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H2 separation membrane reactor for further CO2 enrichment and energy recovery
Energy xxx (2015) 1e7
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Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery Kyung-Ran Hwang*, Jin-Woo Park, Sung-Wook Lee, Sungkook Hong, Chun-Boo Lee, Duck-Kyu Oh, Min-Ho Jin, Dong-Wook Lee, Jong-Soo Park** Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
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a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 15 June 2015 Accepted 18 June 2015 Available online xxx
The CCR (catalytic combustion reaction) of the retentate gas, consisting of 90% CO2 and 10% H2 obtained from a CO2/H2 separation membrane reactor, was investigated using a porous Ni metal catalyst in order to recover energy and further enrich CO2. A disc-shaped porous Ni metal catalyst, namely Al[0.1]/Ni, was prepared by a simple method and a compact MCR (micro-channel reactor) equipped with a catalyst plate was designed for the CCR. CO2 and H2 concentrations of 98.68% and 0.46%, respectively, were achieved at an operating temperature of 400 C, GHSV (gas-hourly space velocity) of 50,000 h1 and a H2/O2 ratio (R/ O) of 2 in the unit module. In the case of the MCR, a sheet of the Ni metal catalyst was easily installed along with the other metal plates and the concentration of CO2 in the retentate gas increased up to 96.7%. The differences in temperatures measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet in the MCR. The disc-shaped porous metal catalyst and MCR configuration used in this study exhibit potential advantages, such as high thermal transfer resulting in improved energy recovery rate, simple catalyst preparation, and easy installation of the catalyst in the MCR. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Catalytic combustion CO2 enrichment CO2/H2 membrane Metal catalyst Micro-channel reactor
1. Introduction CCS (Carbon capture and storage) is a group of technologies aimed at mitigating the global warming effect by capturing up to 90% of the CO2 emitted during the combustion of fossil fuels in power plants and various industrial processes [1e3]. The CO2 is captured and separated using one of three methods, namely precombustion capture (pre-CCS), post-combustion capture, and oxyfuel combustion. The captured CO2 is then transported and sequestrated in selected geological rock formations [1,3]. Among the CCS technologies available, pre-CCS using CO2/H2 separation membranes located within integrated gasification and IGCC (combined cycle power generation) units has been considered to be a relatively cost efficient and viable technology. This methodology has the following advantages: (1) IGCC yields a concentrated CO2 mixture, (2) the CO2 can be captured at a high pressure and (3) the separated high-purity H2, carbon-free fuel, can also be used for other purposes in the power plant where it is produced [4e6].
* Corresponding author. Tel.: þ82 42 860 3504; fax: þ82 42 860 3495. ** Corresponding author. Tel.:þ82 42 860 3664; fax: þ82 42 860 3495. E-mail addresses: [email protected] (K.-R. Hwang), [email protected] (J.-S. Park).
Recently, various type of H2/CO2 separation membranes and their modules have been developed for pre-CCS applications [7e9]. Berkel et al. constructed a membrane module including a 1 m long Pd membrane and the module generated about 12 Nm3/m2/h of hydrogen at 400 C and 30 bar of a gas mixture from a natural gas combined cycle with assistance of N2 sweep gas. The retentate was composed of above 80% in dry CO2-levels balanced by remaining CO and H2 [7]. Lee et al. investigated the combined WGS (water-gas shift) reaction and membrane tests for pre-CCS in a coal gasifier and reported that using the high-pressure membrane module equipped with a planar-type PdeAu membrane, simultaneous CO2 and H2 enrichments of 92.1% and 99.8%, respectively, were achieved [8]. As mentioned previously, the retentate, which may contain up to 90% CO2, is finally sequestrated at a safe site. However, besides CO2, the sequestrated retentate gas typically contains up to 10% of combustible gases, mainly H2 which usually goes unutilized [8]. We tried to focus on recovery of energy from the combustible gases in the retentate to heat up the main stream in a combined IGCC/preCCS reactor system. In general, the operating temperatures are changed up and down along the unit operations of IGCC incorporated with the pre-CCS using the membrane system, as shown in Fig. 1(a). As we know, several coolers and heaters are present between the various process units to recover energy and generate
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Please cite this article in press as: Hwang K-R, et al., Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.067
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Fig. 1. Block diagram of the combined IGCC/pre-CCS system using the membrane system (a) and block diagram of the advanced IGCC/pre-CCS system using the membrane and energy recovery systems (b).
steam for the WGS reaction. Among the various unit operations, the heater that is used for achieving operation temperatures of the membrane reactor, 350e450 C, requires the largest amount of energy [10]. Herein, if the dewatered stream is heated to the operating temperature of the membrane reactor using the heat of combustion reaction of combustible gases in retentate alone without any assistance from the heater (Fig. 1(b)), the overall energy recovery of the process could be improved. Furthermore, by utilizing the combustible gases in the retentate, CO2, which constitutes approximately 90% of the initial retentate, could be enriched up to 99%, which would be more favorable for sequestration. The CCR (catalytic combustion reaction) has been reported to be an effective method for obtaining energy or eliminating it from a H2-rich or lean mixture [11e15]. Zhang et al. prepared a monolithic Pt-based catalyst with a mesoporous ceramic coating for used in the CCR of lean H2 [14]. The Pt/Ce0.6Zr0.4O2/MgAl2O4 catalyst showed excellent catalytic activity for H2 combustion. Recently, in order to obtain high surface area of the catalyst and improve the thermal conductivity of the catalytic support for applications in exothermic CCRs, the ceramic monolith support in the Pt-doped CCR catalyst has been replaced with porous Ni-foam [15]. However, alumina, which has a lower thermal conductivity than metals, is still used in the preparation steps for the catalysts, involving some limitations such as peeling of the catalyst layer and lack of uniformity. In this study, we have investigated for the first time, the catalytic combustion of the retentate gas obtained from the CO2/H2 separation membrane reactor, with the goal of recovering energy and further concentrating CO2 in the retentate for pre-CCS. A discshaped porous Ni metal catalyst has been developed and a compact MCR (micro-channel reactor) has been designed for the CCR of a simulated retentate gas.
