hydrolysis of the hemicellulose, followed by enzymatic hydrolysis of the remaining
lignocellulose complex (Aden et al. 2002). Using only the experimentally achieved process
parameters and a feedstock cost of $53/dry ton, the calculated minimum ethanol selling price
(MESP) is $2.70/gal. The major barrier issues that are being addressed in the research and
development (R&D) programs are optimal composition of the biomass, hemicellulose
decomposition, recalcitrance of cellulose, fermentation strain robustness, and lignin utilization.
Taken together, the future MESP is forecast to be similar to that of corn ethanol.
Thermochemical Conversion of Lignocellulosics to Fuels
Thermochemical routes involve gasification of the biomass to a syngas followed by catalytic
conversion of the syngas (H 2 + CO) to produce fuels. Another process involves pyrolysis of the
biomass to produce an oil that can be steam-reformed to synthesis gas for the production of
liquid fuels. Unlike the biological processes above, thermochemical routes are indifferent to the
polymeric composition of the materials; they utilize all of the carbon in the conversion, and build
on over a hundred years of conversion of solid fuels in coke and town gas works.
Syngas has been produced since the 1930s from coal or petroleum coke gasification, and its
conversion to hydrogen and Fischer-Tropsch (F-T) liquid (FTL) hydrocarbons and waxes has
also been achieved at a commercial-scale (Courty et al. 1999). Presently, there are many natural
gas-to-liquids projects based on FTL using remote and shut-in gas resources through either
steam-reforming or partial oxidation technology. FTL hydrocarbons have an Anderson, Shultz,
and Flory (ASF) distribution of carbon chain length with degree of conversion over the catalyst.
Recent advances in the area have allowed the production of high chain lengths (waxes) which are
then hydrocracked to the fuel product. Mixed alcohols can also be produced over F-T-like
catalysts, again with an ASF distribution, however, by recycling the methanol (a C 1 product), the
yield of ethanol is increased together with mixed alcohol products having economic values that
are much greater than their values as fuels.
If a plant to produce hydrogen from biomass via syngas were to be built today and the catalysts
were demonstrated to have a long life, the minimum hydrogen selling price is estimated to be
$1.38/kg, with an intermediate syngas price of $6.88/GJ (Spath and Dayton 2003).
HYDROGEN FROM ELECTROLYSIS OF WATER
Less than 1% of the hydrogen produced in the United States is made with electrolyzers running
off of grid power generated using carbon-based fuels. The bulk of hydrogen production is done
through the steam-reforming of natural gas for use in fertilizer production, and this gas is too
impure to be used in the fuel cells envisioned for transport vehicles. As a result of the recent
focus on a hydrogen economy, many studies have been made of the present status of this small
market technology for electrolytic hydrogen production. Elements of these analyses (Ivy 2004;
NRC and NAE 2004; DOE 2004) are presented here to provide a reference point for solar
hydrogen production.