Optimal Microstructural Design of a Catalytic Premixed FeCrAlloy

1990

Ind. Eng. Chem. Res. 2004, 43, 1990-1998

KINETICS, CATALYSIS, AND REACTION ENGINEERING Optimal Microstructural Design of a Catalytic Premixed FeCrAlloy Fiber Burner for Methane Combustion Daniele Ugues, Stefania Specchia,* and Guido Saracco Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10124 Torino, Italy

New catalytic FeCrAlloy premixed fiber burners for natural gas combustion in domestic applications were developed according to an accurate microstructural design consisting of subsequent optimized steps: (i) optimal preoxidation of the FeCrAlloy fiber mat so as to develop the best possible Al2O3 layer for simultaneous prevention of deeper fiber oxidation at high temperature and optimal catalyst anchoring over the fiber surface, (ii) deposition via the either spray-pyrolysis or wash-coating technique of a previously developed perovskite catalyst (LaMnO3‚ 17MgO), and (iii) extensive testing of the catalyst-burner bonding strength and burner performance. Even after extensive aging under practical operating conditions (repeated on-off cycles and prolonged durations under high-temperature radiating conditions), the best developed burners (i.e., those obtained by spray pyrolysis) exhibit very limited catalyst loss and still remarkably lower (30-50%) NO and CO emissions than noncatalytic, virgin FeCrAlloy burners at the specific power densities of practical interest (150-650 kW/m2). 1. Introduction In most high-temperature environments of technological interest, protection of an underlying metallic structure is most often accomplished by the formation of a continuous scale over the surface; this layer serves as a barrier to prevent corrosive compounds from further penetrating into the remaining unoxidized metal. Long-term protection is generally associated with a scale growing by a diffusion-controlled mechanism; in fact, the rate of scale growth decreases with reaction time as the diffusion distance (scale thickness) increases. The chemical compositions of alloys designed for high-temperature applications have generally been established so that protective layers of either Cr2O3 or Al2O3 are formed. Both of these oxides offer excellent protective capabilities because of their slow growth rates and thermodynamic stability.1-4 For applications at very high temperatures (above around 1100 °C), the loss of chromium from the scale as volatile CrO3 becomes increasingly significant; therefore, alumina-forming alloys are preferred.5-10 In this context, alloys based on the Fe-Cr-Al system, generally termed FeCrAlloys, have become particularly attractive in the fabrication of long-lasting gas burners and industrial heaters and in other major high-temperature applications.11-13 Moreover, natural gas or liquid petroleum gas (LPG) premixed burners (Figure 1), based on metal fiber FeCrAlloy supports, offer low environmental impacts and good resistance to flashback phenomena because of their enhanced flame stability; high radiation efficiency with enhanced emissivity [so as to * To whom correspondence should be addressed. Tel.: +39. 011.5644608. Fax: +39.011.5644699. E-mail: stefania.specchia@ polito.it.

increase, especially at low specific heat power inputs (Figure 1b), the radiative fraction of heat exchanged, to the benefit of boiler efficiency]; very low pressure drops; and good resistance to thermal and mechanical shocks, thus guaranteeing long-term performance stability under normal operating conditions.14 For these alloys, exposure to very high temperatures, results in the preferential migration of aluminum atoms toward the interface with the environment, providing Al enrichment in the surface layers. There, the reaction between aluminum and oxygen produces an alumina film covering, protecting the steel matrix and preventing further deterioration effects due to oxidation processes at high temperature. In the mentioned field of domestic and industrial premixed gas burners, an interesting opportunity is to use this type of alloy as a support for a specific catalytic layer directed toward increasing the efficiency of the combustion and reducing the formation of pollutant compounds.15 The catalyst is usually a ceramic material that exhibits a poor tendency to adhere directly to the metal surface. Therefore, the interposition of an intermediate coating able to bond with both the metal substrate and the catalyst layer is generally required. For this specific purpose, in the FeCrAlloy system, the thermally grown alumina layer might act not only as a barrier for oxygen, but also as an effective bond coating. This coating is indeed intrinsically adherent to the metal substrate (as it is generated on its surface through simple oxidation reactions) and might offer, simply because it is a ceramic, a suitable surface for catalyst deposition purposes. This paper aims to demonstrate that, by properly selecting the operating temperature and the chemical composition of the oxidative environment (oxygen par-

