Underground Coal Gasification Research Articles

Challenges and Opportunities for End-of-Life Coal Mine Sites: Black-to-Green Energy Approach

This paper presents the possibilities of adapting active mines to generate green energy after their closure using their resources and/or infrastructure. For this purpose, firstly, the temporal horizon of selected mines in Poland was determined, its basic assumption being the analysis of the current state. In the research, 18 mining plants operating within 12 mines in the Upper Silesian Coal Basin (USCB) were analyzed. The analyzed mines belong to three of the five largest hard coal producers in Poland, and the main object of exploitation is hard coal of energy types. Severe restrictions or even abandonment of further investments in the development of the coal mining industry were taken into consideration (regarding the construction of new shafts or the development of new exploitation levels). When determining the temporal horizon, the challenges that hamper the exploitation based at the levels of natural hazards and depth of exploitation in each mine were considered. Secondly, the criteria for the adaptation of active mines to generate energy are presented. The possibility of using the resources and infrastructural potential of active mines to produce geothermal energy from water, extracting coalbed methane (CBM), and processes of underground coal gasification (UCG) are analyzed. Finally, for a selected example—generating energy from underground coal gasification in Polish mine conditions—a structural analysis of the criteria was performed using the MICMAC method, as the Central Mining Institute has an extensive experience in the development of underground coal gasification trials in coal mines. Based on expert analysis and using structural analysis, the criteria important for UCG were selected. As demonstrated in the article, the MICMAC method can be applied in other scenarios with different criteria to implement new technologies in coal mines.

Environmental evaluation study of toxic elements (F, Zn, Be, Ni, Ba, U) in the underground coal gasification (UCG) residuals

Underground coal gasification (UCG) is considered to be an important means of coal utilization. However, the stability of underground coal gasification residuals has always been the focus of people’s attention. In this paper, based on the sequential chemical extraction and thermodynamic equilibrium calculation, the chemical forms and transformation mechanism of the toxic elements (TEs) were investigated. Moreover, the risk level of toxic elements to underground environment was also studied in this paper. Results showed that Zn and Ni are thiophilic elements and favored to react with S to generate ZnS and NiS in the UCG pyrolysis stage. In the 900–1300 °C reduction ash, the toxic elements (Zn, Be, Ni, Ba, and U) in the residual speciation (F4) have the large percentage. For the oxidation residuals, Be and U are intimately related with the Fe–Si–Al–O–Be–U mineral system, and F exists in the form of CaF2. According to the risk assessment code (RAC), the harmful degree of toxic elements to the underground environment decreases with rises of reaction temperature. When the reaction temperature reaches 1200 °C, the harmful degree of toxic elements to the underground environment is range in the Low risk, indicating that the underground residuals have reached the stability. The results can provide suggestions for future handing of the underground coal gasification residuals.

Thermodynamic equilibrium simulation on the synthesis gas composition in the context of underground coal gasification

Underground coal gasification is a promising technology involved with the in-situ conversion of coal resources into a high-quality synthesis gas with low cost. A thermodynamic equilibrium simulation was conducted to analyze the effects of the reaction temperature, pressure and ratio of oxygen/steam on the synthesis gas composition in the context of underground coal gasification. The results show that the increase of the gasification pressure promotes the formation of CH4 while inhibits the formation of H2 and CO in the synthesis gas under different temperatures. And the gasification pressure for the maximum low heating value (LHV) of synthesis gas shifts to larger pressures with the increasing gasification temperatures, and the corresponding maximum LHV is 7.87 MJ/m3 at the pressure of 5.6 MPa when the gasification temperature is set as 800 °C. The increase of the gasification temperature leads to an increase in the contents of H2 and CO and a decrease in CH4 content in the equilibrium synthesis gas. The gasification temperature shifts to higher values to reach the maximum LHV of synthesis gas at elevated pressures except for the slight increase of LHV within the temperature range of 700–1000 °C at 0.1 MPa. Further analysis on the effect of the ratio of oxygen/steam indicates that increasing the moisture content in the gasification agent significantly results in the enhancement of H2 and CH4 contents in the synthesis gas as well as its LHV.

