Large-scale Experimental Investigations to Evaluate the Feasibility of Producing Methane-Rich Gas (SNG) through Underground Coal Gasification Process. Effect of Coal Rank and Gasification Pressure

Krzysztof Kapusta 1,* , Marian Wiatowski 1 , Krzysztof Stańczyk 1, Renato Zagorščak 2 and Hywel Rhys Thomas 2 1 Główny Instytut Górnictwa (Central Mining Institute), 40-166 Katowice, Poland; mwiatowski@gig.eu (M.W.); kstanczyk@gig.eu (K.S.) 2 Geoenvironmental Research Centre (GRC), School of Engineering, Cardiff University, Cardiff CF24 3AA, UK; ZagorscakR@cardiff.ac.uk (R.Z.); thomashr@cardiff.ac.uk (H.R.T.) * Correspondence: kkapusta@gig.eu; Tel.: +48-32-3246535; Fax: +48-32-3246522


Introduction
Meeting the challenges of energy security and ensuring competitive energy costs is more important than ever. These two main goals are extremely important for maintaining security of energy supply in many parts of the world. Despite current trends towards a transition to renewable energy, fossil fuels and especially coal will continue to be the main sources of energy in the near future. Coal is the largest fuel in the global industrial energy mix, but there are significant regional differences. Coal is by far the main fuel used in industry in China and India [1]. Deployment of Carbon Capture and Storage (CCS) technologies could allow for making a distinction between coal use and the emissions from its combustion. CO 2 injection enhanced oil recovery (EOR) and carbon storage in shales are expected to be a promising method. The gas injection EOR has the win-win effect on CCS when carbon dioxide is applied to stimulate the oil reservoir [2]. Such technologies, along with a significant reduction in the total demand for coal, are nowadays an important feature in scenarios for the sustainable development of energy systems. Both reactions are favored by the increased pressure [29,30]. Gasification pressure depends on the seam depth, which affects the hydrostatic pressure in the coal seam, and hence the allowable range for the pressure in the underground reactor. The hydrostatic pressure increases with depth at about 0.01 MPa/m for fresh water and 0.012 MPa/m for a saturated saline aquifer [31]. Consequently, at 100 m depth the hydrostatic pressure is approximately 1 MPa, and at 1000 m depth it increases to approximately 10 MPa. Such difference in pressure has a significant effect on the permissible operating conditions, and thus on the composition and conditions of the product gas. As CH 4 formation is based on hydrogen production, the amount of water (steam) and hydrogen available to the gasification process is critical. Typically, the product gas from UCG has higher methane content than the product from the various surface gasifiers. Lower temperatures involved in some parts of the UCG cavity/channel and longer gas residence times, particularly at high pressures, and catalytic effects of post-gasification ash are expected to play an important role [32]. The presence of larger concentrations of methane would typically be an advantage in power generation or natural gas synthesis operations, but can be disadvantageous in synthesis reactions. While underground coal gasification has been developed and tested over the past 80 years at numerous locations world-wide, the bulk of this experience has been in relatively shallow coals, i.e., <200 m burial depth. In Europe the main UCG interest was mostly in deep unmineable coals (bituminous rank sometimes lying in relatively thin seams) compared with those in the former Soviet Union (USSR), the USA and Australia, which have all been in shallower deposits, of which a high proportion have been of lower rank coals (particularly of high volatile bituminous and subbituminous coals). As a result, the number of operations carried out in deep seams is very limited [3].
This article presents results of an experimental study on methane-oriented UCG. Experimental simulations of UCG with oxygen and water using large bulk samples of Welsh semi-anthracite and Polish bituminous coal were conducted in a high pressure ex situ laboratory installation. Methane efficiency, gas production rates and temperature profile distribution in the seam were monitored during the multiday gasification experiments. Gasification resulted in relatively high methane yields and the maximum CH 4 concentration (average) obtained during the whole experimental campaign was: 20.6% vol. The study revealed that not only gasification pressure, but also coal rank had a significant impact on methane formation. Therefore, the feasibility of methane-rich gas production through underground gasification of the two coals used was demonstrated.

