Hydrogen Production from Methane Steam Reforming in Combustion

Article pubs.acs.org/IECR

Hydrogen Production from Methane Steam Reforming in Combustion Heat Assisted Novel Microchannel Reactor with Catalytic Stacking Chun-Boo Lee,†,‡ Sung-Wook Lee,†,‡ Dong-Wook Lee,† Shin-Kun Ryi,† Jong-Soo Park,†,* and Sung-Hyun Kim‡,* †

Energy Materials and Convergence Research Department, Korea Institute of Energy Research (KIER), 102 Gajeong-ro, Yuseong-Gu, Daejeon 305-343, South Korea ‡ Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-701, South Korea ABSTRACT: In this study, we further investigate the application of an MCR containing a nickel metal catalyst with variable number of stacks for hydrogen production via MSR. Microchannels were contained in the MCR one side of the nickel metal catalyst with variable number of stacks for producing hydrogen to generate the necessary heat for the endothermic MSR and the combustion reaction was performed on the other side of the MCR. The catalyst used in this study for the MSR reaction, which was a prepared Pd[0.1]-Al[0.3]/Ni catalyst. The methane conversion and hydrogen production mole ratio were 94.7% and 9.39 mol h−1, respectively, and the steam/carbon (S/C) ratio was 3 at 650 °C, GHSV = 10 000 h−1. The promise of a feasible simplified system for hydrogen production from the MSR was confirmed. If a hydrogen separation membrane is placed between the MSR elements in an MCR, the forward reaction will be preferentially promoted by the rapid removal of the hydrogen produced through the MSR reaction. reforming. Cai et al.13 studied the ethanol steam reforming reaction over a supported Ir/CeO2 catalyst in a microchannel structured reactor. They obtained a hydrogen yield exceeding 40 LH2/(gcat h) with an ethanol conversion of 65%. Their research used a catalytic coating inside the microchannels. However, when coating microchannel plates, a thin layer and homogeneous film with stable adhesion over the surface of the microchannels is needed to inhibit the peeling of the catalyst at the relatively high temperature required.5 For this reason, a different method is needed for loading the catalysts into the MCR. In our previous studies,5,11,14 we developed a porous nickel metal catalyst for the MSR reaction that exhibited a high methane conversion and stable hydrogen production. This nickel metal catalyst has various advantages, such as good catalytic performance for the MSR reaction. It is easy to prepare, and the number of stacks can be modulated. Moreover, we prepared and tested a multimembrane reformer for the direct production of hydrogen via MSR.14 We developed high hydrogen permeation flux and perm-selectivity membranes and tested them in several situations.15 In particular, we designed and prepared an MCR containing a nickel metal catalyst and investigated the potential application of an on-board MSR for hydrogen production using an assisted combustion reaction.5 In the present work, we further investigate the application of an MCR containing a nickel metal catalyst with variable number of stacks for hydrogen production via MSR. Microchannels were contained in the MCR one side of the nickel metal catalyst with variable number of stacks for

1. INTRODUCTION Hydrogen is a very important industrial gas with many chemical reactions and future applications. It is a promising clean fuel that can be produced from sources such as natural gas, coal, and biomass. Hydrogen energy is drawing attention as an upcoming alternative energy source. It is mostly produced from the hydrocarbon reforming process. Methane steam reforming (MSR) is a well-known industrial hydrogen production process. Recently, it has been studied in mesoscale reactors1,2 and portable and on-board fuel processors.3,4 Fuel cells are a very promising technology for electrical power generation in stationary, mobile, and portable applications. However, use of a conventional fixed-bed reactor for MSR has some problems: because the reaction is highly endothermic and heat transfer is typically poor, a substantial amount of heat transfer surface is usually needed, and hence, large reactors are required.5 In addition, deactivation of Ni-based catalysts is a concern. Several investigators have recently reported on the high activity and stability of Ni catalysts.6−9 The microchannel reactor (MCR) has been recognized as a promising technology for small scale, compact, and mobile hydrogen production systems offering easy integration into existing installations, along with high heat and mass transfer coefficients, a high surface area to volume ratio, higher conversion and selectivity, as well as a fast response.6,10 However, there is still an issue that remains to be solved, namely, the catalyst coating on the metal surface.11 Many different catalysts have been used to evaluate the steam reforming of hydrocarbons in an MCR. Casanovas et al.12 developed an MCR that can be used to produce hydrogen from ethanol steam reforming. The results revealed that ethanol has been processed independently in separate zones of a MCR for total oxidation and steam © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14049

