Ind. Eng. Chem. Res. 2010, 49, 10917–10923
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Investigation on the Combined Operation of Water Gas Shift and Preferential Oxidation Reactor System on the kW Scale Martin O’Connell,*,† Gunther Kolb,† Karl-Peter Schelhaas,† Jochen Schuerer,† David Tiemann,† Steffen Keller,† Dorothee Reinhard,† and Volker Hessel†,‡ Institut fu¨r Mikrotechnik Mainz GmbH, Carl-Zeiss-Strasse 18-20, 55129 Mainz, Germany, and Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology (TU/e), Den Dolech 2, Postbus 513, 5600 MB, EindhoVen, The Netherlands
A 5 kWel water gas shift reactor was integrated with a 5 kWel preferential oxidation reactor for the purposes of reducing the carbon monoxide levels in a reformate exit stream to levels below 100 ppm. The integrated system worked best at partial load with CO concentrations being reduced to 40 ppm at 60% load level and S/C ) 3.2. 1. Introduction Integrated fuel cell systems for both mobile and stationary applications have become of great interest in the recent past for many wide and varied reasons.1 The main drivers for the development and commercialization of fuel cell systems are environmental and socio-economic. The pressure to reduce the environmental impact of energy generation from fossil fuel sources and the desire to ensure a secure energy supply line so as to lessen the influence of external factors have necessitated the development of such systems, among others, in recent years. Numerous fuel processor systems are currently under development for both stationary and mobile applications. In the case of application and integration with a low temperature PEM fuel cell stack, then it is necessary to purify the H2-rich gas originating from a reformate to levels under 10 ppm so as to avoid the poisoning of the electrocatalyst of the fuel cell anode. Dokupil et al.2 have reported on a 300 W fuel processor system which used LPG as feedstock. The CO purification module developed therein consisted of two adiabatic WGS reactors and a PROx reactor which was capable of reducing 12 vol% CO in a reformate exit stream to levels under 50 ppm. A 2 kW system was developed by Cipiti and co-workers,3 also utilizing a LPG feedstock. Here a one stage shift reactor, operating at 330 °C, and a PROx reactor were integrated, as part of a complete unit. The shift reactor was capable of converting some 10% CO down to 0.8%. However, due to difficulties in thermal management, 2000 ppm was observed as the residual output downstream the PROx reactor. As these authors themselves stated, many requirements for such systems have been achieved but also many challenges need to be addressed. In the field of systems based on gasoline, a major motivation has been the development of such systems for application in auxiliary power units (APU).4 A two stage shift system was coupled to an external PROx reactor, using ATR of gasoline as the reformate generation, by Qi and colleagues.5 CO abatement to levels under 1000 ppm was achieved. A gasoline-based system containing two shift reactors and two PROX reactors was developed by General Motors6 and was capable of reducing CO levels to under 100 ppm for a system which had an electrical power output of 27 kW. Another two stage shift system, integrated with a one stage PROx reactor, which was based on * Corresponding author E-mail:
[email protected]. † Institut fu¨r Mikrotechnik Mainz GmbH. ‡ Eindhoven University of Technology.
