SULPHUR POISONING OF THE ACTIVE MATERIALS USED IN SOFCS

F/01/00222/REP URN 04/559

ContractorRolls-Royce plc

Prepared byR.H. Cunningham1, M. Fowles2, R.M. Ormerod3 and J. Staniforth3

1 - Rolls-Royce Fuel Cell Systems Ltd 2 - Synetix Ltd

3 - Keele University

The work described in this report was carried out under contract as part of the DTI New and Renewable Energy Programme, which is managed by Future Energy Solutions. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of the DTI or Future Energy Solutions.

First Published 2004 © Rolls-Royce plc 2004

dti

EXECUTIVE SUMMARY

Project objectivesAt the outset the project had a number of objectives each of which were crucial in the development of the Rolls-Royce Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) concept and in bringing it to a state of readiness for field trial demonstrations. The general objectives were:

• To determine the amount of sulphur that would lead to an instant loss of fuel cell performance.

• To determine what amount of sulphur would not lead to any damage.• To determine whether or not the damage caused by sulphur was reversible, and if

so how can the fuel cell performance be recovered.• To develop a strategy to protect the fuel cell from poisoning by: changing the

operating conditions; processing the natural gas before it enters the fuel cell; the use of off-the shelf catalysts.

• To determine the operational envelope in terms of minimising carbon deposition.

Summary of workThe project work was divided between four partners: Rolls-Royce as the lead partner was in charge of the project management. Rolls-Royce was responsible for:

• The manufacture and supply of their IP-SOFC modules for both in-house testing to the other partners.

• For defining the likely IP-SOFC operating conditions.• For in-house testing of more complex fuel cell arrangements.As the academic partner, Keele University was responsible for fundamental tests on small fuel cell arrangements to investigate the effect of sulphur poisoning on the anode, and sulphur and carbon poisoning on the reforming catalyst.

As a global business that develops, manufactures and sells catalysts and catalyst technology, Synetix was responsible for:

• Providing expertise and advise on catalysts.• Supplying small amounts of catalysts.• Providing advise on sulphur removal technologies.

In its role to promote and assist the efficient use of natural gas in France, Gaz de France (GdF) wishes to help manufacturers and users exploit SOFC technology. As such GdF’s role in the project was to:

• Test fuel cells under near commercial conditions.• Give advice on natural gas composition and commercial operation conditions.

Summary of main resultsAddition of hydrogen sulphide to methane / steam mixtures was found to cause deactivation of the reforming catalyst. The rate of deactivation was inversely proportional to temperature and directly proportional to hydrogen sulphide concentration. However even at a temperature of 1000°C and a total hydrogen sulphide concentration of 1ppm, the reforming catalyst suffered a 21% drop in activity after only 240 minutes. On cutting the hydrogen sulphide from the fuel supply

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the catalyst activity was found to increase almost immediately. This recovery process however was found to be dependent on temperature; at temperatures below 800°C sulphur poisoning was found to be irreversible.As with the reforming catalyst, the rate of deactivation of the fuel cell anodes resulting from exposure to hydrogen sulphide was found to increase with the concentration of sulphur added and decrease with temperature. Very little difference in sulphur tolerance was observed between the two anode formulations studied, although this was to be expected, as both were nickel-based. As with the reforming catalyst, the loss in catalytic activity of the anodes resulting from sulphur poisoning was found to be partially reversible at high temperature.

IP-SOFC modules operating on humidified hydrogen, doped with hydrogen sulphide suffered extremely rapid and severe deactivation when the hydrogen sulphide concentration was similar to that in UK natural gas: for a deactivation rate of 0.75% per 1000h (6.4% per year), the maximum permissible sulphur concentration in the fuel gas would be 18ppb. Cell performance was found to recover significantly when the hydrogen sulphide stream was stopped: >90% of the initial cell performance could be recovered following exposure to 2ppm hydrogen sulphide. With simulated fuel mixtures doped with hydrogen sulphide no degradation was observed as a result of sulphur addition over a period of 200h.

Problems associated with cell leakage encountered during the course of the programme prevented longer-term testing from being carried out, however the results obtained were sufficient to be used to calculate suitable methods of desulphurisation to meet the fuel cell requirements. Based on these results Synetix have recommended the use of a combination of zinc oxide and a copper-zinc oxide absorbent in the Rolls- Royce system to achieve the required level of desulphurisation. The use of zinc oxide as a desulphurisation method proved to be partly successful in reducing the degree of deactivation of the fuel cell anode material when fuel mixtures containing 5ppm hydrogen sulphide were used.

Tests showed that the module porous support material is partially active towards methane pyrolysis, but only at high temperatures; the small amount of carbon produced at high temperatures mainly comes from gas-phase pyrolysis. Results indicated that the support material should not be exposed to raw methane at temperatures above 850°C. The support material was however more susceptible to carbon formation when exposed to natural gas or higher hydrocarbons: significant carbon deposition occurs with natural gas at temperatures above 720°C. This demonstrates the need for pre-reforming in the Rolls-Royce system. The support material was found to be active for steam reforming and partial oxidation of methane at temperatures above 750°C, however no evidence of carbon formation was observed even when O/C ratios as low as 1 were used. Long term tests of the module support material demonstrated that the addition of oxidants to the fuel feed reduced the quantity of carbon deposited to approximately 3% of that produced when raw methane was exposed to the support at the same temperature.

Conclusions and recommendationsBoth the reforming material and the fuel cell anodes used in the Rolls-Royce IP- SOFC design are susceptible to poisoning through sulphur compounds in the fuel feed. The susceptibility increases with sulphur concentration, but the safe concentrations of sulphur that result in little or no deactivation are far lower than the

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concentrations typically found in natural gas. The results demonstrate a clear need for desulphurisation of the fuel to be included in the fuel cell system. The materials’ tolerance to sulphur does increase with operating temperature, but not enough to remove the need for desulphurisation. To keep degradation due to sulphur poisoning at an economically sensible level, sulphur concentrations no greater than 18ppb may be required at the fuel cell inlet, however this figure was however calculated from short-term tests performed using relatively high concentrations of sulphur. To improve our confidence in this figure, longer-term tests using lower sulphur concentrations should be performed; this should be accompanied by long term testing incorporating the suggested desulphurisation method

To prevent carbon formation on the support material, the ceramic components used in the IP-SOFC design should not be exposed to raw methane at temperatures greater than 850°C, and natural gas at temperatures above 720°C. The lower-temperature operating window for natural gas clearly demonstrates the need for pre-reforming in the Rolls-Royce system. Mixing anode off gas (AOG) with raw fuel reduces the rate of carbon formation on the ceramic components by a factor of 30. Following on from this programme suitable methods of natural gas pre-reforming should be investigated for incorporation into the system.

