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Requirements for coal quality in coal liquefaction. Coal liquefaction is a hot topic in the field of coal chemistry at present, and many coal mines are eager to try it out. However, it should be noted that coal liquefaction has certain requirements regarding coal quality – not all types of coal can be used for liquefaction. Coal liquefaction is divided into direct liquefaction and indirect liquefaction. These two liquefaction methods have different requirements regarding coal quality. 1) Basic requirements for coal quality in direct liquefaction (1) The ash content in coal should be low, generally less than 5%; therefore, the raw coal needs to be washed and processed to produce refined coal suitable for liquefaction. Coal has a high ash content, which affects oil yield and the proper operation of the system. The ash composition of coal also has an impact on the liquefaction process. Elements such as Fe, Co, and Mo in the ash facilitate liquefaction by acting as catalysts; whereas elements like Si, Al, Ca, and Mg in the ash are detrimental to liquefaction, as they tend to cause scaling, which hinders heat transfer and affects normal operation. They can also lead to blockages and wear in piping systems, reducing the service life of the equipment. (2) The coal should have good grindability. Because direct coal liquefaction requires first grinding coal into coal powder with a particle size of around 200 mesh, drying it to a moisture content of less than 2%, and preparing it into oil-coal slurry; thereafter, a hydrogenation reaction is carried out under high temperature and pressure. If the grindability is poor, energy consumption is high, equipment wear is severe, and there is heavy consumption of spare parts and materials, which increases production costs. At the same time, the moisture content of coal is required to be low. High moisture levels are unfavorable for grinding and for preparing oil-rich coal slurry, thereby increasing investment and production costs. (3) The higher the hydrogen content in coal and the lower the oxygen content, the better; this can reduce the amount of gas required for hydrogenation as well as the amount of wastewater generated, thereby improving economic efficiency. (4) The lower the content of impurity atoms such as sulfur and nitrogen in coal, the better, in order to reduce the costs associated with oil processing and upgrading. (5) The composition of coal rock is also a key indicator for liquefaction. The higher the silkiness component, the better the liquefaction properties of the coal; a high vitrinite content results in poor liquefaction activity. Therefore, the coals that can be used for direct liquefaction are generally younger coal types such as lignite and bituminous coal, and not all of these coal grades can be directly liquefied. Shenhua’s non-stick coal, long-flame coal, and Yunnan Xianfeng’s lignite are all good coal types for direct liquefaction. The indirect liquefaction of coal involves gasifying the coal to produce a feed gas consisting of H2CO, which is then combined with a catalyst under certain pressure and temperature conditions to yield liquid oil; therefore, the requirements regarding the quality of the coal are relatively lower. 2) Requirements for coal quality in indirect liquefaction: (1) The ash content of the coal must be below 15%. Of course, the lower it is, the more favorable it is for gasification as well as for liquefaction. (2) Coal should have good grindability and low moisture content. Regardless of the gasification process used, powder production is an important step. (3) For the process of producing gas from water-coal slurry, the coal must have good slurry-forming properties. The solid concentration of water-coal slurry should be above 60%. (4) Requirements for the ash fusion point of coal. Fixed-bed gasification requires a higher ash fusion temperature for coal, with an ST value generally not less than 1250°C; fluidized-bed gasification requires an ash fusion temperature of ST below 1300°C. Although indirect liquefaction has a wider range of applicability to coals, and different gasification methods need to be chosen for various types of coal, it is necessary to wash and process the raw coal in order to reduce its ash and sulfur content.