24
Ind. Eng. Chem. Res. 2000, 39, 24-33
Development of a Methane Premixed Catalytic Burner for Household Applications Isotta Cerri, Guido Saracco,* Francesco Geobaldo, and Vito Specchia Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy
A catalytic premixed burner prototype for domestic-boiler applications was developed on the basis of a perovskite-type catalyst (LaMnO3) deposited over a FeCrAlloy fiber panel. An economic and simple catalyst-deposition route, based on in situ pyrolysis of suitable precursors, was conceived and optimized on purpose. Finally, a catalytic burner and a reference noncatalytic one were comparatively tested in a pilot plant (maximum power, 30 kW, corresponding to about 2000 kW/m2). The catalytic burner allowed a strong reduction of CO and unburned hydrocarbon (HC) emissions to very low and acceptable levels (down to 3-5 times lower than those of the noncatalytic burner) when operated below 800 kW/m2. In these conditions, the NOx emissions remained quite acceptable and practically unaffected by the presence of the catalyst. 1. Introduction The catalytic combustion of methane is currently investigated in a variety of international research programs, thanks to its numerous potential applications (e.g., boilers, process heaters, reciprocating engines, gasturbine cycles, etc.). The primary goal of the present contribution is the development of a catalytic premixed fiber burner for household applications, characterized by a markedly lower environmental impact in terms of NOx, CO, and HCs (hydrocarbons), compared to its noncatalytic counterparts. In a fully premixed fiber mat burner the air-methane mixture is fed to a porous panel (see, e.g., Figure 1a) and, as a function of the local momentum of the gas mixture through the panel, the combustion may occur according to different regimes. At low surface heat powers (Q) and excesses of air (Ea), the combustion mostly occurs in a thin layer within the permeable panel (radiant or flameless regime, Figure 1b); the burner outlet surface (burner deck) reaches temperatures varying from 700 to 900 °C, depending on both Q and Ea values, and glows flamelessly. As a consequence, thermal energy is readily exchanged by radiation, which entails low flame temperatures and consequently low NOx formation.1 However, earlier studies about a noncatalytic FeCrAlloy-type burner2 enlightened that methane combustion was incomplete and responsible for high CO and HC concentrations, especially when operating below 400 kW/m2. Conversely, at high Q and Ea values the local momentum is sufficient to blow the combustion out of the burner, thereby establishing a flame front (blue-flame regime, Figure 1d) immediately close to the relatively cold burner deck (200-300 °C). In this case, the thermal energy is transferred less intensively to the heat sink by convection, so that the flame temperature rises, thus favoring the methane combustion completeness (lower CO and HC emissions), but leading to a higher NOx generation. It has to be emphasized that the change from one regime to the other, occurring at intermediate Ea and Q values, does not happen at once * To whom correspondence should be addressed. Tel./Fax: 39+11-5644654/99. E-mail:
[email protected] all over the burner deck, but so gradually that it may be considered as a third regime (transition regime, Figure 1c), characterized by the simultaneous presence of both short blue flames and radiating zones over the burner deck. Mainly because of their superior efficiency and their comparatively low environmental impact,3 premixed fiber burners recently gained several industrial or civil applications.4-7 A promising application opportunity for such burners lies in domestic power-modulating boilers. The goal is to assemble a boiler capable of coping with variable hot water requests: from about 2-3 kW (160240 kW/m2) for apartment heating up to 25 kW (2000 kW/m2) for sanitary purposes, so as to produce hot water with time delays compatible with the users’ comfort. In this context, a continuous power-modulation system has to be preferred over an on-off regulation one for either energy savings (the energy demand can be followed readily with no need for hot water accumulation, which would lead to some heat loss) or longer materials durability (reduced thermal fatigue). At the same time, pollutant emissions (CO, HCs, and NOx) must remain acceptable over the whole heat power range. Currently, this does not seem to be viable with FeCrAlloy fiber mats, owing to the high and unacceptable CO and HC emissions they entail at the low Q values as mentioned earlier. The deposition of proper oxidizing catalysts on the fiber burner, might reduce this drawback. Catalysts based on the noble metals (mainly Pt) were deposited on ceramic8,9 and metallic10,11 fiber burners, but did not lead to encouraging results in a long period due to catalyst sintering, volatilization, and sulfur poisoning. Some perovskite catalysts (A1-xAx′B1-yBy′O3(δ, where A, A′ ) La, Sr, Ba, etc. while B, B′ ) Co, Mn, Cr, etc.) seem to be a favorable alternative to the noble metals by guaranteeing a higher thermal, chemical, and physical stability and resistance to the poisoning effect of sulfur compounds used as odorizers (e.g., tetrahydrothiophene).12 Some perovskites, such as LaMnO3 studied later on, are also quite dark in color and should therefore ensure high emissivity. This paper concerns the development of a catalytic burner based on the combination of a FeCrAlloy fiber
10.1021/ie990425y CCC: $19.00 © 2000 American Chemical Society Published on Web 01/04/2000
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 25
2. Materials and Methods
Figure 1. The NIT 100s fiber burner: (a) close view of the NIT100s burner at room temperature. Firing modes of the burner fed with a methane-air mixture and 30% excess of air; (b) radiant or flameless regime; (c) transition regime; (d) blue-flame regime.
