DEACTIVATION AND DURABILITY OF THE CATALYST FOR HOTSPOT™
NATURAL GAS PROCESSING
ETSU F/02/00173/REP
Contractor:Johnson Matthey Technology Centre
Prepared byR Ebner
S Ellis S Golunski
The work described in this report was carried out under contract as part of the New and Renewable Energy Programme, managed by the Energy Technology Support Unit (ETSU) on behalf of the Department of Trade and Industry. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of ETSU or the Department of Trade and Industry.
Johnson Matthey is developing its autothermal fuelprocessing technology (HotSpot) for a number of different fuels and applications. Although globally most effort has been directed at mobile applications, the market for residential fuel-cell systems is more likely to emerge in the short term. In the commercialisation of such reforming systems, lifetime and maintenance become important issues. Although the external construction and the internal engineering of the HotSpot reactor assembly are highly robust, the durability of the catalyst bed during natural gas processing is largely untested.
The aims of this project were to- establish the relationship between the condition of
the existing catalyst and its performance during natural-gas processing
- produce a new catalyst with improved durability
The objectives were to- rank the factors that contribute to de-activation of
the existing catalyst- understand the underlying mechanisms of de
activation- predict rate of de-activation- improve the durability of the catalyst
Potential causes of deactivation of the natural-gas reforming catalyst were considered, namely temperature excursions, contact with steam, presence of sulphur compounds, build up of carbon deposits, duration of running, and reducing or oxidising species in the gas phase. These factors were included in the design of a matrix of accelerated ageing experiments. After pre-treatment, each sample was characterised, and tested under HotSpot conditions. Statistical analysis of the results led to a ranking of the deactivating factors, with the most destructive being examined more closely. Based on the emergent understanding of the physical and chemical mechanisms of de-activation, new catalyst formulations were prepared and evaluated.
The statistical evaluation highlighted two main areas for catalyst improvement - thermal stability and sulphur tolerance of the catalyst surface. However, the
2
tests also demonstrated that the physical durability of the catalyst bulk was also a key issue.
The threshold sulphur tolerance of the catalyst was determined, and the associated rate of de-activation was established. Further experiments revealed that sulphur selectively inhibits the methane+H2O reaction, but the effect is reversible.
The surface of the catalyst was successfully stabilised against further thermal degradation, but it was not possible to make it immune to sulphur poisoning. The bulk problems of crumbling, attrition and settling were overcome by supporting the catalyst on a continuous substrate.
Conclusions : The statistical method (known as Plackett Burman)used here is very effective in rapidly screening and ranking the causes of catalyst de-activation. It is made more powerful by complementary testing and characterisation, which allow more fundamental information about the catalyst condition to be derived.
During use, the existing granular catalyst suffers from- physical disintegration- sulphur poisoning (of the steam reforming
reaction)- decline in active surface area (by sintering).
The most effective strategies for ensuring long lifetime of the catalyst bed are- pre-sintering of the catalyst surface- coating the catalyst onto a continuous support- operating at sulphur levels below 10 ppm (by
volume).
Recommendations : (i) The Plackett Burman method should be more widely applied to the study of catalyst durability.
(ii) The HotSpot natural-gas reformer should be used in conjunction with an upstream sulphur-trap.
(iii) Future work should be directed at identifying the most appropriate substrate, and at designing a mass-manufacturing process for coating presintered catalysts onto the preferred substrate
2. Statistical evaluation of catalyst deactivation.............................................. 2
2.1 Process of natural gas reforming............................................................ 22.1.1 Catalytic reactions during HotSpot reforming................................ 22.1.2 Sources of deactivation .................................................................. 22.1.3 Summary of deactivating factors.................................................... 3
2.2.3.1 Setting up the design................................................................ 52.2.3.2 Effects...................................................................................... 52.2.3.3 Errors........................................................................................ 62.2.3.4 Significance of effects.............................................................. 6
2.4.1 Pre-treatment of catalyst..................................................................82.4.2 Testing of catalyst........................................................................... 82.4.3 Catalyst characterisation................................................................. 9
2.4.3.1 BET method............................................................................. 92.4.3.2 CO chemisorption...................................................................102.4.3.3 Temperature programmed reduction (TPR)............................10
2.5. Results of statistical evaluation............................................................102.5.1 Surface characterisation.................................................................102.5.2 Catalyst performance.....................................................................112.5.3 Results of the Plackett - Burman method......................................132.5.4 Ranking of de-activating factors....................................................13
3. Mechanisms of catalyst deactivation..........................................................143.1 HotSpot simulation tests.......................................................................143.2 Additional ageing of samples................................................................14
3.2.1 Effect of sulphur at different temperatures....................................153.2.2 Regeneration of poisoned catalyst.................................................163.2.3 Thermal degradation......................................................................16
3.3 Sulphur poisoning.................................................................................173.3.1 Effect of different sulphur concentrations.....................................173.3.2 Steam reforming capacity of the catalyst.......................................183.3.3 Lifetime prediction.........................................................................19
Global warming and the associated risk of dramatic climate change have become major issues in our society. Of the many solutions being considered, more efficient use of existing fuels is seen as a key strategy for reducing CO2 emissions in the short-term.
