Comparison of three coal gasification technologies
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1. Lurgi Pressurized Gasifier Lurgi furnace is a fixed bed pressurized gasifier. Strictly speaking, the Lurgi pressurized gasifier belongs to the first generation of coal gasification technology, but it has been continuously improved since its invention and is still used on a large scale in South Africa. Lurgi gasification process has the following characteristics: (1) Use granular coal with a particle size between 5 and 50mm ； (2) It is possible to gasify various coals from lignite to anthracite, but there are certain requirements for the thermal stability, mechanical strength, binder and other performance indicators of the raw materials. ； (3) Operating pressure from 2 to 3MPa ； (4) When gasifying bituminous coal, CO in the crude gas: 15%～25% ； CO2: 24%～34% ； H2: 34%～40% ； CH4: 9%～13% ； (5) Top gas temperature 250~350℃ ； (6) Gas production capacity of a single furnace is 30,000~50,000Nm3/h ； (7) The efficiency of cold gas can reach 80%. It can be seen from the above process characteristics that the gas temperature of Lurgi is low, the CH4 and tar content in the gas are high, and crude gas purification and tar treatment units are unavoidable, resulting in prominent environmental issues. From the perspective of gas composition, Lurgi is the most suitable gas production technology for direct reduction. As long as the gas is decarbonized, it can be directly used by the reduction shaft furnace. 2. Texaco coal-water slurry gasifier Texaco furnace is a coal gasification technology developed by the American company Texaco on the basis of heavy oil gasification. It is the second-generation entrained bed gasification process with the most commercial performance at present. It has the advantages of high pressure, rich operation and operation experience, high gasification temperature, and high effective components of coal gas. The main technical features are as follows: (1) The feed material is coal-water slurry made from more than 75% to 200 mesh pulverized coal, and the mass fraction of pulverized coal in the coal slurry is 65 to 70%. In theory, Texaco can be used for gasification of various coals, but experience shows that the most suitable coal type should be one with an ash melting point of about 1300°C and an ash content of less than 20%. ； (2) Gasification pressure from 2.6 to 8.4MPa ； (3) The carbon conversion rate is above 95%, and the cold gas efficiency can reach above 70% ； (4) The (CO+H2) effective gas composition in dry coal gas is more than 80%, CO accounts for about 49%, H2 accounts for about 31%, and CO2 accounts for about 18% (Datong Coal) ； (5) The gasification temperature reaches 1300~1400℃, and the crude gas temperature after water quenching is 200~260℃. If a heat energy recovery gasifier is used, the temperature of the crude gas will drop from 1370°C to 400°C after heat exchange.℃ ； (6) It adopts the equipment form of single nozzle and hot fireplace. The average nozzle life is 1500h and the refractory life is 1 to 2 years, so a backup system must be installed. ； (7) The oxygen consumption to produce 1000Nm3 (CO+H2) effective gas is about 400Nm3, and the coal consumption is about 640kg ； (8) The maximum daily coal processing capacity of a single furnace can reach 2,000t. It can be seen from the above characteristics that in terms of production scale, the Texaco furnace can fully meet the needs of direct reduction. To produce direct reduced iron from Texaco gas, in addition to decarbonizing the gas, it is necessary to transform the CO in the gas to increase the hydrogen content in the reducing gas. 3. Shell Dry Coal Pulverized Gasifier The Shell gasification process was originally called the Shell-Koppers method. It is a second-generation dry coal pulverized gasification process developed by combining Shell’s experience in high-pressure oil gasification and Koppers’ experience in coal gasification. The main technical features are as follows: (1) The feed material adopts dry coal powder with 90% less than 100μm. ； (2) Gasification pressure from 2 to 4MPa ； (3) The carbon conversion rate can reach more than 98%, and the cold gas efficiency can reach more than 80% ； (4) The (CO+H2) effective gas composition in dry coal gas is more than 88%, CO accounts for about 69%, H2 accounts for about 19%, and CO2 accounts for about 10% (Datong Coal) ； (5) The gasification temperature reaches 1400~1600°C. The high-temperature gas leaving the gasification furnace is first quickly cooled to 900°C by the cooled downstream crude gas, and then the heat is recovered through the syngas cooler and the gas temperature is reduced to about 350°C. ； (6) It adopts the form of equipment with multiple nozzles and water-cooled walls. The service life of the nozzles currently exceeds 16,000 hours. There is no need to replace refractory materials, so it can be operated without furnace preparation. ； (7) The oxygen consumption to produce 1000Nm3 (CO+H2) effective gas is about 330~360Nm3, and the coal consumption is about 600kg ； (8) The maximum daily coal processing capacity of a single furnace can reach 2,400t. Since the pulverized coal slurrying process is omitted, Shell has no slurrying requirements for coal types, so the coal types have wider applicability. The effective gas content in the gas obtained by Shell coal gasification is the highest among the three gasification processes. Under ideal conditions, the reduction potential of the gas without decarbonization is close to the requirements of the direct reduction process. However, the H2/CO ratio in the gas is only about 0.3, so CO conversion is also necessary. In addition, compared with coal-water slurry gasification, the oxygen consumption of dry coal gasification can be reduced by 15%, the effective gas composition is increased by 6% to 8%, and the coal consumption is reduced by 2% to 4%. 4. Conclusion Based on the analysis of the technical characteristics of the above three coal gasification processes, from the perspective of gas quality, the Lurgi pressurized fixed bed gasifier is the most suitable technical solution for direct reduction gas source selection. However, from the perspective of resource limitations, production capacity, consumption indicators and environmental protection, Lurgi is the least technologically advanced process among the three coal gasification processes. Compared with the Shell process, the advantages of Texaco are mature production operation technology and high localization rate of equipment, so it has obvious advantages in investment and operating costs. However, due to the innate technological advancement of Shell, the development of Shell technology in the world, especially in China, has been catching up in recent years. In the past three years, there are more than 15 sets of Shell gasifiers under construction and in operation in China. Therefore, the operation and production technology of Shell furnaces will soon make great progress. What is lacking is that the equipment investment cost is too high.
