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.
hydrolysis <strong>of</strong> the hemicellulose, followed by enzymatic hydrolysis <strong>of</strong> the remaining<br />
lignocellulose complex (Aden et al. 2002). Using only the experimentally achieved process<br />
parameters and a feedstock cost <strong>of</strong> $53/dry ton, the calculated minimum ethanol selling price<br />
(MESP) is $2.70/gal. The major barrier issues that are being addressed in the research and<br />
development (R&D) programs are optimal composition <strong>of</strong> the biomass, hemicellulose<br />
decomposition, recalcitrance <strong>of</strong> cellulose, fermentation strain robustness, and lignin utilization.<br />
Taken together, the future MESP is <strong>for</strong>ecast to be similar to that <strong>of</strong> corn ethanol.<br />
Thermochemical Conversion <strong>of</strong> Lignocellulosics to Fuels<br />
Thermochemical routes involve gasification <strong>of</strong> the biomass to a syngas followed by catalytic<br />
conversion <strong>of</strong> the syngas (H2 + CO) to produce fuels. Another process involves pyrolysis <strong>of</strong> the<br />
biomass to produce an oil that can be steam-re<strong>for</strong>med to synthesis gas <strong>for</strong> the production <strong>of</strong><br />
liquid fuels. Unlike the biological processes above, thermochemical routes are indifferent to the<br />
polymeric composition <strong>of</strong> the materials; they utilize all <strong>of</strong> the carbon in the conversion, and build<br />
on over a hundred years <strong>of</strong> conversion <strong>of</strong> solid fuels in coke and town gas works.<br />
Syngas has been produced since the 1930s from coal or petroleum coke gasification, and its<br />
conversion to hydrogen and Fischer-Tropsch (F-T) liquid (FTL) hydrocarbons and waxes has<br />
also been achieved at a commercial-scale (Courty et al. 1999). Presently, there are many natural<br />
gas-to-liquids projects based on FTL using remote and shut-in gas resources through either<br />
steam-re<strong>for</strong>ming or partial oxidation technology. FTL hydrocarbons have an Anderson, Shultz,<br />
and Flory (ASF) distribution <strong>of</strong> carbon chain length with degree <strong>of</strong> conversion over the catalyst.<br />
Recent advances in the area have allowed the production <strong>of</strong> high chain lengths (waxes) which are<br />
then hydrocracked to the fuel product. Mixed alcohols can also be produced over F-T-like<br />
catalysts, again with an ASF distribution, however, by recycling the methanol (a C1 product), the<br />
yield <strong>of</strong> ethanol is increased together with mixed alcohol products having economic values that<br />
are much greater than their values as fuels.<br />
If a plant to produce hydrogen from biomass via syngas were to be built today and the catalysts<br />
were demonstrated to have a long life, the minimum hydrogen selling price is estimated to be<br />
$1.38/kg, with an intermediate syngas price <strong>of</strong> $6.88/GJ (Spath and Dayton 2003).<br />
HYDROGEN FROM ELECTROLYSIS OF WATER<br />
Less than 1% <strong>of</strong> the hydrogen produced in the United States is made with electrolyzers running<br />
<strong>of</strong>f <strong>of</strong> grid power generated using carbon-based fuels. The bulk <strong>of</strong> hydrogen production is done<br />
through the steam-re<strong>for</strong>ming <strong>of</strong> natural gas <strong>for</strong> use in fertilizer production, and this gas is too<br />
impure to be used in the fuel cells envisioned <strong>for</strong> transport vehicles. As a result <strong>of</strong> the recent<br />
focus on a hydrogen economy, many studies have been made <strong>of</strong> the present status <strong>of</strong> this small<br />
market technology <strong>for</strong> electrolytic hydrogen production. Elements <strong>of</strong> these analyses (Ivy 2004;<br />
NRC and NAE 2004; DOE 2004) are presented here to provide a reference point <strong>for</strong> solar<br />
hydrogen production.<br />
203