Article pubs.acs.org/IECR
Calcium Looping Process for Clean Coal Conversion: Design and Operation of the Subpilot-Scale Carbonator Nihar Phalak, Shwetha Ramkumar, Niranjani Deshpande, Alan Wang, William Wang, Robert M. Statnick, and Liang-Shih Fan* 125 Koffolt Laboratories, William G. Lowrie Department of Chemical and Biomolecular Engineering, 140 West 19th Avenue, The Ohio State University, Columbus, Ohio 43210, United States ABSTRACT: The calcium looping process (CLP), which is being developed at The Ohio State University (OSU), is a clean coal technology for the production of hydrogen (H2) and electricity from coal-derived syngas. It integrates the water−gas shift (WGS) reaction with in situ removal of carbon dioxide (CO2) and other contaminants like sulfides and halides, thus resulting in the production of high-purity H2. The in situ removal of CO2 drives the equilibrium-limited WGS reaction forward. The CLP has the potential to reduce the overall footprint of a coal-to-H2 process because of the integration of several unit operations in a single-stage reactor. The high-temperature operation and the different exothermic reactions involved provide various sources of heat, which, when integrated appropriately, result in a process with low energy penalty. Prior work conducted in a fixed-bed reactor has shown that high carbon monoxide (CO) conversions and high H2 purities can be obtained, depending on the operating pressures. On the basis of the encouraging results obtained from the fixed-bed reactor, a subpilot-scale fluidized-bed reactor (carbonator) has been designed and constructed at OSU. In this work, the design of this reactor has been detailed and some operational results have been provided.
1. INTRODUCTION The United States has the world’s largest coal reserves, and growing energy demand is expected to drive coal production by approximately 20% in the next 2 decades.1 In the U.S., coal is primarily used for electricity generation; coal-fired power plants use >90% of the coal mined and produce ∼50% of the nation’s electricity.2,3 However, because it is a carbon-intensive fuel, energy generation from coal leads to emission of a huge amount of carbon dioxide (CO2). As a result, there have been extensive efforts in the research and development of carbon capture and sequestration (CCS) technologies for application in coal-based energy-generation systems.4−6 In particular, the application of CCS to coal-gasification systems provides a high-efficiency route to produce clean energy in the form of electricity and/or hydrogen (H2).6,7 CCS applied to coal-gasification systems entails precombustion CO2 capture, which can be achieved using different technologies involving solvents, solid sorbents, chemical looping, membranes, etc.7 The calcium looping process (CLP) developed at The Ohio State University (OSU) is a calcium-sorbent-based CO2 capture process that enables the production of high-purity H2 from coal-derived syngas.6−8 Along with H2 production and CO2 capture, the CLP also integrates the removal of other acidic gaseous impurities like hydrogen sulfide, hydrogen chloride, carbonyl sulfide, etc., in a single-stage reactor and obviates the need of a water−gas shift (WGS) catalyst.9 It was also found that the presence of the calcium sorbent reduces the excess steam requirement of the WGS reaction.10 The process scheme, different reactions occurring in the CLP, and the results from the bench-scale fixed-bed reactor tests have been reported elsewhere.9−11 It was found that ∼70% (by volume) H2 purity could be achieved at atmospheric pressure and 600 °C; at higher operating pressures (21 atm), >99% purity H2 could be produced.9,10 © 2012 American Chemical Society
The concept of using a calcium-based CO2 acceptor for enhancing H2 production from syngas through the WGS reaction and from methane (CH4) through the steam-reforming reaction has been investigated by several researchers.12−16 Among different CO2 acceptors, calcium sorbents provide more favorable thermodynamics and high reaction rates and also exhibit superior capture capacity.17 Naturally occurring CaO precursors such as limestone and dolomite are abundantly available and inexpensive, which further enhances the economic viability of the process.18 However, the regenerability of CaO poses a significant challenge. The phenomenon of decreasing CO2 capture capacity of CaO over multiple cycles has been