2. Experimental 2.1. Preparation of the Ni metal catalyst for CCR A disc-shaped Al[0.1]/Ni metal catalyst (about 50 mm in diameter and 2 mm in thickness) was prepared for catalytic H2 combustion. Firstly, the Ni powder was impregnated with Al (0.1 wt.%) by incipient wetness impregnation method over pre-dried Ni metal powder (Vale Inco Pacific Ltd.) with an aqueous solution of Al(NO3)3$9H2O (SigmaeAldrich Co. Ltd) to prepare a cermet system
for preventing the sintering of metals, which may occur at relatively high temperatures. The impregnated powder was dried at 120 C for 12 h and subsequently treated with a 75% H2/He mixture at 450 C for 30 h to remove impurities. Thereafter, the powder was ground with a ball-mill for 2 h. The prepared Al[0.1]/Ni powder was compressed into a cylindrical metal mold without any binder at a pressure of 30 MPa. The compressed disc-shaped catalyst was further sintered at 900 C for 2 h under H2 flow to improve its mechanical strength. The prepared catalyst was characterized by XRD (X-ray powder diffraction, D/MAX IIIC). The surface images were obtained by FE-SEM (field emission scanning electron microscopy, HITACHIS-4700) and pore-related information such as pore size, total volume and porosity were obtained by mercury porosimetry (Autopore IV 9500).
2.2. CCR test in a unit module A schematic diagram of the CCR, cross-sectional image of the unit module installed with the disc-shaped metal catalyst, and photographs of metal catalyst and two types of O-rings are shown in Fig. 2(a,b and c), respectively. A gas mixture of 10% of H2 in CO2, which is similar to the composition of a retentate gas from the H2/ CO2 separation membrane module, was fed to the reactor using MFC (mass flow controllers, Brooks Instrument). The product was passed through a cold-trap to remove the liquid water, formed during the H2 combustion reaction before the gas entered into a GC (gas chromatography, Agilent 6890N) system. A K-type thermocouple was positioned on the upper surface of the metal catalyst in the unit module to control the reaction temperature. Two additional thermocouples, one at the inlet and the other at the outlet of reactor, were placed to monitor the temperatures before and after the reaction. A mixture containing 10% H2 in N2 was passed through the reactor at a constant flow rate of 100 ml/min while heating the reactor to the reaction temperature (350 C) and cooling it down to room temperature. No other activation procedures were applied for the metal catalyst. All experimental were carried out in atmospheric pressure. The product gas was analyzed using an on-line GC equipped with two thermal conductivity detectors and two columns (HP-MOLSIV and Carboxen1010). As shown in Fig. 2(b), the CCR occurred as the reactant traveled vertically through the metal catalyst. The product gas was vented to the supporter, which was positioned just below the catalyst, and then passed through the outlet port. Two metal O-rings were used to seal the flange type module [16], as shown in Fig. 2(b and c). The effects of variables
Please cite this article in press as: Hwang K-R, et al., Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.067
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Fig. 2. An experimental schematic diagram of the apparatus for CCR (a), cross-sectional image of the unit module used for the CCR containing the disc-shaped metal catalyst (b), photograph of the disc-shaped metal catalyst and two types of O-rings (c) and layout of the components and gas path in the MCR and the photograph of an assembled MCR (d).
such as temperature and GHSV (gas-hourly space velocity) on the CCR of H2 were investigated at a ratio of hydrogen to oxygen (R/O) of 2. For comparison, H2 combustion using a specially prepared Pd impregnated Al[0.1]/Ni metal catalyst and without a catalyst were studied. 2.3. CCR test in MCR The layout of the components in the MCR, travel path for the gases and the photograph of an assembled MCR used in this study
are shown in Fig. 2(d). The MCR consisted of a CCR section and a heat exchanger section with inlet and outlet ports for N2, which acted as the heat exchange gas. In addition, the MCR also contained various types of metal plates with micro-channels, which were assembled with diffusion bonding in a hot-press [17]. The metal plates were chemically etched and included half-etched straight channel plates (8 sheets), fully etched 3D mixing channel plates (2 sheets), catalyst holder plates (2 sheets) and separator and cover plates (5 sheets). Each of the plates was 1 mm thick, except for the cover plates. The disc-shaped porous metal catalyst was 89 mm in
Please cite this article in press as: Hwang K-R, et al., Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.067
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diameter and 1 mm in thickness. The assembled MCR with width, length and depth of 200 mm, 290 mm, and 100 mm, respectively, is shown in Fig. 2(d). A leak test of the diffusion ebonded MCR was conducted with H2 at a pressure of 5 bar to confirm that the metal plates were joined well. As shown in Fig. 2(d), a mixture of reactants (CO2 and H2) and O2 entered the reactor at one end and vented at the opposite end. The heat exchange gas, N2, entered the reactor at the opposite end as the reactants and the reactant and heat exchange gas traveled in counter directions for effective heat exchange. The metal catalyst was installed at the center of a fully etched plate and a metal O-ring was used to prevent channeling of reactants not being able to contact the catalyst. The reaction temperature was adjusted with a temperature controller as shown in Fig. 2(d). The MCR was set in an electric furnace instead of a unit module in Fig. 2(a) to carry out the CCR test. The gas flow was controlled using MFCs and the products were analyzed using an online GC system, as described previously in this section.
