Experimental Study of Reverse Underground Coal Gasification

Global and micro/macro structural models simulating the Longwall generator concept for underground coal gasification (UCG) are formulated for several geometric and loading conditions. Dynamic thermal and stress response solutions are computed for axisymmetric boundary value problems represented by a stationary circular cavity model and an expanding radially propagating circular cavity. Static thermoelastic response solutions are presented for circular and elliptical cylindrical presented for circular and elliptical cylindrical cavities in a homogeneous, isotropic medium and layered media. Considerations pertaining to roof collapse and surface subsidence are discussed. The micro/macro modeling investigations include thermorheological representation of Pittsburgh coal at elevated temperature and studies on thermal crack propagation of coal fissures. Finally, the relevance of stress and temperature profiles, fracture permeabilities, and fissure response is discussed in relation to the propagation and stability of the UCG process. Introduction Fossil fuels are currently the main energy resource for the U.S. with coal reserve estimates ranging from 3 to 4 trillion tons. Only one-sixth of this coal can be extracted by state-of-the-art mining techniques. It is anticipated that about one-third of the U.S. total coal energy reserves, particularly thin coal beds and thick-seam deep coal, can be recovered by underground coal gasification (UCG). This process entails ignition of a coal seam and circulation of gas through a controlled path. The resulting chemical reactions produce a gas with calorific value, depending on several process parameters and variables. Their optimization parameters and variables. Their optimization requires sophisticated analytical simulations of the structural, heat transfer, fluid flow, and reaction kinetics aspects along with controlled laboratory experiments and field tests. Descriptions and appraisals of various underground coal pregasification schemes and gasification processes mad a discussion of significant problems, processes mad a discussion of significant problems, such as roof collapse, gas leakage, water control, and surface subsidence, etc., have been given in a detailed report by Arthur D. Little, Inc.. Reports on UCG experiments in the Soviet Union, Great Britian, and other countries have been summarized in excellent monographs. The outlook for UCG in the U.S. was discussed recently at the First Annual UCG Symposium conducted by the Laramie Energy Research Center in Wyo. It appears that several candidate concepts and solutions exist, depending (on the type of coal, coal seam thickness, overburden, and environmental requirements. The concepts currently being investigated by ERDA's research centers and laboratories includethe vertical well to well linking experiments at Hanna, Wyo., for medium seams (Laramie Energy Research Center),the packed-bed concept for thick coal seams (Lawrence Livermore Laboratory), andthe Longwall generator concept for thin seams (Morgantown Energy Research Center). This paper presents results obtained from analytical structural and thermal simulations associated moth the Longwall generator UCG concept. Temperature and stress response solutions are presented for different boundary value problem representations. The results of this study provide fundamental insight to the interpretation of UCG global and micro/macro mechanisms. UCG LONGWALL GENERATOR CONCEPT AND MECHANISMS Morgantown Energy Research Center presently is developing the Longwall generator concept to gasify thin-seam eastern coals (Fig. 1). In this concept, directional holes (6-in. diameter) are drilled from the ground surface with horizontal holes, 500 ft in length, entering through the 6-ft-thick coal seam (1,000-ft overburden) and returning vertically back to the coal surface. The gasification reaction zone propagates horizontally in the coal propagates horizontally in the coal maximum-permeability direction between the parallel horizontal holes drilled approximately in the coal butt-cleat direction. JPT P. 413

This study investigates the influence mechanism of key factors on the heating value of syngas during underground coal gasification (UCG) and proposes an optimization path for enhanced energy conversion efficiency based on typical global field test data. Integrating data review and pattern analysis, it systematically explores the influence of core factors, including coal seam characteristics, reactor structure, and gasification agent ratio. It is found that the relationship between syngas heating value and coal rank is not simply linear, with representative heating values ranging from 4.13 to 11.96 MJ/m3. Medium-rank coal, characterized by “medium volatile matter and low ash content”, yields high-heating-value syngas when paired with air/steam as the gasification agent. Shaftless reactor structures demonstrate superior overall performance compared to shaft-based designs, with the representative heating value improving from 3.83 MJ/m3 to 7.8 MJ/m3. The combination of U-shaped horizontal wells with the Controlled Retracting Injection Point (CRIP) technology improves the heating value. Effective control over the syngas heating value can be achieved by optimized composition and ratio of the gasification agent, with representative value of 9.10 MJ/m3 in oxygen-enriched steam gasification compared to 4.28 MJ/m3 in air gasification. Based on an evaluation of data fluctuation characteristics, the significance ranking of the factors is as follows: gasification agent, coal rank, and reactor structure. Consequently, an engineering optimization path for enhancing UCG syngas heating value is proposed: prioritize optimizing the composition and ratio of the gasification agent as the primary means of heating value control; on this basis, rationally select coal rank resources, focusing on process compatibility to mitigate performance fluctuations; and then incorporate advanced reactor structures to construct a synergistic and efficient gasification system. This research can provide theoretical support and data references for engineering site selection, process design, and operational control of UCG projects.

