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Al2O3
Applied Catalysis A: General 148 (1997) 325-341
Catalytic combustion of gasified biomass over Pt/Al,O, Marcus F.M. Zwinkels *, G.M. Eloise Heginuz, Bjijrn H. Gregertsen, Krister Sj&trijm, Sven G. J&-b Kungl Tekniska Hiigskolan (Royal Institute of Technology), Department of Chemical Engineering and Technology, Chemical Technology, S-100 44 Stockholm, Sweden
Received 23 February 1995: revised 8 March 1996; accepted 9 March 1996
Abstract Catalytic combustion of gasified biomass over Pt/Al,O, was investigated to clarify its dependence on the fuel gas composition and the air-fuel ratio. An experimental study was performed using synthetic mixtures of the main components in gasified biomass: carbon monoxide, hydrogen, methane, carbon dioxide, water, benzene (model aromatic compound), ammonia, and inert (argon or nitrogen). Results showed that carbon monoxide and hydrogen ignited simultaneously over the catalyst used, with methane having a higher ignition temperature than that of carbon monoxide and hydrogen. The ignition temperature for carbon monoxide and hydrogen decreased as the oxygen-fuel ratio and inert concentration increased. The presence of water vapor in the feed had an inhibiting effect on the ignition of carbon monoxide, whereas the opposite was observed for methane. Methane ignition was favored by the presence of carbon monoxide and hydrogen, not only because their combustion heated the catalyst but also because it attenuated the oxygen inhibition of methane combustion under certain conditions. These effects were explained by co-adsorption phenomena involving fuel components and oxygen. Ammonia was found not to influence the combustion of the other components and was completely converted to nitrogen monoxide over the used catalyst. Keywords: Catalytic combustion; Low heating value gas; Platinum; Gasified biomass
1. Introduction Catalytic fuel combustion is gaining ground as a technology which enables strong reduction of nitrogen oxide emissions (NO,) from combustion processes.
* Corresponding author. Tel.: (+ 46-8) 7908254; fax: (+46-g) 108579; e-mail: [email protected]. 0926-860X/96/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PZZSO926-860X(96)00114-7
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In a catalytic combustor, a premixed lean air-fuel mixture is ignited by a catalyst, which enables complete combustion at maximum temperatures far lower than possible in conventional gas-phase combustors. Consequently, the formation of thermal NO, can almost completely be circumvented, while the combustion efficiency is high. Catalytic combustion has proved to be an effective technology for avoiding NO, generation from fuels with low nitrogen contents, such as natural gas. However, for countries that lack large fossil fuel reserves, biofuels are considered to be a promising alternative. Such fuels have much higher nitrogen contents, which during combustion are converted to NO, (fuel NO,). Fuel NO, accounts for a significant part of the total NO, formed during combustion of nitrogen-containing fuels. Direct combustion of biomass, being a solid fuel, generates a number of pollutants. The level of these pollutants can be reduced by using a two-step process for the generation of thermal energy from biomass. The first step is the gasification of the fuel in an oxygen-deficient atmosphere. In the second step, the main product of this gasification, a low heating value gas (LHV), is combusted. The main advantage of this approach is that a gas combusts more readily than a solid fuel. Moreover, a hot-gas cleaning step can be introduced after the gasifier. The gas from a biomass gasifier contains up to a few thousand ppm of ammonia. This ammonia is largely converted into NO, in conventional combustors. Catalytic combustion may enable conversion of the ammonia to molecular nitrogen instead [ 1,2]. Besides the favourable conversion of ammonia to nitrogen, the catalyst stabilizes the combustion of the LHV gas. An LHV gas, such as gasified biomass, is a complex mixture containing carbon monoxide, hydrogen, carbon dioxide, methane and other light hydrocarbons, water, ammonia, nitrogen, and heavier aromatic hydrocarbons (tars). The presence of all these components is likely to influence the behaviour of a combustion catalyst and an understanding of the interactions between the LHV gas components during catalytic combustion is desirable. So far, most of the reported work on catalytic combustion has focused on methane and natural gas [3-71. The effect of water and carbon dioxide on methane combustion over palladium catalysts was recently reported: Burch et al. [8] found that both water and carbon dioxide had an inhibiting effect, this effect being strongest for water, which was in agreement with the observations from Ribeiro et al. [9]. Cullis et al. showed that carbon monoxide and water inhibited methane combustion, whereas no inhibition was observed for carbon dioxide [lo]. Platinum catalysts have also been studied extensively in methane combustion. It was shown that methane combustion over platinum is a complex process depending among other things on the pretreatment history of the catalyst [ 1 l] and the oxygen-methane ratio [ 121. The aim of the five studies mentioned above was to investigate the effects of conditions and reaction products on the combustion of methane. Groppi et al.
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[ 131 studied catalytic combustion of an LHV gas, represented by carbon monoxide-hydrogen mixtures. They found carbon monoxide to be more readily combusted than hydrogen over manganese-containing hexa-aluminate catalysts and they observed no differences between combustion of the mixture and of the two separate components. In summary, the components in an LHV gas, like gasified biomass, may affect the combustion of such a gas over a catalyst in complex ways, depending on test conditions. For the proper design of catalytic combustors using gasified biomass, a more complete understanding of these influences is required. The above-mentioned studies on methane combustion give certain information that is valuable for the understanding of catalytic combustion of LHV gases. However, to obtain a more complete picture of the possible interactions we studied combustion of mixtures with most of the important components in LHV gases. In particular, the influences of fuel gas composition and air-fuel ratio are discussed in this paper. Moreover, the role of ammonia is investigated, both with respect to its conversion and with respect to the influence ammonia may have on the combustion of other fuel components.
2. Experimental 2.1. Catalyst properties Combustion experiments were conducted using a commercial Pt/Al,O, catalyst, provided by Perstorp AB (NX-45, Perstorp, Sweden). This catalyst, commercially used for oxidation of carbon monoxide and volatile organic compounds from industrial processes, had a surface area of 244 m2/g and was supplied as spherical pellets, 5 mm in diameter. The Pt loading of the catalyst was 0.14 wt.-%. The Pt was deposited to give an egg-shell catalyst, minimizing internal diffusion limitations. 2.2. Chemisorption
experiments
The Pt dispersion of the fresh catalyst was determined by hydrogen-oxygen titration using a Micromeritics TPD/TPR 2900 temperature-programmed chemisorption instrument equipped with a thermal conductivity detector. The hydrogen-oxygen titration measurement was carried out according to the method developed by Benson and Boudart [14]. The same instrument was used to perform temperature programmed desorption (TPD) of carbon monoxide. In both cases the samples were prereduced in-situ with hydrogen, after which titration (H2/02) or saturation (CO) was performed at ambient conditions. TPD of CO was done in a helium flow of 50 cm3/min with a heating rate of lO”C/min. Water desorbing during TPD experiments was removed in a cold trap filled with a mixture of liquid nitrogen and 2-propanol.
