CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification

Accepted Manuscript CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification

Geeta Kumari, Prabu Vairakannu PII:

S0360-5442(17)31878-9

DOI:

10.1016/j.energy.2017.11.027

Reference:

EGY 11815

To appear in:

Energy

Received Date:

31 May 2017

Revised Date:

08 October 2017

Accepted Date:

05 November 2017

Please cite this article as: Geeta Kumari, Prabu Vairakannu, CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification, Energy (2017), doi: 10.1016/j.energy.2017.11.027

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Two stage underground coal gasification is carried out using CO2-air as the gasifying medium The average calorific value of 217 and 260 kJ/mol of syngas is obtained using high ash and low ash Indian coal The effect of O2/air molar ratio and CO2/oxidant molar ratio on syngas is studied

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CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification Geeta Kumari, Prabu Vairakannu* Department of Chemical Engineering Indian Institute of Technology Guwahati, Assam 781039, INDIA *Corresponding

author. Tel.: +91 361 258 2279; fax: + 91 361 2582291 E-mail address: [email protected]

Abstract CO2-air is a potential gasifying-oxidizing medium for two stage mode of underground coal gasification (UCG). The existing literature deficits the UCG studies on the use of CO2-air as a gasifying medium especially for high ash Indian coals. Thus, the present study investigated the viability of utilizing the CO2-air as a gasifying medium for high ash Indian coals using a laboratory scale borehole gasification set-up in a two-stage gasification mode of operation. A typical Indian coal having 42% ash and a North East Indian coal having 4% ash are used for the borehole gasification experiments. The effect of the process parameters such as the molar ratio of O2/air and the molar ratio of CO2/oxidizing agent on the product gas composition is evaluated. The results show that the gasification of low ash coal and high ash coal produced product gas with a calorific value as high as 260 kJ/mol and 214 kJ/mol, respectively. CO2-air based gasification necessitates the presence of oxygen in the feed gas with O2/air ratio of 0.1 and 1 for the low and the high ash coal, respectively. Repeatability experiments show 7% and 14% of error in the calorific values of syngas for low and high ash coals, respectively. Keywords: Underground coal gasification (UCG); CO2-air gasification, syngas, calorific value, high ash coal, two stage gasification. 1. Introduction Coal is a primary fossil fuel in India and plays an important role in energy sectors [1]. A significant portion of Indian coal resources is found deep underground and, the conventional mining process is not feasible for the exploitation of deep coal seams economically. Underground coal gasification (UCG) is a promising technology, which avoids mining process, ash disposal, coal transportation, etc. [2]. It is a clean coal technology, which involves the in-situ conversion of coal deposits into a

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high quality syngas. The product gas (syngas) contains a mixture of hydrogen, carbon mono-oxide, methane and higher hydrocarbons. This syngas can be used for energy production or manufacturing of various chemical feed stocks [3][4]. Minimal environmental pollution, high fuel resource utilization and low operating cost are the main advantages of a UCG process [5]. Sir William Siemens, Germany invented the UCG technology for the utilization of deep coal seams. Later, several countries such as Australia, Canada, South Africa and India showed their interest in this technology for the economic recovery of deep ground coal resources [6][7]. In Poland, UCG field trials were conducted in “Barbara” coal mine at a shallow depth of 30 m [8]. Several UCG commercial projects and pilot plants are being planned worldwide for reducing CO2 emissions, which include the capture and reinjection of CO2 into UCG cavity [9]. The recycling of CO2 gas in a UCG operation can reduce the emission of CO2 per unit of electricity production [10][11]. UCG process produces syngas in an underground coal seam with the supply of suitable oxidizing/gasifying agent through injection wells. A set of simultaneous reactions such as pyrolysis, combustion and gasification is progressed in the borehole between the drilled injection and production wells. Typical oxidizing/gasification mediums such as oxygen, air and steam are injected into the coal seam and the generated syngas (H2 & CO) with CH4 and other hydrocarbons are captured through the production well. These gases can be used as a fuel or a chemical feedstock after a proper treatment for removing impurities [12][13]. The major chemical reactions taking place in a cavity zone are as follows. 1 𝐶 + 𝑂2→2𝐶𝑂 2 𝐶 + 𝑂2→𝐶𝑂2

(𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛)

(1)

(𝐶𝑜𝑚𝑝𝑙𝑒𝑡𝑒 𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛)

(2)

𝐶 + 𝐻2𝑂→𝐶𝑂 + 𝐻2

(steam gasification)

(3)

𝐶 + 2𝐻2→2𝐶𝐻4

( methanation reaction)

(4)

(Boudouard reaction)

(5)

𝐶 + 𝐶𝑂2→2𝐶𝑂

Coal → C𝐻4 + 𝐻2O + CO + C𝑂2 + 𝐻2 + HC’s (pyrolysis reaction )

(6)

The reactions (1), (2) and (4) are exothermic in nature and the other reactions are endothermic.