previously reported.19,20 This occurs because of sorbent sintering at high temperatures during calcination.21−24 Because the sorbent is used over multiple cycles, repeated sintering worsens its performance. To address this issue, several methods25 have been suggested such as hydration of the sorbent,26−31 synthesis of novel sorbents by physical and chemical modifications,32−35 pretreatment methods like thermal reactivation,36 etc. As outlined in a prior work,9 the CLP includes an additional step for hydrating CaO, making it a three-stage process comprised of carbonation, calcination, and hydration. The hydrated CaO helps to achieve good CO2 capture and high H2 purity in the carbonator over multiple cycles.9 Because the sorbent is hydrated prior to each subsequent carbonation, the sorbent entering the CLP carbonator is in the form of calcium hydroxide [Ca(OH)2]. The carbonator operating conditions are such that dehydration of Ca(OH)2 Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 9938
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one such section has also been provided in Figure 1. Each section has ports for measuring the temperature and pressure and for drawing product gas for analyses. The entire assembly has been constructed using stainless steel 304. To achieve the desired operating temperature, high-temperature ceramic heaters (OMEGA Engineering, Inc.) are used along the length of the riser. Appropriate ceramic wool insulation has been provided to minimize heat losses. Multiple type K thermocouples monitor the temperature of the riser. These thermocouples also provide feedback to the ceramic heaters so that the desired operating temperature is achieved and maintained throughout the experiment. The feedback process control loop has been programmed in LabVIEW (National Instruments, Inc.). Figure 2 provides a snapshot of the reactor. The calcium sorbent is continuously delivered to the reactor using a Schenck Accurate midrange volumetric hopper and screw feeder. The revolutions per minute (rpm) of the motordriven screw decide the solids feed rate, and it is kept constant throughout the experiment. The solids feeding section also consists of two pneumatically operated gate valves, which open and close alternately and, thus, imitate a double-dump valve. This action prevents the gas entering the reactor from escaping through the solids feeding section. The opening and closing sequence of the valves has also been programmed in LabVIEW, and the time intervals in the sequence can be specified by the operator. The screw feeder feeds the sorbent in the stand pipe containing these valves. An inclined section then carries the solids into the reactor. A gas mixing panel (shown in Figure 3), designed and constructed by Swagelok Company, is used to deliver the reactant gases to the reactor. The gas mixing panel consists of independent mass flow controllers (MFCs; Brooks Instruments) for CO, H2, CO2, and N2. The MFCs are controlled using a dedicated digital read-out box. The mixing panel has been provided with a manual ball valve and a pneumatically
is favored, and the resulting steam is available for the WGS reaction.9 In the majority of the work reported in the literature, CaO obtained from natural precursors has been investigated.12−16 In this work, Ca(OH)2 has been directly used as a CO2 acceptor to enhance the noncatalytic WGS reaction, as would be the case in the three-stage CLP.9 The experiments have been performed in a subpilot-scale fluidized-bed carbonator, which has been designed on the basis of the data obtained on the bench scale. The results from the operation of this reactor have been reported here.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Pure gasesCO (99.5%), CO2 (99.9%), and H2 (>99.9%)supplied by Praxair were used to simulate a syngas mixture for all of the tests. Nitrogen (N2) was used as the carrier gas. Because large volumetric flow rates of N2 were required, liquid N2 dewars were used as the source of gaseous N2. Ultrahigh-purity (99.999%) helium and argon were used as carrier gases in the micro gas chromatograph (micro-GC). Calibration of the micro-GC was conducted using certified standard gases9.5% CO, 3% CO2, and 5% H2. Compressed air, available at the facility, was used to dilute the vent gas. The sorbent used in all of the tests was commercial-grade, high-calcium Ca(OH)2, obtained from Graymont Inc. The manufacturer-specified composition of the sorbent and other sorbent characteristics have been provided elsewhere.37 2.2. Test Facility. The CLP comprises three reactors: the carbonator, calciner, and hydrator.9 This work details the design and operation of the subpilot-scale carbonator only. The existing setup does not include the calciner and hydrator. The subpilot-scale CLP carbonator is a fluidized-bed reactor designed for once-through testing. Figure 1 provides the detailed design of this reactor. The riser section of the reactor is 11.5 ft height and 4 in. inside diameter. It is mainly composed of four identical flanged sections. A three-dimensional view of