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2.3. Combustion experiments Combustion of a synthetic gas mixture, representing a gasified biomass, was tested in a continuous flow reactor, operating at atmospheric pressure. The synthetic gas mixture is referred to as fuel gas in the following sections. The catalyst sample, typically 8 cm3, was placed in an Inconel reactor situated in a tubular furnace, which was used to heat the fuel gas-oxidant mixture prior to entering the catalyst bed and was programmed to increase the temperature of the gas fed to the catalyst bed by 2”C/min. The space velocity was 50,000 hh’. All reaction products were analyzed on-line by a Balzers QMG 421 quadrupole mass spectrometer. The rapid analysis provided by the mass spectrometer was needed to follow the fast ignitions observed during the experiments, in particular for hydrogen and carbon monoxide. A synthetic bottled mixture of four of the main components of gasified biomass was used in all experiments: 24.9 mol-% methane, 30.1 mol-% carbon monoxide, 28.1 mol-% carbon dioxide, and 16.9 mol-% hydrogen. This mixture is representative of the ratios of these four components in gasified biomass. The above mixture was added to a 20 mol-% oxygen-in-argon mixture, giving oxygen-fuel ratios between 2.7 and 10.1, i.e. lean mixtures in all cases. The oxygen-fuel ratio is the supplied amount of oxygen divided by the stoichiometric amount of oxygen. In some experiments water and benzene were added. In these experiments, mixtures with an oxygen-fuel ratio of 2.7 were used, giving the concentrations shown in Table 1. In one experiment the oxygen concentration was decreased by 50% by adding extra argon, thus keeping the other concentrations constant. Methane and benzene were combusted both in the fuel gas and as sole combustible components to elucidate the influence of the presence of the other fuel gas components. Ammonia was added in one experiment in a concentration (see Table 1) that represents about 100 ppm in the fuel gas. In this experiment air was used instead
Table 1 Composition of test gas mixture corresponding to an oxygen-fuel benzene, and ammonia added in certain experiments Component
Concentration
W
H,O Benzene
2.3% 2.7% 2.7% 1.6% 18.6-18.9% balance 0.7% 600 ppm
NH,
15 ppm
co co2 H* 02
AP
a N, in the experiments
ratio of 2.7 and the concentrations
during which NO, analysis was performed.
of water,
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of the oxygen-argon mixture, which was needed for the chemiluminescence NO, analysis (Eco Physics CLD 700). A second experiment in air was conducted, both to measure the NO, produced from nitrogen from the air, and to investigate the effect of ammonia on the conversion of the other components.
3. Results 3. I. Chemisorption
experiments
The platinum dispersion of the fresh catalysts, determined by Hz/O, titration, was 27%. The results of carbon monoxide TPD from a fresh catalyst are shown in Fig. 1. It is clear that carbon monoxide is strongly chemisorbed at catalyst temperatures below 250°C although a desorption onset is observed between 100 and 150°C. This is important since this is the temperature range of interest for the combustion experiments, as will be shown in the following sections. 3.2. Injluence of the oxygen-fuel
ratio on fiel gas combustion
Several combustion experiments were conducted with a gas mixture of the composition described in the previous section, corresponding to an oxygen-fuel ratio of 2.7. Fig. 2a and b show the conversions of the combustible components in the fuel gas versus the gas inlet temperature and catalyst temperature respectively. In many of the plots shown in this section, conversion is depicted as a function of either gas inlet temperature or catalyst bed temperature,
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(“C) catalyst.
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Fig. 2. Conversion of H,, CO, and CH, in the fuel gas versus (a) gas inlet temperature temperature gas at an oxygen-fuel ratio of 2.7.
and (b) catalyst bed
depending on the phenomenon which is to be elucidated. Note that the results given in Fig. 2a and b are extracted from one and the same experiment; they are only plotted in two different ways to focus on the different aspects of the results. For example, an increase of a few degrees in the gas inlet temperature, during ignition of the catalyst, gave rise to an increase in the catalyst bed temperature of hundreds of degrees, which is seen in the insert in Fig. 2a. This rapid rise in catalyst bed temperature explains the differences between the two sets of curves. The conversion plots shown should thus be seen as transient data. Fig. 2a clearly shows a simultaneous ignition of hydrogen and carbon monoxide at 155°C. The ignition curves are steep and almost 100% conversion is reached quickly. The ignition temperature (expressed here as T.o) for methane
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was approximately 5°C higher with respect to the gas inlet temperature. However, when the catalyst temperature is considered (Fig. 2b), the difference in ignition temperature between hydrogen and carbon monoxide on the one hand, and methane on the other was nearly 500°C. This is a result of the catalyst being heated up by the combustion of hydrogen and carbon monoxide. During the time needed to heat the inlet gas from 150 to 2OO”C, the catalyst bed temperature rose from 180 to 900°C. Methane conversion levelled off at around 90% in nearly all experiments, which is probably due to external diffusion limitation. When, upon further temperature increase, homogeneous reactions became significant, conversion of methane rose to 100%. An interesting observation was made in a preliminary experiment in which the fuel gas was not added to the feed before the temperature was about 20°C below the ignition temperature. This procedure led to immediate ignition of the catalyst and final conversions were reached within a few seconds. With respect to the gas inlet temperature, the near-simultaneous ignition of all combustible components in the fuel gas, which was seen at an oxygen-fuel ratio of 2.7, was no longer observed when this ratio was increased. Fig. 3 shows the conversion of hydrogen, carbon monoxide, and methane as well as the catalyst bed temperature as a function of the gas inlet temperature at oxygen-fuel ratio 4.3. It is clear that the ignition now occurs in two steps. Carbon monoxide and hydrogen still ignited simultaneously, as under all studied conditions, but the ignition occurred at a gas inlet temperature that was nearly 20°C lower than for the mixture with an oxygen-fuel ratio of 2.7. However, the T,, of methane (gas inlet) increased from 157 to 239°C when the oxygen-fuel ratio was increased
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332
from 2.7 to 4.3. The two-step ignition is further demonstrated by the catalyst bed temperature curve. Two steep increases are clearly seen upon the ignitions of hydrogen and carbon monoxide on the one hand and methane on the other. The oxygen-fuel ratio was increased even further, the results of which are given in Fig. 4, showing the catalyst bed temperatures needed for 10% CT,,) conversion of carbon monoxide and methane in the fuel gas as a function of the oxygen-fuel ratio. For carbon monoxide, the value of T,, for carbon monoxide first decreased with increasing oxygen-fuel ratio, but at higher values of this ratio the downward trend levelled off. A similar trend was seen with respect to the gas inlet temperature. However, the T,, for methane strongly increased with increasing oxygen-fuel ratio, with the upward trend levelling off at high values of this ratio. In all the above experiments the oxygen-fuel ratio was varied by controlling the ratio between the synthetic fuel gas and the oxygen-argon mixture. However, in one experiment the oxygen-fuel ratio was decreased by replacing 50% of the oxygen by argon, giving an oxygen-fuel ratio of 1.3. Fig. 5 shows the conversion of the combustible components in the fuel as a function of the catalyst bed temperature at the decreased oxygen concentration. This decrease caused a rise in values for T5a for hydrogen and carbon monoxide with respect to the gas inlet and catalyst bed temperatures of 40 and 8°C respectively. The addition of inert to the fuel gas, simulating a lower heating value of the fuel, thus increased the preheat temperature required for ignition of hydrogen and carbon monoxide. As at normal oxygen concentration, methane ignition occurred nearly simultaneously with hydrogen and carbon monoxide, with a TSO of 195°C (gas inlet
600
190
180 500 _0 0" 7 2 P 400
a c
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0ygen:fuel Fig. 4. Catalyst ignition oxygen-fuel ratio.
temperature
ratio (mol/mol,,Oich)
(I’,,,) for CO (triangles)
and CH,
(squares)
in the fuel gas versus
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1.0
0.6
‘;‘
c 0.6 .o I z 0.4 8 0.2
0.0
0
400
600
Gas inlet temperature
(“C)
Fig. 5. Conversion of combustible components in the fuel gas versus catalyst bed temperature at 50% decreased oxygen concentration. Fuel component concentrations as in Table 1, oxygen-fuel ratio 1.3. CH, conversion at normal oxygen concentration is given for comparison.
temperature). More interesting are the conversion versus catalyst bed temperature plots for methane at normal and decreased oxygen levels, given in Fig. 5. These plots show that methane ignition is favored by the lower oxygen concentration. 3.3. Methane combustion
in the presence and absence of Hz, CO, and CO,
The combustion behavior of methane in the presence and absence of the other components in the fuel gas is shown in Fig. 6, giving the methane conversion both versus gas inlet temperature and catalyst bed temperature for two different methane concentrations. In Fig. 6a, the methane concentration is 1.2%, both with and without the other components, which corresponds to an oxygen-fuel ratio of 5.5. The methane concentration and oxygen-fuel ratio in Fig. 6b are 2.3 and 2.7, respectively. Combustion of methane in the fuel gas is much easier than for methane alone, which follows from the differences in gas inlet ignition temperature being about 200°C and 300°C for methane concentrations of 1.2 and 2.3%, respectively. At the lowest of these two concentrations methane ignites at a 300°C (cf. Fig. 4) higher catalyst temperature than carbon monoxide and hydrogen. This difference may be partly due to the heat released by the combustion of carbon monoxide and hydrogen, which helps to heat up the catalyst to a temperature high enough for methane combustion. This is clear when the methane conversion versus catalyst bed temperature plot is regarded. There is no significant difference for
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0.8
200
0
400
Gas inlet /catalyst
600
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bed temperature
1000
(“C)
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-.-.---.- In fuel gas
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g .e 9 s 0
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0.2 -
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(b)
0.0
t0
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Gas inlet / catalyst
600
bed temperature
800
1000
(“C)
Fig. 6. Conversion of CH, versus gas inlet temperature and catalyst bed temperature in the presence and absence of CO, H,, and CO,. (a) Methane concentration 1.2% in both cases, i.e. oxygen-fuel ratio 5.5 for fuel gas. (b) Methane concentration 2.3% in both cases, i.e. oxygen-fuel ratio 2.7 for fuel gas.
