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This paper presents a coupled thermo-mechanical model to investigate the ground response during underground coal gasification (UCG). The model incorporated the temporal and spatial development of temperature, the gradual growth of the cavity, and temperature-dependent material properties. Model verification was made against two benchmarks to acquire the confidence for the predictive purpose. The first exercise demonstrated the correctness of the model implemented in COMPASS. The second exercise showed that using the ash-filled cavity to represent null or empty zones is a good option in the numerical modeling and provided highly comparable results to other models. Based on the Hanna UCG trial, different cases were simulated to investigate the effects of the cavity size in the coal seam and the thermal expansion coefficient of the caprock and base rock on key features that take place during the process of UCG. A maximum temperature in the range of 1200–1500 ℃ was induced by the gasification of coal, and a cavity with a maximum length of 13.5 m was formed after 30 days of simulation. Meanwhile, small vertical displacement in the range of -5–12 mm took place near the cavity because of the thermal expansion of the geologic materials and the reduction of the overall weight with the creation of the cavity. In addition, it was found the thermal expansion coefficients can influence the thermo-mechanical response of geologic materials, but the effects were insignificant when its order of magnitude was smaller than 10-6 K-1.

This paper presents a coupled thermal-hydraulic-chemical numerical model to study the temperature development and solid–gas conversion during underground coal gasification (UCG). Theoretical formulations are based on the porous medium approach, involving the gas flow, solid mass loss, and heat flow, and the chemical reactions driving the phase change and transformation of gaseous species are considered by the kinetics model. A modified empirical relationship is proposed for the increase of porosity due to combustion and gasification, and the corresponding variation of permeability is expressed by a semi-empirical relationship. A numerical solution is obtained via the coupling of the transport model and the kinetics model for the sink/source terms due to chemical reactions. Verification tests have been conducted and compared with alternative numerical solutions to build confidence in the accuracy of the implementation of the model. Evaluation of the model against literature benchmarks including temperature development and syngas composition variation indicates that the coupled model can predict the UCG process.

This paper deals with the characterisation of inorganic constitutions generated at various operating conditions in the context of underground coal gasification (UCG). The ex situ small-scale experiments were conducted with coal specimens of different rank, from the South Wales Coalfield, Wales, UK, and Upper Silesian Coal Basin, Poland. The experiments were conducted at various gaseous oxidant ratios (water: oxygen = 1:1 and 2:1), pressures (20 bar and 36 bar) and temperatures (650°C, 750°C and 850°C). Increasing the amount of water in the oxidants proportionately decreased the cationic elements but increased the concentrations of anionic species. The temperature played minor impact, while the high-pressure experiments at temperature optimum to produce methane-rich syngas (750°C) showed significant reduction in cationic element generation. However, both coal specimens produced high amount of anionic species (F, Cl, SO4 and NO3). The “Hard” bituminous coal from Poland produced less gasification residues and condensates than the South Wales anthracitic coal due to its higher reactivity. The inorganic composition found in the solid residue was used in the theoretical calculation to predict the dissolved product concentrations when the solid residue interacts with deep coal seam water in the event of UCG cavity flooding. It was evident from the solubility products of the Cr, Ni and Zn that changes in the groundwater geochemistry occur; hence, their transportation in the subsurface must be studied further. Supplementary Information The online version contains supplementary material available at 10.1007/s11356-021-15780-8.

In this paper, a flexible numerical framework to provide thermal performance assessment for the underground buried cables, considering different geological and meteorological conditions, has been presented. Underground cables tend to retain the heat produced in the conductor, so complex coupled thermo-hydraulic response of the porous medium surrounding the cables needs to be assessed to prevent cable overheating and the associated reduction in cable capacity for carrying current. Applying a coupled thermo-hydraulic model within the developed numerical framework to conduct a health assessment on a subset of National Grid Electricity Transmission’s underground cables, this study provides novel insights into the thermal behaviour of buried circuits. The results indicate that backfill and surrounding native soil have the dominant effect on the thermal behaviour of cables, while the amount of precipitation and ambient temperature were found to have less impact on cable’s thermal behaviour. The findings strongly infer that the nature of the overloading which is undertaken in practice would have no ongoing negative impact, suggesting that more frequent or longer duration overloading regimes could be tolerated. Overall, this study demonstrates how the developed numerical framework could be harnessed to allow safe rating adjustments of buried transmission circuits.