Figure 1. Schematic of the subpilot-scale CLP carbonator at OSU. 9939
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The gaseous mixture delivered by the gas mixing panel is preheated to a suitable temperature before its injection into the reactor. The gas preheater is a stainless steel helical coil (∼25 ft length and ∼0.6 in. inside diameter) surrounded by a hightemperature ceramic heater. Two such preheaters are used in series. The preheated gas is then mixed with steam, produced using a combination of a high-precision Optos 3HM water pump (Eldex Laboratories) and a HGA-S steam generator (Micropyretics Heaters International). The steam generator is capable of producing steam up to a maximum temperature of 500 °C. The steam−gas mixture then enters the reactor through a sintered haste alloy plate. The sintered plate is porous enough to allow minimum pressure drop across it; however, it has suitable strength to support the weight of the solids in the reactor. A slipstream of the gas exiting the reactor is drawn through one of the gas sampling ports provided on the riser. The gas sampling port contains a 2-μm Swagelok in-line filter to prevent the solids from entering the gas analyses equipment, the microGC (Varian). An independent diaphragm micropump (KNF Neuberger) has been installed to extract the sample gas from the port. The pump allows sample gas in excess of 1 L/min to be delivered to the micro-GC. Prior to entering the micro-GC, the product gas is passed through a desiccant bed (anhydrous calcium sulfate) to remove the moisture. The gas−solid mixture from the reactor is diluted with air before it enters the Donaldson Torit downflow baghouse. The air dilution cools the exit gas to a suitable temperature for the baghouse and also maintains the gas composition outside the explosion limits. The solid-free gas from the exit of the baghouse is vented out of the facility. The reacted solids are recovered from the baghouse for analysis. 2.3. Operating Procedures. The reactor was heated to the operating temperature and maintained at that temperature. The feed gas to the reactor was also preheated to the same temperature. During preheating, only N2 was flowed through the reactor. Once the desired temperatures were attained, the reactive component (CO or a CO, CO2, and H2 mixture) was introduced in the feed gas and the sorbent was injected using the screw feeder. The product gas was analyzed using the micro-GC. Prior to each experimental run, the micro-GC was calibrated using certified standard gases. On the basis of the
Figure 2. Snapshot of the subpilot-scale CLP carbonator at OSU.
operated diaphragm valve for each gas. Gas cylinders are used to supply the gases to the gas mixing panel. For the flammable gasesH2 and COthe gas cylinders are stored in gas cabinets (Air Liquide) provided with independent ventilation. Automatic switchover valves (Scott Specialty Gases) have been installed for each gas to enable cylinder changeover during an experiment. For the safety of the operators, flash arrestors, check valves, and maximum-flow-limit shut-off valves have been installed in the gas lines.
Figure 3. Assembly of the gas mixing panel. Reproduced with permission from Swagelok Company. 9940
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results obtained from the bench-scale tests,9,10 all of the experiments in the current work were conducted at 600 °C. The pressure in all of the experiments was 1 atm. At the completion of every experimental run, solids from the baghouse were collected and analyzed for their CO2 content using a thermogravimetric analyzer. The data obtained from the microGC were also analyzed. The data obtained from gas analyses were in good agreement with the data obtained from solids analyses.
