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Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification
Journal of CO₂ Utilization 21 (2017) 177–190
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Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification Geeta Kumari, Prabu Vairakannu
MARK
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Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam 781039, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Underground coal gasification (UCG) CO2 gasification Thermo gravimetric studies Syngas
Oxidizing/gasifying agents play an essential role in the economy of underground coal gasification (UCG). The selection of feed gases to UCG depends on the prevailing conditions and inherent properties of a coal seam. Steam based UCG operation would lead to transportation and operational difficulty for efficient gasification. Alternatively, CO2 gas is a potential gasifying medium for coal gasification and this option would ensure the reutilization of waste CO2 gas in a UCG operation. Thus, in the present study, borehole coal combustion and gasification experiments are carried out to simulate CO2 enhanced UCG system using a low ash coal originating from the Northeast region of India. UCG experiments are simulated in a laboratory scale using CO2/O2 gases and the results are compared with pure oxygen and oxygen enriched air based UCG operation. The composition and calorific value of syngas are analysed for various gasifying agents. The CO2/O2 borehole experimental studies show that a CO enriched product gas (∼40 vol.%) can be generated under dried coal seam conditions. Thermogravimetric analysis (TGA) studies show that the rate of pyrolysis is higher under a CO2 atmosphere as compared to N2 atmosphere. Further, the TGA results show that the generated CO2 pyrolysis products led to substantial char gasification even at low temperatures. The reactivity of tar with CO2 enhanced the calorific value of the product gas due to dry reforming reactions. Also, the presence of suitable inorganic species in the coal progressed the CO2 gasification through ash catalysis.
1. Introduction There is renewed interest in the studies of underground coal gasification (UCG) due to energy scarcity problems and depletion of conventional fuel resources. In order to exploit deep coal resources economically, numerous research studies on UCG are ongoing for commercialization of this technology. UCG technology has high potential to generate clean energy through carbon capture and storage (CCS) [1,2]. The selection of oxidizing/gasifying medium for a UCG process depends on the prevailing conditions, inherent properties and surrounding strata of the coal seam and the syngas usage. Also, various oxidizing mediums such as air, O2, O2 enriched air, steam/O2, O2/CO2 are the potential options for feed gas in an in-situ gasification process. Air as an oxidizing medium eliminates the operating cost of air separation unit (ASU). However, the use of air may reduce the prevailing temperature of a UCG process and it would result in inefficient gasification. Particularly, in high ash coal seams, air gasification produces a less calorific fuel gas [3] and the combustion flame front in these coal seams may not be sustained in air atmospheric conditions. Furthermore, in high pressure UCG operations, the compression cost of air is
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Corresponding author. E-mail address: [email protected] (P. Vairakannu).
http://dx.doi.org/10.1016/j.jcou.2017.06.021 Received 22 May 2017; Received in revised form 12 June 2017; Accepted 28 June 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
expensive and it might be equivalent to the cost of oxygen separation from air. Therefore, air gasification may be suitable for a shallow UCG process. Oxygen gasification is found to be suitable for a high ash coal seam and it is more advantageous in terms of CCS [3]. Oxygen fed UCG operation may assure the production of high calorific product gas with sustained combustion operation in high ash coal seams. However, the energy penalty of an air separation unit (ASU) for O2 gasification may be high. Oxygen enriched air as a gasifying medium may reduce the operating cost of ASU and further, ensures the flame stability in UCG process. However, the generated flue gas contains a significant amount of nitrogen, which is to be separated for implementation of CCS. Superheated steam/O2 is a suitable gasifying medium. However, it creates operational difficulty during UCG process. The generation of superheated steam (∼400 to 600 °C) is an energy intensive process and a considerable heat loss may occur at injection feed wells of deep underground coal seams. A proper insulation of these injection pipelines is required for efficient gasification. In high ash coal seams, ash heap in cavity may reduce the temperature of feed steam and it would result in inefficient gasification. CO2/O2 is one of the potential options for the gasifying medium of
Journal of CO₂ Utilization 21 (2017) 177–190
G. Kumari, P. Vairakannu
UCG operation. CO2/O2 feed gas may avoid transportation problems of superheated steam to deep coal seams. However, a high concentration of CO2 feed gas may extinguish the flame front in a UCG cavity and thus, an optimal concentration of CO2/O2 should be maintained in a UCG process. The use of CO2 as a gasifying medium for UCG reduces greenhouse gas emission. Coal based power plants emit CO2 gas to atmosphere, which causes global warming. A lot of research is undergoing for the capture and storage of carbon gases in geological reservoirs, deep sea and coal seams. Alternatively, CO2 gas can be reutilized as a gasification agent for the conversion of solid fuel into calorific gases. Deep coal seams can be exploited through UCG technology using O2/CO2 as gasifying medium. However, there is a need for the production of pure CO2 gas to supply continuously into UCG feed well. This would be feasible if UCG power plants are integrated with carbon capture technologies such as pre combustion, post combustion and oxy-fuel combustion. In oxy-fuel combustion technology, UCG syngas is burnt with O2 gas and as a consequence, pure CO2 gas can be obtained as a flue gas stream. Alternatively, pure CO2 can also be produced using advanced technologies such as fuel cell system and chemical looping combustion system. Fuel cell system inherently separates oxygen from air through semipermeable membranes and the resultant flue gas would be a mixture CO2 and H2O [4,5]. In case of chemical looping combustion, metal oxide reaction separates oxygen from air. Further, the resultant metal oxides react with syngas and produce pure CO2 gas. This gas can be recycled for the supply of sustained feed gas to UCG operation. Thus, CO2 fed UCG needs carbon capture system for the sustained production of CO2 stream for the supply of feed gas to injection well. In our earlier studies [6], CO2 recycling was proposed for UCG process and net thermal efficiency of the proposed CO2 fed UCG power plants with CCS is estimated. The results have shown that the CO2 based UCG power plant achieved a high net thermal efficiency as compared to steam based UCG power plants. Several reports are available for CO2 fed coal gasification studies. Irfan et al. [7] reported a brief review on coal gasification studies, which was carried out under CO2 atmosphere. They explained the effect of pressure, temperature, coal rank, catalysts and feed gas composition on coal gasification. It is concluded that CO2 as a gasifying medium would increase the thermal efficiency of a boiler. A numerous thermo gravimetric analyser (TGA) studies on CO2 gasification is reported in literature. Mandapati et al. [8] carried out CO2 gasification of Indian coal chars with different inherent properties using a TGA. Three types of reaction models are used to study the reaction kinetics of coals and reported that the random pore model is best suited in the modelling studies. Zhang et al. [9] carried out CO2 gasification using a bituminous coal (Wyodak, Powder River basin) in a fixed bed reactor, which was operated at atmospheric pressure conditions in the presence of Fe2CO3 catalyst. They discussed the effect of iron catalyst on CO2 gasification using a TGA and concluded that a less activation energy (58.3 kJ/mol) is estimated for the CO2 gasification in the presence of iron catalyst as compared to the non-catalytic conditions (92.7 kJ/mol). Naidu et al. [10] carried out CO2 gasification kinetic studies on a low rank coal and biomass using a TGA in both isothermal and non-isothermal conditions. They estimated activation energy using different kinetic models and found that the reactivity of CO2 with coal is enhanced in the presence of calcium as a catalyst. Jayaraman et al. [11] carried out a series of experiments in steam and CO2 gasification of high ash Indian coals at different heating rates (40, 100, 500, 800 and 1000 K/min) using a TGA. They concluded that smaller particles have higher CO2 gasification rate and char formed at higher heating rate has better gasification efficiency. They found the activation energy in the range of 138–193 kJ/mol and 122–177 kJ/mol for the CO2 and steam gasification, respectively [11]. Urych et al. [12] studied the kinetics of coal pyrolysis under different heating rates to simulate UCG process and concluded that the kinetic parameters such as activation energy, Arrhenius constant are increased with increase in the heating rate. Silbermann et al. [13] carried out experimental kinetic studies on CO2
gasification using a TGA with an average heating rate of 200 K/min using deep mined coals, which are at a depth in the range of 700 to 800 m in Western Canadian Sedimentary Basin. They found that the coal ash contents and its suitable composition enhanced the char reactivity. Lahijani et al. [14] discussed the importance of Boudouard reaction, which converts CO2 gas into a calorific valuable CO gas. They performed CO2 gasification experiments using biomass chars in a TGA and studied catalytic activity of alkali, alkaline earth and transition metal salts on the char-CO2 reactivity. Their result shows the catalytic activities of alkali for carbon conversion in the order of Na > Ca > Fe > K > Mg. The existing literature on TGA experiments reveals that the activation energy is higher for CO2 gasification and it can be reduced with the addition of suitable catalysts or through inherent ash catalysis process. Stanczyk et al. [15] carried out several ex-situ UCG experiments using oxygen, air and oxygen enriched air in a pilot plant scale level. They carried out a 50 and 30 h long duration UCG experiments using lignite and hard coals in ex-situ conditions, respectively. The gasification of lignite and hard coal produces a product gas with a calorific value of 4.8 MJ/m3 at 4:2 M ratio of O2/air and 5.74 MJ/m3 at 2:3 M ratio of O2/air, respectively. They concluded that air as a gasifying medium resulted in a low quality syngas as compared to oxygen and oxygen enriched air gasification. Stanczyk et al. [16] carried out ex-situ UCG experiments using a hard coal with oxygen and steam as the gasifying medium for hydrogen production. They obtained hydrogen gas with an average composition of 15.28% and 53.77% for oxygen and steam gasification, respectively. Wiatowski et al. [17] performed ex-situ UCG experiments using a hard coal at 0.5 MPa with oxygen and oxygen enriched air. They concluded that a high pressure UCG operation leads to the production of methane and carbon monoxide at high concentration using oxygen as the gasifying medium. Kapusta et al. [18] carried out UCG experiments using a high moisture lignite coal. They concluded that the high moisture lignite coal seam can be exploited through the in-situ gasification process using oxygen as the gasifying medium. These studies show the feasibility of utilization of steam/O2, air, O2 and O2 enriched air as the UCG feed gas streams. However, a detailed study on CO2 fed UCG is essential in order to explore the interaction of CO2 gas under volatile matter enriched coal (fresh coal) and tar deficient coal seam conditions. Several literature are available in the context of CO2 gasification of solid fuel for conventional fixed bed and fluidized bed gasifier technologies. But, very few studies are available for the CO2 gasification in the context of UCG. Falshtynskyi et al. [19] discussed various parameters of borehole gasification for a thin coal seam field in Solenovsk for UCG technology. The product gas composition is predicted through energy and material balance calculations for various combinations of oxidizing and gasifying mediums. They estimated a syngas composition of 52.9% CO and 11% H2 gas for a feed mixture of CO2, O2 and N2. Duan et al. [20] proposed a method of CO2 recycling in UCG process. They proposed a two stage method for the optimization of UCG process parameters in CO2 recycling conditions and concluded that the feed CO2 gas reduces the reaction temperature drastically as compared to steam. Mocek et al. [21] carried out a pilot scale UCG experiment in a duration of 60 days in Wieczorek mine, Poland using oxygen, air and CO2 as the gasifying mediums. They concluded that the operating pressure is a major parameter for the efficient gasification of the coal insitu conditions. It is reported that the percentage of calorific value gases such as H2, CO, CH4, N2 and CO2 is 8%, 15%, 2% and 5%, respectively for air and CO2 as gasifying mediums. Konstantinou et al. [22] carried out UCG experiments using bituminous coals with operating pressures in the range of 1 to 3 MPa using a mixture of steam and CO2. They concluded that an increase in the operating pressure of the UCG resulted in high gasification rate and efficiency. It is found that the feed gases of H2O/CO2 at a mass ratio of 2:1 produces the syngas composition of 17.4% CO, 3% CH4, and 42.1% H2. The existing literature lacks a detailed study on the CO2 gasification for the UCG technology. 178
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condition of the coal blocks prevents gas leakage in the borehole cavity. The gap between upper and lower blocks are completely joined using a sealing agent (M-seal) and further moulded with clay sand before starting the combustion experiments. Therefore, gas leakages between the blocks is avoided during the combustion. However, the minimal leakage during combustion is arrested by using china clay at appropriate locations. As the gas leakage is arrested immediately, the composition of the product gas would not get affected. Fig. 2 shows the schematic diagram of the laboratory scale experimental set-up, which simulates the UCG conditions. The inlet hole of the coal block is connected to a 5 mm stainless steel pipe, which carries the feed gas from a gas cylinder. CO2 and O2 gases are sent to the borehole using a TJunction, where both the gases get mixed. The mixed gas is sent to the borehole through the stainless steel pipe. The gas flow can be controlled using a rotameter with a proper adjustment of the flow rate. The temperature inside the linking channel is measured using a set of thermocouples, which are fixed at various locations of the horizontal pathway. The thermocouples of K-type are kept at four locations of the borehole channel to measure the temperature of the reaction zones. The thermocouples are inserted into the borehole reaction zone through the injection and production wells. The reaction zone temperatures are measured and recorded using an Agilent data logger. The UCG experiments with different oxidizing/gasifying mediums such as O2/CO2, pure oxygen, oxygen enriched air are conducted to analyse the progress of reactions and chemical composition of the product gas. The coal block is ignited using a small piece of camphor near the injection point, which is 5 cm away from the inlet hole of the coal block. The established fire front at the injection point propagates towards the outlet hole. The production hole of the coal block is connected to a gas sample collector. Tar vapours in the product gas get condensed as it comes out of the borehole. It forms a semisolid content and is collected in a beaker at the outlet pipe. The product gas samples are collected at every 15 min time interval using vacutainer tubes. The composition of these gas samples is further analysed using a gas chromatograph (GC). The product gas consisting of CO, H2, CO2, CH4, C2H6 and other lighter hydrocarbons along with un-burnt O2, N2 etc. is analysed. A thermal conductivity detector (TCD) is used for the analysis of gas samples using helium as a carrier gas in the GC. A packed column (carbo-sieve S-II) is used in the GC for the product gas analysis. The GC is operated with the carrier gas at a flow rate of 16 ml/min with an oven temperature of ramping between 40 and 250 °C. Further, a post gasification cavity analysis is carried out using the coal samples, which are collected at various locations of the burnt coal block. These samples are characterized using a Fourier transform infrared spectroscopy (FTIR) and also by proximate analysis.