May 8, August August August

2013 21, 2013 21, 2013 21, 2013

dx.doi.org/10.1021/ie402350a | Ind. Eng. Chem. Res. 2013, 52, 14049−14054

Industrial & Engineering Chemistry Research

Article

Figure 1. Microchannel reactor. (a) Photographs of the microchannel reactor (MCR); (b) photograph of the plates; (c) schematic drawing of the configuration.

Figure 2. Process flow sheet for methane steam reforming (MSR) using the MCR.

2. EXPERIMENTAL SECTION

producing hydrogen to generate the necessary heat for the endothermic MSR, and the combustion reaction was performed on the other side of the MCR. Here, we focus on the possibility of highly hydrogen production concurrently with highly methane conversion by using a MCR.

2.1. Preparation of Steam Reforming Catalyst. Nickel powder, purchased from Vale Inco Pacific Ltd., was used as the raw material for the steam reforming (SR) metal catalyst support. The particles were smooth and spherical, with a diameter of ∼3.0 14050



Article

Figure 3. SEM images of prepared Pd[0.1]-Al[0.3]/Ni catalyst surface (a) and cross-section (b).

PLOT fused silica capillary column (30 m × 0.53 mm, SUPELCO) and thermal conductivity detector (TCD). The reformate gas flow rate was measured using a soap-bubble flow meter (Gilibrator 2, SENDIDYNE).

μm and nickel content of 99.7%. The SR catalyst, Pd−Al/Ni, was prepared by the incipient wetness impregnation method using predried nickel metal powder with an aqueous solution containing a mixture of aluminum nitrate (Al(NO3)39H2O, Aldrich) and Pd (Pd(NO3)2, PMRESEARCH).14 The contents of Pd and Al were 0.1 wt % and 0.3 wt %, respectively, relative to the total weight of the Ni support. The SR catalyst powder was pretreated with 90% H2/10% Ar at 450 °C for 30 h to eliminate any impurities. Afterward, it was compressed without a binder in a 50 mm diameter metal cylindrical mold using a lab-made press under high pressure (333 MPa). The compressed support was heated at 700 °C under H2 for 2 h to improve its mechanical strength. 2.2. Microchannel Reactor Design. Figure 1 shows the MCR (a), the microchannel plates (b), and the MCR configuration (c) used in this study. The MCR is made of stainless steel (SS) 316L and comprised two reaction parts: the combustion reaction part with fuel and air inlets to release the heat produced during MSR was used for maintaining the heat demand for the endothermic SR side of the MCR, and the MSR part.5 The MCR was made by diffusion bonding (CorHex Corp.) 62 plates (9 different plates). The microchannel plates were all etched by the chemical method, including half-etched straight channel plates (22 plates of 2 different), 3D mixing channel plates (9 plates of 2 different), and separator plates (5 plates), catalysts hold plates (24 plates of 2 different) and cover plates (2 plates). The plates had dimensions of 200 mm length × 60 mm width × 1.0 mm height (0.5 mm depth) except for the catalyst holding plates and 3D mixing channel plates with dimensions of 200 mm length × 60 mm width × 1.0 mm height (1.0 mm depth). The catalysts were placed in the MCR catalyst hold plates and tightened with metal O-rings. 2.3. SR Reaction of Methane in the MCR. A detailed schematic diagram of the MSR reaction in the MCR is shown Figure 2. For the temperature measurement, five K-type thermocouples were installed. In the air and fuel feeding part, a thermocouple (TC) was used. A second TC was used to measure the temperature at the outlet of the combustion reaction, a third TC measured the SR reactant inlet temperature, a fourth TC measured the reformate outlet temperature, and a fifth one measured the MCR skin temperature.5 The MSR in the MCR was evaluated with respect to several variables, including the temperature and gas hourly space velocity (GHSV). It was performed at a steam/CH4 ratio of 3 under atmospheric pressure. The gases were fed by a mass flow controller (MFC, Brooks 5850E series) and the water was fed with a steam generator via a high pressure micropump (Minichemi pump, 1− 20 mL/min, NS). The reformate gas was passed through an SS cold trap to remove any remaining water and analyzed by an online GC (6890, Agilent) equipped with a carboxen 1010