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Several literatures reported the use of oxygen, oxygen enriched air and steam as a gasifying medium for UCG operation. Stanczyk et al. [14] carried out the experimental studies on UCG using lignite and hard coal with oxygen, air and oxygen-enriched air as the gasifying medium. They found that the use of pure air as a gasifying medium led to inefficient gasification. However, their results show that a syngas with reasonable calorific values of 4.18 MJ/m3 and 5.74 MJ/m3 can be produced with oxygen enriched air as a feed gas using the lignite and the hard coal, respectively. Thorsness et al. [15] conducted several UCG trials using air, oxygen and steam as the gasifying medium at Hoe creek site, the USA. Their results show that the steam-O2 feed gas produced a high quality syngas with a calorific value of 8.1 MJ/m3 whereas air as a feed gas reduced the heating value of syngas by 50% (4.3 MJ/m3). Laciak et al. [16] conducted a 63-hour duration laboratory scale UCG experiment using air as the primary gasifying medium and obtained the product gas with a calorific value of 3.27 MJ/Nm3. Gur et al. [17] carried out the UCG experiments using Turkish lignite coal with air, oxygen and steam as the gasifying medium. They obtained a product gas with high calorific value in the range of 6–9 MJ/ Nm3 during oxygen combustion stage and 4.6 MJ/m3 under O2/air gasification period. Thus, oxygen enriched air as a feed gas may be suitable to generate a syngas with a medium calorific value in the range of 4-5 MJ/Nm3. However, the production of high quality syngas using either air or oxygen-enriched air is not feasible due to the nitrogen dilution of syngas. Further, the blend of air/steam or air/CO2 as the oxidizing/gasifying medium is not a viable option since these gases are fire extinguishing agents (N2 (in air), steam, CO2). These drawbacks can be rectified by employing two stage gasification method in the UCG technology. Two stage gasification method ensures the availability of coal surface area in borehole seam for the injected gasifying medium. Also, this method establishes a high temperature reaction zone for efficient gasification. Yang et al. [18] carried out a field UCG test with the implementation of two stage gasification method for hydrogen production using air and steam as the gasifying medium. They obtained an average of 50% hydrogen (volume) in the syngas and concluded that the two stage gasification method is highly suitable for a UCG seam with a long channel and multiple injection points for large scale hydrogen production. Yang et al. [19] conducted pilot scale UCG experiments in Liuzhuang Colliery, Tangshan using air-steam as the gasifying medium. They implemented two stage gasification technique and obtained a syngas having 4.25 MJ/m3 heating value. Liu et al. [20] conducted laboratory scale UCG experiments using two stage gasification