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CONTENTS

1 INTRODUCTION................................................................................................. 12 SULPHUR OPERATING ENVELOPE FOR THE IP-SOFC..............................4

2.1 Thermodynamic stability of sulphur compounds..........................................42.2 Effect of sulphur on the reforming catalyst...................................................42.3 Effect of sulphur on the anode.......................................................................72.4 Effect of sulphur on the current collector......................................................82.5 Tests on simple SOFC arrangements under load...........................................8

2.5.1 Humidified hydrogen.............................................................................82.5.2 Simulated operating conditions............................................................ 10

3 METHODS OF DESULPHURISATION............................................................123.1 Design of desulphurisation..........................................................................123.2 Testing of desulphurisation.......................................................................... 13

4 CARBON DEPOSITION ON MODULE SUPPORT........................................144.1 Background..................................................................................................144.2 Methane pyrolysis on ceramic support........................................................ 144.3 ^-Butane pyrolysis on ceramic support........................................................ 154.4 Pyrolysis of natural gas on ceramic support................................................15

5 REFORMING CHARACTERISTICS OF SUPPORT........................................165.1 Activity of support for methane steam reforming........................................165.2 Activity of support for methane partial oxidation........................................ 165.3 Activity of support for dry reforming.......................................................... 165.4 Prediction of likely operating conditions and validation of a model............ 165.5 Propensity for carbon formation under simulated operating conditions.....17

5.5.1 Experimental findings..........................................................................175.5.2 Comparison with thermodynamic predictions..................................... 17

7 CURRENT COLLECTOR & REFORMING CATALYST - CARBONTOLERANCE..............................................................................................................19

7.1 Current collector...........................................................................................197.2 Reforming catalyst....................................................................................... 19

8 CONCLUSIONS..................................................................................................20

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1 INTRODUCTIONRolls-Royce has been developing its Integrated Planar Solid Oxide Fuel Cell (IP- SOFC) technology since 1993. The aim is to bring the IP-SOFC to a state of readiness for field trial demonstrations of single-system modules by gas and electricity utilities by 2007. This requires scale-up in both stack rating and component production volume, simultaneously achieving reliability, durability and performance targets commensurate with this aim. The IP-SOFC technology consists of an array of series connected fuel cells fabricated on a porous ceramic support tube. This basic unit is known as a module, which is the fundamental building block of the SOFC stack. Individual modules can be manifolded together to form a bundle, and these can be further manifolded to eventually build up a multi-kilowatt stack.

Two factors that affect the reliability, durability and performance of the IP-SOFC modules are sulphur poisoning and carbon deposition. Sulphur poisoning and carbon deposition both exert a strong influence on the reliability, durability and performance of the IP-SOFC modules. However, fuel processing adds both hardware and operating costs, and impacts on system design.It is generally accepted that some measure of desulphurisation is required when operating SOFCs on sulphur bearing hydrocarbon fuels. The published results of the Siemens-Westinghouse demonstration in The Netherlands show that SOFC fuel desulphurisation using activated carbon is not an appropriate technique [1] . The demonstration initially used a simple desulphurisation unit, leading to sulphur poisoning of the anodes with a performance loss of the fuel cell stack. It therefore proved necessary to replace the activated carbon with a higher quality material. The replacement activated carbon contained heavy metal additives and had to be replaced every six weeks, creating special waste and introducing high maintenance costs.

The Rolls-Royce design differs from that of Siemens-Westinghouse and will more than likely be operated under different conditions that lend themselves to other means of sulphur removal. Although information exists in the open literature regarding sulphur effects in other SOFC designs, it was recognised that Rolls-Royce could not take forward its own development and demonstration programme simply by relying on open literature results from investigators on other systems and materials. For example, it is well known that temperature plays an important role in the sulphur poisoning of nickel based reforming catalysts [2] .Before fixing a method of desulphurisation it is therefore imperative to determine the operating envelope of the IP-SOFC module with regard to sulphur poisoning, as this will influence the choice of desulphurisation method.

The effect of sulphur addition to the fuel gas was investigated over the reforming catalyst. The catalyst, supplied by Synetix had been developed in a previous DTI/EPSRC ACCP LINK project: “Development of an Internally Reforming SOFC Module”. The tolerance of the catalyst towards sulphur was investigated over a range of temperatures and sulphur concentrations using methane steam reforming as a model reaction to determine the catalyst’s activity.

[1] J. Sukkel, in: Proc. 4th Eur. Solid Oxide Fuel Cell Forum, ed. A.J. McEvoy, 2000, Vol. 1, p159[2] M.V. Twigg, in: Catalyst Handbook (Second edition), Chapter 4, Wolfe Publishing Ltd,

London (1989)

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Fundamental studies of the sulphur tolerance of two Rolls-Royce anode formulations were performed over a range of temperatures and sulphur concentrations. These were performed on the powdered anode without any electrical load current. As with the reforming catalyst, methane steam reforming was used as a model reaction to quantify any loss in activity. The ability of the anodes to recover from sulphur poisoning was also quantified. A limited study was also performed on the current collector’s tolerance to sulphur.The tolerance of the fuel cells to sulphur was also measured using IP-SOFC modules under electrical current loads. Sulphur tolerance was investigated for fuel cells operating under humidified hydrogen and simulated fuel mixtures representative of the predicted operating conditions of the Rolls-Royce system. The fuel-gas composition was determined for different operating conditions using the Rolls-Royce IP-SOFC global model previously developed during the LINK programme. From these test results the level of desulphurisation required to achieve the maximum tolerable fuel cell deactivation rate was calculated. The results were then used to devise practical methods for desulphurisation.