Fuel cells are likely to have a major role to play. They convert chemical energy into electricity, without generating excessive waste heat. Furthermore, there is now a widespread conviction that mass-market fuel-cell systems will begin to be commercialised in the next five years. This, in no small part, is due to the rapid progress that has been made recently in the development of the solid polymer fuel cell and in the ancillary hydrogen- generation technology.
Although globally most effort has been directed at mobile applications, the market for residential fuel-cell systems (fuelled by natural gas) is more likely to emerge in the short term. One reason for this is that several of the performance and production targets (including cost; see Table 1) are less demanding.
Table 1 Approximate cost and lifetime targets for stationary and transport applications [1]
Johnson Matthey (JM) has shown on micro-reactor and full scale that a feed of natural-gas + air + water can be converted to hydrogen at moderate temperatures by its HotSpot™ reforming process. In common with HotSpot technology developed for mobile methanol-reforming, the conversion of natural gas occurs by a combination of steam-reforming and partial oxidation, so that hydrogen is released from both the methane and the water molecules. The first phase of research and development culminated in a 5 kWe prototype natural-gas processor (with integrated CO clean-up) being delivered to PlugPower in the United States, where it was successfully tested in a fuel cell system in December 1998.
Several manufacturing and energy-supply companies have ambitious plans to launch the sale of embedded power sources, for domestic installation, in the near future. For this to happen, many design and performance issues will have to be resolved - these will include lifetime and maintenance of all the system components. If HotSpot is to become the preferred choice of natural-
6
gas processor for such systems, its ability to meet the stringent lifetime target (Table 1) will have to be proven.
This project represents the first steps towards achieving long-term durability of the HotSpot catalyst. It begins with a rapid screening of the possible causes of deactivation, followed by a more detailed study of those causes that emerge highest in the order of ranking. Based on this knowledge, fundamental improvements in the design of the catalyst bed and in operation of the reforming process have been identified.
2. STATISTICAL EVALUATION OF CATALYSTDEACTIVATION
2.1 Process of natural gas reforming
The feed gas to the HotSpot reformer consists of natural gas (the major constituent of which is methane), air and steam. In principle, these gases can undergo several different reactions in an adsorbed state on the catalyst surface, of which the most likely are shown in Table 2.
Table 2 Reactions on the surface of a natural gas reforming catalyst [2^ [3]
2.1.1 Catalytic reactions during HotSpot natural-gas reforming
In practice, the HotSpot process combines exothermic partial oxidation and water-gas shift with endothermic steam reforming on the same catalyst particles, ensuring effective heat transfer without the need for mechanical heat exchange. The relative contributions of the reactions can be estimated by measuring the H2/CH4 ratio achieved during self-sustaining operation. A ratio of 2 is observed for pure partial oxidation, where the only source of H2 molecules is each CH4 molecule. When the H2/CH4 ratio is >2, it indicates that steam is being converted as well.
2.1.2 Sources of deactivation
There are three primary sources of deactivation inside a reformer :- Physical/chemical conditions under which the process is carried out- Local environment caused by the reactions taking place- Poisons in the feed gas.
7
The HotSpot natural-gas reformer works at pressures between 1 and 5 bara, at temperatures between 600 and 800°C. The upper limit of the temperature range is high enough to induce loss of surface area, by the movement and coalescence of microscopic catalyst particles. This process, commonly known as sintering, continues for as long as the catalyst resides above the threshold temperature. Furthermore, the presence of steam and the relative amounts of reductants and oxidants in the vicinity can affect the rate of sintering.
As its name implies, natural gas does not have a fixed composition, but varies according to origin (see Table 3). However, despite these variations, the potential poisons that it contains can be classed in just two categories :(i) higher hydrocarbons (ethane, propane etc) - these are liable to deposit
coke-like species on the catalyst surface ;(ii) sulphur compounds - these are liable to form H2S, which is an almost
Table 3 Content of natural gas from different sources (composition expressed as %, unless otherwise stated)
8
2.1.3 Summary of de-activating factors
Having considered the potential sources of de-activation during HotSpot natural-gas reforming, the 7 factors listed in Table 4 were assessed in this study.
A statistical method was used to screen the selected factors efficiently and rapidly.
