assessment of advanced catalyst performance and

ASSESSMENT OF ADVANCED CATALYST PERFORMANCE AND FABRICATION OPTIONS

FOR A COMPACT STEAM REFORMER

ETSU F/02/00180/REP

DTI PUB URN 01/1163

ContractorAdvantica Technologies Ltd.

Prepared byA L Dicks, P Goulding, S L Jones, R Judd, K Pointon

A. Gough (Newcastle University)

The work described in this report was carried out under contract as part of the DTI Sustainable Energy Programmes. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of the DTI.

First published 2001 © Crown Copyright 2001

I

EXECUTIVE SUMMARY

Objectives

This project sought to establish the feasibility in a proof-of-concept evaluation of the Advantica (formerly BG Technology) compact reformer. The aim was to build on previous work on the compact reformer, by addressing the following critical objectives:

• Improvement of the adherence, durability and degradation rates of the thin-layer reforming and combustion catalysts under realistic operating conditions

• Use of computer modelling to assess likely effects in the reforming reactor of catalyst degradation at different rates, and assess possible ways of avoiding consequential damage.

• Evaluation of the likely options for the manufacture of a commercial compact reformer

• Production of an outline system design suitable for eventual implementation in a prototype reformer.

Background and need for work

There is increasing interest worldwide in the development of innovative fuel processing technologies for fuel cell systems, and a growing interest in the use of compact chemical reactors for a variety of applications. The recent surge in development of fuel cells systems, especially for small-scale power generation applications has spurred development of a variety of fuel processing options. The potential high power density of the Compact reformer concept could lead to major applications in fuel cell systems for stationary and transport use. Although the basic concepts have been demonstrated in the previous projects, there were still areas of uncertainty. This project set out to address these uncertainties and provide a sound basis for managing the risk of further development of this technology.

The project aimed to provide the following benefits:

• Proof of the compact reformer concept.• An assessment of the issues affecting the lifetime of catalysts used, and to

identify any other critical success factors.• System design data for integrating the compact reformer into a PEM fuel

cell system• System design data for using the compact reformer for hydrogen

generation• A sound basis for further development and commercialisation of the

compact reformer by the industrial partners based in the UK

I

Work Programme

Over 1000 distinct sols, washcoats and catalysts have been produced in the course of this project. As many of these have been based around an alumina support, much emphasis has been placed on investigating the properties of the alumina sols and washcoats with a view to later addition of active catalytic metals and oxides, either by impregnation or incorporation into the sol or washcoat direct.

A detailed study has enabled the necessary concentrations and acidities required to give rise to coatings that are both adherent post-firing and of controllable thickness. These sols and washcoats have been used as the base for nickel and ruthenium based reforming catalysts, and for palladium based combustion catalysts. Coating was initially done on coupons of 316L stainless steel or 3220H nicrofer to determine catalyst adhesion. Having established which of the formulations were most adherent, the material was coated onto 3cm lengths of 3.1mm o.d. rods for reactivity testing.

The next stage was coating of the most promising formulations into 8mm x 2mm x125mm straight-through reactor channels within diffusion bonded blocks. These were constructed from heat exchanger materials provided by Chart.

The final stage of the process took the final selected formulations and coated them into the channels of 5-channel proof of concept reactor, with alternating combustion and reforming channels. Three similar reactors were tested, ultimately for periods in excess of 100 hours, and at reaction temperatures from 600°C to 800°C.

In parallel with the experimental work, reactor and system modelling was carried out. Both the simplified models of earlier projects were revisited to seek to achieve more meaningful results. This project also required the modelling of alternative reactor configurations. Counter-current versions of both the simpler models were developed and compared. The first of the simplified reactor models was constructed in the AspenPlus flowsheeting environment. The model was made up of five stages represented by cooled- wall plug-flow reactor units for either channel. These reactor units were cooled or heated by a very large flow of coolant such that the low thermal resistance between the channels was modelled. The coolant flows were linked across the channels so as to create co-current or counter-current flow. The reactor dimensions are chosen to be similar to those used for single channel combustion and reforming experiments at Newcastle. The activation energies were taken directly from differential reactor experiments on the early thin-coat catalysts.

The second of the simplified reactor models was built in a spreadsheet environment. This offered the advantages of freedom in structuring the reaction kinetics and speed of creating many stages in the model to give good resolution. The model splits the channels into a large number of cells. Each cell starts by calculating the composition of the reforming and combustion

II

flows. From the composition in each channel the local reaction rates are calculated. The enthalpy and heat capacity of each component are calculated from polynomials based on the previous temperature and summed to give a total enthalpy and heat capacity. The new temperature is calculated from enthalpy change and the local heat capacity.

For the counter-current model an iteration is required as each cell only has information to perform calculations for one or other of the channels. This is achieved through a simple substitution method by separating the initial and calculated temperature profiles. Following an initial guess of temperature in each cell, the calculated temperature is repeatedly substituted until convergence is reached.

In addition to the reactor modelling, flowsheet analyses have been carried out and consideration given to the manufacturing issues associated with the fabricating a compact reformer. Flowsheeting has enabled us to review the system operating conditions. This has helped to reduce the challenges for materials and catalysts, particularly the operating temperature. The review of operating conditions attempted to:

• establish the penalties to a process incorporating Compact Reformer of relaxing its operating conditions.

• identify ‘process fixes’ that can mitigate these penalties.

• determine the optimum initial conditions for design based on a trade-off of penalties and performance.

Consideration of manufacturing issues led to the choice of internal reactor hardware configuration and the methods of catalyst manufacture and deposition.

Summary of Achievements

This project has demonstrated the proof of the compact reformer concept in three complete small-scale reactor blocks. Highly active and durable catalysts for steam reforming of natural gas and catalytic combustion of natural gas, both developed within this project, were prepared and deposited inside the channels of the compact heat exchanger hardware. The latter was fabricated by a leading heat exchanger manufacturer, and demonstrates that the unit could be manufactured commercially. The successful tests of the small reactors has taken the technology much further than the original project scope, which sought only to show that active and durable catalyst could be coated within the hardware.

The adherence of catalyst appears to be less dependent on the surface preparation of the catalyst substrate than other factors associated with the catalyst preparation and deposition. Whilst good performance for each of the catalyst types has been obtained over 100 hour tests using pure methane as fuel (>90% conversion), insufficient data has been obtained to establish the lifetime of the catalysts in a natural gas fuelled system. Results so far indicate

ill

that some degradation occurs during the first 100 hours, and this may be associated with sintering of the catalyst. Further work, in particular tests of longer duration, will be needed fully to assess the issues affecting lifetime. System data has been obtained for integrating the compact reformer into a PEMFC system. Operating at moderately elevated pressure and applying recycle of the anode off gas to the reformer ensures a high system efficiency (ca. 40%) even when the reformer is operating at relatively moderate temperatures (600-650°C). The latter will help to reduce the costs of the reactor materials thereby assisting towards the evolution of a commercial product.

Conclusions

The present project has led to a proof-of-concept Compact Reformer, which is a substantial advance on previous work. This is based upon novel compact heat exchanger technology with non-optimised catalysts for combustion and reforming. The main conclusions from the work are:

• Catalyst materials for promoting the steam reforming reaction and the combustion reaction can now be prepared and coated onto compact heat exchanger hardware.

• Catalyst compositions have been established which achieve short-term targets (>100 hours) for activity and durability (adhesion), in heat exchanger hardware.

• Tests have shown that both catalysts are active enough to enable target power densities in excess of 10kW/m2 to be achieved.

• Micro-reactors have been established for testing of both steam reforming and combustion catalysts. A test facility at Newcastle University has been modified to allow the testing of complete compact reformer reactor blocks.

• Two reactor models have been developed which enable the temperature profiles within the compact reformer to be predicted for given operating conditions. There is good agreement between the two models, and the data generated by the models should help to establish the balance between the two reactions once more experimental data has been obtained for the catalyst activities.

• Methods have been developed for coating each type of catalyst into completely fabricated diffusion-bonded test reactors. Each reactor is built up of channels of 1mm width or less.

• Tests have been carried out in which both steam reforming and combustion have been demonstrated side by side. The diffusion bonded test reactors behaved satisfactorily as complete compact reformers.

Recommendations

Although the project addressed the operating conditions of the reformer in a stationary PEMFC application, no provision was made in the present project

iv

for testing the proof-of-concept in a complete fuel processing system. This will need to be addressed in the future. There are a number of key areas where progress needs to be made on the compact reformer unit itself, in parallel with overall system development.

Catalyst Development

This project has led to a greater understanding of the nature of catalysts coated on stainless steel. This has culminated in the development of combustion and reforming catalysts that are suitably adherent and display outstanding activity in laboratory tests. Catalyst development now needs to continue, focusing on:• Development of new catalyst contenders

• Catalyst durability - more longer term tests

• Adhesion of the catalysts should be established under harsh conditions such as thermal cycling and the necessary development carried out should this be unsatisfactory with the current catalysts.

• Assessment of coating methods

• Catalyst coating has centred on washcoat technology thus far. Further approaches such as flame-assisted deposition and variants should be investigated. A key aim here will be to develop the means to coat the heat exchanger components before assembly.

• Development of catalyst reproducibility/control

• Study of the kinetics of the reforming and combustion reactions over favoured catalysts

• Establish envelope of carbon-free operation of favoured reforming catalysts

Manufacturing

Issues surrounding the cost and manufacturability of compact reformer components need to be addressed, in particular:• The manufacturability of heat exchangers as a function of design concept

and of material.

• Cost versus scale of various heat exchanger concepts.

• Methods for regenerating spent CR units.

