Sulfur removal technology from coal

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This post was last edited by liaifeng on 2018-9-3 11:18. Technologies for removing sulfur from coal. Coal is the most abundant fossil fuel resource in the world, accounting for over 70% of the world’s total fossil fuel reserves. Currently, coal accounts for about 30% of the world’s primary energy consumption. According to projections by the World Energy Council, coal’s role as an important component of primary energy sources is not expected to change for a long time; it is anticipated that by 2020, coal will account for 33.7% of the world’s primary energy consumption. China is a major country in terms of coal production and consumption. At present, coal supplies 75% of the country’s primary energy needs, and it will remain the main source of primary energy in China for the next few decades. As an energy source, coal has made significant contributions to human development. However, its exploitation and use have also given rise to a range of pollution problems, particularly the visible and potential hazards caused by the smoke generated during combustion, which threaten ecological balance and human survival. 　　The four major global environmental problems—atmospheric particulate matter, acid rain, the greenhouse effect, and depletion of the ozone layer—have seriously affected human living conditions due to rapid economic development. Air pollution is directly related to energy production and utilization; in particular, the exploitation of coal is a major source of smoke, acid rain, and the greenhouse effect. The depletion of the ozone layer is also closely linked to the methane (CH4) emissions generated during coal mining. 　　In fact, coal, like natural gas and oil, is not a source of pollution in itself; it is only due to outdated technologies for its clean use and conversion that **environmental pollution becomes severe when coal is used as the primary energy source. To date, there is no effective solution to the environmental problems caused by the direct combustion of coal. Therefore, the fundamental solution to the problems of low coal utilization efficiency and severe pollution is to utilize coal resources in a rational, clean, and efficient manner, as well as to vigorously develop and apply clean coal technologies – this is incorrect! No reference source found. It is very important and necessary to discuss and analyze flue gas purification technologies. 　　1.1 The environmental impact of coal utilization Coal has contributed to the rapid economic growth of our country during its use, but the efficiency of coal utilization is relatively low, resulting in certain negative impacts on the environment. Qu Xiao’e evaluated environmental pollution in China from 1990 to 2009, using pollution indicators such as industrial SO2 emissions, industrial smoke emissions, and industrial dust emissions. She found that the energy consumption structure based on coal, while supporting economic development, also led to severe environmental pollution. 　　Most scholars study the impact of coal utilization on the atmospheric environment. Some scholars conduct research from a qualitative perspective. Dong Xueling and Liu Daman believe that the dust emitted from coal burning contains a large amount of potentially harmful substances, especially inhalable particles with a particle size of less than 10 μm, which can reduce atmospheric visibility and thus pollute the atmospheric environment. Lu Zhongxian and Tang Yongshun believe that coal generates large amounts of coal dust and particulates during extraction, and the harmful gases emitted during combustion can pollute the atmospheric environment. Xu Ruilin and Wang Tijian believe that air pollution in Jiangsu Province is of the bituminous coal type. They conducted statistics on the urban air quality in Jiangsu and concluded that, in terms of their respective loads, the pollutants are ranked as follows: dust deposition, total suspended particles, SO2, and NOX. Sun Nan believes that the use of coal leads to an increase in carbon dioxide concentrations, thereby causing changes in the global climate. Liu Haibin and Guo Zhengquan believe that the increase in carbon dioxide emissions requires our country to enhance the utilization of coal resources. 　　1.2 Overview of Sulfur Removal Technologies in Coal 　　1.2.1 Necessity 　　Apart from about half being used for power generation, nearly 30% of China’s coal is used in industrial boilers, while 6% is used for domestic heating. Luo Jiaoying pointed out that in our country, coal is mainly used through direct combustion, and this process generates large amounts of harmful pollutants such as dust, SO2, CO2, and NOX, causing severe pollution to the atmospheric environment. 　　