Energy & Fuels 2007, 21, 2763-2768
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Effect of Coal Rank on Steam Gasification of Coal/CaO Mixtures Shiying Lin,*,† Yin Wang,‡ and Yoshizo Suzuki‡ Japan Coal Energy Center, 2-14-10 Mita, Minato-ku, Tokyo 108-0073 Japan, and Clean Gas Research Group, Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba 305-8569, Japan ReceiVed March 6, 2007. ReVised Manuscript ReceiVed May 26, 2007
The effect of coal rank on hydrogen generation during the reaction of coal/CaO and char/CaO mixtures with high-pressure steam was investigated using a flow-type reactor, and the results were compared with the results for the gasification of the same coal and char sample in the absence of CaO. For the coal/CaO mixture, reactions of coal, CaO and CO with steam, and CO2 absorption by CaO occurred simultaneously in the reactor and H2 was the primary gas product. CO2 was fixed by CaCO3, and CO was completely converted to H2. The rates of H2 generation per mole of carbon in the coal (char) samples decreased in the order of lignite > sub-bituminous >bituminous. The rate of H2 generation seemed to be affected by both the reactivity and the carbon content of the coal.
Introduction Hydrogen is expected to serve as a clean source of energy, and its use will thus protect the environment, which has been polluted by human energy consumption for industrial and domestic uses. The International Association for Hydrogen Energy has advanced the view that, through the use of fuel cell technology, hydrogen can be used to produce power more efficiently than through traditional methods and serve as a clean fuel for transportation. However, hydrogen is a secondary energy and must be produced using some other energy source, such as nature and fossil energies.1 Therefore, the development of highly efficient technology for H2 production is important. The HyPr-RING method (hydrogen production by reactionintegrated novel gasfication) is a highly efficient method of producing H2 from coal in a single reactor (gasifier).2,3 Lime (CaO) was used to completely absorb CO2 in a gasifier. Once the CO2 is removed, CO is completely converted to H2 in the gasifier and the product gas containing high hydrogen content is produced in a single step in the gasifier. Lin et al.4 have reported the production of 82% H2, along with a small amount of methane (17%), in a single reactor; the concentrations of CO and CO2 in the product gas were less than 3%. Coal gasification (which includes pyrolysis) is the primary reaction in the gasifier. Other reactions that may occur are CaO hydration to Ca(OH)2, CO2 absorption by CaO and Ca(OH)2, the CO shift reaction to produce H2, and H2S absorption by CaO. Because CaO, Ca(OH)2, and CaCO3 have an eutectic * To whom correspondence should be addressed: Clean Gas Research Group, Energy Technology Research Institute, AIST, 16-1, Onogawa, Tsukuba 305-8569, Japan. Telephone: +81-298-61-8224. Fax: +81-29861-8209. E-mail:
[email protected]. † Japan Coal Energy Center. ‡ National Institute of Advanced Industrial Science and Technology. (1) Bisio, A.; Boots, S. Encyclopedia of Energy Technology and the EnVironment; John Wiley and Sons, Inc.: New York, 1995; Vol. 3. (2) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Kagaku Kogaku Ronbunshu 1999, 27 (4), 520. (3) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy ConVers. Manage. 2005, 46, 869-880. (4) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2004, 83, 869874.
melting point as low as 900 K,5 coal gasification in a HyPrRING gasifier is carried out at a relatively low temperature to avoid clinker generation in the gasifier. As we reported in a previous study,6 char gasification with steam is the control reaction in the integration of reactions in the gasifier. In our previous study,6 we found that the addition of CaO as a CO2 sorbent in steam gasification of coal increased H2 generation by a factor of more than 1.5 times compared with coal-only gasification. We also investigated the effects of temperature and pressure on the steam gasification of char. However, because different ranks of coal have different properties, the effect of the coal rank must also be investigated if we are to develop the best operating conditions for this new process. In this study, we examined the effect of CaO addition on coal gasification using coals of different rank: lignite, subbituminous coal, and bituminous coal. The gas products obtained from the gasification of coal and coal/CaO mixtures were investigated at 873-973 K and 3.0 MPa. A high-pressure flowtype reactor with a fixed bed was used for measuring gas products and reaction rates. Thermodynamic analysis was used to determine the most suitable conditions for the reaction system. Thermodynamic Study Equilibrium compositions of gas products from coal (C, H, O)/H2O/CaO reaction systems were calculated using HSC Chemistry 4.0 software based on thermodynamic data7 and are shown in Figure 1 for lignite, sub-bituminous, and bituminous coals. H2 and CH4 were the main gas products, and the CO and CO2 components decreased with an increasing pressure, approaching zero at a pressure above 3 MPa. This is because CaO absorbs CO2 and enhances the CO shift reaction to produce H2, as described in detail in our previous study.6 Comparing the equilibrium compositions of the gas products for lignite, sub-bituminous, and bituminous coals indicates that (5) Curran, G. P.; Fink, C. E.; Corin, E. Proceeding of the 8th Synthetic Pipeline Gas Symposium, Chicago, IL, 1976. (6) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Fuel 2002, 81, 20792085. (7) Barin, I. Thermochemical data of pure substances. VCH, Verlagsgesellschaft mbH, D-6940 weinbeim (Federal Republic of Germany), 1989.