the cases in which the other fuel gas components are present or absent. This situation is different though for the conditions prevailing in Fig. 6b. In this case, methane ignites simultaneously with carbon monoxide and hydrogen (cf. Fig. 2a) and here the methane conversion versus catalyst bed temperature shows a distinction between the cases with the other fuel gas components present or absent. When catalyst bed temperatures are compared, the methane ignition temperature is decreased by 100°C due to the presence of carbon monoxide and hydrogen. Hence, the combustion of carbon monoxide and hydrogen seems to have a mechanistic effect on the combustion of methane when the ignition of all three components occurs simultaneously.
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0.8
200
400
Gas inlet I catalyst
600 bed temperature
800 (“C)
Fig. 7. Conversion of CH, in the fuel gas versus gas inlet temperature and catalyst bed temperature presence and absence of 0.7% water vapor in the feed. Oxygen-fuel ratio 2.7.
in the
3.4. Influence of water and benzene on fuel gas combustion The effect of the addition of 0.7% water vapor to the feed is shown in Fig. 7, depicting the conversion of methane in the fuel gas as a function of the gas inlet temperature and catalyst bed temperature in the presence and absence of water vapor in the feed. It is shown that the gas inlet ignition temperature for methane slightly increased when water vapor was added to the feed. On the other hand, the catalyst bed ignition temperature was lowered by nearly 200°C due to the presence of water in the feed. The simultaneous addition of water vapor and benzene to the feed was studied in one experiment, the results of which are depicted in Fig. 8, which gives the conversions of all combustible components in the fuel gas mixture as a function of the gas inlet temperature. This figure also shows the temperature dependence of the conversion of benzene when fed as sole combustible component. Two observations can be made, the first being that benzene combustion was enhanced by the presence of the other combustible components in the fuel gas mixture. Benzene ignited simultaneously with carbon monoxide and hydrogen at a temperature about 20°C lower than it did in the absence of the other components. The second observation is that the presence of benzene appeared to have no significant influence on the catalyst bed ignition temperatures for hydrogen, carbon monoxide, and methane. However, the gas inlet ignition temperatures increased by 15-20°C as a result of the benzene addition.
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0.2
150
200
Gas inlet
250
temperature
300
350
(“C)
Fig. 8. Conversion of benzene and other combustible fuel gas components versus gas inlet temperature in the presence (oxygen-fuel ratio 2.5) and absence of CO, H,, CH,, H,O, and CO,. Benzene concentration 600 ppm in both cases.
3.5. InfZuence of ammonia The combustion of the fuel gas in air in the presence of ammonia yielded an ignition temperature for carbon monoxide and hydrogen of 138°C with respect to the inlet gas, which can be compared with an ignition temperature of 142°C under the same conditions without ammonia. The effect of ammonia on the combustion of the other components is thus very weak, though it needs to be pointed out that the ammonia concentration in the air-fuel mixture was 15 ppm only. The difference in ignition temperature for CO/H, between air and the oxygen-argon mixture is about 10°C and is probably due to a difference in oxygen concentration. The nitric oxide concentration during combustion of the ammonia-containing fuel gas was about 15 ppm, whereas no nitric oxide was formed during combustion of an ammonia-free fuel gas. Thus, all the ammonia in the fuel gas was converted to nitric oxide over the studied platinum catalyst. No nitrogen dioxide formation was observed.
4. Discussion The results given in the previous section show that the composition feed has a noticeable influence on the combustion behaviour.
of the
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4.1. Effect behaviour
of fuel
gas composition
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and oxygen-fuel
337
ratio on combustion
The simultaneous ignition of carbon monoxide and hydrogen, observed in all combustion experiments, is a phenomenon that can be explained by carbon monoxide inhibition of oxidation over the platinum catalyst. A negative reaction order for carbon monoxide due to its strong chemisorption on platinum group metals in oxidation reactions has been reported earlier [ 15-171, the effect being strongest at the highest carbon monoxide concentrations. In fact, at low concentrations, the reaction order for carbon monoxide is shifted from negative to positive, as shown by Wei [15]. He observed a maximum in the carbon monoxide oxidation rate over platinum at a few tenths of a percent of carbon monoxide. Similar observations were made by Oh and Eickel in their investigation of carbon monoxide oxidation over rhodium catalysts [17]. This behavior in carbon monoxide oxidation agrees with the trend in Fig. 4. The ignition temperature decreases at higher oxygen-fuel ratios, since the inhibition by carbon monoxide becomes less strong. The fact that the downward trend levels off may be related to the shift from a regime with high surface carbon monoxide surface coverage (negative order in carbon monoxide) to a regime with low carbon monoxide surface coverage (positive reaction order). The carbon monoxide concentrations corresponding to the oxygen-fuel ratios of 5.5 and 10.0 in our experiments are 1.4 and 0.7%, respectively. Given the presence of hydrogen in our case, our observations seem to confirm the assumptions made by Wei. The strong adsorption of carbon monoxide below 250°C was demonstrated in the carbon monoxide TPD, given in Fig. 1. The carbon monoxide that covers the surface inhibits the adsorption of hydrogen and oxygen and the oxidation of hydrogen is strongly retarded. The oxidation of hydrogen cannot start until carbon monoxide starts to desorb to a significant extent, which lowers the carbon monoxide surface coverage and enables adsorption of hydrogen and/or oxygen. Hydrogen is easily oxidized at temperatures below ambient in the absence of carbon monoxide over this catalyst [18]. Moreover, when the catalyst was heated to 20°C below the ignition temperature in the absence of the fuel gas, ignition took place immediately after addition of the fuel to the feed. This procedure apparently led to the circumvention of inhibition by carbon monoxide. The platinum surface was covered with oxygen which momentarily reacted with the hydrogen in the fuel gas. The heat released by this reaction was sufficient to heat the catalyst to a temperature above the carbon monoxide ignition point. Consequently, the carbon monoxide was converted rapidly and could not inhibit the adsorption and subsequent reaction of hydrogen. This shows how strong the effect of the presence of carbon monoxide is for the catalytic combustion of the fuel over platinum catalysts. Hence, the carbon monoxide content of the gas will be an important factor, determining the ignition temperature of LHV gases over such catalysts.
