VOLUME 22, NUMBER 6
NOVEMBER/DECEMBER 2008
Copyright 2008 by the American Chemical Society
Articles Catalytic Steam Gasification Reactivity of HyperCoals Produced from Different Rank of Coals at 600-775 °C 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 April 8, 2008. ReVised Manuscript ReceiVed August 22, 2008
HyperCoal is a clean coal with ash content 750 °C (1) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 6137–6146. (2) Molina, A.; Mondragon, F. Fuel 1998, 77, 1831–1839. (3) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438–42. (4) Nahas, N. C. Fuel 1983, 62, 239–241. (5) Juntgen, H. Fuel 1983, 62, 234–238. (6) Veraa, M. J.; Bell, A. T. Fuel 1978, 57, 194–200. (7) McKee, D. W.; Chatterji, D. Carbon 1975, 13, 381–390. (8) McKee, D. W.; Spiro, C. L.; Kosky, P. G.; Lamby, E. J. Fuel 1983, 62, 217. (9) Kayembe, A.; Pulsifer, A. H. Fuel 1976, 55, 211–216. (10) Miura, K.; Aimi, M.; Naito, T.; Hashimoto, K. Fuel 1986, 65, 407– 11. (11) Takarada, T.; Ichinose, S.; Kato, K. Fuel 1992, 71, 883–887. (12) Wigmans, T.; Elfring, R.; Moulijn, J. A. Carbon 1983, 21 (1), 1– 12.
10.1021/ef800243m CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
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Table 1. Properties of Coals and HyperCoals. elemental analysis (wt % daf) coal
ash (wt %, db)
C
H
N
S
O
Oaky Creek coal Oaky Creek HPC Pasir coal Pasir HPC Mulia coal Mulia HPC
8.5 0.05 4.9 0.06 3.2 0.01
86.6 87.8 73.5 79.4 65.5 77.3
5.2 5.3 5.3 5.7 5.0 5.3
2.5 2.4 1.9 1.1 0.9 1.0
0.7 0.5 0.2 0.3 0.1 0.2
4.9 4.0 19.1 13.5 28.6 16.2
is difficult because of heat requirements.1 One must supply steam at 750 °C or heat the gasifier externally to maintain 750 °C. Addition of catalyst can significantly enhance the steam gasification reaction, and commercially acceptable gas production rates may be achieved at temperatures lower than 800 °C.3,4 The most favored catalysts are alkali metal salts, especially K2CO3.5-12 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 was difficult.3,13-16 To overcome the problem of loss of catalyst, our research group has developed a process to remove ash from coal by solvent extraction.17 The extracted coal named HyperCoal has less than 500 ppm of ash. Because of its ash-less characteristics, a catalytic gasification process for coal may be developed using HyperCoal as a feed material leading to low gasification temperature, easy recovery, and recycling of catalyst and high H2 selectivity. The major points reported in our previous study18,19 were high gasification rates at temperatures as low as 775 °C, no catalyst deactivation, feasibility of catalyst recovery and recycling, and H2 selectivity. In this study the gasification reactivity of HyperCoals produced from a high-rank coal, a sub-bituminous coal and a lowrank brown coal have been compared. Gasification rates in the 600∼775 °C temperature range were obtained to investigate the effect of temperature and rank of original coal on reactivity. Experimental Procedures A bituminous coal, Oaky Creek (OC) from Australia; a subbituminous coal, Pasir (PAS); and a brown coal, Mulia (MUL), both from Indonesia, were selected for the investigation. HyperCoals (HPC) were produced by the extraction of the coal with 1-methylnaphthalene at 360 °C and subsequently separating the extract (HyperCoal) from the solvent and named OCHPC, PasHPC, and MulHPC, respectively. The extraction yields depended on the rank of the coal and were 66, 37.8, and 35.8% for OC, PAS, and MUL coals, respectively. The HPC production method has been described in detail elsewhere.17 The properties of the coals and HPCs are shown in Table 1. The experimental setup is shown in Figure 1. Physical mixing procedure was used for catalyst mixing. The desired amount of K2CO3 was mixed with 15 mg (daf) of HPC in an agate mortar as solid particles. A common procedure is to mix K2CO3 as an aqueous solution for homogeneous dispersion; however, HPC does not