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Figure 1. Scheme of a premixed gas burner: (a) blue flame regime (high specific power input, e.g., >700 kW/m2), (b) radiant regime (low specific power input, e.g., 700 kW/m2) with the following cycle: ignition, 5 min of radiant mode at 300 kW/m2, 3 min of blue flame mode at 750 kW/m2, 5 min at 300 kW/m2, 3 min stop. Thermal cycling was performed for up to 1000 h. Finally, the premixed catalytic burners were then tested, both in aged and fresh status, to evaluate the catalytic contribution to the reduction of CO and NOx emissions in the flue gases (measured by a multiplegas analyzer by Hartmann & Brown including an infrared analyzer for CO and NO, as well as a paramagnetic analyzer for O2) in the same test rig as used for the thermal aging test, at different specific power input values in the 150-650 kW/m2 range and at different excess air levels (Ea ) 2-70%). 3. Results and Discussion 3.1. Preoxidation Treatment Optimization. The thermal growth of oxide layers on FeCrAlloys should be aimed at achieving a compact and adherent film that acts as a barrier preventing the substrate from corrosive media penetration. However, in this case, the aim was to develop an effective bond coating that is also suitable as a catalyst anchoring. Therefore, the study was focused on the formation of an R-alumina layer that was strictly anchored to the basic alloy, but that also exhibited an enhanced specific external surface area and roughness so as to provide grip opportunities to the catalyst layer to be deposited. Many studies have been reported in the open literature concerning the properties of oxide-metal interface,

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Figure 2. Typical oxide morphologies on FeCrAlloy (Kanthal APM) after 10 min of isothermal oxidation treatment in calm air at (a) 900, (b) 1000, (c) 1100, and (d) 1200 °C. Transverse sections.

and several mechanisms have been proposed to explain which parameters are either favorable or detrimental to interface adhesion.21-24 Unfortunately, the higher the adhesion level at the oxide-metal interface, the smoother and denser the structure of the oxide developed. In other words, all of the studies mentioned agree that the presence of cavities along the metal-oxide interface is the clearest sign of weak adhesion between the thermally grown oxide and the underlying metal. Furthermore, most theories on the spalling of thermally grown oxide films are actually based on the assumption that stresses due to differential thermal expansion preferentially concentrate at the interface discontinuities and, from there, delamination cracks easily propagate, thus ultimately leading to the complete exfoliation of the surface layer.25-28 In our work, the effect of temperature was studied in several oxidation runs between 900 and 1200 °C and investigated in terms of the morphological features of the thermally grown oxide; however, significant modifications in such specimen morphologies could be visually assessed through SEM analyses on specimens oxidized at distinctly different temperatures, at least on a 100 °C base increase. The SEM observations made in the present study clearly show that short-time (10min) oxidation treatments in air provided the following oxide morphologies and adhesion properties depending on the operating temperature (Figure 2): At 900 °C, the oxides had a weak whisker morphology29-32 and the adhesion between the oxide layer

and the metal matrix was poor (Figure 2a). At 1000 °C, the morphology consisted of sharp-edged grains with slightly enhanced adhesion properties but with pronounced cavities at the oxide-metal interface (Figure 2b). At 1100 °C, the oxides had a rounded grain morphology with good adhesion levels at the interface with the metal substrate (Figure 2c). At 1200 °C, the oxides had completely globular grains morphology and the best achievable adhesion properties (Figure 2d). Exposure to temperatures lower than 900 °C favors the formation of chromium oxide rather than the aluminum oxide,33 which is completely unacceptable given that chromium tends to leave the protective layer by evaporation once higher temperatures are imposed. Conversely, preoxidation at temperatures higher than 1200 °C is not recommended, as such conditions are dangerously close to the operating limits generally assumed for FeCrAlloys and would therefore lead to bulk metal oxidation. The oxidative pretreatment carried out at temperatures ranging between 900 and 1200 °C clearly demonstrated that it was not feasible to achieve an oxide structure exhibiting both good adhesion with the metallic substrate and a high surface area and roughness through variations of the oxidation temperature alone, under an oxidizing atmosphere of constant composition (air). Indeed, the 1200 °C thermally grown oxide showed a smooth surface not very suitable for hosting the catalyst deposit. However, this oxide structure showed the highest adherence at the metal/oxide interface.