Table 1a Proximate analysis of the coal Content
Weight, (%)
Moisture Volatile matter Ash Fixed carbon
7 30 9 54
Therefore, in the present study, UCG borehole combustion experiments are performed with CO2/O2 as gasifying agents using a less ash coal and the results are compared with O2 enriched air and pure oxygen based gasification systems. The gas composition and calorific values of the product gas are measured and analysed. The experimental results show the feasibility of CO2/O2 UCG gasification under fresh coal as well as dry coal seam conditions. The pyrolysis and CO2 gasification of the less ash coal are also studied using a TGA. 2. Thermo-gravimetric analysis (TGA) and borehole gasification studies A 9% ash content coal from Bapung coal mines, Assam is used in the present study. Atmospheric pressure TGA studies under a nitrogen and CO2 gas atmosphere are performed. Proximate of the coal samples are carried out as per Indian standard IS: 1350 (Part 1)-1984. The ultimate analysis is performed using CHONS analyzer. The results are shown in Tables 1a and 1b . 2.1. TGA experimental procedure In the present study, isothermal and non-isothermal TGA studies are performed. In the non-isothermal study, a 10 to 14 mg of coal sample is taken in an alumina crucible and is heated to 1000 °C for a period of 100 min at a heating rate of 10 °C/min. TGA experiments are carried out under CO2 and N2 atmosphere. In the isothermal condition, the coal samples are heated initially at a heating rate of 33 °C/min. As the temperature reached 1000 °C in 30 min, it is maintained at the isothermal conditions for an hour. 2.2. UCG borehole gasification Fig. 1 shows the laboratory scale UCG experimental set up. The UCG experiments are carried out as per the methodology of the earlier studies [3]. The coal blocks of required size are sliced using wood files and cutters. A 6 mm semi cylindrical curved horizontal pathway (borehole channel) is created on the bottom surface of the coal block (Fig. 1a). The horizontal pathway simulates a linkage channel between injection and production wells of a real UCG seam. The sliced coal blocks in an enclosed position (Fig. 1b) are kept on a platform of refractory bricks, which withstand a high reaction temperature. Fig. 1c shows a set of upper and lower coal blocks in a clay moulded condition. This simulated UCG experimental setup is kept in a tightly packed wooden box (Fig. 1d). Further, a considerable quantity of clay and sand is fed into the top of the coal blocks and this loaded condition simulates the effect of overburden stress on the coal block. Also, the tightly packed
3. Result and discussion 3.1. TGA studies During the borehole combustion experiments, it is noted that a high quantity of tar content is obtained along with the product gas. Through the outlet pipe of the borehole setup, the tar vapours of the product gas get condensed and, are collected as a semisolid substance. The collected tar at the outlet of the borehole is analysed using the TGA. The CO2 reactivity of tar is evaluated and the results are compared with the reactivity of tar under N2 atmosphere. The mass loss during the nonisothermal TGA gasification of the raw coal samples under N2 and CO2 atmosphere is shown in Fig. 3. The reduction in the weight of the coal sample is observed due to the liberation of inherent moisture (due to drying), volatile matter (due to pyrolysis) and fixed carbon (due to gasification). The coal sample mass loss per degree Celsius is estimated and is shown in Fig. 3a. It can be seen that thermal degradation starts at 125 °C, which is an indication of the removal of moisture. The pyrolysis reaction occurs at the temperatures in the range of 400 to 600 °C. One can observe that the rate of pyrolysis is rapid under CO2 atmosphere. It
Table 1b Ultimate analysis of the coal (dry ash free basis) Content
Weight%
Carbon Hydrogen Nitrogen Sulfur Oxygen
74.7 5.7 1.3 3.7 14.6
179
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G. Kumari, P. Vairakannu
Fig. 1. UCG laboratory scale experimental set-up (a) sliced coal block, (b) upper and lower block in enclosed position, (c) coal block in moulded condition and (d) moulded coal block with overburden in enclosed position.
Fig. 2. Schematic diagram of the UCG experimental set-up.
min. Hence, the TGA study reveals that the CO2 pyrolysis and gasification of coal occur at 400 °C and 850 °C, respectively. Fig. 4 shows the TGA curve of coal reactivity at 1000 °C under the CO2 atmosphere. Initially, the coal sample is heated at a rate 33 °C/min for the first 30 min. When the temperature reached 1000 °C, the isothermal condition is maintained for the next 60 min. A linear drop in the mass of the coal sample is observed. The solid residue of 10.3% (point ‘G’) is obtained and it contains purely of ash content. It is estimated that a mass reduction occurs at a rate of 0.505%/min at 1000 °C due to Boudouard reaction. In order to examine the reactivity of tar with CO2, the TGA studies are carried out under the inert and CO2 atmosphere. During the borehole combustion experiments, the high temperature product gas in the combustion zone losses its thermal energy in the adjacent coal zones of the borehole pathway. This heat liberates the volatile matter and tar content (high hydrocarbons), which are carried along with the syngas. At the outlet hole, the tar vapours are get condensed. The obtained semisolid tar content is used to analyse its pyrolysis behaviour under the CO2 atmosphere in the TGA studies. Fig. 5a shows the TGA curves of tar content reactivity under both N2 and CO2 atmosphere. It is shown that a significant mass loss occurs up to a temperature of 337 °C under both N2 and CO2 atmosphere. It is found that a 70% of mass is evaporated as moisture and light gases in both the cases. The remnant 30% (point ‘H’) of mass may contain medium and higher hydrocarbons, which begin to react under the CO2 atmosphere with further increase in the temperature. In Fig. 5b, the difference between the retained mass of
is estimated that a 0.3 percent of mass loss per °C occurs under N2 atmosphere whereas it is 0.5 under CO2 atmosphere. It is also noticed (Fig. 3b) that a residual mass difference between the N2 and CO2 atmosphere is estimated as 8% (difference in the retained mass between the points ‘A’ and ‘B’) at 400 °C and it is significantly increased to 20% (difference in the retained mass between the points ‘C’ and ‘D’) at 600 °C. It is evident that the tar radicals reacted with CO2 and converted into gaseous molecules. However, under N2 inert atmosphere, the tar vapours can crack and form a low molecular gas only at a high temperature. It is interesting to note that the mass retained at 600 °C under CO2 atmosphere is only about 50.64% (point ‘D’), which indicates a loss of 12.36% fixed carbon content as compared to the initial weight of the original coal sample. The final mass residue under N2 atmosphere is weighed as 62% (point ‘E’), which is purely a mixture of ash (9%) and fixed carbon (54%). Therefore, the moisture (7%) and volatile matter (30%) content get completely vaporized under N2 atmosphere. In CO2 atmosphere, an estimate of 36.37% (point ‘F’) of solid residue is observed and this contains 27.37% of fixed carbon and 9% of ash. Hence, approximately a 26.63% of fixed carbon is converted into gaseous matters, which may be due to Boudouard reaction (Eq. (1)) and methanation reaction.
C+ CO2 → 2CO
(1)
It can be seen from Fig. 3a that a significant mass reduction of fixed carbon occurs at the temperature range between 850 °C–1000 °C and a 50% conversion of fixed carbon is estimated for a heating rate of 10 K/ 180
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Fig. 3. TGA analysis carried out of coal in N2 and CO2 atmosphere under non-isothermal conditions (a) mass loss/ °C and (b) mass retained%.
Fig. 4. TGA analysis of the coal carried out under CO2 atmosphere in isothermal conditions between 30 and 100 min.
remnant final mass at 850 °C is calculated as 8.8% (point ‘M’). A reduction of 6.04% (difference in mass reduction between points ‘M’ and ‘N’) mass is observed in the temperature interval between 850 °C to 1000 °C due to the CO2 gasification of higher hydrocarbons and aromatics. In total, it is estimated that a 16% of tar content (point ‘O’ in Fig. 5b) gets converted into gaseous matter due to the reactivity of CO2. Energy-dispersive X-ray spectroscopy (EDX) analysis of the ash samples is performed in order to identify the chemical composition of ash. The coal ash may decrease the reactivity due to the deposition of ash on char surface and resists the diffusion of gas molecules. However, certain minerals in coal act as a catalyst for the reaction of CO2 with char. EDX analysis (Table 2) shows the presence of significant amount of inorganic species such as Ca, Fe, Mg, K, which may act as catalysts for the CO2 gasification. The existing literature supports the catalyst
the coal samples under N2 and CO2 atmosphere is plotted against the operating temperature. At a temperature interval of 300 to 472 °C, a significant mass reduction difference of 9% (difference in mass reduction between the points ‘I’ and ‘J’) is observed between the TGA results, which are carried out under the CO2 and N2 atmosphere. This confirms the reactivity of low molecular hydrocarbon tar with CO2 gas. One can also notice that the difference in the mass reduction of the coal samples under the N2 and CO2 atmosphere is identical between 500 and 850 °C (point between ‘J’ and ‘K’), which shows that the mass loss is only due to the thermal cracking of higher hydrocarbons. The retained final mass under N2 atmosphere is estimated as 16.1% (point ‘L’ in Fig. 4a). This shows that an approximately 14% (difference in mass reduction between points ‘L’ and ‘H’) of the tar content is vaporized at the temperature interval of 400 to 1000 °C. In the CO2 atmosphere, the 181
Journal of CO₂ Utilization 21 (2017) 177–190
G. Kumari, P. Vairakannu
Fig. 5. (a) TGA analysis of the tar carried out under CO2 and N2 atmosphere, (b) difference in mass reduction of tar between N2 and CO2 atmosphere under non isothermal conditions.
role of these ash species on the CO2 gasification. Ca2+ ions plays a catalyst role on the CO2 gasification [20]. Also the presence of calcium and magnesium in the ash content increases the coal reactivity [21]. During the CO2 gasification, a fraction of H+ ions, which is present in the carboxylic acid groups, gets exchanged with metal cations such as K+, Na+, Ca2+, Mg2+ and Fe3+. The presence of these metal cations increases the CO2-char reactivity [23,24]. Also, it is reported that the gasification reactivity is increased in the presence of iron catalyst as compared to catalyst free basis. Si present in the form of Si-O increases the reactivity of coal under the CO2 atmosphere in the temperature range of 700 to 800 °C [25,26]. Therefore, the presence of substantial amount of these inorganic species of ash content may promoted the insitu coal CO2 gasification.
