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An experimental ex-situ study of the suitability of a high moisture ortho-lignite for underground coal gasification (UCG) process
Fuel 179 (2016) 150–155
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
An experimental ex-situ study of the suitability of a high moisture ortho-lignite for underground coal gasification (UCG) process Krzysztof Kapusta ⇑, Marian Wiatowski, Krzysztof Stan´czyk Główny Instytut Górnictwa (Central Mining Institute), Plac Gwarków 1, 40-166 Katowice, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A multi-day experimental simulation
of UCG using large bulk samples of ortho-lignite was conducted. The average moisture content of the coal was 46.5 wt%, and its calorific value was 12.6 MJ/kg. The overall process energy efficiency was estimated at 59%, and the average gas calorific value was approximately 7.2 MJ/N m3. The study demonstrated that oxygenblown UCG of high moisture lignites may be a feasible option.
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 22 March 2016 Accepted 22 March 2016 Available online 30 March 2016 Keywords: Underground coal gasification UCG Lignite
a b s t r a c t An experimental simulation of underground coal gasification (UCG) using large bulk samples of ortho-lignite was conducted in an ex-situ laboratory installation. The main goal of the experiment was to evaluate the suitability of the high-moisture lignite for UCG. The average moisture content of the coal feed was 46.5 wt%, and its calorific value was 12.6 MJ/kg. Changes in the gas composition, the gas production rates, and the distribution of temperatures in the artificial coal seam were measured over the course of the experiment. During the 120 h UCG trial, gas with an average calorific value of approximately 7.2 MJ/N m3 was produced. The overall energy efficiency of the process was estimated at 59%. The study results demonstrated that exploitation of the high-moisture lignite deposits using oxygen-blown UCG may be a feasible option. Therefore, this technique may be considered as an attractive option for the extraction of low-rank Miocene ortho-lignites, large deposits of which are located in Poland and in many other countries around the world. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction A significant proportion of the world’s coal reserves are composed of low-value coals, predominantly lignites and high-ash bituminous coals. Lignite, the recoverable reserves of which are estimated at 18% of the total global coal reserves, remains a crucial contributor to the energy supply in many countries [1]. Specific physicochemical properties of these low-rank coals impose many ⇑ Corresponding author. Tel.: +48 32 3246535; fax: +48 32 3246522. E-mail address: [email protected] (K. Kapusta). http://dx.doi.org/10.1016/j.fuel.2016.03.093 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
restrictions on their handling and utilisation. Lignites usually are characterised by high moisture contents, reaching up to 60 wt%, and consequently poor calorific values [2]. Lignites are also very susceptible to spontaneous ignition. Because these properties considerably increase the risk and cost of transportation, lignites must be used in close proximity to their mining sites. Underground coal gasification (UCG) is a technique that converts coal underground (in situ) into a valuable gas product [3–7]. Because the main gas components are H2, CO, CH4, and CO2, it may be used directly for electricity and heat production or as a feedstock for chemical synthesis (syngas) [8,9]. In its
K. Kapusta et al. / Fuel 179 (2016) 150–155
simplest configuration, UCG involves preparation of two vertical boreholes into the coal seam, one for the supply of oxidants (oxygen, air and steam), and another for the gas recovery (production well), achievement of hydraulic connectivity between them (horizontal drilling) and efficient seam ignition. The gas is produced as a result of a series of primary exothermic and endothermic reactions between coal and supplied oxidants and many secondary physicochemical phenomena, including gas-phase reactions and coal pyrolysis. The UCG process has recently attracted a considerable attention as an alternative to the traditional mining methods. UCG may provide a convenient source of energy from coal seams for which traditional coal extraction techniques are economically, technically or environmentally not feasible [10]. Although UCG has been studied internationally for a long time, there are still many technological difficulties to be solved. Regarding the UCG of low-rank coals, most of the global research activities were focused on hard lignite (meta-lignite). In contrast, Polish lignite resources are limited almost exclusively to soft lignite (ortho-lignite). The feasibility of UCG of high-moisture ortho-lignites is one of the issues to be scientifically explained [11]. There are 90 well documented lignite deposits in Poland, distributed into eight regions. Their total geological resources are estimated at approximately 36,132 million tonnes. The Miocene ortholignites, due to their abundance and geological conditions, are of dominant industrial importance. These ortho-lignites are characterised by high moisture content (>50%) and relatively low calorific value and are mined exclusively using the open-cast method. Because Polish lignite deposits are usually of shallow depth, many of them do not meet the site selection criteria for UCG [12]. Polish ortho-lignites are primarily used for the electricity production (approximately 65.0 million tonnes per