A novel method for predicting permeability in quartz-bearing rocks at high temperatures during underground coal gasification

Prediction-assessment-optimization of water influx in underground coal gasification: A systematic method

Assessing pollutant leaching and migration from underground coal gasification residual coke

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.

Against the background of global energy transformation and low-carbon development, numerous difficult-to-mine coal resources (e.g., deep, thin coal seams and low-quality coal) remain underdeveloped, leading to potential resource waste. This study systematically summarizes the feasibility of developing these resources via underground coal gasification (UCG) technology, clarifies its basic chemical/physical processes and typical gas supply/gas withdrawal arrangements, and establishes an analytical framework covering resource utilization, gas production quality control, environmental impact, and cost efficiency. Comparative evaluations are conducted among UCG, surface coal gasification (SCG), natural gas conversion, and electrolysis-based hydrogen production. Results show that UCG exhibits significant advantages: wide resource adaptability (recovering over 60% of difficult-to-mine coal resources), better environmental performance than traditional coal mining and SCG (e.g., less surface disturbance, 50% solid waste reduction), and obvious economic benefits (total capital investment without CCS is 65–82% of SCG, and hydrogen production cost ranges from 0.1 to 0.14 USD/m3, significantly lower than SCG’s 0.23–0.27 USD/m3). However, UCG faces challenges, including environmental risks (groundwater pollution by heavy metals, syngas leakage), geological risks (ground subsidence, rock mass strength reduction), and technical bottlenecks (difficult ignition control, unstable large-scale production). Combined with carbon capture and storage (CCS) technology, UCG can reduce carbon emissions, but CCS only mitigates carbon impact rather than reversing it. UCG provides a large-scale, stable, and economical path for the efficient clean development of difficult-to-mine coal resources, contributing to global energy structure transformation and low-carbon development.

Abstract Underground coal gasification (UCG) is an in-situ and non-conventional method for extracting coal by injecting steam, air, and oxygen into the target coal seam. The resultant gas from this process is transported via production pipes to a gas processing facility, where it can be used for electricity generation and the production of synthetic gas (syngas). The UCG process creates cavities within the coal seam, which requires groundwater as a natural barrier to prevent gas loss. However, groundwater entering the cavities can lead to a drawdown of groundwater head, and if this drawdown is excessive, subsidence may occur. This study aims to predict groundwater intrusion in the gasification zone and its effects on the aquifers above and below the gasification zone. UCG modeling was performed with a coal hydraulic conductivity value of 4.5 x 10 −8 m/s and operational pressure variations of 10 bar and 20 bar. The hydrostatic pressure on the target coal is 32 bar. Simulations were conducted over 144 days or for the gasification of 100 meters (10 grid models) of coal. Simulation results with coal conductivity of 4.5 x 10-8 m/s and an operating pressure of 10 bar, the model predicts a groundwater intrusion of 61.96 m 3 /day into the gasification zone, 0.0998 m 3 /day from the upper aquifer, and 0.1088 m 3 /day from the lower aquifer, causing a drawdown of 1.42 meters in the upper aquifer and 2.47 meters in the lower aquifer. At operating pressure 20 bar, predictions are 27.34 m 3 /day intrusion into the gasification zone, 0.0803 m 3 /day from the upper aquifer, and 0.1052 m 3 /day from the lower aquifer, resulting in a drawdown of 1.28 meters in the upper aquifer and 2.16 meters in the lower aquifer.

Impact of O2-enriched CO2 blending ratios on energy efficiency and carbon abatement in underground coal gasification

Research progress on numerical simulation methods and models for underground coal gasification

The potential of underground coal resources in Indonesia has not been fully exploited yet, necessitating new mining methods, one of which is underground coal gasification (UCG). The Asam-asam Basin is one of Indonesia's coal-producing basins that warrants further research regarding the potential development of UCG in the area. This study identifies the geological criteria of coal seams, such as seam thickness, depth, rank, seam dip, surrounding rock types, geological structure, and coal inventory located in the Asam-asam area.Data were collected through core drilling and well logging analysis at 4 boreholes (MIA004–MIA007). The results indicate that the area contains 9 coal seams: A, B, C, D, E, F, G, H, and I. Coal seam thickness varies from 0.49 meter to 18.95 meters, with surrounding rocks predominantly consisting of claystone, and some seams interbedded by sandstone and siltstone. The coal rank is primarily high volatile bituminous. Seams B, C, F, G, and H meet the coal inventory criteria. Evaluation using analytical hierarchy process (AHP) method indicates that Seam F has the highest potential for UCG development, with an average score of 0.78.

A real options approach for cost benefits assessment of power generation from underground coal gasification with CCS

Risk assessment and kinetic characteristics of syngas explosion in underground coal gasification for hydrogen production

A new paradigm for safe and accurate design of underground coal gasification coal pillars based on physics-informed neural networks

Product characterization in pyrolysis and reduction zone of underground coal gasification with implication on CO2 utilization

Pore structure evolution of semi-coke under steam atmosphere in the context of deep underground coal gasification

As a major source of carbon emissions, the carbon-based power generation industry requires a scientifically robust environmental performance evaluation system to facilitate its green transition and sustainable development. Focusing on unique transition dynamics across four carbon-based power generation formats, this study compares environmental dimension indicators across typical ESG evaluation frameworks and proposes an innovative evaluation index model of environmental performance based on common metrics, with a particular emphasis on their contribution potential to the “Dual Carbon Goals”. The framework’s core innovation lies in its Dual Carbon-focused indicator system, which evaluates three critical indicators overlooked by mainstream ESG methodologies. It extends to include upstream/downstream processes, addressing gaps in current evaluation systems. The findings reveal that core environmental issues, such as climate change, pollution emissions, and resource utilization, exhibit broad commonality in ESG evaluations. Among the assessed indicators, carbon emission intensity carries the highest weight, underscoring its centrality in each power generation sector’s efforts to align with the Dual Carbon Goals. Furthermore, the analysis demonstrates that underground coal gasification combined cycle power generation has a relatively favorable environmental performance, ranking slightly below natural gas combined cycle but above shale gas combined cycle power generation. In contrast, traditional coal-fired power generation exhibits significantly poorer environmental outcomes, highlighting both the efficacy of technological upgrades in reducing emissions and the urgent need for transitioning away from conventional coal-based power.

