First published in Cornerstone, Volume 4, Issue
By Wang Fuchen, Professor, Associate Dean, School of Resources and Environmental Engineering, East China University of Science and Technology
Yu Guangsuo, Professor, Director, Institute of Clean Coal Technology, East China University of Science and Technology
Guo Qinghua, Associate Professor, Institute of Clean Coal Technology, East China University of Science and Technology
Coal is utilized in three ways in China: direct combustion (through coal-fired power plants and industrial boilers), coking, and gasification. Among these three methods, coal gasification is the cleanest option, and the most complex. Coal gasification accounts for 5% of China’s total coal consumption; it is a core technology in efficient and clean coal conversion, and important in the development of coal-based bulk chemicals (chemical fertilizers, methanol, olefins, aromatics, ethylene glycol, etc.), coal-based clean fuel synthesis (oil, natural gas), advanced integrated gasification combined-cycle (IGCC) power generation, polygeneration systems, hydrogen production, fuel cells, direct reduction iron-making, and other process industries. Coal gasification is not only the foundation for the modern coal chemical industry, and widely used in the oil refining, power generation, and metallurgical industries, it is the common key technology of these industries.
R&D PROCESS FOR COAL GASIFICATION TECHNOLOGY IN CHINA
Research and development on China’s coal gasification technology began in the late 1950s. Government support has resulted in many new developments over the last 30 years, including:
During the 9th–12th Five Year Plans the East China University of Science and Technology carried out several coal gasification projects, including:
Moreover, Thermal Power Research Institute of State Power Corporation and others have further developed dry pulverized coal gasification technology with an industrial demonstration plant using a 2000-t/d single gasifier for power generation with a 250-MW IGCC.
The Institute of Coal Chemistry, Chinese Academy of Sciences, has developed an industrial demonstration plant for fluidized bed oxygen/steam-blown synthesis gas (syngas) production. Its bituminous coal capacity is 200 t/d (normal pressure).
Tsinghua University has also established an experimental unit for multi-stage oxygen-fed entrained bed gasification. Tsinghua University and Shanxi Fengxi Fertilizer Industry (Group) Ltd. have jointly developed the Tsinghua gasifier. The first-generation gasifier adopted the refractory brick structure and oxygen stage-fed entrained bed gasification, with seven gasifiers that are in or are about to enter operation; the second-generation gasifier adopted a membrane wall structure that reduces operating costs and broadens coal adaptability.Currently, 28 gasifiers are under construction and 7 gasifiers are operating.
At present, the OMB coal-water slurry gasification technology is the most widely used, especially in China; this is also China’s first large-scale domestically built coal gasification system.
OPPOSED MULTI-BURNER COAL-WATER SLURRY GASIFICATION TECHNOLOGY
East China University of Science and Technology established China’s first large-scale cold-model entrained-flow gasifier unit with the support of government and industry. Researchers studied the refractory brick, burner, and other issues, gaining an in-depth understanding of the principle, flow field, mixing process, and burner atomization mechanism of the coal-water slurry gasification process. This research resulted in a proposal to develop a multi-burner coal-water slurry gasification technology plan. Figure 1 depicts a schematic of the OMB coal-water slurry gasification process. The technology involves processing syngas from raw materials such as pure oxygen and coal-water slurry. The technical characteristics of the technology include: (1) OMB coal-water slurry entrained-flow gasifier and compound-bed gas washing and quenching equipment; (2) three-unit combination comprising the mixer, cyclone separator, and water scrubber of the preliminary purification process for syngas; (3) direct heat exchange-type wastewater treatment and heat recovery technology for evaporative separation.
The OMB coal-water slurry gasifier (Figure 2) has four symmetrical burners, located at the upper part of the gasifier chamber. This type of opposed impact gasifier overcomes the flaw of irrational residence time distribution in the single-burner coal-water slurry gasifier, as well as short residence time of partial reaction materials in the gasifier. The result is an improvement in gasification efficiency. Evidence from the research shows improvements with a high carbon conversion rate, low oxygen consumption, and less coal consumption.
In comparison to the single-burner gasifier, the OMB gasifier has obvious advantages in large-scale gasification. At present, the OMB coal-water slurry gasifier has been adopted for the Inner Mongolia Rongxin Chemical Company, the largest-scale coal-water slurry gasification plant in the world.