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method with oxygen enriched air as the gasifying medium. They obtained a product gas with calorific value in the range of 5.31 to 10.54 MJ/Nm3 at various molar ratios of O2/air. Thus, the two stage gasification method can be employed for air-CO2 and air-steam mixture for the economic recovery of deep coal seams. Air as a gasifying medium may be economic to UCG as it eliminates the energy penalty of an air separation unit for oxygen separation. The available literature shows the detailed studies on utilizing air as the gasifying medium under blended conditions with other gases such as air-O2, airsteam and air-O2-steam for UCG operations. However, two stage gasification of UCG using airCO2 is not yet studied especially for high ash Indian coals. Our earlier studies [21] show the feasibility of CO2-O2 gasification of high ash and low ash Indian coals for the production of high quality syngas. In the present study, air-CO2 based laboratory scale UCG experiments are conducted using a borehole combustion setup in a two stage gasification mode of operation. The results show that a sustained high quality syngas can be produced under CO2-air atmosphere using typical high ash Indian coals. 2. UCG experimental methodology Low ash coal (~4% ash) is sourced from Bapung coal mines (Assam) and, high ash coal (~42% ash) is brought from Jharia coal mines (Dhanbad), India. The proximate and ultimate analyses are carried out as per the Indian standard (IS) methods. Table 1 shows the proximate and ultimate analysis of the coals. Low ash coal has huge gaseous matters of 42% (wt. %) whereas high ash coal contains 19% volatile matter (wt. %) with less calorific value. A bomb calorimeter is used to estimate the calorific value of low and high ash coals as per Indian Standard methods (IS: 13591959). Figure 1 shows the photograph of the UCG experimental setup and, the experimental methodology is described in our earlier studies [22]. The coal blocks are sliced using a steel cutter and a rotary blade. A semi cylindrical borehole of 7 mm diameter is grooved longitudinally at the center of the coal block. The coal blocks have the dimensions of length, width and height of 45 cm, 20 cm and 12 cm, respectively. The sliced coal blocks are placed on a platform of refractory bricks for reducing the heat loss during the combustion phenomena. Two coal blocks are kept one over the other connecting its grooved semi-cylindrical slot into a borehole for combustion and gasification. The joining ends of both the blocks on all sides are sealed using china clay and sand mud to arrest the leakage of syngas. A stainless steel pipe of 5 mm diameter is used to supply

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oxygen and CO2 gas into the borehole cavity. A piece of camphor is used for the ignition of the coal blocks. The coal blocks are ignited near the injection hole and, the established combustion front propagated towards the outlet end of the borehole. The temperature inside the borehole is measured using four “K” type thermocouples, which are kept at a distance 8 cm apart from each other. The product gas is collected at every 10 minutes interval of the experiments using vacutainer tubes and, the collected gas samples are analyzed using a gas chromatograph (GC). The calorific value of the syngas is calculated using the heat of combustion of each product gas. H2 + 0.5 O2 → H2O (g)

ΔHr = ‒ 241.8

kJ mol

(7)

CO + 0.5 O2 → CO2

kJ ΔHr = ‒ 282.8 mol

(8)

CH4 + 2 O2 →CO2 + 2H2O(g)

ΔHr = ‒ 802

𝐶2H + 3.5 O2 →2CO2 + 3H2O(g)

ΔHr = ‒ 1428

6

kJ mol kJ mol

Lower heating value (LHV) of the product gas = ∑(ni ΔHr ) i

(9)

(10) (11)

Where ‘ni’ is the mole fraction of the gas species and ΔHr is the heat of combustion of the gas. i

In the present study, the laboratory scale UCG experiments are carried out in a two stage gasification mode. During the first stage (combustion stage), the oxidants such as air and O2 are supplied to the borehole for the progress of combustion reaction. When a high temperature flame front (~1000 °C) is established inside the borehole, the supply of oxidant (air) is switched off. Then, CO2 gas is injected immediately (second stage) for the progress of dry reforming and Boudouard reaction. Once the temperature of the combustion front drops below 800°C, the supply of CO2 gas is switched to air. This procedure is repeated cyclically to ensure the existence of high temperature zones and high surface area of coal for efficient gasification. In the present study, six experiments are carried out using the low ash and the high ash coals. The effect of process parameters such as air/O2 molar ratio, CO2/oxidizing agent molar ratio and initial combustion period on the product gas composition is analyzed. 3. Result and Discussion

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The use of air as a gasifying medium in the UCG borehole combustion experiments reduced the combustion zone temperature as low as 600ºC. As a result, a low quality syngas is produced. Therefore, the experiments are carried out with the addition of pure oxygen into the air stream for establishing high temperature zones during the first stage of the UCG experiment. Each UCG experiment has undergone three different period of reaction phases such as (i) initial O2 combustion, (ii) air-O2 combustion and (iii) two stage air-O2-CO2 gasification. The initial O2 combustion phase is the ignition period, which establishes a well sustained combustion front in the borehole. Then, the concentration of O2 in the feed gas is reduced by increasing the flow rate of air to the extent that the temperature of the borehole is maintained as high as 700°C during the O2-air combustion phase. It is noted from the experimental results that the syngas is produced with a significant quantity of H2 and CH4 due to the progress of CO2 dry reforming and cracking reactions. Dry reforming reactions would progress only at high temperature conditions (>1000°C). However, activated char can act as a catalyst for the CO2 dry reforming reaction [23] and reduces the activation energy. Devolatilized coal char near the adjacent zones of a combustion front in the borehole would serve as a catalytic site for the dry reforming reaction under UCG conditions. Table 2 shows the average values of the composition and the calorific value of the product gas during each run of the experiments. 3.1. Repeatability of the experiments Two experiments (Expt. #3 & Expt. #6) are repeated under similar operating conditions for examining the reliability of the obtained data. The repeatability of the experimental results can be examined by comparing the calorific values of Expt. #2 and #3 for low ash coals and Expt. #5 and #6 for high ash coals. Table 3 shows the error analysis of these experiments and the percentage error is calculated as follows, % error = Difference in calorific value of syngas between the repeatability expt. original expt. syngas calorific value (12)