Insights into solid-gas conversion and cavity growth during Underground Coal Gasification (UCG) through Thermo-Hydraulic-Chemical (THC) modelling

This paper presents the application of a thermal-hydraulic-chemical model to simulate (1) a laboratory small-scale borehole combustion experiment using high-ash-content coal and (2) an ex-situ large-scale UCG experiment using low-moisture-content hard coal. The focus was to study temperature development, syngas composition, pore structure alteration of coal, and cavity growth pertinent to the UCG process. Both numerical simulations reproduced the temperature development trends along the gasification channel. Compared with the measured cavity with a horizontal length of 15 cm and a maximum vertical extension of 5.4 cm that was created in the 10-h-long borehole combustion experiment, a similar simulated cavity was formed in the first simulation, which had a horizontal length of 10 cm and a maximum vertical extension of 5.1–5.7 cm. This simulation indicated that the proposed model can be used to estimate the cavity growth with the process of UCG due to solid-gas conversion. In the second simulation, the simulated composition of syngas showed good agreement with the experimental results. It also revealed that fierce gasification and combustion mainly occurred close to the inlet, and thus created an L-shaped cavity in the 20-h-long UCG process. Moreover, the UCG process was distinctly divided into three stages from the perspective of solid loss and pore structure alteration at a representative node owing to thermal expansion, compressional shrinkage, pyrolysis, and gasification and combustion. The second stage of 3–11 h was the main period for combustion and gasification of the UCG process, namely, 70% of the solid mass was converted into syngas at temperatures above 1320 K, and the porosity increased linearly with time and the permeability showed exponential growth. Furthermore, a parametric sensitivity study based on the second simulation indicated that the simulated cavity growth of UCG was sensitive to the kinetics of char combustion, while the assumed pyrolyzed production ratios in the model were not of significance to the cavity growth during UCG. It is claimed that the coupled thermo-hydraulic-chemical model adopted in this paper provides novel insights into the UCG reactor's dynamic behavior, including solid-gas conversion and cavity growth.

Influence of scale and atmosphere on the pyrolysis properties of large-scale bituminous coal

In order to explore the influence of scale and atmosphere on the pyrolysis properties of coal in the underground coal gasification process, bituminous coal samples measuring 5, 8, and 10 cm were separately pyrolyzed in N2 and CO2, and the functional groups and pore structure at various scales were also analyzed. The results indicated that the main influences on large-scale bituminous coal pyrolysis were both the inner and outer temperatures, and the inner temperature was lower than the outer temperature. The production rate of pyrolysis tar in CO2 is higher than that in N2. The pyrene, naphthalene, and benzene concentrations in the pyrolysis tar decreased, but the indene increased with the increase of scale. The specific surface area and pore volume of pyrolysis coke were significantly increased in CO2 with the increase of scale. The average pore diameter of pyrolysis coke showed a decreasing trend with the increase of scale. In a N2 atmosphere, the average pore diameter and the most probable diameter were both greater than those in CO2. The −OH, CH2, and CH3 concentrations in the coke in CO2 were lower than those in N2. The larger the scale, the more −OH was contained in the coke.

Removal of pollutants from aqueous solutions by coals and residual cokes obtained from simulated underground coal gasification experiments

Groundwater pollution caused by underground coal gasification (UCG) as well as its remediation has attracted significant public attention. In this paper, model gasification experiments on three types of coals with different coalification degrees were carried out using a UCG model test system. The residual coal and coke samples were characterized via SEM, low temperature N2 adsorption, and FT-IR. Thereafter, their adsorption of phenol and Cr(VI) from simulated wastewater as well as the purification of gas washing water were investigated. Compared with the raw coals, the UCG residual cokes presented a higher specific surface area (SBET) of 44.99–381.51 m2/g and total pore volume (Vt) of 0.040–0.228 cm3/g; Cokes obtained from brown coal gasification had the highest SBET and Vt values. Parts of O-containing groups were preserved after the gasification. Phenol and Cr(VI) were removed by all types of coals and residual cokes. The removal rate of phenol increased with SBET and Vt, and cokes presented higher phenol adsorption capacities than those of raw coals. The O-containing groups significantly influenced and favored the removal of Cr(VI). Brown coal and its UCG coke had the highest adsorption capacities, with phenol removal rates of 17.71% and 72.44%, and presented the highest adsorption capacities, with Cr(VI) removal rates of 29.02% and 21.77%, respectively. When UCG residual cokes were used, the removal rates of total organic carbon (TOC) in both types of gas washing water exceeded 82%. Almost all the Cr and Se and some of the Ni were removed from gas washing water.