Process

Development of reformer technology cannot be carried out in isolation from the rest of the PEMFC system since process integration will ultimately be required to obtain maximum system efficiency. The operating reformer parameters should be refined and optimised both for incorporation into a

v

stand-alone fuel processor and also as part of an integrated fuel cell system. Control and operating procedures also need to be addressed.

vi

Contents

1 Introduction.............................................................................................. 1

2 Aims and Objectives................................................................................32.1 Coated catalyst development and testing...............................................32.2 Reactor modelling..................................................................................32.3 Manufacture and Construction of the Compact reformer......................32.4 System design........................................................................................3

3 Coated catalyst development and testing...............................................43.1 Introduction............................................................................................43.2 Reforming Catalyst Development..........................................................53.3 Combustion Catalyst Development.......................................................53.4 Reactivity Testing..................................................................................5

3.4.1 Steam Reforming Activity Testing..........................................63.4.2 Combustion Activity Testing.................................................. 73.5 Results and Discussion.......................................................... 83.5.1 Reforming Catalysts...............................................................83.5.2 Combustion Catalysts..........................................................10

4 Reactor modelling.................................................................................. 134.1 Introduction.......................................................................................... 134.2 Flowsheeting Package Model.............................................................. 134.3 Model Structure ................................................................................... 134.4 Reaction Kinetics................................................................................. 144.5 Results.................................................................................................. 154.6 Spreadsheet Model............................................................................... 164.7 Model Structure................................................................................... 174.8 Reaction Kinetics................................................................................. 174.9 Discussion............................................................................................23

5 System Design.........................................................................................245.1 Background..........................................................................................245.2 Initial Position......................................................................................245.3 Testing Alternative Operating Conditions...........................................275.4 Conclusions..........................................................................................29

6 Manufacturing Evaluation and Proving of Catalyst Coating withinCHE Hardware............................................................................................. 30

6.1 Manufacturing Evaluation...................................................................306.2 Testing of the Multi-channel Reactor..................................................31

6.2.1 Run 1....................................................................................326.2.2 Run 2....................................................................................336.2.3 Run 3....................................................................................35

7 Conclusions.............................................................................................38

vll

8 Recommendations..................................................................................398.1 Catalyst Development..........................................................................398.2 Manufacturing......................................................................................398.3 Process.................................................................................................40

9 References...............................................................................................41

Appendix A. Linking Catalyst Activity with Process Parameters............421 Background............................................................................................422 Introduction............................................................................................423 Channel dimensions...............................................................................444 Effect of conversion................................................................................445 Effect of Pressure...................................................................................466 Effect of Stoichiometry..........................................................................47

Appendix B. The Fuel Cell Model ...............................................................48

viii

1 INTRODUCTIONThere is increasing interest world wide in the development of innovative fuel processing technologies for fuel cell systems, and a growing interest in the use of compact chemical reactors for a variety of applications. The recent surge in development of PEM fuel cells systems, especially for domestic and other small scale power generation applications has spurred several new developers to enter the field, offering a variety of fuel processing options, including various options for reducing the size of the reformer and fuel processor Similarly a growth in interest in fuel cells for transportation has led several organisations to develop reactors to enable various liquid fuels such as methanol and gasoline to be reformed on-board vehicles.

The compact reformer concept grew out of the area of process intensification championed at Newcastle University by the group led by Prof. Colin Ramshaw. The potential high power density of the CR concept could lead to major applications in fuel cell systems for stationary and transport use.

The basic idea of the compact reformer is to catalytically activate both sides of a compact heat exchanger (CHE) - one side for combustion to provide heating, the other for reforming methane to produce syngas - by applying thin adherent coatings of catalyst. The thin coating results in small conduction and diffusion path lengths that largely eliminate heat and mass transfer restrictions associated with conventional radiant tube reformers. This allows an improved utilisation of the intrinsic reforming catalyst kinetics.

In previous DTI supported projects [1,2], combustion and reforming microreactors were constructed and have been used to demonstrate that the required catalyst activities can be achieved. Techniques (based on slurry coating and sol-gel technology) for coating the catalyst onto the channels of the CHE have been developed, and methods of improving the adhesion of the coating were investigated. Although the basic concepts have been demonstrated, there were still a number of uncertainties that must be addressed in order to prove the concept of the Compact Reformer. It was the aim of the research reported here, to address these uncertainties and provide a sound basis for managing the risk of further development of this technology. The last project [2] ended in July 1997; since then, Advantica (as BG Technology) has independently funded further work at the Universities of Newcastle and Warwick, concerned with catalyst fabrication and testing, development of a hybrid reformer, and an assessment of reactor fabrication.

In the Compact Reformer development several issues have been addressed and in good progress has already been made in the following areas:• The necessary activity to achieve a target heat flux of 10kW/m2 has been

demonstrated separately in combustion and reforming micro-reactors.

• Adherence of catalyst (for both reforming and combustion catalyst) has been satisfactory for 100 hours of operation and sufficiently thick coatings can be laid down with a medium of viscosity low enough to pass into the narrow passages of compact heat exchangers. However these properties had yet to be satisfactorily demonstrated using real compact

1

heat exchanger material appropriate to the compact reformer, and with correspondingly high catalyst activities. Indeed, it has been remarked that highly adherent catalysts have often been found to show insufficient activities, and active catalysts have proved difficult to coat satisfactorily.

• Initial activity of the reforming catalyst has been low but appeared to be dependent on hydrogen addition to the feed. This indicates that there is great potential for optimising the catalyst formulation and system design.

• Complementary computer models for regression of experimental rate data and modelling of reactor kinetics have been developed. With the data used for modelling, the rate of reforming is limited by the combustion kinetics and, thus, there is no risk of thermal runaway of the Compact Reformer.

• A hybrid reformer had been developed and demonstrated for a short time at Newcastle University. This was to be used in the present project to evaluate the basic concept of the Compact Reformer, before proceeding to coat heat exchanger hardware.

Thus at the start of the present project, compact reformer development had reached a critical stage. Some aspects of the technology had been demonstrated but significant uncertainties still remained. The overall aim of the new phase of development was to address the remaining critical issues and establish a firm basis on which future development and scale-up could take place.

This project has been completed by a partnership consisting of Advantica Technologies Ltd and Alstom, with technical support from Newcastle University, Chart Heat Exchangers and Alstom Research and Technology.

2

2 AIMS AND OBJECTIVESThe overall aim of this project was to evaluate the compact reformer concept by tackling specific issues that need to be resolved before the feasibility of the technology can be demonstrated. As such the output of the project can best be described as a pre-prototype demonstration of the compact reformer. These critical issues are embodied in the following objectives:

2.1 Coated catalyst development and testingTo measure and improve on the adherence, durability and degradation rates of the thin-layer reforming and combustion catalysts for the compact reformer under realistic operating conditions. Catalysts were required which could be coated practically within the narrow channels of compact heat exchanger hardware.

The target for this project was to demonstrate or project catalyst performances (above 90%) for both reforming and combustion over a period of 1000 hours.

2.2 Reactor modellingTo use computer modelling to assess likely effects in the reforming reactor of catalyst degradation at different rates, and assess possible ways of avoiding consequential damage, due for instance to ‘hot-spots’ where reforming catalyst has deteriorated. This was to include a brief examination of the use of the device as a partial-oxidation reactor.

The target here was to provide a means of achieving a lifetime of the concept in excess of 1000 hours.

2.3 Manufacture and Construction of the Compact reformerTo provide an evaluation of the likely options for the manufacture of a commercial compact reformer, taking into account process costs and possible novel techniques for fabrication. To design and construct a compact reformer block which would at as a realistic precursor to a real CR unit. This addressed novel techniques for fabrication and as such took a major step beyond previous projects. A major deliverable in the current project required the design, building, coating and testing of a complete proof-of-concept unit based on compact heat exchanger hardware. The catalyst lifetime objectives were finally tested in this proof of concept reactor.

2.4 System designTo produce an outline system design suitable for eventual implementation in a proof-of-concept prototype reformer.

3

3 COATED CATALYST DEVELOPMENT AND TESTING

3.1 IntroductionCatalyst science lies at the heart of the compact reformer concept. It has been recognised through the previous work that without highly active and durable catalyst coatings, the concept is not feasible. A huge effort has therefore been expended on the catalyst development work, which has included preparation, characterisation and testing. Over 1000 distinct sols, washcoats and catalysts have been produced in the course of this project. As many of these have been based around an alumina support, much emphasis has been placed on investigating the properties of the alumina sols and washcoats with a view to later addition of active catalytic metals and oxides, either by impregnation or incorporation into the sol or washcoat direct.

Before starting work on the catalyst design and preparation, a very detailed study was made of surface finishes, ranging from priming with alumina and ceria sols at prepared at different pH values, through to grinding at different grits, polishing, etching and priming with manganese oxide. The outputs of this review determined the starting points for the coating exercise, which followed. There was considerable concern at the start of the project that coating effectiveness would be strongly dependent on the surface properties and prior treatment history. It was important that an understanding of the impact of such parameters was gained prior to beginning catalyst coating. The nature of the surface is also related to the fabrication of the heat exchanger hardware itself, discussed in section 6.

Alumina coatings have been made from proprietary Disperal P2, “classic” Disperal and a developmental 10/2 grade from Condea, either as sols or washcoats when used in conjunction with Martinel Trihyde ol-107. A detailed study has enabled the necessary concentrations and acidities required to give rise to coatings that are both adherent post-firing and of controllable thickness. These sols and washcoats have been used as the base for nickel and ruthenium based reforming catalysts, and for palladium based combustion catalysts.

Coating was initially done on coupons of 316L stainless steel or Nicrofer 3220H to determine catalyst adhesion.