mat (Figure 1a) and perovskite catalysts, selected on the grounds of previous research efforts.13,14 Materials compatibility was assessed and a suitable catalystdeposition technique on the FeCrAlloy fiber mat (NIT100S by ACOTECH, Belgium) was developed and optimized. Finally, specific pilot-plant tests (30-kW maximum power) enlightened quite encouraging properties of the catalytic burner as opposed to its noncatalytic counterpart.
2.1. FeCrAlloy. The NIT100S fiber burner is made of FeCrAlloy, a high-temperature resistant alloy composed of (percentage by weight) Cr ) 20%, Al ) 5.0%, Y ) 0.1%, Si ) 0.3%, trace elements (Cu, C, Mn) e0.1%, and Fe ) balance. The choice of this refractory steel was dictated mainly by its outstanding resistance at temperatures up to 1200 °C in air. This particular behavior strictly depends on the formation of a thin, protecting R-alumina layer on the fiber surface. Moreover, the presence of yttrium plays a strategic role, allowing tight bonding between the protective alumina layer and the metal core.15,16 In the NIT100S burner, the metal fibers have a rectangular section (about 10 × 20 mm) and are knitted to form a mattress (thickness about 2 mm), according to a warp-and-weft scheme (Figure 1a). Beyond small burner specimens (15 × 20 mm on average), some FeCrAlloy sheet samples (thickness, 50 µm; size, 15 × 20 mm) were also considered for aging and catalyst-deposition tests, so as to provide a more accurate characterization of the materials. In fact, these last flat samples allowed us to investigate, by X-ray diffraction (PW1710 Philips diffractometer equipped with a monochromator on the diffracted-beam CuKR radiation), the nature and, qualitatively, the relative amount of the oxides covering the sample surface after specific thermal treatments or catalyst-deposition treatments. A good knowledge of the status of the FeCrAlloy surface is essential for catalyst-deposition purposes because it necessarily influences properties such as catalyst-support adhesion and possible interaction. All the samples were degreased in acetone with ultrasound and weighted: afterward, to investigate the growth quality and rate of the alumina layer, hightemperature oxidation in air at various temperatures (900, 1000, 1100, and 1200 °C) was accomplished in an electric oven. The growth of the oxide layer was checked every 2 h by nondestructive analyses on samples taken temporarily out of the treatment oven. Such analyses consisted of weight measurement by a microbalance (AE100 Mettler) and XRD analyses (just for the sheet samples). Finally, some samples were studied using scanning electron microscopy (SEM 525M with a Philips EDAX probe PW9100). 2.2. Catalyst Preparation and Characterization. Different perovskite-type oxides belonging to the LaCr1-xMgxO3 (0 < x < 0.5)13 and LaMn1-xMgxO3 (0 < x < 0.5)14 systems were studied earlier. Among these, the LaMnO3 perovskite was selected here, owing to its higher activity. LaMnO3 samples, supported (30 wt %) on R-alumina powders (size, 38-63 µm), were prepared to investigate potential interactions between the catalyst and support. In an attempt to reduce catalyst deactivation by solid-state reaction with the support (see section 3.2.1), catalysts were prepared by depositing the LaAlO3 itself (30 wt %) on the Al2O3 powder, prior to the LaMnO3 deposition. In all the above cases, the preparation procedure was based on the so-called “citrate method”.17 For the LaMnO3/Al2O3 or the LaMnO3/LaAlO3/Al2O3 catalysts, stoichiometric amounts of the LaMnO3 precursor salts, mainly nitrates with the only exception of manganese acetate, were blended with the support powder (Al2O3 or LaAlO3/Al2O3), adding about 30 wt % of glycerine (as a reducing agent) and 40 wt % of water. Such suspensions were slowly heated to the beginning of the NOx
26
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
