What are the advantages of underground coal gasification technology?

Reposted answer: Underground Coal Gasification (UCG) is a technology that involves the controlled combustion of coal underground; through thermal and chemical processes applied to the coal, combustible gases are generated, representing a new approach for developing clean energy sources as well as producing chemical raw materials. Its essence is to extract only the energetic components from coal, leaving pollutants such as ash and slag underground. Coal in-situ gasification technology integrates various processes such as shaft construction, coal mining, and conversion, **improving the efficiency and level of utilization of coal resources. It has received significant attention from countries around the world and is regarded as a new generation of coal mining methods. As early as 1979, the United Nations’ \"World Coal Outlook Conference\" clearly stated that underground coal gasification is an important approach to fundamentally solving a range of technical and environmental problems associated with traditional methods of coal mining and use. At present, industrial trials of coal underground gasification have achieved initial results both domestically and internationally, and the production of gasified coal gas on an industrial scale has taken place in countries such as Russia and the United States, as well as in regions in China like Shandong and Hebei. Our country is rich in coal resources. At present, not only is the utilization rate low, but surface mining also has a significant impact on the environment and safety. Converting underground coal resources into usable coal and other products through coal gasification is one of the important ways to address energy issues. 　　I. Current Research Status at Home and Abroad The former Soviet Union was the first country in the world to conduct field tests on underground gasification, and it is also one of the countries where industrial applications of underground gasification have been successful. In 1935, the former Soviet Union established five test sites in the suburbs of Moscow, as well as in Donbas and Kuzbas. By 1936, it moved from the experimental phase to industrial testing. In 1940, gasification stations in Donbas and the suburbs of Moscow managed to solve the problem of underground gasification without wells on a technical level, making them the first industrial facilities for underground coal gasification. To explore gasification methods, by the end of the 1960s, the former Soviet Union had built 12 plants, with the gas produced being used for power generation or as industrial fuel gas. 　　After World War II, coal in-situ gasification sparked great interest in the United States. In 1946, the United States first conducted tests in shallow coal seams in Alabama. During the energy crisis in the United States in the 1970s, 28 universities and research institutions were organized to carry out large-scale, planned research work in Wyoming. The Rocky Mountain-1 experiment was completed between 1987 and 1988, yielding results such as larger furnace sizes, increased production capacity, reduced costs, and higher calorific value of the gas produced. 　　The UK resumed testing in 1949, and by 1956 a total of six tests had been conducted. Britain, France, Germany, Belgium, and many countries in Eastern Europe focused their efforts in the 1970s on deep coal seams located several kilometers below the surface, where mining is difficult, with the ultimate goal of establishing joint ventures for underground coal gasification power plants. In 1988, six EC member states established a European Coal Underground Gasification Group. The first test of underground gasification in deep coal seams was conducted in Alcorisa, Spain, in June 1997; this test was completed in December 1998, with a total of 301 hours of gasification taking place. At present, the results of these tests and the related technologies still need further validation before they can be applied in large-scale industrial production. 　　Coal in-situ gasification experiments in our country also began early, but technological development mainly took place after the 1980s. From 1958 to 1962, China conducted experiments on underground coal gasification under natural conditions in many mining areas such as Datong, southern Anhui, and northern Shenyang, achieving certain results. In 1985, China University of Mining and Technology conducted field tests in the abandoned coal pillars at Mazhuang Mine in Xuzhou. Subsequently, the university developed a new process for underground coal gasification suitable for China’s coal mines, characterized by \"long tunnels, large cross-sections, and two stages\" of gasification. It carried out semi-industrial trials for underground coal gasification at Xinxiu Mine and Xinhe No. 2 Mine in Xuzhou, as well as an industrial-scale trial at Liuzhuang Coal Mine in Tangshan, Hebei. 　　