2.2.1 Experimental Design
The traditional "one-factor-at-a-time" approach to the study of catalysts provides an estimate of the effect of a single variable, while all the other variables are fixed. Although simple, this approach can be lengthy and, because it assumes that the variables act additively, interactions may not be detected. The use of statistical methods is less widespread, partly because they are poorly understood by many experimental scientists. However, these methods have considerable advantages over the "one-factor-at-one-time" approach.
In a full factorial design, high precision can be achieved and interactions between the variables can be detected, but at the cost of a large number of experiments. However, the number can be reduced by employing specially developed designs.
2.2.2 Factorial design
In a general factorial design, an investigator selects a fixed number of levels (= L) for each of a number of variable factors (= K) and then runs experiments with all possible combinations. If there are L1 levels for the first
9
variable, L2 for the second and LK for the Kth, the complete array of experiments runs is called a ‘Li x L2 x ...x LK factorial design’.
2.2.3 Plackett Burman method
For general screening of a relatively large number of variables (>4) a two- level (2k) factorial design is powerful because of the manageable number of experiments required to yield a large amount of information. Such a design cannot examine every effect of a specific factor but it can indicate major trends (including compounding effects) and so highlight the most appropriate direction for further experiments. [8]In this project the 7 variables shown in Table 4 were examined. A complete factorial design would require 27 = 128 sets of experiments. Since each set (see Section 2.4 below) required 6 staff days on average, it would take one person about 750 days to complete the screening. Instead, the method of R. Plackett and J.P. Burman [9] - a multi-factor experimental design - allowed the screening to be reduced to 12 sets of experiments.
2.2.3.1 Setting up the designTable 5 shows the experimental design matrix used in this project. Each of the variables was set at two levels, " + " denoting high level and " - " denoting low level. In practice, the ‘levels’ were the two extremes of a realistic range for each variable. For example, the + and - levels set for temperature were 400oC and 800oC respectively, reflecting the range inside the HotSpot catalyst bed during operation. Inspection of the matrix shows that a specific variable appears 6 times at its high level and 6 times at its low level. Although, the matrix has definite symmetry (as shown in Table 5), the experiments were carried out in a random order.
Table 5 Plackett-Burman matrix for determining the effects of up to 11 variables at two levels
10
2.2.3.2 EffectsThe effect of a variable on the chosen response is simply the difference between the average value of the response for the six runs at high level and the average value of the response for the six runs at low level. In this project, the chosen responses were (i) total surface area, (ii) exposed metal area, and(iii) catalytic performance.
Mathematically, the effect of ‘a’ (Ea) is shown as :
R at (+) VRat (-)--------^------ — (1)66where R is the measured response.
In this calculation only the effect of A is included, since the effect of the other variables cancels out. This can be understood by considering variable B - if A is on high level, B is three times on high and three times on low level.
Ea =
2.2.3.3 Calculation of errorSome variables included in the matrix are not assigned any experimental value. The effects of these so called ‘dummy variables’are calculated in the same way as the effects of real variables. If there are no interactions, and all levels are reproduced perfectly, the effect shown by a dummy variable should be 0. In reality, the effect is often not equal to 0 because of errors in measuring the response. At least three such dummy variables will provide adequate confidence, but more can be used if fewer variables need to be studied. Equation 2 shows how the dummy effects are combined to estimate the variance of an effect:
Veff: variance of an effect (2)
Edummy: effect shown by a dummyn: number of dummy variables
The relationship between the variance of an effect and the standard error (S.E) of an effect is shown in Equation 3:
S-E-effecct = V Veffect (3)
Vi2dummy )
effect n
2.2.3.4 Significance of effectsTo determine the significance of each effect, it has to be compared with the value for the standard error. An effect is valid if it exceeds the standard error (see Table 6).
size of effect confidence levelStandard deviation 68 %Two times standard deviation 95 %
Table 6 confidence levels in a gaussian normal distribution [13]
11
2.3 Measurement Plan
Each of the de-activating factors (from Table 4) was assigned to one of the variables in the design matrix (in Table 5), and real conditions were defined for the two levels. The outcome is shown in Table 7. The matrix could then be re-written to show the exact conditions used during 12 different pretreatments of the catalyst (Table 8).
Variable - + Commentsa Temperature 400oC 800°C Temperature range during
natural gas reforming.b steam concentration 0 % 10 % The high steam level is in the
range for HotSpot reforming.c SO2 concentration 0 ppm 5 ppm 5 ppm is the threshold of the
sulphur trap before the natural gas reformer
d Dummy Spare value to calculate the measurement error
e Reducing strength 1% O2 1% H2 Examination of surface oxidation state
f Methaneconcentration
0 % 2 % Examination of any residual effect of methane on the catalyst
g Dummyh Dummyi Duration 2 h 20 h Determination of sintering &
cumulative poisoning effects1 Dummyk Dummy
Table 7 Assigned variables and their levels
Originally, variable ‘h’ (Table 7) was ‘higher hydrocarbons’, which required the inclusion of ethene at the + level. However, due to the construction of the apparatus used for pre-treating the catalyst, the ethene was found to be converted into carbon before it reached the catalyst. As a result ‘h’ was reassigned as a dummy variable, and the effect of higher hydrocarbons was assessed by switching from methane to a coking mix (see section 2.4.2) during testing of the pre-treated catalyst.