mix with water, and it was not possible to follow the procedure of aqueous dispersion. In addition, the homogeneity of catalyst mixing is assumed not to be a major concern because of the reported mobile nature of potassium (K) at high temperature in (13) Bruno, G.; Buroni, M.; Carvani, L.; Piero, D.; Passoni, G. Fuel 1988, 67, 67–72. (14) Kuhn, L.; Plogmann, H. Fuel 1983, 62, 205–208. (15) Huhn, F.; Klein, J.; Juntgen, H. Fuel 1983, 62, 196–199. (16) Formella, K.; Leonhardt, P.; Sulimma, A.; van Heek, K. H.; Juntgen, H. Fuel 1986, 65, 1470–72. (17) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947–967. (18) Sharma, A.; Takanohashi, T.; Morishita, K.; Takarada, T.; Saito, I. Fuel 2008, 87, 491–97. (19) Sharma, A.; Takanohashi, T.; Saito, I. Fuel 2008, 87, 2866–2690.
Figure 1. Schematic of experimental set-up for catalytic steam gasification of HyperCoals.
the presence of carbon.11,12,18 The particle size of coal and HPC samples was under 75 µm. Experiments were carried out in a thermogravimetric (TG-DTA 2020S, MAC) apparatus with about 20 mL/min argon (Ar) as TG carrier gas flowing from the bottom. At the start of the experiment, 110 mL/min Ar was mixed with 20 mL/min oxygen (O2) and was flowed from the top into the TG-DTA. At the same time, 0.08 mL/ min of water was pumped by a HPLC pump to a steam generator held at 250 °C. About 130 mL/min Ar was also flowed to the steam generator as a steam carrier gas. This provides a 50 vol % steam/ argon gas mixture. A 4-way valve at the inlet of the TG-DTA was used to change (Ar + O2) flow to (Ar + steam) flow. The flow lines were kept at 250 °C by using ribbon heaters. First, about 15 mg of sample was heated in (O2 + Ar) flow for 10 min at 200 °C to remove moisture and to reduce the swelling propensity of the HPC by mild preoxidation. After 10 min hold at 200 °C, the gas was switched to pure Ar flow and heated to the desired temperature at 20 K/min. When the desired temperature was reached, without any hold time the argon gas was switched to 50% steam/argon mixture, giving an expected 0.5 atm of partial pressure of steam 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. The gas evolution rate was measured by a film flow meter.
Results and Discussion Gasification Rate. Figure 2a shows a typical weight loss curve for PasHPC pyrolyzed in argon up to 700 °C followed by steam gasification at 700 °C and 0.5 atm partial pressure of steam with K2CO3. In our previous report, we presented results of experiments carried out with 0, 3, 6, 12, and 20 wt % (HPC daf basis) catalyst loading.18,19 The catalyst loadings used in this study were 10% for OCHPC, 12% for PasHPC, and 12% for MulHPC. All experiments were carried out at atmospheric pressure. The weight loss curve can be roughly divided into three stages; moisture removal or drying stage, devolatilization stage, and fixed-carbon gasification stage.1,2 The initial weight loss during heating from room temperature to 200 °C and holding for 10 min in O2 + Ar mixture is mainly due to the loss of the contained moisture in the sample and probably some solvent (1-MN) that remained during the HPC production. In some samples, weight gain was also observed during the preoxidation period. Because of extremely high swelling propensity of HPC, for 15 mg and higher sample amounts, sample flowed out of the crucible of the TG-DTA. When experiments were carried out with 4 mg, sample showed swelling but did not flow out of the crucible, and gasification was done successfully.18 However, 4 mg is too little of an amount to obtain any reliable results. Therefore, 15 mg and higher amounts were used for experiments after preoxidation treatment. In actual process,
Catalytic Steam Gasification ReactiVity of HyperCoals
Energy & Fuels, Vol. 22, No. 6, 2008 3563
Figure 2. Weight loss profile of (a) PasHPC pyrolyzed in argon up to 700 °C followed by steam gasification, (b) OCHPC, PasHPC, and MulHPC pyrolyzed at 700 °C in argon.
