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Author/Source: Peng Aihua1, Jiang Tao2, Zhao Yonghua1, Nie Yimin1, Chen Qi2 (1. Hubei Shuanghuan Technology Co., Ltd., Yingcheng, Hubei 43240). Date: 2009-1-13. Hubei Shuanghuan Technology Co., Ltd. is a large-scale integrated enterprise that produces synthetic ammonia using heavy oil as raw material, and also manufactures soda ash and ammonium chloride through the combined alkali production process. In 2005, Shuanghuan Company produced 200 kt of synthetic ammonia, as well as 600 kt each of soda ash and ammonium chloride. Due to the rising prices of crude oil in both international and domestic markets, companies were forced to change their raw material sources. After thorough research and evaluation, Shuanghuan Company decided to adopt Shell’s gasification technology to switch from using oil as a raw material for ammonia production to coal. In June 2001, the project was approved for implementation by the former Ministry of Economy and Trade, and it was included in the sixth batch of national debt-funded projects that year. 1 Feasibility analysis of producing synthetic ammonia using Shell’s gasification technology. At present, the more advanced gasification technologies available internationally and domestically include Texaco’s water-coal slurry gasification technology and Shell’s pulverized coal gasification technology. The Texaco water-coal slurry gasification technology has been adopted by several companies in China, and more companies are investing in its implementation. After years of operation, this process has accumulated a great deal of experience; numerous studies have been conducted both domestically and internationally in terms of technology as well as equipment. Therefore, the use of Texaco water-coal slurry gasification technology is highly mature and reliable. However, this technology also has some drawbacks: first, the refractory bricks are expensive and have a short service life; the bricks exposed to heat must be replaced annually ； Second, the burner has a short lifespan, and it must be inspected and replaced every two months ； Third, it has a high oxygen consumption rate; the content of useful gases CO + H2 is only around 80%, and there are certain requirements regarding the type of coal used ； Fourth, ash and slag have a high carbon content, making them difficult to treat. The dry powder gasification technology developed by Shell has the following advantages over Texaco’s water-coal slurry gasification technology: first, a membrane-type water wall is used in place of refractory bricks, which extends the continuous operation time of the gasifier ； Second, the burner has a long service life, usually over one year ； Third, the content of the active gases CO + H2 in the gas reaches 90%, with a carbon conversion rate of >99% ； Fourth, it has a high gasification temperature; the carbon content in the slag and ash is low, making it easy to handle and causing no environmental pollution ； Fifth, there are no special requirements regarding the type of coal. Currently, the 253 MW unit at the Dutch Nuon power plant (formerly Demkolec power plant), which uses this technology to produce syngas for gas-steam combined cycle power generation, has had a stable operation of its gasification section since it came online in 1993. After the defects in its generator set were resolved, the power plant began commercial operation in 1997. Although Shell’s powder gasification technology has not yet been used for ammonia synthesis abroad, since the main components of the crude gas produced are CO, H2, N2, CO2, H2S, etc., it can meet the requirements for raw gas in ammonia synthesis after appropriate purification. To date, 19 coal gasification units have signed technology introduction contracts with Shell in China. They are used respectively for the production of methanol, synthetic ammonia, and hydrogen, among others. In addition to the plant in Hubei Shuanghuan, which has already started operating with a coal input of 1,000 tons per day, three plants owned by Sinopec, each with a coal input of 2,000 tons per day, will also begin operations one after another in 2006. 2 Introduction to the Shell gasification process: Coal from the coal yard and limestone are mixed in a certain proportion using a weighing feeder, then fed into a coal grinder for mixing, and dried with hot air. After being filtered by a bag filter, the dried coal powder is stored in a coal powder silo. The coal powder coming out of the powder bin passes through a lock-hopper device, is pressurized to 4.2 MPa by nitrogen, and is sent to the burner using nitrogen as a driving force, where it enters the gasification furnace along with steam and oxygen to undergo a partial oxidation reaction. The reaction temperature is 1500–1700°C, and the pressure is 3.5 MPa. The syngas exiting the gasification furnace is cooled to 900°C by cold gas supplied by a quenching compressor at the top of the furnace. After heat is recovered through the quencher heat exchanger, the pipeline heat exchanger, the return chamber heat exchanger, and the syngas cooler, the temperature of the syngas drops to 330°C. It then enters a high-temperature, high-pressure ceramic filter to remove 99% of the fly ash from the syngas. The gas exiting the ceramic filter enters the Venturi scrubber and scrubbing tower, where the remaining ash is removed using high-pressure process water and the temperature of the syngas is reduced to 