The main sulphur compounds present in natural gas are organo-sulphur compounds added as odourants. The concentration and type of odourant used varies from country to country, and the concentration can vary quite significantly over time. Data were collected on the sulphur levels and types of odorants used in a number of countries.

For the experimental work carried out in the project hydrogen sulphide addition to the fuel gas was used to represent the sulphur concentrations present in natural gas. Prior to this, thermodynamic calculations were performed to verify that, at the predicted SOFC operating temperature (850-950°C), typical stenching agents would readily decompose to hydrogen sulphide and hydrocarbons.In addition to sulphur issues it was also necessary that Rolls-Royce extended its studies on the effects of carbon deposition in the IP-SOFC design. The design has four areas where carbon deposition from hydrocarbon fuel mixtures is a potential problem: the inert ceramic support, the internal reforming catalyst, the anode, and the pre-heating arrangement. This part of the project built on the LINK project, however carbon deposition on the reforming catalyst and the support material had not been previously investigated in the LINK programme.

The tolerance of the anodes towards hydrocarbons was already covered in the previous LINK programme, and Keele University had measured carbon deposition on different anode formulations for a range of operating conditions. In this report, the susceptibility of the ceramic support and the reforming catalyst to carbon formation are reported. A number of parameters affect the propensity for carbon formation including the operating temperature and the fuel to oxidant ratio.

The effect of higher hydrocarbons present in natural gas also had to be considered as a major issue, in addition to establishing an operation envelope for avoiding carbon deposition for methane over both the reforming catalyst and the support materials. This has to be done over a range of operating conditions (temperature and methane / oxidant ratio). Pyrolysis of natural gas components can also lead to carbon formation, and is of particular interest with respect to the fuel preheating and oxidant recirculation arrangement in the Rolls-Royce system. Pyrolysis characteristics were therefore of major importance in this project. Material choice and operating temperature may necessitate fuel pre-reforming to remove higher hydrocarbons, but

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as with sulphur removal this adds cost and system complexity. The operating envelope where carbon deposition can be avoided therefore must be determined for the IP-SOFC system.

The susceptibility to carbon deposition of the SOFC support material and reforming catalyst was measured at various temperatures and fuel compositions. These results were used to predict a suitable operating window to prevent carbon build up. In addition to methane the susceptibility of the support material to carbon formation was measured with natural gas and ^-butane, the role of the ^-butane was to represent the higher hydrocarbons present in natural gas and to quantify the relative propensity of each fuel gas to form carbon.

Thermodynamic calculations verified the assumption that within the predicted fuel cell temperature-range the organo-sulphur compounds used as stenching agents all readily decompose to hydrogen sulphide. This result enabled the use of hydrogen sulphide for sulphur tolerance tests in the programme, thus simplifying equipment design.

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2 SULPHUR OPERATING ENVELOPE FOR THE IP-SOFC

2.1 Thermodynamic stability of sulphur compoundsThe main sulphur compounds present in natural gas are organo-sulphur compounds added as odourants. However, the concentration and type of odourant used varies from country to country, and the concentration can vary quite significantly over time. Data relating to this is given in Table 2.2. The compounds that are mainly used are: dimethyl sulphide (DMS), diethyl sulphide (DES), ethyl mercaptan, f-butyl mercaptan and tetrahydrothiophene (THT). In the United Kingdom a mixture of ethyl mercaptan and DES is mainly used, while across Europe the main odorant is THT. In the USA the main odourant used is f-butyl mercaptan. Thermodynamic calculations using Simsci PRO/II demonstrated that at the typical SOFC operating temperature (~900°C) all organo-sulphur compounds used in natural gas decomposed to hydrogen sulphide and carbonyl sulphide. Table 2.1 shows the results for the thermal decomposition of organo-sulphur compounds in a helium carrier stream and in a gas mixture typical of the Rolls-Royce SOFC fuel-supply-tube (FST) inlet composition.Table 2.1 - Thermal decomposition of 200ppm organo-sulphur compounds at 900°C in helium carrier gas and simulated fuel mixtureCompound % Decomposition

Helium carrier Simulated fuel mixture (20%CH4, 10%CO, 15%CO2, 15%H2, 40%H2O)

DES 100 100DMS 100 100Ethyl mercaptan 84.6 100f-Butyl mercaptan 15.5 100Thiophene 0.5 100THT 100 100

These results impacted on the level of complexity of the subsequent studies to determine the sulphur tolerance of the IP-SOFC components. At typical fuel cell operating temperatures hydrogen sulphide was representative of the sulphur additives commonly used in natural gas.

2.2 Effect of sulphur on the reforming catalystThe reforming catalyst used in the study was a precious metal supported on the Rolls- Royce ceramic substrate in the form of extruded rods, and had been developed in the previous LINK programme. Methane steam reforming was used as a model reaction to determine the catalyst’s activity and tolerance to sulphur. The catalyst activity was measured over a range of temperatures from 700-1000°C and a range of sulphur concentrations ranging from sulphur-free fuel to 20ppm sulphur. The steam/methane ratio used for the study was 1.0. The reforming catalyst underwent deactivation at all temperatures except 1000°C even when no H2S was present in the gas stream. This is indicative of carbon formation caused by the low steam/methane ratio (1.0) used for these tests. At the time when these tests were carried out in the Keele University test rig it was not possible to use a higher steam/methane ratio. However it is clear from the results (Table 2.3) that the introduction of H2S into the gas stream did increase the rate of catalyst deactivation. Temperature also had an influence on sulphur tolerance, with the reforming catalyst being more tolerant to sulphur at higher temperatures. However even at 1000°C, the lowest concentration of H2S (1ppm) led to a 21% drop

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Table 2.1 - Odour addition to natural gas across EuropeFrance United Kingdom Germany Belgium The Netherlands Italy Spain Denmark