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Figure 4. FeCrAlloy thin sheet oxidized at 1200 °C for 10 min in a 0.5 vol % O2 atmosphere.

Figure 3. FeCrAlloy thin sheet oxidized at 1200 °C for 10 min in (a) 2 and (b) 0.5 vol % O2 atmosphere.

Therefore, the further steps of the oxide microstructure design were made starting from modifications of the procedure at 1200 °C. The aim was to attain the best compromise between adhesion properties and surface “roughness”. This balance could be achieved effectively through a decrease in the oxygen content of the oxidative pretreatment environment. An atmosphere poor in oxygen content should favor the formation of a porous and sharp-edged oxide. The oxygen content was varied in the concentration range 0.5-10 vol % (Figure 3). Intermediate oxygen concentrations (1-10 vol %) resulted in the development, on the same specimen, of mixed oxide structures ranging from a globular morphology to a sharp-edged and porous one (Figure 3a; as an example, the O2 content in the atmosphere for this sample was 2 vol %). These operating conditions did not ensure sufficient control and reproducibility of the oxide morphology. Under such conditions, the development of mixed structures with either globular or sharp grains or even whisker structures was frequently observed. Conversely, the oxidation process carried out at 1200 °C in 0.5 vol % oxygen (Figure 3b; O2 content in the atmosphere of 0.5 vol %) led to the formation of the ideal oxide structure: well adherent at the metal-oxide interface and porous at its external surface. Figure 4, in particular, shows the cavity-free metal-oxide interface and the several oxide protrusions of the oxide surface, which

Figure 5. FeCrAlloy basic knitted mattress oxidized at 1200 °C for 10 min in a 0.5 vol % O2 atmosphere.

clearly favor mechanical keying with the catalyst layer to be deposited. The FeCrAlloy oxidation develops through two stages: The first involves the fresh metal surface in contact with the process atmosphere. Because the aluminum atoms are uniformly distributed on such a surface, they react with oxygen all over the surface and develop into a continuous oxide layer covering the whole substrate surface. After the formation of this first compact and uniformly distributed layer of oxide grains, a second stage takes place where the contact between aluminum and oxygen usually occurs through Al atom diffusion into the oxide structure. Such diffusion in a compact alumina layer generally occurs along the oxide grain boundaries because of a short-circuit process; therefore, once the aluminum atoms reach the gasoxide interface, they are accumulated in preferential sites: the oxide grain boundaries. As a consequence, during the second oxidation stage, localized, unevenly distributed oxide growth occurs. Once the initial grain nucleus is formed, it tends to grow into either a globular or protruding grain, depending on the thermochemistry of process atmosphere. In a 0.5 vol % O2 atmosphere, growth of the protruding oxide grains was observed to be the only one favored under these conditions. However, the explanation of this behavior would require a deeper physicochemical study that is far beyond the scope of our investigation.

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Figure 6. FeCrAlloy basic knitted mattress oxidized at 1200 °C for 10 min in a 0.5 vol % O2 atmosphere. Oxide fracture detail: (a) outer fiber of the mattress, (b) inner fiber of the mattress.