After 2000, the new coal underground gasification process featuring \"long and large channels\" achieved good results in places such as Suncun, Xiezhuang, Zhangzhuang, Ezhuang in Xinwen, Shandong; Xiyang in Shanxi; and Tiefa and Fuxin in Liaoning. The gas produced through this process is used for residential heating, power generation, as well as in the production of ammonia and methanol. At present, the Suncun gasification station produces 6*104 m3 of gas per day. In addition to supplying more than 10,000 households, it is equipped with 2 generators rated at 400 kW each, generating an average of 18*104 kW•h of electricity per month. The Xiazhuang gasification station produces 4*104 m3 of gas per day, which is used to supply over 6,000 households ； Zhangzhuang Gasification Station, with a designed daily production capacity of 15*104 m3; it is planned to adopt a new technology for the one-step synthesis of dimethyl ether, aiming to build a facility with an annual production capacity of 2000 tons of dimethyl ether ； The first phase of the Ezhuang gasification plant has a daily gas production capacity of 10*104 m3, which is used to supply more than 1,000 households in the mining area for domestic use, as well as to power a 400kW generator set that has been installed there. 　　II. Principles of underground coal gasification The principles of underground coal gasification are the same as those of surface gasification, and the composition of the gas produced is also essentially identical. However, the process differs: in surface gasification, the reaction takes place within coal blocks in a gasifier, whereas in underground gasification, it occurs in gasification channels within the coal seam. The coal layer at one end of the inlet hole of the gasification channel is ignited, and a gasifying agent (air, oxygen, water vapor, etc.) is blown in through the hole. After the coal seam burns, depending on temperature and chemical reactions, three zones are formed in the gasification channel: the oxidation zone, the reduction zone, and the carbonization/drying zone. After passing through 3 reaction zones, gas mainly containing combustible components CO, H2, and CH4 is formed. These 3 reaction zones move gradually toward the outlet along the direction of the gas flow, thereby maintaining the continuous progress of the vaporization reaction. The main components of an underground gasification furnace are the construction of inlet and outlet openings as well as the creation of gasification channels. Depending on the method used to construct these channels, coal underground gasification can be divided into shaft-type and shaftless types; the former uses manually excavated tunnels as gasification channels, while the latter uses drill holes for this purpose. 　　III. Technologies related to underground coal gasification Technologies related to underground coal gasification can be classified, according to the production stage and technical type, into technologies such as coal seam exploration, construction of underground coal gasification reactors, control of underground coal gasification, and treatment of the gasified coal gas. 　　(1) Coal seam exploration techniques: Similar to other exploration techniques, these rely on geology, 2D and 3D seismic surveys, as well as drilling, to study the distribution and thickness of coal seams underground. At present, the main method of coal gasification in our country is wellbore gasification (that is, gasification through artificial tunnels), which involves using abandoned coal mines for underground gasification. The distribution of coal layers is usually already known during coal mining, so this aspect is not very important in current underground coal gasification processes in our country. However, with the development of borehole-based gasification, independent coal layer exploration techniques for such gasification methods will become crucial. The existing exploration technologies are fully capable of meeting the needs of coal layer exploration. 　　(2) Furnace construction technology: Underground gasification furnaces are divided into two types: “well-type” and “well-less type”. “\"Wellless\" gasification refers to a process in which the gasification channel is created through drilling; it features a simple construction process and a short construction period, and can be used for gasifying coal seams at deep depths or underwater. Wellless reactors are used abroad, but the narrow gasification channels reduce the gas output volume, and drilling costs are high. All the gasification furnaces currently built in China use the shaft-type design; these furnaces are constructed within active coal mines, with tunnels extending from the mine shafts into the coal layers to be gasified. Once the underground passages for the gasification furnaces are completed, a sealed wall is built in the passage connecting the furnace to the mine tunnels, after which the furnace is ignited. “The \"well-type\" gasification method can make use of existing vertical shafts and tunnels, reducing the investment required for building gasification furnaces. It is also possible to utilize the coal pillars remaining underground in old mines (as a form of waste utilization). The gasification channels are large, which facilitates large-scale production and results in lower gasification costs. Due to the need for tunnel construction, shaft-type gasification is not suitable for gasifying deep coal resources, as high ground stresses and temperatures are present. 　　