Table 2 Energy dispersive X ray spectroscopy (EDX) analysis of the coal ash. Elements
Wt, %
O Si Al Fe Ca S Ti Mg K Total
50.5 18.6 17.6 5.8 3.1 1.8 1.7 0.6 0.3 100
3.2. Oxygen enriched air gasification based borehole combustion Table 3 shows the details of experiments and the summary of Table 3 Details of the conducted experiments and summary of the results. Serial No.
Fuel
Gasifying medium
Time Duration (min)
Maximum H2 (%)
Average H2 (%)
Maximum CO (%)
Average CO (%)
Maximum CH4 (%)
Average CH4 (%)
Maximum calorific value (kJ/mol)
Average calorific value (kJ/mol)
1
Coal
220
6
4
6
4.5
6.9
4.7
112
106
2 3
Coal Coal (Fresh) Coal (Dry)
Oxygen enriched air Oxygen CO2/O2
180 210
30 26
17.52 21.6
11 41
5.76 28.69
27 16
22.12 9.68
298 285
243.72 212.55
CO2/O2
210
26
20.34
39
30.85
9
8.03
234
202.20
4
182
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Fig. 6. Temperature profile of the borehole during O2 enriched air gasification.
obtained results during the borehole gasification. The first experiment is carried out using air as a cheaper oxidizing medium. During the air gasification based borehole combustion experiment, an unsustained combustion flame front is observed. Thus, an oxygen enriched air gasification experiment is carried out and it ensured the sustainability of the combustion front in the borehole cavity. However, a maximum temperature of 800 °C is observed in the reaction zone and this may not be sufficient for the efficient gasification. Fig. 6 shows the temperature profile of the oxygen enriched air gasification. A 3.5-h experimental run is carried out and air is used as a gasifying medium in the first 90 min of the borehole combustion. It is observed that a maximum temperature of 600 °C is noted, which is not favourable for the syngas production. Thus, oxygen gas is supplied along with air and the reaction temperature is raised to 800 °C. One can see that the sustained combustion front exists at the first thermocouple region and it does not even reach the second thermocouple zone, which shows a less temperature of 300 °C during the entire run of the experiment. Air is supplied at 0.2 litre per minute (lpm) initially and oxygen gas is introduced at an equal flow rate of air after 90 min of the air gasification. The O2/air molar ratio of 1 is maintained for the first 10 min in order to enhance the combustion rate. Then, the molar ratio is gradually decreased to 0.28 in order to estimate the optimum conditions of gas flow rate for the efficient gasification. Fig. 7 shows the composition and calorific value of the product gas, which is obtained during the oxygen enriched air gasification. With increase in the flow rate of air, the combustion rate is enhanced and resulted in a higher concentration of CO2 gas at the time interval between 120 and 180 min (Fig. 7b). An increase in the concentration of calorific value gases such as H2, CO, CH4 (Fig. 7a) in the product gas is observed at the O2/air molar ratio of 0.33. The calorific value of the product gas at this molar ratio is estimated as 115 kJ/mol (Fig. 7c). A 2 to 18% of unutilized oxygen is observed (Fig. 7b) in the product gas due to bypassing of the feed gas from the combustion zone. A reverse trend gas composition between CO2 and N2 gas is observed with an increase in the O2/air molar ratio. It is found that an estimate of CO2, O2 and N2 concentration of the product gas in the range of 43–50%, 5–8% and 30–35%, respectively, is observed. Each of the calorific value gas is found in the range of 6–7% in the product gas at this feed molar ratio of O2/air (0.33). With the reduction in the O2/air molar ratio to 0.28, a drop in CO2 gas to 28% and a rise in O2 and N2 to 13% and 45%, respectively in the product gas due to inefficient combustion. Therefore, the O2/air molar ratio of 0.33 is found as an optimum ratio for this coal. Although the oxygen input to the feed stream increases the calorific value of the product gas, it does not show a significant rise in the heating value and only an average calorific value of 90 kJ/mol (Fig. 7c) is estimated at the optimum feed
ratio. 3.3. Pure oxygen gasification based borehole combustion A pure oxygen based UCG gasification experiment is carried out using a coal block of length 30 cm for a duration of 3 h. The temperature and concentration profiles of the experiment are shown in Figs. 8 and 9, respectively. Four thermocouples are kept at an equal distance intervals of 8 cm apart in the borehole. The first thermocouple at 5 cm from the entrance of the inlet hole shows a temperature of 1000 °C for the first 40 min and gradually decreases to 700 °C. This is due to the movement of combustion front away from the first thermocouple region in the axial direction of the borehole with an increase in the flow rate of oxygen from 0.2 lpm to 0.3 lpm. This can be also observed in a sudden rise of temperature simultaneously in the second thermocouple at the 60th min. Further, the combustion front progressed towards the third thermocouple region (which is 21 cm from the inlet hole) with an increase in the flow rate of O2 to 0.5 lpm at the 90th minute. However, the combustion front did not reach the fourth thermocouple region, which shows a less temperature in the range of 200 to 400 °C at the end of 3 h. It can be seen from the temperature profile that a well sustained fire front is established with a lengthy combustion zone of about 16 cm (between the first three thermocouples). A 25–30% of methane gas is evolved during the UCG experiment and it increased the calorific value of the product gas (Fig. 9a). The liberation of volatile matter and cracking of tar may be the major sources for methane generation. A significant amount of liquid tar is obtained along with the product gas, which is collected as a separate stream. Hydrogen gas is generated as a result of pyrolysis and steam (inherent moisture) char gasification reaction. It can be seen that the production of hydrogen gas is not stable (6 to 29%), which may be due to the movement of the combustion front to different locations in the borehole. This leads to the variation in the rate of volatile matter removal with increase in the flow rate of oxidant. A 2–10% of CO gas is generated due to pyrolysis and incomplete combustion of the char. The rise in CO2 concentration shows the enhanced combustion (Fig. 9b) at higher O2 flow rate. A high calorific gas of 298 kJ/mol is obtained at an O2 flow rate of 0.4 lpm. The pure oxygen based UCG produces an average calorific value of syngas 200 kJ/mol (Fig. 9c), which is higher than the calorific value of O2 enriched air gasification syngas. A significant amount of unreacted O2 about 10–25% is observed in the product gas. This shows the bypass of oxygen flow inside the cavity. Daggupati et al. [27] and Stanczyk et al. [16] reported a significant amount of unreacted O2 with 28% and 7%, respectively, during the pure oxygen gasification experiments. This may be due to insufficient 183
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Fig. 7. Concentration profile of (a) combustible gas and (b) Non-combustible gas of the product gas, and (c) calorific value of the product gas and molar ratio of O2/air during the O2 enriched air gasification.
small, a high flow rate of feed gas resulted in a less residence time, which led to the release of unreacted O2.
surface area on the borehole of the cavity for the reaction. Also, the atmospheric pressure UCG conditions may not utilize the excess oxygen in the gas phase combustion reactions. Another factor for the unspent O2 may be due to a less residence time of reactants in the cavity. As the length of the coal block in the laboratory scale experimental set-up is
Fig. 8. Temperature profile of the borehole during pure O2 gasification.
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Fig. 9. Concentration profile of (a) combustible gas and (b) Non-combustible gas of the product gas and, (c) calorific value of the product gas and flow rate during the O2 during pure oxygen gasification.
Fig. 10. Composite flow rate of the feed gases at various molar ratios during CO2/O2 borehole gasification of dry coal (pure O2 burnt coal) and fresh coal block under identical flow parameter conditions.
condition is maintained in both the experiments. The flow rate of CO2 and O2 gas is maintained in such a way that each molar ratio of CO2/O2 is kept constant for a certain duration of time and further increased in order to explore the effect of feed gas composition on the product gas. The molar ratio of CO2/O2 is varied both increasing and decreasing order in the range of 0.4 to 0.67 in order to evaluate the optimum feed ratio. Initially, the feed gas ratio (CO2/O2) is maintained at 0.4 for 45 min and increased to 0.6, which is maintained for 60 min and further it is reduced to 0.4. Fig. 11a and 11b show the temperature profile of fresh and dry coal CO2 gasification, respectively. One can notice the existence of high temperature flame front (> 1000 °C) during the entire run of the fresh coal combustion experiment due to the presence of high volatile matter. On the other hand, a high temperature profile is attained in the tar
3.4. CO2/O2 gasification based borehole combustion Two CO2/O2 gasification experiments are carried out at various feed ratios of O2/CO2 in order to find out the optimum ratio for the efficient gasification. As the tar content is produced in a significant quantity during the pure O2 combustion experiment, it would be interesting to explore its effect on the CO2 gasification conditions. Thus, the O2eCO2 gasification experiments are carried out in a fresh coal block as well as in the O2 burnt coal block (tar depleted condition) of the previous experiment (pure O2 experiment) under identical operating conditions. Each experiment is conducted for a period of 4 h. It is found difficult to reignite the tar deficient O2 burnt coal block due to the lack of volatile matter. Fig. 10 shows the composite flow rate conditions of feed gases such as O2 and CO2 at various molar ratios. A similar feed gas flow rate 185
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Fig. 11. Temperature profile of (a) fresh coal, and (b) dry coal block during CO2/O2 gasification.
3.4.1. Fresh coal CO2/O2 gasification During the first 45 min of the fresh coal CO2 gasification at a molar ratio (CO2/O2) of 0.4, it is seen that the CO/H2 ratio of the product gas is gradually increased to 1.8. The product gas (Fig. 12a) containing a higher proportion of CO (∼40%) than H2 (∼25%), shows the progress of CO2-tar reforming reaction [29]. In the initial stages of the experiment, the tar enriched fresh coal containing high molecular weight hydrocarbons and aromatics resulted in a high concentration of CO as per the reactions (2–4). The dry reforming of hydrocarbons with CO2 is an endothermic reaction and the simultaneous progress of char-oxygen burning reaction in the cavity provides heat for the dry reforming gasification reaction. With increase in the feed CO2/O2 molar ratio to 0.6, the CO/H2 ratio of syngas decreases. It is found that the concentration of CO and H2 in the product gas gradually decreases (∼20% of each) with an increase in the molar feed rate of CO2 and O2. With further reduction in the CO2/O2 molar ratio to 0.4, an increase in the volume of CO (∼28%) and H2 (∼25%) is observed and the ratio of CO/H2 of the product gas is estimated approximately as one. Therefore, the optimum molar ratio of CO2/O2 for the efficient CO2 dry reforming is estimated as 0.4.
deficient dry coal gasification, only after 90 min of the combustion period. In the fresh coal block experiment, the O2 gasification is carried out for the first 30 min at a flow rate of 0.5 lpm in order to raise the temperature of borehole zones. Fig. 12a and 12b show the composition of calorific and non-calorific product gases, respectively for the fresh coal CO2/O2 gasification. Fig. 13a and 13b show the composition of calorific and non-calorific product gases, respectively for the dry coal CO2/O2 gasification. Figs.12c and 13c show the calorific value of the product gas during the CO2/O2 gasification under the fresh coal and the tar deficient coal conditions, respectively. The syngas containing CO and H2 is generated due to the dry reforming of tar with CO2 as per the following reaction [28]. The liberated methane and higher hydrocarbon gas undergo the dry reforming reaction as follows, (2)
CH 4 + CO2 → 2CO + 2H2
Cn H2n + 2 + nCO2 → 2nCO +
2n + 2 H2 2
(3)
In similar to hydrocarbons, the aromatics, which is present in the tar content, may also undergo the dry reforming reaction as follows [28].
