The following are two broad routes for the production of energy or chemical feedstock from biomass:

Biological: Direct biophotolysis, indirect biophotolysis, biological reac­tions, photofermentation, and dark fermentation are the five major biologi­cal processes.

Thermochemical: Combustion, pyrolysis, liquefaction, and gasification are the four main thermochemical processes.

Thermal conversion processes are relatively fast, taking minutes or seconds to complete, while biological processes, which rely on enzymatic reactions, take much longer, on the order of hours or even days. Thus, for commercial use, thermochemical conversion is preferred.

Gasification may be carried out in air, oxygen, subcritical steam, or water near or above its critical point. This chapter concerns hydrothermal gasification of biomass above or very close to the water’s critical point to produce energy and/or chemicals.

Conventional thermal gasification faces major problems from the forma­tion of undesired tar and char. The tar can condense on downstream equip­ment, causing serious operational problems, or it may polymerize to a more complex structure, which is undesirable for hydrogen production. Char residues contribute to energy loss and operational difficulties. Furthermore, very wet biomass can be a major challenge to conventional thermal gasification because it is difficult to economically convert if it contains more than 70% moisture. The energy used in evaporating fuel moisture (2257 kJ/kg), which effectively remains unrecovered, consumes a large part of the energy in the product gas.

Gasification in supercritical water (SCWG) can largely overcome these shortcomings, especially for very wet biomass or organic waste. For example, the efficiency of thermal gasification of a biomass containing 80% water in conventional steam reforming is only 10%, while that of hydrothermal gasifica­tion in SCW can be as high as 70% (Dinjus and Kruse, 2004). Gasification in near or supercritical water therefore offers the following benefits:

• Tar production is low. The tar precursors, such as phenol molecules, are completely soluble in SCW and so can be efficiently reformed in SCW gasification.

• SCWG achieves higher thermal efficiency for very wet biomass.

• SCWG can produce in one step a hydrogen-rich gas with low CO, obviating the need for an additional shift reactor downstream.

• Hydrogen is produced at high pressure, making it ready for downstream commercial use.

• Carbon dioxide can be easily separated because of its much higher solubility in high-pressure water.

• Char formation is low in SCWG.

• Heteroatoms like S, N, and halogens leave the process with aqueous efflu­ent, avoiding expensive gas cleaning. Inorganic impurities, being insoluble in SCW, are also removed easily.