Description of the UCG Test Stand
The schematic view of the surface installation (ex situ) used for the underground gasification experiments is presented in Figure 1.
Polish bituminous coal were conducted in a high pressure ex situ laboratory installation. Methane efficiency, gas production rates and temperature profile distribution in the seam were monitored during the multiday gasification experiments. Gasification resulted in relatively high methane yields and the maximum CH4 concentration (average) obtained during the whole experimental campaign was: 20.6%vol. The study revealed that not only gasification pressure, but also coal rank had a significant impact on methane formation. Therefore, the feasibility of methane-rich gas production through underground gasification of the two coals used was demonstrated.
The main part of the installation is a gasification reactor in which the underground geological conditions of the coal seam are reproduced. The installation enables the simulations of the underground coal gasification process on the surface (ex situ), in an artificial coal seam (maximum seam length 3.5 m, cross-section 0.41 × 0.41 m). Tests can be carried out using gasification media such as oxygen, air, steam, CO 2 and mixtures thereof. Nitrogen is used as a safety agent for inertizing and cooling down the reactor after gasification. The maximum gasification pressure is~50 atm and the temperature is 1600 • C. The raw UCG gas is washed with water to lower its temperature, remove solid particles and condense high-boiling tar components. The subsequent stages of gas treatment include the separation of aerosols. The gas produced is finally neutralized in a combustion chamber fed with natural gas. The concentrations of the main gaseous components are analyzed by means of gas chromatography Energies 2020, 13, 1334 4 of 14 (GC). The Agilent 3000A Micro GC (Agilent Technologies, Santa Clara, CA, USA) is used for these purposes. The gas product was sampled every 30 min.

Coal Samples and Preparation of the Artificial Seam
The coal samples for the UCG tests were gathered from two different locations. The first selection of coal blocks was obtained from an open cast coal mine in the South Wales Coalfield, UK. An average thickness of the coal seam was 1.2 m and the sampling location was 88 m below the ground level. This sample was marked as "Six Feet" (semi-anthracite). The second selection of blocks was obtained from the "Wesoła" mine in Upper Silesia Basin, Poland (bituminous coal). The sampling location was 950 m below the ground level from a coal seam of an average thickness of 5 m. Results of proximate and ultimate analyses for the coals under study are presented in Table 1. The raw coal samples after initial processing were used to create a continuous artificial coal seam with a total length of 3.05 m, width 0.41 m and thickness 0.41 m. The cross sections of the UCG reactor for the UCG tests and details of thermocouples are presented in Figure 2.

Experimental Campaign and Test Procedure
For each of the coal type under study, the gasification tests were conducted under two distinct pressure regimes-20 and 40 bar with the main aim to assess feasibility of the methane-rich gas production. The general process assumptions for the gasification tests are presented in Table 2. The oxidant supply rates (Nm 3 /h) over the course of the experiments are presented in Figure 3. Since the main aim of the study was to investigate the effect of coal rank and gasification pressure on CH 4 formation, the oxidant supply rates were the same in each experiment. The supply rates were established based on previous gasification tests and adapted to the given rector's geometry. Oxygen Od a , % 0.50 ± 0.05 7.65 ± 0.1 16 Specific Gravity, g/cm 3 1.35 ± 0.028 1.40 ± 0.018 The raw coal samples after initial processing were used to create a continuous artificial coal seam with a total length of 3.05 m, width 0.41 m and thickness 0.41 m. The cross sections of the UCG reactor for the UCG tests and details of thermocouples are presented in Figure 2.   vertical cross-section [30]. Reproduced from [30], Elsevier: 2019 Distribution of temperatures during the gasification process was controlled by 10 high temperature thermocouples (Pt10Rh-Pt). The left side of Figure 2b shows vertical cross-section of the reactor. It can be seen, that the thermocouples are located in the insulating layer of the reactor and do not reach the coal seam. Such location of the thermocouples is necessary to protect them from direct contact with oxidizers. The distance of the thermocouples from the bottom and roof of the artificial seam was about 2 cm.

