(SNG) Through Underground Coal Gasiﬁcation Process

energies

Article 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 Sta ´nczyk 1, Renato Zagoršˇcak 2 and Hywel Rhys Thomas 2

1 Główny Instytut Górnictwa (Central Mining Institute), 40-166 Katowice, Poland; [email protected] (M.W.); [email protected] (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: [email protected]; Tel.: +48-32-3246535; Fax: +48-32-3246522

Received: 10 February 2020; Accepted: 10 March 2020; Published: 13 March 2020

Abstract: 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.

Keywords: underground coal gasiﬁcation; UCG; methanation; methane; SNG

1. 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. CO2 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.

Energies 2020, 13, 1334; doi:10.3390/en13061334 www.mdpi.com/journal/energies Energies 2020, 13, 1334 2 of 14

The increased demand for coal will eventually lead to the mining of coal seams lying deeper into the ground. Conventional underground coal mining becomes more difficult, more dangerous and more expensive as mining depth increases. Since the early 1930s, Underground Coal Gasification (UCG) has promised a revolution in the safe and economic recovery of vast reserves of otherwise unmineable coal [3–6]. In the past decades, advances in the key technologies for drilling, completion and monitoring have transformed how UCG can be undertaken at great depth. This, combined with current issues of energy security and the need to reduce the environmental footprint, has initiated a huge global resurgence of interest in UCG. UCG may provide a convenient source of energy from coal seams for which traditional coal extraction techniques are economically, technically or environmentally infeasible. Many studies have shown the potential advantages of UCG over the conventional mining methods, such as the increase in coal utilization efficiency and the improvement of economic performance with simultaneous minimization of environmental emissions [7–11]. During UCG coal is converted into gaseous products directly in the underground conditions (in situ). There are several operational techniques for the UCG, which are explained in detail elsewhere [12,13]. The UCG involves injecting gasifying media into the ignited coal seam through a surface well. The product gas mainly contains H2, CH4, CO, CO2 and can be used both as a chemical feedstock (syngas) and as fuel for power generation [14,15]. From the chemical and thermodynamic point of view the UCG process runs analogically to gasification in the surface reactors. Typical gasification media are oxygen, air and steam. The final product gas consists mainly of hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. Composition and calorific value of the product gas depend largely on the gasifying agent employed, thermodynamic conditions of the operation, coal rank and local hydrogeological conditions [16–20]. Several examples exist in literature demonstrating that under appropriate control of the process, the UCG could be oriented on the production of a specific product, such as hydrogen [21–27]. Methane, the main component of natural gas (NG), is one of the most desirable UCG products that significantly contributes to the calorific value of gas. Modern techniques for extracting natural gas from geological deposits are based on physical processes. For example, hydraulic fracturing techniques are commonly used to increase the permeability of shale gas deposits for effective gas recovery [28]. Unlike the techniques used in NG production, the UCG process involves both physical (drilling) and chemical processes. During the UCG process, methane is formed in a methanation reaction (gas phase):

CO + 3H CH +H O ∆H = 206 kJ/mol 2 → 4 2 − and through a direct hydrogenation of solid carbon (hydrogasiﬁcation reaction):

C + 2H CH ∆H = 91.0 kJ/mol 2(g) → 4(g) − 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 CH4 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. Energies 2020, 11, x FOR PEER REVIEW 3 of 14

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 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 Energies 2020, 13, 1334 3 of 14 significant impact on methane formation. Therefore, the feasibility of methane-rich gas production through underground gasification of the two coals used was demonstrated. While underground coal gasification has been developed and tested over the past 80 years at 2.numerous Materials locations and Methods 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 2.1.(bituminous Description rank of sometimes the UCG Test lying Stand in relatively thin seams) compared with those in the former Soviet UnionThe (USSR), schematic the USAview andof the Australia, surface installation which have (ex all beensitu) used in shallower for the underground deposits, of which gasification a high experimentsproportion have is presented been of lower in Figure rank 1. 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 p – pressure detector simulations of UCG with oxygen and waterT – temperature using large measurement bulk samples of Welsh semi-anthracite and Polish bituminous coal were conductedF in– flow a high rate measurement pressure ex situ laboratory installation.(8) Methane GC – gas chromatograph efficiency, gas production rates and temperature profile distribution in theGC seam were monitored during p,T,F p,T,F p,T,F p,T,F p, T the multiday gasification experiments. Gasification resulted in relatively high methane yields and p,T p,T T p,T p,T p,T the maximum CH concentration (average) obtained during the whole experimental campaign was: 4 T8 T9 T10 T11 T12 T13 T14 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 throughAirflow Coal seam underground gasification of the two coals used was demonstrated. H2O O2 Air N2 Steam p,T,F 2. Materials and Methods T1 T2 T3 T4 T5 T6 T7 Drain Drain Drain 2.1. Description of the UCG Test Stand (1) (2) (3) (4) (5) (6) (7) The schematic view of the surface installation (ex situ) used for the underground gasification experiments is presented in Figure1.

Figure 1. Scheme of the ex-situ high pressure underground coal gasification (UCG) installation: (1) gasification reagents, (2) gasification reactor (3) water scrubber, (4) air cooler, (5,6) separators, (7) thermal combustor, (8) gas treatment module prior to gas chromatography (GC) analysis [30]. Reproduced from [30], Elsevier: 2019.

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, CO2 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.

2.2. 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 Table1.

Table 1. Proximate and ultimate characteristics of coals used for the gasiﬁcation tests.