achieve improved CO2 enrichment and energy recovery. The results of the CCR in the unit module at an R/O of 2.0, GHSV of 50,000 h1, and an operating temperature of 350 C are presented in Fig. 5(a and b). After the combustion of lean H2 over the Al[0.1]/Ni metal catalyst, CO2 was enriched up to 97.6%, whereas small quantity of H2 and O2 remained, as shown in Fig. 5(a). The difference in the temperature measured before and after the CCR at the outlet of the unit was 36 C, implying that significant amount of energy was produced by combustion of lean H2, which can be recovered and reused. In addition, traces of CO and CH4 were also detected. This result can be explained by considering the following reversible reactions (Eqs. (1) and (2)):
3. Results and discussion
In other words, CH4 and CO were formed from methanation and/or reverse WGS reactions, respectively. It is worth noting that the reactions did not take place in the blank test (not shown here), where CCR was carried out in the absence of catalyst under the same reaction conditions. When the Pd[0.1]-Al[0.1]/Ni metal catalyst was used (Fig. 5(b)), the CO2 concentration increased slightly, whereas the H2 and O2 concentrations decreased to about half of the values obtained with the Al[0.1]/Ni catalyst, under identical reaction conditions. This implies that precious metals assist the CCR of lean H2 as well documented in the literature [12]. Traces of CO and CH4 were also detected. However, Ni in the metallic state
3.1. Textural properties of the Al[0.1]/Ni metal catalyst The XRD pattern of the prepared disc-type metal catalyst is shown in Fig. 3. The diffraction peaks at 2q ¼ 44.5 (111), 51.8 (200) and 76.4 (222) are characteristic of nickel metal. However, the diffraction peaks corresponding to the impregnated Al were not detected, which may have been due to the undetectable levels of Al. The pore size distribution and FE-SEM image of the disc-type metal catalyst are shown in Fig. 4(a and b). The Al[0.1]/Ni metal catalyst had two pore sizes of 0.6 and 184 mm. In addition, the porosity, total pore volume, and average pore size of the porous metal catalyst were 39.4%, 0.0741 ml/g and 619.3 nm, respectively. Since the metal catalyst was prepared by compressing the metal powder in the absence of binders and then sintered at high temperature, pores were observed between the spherical Ni powders, as shown in the FE-SEM image. The CCR occurred as the reactants passed through the pores of the metal catalyst, as shown in Fig. 2(b).
CH4 þ H2O 4 CO þ 3H2, DH ¼ 205.8 kJ/mol (steam reforming of CH4 /methanation) (1) CO þ H2O 4 CO2 þ H2, DH ¼ 41.2 kJ/mol (WGS/reverse WGS)(2)
3.2. CCR in the unit module We carried out the CCR of a simulated retentate gas, containing 90% CO2 and 10% H2, from CO2/H2 separation membrane reactor to
Fig. 3. XRD pattern of the disc-shaped metal catalyst.
Fig. 4. Pore size distribution (a) and FE-SEM image (b) of the porous metal catalyst.
Please cite this article in press as: Hwang K-R, et al., Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.067
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Fig. 6. Effect of reaction temperature on the CCR in the unit module at R/O ¼ 2.0 and GHSV ¼ 50,000 h1 over the Al/Ni catalyst (a) and effect of GHSV on the CCR in the unit module at R/O ¼ 2.0 and 350 C over the Al/Ni catalyst (b). Fig. 5. Results of the CCR of H2 in the unit module using a mixture of CO2 (90%) and H2 (10%) at R/O ¼ 2.0, GHSV ¼ 50,000 h1 and 350 C with (a) Al/Ni catalyst and (b) Pd/Al/ Ni catalyst.
exhibited good catalytic activity for CCR, comparable to that of the Ni catalyst containing Pd. This is consistent with the results of the study by Li et al. [18]. They found that the presence of metallic Ni in the Ni/Al2O3 catalyst promoted the combustion activity of CH4 under specific reaction conditions. Therefore, we suggest that Al [0.1]/Ni metal is an effective catalyst for the CCR of H2 in the retentate gas from the membrane reactor. The effect of reaction temperature on the CCR over the Al[0.1]/Ni catalyst at R/O of 2.0 and GHSV of 50,000 h1 is shown in Fig. 6(a). A slight increase in the concentration of CO2 was observed with an increase in the reaction temperature. At a reaction temperature of 400 C, the concentration of CO2 reached 98.68%, which was similar to the results obtained for the CCR using Pd[0.1]-Al[0.1]/Ni as the catalyst, as shown in Fig. 5(b). On the other hand, the concentrations of H2 and O2 decreased monotonically as the reaction temperature increased. For example, the concentrations of H2 and O2 at 320 C were 1.63% and 2.03% at 320 C, respectively, whereas the corresponding values were 0.46% and 0.38%, respectively, at 400 C. The maximum amounts of CH4 and CO were detected at 350 C. It is difficult to explain the relationship between the reaction temperature and the amount of trace compounds generated from the sidereactions, because one of the side-reactions is exothermic, whereas the other one is endothermic, as shown in Eqs. (1) and (2). However, the porous Ni metal catalyst appears to contribute lightly to methanation and/or the WGS reactions [17,19]. The effect of GHSV on the catalytic performance and difference in the temperature measured before and after the CCR is presented
in Fig. 6(b). A higher value of GHSV resulted in a shorter residence time in the reactor, which may be insufficient for the reactants to undergo complete CCR. As expected, for a GHSV of 10,000 h1, CO2 was enriched up to 98.7% and no O2 was detected, indicating that the residence time was sufficient for the complete consumption of O2 as an oxidant. However, H2 of 0.71% was detected, which might have originated from the reversible side-reactions. Further, while the concentration of CO2 decreased slightly from 98.7% at 10,000 h1 to 97.3% at 100,000 h1, there was no significant difference in the gas composition and concentration of the enriched CO2 above a GHSV value of 50,000 h1. The differences in temperatures measured before and after CCR at the outlet of the reactor increased steadily as a function of GHSV (10 C, 36 C and 63 C at 10,000 h1, 50,000 h1 and 100,000 h1, respectively). This suggests that the CO2 enrichment(greater than 97.3%) and a large amount of heat of reaction could be obtained via the CCR of lean H2 in the retentate (90% CO2 and 10% H2) using Al[0.1]/Ni metal catalyst at a GHSV of 100,000 h1, operating temperature of 350 C, and R/O of 2.