VariableNo. a B c e f i
1 400 °C 10% 5 ppm 1% O2 0% 2 h2 800 °C 0% 0 ppm 1% H2 0% 2 h3 800 °C 10% 5 ppm 1% O2 0% 20 h
12
4 800 °C 10% 0 ppm 1% O2 2% CH4 20 h5 400 °C 10% 5 ppm 1% H2 2% CH4 2 h6 800 °C 0% 5 ppm 1% H2 0% 20 h7 400 °C 0% 5 ppm 1% H2 2% CH4 20 h8 400 °C 10% 0 ppm 1% H2 0% 20 h9 400 °C 0% 0 ppm 1% O2 2% CH4 20 h10 800 °C 0% 5 ppm 1% O2 2% CH4 2 h11 400 °C 0% 0 ppm 1% O2 0% 2 h12 800 °C 10% 0 ppm 1% H2 2% CH4 2 h
Table 8 Conditions used during pre-treatment of catalyst
2.4 Experiments
Experiments to examine the deactivation and durability of the HotSpot catalyst were divided into three main parts (Figure 1) :- pre-treatment (carried out in a hydrothermal-ageing rig)- catalyst characterisation (of surface and bulk properties)- catalyst testing under HotSpot conditions.
Coking Mix as Fuel
Methane as Fuel
Oxidizing with 1% 02 in N2
BETTPR
CO chemisorption
fresh catalyst
Test rig
Temperature Steam concentration
S02 Methane
Reducing strength (H2/02) Duration
Hydrothermal Ageing Rig
Figure 1 Sequence of deactivation experiments
2.4.1 Pre-treatment of catalyst
The catalyst was pre-treated in a hydrothermal ageing rig, capable of holding about 30 g of catalyst in one charge. The catalyst was loaded into a porous glass-fibre paper box, which was mounted at the centre of a ceramic tube, through which the appropriate gas mixture was passed. The catalyst was
13
heated to the temperature specified in Table 8, and held there for the required duration. The catalyst was then allowed to cool under the gas mixture, before being stabilised with a mixture of 1 % O2 in N2 at room temperature.
2.4.2 Testing of catalyst
Fresh and aged granular catalyst samples (grain size between 355 gm and 800 gm) were tested in a small scale HotSpot reactor, using a bed mass of 25 g (21 cm3). Firstly, a pre-heated single-phase mixture of methane, steam and air was fed to the reactor. (Methane was used instead of natural gas to allow the de-activating factors in the pre-treatment to be assessed unambiguously.) The methane feed could then be switched to a synthetic natural-gas (‘coking mix’), which comprised 7% ethane, 3% propane and 90% methane, to observe the effects of higher hydrocarbons.
Three thermocouples were used to measure the temperature at different points within the catalyst bed. The pressure was measured at the inlet and outlet of the reactor. The product stream (reformate) was cooled down to condense out the water, and the flow-rate was measured with a dry flowmeter. The dry reformate was analysed with a gas chromatograph, which detected H2, CH4, CO2, N2 and CO.
The first experimental task was to find the maximum H2/CH4-ratio at the highest possible flow rate - the threshold to the so-called ‘kinetic regime’. At this point every active site of the catalyst is expected to be functioning. Therefore, any blocking or poisoning of the sites will be immediately reflected by a decline in performance.
During the experiments the H2O/CH4 ratio was kept at 3 and the O2/CH4 ratio was set to maintain a maximum temperature of about 650°C in the catalyst bed. These conditions were based on thermodynamic models, which predicted high conversion and a high ratio for H2-produced/CH4-consumed. For a fresh catalyst, the kinetic regime was reached at the feed-rates shown in Table 9. Under these conditions, the bed temperature was stable at just under 650oC, the H2/CH4 ratio was 2.63, and the CH4 conversion was 91 %.
CH4 1.85 l/minAir 6.42 l/minh2o 3.83 l/min
Table 9 Standard feed rates
All the pre-treated catalyst samples were initially exposed to the feed stream shown in Table 9, but then the air feed-rate was adjusted to establish a temperature of 600 - 650 °C in the catalyst bed.
2.4.3 Catalyst characterisation
Well established methods for characterising catalysts were used to measure- overall surface area
14
exposed metal area surface and bulk reducibility.
