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Deactivation of Ca-Based Sorbents by Coal-Derived Minerals during Multicycle CO2 Sorption under Elevated Pressure and Temperature Koji Kuramoto,† Sayaka Shibano,† Shinji Fujimoto,† Tatsuya Kimura,† Yoshizo Suzuki,*,† Hiroyuki Hatano,† Lin Shi-Ying,†,‡ Michiaki Harada,‡ Kayoko Morishita,§ and Takayuki Takarada§ Clean Fuel Research Group, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, Center for Coal Utilization Japan (CCUJ), 7th Floor, Sumitomo Gaien Building, 24 Daikyocho, Shinjuku-ku, Tokyo 160-0015, Japan, and Department of Biological & Chemical Engineering, Gunma University, 1-5-1 Tenjin, Kiryu 376-8515, Japan
Deactivation of Ca-based sorbents by coal-derived minerals during multicycle CO2 sorption reactions at elevated temperature and pressure was investigated using a laboratory-scale horizontal-tube reactor. The sorbents tended to undergo a solid-solid reaction with coal-derived ash components such as silicon (Si) and aluminum (Al) during multicycle CO2 sorption with an intermediate hydration stage. This reaction formed complex inorganics, such as mayenite (Ca12Al14O33), calcium silicate (Ca2SiO4), and spurrite (Ca5(SiO4)2CO3), and substantially decreased the sorbents’ CO2 sorption ability. Interaction with the coal-derived minerals was significant during the multicycle calcination-carbonation reaction only when the sorbents were subjected to the intermediate hydration treatment. This result suggests that steam enhanced the solidsolid interaction between the minerals and the sorbents at elevated temperature (>873 K) and pressure (6.0 MPa) and that the interaction should be avoided by selecting proper reaction conditions for efficient utilization of the Ca-based sorbents. 1. Introduction In the next decade, hydrogen may become an important energy carrier for sustained energy utilization with minimized adverse impact on the environment. It can be used as a fuel in the combustion process for thermal energy demand or fuel cells for electricity without any carbon emissions as well as other pollutants and harmful matters. Most hydrogen production occurs by steam reforming of a hydrocarbon-steam mixture to H2 and CO2, and the conventional industrial H2 production process is mainly based on the partial oxidation of natural or gas coal.1 This process practically requires two reaction steps as
C + H2O f CO + H2
(1)
CO + H2O f CO2 + H2
(2)
Here, water-carbon reaction equation (1) is endothermic and its chemical equilibrium favors the formation of H2 and CO at elevated temperature (practically above 1273 K), while water-gas shift reaction equation (2) occurs in the shift conversion reactor operated below 673 K in the presence of catalysts. Then, the gasification process requires CO2 separation (e.g., pressure-swing adsorption) for purification of the product H2, which is an energy-consuming option. We have proposed the HyPr-RING (hydrogen production by reaction integrated novel gasification) process, which is a novel process for producing hydrogen in high yield with little release of CO2 by thermochemical decomposition of water with coal in the presence of Ca* To whom correspondence should be addressed. Tel.: +81 298 61 8073. Fax: +81 298 61 8209. E-mail:
[email protected]. † AIST. ‡ CCUJ. § Gunma University.