adiabatic reactors with intercooling, was developed by Severin et al which was capable of delivering 3 kWel. 160 ppm of CO was observed downstream of the PROx reactor.7 CO levels of down to 65 ppm were measured by Bowers et al.8 for another setup containing a two stage shift and a final PROx cleanup section. Diesel is also another major logistic fuel which has been investigated as the reforming feedstock for various applications.9 A complete 5 kW system was assembled by Rosa et al which contained a one stage shift and a one stage PROx reactor.10 The CO content of the reformate was 7.4 vol % and was reduced to levels between 0.3 and 1 vol % after the WGS reactor and to concentrations less than 100 ppm after the PROx reactor. As part of a conceptual study which was aimed at the MW scale,11 medium and low temperature shift reactors were developed together with a PROx reactor. Having started with marine diesel fuel as the feedstock, 16% CO dry concentration in the reformate flow was converted down to below 10 ppm after the PROx reactor. In the integration presented here, microstructured plate heat exchanger reactor technology was applied, which confers a series of advantages compared to standard technology such as fixed catalyst beds, namely lower pressure drop and enhanced heat and mass transfer.12 On coating plate heat exchangers with catalyst, the resultant heat generated by exothermic reactions may be removed thus enhancing the overall thermal management of the reactor.1 A complete fuel processor, based on microchannel plate technology and designed for a power equivalent of 5 kWel was presented by Kolb et al.13 The CO cleanup reactors consisted of high temperature and low temperature WGS reactors together with a one stage PROx reactor. The reformate, originating from the ATR of isooctane, contained 9.4 vol% CO (d.b.) which was converted to 200 ppm downstream the PROx reactor. Individual CO cleanup reactors were previously reported,14 a one stage WGS and a PROx reactor respectively, which were also designed for a power equivalent of 5 kWel and evaluated using surrogate diesel reformate. CO concentrations as low as 20 ppm were observed at the exit of the PROx reactor. Herein, the evaluation of these WGS and PROx reactors in combined operation are reported. Experiments were performed at S/C ratios of 3.2 and 3.8. Careful control of the system was necessary so as to achieve CO concentrations of less than 100 ppm at the exit of the PROx reactor.
10.1021/ie1005614 2010 American Chemical Society Published on Web 06/02/2010
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Figure 1. Simplified experimental scheme of the setup for the first level integration of the WGS and PrOx reactors.
2. Experimental Section A detailed description of the manufacture of the reactors is reported elsewhere,14 but a brief description is included here. The etched, stainless steel plates were coated with catalyst by Johnson Matthey Fuel Cells (both catalyst manufacture and coating performed by JM). The reactors were stapled and sealed by laser welding before the manifolds were attached to the plate stack also by laser welding. A simplified scheme of the test-rig applied for the integrated testing can be seen in Figure 1. Reformate surrogate, nitrogen, carbon dioxide, carbon monoxide, and hydrogen were dosed separately, mixed, and preheated in an electrical gas heater. Water, which was dosed separately, was evaporated in a dedicated evaporator and superheated in a separate electrical gas-heater. This was then added to the feed flow. For the sake of simplicity, the WGS reactor was cooled with nitrogen instead of cathode off-gas surrogate which would later be the cooling medium in a practical system. This nitrogen was preheated in another gas-heater. Finally the start-up gas, which was fed to the WGS reactor, was substituted with air. This was then preheated in another three gas heaters with 5 kW of heating power each, all of which were switched in parallel. Air was then added to the WGS reactor product downstream, and this then constituted the feed of the PrOx reactor. No start-up gas was used for the PrOx reactor because the single reactor testing14 had shown that heating by steam condensation, of the WGS product, on the reformate side was sufficient to preheat the reactor. The PrOx reactor was cooled with water. The reactors as mounted into the test rig can be seen in Figure 2. The flow rates for 100% load level (LL) have been communicated in a previous publication.14 Twenty-nine temperatures were monitored inside the reactors, and the values were gathered by a data acquisition system. Additionally, it turned out to be important to measure some
Figure 2. Integrated WGS and PROX reactors installed in the testing facility at the commencement of testing.
temperatures at the outer surface of the reactors because the behavior during start-up could be better understood by these means. The exact positions of the thermocouples are given in Figures 3 and 4. For ease of understanding, the following temperatures were chosen as being representative of the overall, integrated system even though many more were measured: (a) T 401 which was the temperature at the PROX reactor inlet zone, (b) T 404 which was the temperature at the PROX reactor outer zone, (c) T 5053 which was the temperature at the PROX outlet, cooling side (not in the figure), (d) T 423 which was the temperature at the WGS surface inlet (not in the figure), (e) T 413 which was the temperature at the WGS reactor inlet zone, and (f) T 418 which is the temperature at the WGS reactor outlet zone. The measurements of gas compositions were performed with micro-GC and downstream the PrOx reactor with an IR-sensor for CO after water removal, as was extensively described elsewhere.14 In some cases additional analyses was performed with conventional GC. Because the majority of data came from the Micro GC and the IR sensor, they are referred to on a dry basis (d.b.).