Experimental Campaign and Test Procedure
For each of the coal type under study, the gasification tests were conducted under two distinct pressure regimes-20 and 40 bar with the main aim to assess feasibility of the methane-rich gas production. The general process assumptions for the gasification tests are presented in Table 2. The oxidant supply rates (Nm 3 /h) over the course of the experiments are presented in Figure 3. Since the main aim of the study was to investigate the effect of coal rank and gasification pressure on CH4 formation, the oxidant supply rates were the same in each experiment. The supply rates were established based on previous gasification tests and adapted to the given rector's geometry.   [30]. Reproduced from [30], Elsevier: 2019 Distribution of temperatures during the gasification process was controlled by 10 high temperature thermocouples (Pt10Rh-Pt). The left side of Figure 2b shows vertical cross-section of the reactor. It can be seen, that the thermocouples are located in the insulating layer of the reactor and do not reach the coal seam. Such location of the thermocouples is necessary to protect them from direct contact with oxidizers. The distance of the thermocouples from the bottom and roof of the artificial seam was about 2 cm. The coal seams were ignited using a pyrotechnic charge. The pyrotechnic charge was located inside the gasification channel on the bottom of the coal seam at a distance of approx. 1 m from the face of the coal seam.  The coal seams were ignited using a pyrotechnic charge. The pyrotechnic charge was located inside the gasification channel on the bottom of the coal seam at a distance of approx. 1 m from the face of the coal seam.

Calculations
The main process parameters are calculated using the following methodologies:  Gas yield The total gas yields during the particular stages of gasification are calculated by integrating the curves of the gas production rates. 2 Average gas production rate The average gas production rate was calculated by dividing the yield of gas by duration of the stage. 3 Energy in process gas The total energy in process gas at each stage of the gasification was calculated by multiplying the gas yield by the corresponding average gas calorific value. 4. Average reactor power.
This value was calculated as the ratio of energy to time at each stage of the gasification experiment. 5 Gasification rate Based on the gas composition and its yield the amount of carbon contained in the process gas was calculated. The mass of coal gasified during the particular stages was calculated based on technical and elemental analysis of the raw coal.

Calculations
The main process parameters are calculated using the following methodologies:

Gas calorific value
The gas product was sampled every 30 min. The calorific value (Q, MJ/Nm 3 ) of the gas product is calculated on the basis of the gas composition, according to the following equation:

1.
Gas yield The total gas yields during the particular stages of gasification are calculated by integrating the curves of the gas production rates.

2.
Average gas production rate The average gas production rate was calculated by dividing the yield of gas by duration of the stage.

Energy in process gas
The total energy in process gas at each stage of the gasification was calculated by multiplying the gas yield by the corresponding average gas calorific value.
This value was calculated as the ratio of energy to time at each stage of the gasification experiment.

Gasification rate
Based on the gas composition and its yield the amount of carbon contained in the process gas was calculated. The mass of coal gasified during the particular stages was calculated based on technical and elemental analysis of the raw coal.

6.
Energy efficiency The gasification efficiency was calculated by dividing the energy contained in the process gas by energy contained in the mass of gasified coal.

Gas Production Rates
The evolution of product gas over the course of the gasification experiments with "Six Feet" and "Wesoła" coals, conducted at 20 and at 40 bar are presented in Figures 4 and 5, respectively. As can be seen from the presented graphs, the values of the gas production rates were changeable in all gasification experiments, with the maximum values approximately 10 Nm 3 /h. The oscillations observed in gas production rates usually reflect changes in gasification conditions inside the coal seam, resulting from the heterogeneity of coal properties and gas flow disturbances due to enlargement of the cavity and spalling of roof material. Such phenomena are typical for the UCG process. In the UCG experiments carried out at 40 bar, a gradual increase in gas production rate was observed during the entire gasification process, and the values of the produced gas volumes were characterized by smaller fluctuations in time. This proves the positive effect of the increase in gasification pressure on the stabilization of the quantitative parameters of the gas produced during UCG. 6 Energy efficiency The gasification efficiency was calculated by dividing the energy contained in the process gas by energy contained in the mass of gasified coal.