Engineering properties of four different rocks after heat treatment

The effect of high temperature on engineering properties of rocks has recently become an important topic for geotechnical engineering applications due to the development in geothermal energy extraction, underground coal gasification, disposal of nuclear waste and deep mining applications. This study aims to investigate the engineering properties of four rocks heated at high temperatures and to identify the corresponding mineralogical changes. Two types of sandstone (Beaudesert sandstone and Jimboomba sandstone), basalt and argillite were subjected to a range of temperatures from 25 to 800 °C and their properties such as porosity and point load index were studied through a series of laboratory experiments. The physical and mineralogical alteration of the heated rock specimens were also investigated to better understand the thermally induced processes that occurred in each rock type using the X-Ray Diffraction, Thermogravimetry (TG), Differential Scanning Calorimetry (DSC) and Derivative Thermogravimetry (DTG) techniques. The obtained data indicated the important role of mineral composition in the thermal behaviour of the studied rocks. Noticeable changes in the mineral compositions at the threshold temperature were observed for each rock type. This behaviour was confirmed with TG/DSC/DTG analysis. This temperature-related process resulted in the formation of cracks, an increase in porosity and a decrease in strength. However, the effects of high temperatures on engineering properties were found to be different in the two sandstone types. While Beaudesert sandstone underwent significant weakening at high temperatures, only a slight change in the strength of Jimboomba sandstone was recorded. The threshold temperatures were identified as 400 °C, 400 °C, 500 °C and 300 °C, for the tested Beaudesert sandstone, Jimboomba sandstone, basalt and argillite samples, respectively. The critical temperatures (obtained using a damage coefficient) were found as 570 °C for both Beaudesert sandstone and basalt, and 440 °C for argillite. Analysis of the laboratory data revealed the difference between the threshold temperatures and the critical temperatures. It is recommended to use both threshold temperature and critical temperature parameters to provide more accurate estimations of the temperature range at which rock undergoes significant changes.

Life cycle energy consumption and GHG emissions of hydrogen production from underground coal gasification in comparison with surface coal gasification

Developing underground coal gasification (UCG)-based hydrogen production (UCG-H2) is expected to alleviate hydrogen supply and demand contradiction, but its energy consumption and environmental impact need to be clarified. In this paper, comparative study of energy consumption and greenhouse gas (GHG) emissions between UCG-H2 and typical surface coal gasification (SCG)-based hydrogen production (SCG-H2) is carried out using life cycle assessment method. Result shows energy consumption of UCG-H2 is only 61.2% of that of SCG-H2, which is 1,327,261 and 2,170,263 MJ respectively, reflecting its obvious energy saving advantage. 80% capture rate can achieve an appropriate balance between energy consumption and emissions. Under this capture rate, emissions of UCG-H2 and SCG-H2 are roughly equivalent, which are 207,582 and 197,419 kg CO2-eq respectively. Scenario analysis indicates energy consumption in hydrogen industry can reduce by 38.8% when hydrogen production is substituted by UCG with CCS to fully meet demand of 21 Mt in 2030.

Development of gasification agent injection tool for underground coal gasification

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.

Model Predictive Control and Adaptive Kalman Filter Design for an Underground Coal Gasification Process

The control of a nonlinear system such as an underground coal gasification (UCG) process is a challenging task. Several nonlinear design approaches are implemented to improve the tracking performance of the UCG process, however, the nonlinear techniques make implementation complex and computationally inefficient. In this work, a constrained linear model predictive control (MPC) is designed for the UCG process to track the desired trajectory of the heating value, while satisfying actuator constraints pertaining to UCG. The unknown states required for MPC design are reconstructed by using linear adaptive Kalman filter (AKF) and unscented Kalman filter (UKF). The design of MPC and AKF is based on the quasi-linear model of the UCG process. A fair comparison between different control strategies is conducted which include MPC– AKF, MPC– UKF, MPC– gain scheduled modified Utkin observer (GSMUO) and dynamic integral sliding mode control (DISMC)–GSMUO. The quantitative analysis and simulation results show that MPC- AKF outperforms its counterparts by yielding the least tracking error and average control energy. This conclusion holds, even in the presence of an external disturbance, parametric variations, and measurement and process noises. Moreover, MPC- AKF yields 51%, 44% and 46% improvement in absolute relative root-mean-squared error with reference to MPC– UKF, MPC– GSMUO and DISMC–GSMUO, respectively. A quantitative analysis has also been carried for AKF and UKF, which shows that the performance of AKF is more robust against changes in the initial values of measurement and process covariances.