150°C; thereafter, it is sent out of the gasification unit’s boundary area to the sulfur-resistant conversion unit. The washed synthetic gas, along with the synthetic gas coming out of the ceramic filter, is mixed and cooled to 200°C before entering the quench gas compressor; after compression, it is sent back to the top of the gasifier. The slag generated within the gasification furnace flows along the walls of the furnace to the slag pool at its bottom; there it solidifies into glass-like slag upon contact with water. Subsequently, it is transferred via collectors, slag breakers, and slag hoppers to the slag dewatering tank on a scheduled basis. After that, it is removed using slag skimmers and sent to a slag disposal site, where it is sold as a commodity. The fly ash collected in the high-temperature and high-pressure filter is stripped in a fly ash stripping tower and cooled to 100°C before being fed into the fly ash storage tank. As needed, part of the fly ash is returned to the coal grinder, while the rest is sold as a commodity. The water inside the membrane-type water wall of the gasifier and in the syngas cooler is forced to circulate by pumps; the 5.4 MPa saturated steam generated enters the drum, and after steam-water separation, the steam proceeds to the steam main while the water continues to circulate. The process flow diagram is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/061220847016078.jpg Figure 1: Schematic diagram of the Shell gasification process. 3. Main design parameters. 3.1 Chemical composition of coal: The type of coal selected for our company’s design is Xinmi coal from Henan Province. Its chemical composition (on a dry, ash-free basis) is as follows: C 88.86%, H 3.95%, O 4.80%, N 1.83%, S 0.55%, Cl 0.01%. Its lower heating value is 24.98 MJ/kg, and its ash melting point is 1410 °C. 3.2 Key consumption figures and technical characteristics: The gasification unit is designed to produce 200 kt of synthetic ammonia per year; thus, the effective gas output of CO + H2 is 55,000 m3/h. The following quantities are required: Coal – 39.8 t/h; Limestone – 2.48 t/h; Oxygen (99% purity) – 20,400 m3/h; Ultra-high pressure nitrogen (80°C, 7.2 MPa) – 12,000 m3/h; High-pressure nitrogen (40°C, 5.2 MPa) – 7,000 m3/h; Low-pressure nitrogen (40°C, 0.6 MPa) – 4,500 m3/h; Steam (265°C, 4.5 MPa) – 2 t/h; By-product steam (265°C, 5.4 MPa) – 54 t/h. The gas composition exiting the gasification unit is as follows: CO 55.3%, N2 9.0%, H2 19.1%, CO2 2.2%, H2O 14%, H2S 1000×10^-6. The temperature is 150°C and the pressure is 3.25 MPa. 4 Introduction to Major Equipment: After the project was approved, in order to save on costs, the company adopted a policy of importing key equipment (such as critical instruments, valves, and materials), while sourcing other equipment domestically. The internal components of the main equipment, namely the gasification furnace and the syngas cooler, are manufactured by foreign manufacturers, while the casing is produced domestically; the internal components and the casing are assembled at domestic factories ； All other equipment is manufactured in China. 4.1 Gasifier body The gasifier body includes the gasifier, gas transfer pipes, and syngas cooler (also known as a waste heat boiler). It consists of an outer shell and inner components; the outer shell withstands static pressure but not high temperatures, while the inner components form a vaporization space, a slag collection space, and a gas delivery space. The basic structural form of the internal components is a membrane-type water wall structure. The internal components are manufactured by the Spanish company BPE. The casing is manufactured by Jinzhou Heavy Machinery Factory, and the internal components together with the casing are assembled by Jinzhou Heavy Machinery Factory. The three major components are transported to the construction site by waterway and then installed and aligned. 4.2 Quenching gas compressor: Two centrifugal compressors are selected, one in operation and one as a backup. Due to the high temperature of the gas (inlet temperature of 200°C), high CO content, as well as the presence of H2S and a small amount of dust, the equipment must be capable of withstanding high temperatures, resistant to H2S corrosion, and have good sealing properties. This device was manufactured by Jinxi Chemical Machinery Factory. 4.3 High-temperature and high-pressure fly ash filter: A vertical filter with an inner diameter of 4750 mm is used, comprising 12 filter units, with 48 filter tubes in each unit. The filtering element is a sintered ceramic tube produced by the German company Pall Schumacher, while the housing is manufactured by Shanghai Boiler Factory. The filter elements are assembled on-site. Due to the high gas temperature, the shell wall thickness is 100 mm, and the total weight of this equipment is approximately 200 tons. 4.4 Coal Grinding System The coal grinding system includes coal grinders, circulation fans, hot air furnaces, and bag filters. The design features two coal grinding systems, one in operation and one as a backup; each system has a capacity of approximately 57 t/h, which is sufficient to meet the requirements of full-load production. The coal grinder is of medium-speed type and was manufactured by Beijing Electric Power Equipment Factory. 