Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 330

Metal catalysts have a severe corrosion effect at the temperatures needed
to secure high yields of hydrogen. To overcome this problem, Antal et al.
(2000) used carbon (e.g., coal-activated and coconut shell-activated carbon
and macadamia shell and spruce wood charcoal). The carbon catalysts
resulted in high yields of gas without tar formation.

9.4.3 Residence Time
A longer residence of the reactants in the reactor gives a better yield. Lu
et al. (2006) experimented with 2% (by weight) sawdust and 2% carboxy-
methyl cellulose (CMC) in a flow reactor at 650 C and 25 MPa. Mettanant
et al. (2009b) experimented with 2% rice husk in a batch reactor under the
same conditions. Both found a steady increase in hydrogen and a moderate
increase in methane ( Figure 9.8 ) when the residence time was increased by
three times and six times, respectively. Total organic carbon in the liquid
product decreases with residence time, whereas carbon and hydrocarbon gas-
ification efficiencies increase. This implies that a longer residence time is
favorable for SCW biomass gasification. The optimum residence time,
beyond which no further improvement in conversion efficiency is possible,
depends on several factors. At a higher temperature,

the residence time

required for a given conversion is shorter.

9.4.4 Solid Concentration in Feedstock
Unlike in other gasification methods, solids fraction in the feed have an
important effect on the gasification in SCW. Thermodynamic calculations

18
16
14
12
10
8
6
4
2
0

10 20 30 40 50 60

Residence time (min)

H 2 CH 4 CO CO 2

FIGURE 9.8 Effect of residence time on the gasification of 2% rice husk in SCW at 650 C,
30 MPa in a batch reactor.