Having established which of the formulations were most adherent, material was coated onto 3cm lengths of 3.1mm o.d. rods of the same materials for reactivity testing.

The next stage was coating of the most promising formulations into 8mm x 2mm x125mm straight-through reactor channels within diffusion bonded blocks. These were constructed from heat exchanger materials provided by Chart.

The final stage of the process took the final selected formulations and coated them into the channels of 5-channel proof-of-concept reactor, which consisted of similar external channel dimensions to the block described above, with internal channel designs aimed at maximising exposed surface area and inter

4

channel heat transfer. The five channels consisted of three combustion and two reforming reactors arranged alternately.

3.2 Reforming Catalyst DevelopmentAt the outset of the work, considerable effort was dedicated to repeating and developing the “WA” Ni/Cr/alumina preparation of the previous ETSU project [2]. Work in the current project has found WA to adhere well to the substrate but poorly within itself. Although described as a washcoat in the report [2], WA more closely represents a saturated solution of nickel nitrate, together with some chromium nitrate within a partially gelled dilute washcoat.

Subsequently, the so-called “high nickel” catalyst of the first ETSU project [1] was investigated. This showed little difference in activity compared to WA, and adhered very well at high and controllable thickness (10-20 gm from one coating pass, up to 40 gm for three passes). The high nickel composition has now been shown to coat the internal structure of simple single channel reactors very well, without blockage.

The high nickel catalyst has a very high initial activity, but reactivity tests showed some degradation of reformer catalyst activity over the first few hours in use. This is not unusual even for conventional pelleted catalysts. For ultimate deployment of the Compact Reformer, however, lifetimes of tens of thousands of hours are needed; consequently stabilization of the activity is required. Work therefore concentrated on the addition of kaolin to a modified nickel-alumina sol, and also the use of alternative sol bases such as ceria and zirconia. This approach has met with some success as illustrated by the catalyst lifetimes, which will be reported for the 5-channel reactor system.

A study has also been made of WA coatings heated to different temperatures, from the standard 600°C up to 900°C. Again, adhesion differed little between the preparations, with no apparent gross sintering of the nickel-based oxide. Finally, a highly active commercial steam reforming catalyst developed in our own laboratories was investigated. The proprietary nickel-based LH catalyst was ground to a powder and attempts made to suspend it in ceria and alumina sols. All such attempts failed in that the coatings were poorly adherent.

3.3 Combustion Catalyst DevelopmentRepeated and modified preparation of the WPd(1) and WPd(3) Palladium based combustion catalysts of the previous ETSU study [2] were found to be highly adherent and active. Further description of these preparations and the nomenclature can be found in this earlier report [2].

3.4 Reactivity TestingActivity testing of reforming catalysts deposited onto stainless steel rods has been carried out using a simple micro-reactor, a test procedure used for several years at Advantica. This test has also been developed to allow testing of combustion catalysts.

5

3.4.1 Steam Reforming Activity TestingThe experimental arrangement for testing of reforming catalyst activity is illustrated in Figures 1 and 2. The sample, coated on the outside of a 3cm length of rod, as described earlier, was mounted in a stainless steel microreactor of 4mm i.d. and tested in a dilute reaction mixture at high space velocity. Reaction rates were obtained from the gas flow rates and compositions determined by gas chromatography of the effluent gases.Samples were activated in dry 330 ml/min 5%H2/95%He for at least 3 hours at 650°C. However, some thin film catalysts have been shown to give a rapid initial deactivation so that in previous projects the samples were pre-aged before testing. Time constraints meant that this procedure would not be possible for the large numbers of catalysts in this project. Instead, on the assumption that a thermal event is the most significant, some samples were treated at 150°C above the nominal test temperature for 1hour in order to minimise activity change during the duration of the activity test.

The test gas (5%CH4/10%H2O/2%H2/83%He) was set at nominally 1000-2250 ml/min, the relative amounts of CO, CO2 and CH4 in the reactor exhaust defining the reaction rate. The rate of the reforming reaction was measured at 650°C, 700°C, 600°C and finally a second time at 650°C. All temperature ramping was conducted at 20°C/min and the sample was equilibrated at each temperature for one hour before rate measurements were made.

6

Figure 1Block Diagram of the Experimental Rig

HeatedGasBlender Humidifier Reactor in

furnaceGasanalysis

Figure 2Cross Section of Tubular Micro-Reactor

Reactant gas Product gas

4

Stainless steel tube

Test piece Alumina spacer Thermocouple

3,4.2 Combustion Activity TestingA suitable test for combustion catalyst activity was developed as part of this project. This involved modifying an existing rig for the purpose and ensuring that approximately differential conditions could be set up. This required ensuring a nominal conversion of 10% or less with minimal temperature variation along the sample.

The sample was mounted in a shortened version of the microreactor shown in Figure 2 in a similar manner to that described above for reforming samples. Heating of the reactor was achieved with a 750W cartridge heater mounted along with the reactor within a substantial stainless steel block. No activation of the catalyst was necessary. Pre-treatment of the catalyst consisted of exposure to the test gas at the maximum nominal temperature likely to be experienced in the compact reformer for 1 hour.

The test gas was 500-1500ml/min l%CH4/4%02/95%He, the relative amounts of CO, CO2 and CH4 in the reactor exhaust defining the reaction rate. The rate of combustion reaction was measured whilst cooling from 800°C to 600°C. The sample was then cooled to 500°C before measuring the rates, whilst heating from 600°C to 800°C. All temperature ramping was at 30°C/min and

7

the sample was equilibrated for 15 minutes at each temperature before rate measurements were made.

3.5 Results and Discussion

3.5.1 Reforming CatalystsMeasured methane consumption rates were extremely high in many cases so that differential conditions were generally not approached (conversions as high as 64% were observed). Since target conversions had been exceeded, activities were obtained by applying the appropriate integrated rate law (see Appendix 1). The weight-normalised activities of over 30 different samples were measured as 1st-order rate constants over the temperature range 600- 700°C.

Measured rates at 650°C at the end of the test procedure were frequently slightly lower than those measured at the start. This may indicate a continued deactivation over the timescale of the test procedure (3 hours). It should be borne in mind therefore that the activities measured here are initial activities, many of the catalysts requiring further development in order for this activity to be retained.

The response of catalyst activity to temperature is given for the WA3 catalyst in Figure 3. Deactivation of the catalysts meant that reliable activation energies were obtained in only a few cases. Generally, the activation energies for the reaction over these catalysts were rather low (44-71kJ/mol) but higher than that observed previously over an LH pellet in this temperature range (28- 29kJ/mol). This may indicate a significant amount of diffusion control even in the thin layers since the activation energy is significantly lower than expected for the activation of the rate-determining step (c.f 100-135 kJ/mol for CH4 adsorption on Ni and that of 13.4 kJ/mol implied over this temperature range by the approximate T1'75 dependence of diffusion coefficients).

A series of catalysts was tested on both stainless steel and nicrofer. If the substrates serve only as a base for the catalyst layer and any transfer of ions or reaction intermediates between substrate and catalyst is insignificant, it would be expected that measured activities would not be influenced by the nature of the substrate. This was indeed found to be the case.

8

Figure 3. Arrhenius Plots for WA3 Catalyst

-6.5-------0.000124 0.000128 0.000132 0.000136 0.000140

1/RT (mol/J)

A sufficient range of catalyst thicknesses had been obtained with WAS and high nickel catalysts to examine the impact of catalyst thickness on apparent activity.Results hinted that apparent activity may fall somewhat with thickness consistent with some diffusion control within the catalyst, but the data are not good enough to make this conclusion reliably. Certainly, there is no strong correlation. This suggested that activity expressed per unit geometric area would be strong function of thickness. This was indeed so and therefore there may be scope for obtaining higher power densities with these catalysts by increasing the catalyst loading (Figure 4).

Figure 4. Effect of Thickness on Reforming Activity at 650°C

Coating weight (g/m2)

• WA3• High Ni

9

Examination of the effect on the catalyst of various pre-treatments indicates that generally the results are consistent with the thermal history being important, the activity decreasing with increasing maximum temperature.

Catalysts containing purely ruthenium as their active component were disappointing in their performance. These were much less active than the nickel-containing catalysts and would be incapable of delivering meaningful reaction rates in the compact reformer. Combined Ni/Ru catalysts gave much higher activities, but there was no evidence that these had more activity than would be expected to result from the Ni component.

At 650°C, the best reforming activity gave an activity around 0.13 mol/s/g/bar. At the coating weights present on these samples, this activity implies a heat transfer rate of around 23kW/m2 in the compact reformer operating at 90% conversion and atmospheric pressure with a 4:1 steam: methane ratio. Thus adequate initial reforming activity has been demonstrated for the target 10kW/m2 compact reformer.

3.5.2 Combustion CatalystsAn example of the effect of temperature on 1st-order rate constant for combustion of methane is given in Figure 5. These Arrhenius plots were characterised by large hysteresis between measured curves during heating and those during cooling. Since the active component of all three combustion catalysts was palladium, the most likely cause of the observed behaviour is the PdO/Pd transition. Data given in the literature suggests that the temperature of decomposition of supported PdO is controlled thermodynamically and can be given as:

Tdecomp (K)16340

(14.4952 - Ln(OJ ))

where PO is the partial pressure of oxygen. Under the experimentalconditions, this temperature is around 695°C. Thus, the catalyst is initially in the lower-activity metallic form at 800°C, the rate constant decreasing with temperature in the expected manner. Recombination to the higher activity PdO occurs at lower temperatures than the original decomposition and is known to be dependent on the catalyst composition.

