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Catalytic combustion of gasified refuse-derived fuel
Applied Catalysis B: Environmental 45 (2003) 1–11
Catalytic combustion of gasified refuse-derived fuel Henrik M.J. Kušar∗ , Anders G. Ersson, Sven G. Järås Kungl Tekniska Högskolan, Department of Chemical Engineering and Technology/Chemical Technology, SE-100 44 Stockholm, Sweden Received 10 November 2002; received in revised form 21 February 2003; accepted 26 February 2003
Abstract The catalytic combustion of gasified refuse-derived fuel (RDF), i.e. a low heating-value (LHV) gas containing H2 , CO and CH4 as combustible components, has been studied and compared with the combustion of methane. Two metal oxide catalysts, i.e. a spinel and a hexaaluminate, and three noble metal catalysts were tested. The results show that the Pd-based catalysts were the most active both for the gasified waste, i.e. RDF and methane. Incorporating an active support such as LaMnAl11 O19 enhances the catalytic activity for methane in gasified waste. Substituting Mn into the crystal lattice of the spinel also increased the catalytic activity for H2 and CO, while the methane activity remained low. The formation of NOX from fuel-bound nitrogen was investigated by adding NH3 to the gas stream. The metal oxide catalysts showed a higher selectivity for oxidising NH3 into N2 than the catalysts containing precious metals. The spinel materials have high thermal stability and are comparable to the hexaaluminates confirming that they could be promising as washcoat materials to avoiding sintering at high temperatures. © 2003 Elsevier B.V. All rights reserved. Keywords: Catalytic combustion; Gasified waste; Gasified biomass; RDF; NH3 ; NOX ; Hexaaluminate; Spinel; Pd; Pt
1. Introduction Until recently, most combustible wastes have been deposited in landfills. However, the European Parliament has created new directives [1], which prohibit the deposition of combustible waste material in landfills from the year 2002, and organic waste from the year 2005. Combustion of waste both reduces the volume of waste that has to be deposited and utilises the energy content of the waste. Hence, interest has increased for the use of waste as fuel. Normally waste is combusted directly in an incinerator and the product is mainly heat. In order to achieve a higher electricity/heat ratio, an integrated combined cycle (IGCC) may be used. In an IGCC plant, the waste is gasified ∗ Corresponding author. Tel.: +46-8-7906604; fax: +46-8-108579. E-mail address: [email protected] (H.M.J. Kušar).
and the produced gas is combusted in a gas turbine and the exhaust heat is utilised in a steam turbine [2]. The gas produced from the gasification step is a low heating-value (LHV) gas that mainly contains H2 , CO, and CH4 as combustible components. The low heating-value of the gas, i.e. 18.5 MJ/kg, compared to natural gas, 48 MJ/kg, makes it more difficult to combust in a conventional gas turbine combustion chamber. Catalytic combustion could be a solution for achieving a stable incineration of the gasified waste. High temperature catalytic combustion is an alternative to ordinary flame combustion, and can achieve ultra-low emissions of hydrocarbons, carbon monoxide and nitrogen oxides and has been reviewed by several authors [3–7]. The catalytic combustion chamber allows stable combustion outside the normal flammability limits, i.e. the adiabatic flame temperature can be lowered below the critical temperature for formation of thermal NOX (>1500 ◦ C). Moreover, a
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catalytic system performs with less thermo-acoustics and variations during combustion. Several metal oxides have been proposed as combustion catalysts in the literature [8]. Substituted hexaaluminates, developed by Machida et al. [9], are among the most promising materials for catalytic combustion, due to their catalytic activity as well as their thermal stability. Catalytic combustion of gasified biomass has been studied over different catalyst materials. For the catalytic combustion of LHV gas, substituted hexaaluminates have proven to be a viable substitute for noble metals. The problem with high ignition temperatures found for methane is avoided as the fuel components H2 and CO ignite at much lower temperatures over these catalysts [10–12]. Especially the Mn-substituted hexaaluminate (LaMnAl11 O19 ) has shown high activity for combustion of LHV gas. Spinels are another interesting group of materials with possible use as combustion catalysts. These materials have shown high thermal stability as well as promising activity in oxidation reactions [13,14]. The spinel structure allows for several different substitutions of ions that could increase the catalytic activity. Thormählen et al. [15] have studied the oxidation of CO over a cobalt spinel for abatement of car exhaust emissions, in the presence of similar compounds as are found in LHV gases. The waste material, referred to refuse-derived fuel (RDF) contains a mixture of paper wastes, plastics, mainly polyethylene, forest residues and household wastes. The main difference compared to biomass is a higher ash content for waste. Thus, RDF contains more metals and chlorine that could be poisonous to a catalyst. However, as for biomass, it may contain significant amount of fuel-bound nitrogen and sulphur compounds. Fine-tuning the gasification process could significantly lower the amounts of the above-mentioned compounds in the gasification gas [16]. Small amounts of fuel-bound nitrogen will always be present in the gas. Hence, using gasified waste or gasified biomass, one of the most important issues is to reduce the fuel NOX formed from fuel-bound nitrogen compounds, e.g. NH3 and HCN. Earlier studies have shown that catalytic combustion could be an alternative for selectively oxidising the fuel-bound nitrogen compounds into N2 . Several authors have reported that the selectivity of NH3 to
N2 is around 60% under lean conditions, but varies strongly with temperature [17,18]. The present work reports investigations on the use of spinel and hexaaluminate materials, both as active phase and impregnated with precious metals, as catalysts for the combustion of gasified RDF. The materials were tested with emphasis on the ignition characteristics of the main combustible components in the gasified waste, H2 , CO and CH4 , and compared to combustion of pure methane. The conversion of NH3 into N2 and the formation of NOX were also studied. 2. Experimental 2.1. Catalyst preparation The preparation of LaMnAl11 O19 , MgAl2 O4 and manganese-doped spinel (MgMn0.25 Al1.75 O4 ) was performed by the carbonate co-precipitation described elsewhere [19,20] (see Table 1). Metal nitrates (Al(NO3 )3 ·9H2 O, Mn(NO3 )2 ·4H2 O, La(NO3 )3 ·6H2 O and Mg(NO3 )2 ·6H2 O) were dissolved in water and added to an ammonium carbonate solution ((NH4 )2 CO3 ). The pH of the ammonium carbonate solution was held constant at around 8.5 by addition of ammonia solution. The formed precipitate was then centrifuged, washed with acetone, and dried 12 h at 70 ◦ C. The precipitate was consecutively calcined at 1000 and 1200 ◦ C for 4 h each in dry air. In order to test the thermal stability of the materials a fraction of the powder was aged at 1400 ◦ C in 15% steam for 10 h. After each calcination, samples were taken for analysis. The calcined powders were crushed, mixed with ethanol and ball-milled for 24 h. Cordierite monoliths, 400 cpsi Corning, with a diameter of 14 mm and a length of 10 mm, were dip-coated with Table 1 The different catalysts used in the tests, sample names, washcoat composition and precious metal content Sample name