C7 H8 + 7CO2 → 14CO + 4H2
(4)
3.4.2. Dry and volatile deficient coal CO2/O2 gasification In the dry coal gasification, it is observed a high CO/H2 ratio (∼3) at the beginning stage of the CO2 gasification due to the progress of Boudouard reaction under dry and tar deficient conditions. With an increase in the CO2/O2 feed ratio to 0.67, the CO/H2 ratio of syngas decreases gradually to unity. This might be due to the high flow rate of feed gas, which leads to a less residence time for the gas reactants in the cavity. Therefore, the heterogeneous reactions between the solid (char) and gas (CO2/O2) reactants is inefficient and therefore, a less production of CO gas is observed. A sustained production of hydrogen gas is observed due to the dry reforming of residual methane of coal with CO2. However, after 165 min of the experiment, the COeH2 ratio increases to 2 due to the decline in the production of hydrogen gas
However, the syngas produced during the dry reforming may undergo methanation reaction as follows (Eq. (5)),
CO + 3H2 → CH 4 + H2 O
(5)
The produced CO gas may undergo the water gas shift reaction and would produce H2 as follows (Eq. (6)),
CO + H2 O→ H2 + CO2
(6)
Fig. 14 shows the comparison of CO/H2 ratio of the produced syngas for the fresh and dry coal CO2 gasification. The trend of CO2/O2 gasification in the fresh coal and dry coal is explained in the subsequent section. 186
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Fig. 12. (a) Concentration profile of (a) combustible gas (b) Non-combustible gases of the product gas, (c) calorific value of the product gas during the CO2/O2 fresh coal block borehole gasification.
dissembled and the heat affected zones of the coal block is analysed. Fig. 15 shows the photograph of the burnt coal block. The cavity region can be seen with a carbon deposited thermocouple. One can also notice the tar deposition near the cavity zone. Four coal samples (zone #1 to zone #4 as shown in Fig. 15) at different heat affected zones of the block are collected and analysed. Table 4 shows the proximate analysis of these heat affected zones of the burnt coal. In the inlet zone (before cavity region), a 5% reduction of volatile matter and a complete removal of moisture are estimated due to solid heat conduction of coal from the combustion zone towards the direction of inlet hole. The analysis of ash, which is deposited on the cavity region, shows 84% ash and 13% of fixed carbon. It reveals the possibility of coal spalling (shredding of coal particles from the upper zone into the combustion cavity) during the UCG process. The analysis of coal char in the cavity region (zone #1) between the first and third thermocouple region shows a complete removal of volatile matter and moisture. A high percentage of fixed carbon (∼80%) is estimated due to the depletion of volatile matter during the combustion in the cavity region. Fourier transform infrared spectroscopy (FTIR) analyses are carried out to determine the functional groups of the coal samples. Fig. 16 shows the FTIR spectra of the various heat affected coal samples. The FTIR of raw coal shows the presence of functional group such as eOH, eCH (aliphatic), eC]C, eCeO, eS]O and eCH (aromatic). The strong broad absorption band appeared in the range of 3600–3000 cm−1 signifies the stretching vibration of eOH groups. One can notice a significant reduction of this band with respect to temperature magnitude of the hot zones. The zone #1 coal has insignificant
(∼10% as shown in Fig. 13a). It may be due to the depletion of methane gas (∼3%) in the coal block, which reduces the hydrogen production. On the other hand, one can see that the presence of high methane content (∼10%) in the syngas during the fresh coal CO2 gasification resulted in the continuous production of hydrogen due to the dry reforming reaction. During the entire run of the experiment, a significant amount of tar is produced. An average volume of 50 ml tar is obtained as a condensed product. In summary, the gas concentration and calorific value of the product gas is estimated for each experiment (Table 3). A maximum calorific value (∼298 kJ/mol) of the product gas is obtained during the pure O2 gasification and an equivalent calorific value is gained in the CO2/O2 gasification using a fresh coal block (Figs. 12c and 13c). In the tar deficient conditions, a high calorific value of 234 kJ/mol is obtained during the CO2/O2 gasification. An average of 20% and 29% of H2 and CO, respectively, in the product gas is estimated during the CO2/O2 gasification. The high percentage of CO in the product gas is due to the dry reforming reaction of CO2 with methane and high hydrocarbon gases. It can be seen that a higher percentage (average 22.12%) of methane gas gets evolved during the pure O2 gasification where it is noted as 8.5% during the CO2/O2 gasification. This is due to the conversion of methane into syngas in the dry reforming reaction (Table 3).
3.5. Post-gasification studies A post gasification study is carried out in the burnt coal block of dry coal CO2 gasification experiment. The experimental set-up is 187
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Fig. 13. Concentration profile of (a) combustible gas (b) non-combustible gas of the product gas, and (c) calorific value of the product gas during the CO2/O2 dry coal block borehole gasification.
−OH group as compared to other zones, which confirms the disruption and conversion of −OH bond into hydrogen and oxygen containing compounds. The absorption band length 3000 − 2800 cm−1 denotes the weak stretching vibrations of eCH bond of aliphatic hydrocarbons, which appears almost negligible in the zone #1 coal as compared to other zone coal samples. The weak band stretching vibration are observed at the band length 1300–1100 cm−1 shows the presence of eCeO bond, which is insignificant in the zone #1 coal as compared to the rest of three zones. FTIR and proximate analyses studies show a complete removal of volatile matter in the cavity region and therefore, the prevailed conditions enhanced the rate of Boudouard reaction, which resulted in a 40% of CO production. The analysis of coal sample (zone #2) between the cavity and the adjacent zone, which is 3 cm length in the axial direction towards the outlet hole, shows a similar
composition as that of coal char in the cavity region. Also, the FTIR analysis of the zone #2 coal sample shows a slightly higher quantity of eOH group as compared to the zone #1 sample. This seems that the zone #2 is a highly heat affected region, which is near the cavity zone. The aromatics and higher hydrocarbon present in this region might undergoes the dry reforming reaction and produces the syngas effectively. The FTIR spectra also shows the presence of aromatic compounds at a band length 900–700 cm−1 with weak stretching vibration. The functional groups are depleted proportionately with increase in the prevailing hot conditions of the borehole cavity zones. The strong absorption band appeared in the range of 1800–1500 cm−1 signifies the stretching vibrations of eC]O, which may be the compounds of aryl esters and carboxylic acid. It appeared with sharp intensity in the zone Fig. 14. CO/H2 ratio of the syngas during the CO2/O2 gasification under the fresh coal and dry coal conditions.
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Fig. 15. Photograph of the post CO2/O2 gasification cavity analysis [Zone #1: combustion cavity, Zone #2: cavity end region, Zone #3: Adjacent zone to cavity, Zone #4: Near outlet zone] [TC1, TC2, TC3, TC4 are thermocouple positions along borehole pathway].
Table 4 Post-combustion cavity analysis of the burnt coal block (obtained after O2/CO2 feed gasification under tar deficient conditions). Sample
Moisture (%)
Ash content (%)
Volatile matter (%)
Fixed carbon (%)
Original coal Fallen/spalled coal At oxygen inlet (just before cavity) Cavity region (zone #1) Cavity end region (zone #2) Adjacent zone to cavity in axial direction (zone #3) Near outlet zone (zone #4)
7 0 0 0 0 1 1
9 84 11 18 20 12 13
30 3 25 0 0 20 25
54 13 64 82 80 67 61
exposed to high temperature zones, the heavier fractions of the tar content are not cracked. As the temperature of the product gas get decreased at the outlet hole, these heavier fractions get condensed in the outlet pipe and are collected. The coal contains 3.7% of sulphur (ultimate analysis) and the ultimate and EDAX ash analysis show the existence of sulphur in both organic and inorganic forms. A band length of 1100–900 cm−1 of FTIR spectra signifies S]O stretching vibration. It shows a higher proportion of S]O in the zone #4 as compared to the zone #1 sample. It reveals that the coal sulphur is burnt in the combustion zone and would get liberated along with the product gas in the form of SO2.
#3 and zone #4 coal samples. This shows the presence of significant amount of ester and acid groups in the outlet zone since it is not exposed to high temperature. Also, the other functional groups such as eOH, −CH (aliphatic and aromatic) and eC]C are present in a significant quantity in these zones. And these results are reflected in the proximate analysis of the coal samples with the presence of significant amount of volatile matter (∼20-25%). The generated thermal energy in the reaction zone (zone#1) due to the combustion reaction, get consumed in the gasification zone (zone#2) for the progress of dry reforming reaction. And, the residual energy, which is left and insufficient for the gasification, is retained in the form of sensible heat of the product gas. This remnant heat liberates the volatile matters, which is present in the zone #3 and zone #4. The high molecular weight hydrocarbons and aromatics in these regions get vaporized and are carried along with the product gas to the outlet hole. As these gases are not
4. Conclusions The laboratory scale UCG experiments are performed using O2 Fig. 16. FTIR analyses of the various heat affected zones in dry coal obtained during CO2/O2 based borehole post gasification studies.
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enriched air, pure O2 and O2/CO2 as the gasifying medium with a less ash coal. The following conclusions can be drawn from the present study. (i) The TGA studies of the coal under non-isothermal condition show a higher rate of volatile matter removal under the CO2 atmosphere as compared to N2 atmosphere. This shows the reactivity of tar content with CO2 at 400 °C to 600 °C. The CO2 gasification of char predominates at above 850 °C. (ii) The TGA studies of the obtained tar under non-isothermal conditions ensure the reactivity of CO2 with aromatics and high hydrocarbon content of tar at 850 °C. (iii) The oxygen enriched air gasification severely reduces the temperature of combustion zone and thereby diminishes the quality of product gas as compared to the pure O2 and CO2/O2 gasification conditions. (iv) The ash analysis shows the presence of suitable catalyst species such as Ca, Fe, Mg and K, which may enhanced the CO2 gasification of coal. (v) In the fresh coal seam conditions, the dry reforming of hydrocarbon and aromatic content of the coal with CO2 produces a sustained syngas stream with an equi-molar proportion of CO and H2. (vi) The borehole CO2/O2 gasification in a dried coal seam favours a 40% of CO production through the Boudouard reaction under high temperature conditions. The CO2 fed gasification increases the CO/ H2 ratio of syngas in dry and tar deficient conditions. The present study brings out the impact of CO2 gas on the product gas composition in terms fixed carbon and volatile matter reactivity in the borehole combustion setup. Further, a detailed experimental study on O2/CO2 based UCG is required to analyse the effect of parameters on the gasification efficiency. Acknowledgements Dr. V. Prabu, the corresponding author of this article expresses his gratitude to SERB, Department of Science and Technology, Government of India, for their financial support to the laboratory scale experimental studies on UCG (Sanction letter No.: SR/FTP/ETA-220/2013). References [1] S.J. Self, B.V. Reddy, M.A. Rosen, Review of underground coal gasification technologies and carbon capture, Int. J. Energy Environ. Eng. 3 (2012) 16. [2] A.W. Bhutto, A.A. Bazmi, G. Zahedi, Underground coal gasification: from fundamentals to applications, Prog. Energy Combust. Sci. 39 (2013) 189–214. [3] V. Prabu, S. Jayanti, Laboratory scale studies on simulated underground coal gasification of high ash coals for carbon neutral power generation, Energy 46 (2012) 351–358. [4] P. Vairakannu, G. Kumari, CO2-Oxy underground coal gasification integrated proton exchange membrane fuel cell operating in a chemical looping mode of reforming, Int. J. Hydrogen Energy 41 (2016) 20063–20077. [5] V. Prabu, Integration of in-situ CO2-oxy coal gasification with advanced power generating systems performing in a chemical looping approach of clean combustion, Appl. Energy 140 (2015) 1–13.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification Geeta Kumari, Prabu Vairakannu
MARK
⁎
Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam 781039, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Underground coal gasification (UCG) CO2 gasification Thermo gravimetric studies Syngas
Oxidizing/gasifying agents play an essential role in the economy of underground coal gasification (UCG). The selection of feed gases to UCG depends on the prevailing conditions and inherent properties of a coal seam. Steam based UCG operation would lead to transportation and operational difficulty for efficient gasification. Alternatively, CO2 gas is a potential gasifying medium for coal gasification and this option would ensure the reutilization of waste CO2 gas in a UCG operation. Thus, in the present study, borehole coal combustion and gasification experiments are carried out to simulate CO2 enhanced UCG system using a low ash coal originating from the Northeast region of India. UCG experiments are simulated in a laboratory scale using CO2/O2 gases and the results are compared with pure oxygen and oxygen enriched air based UCG operation. The composition and calorific value of syngas are analysed for various gasifying agents. The CO2/O2 borehole experimental studies show that a CO enriched product gas (∼40 vol.%) can be generated under dried coal seam conditions. Thermogravimetric analysis (TGA) studies show that the rate of pyrolysis is higher under a CO2 atmosphere as compared to N2 atmosphere. Further, the TGA results show that the generated CO2 pyrolysis products led to substantial char gasification even at low temperatures. The reactivity of tar with CO2 enhanced the calorific value of the product gas due to dry reforming reactions. Also, the presence of suitable inorganic species in the coal progressed the CO2 gasification through ash catalysis.