Binary TiO 2/RGO Photocatalyst for Enhanced Degradation of Phenol and its Application in Underground Coal Gasification Wastewater Treatment

In this work, we synthesize the TiO2/RGO binary nanohybrids through the sol-anaerobic calcination method, the TiO2 nanoparticles belonging to anatase type were evenly loaded on RGO surface in the form of Ti-O-C bond. The enhanced photocatalytic performance can be ascribed to the existence of RGO that can effectively promote the transfer of photo-induced electron-hole pairs to generate more hydroxyl radicals (·OH) and superoxide anion (O2·-) degrading phenol. The TiO2/RGO nanocomposite with weight ratio of RGO to TiO2 (37.0% to 55.7%) has achieved maximum degradation of phenol from aqueous solution, and about 97.87% of phenol has been removed within 180 min under UV-light illumination. Through GC-MS tracking of phenol degradation process, it can be observed that long-chain intermediates will be generated, and these intermediates will eventually be degraded, confirming that phenol can be completely removed. Additionally, around 67% TOC of simulated underground coal gasification (UCG) wastewater can be removed after 10 h degradation using TiO2/RGO binary composites as photocatalysts. All these results demonstrate that the binary nanohybrids possess great potential for the organic contaminants destruction from the actual coal chemical wastewater.

Effect of oxygen and steam on gasification and power generation in industrial tests of underground coal gasification

To optimize the gasification process and improve the underground coal gasification (UCG) technology for power generation, a pilot project of UCG integrated with the power generation was built in the abandoned mine “Ankou”. The UCG industrial tests with different oxygen concentrations of oxidant along with the pulsed injection method, and power generation experiments using UCG syngas were conducted. The results indicate that a decrease in the rate of air blowing for a short time exhibited a limited influence on syngas production, but a change in the amount of oxygen injection presented an obvious effect. With the increase in the oxygen concentration in the agent, the H2 content, combustible components and calorific value of syngas, flow ratio, and the consumption of coal, steam and oxygen increased, along with a rise in the gasification efficiency after an initial drop. The oxygen-enriched gasification trials showed higher air consumption, lower consumption of coal, steam and oxygen, and a low combustible components and calorific value of syngas compared to the gasification tests of the oxygen-enriched steam. The evaporation of ground water and coal moisture in the UCG panel provided steam for the oxygen-enriched gasification process, and its utilization rate was about 6–32 wt%. From the characteristics of UCG syngas, the technical transformations such as exhaust gas booster combustion control, high energy ignition, low pressure intake, expansion of the air, and gas inlet pipe diameter were carried out on the existing gas generator sets. The effectiveness of the measures was verified by the power generation experiments.

Application of geostatistical methods in spatio-temporal modelling of temperature changes of UCG experimental trial

Underground coal gasification (UCG) technology enables coal to the extracted from underground seams that are too deep or affected by tectonic disturbances. In the underground, coal is transformed into syngas with gasification agents and subsequently extracted to the surface. The paper describes the use of geostatistical methods of modelling for the analysis of the UCG process. Based on the geostatistical theory and utilization of Isatis's geostatistical environment, a spatio-temporal model of temperature changes in the experimental generator during the UCG process in laboratory conditions was constructed. For the construction of this model, data from the experiment were used. From the measured data, temperatures obtained from thermocouples placed in the experimental generator were processed. A spatio-temporal model of temperature changes or temperature slices obtained from this model was used to analyze and subsequently calculate the combustion front movement during the experiment. Individual time slices of temperatures were analyzed in ArcGIS-ArcMap environment. Based on the visualization of these slices, an animation of temperature propagation and movement of temperature zones in time and space was created, which represents a view of the resulting process dynamics. It is clear from the model results that shortly after the experiment, two oxidation zones were created - left and right. From the calculated motion vectors of the front, a cyclic movement in the clockwise direction of the left oxidation zone is evident, with the highest movement velocity occurring at approximately half the duration of the experiment.

