Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 327

The next step involves separation of the reaction products. The solubility
of hydrogen and methane in water at low temperature but high pressure is
considerably low, so they are separated from the water after cooling while
the carbon dioxide, because of its high solubility in water, remains in the liq-
uid phase. For complete separation of CO 2 , the gas may be scrubbed with
additional water (refer to Figure 9.15 later in this chapter). The gaseous
hydrogen is separated from the methane in a pressure swing adsorber. The
CO 2 -rich liquid is depressurized to the atmospheric pressure, separating the
carbon dioxide from the water and unconverted salts.
9.4 EFFECT OF OPERATING PARAMETERS ON SCW
GASIFICATION
The product of gasification is defined by its yield and composition which are
influenced by a number of gasifier design and operating parameters. For
proper design and operation of an SCW gasifier, a good understanding of the
influence of the following parameters is important:
Reactor temperature
Catalyst use
Residence time in the reactor
Solid concentration in the feed
Heating rate
Feed particle size
Reactor pressure
Reactor type
9.4.1 Reactor Temperature
Temperature has an important effect on the conversion, the product distribu-
tion, and the energy efficiency of an SCW gasifier, which typically operates
at a maximum temperature of nearly 600 C. The overall carbon conversion
increases with temperature; at higher temperatures hydrogen yield is higher
while methane yield is lower. Figure 9.7 shows the temperature dependence
of gasification efficiency and product distribution in a reactor operated at
28 MPa (30-s residence, 0.6-M 1 glucose) (Lee et al., 2002). We see that the
hydrogen yield increases exponentially above 600 C, while the CO yield,
which rises gently with temperature, begins to drop above 600 C owing to
the start of the shift reaction Eq. (7.52).
Gasification efficiency is measured in terms of hydrogen or carbon in the
gaseous phase as a fraction of that in the original biomass. Carbon conver-
sion efficiency increases continually with temperature, reaching close to
1 M - mol/liter