year). Similar lignite utilisation patterns are also characteristic for other European ligniteproducing countries, of which Slovenia, Bulgaria and Romania have recently shown the most intensive research activity in the development of UCG technology [13–15]. The issue of the suitability of high-moisture lignites for UCG is still questionable. The ex-situ simulations of UCG using large samples of Polish ortho-lignites (length of 2.5 m) revealed that the coal moisture content is one of the crucial parameters determining the gasification conditions, the quality and amounts of the products, as well as the suitability of lignite for UCG [11,16,17]. The average calorific value for the oxygen-blown laboratory UCG experiment with ortho-lignite (moisture content 53.0 wt%) was 5.2 MJ/N m3 [16]. Gasification with an oxygen-enriched air (OEA) resulted in a considerable deterioration of the gas calorific value to 4.18 MJ/N m3 for the optimum established oxygen/air ratio [11]. The high moisture content in the tested lignite resulted in a very poor thermal efficiency (20%) and low gas quality because a considerable amount of thermal energy was consumed for water evaporation. This article presents the result of an experimental study on the suitability of one of the Polish ortho-lignites for the UCG. The main differences with respect to the previous experimental work [16,17] were considerably larger dimensions of the artificial coal seam. A multi-day oxygen-blown UCG experimental simulation was conducted in a laboratory ex-situ installation designed for tests with large bulk samples. 2. Materials and methods 2.1. Description of the experimental installation Large-scale surface installation was used to experimentally simulate the underground coal gasification process in the laboratory conditions (Fig. 1). An essential part of the installation is a gasifica-
151
tion chamber, where the underground geological conditions of the coal seam are reproduced. The maximum length of the artificial coal seam is approximately 7 m. Oxygen, air and steam can be used as gasification reagents, supplied individually or as a mixture. Nitrogen is used as a safety agent for inertising and cooling down the reactor after gasification. The experimental set-up was designed to conduct UCG tests under an atmospheric pressure regime. The raw UCG-derived gas is subject to scrubbing with water to reduce its temperature, remove particulate matter and condense high boiling tar components. The subsequent gas treatment step involves separation of aerosols. The produced gas is finally burnt in a natural gas-fuelled thermal combustor. The concentrations of the main gaseous components are analysed using the gas chromatography (GC) technique. An Agilent 3000A Micro GC device is used for these purposes. The distributions of temperature fields during the experiments are recorded by thermocouples (Pt10Rh-Pt) installed directly in the various zones of the reaction chamber. The inlet and outlet gas temperatures (T) and pressures (p) are also monitored as the crucial operational parameters.
2.2. Lignite characteristics and preparation of the artificial coal seam The bulk samples of lignite for the UCG trial were obtained from the Turów deposit located in south-western Poland. The deposit is currently extracted by the Turów Mine using the open-cast method. This deposit of ortho-lignite (Miocene age) is characterised by a high moisture content and by a relatively low calorific value (Table 1). The raw lignite samples, each having dimensions of approximately 0.9 m 0.9 m 1.5 m after initial processing, were used to create a continuous artificial coal seam of the total length of 5.7 m, width of 0.8 m and thickness of 0.8 m. The gasification channel was drilled along the bottom part of the seam (Fig. 2). The dimensions of the channel were 0.1 m 0.1 m. Sand was used to fill the voids between the reactor’s walls and the coal seam and for the preparation of the roof stratum. 27 thermocouples were installed inside the reactor to record the temperature profiles during gasification (Fig. 2). The Nos. 1–6, 9–13, 15–20 thermocouples were located inside the coal seam. The Nos. 22–28 thermocouples were located inside the overburden sand stratum, and the thermocouples denoted as 14 and 21 were installed in the sand layer, behind the coal seam.
2.3. Experimental procedure The coal seam was ignited using a pyrotechnic charge, placed in the gasification channel at a distance of approximately 1 m from the face of the coal seam. The gasification process was started by adding oxygen (99.5% purity) into the ignited coal seam. An initial oxygen supply rate was 3 m3/h and was gradually increased over the course of experiment, up to the maximum value of 6 m3/h in the final phase of the gasification process. The oxygen flow rate modulations were necessary to sustain the optimal thermodynamic conditions for gasification reactions. Due to the overstoichiometric water content in the raw coal, no water addition was required. The gasification process was conducted under near-ambient pressure conditions. Concentrations of the syngas components (H2, CO, CO2, CH4, N2, O2, C2H6, and H2S) were analysed in one hour time intervals. The laboratory simulation of the UCG lasted for 120 h.
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to thermal combustor F
O2
Coal seam
p,T
H2O
p,T
Air N2
7.4 m
(4)
(1)
(2) (6) (3)
(5)
GC
Fig. 1. Schematic view of the laboratory UCG installation: (1) reagents supply system, (2) gasification chamber, (3) water scrubber, (4) gas cooler, (5) separator, and (6) filters, GC – gas chromatograph.