based CO2 sorbents. The major reactions participating in the process are hydration and CO2 sorption reactions of Ca-based sorbents
CaO + H2O f Ca(OH)2
(3)
Ca(OH)2 + CO2 f CaCO3 + H2O
(4)
as well as the above-mentioned water-carbon reaction equation (1) and the water-gas shift reaction equation (2). These two reactions (eqs 3 and 4) produce reaction heat, which compensates for the heat required for the endothermic water-carbon reaction, so that it is not necessary to feed air or oxygen for partial combustion. Another merit of this process is that both reactions of eqs 1 and 2 take place in a single reactor at a relatively milder temperature (approximately 973 K) than the conventional water-carbon reaction process due to the catalytic effects of Ca-based sorbents.4 In addition, Ca-based sorbents are converted to CaCO3 through the in situ sorption of CO2 produced via eq 2, which results in the substantial increase in the H2 yield in product gas as
C + CaO + 2H2O f 2H2 + CaCO3
(5)
The detailed description on the concept of our process and experimental examinations were presented elsewhere.2,3 Efficient CO2 sorption with the Ca-based sorbents during gasification in a single reactor requires elevated pressure. Lin et al.2 demonstrated the steam gasification of various carbonaceous resources, including coal, under a wide range of pressures (>20 MPa) and temperatures (>923 K) using a batch autoclave reactor. They obtained high conversions of coal to hydrogen (typically ca. 80 vol % of H2 in the gaseous product) with complete fixation of CO2 with Ca(OH)2. Thus, calcium compounds were found to behave as efficient CO2 sorbents under
10.1021/ie030159v CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003
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Figure 1. Schematic of the laboratory-scale horizontal-tube reactor system used for the multicycle CC and CHC tests of pure sorbents and sorbent/HTA mixtures under elevated pressure.
these gasification temperatures in a pressurized reactor. Wang and Takarada4 investigated the catalytic role of Ca(OH)2 in the steam gasification of low-rank coals above supercritical conditions. Upon the addition of Ca(OH)2, they observed efficient fixation of CO2 and enhanced decomposition of tar and char during the gasification. They concluded that the Ca-based sorbents play a crucial role in the proposed process for promoting steam gasification as well as for efficient CO2 fixation in the reactor. In the HyPr-RING process, the active Ca-based sorbent is supposed to be recycled to minimize the release of CO2 and the need for recharging with fresh sorbents. Therefore, it is essential that the Cabased sorbents retain their physical constitution and chemical reactivity for multicycle CO2 sorption throughout the alternating calcination and carbonation reactions. Kuramoto et al.5 investigated the multicycle CO2 sorption characteristics of Ca-based sorbents and demonstrated that both the reactivity and the durability of the sorbents during the multicycle carbonation-calcination reaction are maintained by an intermediate vaporphase hydration treatment conducted prior to carbonation. When the sorbents are introduced, they react with steam and CO2 as well as with coal-derived volatiles and char in the gasifier. The mineral matter present in the coal particles is known to evolve during high-temperature processes such as gasification and combustion.6 This frequently causes serious deposition on and fouling of the heat exchanger or defluidization due to the formation of agglomerates of the bed material and melted ash.7 In the HyPr-RING process, the mineral matter may attach to and then react with the Ca-based sorbents during gasification,8 which might affect the CO2 sorption characteristics of the sorbents. This possibility prompted us to examine the solid-solid interaction between the sorbents and coal-derived minerals during multicycle CO2 sorption to see whether the interaction affects the reactivity and durability of the sorbents at elevated temperature and pressure. In this work, we used a pressure-resistant horizontal-tube reactor system to examine the influence of the coal-
Figure 2. CO2 sorption behavior of pure sorbents and sorbent/ HTA mixtures as a function of the cycle number in the CC and CHC tests at different carbonation temperatures. Samples were calcined in N2 at 1173 K under ambient pressure for 10 min in the calcination stage. Hydration and carbonation were performed at 873, 923, and 973 K under 6.0 MPa. Table 1. Elemental Composition of the HTA Sample element HTA, wt %
Si
Al
Fe
Mg
Ca
17.2
12.3
5.2
2.2
10.3
derived minerals on the CO2 sorption characteristics of Ca-based sorbents during multicycle calcination-carbonation (CC) and calcination-hydration-carbonation (CHC) reactions under elevated pressure. The residues sampled after the multicycle reactions were subjected to X-ray diffraction (XRD) and scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) analyses to clarify the interaction between the minerals and the sorbents during the reactions.
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Figure 3. Typical surface morphology and local EDX spectra of residues from the CC (a) and CHC (b) reactions.