3. RESULTS AND DISCUSSION 3.1. Catalyst and Microchannel Reactor. SEM images of the catalyst (diameter, 50 mm; thickness, 1.2 mm) are shown in Figure 3. The catalyst plate has very small, uniform pores and a smooth surface. Our previous study,14 which confirmed the results previously reported in the literature, showed that the Pd[0.1]-Al[0.3]/Ni catalyst offered good performance and stability for the SR reaction. The pore size, total pore volume, and porosity of the catalyst powder (see Table 1) were reduced Table 1. Properties of Catalyst Powder and Pressed Catalyst, Measured by Mercury Porosimetry and BET

catalyst powder pressed catalyst

porosity (%)

avg. pore size (nm)

total pore vol. (mL/g)

BET surface area (m2/g)

74.1

693.0

0.371

0.3687

35.5

276.1

0.064

0.6564

by 52.1%, 60%, and 82.7%, respectively, compared with those of the thermally treated nickel powder. Moreover, the thermal conductivity of the catalyst was 59.1 W/m/K, lower than that of the nickel metal powder (90.9 W/m/K) but still much higher than that of commercial ceramic-based catalysts.5,15,16 In our previous study,5 an MCR containing a metal catalyst was used to investigate the potential application of on-board MSR to hydrogen production. Rapid permeation of the gas through the porous nickel plate occurred. The hydrogen permeation rate was ∼2.5 times higher than that of CO and CO2.11 The plate type catalyst was inserted between 3D mixing channel and catalyst holding plates. The newly developed technique for varying the number of stacks is a very difficult method, mainly because of the different thicknesses of the catalysts and commercial MCR plates. To address this, a metal Oring sealing mechanism was used to make it easier to install the catalyst in the MCR. Metal O-rings have been developed as a secure method of sealing for gases or liquids. They seal over a wide range of pressures, temperatures, and tolerances, normally require very little room, are light in weight, offer differing amounts of compression, and are cost-effective.17 Figure 4 shows the photographs of cross sections of the microchannel plates, catalysts, and metal O-rings located in the middle of the diffusion bonded MCR. Twelve catalysts and metal O-rings were used. The metal O-ring sealing mechanism was employed to vary the number of stacks and ensure MCR sealing. The catalysts were 14051



Article

the SMR reaction by contrillong the flow rates of air and combustible gases. Fuel could be changed from hydrogen to methane or retentate gas a mixture of hydrogen and methane.5 If this kind of reactor applies to the fuel cell system to remove the need for an electro-device to start up the fuel cell system. 3.3. Methane Steam Reforming (MSR) in the Microchannel Reactor (MCR). Steam reforming is a well-known technology operated industrially on a large scale for hydrogen production, and several detailed reviews of this technology have been published.18−20 Below are the reactions that occur during this process, namely the reforming, water−gas shift (WGS), and methanation (MTN) reactions: CH4 + H 2O ↔ CO + 3H 2 , Figure 4. Photographs showing the cross-sectional views of the microchannel reactor (MCR).