2.4.3.1 BET methodDecline in catalytic performance is often due to loss of surface area, which can be induced by high temperature and the presence of certain gas species (particularly steam). The BET method (named after Branauer, Emmett and Teller[10]) was used to detect changes in the surface area, resulting from the catalyst pre-treatments. In this method an inert gas is condensed on the catalyst to form a single overlayer[11]. The amount of gas retained simply reflects the accessible area (including pores, channels and fissures) and not the chemical nature of the surface.2.4.3.2 CO chemisorptionIn the case of precious metal catalysts, the active sites are often located on the metal surface. Catalyst de-activation can occur through a decline in the exposed metal area, even though the total surface area of the catalyst may not change perceptibly. This can be measured by using a simple probe molecule, which will bond to each exposed metal atom in a predictable way. In this project, carbon monoxide was used as the probe molecule.
2.4.3.3 Temperature programmed reduction (TPR)The reducibility of the fresh and pre-treated catalysts was monitored by a TPR technique, in which a sample was exposed to a gas mixture of 10% H2 in N2, whilst the temperature was increased linearly. Chemical reduction of the catalyst was detected by measuring the consumption of hydrogen[12]. The method is sensitive enough to detect reducible surface species, such as sulphates or sulphites formed by exposure of the catalyst to gas-phase sulphur.
2.5. Results of statistical evaluation
2.5.1 Surface characterisation
Three samples of the fresh catalyst were characterised by the BET method and by CO chemisorption (Table 10). One of the samples was then subjected to activity testing, before being characterised again. As shown in Table 10, both the metal area and the total surface area had declined substantially during testing, even though the performance had not shown signs of deactivation (see below).
Table 10 BET surface area and CO/metal ratio of fresh catalyst
15
The surface characterisation of the pre-treated catalysts is summarised inTable 11.
Variable BET CO/metalratio
No. Temperature Steam cone. 802 cone. Reducingstrength
Methanecone.
Duration m2/g CO / metal atom
1 400 °C 10% 5 ppm 1%02 0% 2 h 88 8 1.1452 800 °C 0% 0 ppm 1% H2 0% 2 h 76 3 0.3133 800 °C 10% 5 ppm 1%02 0% 20 h 46 8 0.1374 800 °C 10% 0 ppm 1%02 2% 20 h 39 2 0.5825 400 °C 10% 5 ppm 1% H2 2% 2 h 89 0 0.8756 800 °C 0% 5 ppm 1% H2 0% 20 h 70.2 0.1047 400 °C 0% 5 ppm 1% H2 2% 20 h 89 2 0.6908 400 °C 10% 0 ppm 1% H2 0% 20 h 86 9 0.7109 400 °C 0% 0 ppm 1%02 2% 20 h 89 3 0.66810 800 °C 0% 5 ppm 1%02 2% 2 h 69 7 0.13211 400 °C 0% 0 ppm 1%02 0% 2 h 92.7 1.17012 800 °C 10% 0 ppm 1% H2 2% 2 h 56 7 0.261
Table 11 Surface properties of pre-treated catalyst samples
2,5,2 Catalyst performance
The fresh and pre-treated catalyst samples were subjected to a sequence of activity tests (summarised in Figure 2). In phase 1, the reformate was analysed after the reactor first reached a steady state, with methane as the fuel. After 1 hour, the reactor was stopped and allowed to cool for an hour, before being re-started (phase 2). An hour later, the fuel was switched to synthetic natural gas (phases 3 and 4), before reverting back to methane (phase 5).
Timing of key reading in each phase
cooldown
1 2 3 4 5 6 time in h
Figure 2 Test phases
In Figure 2, the position of the arrows indicates the key analysis point in each phase. From the analytical measurements, the ratio of H2-formed/CH4- consumed was calculated, and the value was used as the ‘response’ (R) in
16
the statistical analysis (Table 12). The reformate was also analysed at other times in each phase, and the results were converted to ‘responses’, which are plotted out in full in Figure 3.
From these values the statistical ‘effect’ of each variable was calculated using Equation 1 (section 2.2.3.2). The results are presented in Table 13, together with the calculated ‘effects’ of the same variables on physical properties of the catalyst. The ‘effects’ on hydrogen consumption during TPR are not included, because of the high standard deviation.
above the standard deviationabove two times the standard deviatior
Table 13 Statistical effects of each variable
2,5,4 Ranking of de-activating factors
A different order for the effect of the de-activating factors was observed when physical properties (total surface area or exposed-metal area) were used as the ‘response’, compared to when catalytic performance was used :-
loss of total surface :high temperature » presence of steam > long exposure > all others loss of exposed-metal area : high temperature » all others loss of catalytic performance :presence of sulphur > high temperature > long exposure > all others
Clearly, the critical ‘response’ is catalytic activity. However, comparison of the above ranking orders provides additional information, which would not have been available from the catalytic measurements alone. For example, high temperature clearly induces a loss in total surface area, and an associated decline in the amount of metal left exposed. This re-construction of the surface results in a measurable decline in catalytic performance, but it is not as dramatic as when the catalyst has previously been exposed to sulphur (note how the de-activated samples fall into one of two categories, in Figure 3). The implications are that active sites are lost during surface reconstruction, and that those remaining are then susceptible to blocking by sulphur species. This retained-sulphur was detected by TPR, which revealed an additional hydrogen-consumption peak.