According to the 2013 China Environmental Status Report, in 2013, among the 473 cities where precipitation was monitored, 44.4% of them experienced acid rain, and 27.5% of them had an acid rain frequency of over 25%. Among them, the proportions of cities where the annual average precipitation pH was below 5.6 (acid rain), below 5.0 (moderately severe acid rain), and below 4.5 (severe acid rain) were 29.6%, 15.4%, and 2.5%, respectively. The areas affected by acid rain across the country are concentrated along the Yangtze River and to its south, including most of Jiangxi, Fujian, Hunan, and Chongqing, as well as the Yangtze River Delta, Pearl River Delta, and southeastern Sichuan. The area affected by acid rain accounts for about 10.6% of the country’s total area. Acid rain in China is of the sulfuric acid type; SO2 emitted from sources such as coal-fired power plants, which are elevated sources, can be dispersed by the atmosphere to distant areas, where it contributes to the formation of acid rain. The combustion of large amounts of high-sulfur coal is the main cause of severe acid rain pollution in the southwest and central-south regions. The environmental harms caused by acid rain include forest degradation, lake acidification, fish deaths, a decline in aquatic species populations, soil acidification and impoverishment in agricultural fields, increased contamination by toxic heavy metals, significant reductions in crop yields of grains, vegetables, and fruits, as well as damage to buildings, bridges, and cultural relics. 　　1.2.2 Sufficiency The technologies for controlling SO2 emissions can be mainly divided into pre-combustion desulfurization (coal washing technology), in-combustion desulfurization (in-furnace sulfur fixation), and post-combustion desulfurization (Flue Gas Desulfurization, FGD). Desulfurization before combustion can remove most of the inorganic sulfur in coal, but it is ineffective against organic sulfur; only by combining it with other desulfurization techniques can SO2 emissions be brought within environmental standards ； Desulfurization during combustion can simplify the purification process and improve heat utilization efficiency, but it results in lower desulfurization efficiency, increased dust emissions, and the sulfates produced by sulfur fixation decompose at the high temperatures in coal-fired boilers ； Flue gas desulfurization after combustion is a technology that is relatively mature, stable, and efficient; it is currently the most effective and widely used desulfurization method for controlling SO2 emissions in the atmosphere. 　　2. Technologies for removing sulfur from coal before combustion The sulfur components in coal are separated using certain methods prior to its combustion or conversion; this allows for a reduction in the sulfur content during combustion, thereby decreasing the sulfur dioxide pollution generated by coal burning. The methods employed include physical methods, chemical methods, and microbial methods. 　　2.1 Physical desulfurization technologies To date, physical desulfurization technologies are the only industrialized methods for coal purification. The jigging method, heavy medium coal washing method, and flotation method widely used in China all belong to physical purification methods. The sorting process that separates the product from the waste residues is the key step in the physical purification of coal. Physical methods can be used to remove clay, shale, and pyrite sulfur from raw coal. By crushing coal, impurities bound by non-chemical bonds are separated from the coal. Subsequently, by utilizing the difference in specific gravity between the organic components that make up coal (the basic microscopic structure of coal) and the denser mineral impurities, it is sometimes possible to separate them by also taking advantage of differences in their surface wettability, magnetism, and electrical conductivity. The main methods include: modern flotation, heavy liquid enrichment, magnetic separation, electrostatic separation, coagulation, and fine coal–heavy medium cyclone separation. 　　Figure: Physical coal purification system 2.2 Chemical desulfurization techniques Chemical desulfurization techniques involve the use of strong bases, strong acids, and strong oxidizing agents to remove sulfur from coal through processes such as chemical reduction, nitrogen extraction, and pyrolysis. Common methods include the Meyers method, CaCl2 oxidation, NaOH melting, (NH4)2O method, and H2O2 oxidation. Although these chemical methods can remove almost all inorganic sulfur as well as much of the organic sulfur present in coal, they present challenges such as complex process conditions and high costs. Sometimes these processes need to be carried out under specific acid-base conditions, and at times even at high temperatures. For coking coal, it has a significant impact on the properties of the coal, such as causing a decrease in its cohesion and calorific value. It is currently limited to laboratory research. 　　