Coal Sample No. Parameter “Six Feet” Semi-Anthracite “Wesoła” Hard Coal As Received r 1 Total Moisture Wt , % 1.15 0.40 3.60 0.40 ± ± r 2 Ash At , % 4.61 0.30 8.74 40 ± ± 3 Volatiles Vr, % 9.92 0.12 27.67 0.50 ± ± r 4 Total Sulphur St , % 1.55 0.04 0.31 0.02 ± ± 5 Calorific Value Q r, kJ/kg 33,416 220 28,798 200 i ± ± Analytical 6 Moisture Wa, % 0.84 0.30 2.18 0.27 ± ± 7 Ash Aa, % 4.62 0.30 8.87 0.63 ± ± 8 Volatiles Va, % 9.95 0.13 28.08 0.92 ± ± a 9 Heat of Combustion Qs , kJ/kg 34,414 228 30,317 161 ± ± 10 Calorific Value Q a, kJ/kg 33,527 221 29,258 201 i ± ± 11 Total Sulphur Sa, % 1.55 0.04 0.31 0.08 ± ± a 12 Carbon Ct , % 87.31 0.66 75.35 1.13 ± ± a 13 Hydrogen Ht , % 3.97 0.28 4.61 0.40 ± ± 14 Nitrogen Na, % 1.29 0.12 1.20 0.22 ± ± 15 Oxygen O a, % 0.50 0.05 7.65 0.1 d ± ± 16 Specific Gravity, g/cm3 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 Figure2.

2.3. 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 Table2. The oxidant supply rates (Nm3/h) over the course of the experiments are presented in Figure3. 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. Energies 2020, 11, x FOR PEER REVIEW 4 of 14

analyzed by means of gas chromatography (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.

2.2. 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.

Table 1. Proximate and ultimate characteristics of coals used for the gasification tests.

Coal Sample No. Parameter “Six Feet” “Wesoła” Hard Coal Semi-Anthracite As Received 1 Total Moisture Wtr, % 1.15 ± 0.40 3.60 ± 0.40 2 Ash Atr, % 4.61 ± 0.30 8.74 ± 40 3 Volatiles Vr, % 9.92 ± 0.12 27.67 ± 0.50 4 Total Sulphur Str, % 1.55 ± 0.04 0.31 ± 0.02 5 Calorific Value Qir, kJ/kg 33,416 ± 220 28,798 ± 200 Analytical 6 Moisture Wa, % 0.84 ± 0.30 2.18 ± 0.27 7 Ash Aa, % 4.62 ± 0.30 8.87 ± 0.63 8 Volatiles Va, % 9.95 ± 0.13 28.08 ± 0.92 9 Heat of Combustion Qsa, kJ/kg 34,414 ± 228 30,317 ± 161 10 Calorific Value Qia, kJ/kg 33,527 ± 221 29,258 ± 201 11 Total Sulphur Sa, % 1.55 ± 0.04 0.31 ± 0.08 12 Carbon Cta, % 87.31 ± 0.66 75.35 ± 1.13 13 Hydrogen Hta, % 3.97 ± 0.28 4.61 ± 0.40 14 Nitrogen Na, % 1.29 ± 0.12 1.20 ± 0.22 15 Oxygen Oda, % 0.50 ± 0.05 7.65 ± 0.1 16 Specific Gravity, g/cm3 1.35 ± 0.028 1.40 ± 0.018

The raw coal samples after initial processing were used to create a continuous artificial coal Energiesseam with2020, a13 total, 1334 length of 3.05 m, width 0.41 m and thickness 0.41 m. The cross sections of the 5UCG of 14 reactor for the UCG tests and details of thermocouples are presented in Figure 2.

T13 T12 T11 T10 T9

3.05m

Coal seam Inlet

0.25m 0.5m 0.5m 0.5m 0.5m 0.5m 0.5m 0.25m T6 T5 T4 T3 T2

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Fire channel

Roof thermocouples (T9-T13) Bottom thermocouples (T2-T6) (b)

Figure 2. 2. CrossCross sections sections of of the the ex-situ ex-situ reactor reactor prepared prepared forUCG for UCG tests: tests:(a) side (a cross-section,) side cross-section, (b) vertical (b) verticalcross-section cross-section [30]. Reproduced [30]. Reproduced from [from30], Elsevier: [30], Elsevier: 2019 2019 Distribution Distribution of temperatures of temperatures during during the thegasiﬁcation gasification process process was was controlled controlled by 10 by high 10 temperaturehigh temperature thermocouples thermocouples (Pt10Rh–Pt). (Pt10Rh The–Pt). left The side left of sideFigure of2 Figureb shows 2bvertical shows vertical cross-section cross- ofsection the reactor. of the reactor. It can be It seen, can be that seen, the that thermocouples the thermocouples are located are locatedin the insulating in the insulating layer of the layer reactor of the and reactor do not and reach do the not coal reach seam. the Such coal location seam. Such of the location thermocouples of the theis necessaryrmocouples to protect is necessary them fromto protect direct them contact from with direct oxidizers. contact The with distance oxidizers. of the thermocouplesThe distance of from the thermocouplesthe bottom and from roof ofthe the bottom artiﬁcial and seam roof wasof the about artificial 2 cm. seam was about 2 cm.