The pre-filmed structure is adopted for the process burner of the OMB gasifier. In comparison to the GE (Texaco) burner, the biggest difference is that the pre-filmed burner avoids the premixing of central oxygen and coal-water slurry in the secondary channel by reducing the central oxygen channel. The pre-filmed burner’s advantages are good atomization performance, simple structure, low velocity of coal-water slurry outlet, and its ability to reduce or avoid wear and tear. The demonstration proves this new type of burner has excellent technological results and long service life. At present, the service life of pre-filmed process burners can reach about 90 days on the average. At Yankuang Cathay Pacific Co., Ltd, the longest service life of such a burner was 152 days.
Syngas Washing and Quenching System
Raw syngas produced in the gasification process at a high temperature with a large quantity of slag enters the washing and quenching chamber located below the gasification chamber for quenching, washing, and humidification. The compound-bed washing and cooling chamber contains a spray bed and a bubbling bed. The spray bed is formed by the washing and quenching ring and the dip tube, and the bubbling bed formed between the bubble breaker and the metal shell. This type of washing and quenching chamber abandons the traditional riser. With several bubble breakers installed in the bubbling area, the effects of air bubble breakup and gas-phase dispersal are realized, promoting the formation of a homogeneous gas-liquid mixture, the reduction of liquefied gas, and slag separation through sedimentation. Industrial plants demonstrate the advantages of the spray-bubbling compound bed in terms of washing and cooling efficiency, load adaptability, and operational stability.
Preliminary Purification System for Syngas
The preliminary purification of syngas based on the OMB gasification process adopts the idea of purification in stages; the ash carried by the syngas is passed through the mixer and the cyclone separator for elementary separation. Subsequently, the ash undergoes further separation of fine particles in the water scrubber. This reduces system pressure drop, prevents clogging of the purification system, and greatly reduces the solid content (<1 mg/Nm) of the syngas in the system. The operation results indicate a system pressure drop upon preliminary purification of the syngas in stages of ≤0.1 MPa. The amount of ash content in the syngas from the scrubber is low and it can directly enter the transformation section without any pre-transformation after the separation of coarse particles in the cyclone separator. Other benefits are improving the water quality at the bottom of the washing tower without any blockage of the quench ring, and preventing the phenomenon of pressure drop increasing in the conversion furnace catalyst.
Wastewater Treatment System
The evaporative hot water tower is key equipment for the wastewater treatment system in the OMB gasification process. The black water undergoes flash evaporation upon a reduction in pressure in the evaporation room of the hot water tower. The steam enters the hot water chamber for direct heat transfer with the gray water and results in an improved heat transfer effect. In addition, this prevents fouling. The operation results show that the temperature between the flash steam exported from the evaporative hot water tower and high-temperature gray water is with a temperature difference of <4°C. The smaller design of the system reduces the need for pumping. Consequently, in comparison to single-burner gasification process, there is less rotating equipment which improves operational reliability.
Continuous Feeding Operation Under Pressure and Online No-Fluctuation Switching of Gasifiers
A set of independent feed systems (including an oxygen and a coal slurry feed) is used for each set of opposed process burners of the OMB coal-water slurry gasifier. When a pair of burner feeding systems malfunctions, the work can be suspended to carry out repairs, and the pair of feeding systems can reoperate again after the malfunction is fixed. Throughout the entire process, the other pair of burner feeding systems maintains normal operation, and ensures that the gasifier is only working under a reduced-load condition without the need to fully stop the whole process, thereby greatly reducing the risk of stoppage.
The online no-fluctuation switching of the gasifiers can realize no fluctuations in the upstream and downstream load capacities during switching operations. During the switching process, the continuous feed feature under pressure using this type of gasification technology has advantages. Stoppage and commencement of operations for the two pairs of burners can be carried out successively through the in-operation gasifier and the active-standby gasifier, thus achieving the switching of gasifiers. This mode of operation greatly improves the operational stability of the gasification system and significantly reduces the consumption of raw materials in the switching process.
