Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 329

TABLE 9.2 Three Categories of Hydrothermal Gasification of Biomass

Based on the Target Product ( T c B

374.29 C)

Temperature Range Catalyst Use Desired Product

500 C)

High temperature ( Not needed Hydrogen-rich gas

.

Medium temperature (T c to 500 C) Needed Methane-rich gas

Low temperature ( T c ) Essential Other gases from smaller

organic molecules

,

simple organic compounds as its feedstock. The last two groups, because of
their low-temperature operation, need catalysts for reactions.

9.4.2 Catalysts
An effective degradation of biomass and the gasification of intermediate products
of thermal degradation into lower-molecular-weight gases like hydrogen require
the SCW reactor to operate in the high-temperature range ( 600 C). The higher

the temperature, the better the conversion, especially for production of hydrogen,
but lower the SCW's energy efficiency. A lower gasification temperature is there-
fore desirable for higher thermodynamic efficiency of the process.
Catalysts help gasify the biomass at lower temperatures, thereby retain-
ing, at the same time, high conversion and high thermal efficiency.
Additionally, some catalysts also help gasification of difficult items like the
lignin in biomass. Watanabe et al. (2003) noted that the hydrogen yield from
lignin at 400 C and 30 MPa is doubled when a metal oxide (ZrO 2 ) catalyst
is used in the SCW. The yield increases four times with a base catalyst
(NaOH) compared to gasification without a catalyst. The three principal
types of catalyst used so far for SCW gasification are: (1) alkali, (2) metal,
and (3) carbon-based.
An important positive effect of catalysts in SCWG is the reduction in
required gasification temperature for a given yield. Minowa et al. (1998)
noted a significant reduction in unconverted char while gasifying cellulose
with a Na 2 CO 3 catalyst at 380 C. Base catalysts (e.g., NaOH and KOH)
offer better performance, but they are difficult to recover from the effluent.
Some alkalis (e.g., NaOH, KOH, Na 2 CO 3 ,K 2 CO 3 , and Ca(OH) 2 ) are also
used. They, too, are difficult to recover.
The special advantage of metal oxide catalysts is that they can be recov-
ered, regenerated, and reused. Commercially available nickel-based catalysts
are effective in SCW biomass gasification. Among them, Ni/MgO (nickel
supported on an MgO catalyst) shows high-catalytic activity, especially for
biomass (Minowa et al., 1998).

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