This paper presents the baseline geochemistry and hydrogeology of a site in the South Wales Coalfield. Three deep exploration boreholes were drilled up to 650 m for that purpose and investigated together with the surrounding surface environment. The present study reports (i) General geological description, mineralogy and chemical composition (ii) Geochemistry of the coal seam water, surface waters and water from a shallow water table (iii) Periodic monitoring of temperature, water level and (iv) Adsorbed methane on rock samples and free gas concentrations acquired from three exploration boreholes. Rock specimens from the deep (425 m to 656 m) contained a minor amount of clay minerals such as clinochlore and kaolinite. Along with the major sandstone constitutions such as quartz and feldspar, rutile was also found in minor quantities. The trace elements such as arsenic, barium and thorium were found in various depths. The water temperature was stable around 13.5 °C (at 240 m) in Borehole 1, 15.6 °C (at 255 m) in Borehole 2 and 18.1 °C (at 453 m) in Borehole 3 throughout the monitoring period. The thermal profiling indicated the temperature gradient of 1.5 °C/100 m with maximum water temperatures at 22 to 24 °C which shows the thermal energy potential of the site. The low water temperature gradient in deep strata below 580 m, where the coal seams were encountered, indicated fluid movement in the deep coal seam aquifers. The gas analysis results showed the availability of the methane in the coal seams, and the water chemistry from the boreholes also exhibited the similar characteristics of the methane-producing wells around the world. The water chemistry of the shallow aquifers at the site was different from the water chemistry of the aquifers near the coal seam. The water quality measurements demonstrated the surface water quality was not affected by the drilling operations. Overall, this paper provides insights into the geochemistry and hydrogeology of the South Wales Coalfield that represents the environmental baseline data for any future industrial exploration or development activities.

The experimental analysis aimed at investigating the high-pressure (sub- and super-critical) CO2 sorption behaviour on two high-rank coals of different sizes is presented in this paper. Coals from the same seam (9ft seam), but from depths of 150 m (BD coal) and 550 m (AB coal) and different locations of the South Wales (UK) coalfield, known to be strongly affected by tectonically developed fracture systems, are employed for that purpose. Hence, the sorption behaviour of powdered (0.25-0.85 mm, 2.36-4.0 mm) and core samples obtained from locations associated with the deformation related changes is analysed in this paper to assess the CO2 storage potential of such coals. The results show that the coals exhibit maximum adsorption capacities up to 1.93 mol/kg (BD coal) and 1.82 mol/kg (AB coal). No dependence of the CO2 maximum sorption capacity with respect to the sample size for the BD coal is observed, while for the AB coal the maximum sorption capacity is reduced by more than half between the powdered and core samples. The CO2 sorption rates on BD coal decrease by a factor of more than 9 from 0.25-0.85 mm to 2.36-4.0 mm and then remain relatively constant with further increase in sample size. The opposite is observed for the AB coal where sorption rates decrease with increasing sample size, i.e. reducing by a factor of more than 100 between the 0.25-0.85 mm and core samples. The differences in behaviour are interpreted through the structure each coal exhibits associated with the burial depths and sampling locations as well as through the minor variations in ash contents. This study demonstrates that anthracite coals, having experienced sufficient deformation resulting in changes in fracture frequency, can adsorb significant amounts of CO2 offering great prospect to be considered as a CO2 sequestration option.

