Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 326

Combustion air
+ CH 4

Flue gas

Product
gas

SCW reactor

Heat exchanger

Phase
separator

Product

Clean
water

Waste effluent pump

FIGURE 9.6 Schematic of a pilot plant for SCWG of biomass.

It is then pumped to the required supercritical pressure. Alternatively, water
may be pressurized separately and the biomass fed into it. In any case, the
feedstock needs to be heated to the designed inlet temperature for the gas-
ifier, which must be above the critical temperature and well above the
designed gasification temperature because the enthalpy of the water provides
the energy required for the endothermic gasification reactions. This tempera-
ture is a critical design parameter.
The sensible heat of the product of gasification may be partially recov-
ered in a waste heat-recovery exchanger and used for partial preheating of
the feed ( Figure 9.6 ). For complete preheating, additional heat may be
obtained from one of the following:

Externally fired heater ( Figure 9.6 )
Burning of a part of the fuel gas produced to supplement the external fuel
Controlled burning of unconverted char in the reactor system (refer to
Figure 9.12 later in this chapter)

After gasification, the product is first cooled in the waste heat-recovery
unit. Thereafter, it cools to room temperature in a separate heat exchanger
by giving off heat to an external coolant.