Since the sample had been cooled to 500°C, recombination would be expected to be as complete as possible before the heating cycle. Thus the rate constant measured at 600°C was higher during the heating cycle than that during the cooling cycle, but with limited scope for further increase with temperature due to the impending decomposition. The maximum activity was generally seen at 650°C.

The thickness of WPD4 catalyst was varied sufficiently for the influence of coating weight to be discerned. The activity expressed per unit geometric area increased with coating weight in the same manner as demonstrated by the

10

reforming catalysts (Figure 6). Consequently, higher reaction rates may be obtainable with higher catalyst loadings (adhesion permitting).

At 650°C, the best combustion activity observed was for WPD1 catalyst - 0.043 mol/s/g/bar. At the coating weight present on this sample, this activity implies 22kW/m2 in the compact reformer operating at 90% conversion and atmospheric pressure with 10% excess air. Thus adequate combustion activity has been demonstrated for a 10kW/m2 compact reformer.

These tests have indicated high initial activities for WA, high-nickel and WP catalysts. In the case of both reforming and combustion catalysts there is clear evidence of mass transfer limitations even in these relatively thin layers. For the combustion catalyst, activity is influenced greatly by the Pd/PdO transition.

Figure 5. Arrhenius Plot for WPD1 Catalyst

0.000110 0.000120 0.000130 0.000140

1/RT (mol/J)

• LI cooling — *— LI heating

11

Figure 6. Effect of Thickness on Combustion Activity

Coating weight (g/m2)

• WPD4(650°C)• WPD4(800°C)

Extrapolation of measured reaction rates to isothermal compact reformer channels gives power densities that compare well with the 10kW/m2 target (Table 1), and gave a sound basis for coating of the proof-of concept reactor blocks as described in Section 6. Considerable effort was still required however to tailor catalyst rheologies such that a coating method could be developed to coat the tortuous and narrow internal channels of these proof of concept diffusion bonded units.

Table 1. Projected power densities of Compact Reformer

Catalyst Feed Power density kW/m2 @ 650°C, 1 bar, 90% conversion

WA 3:1 steam/methane 33

High-Nickel 3:1 steam/methane 35

WPD1 Methane/10% excess 22air

12

4 REACTOR MODELLING

4.1 IntroductionPrior to 1998, reactor-modelling work had been conducted using CFD techniques in a one-dimensional model that tried to take account of diffusion, mixing, boundary layer, reaction kinetics and temperature. With the CFD codes available at the time, there was uncertainty in the complete convergence of the model and hence a lack of confidence in the results. In 1998 it was decided that this approach was too complex given that so little was understood and that a simpler modelling approach would yield information relevant to the state of understanding of the reaction chemistry and the design of the reactor.

By the second ETSU project, two other modelling approaches had been tried with the primary aim of predicting a temperature profile for two single channels in co-current flow. These models both restricted themselves to modelling just the reaction kinetics in a simple flow and heat exchange environment. Neither model produced usable results at that time, but did illustrate that the simplified approach could produce the right kind of results.

For the current project, both the simplified models were revisited to seek to achieve more meaningful results. This project also required the modelling of alternative reactor configurations. The only alternative to co-current flow without switching to three-dimensional modelling is counter-current flow. Counter-current versions of both the simpler models were developed and compared.

4.2 Flowsheeting Package ModelThe first of the simplified reactor models was constructed in the AspenPlus flowsheeting environment. This had the advantages of being relatively simple to construct; using robust physical properties, heat balance and convergence methods provided automatically by the package. It had drawbacks in that it had a fixed selection of kinetic equations and it was impractical to work with a large number of steps.

4.3 Model StructureThe model was made up of five stages represented by cooled-wall plug-flow reactor units for either channel. These reactor units were cooled or heated by a very large flow of coolant such that the low thermal resistance between the channels was modelled. The coolant flows were linked across the channels so as to create co-current or counter-current flow as illustrated in Figure 7.

As the model uses plug-flow reactors, the extent of the reaction depends on residence time, itself a function of the reactor diameter and length, which must be provided. Flows, composition, inlet temperatures and pressures are also set along with the steam to carbon ratio of the reforming side feed. The other key variables in the model are the pre-exponential factors and activation energies for the three kinetic reactions specified.

13

Figure 7 Flow configurations for the compact reformer model

Co-current Structure

Counter-current Structure

4.4 Reaction KineticsThe reactions in the model all utilised an Arrhenius-type expression provided by the package:

^ e- ^ Z C^'

where: A = pre-exponential factor

E = activation energy

R = gas law constant

T = absolute temperature

Ci = concentration of ith component

ai = exponent of ith component

In the reforming side of the model, kinetic expressions were entered for the reforming and shift reactions. The reforming reaction was first order in methane and the shift reaction was first order in carbon monoxide. In the combustion side of the model a kinetic expression was entered that was first order in methane and in oxygen.

14

4.5 ResultsThe following were used as input data to both co-current and counter-current models:

Table 2. Input data for flowsheeting package model

Reforming Combustion

Reactor length (m) 0.06 0.06

Reactor diameter (m) 0.0028 0.0028

CH4 feed flow (mol/hr) 0.165 0.25

Inlet temperature (°C) 750 750

Steam: carbon 4

Excess air (%) 10

Pre-exponential factors (/s) 14000 (reforming) 1925000

1051 (shift)

Activation energies (kJ/kmol) 38335 (reforming) 54280

14.34 (shift)

The reactor dimensions are chosen to be similar to those used for single channel combustion and reforming experiments at Newcastle. The activation energies were taken directly from differential reactor experiments on the early thin-coat catalysts. The pre-exponential terms are required in a different form to those produced from the lab and it was not clear how any conversions could be made. Thus, these values were adjusted empirically to achieve a similar result to that achieved with the co-current spreadsheet model. The results for both co- and counter-current models are shown in Figures 8 and 9:

15

Figure 8. Results from Co-current Flowsheet Model

Temperature

Combustion conv

Reforming conv

Position in Reactor

Figure 9. Results from Counter-current Flowsheet Model

Temperature Combustion conv Reforming conv

Position in Reactor

4.6 Spreadsheet ModelThe second of the simplified reactor models was built in a spreadsheet environment. This offered the advantages of freedom in structuring the reaction kinetics and speed of creating many stages in the model to give good resolution. Also, once built, the model provides graphical results in response

16

Con

vers

ion

frac

tion

Con

vers

ion

frac

tion

to input changes very quickly compared to the flowsheet. The drawbacks were that there is no self-checking of the structure and heat balances; all properties have to be calculated from simple polynomials and any necessary convergence routines have to be designed and programmed. These factors lead to a need to be very careful and rigorous in designing and programming the spreadsheet to limit introducing unexpected errors.

4.7 Model StructureThe model splits the channels into a large number of cells. Each cell starts by calculating the composition of the reforming and combustion flows. From the composition in each channel the local reaction rates are calculated. The enthalpy and heat capacity of each component are calculated from polynomials based on the previous temperature and summed to give a total enthalpy and heat capacity. The new temperature is calculated from enthalpy change and the local heat capacity.

For the counter-current model iteration is required as each cell only has information to perform calculations for one or other of the channels. This is achieved through a simple substitution method by separating the initial and calculated temperature profiles. Following an initial guess of temperature in each cell, the calculated temperature is repeatedly substituted with the guess until convergence is reached.

The model can accept input parameters of flows, composition, inlet temperatures, pressures and steam to carbon ratio of the reforming side feed. The overall length of the reactor can also be set. The key terms for the kinetic equations, the pre-exponential factors and activation energies, are only required for the reforming and combustion reactions as the shift reaction is assumed to be at equilibrium. However, as these terms are supplied from the Advantica lab on a volume of catalyst basis, the thickness of the catalyst in each channel must also be supplied.

4.8 Reaction KineticsReforming ReactionThe reforming rate at the prevailing temperature is calculated using:

.Rate = ^,pc^4 e(1 -

Where: A = pre-exponential factor

Pcha = partial pressure CH

E = Activation Energy

R = gas constant

Kref= equilibrium constant for reforming

T = absolute temperature

17

K is a measure of how far the reaction is from equilibrium and the inclusion of this term causes the reaction rate to reduce as equilibrium is approached.

KpH2 PCO

Pch4 Ph2o

The equilibrium constant is calculated from data taken from literature:

Kref = ^30.229-

Shift ReactionTo resolve the shift reaction, it is assumed that it is at equilibrium (studies have shown this to be approximately true). If, at equilibrium, x mols of CO are assumed to have reacted then:

K = (pco2 +x) (ph2 +x)

(PCO - xXpH2O - x)

The composition takes account of the reforming reaction that has already occurred.

As the reaction is assumed to be at equilibrium, K can be calculated from published data as:

KsAft = ^-3-957+4^

The equation may then be rearranged as a quadratic and a solution for x obtained.

The new composition of the reforming stream is obtained by adding/subtracting the reaction rates from the initial component flow:

H2 Initial + 3(Reform rate) + x

CH4 Initial - Reform rate

CO Initial + Reform rate - x

CO2 Initial + x

H2O Initial - Reform rate - x

Combustion ReactionThe combustion reaction rate is an Arrhenius equation that is first order with respect to methane and oxygen:

Rate = ApcH4 Po2 e-Rr

The new composition of the combustion stream is obtained by adding/subtracting the reaction rates from the initial component flow:

18

CH4 Initial - Combustion rate

CO2 Initial + Combustion rate

O2 Initial - 2(Combustion rate)

H2O Initial + 2(Combustion rate)

New Temperature Calculation

To calculate a new temperature, first the enthalpies of each component are calculated and summed using polynomials in temperature and multiplying by molar flow rate.