3.3. CCR in the MCR The MCR has many advantages such as excellent heat and mass transfer characteristics, which result in fast response and rapid heat conductivity and stackability with a large surface-area-to-volume ratio. Therefore, it is a promising configuration for a smallcapacity reactor [17]. Furthermore, diverse reactions in which heat transfer is key concern can be combined in an MCR. Owing to its unique advantages, many MCR studies have been performed in the fields of combustion [12,20], reforming, and WGS reactions
Please cite this article in press as: Hwang K-R, et al., Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.067
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Fig. 7. Results of CCR of H2 in the unit MCR with the Al/Ni catalyst using a mixture of CO2 (90%) and H2 (10%) at R/O ¼ 2.0, GHSV ¼ 50,000 h1 and 350 C.
[17,21,22]. Herein, the method of installing the catalyst into the MCR is a crucial issue. In the present work, a sheet of metal catalyst was installed in the MCR unit, which is a combination of a CCR unit and heat exchanger, as shown in Fig. 2(d). This method, incorporating the thin metal catalyst as a disc within the MCR, can make the reaction more efficient with less by-product formation due to better control of hot spots, which can be caused by highly exothermic and very fast combustion reaction. Note that with this method, the metal catalysts could be easily stacked like plates in the MCR. The results of the CCR for a mixture of CO2 (90%) and H2 (10%) in the MCR are shown in Fig. 7. N2 gas, at a flow rate of 1000 ml/min, was used to recover the heat of the CCR. CO2 was enriched up to 96.7%. In addition, traces of H2, O2, CO and CH4 were observed after the CCR. This result is consistent with the results of the unit module test. The differences in temperature measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet. This temperature difference between product gas and N2 arises from heat loss due to radiation. The heat loss by radiation can be minimized by stacking multiple MCRs and this could lead to the recovery of a large amount of energy. The energy balance in MCR was calculated by using thermodynamics properties of gases [23] on the assumption that (1) the catalytic conversion of H2 in retentate is 97%, (2) the temperature of flue gas goes up to 400 C, after H2 combustion at 350 C and 50,000 h1 (3) the temperature of main stream (a mixture of H2 60% and CO2 40% and) for heat recovery is 60 C at the inlet of MCR and (4) the heat loss of MCR system is 10%. Refer to Fig. 1(b). As a result, at a main stream flow rate of 5000 ml/min, the temperature of the main stream entering H2/CO2 membrane reactor goes up to 616 C, which means that the dewatered main stream can be heated over the operating temperature of the membrane reactor using the heat of combustion reaction of combustible gases in retentate alone without any assistance from the heater. Consequently, this study suggest a possible approach to the advanced combined IGCC/pre-CCS system by adopting the multi-MCR in which the process allows further enrichment of CO2, which would be beneficial for CO2 sequestration, in addition to utilizing the energy of the combustible gas in the retentate.
4. Conclusions The CCR of the retentate gas, consisting of 90% CO2 and 10% H2 obtained from a CO2/H2 separation membrane reactor, was investigated using a porous Ni metal catalyst in order to recover energy
and further enrich CO2. A disc-shaped porous Ni metal catalyst, namely Al[0.1]/Ni, was prepared by a simple method and a compact MCR (micro-channel reactor) equipped with a catalyst plate was designed for the CCR. The Al/Ni catalyst, which is a non-precious metal catalyst, was found to be effective for the CCR of lean H2 in the retentate gas in the unit module and also in the MCR. CO2 and H2 concentrations of 98.68% and 0.46%, respectively, were achieved at an operating temperature of 400 C, GHSV (gas-hourly space velocity) of 50,000 h1 and a H2/O2 ratio (R/O) of 2 in the unit module. In the case of the MCR, a sheet of the Ni metal catalyst was easily installed along with the other metal plates and the concentration of CO2 in the retentate gas increased up to 96.7%. The differences in temperatures measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet in the MCR. The disc-shaped porous metal catalyst and MCR configuration used in this study exhibit potential advantages, such as high thermal transfer resulting in improved energy recovery rate, simple catalyst preparation, and easy installation of the catalyst in the MCR. Consequently, this study suggest a possible approach to the advanced combined IGCC/pre-CCS system by adopting the multiMCR in which the process allows further enrichment of CO2, which would be beneficial for CO2 sequestration or use of C1 chemistry, in addition to utilizing the energy of the combustible gas in the retentate.