Washcoat
Precious metal content
Cordierite LMHA Pd/LMHA MnMAS Pd/MAS Pt/MAS
None LaMnAl11 O19 LaMnAl11 O19 MgMn0.25 Al1.75 O4 MgAl2 O4 MgAl2 O4
None None 2.0 wt.% Pd None 2.0 wt.% Pd 2.0 wt.% Pt
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the catalyst slurry. The coated monoliths were dried for 3 h at 70 ◦ C; this procedure was repeated until a washcoat layer corresponding to 17 wt.% of the catalyst had been applied. The monoliths were finally calcined at 1000 ◦ C for 4 h. The catalysts containing noble metals were prepared by impregnation of the washcoated monoliths with a palladium or platinum nitrate solution, and calcined at 1000 ◦ C for 4 h. 2.2. Catalyst characterisation Information about the crystalline structures was obtained from powder X-ray diffraction (XRD) using a Siemens Diffraktometer 5000, scanning 2θ from 10 to 90◦ using monochromatised Cu K␣ radiation. The specific surface area and pore size distribution were measured according to the Brauner–Emmet–Teller (BET) method, with N2 adsorption at liquid nitrogen temperature on a Micromeritics ASAP 2010. 2.3. Gasification experiments In order to determine a suitable gas composition for the catalyst tests, a number of gasification experiments were carried out, using waste. The gasification tests were carried out in a laboratory-scale, isothermal, atmospheric fluidised bed gasifier described elsewhere [16]. The waste material, in the form of crushed tablets, was fed from the top with a screw-feeder and the bed material used was quartz. The reactor has external heating and the oxygen content in the fluidising gas may be varied independently of the temperature. The waste material was gasified at several temperatures. The gas composition of the gasified waste was compared to the gas composition of previous studies of gasified biomass under similar conditions. The differences in the product gas composition between the waste and the biomass were concluded to be small. In Table 2, a typical composition of the gasified waste is given as well as the composition of the synthetic gas used for the catalytic activity tests in this work. 2.4. Activity tests The measurements of the catalytic activity of the different catalysts were carried out in a tubular flow reactor equipped with a temperature-programmed furnace. In the experiments the temperature was increased
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Table 2 Gas compositions of typical gasified waste and synthetic gasified waste Gas compound
Typical gasified waste (%)
Synthetic gas composition (mol%) used for the activity tests, λ = 3
N2 O2 H2 CO CO2 CH4 C2 –C12 H2 O NH3 H2 S
44.5 0.0 10.2 14.7 13.8 4.6 1.0 11.2 0.165 0.20
83.0 8.2 1.1 1.6 1.3 0.5 0.0 2.5 0.034 0.0
by 5 ◦ C/min from 100 to 900 ◦ C and the space velocity corresponded to 100,000 h−1 . The tests were carried out using a synthetic gas mixture simulating gasified waste (see Table 2) with the air/fuel ratio (λ) equal to 3. To simulate the water content of the gasification gas water was added, pumped by a syringe pump and evaporated prior to the reactor inlet and mixed with air in the preheating zone of the reactor.
3. Results 3.1. Surface area and crystal phases The BET surface areas and the crystal phases for the different washcoat materials after calcinations at different temperatures are shown in Table 3, where the MAS shows the largest surface area of the materials after calcination up to 1200 ◦ C, which is the temperature used for the monoliths in the experiments. However after ageing at 1400 ◦ C the LMHA shows a larger surface area, for more detailed information [21]. 3.2. Gasified waste combustion The temperature for 10% conversion (T10 ) was used as a measurement of the ignition for the various fuel components (Fig. 1). The Pd/LMHA catalyst showed the highest activity for combustion of the synthetic gasified waste (Fig. 2); H2 ignited first, immediately followed by CO at 200 ◦ C. The conversion increased rapidly up to 90% for H2 and to full conversion for
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Table 3 BET surface area and crystal phase after calcinations at 1000 ◦ C, 1200 ◦ C (for 4 h) and 1400 ◦ C (for 10 h in 15% steam) for the washcoat materials Catalyst
LMHA MAS MnMAS
BET surface area (m2 /g)
Crystal phase
1000 ◦ C
1200 ◦ C
1400 ◦ C
1000 ◦ C
1200 ◦ C
1400 ◦ C
63.5 85.3 11.2
24.9 32.9 9.4
10.7 7.5 8.1
Amorphous MgAl2 O4 , MgO MgAl2 O4
LaMnAl11 O19 MgAl2 O4 , MgO MgAl2 O4
LaMnAl11 O19 MgAl2 O4 MgAl2 O4
CO. For methane, T10 was 330 ◦ C and the conversion increased steadily reaching 95% conversion at 720 ◦ C. The Pd/MAS catalyst (Fig. 3) showed similar ignition characteristics as the Pd/LMHA for both H2 and CO. However, the conversion of methane was lower, T10 = 490 ◦ C, and 95% conversion was not reached until 775 ◦ C. The Pt/MAS (Fig. 4) showed a much higher ignition temperature for all fuel components compared to the palladium-based catalysts. The order of ignition was reversed, compared to the Pd catalysts, starting with CO followed by H2 and CH4 . In spite of the higher ignition temperature, 95% conversion of methane in the gas mixture was reached at similar temperatures as for the Pd/MAS catalyst. The metal oxide catalysts, LMHA (Fig. 5) and MnMAS (Fig. 6) showed similar activities for combustion of the synthetic gasified waste. For the LMHA, the ignition sequence was CO followed by H2 both with a T10 around 450 ◦ C. The methane had a T10 at 620 ◦ C. However, for the MnMAS, the H2 ignited
before CO, with T10 at 420 and 470 ◦ C, respectively. Methane ignites at a higher temperature over MnMAS than over LMHA. For both catalysts, 95% conversions of all components were reached at 770 ◦ C. Compared to the blank test, uncoated cordierite shown in Fig. 7, all catalysts gave an increased activity. The conversion of CO and H2 increased considerably more slowly for the metal oxide catalysts than for the precious metal-containing catalysts. 3.3. Methane combustion Combustion of methane was tested over the different catalysts in order to compare their activity for gasified waste with their activity for natural gas. The results from the methane tests are presented in Fig. 8. The Pd catalyst showed a superior activity as could be expected. The Pd/MAS showed the highest activity in 400–700 ◦ C range, i.e. when PdO is present. At temperatures above 700 ◦ C, the Pd/LMHA showed the
Fig. 1. The temperatures at which 10% conversion (T10 ) are reached for the fuel components in the gasified waste (H2 , CO and CH4 ) and for pure methane.