2.3. Experimental Campaign and Test Procedure Table 2. Experimental assumptions for the methane-oriented ex situ UCG tests. For each of the coal type under study, the gasification tests were conducted under two distinct Experiment No. Parameter pressure regimes—20 and 140 bar with the 2 main aim to assess 3 feasibility of the methane 4 —rich gas production.Coal Type The generalsemi-anthracite process assumptions semi-anthracite for the gasification bituminous tests are presented bituminous in Table 2. The “Six Feet”3 deposit “Six Feet” deposit oxidant supplyOrigin rates (Nm /h) over the course of the“Wesoła” experiments coal (Upper Silesia, are Poland)presented “Wesoła” in coalFig (Upperure 3. Silesia, Since Poland) the (South Wales, UK) (South Wales, UK) main aim of the study was to investigate the effect of coal rank and gasification pressure on CH4 Gasiﬁcation Reagent O2/H2OO2/H2OO2/H2OO2/H2O formation,Gasiﬁcation Pressure, the oxidant Bar supply20 rates were 40 the same in each 20 experiment. The supply 40 rates were establishedExperiment Duration, based hon previous96 gasification 96 tests and adapted 96to the given rector’s geometry. 96

The coal seamsTable 2. were Experimental ignited usingassumptions a pyrotechnic for the methane charge.-oriented The pyrotechnic ex situ UCG chargetests. was located inside the gasiﬁcation channel on the bottom of the coalExperiment seam at aNo. distance of approx. 1 m from the Parameter face of the coal seam. 1 2 3 4 Coal Type semi-anthracite semi-anthracite bituminous bituminous “Wesoła” coal “Wesoła” coal “Six Feet” deposit “Six Feet” deposit (South Origin (Upper Silesia, (Upper Silesia, (South Wales, UK) Wales, UK) Poland) Poland) Gasification Reagent O2/H2O O2/H2O O2/H2O O2/H2O Gasification Pressure, Bar 20 40 20 40 Experiment Duration, h 96 96 96 96

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9 Oxygen Water 8

(kg/h) - water

, 5

- gases ) 3

3 2

( Gasification agent supply rate Oxygen Oxygen + water 1

0 0 10 20 30 40 50 60 70 80 90 100 Time (h)

Figure 3. Supply rates of the gasiﬁcation reagents during the UCG tests.

2.4. Calculations Figure 3. Supply rates of the gasification reagents during the UCG tests. The main process parameters are calculated using the following methodologies: The coal seams were ignited using a pyrotechnic charge. The pyrotechnic charge was located Gasinside caloriﬁc the valuegasification channel on the bottom of the coal seam at a distance of approx. 1 m from the face of the coal seam. The gas product was sampled every 30 min. The caloriﬁc value (Q, MJ/Nm3) of the gas product is calculated2.4. Calculations on the basis of the gas composition, according to the following equation:

TheQ = main(H ) process10.78 +parameters(C H ) 63.38are calculated+ (CH ) using35.895 the+ following(CO) 12.64 methodologies:+ (H S) 23.34, 2 × 2 6 × 4 × × 2 × Gas calorific value where: (H ), (C H ), (CH ), (CO), (H S) are the mole fractions of particular components, 10.78, etc. are The2 gas2 product6 was4 sampled2 every 30 min. The calorific value (Q, MJ/Nm3) of the gas product heating values of the particular components (MJ/Nm3). is calculated on the basis of the gas composition, according to the following equation: 1. Gas yield Q = (H2) × 10.78 + (C2H6) × 63.38 + (CH4) × 35.895 + (CO) × 12.64 + (H2S) × 23.34, The total gas yields during the particular stages of gasification are calculated by integrating the where: (H2), (C2H6), (CH4), (CO), (H2S) are the mole fractions of particular components, 10.78, etc. are curves of the gas production rates. heating values of the particular components (MJ/Nm3). 2. Average gas production rate 1 Gas yield The average gas production rate was calculated by dividing the yield of gas by duration of The total gas yields during the particular stages of gasification are calculated by integrating the the stage. curves of the gas production rates. 3. 2 EnergyAverage in process gas production gas rate TheThe total average energy gas in processproduction gas atrate each was stage calculated of the gasification by dividing was the calculated yield of gas by multiplyingby duration theof the gasstage. yield by the corresponding average gas calorific value. 3 Energy in process gas 4. Average reactor power. The total energy in process gas at each stage of the gasification was calculated by multiplying This value was calculated as the ratio of energy to time at each stage of the gasification experiment. the gas yield by the corresponding average gas calorific value. 5. 4. AverageGasification reactor rate power. This value was calculated as the ratio of energy to time at each stage of the gasification Based on the gas composition and its yield the amount of carbon contained in the process gas was experiment. calculated. The mass of coal gasified during the particular stages was calculated based on technical and5 elementalGasification analysis rate of the raw coal. 6. EnergyBased e ffionciency 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 technicalThe gasification and elemental efficiency analysis was calculatedof the raw bycoal. dividing the energy contained in the process gas by energy contained in the mass of gasified coal.