2. Experimental Section 2.1. Samples. Japanese bituminous coal (Taiheiyo) was employed as the parent coal for the ash sample. High-temperature ash (HTA) was prepared by burning the coal with air in a fluidizing column at 1088 K for 120 min. The elemental composition of the sample is listed in Table 1. Reagent-grade calcium carbonate (CaCO3; Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used as the CO2 sorbent. Sorbent particles with diameters of ca. 5 µm were well mixed mechanically with HTA (1/1, w/w). 2.2. Multicycle CC and CHC Tests for Pure Sorbents and Sorbent/HTA Mixtures under Elevated Pressure. The multicycle CC and CHC reactions were carried out under elevated pressure in a pressure-resistant horizontal-tube reactor system (Figure 1). In the CHC tests, the sorbent/HTA mixtures were first heated to the calcination temperature of 1173 K under atmospheric pressure and then kept at that
temperature for 10 min under a N2 flow of ca. 250 stdcm3/min (calcination stage). Then, the reactor pressure was increased to 6.0 MPa, and the temperature was lowered to the prescribed values of 873, 923, and 973 K. When the reactor reached steady state, a mixture of carrier gas (N2; ca. 250 std-cm3/min) and steam (ca. 0.2 g/min) was introduced into the reactor for 10 min (intermediate hydration stage). After the hydration stage, the CO2-containing reactant gas (ca. 20 vol % CO2 in N2) was introduced into the reactor at 250 std-cm3/ min for 25 min (carbonation stage). After exposure of the sorbents to the CO2-containing gas under the given pressure, the CO2 remaining in the reactor was swept out as rapidly as possible with N2 gas, and the system pressure was reduced to atmospheric pressure. Then the carbonated sample was reheated to the calcination temperature under a N2 flow. During the heating of the sorbents to the calcination temperature, the carbonated sorbents (CaCO3) thermally decomposed, releasing CO2.
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During this second calcination stage, the volumetric flow rate of the effluent gas and the transient variations in the CO2 concentration in the effluent gas were simultaneously measured at the reactor exit. From the variations of the CO2 concentration with time and the flow rate of the effluent, the amount of CO2 sorbed by the sorbents during the carbonation stage was quantified. This sequence was repeated several times for the CHC tests. In the CC tests, the CC reactions were repeated without the intermediate hydration treatment. Note that amount of CO2 released during calcination of a given amount of reagent-grade CaCO3 (purity, >99.5%) was estimated through the above procedure and the deviation between measured and theoretical values was confirmed to be less than 5 mol %. The residues obtained after the CHC and CC tests were subjected to X-ray diffractometric analysis (RINT 2000, Rigaku Corp., Tokyo, Japan). The residues’ microscopic morphology and local mineral composition were observed with a scanning electron microscope (SS550, Shimadzu Corp., Kyoto, Japan) equipped with an EDX analyzer (EDAX; EDAX Corp., Tokyo, Japan). 3. Results and Discussion 3.1. Multicycle CO2 Sorption Characteristics of Ca-Based Sorbents with and without HTA. We compared the multicycle CO2 sorption characteristics of pure Ca-based sorbents in the CC and CHC tests with the characteristics of sorbent/HTA mixtures (Figure 2). Here, the conversion is expressed as XCO2/CaO, which is defined as the molar ratio of CO2 sorbed to CaO initially loaded. With the pure sorbents, more than 85% of the calcium was constantly used for CO2 sorption in the multicycle CHC tests at the temperatures tested, although in the CC test, XCO2/CaO decreased monotonically with the cycle number owing to sintering and crystal growth of the sorbents themselves during high-temperature calcination.5 For sorbent/HTA mixtures, XCO2/CaO in the CHC tests decreased with the cycle number. This deactivation of the sorbents became significant at the higher temperatures (only about 20% of the loaded CaO was used for CO2 sorption in the fifth cycle at a calcination temperature of 973 K). In the CC tests, the variation in the CO2 sorption ability of the sorbent/HTA mixture with the cycle number was qualitatively similar to the variation for pure sorbent. These results clearly suggest that the solid-solid interaction between the Ca-based sorbents and coal-derived minerals was enhanced by the intermediate hydration treatment during the multicycle CC reactions. 