1. Introduction There is renewed interest in the studies of underground coal gasification (UCG) due to energy scarcity problems and depletion of conventional fuel resources. In order to exploit deep coal resources economically, numerous research studies on UCG are ongoing for commercialization of this technology. UCG technology has high potential to generate clean energy through carbon capture and storage (CCS) [1,2]. The selection of oxidizing/gasifying medium for a UCG process depends on the prevailing conditions, inherent properties and surrounding strata of the coal seam and the syngas usage. Also, various oxidizing mediums such as air, O2, O2 enriched air, steam/O2, O2/CO2 are the potential options for feed gas in an in-situ gasification process. Air as an oxidizing medium eliminates the operating cost of air separation unit (ASU). However, the use of air may reduce the prevailing temperature of a UCG process and it would result in inefficient gasification. Particularly, in high ash coal seams, air gasification produces a less calorific fuel gas [3] and the combustion flame front in these coal seams may not be sustained in air atmospheric conditions. Furthermore, in high pressure UCG operations, the compression cost of air is
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Corresponding author. E-mail address: [email protected] (P. Vairakannu).
http://dx.doi.org/10.1016/j.jcou.2017.06.021 Received 22 May 2017; Received in revised form 12 June 2017; Accepted 28 June 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
expensive and it might be equivalent to the cost of oxygen separation from air. Therefore, air gasification may be suitable for a shallow UCG process. Oxygen gasification is found to be suitable for a high ash coal seam and it is more advantageous in terms of CCS [3]. Oxygen fed UCG operation may assure the production of high calorific product gas with sustained combustion operation in high ash coal seams. However, the energy penalty of an air separation unit (ASU) for O2 gasification may be high. Oxygen enriched air as a gasifying medium may reduce the operating cost of ASU and further, ensures the flame stability in UCG process. However, the generated flue gas contains a significant amount of nitrogen, which is to be separated for implementation of CCS. Superheated steam/O2 is a suitable gasifying medium. However, it creates operational difficulty during UCG process. The generation of superheated steam (∼400 to 600 °C) is an energy intensive process and a considerable heat loss may occur at injection feed wells of deep underground coal seams. A proper insulation of these injection pipelines is required for efficient gasification. In high ash coal seams, ash heap in cavity may reduce the temperature of feed steam and it would result in inefficient gasification. CO2/O2 is one of the potential options for the gasifying medium of
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G. Kumari, P. Vairakannu
UCG operation. CO2/O2 feed gas may avoid transportation problems of superheated steam to deep coal seams. However, a high concentration of CO2 feed gas may extinguish the flame front in a UCG cavity and thus, an optimal concentration of CO2/O2 should be maintained in a UCG process. The use of CO2 as a gasifying medium for UCG reduces greenhouse gas emission. Coal based power plants emit CO2 gas to atmosphere, which causes global warming. A lot of research is undergoing for the capture and storage of carbon gases in geological reservoirs, deep sea and coal seams. Alternatively, CO2 gas can be reutilized as a gasification agent for the conversion of solid fuel into calorific gases. Deep coal seams can be exploited through UCG technology using O2/CO2 as gasifying medium. However, there is a need for the production of pure CO2 gas to supply continuously into UCG feed well. This would be feasible if UCG power plants are integrated with carbon capture technologies such as pre combustion, post combustion and oxy-fuel combustion. In oxy-fuel combustion technology, UCG syngas is burnt with O2 gas and as a consequence, pure CO2 gas can be obtained as a flue gas stream. Alternatively, pure CO2 can also be produced using advanced technologies such as fuel cell system and chemical looping combustion system. Fuel cell system inherently separates oxygen from air through semipermeable membranes and the resultant flue gas would be a mixture CO2 and H2O [4,5]. In case of chemical looping combustion, metal oxide reaction separates oxygen from air. Further, the resultant metal oxides react with syngas and produce pure CO2 gas. This gas can be recycled for the supply of sustained feed gas to UCG operation. Thus, CO2 fed UCG needs carbon capture system for the sustained production of CO2 stream for the supply of feed gas to injection well. In our earlier studies [6], CO2 recycling was proposed for UCG process and net thermal efficiency of the proposed CO2 fed UCG power plants with CCS is estimated. The results have shown that the CO2 based UCG power plant achieved a high net thermal efficiency as compared to steam based UCG power plants. Several reports are available for CO2 fed coal gasification studies. Irfan et al. [7] reported a brief review on coal gasification studies, which was carried out under CO2 atmosphere. They explained the effect of pressure, temperature, coal rank, catalysts and feed gas composition on coal gasification. It is concluded that CO2 as a gasifying medium would increase the thermal efficiency of a boiler. A numerous thermo gravimetric analyser (TGA) studies on CO2 gasification is reported in literature. Mandapati et al. [8] carried out CO2 gasification of Indian coal chars with different inherent properties using a TGA. Three types of reaction models are used to study the reaction kinetics of coals and reported that the random pore model is best suited in the modelling studies. Zhang et al. [9] carried out CO2 gasification using a bituminous coal (Wyodak, Powder River basin) in a fixed bed reactor, which was operated at atmospheric pressure conditions in the presence of Fe2CO3 catalyst. They discussed the effect of iron catalyst on CO2 gasification using a TGA and concluded that a less activation energy (58.3 kJ/mol) is estimated for the CO2 gasification in the presence of iron catalyst as compared to the non-catalytic conditions (92.7 kJ/mol). Naidu et al. [10] carried out CO2 gasification kinetic studies on a low rank coal and biomass using a TGA in both isothermal and non-isothermal conditions. They estimated activation energy using different kinetic models and found that the reactivity of CO2 with coal is enhanced in the presence of calcium as a catalyst. Jayaraman et al. [11] carried out a series of experiments in steam and CO2 gasification of high ash Indian coals at different heating rates (40, 100, 500, 800 and 1000 K/min) using a TGA. They concluded that smaller particles have higher CO2 gasification rate and char formed at higher heating rate has better gasification efficiency. They found the activation energy in the range of 138–193 kJ/mol and 122–177 kJ/mol for the CO2 and steam gasification, respectively [11]. Urych et al. [12] studied the kinetics of coal pyrolysis under different heating rates to simulate UCG process and concluded that the kinetic parameters such as activation energy, Arrhenius constant are increased with increase in the heating rate. Silbermann et al. [13] carried out experimental kinetic studies on CO2
gasification using a TGA with an average heating rate of 200 K/min using deep mined coals, which are at a depth in the range of 700 to 800 m in Western Canadian Sedimentary Basin. They found that the coal ash contents and its suitable composition enhanced the char reactivity. Lahijani et al. [14] discussed the importance of Boudouard reaction, which converts CO2 gas into a calorific valuable CO gas. They performed CO2 gasification experiments using biomass chars in a TGA and studied catalytic activity of alkali, alkaline earth and transition metal salts on the char-CO2 reactivity. Their result shows the catalytic activities of alkali for carbon conversion in the order of Na > Ca > Fe > K > Mg. The existing literature on TGA experiments reveals that the activation energy is higher for CO2 gasification and it can be reduced with the addition of suitable catalysts or through inherent ash catalysis process. Stanczyk et al. [15] carried out several ex-situ UCG experiments using oxygen, air and oxygen enriched air in a pilot plant scale level. They carried out a 50 and 30 h long duration UCG experiments using lignite and hard coals in ex-situ conditions, respectively. The gasification of lignite and hard coal produces a product gas with a calorific value of 4.8 MJ/m3 at 4:2 M ratio of O2/air and 5.74 MJ/m3 at 2:3 M ratio of O2/air, respectively. They concluded that air as a gasifying medium resulted in a low quality syngas as compared to oxygen and oxygen enriched air gasification. Stanczyk et al. [16] carried out ex-situ UCG experiments using a hard coal with oxygen and steam as the gasifying medium for hydrogen production. They obtained hydrogen gas with an average composition of 15.28% and 53.77% for oxygen and steam gasification, respectively. Wiatowski et al. [17] performed ex-situ UCG experiments using a hard coal at 0.5 MPa with oxygen and oxygen enriched air. They concluded that a high pressure UCG operation leads to the production of methane and carbon monoxide at high concentration using oxygen as the gasifying medium. Kapusta et al. [18] carried out UCG experiments using a high moisture lignite coal. They concluded that the high moisture lignite coal seam can be exploited through the in-situ gasification process using oxygen as the gasifying medium. These studies show the feasibility of utilization of steam/O2, air, O2 and O2 enriched air as the UCG feed gas streams. However, a detailed study on CO2 fed UCG is essential in order to explore the interaction of CO2 gas under volatile matter enriched coal (fresh coal) and tar deficient coal seam conditions. Several literature are available in the context of CO2 gasification of solid fuel for conventional fixed bed and fluidized bed gasifier technologies. But, very few studies are available for the CO2 gasification in the context of UCG. Falshtynskyi et al. [19] discussed various parameters of borehole gasification for a thin coal seam field in Solenovsk for UCG technology. The product gas composition is predicted through energy and material balance calculations for various combinations of oxidizing and gasifying mediums. They estimated a syngas composition of 52.9% CO and 11% H2 gas for a feed mixture of CO2, O2 and N2. Duan et al. [20] proposed a method of CO2 recycling in UCG process. They proposed a two stage method for the optimization of UCG process parameters in CO2 recycling conditions and concluded that the feed CO2 gas reduces the reaction temperature drastically as compared to steam. Mocek et al. [21] carried out a pilot scale UCG experiment in a duration of 60 days in Wieczorek mine, Poland using oxygen, air and CO2 as the gasifying mediums. They concluded that the operating pressure is a major parameter for the efficient gasification of the coal insitu conditions. It is reported that the percentage of calorific value gases such as H2, CO, CH4, N2 and CO2 is 8%, 15%, 2% and 5%, respectively for air and CO2 as gasifying mediums. Konstantinou et al. [22] carried out UCG experiments using bituminous coals with operating pressures in the range of 1 to 3 MPa using a mixture of steam and CO2. They concluded that an increase in the operating pressure of the UCG resulted in high gasification rate and efficiency. It is found that the feed gases of H2O/CO2 at a mass ratio of 2:1 produces the syngas composition of 17.4% CO, 3% CH4, and 42.1% H2. The existing literature lacks a detailed study on the CO2 gasification for the UCG technology. 178
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condition of the coal blocks prevents gas leakage in the borehole cavity. The gap between upper and lower blocks are completely joined using a sealing agent (M-seal) and further moulded with clay sand before starting the combustion experiments. Therefore, gas leakages between the blocks is avoided during the combustion. However, the minimal leakage during combustion is arrested by using china clay at appropriate locations. As the gas leakage is arrested immediately, the composition of the product gas would not get affected. Fig. 2 shows the schematic diagram of the laboratory scale experimental set-up, which simulates the UCG conditions. The inlet hole of the coal block is connected to a 5 mm stainless steel pipe, which carries the feed gas from a gas cylinder. CO2 and O2 gases are sent to the borehole using a TJunction, where both the gases get mixed. The mixed gas is sent to the borehole through the stainless steel pipe. The gas flow can be controlled using a rotameter with a proper adjustment of the flow rate. The temperature inside the linking channel is measured using a set of thermocouples, which are fixed at various locations of the horizontal pathway. The thermocouples of K-type are kept at four locations of the borehole channel to measure the temperature of the reaction zones. The thermocouples are inserted into the borehole reaction zone through the injection and production wells. The reaction zone temperatures are measured and recorded using an Agilent data logger. The UCG experiments with different oxidizing/gasifying mediums such as O2/CO2, pure oxygen, oxygen enriched air are conducted to analyse the progress of reactions and chemical composition of the product gas. The coal block is ignited using a small piece of camphor near the injection point, which is 5 cm away from the inlet hole of the coal block. The established fire front at the injection point propagates towards the outlet hole. The production hole of the coal block is connected to a gas sample collector. Tar vapours in the product gas get condensed as it comes out of the borehole. It forms a semisolid content and is collected in a beaker at the outlet pipe. The product gas samples are collected at every 15 min time interval using vacutainer tubes. The composition of these gas samples is further analysed using a gas chromatograph (GC). The product gas consisting of CO, H2, CO2, CH4, C2H6 and other lighter hydrocarbons along with un-burnt O2, N2 etc. is analysed. A thermal conductivity detector (TCD) is used for the analysis of gas samples using helium as a carrier gas in the GC. A packed column (carbo-sieve S-II) is used in the GC for the product gas analysis. The GC is operated with the carrier gas at a flow rate of 16 ml/min with an oven temperature of ramping between 40 and 250 °C. Further, a post gasification cavity analysis is carried out using the coal samples, which are collected at various locations of the burnt coal block. These samples are characterized using a Fourier transform infrared spectroscopy (FTIR) and also by proximate analysis.