It can be seen that a maximum variation of 7% and 14.6% is estimated in the calorific values of the syngas during the CO2 gasification of the low ash coal and the high ash coal, respectively. This variation may be due to the heterogeneity nature and the non-similarity in the dimensions of the coal blocks.

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3.2. Effect of oxidizing agents (O2 and O2 enriched air) on product gas composition The effect of oxidizing agents such as O2 and air on the product gas composition can be seen during the first two phases of the experiments (O2 and air-O2 combustion). In the case of the low ash coal, these effects are observed during the first two runs of Expt. #1, #2 and #3. Figure 2 shows the composition and the calorific value of the product gas during the low ash coal gasification (Expt. #1). It can be observed a large deviation in the profile of the product gas composition at each 15 minutes period during the CO2 gasification period (Figure 2). This is due to the two stage gasification mode of UCG operation. The alternating air combustion and CO2 gasification stages led to the cyclic production of low and high calorific value syngas, respectively. During the pure O2 combustion period, a high temperature (~1200°C) flame front is established and, a syngas having the average calorific value in the range of 115-134 kJ/mol (Table 2: run#1a, #2a, #3a) is obtained. With the addition of air to the O2 feed stream (O2 enriched air combustion period), the calorific value of the product gas is reduced (run #1b, #2b, #3b). Thus, the O2-enriched air feed gas reduced the calorific value of the syngas, which is equivalent to 75% of the heating value of the pure O2 combustion based syngas. Figure 3 shows the temperature profile of the borehole during the low ash coal gasification (Expt. #1). One can notice the corresponding decrease in the temperature of the borehole from 1200ºC to 800ºC due to the air dilution of the feed gas. The average percentage composition of hydrogen in the syngas is increased from 9% to 14.7% while shifting the feed gas from the O2 combustion phase to the air-O2 phase. This shows that a significant quantity of hydrogen gas also burnt during the pure O2 feed phase. In the case of high ash coals, it is found difficult for initial ignition at the low flow rates of O2 from 0.2 to 0.4 lpm (Exp. #4, #5, #6). However, a sustained combustion front is established at 0.5 lpm of O2 gas and, a syngas with calorific value in the range of 122-130 kJ/mol is produced (run #4a, #5a, #6a). The temperatures in the range of 800°C to 1000°C is measured in the high ash coal cavity; however, it is less as compared to low ash coals due to the accumulation of ash layer in the cavity. Further, during the O2-air combustion phase, the heating value of the product gas is estimated in the range of 72-122 kJ/mol (run #4b, #5b, #6b). The calorific value gases such as H2, CH4 and CO having each of about 5 to 10% (vol. %) are found in the product gas. The addition of air with O2 at 1:1 ratio led to the reduction of the calorific value of the product gas in the range of ½ to ¾ of the heating value of the pure oxygen based syngas.

