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more hydrogen at a lower temperature than pyrolysis or gasification [2,3,6,83,91]
(Kim and Mitchell, 2012, pers. comm.). SCW gasification also does not require
drying, sizing, and other methods of feed preparations, thereby costing less for the
overall process. The temperature of the pyrolysis and gasification process can be
reduced if the gases coming out of these processes are further steam reformed. This,
however, adds to the overall cost. The rates for the low-temperature processes such
as anaerobic digestion and fermentation can be enhanced with the use of suitable
microbes and enzymes. The development of future hydrogen economy will require
further research in the improvement of these technologies.
Biomass generally contains three important components: cellulose, hemicellulose, and lignin. Both cellulose and hemicellulose (collectively called homocellulose) are easy to hydrolyze, decompose, dehydrogenate, decarboxylate, and reformed
as shown by numerous studies mentioned earlier. Lignin component is generally
toughest to convert. Yamaguchi et al. [123,138] studied lignin gasification in SCW.
They indicated that lignin gasification involves three steps: (1) lignin decomposition
to alkylphenols and formaldehyde in SCW, (2) gasification of alkylphenols and formaldehyde over a catalyst, and (3) formation of char from formaldehyde. They showed
that SCW gasification is a promising technique to reduce the lignin gasification
temperature. They also studied lignin gasification with three different catalysts at
400°C—RuCl 3 /C, Ru(NO)(NO 3 ) 3 /C, and RuCl 3 /C—and found that the order of gasification activity was Ru/C = Ru(NO)(NO 3 ) 3 /C > RuCl 3 /C. Extended x-ray absorption fine structure (ExAFS) analysis showed that during lignin gasification in SCW,
ruthenium particle sizes in Ru(NO)(NO 3 ) 3 /C and Ru/C catalysts were smaller than
that in the RuCl 3 /C catalyst. The study concluded that the ruthenium catalysts with
smaller particle size of metal particles were more active for the lignin gasification.
Lignin is one of the major fractions of woody biomass that is a polymer of aromatic
compounds such as coniferyl alcohol, sinapyl alcohol and, coumaryl alcohol, and it
constitutes about 30 wt% and 40% of energy of woody biomass. Yamaguchi et al.
[123,138] examined the effects of various noble and transition metal catalysts and titania and activated carbon supports on lignin conversion and hydrogen production rates.
The results showed that for the lignin gasification, the activity order followed ruthenium > rhodium > platinum > palladium > nickel, whereas the hydrogen production
rate followed the order palladium > ruthenium > platinum > rhodium > nickel. Both
titania and activated carbon provided stable support. Hydrogen production rate from
lignin increased with temperature and shorter residence time.
Byrd et al. [124] examined a two-stage process to obtain clean fuels from
switchgrass. In the first stage, subcritical hydrothermal liquefaction of switchgrass
was carried out to obtain biocrude that did not contain some of the inorganic and
other undesirable elements. In the second stage, catalytic gasification of biocrude in
SCW was carried out to obtain clean syngas dominant in hydrogen. Biocrude contained many oxygenated hydrocarbons of varying molecular structure and weights,
including lignin-derived products and sugars and their decomposition products. The
supercritical gasification of biocrude was carried out at 600°C and 250 atm pressure.
Nickel, cobalt, and ruthenium catalysts were prepared on titania, zirconia, and magnesium aluminum spinel supports. Magnesium aluminum spinel structure did not
work. Over time, zirconia-supported catalyst plugged the reactor, although Ni/ZrO 2