Table 1 Basic physical and chemical parameters of the Turów lignite. Parameter
Value
As received 1 2 3 4
Total moisture Wrt (%) Ash Art (%) Total sulphur Srt (%) Calorific value Qri (kJ/kg)
46.52 3.18 0.15 12,656
Analytical 5 6 7 8 9 10 11 12 13
Moisture Wa (%) Ash Aa (%) Volatiles Va (%) Calorific value Qai (kJ/kg) Total sulphur Sa (%) Carbon Cat (%) Hydrogen Hat (%) Nitrogen Na (%) Oxygen Oad (%)
10.17 5.34 44.90 22,920 0.26 60.69 4.60 0.57 18.61
Flow rate, m3/h
12
Ash fusion temperatures
14 15 16 17
Product gas Oxygen
14
No
8 6 4 2 0
12
24
36
48
Oxidising
Reducing
1050 1260 1420 1450
1030 1170 1220 1340
3.1. Gas production rate and gas composition As shown in Fig. 3, the value of the gas production rate changed over the course of the experiment. In the initial stage of the gasification process (up to 12 h), relatively low gas yields were obtained, which can be attributed to a low gasification temperature and very limited area of the pyrolysis zone. This stage was characterised by
1.25 m
23
16
8
9
84
96
2.25 m
24
3.25 m
17
25
4.25 m
26
18
19
10
11
12
Gasification channel
Inlet 1
2
108
120
a high CO2 content and low calorific value of the product gas (5 MJ/N m3), which suggests that the coal combustion was a dominating chemical reaction (Fig. 4). An increase in the oxygen supply rate to the value of 5 m3/h resulted in the significant improvement of thermodynamic conditions for the gasification reactions. As a consequence, both gas production rate and the content of combustible gas components significantly increased. Between 16 and 72 h, very stable gasification parameters were observed. This period was characterised by a good-quality product gas with an average calorific value of 8 MJ/N m3, although some fluctuations in the gas yield were observed (Fig. 3). The gradual deterioration of the gas quality gave rise to a further increase in the oxygen supply rate at approximately 92 h of the gasification
5.25 m
27
6.25 m
20
28
21
0.8 m
15
72
Fig. 3. Gas production and oxygen supply rates over the gasification experiment.
3. Results and discussion
22
60
Time, h
Conditions
Sintering (°C) Softening (°C) Melting (°C) Flow (fluid) (°C)
0.25 m
10
3
4
13
Outlet
Wylot 5
14
6
5.7 m
Fig. 2. Location of thermocouples during the UCG experiment (proportions are not respected).
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K. Kapusta et al. / Fuel 179 (2016) 150–155
90
12
70 60 50 40 30 20
10 3
Concentration, %vol.
80
Calorific value, MJ/Nm
CO2 H2 N2 CH4 CO
8 6 4 2
10 0 0 0
12
24
36
48
60
72
84
96
108 120
0
12
24
36
48
60
72
Time, h
Time, h
(a)
(b)
84
96
108 120
Fig. 4. Gas composition (a) and gas heating value (b) vs. time.
run, which resulted again in the improvement of process conditions, gas quality and gas yields. The gasification trial was terminated at 120 h as a consequence of the seam roof collapse and resulting uncontrollable gas flow through the simulated coal seam. The average gas composition and gas calorific value obtained for the gasification experiment are presented in Table 2. The average composition of the gaseous end product is characterised by relatively high hydrogen and methane contents, i.e., 29.88 vol.% and 5.2 vol.%, respectively. Previous experimental studies [12] showed that in-seam gasification of wet Miocene lignites results in a relatively low gas quality (calorific value of approximately 5 MJ/N m3) and in a poor overall process efficiency. The average gas calorific value of 7.2 MJ/N m3 obtained in this experiment significantly exceeds the values from the earlier UCG tests on similar lignites. However, it should be noted that such results were obtained for a relatively high oxygen rates, compared with field pilot tests, and the results may not be wholly representative of scale up performance. Another important issue is the gasification pressure, the value of which was close to atmospheric over the entire course of the experiment. The field scale UCG operations are conducted at high pressures (approximate to hydrostatic) to increase the process efficiency and to improve the gas quality by methane formation [6]. From the experimental results, it can be concluded that the crucial issue determining the UCG of high-moisture lignites may be the geometry of the underground generator. The previous UCG trials on high-moisture lignites were performed in shorter coal seams, resulting in significant losses of thermal energy for the evaporation of water. In the longer gasification channels, steam is able to condense onto cooler sections of the gasification channel. The heat of condensation is therefore recovered and made available for the gasification process.
Table 2 Average gas composition and gas calorific value (dry basis). Parameter
Value
H2, vol.% CO, vol.%. CO2, vol.%. CH4, vol.%. N2, vol.%. O2, vol.%. C2H6, vol.%. H2S, vol.%. Gas calorific value (MJ/N m3)
29.8 15.5 45.3 5.2 3.7 0.2 0.14 0.08 7.2
3.2. Material and energy balance data According to the material balance calculations, approximately 20% (947 kg) of the raw coal feed was consumed during the gasification trial, with an average consumption rate of 7.9 kg of coal/h (Table 3). Such a relatively low seam recovery rate was due to the necessity of the premature termination of the process, after 120 h. The rest of the coal feed was left as a coal char or partially dried coal, as confirmed by large quantities of water condensate produced during the experiment (690 l) as well as by the postgasification examination of the UCG cavity. The large amounts of the collected water condensate indicate a very intensive evaporation of lignite moisture prior to gasification of the carboniferous material. According to the energy balance estimations, the total gross energy efficiency of the process was 58.8% (calculated as a ratio of the energy input in coal to the energy output in gas). This relatively high value is similar to that obtained during the UCG of some hard coals [18,19]; this similarity may confirm the applicability of this technique for the extraction of high-moisture lignites.