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3.3. Effect of O2/air molar ratio on product gas composition Exp. #1, #2, #3 are carried out at various O2/air molar ratios of 1, 0.67 and 0.11 using the low ash coal blocks. At the O2/air molar ratio of 1 and 0.67, the product gas is obtained with a heating value of 245 kJ/mol (run #1c) and 178 kJ/mol (run #1d), respectively. The specific coal block of Expt. #1 is comparatively shorter in length and thus, the residence time of the gaseous reactants in the active gasification zones decreased gradually with the propagation of the flame front towards downstream of the borehole. As a result, the calorific value of the product gas declined at the end of Expt. #1 (run #1d) at 0.67 O2/air molar ratio. In Expt. #2, it can be seen that at 0.11 molar ratio of O2/air, a high calorific value of syngas (243 kJ/mol) is estimated (run #2c) during the CO2 gasification period. Figure 4 shows the composition and the calorific value of the product gas at constant O2/air feed molar ratio during the low ash coal gasification (Expt. #2). It can be seen that an equimolar proportion of methane and hydrogen is produced until the end of 5 hour of the experiment at 0.2 lpm of CO2. The cracking and reforming reactions between tar and CO2 increased the production rate of H2 and CH4 with each gas of 18.5% molar composition in the syngas (run #2c). Also, it is found that the adequate temperature in the range of 800 to 900°C for gasification can be maintained in the borehole at the O2/air molar ratio as low as 0.11. Hence, further experiments are carried out at 0.11 molar ratio of O2/air. The effect of O2/air molar ratio on the product gas composition of the high ash coal is studied in Expt. #4, #5 and #6. It is observed that the syngas quality decreases with reduction in the molar ratio of O2/air. The average calorific value of the product gas in the range of 189-200 kJ/mol (run #4c, #5c, #6c) is estimated at the O2/air molar ratio of 1. Figure 5 shows the composition and the calorific value of syngas during the high ash coal gasification (Expt. #4). The calorific value gases such as CH4, H2 and CO are estimated in the range of 10-20%, 12-35% and 5-20%, respectively (Fig. 5a). The results of Expt. #4 show the production of a high quality syngas with calorific value in the order of 200 to 250 kJ/mol (Fig. 5b) at the O2/air molar of 1. Expt. #5 is carried out at various O2 /air molar ratios of 1, 0.67, 0.43, 0.25 and 0.11. Figure 6 shows the composition and the calorific value of the product gas during high ash coal gasification of Expt. #5. The syngas with hydrogen and methane concentrations as high as 20% and 15% is produced, respectively (Figure 6a). It can be observed that the calorific value of the gases decreased continuously with the decrease in the O2/air ratio (Figure 6b). Figure 7 shows the temperature profile of the high ash coal borehole

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gasification during Expt. #5. With the decrease in the molar ratio of O2/air, the temperatures of the borehole dropped as low as 600°C. At 0.11 molar ratio of O2/air, the syngas calorific value of 138 kJ/mol (run #5g) is estimated for the high ash coals whereas the calorific value is almost doubled (~260 kJ/mol) at this molar ratio for the low ash coals (run #2c). Thus, it is evident that the presence of significant proportion of volatile matter in the low ash coal generated a high quality syngas as compared to high ash containing coals. 3.4. Effect of CO2/oxidant molar ratio on product gas composition The effect of CO2/oxidant molar ratio on the product gas composition of the low ash coal is studied in Expt. #3. Figure 8 shows the calorific value of the product gas for Expt. #3. The CO2/oxidant molar ratio in the feed gas is varied in the range of 0.2 to 0.5 (run #3c to run #3f). It can be seen that the calorific value of the syngas decreased gradually with increase in the CO2/oxidant molar ratio. At 0.5 molar ratio of CO2/oxidant, a syngas with a lowest average calorific value of 65 kJ/mol is produced (run #3f). It can be concluded that the optimum flow rate of CO2 is 0.2 lpm for the O2/air molar ratio of 0.11 for the low ash coals. Whereas, in the case of high ash coals, with the increase in the CO2 flow rate to 0.3 lpm, the combustion front extinguished in the borehole. Thus, the optimal molar ratio of CO2/oxidant is 0.2 for the high ash coals at the O2/air molar ratio of 1. 4. Conclusions The CO2-air based in-situ coal gasification is simulated in the laboratory scale borehole combustion experiments under two stage operational mode. The results show that the combination of CO2/air is a potential feed gas for the effective utilization of deep coal resources. The following conclusions can be drawn from the present study. (i) The CO2-air gasification of the low ash coal produced a syngas with high calorific value in the range of 245 - 260 kJ/mol. (ii) The gasification of the high ash coal using the CO2-air feed gas generated a syngas with heating value in the range of 197-214 kJ/mol. (iii) The CO2-air gasification required a minimal supply of O2 along with feed gas for sustaining the temperature in the range of 700-800ºC in the reaction zones. High ash coal and low ash coal require 1 and 0.1 molar ratio of O2/air during the borehole gasification, respectively.