10.1021/ef070116h CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007
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Lin et al. Table 1. Proximate and Ultimate Analyses of Coals Used in This Study proximate analysis (wt %)
ultimate analysis (wt %, daf)
coals
W
A
VM
FC
C
H
O
N
Wyoming Adaro Taiheiyo Datong Ebenezer
15.2 12.6 4.8 4.6 1.73
7.0 1.6 7.6 6.4 12.9
46.6 40.6 48.8 24.3 36.5
31.2 45.2 38.8 64.6 48.9
69.5 74.2 77.3 81.2 81.6
5.0 5.1 6.6 4.7 6.1
24.4 19.5 14.8 13.0 10.8
1.0 1.2 1.3 1.0 1.5
Sub-bituminous and bituminous coals produced more H2 at low pressure than lignite because they had comparatively high carbon contents to react with H2O to produce H2 during gasification. As the pressure increased, CH4 was generated in the system, which caused H2 production to decrease. Because bituminous and sub-bituminous coals generated more CH4 as the pressure increased, the decrease in H2 production for these coals was greater than that for lignite. The low production of CH4 by lignite was probably due to the fact that its oxygen content was higher than that of the other coals. The presence of O enhanced the decomposition of CH4 into H2.
CH4 + 1/2O2 + H2O + CaO f 3H2 + CaCO3
(1)
Experimental Section Sample Preparation. Five coals were use in this study: Wyoming coal (lignite, U.S.A.), Adaro coal (lignite, Indonesia), Taiheiyo coal (sub-bituminous, Japan), Datong coal (bituminous, China), and Ebenezer coal (bituminous, Australia). Table 1 shows the proximate and ultimate analyses for these coals. The coals were crushed and sieved to approximately 0.075-0.125 mm for the experiments. Powdered CaO was used (Kishida Reagent Chemicals, Osaka, Japan). Pressurized Reactor. Figure 2 is a schematic of the experimental apparatus. The fixed-bed reactor, made of Hastelloy tube, was 100 mm in length, with an inner diameter of 10 mm, and was contained within a pressure vessel. Pressure in the reactor was controlled by means of a nitrogen gas cylinder, and the temperature was controlled with an electric furnace. Nitrogen gas was introduced at 0.2 L/min during the pyrolysis stage of each experimental run. High-pressure and high-temperature steam was introduced at the top of the reactor, passed through the fixed bed, and allowed to flow out through the
Figure 1. Equilibrium compositions of gas products from lignite, subbituminous, and bituminous coals in coal (C/H/O)/CaO/H2O mixtures.
sub-bituminous and bituminous coals produce more H2 than lignite at low pressure. With an increasing pressure, H2 production decreased for all three coal ranks and the decreases for sub-bituminous and bituminous coals were larger than the decrease for lignite. Consequently, at 3.0 MPa, H2 production values for these three coals were similar. However, the H2/CH4 ratios at 973 K and 3.0 MPa for the three coals were quite different: 13.8 for lignite, 7.0 for sub-bituminous, and 6.5 for bituminous coals.
Figure 2. Schematic diagram of the high-pressure fixed-bed reactor.
