Overview of indirect coal liquefaction technology

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The indirect liquefaction of coal was first proposed in 1923 by the German Royal Coal Research Institute, which is why it is also known as Fischer-Tropsch synthesis. The principle involves using coal as a raw material to first produce syngas (CO + H2) through gasification, and then using this syngas as a feedstock to synthesize liquid hydrocarbon products via catalysis (F-T synthesis). Currently, the main techniques for the indirect liquefaction of coal include the following: (1) Solvent-Refined Coal process (SRC). It is a coal direct hydrogenation liquefaction process developed in 1962 by the Office of Coal Research (OCR) in collaboration with Speneev Chemical Company. Initially created to make use of America’s high-sulfur coals in an environmentally friendly manner, it is a coal liquefaction technology designed to produce heavy fuel oils. No catalyst is used, and the reaction conditions are relatively mild; the sulfur compounds present in the coal itself are utilized to convert the coal into SRC-1, a solid substance with low ash and sulfur content at room temperature. Later, the process was further improved by introducing a residual residue recycling cycle and a vacuum distillation method for solid-liquid separation, resulting in a heavy fuel oil that remains liquid at room temperature, namely SRC-II. (2) Hydrogen-supplying solvent method (EDS). Exxon Research and Engineering in the United States was the first to develop a coal liquefaction process using a hydrogen supply solvent in 1966. No catalyst is added to the liquefaction reaction mixture either, thereby avoiding the toxic effect of minerals in coal on the catalyst and extending the service life of the high-performance active catalyst. Its difference from the SRC method is that the circulating solvent is subjected to catalytic hydrogenation separately, thereby improving the oxygen supply capacity of the solvent and increasing the liquefaction oil yield; the main products are light oils and medium oils. (3) Hydrogen-coal method (H-Coal). The H-Coal process, developed by the American hydrocarbon company HRI based on the H-Oil process, is entirely different from the SRC and EDS processes. It utilizes highly active catalysts and bubbling bed reactors, which results in a significant increase in both the liquefaction conversion rate and the yield of liquid products. Additionally, it improves the quality of the liquefied crude oil, as well as reducing the content of heteroatoms in the liquefied oil. (4) German IGOR process. The integrated process for coal hydroliquefaction and hydrorefining, developed by the German company for environmental protection and raw material recycling in collaboration with DMT (German Institute for Mining and Metallurgical Technology) based on traditional German techniques, allows the liquefied coal produced through this process to be used directly as feedstock suitable for hydrocracking and catalytic reforming processes. This approach replaces the traditional coal liquefaction method, in which synthetic oil obtained from coal hydroliquefaction required additional hydrorefining steps. Later, the IGR process combined coal slurry hydrogenation with the hydrogenation refining of liquefied crude oil, which not only simplified the process but also yielded refined oil with a very low content of heteroatoms, representing the development direction of direct coal liquefaction technology. (5) Russian low-pressure hydrogenation liquid process. Developed jointly by the **Academy of Sciences**, the **Institute of Combustibles**, and the Tula Coal Company of the former Soviet Union, this process takes advantage of the experience gained in the hydrogenation liquefaction of lignite and coal tar, as well as the abundant resources of lignite. It employs a hydrogenation process using a coal slurry phase along with highly active copper-based catalysts, thereby reducing the pressure required for the hydrogenation reaction and increasing the yield of oil products. (6) Coal catalytic two-stage liquefaction (CTSL) process. A coal liquefaction process developed in 1982 by the American hydrocarbon company HRI. Its features are as follows: both the first and second stages of coal liquefaction are equipped with highly active hydrogenation and hydrocracking catalysts. The two reactors are separate yet closely connected, allowing for independent control of their respective reaction conditions, which ensures that coal liquefaction operates under optimal conditions at all times. This process yields a high yield of liquefied coal oil, around 80%, while the cost is 17% lower compared to the one-stage coal liquefaction process. This combination facilitates an excellent balance between the technical aspects and economic efficiency of coal liquefaction, resulting in a significant improvement in the quality of the produced oil. (7) HTI process for coal. Developed by building on the two catalytic liquefaction methods and the H-Coal method, it utilizes a slurry bed reactor developed in recent years as well as a patented iron-based catalyst that requires only a small amount of HTI. Its advantage lies in relatively mild reaction conditions; an on-line hydrogenation fixed-bed reactor is connected in series after the high-mixing separator to carry out hydrogenation refining of the liquefied oil ； Solid-liquid separation is carried out using critical solvent extraction to recover heavy oil from the liquefied residue to the greatest extent possible, thereby significantly increasing the yield of liquefied oil. (8) Japanese NEDOL coal liquefaction process. The bituminous coal liquefaction process, developed by the National Institute of Advanced Industrial Science and Technology of Japan (NEDO) in the early 1980s, incorporates the technical insights from the U.S. EDS process and a new German process; it involves pre-hydrogenating the cyclic solvent used to prepare coal slurry in order to enhance the hydrogen-supplying capacity of this solvent. The solid-liquid mixture resulting from the liquefaction reaction is separated using vacuum flash evaporation, which simplifies the process and facilitates scale-up. The use of inexpensive iron-based catalysts such as pyrite in the coal liquefaction process also reduces the costs associated with coal liquefaction. It also enables the coal liquefaction reaction to proceed under milder conditions; the quality of the resulting liquefied product is higher than that obtained using the U.S. EDS process, while the operating pressure is lower than that of the new German coal liquefaction process. (9) Coal co-treatment process. There are two types: coal/oil co-treatment and coal/waste plastic co-treatment. In the coal/oil co-treatment process, raw coal is combined with heavy oils such as petroleum residue, oil sands asphalt, or petroleum tar for hydroliquefaction to produce oil. This is essentially a combination and development of the advanced processing technologies for heavy oil products in the petroleum refining industry and direct coal liquefaction technology. The coal/waste plastic co-treatment process involves using raw coal along with organic polymer waste materials such as waste plastics and waste rubber for hydroliquefaction to produce oil. The principle behind this coal co-treatment process is that the hydrogen-rich components in heavy oils, waste plastics, and rubber can serve as sources of active hydrogen during the liquefaction process, thereby stabilizing the free radical “fragments” generated during coal pyrolysis. This process can significantly reduce the consumption of hydrogen solvents and hydrogen gas. It not only allows for the simultaneous processing of coal, residue, and waste plastics but also increases the conversion rate of the liquefaction feedstock, as well as the yield and quality of the liquefied oil products. Therefore, the co-treatment process for coal has greater development prospects than coal hydrogenation liquefaction alone. (10) Shenhua coal liquefaction process. The solvent-full hydrogenation coal liquefaction process developed by Shenhua Group combines the advantages of the US HTI process with those of the Japanese TOP-NEDOL process in order to improve the balanced operation of coal liquefaction units. Coal slurry is mixed with a catalyst and fed into the coal liquefaction reactor, where the coal is converted into light hydrocarbons through two stages of reaction. After high and low pressure flashing, the heaviest components are separated out in a vacuum distillation tower; the residue contains 50% solid particles. All the remaining fractions from coal liquefaction are sent to a stabilization hydrogenation unit for further processing. The resulting products are then sent to a distillation tower to be separated into light, medium, and heavy fractions. All the heavy fractions, along with a small amount of medium fractions, are mixed together and recycled back to the coal liquefaction unit to be used in preparing the coal slurry. The light fractions and most of the middle fractions require further processing. The stable hydrogenation unit employs IFP’s T-STAR process, which is characterized by the ability to switch catalysts online, as well as the use of a bubbling bed reactor that allows for relatively relaxed constraints on feedstock.