Evolution of permeability and microscopic pore structure of sandstone and its weakening mechanism under coupled thermo-hydro-mechanical environment subjected to real-time high temperature

Thermal damage mechanisms and the characteristics of performance deterioration of a repository host rock is critical for evaluating the potential of an underground coal gasification project. Previously, many studies have primarily focused on the microstructural evolution of various rock types after high-temperature treatment only. However, relatively little is known regarding the coupled thermo-hydro-mechanical (THM) behavior of rock, which is frequently encountered in geo-engineering underground regions with high-temperatures. In this study, using a self-developed universal tester with the ability of THM coupling of high temperature and high pressure, permeability evolution in sandstone under real-time high temperature (20–700 °C) and triaxial stress (hydrostatic pressure = 25 MPa) were observed. The microphysical parameters of these specimens subjected to the THM environment were then measured, and the corresponding morphological evolution processes were also assessed using micro-computed tomography (MCT). Further, for the purpose of contrast, we have also determined the microphysical parameters for the sandstone after heat treatment only. The results show that the evolution of the microscopic structures within the sandstone under the coupled THM condition is vastly different from those subjected to heat treatment only. The experimental results in this study can provide theoretical guidance for the stability of surrounding rock of a combustion cavity during in-situ coal gasification.

Unsteady Heat Conduction Study During Underground Coal Gasiflcation For Constructing an In-situ Pyrolysis Project.(Dept.M)

Underground coal gasification (UCG) Is a potential coal utilization technology, receiving renewed interest around the world. In the last .five years there has been Increasing Interest In tile world energy and mining Industry for tile development of a new operating underground coal gasification systems to generate synthetic gas from underground coal seams. In this paper, the problem of unsteady heat conduction In the underground coal and rock layers during underground gasification Is considered. The heating of coal results In releasing considerable amount of gases mainly hydrogen and methane which would be collected and pumped through pipelines for use in different Industrial applications. In the present analysis, a spherical type heat source having a surface temperature of (1000OC, 1500OC, 1750OC and 2000OC) is used for heating the coal layer for achieving tile pyrolysis process. The differential equations describing the unsteady heat conduction in both coal and rock layers are solved numerically using the discretization method (control volume formulation). A computer program Is constructed for calculating the temperature fields and the gasified volumes of mineral coal as well as the heating velocity in the coal seams. In the experimental part, a model of an underground coal layer Is constructed. To validate the numerical model, an electrical spherical heat source Is Inserted directly into the coal layer for heating. The temperature distribution In coal layer is measured In different locations using the Thermocouples. The results show that the temperature profiles and the maximum attainable temperature in the coal seams depend on the time and the surface temperature of the heating reactor. Furthermore, the gasified volume of coal increase with the increase of both the surface temperature of the heating source and the running time. It is found also that the heating velocity in the underground coal. Lies between the mean value (2 to 10-3 K/min.) and increase with the time to reach a maximum value, after this it decrease to approach zero at the steady-state conditions. This study indicates that geometry, position, surface temperature and distribution of heating sources are very important parameters which should be carefully chosen for realizing an efficient pyrolysis of underground coal and avoiding thereby the combustion of a part of the coal seams. The analysis indicated that the best temperature is between (700-800oC) at which the high efficiency of pyrolysis process, the analysis might be useful in constructing an in-situ pyrolysis project for utilizing the underground coal seams.

Underground Coal Gasification (UCG) -A Pilot Study at Certain Area of Jamalganj Coal Basin, Joypurhat District, Bangladesh

Underground Coal Gasification (UCG) -A Pilot Study at Certain Area of Jamalganj Coal Basin, Joypurhat District, Bangladesh

In situ sorption phenomena can mitigate potential negative environmental effects of underground coal gasification (UCG) - an experimental study of phenol removal on UCG-derived residues in the aspect of contaminant retardation