Although the design Ca:C was set at 1.3, calculation of the actual Ca:C “delivered” to the reactor was performed post-experiment. This is because of the variability introduced during operation of the solids feeder. As stated earlier, the screw feeder is a volumetric-based feeder, and prior to each experiment, the feeder was calibrated for the mass-based solids feed rate. However, during the course of the experiment, this feed rate (being a function of the bulk density of the solids) can fluctuate because of the way the solids “pack” themselves in the storage hopper, etc. Hence, at the end of each experiment, the mass of the sorbent left in the hopper was determined and subtracted from the initial amount of sorbent fed to the hopper. The difference is equal to the mass of the sorbent delivered to the reactor over the duration of the experiment. Knowing the duration of the experiment, the actual (average) sorbent feed rate can be calculated, which leads to a slightly different value of the Ca:C. When the Ca:C values were set at 1.3 and 2, postexperiment calculations indicated that the actual inlet Ca:C values during the experiment were 1.5 and 2.3, respectively. From Figure 4, it can be seen that a change in the Ca:C did not significantly alter CO conversion. In both cases, approximately 45−50% CO conversion was achieved. However, results from the tests conducted previously at OSU indicate that the CO2 removal achieved is a strong function of the Ca:C.37 Because CO2 capture drives the equilibrium-limited WGS reaction in the CLP carbonator,6,9,10 it is reasonable to expect a similar trend here: higher the Ca:C, higher the CO conversion. Because such a trend was not observed, it is hypothesized that, for the given scale of operation, the difference between the two Ca:C that were selected for investigation is not large enough to significantly affect CO conversion. 100% CO2 capture was, however, observed in all of the experiments that were conducted. Figure 4 also shows that CO conversion is low initially and then increases as the experiment progresses, ultimately reaching a steady value. The initial low conversion is mainly due to the large response time of the micro-GC toward CO. The start-up of the experiment, where the system is essentially in an unsteady state, also contributes to this profile. A similar profile was observed in all of the experiments. For the case where the Ca:C of 2.3 was used, sudden drops in conversion can be observed at 20 and 40 min. These were due to the clogging of the gas sampling line by the sorbent. The sample gas flow to the micro-GC was momentarily suspended for cleaning of the line, which affected the concentration of CO. Another phenomenon that is believed to have hampered CO conversion in the experiments was the contact of “cold” solids with hot gas. As stated earlier, the current design of this system allows for once-through testing only. Thus, the solids fed to the reactor were at room temperature, while the preheated gas was fed at operating temperature (600 °C). At the point of contact of the gas and the solid sorbent, a temperature drop is inevitable. Lower temperatures will lead to poor kinetics, especially for the carbonation reaction. 24 Because the carbonation reaction is expected to drive the WGS reaction, this ultimately affects the final conversion of CO. If this phenomenon is significantly more important, then the Ca:C will cease to have a bearing on the final CO conversion. If a significantly higher Ca:C (5−10 or higher) would have been used, a higher CO conversion may have been possible. However, these experiments were not performed for two reasons. First, from the point of view of the overall process, a higher Ca:C will increase the solids circulation among the
3. RESULTS AND DISCUSSION 3.1. Effect of the Calcium-to-Carbon Mole Ratio (Ca:C). The Ca:C, which quantifies the calcium loading in the carbonator, was investigated, and Figure 4 shows the results.
Figure 4. Effect of the Ca:C on CO conversion.
Two Ca:C1.5 and 2.3were used. The Ca:C is calculated based on the carbon entering the feed gas. For Ca:C investigation, the experiments were performed using CO, and hence the ratio of moles of Ca(OH)2 to moles of CO is the Ca:C. Because both of the molar flow rates were kept constant throughout the experiment, the Ca:C at the reactor inlet is constant. Similarly, in experiments in which the feed gas contained both CO and CO2 (see section 3.4), the Ca:C was calculated as the ratio of moles of Ca(OH)2 to combined moles of CO and CO2. The selection of the Ca:C was based on the results obtained in prior work conducted at OSU.37 In that work,37 the effect of the Ca:C on the extent of CO2 removal from coal combustion flue gas was investigated, and it was found that the CO2 removal achieved is a strong function of the Ca:C. For example, using Ca(OH)2, it was found that 50% CO2 removal is possible at Ca:C ∼ 0.7, and for 90% and 100% CO2 removal, the Ca:C values required are ∼1.3 and ∼1.6, respectively.6,37 In the present