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The present study shows the feasibility of utilization of CO2 (greenhouse gasification agent) and air (cheap oxidizing medium) for in-situ gasification of deep coal resources. Further, a field scale study is required for commercializing this technology. References [1]

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Figure caption

Figure 1. Photographic view of (a) sliced coal block with centre borehole, (b) clay moulded condition of coal blocks and (c) complete borehole experimental set-up

Figure 2. (a) Composition and (b) calorific value of the product gas during two stage gasification of low ash coal with the variation of O2/air feed molar ratio (Expt. #1)

Figure 3. Temperature profile of the borehole during the two stage gasification of low ash coal with the variation of O2/air feed molar ratio (Expt. #1) (TC1, TC2, TC3, TC4 – thermocouples at first, second, third and fourth position in the borehole, respectively)

Figure 4. (a) Composition and (b) calorific value of the product gas during the two stage gasification of low ash coal with constant O2/air feed molar ratio (Expt. #2)

Figure 5. (a) Composition and (b) calorific value of the product gas at 1 and 0.67 O2/air feed molar ratio during the two stage gasification of high ash coal (Expt. #4)

Figure 6. (a) Composition and (b) calorific value of the product gas at variable O2/air feed molar ratios during the two stage gasification of high ash coal (Expt. #5)

Figure 7. Temperature profile of the borehole at variable O2/air feed molar ratios during two stage gasification of high ash coal (Expt. #5) (TC1, TC2, TC3 – thermocouples at first, second and third position in the borehole, respectively)

Figure 8. Calorific value of the product gas and flow rate of feed O2 and CO2 gas during CO2/O2 gasification during two stage gasification of low ash coal with variable CO2/oxidant molar ratio (Expt. #3)

ACCEPTED MANUSCRIPT

(a)

(b)

(c)

Figure 1. Photographic view of (a) sliced coal block with center borehole, (b) clay moulded condition of coal blocks and (c) complete borehole experimental set-up

ACCEPTED MANUSCRIPT

Gas composition (vol %)

(a) 35 H2 CO CH4 C2H6

30 25 20 15 10 5 0 1

2

3

Time (hrs)

4

5

6

(b) Calorific value-O2/Air combustion

Calorific value-CO2 gasification

O2 flow rate

Air flow rate

350

0.7

300

0.6

250

0.5

200

0.4

150

0.3

100 50

0.2 Pure O2 combustion

O2/Air combustion

0.1

CO2 gasification

0

0 0

1

2

3

Time (hrs)

4

5

6

Figure 2. (a) Composition and (b) calorific value of the product gas during two stage gasification of low ash coal with the variation of O2/air feed molar ratio (Expt. #1)

Feed gas flow rate (lpm)

Calorific Value (kJ/mol)

0

ACCEPTED MANUSCRIPT

O2/Air combustion

1400

Temperature (˚C)

1200

CO2 gasification

TC1 at 5 cm from inlet TC3 at 10 cm from TC2

TC2 at 10 cm from TC1 TC4 at the oulet hole

2

4

1000 800 600 400 200 0 0

1

3

5

6

Time (hrs)

Figure 3. Temperature profile of the borehole during the two stage gasification of low ash coal with the variation of O2/air feed molar ratio (Expt. #1) (TC1, TC2, TC3, TC4 – thermocouples at first, second, third and fourth position in the borehole, respectively)

Gas composition (vol %)

ACCEPTED MANUSCRIPT

(a)

35 H2 CO CH4 C2H6

30 25 20 15 10 5 0 0

1

2

3

4

Time (hrs)

5

6

7

Calorific value-O2/Air combustion O2 Flow rate CO2 flow rate

Calorific value-CO2 gasification Air flow rate

350

1.2

300

1

250

0.8

200

0.6

O2/Air combustion

150

CO2 gasification

0.4

100

0.2

50 0

0 0

1

2

3

Time (hrs)

4

5

6

7

Figure 4. (a) Composition and (b) calorific value of the product gas during the two stage gasification of low ash coal with constant O2/air feed molar ratio (Expt. #2)

Feed gas flow rate (lpm)

Calorific value (kJ/mol)

(b)

Gas composition (vol %)

ACCEPTED MANUSCRIPT

(a)

