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Hydrothermal Processes in Subcritical Water
on the temperature and pressure of water, the contact time, and the catalyst if it
is present. High solubility of intermediates in water, particularly at high temperature and pressure, allows further organic reactions to occur in aqueous media
and prevents the formation of tar and coke. The reactive species originating from
biomass (or other species) are diluted by solvation in water, thereby preventing
polymerization to unwanted products. These conditions also lead to the formation of high gas yield at relatively low temperatures. The HTG process is thus
the process of gaseous fuel generation in an aqueous medium, which differs from
“steam gasification” in which solids react with gaseous steam to produce a set of
gaseous products.
The goal of HTG under subcritical conditions is to obtain high quality and
yield of fuel gas. Two most important components of fuel gas are hydrogen and
methane. As discussed earlier, steam gasification and reforming generates gas
with high hydrogen concentration. Thermochemical formation of methane is possible only by low-temperature hydrothermal route since in conventional steam or
oxygen gasification process, temperatures are generally too high for the methane
production from biomass. The HTG under subcritical conditions can be divided
into two parts: (1) low-temperature APR and its derivative technologies, and
(2) high-temperature catalytic gasification. The APR and its derivative technologies are discussed in Chapter 6. Here we address the subject of high-temperature
catalytic gasification.
At higher temperatures up to supercritical temperature, in the presence of a catalyst, biomass or organic compounds are gasified mainly to methane and carbon
dioxide. In the absence of a catalyst, this region of temperature (250°C to critical
temperature, 374°C) is also called HTL region wherein carbohydrates are liquefied to various organic products. In the catalytic HTG process, the heat recovery is
important for an efficient operation. The catalytic HTG process converts biomass/
water slurry into fuel gas and water that are subsequently separated. The gaseous
fuel can be used for heat, power, or the generation of various chemicals. The role of
catalysts on HTG is described in Section 5.4.1.
5.4.1 CATAlySTS For hTg
The HTG can be divided into three regions depending on the range of temperature
[6–11,95–105]. Osada et al. [98–101] identified region 1 as the one with a temperature
range of 500°C–700°C; supercritical water in which biomass decomposes and the
activated carbon can be used to avoid char formation or alkali catalyst to facilitate
water–gas shift reaction. In this region, very little solids are remained and the main
product of the gasification is hydrogen. In region 2, where the temperature range
is 374°C–500°C that is again in the supercritical region, biomass hydrolyzes and
metal catalyst facilitates gasification. In this region, the main product is hydrogen
with some carbon dioxide, carbon monoxide, and methane. Both regions 1 and 2
producing fuel in supercritical water are discussed in Chapter 10.
In this section, we focus on region 3 where temperature is below the critical temperature of 374°C. In this case, biomass hydrolysis is slow and catalysts are required
for gas formation. In the subcritical region, the gas product distribution will be