work, to begin with, a Ca:C of 1.3 was chosen, which meant that if all of the CO sent to the carbonator got converted via the WGS reaction, then 90% of the CO2 produced would be captured. However, from the fixed-bed tests conducted earlier, it was observed that a CO conversion of 70% could be achieved at 600 °C and 1 atm.9,10 Moreover, this conversion was recorded during the initial period of the fixed-bed experiment, when the Ca:C is practically infinite, making it the maximum conversion possible. (For a fixed-bed experiment, where breakthrough curves are obtained for gas conversion, a parameter like the Ca:C cannot be calculated; nonetheless, because all of the sorbent is fresh when the experiment begins, the Ca:C can be said to be infinite.) Hence, it was expected that CO conversion would never exceed 70%. Assuming that 70% CO conversion is achieved, the effective operating Ca:C in the reactor would exceed 1.6, which is sufficient to achieve 100% CO2 capture. 9941
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excess steam also aided in suppressing the Boudouard reaction. In the Boudouard reaction, CO cracks to form CO2 and carbon (C), as follows:
different units of the CLP, leading to less attractive process economics and other solids handling and waste disposal issues.38 As a result, there exists a trade-off between a superior performance in the carbonator and the overall feasibility of the CLP. Second, as outlined above, the current experimental setup posed some unavoidable limitations, and hence future work is expected to overcome them so that more accurate conclusions can be drawn. 3.2. Effect of the Steam-to-Carbon Mole Ratio (S:C). As described earlier in section 2.2, “excess steam” could be added to the reactor using a water pump−steam generator combination. Hence, to investigate the effect of the S:C, the experimental results of two runswith and without excess steamwere compared. Calculation of the S:C was based on the total steam available for the reaction including the steam resulting from dehydration of Ca(OH)2. For example, when the Ca:C was maintained at 1.5 and 1.5 mol of excess steam was supplied per 1 mol of C fed to the reactor, the overall S:C was 3. In the absence of excess steam, the overall S:C was the same as the Ca:C (=1.5). The effect of the S:C on H2 purity is shown in Figure 5. When no excess steam was supplied and dehydration of
2CO(g) → CO2 (g) + C(s)
This reaction is thermodynamically feasible in the operating conditions used in the CLP carbonator in this work.39 Occurrence of the above reaction is undesirable for the CLP because it results in a loss of the reactant CO. In the absence of the Boudouard reaction, a one-to-one ratio exists between the moles of CO reacted and the moles of H2 produced. It is difficult to accurately quantify the extent of the Boudouard reaction because the product CO2 gets captured by the calcium sorbent and the C is left behind in the reactor, mainly as a deposition on the reactor walls. The difference in the total CO conversion and CO converted to H2, as obtained from the micro-GC data, provided an indication of the extent of the Boudouard reaction. In the case where excess steam was supplied to the reactor, the difference between the two conversions was found to be negligible. However, approximately 15% of the CO that was converted did not result in the production of H2 when no excess steam was supplied. Figure 6
Figure 5. Effect of the S:C on H2 purity in the product gas (Ca:C = 1.5).
Ca(OH)2 was the only source of steam for the WGS reaction, a H2 purity of 37% was obtained. The H2 purity increased to 49% when excess steam was supplied to the carbonator. This result is expected as per Le Chatelier’s principle: an increase in the concentration of one of more reactants drives the reaction in the forward direction. Replicate runs were conducted and confirmed the reproducibility of these results. Figure 4, which shows the effect of the Ca:C on CO conversion, also inherently represents the effect of the S:C on CO conversion and, hence, H2 purity. In Figure 4, the S:C is equal to the Ca:C in each case. However, unlike Figure 5, no significant difference in CO conversion (and, hence, H2 purity) is observed. When the results from Figures 4 and 5 are compared, it can be hypothesized that the enhancement provided by externally supplied steam is superior to the one provided by increasing the supply of steam via increments in the moles of Ca(OH)2. The externally supplied excess steam is readily available for the WGS reaction, while in the other scenario, Ca(OH)2 has to decompose to provide the steam in situ. When the steam is readily available, it is likely to be well distributed throughout the reactor and causes a more pronounced effect on the forward reaction. 3.3. Carbon Formation via the Boudouard Reaction. In addition to enhancing the product H2 purity, the presence of
Figure 6. (a) Sorbent fed to the reactor: Ca(OH)2. (b) Solids recovered from the reactor wall after completion of the experiment.