Furthermore, this treatment was found to be very accurate and quite reproducible, thus making it amenable to an industrial process. In addition to these merits, as diffusion processes and oxidation reactions at 1200 °C are rapid, the use of this temperature, notwithstanding the high costs for reaching and maintaining it, allows a short process time to be achieved, which is also highly desirable from an industrial point of view, as discussed earlier. In this case, the technological goal of an optimal surface capable of hosting the catalyst prevails over the industrial process costs: the investment costs for an industrial high-temperature furnace are partially recovered with the low costs derived from the short time required for the preoxidation process and, as a consequence, the high productivity. Once the pretreatment for catalyst deposition on a metal foil of FeCrAlloys had been optimized, the study was focused on the transfer of these achievements to the fiber structures, which are the most interesting materials for gas premixed burner fabrication. Because metallic fibers have a higher surface-tovolume ratio than thin metal foils, the diffusion and oxidation processes are magnified in the oxidation of the FeCrAlloy basic knitted fibers. In this case, the 10-min process time ensured a consistent thickness of the grown oxide and a gas-oxide interface exhibiting a good porosity level with protruding grains (Figure 5), just slightly more rounded than the similar ones developed on the thin foils. However, this rounding effect of the

Figure 7. FeCrAlloy fibers of an NIT 200/S mat subjected to deposition of LaMnO3‚17MgO by different techniques: (a) washcoating, (b) spray pyrolysis.

Figure 8. Magnification of an FeCrAlloy fiber of an NIT 200/S mat subjected to deposition of LaMnO3‚17MgO by the spraypyrolysis technique.

oxide grains did not seemed to be detrimental for the clamping of the deposited catalyst layer. On the other hand, the oxide metal interface on the various fibers was less uniform (Figure 6). This is probably due to the high concentration of surface defects and stress levels typical of the cold drawn fibers. The temperature, time, and oxygen partial pressure parameters optimized for the pretreatment in this way

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Figure 9. Catalyst weight loss on catalytic FeCrAlloy NIT 200/S mat samples prepared by the wash-coating and spray-pyrolysis techniques after prolonged mechanical stress.

Figure 11. Performance of FeCrAlloy NIT 100/S premixed burners with LaMnO3‚17MgO catalyst deposited by the spraypyrolysis and wash-coating techniques, both fresh and aged, compared with that of a noncatalyzed premixed burner: (a) CO emissions, (b) NO emissions. (Ea ) 10%, dry gases, actual O2 concentration.)

Figure 10. FeCrAlloy NIT 100/S fiber mat sample with LaMnO3‚ 17MgO catalyst deposited by spray pyrolysis: (a) fresh status, (b) after 1000 h of thermal aging.

were found to be as follows: T ) 1200 °C, O2 content ) 0.5 vol %, and t ) 10 min. 3.2. Catalyst Deposition and Catalytic Burner Performance Tests. FeCrAlloy mats NIT 100/S and 200/S, pretreated according to the selected oxidation pathway, were subjected to deposition of the best developed catalyst (LaMnO3‚17MgO) according to the wash-coating and spray-pyrolysis techniques described in section 2. Figure 7 shows catalyst layers deposited on the metallic knitted fibers using the two different deposition techniques: the spray-pyrolyzed catalytic layer (in

image b) seems more uniform, more adhesive, more porous, and less affected by the presence of superficial fractures and fragments than the wash-coated catalytic layer (image a). In particular, Figure 8 shows a magnification of a covered fiber. From this image, it is possible to clearly distinguish the metal core fiber, the thermally grown aluminum oxide layer, and the catalyst layer (catalyst thickness of about 1-2 µm). It is worth anticipating that the highly porous nature of the spraypyrolyzed catalyst layer should be a positive feature in terms of catalytic activity maximization. Vibration tests were performed on the prepared catalytic premixed metallic burners so as to verify the adhesion and the inherent cohesion of the catalyst layer and the bond coating. The results of the vibration tests showed an asymptotic loss of weight in the catalyst (Figure 9) due to mechanical stress after about 10 cycles (i.e., about 1 h of vibration, which would correspond to about 1 year of real working conditions for a household burner according to preliminary calculations); no further loss of coating occurred. More specifically, the specimens catalyzed by wash-coating showed a more pronounced asymptotic loss of catalyst, up to 0.9 wt % compared to 0.5 wt % for the sprayed samples. This confirms spray pyrolysis as the more appropriate technique for catalyst deposition, as it ensures a more homogeneous and better adhered catalyst layer over the metallic fibers. As mentioned earlier, thermal shock resistance tests were also performed in which the catalytic burners were subjected to repeated on-off cycles in the boiler test rig.15 A deep SEM investigation of the stressed catalytic burners after the thermal shock resistance tests (Figure 10) showed that the differences between the fresh and