This paper presents an analysis of contaminants generated from large-scale, laboratory-based, underground coal gasification (UCG) experiments using a high-rank coal from the South Wales Coalfield. The experiments were performed at atmospheric and elevated pressures (30 bar) by varying the oxidants’ composition. The experiments were designed to predict the amount of produced water and contaminants generated at each stage of the operating conditions. The mass balance of water supplied and produced in the experiments was accounted for. Chemical analyses of produced water, char and ash contents were performed to quantifytheinorganicandorganicchemicalparameters. Mostof the contaminant concentrationsinthe produced water from the 30-bar pressure experiment were lower than the concentrations generated from the atmospheric pressure experiment. The measured concentrations of theinorganicchemicalspeciesandtheinorganicparameters of the coal seam water from the South Wales Coalfieldwereusedintheoreticalcalculationstopredict\udthe dominant equilibrium species concentrations in a hypothetical scenario of effluent contaminated groundwater.Thebiodegradationoforganiccontaminantssuch as phenol, benzene and sorbed fractions of inorganic contaminants from the produced water on iron oxide in theashresiduewaspredictedusingexistingbiotransformation kinetics and surface complexation models, respectively. The biodegradation of phenol and benzene would be a slow process even at optimum conditions and the iron oxide left in the cavity can act as a sorbent for a few inorganic species. The evidence from the present study suggests future work towards (i) developing an appropriate water treatment process during gas cleaning, (ii) operational procedure (pressure and proportions of oxidant) and (iii) developing UCG-specific experimental prediction of contaminant transportation and transformation kinetics.\udKeywords High-rankcoal.Undergroundcoal gasification.Highpressure.Producedwater.Inorganic organiccontaminants.Ash

This work presents the state-of-the-art review of investigations related to the adsorption process, adsorption models, experimental adsorption results, and influencing factors, considering the main contaminants produced by underground coal gasification (UCG) technology as adsorbates and the various rocks and soils surrounding the UCG cavity as adsorbents. Based on the literature reviewed, it is found that claystone, coal, coal char, shale, and clay materials present a good prospect for effective phenol adsorption; coal, coal char, shale, and clay materials can also remove benzene and some heavy metals from aqueous solutions. However, their performance varies under the effect of the influencing factors, such as the initial concentration of adsorbates in solution, the pH of the solution, the temperature and contact time controlled in the adsorption process, and the adsorbent dosage. A preliminary assessment of the potential of rocks and soils to act as natural buffers in UCG application is provided. The impact of UCG process on the adsorption of contaminants on the surrounding strata together with the major challenges and future perspectives are highlighted and outlined, to identify knowledge deficiencies regarding the retardation of UCG contaminants using the natural buffers. The prospect of surrounding strata as natural buffers can benefit the site selection, design, and commercialization of UCG.

Underground coal gasification (UCG) has a great potential to be accompanied by the technology of (enhanced) coal bed methane ((E)CBM) and carbon capture and storage (CCS), enhancing coal energy efficiency and mitigating climate change. This study aims to assess the environmental impact of three field-scale UCG hypothetical sites targeting deep-buried seams during the UCG operational stage in the combined CBM-UCG application. For this purpose, a risk assessment methodology is developed with consideration of the geomechanical and hydrogeological impact of UCG operations on the overlying strata by analogy to longwall mining operations. A series of numerical simulations are conducted to investigate the effect of variations in the saturation conditions around the UCG reactor, the change of rock permeability, and the UCG reactor operation duration on the propagation of heat, moisture and gaseous chemicals. The results demonstrate that CBM-UCG activities in the three sites are not likely to cause negative impacts on the environment during the UCG process when the behaviour of the reactor and the change of the surrounding strata are in control as expected. This work can provide insights in terms of environmental decision making, regulation and management to prevent reduction in gasification efficiency, to conduct the post-UCG (E)CBM process and to mitigate the UCG gas leakage and contamination of surrounding aquifers, highlighting where additional information is required in CBM-UCG activities.

