Energy Fuels 2009, 23, 4887–4892 Published on Web 08/25/2009
: DOI:10.1021/ef900461w
Effect of Steam Partial Pressure on Gasification Rate and Gas Composition of Product Gas from Catalytic Steam Gasification of HyperCoal Atul Sharma,* Ikuo Saito, and Toshimasa Takanohashi Advanced Fuel Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba, Ibaraki, Japan Received May 15, 2009. Revised Manuscript Received July 22, 2009
HyperCoal was produced from coal by a solvent extraction method. The effect of the partial pressure of steam on the gasification rate and gas composition at temperatures of 600, 650, 700, and 750 °C was examined. The gasification rate decreased with decreasing steam partial pressure. The reaction order with respect to steam partial pressure was between 0.2 and 0.5. The activation energy for the K2CO3-catalyzed HyperCoal gasification was independent of the steam partial pressure and was about 108 kJ/mol. The gas composition changed with steam partial pressure and H2 and CO2 decreased and CO increased with decreasing steam partial pressure. By changing the partial pressure of the steam, the H2/CO ratio of the synthesis gas can be controlled.
extracted coal, here called HyperCoal (HPC), has less than 500 ppm of ash. Because of its ashless characteristics, a catalytic gasification process for coal may be developed using HyperCoal as a feed material leading to low gasification temperature and easy recovery and recycling of the catalyst. In our first study,13 we reported high gasification rates at temperatures as low as 775 °C with no catalyst deactivation, feasibility of catalyst recovery and recycling, and H2 selectivity from catalytic gasification of Oaky Creek HyperCoal. In the second study,14 the effect of catalyst addition on the gasification reactivity of HyperCoal and its parent coal was compared at 700 and 775 °C, and it was found that HyperCoal and coal have nearly same rates at 700 and 775 °C but at different catalyst loadings. In the following study,15 gasification rates of HyperCoals prepared from three different ranks of parent coals in the 600-775 °C range were compared. In a recent study,16 the effect of the catalyst mixing procedure and catalyst loading ratio on the gasification rate was examined. Production of synthesis gas (H2/CO) at such low temperatures would also be an attractive application for HyperCoal. However, the results of our previous studies13-16 showed that, irrespective of gasification temperature and coal type, the product gas from the K2CO3-catalyzed steam gasification of HyperCoal contained H2 and CO2 as the major products gases with little CO and was suitable for H2 production. In this study, the effect of steam partial pressure on reaction rate and on composition of the gas produced from catalytic steam gasification of HyperCoal has been investigated. The parameters investigated were gasification temperatures of 600, 650, 700, and 750 °C, catalyst loading of 50 wt %, and steam partial pressures of 0.5, 0.1, and 0.02 atm.
Introduction Gasification is a primary conversion route to produce synthesis gas (H2 and CO) from coal.1-12 Gasification is an endothermic reaction and requires temperatures above 1000 °C to achieve acceptable rates for commercial application.1-6 The product gas obtained at such high temperatures usually has low H2/CO (0.5-0.7). The catalytic gasification of coal has been widely considered to be an effective means to decrease the gasification temperature and control the composition of the product gas.8-12 The most favored catalysts are alkali metal salts especially K2CO3.2-16 However, the major drawback in catalytic gasification of coals is the interaction of catalyst with the mineral matter (ash) present in the coals leading to the formation of compounds from which recovery of the catalyst is difficult.1-8,12 To overcome the problem of loss of catalyst, our research group has developed a process to remove mineral matter from coal by solvent extraction.13-17 The solvent *Corresponding author. E-mail:
[email protected]. (1) Takarada, T; Tamai, Y; Tomita, A. Fuel 1985, 64, 1438–42. (2) Nahas, N. C. Fuel 1983, 62, 239–241. (3) Veraa, M. J.; Bell, A. T. Fuel 1978, 57, 194–200. (4) McKee, D. W.; Chatterji, D. Carbon 1975, 13, 381–390. (5) Kayembe, A; Pulsifer, A. H. Fuel 1976, 55, 211–216. (6) Wigmans, T; Elfring, R; Moulijn, J. A. Carbon 1983, 21 (1), 1–12. (7) Formella, K; Leonhardt, P; Sulimma, A; van Heek, K. H.; Juntgen, H. Fuel 1986, 65, 1470–72. (8) Miura, K; Aimi, M; Naito, T; Hashimoto, K. Fuel 1986, 65, 407– 11. (9) Kwon, T. W.; Kim, J. R.; Kim, S. D.; Park, W. H. Fuel 1989, 68, 416–421. (10) Lee, J. W.; Kim, S. D. Fuel 1995, 74, 1387–1383. (11) Schumacher, W.; Muhlen, H. J.; van Heek, K. H.; Juntgen, H. Fuel 1986, 65, 1360–1363. (12) Chin, G.; Liu, G.; Dong, Q. Fuel 1987, 66, 859–863. (13) Sharma, A; Takanohashi, T; Morishita, K; Takarada, T; Saito, I. Fuel 2008, 87, 491–97. (14) Sharma, A; Takanohashi, T; Saito, I. Fuel 2008, 87, 2686–2690. (15) Sharma, A; Saito, I.; Takanohashi, T. Energy Fuels 2008, 22 (6), 3561–3565. (16) Sharma, A; Kawashima, H.; Saito, I.; Takanohashi, T. Energy Fuels 2009, 23 (4), 1888–1895. (17) Okuyama, N; Komatsu, N; Shigehisa, T; Kaneko, T; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947–967. r 2009 American Chemical Society