3.3. Temperature distribution Based on the temperature data gathered from the 27 thermocouples, two-dimensional temperature profiles were plotted, representing the temperature distribution in a longitudinal crosssection of the seam at selected times of the gasification process (Fig. 5). The values of the temperature between the nodes of the measurement network (Fig. 2) were approximated by mathematical interpolation. The obtained 2D temperature profiles illustrate the propagation of gasification front and development of the UCG cavity over the course of the experiment. The maximum temperatures measured in the coal seam during the gasification were approximately 1000 °C. These maxima were observed in the Table 3 Main material and energy balance data. Parameter
Value
Estimated total coal consumption (kg) Average coal consumption rate (kg/h) Average gas production rate (N m3/h) Water condensate captured (kg) Average reactor power (kW) Gross energy efficiency (%)
947 7.9 8.1 690 13.9 58.8
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K. Kapusta et al. / Fuel 179 (2016) 150–155
Height, m
1
1,000 0C
Coal seam
0.5
900 0C
800 0C
0 0
1
2
3
Height, m
1
(a)
4
5
6
7
600 0C
Coal seam
0.5
500 0C
400 0C
0 0
1
2
3
1
Height, m
700 0C
(b)
4
5
6
7 300 0C
200 0C
Coal seam
0.5
100 0C
0
0 0C
0
1
2
4
3
5
6
7
Length of gasification chamber, m
(c)
Fig. 5. Temperature distribution in the reactor for times: (a) 20 h, (b) 70 h and (c) 110 h of the experiment.
oxidation zone, at a distance of approximately 1 m from the face of the coal seam (ignition point). Fig. 5c illustrates the situation that led to the early termination of the experiment. Disturbances in the gas flow were caused by the partial blockage of the gasification channel after the roof collapse, resulting in moving the oxidation zone towards the inlet and development of the process in a very limited space. As a consequence, the gaseous products were combusted in the oxygen-rich atmosphere, leading to a sharp deterioration of the gas calorific value. As can be concluded from the temperature plots, the gasification process was operated below the ash melting temperatures for both oxidation and reduction zones (1420 and 1220 °C, respectively). This behaviour may be advantageous for the UCG operation because no slag is formed underground, which could contribute to the lowering of hydraulic connectivity between the injection and production wells. However, the slag formation may prevent inorganic contaminants against leaching by groundwater from mineral residues in the post gasification phase.
configuration promotes the process of steam condensation underground, leading to the recovery of thermal energy and making it available for the process (heating up of the unreacted coal). Recuperation of the physical heat of gas on the surface is another option that can be considered for the improvement of the overall process performance. The study proved that the poor mechanical strength of lignite may by a limiting factor for the exploitation of lignite deposits by UCG because it may result in roof collapse and uncontrollable gas flow through the underground part. Hence, for safety and operational reasons, a reliable geomechanical analysis of the stability of overlying strata is necessary when planning UCG in such deposits.
4. Conclusions
References
The experiment performed demonstrated that exploitation of high-moisture lignites through the UCG technique may be a feasible option. Using oxygen as the gasification reagent, gas with maximum calorific value of approximately 10 MJ/N m3 and with an overall energy efficiency of approximately 59% was produced. Therefore, this option may be considered encouraging for low-rank Miocene ortho-lignites, large deposits of which are located in Poland and in many other European countries. The over-stoichiometric water content in the lignite seam eliminates the need for supplying of additional reagent water, at least at the early stages of gasification process. The excessive water, however, may result in a decrease in the gasification efficiency due to considerable heat loss for the evaporation of coal moisture. The obtained results suggest that the key issue for the improvement of process performance in lignite deposits may be the appropriate length of the gasification channels. The proper reactor
[1] World Energy Resources: 2013 Survey. World Energy Council; 2013. [2] Stracher GB, editor. Geology of coal fires: case studies from around the world. Geological Society of America; 2007. [3] Shafirovich E, Varma A. Underground coal gasification: a brief review of current status. Ind Eng Res 2009;48:7865–75. [4] Gregg DW, Edgar TF. Underground coal gasification. AIChE J 1978;24(753):781. [5] Couch GR. Underground coal gasification. London: IEA Clean Coal Centre, International Energy Agency; 2009. [6] Bhutto AW, Bazmi AA, Zahedi G. Underground coal gasification: from fundamentals to applications. Prog Energy Combust Sci 2013;39:189–214. [7] Khadse A, Qayyumi P, Mahajani S, Aghalayam P. Underground coal gasification: a new clean coal utilization technique for India. Energy 2007;32:2061–71. [8] Nakaten N, Schlüter R, Azzam R, Kempka T. Development of a techno-economic model for dynamic calculation of cost of electricity, energy demand and CO2 emissions of an integrated UCG-CCS process. Energy 2014;66:779–90. [9] Wang Z, Huang W, Zhang P, Xin L. A contrast study on different gasifying agents of underground coal gasification at Huating Coal Mine. J Coal Sci Eng (China) 2011;17:181–6. [10] Burton E, Friedmann J, Upadhye R. Best practices in underground coal gasification. Lawrence Livermore National Laboratory; 2006.
Acknowledgments The research presented in this paper was performed as a part of the project: Elaboration of coal gasification technology for a high efficiency production of fuels and electricity, supported by the Polish National Centre for Research and Development under contract no. SP/E/3/7708/10.