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Fig. 2. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pd/LMHA. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 3. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pd/MAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 4. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pt/MAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 5. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over LMHA. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 6. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over MnMAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 7. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over cordierite. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 8. Laboratory-scale activity tests, total oxidation of methane over 2% Pd/MAS (䉭), 2% Pd/LMHA (䉱), 2% Pt/MAS (䊊), LMHA (䊏), MnMAS (䊐) and cordierite (×).
highest activity followed by the LMHA catalyst. The Pt/MAS catalysts showed much lower activity and the MnMAS showed almost no increase in activity compared to the uncoated cordierite. 3.4. Fuel-NOX formation Of much concern in the catalytic combustion of gasified waste or biomass is the conversion of fuel-bound nitrogen components into undesired NOX . In the present study, NH3 was used to simulate the nitrogen components in the fuel. The oxidation of NH3 in the synthetic gasified waste over the various catalysts is shown in Fig. 9a. Generally the oxidation of NH3 follows CO and H2 . The ignition order is Pd/LMHA < Pd/MAS < Pt/MAS < LMHA < MnMAS < cordierite. The yields of NO and NO2 are shown in Fig. 9b and c, respectively. Generally NO2 makes up a small fraction of the total NOX . The Pt/MAS showed the highest affinity for NO2 formation, starting to form NO2 immediately after the ignition of NH3 and reaching its maximum (17%)
at 580 ◦ C. The Pd/MAS catalyst showed the second highest formation of NO2 , the formation takes off at around 400 ◦ C reaching its maximum (13%) at 620 ◦ C. The Pd/LMHA catalyst showed significantly less NO2 formation (3%) compared to the Pd/MAS. Surprisingly the LMHA catalyst showed a much higher NO2 formation, starting at 500 ◦ C and reaching a maximum of 11% at 720 ◦ C. Both the MnMAS and the cordierite produced very minor amounts of NO2 . For all catalysts, a sharp cut-off in NO2 formation occurs at 770 ◦ C. The noble metal catalysts produced the most NO, reaching up to 95% NO yield. Generally, all catalysts start NO formation immediately after the ignition of NH3 . For all catalysts, except the Pd/LMHA, a sharp decrease in NO formation takes place at around 770 ◦ C. At temperatures above 770 ◦ C, NO formation stabilises at around 50% yield NO for Pd/MAS, MnMAS, LMHA and the uncoated cordierite. This behaviour does not take place for Pd/LMHA, which steadily gives a higher NO yield (75%). The Pt/MAS shows a third behaviour, and increases the NO yield
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Fig. 9. Laboratory-scale activity tests of synthetic gasified waste showing NH3 conversion (a), NO yield (b), and NO2 yield (c) over 2% Pd/MAS (䉭), 2% Pd/LMHA (䉱), 2% Pt/MAS (䊊), LMHA (䊏), MnMAS (䊐) and cordierite (×).
after the dip at 770 ◦ C. For the uncoated cordierite, a peak in NO formation occurs at 710 ◦ C, i.e. during the ignition of NH3 . 4. Discussion 4.1. Activity tests The palladium catalysts showed the highest activity for methane combustion of all the tested catalysts both
for pure methane and for the methane in the gasified waste. Comparing the catalytic combustion of methane in the gasified waste with that of pure methane, for the former, Pd/LMHA showed higher activity than Pd/MAS over the whole temperature range, igniting the methane at 160 ◦ C lower temperature. Both the Pd-based catalysts (see Figs. 2 and 3) showed the PdO to Pd transformation at around 500 and 600 ◦ C, respectively, this is almost 50–150 ◦ C lower temperature than for pure methane (Fig. 8). The explanation is most likely, that only the inlet temperature of the catalyst
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is measured, the surface is probably much warmer since H2 and CO ignites before CH4 and causes an adiabatic temperature rise. When instead comparing pure methane the Pd/LMHA increased its ignition temperature by 120 ◦ C while the Pd/MAS decreased the temperature by 50 ◦ C. Burch et al. [22] have studied the inhibition of H2 O and CO2 for methane combustion over Pd catalysts and found that water is a strong inhibitor even at low partial pressures. For the combustion of synthetic gasified waste the amounts of H2 O and CO2 are larger than the amount of methane itself (see Table 2). This could explain why methane in the gas mixture is harder to combust over Pd/MAS than when alone. Interesting is that the Pd/LMHA instead lowers its ignition temperature for the methane in the gas mixture, showing no inhibition by H2 O, this is probably as mentioned above due to the adiabatic temperature rise from H2 and CO. For combustion of pure methane, instead the Pd/MAS was the best catalyst at temperatures below 750 ◦ C, above this temperature the PdO is reduced to Pd and consequently the activity decreases. At temperatures above 750 ◦ C, Pd/LMHA was the most active. Similar results have been reported; the catalytic activity of Mn-substituted hexaaluminates is effective in compensating for the drop in activity of Pd so that a stable combustion reaction can be attained in the whole temperature range [23]. The higher activity in the low-temperature region of the Pd/MAS catalyst is most likely connected to its larger surface area compared to Pd/LMHA. When the activity of the supported palladium decreases, either due to water inhibition or as the PdO decomposes into metallic Pd, the activity of the support becomes more important for the overall activity. Hence as the LMHA is more active than the MAS for combustion of pure methane, the overall activity for Pd/LMHA will be higher than for Pd/MAS. Moreover, Sohn et al. [24] have shown that a change in oxidation state of Pd occurs after high-temperature treatments of Pd on Mn-substituted hexaaluminates that also could affect the activity. All the other catalysts showed much lower activity for methane combustion, both for the methane in the gasified waste as well as for pure methane. For the gasified waste the non-Pd catalysts ignite the methane at similar temperatures, T10 are all above 600 ◦ C. For combustion of pure methane, however, LMHA showed a higher activity than the other non-Pd catalysts followed by Pt/MAS and Mn-