presents a comparison between the sorbent that was fed in the reactor and the sorbent that was recovered from the walls of the reactor after the experiment was completed. The difference is clearly visible: the sorbent in Figure 6b is discolored because of 9942
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absence of a catalyst. The results from the experiments conducted using a syngas-like mixture show that the equilibrium limitation imposed by the presence of H2 and CO2 in the feed gas is not significant and CO conversions are similar to those observed in tests performed with CO only. In all of the experiments conducted, no CO2 was detected in the product gas. Previously conducted tests on the bench-scale fixed-bed reactor indicate that ∼70% purity H2 can be obtained at 600 °C and 1 atm at S:C = 1. The superior performance in the fixed-bed reactor is likely due to better gas−solid contact, very high Ca:C, etc. The scale-up factor is also believed to have an effect on the performance.
the C formed during the experiment. It is believed that the presence of more steam favors the occurrence of the WGS reaction over the Boudouard reaction. 3.4. Testing with a Syngas-like Mixture. For the parametric investigations discussed in the preceding sections, the experiments were conducted with only CO as the reactive component in the feed gas. Experiments using the major syngas constituentsCO, CO2, and H2in the feed gas were also performed. The result from one such experiment is shown in Figure 7. The composition of the feed gas used in this
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AUTHOR INFORMATION
Corresponding Author
*Telephone: (614)-688-3262. Fax: (614)-292-3769. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial assistance provided by the National Energy Technology Laboratory (NETL) of the U.S. Department of Energy (DOE) and the Ohio Coal Development Office of the Ohio Department of Development. Assistance of IWI, Inc., during construction of the reactor is also gratefully acknowledged.
Figure 7. Product gas composition for tests conducted with a syngaslike mixture (S:C = 3; Ca:C = 1.5).
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experiment is provided in Table 1. This composition is based on a typical syngas composition derived from a GE entrained
(1) U.S. Energy Information Administration. Annual Energy Outlook 2011; DOE/EIA-0383; 2011. Available at http://www.eia.gov/ forecasts/aeo/pdf/0383(2011).pdf. (2) U.S. Energy Information Administration. Official of Oil, Gas and Coal Supply Statistics. Annual Coal Report 2009; DOE/EIA-0584; 2009. Available at http://www.eia.gov/cneaf/coal/page/acr/acr.pdf (3) U.S. Energy Information Administration. Official Energy Statistics from the U.S. Government. Net Generation by Energy Source by Type of Producer; 2007; http://www.eia.doe.gov/cneaf/electricity/epm/ table1_1.html. (4) Ciferno, J.; Fout, T.; Jones, E.; Andrew, P.; Murphy, J. T. Capturing Carbon from Existing Coal-Fired Power Plants. Chem. Eng. Prog. 2009, 33−41. (5) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 capture technologyThe U.S. Department of Energy’s Carbon Sequestration Program. Int. J. Greenhouse Gas Control 2008, 2, 9−20. (6) Fan, L.-S. Chemical Looping Systems for Fossil Energy Conversions: John Wiley & Sons: Hoboken, NJ, 2010. (7) Li, F.; Fan, L.-S. Clean coal conversion processesprogress and challenges. Energy Environ. Sci. 2008, 1, 248−267. (8) Ramkumar, S.; Iyer, M. V.; Fan, L.-S. (The Ohio State University, Columbus, OH). Calcium Looping Process for High Purity Hydrogen Production. International Patent Application PCT/US2008/039783, 2008. (9) Ramkumar, S.; Fan, L.-S. Thermodynamic and Experimental Analyses of the Three-Stage Calcium Looping Process. Ind. Eng. Chem. Res. 2010, 49, 7563−7573. (10) Ramkumar, S.; Fan, L.-S. Calcium Looping Process (CLP) for Enhanced Noncatalytic Hydrogen Production with Integrated Carbon Dioxide Capture. Energy Fuels 2010, 24, 4408−4418. (11) Ramkumar, S.; Iyer, M. V.; Fan, L.-S. Calcium Looping Process for Enhanced Catalytic Hydrogen Production with Integrated Carbon Dioxide and Sulfur Capture. Ind. Eng. Chem. Res. 2011, 50, 1716− 1729. (12) Brun-Tsekhovoi, A. R.; Zadorin, A. N.; Katsobashvili, Y. R.; Kourdyumov, S. S. The Process of Catalytic Steam Reforming of
Table 1. Composition of the Feed Gas Used in the Experiment with a Syngas-like Mixture gas
volume %
N2-free volume %
CO H2 CO2 N2
6.5 6.17 2.5 balance
42.8 40.7 16.5
REFERENCES
flow gasifier.40 The S:CO was maintained at 3, due to which the overall S:C reduced to ∼2.2. The overall S:C is lower in this case because the C contributed by CO2 is also considered in the calculation. As can be seen from Figure 7, the final product contained only H2 and CO. All of CO2that supplied in the feed and that generated in situwas captured. On a N2- and steam-free basis, the product was found to contain ∼70% H2 and balance CO. On the basis of the data obtained from the micro-GC, it was found that CO conversion was 50%, which is similar to the conversion observed in the tests conducted using CO only. Theoretically, with all other conditions remaining the same, the presence of H2 and CO2 in the feed should lower CO conversion by Le Chatelier’s principle. However, because such an effect was not observed, it can be concluded that the presence of a CO2 acceptor helps to overcome this limitation.
