Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 343

Product

Biomass

SCW
gasifier

Separator

Oxygen/Air

Char Slurry

pump

Combustor

Heat exchanger

HP pump

Product

Water

FIGURE 9.12 A conceptual system for combustion of residual carbon deposited on solid
catalysts to provide heat for SCWG of biomass.

dissolved oxygen and solid char may be lower than that between gas and char.
This, along with its high-density feature, may allow the SCW to conduct the
combustion reaction quickly and efficiently. Another advantage of low-
temperature combustion is that it avoids formation of toxic by-products.

9.7.6 Design of Gas Liquid Separator System

In an SCWG system, the product gas mixture is separated from water in two
stages. In the first stage, initial separation takes place in a high-pressure but
low-temperature separator. In the second stage, final separation occurs under
low pressure and low temperature.
At low temperatures (25 100 C), hydrogen or methane has very low sol-

ubility (0.001 0.006) in water, even at high pressure ( Figure 9.13 ). So the

bulk of the hydrogen is separated from the water when cooled. From
Figure 9.14 one notes that the solubility of carbon dioxide is an order of
magnitude higher (0.01

0.03) than that of hydrogen ( Figure 9.13 ) at this
low temperature and high pressure. Figure 9.15 shows one scheme where
hydrogen separated using its low solubility features. Other gases like CO 2
are also separated from the water but to a limited extent.
This feature can be exploited to separate the hydrogen from the carbon diox-
ide, but the CO 2 's equilibrium concentration may not be sufficient to dissolve it
entirely in the high-pressure water. Additional water may be necessary to dissolve
all of these gases except hydrogen so that the hydrogen alone remains in the gas
phase (S1, Figure 9.14 ). The equilibrium concentration of these gases in water
can be calculated from the equation of state, such as Peng

Robinson or SAFT.