Second, the heat capacities of each component are calculated also using polynomials in temperature and multiplying by molar flow rate. These are summed to give a total heat capacity for both reforming and combustion streams. Then:

_ rji H0 ~ Hnewnew — T i + c

Where T = temperature of previous step

H0 = total enthalpy of all initial inlet gases

H^ = total enthalpy of all gases in current step

c = total heat capacity of all gases in currentstep

Note that the change in temperature is referred to the previous temperature, but the enthalpy change is referred to the initial inlet enthalpy.

Step Size

The method of calculation in the model does not require the use of a residence time; hence the step size is dependant only on the activity of the catalyst. Defining the kinetic terms relative to catalyst volume requires an assumption of channel width and length. The length is supplied by the user and the channel width is taken to be 1m, although this has no significance for the temperature profile calculated. The model results are also independent of channel depth. The explanation for this is as follows:

Consider two channels of the same width, but one has twice the height or width of the other. The molar flow of feed gas into the two channels is the same. A packet of gas of length dx will have twice the volume in the second channel as in the first, but will also have twice the residence time. If thereaction has a rate of r mols/s then the concentration of component i leaving

N ~ ^the element in the narrower channel will be:- dx

19

In the wider channel it takes twice as long for the entering gas to leave and so2Fi - 2ri

twice the flow will enter giving an outlet concentration of - 2 dx

Where: F = the molar flow rate of component i into the channel.

Results - Co-current Case

The initial experiments with the spreadsheet model were performed to see how it behaved with similar input data to the flowsheet model. Much larger gas flow rates were used, as the flow rates from the flowsheet model for a single 2.8mm channel were so small as to cause problems with the maths. As the spreadsheet model has no concept of channel diameter, flows on the reforming and combustion sides were set in the same ratio and equivalent to a reactor square cross-section with sides of about 15cm.

The kinetic terms were evaluated in early experiments on thin coat catalysts in the differential reactor, and are shown in Table 3. Unlike the flowsheet model, the spreadsheet model could be designed to accommodate the form of data produced in the lab. Results from the co-current spreadsheet model are shown in Figure 10. Catalyst thicknesses to achieve these results were 850pm on the reforming side and 750pm on the combustion side.

Table 3. Input data for the spreadsheet model

Reforming Combustion

Reactor length (m) 0.3 0.3

Reactor diameter (m) Not relevant Not relevant

CH4 feed flow (mol/hr) 212 321

Inlet temperature (°C) 750 750

Steam: carbon 4

Excess air (%) 10

Pre-exponential factors (mol/s/bar/m3)

1540000 9925000

Activation energies (J/mol) 38335 54280

20

Figure 10. Results from co-current spreadsheet model

Temperature Reforming conv Combustion conv

Position in Reactor

The speed of experimentation in the spreadsheet model allowed identification of some conditions, which will give a prediction of an even temperature distribution along the channel. However, this can only be achieved by changing the kinetic terms for the catalysts.

The above results show that both models can produce similar output with some manipulation of the data. However, the above result is not a practical one, as the catalyst thickness would almost completely close the channels. Also the predicted heat flux is much higher than the 10 kW/m2 sought for the Compact Reformer. Using 50pm catalyst thicknesses and adjusting flows to give 10 kW/m2 gives the results shown in Figure 11.

21

Con

vers

ion

frac

tion

Figure 11. Results from spreadsheet model with 50pm catalyst and target power density of 10kW/m2


Position in Reactor

Results - Counter-current Case

Using the same values as in the first co-current model above gives the results shown in Figure 12.

Figure 12. Results from Counter-current spreadsheet model


Position in Reactor

22

Con

vers

ion

frac

tion

Using the same values as the second co-current case above gives the following results.

Figure 13. Results from spreadsheet model (counter-current case) with 50pm catalyst and target power density of 10kW/m2

Position in Reactor

4.9 DiscussionThe conditions that give agreement between the two models for the co-current case do not lead to complete agreement in the counter-current case. The flowsheet model predicts much higher temperatures in the counter-current case, which results in higher conversions of both feeds. It was not possible to conduct further experimentation with the flowsheet model owing to expiry of the software licence.

In all cases presented here the conversion of combustion feed is relatively low. This appears to mainly be a function of low activity of the combustion catalyst. Later catalysts are known to have much higher activities, but these have yet to be quantified. Higher activity combustion catalyst will tend to push up reactor temperatures, but this can be constrained by reducing the combustion feed.

When more data on catalyst activities is available, the spreadsheet models will be useful in assessing which can be matched together and what bounds on feed flows can be tested.

23

5 SYSTEM DESIGN

5.1 BackgroundThroughout the early work on the Compact Reformer assumptions were made about the operating conditions, based on a projection of the target marketplace for the Compact Reformer being fuel cells and hydrogen generation at pressure. This led to the specification for operation at up to 10 bar. To achieve reasonable conversion of methane to hydrogen at such pressures a high temperature is required. Thus, operation at 850°C or greater was expected to be necessary. Materials selection for these conditions concluded that Incoloy 800HT would be required for the heat exchanger substrate, principally from the point of view of strength at these conditions, but also for its corrosion resistance should the planned protective coatings be breached.

If we are to be successful in the Compact Reformer development it is vital that we find the fastest effective route to market. This will be assisted by reducing the challenges for materials and catalysts in its development. For this reason a review of operating conditions was carried out to:

• establish the penalties to a process incorporating Compact Reformer of relaxing its operating conditions.

• identify ‘process fixes’ that can mitigate these penalties.• determine the optimum initial conditions for design based on a

trade-off of penalties and performance.

5.2 Initial PositionIn the second DTI funded project (2) system modelling had concluded that the optimum pressure for Compact Reformer operation lay around 3 bara for that system based on electrical generation efficiency. As the table below shows, the electrical efficiencies achieved for optimised flowsheets (i.e. all the heating and cooling efficiently satisfied) were around 42%.Table 4. Results from Earlier System Modelling

Stack Size (number of cells) 1500 1500 1500 1500 1500

Operating Pressure (bar) 1.7 3.1 6.1 3.1 6.1

Operating Temperature (°C) 90 90 90 90 90

Steam/Carbon Ratio 3 3 3 3 3

Air Stoichiometry (Fuel cell) 2 1.5 1.5 1.5 1.5

Air Stoichiometry (CR combustor) 1.2 1.2 1.2 1.2 1.2

Combustion Air Pre-heat Temp (°C) 600 600 600 700 700

Stack Efficiency (%) 56 64 63 64 63

Electrical Efficiency (%) 35 42 39 42 39

Overall Efficiency (%) 81 81 80 81 80

24

For the purposes of rapid comparison modelling, a fully optimised flowsheet is not convenient as changing heat flow in the system quickly requires the heat recovery to be re-optimised. Thus, this work was performed with an open flowsheet as shown in Figure 14, where only heaters and coolers exist and no attempt is made to match them together.

Natural

Natural

Figure 14. ‘Open’ Version of Flowsheet Used for Earlier Work

An exergy analysis of this flowsheet suggested inefficiencies around the fuel cell heat recovery system. To mitigate this known issue, the anode off gas, which still contains unutilised methane and hydrogen, was re-routed to provide fuel for the Compact Reformer combustor. The results of this change are shown in Table 5 and the change of layout is shown in Figure 15.

The flowsheet has a fixed gas feed to the reformer. All other flows adjust as required. Re-routing the anode off gas reduces the power output for a given gas feed, as the parasitic load of driving the fuel cell air compressor is not now supplemented by a turbo-expander. However, the reduction in gas feed to the Compact Reformer combustor results in more efficient generation overall.

25

Table 5. Results from Rerouting Anode Off gas

Case Original Revised

Operating Pressure (bara) 3 3

Reformer Temperature (°C) 750 750

Steam/Carbon Ratio 3 3

Air Stoichiometry (Fuel cell) 2 2

Air Stoichiometry (CR combustor) 1.1 1.1

Combustion Air Pre-heat Temp (°C) 600 600

Stack Efficiency (%) 59.2 59.2

Net Power Output (kW) 223.1 192

Electrical Efficiency (%) 36.6 39.5

Figure 15. Flowsheet with Re-routed Anode Off Gas

These efficiency figures are only credible if the flowsheet is still able to satisfy all the heat requirements of the various streams. The potential for this can be checked without designing the heat recovery system by using composite

26

curves. The composite curves for the revised flowsheet (1.8 bara, 650°C, St:C 3.2) are shown in Figure 16 and show self-sufficiency in heat.

Figure 16. Composite heating/cooling curves for the revised flowsheet

5.3 Testing Alternative Operating ConditionsThe efficiency of power generation by the fuel cell in this flowsheet is primarily linked to the partial pressure of hydrogen in the feed gas. (A full explanation of the fuel cell model is given in Appendix A.) According to the equilibrium of the steam reforming reactions, the partial pressure of hydrogen will increase with:• increasing temperature.

• decreasing pressure.

• increasing steam:carbon.

The targets for less severe operating conditions are the lowest pressurepossible and 650 or even 600°C. A series of flowsheets were run to examine what efficiencies are achieved and the effect of steam: carbon ratio. The results are given in Table 6.

27

Table 6. Results for Alternative Operating Conditions

Case Base 1 2 3 4 5 6 7 8 9

Operating Pressure (bara)

3 3 3 3 1.8 1.8 1.8 1.8 1.8 1.8

Reformer Temperature (°C)

750 650 650 650 650 650 650 600 600 600

Steam/Carbon Ratio 3 3 4 5 3 3.2 4 3 4 5

Air Stoichiometry (Fuel cell)

2 2 2 2 2 2 2 2 2 2

Air Stoichiometry (CR combustor)

1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1

Combustion Air Preheat Temp (°C)

600 600 600 600 600 600 600 600 600 600

Stack Efficiency (%) 59.2 59.2 59.2 59.2 59.2 59.2 59.2 59.2 59.2 59.2

Net Power Output(kW)

192 146 165 178 187 191 203 150 173 189

Electrical Efficiency(%)

39.5 34.7 39.2 41.1 44.3 44.7 44.9 35.6 40.9 44.9

Internal Heat Sufficiency

Yes - - - Yes Yes No Yes Yes No

Cases 1-3 were run at 3 bar, which is not the target pressure, but served to demonstrate that the shift in the equilibrium from reducing temperature could be reversed by increasing steam:carbon.