3.2. Physical and Chemical Characteristics of the Solid Residues from the CC and CHC Tests. The surface morphology and chemical compositions of the residues sampled after the fifth carbonation stage in the CC and CHC tests at a carbonation temperature of 973 K were analyzed with SEM-EDX (Figure 3). In the residue from the CC tests, most of the sorbent particles (e.g., region 01) remained facetted, and the particles resemble the pristine CaCO3 sample in shape and scale. In these regions, dominant peaks corresponding to calcium, carbon, and oxygen are visible in the EDX spectrum. In the ash region (regions 02-05), by contrast, the main elements of coal ash, such as silicon and aluminum, dominate, and the Ca peak is small relative to the peaks of the ash components. This result may indicate that, even though the sorbents and ash par-
Figure 4. XRD patterns for residues of the sorbent/HTA mixture from the multicycle CC (a) and CHC (b) reactions: calcination at 1173 K in N2 flow for 10 min at 0.1 MPa, vapor-phase hydration (in CHC mode) at 973 K and 6.0 MPa for 10 min, and carbonation at 973 K and 6.0 MPa with reactant gas (20 vol % CO2 in N2).
ticles were jumbled up, the sorbents remained “liberated” from ash particles after the multicycle CC reactions. In the residue from the CHC tests, such facetted sorbent particles could no longer be found. Apparently, partial fusion of the sorbents occurred during these tests, and neighboring sorbents and ash particles coalesced, possibly around the eutectic point of the calcium compounds.5,9,10 The local EDX spectrum patterns obtained from several locations in the CHC residue indicate that Si, Al, and Mg tend to be fused with the Ca-based sorbents. Under such circumstances, the local chemical interaction between the minerals and sorbents is significant. The residues collected after the fifth carbonation cycle in the CC and CHC tests at 973 K were subjected to XRD analysis (Figure 4). The peaks stemming from calcium carbonate (CaCO3: product in carbonation stage), calcium oxide (CaO: unreacted sorbents), and quartz (SiO2: main component of coal ash) are visible in the XRD pattern for the residue from the CC test. However, the complicated XRD pattern for the residue from the CHC tests suggests the formation of complex inorganic compounds; the peaks can be assigned to mayenite (Ca12Al14O33), calcium silicate (Ca2SiO4), spur-
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Figure 5. Mechanism of solid-solid interaction between Ca-based sorbents and included minerals of coal particles during the HyPrRING process.
rite (Ca5(SiO4)2CO3), and brownmillerite (Ca2(Al, Fe)2O4) as well as to CaCO3 and CaO. These results clearly indicate that high-pressure steam, which is supposed to be used for coal gasification as well as for hydration of the Ca-based sorbents in our process, plays a crucial role in enhancing the solid-solid interaction between the minerals and the sorbents in the temperature range tested (>873 K). When coal particles are introduced into a HyPr-RING gasifier, the carbonaceous portion is consumed, and the particles shrink as the gasification proceeds (Figure 5). This shrinkage releases the included minerals from the particles to the coal surface, which in turn increases contact between the minerals and the Ca-based sorbents surrounding the particles. The minerals released from the coal react with the sorbents to form complex inorganics and partially deactivate the sorbents. The rate of formation of the inorganics during the solidsolid reaction should be low.11 The proper choice of the coal particle size, residence time, and reaction conditions is crucial and may reduce the solid-solid interaction. Therefore, it is important to clarify the characteristics of ash evolution from coal particles as well as the kinetics of the solid-solid reaction between the included minerals and the sorbents in the present range of pressure and temperature to determine suitable reaction conditions for efficient HyPr-RING operation. 