Energies 2020, 11, x FOR PEER REVIEW 7 of 14

Energies 2020, 11, x FOR PEER REVIEW 7 of 14 6 Energy efficiency 6 EnergyThe gasification efficiency efficiency was calculated by dividing the energy contained in the process gas by energyThe gasification contained efficiency in the mass was of calculated gasified coal. by dividing the energy contained in the process gas by energy contained in the mass of gasified coal. Energies3. Results2020, 13 ,and 1334 Discussion 7 of 14 3. Results and Discussion 3.1. Gas Production Rates 3. Results and Discussion 3.1. GasThe Production evolution Rates of product gas over the course of the gasification experiments with “Six Feet” and 3.1.“Wesoła GasThe Production evolution” coals, Ratesconducted of product at gas 20 over and theat 40 course bar are of thepresented gasificati inon Figure experiments 4 and Figure with “ 5,Six respectively. Feet” and “WesołaAs can ”be coal seens, conductedfrom the presented at 20 and graphs, at 40 bar the are values presented of the gasin Figure production 4 and rates Figure were 5, respectively. changeable in The evolution of product gas over the course of the gasification experiments3 with “Six Feet” Asall can gasification be seen from expe theriments, presented with graphs, the maximum the values values of the approximatelygas production 10rates Nm were/h. Thechangeable oscillations in andobserved “Wesoła” in coals,gas production conducted rates at 20 usually and at reflect 40 bar changes are presented in gasification in Figures conditions4 and5, respectively. inside the coal all gasification experiments, with the maximum values approximately 10 Nm3/h. The oscillations Asseam, can be resulting seen from from the presented the heterogeneity graphs, the of values coal properties of the gas production and gas flow rates disturbance were changeables due to observed in gas production rates usually reflect changes in gasification conditions3 inside the coal in allenlargement gasification of experiments, the cavity and with spalling the maximum of roof material. values approximately Such phenomena 10 Nm are/h. typical The oscillations for the UCG seam, resulting from the heterogeneity of coal properties and gas flow disturbances due to observedprocess. in In gas the production UCG experiments rates usually carried reflect out changes at 40 bar, in a gasification gradual increase conditions in gas inside production the coal rate seam, was resultingenlargement from of the the heterogeneity cavity and spalling of coal properties of roof material. and gas Such flow phenomena disturbances are due typical to enlargement for the UCG of process.observed In theduring UCG the experiments entire gasification carried outprocess, at 40 bar,and athe gradual values increase of the producedin gas production gas volumes rate waswere thecharacterized cavity and spalling by smaller of roof fluctuations material. Such in time. phenomena This proves are typical the positive for the effect UCG of process. the increase In the in UCGobserved experiments during carriedthe entire out gasification at 40 bar, a gradualprocess, increase and the in values gas production of the produced rate was gas observed volumes during were characterizedgasification pressure by smaller on the fluctuations stabilization in time.of the Thisquantitative proves theparameters positive of effect the gas of produced the increase during in theUCG. entire gasification process, and the values of the produced gas volumes were characterized by smallergasification fluctuations pressure in on time. the This stabilization proves the of positivethe quantitative effect of theparameters increase of in the gasification gas produced pressure during on theUCG. stabilization of the quantitative parameters of the gas produced during UCG. 11 11

11 11 10 10

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( 10( 10

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9 9 8 8

Gas production Gas production Oxygen Gas production Oxygen Oxygen + water Oxygen + water 8 8

Gas production Gas production 7 Oxygen Gas production 7 Oxygen 0 10 20 30 40Oxygen50 +60 water70 80 90 100 0 10 20 30 40Oxygen50 +60 water70 80 90 100 Time (h) Time (h) 7 7 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) Figure 4. Gas production(a) rates over the course of gasification experiments(b )at 20 bar: (a) “Six Feet” semi-anthracite, (b) “Wesoła” hard coal. FigureFigure 4. 4.Gas Gas production production rates rates over over the the course course of of gasiﬁcation gasification experiments experiments at at 20 20 bar: bar: (a(a)) “Six “Six Feet” Feet” semi-anthracite,semi-anthracite, (b ()b “Wesoła”) “Wesoła hard” hard coal. coal.

11 11

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Gas production Gas production 7 Oxygen Oxygen Oxygen + water Oxygen + water

Gas production Gas production 8

7 Gas production 7 Oxygen 6 Oxygen 0 10 20 30 40Oxygen50 +60 water70 80 90 100 0 10 20 30 40Oxygen50 +60 water70 80 90 100 Time (h) Time (h) 7 6 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b)

FigureFigure 5. Gas5. Gas production production(a) rates rates over over the the course course of gasificationof gasification experiments experiments(b at) 40at bar:40 bar: (a) ( “Sixa) “Si Feet”x Feet” semi-anthracite, (b) “Wesoła” hard coal. semi-anthracite,Figure 5. Gas production (b) “Wesoła” rates hard over coal. the course of gasification experiments at 40 bar: (a) “Six Feet” semi-anthracite, (b) “Wesoła” hard coal. The average gas production rates and gas yields per mass of coal consumed for all UCG experiments are presented in Table3. For both coals used, the gas production rates are a ffected 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 Energies 2020, 13, 1334 8 of 14 of “Six Feet” semi-anthracite resulted from better gasification conditions due to higher calorific value and lower ash content in the gasified sample.