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The heat generated by the combustion of hydrogen and carbon monoxide helps to heat up the catalyst, which facilitates ignition of methane in the fuel gas. Fig. 6a and b show that the gas inlet ignition temperature for methane is decreased by 200°C and 300°C for methane concentrations of 1.2% and 2.3%, respectively, due to the presence of carbon monoxide and hydrogen. These two methane concentrations represent two cases. In the first case of 1.2% methane (oxygen-fuel ratio 5.5 for the fuel gas), methane ignites at 250°C higher gas inlet temperature than carbon monoxide. This means that when methane ignites, carbon monoxide and hydrogen conversion are nearly 100%. The presence of carbon monoxide and hydrogen appears not to have any influence on the ignition of methane under these conditions, judging from the conversion versus catalyst bed temperature plots. In the second case of 2.3% methane (oxygen-fuel ratio 2.7 for the fuel gas) the methane ignites simultaneously with carbon monoxide and hydrogen (cf. Fig. 2a). In this case an enhancement of the methane ignition due to the presence of carbon monoxide and hydrogen is observed when the conversion versus catalyst bed temperature plots are regarded. The two different cases shown here represent two ignition processes. In the case with low fuel concentration, methane ignition takes place when carbon monoxide and hydrogen conversion are nearly 100%. This means that ignition occurs on a platinum surface with high oxygen coverage, as is the case when methane is combusted alone [19]. Hence, carbon monoxide and hydrogen combustion only help to heat up the catalyst, which explains that no difference is observed between the conversion versus catalyst bed temperature plots in the presence and absence of the other fuel gas components. In the case with high fuel concentration, methane ignites on a surface where carbon monoxide and hydrogen ignition also takes place. This means that the oxygen coverage will be lower, due to carbon monoxide and hydrogen combustion, which facilitates the adsorption of methane. The enhanced adsorption of methane increases the reaction rate, which is seen in the conversion versus catalyst bed temperature plots in Fig. 6b. The catalyst bed ignition temperature of methane is lowered by more than 100°C due to the presence of carbon monoxide and hydrogen. If carbon monoxide and hydrogen combustion had no mechanistic effect, the conversion versus catalyst bed temperature plots would overlap, as in Fig. 6a, which is not seen in this case. The decrease in oxygen concentration in the feed, by adding extra argon, increased the ignition temperature for carbon monoxide and hydrogen; compare Fig. 2b and Fig. 5. This observation is in agreement with the assumption of a high carbon monoxide coverage of the platinum surface at low temperatures and low conversions. The decrease in ignition temperature of the catalyst bed for methane combustion resulting from the lowered oxygen concentration is in agreement with the assumption of an oxygen-covered surface during methane combustion. This is further supported by the results given in Fig. 4, which shows that the ignition of methane occurs at
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higher catalyst bed temperatures with increasing oxygen-fuel ratios, i.e. lower fuel concentration. Addition of 0.7% water vapor to the feed increased the ignition temperatures for the combustible components only by about 10°C with respect to the gas inlet temperature. On the other hand, Fig. 7 showed that the methane ignition temperature decreased due to water addition to the feed, when regarding the catalyst bed temperature. An inhibiting effect by water on methane combustion over palladium catalysts was recently demonstrated [8,9]. However, this inhibition decreased with increasing temperature and was almost negligible at a catalyst temperature of 450°C [S]. In our experiments at oxygen-fuel ratio 2.7, methane ignited at gas inlet temperatures that were only a few degrees higher than carbon monoxide and hydrogen. Under such conditions, both reactants and products, i.e. water and carbon dioxide were present, creating a rather complex mixture, which made it difficult to predict possible mechanisms and platinum surface states. Therefore, a more detailed investigation is needed to clarify the observed influence of water. The addition of 600 ppm benzene to the feed showed that benzene, being a model aromatic compound, can be readily combusted over the platinum catalyst studied (Fig. 8). The presence of the other combustible components decreases the ignition temperature for benzene by about 20°C. The final conversion that was achieved, about 90%, is likely to be controlled by external diffusion limitations. Similar mass transfer limitations in benzene oxidation over monolithic platinum catalysts were observed by Barresi and Baldi [20]. They proposed a mechanism for benzene oxidation in air in which gaseous benzene reacts with adsorbed oxygen. Our results show that benzene hardly influenced the ignition of hydrogen and carbon monoxide. It is not clear if the mechanism proposed by Barresi and Baldi is valid for a carbon monoxide-covered platinum surface. 4.2. Technological
implications
Carbon monoxide dominates the ignition of the gas mixture studied, and it is desirable to have a thorough knowledge of the carbon monoxide combustion kinetics in an LHV gas under all realistic conditions. The hydrogen is completely converted once the carbon monoxide ignites. Methane, the gaseous hydrocarbon that is most difficult to oxidize, is converted at an inlet temperature over 200°C lower in the presence of carbon monoxide and hydrogen than in the absence of these compounds. This is important since it determines the necessary preheating temperature in industrial applications. The platinum catalyst studied was very active in oxidation of benzene, the model aromatic compound. However, actual gasified biomass contains other heavier aromatic compounds, such as naphthalene, which have to be oxidized. Another study in our laboratory showed that platinum was very active in the oxidation of naphthalene as well, in the presence of carbon monoxide [21].
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As mentioned in Section 3.2, the inhibiting action of carbon monoxide during ignition of the fuel gas was circumvented by heating the catalyst in the absence of the fuel gas. This observation should be borne in mind during the design and development of start-up procedures of actual catalytic combustors fueled with carbon monoxide-containing LHV gas. The presence of ammonia did not influence the combustion of the other fuel components under the conditions studied. However, ammonia levels that are more than 10 times higher can be expected under actual operation, in which case the influence of ammonia may be stronger. This is likely to be dependent on the type of catalyst used. The platinum catalyst in this study converted all ammonia to nitric oxide. Metal oxides catalysts may offer an alternative with a high selectivity for molecular nitrogen formation instead [ 11. The use of a combination of different types of catalysts may be an interesting approach. This selective conversion of ammonia in gasified biomass to nitrogen, and not to nitrogen oxides, is currently being studied in our laboratory; the results will be published separately [22].
5. Conclusion The presented results clearly demonstrate the influences on combustion behaviour of the fuel gas composition and the oxygen-fuel ratio. Still, a more complete mapping of these effects is necessary if we are to gain a thorough understanding of the behavior of such a complex LHV gas. Such understanding is required for the design of catalytic combustors using LHV gases from biomass or coal gasifiers. Combustion catalysts for this application should not only be active for oxidation of the combustible components in the fuel, they should also have a low selectivity for the conversion of ammonia to nitrogen oxides. Moreover, they should be insensitive to tar compounds that are present in the product gas from coal or biomass gasifiers. These issues are now investigated as part of the on-going work on catalytic combustion in our laboratory.
Acknowledgements This work was supported by the Thermal Engineering Research Institute (VZrmeforsk) and the Swedish National Board for Industrial and Technical Development (NUTEK). We thank Perstorp AB for providing the platinum catalyst and P. Govind Menon for valuable discussions and comments.
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References [l] R. Prasad, L.A. Kennedy and E. Ruckenstein, Comb. Sci. Techn., 27 (1981) 45. 121 W.D. Clark, B.A. Folsom, W.R. Seeker and C.W. Courtney, Trans. ASME, 104 (1982) 120. [3] D.L. Trimm, Appl. Catal., 7 (1983) 249. [4] R. Prasad, L.A. Kennedy and E. Ruckenstein, Catal. Rev.-Sci. Eng., 26 (1984) 1. [5] L.D. Pfefferle and W.C. Pfefferle, Catal. Rev.-Sci. Eng., 29 (1987) 219. [6] H. Arai and M. Machida, Catal. Today, 10 (1991) 81. [7] M.F.M. Zwinkels, S.G. JZt%, P.G. Menon and T.A. Griffin, Catal. Rev.-Sci. Eng., 35 (1993) 319. [8] R. Burch, F.J. Urban0 and P.K. Loader, Appl. Catal. A, 123 (1995) 173. [9] F.H. Ribeiro, M. Chow and R.A. Dalla Betta, J. Catal., 146 (1994) 537. [lo] C.F. Cullis, T.G. Nevell and D.L. Trimm, J. Chem. Sot. Faraday Trans. 1, 68 (1972) 1406. [ll] P. Briot, A. Aucroux, D. Jones and M. Primet, Appl. Catal., 59 (1990) 141. [12] R. Burch and P.K. Loader, Appl. Catal. B, 5 (1994) 149. [13] G. Groppi, A. Belloli, E. Tronconi and P. Forzatti, in JECAT ‘95, Lyon-Villeurbanne, 26-28 April 1995, Vol. 1, p. 257. [14] J.E. Benson and M. Boudart, J. Catal., 4 (1965) 704. [151 J. Wei, in D.D. Eley, H. Pines and P.B. Weisz (Editors), Advances in Catalysis, Vol. 24, Academic Press, London, 1975, p, 57. [16] Y.-F. Yu Yao, J. Catal., 87 (1984) 152. [17] S.H. Oh and CC. Eickel, J. Catal., 112 (1988) 543. [181 0. Augustsson, personal communication, Perstorp AB, Perstorp, Sweden, 1995. [19] S.H. Oh, P.J. Mitchell and R.M. Siewert, J. Catal., 132 (1991) 287. [20] A.A. Barresi and G. Baldi, Chem. Eng. Sci., 47 (1992) 1943. [21] J. Carnd, M. Berg and S.G. &rls, Fuel, in press. [22] E.M. Johansson, S.G. JXrLs and P.G. Menon, in preparation.