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Figure 3. Temperature variation during the experiments.
bottom of the reactor to a condenser for the separation of gas products and water. Gas products were passed through a gas filter before entering the analyzer. A water tank under the reactor was used to rapidly cool the reactor after each reaction. Experimental Procedures. Approximately 0.5 g of coal or char was used for the coal- or char-only experiments, and a mixture of 0.5 g of coal or char with 1 g of CaO was used for coal/CaO mixture experiments. Experimental material was placed in the reactor, and the pressure and temperature were increased. The bed temperature was measured with a K-type thermocouple. The heating rate was about 7 K/min. When the pressure and temperature stabilized at the target values, steam at high pressure and temperature was injected into the reactor at 0.3 L/min to start the reactions. The concentrations of H2, CH4, CO, and CO2 in the product gas were continuously measured with a MicroGC gas chromatograph (Chrompack CP2002, The Netherlands). Figure 3 shows the temperature ranges during the experiment. During the heating phase, before steam gasification, coal pyrolysis took place. Therefore, the reactant for the steam gasification was a char (referred to as direct char hereafter). To determine the characteristics of the direct char for each type of coal used in this study, each coal was converted to direct char under the same experimental conditions to the point of gasification and the reactor was then quickly cooled in the water tank to stop the reaction from proceeding. For a comparison, char was also made at a higher temperature (1123 K) in a fluidized bed under a nitrogen atmosphere (referred to as 1123 K char hereafter). The compositions of the direct chars and the 1123 K chars were determined with a CHN analyzer and are shown in Table 2. During steam gasification of the coal/CaO mixture, CaO absorbs the CO2 generated by coal gasification to form CaCO3 and some CaO absorbs steam to form Ca(OH)2. Changes to the solids during the steam gasification of coal/CaO mixtures were discussed in a previous publication.6
Results and Discussion Product Profile for Coal (Direct Char) and 1123 K Char Steam Gasification. Profiles of the gas product formed in the absence of CaO for Taiheiyo coal (direct char) and 1123 K char are shown in parts a and b of Figure 4, respectively. In the coal experiment, only a small amount of H2 was generated as a result of pyrolysis of the sample material during the heating phase (Figure 4a). At the gasification temperature (923 K), steam was introduced into the reactor and H2 generation increased rapidly, owing to steam gasification. CH4 was generated mainly during coal pyrolysis in the heating phase, but a little CH4 was also generated during the steam gasification phase. Other gases, CO
Figure 4. Amounts of gas products obtained from (a) Taiheiyo coal (direct char) and (b) 1123 K char during the heating and gasification phases in the absence of CaO. Product amounts are given in mol/g of coal and mol/g of 1123 K char. Table 2. Properties of Direct Char and 1123 K Char ultimate analysis (wt %, daf) C
H
O
N
Wyoming Adaro Taiheiyo Datong Ebenezer
Direct Chars (Coal Only, 923 K, 3.0 MPa) 81.8 2.5 14.5 81.0 2.4 15.6 83.1 2.5 13.0 86.7 2.5 9.9 85.3 2.5 10.6
1.1 1 1.4 0.9 1.7
Wyoming Adaro Taiheiyo Datong Ebenezer
Direct Chars (Coal/CaO, 923 K, 3.0 MPa) 80.7 2.4 15.8 81.1 2.4 15.2 84.7 2.6 11.6 82.2 2.7 13.6 82.6 2.8 12.8
1.2 1.3 1.1 1.6 1.8
Wyoming Adaro Taiheiyo Datong Ebenezer
1123 K Chars (1123 K, 0.1 MPa) 93.5 1.3 4.2 96.8 1.5 0.5 95.6 1.5 1.7 96.8 1.3 1.0 98.3 0.9 0.3
0.9 1.1 1.1 0.8 0.6
and CO2, were also generated from coal during both the pyrolysis and gasification stages.
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Figure 5. Amounts of gas products obtained from (a) Taiheiyo coal (direct char) and (b) 1123 K char during the gasification phase in the absence of CaO. Product amounts are given in mol/mol of carbon in direct char and 1123 K char.