Table 1a Proximate analysis of the coal Content
Weight, (%)
Moisture Volatile matter Ash Fixed carbon
7 30 9 54
Therefore, in the present study, UCG borehole combustion experiments are performed with CO2/O2 as gasifying agents using a less ash coal and the results are compared with O2 enriched air and pure oxygen based gasification systems. The gas composition and calorific values of the product gas are measured and analysed. The experimental results show the feasibility of CO2/O2 UCG gasification under fresh coal as well as dry coal seam conditions. The pyrolysis and CO2 gasification of the less ash coal are also studied using a TGA. 2. Thermo-gravimetric analysis (TGA) and borehole gasification studies A 9% ash content coal from Bapung coal mines, Assam is used in the present study. Atmospheric pressure TGA studies under a nitrogen and CO2 gas atmosphere are performed. Proximate of the coal samples are carried out as per Indian standard IS: 1350 (Part 1)-1984. The ultimate analysis is performed using CHONS analyzer. The results are shown in Tables 1a and 1b . 2.1. TGA experimental procedure In the present study, isothermal and non-isothermal TGA studies are performed. In the non-isothermal study, a 10 to 14 mg of coal sample is taken in an alumina crucible and is heated to 1000 °C for a period of 100 min at a heating rate of 10 °C/min. TGA experiments are carried out under CO2 and N2 atmosphere. In the isothermal condition, the coal samples are heated initially at a heating rate of 33 °C/min. As the temperature reached 1000 °C in 30 min, it is maintained at the isothermal conditions for an hour. 2.2. UCG borehole gasification Fig. 1 shows the laboratory scale UCG experimental set up. The UCG experiments are carried out as per the methodology of the earlier studies [3]. The coal blocks of required size are sliced using wood files and cutters. A 6 mm semi cylindrical curved horizontal pathway (borehole channel) is created on the bottom surface of the coal block (Fig. 1a). The horizontal pathway simulates a linkage channel between injection and production wells of a real UCG seam. The sliced coal blocks in an enclosed position (Fig. 1b) are kept on a platform of refractory bricks, which withstand a high reaction temperature. Fig. 1c shows a set of upper and lower coal blocks in a clay moulded condition. This simulated UCG experimental setup is kept in a tightly packed wooden box (Fig. 1d). Further, a considerable quantity of clay and sand is fed into the top of the coal blocks and this loaded condition simulates the effect of overburden stress on the coal block. Also, the tightly packed
3. Result and discussion 3.1. TGA studies During the borehole combustion experiments, it is noted that a high quantity of tar content is obtained along with the product gas. Through the outlet pipe of the borehole setup, the tar vapours of the product gas get condensed and, are collected as a semisolid substance. The collected tar at the outlet of the borehole is analysed using the TGA. The CO2 reactivity of tar is evaluated and the results are compared with the reactivity of tar under N2 atmosphere. The mass loss during the nonisothermal TGA gasification of the raw coal samples under N2 and CO2 atmosphere is shown in Fig. 3. The reduction in the weight of the coal sample is observed due to the liberation of inherent moisture (due to drying), volatile matter (due to pyrolysis) and fixed carbon (due to gasification). The coal sample mass loss per degree Celsius is estimated and is shown in Fig. 3a. It can be seen that thermal degradation starts at 125 °C, which is an indication of the removal of moisture. The pyrolysis reaction occurs at the temperatures in the range of 400 to 600 °C. One can observe that the rate of pyrolysis is rapid under CO2 atmosphere. It
Table 1b Ultimate analysis of the coal (dry ash free basis) Content
Weight%
Carbon Hydrogen Nitrogen Sulfur Oxygen
74.7 5.7 1.3 3.7 14.6
179
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Fig. 1. UCG laboratory scale experimental set-up (a) sliced coal block, (b) upper and lower block in enclosed position, (c) coal block in moulded condition and (d) moulded coal block with overburden in enclosed position.
Fig. 2. Schematic diagram of the UCG experimental set-up.
min. Hence, the TGA study reveals that the CO2 pyrolysis and gasification of coal occur at 400 °C and 850 °C, respectively. Fig. 4 shows the TGA curve of coal reactivity at 1000 °C under the CO2 atmosphere. Initially, the coal sample is heated at a rate 33 °C/min for the first 30 min. When the temperature reached 1000 °C, the isothermal condition is maintained for the next 60 min. A linear drop in the mass of the coal sample is observed. The solid residue of 10.3% (point ‘G’) is obtained and it contains purely of ash content. It is estimated that a mass reduction occurs at a rate of 0.505%/min at 1000 °C due to Boudouard reaction. In order to examine the reactivity of tar with CO2, the TGA studies are carried out under the inert and CO2 atmosphere. During the borehole combustion experiments, the high temperature product gas in the combustion zone losses its thermal energy in the adjacent coal zones of the borehole pathway. This heat liberates the volatile matter and tar content (high hydrocarbons), which are carried along with the syngas. At the outlet hole, the tar vapours are get condensed. The obtained semisolid tar content is used to analyse its pyrolysis behaviour under the CO2 atmosphere in the TGA studies. Fig. 5a shows the TGA curves of tar content reactivity under both N2 and CO2 atmosphere. It is shown that a significant mass loss occurs up to a temperature of 337 °C under both N2 and CO2 atmosphere. It is found that a 70% of mass is evaporated as moisture and light gases in both the cases. The remnant 30% (point ‘H’) of mass may contain medium and higher hydrocarbons, which begin to react under the CO2 atmosphere with further increase in the temperature. In Fig. 5b, the difference between the retained mass of
is estimated that a 0.3 percent of mass loss per °C occurs under N2 atmosphere whereas it is 0.5 under CO2 atmosphere. It is also noticed (Fig. 3b) that a residual mass difference between the N2 and CO2 atmosphere is estimated as 8% (difference in the retained mass between the points ‘A’ and ‘B’) at 400 °C and it is significantly increased to 20% (difference in the retained mass between the points ‘C’ and ‘D’) at 600 °C. It is evident that the tar radicals reacted with CO2 and converted into gaseous molecules. However, under N2 inert atmosphere, the tar vapours can crack and form a low molecular gas only at a high temperature. It is interesting to note that the mass retained at 600 °C under CO2 atmosphere is only about 50.64% (point ‘D’), which indicates a loss of 12.36% fixed carbon content as compared to the initial weight of the original coal sample. The final mass residue under N2 atmosphere is weighed as 62% (point ‘E’), which is purely a mixture of ash (9%) and fixed carbon (54%). Therefore, the moisture (7%) and volatile matter (30%) content get completely vaporized under N2 atmosphere. In CO2 atmosphere, an estimate of 36.37% (point ‘F’) of solid residue is observed and this contains 27.37% of fixed carbon and 9% of ash. Hence, approximately a 26.63% of fixed carbon is converted into gaseous matters, which may be due to Boudouard reaction (Eq. (1)) and methanation reaction.
C+ CO2 → 2CO
(1)
It can be seen from Fig. 3a that a significant mass reduction of fixed carbon occurs at the temperature range between 850 °C–1000 °C and a 50% conversion of fixed carbon is estimated for a heating rate of 10 K/ 180
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Fig. 3. TGA analysis carried out of coal in N2 and CO2 atmosphere under non-isothermal conditions (a) mass loss/ °C and (b) mass retained%.
Fig. 4. TGA analysis of the coal carried out under CO2 atmosphere in isothermal conditions between 30 and 100 min.
remnant final mass at 850 °C is calculated as 8.8% (point ‘M’). A reduction of 6.04% (difference in mass reduction between points ‘M’ and ‘N’) mass is observed in the temperature interval between 850 °C to 1000 °C due to the CO2 gasification of higher hydrocarbons and aromatics. In total, it is estimated that a 16% of tar content (point ‘O’ in Fig. 5b) gets converted into gaseous matter due to the reactivity of CO2. Energy-dispersive X-ray spectroscopy (EDX) analysis of the ash samples is performed in order to identify the chemical composition of ash. The coal ash may decrease the reactivity due to the deposition of ash on char surface and resists the diffusion of gas molecules. However, certain minerals in coal act as a catalyst for the reaction of CO2 with char. EDX analysis (Table 2) shows the presence of significant amount of inorganic species such as Ca, Fe, Mg, K, which may act as catalysts for the CO2 gasification. The existing literature supports the catalyst
the coal samples under N2 and CO2 atmosphere is plotted against the operating temperature. At a temperature interval of 300 to 472 °C, a significant mass reduction difference of 9% (difference in mass reduction between the points ‘I’ and ‘J’) is observed between the TGA results, which are carried out under the CO2 and N2 atmosphere. This confirms the reactivity of low molecular hydrocarbon tar with CO2 gas. One can also notice that the difference in the mass reduction of the coal samples under the N2 and CO2 atmosphere is identical between 500 and 850 °C (point between ‘J’ and ‘K’), which shows that the mass loss is only due to the thermal cracking of higher hydrocarbons. The retained final mass under N2 atmosphere is estimated as 16.1% (point ‘L’ in Fig. 4a). This shows that an approximately 14% (difference in mass reduction between points ‘L’ and ‘H’) of the tar content is vaporized at the temperature interval of 400 to 1000 °C. In the CO2 atmosphere, the 181
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Fig. 5. (a) TGA analysis of the tar carried out under CO2 and N2 atmosphere, (b) difference in mass reduction of tar between N2 and CO2 atmosphere under non isothermal conditions.
role of these ash species on the CO2 gasification. Ca2+ ions plays a catalyst role on the CO2 gasification [20]. Also the presence of calcium and magnesium in the ash content increases the coal reactivity [21]. During the CO2 gasification, a fraction of H+ ions, which is present in the carboxylic acid groups, gets exchanged with metal cations such as K+, Na+, Ca2+, Mg2+ and Fe3+. The presence of these metal cations increases the CO2-char reactivity [23,24]. Also, it is reported that the gasification reactivity is increased in the presence of iron catalyst as compared to catalyst free basis. Si present in the form of Si-O increases the reactivity of coal under the CO2 atmosphere in the temperature range of 700 to 800 °C [25,26]. Therefore, the presence of substantial amount of these inorganic species of ash content may promoted the insitu coal CO2 gasification.