2.3 Biological desulfurization technology The principle of microbial desulfurization relies on the ability of certain acid-tolerant and heat-resistant microorganisms to absorb Fe3+ and SO42- during their growth, thereby promoting the oxidation and decomposition of pyrite and facilitating the removal of sulfur; the sulfur removal rate can exceed 90%. The main methods of microbial desulfurization include leaching, surface oxidation, and extraction techniques. However, the desulfurization reaction takes a long time, making it difficult to meet the requirements of industrial desulfurization. Compared to physical and chemical methods, the microbial method requires less investment and lower operating costs; it can effectively remove sulfides that are finely distributed within coal in a selective manner. 　　2.4 Summary Among the three methods of desulfurization—physicochemical, microbial, and physical methods—the physical method is widely used in industry due to its low cost ； Microbial methods have attracted widespread attention both domestically and internationally, but significant technical research and development are still required before they can be used in industry ； Chemical laws are more commonly used in laboratories. In future development, I believe that the effective combination of these three methods based on different coal qualities would be a good direction to pursue. 　　3. In-furnace desulfurization technology The principle of this process is that fuel and calcium carbide powder, which serves as an absorbent, are fed into the combustion chamber at the same time. The airflow causes the fuel particles, calcium carbide powder, and ash to be vigorously stirred within the circulating fluidized bed, filling the combustion chamber. Inside the combustion chamber, calcium carbide decomposes into calcium oxide, which then combines with sulfur dioxide to form calcium sulfate. The temperature in the boiler’s combustion chamber should be maintained at around 850–900°C to ensure optimal reaction conditions. 　　A circulating fluidized bed (CFB) is generally used. The principle behind this system is that the fuel and desulfurizing agent are fed in from the lower part of the combustion chambers located on the side walls of the furnace, while the primary air is introduced through the air distribution plates at the bottom of the furnace. The secondary air is fed in from the middle of the combustion chambers on the side walls. The coal particles mix intensely with the air to create a \"fluidized\" combustion process. A cyclone separator and a material return device are installed at the outlet of the furnace; these devices send the separated materials back into the furnace for further combustion, thereby enabling material circulation. This extends the time for combustion and desulfurization, achieving complete combustion and effective desulfurization. The flue gas is then sent to the rear heating surfaces, just as in a pulverized coal furnace. 　　The main reactions in the coal desulfurization process in a circulating fluidized bed furnace are the decomposition of CaCO3 and the sulfidation reaction between CaO and SO2. The decomposition of CaCO3 will directly affect the activity of the desulfurization agent, thereby impacting the efficiency of desulfurization. Currently, in research on the calcination and decomposition of limestone, it has not yet been determined by what chemical reaction mechanisms the decomposition process is controlled, nor how it can be controlled. 4 Principles and Technologies of Post-Combustion (Flue Gas) Desulfurization
4.1 Overview
Flue gas desulfurization processes can be classified into three types: wet, dry, and semi-dry methods. Wet flue gas desulfurization involves washing the flue gas with a liquid absorbent to absorb SO2 contained in the gas ； Dry desulfurization involves the removal of SO2 from flue gas using solid powdered or granular absorbents, adsorbents, or catalysts ； The semi-dry method is a desulfurization technique that lies between the wet and dry methods. The following mainly introduces several flue gas desulfurization technologies that are widely used in industry as well as those that have been studied and utilized in recent years. 　　4.2 Several Flue Gas Desulfurization Processes 4.2.1 Limestone/Lime-Gypsum Wet Process The limestone/lime-gypsum process is a representative technique among wet flue gas desulfurization methods and is the most widely used in industry. This technology uses limestone or lime slurry to absorb SO2 from flue gas in a wet scrubber. The main reactions that take place are as follows:
Lime method: SO2 + CaO + H2O → CaSO3·H2O.