MAS, which had activities similar to that of uncoated cordierite. For the other fuel components in the gasified waste, i.e. H2 and CO, the ignition temperatures were much lower than for methane. The H2 and the CO components both ignite at similar temperatures for all catalysts. Interesting is that H2 only reaches 90% conversion over the precious metal catalysts, while CO is completely converted, most probably the reactions become diffusion-controlled in this region. The ignition order of the two components was Pd/MAS = Pd/LMHA < Pt/MAS < LMHA = MnMAS < cordierite. The MnMAS showed as good activity for H2 and CO as LMHA, though not for methane. Both the manganese-substituted catalysts exhibit a shoulder at the ignition curve (Figs. 4 and 5) for the LMHA at 700 ◦ C and for MnMAS already at 550 ◦ C, this could be due to mass-transfer limitation in a diffusion-controlled region. Moreover, MnMAS was the only catalyst which showed a considerable difference in the reactivity for H2 and CO, i.e. reacting the H2 at lower temperatures than the CO. A similar reactivity order was found for the Pd catalysts, the reversed order was observed for Pt/MAS and LMHA, although the differences were very small between the ignition temperatures for CO and H2 . Groppi et al. [25] have found a similar reaction order for LMHA. As mentioned above NH3 was added to the fuel to simulate fuel-bound nitrogen, in order to evaluate the formation of fuel NOX over the catalysts. The oxidation of NH3 follows closely that of H2 and CO for all catalysts. That H2 and CO both are found in much larger quantities than NH3 produces the required energy for the NH3 activation. Generally the selectivity for NOX is proportional to the oxidation activity of the catalyst. The platinum catalyst converts most NH3 into NOX of all the catalysts, which corresponds well with the fact that Pt catalysts are used for nitric acid production [26]. The palladium catalysts also have a high conversion into NOX . The washcoat material seems to affect the conversion to NOX , for the Pd/LMHA the NO2 yield is significantly lower than for the Pd/MAS, while the difference for NO is less. Lietti et al. [17] suggested that, at high temperatures, homogeneous reactions start to take over, reducing the NOX formed down to 50–60%. In the present study, this was observed at temperatures above 770 ◦ C. The
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non-catalytic nature of the reaction was confirmed by combustion over the uncoated cordierite, showing a similar decrease in NOX formation as the catalysts. This clearly shows the problem with designing a catalytic combustion system which can selectively oxidise NH3 to N2 and still achieve high conversions of the fuel components. For solving the problem Burch and Southward [27] have recently proposed a selective oxidation process for decreasing the NOX formation from fuel-bound nitrogen. Further work has to be done to solve the fuel-NOX problem for gasified RDF in order to achieve ultra-low emissions.
5. Conclusions • Palladium-based catalysts were the most active for all components in the gasified waste. However, the differences were largest regarding the methane component. • Pd/LMHA showed a higher activity for the methane component in the gasified waste compared to pure methane, the opposite was found for Pd/MAS. The H2 O found in gasified waste inhibits the Pd activity, hence the lower activity of Pd/MAS. For Pd/LMHA the support material has an activity that compensates for the inhibition of the Pd. • MnMAS showed good activities for H2 and CO well comparable to those of LMHA. Incorporating manganese atoms in the spinel lattice did not enhance the catalytic activity for the methane component as it did for the hexaaluminates. • The precious metal catalysts readily oxidised the ammonia into NOX . The metal oxide catalysts showed lower NOX yields. • The spinel materials show high thermal stability and can preferably be used as washcoat material avoiding sintering at high temperatures.
Acknowledgements Special thanks to P.G. Menon for valuable advice and improving the manuscript, and to the Swedish National Energy Administration (STEM) for funding this work within the research programme “Energy from Waste”, project P-10547.
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References [1] Directive 1999/31/EC of 26 April 1999 on the landfill of waste, OJ L 182, 16 July 1999, p. 1. [2] A.V. Bridgewater, Fuel 74 (1995) 631. [3] D.L. Trimm, Appl. Catal. 7 (1983) 249. [4] R. Prasad, L.A. Kennedy, E. Ruckenstein, Catal. Rev. Sci. Eng. 26 (1984) 1. [5] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev. Sci. Eng. 29 (1987) 219. [6] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev. 32 (1990) 51. [7] E.M. Johansson, D. Papadias, P.O. Thevenin, A.G. Ersson, R. Gabrielsson, P.G. Menon, P.H. Björnbom, S.G. Järås, in: J.J. Spivey (Ed.), Catalysis—Specialists Periodical Reports, vol. 14, Royal Society of Chemistry, Cambridge, 1999, p. 183. [8] M.F.M. Zwinkels, S.G. Järås, P.G. Menon, T.A. Griffin, Catal. Rev. Sci. Eng. 35 (1993) 319. [9] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [10] G. Groppi, L. Lietti, E. Tronconi, P. Forzatti, Catal. Today 45 (1998) 159. [11] E.M. Johansson, K.M.J. Danielsson, A.G. Ersson, S.G. Järås, J. Eng. Gas Turbines Power 124 (2002) 235. [12] E.M. Johansson, S.G. Järås, in: Proceedings of the Symposium on Catalytic Combustion, vol. 42, Division of Petroleum Chemistry, American Chemical Society, San Francisco, 1997, p. 146. [13] P.E. Marti, M. Maciejewski, A. Baiker, Appl. Catal. 4 (1994) 225. [14] M.F.M. Zwinkels, S. Druesne, P.G. Menon, E. Björnbom, S.G. Järås, Ind. Eng. Chem. Res. 37 (1998) 391. [15] P. Thormählen, E. Fridell, N. Cruise, M. Skoglundh, A. Palmqvist, Appl. Catal. B 31 (2001) 1. [16] P. Vriesman, E. Heginuz, K. Sjöström, Fuel 79 (2000) 1371. [17] L. Lietti, C. Ramella, G. Groppi, P. Forzatti, Appl. Catal. B 21 (1999) 89. [18] M. Johansson, S. Järås, Catal. Today 47 (1998) 1. [19] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, Appl. Catal. A 104 (1993) 101. [20] A. Ersson, M. Johansson, S. Järås, in: B. Delmon, et al. (Eds.), Studies in Surface Science and Catalysis, vol. 118, Elsevier, Amsterdam, 1998, p. 601. [21] A.G. Ersson, H.M.J. Kušar, S.G. Järås, Catalytic combustion of diesel fuel, submitted for publication. [22] R. Burch, F.J. Urbano, P.K. Loader, Appl. Catal. A 123 (1995) 173. [23] K. Sekizawa, M. Machida, K. Eguchi, H. Arai, J. Catal. 142 (1993) 655. [24] J.M. Sohn, S.K. Kang, S.I. Woo, J. Mol. Catal. A: Chem. 186 (2002) 135. [25] G. Groppi, A. Belloli, E. Tronconi, P. Forzatti, Catal. Today 29 (1996) 403. [26] J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 17, Wiley, New York, 1996, p. 89. [27] R. Burch, B.W.L. Southward, J. Catal. 198 (2001) 286.