This paper presents the results of an experimental investigation on gas flow and Klinkenberg effect in coal. An anthracite coal sample is subjected to a range of effective stress conditions in order to investigate a general trend of coal permeability reduction with an increase in effective stress. Based on the measured values of permeability at different mean gas pressures for constant effective stress conditions, intrinsic values of permeability in the range 0.2 – 1.5 mD and gas slip factors are determined using the Klinkenberg plot. Using measured gas permeability and calculated intrinsic permeability values, an assessment of the mean gas pressure required to minimize the Klinkenberg effect is conducted.

Deep coal reserves represent a valuable resource, both in terms of their potential for the sequestration of anthropogenic carbon dioxide and for energy extraction through underground coal gasification (UCG). This paper looks at the current field and research status of these technologies and, in line with on-going work at Cardiff University’s Geoenvironmental Research Centre (GRC), focuses special attention on the role of numerical modelling in advancing the research agenda. In response to poor carbon dioxide injectivity experienced in carbon sequestration trials and order-of-magnitude permeability losses caused by sorption induced coal swelling, the direction of work at the GRC is to improve the current understanding of the mechanical response of coal using a coupled thermal, hydraulic, chemical, mechanical modelling framework. Hence, the current focus of theoretical developments in this area are summarized. In relation to UCG, on-going developments to a comprehensive numerical model are discussed. The ultimate aim is to identify and quantify coupled mechanisms controlling syngas composition and production rate, heat and mass transport, geochemical reactions, geomechanical responses, and cavity growth to subsequently improve the state-of-the-art towards minimizing geoenvironmental risks.

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.

The results of ex-situ small-scale laboratory tests performed in a bespoke batch reactor simulating coal gasification to find the most optimal experimental conditions for producing methane-rich syngas in the context of UCG are presented in this paper. The influence of gaseous reactants (oxygen and steam), their supply rates and thermodynamic conditions (temperatures of 650 °C, 750 °C, 850 °C and pressures of 20 bar and 36 bar) on the gasification of semi-anthracite (South Wales coalfield) and bituminous (Silesian basin) coals is investigated. Increasing the gasification pressure from 20 bar to 36 bar and doubling the amount of steam with respect to oxygen benefit the methane generation. Although temperature increase from 650 °C to 850 °C also benefits methane generation, gasification at 750 °C provides the most optimal conditions for methane-rich syngas production. Overall, the highest methane generation occurs at 750 °C, 36 bar and H2O:O2 = 2:1 yielding peak methane concentrations of 44.00 vol% and 35.55 vol%, and average methane concentrations of 15.34 vol% and 14.64 vol% for the semi-anthracite and bituminous coals, respectively. These findings demonstrate that an increase in coal rank favours the methane generation. Owing to high methane content, the syngas produced at such conditions contains the highest calorific value, although the generation of hydrogen and carbon monoxide is reduced in comparison to the experiments conducted at 850 °C. This study shows that gasification of bituminous and semi-anthracitic coals at elevated pressures can provide stable generation of methane-rich syngas whose quality can be controlled by the gasification temperature through the dynamics of steam and O2 supply rates.

This paper presents the results obtained through continuous and simultaneous measurements of gas flow, temperature and coal deformation during high-pressure gas injection in high-rank coal samples, obtained from the South Wales coalfield (UK). The results demonstrate that CO2 flow rates experience an initial decline due to internal coal swelling, followed by the flow rate recovery and global coal swelling. As the flow of high-pressure CO2 induces measurable temperature drop within the sample related to the Joule-Thomson cooling, the changes induced by the variations in thermal state of the system are associated with abrupt shift in coal response to reactive gas flow. However, subsequent injections of He and N2 show that the changes induced by CO2 sorption on coal permeability to gases are irreversible. This work demonstrates the importance of considering the coupled reactive gas and heat transport, and consequent coal deformation mechanisms while assessing the storage potential of coal seams.