ABSTRACT

This chapter addresses various aspects of coal-water chemistry under high-temperature and high-pressure conditions. It examines these three roles of water in coal–water interactions under subcritical conditions. High-temperature water, due to its properties, can act as a reactant and a catalyst for a second pathway to cascade organic molecular transformation of biomass (and to some extent coal) that leads to refined biomass, oil, or synthetic gaseous fuel. One of the indicators that water becomes polar to nonpolar solvent as temperature increases is the solubility of various inorganic salts and organic acids in water. Hydrothermal carbonization is a thermochemical conversion process to convert biomass into a solid, coal-like product in the presence of liquid water. This process is often called a wet or hydropyrolysis process and results in the production of “hydrochar” that has high carbon content and low oxygen content compared to original biomass.