4. CONCLUDING REMARKS Simultaneous CO2 capture and enhanced H2 production were investigated in the presence of a calcium-based CO2 acceptor in the subpilot-scale CLP carbonator. Parametric investigation revealed that, at Ca:C = 1.5 and S:C = 3, a H2 purity of 50% can be achieved via the WGS reaction at 600 °C and 1 atm in the 9943
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Industrial & Engineering Chemistry Research
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Hydrocarbons in the Presence of a Carbon Dioxide Acceptor. Hydrocarbon Energy Progress VII, Proceedings of the 7th World Hydrogen Energy Conference, Moscow, Sept 1988; Veziroglu, T. N., Protsenko, A. N., Eds.; Pergamon Press: New York, 1988; Vol. 2, p 885. (13) Han, C.; Harrison, D. P. Simultaneous Shift Reaction and Carbon Dioxide Separation for the Direct Production of Hydrogen. Chem. Eng. Sci. 1994, 49, 5875−5883. (14) Balasubramaniam, B.; Ortiz, A. L.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from methane in a single step. Chem. Eng. Sci. 1999, 54, 3543−3552. (15) Johnsen, K.; Ryu, H. J.; Grace, J. R.; Lim, C. J. Sorptionenhanced steam reforming of methane in a fluidized bed reactor with dolomite as CO2-capture. Chem. Eng. Sci. 2006, 61, 1195−1202. (16) Sircar, S.; Lee, K. B. Sorption Enhanced Reaction Concepts for Hydrogen Production: Materials & Processes: Research Signpost: Trivandrum, India, 2010. (17) Ochoa-Fernandez, E.; Haugen, G.; Zhao, T.; Ronning, M.; Aartun, I.; Borresen, B.; Rytter, E.; Ronnekleiv, M.; Chen, D. Process design simulation of H2 production by sorption enhanced steam methane reforming: evaluation of potential CO2 acceptors. Green Chem. 2007, 9, 654−662. (18) Harrison, D. P. Sorption-Enhanced Hydrogen Production: A Review. Ind. Eng. Chem. Res. 2008, 47, 6486−6501. (19) Curran, G. P.; Fink, C. E.; Gorin, E. Carbon Dioxide-Acceptor Gasification Process. Studies of Acceptor Properties. Adv. Chem. Ser. 1967, 69, 141. (20) Barker, R. The reversibility of the reaction CaCO3 = CaO + CO2. J. Appl. Chem. Biotechnol. 1973, 23, 733−742. (21) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. The effect of CaO sintering on cyclic CO2 capture in energy systems. AIChE J. 2007, 53, 2432−2442. (22) Iyer, M. V.; Gupta, H.; Sakadjian, B. S.; Fan, L.-S. Multicyclic Study on the Simultaneous Carbonation and Sulfation of High Reactivity CaO. Ind. Eng. Chem. Res. 2004, 43, 3939. (23) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308−315. (24) Bhatia, S. K.; Perlmutter, D. D. Effect of the product layer on kinetics of the CO2-lime reaction. AIChE J. 1983, 29, 79−86. (25) Yu, F.; Phalak, N.; Sun, Z.; Fan, L.