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Author/Source: Xu Shisen, Ren Yongqiang, Xia Juncang, Wang Baomin, Li Xiaoyu, Zhang Dongliang, **, Cheng Jian, Liu Yuan, Li Mingliang (Xi’an Thermal Power Research Institute, Xi’an, Shaanxi; Date: 2009-2-23) Introduction In China, energy shortages and environmental pollution are important factors that hinder economic development. Our country possesses abundant coal resources and is a developing ** that relies on coal as its primary energy source. Coal resource consumption accounts for about 75% of total energy consumption. About 34% of that coal is used for power generation. The fact that coal remains the dominant source of energy in our country is not likely to change significantly for a long time to come. This also fully demonstrates the importance and urgency of China’s development of clean coal technology. Coal gasification is one of the main ways to utilize coal in a clean and efficient manner. Long-term production experience shows that, among the various technologies for converting coal into more convenient forms of energy and products, gasification should be the preferred processing method. Urban gas, feed gas for chemical synthesis, gas for advanced combined-cycle power generation, and integrated utilization systems are the main directions for the development and application of gasification. Based on the relative flow pattern of coal and the gasifying agent, gasifiers can be divided into three types: counterflow, counterflow with co-current, and co-current. Corresponding to these three methods are fixed-bed, fluidized-bed, and gas-flow-bed gasification reactors. Among them, the pressurized oxygen-blown fluidized bed gasifier with dry feed possesses advantages such as high processing capacity per unit reactor, wide adaptability to different coal types, high carbon conversion rate, and good load regulation capabilities, representing the future development direction of coal gasification technology. With the support of the Ministry of Science and Technology, Xi’an Thermal Power Research Institute began researching dry coal powder fluidized bed gasification technology in 1994, and developed a new type of two-stage pressurized fluidized bed gasifier. In 1997, a test unit with a capacity of 0.7 t/d was built, and pressurized gasification tests were conducted on 14 typical Chinese thermal coal types. In 2004, a pilot plant with a coal processing capacity of 36–40 t/h (10 MWth) was built, and gasification tests using four types of coal powder were conducted. 1 Problems of dry-feed gasification furnaces. Pressurized oxygen-blown fluidized bed gasification furnaces that use dry coal powder as feed have advantages such as high processing capacity per furnace, wide adaptability to different types of coal, high carbon conversion rates, and good load regulation capabilities. They are advanced coal gasification technologies that have been proven effective for large-scale operation, and represent the direction in which coal gasification technology will develop in the future. However, almost all currently commercialized dry coal powder pressurized gasification technologies use primary gasification. To cause the molten ash in the hot gas at the outlet of the gasifier to coagulate, quenching is commonly used to achieve this purpose. This process has the following disadvantages: (1) The efficiency of cold gas is low ； (2) It is necessary to add a circulating cold gas pump, which increases power consumption ； (3) Due to the high gas flow rate, the size of the gas cooler, dust removal, and water washing units becomes excessively large. To address these issues, Xi’an Thermal Power Research Institute developed a new type of two-stage dry coal powder pressurized fluidized bed gasifier in 1997. 2 Structure of the two-stage gasifier The structure of the two-stage gasifier is shown in Figure 1. http://www.yf116.cn/jishuwang/upload/060510915467965.jpg Figure 1: Schematic diagram of the structure of the two-stage gasification furnace. The outer shell of this gasification furnace is an upright cylinder; the furnace chamber is divided into an upper chamber and a lower chamber. The lower chamber serves as the first reaction zone, being a cavity that is narrow at both ends and wider in the middle. Four symmetrical nozzles for feeding pulverized coal, water, and oxygen are located on the side walls of the lower chamber. The slag discharge port is situated at the bottom of the lower chamber, in the high-temperature area, and liquid slag discharge is