Catalytic combustion of gasified biomass over Pt/Al,O, Marcus F.M. Zwinkels *, G.M. Eloise Heginuz, Bjijrn H. Gregertsen, Krister Sj&trijm, Sven G. J&-b Kungl Tekniska Hiigskolan (Royal Institute of Technology), Department of Chemical Engineering and Technology, Chemical Technology, S-100 44 Stockholm, Sweden
Received 23 February 1995: revised 8 March 1996; accepted 9 March 1996
Abstract Catalytic combustion of gasified biomass over Pt/Al,O, was investigated to clarify its dependence on the fuel gas composition and the air-fuel ratio. An experimental study was performed using synthetic mixtures of the main components in gasified biomass: carbon monoxide, hydrogen, methane, carbon dioxide, water, benzene (model aromatic compound), ammonia, and inert (argon or nitrogen). Results showed that carbon monoxide and hydrogen ignited simultaneously over the catalyst used, with methane having a higher ignition temperature than that of carbon monoxide and hydrogen. The ignition temperature for carbon monoxide and hydrogen decreased as the oxygen-fuel ratio and inert concentration increased. The presence of water vapor in the feed had an inhibiting effect on the ignition of carbon monoxide, whereas the opposite was observed for methane. Methane ignition was favored by the presence of carbon monoxide and hydrogen, not only because their combustion heated the catalyst but also because it attenuated the oxygen inhibition of methane combustion under certain conditions. These effects were explained by co-adsorption phenomena involving fuel components and oxygen. Ammonia was found not to influence the combustion of the other components and was completely converted to nitrogen monoxide over the used catalyst. Keywords: Catalytic combustion; Low heating value gas; Platinum; Gasified biomass
1. Introduction Catalytic fuel combustion is gaining ground as a technology which enables strong reduction of nitrogen oxide emissions (NO,) from combustion processes.
* Corresponding author. Tel.: (+ 46-8) 7908254; fax: (+46-g) 108579; e-mail: [email protected]. 0926-860X/96/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PZZSO926-860X(96)00114-7
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In a catalytic combustor, a premixed lean air-fuel mixture is ignited by a catalyst, which enables complete combustion at maximum temperatures far lower than possible in conventional gas-phase combustors. Consequently, the formation of thermal NO, can almost completely be circumvented, while the combustion efficiency is high. Catalytic combustion has proved to be an effective technology for avoiding NO, generation from fuels with low nitrogen contents, such as natural gas. However, for countries that lack large fossil fuel reserves, biofuels are considered to be a promising alternative. Such fuels have much higher nitrogen contents, which during combustion are converted to NO, (fuel NO,). Fuel NO, accounts for a significant part of the total NO, formed during combustion of nitrogen-containing fuels. Direct combustion of biomass, being a solid fuel, generates a number of pollutants. The level of these pollutants can be reduced by using a two-step process for the generation of thermal energy from biomass. The first step is the gasification of the fuel in an oxygen-deficient atmosphere. In the second step, the main product of this gasification, a low heating value gas (LHV), is combusted. The main advantage of this approach is that a gas combusts more readily than a solid fuel. Moreover, a hot-gas cleaning step can be introduced after the gasifier. The gas from a biomass gasifier contains up to a few thousand ppm of ammonia. This ammonia is largely converted into NO, in conventional combustors. Catalytic combustion may enable conversion of the ammonia to molecular nitrogen instead [ 1,2]. Besides the favourable conversion of ammonia to nitrogen, the catalyst stabilizes the combustion of the LHV gas. An LHV gas, such as gasified biomass, is a complex mixture containing carbon monoxide, hydrogen, carbon dioxide, methane and other light hydrocarbons, water, ammonia, nitrogen, and heavier aromatic hydrocarbons (tars). The presence of all these components is likely to influence the behaviour of a combustion catalyst and an understanding of the interactions between the LHV gas components during catalytic combustion is desirable. So far, most of the reported work on catalytic combustion has focused on methane and natural gas [3-71. The effect of water and carbon dioxide on methane combustion over palladium catalysts was recently reported: Burch et al. [8] found that both water and carbon dioxide had an inhibiting effect, this effect being strongest for water, which was in agreement with the observations from Ribeiro et al. [9]. Cullis et al. showed that carbon monoxide and water inhibited methane combustion, whereas no inhibition was observed for carbon dioxide [lo]. Platinum catalysts have also been studied extensively in methane combustion. It was shown that methane combustion over platinum is a complex process depending among other things on the pretreatment history of the catalyst [ 1 l] and the oxygen-methane ratio [ 121. The aim of the five studies mentioned above was to investigate the effects of conditions and reaction products on the combustion of methane. Groppi et al.
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[ 131 studied catalytic combustion of an LHV gas, represented by carbon monoxide-hydrogen mixtures. They found carbon monoxide to be more readily combusted than hydrogen over manganese-containing hexa-aluminate catalysts and they observed no differences between combustion of the mixture and of the two separate components. In summary, the components in an LHV gas, like gasified biomass, may affect the combustion of such a gas over a catalyst in complex ways, depending on test conditions. For the proper design of catalytic combustors using gasified biomass, a more complete understanding of these influences is required. The above-mentioned studies on methane combustion give certain information that is valuable for the understanding of catalytic combustion of LHV gases. However, to obtain a more complete picture of the possible interactions we studied combustion of mixtures with most of the important components in LHV gases. In particular, the influences of fuel gas composition and air-fuel ratio are discussed in this paper. Moreover, the role of ammonia is investigated, both with respect to its conversion and with respect to the influence ammonia may have on the combustion of other fuel components.
2. Experimental 2.1. Catalyst properties Combustion experiments were conducted using a commercial Pt/Al,O, catalyst, provided by Perstorp AB (NX-45, Perstorp, Sweden). This catalyst, commercially used for oxidation of carbon monoxide and volatile organic compounds from industrial processes, had a surface area of 244 m2/g and was supplied as spherical pellets, 5 mm in diameter. The Pt loading of the catalyst was 0.14 wt.-%. The Pt was deposited to give an egg-shell catalyst, minimizing internal diffusion limitations. 2.2. Chemisorption
experiments
The Pt dispersion of the fresh catalyst was determined by hydrogen-oxygen titration using a Micromeritics TPD/TPR 2900 temperature-programmed chemisorption instrument equipped with a thermal conductivity detector. The hydrogen-oxygen titration measurement was carried out according to the method developed by Benson and Boudart [14]. The same instrument was used to perform temperature programmed desorption (TPD) of carbon monoxide. In both cases the samples were prereduced in-situ with hydrogen, after which titration (H2/02) or saturation (CO) was performed at ambient conditions. TPD of CO was done in a helium flow of 50 cm3/min with a heating rate of lO”C/min. Water desorbing during TPD experiments was removed in a cold trap filled with a mixture of liquid nitrogen and 2-propanol.