The aim of the study was to investigate the sorption interactions between phenol and materials obtained from four different underground coal gasification (UCG) ex-situ simulations. These interactions are significant in terms of the impact of the UCG on the groundwater environment. Sorption parameters were determined for two sample types: raw coal mined from the coal-bed and then subjected to the gasification process; and char residue acquired from the cavity formed as a result of the UCG processes. Laboratory-scale tests were carried out using deionized water and aqueous solutions with increasing concentrations of phenol (from 50 mg/dm3 to 2000 mg/dm3) at 298 K. On the assumption of physical interactions (non-specific physisorption) and due to a nonlinear mass distribution of adsorbed substances as a function of equilibrium concentration, the Freundlich isotherm model was applied to describe adsorption phenomena. The isotherms have good fitting (R2 from 0.5716 to 0.9811). Relatively high percentage phenol removal efficiency was observed for all tested chars (from 17.0% to 99.8% for the 1.0–2.5 mm fraction and from 6.9% to 99.6% for the 10.0–12.5 mm fraction). Additionally, the sorption characteristics was used to evaluate the retardation coefficients. The largest delay in the organic pollutant migration in the environment around a UCG reactor occurs for phenol transport in the layer of the post-process char from ‘Wesoła’ after 40 bar pressure experiment.

A numerical investigation into the environmental impact of underground coal gasification technology based on a coupled thermal-hydro-chemical model

This study aims to improve the understanding of the potential environmental impact of the underground coal gasification (UCG) process based on a numerical investigation of the gaseous and dissolved chemicals’ transport in the strata surrounding the UCG reactor. A coupled thermal-hydro-chemical (THC) model in the framework of COMPASS code is employed and further developed for this purpose. The model is compared against analytical solutions in the verification process and applied against a former UCG trial (Hoe Creek UCG programme) for validation purpose. The numerical model is then applied to investigate the potential environmental impact of UCG in three different geological materials (sandstone, coal, and shale) in two steps: the potential gas propagation during UCG process and the solute transport in UCG decommissioning stage. The results indicate that the gas propagation is limited to a near-cavity area in geological strata with low permeability and high air entry value such as shale, which is mainly attributed to the dry zone generated by the high temperature, while a larger gas plume is observed for materials with poor water retention characteristics, such as coal and sandstone. Besides, the gas propagation is retarded to some extent due to the reducing effect of high pressure on gas diffusion. The propagation of dissolved chemicals via diffusion is limited to near-cavity areas, i.e. 2 m, in three studied materials after 10 years. Based on the improved understanding of the various chemicals’ migration in different materials, by selecting the target coal seam surrounded by strata layers with high air entry value and low permeability, the gas losses from the reactor during the UCG reactor operation stage can be minimized and the potential negative environmental impact of ash leaching and solute propagation in UCG decommissioning stage can be avoided. This work also provides further insight and suggestions into the importance of investigating the adsorption capacities of different geological media to evaluate their performance as a natural cleaning system.

MODELING OF MATERIAL BALANCE FROM THE EXPERIMENTAL UCG

The underground coal gasification is a continually evolving technology, which converts coal to calorific gas. There are many important parameters in this technology, which are difficult to measure. These parameters include the underground cavity growth, amount gasified coal, and the leakage of input and output gaseous components into the surrounding layers during the coal gasification process. Mathematical modeling of this process is one of the possible alternatives for determining these unknown parameters. In this paper, the structure of the mathematical model of laboratory underground coal gasification process from the material balance aspect is presented. The material balance consists of mass components entering and leaving from the UCG process. The paper shows a material balance in the form of a general mass balance and atomic species balance. The material balance was testing by six UCG laboratory experiments, which were realized in two ex-situ reactors.

A quantitative assessment of the environmental sustainability of UCG and CO2 storage

In this study, an innovative numerical model was developed to quantify the environmental sustainability situation of in situ underground coal gasification (UCG) and the CO2 storage process, which is expressed in terms of the environmental sustainability index (ESI). This approach is based on four environmental indicators: rock and soil, groundwater, surface water, and atmosphere. Based on the ESI values, the methodology proposed herein is used to classify the environmental sustainability state of the UCG process and its corresponding threshold limit value. Finally, the developed mathematical model was applied to possible European coal deposits, specifically in a Bulgarian coal basin. Research efforts have focused on the development of a mathematical model for environmental impact assessments to pave the way for full-scale trial and commercial applications.