Figure 3 depicts a typical load variation curve of two OMB coal-water slurry gasifiers (one in operation and one on standby) in the switching process. The figure shows that, where there is an increase of approximately 15% production capacity in the air separation unit, the gasification plant can guarantee the completion of the switching operation between two sets of gasifiers under a minimum production capacity of 85% of the downstream gas supply. The entire system is smooth and controllable throughout the switching process.
For each numbered point:
Application of OMB Coal-Water Slurry Gasification Technology Project
China’s first large-scale coal gasification technology project was established in Yankuang Cathay Pacific Chemical Co., Ltd. in 2005. Using OMB technology provided a viable alternative and reduced the monopoly on advanced coal gasification technology by international multinational companies.
In 2014, the Inner Mongolia Rongxin Chemical Company conducted a successful test run of its gasification plant. The plant has three OMB coal-water slurry gasifiers with single furnace capacity of 3000 t/d of coal. This coal-water slurry gasification gasifier has the largest coal capacity per gasifier in the world. Since 2015 two gasifiers have operated at full capacity and are currently operating without any gasifier problems.
Compared to other coal-water slurry gasification technologies from overseas, the OMB coal-water slurry gasification technology has greater advantages in areas such as large-scale single-furnace processing, system performance indicators, stability and reliability, and patent licensing fees. The OMB technology is operating with 60 coal-water slurry gasifiers with a further 68 under construction in China and in the U.S. and South Korea. The maximum design capacity of a single gasifier has reached 3150 t/d of coal (dry basis).
East China University of Science and Technology concluded a technology licensing contract with Valero Energy Corporation, the largest oil refining company in the U.S., in 2008. The technology licensing fee amounts to more than RMB100 million. In September 2016, another technology license was implemented with Korea’s TENT Company.
The OMB coal-water slurry gasification technology advantages include high carbon conversion rate, facilitation of large-scale processing, and stable and safe operations. The OMB coal-water slurry gasification technology is one of the three internationally recognized coal gasification technologies, ranking with those of Shell and GE.
PULVERIZED COAL GASIFICATION TECHNOLOGY
Coal-water slurry gasification technology requires coal with better slurry flowing compared to pulverized coal gasification technology, which is more adaptable to a wider range of coals. China has also been developing pulverized coal gasification technology.
Pressurized Two-Stage Pulverized Coal Gasification Technology
Pressurized two-stage pulverized coal gasification technology was developed by Xi’an Thermal Power Research Institute Co., Ltd, which built a 36-t/d pilot plant built in 2005. A demonstration of 2000-t/d dry pulverized coal gasification technology was carried out at the Tianjin 250-MW IGCC Project which began operating in 2012.
Aerospace Furnace (HTL) Gasification Technology
The HTL gasifier employs the single-burner pressurized pulverized coal gasification technology. The technology is applicable for medium-sized gasification plants, with a coal-feed limit of 500–1000 t/d. In 2010, the first demonstration plant was constructed and put into operations in Anhui Linquan Chemical Industry Co., Ltd.
Pulverized Coal Gasification Technology of SE Gasifier
East China University of Science and Technology and Sinopec Group jointly developed the single-burner membrane wall pulverized coal pressurized gasification (SE Gasification Technology). The SE gasification demonstration plant became operational at the end of 2014 and has a daily capacity of 1000 tons of coal.
The gasification coal used is based on a mixture of Guizhou and Shenhua coal with a ratio of 6:4. The fusion temperature of pulverized coal fed to the furnace is approximately 1300°C, and the ash proportion is 16%. The full-load assessment indicators are as follows: oxygen consumption rate, 331 Nm/kNm (CO+H ); coal consumption rate, 569 kg/kNm (CO+H ); carbon conversion rate, 98.3%; efficient gas content, 89%; and ash/slag ratio, about 4:6. There are currently 13 sets of gasifiers being built with a 1500-t/d design capacity for each gasifier.
China’s research and development of coal entrained-bed gasification technology, as well as engineering demonstration, long-term and efficient operation, and further large-scale projects, strongly supported the development of modern coal chemical industry. China possesses the largest coal-slurry gasifier in the world, and coal gasification technologies are internationally recognized. The establishment of large coal-water slurry gasification plants with a daily capacity of 3000 tons of coal is a prelude to a larger-scale demonstration of coal gasification technology. Past, present, and future research has enhanced, and continues to enhance, industrial application of coal gasification technology in China.