The underground coal gasification (UCG) technology is basically mature, but the influence of its own process and tools slows down its industrialization progress. This paper introduced the development and field test of two new UCG coiled-tubing gasification agent injection tools. The test results show that the two kinds of gasification agent injection tools ensure the injection point under control by conducting downhole temperature measurement and ground monitoring jointly. The problem that the tool is burnt by the backfire is solved by designing a backfire prevention device. To realize low pressure drop, the gasification agent flow channel inside the tool is designed optimally to keep the tool pressure drop not more than 0.5 MPa and the system pressure drop not more than 3 MPa. The tool overall has the characteristics of low pressure drop, high temperature resistance, backfire prevention and anti burning to satisfy the demand of the field test. This technology is a new achievement in the development of UCG technology and equipment in China. The research conclusions can provide technical reference for developing a new generation of UCG technology.

Experimental study of underground coal gasification (UCG) of a high-rank coal using atmospheric and high-pressure conditions in an ex-situ reactor

Impact analysis of the oxidant in the process of underground coal gasification

A detailed numerical modelling study was carried out to represent geotechnical aspects of the Wieczorek underground coal gasification (UCG) site in Poland. A coupled thermos-mechanical numerical model was created to represent a single coal burning panel. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. Temperature dependent material properties were assigned to the coupled thermal-mechanical model according to published data. In the model, the burning zone spread about 7.5m laterally after 20 days of burning. Results from the coupled model were used to gauge a worst-case scenario in terms of the potential size of a formed cavity. This data was used within a less computationally expensive mechanical-only numerical model in order to evaluate the ground subsidence caused by the worst-case scenario for single and multiple UCG burning panels. The single panel burning resulted in 23mm of ground subsidence at the top of the model after long term coal burning. The ground subsidence measured at the top of the model, at the center point of the gasification arrangement, was approximately 72mm when five panels were burnt with an edge to edge panel distance of 5m; this was increased to 85mm for seven panels. The numerical modelling results have implications to the industrial application of UCG. Keywords: Numerical model, underground coal gasification, calorific value, subsidence.

Underground coal gasification (UCG) technology integrates three processes: well construction, coal mining and gasification. It not only overcomes the safety problems of existing mine coal mining, but also avoids the pollution of waste gas, waste water and waste residue in the process of surface coal gasification of traditional coal mining. It has the advantages of low gas production cost, high safety and good environmental benefits. However, effective temperature control technology of production wellbore still needs to be developed. Based on the annulus spray method and the combination of insulated oil pipe and spray device, a set of high-temperature gas spray cooling scheme is developed by using NASA variable specific heat calculation formula, considering the variation of specific heat value of main components of high-temperature mixture with temperature, and the finite element model of production wellbore is established by using commercial software (CFX), The temperature field and pressure field of production wellbore are simulated and analyzed. The analysis results show that the developed spray cooling scheme can effectively control the production wellbore temperature, achieve the design goals of slightly lower than 350 °C when the high-temperature gas is output to the ground and no gasification of annulus cooling water. The research methods and results have important practical significance for promoting the development of underground coal gasification process and even engineering application. Underground Coal Gasification (UCG) is a chemical mining method, which conducts controlled in-situ combustion and gasification of underground coal (Gregg D W and Edgar T F, 1978; Shafirovich E et al., 2009; Khan et al., 2015). A series of chemical reactions produce combustible gases such as H2, CO and CH4 (Perkins and Sahajwalla, 2006; Takyi Shadrack Adjei et al., 2023). UCG technology integrates three processes of well construction, coal mining and gasification. It belongs to a special coal mining method and is an important supplement to traditional physical coal mining technology (Lanhe Yang, 2003; Karol Kostúr et al., 2015). It replaces the huge and bulky underground coal mining equipment and ground coal gasification equipment, changes the traditional physical coal mining to chemical coal mining, realizes underground unmanned production, and avoids personal injury and mine accidents. It can not only recover coal resources abandoned in the mine, but also be used to mine thin coal seams, deep coal seams, "three lower" coal seams, and high-sulfur, high-ash, and high-gas coal seams that are difficult for underground workers to mine or are less economical and safe to mine. Moreover, dust pollution caused by coal mining and transportation is avoided, and the gasified gangue and ash remain underground (Rajesh P. Barnwal et al., 2017; Kariznovi, M et al., 2013), which reduces the environmental impact caused by the accumulation of surface solid waste, and prevents surface subsidence to a certain extent (Peng Pei et al., 2016). In a word, it overcomes the safety problems of existing mine coal mining, and also avoids the pollution of waste gas, waste water, waste residue, etc. in the surface coal gasification process of traditional coal mining, and has the advantages of low gas production cost, high safety and environmental protection. Good benefits. UCG technology is suitable for in-situ mining and transformation of difficult-to-mine coal seams, low-grade coal seams, especially deep coal seams, which improves resource utilization and drives the development of traditional industries such as coal, electric power, and chemical industry (Hossein Nourozieh et al., 2010). Therefore, UCG is a new technology of high-carbon resources and low-carbon development of clean energy, and it is also a green coal mining technology that coordinates resources and environment.