preoxidation may not be required if one uses a water-cooled injection system. To have same basis for comparison of gasification rates of OCHPC and OC, all samples were subjected to preoxidation, and other experimental conditions were also kept same. However, higher sample loading may introduce mass diffusion effects. In our previous paper19 we reported the results of the experiments with 15, 25, and 35 mg sample amounts to investigate the effect of sample loading and H2 inhibition on rates. The results showed that conversion profiles are similar for 15, 25, and 35 mg sample weights (bed heights) suggesting that the gasification rate is independent of sample weight or bed height and ruled out the possibility of mass diffusion effects on rates at least in the early stages. When the desired temperature is reached, argon was switched to steam/argon mixture for gasification reaction. The coal/HPC conversion on dry, ash, catalyst, and volatile free basis (dacvf) (from here on called char conversion) was calculated during the fixed-carbon gasification stage by the following equation:10-12,18 X(char conversion, % dacvf) )
W0 - W × 100 W0 × (1 - Wash - Wcat) (1)
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 775 °C, respectively), W is weight at any gasification time (db, mg, >39 min), Wash is the weight fraction of ash content in coal or HPC, and Wcat is the weight fraction of catalyst content. Results only in the char gasification stage will be discussed further. As the reactivity has been compared only during the char gasification stage, char samples were prepared for XRD measurements. Figure 2b shows the weight loss curve during the pyrolysis of OCHPC, PasHOC, and MulHPC at 700 °C in argon with a hold time of 5 min to obtain char. All the experimental conditions were the same as those in Figure 2a except the gasification part. The XRD profile of raw HPCs and the chars prepared at 700 °C from the corresponding HPCs are shown in
Figure 3. XRD profiles of OCHPC, PasHPC, MulHPC, and their chars with and without K2CO3.
Figure 4. Effect of catalyst addition on gasification reactivity of OC coal and OCHPC.
Figure 3. Figure 3a shows the profile of raw HPC, Figure 3b shows the profiles of HPC + K2CO3, and Figure 3c shows profiles of HPC chars. The sharp peaks in Figure 3, panels b and c, are from K2CO3 and K2O. Figure 4 shows the effect of catalyst addition on OC coal and OCHPC at 775 °C. It can be observed that, without catalyst, rates of OC coal and OCHPC were nearly same. Upon addition of catalyst, the rates increased significantly for both OC coal and OCHPC. Effect of Rank of Parent Coal and Temperature on Gasification Rate. To understand the effect of parent coal rank from which HPC was prepared on the gasification rate, experiments were carried out with OCHPC, PasHPC, and MulHPC. OCHPC was produced from a bituminous coal (C ) 86.6%), PasHPC was from a sub-bituminous coal (C ) 72%), and MulHPC was from a brown coal (C ) 65%). The detailed properties of the original coals and the corresponding HPCs are given in Table 1. The char gasification conversion profiles
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Figure 5. Potassium-catalyzed steam gasification rates of OCHPC, PasHPC, and MulHPC at 775, 700, 650, and 600 °C. Table 2. Gasification Rate at 50% Conversion 1/t0.5 [Rate (min-1)] T (°C)
OCHPC
PasHPC
MulHPC
775 700 650 600
0.061 0.016
0.091 0.073 0.035 0.013
0.121 0.091 0.043 0.017
of OCHPC, PasHPC, and MulHPC at 775, 700, 650, and 600 °C are shown in Figure 5. At 775 °C, gasification profiles of OCHPC, PasHPC, and MulHPC are similar up to 80% conversion. After 80% conversion OCHPC maintained the initial high conversion rate until complete conversion, whereas both PasHPC and MulHPC showed a significant decrease in conversion rates. For example, MulHPC took only 15 min to reach 80% conversion but needed another 15 min to reach 100% from 80%. PasHPC took 23 min to reach 80% but another 32 min to attain 100% from 80% conversion. OCHPC did not show such a behavior. A similar pattern was also observed for PasHPC and MulHPC at 700 °C. The significant decrease in rates during the late stages for PasHPC and MulHPC may be due to diffusion resistance offered by the surrounding catalyst, especially during the late stages when catalyst is dominant part and/or catalytic graphitization effect, although later, is less likely, as reported by Veraa et al.6 They observed that in addition to catalyst loading and temperature, thermal aging or annealing of the sample could also affect the gasification rate. Gasification rate especially during the later stages of gasification may be adversely affected due to the thermal annealing that is a direct