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Figure 3. Positions of temperature measurement in the water-gas shift reactor.
Figure 4. Positions of temperature measurement in the preferential oxidation reactor.
Initially it was decided to perform the experiments at a S/C ratio of 3.2 (referring to the feed composition of the reformer). However, during the course of the experiments and owing to the fact that the shift catalyst had suffered a slight degradation, the S/C ratio was changed to 3.8 as this allowed for a better performance of the shift reactor and hence lessened the load to the PROx reactor. The ease of performing load change experiments was also enhanced. 3. Results and Discussion 3.1. Steady-State Operation. As was stated in the previous section, the experiments were first run at S/C ) 3.2, at partial loads from 20% to 80% LL (load level). A typical steady state operation, performed at 60% LL, is displayed in Figure 5. On closer inspection, it can be seen that the temperature at the outlet of the WGS reactor was stable at 250 °C together with the inlet temperature of the PROx reactor which was maintained at an
average of 90 °C. As has been previously discussed,14 these temperatures are vital for the overall thermal management and control of each reactor. During the run, the maximum CO concentration reached was 100 ppm with the lowest being 43 ppm. This was a good performance considering that the CO output from the shift reactor amounted to 1.18 vol % (d.b.). This amounted to 99.63% conversion for the PROx reactor. The performance of the integrated system, at the other load levels in steady state conditions, in terms of both WGS and PROx output is summarized in Table 1. As one can see, CO concentrations downstream from the PROx reactor were always under 100 ppm at all levels up to and including 80% LL. However, at 100% LL such levels were never reached as can be seen in Figure 6. This was primarily as a consequence of the relatively high amount of CO that was sent to the PROx reactor from the WGS reactor. This had the effect of increasing the temperature at the inlet of the PROx reactor to over 110 °C
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Figure 5. Steady state operation at 60% load level, S/C ) 3.2, temperatures, PROx cooling flow, and CO concentrations as indicated. Table 1. Summary of the Performance of the Integrated WGS/PROx, at Different Load Levels for Both S/C ) 3.2 and 3.8 % LL
S/C
[CO]WGS(outlet)/ vol % (d.b.)
% CO conversion (WGS reactor)
[CO]PROx(outlet)/ ppm
% CO conversion (PROx reactor)
20 40 60 80 100 20 40 60 80 (run 1) 80 (run 2)
3.2 3.2 3.2 3.2 3.2 3.8 3.8 3.8 3.8 3.8
0.52 0.92 1.07 1.18 1.50 0.55 0.89 1.13 1.10 1.40
95.5 94.5 93.6 91.8 90.2 96.6 95.2 93.7 91.4 91.6
81 49 44 43 471 57 46 56 57 42
98.44 99.47 99.59 99.63 96.86 99.22 99.48 99.50 99.50 99.70
and in the reactor body to over 160 °C. From the individual component testing,14 it was realized that these temperatures were obliged to be at least 25 °C lower for efficient operation of the PROx reactor and for the achievement of a CO conversion so that the target CO amounts were attained. Taking this into consideration, the steam to carbon ratio was then raised to 3.8, and the runs at partial load were repeated. Better conversion levels for the shift were achieved (in analogous comparison with the lower S/C ratio) thus reducing the load for the PROx reactor. Values as low as 1.1% CO were measured at 80% LL compared to the higher levels observed at the lower S/C ratio. CO concentrations after the PROx were always below 100 ppm, occasionally reaching below 50 ppm. It should also be noted that no major differences in behavior were observed in the PROx reactor under the modified operating
conditions i.e. improvements in results can be directly attributed to the performance of the shift reactor. Generally speaking, maximizing CO conversion with less parasitic consumption of H2 is the main goal in the PROX unit. Because the PROX reaction is temperature sensitive, it is crucial to accurately control the temperature profile. Again from the experiences obtained in single unit testing,14 this was best controlled by optimizing the lambda value to 1.88 and by a careful increase of the PROx cooling flow. 3.2. Load-Change Operation. Similar to the experiments in steady-state mode, these runs were first performed at S/C ) 3.2 and at 3.8. Over the course of time it seemed that these experiments were more challenging for the system, especially with regard to load changes from a low load to a higher load. It was more effective to perform these in a stepwise manner
Figure 6. Steady state operation at 100% load level, S/C ) 3.2, temperatures, PROx cooling flow, and CO concentrations as indicated
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Figure 7. Load change performed from 20% LL to 80% LL at S/C ) 3.2, temperatures, PROx cooling flow, and CO concentrations as indicated.
