**6.3 Hydrothermal gasification (HTG)**

HTG operates near or above the critical point of water at 400–600°C and 23–45 MPa. The primary product of HTG is a mixture of non-condensable gases (H2, CO, CH4, and CO2), which can produce syngas enriched with H2. At the critical point (374°C and 22.1 MPa) of water, the conversion efficiency is improved. Biomass polysaccharides split in the presence of supercritical water (SCW). Due to higher reaction temperatures, HTG reactions progress at a faster rate and complete decomposition

#### *Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing DOI: http://dx.doi.org/10.5772/intechopen.105185*

of biomass is achieved. This is a distinctive feature of HTG compared to other hydrothermal treatments (HTC and HTL). One of the problems with HTC and HTL is the difficulty in byproduct treatment due to undesirable byproducts being occasionally dissolved in the liquid fraction. The conversion rate of HTG is typically higher than 80% that decomposes biomass into gaseous products. Consequently, post-treatment of liquid fraction is not required or easily carried out because only a small amount of organic compounds remain in the liquid. Conventional gasification can be effectively employed when biomass is not wet, but it is ineffective when biomass has a high moisture content (> 80%). The syngas of conventional gasification is partially diluted with nitrogen (due to partial oxidation using air) and contains tar. Syngas from HTG is not diluted with nitrogen and do not contain tars. Tar, if produced, remains in the liquid fraction [2, 4, 63].

The HTG performance is strongly dependent on the operating conditions, including biomass characteristics, temperature, pressure, residence time, feedstock concentration, and catalyst. The rate of hydrolysis and decomposition is relatively fast in the HTG process; hence, short residence times are expected to degrade biomass successfully. Optimization of residence times is required for the efficient destruction of biomass organic compounds. Pressure helps maintain single-phase media for HTG to avoid large heat inputs required for phase change. Two-phase systems need a large heat supply to maintain the temperature of the system. The rate of hydrolysis and biomass dissolution can be controlled by maintaining pressure higher than the supercritical pressure, which may enhance favorable reaction pathways for bio-oil or gas yield. Pressure imparts minor or negligible influence on bio-oil or gas yield in supercritical conditions. This is because, in the supercritical region, the effect of pressure on the properties of water is minimal [54, 58, 64].

SCW exhibits a unique property; the density, viscosity, ionic product, and dielectric constant change significantly when water changes from ambient conditions (25°C and 0.1 MPa) to the supercritical condition. At ambient conditions, the dielectric constant of water is about 80 F/m and water is a polar solvent due to a high dielectric constant (a large number of hydrogen bonds). At supercritical conditions (400°C and 25 MPa), the dielectric constant is about 6 F/m; because of the decrease in the number of hydrogen bonds, water begins to display the behavior of a nonpolar solvent that can completely dissolve many organic compounds, hydrocarbons, and gases (such as CO2, CH4, H2, and N2). This results in poor solubility of inorganic polar compounds in SCW. Many rapid homogeneous reactions involving organic compounds occur at supercritical conditions due to the absence of phase boundaries. In subcritical water, inorganic polar compounds (such as NaCl, KCl, and CaSO4) are usually soluble. But these compounds are insoluble in supercritical water and easily separated from the reaction products. SCW exhibits gas-like properties and using SCW as the reaction medium in HTG has several advantages: low viscosity creates a high diffusion coefficient and enhances mass transfer, low density improves the solvation properties, creates a single-phase reaction environment in the reactor by complete miscibility with different organics and gases, enhance mass transfer, prevent poisoning of catalyst (if used) and coke formation and product gas (syngas) does not have tar and has a high heating value. Syngas can be converted to liquid fuels or value-added chemicals via different gas-to-liquid conversions, such as Fischer-Tropsch synthesis or to ethanol and butanol through syngas fermentation using microorganisms [9, 53, 54, 63].
