Conversion of low-grade coals in sub-and supercritical water: A review

Abstract Low-rank coals are very abundant in several regions throughout the world. However, it is difficult to use the low-rank coals directly due to their undesirable characteristics such as high moisture, high ash and high oxygen content. The utilization of low-rank coals has received more and more attention with the increase of consumption of other fossil fuels. Besides the pyrolysis and alcoholysis, the conversion of low-lank coals under hydrothermal conditions (including sub-and supercritical water) for high-quality solid fuel, coal liquefied oil and gaseous fuels has also been widely investigated. In this review, the development of low-rank coal hydrothermal upgrading, the liquid fuel production from coal in sub-and supercritical water and supercritical water gasification of coal for hydrogen production, including partial oxidative gasification, homogeneous/heterogeneous catalytic gasification and related kinetic investigations are discussed. In addition, the element transformation such as N, S and metals during the hydrothermal process are also presented.