Hydrothermal gasification is suitable for wet biomass treatment. When wet biomass is to be gasified, usual thermochemical gasification is not employed due to its high moisture content. Hydrothermal gasification, on the other hand, uses water as reaction medium, and thus wet biomass can be treated without costly, energy-consuming drying pretreatment. Since reactivity of water is high under these conditions, hydrothermal gasification enables quick and almost complete gasification of biomass. Biomethanation is employed to obtain methane gas from wet biomass, but usually it takes a few weeks for the reaction to complete, and treatment of unreacted fermentation sludge and waste water can be a large problem. Reaction time as long as a few weeks results in bulky reactor. Fermentation sludge can be converted to compost, but when sufficient land is not available for the use of the compost, it is just a waste to be treated. In hydrothermal gasification, reaction is completed in a few minutes at longest, and almost complete gasification is possible when reaction condition is properly adjusted. Sometimes, addition of catalysts such as alkali, metal, or carbon catalysts enhances the reaction.

To investigate the reactions taking place in hydrothermal gasification, tube-bomb reactors of the volume of several mL and autoclaves are often used. However, when you want to develop a commercial plant, a continuous reactor such as is shown in Fig. 4.5.2 is a must. Biomass is fed to the reactor at a high pressure, and then heated to the reaction temperature. In the reactor, biomass is gasified under hydrothermal condition, and the effluent is cooled down to the room temperature. Heat released at this time is recovered by the heat exchanger, and used to heat up the feedstock. After reaching the room temperature, the effluent is depressurized to atmospheric pressure, and the product gas is recovered. The continuous reactor is needed because of the large amount of heat needed to attain the hydrothermal condition. This heat sometimes matches the heat of combustion of the biomass to be gasified, and thus heat recovery using a heat exchanger is needed. Only flow reactors allow this heat recovery. In Fig. 4.5.2, heat balance for the ideal case is also shown. Combustion heat of biomass is maintained in the product gas while heat required to attain hydrothermal condition is recovered so that no heat is added from outside during gasification operation. In practice, the efficiency of the heat exchanger cannot be unity, and endothermic reaction results in necessity of additional heat supply to the reactor.