Figure 8. Load change performed from 80% LL to 20% LL at S/C ) 3.2, temperatures, PROx cooling flow, and CO concentrations as indicated.
rather than going directly to the target as stepwise load changes were deemed to be more pragmatic. Figures 7 and 8 show typical load changes from 20% to 80% at S/C ) 3.2. With regards to the upward load change, too much CO is produced too quickly in the shift reactor which leads to increased temperatures in the PROX. As just one such example, T401 reaches values of 120 °C, over 30 °C more than what is considered to be the optimum value. The CO levels are too large initially for the PROX to convert, resulting in an overshoot of some minutes. However, after some time, the CO amount after the PROX decreases to levels less than 100 ppm. The reverse load change, i.e., 80-20%, is successful in the sense that low levels of CO are quickly achieved. However, changing from a high load level to a lower level has (except for rare instances) always proved less challenging than changes in the reverse direction. Similar to steady-state operation, as a result of the large amounts of CO production at the instant of an upward load change, it was decided that lower levels of CO after the PROX could be produced if the load to the PROX was reduced by (a) increasing the temperature of the shift catalyst and (b) operating at an increased S/C ratio of 3.8 to shift the thermodynamic equilibrium. It was found that running the system under these conditions did allow for improved performance of the WGS catalyst and more importantly in this case, allow for smoother (compared to previous) load changes which made CO concentrations downstream the PROX reactor more manageable. From fuel cell testing, brief spikes over 1000 ppm of CO are tolerable as long as the average CO level can be handled.
Figure 9 depicts an upward load changes from 20 to 60% LL and upward to 80%. First of all, the load change compares very favorably with its equivalent under the original parameters. In the latter case, a large overshoot (over 1000 ppm) was measured. Here, up to 250 ppm was measured but only for 30 s or so. Second, close observation of the two temperature curves representing the PROX reactor (i.e., T401 and T404) is very instructive to the reader with respect to the temperature sensitivity of the PROX reactor. Before the load change both of these temperatures are at 90 and 115 °C respectively. After the load change, the CO level into the PROX is too high, increasing to a maximum of 1.5% accompanied by a concomitant rise in the temperature of the PROX at positions T401 and T404 to 108 and 163 °C respectively. The CO concentration after the PROX also rises, up to a maximum of 250 ppm. Effectively, it was easier to control the extra heat generated by the increased CO evolution thus keeping the overshoot of CO levels after the PROX to a minimum. The CO levels entering the PROX decrease relatively quickly to steady values and these PROX temperatures also decrease to 97 and 135 °C. The observed CO concentration after the PROX was now 50 ppm. After a period of time, the load was further increased to 80% LL. Again, similar behavior was observed with regards to both the CO concentration directly after the shift reactor and the aforementioned temperatures in the PROX reactor but after time, the CO level after the PROX does decrease to 70 ppm. The final upward load change is illustrated in Figure 10. As before, the results here were promising. Similar to the load change from 20 to 60% LL, an overshoot to 450 ppm was
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Figure 9. Load change performed from 20% LL to 60% LL and subsequently to 80% LL at S/C ) 3.8., temperatures, PROx cooling flow, and CO concentrations as indicated.