40 H2 CO CH4

35 30 25 20 15 10 5 0

1

2

Time (hrs)3

4

5

6

(b) Calorific value- O2/Air combustion

Calorific value- CO2 gasification

O2 flow rate

CO2 flow rate

350

0.7

300

0.6

250

0.5

200

0.4

150

0.3

100 50

0.2

CO2 gasification

O2/Air combustion

0.1

Time (hrs)

0

0 0

1

2

Time3(hrs)

4

5

6

Figure 5. (a) Composition and (b) calorific value of the product gas at 1 and 0.67 O2/air feed molar ratio during the two stage gasification of high ash coal (Expt. #4)

Feed gas flow rate (lpm)

Calorific value (kJ/mol)

0

ACCEPTED MANUSCRIPT

Gas composition (vol %)

(a)

30 H2 CH4

25

CO C2H6

20 15 10 5 0 1

2

3

4

Time (hrs)

5

6

7

8

(b) Calorific value-Air combustion O2 flow rate CO2 flow rate

300

Calorific value-CO2 gasification Air flow rate

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

250 200 150 100

(c)

50

O2/Air combustion

CO2 gasification

0 0

1

2

3

4

Time (hrs)

5

6

7

8

Figure 6. (a) Composition and (b) calorific value of the product gas at variable O2/air feed molar ratios during the two stage gasification of high ash coal (Expt. #5)

Feed gas flow rate (lpm)

Calorific value (kJ/mol)

0

ACCEPTED MANUSCRIPT

TC2 at 10 cm from TC1

TC3 at 10 cm from TC2

TC4 at the outlet hole

O2/Air combustion

1000

Temperature (˚C)

TC1 at 5cm from inlet

CO2 gasification

900 800 700 600 500 400 300 200 100 0 0

1

2

3

4

5

6

7

8

Time (hrs)

Figure 7. Temperature profile of the borehole at variable O2/air feed molar ratios during two stage gasification of high ash coal (Expt. #5) (TC1, TC2, TC3 – thermocouples at first, second and third position in the borehole, respectively)

Calorific value (O2/air gasification) O2 flow rate CO2 flow rate

Calorific value (CO2 gasification) Air flow rate

300

1.2

250

1

200

0.8

150

0.6

100

0.4

50

0.2

0 0

1

2

3

Time (hrs) 4 Time (hrs)

Feed gas flow rate (lpm)

Calorific Value (kJ/mol)

ACCEPTED MANUSCRIPT

0 5

6

7

8

Figure 8. Calorific value of the product gas and flow rate of feed O2 and CO2 gas during CO2/O2 gasification during two stage gasification of low ash coal with variable CO2/oxidant molar ratio (Expt. #3)

ACCEPTED MANUSCRIPT

Table 1: Proximate and ultimate analysis of coals used in the present study

S. No. Fuel

Proximate analysis (weight %)

Ultimate analysis (weight %)

Moisture content

Volatile Ash matter

Fixed carbon

C

H

N

O

S

Lower Heating Value (MJ/kg)

1

Low ash coal

7

42

4

47

78.3

5.5

2.3

12.0

1.8

31.98

2

High ash coal

1

19

42

38

50.17

2.7

1.8

45.3

0

23.14

Table 2. Composition and calorific values of the product gas during CO2-air based borehole gasification Expt. No.

Ru n No.

Coal used

Total time

Feed flow rate (lpm)

(hours)

Time interval (hours)

O2 CO2 Air Exp.#1

Exp.#2

Exp.#3

Exp.#4

Exp.#5

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d 3e 3f 4a 4b 4c 4d 5a 5b 5c 5d 5e 5f 5g

Low ash coal

6

Low ash coal

6.5

7.5 Low ash coal High ash coal

5.5

7 High ash coal

0.5 0.5 0.5 0.2 0.4 0.2 0.2 0.1 0.1 0.2 0.1 0.4 0.2 0.1 0.1 0.2 0.1 0.3 0.1 0.4 0.1 0.5 0.5 0.5 0.5 0.2 0.4 0.2 0.5 0.5 0.5 0.2 0.4 0.2 0.3 0.2 0.2 0.2 0.1 0.2

0.5 0.5 0.6 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.5 0.5 0.6 0.5 0.5 0.6 0.7 0.8 0.9

