Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 323

Carbon dioxide can be easily separated because of its much higher solu-
bility in high-pressure water.
Char formation is low in SCWG.
Heteroatoms like S, N, and halogens leave the process with aqueous efflu-
ent, avoiding expensive gas cleaning. Inorganic impurities, being insolu-
ble in SCW, are also removed easily.
The product gas of SCWG automatically separates from the liquid con-
taining tarry materials and char if any.

9.3 BIOMASS CONVERSION IN SCW
There are three major routes for SCW-based conversion of biomass into
energy which are as follows:

1. Liquefaction: Formation of liquid fuels above critical pressure (22.1 MPa)
but near critical temperature (300

400 C).
2. Gasification to CH 4 : Conversion in SCW in a low-temperature range
(350 500 C) in the presence of a catalyst.

3. Gasification to H 2 : Conversion in SCW with or without catalysts at higher
(

600 C) temperatures.

.

Here we discuss only the last two gasification options.

9.3.1 Gasification
Supercritical biomass gasification takes place typically at around

500 C)
temperature with catalysts. The biomass decomposes into char, tar, gas, or
other intermediate compounds, which are reformed into gases like CO, CO 2 ,
CH 4 , and H 2 . The process is schematically shown in Figure 9.4 . If the bio-
mass is represented by the general formula C 6 H 12 O 6 , the gasification process
may be described by the following overall reaction:
mC 6 H 12 O 6 1

750 C in the absence of catalysts, and at an even lower (350

500

nH 2 O - wH 2 1 xCH 4 1 yCO zCO 2 (9.3)

1

Gasification in SCW involves, among other reactions, hydrolysis and oxi-
dation reactions. A brief description of these reactions follows.

CO

Char

CO 2

Thermal
decomposition

Biomass Tar Reforming

CH 4
H 2

Gas

FIGURE 9.4 Biomass conversion process.