Table 2 Energy dispersive X ray spectroscopy (EDX) analysis of the coal ash. Elements
Wt, %
O Si Al Fe Ca S Ti Mg K Total
50.5 18.6 17.6 5.8 3.1 1.8 1.7 0.6 0.3 100
3.2. Oxygen enriched air gasification based borehole combustion Table 3 shows the details of experiments and the summary of Table 3 Details of the conducted experiments and summary of the results. Serial No.
Fuel
Gasifying medium
Time Duration (min)
Maximum H2 (%)
Average H2 (%)
Maximum CO (%)
Average CO (%)
Maximum CH4 (%)
Average CH4 (%)
Maximum calorific value (kJ/mol)
Average calorific value (kJ/mol)
1
Coal
220
6
4
6
4.5
6.9
4.7
112
106
2 3
Coal Coal (Fresh) Coal (Dry)
Oxygen enriched air Oxygen CO2/O2
180 210
30 26
17.52 21.6
11 41
5.76 28.69
27 16
22.12 9.68
298 285
243.72 212.55
CO2/O2
210
26
20.34
39
30.85
9
8.03
234
202.20
4
182
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Fig. 6. Temperature profile of the borehole during O2 enriched air gasification.
obtained results during the borehole gasification. The first experiment is carried out using air as a cheaper oxidizing medium. During the air gasification based borehole combustion experiment, an unsustained combustion flame front is observed. Thus, an oxygen enriched air gasification experiment is carried out and it ensured the sustainability of the combustion front in the borehole cavity. However, a maximum temperature of 800 °C is observed in the reaction zone and this may not be sufficient for the efficient gasification. Fig. 6 shows the temperature profile of the oxygen enriched air gasification. A 3.5-h experimental run is carried out and air is used as a gasifying medium in the first 90 min of the borehole combustion. It is observed that a maximum temperature of 600 °C is noted, which is not favourable for the syngas production. Thus, oxygen gas is supplied along with air and the reaction temperature is raised to 800 °C. One can see that the sustained combustion front exists at the first thermocouple region and it does not even reach the second thermocouple zone, which shows a less temperature of 300 °C during the entire run of the experiment. Air is supplied at 0.2 litre per minute (lpm) initially and oxygen gas is introduced at an equal flow rate of air after 90 min of the air gasification. The O2/air molar ratio of 1 is maintained for the first 10 min in order to enhance the combustion rate. Then, the molar ratio is gradually decreased to 0.28 in order to estimate the optimum conditions of gas flow rate for the efficient gasification. Fig. 7 shows the composition and calorific value of the product gas, which is obtained during the oxygen enriched air gasification. With increase in the flow rate of air, the combustion rate is enhanced and resulted in a higher concentration of CO2 gas at the time interval between 120 and 180 min (Fig. 7b). An increase in the concentration of calorific value gases such as H2, CO, CH4 (Fig. 7a) in the product gas is observed at the O2/air molar ratio of 0.33. The calorific value of the product gas at this molar ratio is estimated as 115 kJ/mol (Fig. 7c). A 2 to 18% of unutilized oxygen is observed (Fig. 7b) in the product gas due to bypassing of the feed gas from the combustion zone. A reverse trend gas composition between CO2 and N2 gas is observed with an increase in the O2/air molar ratio. It is found that an estimate of CO2, O2 and N2 concentration of the product gas in the range of 43–50%, 5–8% and 30–35%, respectively, is observed. Each of the calorific value gas is found in the range of 6–7% in the product gas at this feed molar ratio of O2/air (0.33). With the reduction in the O2/air molar ratio to 0.28, a drop in CO2 gas to 28% and a rise in O2 and N2 to 13% and 45%, respectively in the product gas due to inefficient combustion. Therefore, the O2/air molar ratio of 0.33 is found as an optimum ratio for this coal. Although the oxygen input to the feed stream increases the calorific value of the product gas, it does not show a significant rise in the heating value and only an average calorific value of 90 kJ/mol (Fig. 7c) is estimated at the optimum feed
ratio. 3.3. Pure oxygen gasification based borehole combustion A pure oxygen based UCG gasification experiment is carried out using a coal block of length 30 cm for a duration of 3 h. The temperature and concentration profiles of the experiment are shown in Figs. 8 and 9, respectively. Four thermocouples are kept at an equal distance intervals of 8 cm apart in the borehole. The first thermocouple at 5 cm from the entrance of the inlet hole shows a temperature of 1000 °C for the first 40 min and gradually decreases to 700 °C. This is due to the movement of combustion front away from the first thermocouple region in the axial direction of the borehole with an increase in the flow rate of oxygen from 0.2 lpm to 0.3 lpm. This can be also observed in a sudden rise of temperature simultaneously in the second thermocouple at the 60th min. Further, the combustion front progressed towards the third thermocouple region (which is 21 cm from the inlet hole) with an increase in the flow rate of O2 to 0.5 lpm at the 90th minute. However, the combustion front did not reach the fourth thermocouple region, which shows a less temperature in the range of 200 to 400 °C at the end of 3 h. It can be seen from the temperature profile that a well sustained fire front is established with a lengthy combustion zone of about 16 cm (between the first three thermocouples). A 25–30% of methane gas is evolved during the UCG experiment and it increased the calorific value of the product gas (Fig. 9a). The liberation of volatile matter and cracking of tar may be the major sources for methane generation. A significant amount of liquid tar is obtained along with the product gas, which is collected as a separate stream. Hydrogen gas is generated as a result of pyrolysis and steam (inherent moisture) char gasification reaction. It can be seen that the production of hydrogen gas is not stable (6 to 29%), which may be due to the movement of the combustion front to different locations in the borehole. This leads to the variation in the rate of volatile matter removal with increase in the flow rate of oxidant. A 2–10% of CO gas is generated due to pyrolysis and incomplete combustion of the char. The rise in CO2 concentration shows the enhanced combustion (Fig. 9b) at higher O2 flow rate. A high calorific gas of 298 kJ/mol is obtained at an O2 flow rate of 0.4 lpm. The pure oxygen based UCG produces an average calorific value of syngas 200 kJ/mol (Fig. 9c), which is higher than the calorific value of O2 enriched air gasification syngas. A significant amount of unreacted O2 about 10–25% is observed in the product gas. This shows the bypass of oxygen flow inside the cavity. Daggupati et al. [27] and Stanczyk et al. [16] reported a significant amount of unreacted O2 with 28% and 7%, respectively, during the pure oxygen gasification experiments. This may be due to insufficient 183
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Fig. 7. Concentration profile of (a) combustible gas and (b) Non-combustible gas of the product gas, and (c) calorific value of the product gas and molar ratio of O2/air during the O2 enriched air gasification.
small, a high flow rate of feed gas resulted in a less residence time, which led to the release of unreacted O2.
surface area on the borehole of the cavity for the reaction. Also, the atmospheric pressure UCG conditions may not utilize the excess oxygen in the gas phase combustion reactions. Another factor for the unspent O2 may be due to a less residence time of reactants in the cavity. As the length of the coal block in the laboratory scale experimental set-up is
Fig. 8. Temperature profile of the borehole during pure O2 gasification.
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Fig. 9. Concentration profile of (a) combustible gas and (b) Non-combustible gas of the product gas and, (c) calorific value of the product gas and flow rate during the O2 during pure oxygen gasification.
Fig. 10. Composite flow rate of the feed gases at various molar ratios during CO2/O2 borehole gasification of dry coal (pure O2 burnt coal) and fresh coal block under identical flow parameter conditions.
condition is maintained in both the experiments. The flow rate of CO2 and O2 gas is maintained in such a way that each molar ratio of CO2/O2 is kept constant for a certain duration of time and further increased in order to explore the effect of feed gas composition on the product gas. The molar ratio of CO2/O2 is varied both increasing and decreasing order in the range of 0.4 to 0.67 in order to evaluate the optimum feed ratio. Initially, the feed gas ratio (CO2/O2) is maintained at 0.4 for 45 min and increased to 0.6, which is maintained for 60 min and further it is reduced to 0.4. Fig. 11a and 11b show the temperature profile of fresh and dry coal CO2 gasification, respectively. One can notice the existence of high temperature flame front (> 1000 °C) during the entire run of the fresh coal combustion experiment due to the presence of high volatile matter. On the other hand, a high temperature profile is attained in the tar
3.4. CO2/O2 gasification based borehole combustion Two CO2/O2 gasification experiments are carried out at various feed ratios of O2/CO2 in order to find out the optimum ratio for the efficient gasification. As the tar content is produced in a significant quantity during the pure O2 combustion experiment, it would be interesting to explore its effect on the CO2 gasification conditions. Thus, the O2eCO2 gasification experiments are carried out in a fresh coal block as well as in the O2 burnt coal block (tar depleted condition) of the previous experiment (pure O2 experiment) under identical operating conditions. Each experiment is conducted for a period of 4 h. It is found difficult to reignite the tar deficient O2 burnt coal block due to the lack of volatile matter. Fig. 10 shows the composite flow rate conditions of feed gases such as O2 and CO2 at various molar ratios. A similar feed gas flow rate 185
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Fig. 11. Temperature profile of (a) fresh coal, and (b) dry coal block during CO2/O2 gasification.
3.4.1. Fresh coal CO2/O2 gasification During the first 45 min of the fresh coal CO2 gasification at a molar ratio (CO2/O2) of 0.4, it is seen that the CO/H2 ratio of the product gas is gradually increased to 1.8. The product gas (Fig. 12a) containing a higher proportion of CO (∼40%) than H2 (∼25%), shows the progress of CO2-tar reforming reaction [29]. In the initial stages of the experiment, the tar enriched fresh coal containing high molecular weight hydrocarbons and aromatics resulted in a high concentration of CO as per the reactions (2–4). The dry reforming of hydrocarbons with CO2 is an endothermic reaction and the simultaneous progress of char-oxygen burning reaction in the cavity provides heat for the dry reforming gasification reaction. With increase in the feed CO2/O2 molar ratio to 0.6, the CO/H2 ratio of syngas decreases. It is found that the concentration of CO and H2 in the product gas gradually decreases (∼20% of each) with an increase in the molar feed rate of CO2 and O2. With further reduction in the CO2/O2 molar ratio to 0.4, an increase in the volume of CO (∼28%) and H2 (∼25%) is observed and the ratio of CO/H2 of the product gas is estimated approximately as one. Therefore, the optimum molar ratio of CO2/O2 for the efficient CO2 dry reforming is estimated as 0.4.
deficient dry coal gasification, only after 90 min of the combustion period. In the fresh coal block experiment, the O2 gasification is carried out for the first 30 min at a flow rate of 0.5 lpm in order to raise the temperature of borehole zones. Fig. 12a and 12b show the composition of calorific and non-calorific product gases, respectively for the fresh coal CO2/O2 gasification. Fig. 13a and 13b show the composition of calorific and non-calorific product gases, respectively for the dry coal CO2/O2 gasification. Figs.12c and 13c show the calorific value of the product gas during the CO2/O2 gasification under the fresh coal and the tar deficient coal conditions, respectively. The syngas containing CO and H2 is generated due to the dry reforming of tar with CO2 as per the following reaction [28]. The liberated methane and higher hydrocarbon gas undergo the dry reforming reaction as follows, (2)
CH 4 + CO2 → 2CO + 2H2
Cn H2n + 2 + nCO2 → 2nCO +
2n + 2 H2 2
(3)
In similar to hydrocarbons, the aromatics, which is present in the tar content, may also undergo the dry reforming reaction as follows [28].