Experimental Procedures A subbituminous coal, Pasir (PAS) from Indonesia was selected for the investigation. The HyperCoal production method17 and gasification procedure15,16 has been described in detail elsewhere. Briefly, HyperCoal (HPC) was produced by the solvent extraction of the coal with 1-methylnaphthalene at 360 °C and subsequently separating the extract (HyperCoal) 4887
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Energy Fuels 2009, 23, 4887–4892
: DOI:10.1021/ef900461w
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Table 1. Properties of Coal and HyperCoal elemental analysis (wt % daf) coal
ash (wt %, db)
Pasir coal Pasir HyperCoal (HPC)
4.2 0.01
C
H
N
68.2 5.4 1.1 79.5 5.9 1.1
S
O
0.16 25.14 0.29 13.21
Figure 2. Weight loss profile of HPC pyrolyzed in argon up to 650 °C followed by steam gasification.
desired temperature at 20 °C/min in pure argon. When the desired temperature was reached without any hold time, the pure argon gas was switched to the preset steam/argon gas mixture giving an expected 0.5, 0.1, and 0.02 atm of steam partial pressure over the sample. The steam flowing from top comes into contact with the sample in the crucible. The evolved gases flow out together with the purge gas from the side into an ice cooled tar trap to remove tar before injecting to the micro gas chromatograph (Agilent 3000A). The total gas flow rate at the outlet was measured every 3 min by a film flow meter.
Figure 1. Schematic of the experimental setup for catalytic steam gasification of HyperCoal.
from the solvent. The extraction yield was 51% for Pasir coal. The properties of the Pasir coal and HyperCoal produced from Pasir coal are shown in Table 1. HyperCoal has nearly no mineral matter. Because it has almost no mineral matter, all the inorganically associated sulfur will be removed. The only sulfur in HyperCoal will be the organically associated sulfur. A detailed characterization of catalyst, HyperCoal, original coal, and chars using X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy energy dispersive Xray (SEM-EDX) mapping techniques had been carried out and reported elsewhere.13,15,16 The experimental setup is shown in Figure 1. In the present study, all samples for catalytic gasification experiments were prepared with 50% catalyst loading. Catalyst loading was on a dry and ash free weight percent basis of coal and HPC. Both dry mixing and wet mixing methods were investigated. The catalyst mixing method has been described in detail elsewhere.16 Briefly, a desired amount of K2CO3 was added on the top of a measured sample already loaded into a test crucible as solid particles and stirred with a small spatula until white K2CO3 disappears by capturing moisture from the air. A common procedure is to mix K2CO3 as an aqueous solution for homogeneous dispersion,8-12 but it could not be applied as HyperCoal does not mix with water. The particle size of the coal and HyperCoal samples was under 75 μm. The gasification experiments were carried out at 600, 650, 700, and 750 °C and at 0.5, 0.1, and 0.02 atm steam partial pressure. Experiments were carried out in a thermogravimetric (TG-DTA 2020S, MAC) apparatus with about 20 mL/min argon (Ar) as the TG carrier gas flowing from the bottom (Figure 1). At the start of the experiment, 110 mL/min Ar was mixed with 20 mL/min oxygen (O2) and flowed from the top into the TG-DTA. At the same time, a desired amount of water was pumped by an HPLC pump to a steam generator held at 250 °C. Argon was also flowed to the steam generator as a steam carrier gas. By changing the amount of water pumped by the HPLC pump to the steam generator, and the flow rate of argon as the carrier gas, steam partial pressures of 0.5, 0.1, and 0.02 atm were achieved. A fourway valve at the inlet of the TG-DTA was used to change the Ar þ O2 flow to Ar þ steam flow. The flow lines were kept at 250 °C by using ribbon heaters. First, a desired amount of sample was heated in O2 þ Ar flow up to 200 °C and held for 5 min to remove moisture and reduce the swelling propensity of the HyperCoal by mild preoxidation. After a 5 min hold at 200 °C, the gas was switched to pure argon flow for 60 min to remove the O2 from the reaction zone. After a 60 min hold, the sample was heated to the