Figure 10. Load change performed from 40 to 80% LL at S/C ) 3.8, temperatures, PROx cooling flow, and CO concentrations as indicated.
measured for a short period of time after which the load to the PROX reactor was reduced and CO levels after the PROX were found to be 50 ppm. It should also be noted that in all cases, the downward load changes were performed but these were found to be less technically demanding with smooth operational measurements observed.
Acknowledgment
4. Conclusion and Outlook
Literature Cited
In the results presented here, a 5 kW water-gas shift reactor was integrated with a corresponding PROX reactor. Carbon monoxide concentration, downstream the PROX reactor, were reduced to values below 100 ppm, occasionally below 50 ppm. The experimental strategy was amended as the shift catalyst delivered a higher CO load than anticipated to the PROX reactor. These changes were (a) a higher operating temperature for the shift catalyst so that the conversion of the shift catalyst was sufficiently high and (b) increasing the S/C ratio to 3.8. On the other hand, no problems were observed with the 5 kW preferential oxidation reactor. In fact, the PROX reactor was capable of reducing up to 2 Vol % of carbon monoxide to less than 100 ppm (sometimes to values as low as 50 ppm), up to 80% load levels. Load changes were also performed, safely, up to 70% load level. Ideally, a reduction in CO concentration to values under 10 ppm is required and it is believed that the best way to achieve this is to lower the load to the PROx reactor.
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The research presented in this study was performed with funding provided by the European Commission in the scope of the 6th Framework Project, HyTRAN (“Hydrogen and Fuel Cell Technologies for Road Transport”), Contract No. TIP3-CT2003-502577.
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 (9) Lindstro¨m, B.; Karlsson, J.; Ekdunge, P.; De Verdier, L.; Ha¨ggendal, B.; Dawody, J.; Nilsson, M; Pettersson, L. J. Diesel fuel reformer for automotive fuel cell applications. Int. J. Hyd. Energy 2009, 34, 3367. (10) Rosa, F.; Lo´pez, E.; Bricen˜o, Y.; Sopen˜a, D.; Navarro, R. M.; Alvarez-Galva´n, M.-C.; Fierro, J. L. G.; Bordons, C. Design of a diesel reformer coupled to a PEMFC. Cat. Today 2006, 116, 324. (11) Krummrich, S.; Tuinstra, B.; Kraaij, G.; Roes, J.; Olgun, H. Diesel fuel processing for fuel cellssDESIRE. J. Power Sources 2006, 160, 500. (12) Kolb, G.; Hessel, V. Micro-structured reactors for gas phase reactions. Chem. Eng. J. 2004, 98, 1. (13) Kolb, G.; Baier, T.; Schuerer, J.; Tiemann, D.; Ziogas, A.; Specchia, S.; Galletti, C.; Germani, G.; Schuurman, Y. A micro-structured 5 kW complete fuel processor for iso-octane as hydrogen supply system for mobile
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auxiliary power units: Part IIsDevelopment of water-gas shift and preferential oxidation catalysts reactors and assembly of the fuel processor. Chem. Eng. J. 2008, 138, 474. (14) O’Connell, M.; Kolb, G.; Schelhaas, K. P.; Schuerer, J.; Tiemann, D.; Ziogas, A; Hessel, V. The development and evaluation of microstructured reactors for the water gas shift and preferential oxidation reactions in the 5 kW range. Int. J. Hyd. Energy 2009, 35, 2317.
ReceiVed for reView March 09, 2010 ReVised manuscript receiVed May 18, 2010 Accepted May 19, 2010 IE1005614
