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nontransferred torches, the position of the arc is within the torch, and plasma is estab-
lished in the gas flowing through the device.
10.4 GASIFICATION IN AN AQUEOUS ENVIRONMENT:
HYDROTHERMAL BIOMASS CONVERSION
10.4.1
Introduction
Although wet biomass has a very low overall heating value, still products with a high
heating value can be extracted from it by applying advanced conversion processes.
Gasification in hot compressed water is considered a promising technique to convert
such wet streams into medium calorific gas. The produced gas is rich in either
hydrogen or methane depending on the operating conditions and applied catalysis.
By practicing countercurrent heat exchange between the feed stream and the reactor
effluent (see Figure 10.10), high thermal efficiencies can be reached despite the low
dry matter content of the feedstock. Apart from this essential energetic benefit, the
unique properties of hot compressed water are believed to be beneficial for the desired
chemistry. It promotes ionic reaction pathways over radical routes, leading to less char
formation. Generally, organic molecules are reactive in pressurized water at tempera-
tures above 250
C.
H
2
-rich gas
LP G/L
separator
Reactor
Feed tank
Water
Biomass/
water
CO
2
-rich gas
HP G/L
separator
Heat exchanger
LP G/L
separator
Feed pump
Water
FIGURE 10.10
Simplified process scheme of the PDU for SCWG in Enschede,
the
Netherlands.
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