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 7.6 describes the SCWG process using the example of an SCWG plant for gasifying sewage sludge. Biomass is made into a slurry for feeding. It is then pumped to the required supercritical pressure. Alternatively, water may be pressurized separately and the biomass fed into it. In any case, the feedstock needs to be heated to the designed inlet temperature for the gasifier, which must be above the critical temperature and well above the designed gasification temperature because the enthalpy of the water provides the energy

required for the endothermic gasification reactions. This temperature is a criti­cal design parameter.

The sensible heat of the product of gasification may be partially recovered in a waste heat-recovery exchanger and used for partial preheating of the feed (Figure 7.6). For complete preheating, additional heat may be obtained from one of the following:

• Externally fired heater (Figure 7.6)

• Burning of a part of the fuel gas produced to supplement the external fuel

• Controlled burning of unconverted char in the reactor system (refer to Figure 7.12 later in chapter)

After gasification, the product is first cooled in the waste heat-recovery unit. Thereafter, it cools to room temperature in a separate heat exchanger by giving off heat to an external coolant.

The next step involves separation of the reaction products. The solubility of hydrogen and methane in water at low temperature but high pressure is considerably low, so they are separated from the water after cooling while the carbon dioxide, because of its high solubility in water, remains in the liquid phase. For complete separation of CO2, the gas may be scrubbed with additional water (refer to Figure 7.14 later in chapter). The gaseous hydrogen is separated from the methane in a pressure swing adsorber. The CO2-rich liquid is depressurized to the atmospheric pressure, separating the carbon dioxide from the water and unconverted salts.