Results and Discussion Figure 2 shows a typical weight loss curve for HPC pyrolyzed in argon up to 650 °C followed by steam gasification at 650 °C and 0.5 atm partial pressure of steam with 50% K2CO3. In a previous study,16 the gasification rate and gas composition was investigated at 10, 20, 40, and 50% catalyst mixing ratios. It was reported that the gasification rate was affected by the catalyst amount but gas composition was not affected by the catalyst amount. It was also reported that after 50% catalyst loadings rates were almost independent of catalyst amount. Therefore, in the present study only one catalyst mixing ratio, 50% catalyst loading, has been selected. In actual process, 40-50% catalyst loadings would be very high as they may cause problems related to the access of reactant gas to the reaction surface at medium to high conversion levels and lower loadings such as 20-30% would be more appropriate. All experiments were carried out at atmospheric pressure. The weight loss curve can roughly be divided into three stages; the moisture removal or drying stage, devolatilization stage, and fixed-carbon gasification stage. The initial weight loss (up to 200 °C) during heating from room temperature to 200 °C in an O2 þ Ar mixture is mainly due to moisture captured from air. Preoxidation was done to reduce the extremely high swelling propensity of HyperCoal. After the preoxidation stage, the sample was switched to 100% argon for 60 min. The coal/HPC conversion on dry, ash, catalyst, and volatile free basis (dacvf) (from here on called char conversion) was calculated during the fixedcarbon gasification stage by the following equation: X ðchar conversion, % dacvfÞ ¼
W0 -W 100 W0 ð1 -Wash -Wcat Þ
ð1Þ
where W0 is the weight when the gasification begins (db, mg) (weight at t=39, 42, 45, and 49 min for T=600, 650, 700, and 750 °C, respectively), W is the weight at any gasification time (db, mg, >39 min), Wash is the weight fraction of ash content 4888
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interaction has been a major hurdle for developing a commercial process.2,7,12 As reported in our previous reports,13-16 catalyst was recycled four times without observing any significant decrease in rate suggesting that the catalyst deactivation problem can be overcome by using HPC which does not contain minerals. Figure 3b shows the total gas yield and amount of CO, CO2, and H2 gas evolved per gram of char from coal and HPC without and with K2CO3. Formation of CH4 was very small and can be assumed to be negligible for the present discussion. Without K2CO3, the total gas yield and amount of each gas evolved were nearly same for coal and HPC. On addition of K2CO3, the total gas yield increased significantly in comparison to that without a catalyst. In addition, amount of H2 evolved during catalytic gasification was several times higher than that without a catalyst. The amount of carbon evolved as CO and CO2 was higher on addition of catalyst for both coal and HPC suggesting that the amount of char increased on addition of K2CO3. Since the total yield of gas produced without and with catalyst is different, it will be better to compare the fraction of each gas produced on a mole percent basis. The mole percent fraction of each gas was calculated by making the total yield equal to 100 mol, and the fraction of each gas was then calculated from Figure 3b. Figure 3c compares the mole percent fraction of each gas in the produced gas. Without catalyst, the gas composition was H2 58-50%; CO2 43-35%; CO 5-6%. With addition of catalyst, the gas composition was H2 63-62%; CO2 37-35%; CO 3-2%. The H2/CO ratio was 10 without catalyst and increased to 20 and higher on addition of catalyst. The fraction of H2 was nearly 1.8 times the fraction of CO2 when catalyst is used suggesting that the addition of K2CO3 also enhanced the water-gas shift reaction 2 in addition to carbon-steam gasification reaction 1.8-12,18 C þ H2 O f CO þ H2
ð1Þ
CO þ H2 O h H2 þ CO2
ð2Þ
Figure 4 shows the effect of steam partial pressure on gasification rate at 600, 650, 700, and 750 °C. At 600 and 650 °C, experiments were made with 0.5 and 0.1 atm steam partial pressure only as the gasification rates below 0.1 atm partial pressure became quite low. The conversion profiles shown in Figure 4 actually represent the progress of C-H2O reaction 1. Any change observed in conversion profiles shown in Figure 4 indicates the change in reaction 1.8 Reaction 2 is more representative of the gas composition of the produced gas.8,11 The results show that gasification rates decreased with decreasing steam partial pressure. This decrease suggests that lowering of the partial pressure slowed down the rate of reaction 1. For simple understanding and comparison of gasification reactivity, the time needed to reach 50% conversion was obtained from the gasification profiles for all the runs and is shown in Table 2. Rate constants were obtained by inversing this time value. The rate constants thus obtained were plotted against steam partial pressure according to eq 3 to investigate the relationship between the steam partial pressure and reaction rate.8,18
Figure 3. Comparison of (a) gasification profiles, (b) cumulative gas yield, and (c) mole percent fraction of product gas, for coal and HPC at 700 °C with and without K2CO3.