(3) The control technologies for underground coal gasification mainly include the following two aspects. Control processes for the underground coal gasification process. Gasification processes can be classified according to different types of gasifying agents into: air-based continuous gasification, oxygen-enriched gasification, oxygen-enriched–water vapor gasification, hydrogen-enhanced gasification, etc. The control techniques for the gasification process can be further divided into methods such as alternating operation of two (or multiple) furnaces, gas supply at multiple points (moving points), reverse gas supply, pulsating gas supply, and a combination of these approaches.  Coal in-situ gasification monitoring and control technology: The in-situ gasification monitoring and control system can be divided into two main parts, namely the automatic parameter acquisition system and the data analysis system. A measurement and control system is a software package that uses an electronic computer as its core to control the periodic collection of various measurement values such as temperature, pressure, flow rate, and gas composition from peripheral devices, as well as to handle functions like display and data plotting. The computer analysis system features functions such as coal consumption measurement, thermal efficiency statistics, analysis of various relevant parameters, over-limit alarms, process plan suggestions, and automatic feedback control of process parameters. The main function of the analysis system is to organize, analyze, and predict the collected parameters in order to determine the operating condition of the gasification furnace and provide optimized operational parameters, among which system prediction is crucial. System forecasting refers to the prediction of the development and changes in a system composed of multiple factors, or in other words, it is the prediction of how these various factors influence one another and develop in a coordinated manner. Currently, in addition to on-site data collection, theoretical derivations such as mathematical simulations and experimental tests are also important methods for the relevant parameters of coal in-situ gasification control technology. 　　(4) Gas treatment technology: Gas contains large amounts of dust, tar, moisture, carbon dioxide, nitrogen, and other substances. This type of gas is called hot gas or crude gas, and its temperature can reach 200°C. Therefore, it is necessary to purify and recycle the raw gas in order to reduce blockages and corrosion in equipment and pipelines, improve the efficiency of the transportation system, recover valuable chemical products, and at the same time meet the requirements of subsequent processes for feed gas. The task of the gas purification system is to remove harmful impurities from the gas, reduce its temperature, and produce useful by-products. Therefore, gas purification is not only related to the quality of the gas itself, but also to the economic efficiency of the entire gasification process. Gas purification generally serves four purposes:  Cooling;  Dehydration;  Recovery of valuable by-products;  Removal of unwanted harmful impurities. 　　IV. Characteristics of the composition of gas produced by underground coal gasification Rational utilization of the gas generated by underground coal gasification is an important way to further improve its economic efficiency, and the composition of this gas is a key factor affecting its comprehensive utilization. The composition of the purified coal gas obtained using different gasification processes varies greatly; coal gas produced by oxygen-enriched steam underground gasification has higher levels of H2 and CO ； The next is two-stage pulsating vaporization: and it is the case where the nitrogen content in the continuous air components is high, while the content of useful components is low. Due to the different components of gasified coal gas. The calorific value of gas also varies. 　　V. Prospects for the application of underground coal gasification technology The gas produced by underground coal gasification can be used for the following purposes:  For power generation ；  Used for industrial gas ；  Extract pure hydrogen for further use as a reducing gas and in fine chemical products ； Civil gas used in cities ；  Used to produce methane, which is then fed into the natural gas network ；  Used as a raw material gas for chemical synthesis; methanol, ammonia, dimethyl ether, petroleum, etc. can be synthesized from it. At present, the gas produced by underground gasification in our country is mainly used for urban gas supply, power generation, the production of ammonia and dimethyl ether, as well as the extraction of pure hydrogen. 　　