In the 1123 K char experiment, no H2 or CH4 was generated during the heating phase (Figure 4b). When steam was injected to start gasification, H2 generation increased more rapidly than was the case for coal (direct char) gasification (Figure 4a). For example, after 80 min, H2 generation reached 0.048 mol/g for 1123 K char but only 0.024 mol/g for the coal (direct char). This difference is due to the fact that more carbon was available for gasification in the 1123 K char than in the coal. CH4 was generated by decomposition of the volatile during the heating phase and was also generated in the steam gasification phase by the reaction equilibrium and reaction rate
C + H2O f CO + H2
(2)
2H2 + C f CH4
(3)
However, a comparison of parts a and b of Figure 4 shows that CH4 was generated mainly from volatiles in the coal during pyrolysis in the heating phase and little CH4 was generated during gasification. CO2 is generated during coal gasification because the watergas shift reaction that converts CO to H2 is favored at a high steam pressure and low gasification temperature
CO + H2O f CO2 + H2 (water-gas shift reaction) (4) Reactivity of Carbon in the Steam Gasification Stage. To investigate the reactivity of carbon in direct char and 1123 K
Figure 6. Amounts of gas products obtained from the gasification of (a) Taiheiyo coal (direct char)/CaO and (b) 1123 K char/CaO mixtures. Product amounts are given in mol/mol of carbon in direct char and 1173 K char.
char, we changed the units of Figure 4 from moles of gas products per gram of sample to moles of gas products per mole of carbon in the sample (parts a and b of Figure 5). The reactivity (H2 production) of 1123 K char was still about 1.4 times that of direct char, but the difference was much smaller than that shown in Figure 4. The lower reactivity of direct char in the gasification may have been due to the fact that tar remained in the pore of particles and inhibited the diffusion of the reactant gas (steam) into the particle. Effect of CaO Addition on H2 Production during Gasification. Profiles of the gas product obtained during steam gasification of Taiheiyo coal (direct char)/CaO and 1123 K char/ CaO mixtures are shown in parts a and b of Figure 6, respectively. A comparison of Figure 6a with Figure 5a shows that, when steam was injected into the reactor to start gasification of the direct char/CaO mixture, hydrogen production increased much faster than was the case for the direct char in the absence of CaO and more than twice as much H2 was generated in the presence of CaO. The mechanism by which the addition of CaO increases H2 production during coal gasification is not yet completely clear. CaO may act as a catalyst,8 or heat generated by the reaction of CaO with steam may enhance H2 production. We concluded that tar generated during coal pyrolysis may have adhered to
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Figure 8. Hydrogen production from the gasification of (a) direct char/ CaO and (b) 1123 K char/CaO mixtures for various coals.
Figure 7. Hydrogen production from the gasification of (a) direct chars and (b) 1123 K chars of various coals in the absence of CaO.
the surface of CaO particles. The tar (including the tar in the particle pores) may have subsequently undergone a CaOcatalyzed reaction with steam to generate gases in addition to those produced by direct char gasification. The original purpose of the CaO in the process of HyPrRING was to absorb CO2 during coal gasification, but some CO2 nevertheless remained in the gas mixture produced (Figure 6). Because, in this study, we focused on coal and char reactivity in the presence of CaO and used a thin fixed bed, the gases had a short residence time of about 0.4 s in the bed. This residence time was too short for the removal of gas (that is, absorption of CO2 by CaO) but was long enough to allow for the reaction of the coal or char with steam. In a previous study,4 in which we used a comparatively long contact time (4-5 s) between CO2 and the sorbent in a fluidized bed reactor, CO2 was reduced to about 2% under the temperature and pressure conditions used in the current study. A comparison of Figures 5 and 6 shows that the addition of CaO had no effect on either CH4 generation or the decomposition of CH4 by the reaction with steam. Reduction in the amounts of CO and CO2 by the addition of CaO may reduce (8) Lang, R. J.; Neavel, R. C. Fuel 1982, 61, 620-626.
the CH4 component of gases produced as the equilibrium calculation,6 but such reductions were not clearly evident in this experiment. Figure 6b shows gas products obtained from the gasification of the 1123 K char/CaO mixture. H2 generation was greater in the presence of CaO than in the absence of CaO (Figure 5b), but the increase was less than that observed for the gasification of the direct char/CaO mixture (Figure 6a); H2 production from direct char in the presence of CaO was 2.5 times that in the absence of CaO, whereas the increase was only a factor of about 1.4 for 1123 K char. Because the 1123 K char with no tar could undergo steam gasification to generate H2, the increased H2 production in the presence of CaO may have been caused by CaO catalysis of the reaction with steam or by the heat generated by the reaction of CaO with steam. Dependence of H2 Generation on the Coal Rank. Hydrogen generation from coals of various ranks is shown in Figure 7. Figure 7a shows H2 production from direct char in the absence of CaO. The H2 generation behaviors of the chars made from the various coals were quite different. Direct char from Wyoming coal produced the most H2, and direct char from Ebenezer coal produced the least H2. The H2 production decreased in the order of lignite (Wyoming coal) > subbituminous (Taiheiyo coal) > lignite (Adaro coal) > bituminous (Datong coal) > bituminous (Ebenezer coal). This order can be explained by the fact that the gasification reactivity of lowrank coals is higher than that of high-rank coals. Note, however, sub-bituminous Taiheiyo coal produced more H2 than the Adaro coal (lignite). The fact that Taiheiyo coal (sub-bituminous) had
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Figure 9. H2/CH4 ratio in gas products as a function of the carbon content in coal and 1123 K char.
