An experimental campaign on the methane-oriented underground coal gasification (UCG) process was carried out in a large-scale laboratory installation. Two different types of coal were used for the ...oxygen/steam blown experiments, i.e., “Six Feet” semi-anthracite (Wales) and “Wesoła” hard coal (Poland). Four multi-day gasification tests (96 h continuous processes) were conducted in artificially created coal seams under two distinct pressure regimes-20 and 40 bar. The experiments demonstrated that the methane yields are significantly dependent on both the properties of coal (coal rank) and the pressure regime. The average CH4 concentration for “Six Feet” semi-anthracite was 15.8%vol. at 20 bar and 19.1%vol. at 40 bar. During the gasification of “Wesoła” coal, the methane concentrations were 10.9%vol. and 14.8%vol. at 20 and 40 bar, respectively. The “Six Feet” coal gasification was characterized by much higher energy efficiency than gasification of the “Wesoła” coal and for both tested coals, the efficiency increased with gasification pressure. The maximum energy efficiency of 71.6% was obtained for “Six Feet” coal at 40 bar. A positive effect of the increase in gasification pressure on the stabilization of the quantitative parameters of UCG gas was demonstrated.
This paper presents a series of surface experimental simulations of methane-oriented underground coal gasification using hydrogen as gasification medium. The main aim of the experiments conducted was ...to evaluate the feasibility of methane-rich gas production through the in situ coal hydrogasification process. Two multi-day trials were carried out using large scale gasification facilities designed for ex situ experimental simulations of the underground coal gasification (UCG) process. Two different coals were investigated: the “Six Feet” semi-anthracite (Wales) and the “Wesoła" hard coal (Poland). The coal samples were extracted directly from the respective coal seams in the form of large blocks. The gasification tests were conducted in the artificial coal seams (0.41 × 0.41 × 3.05 m) under two distinct pressure regimes - 20 and 40 bar. The series of experiments conducted demonstrated that the physicochemical properties of coal (coal rank) considerably affect the hydrogasification process. For both gasification pressures applied, gas from “Six Feet” semi-anthracite was characterized by a higher content of methane. The average CH4 concentration for “Six Feet” experiment during the H2 stage was 24.12% at 20 bar and 27.03% at 40 bar. During the hydrogasification of “Wesoła" coal, CH4 concentration was 19.28% and 21.71% at 20 and 40 bar, respectively. The process was characterized by high stability and reproducibility of conditions favorable for methane formation in the whole sequence of gasification cycles. Although the feasibility of methane-rich gas production by underground hydrogasification was initially demonstrated, further techno-economic studies are necessary to assess the economic feasibility of methane production using this process.
•Large scale gasification experiments of methane-rich gas production using hydrogen (hydrogasification) were conducted.•The effect of pressure and coal rank on the methane concentrations and gas production rates was confirmed in the study.•The technical feasibility of the underground coal hydrogasification (UCHG) concept was confirmed.
AbstractDespite a body of research carried out on thermally coupled processes in soils, understanding of thermo-osmosis phenomenon in clays and its effects on hydromechanical behavior is incomplete. ...This paper presents an investigation on the effects of thermo-osmosis on hydraulic behavior of saturated clays. A theoretical formulation for hydraulic behavior was developed, incorporating an explicit description of thermo-osmosis effects on coupled hydromechanical behavior. The extended formulation was implemented within a coupled numerical model for thermal, hydraulic, chemical, and mechanical behavior of soils. The model was tested and applied to simulate a soil heating experiment. It is shown that the inclusion of thermo-osmosis in the coupled thermohydraulic simulation of the case study provides a better agreement with the experimental data compared with the case in which only thermal expansion of the soil constituents was considered. A series of numerical simulations are also presented, studying the pore-water pressure development in saturated clay induced by a heating source. It is shown that pore-water pressure evolution can be considerably affected by thermo-osmosis. Under the conditions of the problem considered, it was found that thermo-osmosis changed the pore-water pressure regime in the vicinity of the heater when the value of thermo-osmotic conductivity was larger than 10−12 m2·K−1·s−1. New insights into the hydraulic response of the ground and the pore-pressure evolution due to thermo-osmosis are provided in this paper.