Regulations gas distributed: odour level 2 for gas

< 1% in airgas transported:

odour level 2 for gas < 1% in air

gas distributed: odour level 2 for gas

< 1% in airgas distributed:

odour level 2 for gas < 1% in air

gas distributed: odour level 2 for gas

< 1% in airgas distributed:

odour level 2 for gas < 1% in air

gas distributed: odour level 2 for gas

< 1% in airgas distributed:

odour level 2 for gas < 1% in air

Location of odouraddition

centralised atdistribution points

for gastransportation

centralised at distribution points

for gastransportation

localised at points of distribution for public supply

localised at points of distribution for public supply

localised at points of distribution for public supply

centralised at gas terminals and

complimented at points of distribution

for public supply

localised at points of distribution for public supply

Additivecomplimentary to

existing odouryes yes no no no no no no

products THT Odorant BE THT THT(Scentinel E)

THT THT(Scentinel E)

THT THTIndex of odour

intensityodour index

mg THT/m3 gasodour intensity - - - - - -

Relationship of odour

linear formula (GdF) logarithmic formula(BG)

- - - - - -Nominal amount of

odour (mg/m3 gas)

25(equivalent THT)

16(Odorant BE)

15 20(6)

18 >32(>8)

15 + 10 10-15Permitted limits

(mg/m3 gas) 15 - 40 9 - 60 8 - 30 17 - 34(5.4 - 7.1)

10 - 36>32 (>8) to ? (level not exceeding odour

level 3)8 - 30 7.5 - ?

Type of control analysis analysis smell and analysis smell and analysis smell and/or analysis smell and/or analysis smell followed by analysis

analysis of THTLocation of control on-line, downstream

of odour additionon-line, downstream

of odour addition and distribution

at distribution points at distribution points at distribution points downstream of major odour addition

and at distribution points

downstream of major odour addition

and at distribution points

at points of odour addition and at

distribution pointsFrequency of

controlon-line

every 6 secondson-line

every 6 secondsperiodically

> every 3 monthson-line or

periodically > every 6 months

Electronic alarms yeslocalised and central

yes - - - - - -

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in catalyst activity over a period of only 240 minutes. The conclusion that can be drawn from these experiments is that the concentration of sulphur in the fuel gas entering the reforming catalyst would have to be far lower than 1ppm for a commercial system.

Table 2.3 - Drop in activity of reforming catalyst during methane steam reforming as a function of temperature and hydrogen sulphide concentration

Temp. / °C H2S concentration / ppm

% deactivation (relative to max. conversion)

700 0 261 1002 1003 >96

800 0 461 492 >913 >994 100

900 0 581 552 283 224 415 98

1000 0 <11 212 263 254 245 53

20 67The reversibility of the sulphur poisoning effect was investigated by cutting the hydrogen sulphide supply in the fuel supply at 1000°C. The activity of the catalyst was found to increase almost immediately (Figure 2.1) indicating that the poisoning effect of the H2S is reversible at high temperature. Below 800°C the sulphur poisoning effect was found to be irreversible.

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Figure 2.2 - Recovery of catalyst activity following sulphur exposure

2.3 Effect of sulphur on the anodeShort-term measurements of the anode’s tolerance to sulphur were made in a small- scale test rig using methane steam reforming as a model reaction to measure the anode’s activity. A steam/methane ratio of 2.0 was used for these tests. Two anode formulations were tested: Anode 13 and Anode 19. Figure 2.3 shows the drop in methane steam reforming activity for the two anode formulations at three different temperatures.

• 1ppm• 2ppm• 5ppm O- 1ppm O- 3ppm• - 5ppm

Temperature/°C

Figure 2.3 - Deactivation of Anode 13 after 240 minutes hydrogen sulphide exposure during methane steam reforming (filledpoints Anode: 13; non­filled points: Anode 19)__________________________________________

At low sulphur concentrations the rate of deactivation passes through a maximum at 950°C; at higher concentrations the rate of deactivation is inversely proportional to temperature. In general Anode 19 is more susceptible to sulphur poisoning than

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Anode 13. The results clearly show however that there is very little difference in sulphur tolerance between the two anode formulations and the deactivation rate is rapid for both anodes.Some of the anode’s activity could be recovered following sulphur poisoning if the exposure to sulphur is stopped. Table 2.4 shows the recovery of Anode 13 following exposure to 2ppm sulphur for 120 minutes.

Table 2.4 - recovery of Anode 13 following exposure to 2ppm sulphurTemperature / °C % recovery after 90 minutes

950 631000 70

In addition to the anode material being more stable at higher temperatures, the results demonstrate that more of the anode’s activity can be recovered if the sulphur contamination is ceased. In conclusion, high temperature operation favours a greater tolerance to sulphur contamination.

2.4 Effect of sulphur on the current collectorShort-term tests of the current collector’s tolerance to sulphur were performed. Two current collector formulations have been measured: CC-A and CC-B. Both were used to perform methane steam reforming, which was again used to measure their activity, using a steam/methane ratio of 2.0 at a temperature of 900°C for 19h.The addition of 2.4ppm H2S to the fuel stream resulted in a 35% drop in activity of CC-A; there was no discernable drop in performance for CC-B even when the concentration of H2S in the fuel stream was 5.5ppm. From these tests it appears that one of the current collector formulations is more tolerant to sulphur than the other. Although this may be the case, the use of CC-B would not of course eliminate the need for sulphur removal in the final system, as it has already been proven that the SOFC anode materials are extremely intolerant to sulphur.

2.5 Tests on simple SOFC arrangements under load

2.5.1 Humidified hydrogen

As a consequence of the proposed anode off gas (AOG) recycling in the Rolls-Royce design, the concentration of sulphur in different parts of the system will vary. The Rolls-Royce global model for predicting fuel supply tube (FST) inlet composition - essentially the inlet to the reformer unit, FST outlet composition and AOG outlet composition was used to calculate the flow rates, and sulphur concentrations for typical operating conditions. Table 2.5 presents the sulphur concentrations at various points in the system for typical operating conditions. The concentration of hydrogen sulphide in the raw fuel represents the highest European level (8.2ppm).A standard 7-cell module was tested under humidified hydrogen at 900°C. Under a total flow of 3l.min-1 the module produced a maximum of 12.2W at a total load current of 3.4A. Under a current load of 2.7A, the module power was 11.4W. Following initial characterisation the module was left under a constant current load (1.7A) for a period of 80 hours; no significant change was observed in the total module voltage during this period.