Catalytic combustion of gasified refuse-derived fuel Henrik M.J. Kušar∗ , Anders G. Ersson, Sven G. Järås Kungl Tekniska Högskolan, Department of Chemical Engineering and Technology/Chemical Technology, SE-100 44 Stockholm, Sweden Received 10 November 2002; received in revised form 21 February 2003; accepted 26 February 2003
Abstract The catalytic combustion of gasified refuse-derived fuel (RDF), i.e. a low heating-value (LHV) gas containing H2 , CO and CH4 as combustible components, has been studied and compared with the combustion of methane. Two metal oxide catalysts, i.e. a spinel and a hexaaluminate, and three noble metal catalysts were tested. The results show that the Pd-based catalysts were the most active both for the gasified waste, i.e. RDF and methane. Incorporating an active support such as LaMnAl11 O19 enhances the catalytic activity for methane in gasified waste. Substituting Mn into the crystal lattice of the spinel also increased the catalytic activity for H2 and CO, while the methane activity remained low. The formation of NOX from fuel-bound nitrogen was investigated by adding NH3 to the gas stream. The metal oxide catalysts showed a higher selectivity for oxidising NH3 into N2 than the catalysts containing precious metals. The spinel materials have high thermal stability and are comparable to the hexaaluminates confirming that they could be promising as washcoat materials to avoiding sintering at high temperatures. © 2003 Elsevier B.V. All rights reserved. Keywords: Catalytic combustion; Gasified waste; Gasified biomass; RDF; NH3 ; NOX ; Hexaaluminate; Spinel; Pd; Pt
1. Introduction Until recently, most combustible wastes have been deposited in landfills. However, the European Parliament has created new directives [1], which prohibit the deposition of combustible waste material in landfills from the year 2002, and organic waste from the year 2005. Combustion of waste both reduces the volume of waste that has to be deposited and utilises the energy content of the waste. Hence, interest has increased for the use of waste as fuel. Normally waste is combusted directly in an incinerator and the product is mainly heat. In order to achieve a higher electricity/heat ratio, an integrated combined cycle (IGCC) may be used. In an IGCC plant, the waste is gasified ∗ Corresponding author. Tel.: +46-8-7906604; fax: +46-8-108579. E-mail address: [email protected] (H.M.J. Kušar).
and the produced gas is combusted in a gas turbine and the exhaust heat is utilised in a steam turbine [2]. The gas produced from the gasification step is a low heating-value (LHV) gas that mainly contains H2 , CO, and CH4 as combustible components. The low heating-value of the gas, i.e. 18.5 MJ/kg, compared to natural gas, 48 MJ/kg, makes it more difficult to combust in a conventional gas turbine combustion chamber. Catalytic combustion could be a solution for achieving a stable incineration of the gasified waste. High temperature catalytic combustion is an alternative to ordinary flame combustion, and can achieve ultra-low emissions of hydrocarbons, carbon monoxide and nitrogen oxides and has been reviewed by several authors [3–7]. The catalytic combustion chamber allows stable combustion outside the normal flammability limits, i.e. the adiabatic flame temperature can be lowered below the critical temperature for formation of thermal NOX (>1500 ◦ C). Moreover, a
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catalytic system performs with less thermo-acoustics and variations during combustion. Several metal oxides have been proposed as combustion catalysts in the literature [8]. Substituted hexaaluminates, developed by Machida et al. [9], are among the most promising materials for catalytic combustion, due to their catalytic activity as well as their thermal stability. Catalytic combustion of gasified biomass has been studied over different catalyst materials. For the catalytic combustion of LHV gas, substituted hexaaluminates have proven to be a viable substitute for noble metals. The problem with high ignition temperatures found for methane is avoided as the fuel components H2 and CO ignite at much lower temperatures over these catalysts [10–12]. Especially the Mn-substituted hexaaluminate (LaMnAl11 O19 ) has shown high activity for combustion of LHV gas. Spinels are another interesting group of materials with possible use as combustion catalysts. These materials have shown high thermal stability as well as promising activity in oxidation reactions [13,14]. The spinel structure allows for several different substitutions of ions that could increase the catalytic activity. Thormählen et al. [15] have studied the oxidation of CO over a cobalt spinel for abatement of car exhaust emissions, in the presence of similar compounds as are found in LHV gases. The waste material, referred to refuse-derived fuel (RDF) contains a mixture of paper wastes, plastics, mainly polyethylene, forest residues and household wastes. The main difference compared to biomass is a higher ash content for waste. Thus, RDF contains more metals and chlorine that could be poisonous to a catalyst. However, as for biomass, it may contain significant amount of fuel-bound nitrogen and sulphur compounds. Fine-tuning the gasification process could significantly lower the amounts of the above-mentioned compounds in the gasification gas [16]. Small amounts of fuel-bound nitrogen will always be present in the gas. Hence, using gasified waste or gasified biomass, one of the most important issues is to reduce the fuel NOX formed from fuel-bound nitrogen compounds, e.g. NH3 and HCN. Earlier studies have shown that catalytic combustion could be an alternative for selectively oxidising the fuel-bound nitrogen compounds into N2 . Several authors have reported that the selectivity of NH3 to
N2 is around 60% under lean conditions, but varies strongly with temperature [17,18]. The present work reports investigations on the use of spinel and hexaaluminate materials, both as active phase and impregnated with precious metals, as catalysts for the combustion of gasified RDF. The materials were tested with emphasis on the ignition characteristics of the main combustible components in the gasified waste, H2 , CO and CH4 , and compared to combustion of pure methane. The conversion of NH3 into N2 and the formation of NOX were also studied. 2. Experimental 2.1. Catalyst preparation The preparation of LaMnAl11 O19 , MgAl2 O4 and manganese-doped spinel (MgMn0.25 Al1.75 O4 ) was performed by the carbonate co-precipitation described elsewhere [19,20] (see Table 1). Metal nitrates (Al(NO3 )3 ·9H2 O, Mn(NO3 )2 ·4H2 O, La(NO3 )3 ·6H2 O and Mg(NO3 )2 ·6H2 O) were dissolved in water and added to an ammonium carbonate solution ((NH4 )2 CO3 ). The pH of the ammonium carbonate solution was held constant at around 8.5 by addition of ammonia solution. The formed precipitate was then centrifuged, washed with acetone, and dried 12 h at 70 ◦ C. The precipitate was consecutively calcined at 1000 and 1200 ◦ C for 4 h each in dry air. In order to test the thermal stability of the materials a fraction of the powder was aged at 1400 ◦ C in 15% steam for 10 h. After each calcination, samples were taken for analysis. The calcined powders were crushed, mixed with ethanol and ball-milled for 24 h. Cordierite monoliths, 400 cpsi Corning, with a diameter of 14 mm and a length of 10 mm, were dip-coated with Table 1 The different catalysts used in the tests, sample names, washcoat composition and precious metal content Sample name