K. Kapusta et al. / Fuel 179 (2016) 150–155 [11] Stan´czyk K, Howaniec N, Smolin´ski A, S´wia˛drowski J, Kapusta K, Wiatowski M, et al. Gasification of lignite and hard coal with air and oxygen enriched air in a pilot scale ex situ reactor for underground gasification. Fuel 2011;90:1953–62. [12] Bielowicz B, Kasin´ski JR. The possibility of underground gasification of lignite from Polish deposits. Int J Coal Geol 2014;131:304–18. [13] Durucan S et al. TOPS: technology options for coupled underground coal gasification and CO2 capture and storage. Energy Proc 2014;63:5827–35. [14] Sheng Y et al. Interdisciplinary studies on the technical and economic feasibility of deep underground coal gasification with CO2 storage in Bulgaria. Mitig Adapt Strateg Glob Change 2015:1–33. [15] http://www.ispe.ro/wp-content/uploads/2014/09/COAL2GAS_Kick-OFF_ PRESS RELEASE.pdf [accessed on 8th January 2016].
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[16] Stan´czyk K, Smolin´ski A, Kapusta K, Wiatowski M, S´wia˛drowski J, Kotyrba A, et al. Dynamic experimental simulation of hydrogen oriented underground gasification of lignite. Fuel 2010;89:3307–14. [17] Kapusta K, Stan´czyk K. Pollution of water during underground coal gasification of hard coal and lignite. Fuel 2011;90:1927–34. [18] Wiatowski M, Stan´czyk K, S´wia˛drowski J, Kapusta K, Cybulski K, Krause E, et al. Semi-technical underground coal gasification (UCG) using the shaft method in experimental Mine ‘‘Barbara’’. Fuel 2012;99:170–9. [19] Wiatowski M, Kapusta K, S´wia˛drowski J, Cybulski K, Ludwik-Pardała M, Grabowski J, et al. Technological aspects of underground coal gasification in the experimental ‘‘Barbara” Mine. Fuel 2015;159:454–62.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
An experimental ex-situ study of the suitability of a high moisture ortho-lignite for underground coal gasification (UCG) process Krzysztof Kapusta ⇑, Marian Wiatowski, Krzysztof Stan´czyk Główny Instytut Górnictwa (Central Mining Institute), Plac Gwarków 1, 40-166 Katowice, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A multi-day experimental simulation
of UCG using large bulk samples of ortho-lignite was conducted. The average moisture content of the coal was 46.5 wt%, and its calorific value was 12.6 MJ/kg. The overall process energy efficiency was estimated at 59%, and the average gas calorific value was approximately 7.2 MJ/N m3. The study demonstrated that oxygenblown UCG of high moisture lignites may be a feasible option.
a r t i c l e
i n f o
Article history: Received 28 January 2016 Received in revised form 22 March 2016 Accepted 22 March 2016 Available online 30 March 2016 Keywords: Underground coal gasification UCG Lignite
a b s t r a c t An experimental simulation of underground coal gasification (UCG) using large bulk samples of ortho-lignite was conducted in an ex-situ laboratory installation. The main goal of the experiment was to evaluate the suitability of the high-moisture lignite for UCG. The average moisture content of the coal feed was 46.5 wt%, and its calorific value was 12.6 MJ/kg. Changes in the gas composition, the gas production rates, and the distribution of temperatures in the artificial coal seam were measured over the course of the experiment. During the 120 h UCG trial, gas with an average calorific value of approximately 7.2 MJ/N m3 was produced. The overall energy efficiency of the process was estimated at 59%. The study results demonstrated that exploitation of the high-moisture lignite deposits using oxygen-blown UCG may be a feasible option. Therefore, this technique may be considered as an attractive option for the extraction of low-rank Miocene ortho-lignites, large deposits of which are located in Poland and in many other countries around the world. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction A significant proportion of the world’s coal reserves are composed of low-value coals, predominantly lignites and high-ash bituminous coals. Lignite, the recoverable reserves of which are estimated at 18% of the total global coal reserves, remains a crucial contributor to the energy supply in many countries [1]. Specific physicochemical properties of these low-rank coals impose many ⇑ Corresponding author. Tel.: +48 32 3246535; fax: +48 32 3246522. E-mail address: [email protected] (K. Kapusta). http://dx.doi.org/10.1016/j.fuel.2016.03.093 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
restrictions on their handling and utilisation. Lignites usually are characterised by high moisture contents, reaching up to 60 wt%, and consequently poor calorific values [2]. Lignites are also very susceptible to spontaneous ignition. Because these properties considerably increase the risk and cost of transportation, lignites must be used in close proximity to their mining sites. Underground coal gasification (UCG) is a technique that converts coal underground (in situ) into a valuable gas product [3–7]. Because the main gas components are H2, CO, CH4, and CO2, it may be used directly for electricity and heat production or as a feedstock for chemical synthesis (syngas) [8,9]. In its
K. Kapusta et al. / Fuel 179 (2016) 150–155