function of the duration of time for which sample is held at high temperature. However, they found that samples with high initial catalyst loading always gasify at a faster rate than those with lower loading, and the reason for this difference was the thermal annealing of the sample. Therefore, they concluded that K2CO3 not only increases the gasification rate through its action as a catalyst but also by reducing the thermal annealing or catalytic graphitization of the sample. The rates of all three HPCs decreased with decreasing temperature, although the extent of decrease was different. OCHPC showed significant decrease in conversion rate at 700 °C, and only 60% conversion was achieved in 90 min. In contrast, PasHPC and MulHPC showed only a slight decrease in rate, and nearly 100% conversions were achieved in 50 and 30 min, respectively. At 650 °C, the rate of OCHPC became quite slow, and only 20% conversion could be achieved in 90 min. PasHPC and MulHPC showed noticeable decrease in rates at 650 °C but were still high enough to achieve 100% conversion in 90 and 60 min, respectively. Experiments were not carried out for OCHPC at 600 °C. Both PasHPC and MulHPC showed a significant decrease in rates at 600 °C but were high enough to be nearly same as the rate achieved for OCHPC at 700 °C. From above results it may be concluded that HPCs (PasHPC, MulHPC) produced from low-rank coals (Pasir, Mulia) were more reactive than those (OCHPC) produced from high-rank coal (Oaky Creek). This is similar to the widely reported understanding that low-rank coals are more reactive than highrank coals, and one of the reasons for this is the more-ordered carbon structure in high-rank coals than in low-rank coals.3,10,11,20 To investigate if similar reason could be attributed for the observed difference in reactivity of HPCs, chars were prepared at 700 °C from OCHPC, PasHPC, and MulHPC, and their XRD profiles were measured. The XRD profiles are shown in Figure 3c together with initial HPC [Figure 3, panels a and b] XRD profile. XRD profiles show that the 002 peak for OCHPC (char and coal) is much sharper than the 002 peaks for PasHPC and MulHPC (char and coal). The relatively sharp peak for OCHPC suggests that the carbon structure is more ordered in OCHPC than that in PasHPC and MulHPC. The more ordered carbon structure of OCHPC could be one of the reasons for the observed low reactivity of OCHPC. These results suggest that gasification reactivity of HPC depends on the rank of original coal from which they were produced. Takarada et al.20 reported that the reactivity of K2CO3-loaded coals depends on the coal type, with the lower rank coals having higher reactivity than higher rank coals. The results from the present study are also in accordance with their findings. For simple understanding and comparison of gasification rates of HPCs produced from different rank of original coals, gasification rates were calculated by inversing the time needed to reach 50% conversion, and the results are shown in Table 2. The interesting observations from these tabulated values are (1) rate of MulHPC was about twice that of OCHPC at 775 °C and (2) rates observed at 700 °C for OCHPC were achieved at 600 °C for PasHPC and MulHPC. Takarada et al.11 reported that gasification rates higher than 2 h-1 (0.03 min-1) or about 30 min for conversion are required for commercial application. From Table 2, it can be seen that rates higher than 0.03 min-1 were easily achieved at 650 °C for PasHPC and MulHPC. Thus, the temperatures (