0-0.5 0.5-2.0 2.0-4.0 4.0-6.0 0-0.25 0.25-0.5 0.5-5.5 5.5-6.5 0-0.25 0.25-0.5 0.5-5.5 6.5-7.0 5.5-6.5 7.0-7.5 0-0.5 0.5-0.75 0.75-3.5 3.5-5.5 0-1.75 1.75-2.25 2.25-3.0 3.0-4.0 4.0-5.5 5.4-6.25 6.25-6.75

Molar ratio

Average calorific Value of product gas (kJ/mol)

Product gas composition (%)

O2/air

CO2/(O2+air)

H2

CO

CH4

CO2

O2

N2

C2H6

1 1 0.67 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 1 1 0.67 1 1 0.67 0.43 0.25 0.11

0.2 0.2 0.2 0.4 0.2 0.3 0.4 0.5 0.2 0.2 0.2 0.2 0.2 0.2 0.2

9.1 14.7 23.6 20.8 7.8 6.8 18.4 11.7 6.8 7.2 19.6 11.7 7.4 7.0 10.8 8.1 24.9 19.5 10.8 7.3 14.2 17.2 16.0 12.1 11.5

8.9 6.8 8.8 7.2 7.4 6.8 9.2 7.3 8.6 12.1 7.7 3.5 2.8 2.6 9.0 8.3 9.2 7.6 11.7 6.1 9.9 8.8 12.9 17.2 8.9

8.8 5.5 18.4 11.1 4.8 4.0 19.5 5.8 7.0 3.8 19.2 9.6 6.4 3.6 9.0 5.9 15.5 15.0 7.2 4.6 13.2 12.6 14.8 10.3 9.5

68.5 39.4 42.8 54.9 63.2 27.2 45.8 71.6 73.7 29.5 51.1 81.9 79.8 86.3 65.0 37.0 42.9 48.8 61.0 40.3 58.4 57.0 52.1 55.7 66.3

4.4 7.0 5.3 4.7 12.9 9.4 4.8 3.1 2.7 10.3 4.3 2.7 2.4 2.4 6.1 6.1 5.9 3.8 9.0 8.1 3.5 3.7 4.0 4.2 3.11

26.1 44.3 36.2 34.2 33.6 -

0.29 0.59 1.15 1.25 3.9 1.6 2.3 0.38 1.3 0.6 1.6 1.0 1.0 0.76 0.66 0.38 0.3 0.53 0.37 0.07 0.73 0.65 0.03 0.30 0.5

121.9 107.2 245.8 178.1 134.1 90.4 260.9 101.1 115.6 90.9 246.0 130.3 94.1 64.0 125.0 95.0 214.0 196.8 122.1 72.7 189.7 176.9 194.6 165.9 138.0

Exp.#6

6a 6b 6c 6d 6e 6f

6 High ash coal

0.5 0.5 0.5 0.4 0.3 0.2

0.2 0.2 0.2 0.2

0.5 0.5 0.6 0.7 0.8

0-0.5 0.5-1.25 1.25-1.75 1.75-3.25 3.25-5.25 5.25-6

1 1 0.67 0.43 0.25

0.2 0.2 0.2 0.2

9.45 8.8 15.5 16.5 15.9 16.7

10.5 11.2 8.7 12.0 11.7 10.6

8.6 7.5 15.2 15.5 17.5 14.4

57.0 27.3 52.9 49.8 50.4 54.9

13.9 13.4 6.7 6.1 4.4 3.1

31.4 -

0.59 0.61 0.91 0.1 0.1 0.21

129.9 122.0 197.4 199.8 213.3 190.1

ACCEPTED MANUSCRIPT

Table 3. Percentage error in the calorific value of syngas between the repeated experiments for low and high ash coal

Coal

Run No.

Flow rate (lpm) O2 CO2 Air

Low ash coal (Expt. #2 & #3)

2c & 3c

0.1

0.2

0.9

% error in calorific value of product gas in the repeated experiments 5.7

2d & 3e

0.1

0.4

0.9

6.9

5c & 6c

0.5

0.2

0.5

-4.1

5d & 6d

0.4

0.2

0.6

-12.9

5e & 6e

0.3

0.2

0.7

-9.6

5f & 6f

0.2

0.2

0.8

-14.6

High ash coal (Expt. #5 & #6)