Another important factor to emerge was the poor physical durability of the catalyst granules, which were found to crumble during testing. By optimising the bed size at 25 g, settling and attrition within the catalyst bed were minimised, allowing reliable and reproducible data to be obtained. However, it was clear that scale-up of the bed would require development of a more robust catalyst.
18
3. MECHANISMS OF CATALYST DEACTIVATION
Following on from the statistical analysis, experiments were designed to investigate the underlying mechanisms by which the catalyst de-activates during use in a HotSpot reactor.
3.1 HotSpot simulation tests
A small-scale reactor was constructed, which had the internal features of HotSpot, but held a small enough bed to avoid the problems associated with disintegration of the catalyst granules (Figure 4).
Figure 4 Schematic diagram of small-scale HotSpot test apparatus
3.2 Additional ageing of samples
Further pre-treatments were carried out to establish less ambiguously the causes of deactivation, and to try to reverse them (Table 14)
Question MethodA Can the effects of sulphur
and high temperatures be separated ?
No exposure to S02 until the catalyst reaches its maximum temperature
B Can a poisoned catalyst be regenerated ?
Exposure of seriously de-activated catalyst to H2 at 600°C to remove sulphur
C Is thermal de-activation a continuous process ?
Increasing the duration of pre-treatment
Table 14 Purpose of additional pre-treatments
19
3.2.1 Effect of sulphur at different temperatures
Unlike pre-treatment 10 in the statistical analysis, where it was continuously exposed to SO2, the catalyst was heated to 800°C in 1% O2 and 2% CH4, before 5 ppm S02 was added. As Figure 5 shows, this pre-treatment caused a fall in performance, but this was not as marked as in the case of pre-treatment 10. This suggests that the catalyst retains sulphur more readily at low temperatures.
fresh catalyst Pre-treatment 10 Pre-treatment A
Figure 5 Effect of introducing sulphur at different times during pretreatment
During the first performance test on the catalyst subjected to pre-treatment A, the temperature in the reactor dropped. The air feed was increased to compensate for this, and this had the effect of improving the performance of the catalyst (Figure 6).
^ 2.5
catalyst after pre-treatment A= 0.5
fresh catalyst
no. of start-up
Figure 6 Improvement in performance of de activated catalyst after several start-ups
20
The performance of the de-activated catalyst improved initially with each start-up. After 5 start-ups, it exceeded the performance of the fresh sample, but eventually stabilised at the same level as the fresh catalyst. These results indicate that sulphur is lost from the catalyst with repeated start ups and, surprisingly, that some small amount of sulphur is actually beneficial to performance.
3,2,2 Regeneration of poisoned catalyst
An attempt was made to regenerate rapidly the most severely de-activated sample (ie the product of pre-treatment 3 : 800°C, 10% steam, 5ppm SO2, O2, 20 h) by exposing it for 2 hours to a gas mixture of 1% H2 in N2 at 600°C. These conditions mimic those in the TPR experiment that gave rise to the appearance of a new hydrogen-consumption peak.
Figure 7 shows that this approach was only partially successful. However, the catalyst continued to recover while it was being used. Eventually, its performance stabilised at about 85% of that of the fresh sample. Assuming that most of the sulphur was removed by exposure to H2 at 600°C, these observations that only a small amount of retained sulphur is required to impact substantially on catalytic performance. The 15% deficit in performance, which could not be recovered, is taken to indicate the loss of active sites that occurred by thermally induced re-construction of the surface.
performance versus time
1 2.00
•B 1.50
after regeneration
before regeneration
fresh catalyst
run time in minutes
Figure 7 Improvement in performance of de activated catalyst during operation
3,2,3 Thermal degradation
The effect of the ageing period used in pre-treatment 4 (800°C, 10% steam 1% O2, 2% CH4) was examined, over the range of 2-40 hours.
21
catalyst surface area over time
5
£s ™m 600)€ 50
w 4n ----------< ►
3o 20
1 0 1 5 2
age0 2
iing time5 3
[h]0 35 40 4
Figure 7 Catalyst surface area as function of duration of hydrothermal ageing
As Figure 7 shows, the catalyst surface area fell dramatically at first, but stabilised at 30 m2/g. This loss of surface area was associated with a decline in catalytic performance, which again stabilised at about 80-85% of the fresh sample.