simplest configuration, UCG involves preparation of two vertical boreholes into the coal seam, one for the supply of oxidants (oxygen, air and steam), and another for the gas recovery (production well), achievement of hydraulic connectivity between them (horizontal drilling) and efficient seam ignition. The gas is produced as a result of a series of primary exothermic and endothermic reactions between coal and supplied oxidants and many secondary physicochemical phenomena, including gas-phase reactions and coal pyrolysis. The UCG process has recently attracted a considerable attention as an alternative to the traditional mining methods. UCG may provide a convenient source of energy from coal seams for which traditional coal extraction techniques are economically, technically or environmentally not feasible [10]. Although UCG has been studied internationally for a long time, there are still many technological difficulties to be solved. Regarding the UCG of low-rank coals, most of the global research activities were focused on hard lignite (meta-lignite). In contrast, Polish lignite resources are limited almost exclusively to soft lignite (ortho-lignite). The feasibility of UCG of high-moisture ortho-lignites is one of the issues to be scientifically explained [11]. There are 90 well documented lignite deposits in Poland, distributed into eight regions. Their total geological resources are estimated at approximately 36,132 million tonnes. The Miocene ortholignites, due to their abundance and geological conditions, are of dominant industrial importance. These ortho-lignites are characterised by high moisture content (>50%) and relatively low calorific value and are mined exclusively using the open-cast method. Because Polish lignite deposits are usually of shallow depth, many of them do not meet the site selection criteria for UCG [12]. Polish ortho-lignites are primarily used for the electricity production (approximately 65.0 million tonnes per year). Similar lignite utilisation patterns are also characteristic for other European ligniteproducing countries, of which Slovenia, Bulgaria and Romania have recently shown the most intensive research activity in the development of UCG technology [13–15]. The issue of the suitability of high-moisture lignites for UCG is still questionable. The ex-situ simulations of UCG using large samples of Polish ortho-lignites (length of 2.5 m) revealed that the coal moisture content is one of the crucial parameters determining the gasification conditions, the quality and amounts of the products, as well as the suitability of lignite for UCG [11,16,17]. The average calorific value for the oxygen-blown laboratory UCG experiment with ortho-lignite (moisture content 53.0 wt%) was 5.2 MJ/N m3 [16]. Gasification with an oxygen-enriched air (OEA) resulted in a considerable deterioration of the gas calorific value to 4.18 MJ/N m3 for the optimum established oxygen/air ratio [11]. The high moisture content in the tested lignite resulted in a very poor thermal efficiency (20%) and low gas quality because a considerable amount of thermal energy was consumed for water evaporation. This article presents the result of an experimental study on the suitability of one of the Polish ortho-lignites for the UCG. The main differences with respect to the previous experimental work [16,17] were considerably larger dimensions of the artificial coal seam. A multi-day oxygen-blown UCG experimental simulation was conducted in a laboratory ex-situ installation designed for tests with large bulk samples. 2. Materials and methods 2.1. Description of the experimental installation Large-scale surface installation was used to experimentally simulate the underground coal gasification process in the laboratory conditions (Fig. 1). An essential part of the installation is a gasifica-
151
tion chamber, where the underground geological conditions of the coal seam are reproduced. The maximum length of the artificial coal seam is approximately 7 m. Oxygen, air and steam can be used as gasification reagents, supplied individually or as a mixture. Nitrogen is used as a safety agent for inertising and cooling down the reactor after gasification. The experimental set-up was designed to conduct UCG tests under an atmospheric pressure regime. The raw UCG-derived gas is subject to scrubbing with water to reduce its temperature, remove particulate matter and condense high boiling tar components. The subsequent gas treatment step involves separation of aerosols. The produced gas is finally burnt in a natural gas-fuelled thermal combustor. The concentrations of the main gaseous components are analysed using the gas chromatography (GC) technique. An Agilent 3000A Micro GC device is used for these purposes. The distributions of temperature fields during the experiments are recorded by thermocouples (Pt10Rh-Pt) installed directly in the various zones of the reaction chamber. The inlet and outlet gas temperatures (T) and pressures (p) are also monitored as the crucial operational parameters.
2.2. Lignite characteristics and preparation of the artificial coal seam The bulk samples of lignite for the UCG trial were obtained from the Turów deposit located in south-western Poland. The deposit is currently extracted by the Turów Mine using the open-cast method. This deposit of ortho-lignite (Miocene age) is characterised by a high moisture content and by a relatively low calorific value (Table 1). The raw lignite samples, each having dimensions of approximately 0.9 m 0.9 m 1.5 m after initial processing, were used to create a continuous artificial coal seam of the total length of 5.7 m, width of 0.8 m and thickness of 0.8 m. The gasification channel was drilled along the bottom part of the seam (Fig. 2). The dimensions of the channel were 0.1 m 0.1 m. Sand was used to fill the voids between the reactor’s walls and the coal seam and for the preparation of the roof stratum. 27 thermocouples were installed inside the reactor to record the temperature profiles during gasification (Fig. 2). The Nos. 1–6, 9–13, 15–20 thermocouples were located inside the coal seam. The Nos. 22–28 thermocouples were located inside the overburden sand stratum, and the thermocouples denoted as 14 and 21 were installed in the sand layer, behind the coal seam.