4.5 Forced circulation pumps: Three boiler water forced circulation pumps are installed in accordance with Shell’s requirements; under normal conditions, two of them are in operation while one serves as a backup. When the total circulation volume of hot water falls below the required level, the third pump starts automatically. This pump is characterized by a high inlet pressure (5.0 MPa) and a high temperature (250°C). Manufactured by Shenyang Pump Factory. 4.6 Slag skimmers: The design includes two slag skimmers, one in operation and one as a backup. These slag skimmers are controlled by a program and carry out functions such as slag removal and water drainage; they are manufactured by Baoding Electric Power Equipment Factory. 4.7 Slag crusher: The furnace slag exits the slag lockhopper, is crushed by the slag crusher, and then enters the slag pool; it is manufactured by Beijing Aerospace 11th Research Institute. 4.8 Other equipment: The ventilation devices for coal powder and ash transportation, namely those made of metal sintered materials, are produced by the Northwest Non-ferrous Metals Research Institute. The waste gas is discharged to a flare where it burns after being mixed with fuel gas; the flare was manufactured by Shanghai Institute 711. 32 sets of rapping dust removal devices were provided by Wuhan University of Chemical Technology. 5 Supporting projects for Shell gasification: Since the CO content in the gas produced by Shell gasification is higher than that in oil-based gasification, and due to the premature addition of nitrogen, the amount of raw gas required to produce 200 kt of synthetic ammonia has increased compared to before. The company’s existing gas purification facilities are not sufficient to meet the needs of producing 200 kt of synthetic ammonia; therefore, it has been decided to build a new set of gas purification facilities capable of handling 200 kt/year of synthetic ammonia. This facility will use a gas purification process that involves sulfur-resistant shift reaction, low-temperature methanol washing for desulfurization and decarburization, and methanation. The shift reaction will be carried out in four stages, while the low-temperature methanol washing process will utilize Linde’s patented technology. Since the oxygen consumption in Shell’s gasification process is higher than that in oil gasification, the company’s existing four air separation units with an oxygen production capacity of 3,600 m3/h were insufficient to meet the production requirements. Therefore, a new air separation unit manufactured by Hangyang with an oxygen production capacity of 10,000 m3/h was installed. Two new combined units (one in operation and one as backup) have been added to produce high-pressure and ultra-high-pressure nitrogen at 5.2 MPa and 7.2 MPa, respectively. Two reciprocating oxygen compressors that produce medium-pressure oxygen at 4.0 MPa. Three screw nitrogen compressors produce low-pressure nitrogen at 0.8 MPa. Shell provides the process package and basic design package for the gasification unit, while Linde supplies the basic design package for the low-temperature methanol washing process; China Wuhuan Chemical Engineering Company is responsible for the detailed engineering design. 6 Introduction to the project construction: The three key components for gasification were transported by water to the company’s site in August 2004, and the gasification furnace was installed there in mid-September. Since the company’s design utilizes tower cranes, and based on the layout of the equipment on site, it is necessary to wait until the fly ash filter is in place before lifting the syngas cooler. In October 2004, the fly ash filter was delivered to the company’s site and then lifted into place, followed by the lifting of the syngas cooler. Then, a tower crane is used to assemble the gas delivery conduit and the gas return chamber below. It was then lifted to the upper section and aligned with the gasifier and syngas cooler; the on-site welding work was fully completed by Jinzhou Heavy Machinery Factory at the end of November. All other supporting equipment has been installed, and the site has entered the piping phase. Starting in October 2005, the coal gasification unit underwent pipeline purging, pressure testing for leak detection, individual equipment commissioning, joint commissioning, commissioning of the coal grinding system, drying of the gasifier and syngas cooler, testing of the water vapor systems in the gasifier and syngas cooler, testing of the sequential control systems, testing of the logic and interlock systems, commissioning of the ignition nozzles and startup nozzles, and calibration of the coal powder metering system. By May 2005, all preparatory work was completed, and the unit was ready to start operating with coal feed. Through the joint efforts of Shuanghuan Company, Shell Company, Wuhuan Company, and the equipment manufacturers, various challenges were overcome and one problem after another was resolved; finally, on May 17, 2006, the gasification unit was successfully put into operation during its first trial run. The 10,000 m3/h air separation unit, which is part of the supporting facilities for Shell’s gasification process, came online in June 2004. The sulfur-tolerant shift reaction and low-temperature methanol washing processes were successfully commissioned for the first time in June 2005 using cracking gas from heavy oil gasification. “The total investment in the \"oil to coal\" project exceeds 700 million yuan, of which about 450 million yuan is allocated to coal gasification. Based on a heavy oil price of 3,000 yuan per ton, once coal gasification is put into operation, the cost of synthetic ammonia per ton will decrease by 700 yuan, while the cost of soda ash and ammonium chloride per ton will drop by 300 yuan. This will generate annual savings of 180 million yuan, thereby greatly enhancing the company’s competitiveness. 