used. The inner wall of the lower chamber is equipped with water-cooling walls to recover some of the heat ； The upper furnace chamber serves as the second reaction zone and is relatively long in height. Two symmetric inlets for secondary coal powder and water are provided on the side walls of the upper furnace chamber, and a water-cooled wall for heat recovery is also installed on the inner wall of the upper furnace chamber. During operation, dry coal powder, oxygen (pure oxygen or oxygen-enriched air), and steam are injected into the lower section of the gasification furnace; the amount of coal powder injected accounts for 80%~85% of the total coal amount. Superheated steam is injected into the upper furnace chamber, and this superheated steam carries along the coal powder, with its injection amount accounting for 15%~20% of the total coal amount. The upper furnace of this device serves two purposes: first, it replaces the recycled syngas by rapidly cooling gas at temperatures as high as 1400°C down to around 900°C; second, it utilizes the sensible heat of the gas from the lower furnace for thermal cracking and partial gasification, thereby improving the overall efficiency of the cooled gas and the thermal efficiency. 3 Simulation study: After the concept of the two-stage gasifier was proposed, a gasification process model based on ASPEN PLUS was developed, and its performance was simulated. Figure 2 shows the relationship between cold gas efficiency and two-stage coal feeding ratio. As can be seen from the graph, when the two-stage coal feeding ratio increases from 0 to 0.1, the corresponding cold gas efficiency rises from around 82% to around 86% ； When the two-stage coal feeding ratio reaches 0.15, the carbon conversion rate of the coal drops from 99.5% to around 95%, while the cold gas efficiency can remain above 82%. This indicates that the two-stage dry pulverized coal fluidized bed gasification process has potential for improving the efficiency of cold coal gas. http://www.yf116.cn/jishuwang/upload/060510917056292.jpg Figure 2 shows the trend of cold gas efficiency as the ratio of secondary coal feeding changes. Figure 3 illustrates the trend of gas temperature as the ratio of secondary coal feeding changes; it can be seen from Figure 3 that as this ratio increases, the gas temperature decreases. This is due to a decrease in the amount of oxygen entering the gasifier. As can also be seen from Figure 3, when the secondary coal feeding ratio is 0.1, the temperature in the first stage of the gasifier drops from 1440°C to around 1320°C, while the temperature at the exit of the gasifier falls from around 1440°C to 1110°C. http://www.yf116.cn/jishuwang/upload/060510919077195.jpg Figure 3 shows the trend of gas temperature as a function of the secondary coal feeding ratio. Figure 4 illustrates the trend of the total oxygen-to-coal ratio in the gasifier as a function of the secondary coal feeding ratio. As can be seen from the graph, as the ratio of coal fed in the two stages increases, the oxygen consumption decreases, indicating that two-stage dry pulverized coal fluidized bed gasification may reduce oxygen consumption. As can be seen from the performance analysis of the gasifier above, the two-stage dry coal powder fluidized bed gasifier uses the same feed ratio as the single-stage gasifier; when the feed ratio in the second stage is 0.1, the efficiency of cold gas production increases by approximately 4 percentage points, and the oxygen consumption is 90% of that of the single-stage gasifier. http://www.yf116.cn/jishuwang/upload/060510919502678.jpg Figure 4: Trend of the total oxygen-coal ratio as a function of the secondary coal feeding ratio. The following conclusions can be drawn from the above research. (1) By adopting a two-stage dry coal powder fluidized bed gasification process, the temperature of the outlet gas can be reduced; this makes it possible to omit the gas quenching step in the Shell gasification process, thereby reducing the flow rate of gas entering the gas cooler and decreasing the size of the gas cooler, dust removal, and water washing units. (2) By adjusting the coal feeding ratio, the two-stage dry pulverized coal fluidized bed gasification process can reduce the oxygen consumption during gasification and significantly improve the cold gas efficiency of the gasifier. Compared to the Shell gasification process, it can improve by 2–3 percentage points. 