Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 337

TABLE 9.4 Global Kinetic Rate of Gasification of Model Compound in SCW

Mettanant
et al. (2009b)

Lee et al.
(2002)

Kabyemela
et al. (1997) Blasi et al. (2007)

Feed Waste water from

wood gasifier/TOC

Rice husk Glucose/

COD

Glucose

Reactor Plug Batch Plug Plug

Temperature (K) 723 821 673 873 740 1023 573 673

Residence time (s) 60 120 3600 16 50 0.02 2

Solid content 7 15 g(Toc)/l 9.4 mol/l 0.6 mol/l 0.007 mol/l

Preexponential,
A (s 2 1 )

897 184 897 6 29

Activation energy,
E (kJ/mol)

76 77.4 71 6 3.9 96

k g (s 2 1 ) 0.002 0.003 0.0002 0.006 0.01 0.55 0.15 9.9

9.7 REACTOR DESIGN
Because SCWG is likely to enter the market once its major development bar-
riers are removed this section discusses important considerations for design of
an SCWG reactor. The discussion is based on limited information available in
laboratory units and on the design of thermal gasifiers (see Chapter 8).
The major design parameters for an SCWG reactor are temperature, resi-
dence time, pressure, catalysts, and feed concentration. Important design con-
siderations for auxiliary or support equipment are (1) waste heat-recovery
exchanger and feed-preheating system, (2) the biomass feed system, and
(3) product separation. The following subsections present a brief discussion
of some of the design parameters.

9.7.1 Reactor Temperature
The temperature and pressure of an SCWG must be above the critical value of
374.21 C and 22.089 MPa, respectively. As explained in Section 9.4.7 ,pressure
has a minor effect on biomass conversion, but the effect of gasification tempera-
ture is a major one (see also Section 9.4.1 ).
Because feedstock (biomass and water) must be heated to the reaction tem-
perature using energy from an external source, the lower the designed reactor
temperature, the lower the energy required for feed preheat and the more effi-
cient the process. The gasification temperature should be above 600 Cforarea-
sonable hydrogen yield, but it can be lower if catalysts are used.