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2.3. Combustion experiments Combustion of a synthetic gas mixture, representing a gasified biomass, was tested in a continuous flow reactor, operating at atmospheric pressure. The synthetic gas mixture is referred to as fuel gas in the following sections. The catalyst sample, typically 8 cm3, was placed in an Inconel reactor situated in a tubular furnace, which was used to heat the fuel gas-oxidant mixture prior to entering the catalyst bed and was programmed to increase the temperature of the gas fed to the catalyst bed by 2”C/min. The space velocity was 50,000 hh’. All reaction products were analyzed on-line by a Balzers QMG 421 quadrupole mass spectrometer. The rapid analysis provided by the mass spectrometer was needed to follow the fast ignitions observed during the experiments, in particular for hydrogen and carbon monoxide. A synthetic bottled mixture of four of the main components of gasified biomass was used in all experiments: 24.9 mol-% methane, 30.1 mol-% carbon monoxide, 28.1 mol-% carbon dioxide, and 16.9 mol-% hydrogen. This mixture is representative of the ratios of these four components in gasified biomass. The above mixture was added to a 20 mol-% oxygen-in-argon mixture, giving oxygen-fuel ratios between 2.7 and 10.1, i.e. lean mixtures in all cases. The oxygen-fuel ratio is the supplied amount of oxygen divided by the stoichiometric amount of oxygen. In some experiments water and benzene were added. In these experiments, mixtures with an oxygen-fuel ratio of 2.7 were used, giving the concentrations shown in Table 1. In one experiment the oxygen concentration was decreased by 50% by adding extra argon, thus keeping the other concentrations constant. Methane and benzene were combusted both in the fuel gas and as sole combustible components to elucidate the influence of the presence of the other fuel gas components. Ammonia was added in one experiment in a concentration (see Table 1) that represents about 100 ppm in the fuel gas. In this experiment air was used instead
Table 1 Composition of test gas mixture corresponding to an oxygen-fuel benzene, and ammonia added in certain experiments Component
Concentration
W
H,O Benzene
2.3% 2.7% 2.7% 1.6% 18.6-18.9% balance 0.7% 600 ppm
NH,
15 ppm
co co2 H* 02
AP
a N, in the experiments
ratio of 2.7 and the concentrations
during which NO, analysis was performed.
of water,
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of the oxygen-argon mixture, which was needed for the chemiluminescence NO, analysis (Eco Physics CLD 700). A second experiment in air was conducted, both to measure the NO, produced from nitrogen from the air, and to investigate the effect of ammonia on the conversion of the other components.
3. Results 3. I. Chemisorption
experiments
The platinum dispersion of the fresh catalysts, determined by Hz/O, titration, was 27%. The results of carbon monoxide TPD from a fresh catalyst are shown in Fig. 1. It is clear that carbon monoxide is strongly chemisorbed at catalyst temperatures below 250°C although a desorption onset is observed between 100 and 150°C. This is important since this is the temperature range of interest for the combustion experiments, as will be shown in the following sections. 3.2. Injluence of the oxygen-fuel
ratio on fiel gas combustion
Several combustion experiments were conducted with a gas mixture of the composition described in the previous section, corresponding to an oxygen-fuel ratio of 2.7. Fig. 2a and b show the conversions of the combustible components in the fuel gas versus the gas inlet temperature and catalyst temperature respectively. In many of the plots shown in this section, conversion is depicted as a function of either gas inlet temperature or catalyst bed temperature,
I 0
I 100
200
Catalyst
.
I
300
I
I
.
400
bed temperature
Fig. 1. CO TPD from a fresh Pt/alumina
I
500
(“C) catalyst.
,
I
600
330
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1.0
I
El
= ~5800
0.8 - 2
t; ';‘
.o -c z!z 2 5
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;
600
2.z & s
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0
50
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r-----,
6 ,000
100
Gas
150
inlet
_____..._._....----.
-co
%
200
temperature
/------------
250
(“C)
0
i
---...... C%
:
0.2 -
0.0
(a)
’ 50
-1:
I
100
I
150
200
Gas inlet temperature
250
(72)
0.8
0.0 0
200
400
Catalyst
600
bed temperature
800
1000
(“C)
Fig. 2. Conversion of H,, CO, and CH, in the fuel gas versus (a) gas inlet temperature temperature gas at an oxygen-fuel ratio of 2.7.
and (b) catalyst bed
depending on the phenomenon which is to be elucidated. Note that the results given in Fig. 2a and b are extracted from one and the same experiment; they are only plotted in two different ways to focus on the different aspects of the results. For example, an increase of a few degrees in the gas inlet temperature, during ignition of the catalyst, gave rise to an increase in the catalyst bed temperature of hundreds of degrees, which is seen in the insert in Fig. 2a. This rapid rise in catalyst bed temperature explains the differences between the two sets of curves. The conversion plots shown should thus be seen as transient data. Fig. 2a clearly shows a simultaneous ignition of hydrogen and carbon monoxide at 155°C. The ignition curves are steep and almost 100% conversion is reached quickly. The ignition temperature (expressed here as T.o) for methane
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was approximately 5°C higher with respect to the gas inlet temperature. However, when the catalyst temperature is considered (Fig. 2b), the difference in ignition temperature between hydrogen and carbon monoxide on the one hand, and methane on the other was nearly 500°C. This is a result of the catalyst being heated up by the combustion of hydrogen and carbon monoxide. During the time needed to heat the inlet gas from 150 to 2OO”C, the catalyst bed temperature rose from 180 to 900°C. Methane conversion levelled off at around 90% in nearly all experiments, which is probably due to external diffusion limitation. When, upon further temperature increase, homogeneous reactions became significant, conversion of methane rose to 100%. An interesting observation was made in a preliminary experiment in which the fuel gas was not added to the feed before the temperature was about 20°C below the ignition temperature. This procedure led to immediate ignition of the catalyst and final conversions were reached within a few seconds. With respect to the gas inlet temperature, the near-simultaneous ignition of all combustible components in the fuel gas, which was seen at an oxygen-fuel ratio of 2.7, was no longer observed when this ratio was increased. Fig. 3 shows the conversion of hydrogen, carbon monoxide, and methane as well as the catalyst bed temperature as a function of the gas inlet temperature at oxygen-fuel ratio 4.3. It is clear that the ignition now occurs in two steps. Carbon monoxide and hydrogen still ignited simultaneously, as under all studied conditions, but the ignition occurred at a gas inlet temperature that was nearly 20°C lower than for the mixture with an oxygen-fuel ratio of 2.7. However, the T,, of methane (gas inlet) increased from 157 to 239°C when the oxygen-fuel ratio was increased
1
.o
1000
~__________--------%.
yl. 0.8 ;:-__._r-.-.-.- c
-co ';‘ ----H* c 0.6 ~'~~~~~~~ CH‘l .g _ ____-. T cat fii z 0.4 s
;! ,.j _._._.-.-.”
,/.-~-~-‘-
- 600 n
B
; i
3
- 400 B G e z - 200 a G
1' I
0.2 _._._.-._,_/.0.0 50
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,' ,_.' __._..... .,..__........I I 150 200 250
Gas inlet temperature Fig. 3. Conversion of H,, CO, and CH, temperature at an oxygen-fuel ratio of 4.3.
c - 800 p$
I 300
3500
(‘32)
in the fuel gas, and catalyst
bed temperature
versus
gas inlet
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from 2.7 to 4.3. The two-step ignition is further demonstrated by the catalyst bed temperature curve. Two steep increases are clearly seen upon the ignitions of hydrogen and carbon monoxide on the one hand and methane on the other. The oxygen-fuel ratio was increased even further, the results of which are given in Fig. 4, showing the catalyst bed temperatures needed for 10% CT,,) conversion of carbon monoxide and methane in the fuel gas as a function of the oxygen-fuel ratio. For carbon monoxide, the value of T,, for carbon monoxide first decreased with increasing oxygen-fuel ratio, but at higher values of this ratio the downward trend levelled off. A similar trend was seen with respect to the gas inlet temperature. However, the T,, for methane strongly increased with increasing oxygen-fuel ratio, with the upward trend levelling off at high values of this ratio. In all the above experiments the oxygen-fuel ratio was varied by controlling the ratio between the synthetic fuel gas and the oxygen-argon mixture. However, in one experiment the oxygen-fuel ratio was decreased by replacing 50% of the oxygen by argon, giving an oxygen-fuel ratio of 1.3. Fig. 5 shows the conversion of the combustible components in the fuel as a function of the catalyst bed temperature at the decreased oxygen concentration. This decrease caused a rise in values for T5a for hydrogen and carbon monoxide with respect to the gas inlet and catalyst bed temperatures of 40 and 8°C respectively. The addition of inert to the fuel gas, simulating a lower heating value of the fuel, thus increased the preheat temperature required for ignition of hydrogen and carbon monoxide. As at normal oxygen concentration, methane ignition occurred nearly simultaneously with hydrogen and carbon monoxide, with a TSO of 195°C (gas inlet
600
190
180 500 _0 0" 7 2 P 400
a c
160
0ygen:fuel Fig. 4. Catalyst ignition oxygen-fuel ratio.
