Efficient and clean utilization of coal – gasification technology
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Efficient and Clean Utilization of Coal – Gasification Technology by Zhang Xiaohui (Northwest Research Institute of Chemical Industry, Xi’an, Shaanxi 710600) explains that gasification technology is one of the key technologies for the efficient and clean utilization of coal today. Through an analysis and comparison of several current industrial gasification technologies, the development direction of gasification technology is identified, and some suggestions are put forward for the construction of the energy base in northern Shaanxi and the development of coal chemical industry. Coal gasification technology ； High-efficiency cleanliness ； Development Direction TQ 54 A 1003-5095(2007)11-0023-03 Coal is the most abundant and widely distributed fossil fuel on Earth. China has abundant coal resources but limited oil and gas reserves; coupled with rising oil prices, energy consumption relies even more on coal. Shaanxi Province is one of the key provinces with large reserves of coal resources. The proven coal reserves in northern Shaanxi exceed 200 billion tons, accounting for 99% of the province’s total coal resources. These reserves are abundant, easy to extract, of high quality at low costs, and can be utilized for hundreds of years. To this end, **and Shaanxi Province** have decided to build a large-scale coal-based energy and heavy chemical industry complex in northern Shaanxi, making full use of the region’s coal resources to drive economic development in Shaanxi. Looking at the current situation of energy supply, synthetic ammonia, methanol, as well as future technologies such as direct coal liquefaction and alcohol-ether fuels, all rely on gasification of coal to produce syngas. Across the country, the amount of coal gasified for ammonia synthesis and methanol production alone already exceeds 40 million tons per year ； It is estimated that the amount of coal required for coal-to-oil production will reach hundreds of millions of tons per year in the future ； Industrial direct coal combustion exceeds 400 million tons per year; to address pollution issues, a significant portion of this coal needs to be processed using advanced gasification methods, requiring tens of millions of tons of coal to be gasified each year ； To improve the quality of petroleum products, the refining industry requires 10–20 billion m3 of hydrogen per year. Coal gasification is an economical and reliable method for producing hydrogen; the hydrogen production for petroleum product processing requires 10 million tons of coal per year ； Over the next 20 years, coal-to-oil production is expected to reach tens of millions of tons, requiring an additional installed capacity of over 100 million kW. Advanced gasification technology-based combined cycle power generation systems are planned to be used, with a total annual demand for gasified coal of approximately 100–200 million tons. Therefore, coal gasification is an important technical component and key method for the comprehensive utilization of coal and the development of clean coal technologies; it serves as the cornerstone for advancing important industrial processes such as modern coal chemical industry, coal-to-oil conversion, and fuel gas production. 1 Current Development Status of Coal Gasification Technology 1.1 Classification and Characteristics of Coal Gasification Technologies Based on the way in which coal moves within the gasifier, these technologies can be divided into fixed-bed (moving-bed), fluidized-bed, and pneumatic-bed types; Table 1 outlines the main characteristics of each type of gasification technology. Table 1 Main Characteristics of Gasification TechnologiesGasification Technology: Fixed-bed, Fluidized-bed, Gas-flow bed
Ash Discharge Form: Dry ash, Slag, Dry ash, Ash agglomerates, Slag
Characteristics of Feed Coal: Lump coal, Lump coal, Powder coal, Powder coal, Powder coal/water coal slurry
Particle Size/mm: 13–50, 5–50, 0–8, 0–8, 0.2
Ash Content/%: <20, <15, Unrestricted, Unrestricted, <13
Ash Melting Point/°C: >1,250, <1,300, Unrestricted, Unrestricted, ≤1,350
Operating Pressure/MPa: 2.24, 2.24, 1.0, 0.03–2.5, 2.5–6.5
Operating Temperature/°C: 400–1,200, 400–1,200, 900–1,000, 950–1,100, 1,350–1,700
Coal Gas Temperature: Low, Low, Medium, Medium, High
Oxygen Consumption: Low, Low, Medium, Medium, High
Steam Consumption: High, Low, Medium, Medium, High
Representative Technologies: Lurgi, lurgiBGL, Enders, Ash agglomerates, Shell/Texaco
Fixed-bed pressurized gasification (Lurgi) features high thermal efficiency (or cold coal gas efficiency) and low oxygen consumption. However, it is suitable only for weakly cohesive or non-cohesive lump coal. The coal gas contains substances such as tar and phenols, requiring lengthy purification processes and high investment costs; therefore, it is less commonly used in new gasification projects. Fluidized-bed gasification uses pulverized coal as feedstock (below 6 mm), and the syngas contains almost no tar, phenols, or hydrocarbons. To prevent the material in the bed from sintering due to high ash content, conventional fluidized beds must operate at relatively low temperatures (below 950 °C); therefore, they are only suitable for highly active lignite or sub-bituminous coal. The ash fusion fluidized bed gasification technology developed in our country increases the carbon concentration in the bed by means of selective ash discharge, reduces the risk of slag formation, and raises the operating temperature to 1,100 °C; the range of coal types that can be used has been expanded to include bituminous coal and even anthracite. Fluidized bed technology enjoys increasing attention due to its moderate operating temperature, low investment costs, and wide adaptability to various raw materials. Its drawbacks are the low operational pressure under current technology, a smaller processing capacity per unit compared to fluidized bed systems, and a need to improve the carbon conversion rate due to the carry-away of fine powder. Gas-flow bed gasification uses fine coal powder with a particle size of <0.2 mm as raw material; the coal powder can be fed in a dry state (Shell) or in a slurry form (water-coal slurry, Texaco). It operates at high temperatures, is suitable for almost any type of coal, and the gas produced contains no tar, phenols, or hydrocarbons, offering high gasification efficiency. But at the same time, the investment is high and the oxygen consumption is high. Depending on the coal quality and process characteristics of different raw coals, various gasification processes can be selected. 