This experiment provides a method for ensuring hydrothermal stability during operation. It shows that the catalyst should to be pre-treated to produce a stable surface before use, ie ‘pre-sintering’. This may require a larger bed size (as the pre-sintered catalyst is intrinsically less active than before), but will avoid the major cause of irreversible de-activation.
3.3 Sulphur poisoning
3,3,1 Effect of different sulphur concentrations
At the start of each of these experiments, a fresh catalyst sample was operated at its highest output, as defined in section 2.4.2. After the small- scale HotSpot reactor had reached steady state, H2S was added to the methane feed. Figure 8 shows the effect of different sulphur levels (expressed as molar concentrations of H2S in the methane feed) on the performance of the catalyst. The performance is expressed as a percentage of the ratio of H2-formed/CH4-consumed for the fresh sample.
22
&
performance over time
-100 -50 0 50 100 150 200relative time of H2S feed start [min]
250 300
Figure 8 Effect of H2S on catalytic performance
In each case, when the H2S was introduced, the catalyst bed temperature increased steadily, suggesting that partial oxidation was predominating over steam reforming. To compensate for this, the air feed-rate was reduced. Although this reduced the temperature on the outlet side of the bed and in the middle, the inlet temperature continued to rise (Figure 9).
■£ 600
T bed inletmethane feed
with 6.41 ppm H2ST middle of bed
T bed outlet
relative time of H2S start [min]
Figure 9 Temperature profile of catalyst bed
3,3,2 Steam reforming capacity of the catalyst
The predominance of exothermic partial oxidation in the presence of H2S could imply that sulphur poisons the endothermic steam-reforming contribution to the HotSpot process. To test this hypothesis a micro steam
23
reforming reactor was set up. A gas-feed comprising 13.8% CO2, 2.00% CO, 9.38% CH4, 21.9 % H20, 19.24% H2 and 33.59 % N2 was fed to a 0.2 g bed of catalyst. No oxygen was present in the feed, so steam-reforming was the only likely route to hydrogen production. The catalyst bed was heated at a fixed rate, while the exhaust gas was analysed for methane conversion and hydrogen formation. A fresh catalyst and two pre-treated samples were tested (Table 15).
Pre-treatment C Catalyst heated in hydrothermal ager at 800 °C for 2 h, in 1% 02, 2% CH4;5 ppm S02 included during heating, dwell and cool-down
Pre-treatment D Catalyst heated in hydrothermal ager at 800 °C for 2h, in 1% 02, 2% CH4. 2h;5 ppm S02 added only during dwell time at 800°C
Table 15 catalyst samples for microreactor steam reforming tests
After pre-treatment C, the catalyst was not capable of steam-reforming methane below 650°C - unlike the fresh sample, which began generating hydrogen at 500°C (Figure 10). When the exposure to S02 was limited to a fixed temperature of 800°C (pre-treatment D), suppression of the steamreforming reaction was much less dramatic, confirming that low temperature exposure is the most destructive.
fresh catalyst
average temperature in catalyst bed [°C]
Figure 10 Temperature-programmed steam reforming of methane
3,3,3 Lifetime prediction
As sulphur is the major cause of deactivation, only this factor was taken into account when estimating the useful lifetime of a catalyst. The HotSpot performance of fresh samples of catalyst was tested at various concentrations of H2S (added to the methane). Up to a 20% decline in performance can be correlated quite simply with the gas-phase sulphur concentration. For
24
example the de-activation trace in Figure 11 can be fitted by the following expression :
timemin
396.33( H2S')
ppm
-0,8063
However, once de-activation exceeds 20%, the bed temperature rises substantially, which is likely to induce loss of surface area (even in a presintered sample).
y = 396.33x"R = 0.9889
H2S content in feed gas [ppm]
Figure 11 Time taken for 20% decline in performance, as function of H2S concentration in methane feed
4. CATALYST DEVELOPMENT
In the final phase of this project, the problems of physical and chemical durability were addressed by• applying the catalyst in the form of a washcoat onto a monolithic
substrate;• changing the catalyst formulation, while maintaining the essential
components.
4.1 Washcoated catalyst
Three different washcoat preparations were evaluated. Superficially, each substrate appeared to be coated uniformly. However, HotSpot testing (in the absence of sulphur) revealed substantial differences in performance (Diagram 11). However, one of the monolithic catalysts (‘m2’) matched the
25
performance of the granular catalyst (‘fresh catalyst’), despite containing less than 50% of the mass of used in a packed bed of granular catalyst.
performance at different power outputs
H2/C_inputm1
-B- H2 / CJnput m2
-x- H2 / CJnput m3
fresh catalyst
power output in I/H2 per h
Figure 12 Performance of monolithic catalysts
4.2 Change of catalyst formulation
Granular catalysts were prepared with new formulations, by changinga) precious metal loadingb) proportions of the components in the supportc) number of components in the supportd) preparation route.
