Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 316

the feed increased from 5% to 75%. So, for gasification of very wet biomass,
some other means such as anaerobic digestion (see Section 3.2.2) and hydro-
thermal gasification in high-pressure hot water are preferable because the
water in these processes is not a liability as it is in thermal gasification.
Instead, it serves as a reaction medium and a reactant.
The efficiencies of hydrothermal or anaerobic processes do not decrease
with moisture content of the biomass. For anaerobic digestion and super-
critical gasification, Yoshida et al. (2003) found the gasification efficiency
to remain nearly unchanged, at 31% and 51%, respectively, even when the
moisture in the biomass increased from 5% to 75%. A major limitation of
anaerobic digestion is, however, that it is very slow and most importantly,
it produces methane only, no hydrogen. If hydrogen is the desired product,
as is often the case, an additional step of steam reforming the methane
(CH 4 1

H 2 O CO 3H 2 ) must be added to the anaerobic digestion

5 1

process.
Hydrothermal gasification involves gasification in an aqueous medium at
a very high temperature and pressure exceeding or close to its critical value.
While subcritical water (pressure, P and temperature, T are below their criti-
cal values) has been used effectively for hydrothermal reaction, supercritical
water (SCW) has attracted more attention owing to its unique features. SCW
(P . P c ; T . T c ) offers rapid hydrolysis of biomass, high solubility of inter-
mediate reaction products, including gases, and a high ion product near (but
below) the critical point that helps ionic reaction. These features make SCW
an excellent reaction medium for gasification, oxidation, and synthesis

reactions.
This chapter deals primarily with hydrothermal gasification of biomass in
SCW, while explaining the properties of SCW and the biomass conversion
process in it. The effects of different parameters on SCW gasification and
design considerations for the SCW gasification plants are also presented.

9.2 SUPERCRITICAL WATER
Water above its critical temperature (374.29 C) and pressure (22.089 MPa)
is said to be in supercritical ( Figure 9.1 ) state or simply as SCW. Water or
steam below this pressure and temperature is called subcritical. The term
water in a conventional sense may not be applicable to SCW except for its
chemical formula, H 2 O, because above the critical temperature SCW is nei-
ther water nor steam. It has a water-like density but a steam-like diffusivity.
Table 9.1 compares the properties of subcritical water and steam with those
of SCW, indicating that SCW's properties are intermediate between the liq-
uid and gaseous states of water in subcritical pressure.
Figure 9.1 shows that the higher the temperature, the higher the pressure
required for water to be in its liquid phase. Above a critical point, the line
separating the two phases disappears, and thereby the division between