4. Conclusion In an effort to improve the HyPr-RING process, we investigated the influence of coal-derived ash on the reactivity and durability of Ca-based sorbents for CO2 sorption during multicycle CC and CHC reactions under pressure at various temperatures. We measured the CO2 uptake in each cycle using a laboratory-scale horizontal-tube reactor. We confirmed that Ca-based sorbents were significantly deactivated by the interaction between the sorbents and coal-derived ash in the multicycle CHC reaction under elevated pressure. Steam was found to play a crucial role in enhancing the unfavorable solid-solid interaction. As a consequence of the interaction, complex inorganic compounds such as mayenite (Ca12Al14O33) and calcium silicate (Ca2SiO4) were formed. For efficient CO2 sorption in the reactor as well as for multicycle use of the sorbents in the HyPrRING process, this interaction should be prevented or minimized. Our future work will be aimed at clarifying the kinetics of the solid-solid reaction of calcium components with included minerals and examining the
characteristics of ash evolution from coal particles in the relevant range of pressure and temperature. Acknowledgment Special thanks are extended to Dr. Kinya Sakanishi of the National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, for his valuable comments concerning mineral behavior in hightemperature coal conversion processes. Literature Cited (1) Czuppon, T. A.; Knez, S. A.; Newsome, D. S. Hydrogen. In Encyclopedia of energy technology and the environment; Bisio, A., Boots, S., Eds.; John Wiley & Sons: New York, 1995; Vol. 3, pp 1752-1782. (2) Lin, S.-Y.; Suzuki, Y.; Hatano, H.; Harada, M. Hydrogen Production from Hydrocarbon by Integration of Water-Carbon Reaction and Carbon Dioxide Removal. Energy Fuels 2001, 15, 339. (3) Lin, S.-Y.; Suzuki, Y.; Hatano, H.; Harada, M. Developing an Innovative Method, HyPr-RING, to Produce Hydrogen from Hydrocarbons. Energy Convers. Manage. 2002, 43, 1283. (4) Wang, J.; Takarada, T. Role of Calcium Hydroxide in Supercritical Water Gasification of Low-Rank Coal. Energy Fuels 2001, 15, 356. (5) Kuramoto, K.; Fujimoto, S.; Morita, A.; Shibano, S.; Suzuki, Y.; Hatano, H.; Lin, S.-Y.; Harada, M.; Takarada, T. Repetitive Carbonation-Calcination Reactions of Ca-based Sorbents for Efficient CO2 Sorption under Elevated Temperatures and Pressures. Ind. Eng. Chem. Res. 2003, 42, 975. (6) Gordon, C. Understand Slugging and Fouling during pf Combustion; IEA Coal Research: 1994: Chapter 3. (7) Ishom, F.; Harada, T.; Aoyagi, T.; Sakanishi, K.; Korai, Y.; Mochida, I. Problem in PFBC Boiler (1): Characterization of Agglomerate Recovered in Commercial PFBC Boiler. Fuel 2002, 81, 1445. (8) Katalambula, H.; Bawagan, A.; Takeda, S. Mineral Attachment to Calcium-Based Sorbent Particles during in situ Desulfurization in Coal Gasification Processes. Fuel Proc. Technol. 2001, 73, 75. (9) Curran, G. P.; Fink, C. E.; Gorin, E. CO2 Acceptor Gasification Process. Studies of Acceptor Properties. Adv. Chem. Ser. 1967, 69, 141. (10) Fuerstenau, M. C.; Shen, C. M.; Palmer, B. P. Liquidus Temperatures in the CaCO3-Ca(OH)2-CaO and CaCO3-CaSO4CaS Ternary Systems. 1. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 441. (11) Sango, H.; Miyakawa, T.; Yasue, T.; Arai, Y. Effects of Wet Air on Formation and Thermal Stability of 12CaO‚7Al2O3 (in Japanese). J. Ceram. Soc. Jpn. 1994, 102 (8), 772.
Received for review February 18, 2003 Revised manuscript received April 28, 2003 Accepted May 1, 2003 IE030159V