temperature
ratio (mol/mol,,Oich)
(I’,,,) for CO (triangles)
and CH,
(squares)
in the fuel gas versus
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1.0
0.6
‘;‘
c 0.6 .o I z 0.4 8 0.2
0.0
0
400
600
Gas inlet temperature
(“C)
Fig. 5. Conversion of combustible components in the fuel gas versus catalyst bed temperature at 50% decreased oxygen concentration. Fuel component concentrations as in Table 1, oxygen-fuel ratio 1.3. CH, conversion at normal oxygen concentration is given for comparison.
temperature). More interesting are the conversion versus catalyst bed temperature plots for methane at normal and decreased oxygen levels, given in Fig. 5. These plots show that methane ignition is favored by the lower oxygen concentration. 3.3. Methane combustion
in the presence and absence of Hz, CO, and CO,
The combustion behavior of methane in the presence and absence of the other components in the fuel gas is shown in Fig. 6, giving the methane conversion both versus gas inlet temperature and catalyst bed temperature for two different methane concentrations. In Fig. 6a, the methane concentration is 1.2%, both with and without the other components, which corresponds to an oxygen-fuel ratio of 5.5. The methane concentration and oxygen-fuel ratio in Fig. 6b are 2.3 and 2.7, respectively. Combustion of methane in the fuel gas is much easier than for methane alone, which follows from the differences in gas inlet ignition temperature being about 200°C and 300°C for methane concentrations of 1.2 and 2.3%, respectively. At the lowest of these two concentrations methane ignites at a 300°C (cf. Fig. 4) higher catalyst temperature than carbon monoxide and hydrogen. This difference may be partly due to the heat released by the combustion of carbon monoxide and hydrogen, which helps to heat up the catalyst to a temperature high enough for methane combustion. This is clear when the methane conversion versus catalyst bed temperature plot is regarded. There is no significant difference for
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0.8
200
0
400
Gas inlet /catalyst
600
800
bed temperature
1000
(“C)
1.0
t
-.-.---.- In fuel gas
0.8 -
i
------Alone -7
g .e 9 s 0
vs. Ti, 0.6-
0.4 -
0.2 -
/
-Alone
(b)
0.0
t0
200
400
Gas inlet / catalyst
600
bed temperature
800
1000
(“C)
Fig. 6. Conversion of CH, versus gas inlet temperature and catalyst bed temperature in the presence and absence of CO, H,, and CO,. (a) Methane concentration 1.2% in both cases, i.e. oxygen-fuel ratio 5.5 for fuel gas. (b) Methane concentration 2.3% in both cases, i.e. oxygen-fuel ratio 2.7 for fuel gas.
the cases in which the other fuel gas components are present or absent. This situation is different though for the conditions prevailing in Fig. 6b. In this case, methane ignites simultaneously with carbon monoxide and hydrogen (cf. Fig. 2a) and here the methane conversion versus catalyst bed temperature shows a distinction between the cases with the other fuel gas components present or absent. When catalyst bed temperatures are compared, the methane ignition temperature is decreased by 100°C due to the presence of carbon monoxide and hydrogen. Hence, the combustion of carbon monoxide and hydrogen seems to have a mechanistic effect on the combustion of methane when the ignition of all three components occurs simultaneously.
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0.8
200
400
Gas inlet I catalyst
600 bed temperature
800 (“C)
Fig. 7. Conversion of CH, in the fuel gas versus gas inlet temperature and catalyst bed temperature presence and absence of 0.7% water vapor in the feed. Oxygen-fuel ratio 2.7.
in the
3.4. Influence of water and benzene on fuel gas combustion The effect of the addition of 0.7% water vapor to the feed is shown in Fig. 7, depicting the conversion of methane in the fuel gas as a function of the gas inlet temperature and catalyst bed temperature in the presence and absence of water vapor in the feed. It is shown that the gas inlet ignition temperature for methane slightly increased when water vapor was added to the feed. On the other hand, the catalyst bed ignition temperature was lowered by nearly 200°C due to the presence of water in the feed. The simultaneous addition of water vapor and benzene to the feed was studied in one experiment, the results of which are depicted in Fig. 8, which gives the conversions of all combustible components in the fuel gas mixture as a function of the gas inlet temperature. This figure also shows the temperature dependence of the conversion of benzene when fed as sole combustible component. Two observations can be made, the first being that benzene combustion was enhanced by the presence of the other combustible components in the fuel gas mixture. Benzene ignited simultaneously with carbon monoxide and hydrogen at a temperature about 20°C lower than it did in the absence of the other components. The second observation is that the presence of benzene appeared to have no significant influence on the catalyst bed ignition temperatures for hydrogen, carbon monoxide, and methane. However, the gas inlet ignition temperatures increased by 15-20°C as a result of the benzene addition.
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0.2
150
200
Gas inlet
250
temperature
300
350
(“C)
Fig. 8. Conversion of benzene and other combustible fuel gas components versus gas inlet temperature in the presence (oxygen-fuel ratio 2.5) and absence of CO, H,, CH,, H,O, and CO,. Benzene concentration 600 ppm in both cases.
3.5. InfZuence of ammonia The combustion of the fuel gas in air in the presence of ammonia yielded an ignition temperature for carbon monoxide and hydrogen of 138°C with respect to the inlet gas, which can be compared with an ignition temperature of 142°C under the same conditions without ammonia. The effect of ammonia on the combustion of the other components is thus very weak, though it needs to be pointed out that the ammonia concentration in the air-fuel mixture was 15 ppm only. The difference in ignition temperature for CO/H, between air and the oxygen-argon mixture is about 10°C and is probably due to a difference in oxygen concentration. The nitric oxide concentration during combustion of the ammonia-containing fuel gas was about 15 ppm, whereas no nitric oxide was formed during combustion of an ammonia-free fuel gas. Thus, all the ammonia in the fuel gas was converted to nitric oxide over the studied platinum catalyst. No nitrogen dioxide formation was observed.
4. Discussion The results given in the previous section show that the composition feed has a noticeable influence on the combustion behaviour.
of the
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ratio on combustion
The simultaneous ignition of carbon monoxide and hydrogen, observed in all combustion experiments, is a phenomenon that can be explained by carbon monoxide inhibition of oxidation over the platinum catalyst. A negative reaction order for carbon monoxide due to its strong chemisorption on platinum group metals in oxidation reactions has been reported earlier [ 15-171, the effect being strongest at the highest carbon monoxide concentrations. In fact, at low concentrations, the reaction order for carbon monoxide is shifted from negative to positive, as shown by Wei [15]. He observed a maximum in the carbon monoxide oxidation rate over platinum at a few tenths of a percent of carbon monoxide. Similar observations were made by Oh and Eickel in their investigation of carbon monoxide oxidation over rhodium catalysts [17]. This behavior in carbon monoxide oxidation agrees with the trend in Fig. 4. The ignition temperature decreases at higher oxygen-fuel ratios, since the inhibition by carbon monoxide becomes less strong. The fact that the downward trend levels off may be related to the shift from a regime with high surface carbon monoxide surface coverage (negative order in carbon monoxide) to a regime with low carbon monoxide surface coverage (positive reaction order). The carbon monoxide concentrations corresponding to the oxygen-fuel ratios of 5.5 and 10.0 in our experiments are 1.4 and 0.7%, respectively. Given the presence of hydrogen in our case, our observations seem to confirm the assumptions made by Wei. The strong adsorption of carbon monoxide below 250°C was demonstrated in the carbon monoxide TPD, given in Fig. 1. The carbon monoxide that covers the surface inhibits the adsorption of hydrogen and oxygen and the oxidation of hydrogen is strongly retarded. The oxidation of hydrogen cannot start until carbon monoxide starts to desorb to a significant extent, which lowers the carbon monoxide surface coverage and enables adsorption of hydrogen and/or oxygen. Hydrogen is easily oxidized at temperatures below ambient in the absence of carbon monoxide over this catalyst [18]. Moreover, when the catalyst was heated to 20°C below the ignition temperature in the absence of the fuel gas, ignition took place immediately after addition of the fuel to the feed. This procedure apparently led to the circumvention of inhibition by carbon monoxide. The platinum surface was covered with oxygen which momentarily reacted with the hydrogen in the fuel gas. The heat released by this reaction was sufficient to heat the catalyst to a temperature above the carbon monoxide ignition point. Consequently, the carbon monoxide was converted rapidly and could not inhibit the adsorption and subsequent reaction of hydrogen. This shows how strong the effect of the presence of carbon monoxide is for the catalytic combustion of the fuel over platinum catalysts. Hence, the carbon monoxide content of the gas will be an important factor, determining the ignition temperature of LHV gases over such catalysts.
