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Applications of low rank coals, such as lignites and subbituminous coals, are limited due to several drawbacks, including high moisture content, high transport cost, low heating value, and spontaneous combustibility, etc. Therefore, in many cases low rank coals need some upgrading before it can be extensively used. Hydrothermal treatment, in which coal is treated under a high temperature (300–350 °C) and a high pressure (80–180 kg/cm2) to decrease the O/C atomic ratio in coal, is known as one of the promising upgrading methods for low rank coals.