Biomedical Engineering Reference
In-Depth Information
9.9 ENERGY CONVERSION EFFICIENCY
Matsumura (2002) estimated the energy required for SCWG of water hya-
cinth. His analysis came up with a high overall efficiency. Gasafi et al.
(2008) carried out a similar analysis for sewage sludge that came up with a
much lower efficiency. The energy consumption of these two biomass types
is compared in
Table 9.7
. We note that the energy required to pump and pre-
heat the feed is a substantial fraction of the energy produced in an SCW
plant.
Overall efficiency may depend on the type of feedstock used. Yoshida
et al. (2003) studied options for electricity generation from biomass, includ-
ing SCWG combined cycle, thermal gasification, and direct combustion.
They concluded that the SCWG combined cycle offers the highest efficiency
for high-moisture biomass, but it does not for low-moisture fuels.
9.10 MAJOR CHALLENGES
Commercialization of SCW biomass gasification must overcome the follow-
ing major challenges:
SCWG requires a large heat input for its endothermic reactions and for
maintenance of its moderately high reaction temperature. This heat
requirement greatly reduces energy conversion efficiency unless most of
the heat is recovered from the sensible heat of the reaction product. For
this reason, the efficiency of the heat exchanger and its capital cost
greatly affect the viability of SCWG.
TABLE 9.7
Energy Consumption for Gasification of Biomass
Matsumura et al.
(2002)
Gasafi et al.
(2008)
Investigators
Feedstock
Watery hyacinth
Sewage sludge
Potential energy in feed (MW)
4.44
1.44
Energy in product gas (MW)
3.32
1.38
Electricity consumption in pumping and
others (MW)
0.54
0.05
External energy used for feed preheating
(MW)
1.69
0.33
Net energy production (MW)
1.09
0.99
Overall efficiency (%)
24.5
68.6
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