Overview of current general coal gasification process routes

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1.1 Selection of process solutions my country’s current energy supply is dominated by coal, accounting for about 72% of consumption, most of which is directly burned for the power industry and transportation industry. The proportion of coal used to produce syngas through gasification is very low, and gasification technology is still very backward. In terms of syngas production, atmospheric fixed-bed gasifiers are commonly used in China, using anthracite lump coal or coke as raw materials. The gas furnace series include φ2.74m, φ3.0m and φ3.6m, etc. In addition, there are a small number of Lurgi furnaces. Many years ago, domestic research departments also developed KT furnaces and slag furnaces using pulverized coal as raw materials, and used oxygen-rich continuous gasification processes in atmospheric solid layer gasifiers. However, due to various reasons, these technologies have not yet reached the level of industrial promotion. Although the technology of the atmospheric pressure fixed layer intermittent gasification process is mature and reliable, the equipment is all domestically produced, and the investment is relatively low, it has high energy consumption, high coal quality requirements, requires the use of anthracite lump coal, and low resource utilization. Moreover, it is operated under normal pressure, with low production intensity. During operation, the emission of three wastes is large, and there is certain pollution to the environment. Obviously, there is a big gap compared with foreign coal gasification technology. Foreign coal gasification technology has been industrialized as early as the 1950s. Later, due to the massive development of natural gas and oil, the development of coal gasification stopped for a time. In the early 1970s, there was an energy crisis in the world, and developed countries * * Due to concerns about the tight supply of oil and natural gas, coal gasification technology has been put back on the agenda as an alternative energy source, and research on new coal gasification processes has been accelerated. In the past ten years, many foreign companies have done a lot of work on coal gasification combined cycle power generation technology in order to improve the thermal efficiency of coal-fired power plants and reduce environmental pollution, thus promoting the development of coal gasification technology. At present, a new generation of coal gasification process has been successfully developed that has wide adaptability to coal types, high gasification pressure, large production capacity, high gasification efficiency and less pollution. Among them, representative ones include the SCGP process of the Dutch Shell Company, the coal-water slurry gasification process of the American Texaco Company, the DOW gasification process of the American DOW Chemical Company, the German GSP gasification process, the PRENFLO process (pressurized KT method) and the Lurgi process. Among them, the DOW gasification process is similar to the TEXACO process, but its performance and experience are not as good as TEXACO. ； Although the GSP process and PRENFLO process have better process indicators, they currently only have one set of demonstration devices and little operating experience. ； Although the Lurgi process has many industrial devices and rich operating experience, due to the high CH4 content in coal gasification, the low content of active ingredients (CO+H2), the high tar and phenol content in the gas, and the complicated sewage treatment, it is not suitable as a raw material gas for ammonia synthesis. Therefore, currently, the Shell SCGP process and Texaco's coal-water slurry gasification process are the ones with relatively mature technologies, advanced process indicators and more achievements in the world. The characteristics of Shell’s SCGP process are now introduced as follows:: (1) SCGP gasification process characteristics SHELL Company began to engage in research on coal gasification technology in 1972 based on its successful experience in industrialization of residual oil gasification technology. In 1978, the first pilot plant was built and put into operation in Hamburg, Germany. In 1987, a demonstration unit with a daily coal input capacity of 250 to 400 tons was built near Houston in the United States and was put into operation. A large-scale gasification unit used for combined cycle power generation with a daily coal input of 2,000 tons was completed and put into operation in Buggenum, the Netherlands, in 1993. The gasification unit operates in a single series, and the current operating rate of the unit is over 95%. The production practice of these devices proves that SCGP gasification technology is advanced and mature. Its main features are as follows: (a) It uses dry pulverized coal as feed material and pressurized nitrogen gas transportation, which has good continuity, stable operation of the gasifier, and wide adaptability to coal types. It can be gasified from bituminous coal, lignite to petroleum coke, and has no requirements on the activity of the coal. The adaptable range of the ash melting point of coal can be wider than other gasification processes. It is also suitable for coal types with high ash content, high moisture content and high sulfur content. (b) The gasification temperature is about 1400~1600°C, the carbon conversion rate is as high as over 99%, the product gas is clean, does not contain heavy hydrocarbons, has extremely low methane content, and the effective gas (CO+H2) in the coal gas reaches about 90%. (c) Oxygen consumption is low. Compared with coal-water slurry gasification, oxygen consumption is 15-25% lower, so the investment in supporting air separation equipment can be reduced. (d) The production capacity of a single furnace is large. The gasification pressure of a single furnace currently in operation is 3.0MPa, and the daily coal processing capacity has reached 2,000 tons. (e) High thermal efficiency, cold gas efficiency is 81 to 83% (Texaco cold gas efficiency is about 75%). The remaining 15% of the heat energy is recovered as medium-pressure or high-pressure steam, and the total thermal efficiency is about 98%. (f) The gasifier adopts a water-cooled wall structure without refractory brick lining, requiring less maintenance. There are no transmission parts in the gasifier, and the operation cycle is long, without the need for a backup furnace. (g) The gasifier burner and control system are safe and reliable. The design life of Shell's gasification burner is 8,000 hours. The Demkolec power plant has used the burner for 4 years without any problems. The gasification operation adopts an advanced control system and is equipped with necessary safety interlocks to keep the gasification operation in the best condition. (h) Slag can be used as cement binder or road construction material. The molten slag discharged from the gasifier at high temperature becomes glassy particles after being chilled, which has stable properties and has almost no impact on the environment. Gasified sewage contains less cyanide and is easy to handle. (2) Comparison of two gasification processes SHELL's coal gasification process (SCGP) and TEXACO's coal-water slurry gasification process are two advanced technologies. Mature gasification technology has many years of experience in successful application on an industrial scale. When the two companies developed coal gasification technology, they had different design ideas, and they introduced two technologies to the market. The analysis and comparison of the main links of the two gasification processes are as follows: (a) Applicability of raw materials SCGP can use lignite, sub-bituminous coal, bituminous coal and other coal types as well as petroleum coke as raw materials, or a blend of the two types of coal can be used. And successfully gasified low-quality coal with high ash content (5.7-24.5%, up to 35%), high moisture (4.5-30.7%) and high sulfur content. For areas where the price difference between raw coal and fuel coal is large, it is possible to combine the two into one, which can not only simplify the storage and transportation system but also reduce production costs. Therefore, the choice of coal type is no longer a limiting factor for gasification technology, but an economic factor. Bituminous coal, sub-bituminous coal, petroleum coke and coal liquefaction residue can also be used in coal water slurry gasification, so the adaptability to raw materials is also wide. However, the choice of coal types is limited: Coal types with low moisture content, especially low internal moisture, should be selected to facilitate the preparation of high-concentration coal-water slurry. 2. Select coal types with low ash melting point and suitable ash viscosity. The ash melting point is preferably lower than 1300°C in order to control the appropriate gasification temperature, prolong the service life of the refractory bricks in the furnace, and achieve the purpose of reducing consumption and cost. (b) Coal preparation The coal used as raw material entering the plant is usually an uneven mixture of powder and granules. It needs to be screened, ground and crushed to reach a certain particle size to meet the requirements of transportation and gasification operations. This is an extremely important part. In the SCGP process, while grinding coal to a suitable particle size for gasification, it is dried with hot air using inert gas. The coal particle size when leaving the mill is 90% (wt) 99%, so the discharged ash contains C