Limestone method: SO2 + CaCO3 + H2O → CaSO3·2H2O + CO2. The resulting slurry is then exposed to air, which oxidizes CaSO3 to CaSO4, producing gypsum as a by-product. 　　This process has advantages such as abundant raw material sources, low costs, reliable operation, simple handling, a high calcium utilization rate (>90%), and a high desulfurization efficiency (>90%), and accounts for 85% of current industrial desulfurization units. The gypsum produced as a by-product of this process can be effectively utilized to prevent secondary pollution. To promote the absorption of SO2 and the dissolution of limestone, additives can be used to improve the desulfurization efficiency, reduce the amount of limestone required, and lower the calcium-sulfur ratio. Sulfur removal experiments using inexpensive and readily available organic acid salts as additives showed that organic acids or their corresponding sodium salts enhance the limestone/lime-gypsum process, resulting in a significant increase in the desulfurization efficiency, a substantial extension of the stable operation time, and an increased absorption capacity of the slurry. The liquid-to-gas ratio is an important parameter that affects the performance of desulfurization systems; it can influence factors such as the desulfurization efficiency in the limestone-gypsum wet desulfurization process, as well as the content and concentration of limestone in the slurry. Du Qian and others used a co-current ordered falling film wet desulfurization unit to study the effect of the liquid-to-gas ratio on the desulfurization efficiency in this process. They found that, at a constant liquid-to-gas ratio, the desulfurization efficiency increases as one moves upward, while the rate of increase in desulfurization efficiency decreases with height ； At different liquid-to-gas ratios, the desulfurization rate increases as the liquid-to-gas ratio increases. 　　4.2.2 Seawater desulfurization process The seawater desulfurization process is a wet flue gas desulfurization method that uses natural seawater to remove SO2 from flue gas. The principle behind this process is that the SO2 in the flue gas is absorbed by the seawater, where it undergoes hydrolysis and oxidation in the washing solution. Subsequently, this washing solution is introduced into an aeration tank, where the pH level is increased to prevent the release of SO2 gas. Air is blown into the aeration tank to oxidize SO32- into SO42-. The seawater desulfurization process not only achieves the purpose of desulfurization but also meets emission standards, having a minimal impact on the marine environment; therefore, it facilitates seawater desulfurization for coastal enterprises. 　　Main process flow of seawater desulfurization: A process independently developed by Wang Qingzhang and others from Ocean University of China utilizes white mud, a waste product from alkali plants (mainly composed of CaCO3 and Mg(OH)2), as well as semi-dry flue gas desulfurization ash from power plants (mainly consisting of Ca(OH)2 and CaSO3), to serve as an alkalinity-enhancing additive for seawater desulfurization. This process removes SO2 from flue gases. After desulfurization, the treated seawater is either naturally aerated and oxidized or purified in a comprehensive treatment pond until it meets discharge standards. This process features rapid SO2 absorption, low seawater consumption, and no secondary pollution; it is suitable for coals with varying sulfur contents. As a method that utilizes waste to treat other waste, it is significantly superior to other seawater-based desulfurization processes. 　　By examining the practical experience of the flue gas seawater desulfurization process in Unit 4 (300 Mw) of the Shenzhen West Power Plant, and by drawing on foreign design concepts as well as domestic and international operational experience, Huang Zonghan further improved the relevant equipment, thereby making greater contributions to the plant’s environmental protection efforts. Among them, further improvement directions for the seawater desulfurization process are proposed: further reduction of the aeration tank area by about 20% ； The absorption tower is moving toward smaller sizes ； Eliminate bypass dampers or even do not install any bypass flues, to ensure that the desulfurization system must operate while the unit is running; alternatively, periodically switch the bypass dampers to ensure their flexibility and reliability. 　　The flue gas-seawater desulfurization process is well suited for coastal power plants that burn low-to-medium sulfur coal. It features a simple process, lower investment and operating costs, as well as high desulfurization efficiency. However, it is still uncertain whether the large amounts of seawater discharged as a result of seawater desulfurization pose any potential adverse effects on the marine environment. There has been ongoing debate regarding whether seawater desulfurization technology can be used on a large scale in power plants located in coastal areas. 　　4.2.3 Desulfurization process using rotary spray drying The rotary spray drying method is a typical technique in semi-dry flue gas desulfurization technologies, representing a desulfurization approach that lies between the wet and dry methods. Compared with the wet flue gas desulfurization system, the semi-dry flue gas desulfurization system eliminates the pulp preparation system, and replaces the injection of Ca(OH)2 aqueous solution in the wet system with the injection of CaO or Ca(OH)2 powder along with water mist ； Compared with dry desulfurization systems, it overcomes the disadvantages of low reaction efficiency and long reaction time for the SO2 and CaO reaction in the furnace-based calcium injection method, thereby improving the utilization rate of the desulfurizing agent. The dry solid waste generated by this process has a small volume and is easy to handle, offering good prospects for development. 　　