1.2 Current Status of Gasification Technology 1.2.1 Multi-component Slurry Gasification Technology The multi-component slurry gasification method is a patented technology developed by the Northwest Research Institute of Chemical Engineering. The raw materials for this gasification method can include petroleum coke, petroleum asphalt, coal, various solid residues and liquid residues from petroleum processing, etc. By using appropriate additives and a one-step slurry preparation process, a suitable gasification slurry is produced, which is then reacted with oxygen under high temperature and pressure to generate syngas composed mainly of CO and H2. The multi-material slurry gasification method is advanced, mature, and reliable; it meets environmental standards and has low energy consumption. It has been successfully applied for many years in two enterprises: Zhejiang Fengdeng Chemical Co., Ltd. and the ammonia synthesis plants of Zhejiang Juhua Co., Ltd. 1.2.2 Water-coal slurry gasification technology: This technology belongs to the category of pressurized fluidized bed gasification techniques, and it was developed in the 1950s by Texaco in the United States. Through years of development and improvement, many sets of industrial installations have been established worldwide. Its technical features are: (1) It has strong adaptability to raw coal; no strict requirements are placed on the activity of the coal, but it is not suitable for coal with an excessively high ash melting point, which should generally be below 1,250 ℃ ； (2) High yield of useful components upon gasification (CO+H2≥80%), making it suitable for syngas production ； (3) The gas produced by gasification contains no pollutants such as tar or phenols; the gas is easy to purify, with almost no waste gas emitted, and little wastewater is produced as well. After treatment, it fully meets **the relevant emission standards** ； (4) High carbon conversion rate, generally ranging from 95% to 98% ； (5) High gas production capacity per furnace. In southern Shandong, the Wei River region, Shanghai’s tri-generation systems, and Huainan in Anhui Province, water-coal slurry gasification units have been imported from abroad for the production of synthetic ammonia, methanol, acetic acid, and electricity generation. 1.2.3 Shell Gasification Technology: Shell gasification technology is a patented technology of the Dutch company Shell, and it also belongs to the category of advanced fluidized-bed gasification technologies. This technology primarily involves grinding coal into dry coal powder, which is then injected into a gasification furnace to be burned and produce syngas. Currently, this technology is already used in power plant generation, but its application in large-scale ammonia synthesis production in China has not yet begun. The main features of this technology are: the use of dry coal powder as the gasification feedstock, a water-cooled wall structure in the gasifier, a gasification temperature of 1,400–1,600 °C, a gasification pressure of 3.0 MPa, a carbon conversion rate as high as 99%, a very low methane content in the syngas, and a CO+H2 ratio of 90%. Yueyang Sinopec Shell Gasification Co., Ltd., Yunnan Zhanhua Co., Ltd., and Liuzhou Chemical Industry Group have adopted Shell’s dry coal powder gasification technology for the production of synthetic ammonia, and are currently in the construction phase. 1.2.4 Ende Powder Gasification Technology: The Ende powder gasification technology is a type of powder gasification technique developed by the North Korean ’7·7’ Chemical Plant, based on the atmospheric pressure Winkler gasification technology. Coal for gasification can be lignite, bituminous coal, subbituminous coal, and anthracite with a particle size of 0–10 mm; the gasification agents can be superheated steam along with oxygen or oxygen-enriched air. The gasification temperature is generally around 950 °C, with gasification at atmospheric pressure; the composition of the gas is 80% (CO + H2). Technical features: No tar is produced in the gas, and it offers great operational flexibility ； The production capacity of a single gasification furnace is relatively low, making it unsuitable for the large-scale production of high-pressure syngas. This technology has been adopted for the industrial gas designed by Jingdezhen Coking Gas General Factory. 1.2.5 Circulating Fluidized Bed Gasification Technology The circulating fluidized bed (CFB) gasification technology was developed by LuChi Company in the 1970s; it is primarily used for producing fuel gas and for combined cycle power generation, and it is suitable for supplying low-calorific value fuel gas to cement kilns. Currently, the maximum daily coal processing capacity per furnace is 500 t. Technical features: multiple cycles, carbon particle cyclic gasification; the conversion rate of carbon can reach 95%–98% ； Gases with different requirements can be produced using various gasifying agents ； The production capacity of a single furnace is relatively low. 1.2.6 Ash melting polyfluidized bed gasification technology: The ash melting polyfluidized bed gasifier was developed by the Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences. An industrial demonstration facility has been established at Shaanxi Urbanization Co., Ltd. The gas generation parameters for this process are as follows: gasification at atmospheric pressure, with a gas composition of (CO + H2) at 70%, and a carbon conversion rate of 90%. Technical features: The gasification furnace has a simple structure; it is a single-stage fluidized bed in which the four processes of breaking down coal’s cohesion, removing volatiles, gasifying the coal, and agglomerating the ash can all take place simultaneously ； Wide adaptability to different coal types ； The gasification intensity of atmospheric-pressure gasifiers is moderate; the raw gas contains no tar and only a small amount of phenols, making wastewater treatment simple. No exhaust gas is released, which is highly beneficial for the environment ； All the equipment for this entire system can be fully domestically produced. This technology is currently still in the stage of small-scale industrial demonstration, lacking experience in large-scale industrial application and long-term operation. 