodity in the U.S. Currently, there is currently no other readily available starch-or sugar-based crop in the U.S. from which to ferment ethanol in large quantities. Unless cellulosic biomass feedstocks can be utilized, production is limited.
Cellulosic feedstocks are abundant and do not directly compete with food and feed needs. By developing both biochemical and thermochemical conversion routes, cellulosic ethanol production can use essentially the entire biomass resource base available, and if converted to ethanol, can potentially displace over 50% of our current gasoline usage. While estimated production costs for both biochemical and thermochemical methods are currently higher than gasoline or corn ethanol, substantially more can be done to bring those numbers down.
Although cellulosic ethanol has great promise for addressing our nation’s transportation needs, it does have some limitations, including its reduced energy content compared to gasoline, resulting in lower mpg for today’s vehicles. Ethanol also is not fully compatible with the existing transportation fuel infrastructure and is only suitable as a gasoline replacement. It does nothing to address the need for diesel and jet fuels.
One fuel offering certain advantages over ethanol is butanol (C4H9OH), a member of the alcohol family. It can be produced by a fermentation process similar to ethanol and has a significantly higher energy density (though lower than gasoline). It also has less tendency to absorb water and corrode pipes so it is more compatible with the existing fuel infrastructure. However, butanol is more difficult to produce than ethanol, and the economics and technology remain well behind that of ethanol.
Still another emerging technology is the biochemical conversion of sugars to produce hydrocarbon fuels that are substantially similar to gasoline, diesel and jet fuels. These hydrocarbon fuels would be compatible with the existing fuel infrastructure and have a higher energy density compared to alcohols. The engineered microorganisms used to produce hydrocarbons have a production rate similar to the rate of microbial ethanol production. Since the production of butanol and hydrocarbons can potentially use the same resource base as ethanol, the potential production volume is quite large.
Thermochemical conversion technologies also show considerable promise for synthesis of fuels beyond ethanol. Thermochemical conversion can use a wider range of feedstocks and produce a broader spectrum of fuels. At high temperatures and pressures, this method converts biomass to intermediate liquids or gases, which can then be synthesized into fuels by numerous proven and emerging technologies. Using chemical catalysts, biomass conversion processes have the potential to greatly improve the production rates, as reaction rates often increase exponentially with increasing temperature.
Some thermochemical conversion approaches show considerable promise for producing hydrocarbon fuels similar to gasoline, diesel, and jet fuels, and, therefore, could take advantage of the existing petroleum refining and fuel distribution infrastructure. Since thermochemical conversion technologies can essentially capture the entire feedstock resource base, their potential production volume is also quite large - potentially larger than biochemical conversion because thermochemical processes are generally more robust and flexible with regard to feedstock.
[Adapted from “Advanced Biofuels in the Transition Towards Clean Energy,” by Thomas Foust and Matthew Yung, National Renewable Energy Laboratory, for ME Today.]