4.2.4 Biological desulfurization process Research on the use of microorganisms for desulfurization began alongside research on using microorganisms in mineral processing. In 1947, Colmer and Hinkle discovered and confirmed that chemoautotrophic bacteria could promote the oxidation and dissolution of pyrite present in coal, which is considered to mark the beginning of research on biological hydrometallurgy. 　　The biological method is a newly developed flue gas desulfurization technique that possesses advantages unmatched by other methods: it can operate at normal temperature and pressure, requires low investment, has low energy consumption, and causes no secondary pollution. The principle of microbial flue gas desulfurization is that SO2 in the flue gas is dissolved in water using a wet scrubber or absorption tower, where it is converted into sulfites and sulfates. Under anaerobic conditions and in the presence of an external carbon source, sulfate-reducing bacteria reduce sulfites and sulfates to sulfides. Subsequently, under aerobic conditions, aerobic microorganisms convert these sulfides into elemental sulfur, thereby removing sulfur from the system. 　　Microbial desulfurization processes can be divided into direct methods, indirect methods, and two-step methods, among which the direct microbial desulfurization method is only suitable for removing SQ at low concentrations ； The indirect microbial desulfurization process features relative economic efficiency and convenience, as well as low operating costs, making it suitable for China’s national conditions. The two-step desulfurization process studied by Cork et al. involves converting sulfates/sulfides into elemental sulfur using anaerobic respiration and sulfur-producing bacteria with the photosynthesis of sulfate-reducing bacteria. In the first step, under strict anaerobic conditions, sulfate is converted to H2S by using Desulfobacter postgateii ； In the second step, H2S is converted into elemental sulfur by the strictly anaerobic photolithoautotrophic microorganism Cblorbium limicola. The H2S removal rate using denitrifying bacteria (T. denitrificans) in an anaerobic stirred-tank reactor can reach up to 97%. The step-by-step research on microbial desulfurization processes by Dasu, Lens, and others is further driving its development and progress. 　　The microbial method for flue gas desulfurization has the following advantages: it requires no high temperature, high pressure, or catalysts; operations can be carried out at normal temperature and pressure, resulting in low operating costs and simple equipment requirements. Since autotrophic microorganisms are used for desulfurization, their nutritional requirements are low, and there is no secondary pollution. Therefore, microbial flue gas desulfurization is a biotechnological process with strong practicality and innovative technology; it holds promising prospects and potential, and should receive attention to accelerate its development. 　　Developing microbial flue gas desulfurization technology holds great potential. Wang Yanjin and others believe that in future research, screening for bacterial strains with rapid growth rates and excellent desulfurization capabilities is an essential basic task. They also believe that it is important in future research to use modern genetic engineering techniques to modify certain desulfurization bacteria, enhancing their conversion capabilities in order to obtain highly efficient multi-plasmid bacteria with fast growth and reproduction rates, high activity, and a wide range of tolerance to temperature and pH levels. Additionally, it is crucial to screen for and cultivate desulfurization bacteria with greater adaptability and stability. 　　With the continuous development of biotechnology, microbial flue gas desulfurization technology is bound to make further progress. 　　4.3 Future Development Directions The Venturi scrubber improved by Zhang Jungang and others is capable of effectively removing both dust and sulfur dioxide from coal-fired flue gases, achieving integration of dust removal and desulfurization, which is in line with the current development trends in coal-fired flue gas treatment in China. Among them, the dust removal efficiency for coal-fired flue gas reaches (90–95)%, and the desulfurization efficiency for flue gas with an SO2 concentration of less than 1200 ppm is around 90%. The total resistance of the scrubber is only 1078 Pa or 10 mmH2O, indicating excellent technical performance. An integrated dust removal and desulfurization technology has been successfully developed, which, together with the previously applied integrated desulfurization and denitrification technology, promotes the research and development of desulfurization technologies. 　　5 Current Status and Prospects of Coal Desulfurization Technologies Compared with foreign FGD technologies and other desulfurization methods, the existing domestic desulfurization technologies in China are limited to the desulfurization of flue gases from industrial boilers and furnaces; moreover, their desulfurization efficiency is low and their level of automatic control is insufficient. It is an even greater blank in terms of sulfur recovery. What is even more serious is that our country does not yet possess mature, cost-effective, and widely applicable domestic desulfurization technologies for large coal-fired power plant boilers. 　　We must not relax our standards for ourselves, but we certainly must not be pessimistic either. Regarding the research and development of desulfurization technologies, **the policies are clear: 1. Research and development of technologies that take into account local resource conditions and enable the recovery of sulfur resources are encouraged** ； 2. Encourage research and development of technologies for simultaneous desulfurization and denitrification of flue gas ； 3. Encourage the research and development of technologies and equipment for the treatment, disposal, and resource utilization of desulfurization by-products. 　　Therefore, at present we must strive to absorb and master the advanced technologies introduced, and develop advanced desulfurization technologies with our own intellectual property rights based on the actual conditions of our country’s industry