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Table 2.5 - Predicted sulphur concentrations within an SOFC running under typical operating conditions on European natural gas with 8.2ppm sulphur.

Flow rate / mlmin-1 [H2S] / ppm H2S/H2 flow rate / ml min-1

Raw fuel 408 8.2 33.5AOG recycle 1614 2.7 44.1FST inlet 2022 3.8 77.6FST outlet 2837 2.7 77.6AOG outlet 2838 2.7 77.6Exhaust 1224 2.7 33.5

The module was subjected to two short durability tests under humidified hydrogen, doped with hydrogen sulphide: in the first instance 2ppm, in the second 3.3ppm. Following each test the sulphur contamination was stopped and the fuel cell recovery was measured. Following exposure to 2ppm hydrogen sulphide, 94% of the fuel cell performance could be recovered after 20h operation on humidified hydrogen; following exposure to 3.3ppm hydrogen sulphide, 98% of the performance was restored following a 3h treatment with humidified hydrogen. The results are presented in Figure 2.4. The data were fitted to a first-order rate expression (1) to obtain the rate constant for the deactivation process and the half-life of the anode upon exposure to sulphur.

lnV = - kxt (1)V0

Where: V0 and Vt represent the initial module voltage and the module voltage at time, t respectively; and kx represents the rate constant for the anode deactivation with a sulphur concentration of xppm. The results are summarised in Table 2.6.

Within the European-funded CORE-SOFC programme, a degradation rate of 0.75% per 1000h (6.4% per year) is set as a target. From the kinetic measurements it was calculated that this degradation rate corresponds to a rate constant of 7.53x10"6h-1. From the data presented in Table 2.6 it was calculated that in order to achieve this level of degradation the concentration of hydrogen sulphide in the anode flow stream must not exceed 6ppb; this is equivalent to 18ppb in the raw fuel.

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S’ 3.8

Elapsed time / h

Figure 2.4 - Durability test under sulphur-laced humidified hydrogen

Table 2.6 - Rate constant for deactivation, and anode half-life for 7-cell module operating on sulphurised humidified hydrogen.

[H2S] / ppm ki / h'1 tw / h2.0 2.19 x 10"2 31.63.3 4.35 x 10"2 15.9

2,5,2 Simulated operating conditionsA 7-cell SOFC module was operated on a fuel mixture representing the gas composition of the FST outlet in a system. Following initial IV measurements and an initial stabilisation period under a flow of 31.min'1 humidified H2, the module was operated under a gas mixture representing predicted operating conditions for the Rolls-Royce system. The fuel supply had the following approximate composition: hydrogen, 45%; carbon monoxide, 25%; steam, 20%; carbon dioxide, 10%. A plot of the voltage as a function of time on test is given in Figure 2.5.

Initially 0.2ppm hydrogen sulphide was added to the fuel mixture; this was equivalent to l.Sppm in the raw fuel (equivalent to a lower European sulphur level). The module was initially operated under a current load of 1.0A. During the first few hours of the test the module’s performance was found to increase. This was followed by a temperature cycle, after which the module was subjected to a higher concentration of hydrogen sulphide (0.6ppm - equivalent to 4.3ppm in the raw fuel, the UK natural gas level). With the exception of a 6% drop in performance during the first 30 minutes of the test, the cell performance was found to increase over time such that the current load was gradually increased from 1.0A to 1.7A.The module performance under a simulated reformate mixture was completely different to module performance under humidified hydrogen. However, the module run on humidified hydrogen was operated under a total load current of 1.7A, which is closer to the predicted operating load current in the real system. Initial poorer performance of the module running on reformate forced the use of a lower current density, which may have protected the module from the rapid loss in performance. In

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addition, a large quantity of steam was present in the fuel supply; this may have facilitated the formation of sulphur dioxide and prevented the build up of sulphur on the anodes.

4.3 ppm in methane (standard UK level) (0.5 ppm total)

3> 0.60

> 0.58

1.8 ppm in methane (lowest European level)

(0.2 ppm total)

Elapsed time / h

Figure 2.5 - Performance from a 7-cell module operated on sidphur-doped reformate representing simulated operating conditions__________________

Problems associated with cell leakage encountered during the course of the programme greatly limited the number of modules that were suitable for sulphur tolerance tests. Several modules were in fact tested, however only the two described in this report had satisfactory performance that allowed sulphur tolerance and durability measurements to be performed.

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3 METHODS OF DESULPHURISATION

3.1 Design of desulphurisationThe results described in section 2.4 have shown that the SOFC anodes are extremely intolerant to sulphur. Depending on the operating conditions the concentration of sulphur in the feed gas may have to be less than 18ppb if a degradation rate of less than 0.75% per 1000h is to be realised. Based on this requirement a suitable desulphurisation method has been devised by Synetix that is required to achieve a one-year life.

The process conditions assume: a mean sulphur concentration of 4.3ppm in the raw fuel (UK natural gas); a required sulphur level of less than 18ppb; a raw fuel flow rate of 33g.s-1, natural gas (157m3.h-1); and a lifetime of the unit of one year.Lighter feeds such as natural gas contain sulphur in the form of hydrogen sulphide, mercaptans and simple sulphides. In addition, pipeline natural gas may contain additional stenching agents. Ultra-low levels of desulphurisation are best achieved using a combination of hydrodesulphurisation (HDS) and absorption by a bed of zinc oxide and a copper-zinc-alumina oxide. HDS converts all sulphur containing species to hydrogen sulphide, which are removed by absorption by a bed of zinc oxide. The copper-zinc material is more expensive and has a lower sulphur capacity than zinc oxide, but in a correctly designed bed is able to achieve lower sulphur levels due to a combination of kinetic and thermodynamic equilibrium reasons. Therefore, the copper absorbent is used to ‘polish’ the gas at the exit from the zinc oxide bed to enable the target sulphur slippage to be met.