The remaining cases were run at 1.8 bara, which is the lowest pressure required in the Compact Reformer to push the gases through the rest of the system. Conservative pressure drops have been assumed throughout the system and so there is room to reduce this pressure further, but this should be regarded as a future benefit. Two sets were run: one at 650°C and one at 600°C. In both cases, at low steam:carbon there is an excess of fuel available in the anode off gas. This reduces the control available on the reformer and pushes up the exit temperature on the combustion side. This could be corrected by reducing the gas conversion in the reformer combustion (presently 90%) or by increasing steam:carbon. The latter increases the conversion to hydrogen, which is then used by the fuel cell and does not return as fuel gas to the reformer.

Increasing steam: carbon increases the demand for heat in the flowsheet and will, thus, be limited by the heat available. Table 6 shows where the system remained self-sufficient in heat available. If the system were not selfsufficient then more fuel would need to be burned to provide the heat, so reducing the overall efficiency.

28

5.4 Conclusions1. At a reforming temperature of 650°C, electrical efficiencies as high as 45% may be possible (subject to validity of fuel cell model).2. Reducing reforming temperature to 600°C leads to at least a 4% points reduction in the electrical efficiencies achievable.

29

6 MANUFACTURING EVALUATION AND PROVING OFCATALYST COATING WITHIN CHE HARDWARE

6.1 Manufacturing EvaluationCompact heat exchangers can be made by a variety of techniques [1]. The most advanced methods include fabrication of metallic shims by an etching process, which is akin to the methods used in making printed circuit boards. This allows for complex shapes of flow channels to be designed and constructed by building up several layers of the single shims. Once the shims are formed, they can be bonded either by brazing or by a diffusion bonding process. The latter is a preferred method for materials such as stainless steels and other alloys for high temperature duty. Chart Heat Exchangers are at the forefront of compact heat exchanger manufacture and have produced designs such as the Marbond™ reactor.

At the start of this project considerable discussion took place with Chart regarding the internal structure of the heat exchanger. This is governed by many factors such as the required degree of heat transfer, the materials of construction, the bonding method and the amount of catalyst needed (i.e. the coating thickness) to sustain the required reactions and achieve an adequate lifetime. The target heat fluxes of at least 10kW/m2 meant that the printed circuit design of heat exchanger based on stacked shims was the preferred option. The design subsequently adopted for the project was based on experience that Chart had obtained for other applications in achieving a good reservoir of catalyst within the coated channels of the heat exchanger hardware.

In the original project proposal, a cautious development programme was envisaged progressing from coating of coupons of heat exchanger hardware, through to testing in the “hybrid reactor” and then coating in one complete reactor. It was not intended to test the final reactor as a complete compact reformer demonstration. However, a decision was taken early in the project to move directly from coating of coupons and single-channel reactors to coating complete reactors. Skipping the hybrid reactor step (in which a mixture of coated catalyst and pelleted catalyst could be evaluated) posed some technical risk but this was felt worthwhile in terms of speeding up the general development. The high degree of success that has been achieved in coating has confirmed that this was the correct approach and has enabled the testing of three complete reactors.

Whilst in many ways it would be best to coat catalyst onto the shims before they are assembled and diffusion bonded, the high temperatures that are required for such bonding would place almost impossible requirements on the durability of each catalyst. Whichever approach is adopted, it is essential that the surface preparation of the shims allows for an adherent catalyst coating. In view of the timescale required for development of catalyst coating methods, the philosophy adopted for the present project was to assemble the complete compact reactor and then coat the surfaces after they have been diffusion bonded. The disadvantage of this approach is that it is necessary to ensure an

30

even flow of catalyst wash-coat or sol material through the very narrow channels so that a good uniform coating can be achieved. The realisation that this would be the case, has led to a major effort on development of catalyst coating techniques as described in Section 3. Before this could take place, many coupons of shim material were surface treated (24 methods of surface preparation were tested), in order to establish the preferred method of pretreatment. Despite the considerable time and effort that this took, the general conclusion was that the surface preparation after etching does not materially affect the adherence and durability of the catalysts compared with the other factors identified in the catalyst preparation.

The development of the catalyst coating method started with depositing material onto shim coupons. The adherence of the coatings could be tested and the thicknesses could be checked by using an eddy current device, backed up with electron microscopy. Once confidence had been achieved in coating the single shim coupons, the next stage was to coat the material in single open channels built up of heat exchanger shims. Finally a technique was developed for coating the tortuous channel of the preferred heat exchanger structure. The latter was tested in a “cold” reactor that consisted of shims that had not been diffusion bonded. This reactor could be disassembled once the coating had been applied and dried, so that uniformity of coating could be checked. It was also possible to re-use this test reactor many times. The final check of the viability of manufacturing was to check the performance of coated catalyst within small-scale complete reactors (i.e. reactors incorporating both reforming and combustion catalyst coated onto the preferred channel configuration). Since the most important tests were those on the complete reactor, the results of the single-channel diffusion bonded reactor are not reported here. The results from the final tests of the complete compact reactors are described below.

6.2 Testing of the Multi-channel ReactorThree reactor tests were carried out using the test facility built up at Newcastle University in previous projects. For each test the reactor blocks used were essentially identical with three combustion channels sandwiching two reforming channels. The reactor is shown in unmanifolded and manifolded versions in Figures 17 and 18. The reactor for the third run had narrower combustion channels than the other two reactors. The reactors were manufactured by Chart to a proprietary design that was adopted to meet the project requirements. All three reactors used the same catalyst combinations and the coating was carried out at Advantica. Co-current flow was used for all three runs. Sixteen thermocouples were located in each reactor block to measure the temperature profile during use.

31

Figures 17 and 18. Simple and manifolded versions of the diffusion bonded multi-channel compact reformer blocks

The intention was to run the reactors to achieve 90 % conversion on the reforming side with a 3:1 CH4 : H20 ratio with flow rates that would require a 10 kW/m2 heat input. The reforming heat requirement was calculated assuming that the shift reaction would approach equilibrium, reducing the heat of reaction from the reforming value of 206 kJ/mole to 185 kJ/mole. The total area for heat transfer between reforming and combustion was 40 cm2. The combustion flow rates were calculated to give 10 kW/m2 at 95% conversion, with a 5% excess air. In this case, in addition to the heat transfer surfaces to reforming there were 2 heat transfer surfaces to the external surface of the reactor, giving a total surface of 60 cm2.

These were the first runs to have been carried out with both combustion and reforming running concurrently continuously so there was a learning curve to follow in terms of the experimental set up. The first test failed to achieve any useful results but the second and third runs did demonstrate that both catalysts are capable of running continuously for over 100 hours.

The other major points to come out of the experimental work were that the combustion conversion is particularly sensitive to pressure and the reforming catalyst is stable if kept below 750°C.

The three runs are summarised below and more detailed histories given in the accompanying tables.

6.2.1 Run 1This was an exploratory run to establish conditions for achieving the desired reaction rates and determining how well matched the activities of the two catalysts were. It was also used to identify shortcomings with the feed and product recovery systems.

6.2.1.1 Start Up Procedure.The start up of all 3 runs was essentially the same.

The reactor was heated to 600 °C at 3 °C/min with a nitrogen flow on the reforming side and air (7.5 1/h) on the combustion side. The reforming catalyst was then reduced with hydrogen (2 1/h) for 16 hours before the flow

32

was increased to 5 l/h and the combustion air increased to full rate (59.3 l/h). These flow increases had little effect on the temperatures in the block.

Steam was introduced (40.4 ml/h of water) to the reforming side and after 5 minutes the reforming methane (16.8 l/h) was started. Immediately the methane flow had stabilised, the hydrogen was reduced to 2 l/h. Introduction of the methane caused an immediate drop in the block temperature due to the endothermic reforming reaction starting.

Unfortunately a steam generator problem caused an immediate shutdown of methane and water and the combustion airflow was reduced.

6.2.1.2 Continuation

A second start-up was made 3% hours later when the steam generator furnace had recovered. The reforming conversion quickly stabilised at 57% with the reactor block temperature 608 °C, before the combustion methane was started.

The combustion conversion was initially 86 %, rapidly dropping into the high 70s and the reactor temperature rapidly rose to 650 °C before dropping back to 640 °C

Starting combustion had very little effect on the reforming conversion, despite a marked effect on the temperatures of the reactor block. A similar effect was noted in the other runs and is best illustrated by the start of run 2 (see Figure 19) which went smoothly.

The furnace temperature was ramped to 700 °C At 3 °C/min causing the reactor to rise to 690 °C and the reforming conversion to rise to the mid 80s.

A further trip of the steam-generating furnace during the night caused the reforming side to run for a period without steam, resulting in severe deactivation of the reforming catalyst and the conversion was only 15%. The combustion conversion was 65% with a reactor temperature of 670 °C.

It was decided to continue to run combustion to and increase the temperature until the conversion was over 90%. (This also gave time to check on the steam generator problems1.) The furnace was ramped to 750 °C and the percentage conversion rose to the low 90s with the block temperature at 760°C. Over the next 24 hours the conversion dropped to 72% and the block temperature to 740 °C.

