Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Anaerobic Digestion<br />
One gasification process that has reached widespread application is the use <strong>of</strong> biogas, a mixture<br />
<strong>of</strong> methane and carbon dioxide produced in anaerobic digestion processes. This occurs naturally<br />
in landfills and in the last few decades, landfill gas (LFG) has not only been collected <strong>for</strong> use (<strong>for</strong><br />
safety and environmental reasons), but has also become a major source <strong>of</strong> renewable electricity<br />
generation in Europe and the United States. There is approximately 1 GW <strong>of</strong> installed LFG<br />
capacity in the United States, with units ranging from hundreds <strong>of</strong> kilowatts to 50 MW. There are<br />
also industrial-scale constructed anaerobic digesters <strong>for</strong> urban residues, which have much shorter<br />
residence times than landfills (operating at 35ºC and 50ºC, rather than the ambient ground<br />
temperature), as well as increased resource recovery potentials (IEA 1998).<br />
Liquid Fuels and Hydrogen<br />
The only way to decarbonize light-duty vehicle fuels (about one-third <strong>of</strong> U.S. energy use) is to<br />
remove the fossil carbon by (1) changing to electricity or hydrogen produced from non-carbon<br />
sources, (2) using fossil fuel processes, which capture and sequester the carbon dioxide, or<br />
(3) replacing the fossil carbon with renewable carbon. Decarbonization constitutes a major<br />
research direction <strong>for</strong> biomass programs. The major biomass-derived fuel is ethanol produced<br />
from sugars in Brazil, and from starches in the United States and Europe.<br />
Three countries — Brazil, the United States, and India — account <strong>for</strong> nearly 90% <strong>of</strong> the world’s<br />
ethanol production from biomass (using sugar cane and cereals). Current world ethanol<br />
production is about 0.02 TW. The total production was 21 metric tonnes in 2002 (FAOSTAT<br />
2001) and the rapidly growing U.S. amount was 6.1 metric tonnes in that year. The U.S. statistics<br />
up through 2004 are shown in gasoline gallons equivalent and also as a fraction <strong>of</strong> the total corn<br />
(Zea maize) consumption.<br />
Potentially, the ethanol yield in either process could be increased by 10–20% by converting<br />
residual starch and the hemicellulose and cellulose in the remaining corn solids to ethanol. Other<br />
development ef<strong>for</strong>ts involve higher-value by-products. The industry has demonstrated a very<br />
significant learning curve in the past 25 years, reducing the energy consumption within its<br />
processes, increasing the yield <strong>of</strong> ethanol, and significantly reducing the capital investment to<br />
levels approaching 1.00 $ annual gallon production capacity (IEA 2004). Typical dry mill plant<br />
sizes are about 40–50 million gallons year (IEA 2004); such mills are increasingly being<br />
constructed by farmers’ cooperatives in the grain-growing areas to reduce the costs <strong>of</strong> corn<br />
transportation.<br />
Lignocellulosics to Ethanol by Means <strong>of</strong> Bioconversion<br />
One method <strong>of</strong> converting biomass to bi<strong>of</strong>uels is depolymerization <strong>of</strong> the cellulose and<br />
hemicellulose into their component sugars, followed by bioconversion <strong>of</strong> those sugars to the fuel.<br />
Multiple methods <strong>of</strong> cellulose and hemicellulose depolymerization have been researched. Those<br />
methods include chemical treatments, biological enzyme treatments, and combinations <strong>of</strong> the<br />
two. The combination that has received the most research funding and analysis is dilute-acid<br />
202