It is envisaged that HDS and absorption is carried out in one vessel operating at a single temperature to simplify the overall flow sheet. The natural gas is delivered via a compressor; so two designs have been calculated (at 1bar and 5bar) to encompass the likely operating envelope. The bed volumes required to purify the raw fuel to a 1MW system are given in Table 3.1 together with an estimated cost for the catalysts for the pressurised case.

Table 3.1 - Inlet gas purification designDesign pressure / bar

1 5Operating temperature / °C 350 350HDS volume / m3 (Katalco 61-1) 0.13 0.06ZnO volume / m3 (Katalco 32-4) 1.4 0.28Cu-ZnO volume / m3 (Puraspec 2084) 0.35 0.7Bed AP / mbar 3.4 1.4

Cost of pressurised purificationTotal cost Cost per kW£10,000 £10/kW

It is usual to add some hydrogen to the feedstock before desulphurisation. This is done to provide hydrogen for the conversion of organic sulphur compounds and to inhibit cracking of the hydrocarbon feed on both the HDS catalyst and the feed pre-heater surfaces. For natural gas feeds it is recommended to operate with between 2-5% hydrogen in the hydrocarbon flow, although operation with no additional hydrogen is also possible.

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A source of hydrogen is required to reduce the copper absorbent during start-up. The catalyst should not be exposed to oxidising conditions during normal operation (including start-up, shutdowns and trips). If these operating conditions cannot be readily met, then operation without a copper absorbent should be considered. Synetix can only guarantee sulphur levels of 0.1ppm with a zinc oxide bed, although measured levels of sulphur are usually significantly lower. Activated carbon absorbents will not achieve much less than 0.1 ppm, especially with non hydrogen-sulphide species present in the natural gas.

To achieve ultra low levels of sulphur (<18ppb) the recommended temperature for the desulphurisation is 350°C together with the use of a copper absorbent. If this temperature cannot be achieved, there is some scope for reducing the temperature of operation (to a minimum of 250°C) but the sulphur levels achieved will then depend critically on the sulphur species present in the natural gas.The estimated cost for the desulphuriser unit is £10/kW for a 1MW system, assuming that the desulphuriser unit would be replaced annually. This cost is affordable, and in any case has to be met, as sulphur tolerance measurements show that the consequences of inadequate removal of sulphur impurities from the fuel would be extremely serious for the operation of the fuel cells.

3.2 Testing of desulphurisationSimple tests were carried out at Keele University where zinc oxide was used as a sulphur removal catalyst. The introduction of zinc oxide upstream of the reforming catalyst led to a delay in catalyst deactivation. Due to limitations in the test rig, only a small mass of zinc oxide could be tested, however the results suggested that the zinc oxide had an initial beneficial effect that was gradually overcome by the hydrogen sulphide. It is not clear why this occurred during the relatively short test (20h) as calculations indicated that saturation should not have occurred for at least 8000h. Literature suggests [3] , at an operating temperature of 400°C zinc oxide is capable of reducing an initial hydrogen sulphide concentration of 3250ppm to a final concentration of 30ppb. It was acknowledged that the conditions for this test were not ideal, and it was postulated that side reactions, such as the formation of carbonate limited the life of the zinc oxide. 3

[3] W.F. Elseviers and H. Verelst, Fuel, 78, 601 (1999)

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4 CARBON DEPOSITION ON MODULE SUPPORT

4.1 BackgroundIt is important to fully understand the susceptibility of the ceramic support material to carbon deposition when exposed to hydrocarbon mixtures at high temperature. This will determine the temperature at which the ceramic support could be exposed to raw fuel without significant coking. Natural gas predominantly consists of methane, however, higher hydrocarbons are also present. The activity of the support material towards these higher hydrocarbons also had to be investigated to evaluate the necessity for pre-reforming of the fuel in the final system. The typical composition of North American natural gas is given in Table 4.1.

Table 4.1 - Typical composition of natural gas [4]Component Typical analysis

mol% range (mol%) mass%methane 94.9 87.0 - 96.0 90.2ethane 2.5 1.8 - 5.1 4.4propane 0.2 0.1 - 1.5 0.5Ao-butane 0.03 0.01 - 0.3 0.1M-butane 0.03 0.01 - 0.3 0.1Ao-pentane 0.01 trace - 0.14 0.04M-pentane 0.01 trace - 0.04 0.04>hexanes 0.01 trace - 0.06 0.05nitrogen 1.6 1.3 - 5.6 2.6carbon dioxide 0.7 0.1 - 1.0 1.8oxygen 0.02 0.01 - 0.1 0.04hydrogen trace trace - 0.02 trace

This task was carried out jointly between Rolls-Royce and Keele University: short­term tests were performed at Keele on samples of the support material, and longer- term tests were performed on inactive IP-SOFC modules at Rolls-Royce; all post-test analysis was performed by Keele University.

4.2 Methane pyrolysis on ceramic supportShort-term tests indicated that the ceramic support material was found to be inactive to methane pyrolysis below 850°C. Isothermal tests and subsequent TPO to quantify any surface carbon revealed that at 900°C, 0.2% of the methane passed over the support material was converted to carbon; this increased to 1.0% at 1000°C. From the temperature-programmed experimental data the activation energy for methane pyrolysis was calculated as 502kJ.mol-1. This value is exceptionally high suggesting that at this temperature it is mainly gas-phase pyrolysis that is taking place rather than a surface-catalysed reaction.Longer-term testing on inactive IP-SOFC modules was carried out for 146 hours at 900°C under a flow of 25%CH4/N2. Under these conditions approximately 3% of the methane passing through the modules reacted. Following test the modules were dissected and subjected to TPO analysis to quantify any surface carbon. Seven sections of each module were analysed; carbon was fairly evenly distributed throughout the module. The quantity of carbon deposited on the support was 84 ±

[4] http://www.uniongas.com/NaturalGasInfo/AboutNaturalGas/composition.asp

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38mgcarbon.g-1support. Although this appears to be a very small quantity it should be noted that this accumulated after just one week; over the course of one year the total quantity of carbon deposited could be quite significant.It is clear from these results that the ceramic support material should not be exposed to raw methane at temperatures above 850°C.