6.2.2 Run 2This run used a reactor identical to that in run 1, with the same catalyst combination.

1 Eventually after another trip, the controller was replaced and an interlock installed so that low temperature would trip both water and methane, leaving the reforming side with a low hydrogen purge.

33

6.2.2.1 StartUpReforming catalyst reduction was slightly different in that hydrogen (4 1/h) was put on at 400 °C instead of 600 °C. After 16 hours reduction the reactor was started up following the procedure followed in run 1. The effect on reactor inlet temperatures is well illustrated by Figure 21. This shows the temperatures over the first half of the block as first the reforming is started causing the temperature to drop by 30 °C followed by a sharp rise of over 50 °C to above the furnace temperature, showing that the combustion is supplying all the reforming heat requirements and losing some heat to the furnace. The figure also illustrates that there is little spread between the thermocouples, indicating an even distribution of the reaction between the channels. In contrast the changes in temperatures at the inlet end of the reactor are much larger than those towards the exit, showing that most of the reaction is occurring towards the inlet as would be expected for none-zero order kinetics.

The combustion conversion was over 99% but this caused the reforming conversion to rise from only 60 % to 71%. Clearly heat loss is a major problem with this size of reactor.

Figure 19. Temperatures at inlet end of block during start-up of reforming and then combustion

♦ 'Combustion Inlet C1 ■ 'Reformer Inlet R2


The run was continued for a total of 119 hours before air failure caused a premature end.

The salient features from this test were as follows:

34

• The combustion conversion was very pressure sensitive. Reducing the pressure from 0.75 bar to 0.025 bar after 6 hours on line dropped the conversion from 95% to 75%2.

• Below a furnace temperature of 750 °C, the combustion conversion was less sensitive to temperature than the reforming conversion.

• The initial combustion conversion was above 95 % due to the high pressure. After 6 hours when the pressure was reduced, the conversion was 75 % and gradually declined to below 60% over 48 hours despite raising the furnace temperature to 710°C.

• In contrast the reforming conversion rose from 70% to 86% as the furnace temperature rose and remained high.

• When the furnace temperature was raised to 770 °C after 48 hours, the combustion conversion rose quickly to 95% accompanied by a very rapid increase in the reactor inlet temperature. This is illustrated in Figure 22.

• The increase in temperature resulted in a reforming conversion of over 90%, but over the next few hours it declined rapidly to below 30%.

• The combustion conversion remained stable in the mid 90s until the run was terminated by an air failure after 119 hours.

The combustion catalyst appears remarkably robust. After the air failure caused coke deposition and a dP across the reactor of over 3.3 bar 3, which allowed only a very reduced airflow decoking was attempted. Starting with a furnace temperature of 400°C and gradually raising it to keep the CO2 in the exit gas below 5% eventually removed the blockage. Restoring the normal flow rates of air and methane gave conversions of over 90% initially, declining to 69 % at a reactor inlet temperature of 675°C after a further 50 hours on-line.

6.2.3 Run 3This reactor was slightly different from the previous ones in that, although it had the same heat transfer area, the combustion channels were narrower. The same catalysts were used.

Two major points appeared obvious from the previous run, namely the combustion conversion was very sensitive to pressure and the reformer catalyst to decay rapidly once the temperature rose above 750°C. The objective of this run was to try and keep the temperature below this level and use increased pressure to raise the combustion conversion to 90%. If necessary the reforming pressure would be raised to keep the conversion below 95%.

2 The exit pressure transducer was repaired and showed the high pressure, due to water condensing in the exit valve. Replacing this with a larger model and eventually a lute system enabled the pressure to remain low for the rest of this run.3 The limit of the pressure transducer range.

35

The unit had not been designed for continuous running at pressure on the combustion side so initially the reactor was run at atmospheric pressure outside normal working hours.

6.2.3.1 Start Up

The start up procedure was similar to that for run 2.


The details of the run are shown in Table 7. The run went relatively smoothly until unattended running at pressure was attempted for the first time. This allowed the backpressure to rise and reduce the air to combustion. Fortunately this occurred after 148 hours on-line.The salient points were:

• The combustion catalyst gradually decayed in activity at atmospheric pressure over the first 48 hours.

• Raising the pressure from atmospheric to 40 psig4 raised the conversion from 50% to 90%. This activity was reproduced whenever the pressure was at 40 psig until the run was terminated after 148 hours.

• The reforming catalyst maintained high activity throughout the 148 hours of the run.

Comments Hours on-line

Reactor assembled. Ramped to 400 °C at 3 °C / min with 10 l/h N2 on reforming side and 7.5 l/h air on combustion side.

N2 replaced by 4 l/h of H2 .

Ramping continued to 600 °C .

Water on at 41 ml/h. H2 flow reduced to 2 l/h

Water coming out of reactor. Methane on 16.8 l/h

Combustion methane on (5.9 l/h)

Combustion methane on. Reactor inlet up over 40 °C. First sample shows 100% conversion.

0:00

Furnace set to ramp to 650°C at 0.5°C/min 0:40

Reforming conversion 70% . Combustion 95% 1:10

Reforming 84.4%. Combustion 76.7%. Furnace now steady at 670°C. 19:05

Ramping to 700°C 21:55

4 Pressures within the test reactors were measured in psig (pounds per square inch, gauge pressure). 1 bar gauge pressure is equivalent to 14.7 psig.

36

Reforming 92%. Combustion 76.7% 23:00

Combustion pressure to 20 psig; conversion 74%. Reforming 87% 94:25

Combustion pressure to back to 0.5 psig; conversion 51%. Reforming87%

96:00

Combustion pressure 40 psig. Reactor inlet 687°C 82% conversion. 96:40

Combustion conversion 86.6% reforming 88% 97:00

Furnace set to ramp to 715°C at 0.5°C/min

Pressure down to 0.5 psig for the night. Combustion conversion 53.7%. Reforming 93%. Inlet 706°C.

98:55

All flows and temperatures steady reforming 87.4%. Combustion 56% 136:15

Combustion pressure up to 40 psig 137:05

Reforming 88%. Combustion 91.5% 137:15

Attempting to run overnight at pressure on combustion. 145:00

Reforming conversion 86.5%. Combustion 91.5%

Reforming conversion 87.5%. Combustion 90.5% 148:00

Table 7: Chronology of Run 3 showing that target of 100 hours operation was exceeded

37

7 CONCLUSIONSThe present project has led to a proof-of-concept Compact Reformer, which is a substantial advance on previous work. This is based upon novel compact heat exchanger technology with non-optimised catalysts for combustion and reforming. The main conclusions from the work are:

• Catalyst materials for promoting the steam reforming reaction and the combustion reaction can now be prepared and coated onto compact heat exchanger hardware.

• Catalyst compositions have been established which achieve short-term targets (>100 hours) for activity and durability (adhesion).

• Tests have shown that both catalysts are active enough to enable target power densities in excess of 10kW/m2 to be achieved.

• Micro-reactors have been established for testing of both steam reforming and combustion catalysts. A test facility at Newcastle University has been modified to allow the testing of complete compact reformer reactor blocks.

• Two reactor models have been developed which enable the temperature profiles within the compact reformer to be predicted for given operating conditions. There is good agreement between the two models, and the data generated by the models should help to establish the balance between the two reactions once more experimental data has been obtained for the catalyst activities.

• Methods have been developed for coating each type of catalyst into completely fabricated diffusion-bonded test reactors. Each reactor is built up of channels of 1mm width or less.

• Tests have been carried out in which both steam reforming and combustion have been demonstrated side by side. The diffusion bonded test reactors behaved satisfactorily as complete compact reformers.

Comparing what has been achieved with the original project objectives shows how far the project has evolved. Initially the objectives were concerned with proving the adherence of catalysts for periods of up to 1000 hours in single reactors with a demonstration of the principle in a hybrid reactor. With the support of ETSU, it was decided part-way through the project to proceed straightway to coating of compact heat exchanger hardware. This has been done at the expense of long-term testing, since long-term tests would be only of academic interest if the material could not be deposited within real heat exchanger hardware. We have in fact demonstrated that not only can we deposit catalyst material adherently within real heat exchanger hardware but also that durations of several hundred hours will be possible, for both catalysts. We believe that this approach has made best use of the time and resources available for catalyst/reactor testing, and has enabled us to move from a concept to a practical reactor.

38

8 RECOMMENDATIONSAlthough the project addressed the operating conditions of the reformer in a stationary PEMFC application, no provision was made in the present project for testing the proof-of-concept in a complete fuel processing system. Future work will need to address the issues of integration and control of the Compact Reformer in a complete fuel processing system. There are a number of key areas where progress needs to be made on the compact reformer unit itself, and these developments need to move in tandem with developments to the overall system if the UK is to remain at the forefront of this area.

8.1 Catalyst DevelopmentGreat strides have been made in understanding the nature of catalysts coated on stainless steel within the ‘proof-of-concept’ project, and in addressing issues affecting durability. This has culminated in the development of combustion and reforming catalysts that are suitably adherent and display outstanding activity in laboratory tests. Catalyst development now needs to continue, with several distinct lines of investigation

• Development of new CR catalyst contenders

• Catalyst durability substantially beyond that already achieved will be vital for development of a viable product

• Adhesion of the catalysts will be established under harsh conditions such as thermal cycling and the necessary development carried out should this be unsatisfactory with the current catalysts.

• Assessment of coating methods

• Catalyst coating has centred on washcoat technology thus far. Further approaches such as flame-assisted deposition and variants should be investigated. A key aim here will be to develop the means to coat the heat exchanger components before assembly.