1. TEXACO coal-water slurry gasification process The main structure of the gasifier is coal-water slurry single-nozzle down-spray type. Most of them use water quenching process, but in IGCC power generation projects, the waste boiler process is also used. The maximum capacity of a single furnace can currently reach a daily coal input of 2,000 tons. The operating pressure is mostly 4MPa or 6.5MPa, and a few projects have reached 8.4MPa. The first country to introduce this technology was Shandong Lunan Fertilizer Factory, which was put into production in 1993. Later, several factories used it, and there are currently more than ten. After nearly 30 years of application, Texaco gasification technology is undoubtedly a mature technology. It has been used in my country for more than ten years. From the mastery of technology and proficiency in operation, the localization of equipment and the manufacturing of supporting refractory materials have great advantages. Its main advantage is brought by coal-water slurry, that is, it is easier to increase the pressure. For example, Nanhua's gasifier pressure has reached 8.4MPa, which makes it possible to directly synthesize methanol without compression, saving compression energy consumption. Chen Jiaren, technical consultant of Shenhua Group Coal-to-Liquids Company, pointed out that the main shortcomings of Texaco's gasification technology are also related to coal-water slurry. Coal-water slurry contains 40% water, which reduces its calorific value. Restrictions on coal quality have become stricter, such as coal with poor slurry properties, high ash content, and high ash melting point, which are less economical. ； The gasification efficiency is relatively low, with a carbon conversion rate of approximately 95%. The specific oxygen consumption is the highest among various entrained bed gasification processes, about 400 standard cubic meters/thousand cubic meters (CO+H2) ； Hot furnace walls must be used, refractory bricks cost US$500,000 per furnace, imported ones cost US$1 million, and have a lifespan of less than 2 years. The $150,000 nozzle only lasted about 2 months before being dismantled and repaired. 2. SHELL pulverized coal gasification process The main structure of this gasifier is a multi-nozzle upward waste boiler gasification of dry pulverized coal. All use cold furnace walls, cold gas returns to the furnace to quench hot gas, and waste boilers are used for gas cooling. Gu Zongqin, president of the China Petroleum and Chemical Industry Planning Institute, believes that the main advantages of Shell’s gasification technology are: first, dry coal pulverized feed, wide adaptability to all types of coal used; ； Second, the gasification temperature is high, reaching 1400~1500°C, the carbon conversion rate is high, up to about 98%, the methane content in the product gas is low, and the effective gas content is as high as 90% ； Third, compared with Texaco gasifier, oxygen consumption can be reduced by 15~20% ； Fourth, the production capacity of a single furnace is large, with a daily coal input of up to 2,000 tons. ； Fifth, the gasifier uses water-cooled walls without refractory brick lining, so the maintenance workload is small. ； Sixth, gasification wastewater treatment is simple and zero discharge can be achieved. Because of its advantages, it has attracted some domestic companies to introduce it, and there are currently more than ten companies. The biggest disadvantage of this process is the high investment and high equipment cost. ； The use of waste boilers for heat exchange of syngas increases investment. For chemical production requiring water vapor components, direct water quenching is more reasonable. ； The power consumption required for drying, grinding coal, pressurizing high-pressure nitrogen and returning syngas for quenching is relatively large. 3. Comparison of the two processes TEXACO coal water slurry and SHELL pulverized coal gasification process comparison table No. Project SHELL process TEXACO process 1 Gasification process Air-flow bed Air-flow bed liquid slag discharge Liquid slag discharge 2 Applicable coal types lignite, bituminous coal, petroleum coke, various bituminous coals, petroleum coke 3 Gasification pressure/MPa 2.0~4.0 Commonly used 4.0~6.5 4 Gasification temperature/℃ 1400~1600 1300~1400 5 Gasification agent oxygen 6 Feeding method pulverized dry coal 60% coal-water slurry 7 Maximum coal input per furnace/t·d-1 2000 2000 8 Oxygen amount/m3/k m3 (CO+H2) 330~360 380~430 9 Carbon conversion rate volume fraction/% ≥99 95~98 10 Cold gas efficiency volume fraction/% 80~85 70~76 11 CO+H2 volume fraction in gas/% ~90 ~80
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