4.3 rn-Butane pyrolysis on ceramic supportTo represent higher hydrocarbons in natural gas M-butane pyrolysis was measured over the ceramic support material. The rate of carbon formation at various temperatures is given in Table 4.2 for an M-butane partial pressure of 0.078bar.Table 4.2- Rate of carbon formation resulting from n-butane pyrolysis over ceramic support materia

T / °C Carbon formation /-1 h-1mgcarbon.g support.h

700 4.05800 28.6900 2491000 552

When higher hydrocarbons are present in the gas stream the rate of carbon formation is significant. Assuming that the rate of carbon formation is unaffected by time, extrapolation of these results indicates that even at 500°C there would be approximately a 10% increase in the mass of the ceramic components as a result of carbon deposition in one year. This clearly demonstrates the need for pre-reforming of the raw fuel prior to contact with any heated support surfaces.

4.4 Pyrolysis of natural gas on ceramic supportDesulphurised natural gas is found to react over the ceramic support material at temperatures above 720°C. This is significantly lower than the 850°C measured for methane, and is presumably a result of the higher hydrocarbons; these have been shown to be more reactive on the support material (section 4.3). Pyrolysis of natural gas resulted in carbon formation, and isothermal tests carried out at 900°C and 1000°C showed this to be significant. A 24-hour test carried out at only 600°C gave rise to carbon even although at that temperature the rate of pyrolysis was almost immeasurable.

Undoubtedly the predominant reaction at this temperature is the pyrolysis of higher hydrocarbons, and this confirms the conclusion drawn in section 4.3 for the need to pre-reform the raw fuel in any working SOFC system.

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5 REFORMING CHARACTERISTICS OF SUPPORT

5.1 Activity of support for methane steam reformingThis work presented in sections 5.1 - 5.3 was carried out by Keele University. The rate of methane steam reforming was measured as a function of temperature for a steam/methane ratio of 1.0. The ceramic support was active at temperatures above 850°C with the main products being hydrogen and carbon monoxide, with trace quantities of carbon dioxide. Tests carried out isothermally between 900 - 1000°C followed by TPO analysis showed no evidence of carbon deposition.

5.2 Activity of support for methane partial oxidationThe rate of methane partial oxidation was measured as a function of temperature for an oxygen/methane ratio of 0.5. The ceramic support was active at temperatures above 850°C with the main products being carbon monoxide and steam, indicating that in addition to partial oxidation, methane combustion and the water-gas shift reaction (WGSR) are also significant reactions. Short-term tests (each of three hours) performed isothermally between 800 - 1000°C showed no evidence for the formation of surface carbon.

5.3 Activity of support for dry reforming

The rate of methane dry reforming on the ceramic support was measured as a function of temperature. The reaction only occurs at temperatures above 900°C, and even at 1000°C the activity of the support material for this reaction is extremely low.

5.4 Prediction of likely operating conditions and validation of a modelThe Rolls-Royce IP-SOFC global model previously developed as part of the DTI- funded LINK programme was used to predict the fuel gas compositions to be used in testing in this programme. The global model can be used to predict the total gas flow and composition at various points within the system. Figure 5.1 shows the various components of the fuel cell system.

Figure 5.1 Flow diagram of anode flow stream (AFS) for use inRolls-Royce global model

In the system illustrated in Figure 5.1, the reforming catalyst is positioned between the FST inlet and FST outlet; the fuel cell anode is positioned between the FST outlet and AOG. The model was used to predict the FST inlet compositions for tests carried out

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in section 5.5 and FST outlet compositions for tests carried out in section 2.4. Simulated operating conditions represented fuel utilisations ranging from 70% - 80%, and AOG recycle mass ratios of 5 - 10.

5.5 Propensity for carbon formation under simulated operating conditions

5.5.1 Experimental findings

Inactive modules were tested at Rolls-Royce under conditions that simulated predicted operating conditions. Each module was operated at 900°C under constant conditions for approximately 170h, followed by post-test analysis by Keele University. The quantity of carbon deposited under each operating condition is given in Table 5.1.

Table 5.1 - Carbon deposited on inactive SOFC modules following 170h tests at 900°CSimulated gas mixture Deposited carbon / mg.g'^upportAOG=7, I=2.7A, 75% fuel utilisation 2.53 ± 0.62AOG=5, I=2.7A, 75% fuel utilisation 2.39 ± 0.5614%CH4, 43%H2, 43%CO2 0.27 ± 0.08

The quantity of carbon deposited on the ceramic support at 900°C, exposed to a gas mixture representing simulating operating conditions is significantly less than that deposited when the support is exposed to pure methane at the same temperature (section 4.2). It is clear that the addition of AOG to the raw fuel significantly reduces the rate of carbon formation, however no significant difference was measured between the two AOG ratios studied. The module operated in the absence of steam had significantly less carbon than either of the modules tested with the full AOG mixture. This is somewhat surprising, although it should be noted that due to the high activity of the support material for WGSR, a significant quantity of steam would be present within the module. It is not clear however why less carbon was formed when no water was actively introduced to the module.

5.5.2 Comparison with thermodynamic predictions

The predicted carbon deposition margins under different operating conditions are shown in Figure 5.1. These have been calculated from the Rolls-Royce global model developed in the previous LINK programme. Solid carbon deposition is predicted as the AOG recycle ratio is lowered and as the module temperature decreases. Increasing the fuel utilisation shifts the carbon deposition margin to the left, i.e., the system can be operated with a lower AOG recycle ratio without formation of solid carbon.

Thermodynamic calculations predict that the ceramic support should be free of carbon under the operating conditions studied. With a gas mixture representing 75% fuel utilisation and an AOG ratio of 5, no carbon formation is expected above 640°C. The case for methane pyrolysis is somewhat different. Thermodynamics predict that at 900°C, undiluted methane should be almost completely pyrolysed to hydrogen and solid carbon. In practice at this temperature approximately 3% of the methane passed through the ceramic support material was converted indicating that the reaction kinetics are important at this temperature in determining the level of carbon deposition.