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The heat generated by the combustion of hydrogen and carbon monoxide helps to heat up the catalyst, which facilitates ignition of methane in the fuel gas. Fig. 6a and b show that the gas inlet ignition temperature for methane is decreased by 200°C and 300°C for methane concentrations of 1.2% and 2.3%, respectively, due to the presence of carbon monoxide and hydrogen. These two methane concentrations represent two cases. In the first case of 1.2% methane (oxygen-fuel ratio 5.5 for the fuel gas), methane ignites at 250°C higher gas inlet temperature than carbon monoxide. This means that when methane ignites, carbon monoxide and hydrogen conversion are nearly 100%. The presence of carbon monoxide and hydrogen appears not to have any influence on the ignition of methane under these conditions, judging from the conversion versus catalyst bed temperature plots. In the second case of 2.3% methane (oxygen-fuel ratio 2.7 for the fuel gas) the methane ignites simultaneously with carbon monoxide and hydrogen (cf. Fig. 2a). In this case an enhancement of the methane ignition due to the presence of carbon monoxide and hydrogen is observed when the conversion versus catalyst bed temperature plots are regarded. The two different cases shown here represent two ignition processes. In the case with low fuel concentration, methane ignition takes place when carbon monoxide and hydrogen conversion are nearly 100%. This means that ignition occurs on a platinum surface with high oxygen coverage, as is the case when methane is combusted alone [19]. Hence, carbon monoxide and hydrogen combustion only help to heat up the catalyst, which explains that no difference is observed between the conversion versus catalyst bed temperature plots in the presence and absence of the other fuel gas components. In the case with high fuel concentration, methane ignites on a surface where carbon monoxide and hydrogen ignition also takes place. This means that the oxygen coverage will be lower, due to carbon monoxide and hydrogen combustion, which facilitates the adsorption of methane. The enhanced adsorption of methane increases the reaction rate, which is seen in the conversion versus catalyst bed temperature plots in Fig. 6b. The catalyst bed ignition temperature of methane is lowered by more than 100°C due to the presence of carbon monoxide and hydrogen. If carbon monoxide and hydrogen combustion had no mechanistic effect, the conversion versus catalyst bed temperature plots would overlap, as in Fig. 6a, which is not seen in this case. The decrease in oxygen concentration in the feed, by adding extra argon, increased the ignition temperature for carbon monoxide and hydrogen; compare Fig. 2b and Fig. 5. This observation is in agreement with the assumption of a high carbon monoxide coverage of the platinum surface at low temperatures and low conversions. The decrease in ignition temperature of the catalyst bed for methane combustion resulting from the lowered oxygen concentration is in agreement with the assumption of an oxygen-covered surface during methane combustion. This is further supported by the results given in Fig. 4, which shows that the ignition of methane occurs at
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higher catalyst bed temperatures with increasing oxygen-fuel ratios, i.e. lower fuel concentration. Addition of 0.7% water vapor to the feed increased the ignition temperatures for the combustible components only by about 10°C with respect to the gas inlet temperature. On the other hand, Fig. 7 showed that the methane ignition temperature decreased due to water addition to the feed, when regarding the catalyst bed temperature. An inhibiting effect by water on methane combustion over palladium catalysts was recently demonstrated [8,9]. However, this inhibition decreased with increasing temperature and was almost negligible at a catalyst temperature of 450°C [S]. In our experiments at oxygen-fuel ratio 2.7, methane ignited at gas inlet temperatures that were only a few degrees higher than carbon monoxide and hydrogen. Under such conditions, both reactants and products, i.e. water and carbon dioxide were present, creating a rather complex mixture, which made it difficult to predict possible mechanisms and platinum surface states. Therefore, a more detailed investigation is needed to clarify the observed influence of water. The addition of 600 ppm benzene to the feed showed that benzene, being a model aromatic compound, can be readily combusted over the platinum catalyst studied (Fig. 8). The presence of the other combustible components decreases the ignition temperature for benzene by about 20°C. The final conversion that was achieved, about 90%, is likely to be controlled by external diffusion limitations. Similar mass transfer limitations in benzene oxidation over monolithic platinum catalysts were observed by Barresi and Baldi [20]. They proposed a mechanism for benzene oxidation in air in which gaseous benzene reacts with adsorbed oxygen. Our results show that benzene hardly influenced the ignition of hydrogen and carbon monoxide. It is not clear if the mechanism proposed by Barresi and Baldi is valid for a carbon monoxide-covered platinum surface. 4.2. Technological
implications
Carbon monoxide dominates the ignition of the gas mixture studied, and it is desirable to have a thorough knowledge of the carbon monoxide combustion kinetics in an LHV gas under all realistic conditions. The hydrogen is completely converted once the carbon monoxide ignites. Methane, the gaseous hydrocarbon that is most difficult to oxidize, is converted at an inlet temperature over 200°C lower in the presence of carbon monoxide and hydrogen than in the absence of these compounds. This is important since it determines the necessary preheating temperature in industrial applications. The platinum catalyst studied was very active in oxidation of benzene, the model aromatic compound. However, actual gasified biomass contains other heavier aromatic compounds, such as naphthalene, which have to be oxidized. Another study in our laboratory showed that platinum was very active in the oxidation of naphthalene as well, in the presence of carbon monoxide [21].
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As mentioned in Section 3.2, the inhibiting action of carbon monoxide during ignition of the fuel gas was circumvented by heating the catalyst in the absence of the fuel gas. This observation should be borne in mind during the design and development of start-up procedures of actual catalytic combustors fueled with carbon monoxide-containing LHV gas. The presence of ammonia did not influence the combustion of the other fuel components under the conditions studied. However, ammonia levels that are more than 10 times higher can be expected under actual operation, in which case the influence of ammonia may be stronger. This is likely to be dependent on the type of catalyst used. The platinum catalyst in this study converted all ammonia to nitric oxide. Metal oxides catalysts may offer an alternative with a high selectivity for molecular nitrogen formation instead [ 11. The use of a combination of different types of catalysts may be an interesting approach. This selective conversion of ammonia in gasified biomass to nitrogen, and not to nitrogen oxides, is currently being studied in our laboratory; the results will be published separately [22].
5. Conclusion The presented results clearly demonstrate the influences on combustion behaviour of the fuel gas composition and the oxygen-fuel ratio. Still, a more complete mapping of these effects is necessary if we are to gain a thorough understanding of the behavior of such a complex LHV gas. Such understanding is required for the design of catalytic combustors using LHV gases from biomass or coal gasifiers. Combustion catalysts for this application should not only be active for oxidation of the combustible components in the fuel, they should also have a low selectivity for the conversion of ammonia to nitrogen oxides. Moreover, they should be insensitive to tar compounds that are present in the product gas from coal or biomass gasifiers. These issues are now investigated as part of the on-going work on catalytic combustion in our laboratory.
Acknowledgements This work was supported by the Thermal Engineering Research Institute (VZrmeforsk) and the Swedish National Board for Industrial and Technical Development (NUTEK). We thank Perstorp AB for providing the platinum catalyst and P. Govind Menon for valuable discussions and comments.
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