Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 321

oxidation or for gasification. This feature makes SCW an ideal medium
for destruction of hazardous chemical waste through SCWO.

SCW possesses excellent transport properties. Its density is lower than
that of subcritical water but much higher than that of subcritical steam.
This, along with other properties like low viscosity, low surface tension
(surface tension of water reduces from 7.2 10 2 2 at 25 C to 0.07 at

373 C), and high diffusivity greatly contribute to the SCW's good trans-
port property, which allows it to easily enter the pores of biomass for
effective and fast reactions.
Reduced hydrogen bonding is another important feature of SCW. The
high temperature and pressure break the hydrogen-bonded network of
water molecules.

3

Table 9.1 compares some of these properties of water under subcritical
and supercritical conditions.

9.2.2 Application of SCW in Chemical Reactions
Chemical reactions involve the mixing of reactants. If the mixing is incom-
plete, the reaction will be incomplete, even if the right amounts of reactant
and the right temperature are available. The mixing is better when all reac-
tants are either in the gas phase or in the liquid phase compared to that when
one reactant is in the solid phase and the other is in the gas or liquid phase.
The absence of interphase resistance in a monophase reaction medium
greatly improves the mixing. The conventional thermal gasification of solid
biomass in air or steam involves heterogeneous mixing, and therefore the
gas solid interphase resistance limits the conversion reactions.

SCW allows reactions to take place in a single phase, as most organic
compounds and gases are completely miscible in it. It is thus a superior reac-
tion medium. Because the absence of interphase mass transfer resistance
facilitates better mixing and therefore higher conversion, SCW can be an
excellent medium for the following three types of reactions:

1. Hydrothermal gasification of biomass: SCW is an ideal medium for gasifi-
cation of very wet biomass, such as aquatic species and raw sewage, which
ordinarily have to be dried before they can be gasified economically. SCW
gasification produces gas at high pressure and thus obviates the need for an
expensive product gas compression step for transport or use in combustion.
2. Synthesis reactions: A variety of organic reactions like hydrolysis and
molecular rearrangement can be effectively carried out in SCW, which
serves as a solvent, a reactant, and sometimes a catalyst. There is no need
for acid or base solvents, the disposal of which is often a problem.
3. Supercritical water oxidation: Complete miscibility of oxygen in SCW
helps harmful organic compounds to be easily oxidized and degraded. Thus,
SCW is an attractive means of turning pollutants into harmless oxides.