in coal or HPC, Wcat is the weight fraction of catalyst content. Results only in the char gasification stage will be discussed further. Figure 3a compares the steam gasification profiles of coal and HPC with and without K2CO3 at 700 °C and at 0.5 atm steam partial pressure. Both coal and HPC without catalyst show extremely slow rates. Gasification rates of coal and HPC were nearly same. On addition of K2CO3, the gasification rate of coal and HPC increased dramatically due to the catalytic effect of K2CO3. Nearly 100% conversion was achieved in 50 min for both samples. The results confirmed the wellknown fact2-12 that rate of steam gasification of carbon can be enhanced by the addition of alkali metals. However, with coal loss of catalyst due to the formation of inactive K-mineral compounds by K2CO3-mineral matter
k ¼ k0 PH2 O n
ð3Þ
(18) Levenspiel, O. Chemical Reaction Engineering; John Wiley & Sons: New York, 1972; pp 357-408.
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Figure 4. Effect of steam partial pressure on the conversion rate of K2CO3-catalyzed HPC gasification at different temperatures. Table 2. Time Taken to Achieve 50% Char Conversion t50 [min] steam partial pressure [atm] T [°C]
0.5
0.1
0.02
750 700 650 600
2.1 3.0 6.7 25.0
4.4 6.7 11.4 35.7
12.5 16.4
Figure 5a shows the plot between ln(p) and ln(k). At 600-650 °C, n varied from 0.2-0.3, and at 700-750 °C, n was around 0.5. For noncatalytic steam gasification of carbon, the rate increases in proportion to the partial pressure and the reaction order is first-order with respect to the partial pressure of steam up to atmospheric pressure and zero-order at higher pressures.8-10 For K2CO3-catalyzed char steam gasification, Chin et al.12 reported that the reaction rate was proportional to steam partial pressure while Lee at al.10 reported that the reaction order approaches zero. In the present study, the reaction order with respect to steam partial pressure was 0.2 in 600-650 °C and 0.5 in 700-750 °C. It may be appropriate to conclude that the reaction rate of K2CO3catalyzed steam gasification of char is affected by the steam partial pressure but not to the same extent as in the case of noncatalytic steam gasification. The effect of partial pressure on activation energy can be more appropriately discussed in terms of an Arrhenius plot [ln(k) vs 1/T] at different steam partial pressures.8-12,15,16 Figure 5b shows the Arrhenius plots [ln(k) vs 1/T] in the 600-750 °C temperature range at 0.5 and 0.1 atm steam
Figure 5. (a) Plot between ln(k) and ln(p) showing the effect of steam partial pressure on gasification rate at different temperatures and (b) Arrhenius plots for K2CO3-catalyzed HPC gasification at 0.5 and 0.1 atm steam partial pressure.
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Figure 6. Gas composition of the product gas from K2CO3-catalyzed HPC gasification at different steam partial pressures.
partial pressure. Arrhenius plots at 0.5 and 0.1 atm partial pressure were nearly parallel to each other. Activation energies were 108 and 104 kJ/mol at 0.5 and 0.1 atm steam partial pressures, respectively. The results show that activation energy was independent of the partial pressure and that the decrease in reaction rate with partial pressure is not due to a change in activation energy. In addition, Arrhenius plots [ln(k) vs 1/T] were a straight line suggesting that catalytic steam gasification of HPC is a chemical reaction controlled in the investigated temperature range at 0.5 and 0.1 atm partial pressure. Figure 6 shows the effect of steam partial pressure on gas composition of the produced gas in mole percent at 600, 650, 700, and 750 °C for HPC with catalyst at 0.5, 0.1, and 0.02 atm partial pressure of steam. At 650 and 600 °C, there appears to be very little or negligible effect of partial pressure on the gas composition. At 750 and 700 °C, with decreasing steam partial pressure H2 and CO2 decreased and CO increased. For example, at 700 °C, H2 decreased from 63% at 0.5 atm to 55% at 0.02 atm and CO2 decreased from 33% at 0.5 atm to 25% at 0.02 atm, while CO increased from 3% at 0.5 atm to 19% at 0.02 atm. The results show that gas composition of the synthesis gas at 750 and 700 °C can be changed by changing the steam partial pressure. As mentioned before, the gas composition is primarily determined by reaction 2 which is strongly dependent on the gasification conditions.8-13,18 An observed change in gas composition suggests change in gasification conditions and/or change in reaction 2. In the present case, the only parameter of the gasification conditions that was changed is steam partial pressure. Therefore, the observed change in gas composition with steam partial pressure suggests that reaction 2 is affected by the steam partial pressure. This means that the water-gas shift oxygen exchange rate is affected by the
Figure 7. (a) Correlation between water-gas shift reaction and the gasification rate at different temperatures and partial pressures of steam and (b) effect of steam partial pressure on H2/CO ratio of product gas at 700 and 750 °C.