Washcoat
Precious metal content
Cordierite LMHA Pd/LMHA MnMAS Pd/MAS Pt/MAS
None LaMnAl11 O19 LaMnAl11 O19 MgMn0.25 Al1.75 O4 MgAl2 O4 MgAl2 O4
None None 2.0 wt.% Pd None 2.0 wt.% Pd 2.0 wt.% Pt
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the catalyst slurry. The coated monoliths were dried for 3 h at 70 ◦ C; this procedure was repeated until a washcoat layer corresponding to 17 wt.% of the catalyst had been applied. The monoliths were finally calcined at 1000 ◦ C for 4 h. The catalysts containing noble metals were prepared by impregnation of the washcoated monoliths with a palladium or platinum nitrate solution, and calcined at 1000 ◦ C for 4 h. 2.2. Catalyst characterisation Information about the crystalline structures was obtained from powder X-ray diffraction (XRD) using a Siemens Diffraktometer 5000, scanning 2θ from 10 to 90◦ using monochromatised Cu K␣ radiation. The specific surface area and pore size distribution were measured according to the Brauner–Emmet–Teller (BET) method, with N2 adsorption at liquid nitrogen temperature on a Micromeritics ASAP 2010. 2.3. Gasification experiments In order to determine a suitable gas composition for the catalyst tests, a number of gasification experiments were carried out, using waste. The gasification tests were carried out in a laboratory-scale, isothermal, atmospheric fluidised bed gasifier described elsewhere [16]. The waste material, in the form of crushed tablets, was fed from the top with a screw-feeder and the bed material used was quartz. The reactor has external heating and the oxygen content in the fluidising gas may be varied independently of the temperature. The waste material was gasified at several temperatures. The gas composition of the gasified waste was compared to the gas composition of previous studies of gasified biomass under similar conditions. The differences in the product gas composition between the waste and the biomass were concluded to be small. In Table 2, a typical composition of the gasified waste is given as well as the composition of the synthetic gas used for the catalytic activity tests in this work. 2.4. Activity tests The measurements of the catalytic activity of the different catalysts were carried out in a tubular flow reactor equipped with a temperature-programmed furnace. In the experiments the temperature was increased
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Table 2 Gas compositions of typical gasified waste and synthetic gasified waste Gas compound
Typical gasified waste (%)
Synthetic gas composition (mol%) used for the activity tests, λ = 3
N2 O2 H2 CO CO2 CH4 C2 –C12 H2 O NH3 H2 S
44.5 0.0 10.2 14.7 13.8 4.6 1.0 11.2 0.165 0.20
83.0 8.2 1.1 1.6 1.3 0.5 0.0 2.5 0.034 0.0
by 5 ◦ C/min from 100 to 900 ◦ C and the space velocity corresponded to 100,000 h−1 . The tests were carried out using a synthetic gas mixture simulating gasified waste (see Table 2) with the air/fuel ratio (λ) equal to 3. To simulate the water content of the gasification gas water was added, pumped by a syringe pump and evaporated prior to the reactor inlet and mixed with air in the preheating zone of the reactor.
3. Results 3.1. Surface area and crystal phases The BET surface areas and the crystal phases for the different washcoat materials after calcinations at different temperatures are shown in Table 3, where the MAS shows the largest surface area of the materials after calcination up to 1200 ◦ C, which is the temperature used for the monoliths in the experiments. However after ageing at 1400 ◦ C the LMHA shows a larger surface area, for more detailed information [21]. 3.2. Gasified waste combustion The temperature for 10% conversion (T10 ) was used as a measurement of the ignition for the various fuel components (Fig. 1). The Pd/LMHA catalyst showed the highest activity for combustion of the synthetic gasified waste (Fig. 2); H2 ignited first, immediately followed by CO at 200 ◦ C. The conversion increased rapidly up to 90% for H2 and to full conversion for
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Table 3 BET surface area and crystal phase after calcinations at 1000 ◦ C, 1200 ◦ C (for 4 h) and 1400 ◦ C (for 10 h in 15% steam) for the washcoat materials Catalyst
LMHA MAS MnMAS
BET surface area (m2 /g)
Crystal phase
1000 ◦ C
1200 ◦ C
1400 ◦ C
1000 ◦ C
1200 ◦ C
1400 ◦ C
63.5 85.3 11.2
24.9 32.9 9.4
10.7 7.5 8.1
Amorphous MgAl2 O4 , MgO MgAl2 O4
LaMnAl11 O19 MgAl2 O4 , MgO MgAl2 O4
LaMnAl11 O19 MgAl2 O4 MgAl2 O4
CO. For methane, T10 was 330 ◦ C and the conversion increased steadily reaching 95% conversion at 720 ◦ C. The Pd/MAS catalyst (Fig. 3) showed similar ignition characteristics as the Pd/LMHA for both H2 and CO. However, the conversion of methane was lower, T10 = 490 ◦ C, and 95% conversion was not reached until 775 ◦ C. The Pt/MAS (Fig. 4) showed a much higher ignition temperature for all fuel components compared to the palladium-based catalysts. The order of ignition was reversed, compared to the Pd catalysts, starting with CO followed by H2 and CH4 . In spite of the higher ignition temperature, 95% conversion of methane in the gas mixture was reached at similar temperatures as for the Pd/MAS catalyst. The metal oxide catalysts, LMHA (Fig. 5) and MnMAS (Fig. 6) showed similar activities for combustion of the synthetic gasified waste. For the LMHA, the ignition sequence was CO followed by H2 both with a T10 around 450 ◦ C. The methane had a T10 at 620 ◦ C. However, for the MnMAS, the H2 ignited
before CO, with T10 at 420 and 470 ◦ C, respectively. Methane ignites at a higher temperature over MnMAS than over LMHA. For both catalysts, 95% conversions of all components were reached at 770 ◦ C. Compared to the blank test, uncoated cordierite shown in Fig. 7, all catalysts gave an increased activity. The conversion of CO and H2 increased considerably more slowly for the metal oxide catalysts than for the precious metal-containing catalysts. 3.3. Methane combustion Combustion of methane was tested over the different catalysts in order to compare their activity for gasified waste with their activity for natural gas. The results from the methane tests are presented in Fig. 8. The Pd catalyst showed a superior activity as could be expected. The Pd/MAS showed the highest activity in 400–700 ◦ C range, i.e. when PdO is present. At temperatures above 700 ◦ C, the Pd/LMHA showed the
Fig. 1. The temperatures at which 10% conversion (T10 ) are reached for the fuel components in the gasified waste (H2 , CO and CH4 ) and for pure methane.