ΔH = 205.8 kJ/mol (MSR/MTN)

(1)

CO + H 2O ↔ CO2 + H 2 ,

inserted between etched holder plates (Figure 1(b)) along with the metal O-rings (Figure 1(b)). In this type of stacked catalyst, the reactants (CH4 + H2O) come into contact with the catalysts and react on their surface during their passage through their pores.5 This type of catalyst offers various advantages, such as good catalytic performance for the MSR reaction. 3.2. Hydrogen Combustion in the Microchannel Reactor (MCR). The energy provided during highly exothermic hydrogen combustion was used to satisfy the energy demand for the endothermic MSR reaction, including evaporation of water and heating of the reactants, without the need for a heating furnace. To maximize efficiency, the MCR was insulated to reduce heat loss. The heat needed to start the MCR, initially at room temperature, was provided by virtue of hydrogen combustion over the igniter, a Pt coated mesh catalyst. Hydrogen was used instead of methane because the combustion temperature of the former is lower. As shown in Figure 5, the temperature increased quickly, and the maximum temperature increased with increasing hydrogen and air flow rates. Hydrogen and air were fed at flow rates of 4.46 mol h−1 and 9.5 mol h−1, respectively, where air in excess of the stoichiometric amount was used. After the temperature reached its maximum, the fuel was changed to a mixture of hydrogen and methane. The reaction temperature was controlled as needed for

ΔH = −41.2 kJ/mol (WGS/RWGS)

(2)

CH4 + 2H 2O ↔ CO2 + 4H 2 , ΔH = 164.6 kJ/mol (MSR, sum of (1) + (2))

(3)

CH4 + CO2 ↔ 2CO + 2H 2 , ΔH = 247.0 kJ/mol (methane dry reforming/MTN) (4)

A summary of the reaction kinetics and the corresponding parameters was given by Xu and Froment.20 Reactions 1 and 2 are reversible and are normally at equilibrium, as their rates are very fast. The composition of the product gas from a conventional reformer reactor is therefore governed by thermodynamics. Steam is normally added well in excess of the stoichiometric requirement of reaction 1 so that the equilibrium of reaction 2 moves toward more CO2 production than CO. Steam may be replaced completely (or in part) by CO2, which gives a more favorable H2/CO ratio for some applications. Reactions 1, 3, and 4 are endothermic, while reaction 2 is mildly exothermic. The methane conversion is limited by the thermodynamic equilibrium and is favored at high temperature

Figure 5. Temperature profiles in the microchannel reactor (MCR). 14052



Article

and low pressure. The syngas product composition of the SMR is determined by the equilibria of reactions 1 and 2. The equilibrium constants for both reactions 1 and 2 at all temperatures used in this study were obtained. The MSR using the Pd[0.1]-Al[0.3]/Ni catalyst was carried out in an MCR at 550−650 °C. Figure 6 shows the conversion of

Figure 7. Product distribution and carbon balance at S/C = 3.0 and GHSV = 10 000 h−1. Symbols and solids lines indicate experimental data and equilibrium values, respectively. Brown, hydrogen; green, methane; yellow, carbon dioxide; orange, carbon monoxide. The equilibrium values were calculated using Aspen Plus.

The results presented in Figure 8 show the effect of the GHSV on the MSR reaction in the MCR at 550−650 °C. The methane

Figure 6. Methane conversion and hydrogen production rate as a function of temperature at S/C = 3.0 and GHSV = 10 000 h−1.