2.3. Experimental procedure The coal seam was ignited using a pyrotechnic charge, placed in the gasification channel at a distance of approximately 1 m from the face of the coal seam. The gasification process was started by adding oxygen (99.5% purity) into the ignited coal seam. An initial oxygen supply rate was 3 m3/h and was gradually increased over the course of experiment, up to the maximum value of 6 m3/h in the final phase of the gasification process. The oxygen flow rate modulations were necessary to sustain the optimal thermodynamic conditions for gasification reactions. Due to the overstoichiometric water content in the raw coal, no water addition was required. The gasification process was conducted under near-ambient pressure conditions. Concentrations of the syngas components (H2, CO, CO2, CH4, N2, O2, C2H6, and H2S) were analysed in one hour time intervals. The laboratory simulation of the UCG lasted for 120 h.
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to thermal combustor F
O2
Coal seam
p,T
H2O
p,T
Air N2
7.4 m
(4)
(1)
(2) (6) (3)
(5)
GC
Fig. 1. Schematic view of the laboratory UCG installation: (1) reagents supply system, (2) gasification chamber, (3) water scrubber, (4) gas cooler, (5) separator, and (6) filters, GC – gas chromatograph.
Table 1 Basic physical and chemical parameters of the Turów lignite. Parameter
Value
As received 1 2 3 4
Total moisture Wrt (%) Ash Art (%) Total sulphur Srt (%) Calorific value Qri (kJ/kg)
46.52 3.18 0.15 12,656
Analytical 5 6 7 8 9 10 11 12 13
Moisture Wa (%) Ash Aa (%) Volatiles Va (%) Calorific value Qai (kJ/kg) Total sulphur Sa (%) Carbon Cat (%) Hydrogen Hat (%) Nitrogen Na (%) Oxygen Oad (%)
10.17 5.34 44.90 22,920 0.26 60.69 4.60 0.57 18.61
Flow rate, m3/h
12
Ash fusion temperatures
14 15 16 17
Product gas Oxygen
14
No
8 6 4 2 0
12
24
36
48
Oxidising
Reducing
1050 1260 1420 1450
1030 1170 1220 1340
3.1. Gas production rate and gas composition As shown in Fig. 3, the value of the gas production rate changed over the course of the experiment. In the initial stage of the gasification process (up to 12 h), relatively low gas yields were obtained, which can be attributed to a low gasification temperature and very limited area of the pyrolysis zone. This stage was characterised by
1.25 m
23
16
8
9
84
96
2.25 m
24
3.25 m
17
25
4.25 m
26
18
19
10
11
12
Gasification channel
Inlet 1
2
108
120
a high CO2 content and low calorific value of the product gas (5 MJ/N m3), which suggests that the coal combustion was a dominating chemical reaction (Fig. 4). An increase in the oxygen supply rate to the value of 5 m3/h resulted in the significant improvement of thermodynamic conditions for the gasification reactions. As a consequence, both gas production rate and the content of combustible gas components significantly increased. Between 16 and 72 h, very stable gasification parameters were observed. This period was characterised by a good-quality product gas with an average calorific value of 8 MJ/N m3, although some fluctuations in the gas yield were observed (Fig. 3). The gradual deterioration of the gas quality gave rise to a further increase in the oxygen supply rate at approximately 92 h of the gasification
5.25 m
27
6.25 m
20
28
21
0.8 m
15
72
Fig. 3. Gas production and oxygen supply rates over the gasification experiment.
3. Results and discussion
22
60
Time, h
Conditions
Sintering (°C) Softening (°C) Melting (°C) Flow (fluid) (°C)
0.25 m
10
3
4
13
Outlet
Wylot 5
14
6
5.7 m
Fig. 2. Location of thermocouples during the UCG experiment (proportions are not respected).
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90
12
70 60 50 40 30 20
10 3
Concentration, %vol.
80
Calorific value, MJ/Nm
CO2 H2 N2 CH4 CO
8 6 4 2
10 0 0 0
12
24
36
48
60
72
84
96
108 120
0
12
24
36
48
60
72
Time, h
Time, h
(a)
(b)
84
96
108 120
Fig. 4. Gas composition (a) and gas heating value (b) vs. time.
run, which resulted again in the improvement of process conditions, gas quality and gas yields. The gasification trial was terminated at 120 h as a consequence of the seam roof collapse and resulting uncontrollable gas flow through the simulated coal seam. The average gas composition and gas calorific value obtained for the gasification experiment are presented in Table 2. The average composition of the gaseous end product is characterised by relatively high hydrogen and methane contents, i.e., 29.88 vol.% and 5.2 vol.%, respectively. Previous experimental studies [12] showed that in-seam gasification of wet Miocene lignites results in a relatively low gas quality (calorific value of approximately 5 MJ/N m3) and in a poor overall process efficiency. The average gas calorific value of 7.2 MJ/N m3 obtained in this experiment significantly exceeds the values from the earlier UCG tests on similar lignites. However, it should be noted that such results were obtained for a relatively high oxygen rates, compared with field pilot tests, and the results may not be wholly representative of scale up performance. Another important issue is the gasification pressure, the value of which was close to atmospheric over the entire course of the experiment. The field scale UCG operations are conducted at high pressures (approximate to hydrostatic) to increase the process efficiency and to improve the gas quality by methane formation [6]. From the experimental results, it can be concluded that the crucial issue determining the UCG of high-moisture lignites may be the geometry of the underground generator. The previous UCG trials on high-moisture lignites were performed in shorter coal seams, resulting in significant losses of thermal energy for the evaporation of water. In the longer gasification channels, steam is able to condense onto cooler sections of the gasification channel. The heat of condensation is therefore recovered and made available for the gasification process.