Table 3. Gas production parameters for the UCG experiments conducted. Energies 2020, 11, x FOR PEER REVIEW 9 of 14 Gasification Experiment Gas Production “Six Feet” Semi-anthracite “Wesoła” Hard Coal “Six Feet” Semi-anthracite Wesoła” Hard Coal EnergiesParameter 2020, 11, x FOR PEER REVIEW 9 of 14 the Bouduard reaction20 bar during gasification20 bar of the semi-anthracite,40 bar which is 40fa barvored at higher temperatures.Average Gas the Production Bouduard Rate, reaction during9.0 gasification 9.3 of the semi-anthracite, 9.4 which is favored 9.4 at higher NmIn 3each/h of the four gasification tests, the effect of water injection on the gas quality was evident. temperatures. Gas Yield, Nm3/kg As can be seen in the graphs1.98 presented in 1.77Figure 6 and Figure 1.98 7, water injection resulted 1.70 in a rapid ofIn Coal each Consumed of the four gasification tests, the effect of water injection on the gas quality was evident. increase in methane and hydrogen formation and a decrease in CO2 concentration (limitation of As can be seen in the graphs presented in Figure 6 and Figure 7, water injection resulted in a rapid combustion reactions), irrespective of the coal rank and gasification pressure used. This resulted in a 3.2.increase Product inGas methane Composition and hydrogen and Gas Calorific formation Value and a decrease in CO2 concentration (limitation of significant improvement in the calorific value of gas, which can be concluded from the graphs combustion reactions), irrespective of the coal rank and gasification pressure used. This resulted in a presentedChanges in Figure the product 8 and gasFigure composition 9. for the gasification experiments at 20 and 40 bar are presentedsignificant in improvement Figures6 and 7 in, respectively. the calorific value Average of gas, gas compositions which can be obtained concluded in fro them particular the graphs presented in Figure 8 and Figure 9. gasification experiments70 are presented in Table4. 70 60 60 CO 50 50 2 CO 2 40 70 40 70 30 60 30 60 CO H CO 2 50 20 2 50 20 H CO CO 2 40 2 40 CH 10 4 10 30 30 CH CO H 4 5 20 5 2 20 H CO 2 2 CH 2 10 4 10 C H CH C H 1 2 6 1 4 2 6 H S 5 H S 5 2 0.5 2 0.5

Gas composition (vol.%) Gas composition

Gas composition (vol.%) Gas composition N N 2 2 2 2 0.1 C H C H 1 2 6 1 0.1 2 6 Oxygen Oxygen + water Oxygen + water H S H S Oxygen 2 0.5 2 0.5

Gas composition (vol.%) Gas composition

Gas composition (vol.%) Gas composition N N 2 0 10 20 30 40 50 60 70 80 902 100 0 10 20 30 40 50 60 70 80 90 100 0.1 0.1 Oxygen Oxygen + water Time (h) Oxygen Oxygen + water Time (h) 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time( (h)a) Time( (h)b)

Figure 6. Changes( ain) gas composition over the course of gasification experiments(b) at 20 bar: (a) “Six Feet” semi-anthracite, (b) “Wesoła” hard coal. FigureFigure 6. 6.Changes Changes in in gas gas composition composition over over the the course course of of gasiﬁcation gasification experiments experiments at at 20 20 bar: bar: ( a()a) “Six “Six Feet”Feet” semi-anthracite, semi-anthracite (,b ()b “Wesoła”) “Wesoła” hard hard coal. coal.

70 70 60 60 50 50 CO 70 2 CO 40 70 40 2 60 30 60 30 CO CO 50 50 20 20 CO2 H 40 CH 40 CO 2 4 2 CH H 10 4 30 10 2 30 CO CO 20 20 5 5 H CH 2 4 CH H 10 2 4 C H 10 2 2 C H 2 6 2 6 1 1 5 H S 5 2 N 0.5 0.5 2 N 2 2 (vol.%) Gas composition Gas composition (vol.%) Gas composition C H 2 C H H S 2 6 2 6 2 1 0.1 1 0.1 H S 0.5 2 Oxygen + water N Oxygen Oxygen + water 0.5 Oxygen 2 N 2

Gas composition (vol.%) Gas composition Gas composition (vol.%) Gas composition H S 0.01 2 0.1 0 10 20 30 40 50 60 70 80 90 100 0.1 0 10 20 30 40 50 60 70 80 90 100 Oxygen Oxygen + water Oxygen Oxygen + water Time (h) 0.01 Time (h) 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time( (h)a) Time( (h)b)

FigureFigure 7. Changes 7. Changes( ina) gas in compositiongas composition over over the coursethe course of gasification of gasification experiments experiments(b) at 40 at bar: 40 bar: (a) “Six (a) “Six Feet”Fe semi-anthracite,et” semi-anthracite (b), “Wesoła” (b) “Wesoła hard” hard coal. coal. Figure 7. Changes in gas composition over the course of gasification experiments at 40 bar: (a) “Six AsFeet” can semi beTable- seenanthracite 4. fromAverage, ( theb) gas“ dataWesoła compositions presented,” hard coal. obtained the gas during composition the four wasUCG significantlyexperiments conducted. dependent on both the coal properties and gasification pressure. For both the 20 and 40 bar experiments, gas Table 4. Average gas compositions obtainedAverage during Process the G fouras C UCGoncentra experimentstion, %vol. conducted. from “SixGasification Feet” semi-anthracite Experiment was characterized by higher contents of highly calorific components,Q, MJ/Nm3 CO2 N2 H2 CH4 CO especially methane. As seen from the dataAverage in Table 4Process, the average Gas Concentra methanetion, concentration %vol.C2 H6 H for2S “Six Feet” Gasification“Six Feet” S emiExperiment-anthracite 20 Q, MJ/Nm3 CO36.32 N2 0.4 H219.2 CH15.84 CO27.2 0.69 0.38 11.7 bar C2H6 H2S “Six Feet” Semi-anthracite 20 “Wesoła” Hard Coal 20 bar 36.346.3 0.4 0.7 19.221.6 15.810.9 27.219.5 0.690.64 0.380.37 11.79.2 “Six Feet”bar Semi -anthracite 40 41.6 0.6 14.1 19.1 23.2 1.05 0.32 12.1 “Wesoła” Hardbar Coal 20 bar 46.3 0.7 21.6 10.9 19.5 0.64 0.37 9.2 “Six Feet” Semi-anthracite 40 “Wesoła” Hard Coal 40 bar 41.646.1 0.6 0.7 14.117.7 19.114.8 23.219.3 1.050.94 0.320.51 12.110.4 bar “Wesoła” Hard Coal 40 bar 46.1 0.7 17.7 14.8 19.3 0.94 0.51 10.4

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.