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Fig. 2. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pd/LMHA. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 3. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pd/MAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 4. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over 2% Pt/MAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 5. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over LMHA. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 6. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over MnMAS. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
Fig. 7. Laboratory-scale activity tests, total oxidation of synthetic gasified waste over cordierite. The fuel components ignite separately. (䉭) H2 ; (䊊) CO; (䊐) CH4 .
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Fig. 8. Laboratory-scale activity tests, total oxidation of methane over 2% Pd/MAS (䉭), 2% Pd/LMHA (䉱), 2% Pt/MAS (䊊), LMHA (䊏), MnMAS (䊐) and cordierite (×).
highest activity followed by the LMHA catalyst. The Pt/MAS catalysts showed much lower activity and the MnMAS showed almost no increase in activity compared to the uncoated cordierite. 3.4. Fuel-NOX formation Of much concern in the catalytic combustion of gasified waste or biomass is the conversion of fuel-bound nitrogen components into undesired NOX . In the present study, NH3 was used to simulate the nitrogen components in the fuel. The oxidation of NH3 in the synthetic gasified waste over the various catalysts is shown in Fig. 9a. Generally the oxidation of NH3 follows CO and H2 . The ignition order is Pd/LMHA < Pd/MAS < Pt/MAS < LMHA < MnMAS < cordierite. The yields of NO and NO2 are shown in Fig. 9b and c, respectively. Generally NO2 makes up a small fraction of the total NOX . The Pt/MAS showed the highest affinity for NO2 formation, starting to form NO2 immediately after the ignition of NH3 and reaching its maximum (17%)
at 580 ◦ C. The Pd/MAS catalyst showed the second highest formation of NO2 , the formation takes off at around 400 ◦ C reaching its maximum (13%) at 620 ◦ C. The Pd/LMHA catalyst showed significantly less NO2 formation (3%) compared to the Pd/MAS. Surprisingly the LMHA catalyst showed a much higher NO2 formation, starting at 500 ◦ C and reaching a maximum of 11% at 720 ◦ C. Both the MnMAS and the cordierite produced very minor amounts of NO2 . For all catalysts, a sharp cut-off in NO2 formation occurs at 770 ◦ C. The noble metal catalysts produced the most NO, reaching up to 95% NO yield. Generally, all catalysts start NO formation immediately after the ignition of NH3 . For all catalysts, except the Pd/LMHA, a sharp decrease in NO formation takes place at around 770 ◦ C. At temperatures above 770 ◦ C, NO formation stabilises at around 50% yield NO for Pd/MAS, MnMAS, LMHA and the uncoated cordierite. This behaviour does not take place for Pd/LMHA, which steadily gives a higher NO yield (75%). The Pt/MAS shows a third behaviour, and increases the NO yield
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Fig. 9. Laboratory-scale activity tests of synthetic gasified waste showing NH3 conversion (a), NO yield (b), and NO2 yield (c) over 2% Pd/MAS (䉭), 2% Pd/LMHA (䉱), 2% Pt/MAS (䊊), LMHA (䊏), MnMAS (䊐) and cordierite (×).
after the dip at 770 ◦ C. For the uncoated cordierite, a peak in NO formation occurs at 710 ◦ C, i.e. during the ignition of NH3 . 4. Discussion 4.1. Activity tests The palladium catalysts showed the highest activity for methane combustion of all the tested catalysts both
for pure methane and for the methane in the gasified waste. Comparing the catalytic combustion of methane in the gasified waste with that of pure methane, for the former, Pd/LMHA showed higher activity than Pd/MAS over the whole temperature range, igniting the methane at 160 ◦ C lower temperature. Both the Pd-based catalysts (see Figs. 2 and 3) showed the PdO to Pd transformation at around 500 and 600 ◦ C, respectively, this is almost 50–150 ◦ C lower temperature than for pure methane (Fig. 8). The explanation is most likely, that only the inlet temperature of the catalyst
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is measured, the surface is probably much warmer since H2 and CO ignites before CH4 and causes an adiabatic temperature rise. When instead comparing pure methane the Pd/LMHA increased its ignition temperature by 120 ◦ C while the Pd/MAS decreased the temperature by 50 ◦ C. Burch et al. [22] have studied the inhibition of H2 O and CO2 for methane combustion over Pd catalysts and found that water is a strong inhibitor even at low partial pressures. For the combustion of synthetic gasified waste the amounts of H2 O and CO2 are larger than the amount of methane itself (see Table 2). This could explain why methane in the gas mixture is harder to combust over Pd/MAS than when alone. Interesting is that the Pd/LMHA instead lowers its ignition temperature for the methane in the gas mixture, showing no inhibition by H2 O, this is probably as mentioned above due to the adiabatic temperature rise from H2 and CO. For combustion of pure methane, instead the Pd/MAS was the best catalyst at temperatures below 750 ◦ C, above this temperature the PdO is reduced to Pd and consequently the activity decreases. At temperatures above 750 ◦ C, Pd/LMHA was the most active. Similar results have been reported; the catalytic activity of Mn-substituted hexaaluminates is effective in compensating for the drop in activity of Pd so that a stable combustion reaction can be attained in the whole temperature range [23]. The higher activity in the low-temperature region of the Pd/MAS catalyst is most likely connected to its larger surface area compared to Pd/LMHA. When the activity of the supported palladium decreases, either due to water inhibition or as the PdO decomposes into metallic Pd, the activity of the support becomes more important for the overall activity. Hence as the LMHA is more active than the MAS for combustion of pure methane, the overall activity for Pd/LMHA will be higher than for Pd/MAS. Moreover, Sohn et al. [24] have shown that a change in oxidation state of Pd occurs after high-temperature treatments of Pd on Mn-substituted hexaaluminates that also could affect the activity. All the other catalysts showed much lower activity for methane combustion, both for the methane in the gasified waste as well as for pure methane. For the gasified waste the non-Pd catalysts ignite the methane at similar temperatures, T10 are all above 600 ◦ C. For combustion of pure methane, however, LMHA showed a higher activity than the other non-Pd catalysts followed by Pt/MAS and Mn-