4 Process flow of two-stage dry coal powder gasification technology The process flow of the two-stage dry coal powder gasification technology is shown in Figure 5. This gasification process has the following characteristics. (1) The gasification temperature is between 1400 and 1600°C, the pressure is 3.0 MPa; the carbon conversion rate exceeds 99%, the product gas is relatively clean and free of heavy hydrocarbons, and the proportion of useful gases (CO + H2) in the gas reaches over 90%. High-temperature gasification does not produce condensates such as tar and phenol, does not pollute the environment, and yields gas of high quality. (2) Two-stage gasifier. The dry coal powder is dispersed by the gasifying agent; in a section of the gasifier (at a temperature of 1300–1500°C), the coal undergoes partial oxidation reactions with O2 and H2O, resulting in crude gas composed mainly of CO and H2. A small amount of coal, N2, and steam are fed into the second stage of the gasifier (1000–1200°C), where reactions such as the dry distillation and pyrolysis of coal, the secondary cracking of volatiles, and the decomposition of water vapor take place. The gasification furnace features a water-cooled wall structure that uses slag to resist further slag formation; it has no refractory brick lining, requiring less maintenance, enabling a longer operating cycle, and eliminating the need for a backup furnace. (3) Spray water quenching and cooling. The mixed crude gas formed from the two stages of gasification is quenched to around 900°C at the upper part of the gasifier using spray cooling water, thereby solidifying the molten ash particles entrained in it. The raw gas leaves the gasifier and enters the waste heat boiler, where it exchanges heat with deoxygenated water (4.0 MPa, 40°C), cooling down to around 300°C. (4) The waste heat boiler recovers sensible heat. Waste heat boilers are used to recover the sensible heat from high-temperature gas; they must withstand both high temperatures and pressures, as well as the erosion caused by dust in the gas, resulting in relatively harsh operating conditions. The metal outer shell of the waste heat boiler is a pressure vessel, while its internal components consist of a cylindrical water wall and several layers of coiled water walls. The various layers of the coiled water walls are separated from one another by seals. To remove ash accumulated on the water walls, the waste heat boiler can be equipped with a certain number of pneumatic knocking devices for ash removal, which carry out vibration-based ash removal on a regular or irregular basis. (5) Low oxygen consumption. This process has a relatively low oxygen consumption, thereby reducing the investment required for the air separation unit associated with it. The single furnace has a high production capacity. Moreover, this process features a high thermal efficiency: approximately 83% of the heat energy in the coal is converted into syngas, while about 15% of the heat energy is recovered as high-pressure or medium-pressure steam, resulting in an overall thermal efficiency of around 98%. (6) The upward airflow carries away some liquid slag; to prevent this slag from sticking to the walls during solidification, it is necessary to use rapid cooling to solidify the slag, reducing the furnace temperature instantly below the softening temperature (ST) of the ash and slag. This process uses the method of injecting quenching water for rapid cooling. The slag discharged at high temperature from the gasifier is quenched to form glass-like particles with stable properties, and the slag is removed using a slag lock tank. Gasified wastewater contains low levels of cyanide, making it easy to treat and enabling near-zero emissions. (7) The gasification process is equipped with an advanced control system, including a dedicated process computer control system. To ensure the safety of equipment and operators, necessary DCS and ESD (Emergency Shutdown System) systems are in place to enable the gasification process to operate under optimal conditions. (8) The raw gas exiting the waste heat boiler enters a dry dust collector, where an efficient fly ash filter is used to recover the fly ash. The collected fly ash is then returned to the coal bin at normal pressure after being pressurized (for further gasification). The overall carbon conversion rate is high (around 99%). The removed fly ash goes into a fly ash collection tank, and the fly ash emerging from this tank is sent back to the gasification furnace for reuse. 