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aged samples were absolutely marginal. After aging, the catalyst still appeared quite homogeneously dispersed and well anchored to the fibers, without evidence of sintering or noticeable catalyst detachment. Furthermore, from single-fiber magnifications (not reported here for the sake of brevity), it was possible to notice that the porosity of the aged catalyst layer was very similar to that of the fresh catalytic burner. Finally, the catalytic combustion tests performed in the boiler test rig showed that the presence of a catalyst markedly helped reduce the emissions of pollutant CO and NO, independently of the catalyst deposition route or the aging status of the burner. The spray-pyrolysis deposition technique was once again slightly preferable to the wash-coating technique (Figure 11), possibly as a consequence of the better dispersion of the catalyst and its higher specific surface area. As far as the catalyst aging status is concerned, it has to be noticed that, notwithstanding the worsening of the catalytic performance of the premixed burners upon long thermal aging, a catalytic layer deposited over the firing surface of the burner allows for the maintenance of lower pollutant emissions compared to the noncatalytic premixed burners. This partial loss of catalyst activity cannot be attributed to catalyst loss or macroscopic changes in catalyst microstructure, but is possibly due to some slight but progressive poisoning effect of the perovskite active sites by sulfur compounds originating from the combustion of natural gas odorants. The presence of the MgO promoter in the catalyst composition was actually tailored to minimize this aging mechanism, keeping it at acceptable levels.34 4. Conclusions The goal of the study was mainly the development of innovative catalytic panels for industrial and domestic burners applications. These structured catalytic systems have to be based on an optimized combination of innovative catalysts and a properly pretreated metallic support hosting the catalyst layer. The first part of the research reported here was particularly focused on the preparation of the metallic support on which the actual catalyst layer is to be deposited. An actual design of the oxide morphology was developed to achieve the highest clamping of the deposited catalyst layer onto the support. Toward this end, supports based on FeCrAlloys (thin foils and knitted fiber mattresses) were considered. FeCrAlloys, if exposed at high temperature to an oxidative environment, develop a thermally grown oxide film that protects the metal matrix from corrosion. However, in this case, the oxide film is developed to act as a bond coating between the metal-based support and the deposited ceramic catalytic layer. Therefore, the oxidative pretreatment (10 min at 1200 °C under a mildly oxidizing atmosphere, namely, 0.5 vol % of O2) was defined to achieve formation of an R-alumina layer strictly anchored to the basic alloy and with an enhanced specific surface area for catalyst dispersion purposes. Two catalyst deposition techniques were tested: the spray-pyrolysis and wash-coating procedures. The former performed better than the latter by giving a quite adherent catalytic layer, better catalyst dispersion and activity, and ultimately lower pollutant emissions. As limited catalyst deactivation by sulfur poisoning was noticed after prolonged thermal treatment, it can be