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 results of an extensive experimental analysis aimed at establishing the effects of subcritical and supercritical CO2 sorption on deformation and failure of coals. Two ...high-rank anthracitic coals from the South Wales coalfield, obtained from different locations and depths of 150 m and 550 m, are employed for that purpose. The investigations include i) determination of unconfined compressive strengths and elastic moduli of the cores both non-saturated and saturated with CO2 at 2.1 MPa, 4.3 MPa and 8.5 MPa, ii) assessing the dependence of the parameters obtained on CO2 pressure, iii) analysing the effect of CO2 saturation on failure patterns of the samples tested and iv) determination of the particle size distribution after the failure of the samples. Based on the results of twenty coal specimens tested, it is demonstrated that CO2 sorption reduces the uniaxial compressive strengths and elastic moduli by between 29% and 83% for the range of pressures studied. The reductions observed increase gradually up to 4.3 MPa and then reach a plateau. By accommodating the effect of effective stress on compressive strength values, it is shown that chemical weakening of high rank coals is mostly associated with sorption of subcritical CO2, with negligible impact of supercritical CO2 on further parameter reduction. Inspection of failure patterns during uniaxial compression suggests that non-saturated coal specimens fail through axial splitting with rapid crack propagation and high outburst of coal pieces while the failure of cores subjected to CO2 injection occurs through multiple fractures with negligible material outburst. The post-failure analysis demonstrates that CO2 treated samples disintegrate on smaller particles than non-saturated specimens, as up to 5.6 more CO2 saturated coal pieces passed through the sieves considered in this study than non-saturated pieces. It is claimed that this study presents novel insights into the geomechanical response of high rank anthracitic coals to high pressure CO2 injection.
Display omitted
•Two sets of anthracite coal samples are saturated with sub- and super-critical CO2.•Coals are obtained from different depths and locations in South Wales, UK.•In total, twenty samples are tested for uniaxial compression and sieve analysis.•Reduction of UCS and E between 29% and 83% is a function of CO2 saturation pressure.•CO2-saturated coals disintegrate on smaller particles than non-saturated coals.
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.
•The contaminants produced by underground coal gasification are reviewed.•An assessment of geological strata as potential adsorbents has been carried out.•The main influencing factors of the adsorption process are presented.•The impact of underground coal gasification on the adsorption process is discussed.
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.
•A novel thermal-hydraulic-chemical model is applied to replicate UCG experiments.•Temperature, syngas, and cavity predictions agree with experimental results.•UCG process is divided in three stages based on solid loss and pore structure alteration.•The study shows that cavity growth is sensitive to the kinetics of char combustion.
Display omitted
•A coupled flow-geomechanical model was given to study ground deformation due to UCG.•A field-scale UCG project with the linked vertical wells was numerically ...simulated.•Thermo-mechanical properties of strata and UCG operational conditions were studied.•Cavity with long forward length and short backward length was formed.•Ground deformation was led by combined effects of temperature and cavity evolution.
This paper presents a coupled flow-geomechanical model to study the ground deformation due to geoenergy applications involving complex thermo-hydraulic-chemical–mechanical coupled processes, such as underground coal gasification (UCG). The model is developed by coupling two sub-models, that is, the chemical reactions associated flow model that can predict temperature development and cavity growth and the geomechanical model that considers the temperature-dependent thermo-mechanical properties of geologic materials and the conversion of coal to the cavity. Numerical simulations of a field-scale UCG project with the linked vertical wells (LVW) were conducted through coupled flow-geomechanical modelling. The effects of the thermo-mechanical properties of the strata adjacent to the coal seam and the UCG operational conditions on the temperature distribution, cavity formation, and ground deformation in the operating stage of UCG were studied. Simulated results showed that temperature of over 1200 K would be obtained between the injection well and the production well in the coal seam while the 400 K isotherm only moved less than 2 m vertically in the strata near the UCG reactor. Cavity with long forward length and short backward length was formed between the injection well and the production well. Ground deformation during UCG was caused due to the combined effects of temperature and cavity evolution. The parametric sensitivity study showed that when the initial elastic modulus of the surrounding strata decreased from 4 GPa to 1 GPa, its effect on the ground deformation beyond the right side of the production well was not of significance. The ground deformation was not sensitive when the initial thermal expansion coefficient of the surrounding strata was reduced to the order of magnitude of 1.0-6 (K−1). In addition, adjusting the supply rate of O2 and modifying the location of the horizontal gasification channel can control the ground deformation during UCG. The coupled flow-geomechanical model presented in this paper can be helpful in the safety control of UCG.
The experimental analysis aimed at investigating the high-pressure (sub- and supercritical) CO2 sorption behavior on two high-rank coals of different sizes is presented in this paper. Coals from the ...same seam (9 ft seam) but from depths of 150 m black diamond (BD) coal and 550 m Aberpergwm (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 behavior of powdered (0.25–0.85, 2.36–4.0 mm) and core samples obtained from locations associated with the deformation-related changes is analyzed 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, whereas 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 the BD coal decrease by a factor of more than 9 from 0.25–0.85 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, that is, reducing by a factor of more than 100 between the 0.25–0.85 mm and core samples. The differences in behavior 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.
Display omitted
•An ex-situ experiment of coal gasification under high pressures and temperatures.•Oxygen and steam injected under different molar ratios used as gasification agents.•A comparison ...between semi-anthracite and bituminous coals for syngas production.•Increase in pressure and the amount of steam benefit methane-rich syngas production.•Gasification temperature of 750 °C is the most favourable for methane generation.
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.
PDF