The number of new formulations was limited to 20, and these were screened in the absence and presence of sulphur. The most obvious beneficial effect detected was that of including a pre-sintering step in the preparation. This stabilised the catalyst surface, and prevented further sintering during operation. However, none of the new formulations showed improved resistance to the inhibiting effect of sulphur.
26
5. CONCLUSIONS
The Plackett Burman statistical approach proved to be a very effective method of screening and ranking the causes of catalyst de-activation. This method alone, though, cannot provide an understanding of the chemical and physical changes that the catalyst is undergoing. However, fundamental information about the catalyst condition can be obtained, by combining the statistical analysis with appropriate methods of testing and characterisation.
In the absence of gas-phase sulphur, the standard (granular) HotSpot catalyst produces near equilibrium output from a feed of methane/steam/air. On a microscopic level, the catalyst surface changes by thermally induced sintering, which causes a measurable but not substantial drop in performance. On a macroscopic level, the catalyst granules disintegrate during use, resulting in changes in packing of the catalyst bed. The catalyst can be stabilised by pre-sintering, and by coating onto a continuous substrate. The resulting monolithic bed has the added advantages of low pressure drop, and more effective utilisation of the catalyst (ie higher output per unit mass of catalyst).
In the presence of sulphur (before or during use), the steam-reforming component of the catalyst is selectively inhibited. The initial rate of decline in performance shows a relatively simple dependence on gas-phase sulphur concentration, allowing the time taken for 20% de-activation to be reliably predicted. Subsequent de-activation is more difficult to model because, as partial oxidation begins to predominate, the bed temperature rises - in turn causing more rapid sintering, but also more rapid desorption of the poisoning species. At sulphur concentrations below 10 ppm in the gas phase, the performance initially declines, but then stabilises at a still acceptably high output.
Although sulphur-tolerant components were included in the our new catalyst formulations, no clear improvement was observed. More radical re-design of the catalyst may overcome this inhibition by sulphur, but it may be at the expense of catalyst activity and selectivity. As natural gas often has a low sulphur content (see Table 3), the most pragmatic approach is to accept a certain amount of de-activation. However, if this is continuous, and results in an unacceptably high loss of efficiency, a sulphur-removal stage will need to be fitted prior to the reformer.
The overall conclusion of this project is that a pre-sintered monolithic catalyst is likely to be the most durable, providing the gas-phase sulphur concentration in the feedstream is low (< 10 ppm). To demonstrate the suitability of the HotSpot reformer for domestic applications, rigorous ‘field trials’ will need to be carried out to confirm that the catalyst has a lifetime of several years, and to establish the size and frequency of replacement of the sulphur trap.
27
Although catalyst lifetime is a critical issue in the development and commercialisation of a natural-gas processor, there are other key factors relating to the catalyst that also need to be addressed; these include
• cost and availability of the raw materials• processes for mass manufacture• capital and operating costs of mass-manufacturing processes• re-cycling of the precious metal component• disposal of any waste material.
These so-called ‘lifecycle’ issues should be resolved in the next phase of development of the pre-sintered monolithic catalyst.
28
REFERENCES
[1] Gregor Hoogers, Louise PotterRenewable Energy World January 1999 Pages 50 - 57
[2] Cavallaro, S.; Freni, S."Syngas and electricity production by an integrated autothermal reforming / molten carbonate fuel cell system"Journal of Power Sources Volume 76; 1998; pages 190 - 196 ISBN 0378 - 7753
[4] Trimm, D. L„Handbook of Heterogeneous Catalysis“VCH Verlagsgesellschaft mbH, 1997 Weinheim, Germany Volume 3; pages 1263 - 1282 ISBN 3-527-29212-8
[5] BOC Special GasesAnalysis of BG natural gas
[6] Berkshire Gas, United StatesAnalysis of natural gas
[7] Twigg, Martyn V."Catalyst Handbook" second edition Wolfe Publishing Ltd.1989; London page 227ISBN 0 7234 0857 2
[8] George E. P.; Hunter, William G.; Hunter, J. Stuart„Statistics for Experimenters "John Wiley & Sons 1978, New York 1978 pages 306 - 351 ISBN 0-471-09315-7
[9] Stowe, Robert A.; Mayer, Raymond P.„Efficient Screening of Process Variables“Industrial and Engineering Chemistry Volume 58; 1966 pages 36 - 40