Acknowledgments This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B5-2463).
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Catalytic combustion of the retentate gas from a CO2/H2 separation membrane reactor for further CO2 enrichment and energy recovery Kyung-Ran Hwang*, Jin-Woo Park, Sung-Wook Lee, Sungkook Hong, Chun-Boo Lee, Duck-Kyu Oh, Min-Ho Jin, Dong-Wook Lee, Jong-Soo Park** Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 15 June 2015 Accepted 18 June 2015 Available online xxx
The CCR (catalytic combustion reaction) of the retentate gas, consisting of 90% CO2 and 10% H2 obtained from a CO2/H2 separation membrane reactor, was investigated using a porous Ni metal catalyst in order to recover energy and further enrich CO2. A disc-shaped porous Ni metal catalyst, namely Al[0.1]/Ni, was prepared by a simple method and a compact MCR (micro-channel reactor) equipped with a catalyst plate was designed for the CCR. CO2 and H2 concentrations of 98.68% and 0.46%, respectively, were achieved at an operating temperature of 400 C, GHSV (gas-hourly space velocity) of 50,000 h1 and a H2/O2 ratio (R/ O) of 2 in the unit module. In the case of the MCR, a sheet of the Ni metal catalyst was easily installed along with the other metal plates and the concentration of CO2 in the retentate gas increased up to 96.7%. The differences in temperatures measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet in the MCR. The disc-shaped porous metal catalyst and MCR configuration used in this study exhibit potential advantages, such as high thermal transfer resulting in improved energy recovery rate, simple catalyst preparation, and easy installation of the catalyst in the MCR. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Catalytic combustion CO2 enrichment CO2/H2 membrane Metal catalyst Micro-channel reactor
1. Introduction CCS (Carbon capture and storage) is a group of technologies aimed at mitigating the global warming effect by capturing up to 90% of the CO2 emitted during the combustion of fossil fuels in power plants and various industrial processes [1e3]. The CO2 is captured and separated using one of three methods, namely precombustion capture (pre-CCS), post-combustion capture, and oxyfuel combustion. The captured CO2 is then transported and sequestrated in selected geological rock formations [1,3]. Among the CCS technologies available, pre-CCS using CO2/H2 separation membranes located within integrated gasification and IGCC (combined cycle power generation) units has been considered to be a relatively cost efficient and viable technology. This methodology has the following advantages: (1) IGCC yields a concentrated CO2 mixture, (2) the CO2 can be captured at a high pressure and (3) the separated high-purity H2, carbon-free fuel, can also be used for other purposes in the power plant where it is produced [4e6].
* Corresponding author. Tel.: þ82 42 860 3504; fax: þ82 42 860 3495. ** Corresponding author. Tel.:þ82 42 860 3664; fax: þ82 42 860 3495. E-mail addresses: [email protected] (K.-R. Hwang), [email protected] (J.-S. Park).
Recently, various type of H2/CO2 separation membranes and their modules have been developed for pre-CCS applications [7e9]. Berkel et al. constructed a membrane module including a 1 m long Pd membrane and the module generated about 12 Nm3/m2/h of hydrogen at 400 C and 30 bar of a gas mixture from a natural gas combined cycle with assistance of N2 sweep gas. The retentate was composed of above 80% in dry CO2-levels balanced by remaining CO and H2 [7]. Lee et al. investigated the combined WGS (water-gas shift) reaction and membrane tests for pre-CCS in a coal gasifier and reported that using the high-pressure membrane module equipped with a planar-type PdeAu membrane, simultaneous CO2 and H2 enrichments of 92.1% and 99.8%, respectively, were achieved [8]. As mentioned previously, the retentate, which may contain up to 90% CO2, is finally sequestrated at a safe site. However, besides CO2, the sequestrated retentate gas typically contains up to 10% of combustible gases, mainly H2 which usually goes unutilized [8]. We tried to focus on recovery of energy from the combustible gases in the retentate to heat up the main stream in a combined IGCC/preCCS reactor system. In general, the operating temperatures are changed up and down along the unit operations of IGCC incorporated with the pre-CCS using the membrane system, as shown in Fig. 1(a). As we know, several coolers and heaters are present between the various process units to recover energy and generate
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Fig. 1. Block diagram of the combined IGCC/pre-CCS system using the membrane system (a) and block diagram of the advanced IGCC/pre-CCS system using the membrane and energy recovery systems (b).
steam for the WGS reaction. Among the various unit operations, the heater that is used for achieving operation temperatures of the membrane reactor, 350e450 C, requires the largest amount of energy [10]. Herein, if the dewatered stream is heated to the operating temperature of the membrane reactor using the heat of combustion reaction of combustible gases in retentate alone without any assistance from the heater (Fig. 1(b)), the overall energy recovery of the process could be improved. Furthermore, by utilizing the combustible gases in the retentate, CO2, which constitutes approximately 90% of the initial retentate, could be enriched up to 99%, which would be more favorable for sequestration. The CCR (catalytic combustion reaction) has been reported to be an effective method for obtaining energy or eliminating it from a H2-rich or lean mixture [11e15]. Zhang et al. prepared a monolithic Pt-based catalyst with a mesoporous ceramic coating for used in the CCR of lean H2 [14]. The Pt/Ce0.6Zr0.4O2/MgAl2O4 catalyst showed excellent catalytic activity for H2 combustion. Recently, in order to obtain high surface area of the catalyst and improve the thermal conductivity of the catalytic support for applications in exothermic CCRs, the ceramic monolith support in the Pt-doped CCR catalyst has been replaced with porous Ni-foam [15]. However, alumina, which has a lower thermal conductivity than metals, is still used in the preparation steps for the catalysts, involving some limitations such as peeling of the catalyst layer and lack of uniformity. In this study, we have investigated for the first time, the catalytic combustion of the retentate gas obtained from the CO2/H2 separation membrane reactor, with the goal of recovering energy and further concentrating CO2 in the retentate for pre-CCS. A discshaped porous Ni metal catalyst has been developed and a compact MCR (micro-channel reactor) has been designed for the CCR of a simulated retentate gas.