Statistics show that in China, there are 6244.6*108 tons of coal resources at depths of up to 1500 meters, which are suitable for underground coal gasification. By converting the gas produced from coal gasification into natural gas, an amount of 214.03*1012 cubic meters of synthetic natural gas (methane) can be obtained. Of this amount, 163.74*1012 cubic meters of synthetic natural gas can be produced through underground coal gasification along the West-East Gas Pipeline, while 85.17*1012 cubic meters can be produced in the Shaanxi-Gansu-Ningxia region. It can be seen that coal underground gasification in our country possesses great resource potential. 　　American experts point out that compared to conventional surface gasification processes for producing the same downstream products, coal underground gasification can reduce the cost of producing syngas by 43%, the cost of producing natural gas substitutes by 10%–18%, and the cost of power generation by 27%. Data published by the Leningrad Railway Power Plant Design Institute in the former Soviet Union show that compared to coal-fired power plants, underground gasification power plants require 50% less space, use 30% less metal for boilers, and need 37% fewer employees. In Russia, at Angren and Kansko-Azhikin, the cost of coal and the gas produced through underground gasification was 9.71 and 12.34 rubles per ton respectively in 1989; the cost of the gas obtained from underground gasification, when converted to coal terms, was 5.8 rubles per ton. In Suncun, Shandong Province, China, the cost of water gas produced through underground gasification is 0.193 yuan per cubic meter, with annual profits amounting to 3.75 million yuan, a profit margin of 3.11 million yuan, an investment return rate of 25.83%, and an investment payback period of 3.9 years (Liang Jie et al., 2002). Through economic studies on coal in-situ gasification both domestically and internationally, it can be seen that this method offers advantages over traditional coal mining, such as lower investment costs, faster returns, and reduced expenses. 　　Statistics on the economic benefits of coal and rock in-situ gasification in Russia, the United States, and our country show that although in-situ coal gasification offers certain economic advantages, generally speaking, the production costs for coal gasification under the same calorific value conditions are higher than those of conventional natural gas. As gasification technology continues to improve and costs decrease, large-scale coal gasification will yield even greater economic benefits. Furthermore, with the development of China’s coalbed methane industry, the integrated development and utilization of coalbed methane and underground coal gasification will further reduce costs and enhance the economic efficiency of underground coal gasification. 　　It should be noted that the significance of underground coal gasification lies not only in economic benefits, but also in improving the energy structure and enhancing the safety of coal mining operations. After coal gasification, the ash and slag remain in place, preventing pollution from waste gases, wastewater, and waste residues, and reducing ground subsidence caused by premature coal extraction. This technology can **increase the resource recovery rate, enabling the extraction of coal from areas that are difficult to access using traditional methods – such as marginal coal deposits, deep-seated coal, coal layers under buildings or infrastructure, as well as the protective coal pillars left behind in mines that have been or will be shut down. It also allows for the effective utilization of coal resources located in areas with extremely harsh mining conditions.

Personally, I believe this technology is unsuitable and of no practical value; it can only increase the irreversible pollution that cannot be eliminated, harming future generations.

It’s hard to imagine, especially the control issues. Control gasification through the amount of air intake? Can the gas composition remain stable during such a production process? Also, even if it is successful, vaporization generally takes place at atmospheric pressure; if it is to be used in chemical production, pressure must be applied, and this also results in significant energy consumption. Is this more cost-effective than digging it up and burning it?

This post was last edited by tongshun*n on 2009-10-21 at 16:45. Underground gasification technology is primarily aimed at utilizing the residual coal left after coal mining. The current scale is not large; they are generally used as civilian fuel. Although there are still many issues with using it in the chemical industry under current technical conditions, if the gasification capacity underground can be increased, it holds great promise as an auxiliary technology. Domestic underground gasification technologies have passed provincial-level appraisal.

I think it is a strategic reserve technology, just like coal-to-oil. In this regard, pollution and commercial benefits can be ignored. The only issues are calorific value and controllability.

Oh, I see! Thank you for the advice!