change in steam partial pressure and, with decreasing steam partial pressure, not all CO produced by reaction 1 is converted to CO2 by reaction 2. 4891
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: DOI:10.1021/ef900461w
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Figure 7a shows the relationship between the water-gas shift reaction and the gasification rate at different temperatures and partial pressures of steam. As the gas evolved during the gasification reaction was measured continuously by GC every 3 min, the ratio of the experimentally measured evolution rate of H2, CO2, and CO [rH2/(rCO2 þ rCO)] at 50% conversion was plotted against the apparent rate constant, ln[k] obtained at 50% conversion. For the present discussion, [rH2/(rCO2 þ rCO)] is considered as a parameter representing the extent of oxygen transfer by water-gas shift (WGS) reaction to convert CO to CO2 and will have values between 1 (no oxygen exchange by WGS) and 2 (complete exchange of oxygen by WGS).8 Figure 7a shows that at any given temperature, gasification rate and extent of WGS both decrease with decreasing steam partial pressure. This suggests that, with decreasing steam partial pressure, the gasification rate will decrease and the CO fraction will increase. Figures 4 and 6 show the similar results. Figure 7a also shows another interesting observation. At any given partial pressure (in the present study only 0.5 and 0.1), the extent of WGS reached its peak value at 650 °C, and above and below this temperature, the extent of WGS always decreases. At present, the scientific and/or physical interpretation of this observation is not clear. Some additional data points are needed before any conclusions can be made. Figure 7b shows the change in H2/CO ratio with steam partial pressure at 700 and 750 °C. At a 700 °C gasification temperature, the H2/CO ratio was 21 at 0.5 atm, 9 at 0.1 atm, and 3 at 0.02 atm. These results show that by changing the partial pressure of steam the H2/CO ratio of the synthesis gas can be controlled in a single step at atmospheric pressure. Specifically, in the present study at 700 °C and 0.02 atm steam partial pressure, synthesis gas with H2/CO=3 was obtained, which can be used to produce methane by the FT process which is a well-established commercial process. With the small experimental setup and control instruments used in the present study, a steam partial pressure below 0.02 atm was not
achievable. However, the results do suggest that synthesis gas with H2/CO=2, suitable for DME and methanol production by the FT synthesis process may be produced in a single step at 700 °C by the catalytic HyperCoal steam gasification process. Conclusions The effect of the partial pressure of steam on the gasification rate and on composition of product gas from catalytic steam gasification of HyperCoal at 600, 650, 700, and 750 °C was examined. The following conclusions were drawn. (1) The gasification rate decreased with decreasing steam partial pressure. This was primarily due to a decrease in the reaction rate of the gasification reaction 1 with steam partial pressure. (2) The reaction order did not change linearly with steam partial pressure. The reaction order with respect to steam partial pressure, n, was between 0.2 in 600-650 °C and 0.5 in 700-750 °C. (3) Activation energy was independent of the steam partial pressure in the 0.5-0.1 atm range and was about 104-108 kJ/mol. (4) The gas composition of the product gas changed with steam partial pressure. H2 and CO2 decreased and CO increased with decreasing steam partial pressure. The effect was significant at 700-750 °C and almost negligible in 650-600 °C. The change in gas composition may be attributed primarily to the change in extent of oxygen transfer by the water-gas shift (WGS) reaction to convert CO to CO2. (5) The ratio of the rate of evolution of H2 and CO2 þ CO [rH2/(rCO2 þ rCO)] is a good parameter to correlate gasification rate, extent of water-gas shift (WGS) reaction, and steam partial pressure. (6) By changing partial pressure of steam, the H2/CO ratio of the synthesis gas can be controlled in a single step process.
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