5 Experimental research: Since 1994, the Xi’an Thermal Power Research Institute has been conducting research on dry coal powder fluidized bed gasification technology. With funding from **the power company and the Ministry of Science and Technology, a dry coal powder gasification test unit capable of processing 0.7–1 t/d of coal was built in 1997. Using this unit, pressure gasification tests were conducted on 14 typical Chinese thermal coal varieties; the process flow and the layout of the unit are shown in Figures 5 and 6. http://www.yf116.cn/jishuwang/upload/060510921013262.jpg Figure 5: Flow diagram of the dry coal powder gasification test unit with a capacity of 0.7–1 t/d. The operating parameters and test results for this unit are as follows: Capacity: 600–1000 kg/d; Pressure: 3.0 MPa. Coal powder flow rate in the first stage: 25 kg/h; Oxygen consumption in the first stage: 0.55 m³/kg; Temperature in the first stage: 1360–1500°C. Coal powder flow rate in the second stage: 5 kg/h; Oxygen consumption in the second stage: 0; Temperature in the second stage: 1100–1150°C. Composition of the syngas: CO 56.1%, H2 34.4%, CO2 6.5%, CH4 0.5%, N2 2.0%, and various other gases 0.5%. Carbon conversion rate: 95.9%; Cold gas efficiency: 78%. http://www.yf116.cn/jishuwang/upload/060510924092747.jpg Figure 6: Dry coal powder gasification test unit with a capacity of 0.7–1 t/d. 6. Semi-industrial test facility: With the support of key projects under the “Tenth Five-Year Plan” 863 Program, Xi’an Thermal Power Research Institute, together with six other research institutes and manufacturing units in China, built a semi-industrial dry coal powder pressurized gasification facility in 2004, capable of processing 36–40 t/d of coal (equivalent to 10 MWth). Gasification tests were conducted using four different types of coal powder. Diagrams showing the structure and operation of this facility are provided in Figures 7 and 8. The operating parameters and test results of the semi-industrial plant are as follows: capacity 36–40 t/d, pressure 3.0 MPa, temperature 1360–1600°C, oxygen consumption 0.55 m³/kg. The composition of the syngas is as follows: CO 56.1%, H2 34.4%, CO2 6.5%, CH4 0.5%, N2 2.0%, and various other components including H2S at 0.5%. The carbon conversion rate is 98.3%, and the cold gas efficiency is 82.5%. http://www.yf116.cn/jishuwang/upload/060510925136426.jpg Figure 7: Semi-industrial two-stage dry coal powder pressurized gasification plant with a capacity of 36–40 t/h. http://www.yf116.cn/jishuwang/upload/060510926257976.jpg Figure 8: Schematic diagram of the process in the two-stage dry coal powder pressurized gasification plant. 7 Conclusion: Based on research on the characteristics of fluidized beds and the gasification process, Xi’an Thermal Engineering Research Institute developed a two-stage dry coal powder pressurized fluidized bed gasifier with independent intellectual property rights. With the support of the “Ninth Five-Year Plan” and the 863 Program during the “Tenth Five-Year Plan” period, experimental plants with a capacity of 0.7 t/d were built, as well as semi-industrial plants with a capacity of 36–40 t/h. Various types of coal were also tested using these plants. The test results show that using a two-stage gasifier can reduce the outlet temperature of the gas, and the efficiency of cold gas can be increased by 2–3 percentage points. The reduction in the temperature of the exhaust gas enables the simplification of the process and reduces the geometric dimensions of the gas coolers, dust removal, and washing units. At present, with the support of key scientific and technological projects of Huaneng Group, Xi’an Thermal Power Research Institute has developed a 1,000 t/d dry coal powder fluidized bed gasifier; the process design has been completed, and it will be used in the demonstration project for gasification-based power generation as part of the first phase of \"green coal power\" development. This project is scheduled to be completed and put into operation in 2008. In addition, the Xi’an Thermal Engineering Research Institute is also conducting research on the quenching process for dry coal powder gasification technology, with the aim of applying this technology in the coal chemical industry to help companies reduce their investments and alleviate their financial burdens. References: Xu Shisen, Ren Yongqiang. Two-stage dry coal powder fluidized bed gasifier. Chinese Patent, 012404071.2001-11-30. Xu Yue, Wei Shirang, et al. Simulation of dry coal powder fluidized bed gasifiers using Aspen Plus. Journal of Xi’an Jiaotong University, 2003, 37(7): 692–694.