Underground coal gasification technology can produce hydrogen-rich coal gas, supporting China’s "carbon peak" and "carbon neutrality" goals. As an efficient and clean method of coal utilisation, it is an important supplement to coal mining in China. However, water pollution risks during the process remain a major challenge for its industrialization. This study investigates pore structure development in surrounding rocks and wastewater infiltration in the combustion zone during underground gasification, using thermal damage experiments and wastewater permeability tests, combined with modern characterization techniques and island fractal analysis. Results show that as the temperature increases, crack zones and large cracks gradually form in the surrounding rock samples. As the temperature rises, the fractal dimension first increases and then decreases, while the crack structure becomes simpler. Leachate quality and permeability initially decrease, then increase with temperature. Complex crack structures trap particulate matter, reducing connectivity and permeability, while simple crack structures enhance connectivity and permeability. In surrounding rock samples, trimethylbenzene and ether pollutants are the main contaminants adsorbed in cracks, while disubstituted benzene, carboxylic acids, and esters show a decreasing trend with increasing sampling distance, indicating strong migration potential. Fractal theory and wastewater permeability experiments provide valuable insights into surrounding rock crack complexity, offering theoretical support for wastewater migration during underground coal gasification.

Water injection in underground coal gasification with a horizontal hole: A strategy to prevent steel pipe rupture and enhancing hydrogen production reaction

ABSTRACT Deep coal has the characteristics of high temperature, high pressure, and high stress, which leads to the complexity of the underground reaction process. The theory and technology of shallow underground gasification are no longer suitable for deep coal seam development. Therefore, this paper briefly reviews the principle, process flow, and reaction process of underground coal gasification, and focuses on the development status and research progress of deep underground coal gasification at home and abroad. It is believed that the deep coal gasification process still faces challenges such as complex geological conditions, unclear coupling mechanisms, and an unstable gasification process. Finally, the future of deep underground coal gasification (DUCG) is projected from three aspects: basic theoretical research, gasification technology research, and green development. The results show that foreign research on deep underground coal gasification technology mainly focuses on environmental impact and Carbon Capture, Utilization and Storage(CCUS) integration. At present, large-scale commercial mining has not been realized, and relevant experience provides reference and guidance for domestic DUCG research. Domestic DUCG is in the early stage of development, mostly in laboratory tests and numerical simulations, and lacks typical cases in mines. The DUCG gasification process is mainly Controlled Retraction Injection Point (CRIP), and the gasification agent is recommended to use pure oxygen-steam. However, due to the influence of regional and geological conditions, some conclusions cannot be directly applied. In actual production, it is necessary to improve the key links such as site selection, furnace construction, gas injection, and ignition of underground coal gasification. DUCG is an important direction for the clean utilization of deep coal resources. In the future, the DUCG technology system should be formed from basic engineering, operation, development, comprehensive utilization, etc. and the DUCG technology should be continuously promoted to industrial development.

Exergy analysis of underground coal gasification using supercritical water/carbon dioxide mixture as combined gasifying agent

Purpose. Substantiation of the technological parameters of underground coal gasification based on the established dependence of the efficiency of the goaf formation on the variation in coal seam thickness, in order to determine rational and effective coal gasification modes with a focus on hydrogen production. Methodology. A series of laboratory studies was conducted using a specially designed experimental setup, which made it possible to determine the influence of variable operating characteristics on the gasification process. The trapezoid method was applied to determine the parameters of the goaf of the underground gasifier based on the established geometric data obtained from the opening of laboratory gasifier models. The lower heating value of the producer gas was determined considering the thermodynamic probability of the main gasification reactions within the studied temperature range. Findings. Current trends in the underground coal gasification technology development have been analysed with a focus on hydrogen production as the main energy product. The volumes of coal losses in the near-contour zones of an underground gasifier during gasification of thin seams have been determined. The operation duration parameters of an underground gasifier with the thickness of coal seams from 0.7 to 1.2 m and the lower producer gas combustion heat depending on the temperature regime have been substantiated. Originality. The dependences of the change in the maximum width of the underground gasifier and the value of the expansion of the combustion face length on the coal seam thickness have been identified, which makes it possible to predict the area of the goaf and assess the level of coal losses in the near-contour zones. It has been found that the temperature increase in the reaction zone of the gasifier intensifies the processes of thermochemical conversion of carbon, which leads to an increase in the concentration of hydrogen (H₂) and carbon monoxide (CO) in the composition of synthesis gas, which increases its energy value and suitability for further use in hydrogen energy systems. Practical value. The parameters of operational coal losses in the near-contour zones of an underground gasifier have been substantiated and the duration of the gasification process active phase has been determined. The obtained results make it possible to predict the optimal locations for drilling production wells in order to minimize coal losses in pillars between neighbouring gasifiers. In addition, these data can be used to estimate the volume of coal reserves to be gasified and to calculate the expected producer gas yield, which is key to the feasibility study of the process.