MAS, which had activities similar to that of uncoated cordierite. For the other fuel components in the gasified waste, i.e. H2 and CO, the ignition temperatures were much lower than for methane. The H2 and the CO components both ignite at similar temperatures for all catalysts. Interesting is that H2 only reaches 90% conversion over the precious metal catalysts, while CO is completely converted, most probably the reactions become diffusion-controlled in this region. The ignition order of the two components was Pd/MAS = Pd/LMHA < Pt/MAS < LMHA = MnMAS < cordierite. The MnMAS showed as good activity for H2 and CO as LMHA, though not for methane. Both the manganese-substituted catalysts exhibit a shoulder at the ignition curve (Figs. 4 and 5) for the LMHA at 700 ◦ C and for MnMAS already at 550 ◦ C, this could be due to mass-transfer limitation in a diffusion-controlled region. Moreover, MnMAS was the only catalyst which showed a considerable difference in the reactivity for H2 and CO, i.e. reacting the H2 at lower temperatures than the CO. A similar reactivity order was found for the Pd catalysts, the reversed order was observed for Pt/MAS and LMHA, although the differences were very small between the ignition temperatures for CO and H2 . Groppi et al. [25] have found a similar reaction order for LMHA. As mentioned above NH3 was added to the fuel to simulate fuel-bound nitrogen, in order to evaluate the formation of fuel NOX over the catalysts. The oxidation of NH3 follows closely that of H2 and CO for all catalysts. That H2 and CO both are found in much larger quantities than NH3 produces the required energy for the NH3 activation. Generally the selectivity for NOX is proportional to the oxidation activity of the catalyst. The platinum catalyst converts most NH3 into NOX of all the catalysts, which corresponds well with the fact that Pt catalysts are used for nitric acid production [26]. The palladium catalysts also have a high conversion into NOX . The washcoat material seems to affect the conversion to NOX , for the Pd/LMHA the NO2 yield is significantly lower than for the Pd/MAS, while the difference for NO is less. Lietti et al. [17] suggested that, at high temperatures, homogeneous reactions start to take over, reducing the NOX formed down to 50–60%. In the present study, this was observed at temperatures above 770 ◦ C. The
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non-catalytic nature of the reaction was confirmed by combustion over the uncoated cordierite, showing a similar decrease in NOX formation as the catalysts. This clearly shows the problem with designing a catalytic combustion system which can selectively oxidise NH3 to N2 and still achieve high conversions of the fuel components. For solving the problem Burch and Southward [27] have recently proposed a selective oxidation process for decreasing the NOX formation from fuel-bound nitrogen. Further work has to be done to solve the fuel-NOX problem for gasified RDF in order to achieve ultra-low emissions.
5. Conclusions • Palladium-based catalysts were the most active for all components in the gasified waste. However, the differences were largest regarding the methane component. • Pd/LMHA showed a higher activity for the methane component in the gasified waste compared to pure methane, the opposite was found for Pd/MAS. The H2 O found in gasified waste inhibits the Pd activity, hence the lower activity of Pd/MAS. For Pd/LMHA the support material has an activity that compensates for the inhibition of the Pd. • MnMAS showed good activities for H2 and CO well comparable to those of LMHA. Incorporating manganese atoms in the spinel lattice did not enhance the catalytic activity for the methane component as it did for the hexaaluminates. • The precious metal catalysts readily oxidised the ammonia into NOX . The metal oxide catalysts showed lower NOX yields. • The spinel materials show high thermal stability and can preferably be used as washcoat material avoiding sintering at high temperatures.
Acknowledgements Special thanks to P.G. Menon for valuable advice and improving the manuscript, and to the Swedish National Energy Administration (STEM) for funding this work within the research programme “Energy from Waste”, project P-10547.
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References [1] Directive 1999/31/EC of 26 April 1999 on the landfill of waste, OJ L 182, 16 July 1999, p. 1. [2] A.V. Bridgewater, Fuel 74 (1995) 631. [3] D.L. Trimm, Appl. Catal. 7 (1983) 249. [4] R. Prasad, L.A. Kennedy, E. Ruckenstein, Catal. Rev. Sci. Eng. 26 (1984) 1. [5] L.D. Pfefferle, W.C. Pfefferle, Catal. Rev. Sci. Eng. 29 (1987) 219. [6] Z.R. Ismagilov, M.A. Kerzhentsev, Catal. Rev. 32 (1990) 51. [7] E.M. Johansson, D. Papadias, P.O. Thevenin, A.G. Ersson, R. Gabrielsson, P.G. Menon, P.H. Björnbom, S.G. Järås, in: J.J. Spivey (Ed.), Catalysis—Specialists Periodical Reports, vol. 14, Royal Society of Chemistry, Cambridge, 1999, p. 183. [8] M.F.M. Zwinkels, S.G. Järås, P.G. Menon, T.A. Griffin, Catal. Rev. Sci. Eng. 35 (1993) 319. [9] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [10] G. Groppi, L. Lietti, E. Tronconi, P. Forzatti, Catal. Today 45 (1998) 159. [11] E.M. Johansson, K.M.J. Danielsson, A.G. Ersson, S.G. Järås, J. Eng. Gas Turbines Power 124 (2002) 235. [12] E.M. Johansson, S.G. Järås, in: Proceedings of the Symposium on Catalytic Combustion, vol. 42, Division of Petroleum Chemistry, American Chemical Society, San Francisco, 1997, p. 146. [13] P.E. Marti, M. Maciejewski, A. Baiker, Appl. Catal. 4 (1994) 225. [14] M.F.M. Zwinkels, S. Druesne, P.G. Menon, E. Björnbom, S.G. Järås, Ind. Eng. Chem. Res. 37 (1998) 391. [15] P. Thormählen, E. Fridell, N. Cruise, M. Skoglundh, A. Palmqvist, Appl. Catal. B 31 (2001) 1. [16] P. Vriesman, E. Heginuz, K. Sjöström, Fuel 79 (2000) 1371. [17] L. Lietti, C. Ramella, G. Groppi, P. Forzatti, Appl. Catal. B 21 (1999) 89. [18] M. Johansson, S. Järås, Catal. Today 47 (1998) 1. [19] G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P.L. Villa, Appl. Catal. A 104 (1993) 101. [20] A. Ersson, M. Johansson, S. Järås, in: B. Delmon, et al. (Eds.), Studies in Surface Science and Catalysis, vol. 118, Elsevier, Amsterdam, 1998, p. 601. [21] A.G. Ersson, H.M.J. Kušar, S.G. Järås, Catalytic combustion of diesel fuel, submitted for publication. [22] R. Burch, F.J. Urbano, P.K. Loader, Appl. Catal. A 123 (1995) 173. [23] K. Sekizawa, M. Machida, K. Eguchi, H. Arai, J. Catal. 142 (1993) 655. [24] J.M. Sohn, S.K. Kang, S.I. Woo, J. Mol. Catal. A: Chem. 186 (2002) 135. [25] G. Groppi, A. Belloli, E. Tronconi, P. Forzatti, Catal. Today 29 (1996) 403. [26] J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer Encyclopedia of Chemical Technology, vol. 17, Wiley, New York, 1996, p. 89. [27] R. Burch, B.W.L. Southward, J. Catal. 198 (2001) 286.