• Development of catalyst reproducibility/control

• Study of the kinetics of the reforming and combustion reactions over favoured catalysts

• Establish envelope of carbon-free operation of favoured reforming catalysts

8.2 Manufacturing

The target of cost competitiveness for currently expensive PEMFC systems demands cost reduction in every part of the system. A key requirement of the CR is therefore that it must have minimal impact on system cost. Only in this way will PEMFC manufacturers be induced to embrace the technology. Issues

39

surrounding the cost and manufacturability of CR components need to be addressedin greater detail, in particular:

• Assess manufacturabilty of heat exchangers as a function of design concept and of material.

• Examine cost versus scale of various heat exchanger concepts.

• Develop methods for regenerating spent CR units.

8.3 ProcessDevelopment of reformer technology cannot be carried out in isolation from the rest of the PEMFC system since it will ultimately be required to integrate in order to obtain a maximised system efficiency. A further requirement is therefore to build on previous modelling studies to refine the operating parameters for the reformer both for incorporation into a stand-alone fuel processor and also as part of an integrated fuel cell system. It is also important to explore issues such as how to control the device. Tasks that will be required include:

• Study of operating parameters

• Study of start-up and shut down procedures

• Study of control and operation procedures

40

9 REFERENCES1. B.H. Cooke, Compact Reformer for the SPFC, ETSU Report F/02/0021/REP, 1996

2. P.S. Goulding, A. Gough, M. Deegan, Compact Reformer for the SPFC, ETSU Report F/02/00107/00/REP, 1998

41

Appendix A. Linking Catalyst Activity with Process Parameters

1 BACKGROUNDIn order for values of catalyst activity, obtained from the literature or experimentally measured in our own work, to be expressed in language meaningful with respect to the compact reformer, i.e. in terms of heat transfer rates in kW/m2, several simple spreadsheet tools have been developed. These calculate the average power density expected from an isothermal compact reformer channel, given the catalyst activity, coating thickness, required methane conversion, etc., and assume simple first order kinetics for both reforming and combustion sides.

Given the systems investigated so far1, these spreadsheets have been used to calculate resulting power densities based on activities measured in previous compact reformer projects. These results are specific to the systems in question 2.

Much more detailed modelling of the compact reformer channels will have to be done in future projects. This appendix provides a relatively simple analysis of the factors affecting the power density available from combustion and reforming catalysts and on the implications of the activities measured in this project.

System modelling is being done now to establish operating parameters for the compact reformer. An appreciation of the demands these parameters place on the catalysts is an important piece of information needed to supplement the results of these system studies.

2 INTRODUCTION

Assuming first order kinetics, rate equations describing the reactions are:

dRCH 4dt

= kc(T)• PCH for the combustion reaction

dPCH 4dt

= k (T )• Pcf P ' • P ^1 _ 1 H 2 1 COy -Kj- ' PCH4 ' 1 2O y

for the reforming reaction

where PX is the partial pressure of gas X, Kr is the reforming reaction equilibrium constant, and kc(T) and kr(T) are temperature-dependent rate constants.

42

Thef P 3 • P \1 _ ph2 pco

K • P • P^ Kr 1 CH4 PH2O yterm in the reforming reaction rate equation

accounts for the approach to equilibrium. At equilibrium Kr =f p 3 • pPH2 PCO

P • P ^ PCH4 PH 2O y

so that-----— vanishes. This term comes from the simple theory that Kr isdt

the ratio of forward and reverse reaction rate constants and appears frequently in reforming modelling work. It is not wholly satisfactory, being strictly applicable only for simple reactions where reaction order coincides with molecularity. In the absence of detailed rate measurements on the thin-film catalysts near to equilibrium, it has been adopted for this work. Failure of this approach, unless a drastic failure, will alter the calculated power densities significantly but not radically, because they tend to be dominated by high rates near the start of the channel when the gas composition is far from equilibrium.

The spreadsheet tools simply apply the integrated rate equations to a rectangular channel catalysed with a thin layer and assume:

• First order kinetics as described above

• The channel is isothermal

• Gas moves in plug flow

• The ideal gas equation is an adequate equation of state under compact reformer conditions

• No concentration gradients in the direction normal to the catalysed surface (implying a channel narrow in this direction)

• Shift reaction is at equilibrium at all times

• The enthalpy of reforming and combustion, and the enthalpy of the shift reaction are constant (a good approximation over the temperature range of interest)

• The catalyst displays intrinsic activity regardless of thickness (100% effectiveness factor)

For the resulting power densities to be practically useful, of course, the catalysed channel must be able to transfer the heats of reaction between the two sides of the compact reformer. This analysis can say nothing of this aspect.The results presented in this section are strictly for illustration of general trends, being based on arbitrary rate constants. Consequently, only the relative power densities are important, the absolute power densities being irrelevant.

43

3 CHANNEL DIMENSIONSIt is easy to show that in this simple scheme, the channel height is irrelevant to the power density for both combustion and reforming sides (provided the assumption of uniform concentration normal to the catalyst surface is valid). Consider a channel that achieves a given conversion at a certain flowrate, and more particularly, a thin (constant methane partial pressure) slice of the channel at the inlet. If the channel height is doubled, the residence time in the thin slice is doubled so that twice as many moles of methane are converted as the gas passes through the slice. But there is twice the volume of gas in the slice so that the impact on the partial pressure of methane remains constant. What’s true for the initial slice is true for all other slices along the channel so that the channel achieves the same total conversion at the same flow rate - constant average power density.

Similar arguments can be invoked to show that the channel width and length also have no impact on the average power density obtainable, from which the result follows that the channel shape too is irrelevant (though not to the power density per cubic meter).

4 EFFECT OF CONVERSIONFigure 1 shows that the first-order variation of a reactant along a channel is an exponential one, tending to zero. Note here that residence time is halved for each curve in the direction indicated in this Figure. From the rate equation the local rate, and therefore local power density, are also of similar form. The average power density for a given conversion takes the form of the integral of the curves in Figure 1 and is shown in Figures 2-5. For a given catalyst activity, the effect of reducing the conversion achieved by the channel (by reducing the residence time) is to maintain a higher rate throughout the channel and therefore a higher average power density. The steepness of these curves near equilibrium for the reforming reaction and near complete reaction

Figure 1. First-Order Variation of Partial Pressure Along a Channel (Combustion Reaction)

Dimensionless distance

44

for combustion highlights the trade-off between achieving a high average power density and achieving minimal breakthrough of methane.

45

5 EFFECT OF PRESSURE

Figure 2. Effect of Pressure on Combustion Side (T=650°C, 02:CH4=3, k=10000mol/s/m3/bar, 25micron coating)

Conversion (%)

The general influence of pressure on the average power density is shown for the combustion reaction in Figure 2. Regardless of conversion required power density is proportional to the pressure, a result that arises due to the increase in partial pressure of reactants. A similar trend is evident with the reforming reaction (Figure 3) only at low conversions, the attainment of equilibrium conversion, itself varying with pressure, becoming important at higher conversions.

Figure 3. Effect of Pressure on Reforming Side (T=650°C, s/c=3, k= lOOOOmol/s/m3/bar, 25micron coating)

i

1o-

I0 20 40 60 80 100

Conversion (%)

46

6 EFFECT OF STOICHIOMETRYThe ratio of O2CH4 affects the combustion side as shown in Figure 4. Lower power densities are observed for leaner mixtures, traceable to the dilution of the methane mixture with a reactant not featuring in the rate law. Again a similar trend is seen (Figure 5) for the reforming side only at low conversions, equilibrium considerations dominating at higher conversions.

Figure 4. Effect of Stoichiometry on Combustion Side (T=650°C, P=lbar, k= lOOOOmol/s/m3/bar, 25micron coating)

0 20 40 60 80 100

Conversion (%)

Figure 5. Effect of Stoichiometry on Reforming Side (T=650°C, P=2bar, k= lOOOOmol/ s/m3/bar, 25micron coating)

6

I

1

I

50

45

40

35

30

25

20

15

10

5

0 t------------------ 1------------------ 1------------------ r0 20 40 60 80 100

Conversion (%)

47

Appendix B. The Fuel Cell Model

The fuel cell model embodied in the flowsheet uses a simple approach to calculate its output. The power output from the fuel cell is given by:

Power = F#, Co?n% yew/ yew^

Where : Fh2 = Molar flow of hydrogenLHVh2 = Lower heating value of hydrogen Convn-, = Conversion of hydrogen in cell yew/ = Ce// e^9c/e?/cy yPower = Power conversion efficiency

Unfortunately, the cell efficiency is not known and so an alternative approach is used to obtain the power output and the above equation is then used to calculate the cell efficiency to check that it is reasonable.

From Ohm’s law, if the cell current and voltage are known then the power can be calculated according to:

P = VI

The cell voltage can be obtained from experimental data by assuming a current density for the cell. The data used [Ref 1] is shown in Figure Bl.

Figure Bl. Source Data for Cell Voltage [Ref 1]

PEMtn.Oj.'Hj

FHtyTOIElTHi MX- q.vrt * HC k Pi FILM

NAHM 1 IT

im ?» W *** Mfl m

CURRLW, bA'CITi2

A cell current density of 250 mA/cm2 was assumed and a cell voltage of 0.742 V was taken from the 30/30 curve in Figure Bl. Actual cell current may then be calculated from:

Cell current = 2 Fh2 Com>n2 Fa

Where : Fa = Faraday const.

48

Hydrogen conversion was taken to be 0.85 and the power conversion efficiency to be 0.95.

Reference

1. C. Derouin , R. Sherman, T. Springer, F. Uribe, J. Valerio, T. Zawodinski, M. Wilson, S. Gottesfeld , Fuel Cell Seminar 1990 Phoenix Arizona, proceedings, p 437.

49

assessment of advanced catalyst performance and

Documents