Gas Production Rates
The evolution of product gas over the course of the gasification experiments with "Six Feet" and "Wesoła" coals, conducted at 20 and at 40 bar are presented in Figure 4 and Figure 5, respectively. As can be seen from the presented graphs, the values of the gas production rates were changeable in all gasification experiments, with the maximum values approximately 10 Nm 3 /h. The oscillations observed in gas production rates usually reflect changes in gasification conditions inside the coal seam, resulting from the heterogeneity of coal properties and gas flow disturbances due to enlargement of the cavity and spalling of roof material. Such phenomena are typical for the UCG process. In the UCG experiments carried out at 40 bar, a gradual increase in gas production rate was observed during the entire gasification process, and the values of the produced gas volumes were characterized by smaller fluctuations in time. This proves the positive effect of the increase in gasification pressure on the stabilization of the quantitative parameters of the gas produced during UCG.    Energy efficiency The gasification efficiency was calculated by dividing the energy contained in the process gas by energy contained in the mass of gasified coal.

Gas Production Rates
The evolution of product gas over the course of the gasification experiments with "Six Feet" and "Wesoła" coals, conducted at 20 and at 40 bar are presented in Figure 4 and Figure 5, respectively. As can be seen from the presented graphs, the values of the gas production rates were changeable in all gasification experiments, with the maximum values approximately 10 Nm 3 /h. The oscillations observed in gas production rates usually reflect changes in gasification conditions inside the coal seam, resulting from the heterogeneity of coal properties and gas flow disturbances due to enlargement of the cavity and spalling of roof material. Such phenomena are typical for the UCG process. In the UCG experiments carried out at 40 bar, a gradual increase in gas production rate was observed during the entire gasification process, and the values of the produced gas volumes were characterized by smaller fluctuations in time. This proves the positive effect of the increase in gasification pressure on the stabilization of the quantitative parameters of the gas produced during UCG.   The average gas production rates and gas yields per mass of coal consumed for all UCG experiments are presented in Table 3. For both coals used, the gas production rates are affected by the gasification pressure and the correlation is positive. As seen from the presented data, for the experiments with "Six feet" semi-anthracite, the gas yield expressed as the volume of gas per mass of gasified coal was not significantly dependent on gasification pressure. For "Wesoła" hard coal, the gas yield slightly decreased at higher gasification pressure. Higher gas yields from gasified coal mass during gasification of "Six Feet" semi-anthracite resulted from better gasification conditions due to higher calorific value and lower ash content in the gasified sample.

Product Gas Composition and Gas Calorific Value
Changes in the product gas composition for the gasification experiments at 20 and 40 bar are presented in Figures 6 and 7, respectively. Average gas compositions obtained in the particular gasification experiments are presented in Table 4.
Energies 2020, 11, x FOR PEER REVIEW 9 of 14 the Bouduard reaction during gasification of the semi-anthracite, which is favored at higher temperatures.
In each of the four gasification tests, the effect of water injection on the gas quality was evident. As can be seen in the graphs presented in Figure 6 and Figure 7, water injection resulted in a rapid increase in methane and hydrogen formation and a decrease in CO2 concentration (limitation of combustion reactions), irrespective of the coal rank and gasification pressure used. This resulted in a significant improvement in the calorific value of gas, which can be concluded from the graphs presented in Figure 8 and Figure 9.   the Bouduard reaction during gasification of the semi-anthracite, which is favored at higher temperatures.
In each of the four gasification tests, the effect of water injection on the gas quality was evident. As can be seen in the graphs presented in Figure 6 and Figure 7, water injection resulted in a rapid increase in methane and hydrogen formation and a decrease in CO2 concentration (limitation of combustion reactions), irrespective of the coal rank and gasification pressure used. This resulted in a significant improvement in the calorific value of gas, which can be concluded from the graphs presented in Figure 8 and Figure 9.     As can be seen from the data presented, the gas composition was significantly dependent on both the coal properties and gasification pressure. For both the 20 and 40 bar experiments, gas from "Six Feet" semi-anthracite was characterized by higher contents of highly calorific components, especially methane. As seen from the data in Table 4, the average methane concentration for "Six Feet" Energies 2020, 13, 1334 9 of 14 semi-anthracite was 15.8% vol. at 20 bar and 19.1% vol. at 40 bar. During the gasification of "Wesoła" coal, the methane concentration was 10.9% vol. and 14.8% vol. at 20 and 40 bar, respectively. The higher concentrations of CH 4 in gas produced during the gasification of "Six Feet" sample resulted in relatively higher gas calorific values, i.e., 11.7 and 12.1 MJ/Nm 3 at 20 and 40 bar respectively compared to 9.2 MJ/Nm 3 at 20 bar and 10.4 MJ/Nm 3 at 40 bar during the gasification of "Wesoła" hard coal. The graphs presented in Figures 8 and 9 show that in each of the 4 gasification experiments, the calorific value of the gas increased over time, which reflects the progress of cavity and the gradual improvement of gasification conditions. Deterioration of gas quality was observed at the final stage of the gasification process, regardless of the coal rank and gasification pressure used. This is typical during UCG according to the linear CRIP (Controlled Retracting Injection Point) technique, which is the signal to start the next UCG rector by retracting the linear position of the oxidant injector.

