Anaerobic Digestion
One gasification process that has reached widespread application is the use of biogas, a mixture
of methane and carbon dioxide produced in anaerobic digestion processes. This occurs naturally
in landfills and in the last few decades, landfill gas (LFG) has not only been collected for use (for
safety and environmental reasons), but has also become a major source of renewable electricity
generation in Europe and the United States. There is approximately 1 GW of installed LFG
capacity in the United States, with units ranging from hundreds of kilowatts to 50 MW. There are
also industrial-scale constructed anaerobic digesters for urban residues, which have much shorter
residence times than landfills (operating at 35ºC and 50ºC, rather than the ambient ground
temperature), as well as increased resource recovery potentials (IEA 1998).
Liquid Fuels and Hydrogen
The only way to decarbonize light-duty vehicle fuels (about one-third of U.S. energy use) is to
remove the fossil carbon by (1) changing to electricity or hydrogen produced from non-carbon
sources, (2) using fossil fuel processes, which capture and sequester the carbon dioxide, or
(3) replacing the fossil carbon with renewable carbon. Decarbonization constitutes a major
research direction for biomass programs. The major biomass-derived fuel is ethanol produced
from sugars in Brazil, and from starches in the United States and Europe.
Three countries — Brazil, the United States, and India — account for nearly 90% of the world’s
ethanol production from biomass (using sugar cane and cereals). Current world ethanol
production is about 0.02 TW. The total production was 21 metric tonnes in 2002 (FAOSTAT
2001) and the rapidly growing U.S. amount was 6.1 metric tonnes in that year. The U.S. statistics
up through 2004 are shown in gasoline gallons equivalent and also as a fraction of the total corn
(Zea maize) consumption.
Potentially, the ethanol yield in either process could be increased by 10–20% by converting
residual starch and the hemicellulose and cellulose in the remaining corn solids to ethanol. Other
development efforts involve higher-value by-products. The industry has demonstrated a very
significant learning curve in the past 25 years, reducing the energy consumption within its
processes, increasing the yield of ethanol, and significantly reducing the capital investment to
levels approaching 1.00 $ annual gallon production capacity (IEA 2004). Typical dry mill plant
sizes are about 40–50 million gallons year (IEA 2004); such mills are increasingly being
constructed by farmers’ cooperatives in the grain-growing areas to reduce the costs of corn
transportation.
Lignocellulosics to Ethanol by Means of Bioconversion
One method of converting biomass to biofuels is depolymerization of the cellulose and
hemicellulose into their component sugars, followed by bioconversion of those sugars to the fuel.
Multiple methods of cellulose and hemicellulose depolymerization have been researched. Those
methods include chemical treatments, biological enzyme treatments, and combinations of the
two. The combination that has received the most research funding and analysis is dilute-acid