1.2.7 Lurgi coal gasification by pressure: The Lurgi furnace is a fixed-bed gasification furnace introduced from the German company Lurgi. It uses lump-shaped, weakly bonded, low-quality coal as raw material, with oxygen (air) and steam serving as the gasification agents, to produce gas under pressure. This type of furnace is suitable for producing fuel gas or the feed gas required for indirect liquefaction of oil. Due to the presence of tar and phenolic substances in the produced gas, facilities for methane conversion (or separation) and wastewater treatment are required; as a result, the production process is lengthy and the investment is high. Manufacturers that produce syngas alone rarely use the Ruhr process for gasification. Four factories in our country use this type of furnace. 1.2.8 Dry coal gasification by pressurized gas technique: Although there is still a certain gap between China’s advanced large-scale coal gasification technologies and those abroad, efforts to develop such technologies have accelerated in recent years. Dry coal powder pressurized gasification is a type of high-temperature pressurized gasification; it is expected to achieve a carbon conversion rate of up to 99%, with (CO+H2) levels reaching 92%. It causes little environmental pollution and represents the future direction for coal gasification. Currently, there is no mature dry coal powder pressurized gasification technology available in China, so efforts should be focused on researching and developing this technology in order to create the conditions for its industrial demonstration and practical application. 2 Directions for the Development of Coal Gasification Technology 2.1 Technological Advancement At present, there are various types of coal gasification technologies, each with its own characteristics, and the performance metrics of these technologies vary significantly. Therefore, the development of coal gasification technology should be guided by technological advancement; for instance, dry coal powder pressurized gasification technology boasts advanced performance metrics and is highly regarded by industry professionals, making it a key focus for future research and development in the field of coal chemical technology. 2.2 Diversity of coal types: Coal resources in China are relatively scattered, with significant variations in coal quality; even within the same mining area, different mining sites can have varying coal qualities. Therefore, advanced coal gasification technologies should possess a broad range of adaptability to different coal types, in order to mitigate the adverse effects caused by variations in coal quality. 2.3 Production Capacity: Increasing the production capacity of a single furnace not only reduces investment and saves space, but also lowers operating costs, thereby reducing the cost of the products. 2.4 Increasing pressure: Raising the gasification pressure appropriately helps to improve the production capacity of a single furnace as well as the gasification efficiency, which is of great significance for reducing production costs. Pressurized gasification of coal enables isobaric synthesis in subsequent stages or reduces the compression ratio of syngas, which is much more economical than compressing coal gas. 2.5 Comprehensive utilization of coal energy: Combining coal gasification with power generation and chemical manufacturing to utilize energy in a comprehensive manner can improve the overall thermal efficiency and reduce production costs, such as in IGCC combined-cycle power generation. 2.6 Environmental protection: Coal contains certain inorganic minerals, and during its processing and utilization, waste residues, wastewater, and exhaust gases are generated. Advanced gasification technologies help to reduce the amount of such emissions or make them easier to handle. This also meets the requirements of **’s policies on clean coal utilization, serving as an important guarantee for the sustainable development of the national economy. 3 Recommendations From the analysis of the above coal gasification technologies, it can be seen that dry coal powder pressurized gasification technology, coal slurry gasification, and multi-feedstock slurry gasification technology are the preferred coal gasification methods in current coal chemical applications. As an energy and chemical industry hub in China, Northern Shaanxi is currently seeing the construction of several large-scale ammonia synthesis plants, methanol plants, and power plants. Additional auxiliary chemical plants are also set to be built there in the future. For the construction of these facilities, coal gasification technology will be used to a large extent. Therefore, given the characteristics of the development of this energy and chemical industry hub in Northern Shaanxi as well as the current state of coal gasification technology, it is recommended that Shaanxi Province increase its investment in research and development of new coal gasification technologies as soon as possible, establish research centers for coal chemical technology, and develop patented technologies with independent intellectual property rights to support the development of this energy and chemical industry hub.