7 Operation Status of the Facility 7.1 Operation Status of the Coal Gasification Facility Since its commissioning on May 17, 2006, the Shell coal gasification facility has stopped operating several times due to issues with nitrogen, quench gas compressors, and burner covers. Due to the high degree of automation of the equipment, starting and stopping it is very convenient; its operation is safe and reliable, and the operating conditions remain stable during normal production. Operators only need to pay attention to the execution of the sequential control program and the important parameters, and the ash and slag discharge also proceed smoothly. During the shutdown, the slag accumulation on the water wall of the gasifier and the dust buildup inside the equipment were inspected. It was found that as long as the CO2 level in the syngas is properly controlled, thereby maintaining normal temperature conditions in the gasifier, slag accumulation within it will also be under control. Dust accumulation on the heat exchange surface can be removed automatically through shaking. The maximum load of the device once reached 8.35 kg/s (the designed 100% load is 8.08 kg/s). All process parameters are within normal ranges; the syngas production and the production of usable gas have reached the design values, and all existing heavy oil gasification units have been shut down. Overall, Shell’s powder coal gasification technology offers high gasification efficiency, operational safety and reliability, a high degree of automation, ease of operation, and no environmental pollution, thanks to its dual-loop operation. 7.2 Key consumption indicators: Coal consumption per ton of ammonia = 1.5 t (heat value of coal: 22.49 MJ/kg); Oxygen consumption per ton of ammonia = 830 m3; Steam generated as a by-product per ton of ammonia (at 5.4 MPa) = 2.645 t. Composition of syngas: H2 21%, CO 65%, CO2 2%, N2 11%, H2S 1000 mg/m3; Carbon content in ash = 3%, carbon content in slag = 1%–2%. 7.3 Operation of the gas purification unit: The accompanying gas purification unit is operating properly. In particular, the sulfur-resistant shift process for high-concentration CO was carried out successfully, reducing the CO concentration from 65% to 0.3%. The resulting gas, after desulfurization, decarburization through low-temperature methanol washing, and methanation purification, was sent on to ammonia synthesis. All process parameters of the low-temperature methanol washing system meet the specified requirements, fully satisfying the design specifications. Compared with the original two sets of four-catalyst gas purification units, it saves 40 million yuan in annual operating costs. 8 Experience and Insights: Shell gasification technology is currently the most advanced gasification technology available internationally. However, introducing this technology into production imposes high demands on enterprises. Strict standards must be established and carefully followed at every stage, from equipment ordering and manufacturing, through on-site installation, piping, commissioning, normal operation, to equipment maintenance. Only in this way can long-term, full-capacity, safe, and stable operation of the facility be ensured. Shell has been gradually promoting the localization of equipment in order to reduce investment costs for the company and facilitate the development of powder gasification technology in China. At present, except for some key equipment (such as the internals of gasifiers, the internals of fly ash filters, and the lockhopper valves in the ash handling system), the owners can purchase most of the remaining equipment domestically. Since the gasification process is a solid-handling process that operates under high temperature and pressure, in order to ensure the long-term reliable operation of customer installations, Shell needs to certify the manufacturers of certain key equipment. At the same time, in order to provide customers with a fair market environment and more choices, Shell has been seeking additional qualified suppliers for them, ensuring that there is at least one supplier for key equipment so that owners can make a selection. For example, Shell has now licensed a second supplier of gasifier internals ； For another example, after more than a year of rigorous qualification assessments and on-site tests, Shell certified two domestic suppliers of inflation components; their products have been used in gasification units under construction in China. Some of these products have passed operational tests, demonstrating high reliability and meeting Shell’s technical requirements. Shell is still looking for other qualified suppliers in China in order to localize more key equipment; some cooperation agreements have already been reached and are now in the implementation phase. We believe that in the near future, more key equipment will be produced domestically, thereby further reducing the investment required for Shell’s gasification plants. This will make new contributions to our country’s sustainable development strategy, the advancement of green energy policies, and the improvement of energy efficiency.