Temperature Profiles
The temperature distributions during the experiments in the bottom part and in the roof strata are shown in Figures 10-13. The rate of temperature increase was different between the coals.

Temperature Profiles
The temperature distributions during the experiments in the bottom part and in the roof strata are shown in Figures 10-13. The rate of temperature increase was different between the coals. Temperatures during the gasification of "Wesoła" coal increased more rapidly than during the The gasification of "Six Feet" semi-anthracite yielded significantly lower quantities of CO 2 compared to the gasification of "Wesoła" hard coal. The experiments revealed that the CO 2 content was heavily affected by the gasification pressure and a positive correlation was observed. This can be explained by the intensification of coal combustion reaction (reaction limited by oxidant diffusion) and intensification of methanation reaction leading to the formation of methane and CO 2 as the main products.
The hydrogen and carbon monoxide contents were strongly dependent on both coal properties and gasification pressure. Higher hydrogen yields for "Wesoła" coal were obtained for experiments at both pressures. A negative correlation between hydrogen concentration and gasification pressure was observed, which was caused by the consumption of hydrogen in methanation and hydrogenation reactions favored by the increased gasification pressure. The gasification of "Six Feet" sample generated much more CO than "Wesoła" coal. This may be due to the intensification of the Bouduard reaction during gasification of the semi-anthracite, which is favored at higher temperatures.
In each of the four gasification tests, the effect of water injection on the gas quality was evident. As can be seen in the graphs presented in Figures 6 and 7, water injection resulted in a rapid increase in methane and hydrogen formation and a decrease in CO 2 concentration (limitation of combustion reactions), irrespective of the coal rank and gasification pressure used. This resulted in a significant improvement in the calorific value of gas, which can be concluded from the graphs presented in Figures 8 and 9.

Temperature Profiles
The temperature distributions during the experiments in the bottom part and in the roof strata are shown in Figures 10-13. The rate of temperature increase was different between the coals. Temperatures during the gasification of "Wesoła" coal increased more rapidly than during the gasification of "Six Feet" coal which showed a more gradual increase (potentially due to differences in reactivity, as lower rank coals are more reactive). This is particularly visible for roof strata temperatures. The maximum gasification temperatures during the experiments were approximately 1200 • C and were recorded in the roof strata near the reactor inlet (oxidation zone). It should be emphasized, however, that the actual process temperatures were much higher, but due to the insulating phenomena (refractory materials and ceramic thermocouples casings used), the records had lower values. Another observation is that temperatures in the bottom strata for almost each experiment were about 100 • C lower compared to the roof strata. This confirms that UCG ash and slag produced during the process and molten roof material can effectively insulate against heat conduction to the bottom of the coal seam. The only exception was the "Wesoła" coal gasification test at 40 bar, in which the temperature differences were relatively small. No significant impact of the gasification pressure on temperature distribution was observed during the gasification tests.