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No carbon No carbon -------70%FU, 2.7A-------90%FU, 2.7A

Solid carbon

AOG recycle ratio

Figure 5.1 -Thermodynamic prediction of carbon deposition margins______

These results demonstrate the necessity for experimental determination of the operating envelope if one is to avoid carbon deposition. Although thermodynamic predictions are undoubtedly very useful they cannot of course account for the kinetics of the carbon forming reactions. Both the thermodynamics and the kinetics are of course highly dependant on temperature, and what calculations cannot account for are local temperatures within a module.

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7 CURRENT COLLECTOR & REFORMING CATALYST -CARBON TOLERANCE

7.1 Current collectorFollowing 19h steam reforming at 900°C with a steam/methane ratio of 2.0, the two current collector formulations (CC-A and CC-B) were analysed using TPO for deposited carbon. There was no evidence for carbon formation with either sample. Longer-term tests would have to be performed to confirm this result.

7.2 Reforming catalystThe activity of the reforming catalyst for steam reforming was measured at 950°C using a gas mixture that represented typical FST inlet conditions (25%CH4, 25%H2, 10%CO, 30%CO2, 10%H2O). The test was run for a total of 18h during which time the activity of the catalyst fell by approximately 7.5%, although the bulk of this activity drop occurred within the first 4h. Further tests should be performed to determine the reforming catalyst’s long-term stability to carbon formation.

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8 CONCLUSIONSIn general this programme was successful in meeting most of the objectives set out at the beginning. The communication between the partners was good, especially between Rolls-Royce and the academic partner, Keele University. The geographical location of Keele University in relation to Rolls-Royce was also beneficial, as this facilitated rapid and frequent transportation of samples and test pieces between these two partners. General problems encountered at Rolls-Royce during the course of the programme severely restricted the number of active fuel cell modules that were of a high enough quality to allow long-term testing under operating conditions where sulphur was present. However, short-term durability tests on active modules proved very successful, and resulted in predicted sulphur-tolerance levels for a working system.

Keele University carried out the bulk of the work in determining the rate of deactivation of the anode and the reforming catalyst at various sulphur concentrations. Although the reforming catalyst proved to be more tolerant to sulphur than the anode formulations it was evident that the sulphur concentration in the raw fuel of a working SOFC system would have to be far lower than the levels normally present in pipeline natural gas, either in Europe the or the United Kingdom. The short-term durability tests on active IP-SOFC modules at Rolls-Royce demonstrated that the concentration of sulphur in the raw fuel may have to be as low as 18ppb in the final system if one is to achieve degradation rates of less than 0.75% per 1000h.

Tests at both Rolls-Royce and Keele University demonstrated that sulphur poisoning of both the reforming catalyst and the anode material is reversible, and provided that the operating temperature is high enough, and the materials are subsequently subjected to a sulphur-free gas feed: in excess of 90% of the fuel cells’ original performance could be recovered following exposure to a fuel feed containing 3.3ppm hydrogen sulphide. This result has significant beneficial implications for the system design as it potentially allows for the possibility of temporary partial, or even complete failure of the desulphurisation unit, without causing permanent and irreparable damage to the fuel cell stack.Based on the sulphur-tolerance results and system requirements Synetix have devised a method of gas processing that would achieve the required level of desulphurisation using a combination of zinc oxide and a zinc-copper oxide absorbent. The volume of the desulphuriser has been calculated for both a design pressure of one and five bar. Operating at a higher pressure reduces the volume of the purification unit by almost 50%; the cost of such a unit would therefore also be reduced by a significant amount.The dearth of suitably active modules during the programme meant that Rolls-Royce were unable to supply modules to GdF that had a high enough, and stable performance that would allow them to carry out sulphur tolerance measurements on their test rig. However as a gas supplier, GdF proved to be a useful partner in the programme, supplying information on differences in gas supplies across Europe. GdF have shown their continued support to the Rolls-Royce design and have demonstrated a willingness to investigate sulphur tolerance of the IP-SOFC when more reliable modules can be supplied.The sulphur-tolerance work that has been completed within this programme has gone a long way to increasing our understanding of what the general requirements of the final system will have to be, in terms of the degree of desulphurisation. However

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these conclusions have been drawn from short-term tests. In order to increase the level of confidence in these results, there remains a need for longer-term testing using lower sulphur concentrations that are closer to the levels that can be tolerated by the fuel cells.The susceptibility of the ceramic support material to carbon formation has been characterised both in terms of operating temperature and gas composition. Results have clearly demonstrated the need for natural gas pre-reforming in the system. Although the ceramic support material can be exposed to methane below 850°C without the danger of carbon formation, it is far more intolerant to exposure to higher hydrocarbons. Undiluted natural gas leads to carbon formation on the ceramic support at temperatures as low as 720°C.Diluting the raw fuel with AOG dramatically reduced the rate of coking of the ceramic support, however carbon is still formed. Although tests carried out over the course of 170h showed evidence of some carbon formation it is at this stage not clear whether or not carbon deposition would continue at the same rate unabated over a longer timescale. Tests of a longer duration, of the order of 1000 - 2000h should be performed to investigate this further. The results included in this report also demonstrated the necessity for experimental measurement in addition to thermodynamic calculations. In addition to longer-term testing suitable methods of natural gas pre-reforming should be investigated that are compatible with Rolls-Royce system.

Summary of major findings

• To achieve degradation rates of less than 0.75% per 1000h, concentrations of sulphur in the raw fuel of <18ppb may be necessary.

• Deactivation through sulphur contamination is not permanent. Greater than 90% activity can be recovered following cessation of the sulphur-contamination.

• System operation at pressure reduces the volume of the required purification unit by almost 50%. The calculated cost for such a desulphuriser is £10 per kW.

• The fuel cell ceramic support material cannot be exposed to raw methane above 850°C or natural gas above 720°C. Pre-reforming of natural gas will be required in the final system.

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