a higher hydrogen content than other coals may have contributed to the higher H2 production during coal gasification, as we discussed in the Thermodynamic Study (Figure 1). Figure 7b shows the amounts of H2 produced during gasification of 1123 K chars in the absence of CaO. The 1123 K chars made from the low-rank coals (Wyoming and Taiheiyo) produced more H2 than the corresponding direct chars, but the 1123 K chars made from high-rank coals (Datong and Ebenezer) produced the same amount of H2 as the direct chars. The 1123 K char of Adaro coal (lignite) also had the same reactivity (H2 production) as that of the corresponding direct char. Parts a and b of Figure 8 show H2 production during the gasification of direct char/CaO and 1123 K char/CaO mixtures for various coals. Comparing Figure 8a with Figure 7a indicates that CaO addition enhanced H2 production for all of the coals used in this study. Hydrogen production from direct char/CaO gasification decreased in the order of Wyoming coal > Taiheiyo coal > Adaro coal > Datong coal and Ebenezer coal. The increase of H2 production because of CaO addition was much higher for low-rank coals (lignite and sub-bituminous) than highrank coals (bituminous). A comparison of Figure 8b with Figure 7b indicates that, for 1123 K char/CaO gasification, the increase in H2 production because of CaO addition was smaller than the increase observed for direct char as shown in a comparison of Figure 8a with Figure 7a, especially for the high-rank coals. This result can be explained as follows. The direct chars contained some residual
Lin et al.
tar, which underwent CaO-catalyzed steam gasification to produce H2 in addition to that produced by the gasification of the direct char. However, 1123 K chars, especially those made from high-rank coal, did not contain any residual tar to undergo CaO-catalyzed steam gasification to produce additional H2. H2/CH4 Ratio in the Gas Products. The H2/CH4 ratio in the gas products is an important factor in this work. The H2/ CH4 ratios observed for the gasification of direct char/CaO and 1123 K char/CaO are shown in Figure 9 as a function of the carbon content. For the direct char/CaO gasification, the H2/ CH4 ratio was in the 5-10 range for all of the coals. For the 1123 K char/CaO gasification, the H2/CH4 ratio for high-rank coal increased to about 20 and the ratio for the low-rank coals increased into the 50-70 range. Gasification of the 1123 K char, which contained no tar, produced mainly H2. The gasification reactivity of the 1123 K char made from the low-rank coals may have been higher than the reactivity of the 1123 K made from the high-rank coal. Conclusion Lignite, sub-bituminous, bituminous coals, and their 1123 K chars were used to investigate the effect of the coal rank on H2 production during steam gasification of coal (direct char)/CaO and 1123 K char/CaO mixtures. During the gasification reaction, coal or char, CaO, and CO reacted with steam and CO2 was absorbed by CaO. Hydrogen was the main gas product, along with a small amount of methane. Hydrogen production decreased in the following order: 1123 K char of lignite > coal (direct char) of lignite > coal (direct char) of sub-bituminous >1123 K char of sub-bituminous > coal (direct char) of bituminous >1123 K char of bituminous. The H2 generation rate seemed to be affected by both the reactivity and carbon content of the coal. Hydrogen generation was also enhanced by CaO addition relative to that observed in the absence of CaO. The H2/CH4 ratio in the gas products was largest for 1123 K char obtained from lignite and sub-bituminous coals and was smallest for the lignite and sub-bituminous coals (direct char). The H2/CH4 ratios in the gas products decreased in the order of 1123 K chars of lignite and sub-bituminous coals > 1123 K char of bituminous coals > bituminous coals (direct char) > lignite and sub-bituminous coals (direct char). EF070116H
