Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 325

photograph of PET in ordinary water before and after hydrolysis in SCW into
fine particles of teraphthalic acid in ethylene glycol solution.

9.3.3 SCW Oxidation
SCW that exhibits complete miscibility with oxygen is a homogeneous
reaction medium for the oxidation of organic molecules. This feature of
SCW allows oxidation of harmful or toxic substances at low temperature in
a process known as SCWO or cold combustion. In a typical SCWO unit,
the entire mixture (water, oxygen, and waste) remains as a single fluid
phase with no interphase transport limitations. This allows very rapid and
complete ( . 99.9%) oxidation of the organic wastes to harmless lower-
molecular-weight compounds like H 2 O, N 2 ,andCO 2 . Unlike thermal incin-
eration, SCWO does not produce toxic by-products such as dioxin. This
method of waste treatment is especially attractive for highly dilute toxic
wastes in water.
One important shortcoming of this process is the production of highly
corrosive liquid effluents because chlorine, sulfur, and phosphorous, if pres-
ent in the waste, are converted into their corresponding acids (Serani et al.,
2008). The destruction of polychlorinated biphenyls (PCBs) in SCW, produc-
ing carbon dioxide and hydrochloric acid, may be represented by the follow-
ing simple reaction:
C 12 H 10 2 m Cl m ð mHCl

(9.4)
Conventional thermal incineration uses very high temperature to destroy
by-products like dioxin, but results in the production of another pollutant,
NOx. This is not the case with SCWO owing to its low-temperature opera-
tion (450

PCB Þ 1 ð 19 m Þ= 2O 2 1 ð 5 m Þ H 2 O 12CO 2 1

1 2 5

600 C).

9.3.4 Scheme of an SCWG Plant
A typical SCWG plant includes the following key components:

Feedstock pumping system
Feed preheater
Gasifier/reactor
Heat-recovery (product-cooling) exchanger

Gas liquid separator

Optional product-upgrading equipment

The feed-preheating system is very elaborate and accounts for the majority

( 60%) of the capital investment in an SCW gasification plant.

Figure 9.6 explains the SCWG process using the example of an SCWG
plant for gasifying sewage sludge. Biomass is made into a slurry for feeding.

B