Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 335

6. Combustion of methanol produced by SCWG
7. Anaerobic digestion
SCWG has the distinction of easily separating CO 2 from the product gas.
This makes it an optimal technology for generation of electricity and heat
from biomass when CO 2 emission limits become binding.
Fuel cells have the highest energy conversion efficiency for electricity
generation, but they need hydrogen as their fuel. For hydrogen production,
from very wet biomass, SCW gasification could be an attractive route.
However, the capital costs of a fuel cell and that of a gasification plant have
an important bearing on the economic viability of this generation option.
9.5.2 Waste Remediation
Waste treatment is another SCWG application. As explained in Section 9.3.3 ,in
SCW even highly toxic wastes can be oxidized to harmless disposable residues.
The agricultural industry produces large volumes of nontoxic but unhealthy pro-
ducts such as animal extracts and farm wastes that need to be disposed of
productively. Many of these contain so much moisture that economical combus-
tion or thermal gasification is not possible. Anaerobic digestion is a widely used
alternative, especially in developing countries for production of useful gas
(mostly methane) from animal extracts. Along with methane, anaerobic diges-
tion produces fermentation sludge, which can be used as fertilizer.
Nevertheless, anaerobic gasification is orders of magnitude slower than
thermal and other gasification processes, even with the use of catalysts. As a
result, this makes large-scale commercial operation of anaerobic digesters
difficult. Furthermore, the attractiveness of this method depends on the price
of fertilizer, which can vary as a result of over- or undersupply in the market
(Matsumura, 2002).
SCWG or SCWO is an alternative suitable for waste treatment because it
does not depend on the production of sludge and is much faster than anaero-
bic digestion. Matsumura (2002) noted that SCWG has better energy
efficiency, cheaper gas production, and faster CO 2 payback time (64.8%,
3.05 yen/MJ, and 4.19 years, respectively) in comparison with biomethana-
tion (49.3%, 3.74 yen/MJ, and 5.05 years, respectively).
9.5.3 Chemical Production
Solvents are an important component of many chemical reactions. SCW acts
as a solvent, but can also be a reactant and/or a catalyst. Ordinary subcritical
water is popular as a solvent for reactions, especially because it is inexpen-
sive and easily disposed of. Many organics, however, do not react efficiently
in it. For these reactions, acid or base solvents are needed, which are good