2. Experimental 2.1. Preparation of the Ni metal catalyst for CCR A disc-shaped Al[0.1]/Ni metal catalyst (about 50 mm in diameter and 2 mm in thickness) was prepared for catalytic H2 combustion. Firstly, the Ni powder was impregnated with Al (0.1 wt.%) by incipient wetness impregnation method over pre-dried Ni metal powder (Vale Inco Pacific Ltd.) with an aqueous solution of Al(NO3)3$9H2O (SigmaeAldrich Co. Ltd) to prepare a cermet system
for preventing the sintering of metals, which may occur at relatively high temperatures. The impregnated powder was dried at 120 C for 12 h and subsequently treated with a 75% H2/He mixture at 450 C for 30 h to remove impurities. Thereafter, the powder was ground with a ball-mill for 2 h. The prepared Al[0.1]/Ni powder was compressed into a cylindrical metal mold without any binder at a pressure of 30 MPa. The compressed disc-shaped catalyst was further sintered at 900 C for 2 h under H2 flow to improve its mechanical strength. The prepared catalyst was characterized by XRD (X-ray powder diffraction, D/MAX IIIC). The surface images were obtained by FE-SEM (field emission scanning electron microscopy, HITACHIS-4700) and pore-related information such as pore size, total volume and porosity were obtained by mercury porosimetry (Autopore IV 9500).
2.2. CCR test in a unit module A schematic diagram of the CCR, cross-sectional image of the unit module installed with the disc-shaped metal catalyst, and photographs of metal catalyst and two types of O-rings are shown in Fig. 2(a,b and c), respectively. A gas mixture of 10% of H2 in CO2, which is similar to the composition of a retentate gas from the H2/ CO2 separation membrane module, was fed to the reactor using MFC (mass flow controllers, Brooks Instrument). The product was passed through a cold-trap to remove the liquid water, formed during the H2 combustion reaction before the gas entered into a GC (gas chromatography, Agilent 6890N) system. A K-type thermocouple was positioned on the upper surface of the metal catalyst in the unit module to control the reaction temperature. Two additional thermocouples, one at the inlet and the other at the outlet of reactor, were placed to monitor the temperatures before and after the reaction. A mixture containing 10% H2 in N2 was passed through the reactor at a constant flow rate of 100 ml/min while heating the reactor to the reaction temperature (350 C) and cooling it down to room temperature. No other activation procedures were applied for the metal catalyst. All experimental were carried out in atmospheric pressure. The product gas was analyzed using an on-line GC equipped with two thermal conductivity detectors and two columns (HP-MOLSIV and Carboxen1010). As shown in Fig. 2(b), the CCR occurred as the reactant traveled vertically through the metal catalyst. The product gas was vented to the supporter, which was positioned just below the catalyst, and then passed through the outlet port. Two metal O-rings were used to seal the flange type module [16], as shown in Fig. 2(b and c). The effects of variables
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Fig. 2. An experimental schematic diagram of the apparatus for CCR (a), cross-sectional image of the unit module used for the CCR containing the disc-shaped metal catalyst (b), photograph of the disc-shaped metal catalyst and two types of O-rings (c) and layout of the components and gas path in the MCR and the photograph of an assembled MCR (d).
such as temperature and GHSV (gas-hourly space velocity) on the CCR of H2 were investigated at a ratio of hydrogen to oxygen (R/O) of 2. For comparison, H2 combustion using a specially prepared Pd impregnated Al[0.1]/Ni metal catalyst and without a catalyst were studied. 2.3. CCR test in MCR The layout of the components in the MCR, travel path for the gases and the photograph of an assembled MCR used in this study
are shown in Fig. 2(d). The MCR consisted of a CCR section and a heat exchanger section with inlet and outlet ports for N2, which acted as the heat exchange gas. In addition, the MCR also contained various types of metal plates with micro-channels, which were assembled with diffusion bonding in a hot-press [17]. The metal plates were chemically etched and included half-etched straight channel plates (8 sheets), fully etched 3D mixing channel plates (2 sheets), catalyst holder plates (2 sheets) and separator and cover plates (5 sheets). Each of the plates was 1 mm thick, except for the cover plates. The disc-shaped porous metal catalyst was 89 mm in
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diameter and 1 mm in thickness. The assembled MCR with width, length and depth of 200 mm, 290 mm, and 100 mm, respectively, is shown in Fig. 2(d). A leak test of the diffusion ebonded MCR was conducted with H2 at a pressure of 5 bar to confirm that the metal plates were joined well. As shown in Fig. 2(d), a mixture of reactants (CO2 and H2) and O2 entered the reactor at one end and vented at the opposite end. The heat exchange gas, N2, entered the reactor at the opposite end as the reactants and the reactant and heat exchange gas traveled in counter directions for effective heat exchange. The metal catalyst was installed at the center of a fully etched plate and a metal O-ring was used to prevent channeling of reactants not being able to contact the catalyst. The reaction temperature was adjusted with a temperature controller as shown in Fig. 2(d). The MCR was set in an electric furnace instead of a unit module in Fig. 2(a) to carry out the CCR test. The gas flow was controlled using MFCs and the products were analyzed using an online GC system, as described previously in this section.