Table 4. Average gas compositions obtained during the four UCG experiments conducted.

Average Process Gas Concentration, %vol. Gasiﬁcation Experiment Q, MJ/Nm3 CO2 N2 H2 CH4 CO C2H6 H2S “Six Feet” Semi-anthracite 20 bar 36.3 0.4 19.2 15.8 27.2 0.69 0.38 11.7 “Wesoła” Hard Coal 20 bar 46.3 0.7 21.6 10.9 19.5 0.64 0.37 9.2 “Six Feet” Semi-anthracite 40 bar 41.6 0.6 14.1 19.1 23.2 1.05 0.32 12.1 “Wesoła” Hard Coal 40 bar 46.1 0.7 17.7 14.8 19.3 0.94 0.51 10.4

The higher concentrations of CH4 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/Nm3 at 20 and 40 bar respectively compared to 9.2 MJ/Nm3 at 20 bar and 10.4 MJ/Nm3 at 40 bar during the gasification of “Wesoła” hard coal. The graphs presented in Figures8 and9 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 improvementEnergies 2020 of, 11 gasification, x FOR PEER REVIEW conditions. Deterioration of gas quality was observed at the final stage10 of 14 of the gasification process, regardless of the coal rank and gasification pressure used. This is typical duringEnergies UCG 2020, according 11, x FOR PEER to the REVIEW linear CRIP (Controlled Retracting Injection Point) technique, which10 of is 14 the signal to15 start the next UCG rector by retracting the linear15 position of the oxidant injector.

14 14 15 15 13 13

)

3 14 14 12 12 13

MJ/Nm 13MJ/Nm

)

(

3 11 11 12 12

MJ/Nm MJ/Nm 10 10

(

( 11 11 Oxygen 9 9

Calorific value

10 10Calorific value 8 Oxygen Oxygen + water 8 Oxygen 9 9 Oxygen + water

Calorific value 7 Calorific value 7 8 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Oxygen Oxygen + water 8 Time (h) OxygenTime + (h) water 7 7 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) Figure 8. Changes(a )in gas calorific value over the course of gasification experiments(b) at 20 bar: (a) “Six Feet” semi-anthracite, (b) “Wesoła” hard coal. FigureFigure 8. Changes8. Changes in gasin gas caloriﬁc calorific value value over over the the course course of of gasiﬁcation gasification experiments experiments at at 20 20 bar: bar: ( a(a)) “Six “Six Feet”Feet” semi-anthracite, semi-anthracite (b,) (b “Wesoła”) “Wesoła hard” hard coal. coal.

15 15

14 14 15 15 13 13

)

3 143 14 12 12 13 13

)

MJ/Nm

)

MJ/Nm

(

( 11 11 12 12

MJ/Nm

10 ( ( 10 11 11 9 9

10Calorific value 10 Calorific value Oxygen + water 8 8 9 Oxygen Oxygen + water 9 Oxygen

Calorific value 7 Calorific value 7 Oxygen + water 8 0 10 20 30 40 50 60 70 80 90 100 8 0 10 20 30 40 50 60 70 80 90 100 Oxygen Oxygen + water Oxygen Time (h) Time (h) 7 7 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) FigureFigure 9. Changes 9. Changes in(a gas) in caloriﬁcgas calorific value value over over the coursethe course of gasiﬁcation of gasification experiments experiments(b) at 40 at bar:40 bar: (a) “Six(a) “Six Feet”Fee semi-anthracite,t” semi-anthracite (b) “Wesoła”, (b) “Wesoła hard” hard coal. coal. Figure 9. Changes in gas calorific value over the course of gasification experiments at 40 bar: (a) “Six 3.3.Fee Temperaturet” semi-anthracite Profiles, ( b) “Wesoła” hard coal.

3.3. TemperatureThe temperature Profiles 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. The temperature distributions during the experiments in the bottom part and in the roof strata Temperatures during the gasification of “Wesoła” coal increased more rapidly than during the are shown in Figures 10–13. The rate of temperature increase was different between the coals. gasification of “Six Feet” coal which showed a more gradual increase (potentially due to differences Temperatures during the gasification of “Wesoła” coal increased more rapidly than during the in reactivity, as lower rank coals are more reactive). This is particularly visible for roof strata gasification of “Six Feet” coal which showed a more gradual increase (potentially due to differences temperatures. The maximum gasification temperatures during the experiments were approximately in reactivity, as lower rank coals are more reactive). This is particularly visible for roof strata 1200 °C and were recorded in the roof strata near the reactor inlet (oxidation zone). It should be temperatures. The maximum gasification temperatures during the experiments were approximately emphasized, however, that the actual process temperatures were much higher, but due to the 1200 °C and were recorded in the roof strata near the reactor inlet (oxidation zone). It should be insulating phenomena (refractory materials and ceramic thermocouples casings used), the records emphasized, however, that the actual process temperatures were much higher, but due to the had lower values. Another observation is that temperatures in the bottom strata for almost each insulating phenomena (refractory materials and ceramic thermocouples casings used), the records experiment were about 100 ⁰C lower compared to the roof strata. This confirms that UCG ash and had lower values. Another observation is that temperatures in the bottom strata for almost each slag produced during the process and molten roof material can effectively insulate against heat experiment were about 100 ⁰C lower compared to the roof strata. This confirms that UCG ash and conduction to the bottom of the coal seam. The only exception was the “Wesoła” coal gasification slag produced during the process and molten roof material can effectively insulate against heat test at 40 bar, in which the temperature differences were relatively small. No significant impact of conduction to the bottom of the coal seam. The only exception was the “Wesoła” coal gasification the gasification pressure on temperature distribution was observed during the gasification tests. 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.

