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Author/Source: Wang Shoujian (China Wuhuan Chemical Engineering Company, Wuhan 430079) Date: 2009-3-20 Since the early 1980s, with the research and development of \"clean coal gasification technology,\" significant progress has been made in clean coal gasification processes, which have since been put into commercial use. This technology utilizes advanced fluidized bed reactors, with water-coal slurry and dry coal powder as raw materials, to carry out large-scale single-series pressurized gasification on an industrial scale. It has good gasification parameters, which is beneficial for environmental protection, and has become the mainstream of coal gasification technology. Looking at the development of coal gasification technology, early coal gasification processes primarily used lump coal and small-sized coal as raw materials to produce syngas, which were known as the first-generation coal gasification processes. Powder coal gasification is known as the second-generation gasification process or a clean coal gasification process. The biggest differences between the two generations of coal gasification processes lie in environmental protection, coal type adaptability, and coal utilization efficiency. 1 Environmental Protection: The clean coal gasification process ensures clean operation, safety, and emissions that are free from pollution or result in minimal pollution, thus meeting environmental protection standards. Processes such as Shell’s dry coal powder gasification cause little environmental pollution, with virtually no waste gases emitted during the gasification process ； The molten slag and fly ash discharged by the system have low carbon content, and can be used as building materials such as cement; no pollutants are released during storage ； Vaporized sewage contains no tar, phenols, etc., and is easy to treat. Table 1 shows the pollutant emissions from coal powder preparation, the supporting facility for Shell’s dry coal powder gasification. http://www.nmtech.com.cn/jishuwang/upload/060118942303263.jpg This process embodies the principle of \"clean production\" in terms of environmental protection; emissions of pollutants are strictly controlled in accordance with relevant standards, such as the \"Environmental Air Quality Standards\" and the \"Comprehensive Emission Standards for Air Pollutants\". 2 Coal type adaptability: The clean coal gasification process has a high degree of adaptability to raw materials. For the Texaco water-coal slurry gasification process, apart from lignite with a high moisture content, most types of bituminous coal and petroleum coke can be used as gasification feedstocks, with younger bituminous coal being particularly suitable. There are no strict requirements regarding the particle size, cohesion, and sulfur content of coal. A ash fusion temperature of the coal used for gasification below 1,350°C is favorable for gasification. The ash content in coal should not exceed 20%; it is more economical to use water-coal slurry with a concentration of 60%–65%. Shell’s dry powder gasification process has a wider range of coal compatibility; it can be used with various types of coal, from low-quality lignite and sub-bituminous coal to bituminous coal, as well as petroleum coke. It also allows for the gasification of mixtures of two different coal types. Its tolerance for the ash melting point of coal is greater than that of other gasification processes, enabling it to be used with coals that are high in ash, moisture, and sulfur content. 3 Coal utilization efficiency: The key indicators for evaluating the efficiency of coal gasification include cold coal efficiency, specific oxygen consumption, specific coal consumption, and carbon conversion rate. The efficiency of cold coal is given by η_Cold = calorific value of gas / calorific value of coal used for gasification, % ； When calculating η at low temperatures, the calorific values of coal and gas should be based on the same standard, such as using either the lower or the higher calorific value consistently ； Specific oxygen consumption is the amount of oxygen used (expressed as a percentage) to produce 1,000 m3 of gas (CO+H2) ； Coal consumption per unit of gas produced: amount of coal required to produce 1,000 m3 of (CO+H2) gas, expressed as a percentage of coal ； The carbon conversion rate is given by η_carbon = carbon content in the gas / carbon content in the coal used for gasification, % ； Carbon content in gas = V_gas × (CO% + CO2% + CH4%) × 12 / 22.4. The main performance evaluation indicators for Shell powder coal gasification and Texaco water-coal slurry gasification, such as cold coal efficiency, specific oxygen consumption, specific coal consumption, carbon conversion rate, and effective components of syngas, are shown in Table 2. The typical gas components of the two clean gasification processes are shown in Table 3. 4 Texaco Water-Gas Shift Gasification Process: Texaco developed technology for producing syngas from natural gas and heavy oil as raw materials well before the 1970s. The oil crisis of the 1970s prompted Texaco to seek alternative energy sources and to develop clean gasification technologies. After years of research, the water-coal slurry gasification process was developed. This process uses coal water slurry as the feed, liquid slag discharge, and pressurized gasification in a fluidized bed; the coal water slurry reacts with pure oxygen at high temperature and pressure to produce syngas. Multiple units have been introduced in China in areas such as the Wei River region, southern Shandong, Shanghai Coking, and Huainan. Except for lignite with a high moisture content, most bitumens, petroleum coke, and the like can be used as gasification feedstocks for the Detugou gasifier to produce water-coal slurry with a concentration of 60%–65%; the ash content in the coal should not exceed 20%. Industrial plants use a gasification pressure of 2.8–6.5 MPa, a gasification temperature of 1,300–1,400°C, with CO+H2 levels exceeding 80%. http://www.nmtech.com.cn/jishuwang/upload/060118943362138.jpg5 Shell’s pulverized coal gasification process: This is a unique and advanced clean gasification process developed by Shell. Building on its successful industrial experience with sludge oil gasification technology, Shell began research in 1972. In 1978, the first pilot plant was built and put into operation in Hamburg, Germany. A demonstration unit with a coal feeding rate of 250–400 tons per day was put into operation in Houston, USA, in 1987. In 1993, a large gasification unit with a coal feeding capacity of 2,000 t/d was built at the Danemark Power Plant in the Netherlands. This device is used for combined-cycle power generation and operates in a single series, with an operational rate of over 95%. Shell powder gasification has a wider adaptability to different coal types, and can be used with poor-quality lignite, sub-bituminous coal, bituminous coal, as well as petroleum coke. Even coal with high ash, high moisture, and high sulfur content has little impact on gasification, and the two types of coal can be blended for use. It has a wider range of tolerance for coal ash fusion points compared to other gasification processes. The shell gasification temperature is generally between 1,400 and 1,550°C, with a high proportion of effective components in the syngas, where CO+H2 accounts for over 90%. Thus, the coal consumption for gasification can be reduced. 6 The main differences between the two types of clean coal gasification: In the Texaco gasification process, coal is fed into the gasifier as a suspension containing about 35% water, and this method is only suitable for bituminous coal and hard coal. For lignite, the moisture content in the coal is 55%; to use pumped water-coal slurry, its moisture content must be increased to over 70%, which is not feasible from a technical standpoint. Therefore, coal types such as lignite are not suitable for the fluidized bed process with wet feeding. In Texaco gasifiers, oxygen is required to burn the coal; the water contained in the coal-water slurry is then evaporated and superheated to 1300–1400°C. When the concentration of the coal-water slurry is 65%, that is, in 1.54 kg of such slurry, 1 kg of coal requires 0.54 kg of water to be evaporated, and heating this mixture to 1400°C requires approximately 2,900 kJ of heat. The calorific value of 1 kg of dry hard coal is 25,000 kJ, meaning that 11.6% of the calorific value of the raw coal is wasted. When dry coal powder feed enters the gasifier after drying, much less heat is required to heat and evaporate the moisture in the coal. Refractory bricks are used in the Texaco gasifier; therefore, the ash fusion point of the coal is required