-S. Activation Strategies for Calcium-Based Sorbents for CO2 Capture: A Perspective. Ind. Eng. Chem. Res. 2011, 51, 2133−2142. (26) Kuramoto, K.; Fujimoto, S.; Morita, A.; Shibano, S.; Suzuki, Y.; Hatano, H.; Shi-Ying, S.; Harada, M.; Takarada, T. Repetitive Carbonation-Calcination Reactions of Ca-Based Sorbents for Efficient CO2 Sorption at Elevated Temperatures and Pressures. Ind. Eng. Chem. Res. 2003, 42, 975−981. (27) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Investigation of Attempts of Improve Cyclic CO2 Capture by Sorbent Hydration and Modification. Ind. Eng. Chem. Res. 2008, 47, 2024−2032. (28) Manovic, V.; Lu, D. Y.; Anthony, E. J. Steam Reactivation of Spent CaO-Based Sorbent for Multiple CO2 Capture Cycles. Enivron. Sci. Technol. 2007, 41, 1420−1425. (29) Zeman, F. Effect of steam hydration on performance of lime sorbent for CO2 capture. Int. J. Greenhouse Gas Control 2008, 2, 203− 209. (30) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved LongTerm Conversion of Limestone-Derived Sorbents for in Situ Capture of CO2 in a Fluidized Bed Combustor. Ind. Eng. Chem. Res. 2004, 43, 5529−5539. (31) Martinez, I.; Grasa, G.; Murillo, R.; Arias, B.; Abanades, J. C. Evaluation of CO2 Carrying Capacity of Reactivated CaO by Hydration. Energy Fuels 2011, 25, 1294−1301. (32) Lu, H.; Reddy, E. P.; Smirniotis, P. G. Calcium Oxide Based Sorbents for Capture of Carbon Dioxide at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944−3949. (33) Li, Y.; Zhao, C.; Qu, C.; Duan, L.; Li, Q.; Liang, C. CO2 Capture Using CaO Modified with Ethanol/Water Solution during Cyclic Calcination/Carbonation. Chem. Eng. Technol. 2008, 31, 237−244.
(34) Li, Y.; Zhao, C.; Chen, H.; Duan, L.; Chen, X. Cyclic CO2 capture behavior of KMnO4-doped CaO-based sorbent. Fuel 2010, 89, 642−649. (35) Li, L.; King, D. L.; Nie, Z.; Howard, C. Magnesia-Stabilized Calcium Oxide Absorbents with Improved Durability for High Temperature CO2 Capture. Ind. Eng. Chem. Res. 2009, 48, 10604− 10613. (36) Manovic, V.; Anthony, E. J. Thermal activation of CaO-based sorbent and self-reactivation during CO2 capture looping cycles. Environ. Sci. Technol. 2008, 42, 4170−4174. (37) Wang, W.; Ramkumar, S.; Li, S.; Wong, D.; Iyer, M.; Sakadjian, B. B.; Statnick, R. M.; Fan, L.-S. Subpilot Demonstration of the Carbonation−Calcination Reaction (CCR) Process: High-Temperature CO2 and Sulfur Capture from Coal-Fired Power Plants. Ind. Eng. Chem. Res. 2010, 49, 5094−5101. (38) Connell, D. P.; Phalak, N.; Fan, L.-S.; Statnick, R. M.; Lewandowski, D. A. Design and Economics of Advanced Energy Systems Using Calcium Sorbents for CO2 Capture. Presented at the 36th International Technical Conference on Clean Coal and Fuel Systems, Clearwater, FL, June 5−9, 2011. (39) HSC Chemistry, version 5.0; Outotec Research Oy: Pori, Finland, 2008. (40) U.S. Department of EnergyNational Energy Technology Laboratory. Assessment of Hydrogen Production with CO2 Capture Volume 1: Baseline State-of-the Art Plants; DOE/NETL-2010/1434. Available at http://www.netl.doe.gov/energy-analyses/pubs/BitBase_ FinRep_Rev2.pdf.
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dx.doi.org/10.1021/ie202725w | Ind. Eng. Chem. Res. 2012, 51, 9938−9944