Energies 2020, 13, 1334 10 of 14

The gasification of “Six Feet” semi-anthracite yielded significantly lower quantities of CO2 compared to the gasification of “Wesoła” hard coal. The experiments revealed that the CO2 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 CO2 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 Figures6 and7, 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 Figures8 and9.

3.3. 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 relativelyEnergies small. 2020 No, 11,significant x FOR PEER REVIEW impact of the gasification pressure on temperature distribution11 of 14 was observed during the gasification tests.

1300 1300 1200 1200 Bottom Roof strata 1100 1100 1000 1000

)

900 ) 900

(

800 ( 800 700 700 600 600 500 T2 500 T9 T3 T10

Temperature Temperature 400 400 T4 Temperature T11 300 T5 300 T12 200 T6 200 T13 100 100 0 0 Oxygen Oxygen + water Oxygen Oxygen + water

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h)

(a) (b)

Figure 10.FigureDistributions 10. Distributions of temperatures of temperatures during during “Six “Six Feet” Feet” semi-anthracite semi-anthracite gasification gasiﬁcation at at20 20 bar: bar: (a) (a) seam bottom,seam (bottom,b) roof ( strata.b) roof strata.

1400 1400 1300 Bottom 1300 Roof strata 1200 1200 1100 1100

)

1000 ) 1000

(

900 ( 900 800 800 700 700 600 600 T9 500 500 T10

Temperataure Temperataure 400 T2 400 T11 T3 Temperataure 300 300 T12 T4 200 200 T13 T5 100 100 Oxygen + water Oxygen Oxygen + water T6 Oxygen 0 0

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h)

(a) (b)

Figure 11. Distributions of temperatures during “Wesoła” hard coal gasification at 20 bar: (a) seam bottom, (b) roof strata.

1300 1300 1200 1200 Bottom Roof strata 1100 1100 1000 1000

900 ) 900

)

( o 800 800

( 700 700 600 600 T9 500 500 T10 400 T2 400 T11

Temperature Temperature

Temperature Temperature T3 300 300 T12 T4 T13 200 T5 200 100 T6 100 0 0 Oxygen Oxygen + water Oxygen Oxygen + water

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h)

(a) (b)

Figure 12. Distributions of temperatures during “Six Feet” semi-anthracite gasification at 40 bar: (a) seam bottom, (b) roof strata.

Energies 2020, 11, x FOR PEER REVIEW 11 of 14

1300 1300 1200 1200 Bottom Roof strata 1100 1100 1300 1300 1000 1000 1200 1200

)

900 ) 900 C Bottom Roof strata

C o 1100 1100

(

800 ( 800 1000 1000 700 700

)

900 ) 900

C o 600 600

(

800 ( 800 500 T2 500 T9 700 T3 700 T10

Temperature Temperature 400 400 600 T4 Temperature 600 T11 300 300 500 T5 T2 500 T12 T9 200 T6 T3 200 T13 T10

Temperature Temperature 400 400 100 T4 Temperature 100 T11 300 300 T5 0 T12 0 Oxygen Oxygen + water Oxygen Oxygen + water 200 T6 200 T13 100 0 10 20 30 40 50 60 70 80 90 100 100 0 10 20 30 40 50 60 70 80 90 100 0 0 Oxygen Time (h) Oxygen + water Oxygen TimeOxygen (h) + water

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) Figure 10. Distributions(a) of temperatures during “Six Feet” semi-anthracite gasification(b) at 20 bar: (a) seam bottom, (b) roof strata. Figure 10. Distributions of temperatures during “Six Feet” semi-anthracite gasification at 20 bar: (a) seam bottom, (b) roof strata. Energies 2020, 13, 1334 11 of 14

1400 1400 1300 Bottom 1300 Roof strata 1200 1200 11001400 11001400

) 1300 1300 1000 Bottom ) 1000 C Roof strata

(

1200 o 1200

900 ( 900 1100800 1100800

)

1000 ) 1000 C 700 700

(

900 ( 900 600 600 T9 500800 500800 T10

Temperataure Temperataure 400700 T2 400700 T11 T3 Temperataure 300600 300600 T12 T9 T4 200500 200500 T13 T10

Temperataure Temperataure 400 T5 T2 400 T11 100 T6 T3 Temperataure 100 Oxygen + water 300 Oxygen Oxygen + water 300 Oxygen T12 0 T4 0 200 200 T13 T5 100 0 10 20 30 40 50 60 70 80 90 100 100 0 10 20 30 40 Oxygen50 60+ water70 80 90 100 Oxygen Oxygen + water T6 Oxygen 0 Time (h) 0 Time (h)

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) Figure 11. Distributions(a) of temperatures during “Wesoła” hard coal gasification(b) at 20 bar: (a) seam bottom, (b) roof strata. FigureFigure 11. Distributions11. Distributions of temperatures of temperatures during during “Wesoła” “Wesoła hard” hard coal coal gasiﬁcation gasification at 20 at bar: 20 bar: (a) seam(a) seam bottom,bottom, (b) roof(b) roof strata. strata.