In the underground coal gasification (UCG) process, cavity growth with crack extension inside the coal seam is an important phenomenon that directly influences gasification efficiency. An efficient and environmentally friendly UCG system also relies upon the precise control and evaluation of the gasification zone. This paper presents details of laboratory studies undertaken to evaluate structural changes that occur inside the coal under thermal stress and to evaluate underground coal-oxygen gasification simulated in an ex-situ reactor. The effects of feed temperature, the direction of the stratified plane, and the inherent microcracks on the coal fracture and crack extension were investigated using some heating experiments performed using plate-shaped and cylindrical coal specimens. To monitor the failure process and to measure the microcrack distribution inside the coal specimen before and after heating, acoustic emission (AE) analysis and X-ray CT were applied. We also introduce a laboratory-scale UCG model experiment conducted with set design and operating parameters. The temperature profiles, AE activities, product gas concentration as well as the gasifier weight lossess were measured successively during gasification. The product gas mainly comprised combustible components such as CO, CH4, and H2 (27.5, 5.5, and 17.2 vol% respectively), which produced a high average calorific value (9.1 MJ/m3).

Study on the monitoring method of cavity growth in underground coal gasification under laboratory conditions

A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects

With the depletion of coal reserves all over the world, efforts are being made worldwide to utilize those deep underground coal seams, which are difficult to mine or are not feasible economically. Underground coal gasification (UCG) is a promising technology to achieve that objective and also to reduce the atmospheric pollution that is casued by surface gasification of the mined coal. Based on the previous research performed in this field, it is a well-known fact that the optimum quantity of water influx into coal seam can increase the hydrogen content of the product gas. In our earlier study, a novel method of utilizing humidified oxygen as the gasification agent was conducted for low ash coal gasification. The experimental results had shown the production of substantial quantity of hydrogen in the product gas. In the present study, the laboratory-scale experiments are extended to high-ash Indian coals with increases in the absolute humidity of the feed oxygen by preheating the water (humidifying medium)...

Nonlinear-Control-Oriented Modeling of the Multi-Variable Underground Coal Gasification Process for UCG Project Thar: A Machine Learning Perspective

Underground coal gasification – A numerical approach to study the formation of syngas and its reactive transport in the surrounding strata

Robust tracking of the heating value in an underground coal gasification process using dynamic integral sliding mode control and a gain-scheduled modified Utkin observer

The wastewater from underground coal gasification (UCG) process has extremely complex composition and high concentrations of toxic and refractory compounds including phenolics, aliphatic and aromatic hydrocarbons, ammonia, cyanides, hazardous metals and metalloids. So, the development of biological processes for treating UCG wastewater poses a serious challenge in the sustainable coal industry. The aim of the study was to develop an innovative and efficient wetland construction technology suitable for a treatment of UCG wastewater using available and low-cost media. During the bioremediation process the toxicity of the raw wastewater decreased significantly between 74%-99%. The toxicity units (TU) ranged from values corresponding to very high acute toxic for raw wastewater to non-toxic for effluents from wetland columns after 60 days of the experiment. The toxicity results correlated with the decrease of some organic and inorganic compounds such as phenols, aromatic hydrocarbons, cyanides, metals and ammonia observed during the bioremediation process. The removal percentage of organic compounds like BTEX, PAHs and phenol was around 99% just after 14 days of treatment. A similar removal rate was indicated for cyanide and metals (Zn, Cr, Cd and Pb). Concluded, in order to effectively assess remediation technologies, it is desirable to consider combination of physicochemical parameters with ecotoxicity measurements. The present findings show that wetland remediation technology can be used to clean-up the heavily contaminated waters from the UCG process. Wetland technology as a nature-based solution has the potential to turn coal gasification wastewater into usable recycled water. It is economically and environmentally alternative treatment method.