C7 H8 + 7CO2 → 14CO + 4H2
(4)
3.4.2. Dry and volatile deficient coal CO2/O2 gasification In the dry coal gasification, it is observed a high CO/H2 ratio (∼3) at the beginning stage of the CO2 gasification due to the progress of Boudouard reaction under dry and tar deficient conditions. With an increase in the CO2/O2 feed ratio to 0.67, the CO/H2 ratio of syngas decreases gradually to unity. This might be due to the high flow rate of feed gas, which leads to a less residence time for the gas reactants in the cavity. Therefore, the heterogeneous reactions between the solid (char) and gas (CO2/O2) reactants is inefficient and therefore, a less production of CO gas is observed. A sustained production of hydrogen gas is observed due to the dry reforming of residual methane of coal with CO2. However, after 165 min of the experiment, the COeH2 ratio increases to 2 due to the decline in the production of hydrogen gas
However, the syngas produced during the dry reforming may undergo methanation reaction as follows (Eq. (5)),
CO + 3H2 → CH 4 + H2 O
(5)
The produced CO gas may undergo the water gas shift reaction and would produce H2 as follows (Eq. (6)),
CO + H2 O→ H2 + CO2
(6)
Fig. 14 shows the comparison of CO/H2 ratio of the produced syngas for the fresh and dry coal CO2 gasification. The trend of CO2/O2 gasification in the fresh coal and dry coal is explained in the subsequent section. 186
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Fig. 12. (a) Concentration profile of (a) combustible gas (b) Non-combustible gases of the product gas, (c) calorific value of the product gas during the CO2/O2 fresh coal block borehole gasification.
dissembled and the heat affected zones of the coal block is analysed. Fig. 15 shows the photograph of the burnt coal block. The cavity region can be seen with a carbon deposited thermocouple. One can also notice the tar deposition near the cavity zone. Four coal samples (zone #1 to zone #4 as shown in Fig. 15) at different heat affected zones of the block are collected and analysed. Table 4 shows the proximate analysis of these heat affected zones of the burnt coal. In the inlet zone (before cavity region), a 5% reduction of volatile matter and a complete removal of moisture are estimated due to solid heat conduction of coal from the combustion zone towards the direction of inlet hole. The analysis of ash, which is deposited on the cavity region, shows 84% ash and 13% of fixed carbon. It reveals the possibility of coal spalling (shredding of coal particles from the upper zone into the combustion cavity) during the UCG process. The analysis of coal char in the cavity region (zone #1) between the first and third thermocouple region shows a complete removal of volatile matter and moisture. A high percentage of fixed carbon (∼80%) is estimated due to the depletion of volatile matter during the combustion in the cavity region. Fourier transform infrared spectroscopy (FTIR) analyses are carried out to determine the functional groups of the coal samples. Fig. 16 shows the FTIR spectra of the various heat affected coal samples. The FTIR of raw coal shows the presence of functional group such as eOH, eCH (aliphatic), eC]C, eCeO, eS]O and eCH (aromatic). The strong broad absorption band appeared in the range of 3600–3000 cm−1 signifies the stretching vibration of eOH groups. One can notice a significant reduction of this band with respect to temperature magnitude of the hot zones. The zone #1 coal has insignificant
(∼10% as shown in Fig. 13a). It may be due to the depletion of methane gas (∼3%) in the coal block, which reduces the hydrogen production. On the other hand, one can see that the presence of high methane content (∼10%) in the syngas during the fresh coal CO2 gasification resulted in the continuous production of hydrogen due to the dry reforming reaction. During the entire run of the experiment, a significant amount of tar is produced. An average volume of 50 ml tar is obtained as a condensed product. In summary, the gas concentration and calorific value of the product gas is estimated for each experiment (Table 3). A maximum calorific value (∼298 kJ/mol) of the product gas is obtained during the pure O2 gasification and an equivalent calorific value is gained in the CO2/O2 gasification using a fresh coal block (Figs. 12c and 13c). In the tar deficient conditions, a high calorific value of 234 kJ/mol is obtained during the CO2/O2 gasification. An average of 20% and 29% of H2 and CO, respectively, in the product gas is estimated during the CO2/O2 gasification. The high percentage of CO in the product gas is due to the dry reforming reaction of CO2 with methane and high hydrocarbon gases. It can be seen that a higher percentage (average 22.12%) of methane gas gets evolved during the pure O2 gasification where it is noted as 8.5% during the CO2/O2 gasification. This is due to the conversion of methane into syngas in the dry reforming reaction (Table 3).
3.5. Post-gasification studies A post gasification study is carried out in the burnt coal block of dry coal CO2 gasification experiment. The experimental set-up is 187
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Fig. 13. Concentration profile of (a) combustible gas (b) non-combustible gas of the product gas, and (c) calorific value of the product gas during the CO2/O2 dry coal block borehole gasification.
−OH group as compared to other zones, which confirms the disruption and conversion of −OH bond into hydrogen and oxygen containing compounds. The absorption band length 3000 − 2800 cm−1 denotes the weak stretching vibrations of eCH bond of aliphatic hydrocarbons, which appears almost negligible in the zone #1 coal as compared to other zone coal samples. The weak band stretching vibration are observed at the band length 1300–1100 cm−1 shows the presence of eCeO bond, which is insignificant in the zone #1 coal as compared to the rest of three zones. FTIR and proximate analyses studies show a complete removal of volatile matter in the cavity region and therefore, the prevailed conditions enhanced the rate of Boudouard reaction, which resulted in a 40% of CO production. The analysis of coal sample (zone #2) between the cavity and the adjacent zone, which is 3 cm length in the axial direction towards the outlet hole, shows a similar
composition as that of coal char in the cavity region. Also, the FTIR analysis of the zone #2 coal sample shows a slightly higher quantity of eOH group as compared to the zone #1 sample. This seems that the zone #2 is a highly heat affected region, which is near the cavity zone. The aromatics and higher hydrocarbon present in this region might undergoes the dry reforming reaction and produces the syngas effectively. The FTIR spectra also shows the presence of aromatic compounds at a band length 900–700 cm−1 with weak stretching vibration. The functional groups are depleted proportionately with increase in the prevailing hot conditions of the borehole cavity zones. The strong absorption band appeared in the range of 1800–1500 cm−1 signifies the stretching vibrations of eC]O, which may be the compounds of aryl esters and carboxylic acid. It appeared with sharp intensity in the zone Fig. 14. CO/H2 ratio of the syngas during the CO2/O2 gasification under the fresh coal and dry coal conditions.
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Fig. 15. Photograph of the post CO2/O2 gasification cavity analysis [Zone #1: combustion cavity, Zone #2: cavity end region, Zone #3: Adjacent zone to cavity, Zone #4: Near outlet zone] [TC1, TC2, TC3, TC4 are thermocouple positions along borehole pathway].
Table 4 Post-combustion cavity analysis of the burnt coal block (obtained after O2/CO2 feed gasification under tar deficient conditions). Sample
Moisture (%)
Ash content (%)
Volatile matter (%)
Fixed carbon (%)
Original coal Fallen/spalled coal At oxygen inlet (just before cavity) Cavity region (zone #1) Cavity end region (zone #2) Adjacent zone to cavity in axial direction (zone #3) Near outlet zone (zone #4)
7 0 0 0 0 1 1
9 84 11 18 20 12 13
30 3 25 0 0 20 25
54 13 64 82 80 67 61
exposed to high temperature zones, the heavier fractions of the tar content are not cracked. As the temperature of the product gas get decreased at the outlet hole, these heavier fractions get condensed in the outlet pipe and are collected. The coal contains 3.7% of sulphur (ultimate analysis) and the ultimate and EDAX ash analysis show the existence of sulphur in both organic and inorganic forms. A band length of 1100–900 cm−1 of FTIR spectra signifies S]O stretching vibration. It shows a higher proportion of S]O in the zone #4 as compared to the zone #1 sample. It reveals that the coal sulphur is burnt in the combustion zone and would get liberated along with the product gas in the form of SO2.
#3 and zone #4 coal samples. This shows the presence of significant amount of ester and acid groups in the outlet zone since it is not exposed to high temperature. Also, the other functional groups such as eOH, −CH (aliphatic and aromatic) and eC]C are present in a significant quantity in these zones. And these results are reflected in the proximate analysis of the coal samples with the presence of significant amount of volatile matter (∼20-25%). The generated thermal energy in the reaction zone (zone#1) due to the combustion reaction, get consumed in the gasification zone (zone#2) for the progress of dry reforming reaction. And, the residual energy, which is left and insufficient for the gasification, is retained in the form of sensible heat of the product gas. This remnant heat liberates the volatile matters, which is present in the zone #3 and zone #4. The high molecular weight hydrocarbons and aromatics in these regions get vaporized and are carried along with the product gas to the outlet hole. As these gases are not
4. Conclusions The laboratory scale UCG experiments are performed using O2 Fig. 16. FTIR analyses of the various heat affected zones in dry coal obtained during CO2/O2 based borehole post gasification studies.
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enriched air, pure O2 and O2/CO2 as the gasifying medium with a less ash coal. The following conclusions can be drawn from the present study. (i) The TGA studies of the coal under non-isothermal condition show a higher rate of volatile matter removal under the CO2 atmosphere as compared to N2 atmosphere. This shows the reactivity of tar content with CO2 at 400 °C to 600 °C. The CO2 gasification of char predominates at above 850 °C. (ii) The TGA studies of the obtained tar under non-isothermal conditions ensure the reactivity of CO2 with aromatics and high hydrocarbon content of tar at 850 °C. (iii) The oxygen enriched air gasification severely reduces the temperature of combustion zone and thereby diminishes the quality of product gas as compared to the pure O2 and CO2/O2 gasification conditions. (iv) The ash analysis shows the presence of suitable catalyst species such as Ca, Fe, Mg and K, which may enhanced the CO2 gasification of coal. (v) In the fresh coal seam conditions, the dry reforming of hydrocarbon and aromatic content of the coal with CO2 produces a sustained syngas stream with an equi-molar proportion of CO and H2. (vi) The borehole CO2/O2 gasification in a dried coal seam favours a 40% of CO production through the Boudouard reaction under high temperature conditions. The CO2 fed gasification increases the CO/ H2 ratio of syngas in dry and tar deficient conditions. The present study brings out the impact of CO2 gas on the product gas composition in terms fixed carbon and volatile matter reactivity in the borehole combustion setup. Further, a detailed experimental study on O2/CO2 based UCG is required to analyse the effect of parameters on the gasification efficiency. Acknowledgements Dr. V. Prabu, the corresponding author of this article expresses his gratitude to SERB, Department of Science and Technology, Government of India, for their financial support to the laboratory scale experimental studies on UCG (Sanction letter No.: SR/FTP/ETA-220/2013). References [1] S.J. Self, B.V. Reddy, M.A. Rosen, Review of underground coal gasification technologies and carbon capture, Int. J. Energy Environ. Eng. 3 (2012) 16. [2] A.W. Bhutto, A.A. Bazmi, G. Zahedi, Underground coal gasification: from fundamentals to applications, Prog. Energy Combust. Sci. 39 (2013) 189–214. [3] V. Prabu, S. Jayanti, Laboratory scale studies on simulated underground coal gasification of high ash coals for carbon neutral power generation, Energy 46 (2012) 351–358. [4] P. Vairakannu, G. Kumari, CO2-Oxy underground coal gasification integrated proton exchange membrane fuel cell operating in a chemical looping mode of reforming, Int. J. Hydrogen Energy 41 (2016) 20063–20077. [5] V. Prabu, Integration of in-situ CO2-oxy coal gasification with advanced power generating systems performing in a chemical looping approach of clean combustion, Appl. Energy 140 (2015) 1–13.
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