1300 1300 1200 1200 Bottom Roof strata 1100 1100 1300 1300 1000 1000 1200 1200

900 Bottom ) 900 Roof strata

)

1100 C 1100

( o 800 800 ( 1000 1000 700 700

900 ) 900

)

600 C 600

o C T9

( o 800 800 ( 500 500 T10 700 700 400 T2 400 T11 600 Temperature 600 Temperature Temperature T3 T9 300 300 T12 500 T4 500 T13 T10 200 200 400 T5 T2 400 T11 Temperature Temperature 100 Temperature Temperature 100 T6 T3 300 300 T12 0 T4 0 Oxygen Oxygen + water T13 200 Oxygen Oxygen + water T5 200 100 100 0 10 20 30 40 50 60 70 80 T690 100 0 10 20 30 40 50 60 70 80 90 100 0 0 Oxygen Time (h)Oxygen + water Oxygen TimeOxygen (h) + water 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h) (a) (b) Energies 2020, 11, x FOR PEER REVIEW 12 of 14 FigureFigure 12. Distributions12. Distributions(a) of temperaturesof temperatures during during “Six “Six Feet” Feet” semi-anthracite semi-anthracite gasiﬁcation( gasificationb) at 40at bar:40 bar: (a) (a) seamseam bottom, bottom, (b) roof(b) roof strata. strata. Figure 12. Distributions of temperatures during “Six Feet” semi-anthracite gasification at 40 bar: (a) 1300seam bottom, (b) roof strata. 1300 1200 Bottom 1200 Roof strata 1100 1100 1000 1000

)

C 900 C 900

(

( 800 800 700 700 600 600 500 500 T9 T2 T10 400 400

Temperataure Temperataure Temperataure Temperataure T3 T11 300 300 T4 T12 200 T5 200 T13 100 T6 100 Oxygen Oxygen + water Oxygen Oxygen + water 0 0

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time (h) Time (h)

(a) (b)

FigureFigure 13. Distributions13. Distributions of temperatures of temperatures during during “Wesoła” “Wesoła hard” hard coal coal gasiﬁcation gasification at 40 at bar: 40 bar: (a) seam(a) seam bottom,bottom, (b) roof(b) roof strata. strata.

3.4.3.4. Process Process Balance Balance Calculations Calculations TheThe energy energy and and mass mass balance balance calculations calculations for for the the UCG UCG experiments experiments carried carried out out are are presented presented in in TableTable5. The 5. The study study revealed revealed that that at the at samethe same experimental experimental conditions, conditions, gasiﬁcation gasification of “Wesoła” of “Wesoła coal” 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 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.

Table 5. Summary of the material and energy balance calculations for the four experiments conducted.

Total Gas Energy in Average Reactor Coal Gasification Energy Gasification Experiment Yield, Nm3 Gas, MJ Power, kW Gasified, kg Rate, kg/h Efficiency, % “Six feet” Semi-anthracite 20 bar 864 10,117.5 29.3 436.1 4.5 69.7 “Wesoła” Hard Coal 20 bar 896 8243.2 23.9 504.0 5.3 56.8 “Six feet” Semi-anthracite 40 bar 903 10,890.2 31.5 455.5 4.7 71.6 “Wesoła” Hard Coal 40 bar 903 9364.1 27.1 530.2 5.6 60.8

4. 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 Nm3/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

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.

Table 5. Summary of the material and energy balance calculations for the four experiments conducted.

Total Gas Energy in Average Reactor Coal Gasification Energy Gasification Experiment Yield, Nm3 Gas, MJ Power, kW Gasified, kg Rate, kg/h Efficiency, % “Six feet” Semi-anthracite 20 bar 864 10,117.5 29.3 436.1 4.5 69.7 “Wesoła” Hard Coal 20 bar 896 8243.2 23.9 504.0 5.3 56.8 “Six feet” Semi-anthracite 40 bar 903 10,890.2 31.5 455.5 4.7 71.6 “Wesoła” Hard Coal 40 bar 903 9364.1 27.1 530.2 5.6 60.8

4. 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 Nm3/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 CO2 compared to the gasification of “Wesoła” hard coal and the CO2 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 CH4 and H2 formation and a decrease in CO2 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 CH4 concentrations in the gas obtained at the higher gasiﬁcation pressure.

Author Contributions: K.K.: Conceptualization, Methodology, Writing—original draft, Project Administration, Funding Acquisition; M.W.: Experiments, Visualization; K.S.: Writing—review and editing; R.Z.: Resources, Writing—Review & Editing; H.R.T.: Resources, Writing—Review & Editing. All authors have read and agreed to the published version of the manuscript. Funding: This work was a part of the MEGAPlus project supported by the EU Research Fund for Coal Steel, under the Grant Agreement number 800774—MEGAPlus—RFCS-2017 and Polish Ministry of Science and Higher Education under Grant Agreement No. 3996/FBWiS/2018/2. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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