achieve improved CO2 enrichment and energy recovery. The results of the CCR in the unit module at an R/O of 2.0, GHSV of 50,000 h1, and an operating temperature of 350 C are presented in Fig. 5(a and b). After the combustion of lean H2 over the Al[0.1]/Ni metal catalyst, CO2 was enriched up to 97.6%, whereas small quantity of H2 and O2 remained, as shown in Fig. 5(a). The difference in the temperature measured before and after the CCR at the outlet of the unit was 36 C, implying that significant amount of energy was produced by combustion of lean H2, which can be recovered and reused. In addition, traces of CO and CH4 were also detected. This result can be explained by considering the following reversible reactions (Eqs. (1) and (2)):
3. Results and discussion
In other words, CH4 and CO were formed from methanation and/or reverse WGS reactions, respectively. It is worth noting that the reactions did not take place in the blank test (not shown here), where CCR was carried out in the absence of catalyst under the same reaction conditions. When the Pd[0.1]-Al[0.1]/Ni metal catalyst was used (Fig. 5(b)), the CO2 concentration increased slightly, whereas the H2 and O2 concentrations decreased to about half of the values obtained with the Al[0.1]/Ni catalyst, under identical reaction conditions. This implies that precious metals assist the CCR of lean H2 as well documented in the literature [12]. Traces of CO and CH4 were also detected. However, Ni in the metallic state
3.1. Textural properties of the Al[0.1]/Ni metal catalyst The XRD pattern of the prepared disc-type metal catalyst is shown in Fig. 3. The diffraction peaks at 2q ¼ 44.5 (111), 51.8 (200) and 76.4 (222) are characteristic of nickel metal. However, the diffraction peaks corresponding to the impregnated Al were not detected, which may have been due to the undetectable levels of Al. The pore size distribution and FE-SEM image of the disc-type metal catalyst are shown in Fig. 4(a and b). The Al[0.1]/Ni metal catalyst had two pore sizes of 0.6 and 184 mm. In addition, the porosity, total pore volume, and average pore size of the porous metal catalyst were 39.4%, 0.0741 ml/g and 619.3 nm, respectively. Since the metal catalyst was prepared by compressing the metal powder in the absence of binders and then sintered at high temperature, pores were observed between the spherical Ni powders, as shown in the FE-SEM image. The CCR occurred as the reactants passed through the pores of the metal catalyst, as shown in Fig. 2(b).
CH4 þ H2O 4 CO þ 3H2, DH ¼ 205.8 kJ/mol (steam reforming of CH4 /methanation) (1) CO þ H2O 4 CO2 þ H2, DH ¼ 41.2 kJ/mol (WGS/reverse WGS)(2)
3.2. CCR in the unit module We carried out the CCR of a simulated retentate gas, containing 90% CO2 and 10% H2, from CO2/H2 separation membrane reactor to
Fig. 3. XRD pattern of the disc-shaped metal catalyst.
Fig. 4. Pore size distribution (a) and FE-SEM image (b) of the porous metal catalyst.
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Fig. 6. Effect of reaction temperature on the CCR in the unit module at R/O ¼ 2.0 and GHSV ¼ 50,000 h1 over the Al/Ni catalyst (a) and effect of GHSV on the CCR in the unit module at R/O ¼ 2.0 and 350 C over the Al/Ni catalyst (b). Fig. 5. Results of the CCR of H2 in the unit module using a mixture of CO2 (90%) and H2 (10%) at R/O ¼ 2.0, GHSV ¼ 50,000 h1 and 350 C with (a) Al/Ni catalyst and (b) Pd/Al/ Ni catalyst.
exhibited good catalytic activity for CCR, comparable to that of the Ni catalyst containing Pd. This is consistent with the results of the study by Li et al. [18]. They found that the presence of metallic Ni in the Ni/Al2O3 catalyst promoted the combustion activity of CH4 under specific reaction conditions. Therefore, we suggest that Al [0.1]/Ni metal is an effective catalyst for the CCR of H2 in the retentate gas from the membrane reactor. The effect of reaction temperature on the CCR over the Al[0.1]/Ni catalyst at R/O of 2.0 and GHSV of 50,000 h1 is shown in Fig. 6(a). A slight increase in the concentration of CO2 was observed with an increase in the reaction temperature. At a reaction temperature of 400 C, the concentration of CO2 reached 98.68%, which was similar to the results obtained for the CCR using Pd[0.1]-Al[0.1]/Ni as the catalyst, as shown in Fig. 5(b). On the other hand, the concentrations of H2 and O2 decreased monotonically as the reaction temperature increased. For example, the concentrations of H2 and O2 at 320 C were 1.63% and 2.03% at 320 C, respectively, whereas the corresponding values were 0.46% and 0.38%, respectively, at 400 C. The maximum amounts of CH4 and CO were detected at 350 C. It is difficult to explain the relationship between the reaction temperature and the amount of trace compounds generated from the sidereactions, because one of the side-reactions is exothermic, whereas the other one is endothermic, as shown in Eqs. (1) and (2). However, the porous Ni metal catalyst appears to contribute lightly to methanation and/or the WGS reactions [17,19]. The effect of GHSV on the catalytic performance and difference in the temperature measured before and after the CCR is presented
in Fig. 6(b). A higher value of GHSV resulted in a shorter residence time in the reactor, which may be insufficient for the reactants to undergo complete CCR. As expected, for a GHSV of 10,000 h1, CO2 was enriched up to 98.7% and no O2 was detected, indicating that the residence time was sufficient for the complete consumption of O2 as an oxidant. However, H2 of 0.71% was detected, which might have originated from the reversible side-reactions. Further, while the concentration of CO2 decreased slightly from 98.7% at 10,000 h1 to 97.3% at 100,000 h1, there was no significant difference in the gas composition and concentration of the enriched CO2 above a GHSV value of 50,000 h1. The differences in temperatures measured before and after CCR at the outlet of the reactor increased steadily as a function of GHSV (10 C, 36 C and 63 C at 10,000 h1, 50,000 h1 and 100,000 h1, respectively). This suggests that the CO2 enrichment(greater than 97.3%) and a large amount of heat of reaction could be obtained via the CCR of lean H2 in the retentate (90% CO2 and 10% H2) using Al[0.1]/Ni metal catalyst at a GHSV of 100,000 h1, operating temperature of 350 C, and R/O of 2.