methane and the hydrogen production rate in the MCR with the catalyst as a function of temperature. The methane conversion and hydrogen production mole ratio were 94.7% and 9.39 mol h−1, respectively, and the steam/carbon (S/C) ratio was 3 at 650 °C, GHSV = 10 000 h−1. The hydrogen production increased from 6.37 to 9.39 mol h−1 as the temperature difference was increased from 550 to 650 °C, which is a ∼10 times greater variation than that in our previous study.5 The methane conversion was increased by 83.6% compared to the equilibrium. According to these results, the hydrogen produced via MSR dominantly permeate through the catalyst pores in comparison with other product gases, leading to a shift of the equilibrium,5 and therefore, MCR sustains high performance. In addition, methane and steam mixture as a reactant of MSR is evenly distributed to each plate type catalyst, because the maximum resistance of reactant flow (measured ΔP = 2) occurs at plate type catalysts. The even distribution of reactants also contributes to the high performance of MCR with a unique plate type catalyst. The product distribution and carbon balance are shown as a function of temperature in Figure 7. The experimental hydrogen concentration values were higher than the equilibrium values, while the experimental methane values were lower than the equilibrium values. In addition, the CO2 and CH4 concentrations decreased from 9.1% o 6.5% and from 11.3% to 3.1%, respectively, when the reaction temperature was increased from 550 to 650 °C, while the CO concentration increased from 6.7% to 11.5%. The MSR (eq 1) and WGS (eq 2) reactions proceed simultaneously, with production of four times more hydrogen than carbon monoxide and carbon dioxide. The reason for the increased production is that the differences in the concentration of carbon monoxide and carbon dioxide are not as significant as that for hydrogen. As shown in Figure 7, the carbon balance calculated from the reactant (CH4) and reformates (CO, CO2 and CH4) was 1 ± 0.03, meaning that there was no carbon formed on the catalysts or the microchannels.

Figure 8. Methane conversion and hydrogen production rate as a function of GHSV at S/C = 3.

conversion decreased from 94.69 to 70.15 as the GHSV increased from 10 000 to 70 000 h−1, whereas the hydrogen production increased from 9.4 to 38.4 mol h−1.

4. CONCLUSIONS We successfully constructed and tested an MCR with a variable number of stacks for MSR. The energy provided during the highly exothermic hydrogen combustion was used to satisfy the energy demand for the endothermic MSR on the other side of the MCR. The preferred configuration can be obtained through the proper selection of catalytic stacking and metal O-ring size. In the MCR, the reaction between CH4 and H2O takes place on the surface of the catalyst as the reactants pass through its pores. The maximum methane conversion and hydrogen production rate were 94.7% and 9.39 mol h−1, respectively, at a temperature of 65 °C, a steam/carbon (S/C) ratio of three, and a GHSV of 10 000 h−1. The methane conversion was increased by 83.6% compared 14053



Article

(13) Cai, W.; Wang, F.; Veen, A. V.; Descorme, C.; Schuurman, Y.; Shen, W.; Mirodatos, C. Hydrogen production from ethanol steam reforming in a micro-channel reactor. Int. J. Hydrogen Energy 2010, 35, 1152. (14) Hwang, K.-R.; Lee, C.-B.; Ryi, S.-K.; Lee, S.-W.; Park, J.-S. A multimembrane reformer for the direct production of hydrogen via a steamreforming reaction of methane. Int. J. Hydrogen Energy 2012, 37, 6601. (15) Hwang, K.-R.; Cho, S.-H.; Ihm, S.-K.; Lee, C.-B.; Park, J.-S. Catalytic active filter for water−gas shift reaction. J. Chem. Eng. Jpn. 2009, 42, 1. (16) Baronskaya, N. A.; Yurieva, T. M.; Minyukova, T. P.; Demeeshkina, M. P.; Khassin, A. A.; Sipatrov, A. G. Heat-conducting catalysts for the reactions at medium temperatures. Catal. Today 2005, 105, 697. (17) Parker O-ring Handbook, 2001 edition; Parker Hannifin Corporation: Cleveland, OH, 2001. (18) Rostrup-Nielsen, J. R. Catalytic steam reforming. Catalysis, Science, and Technology; Anderson, J. R., Boudart, M., Eds.; SpringerVerlag: Berlin, 1984; Vol. 5. (19) Kochloefl, K. Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 4. (20) Xu, J.; Froment, G. F. Methane steam reforming, methanation, and water−gas shift: I. Intrinsic kinetics. AIChE J. 1989, 35, 88.