concluded that the developed catalytic burners with optimized microstructures should be capable of guaranteeing safe and hygienic combustion for domestic applications over their lifetime and particularly better performance compared to their commercial noncatalytic counterparts. Literature Cited (1) Kofstad P. High Temperature Oxidation of Metals; John Wiley & Sons: New York, 1996. (2) Ishii, K.; Kohno, M.; Ishikawa, S.; Satoh, S. Effect of rareearth elements on high temperature oxidation resistance of Fe20Cr-5Al alloy foils. Mater. Trans. 1997, 38, No.9, 787. (3) Birks, N.; Rickert, H. The oxidation mechanism of some nickel-chromium alloys. J. Inst. Met. 1962, 91, 308. (4) Moreau, J.; Be´nard, J. The selective oxidation of nickelchromium alloys at high temperature. J. Inst. Met. 1955, 83, 87. (5) Fueki, K.; Ishibashi, H. Oxidation studies on Ni-Al alloys. J. Electrochem. Soc. 1961, 108, 306. (6) Grace, R. E.; Seybolt, A. U. Selective oxidation of Al from an Al-Fe alloy. J. Electrochem. Soc. 1958, 105, 582. (7) Fro¨lich, K. W. Die Zunderung von reinen und von legierten Kupfer. Z. Metalk. 1936, 28, 368. (8) Price, L. E.; Thomas, G. J. Oxidation resistance in copper alloys. J. Inst. Met. 1938, 63, 21. (9) Dennison, J. P.; Preece, A. High temperature characteristics of a group of oxidation resistant copper-based alloys. J. Inst. Met. 1952, 81, 229. (10) Spinedi, P. Ossidazione a temperatura elevata delle leghe rame alluminio. Metall. Ital. 1953, 45, 457. (11) Gulbransen, E. A.; Andrei, K. F. Oxidation studies on the iron-chromium alluminium heater alloys. J. Electrochem. Soc. 1959, 106, 294. (12) Klo¨wer, J.; Brill, U.; Heubner, U. High temperature corrosion behaviour of nickel aluminides: effects of chromium and zirconium. Intermetallics 1999, 7 (10), 1183. (13) Satyanarayana, D. V. V.; Pandey, M. C. The role of active elements in Fe-Cr-Al alloys heating element application. Bull. Mater. Sci. 1995, 18 (3), 207. (14) Saracco, G.; Sicardi, S.; Specchia, V.; Accornero, R.; Guiducci, M.; Tartaglino, M. On the potential of fibre burners to domestic burners applications. An experimental study. Gaswa¨ rme Int. 1996, 45, 24. (15) Cerri, I.; Saracco, G.; Geobaldo, F.; Specchia, V. Development of a methane premixed catalytic burner for household applications. Ind. Eng. Chem. Res. 2000, 39 (1), 24. (16) Messaoudi, K.; Huntz, A. M.; Lesage, B. Diffusion and growth mechanism of Al2O3 scales on ferritic Fe-Cr-Al alloys. Mater. Sci. Eng. 1998, A247, 248. (17) Lee, K. S.; Oh, K. H.; Park, W. W.; Ra, H. Y. Growth of a-alumina oxide film in high temperature oxidation of Fe-20Cr5Al alloy thin strip. Scr. Mater. 1998, 39 (8), 1151. (18) Ugues, D.; Rosso, M. Newly formed alumina scale growth mechanism on iron-chromium-aluminium alloy according to temperature and oxygen partial pressure effects. In Proceedings of EUROMAT 2001; FEMS (Federation of European Material Societies) Corrosion and Protection Section: London, 2001 (CDROM edition). (19) Saracco, G.; Cerri, I.; Specchia, V.; Accornero, R. Catalytic premixed fibre burners. Chem. Eng. Sci. 1999, 54, 3599. (20) Cerri, I.; Saracco, G.; Specchia, V.; Trimis, D. Improvedperformance knitted fibre mats as supports for premixed natural gas catalytic combustion. Chem. Eng. J. 2001, 82, 73. (21) Hindam, H.; Whittle, D. P. Peg formation by short-circuit diffusion in Al2O3 scales containing oxide dispersions. J. Electrochem. Soc. 1982, 129 (5), 1147. (22) Tien, J. K.; Pettit, F. S. Mechanism of oxide adherence on Fe-25Cr-4Al (Y or Sc) alloys. Metall. Trans. 1972, 3, 1587. (23) Howes, V. R. Observation of the metal oxide interface for an Fe-Cr alloy. Corros. Sci. 1968, 8, 221. (24) Howes, V. R. Metal transport in voids at metal-oxide interface. J. Electrochem. Soc. 1969, 116, 1286. (25) Wang, J. S.; Evans, A. G. Measurement and analysis of buckling and buckle propagation in compressed oxide layers on superalloy substrates. Acta Mater. 1998, 46 (14), 4993.

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Received for review October 24, 2003 Revised manuscript received January 23, 2004 Accepted January 27, 2004 IE034202Q