3.3. CCR in the MCR The MCR has many advantages such as excellent heat and mass transfer characteristics, which result in fast response and rapid heat conductivity and stackability with a large surface-area-to-volume ratio. Therefore, it is a promising configuration for a smallcapacity reactor [17]. Furthermore, diverse reactions in which heat transfer is key concern can be combined in an MCR. Owing to its unique advantages, many MCR studies have been performed in the fields of combustion [12,20], reforming, and WGS reactions
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Fig. 7. Results of CCR of H2 in the unit MCR with the Al/Ni catalyst using a mixture of CO2 (90%) and H2 (10%) at R/O ¼ 2.0, GHSV ¼ 50,000 h1 and 350 C.
[17,21,22]. Herein, the method of installing the catalyst into the MCR is a crucial issue. In the present work, a sheet of metal catalyst was installed in the MCR unit, which is a combination of a CCR unit and heat exchanger, as shown in Fig. 2(d). This method, incorporating the thin metal catalyst as a disc within the MCR, can make the reaction more efficient with less by-product formation due to better control of hot spots, which can be caused by highly exothermic and very fast combustion reaction. Note that with this method, the metal catalysts could be easily stacked like plates in the MCR. The results of the CCR for a mixture of CO2 (90%) and H2 (10%) in the MCR are shown in Fig. 7. N2 gas, at a flow rate of 1000 ml/min, was used to recover the heat of the CCR. CO2 was enriched up to 96.7%. In addition, traces of H2, O2, CO and CH4 were observed after the CCR. This result is consistent with the results of the unit module test. The differences in temperature measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet. This temperature difference between product gas and N2 arises from heat loss due to radiation. The heat loss by radiation can be minimized by stacking multiple MCRs and this could lead to the recovery of a large amount of energy. The energy balance in MCR was calculated by using thermodynamics properties of gases [23] on the assumption that (1) the catalytic conversion of H2 in retentate is 97%, (2) the temperature of flue gas goes up to 400 C, after H2 combustion at 350 C and 50,000 h1 (3) the temperature of main stream (a mixture of H2 60% and CO2 40% and) for heat recovery is 60 C at the inlet of MCR and (4) the heat loss of MCR system is 10%. Refer to Fig. 1(b). As a result, at a main stream flow rate of 5000 ml/min, the temperature of the main stream entering H2/CO2 membrane reactor goes up to 616 C, which means that the dewatered main stream can be heated over the operating temperature of the membrane reactor using the heat of combustion reaction of combustible gases in retentate alone without any assistance from the heater. Consequently, this study suggest a possible approach to the advanced combined IGCC/pre-CCS system by adopting the multi-MCR in which the process allows further enrichment of CO2, which would be beneficial for CO2 sequestration, in addition to utilizing the energy of the combustible gas in the retentate.
4. Conclusions The CCR of the retentate gas, consisting of 90% CO2 and 10% H2 obtained from a CO2/H2 separation membrane reactor, was investigated using a porous Ni metal catalyst in order to recover energy
and further enrich CO2. A disc-shaped porous Ni metal catalyst, namely Al[0.1]/Ni, was prepared by a simple method and a compact MCR (micro-channel reactor) equipped with a catalyst plate was designed for the CCR. The Al/Ni catalyst, which is a non-precious metal catalyst, was found to be effective for the CCR of lean H2 in the retentate gas in the unit module and also in the MCR. CO2 and H2 concentrations of 98.68% and 0.46%, respectively, were achieved at an operating temperature of 400 C, GHSV (gas-hourly space velocity) of 50,000 h1 and a H2/O2 ratio (R/O) of 2 in the unit module. In the case of the MCR, a sheet of the Ni metal catalyst was easily installed along with the other metal plates and the concentration of CO2 in the retentate gas increased up to 96.7%. The differences in temperatures measured before and after the CCR were 31 C at the product outlet and 19 C at the N2 outlet in the MCR. The disc-shaped porous metal catalyst and MCR configuration used in this study exhibit potential advantages, such as high thermal transfer resulting in improved energy recovery rate, simple catalyst preparation, and easy installation of the catalyst in the MCR. Consequently, this study suggest a possible approach to the advanced combined IGCC/pre-CCS system by adopting the multiMCR in which the process allows further enrichment of CO2, which would be beneficial for CO2 sequestration or use of C1 chemistry, in addition to utilizing the energy of the combustible gas in the retentate.
Acknowledgments This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B5-2463).
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