Process Balance Calculations
The energy and mass balance calculations for the UCG experiments carried out are presented in Table 5. The study revealed that at the same experimental conditions, gasification of "Wesoła" coal took place at much higher coal consumption rates, i.e., 5.3 kg/h compared to 4.5 kg/h at 20 bar and 5.6 kg/h compared to 4.7 kg/h at 40 bar for "Wesoła" and "Six Feet" coal, respectively. These differences can be explained by the higher reactivity of "Wesoła" sample (lower rank coal). According to energy balance estimates, the "Six feet" coal gasification was characterized by much higher energy efficiency than gasification of the "Wesoła" sample. At 20 bar, the energy efficiency

Process Balance Calculations
The energy and mass balance calculations for the UCG experiments carried out are presented in Table 5. The study revealed that at the same experimental conditions, gasification of "Wesoła" coal Energies 2020, 13, 1334 12 of 14 took place at much higher coal consumption rates, i.e., 5.3 kg/h compared to 4.5 kg/h at 20 bar and 5.6 kg/h compared to 4.7 kg/h at 40 bar for "Wesoła" and "Six Feet" coal, respectively. These differences can be explained by the higher reactivity of "Wesoła" sample (lower rank coal). According to energy balance estimates, the "Six feet" coal gasification was characterized by much higher energy efficiency than gasification of the "Wesoła" sample. At 20 bar, the energy efficiency for "Six Feet" was 69.7% compared to 56.8% obtained for "Wesoła" experiment. For both tested coals, the energy efficiency values increased with pressure and the experiments at 40 bar resulted in 71.6% and 60.8% for "Six Feet" and "Wesoła" coals, respectively. This improvement was mainly due to the higher methane concentrations in the gas obtained during experiments at the higher gasification pressure.

Conclusions
The experiments conducted demonstrated a significant influence of coal properties, and operational pressure on the main process parameters, including gas composition, methane yields and energy efficiency, in particular: The gas production rates were changeable in all gasification experiments, with the maximum values approximately 10 Nm 3 /h. The oscillations reflected the changes in gasification conditions due to the heterogeneity of coal properties and changes in the cavity geometry. A positive impact of gasification pressure increase on the stabilization of quantitative parameters of gas was demonstrated.
The UCG gas composition was significantly dependent on both the coal properties and gasification pressure. For both the 20 and 40 bar experiments, gas from "Six Feet" semi-anthracite was characterized by higher contents of highly calorific components, especially methane. The average methane 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 concentration was 10.9% vol. and 14.8% vol. at 20 and 40 bar, respectively. The gasification of "Six Feet" semi-anthracite yielded significantly lower quantities of CO 2 compared to the gasification of "Wesoła" hard coal and the CO 2 content was heavily affected by the gasification pressure (positive correlation).
The effect of water injection on the gas quality was evident. The water injection resulted in a rapid increase in CH 4 and H 2 formation and a decrease in CO 2 concentration, independently of the coal rank and gasification pressure.
The rate of temperature increase was different between the coals. Temperatures during the gasification of "Wesoła" coal increased more rapidly than during the gasification of "Six Feet" coal which showed more gradual increase. This was potentially due to the differences in reactivity. The maximum gasification temperatures during the experiments were approximately 1200 • C and were recorded in the roof strata near the reactor inlet (oxidation zone).
At the same experimental conditions, gasification of "Wesoła" coal took place at much higher coal consumption rates, i.e., 5.3 kg/h compared to 4.5 kg/h at 20 bar and 5.6 kg/h compared to 4.7 kg/h at 40 bar for "Wesoła" and "Six Feet" coal, respectively. These differences can be explained by the higher reactivity of "Wesoła" sample (lower rank coal).
The "Six Feet" coal gasification was characterized by much higher energy efficiency than gasification of the "Wesoła" sample. At 20 bar, the energy efficiency for "Six Feet" was 69.7% compared to 56.8% obtained for "Wesoła" experiment. For both tested coals, the energy efficiency values increased with pressure and the experiments at 40 bar resulted in 71.6% and 60.8% for "Six Feet" Energies 2020, 13, 1334 13 of 14 and "Wesoła" coals, respectively. This improvement was mainly due to the higher CH 4 concentrations in the gas obtained at the higher gasification pressure.