Table 2 Average gas composition and gas calorific value (dry basis). Parameter
Value
H2, vol.% CO, vol.%. CO2, vol.%. CH4, vol.%. N2, vol.%. O2, vol.%. C2H6, vol.%. H2S, vol.%. Gas calorific value (MJ/N m3)
29.8 15.5 45.3 5.2 3.7 0.2 0.14 0.08 7.2
3.2. Material and energy balance data According to the material balance calculations, approximately 20% (947 kg) of the raw coal feed was consumed during the gasification trial, with an average consumption rate of 7.9 kg of coal/h (Table 3). Such a relatively low seam recovery rate was due to the necessity of the premature termination of the process, after 120 h. The rest of the coal feed was left as a coal char or partially dried coal, as confirmed by large quantities of water condensate produced during the experiment (690 l) as well as by the postgasification examination of the UCG cavity. The large amounts of the collected water condensate indicate a very intensive evaporation of lignite moisture prior to gasification of the carboniferous material. According to the energy balance estimations, the total gross energy efficiency of the process was 58.8% (calculated as a ratio of the energy input in coal to the energy output in gas). This relatively high value is similar to that obtained during the UCG of some hard coals [18,19]; this similarity may confirm the applicability of this technique for the extraction of high-moisture lignites.
3.3. Temperature distribution Based on the temperature data gathered from the 27 thermocouples, two-dimensional temperature profiles were plotted, representing the temperature distribution in a longitudinal crosssection of the seam at selected times of the gasification process (Fig. 5). The values of the temperature between the nodes of the measurement network (Fig. 2) were approximated by mathematical interpolation. The obtained 2D temperature profiles illustrate the propagation of gasification front and development of the UCG cavity over the course of the experiment. The maximum temperatures measured in the coal seam during the gasification were approximately 1000 °C. These maxima were observed in the Table 3 Main material and energy balance data. Parameter
Value
Estimated total coal consumption (kg) Average coal consumption rate (kg/h) Average gas production rate (N m3/h) Water condensate captured (kg) Average reactor power (kW) Gross energy efficiency (%)
947 7.9 8.1 690 13.9 58.8
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Height, m
1
1,000 0C
Coal seam
0.5
900 0C
800 0C
0 0
1
2
3
Height, m
1
(a)
4
5
6
7
600 0C
Coal seam
0.5
500 0C
400 0C
0 0
1
2
3
1
Height, m
700 0C
(b)
4
5
6
7 300 0C
200 0C
Coal seam
0.5
100 0C
0
0 0C
0
1
2
4
3
5
6
7
Length of gasification chamber, m
(c)
Fig. 5. Temperature distribution in the reactor for times: (a) 20 h, (b) 70 h and (c) 110 h of the experiment.
oxidation zone, at a distance of approximately 1 m from the face of the coal seam (ignition point). Fig. 5c illustrates the situation that led to the early termination of the experiment. Disturbances in the gas flow were caused by the partial blockage of the gasification channel after the roof collapse, resulting in moving the oxidation zone towards the inlet and development of the process in a very limited space. As a consequence, the gaseous products were combusted in the oxygen-rich atmosphere, leading to a sharp deterioration of the gas calorific value. As can be concluded from the temperature plots, the gasification process was operated below the ash melting temperatures for both oxidation and reduction zones (1420 and 1220 °C, respectively). This behaviour may be advantageous for the UCG operation because no slag is formed underground, which could contribute to the lowering of hydraulic connectivity between the injection and production wells. However, the slag formation may prevent inorganic contaminants against leaching by groundwater from mineral residues in the post gasification phase.
configuration promotes the process of steam condensation underground, leading to the recovery of thermal energy and making it available for the process (heating up of the unreacted coal). Recuperation of the physical heat of gas on the surface is another option that can be considered for the improvement of the overall process performance. The study proved that the poor mechanical strength of lignite may by a limiting factor for the exploitation of lignite deposits by UCG because it may result in roof collapse and uncontrollable gas flow through the underground part. Hence, for safety and operational reasons, a reliable geomechanical analysis of the stability of overlying strata is necessary when planning UCG in such deposits.
4. Conclusions
References
The experiment performed demonstrated that exploitation of high-moisture lignites through the UCG technique may be a feasible option. Using oxygen as the gasification reagent, gas with maximum calorific value of approximately 10 MJ/N m3 and with an overall energy efficiency of approximately 59% was produced. Therefore, this option may be considered encouraging for low-rank Miocene ortho-lignites, large deposits of which are located in Poland and in many other European countries. The over-stoichiometric water content in the lignite seam eliminates the need for supplying of additional reagent water, at least at the early stages of gasification process. The excessive water, however, may result in a decrease in the gasification efficiency due to considerable heat loss for the evaporation of coal moisture. The obtained results suggest that the key issue for the improvement of process performance in lignite deposits may be the appropriate length of the gasification channels. The proper reactor
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Acknowledgments The research presented in this paper was performed as a part of the project: Elaboration of coal gasification technology for a high efficiency production of fuels and electricity, supported by the Polish National Centre for Research and Development under contract no. SP/E/3/7708/10.
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