to the equilibrium value. Moreover, the conversion decreased from 94.69 to 70.15 as the GHSV increased from 10 000 to 70 000 h−1, while the hydrogen production rate increased from 9.4 to 38.4 mol h−1. These promising results show that a simplified system for hydrogen production using an MSR is feasible. Additional improvement might be possible by placing hydrogen separation membrane between the MSR elements. This would promote the forward reaction by rapid removal of the hydrogen produced.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-42-860-3667. E-mail: [email protected]. *Tel.: +82-2-3290-3297. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the “Energy Efficiency and Resources Technology Development Programs” (Project No. 2011201020005A) of the Ministry of Knowledge Economy, Republic of Korea.

■

REFERENCES

(1) Mei, H.; Li, C.; Ji, S.; Liu, H. Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling. Chem. Eng. Sci. 2007, 62, 4294. (2) Yuan, J.; Ren, F.; Sundén, B. Analysis of chemical-reaction-coupled mass and heat transport phenomena in a methane reformer duct for PEMFCs. Int. J. Heat Mass Transfer 2007, 50, 687. (3) Park, D.-E.; Kim, T.-G.; Kwon, S.-J.; Kim, C.-K.; Yoon, E.-S. Micromachined methanol steam reforming system as a hydrogen supplier for potable proton exchange membrane fuel cells. Sens. Actuators, A 2007, 135, 58. (4) Izquierdo, U.; Barrio, V. L.; Cambra, J. F.; Requies, J.; Güemez, M. B.; Arias, P. L.; Kolb, G.; Zapf, R.; Gutiérrez, A. M.; Arraibi, J. R. Hydrogen production from methane and natural gas steam reforming in conventional and microreactor reaction systems. Int. J. Hydrogen Energy 2012, 37, 7026. (5) Hwang, K.-R.; Lee, C.-B.; Lee, S.-W.; Ryi, S.-K.; Park, J.-S. Novel micro-channel methane reformer assisted combustion reaction for hydrogen production. Int. J. Hydrogen Energy 2011, 36, 473. (6) Wu, P.; Li, X.; Ji, S.; Lang, B.; Habimana, F.; Li, C. Steam reforming of methane to hydrogen over Ni-baded metal monolith catalysts. Catal. Today 2009, 146, 82. (7) Huang, T.-J.; Huang, M.-C. Effect of Ni content on hydrogen production via steam reforming of methane over Ni/GDC catalysts. Chem. Eng. J. 2008, 145, 149. (8) Prasad, D. H.; Park, S.-Y.; Ji, H.; Kim, H.-R.; Son, J.-W.; Kim, B.-K.; Lee, H.-W.; Lee, J.-H. Effect of steam content on nickel nano-particle sintering and methane reforming activity of Ni-CZO anode cermets for internal reforming SOFCs. Appl. Catal., A 2012, 160. (9) Zhai, X.; Ding, S.; Liu, Z.; Jin, Y.; Cheng, Y. Catalytic performance of Ni catalysts for steam reforming of methane at high space velocity. Int. J. Hydrogen Energy 2011, 36, 482. (10) Görke, O.; Pfeifer, P.; Schubert, K. Kinetic study of ethanol reforming in a microreactor. Appl. Catal., A 2009, 360, 232. (11) Ryi, S.-K.; Park, J.-S.; Kim, D.-K.; Kim, T.-H.; Kim, S.-H. Methane steam reforming with a novel catalytic nickel membrane for effective hydrogen production. J. Membr. Sci. 2009, 339, 189. (12) Casanovas, A.; Saint-Gerons, M.; Griffon, F.; Llorca, J. Autothermal generation of hydrogen from ethanol in a microreactor. Int. J. Hydrogen Energy 2008, 33, 1827. 14054


Hydrogen Production from Methane Steam Reforming in Combustion

Recommend Documents