generation and the transformation into a viscous paste. Such paste was then rapidly poured into stainless steel vessels and kept in a stove at 180 °C. In these conditions, a vigorous gas generation (mainly NOx, H2O, and CO2) occurred, leading to the formation of solid foams. These foams were milled in an agate mortar and then calcined for 24 h in air at different temperatures: 900, 1000, 1100, and 1200 °C. During this last treatment the desired perovskite formation took place, as checked by subsequent XRD analysis. The same procedure was used to prepare LaAlO3-coated R-Al2O3 powders which were finally calcined at 1100 °C for 8 h. Further, unsupported nonstoichimetric perovskites characterized by a variable La/Mn ratio (La1-Mn1-′O3 with ,′ ) 0, 0.1, 0.2) were prepared with a final calcination step at 900 °C for 24 h. After each synthesis, the elemental composition of the obtained materials was analyzed by atomic absorption analysis (Perkin-Elmer 1100B spectrophotometer) to verify the La/Mn ratio. The crystalline structure of each catalyst was verified by XRD analysis. Conversely, catalytic activity measurements were performed in a temperature-programmed-oxidation (TPO) apparatus, already described in ref 13, where light-off temperatures could be measured. Briefly, 0.5 g of the catalytic powder, turned into a granular form by pressing, grinding, and sieving (0.20.4 µm), were inserted in a quartz tubular fixed-bed reactor. This reactor was fed with a methane-oxygenhelium mixture (50 Ncm3/min, 2 vol % of methane, 17 vol % of oxygen, helium ) balance). A preliminary 30min preconditioning in air at 850 °C was performed to allow complete gas desorption (e.g., CO2). Then, the airmethane mixture was fed and the CO2 concentration was monitored at temperatures decreasing from 850 °C (at which the methane combustion is complete) down to room temperature at a 5 °C/min rate. The comparison between the various catalysts was accomplished by considering the half-conversion temperature (T50), taken as the index of the catalytic activity. 2.3. Catalyst Deposition on the FeCrAlloy Burner. The catalyst resulting as the suitable one by TPO runs (rhombohedral LaMnO3) was supported on the FeCrAlloy samples (fiber mat and sheet) after preliminary deposition of the LaAlO3 inert phase. A simple and cheap deposition technique was followed. First, samples were preheated at a high temperature, rapidly dipped into the solution containing the precursor salts of the catalyst or the LaAlO3 (metal nitrates and glycerine), then dried by compressed air, and calcined at 900 °C in air. A calcination temperature of 900 °C was selected, not exceeding the highest working temperature (700900 °C) of the burner deck, so as to avoid excessive specific surface area decrease by sintering. The deposition technique was repeated several times to obtain thicker and thicker catalyst or inert phase layers. The optimization of the deposition technique consisted of the selection of the best values of the following key parameters: (1) sample preheating temperature Tp (300 ÷ 1100 °C); (2) concentration of the LaMnO3 or LaAlO3 precursors (0.1-1.2 M); (3) number of deposition cycles (Nc ) 1-15). The procedure to find the optimal catalyst loading on the filter (i.e., number of subsequent deposition cycles) was supported by weight measurements, XRD analyses (for the sheets alone), SEM-EDAX analysis, and TPO tests. Particularly, these last tests were performed on 0.5 g of catalytic panel samples (4 × 20 mm) axially
stacked onto one another in the quartz tubular reactor described earlier, at the same operating conditions used for catalyst pellets except for the feed gas flow rate (lowered here to 10 Ncm3/min). 2.4. Pilot-Plant Testing of the Catalytic Burner. A catalytic burner was prepared with the following procedure. After the precalcination treatment (1100 °C, 3 h) the panel was first deposited with the LaAlO3 phase (Nc ) 5, Tp ) 900 °C, and concentration of the precursors solution ) 0.3 M). Then, the LaMnO3 catalyst was deposited (Nc ) 5, Tp ) 900 °C, and concentration of the precursors solution ) 1 M). Such a catalytic burner and another noncatalytic one, both in a disk shape (diameter, 140 mm), were tested in the pilot plant (maximum power, 30 kW) described in a previous work.2 The air-methane mixture was obtained through an air fan and a Venturi-type mixer capable of precisely controlling the feed stoichiometry. The mixture burned within or close to the premixed burner, placed vertically in the water-cooled combustion chamber. The flue gases leaving the chamber were further cooled in the shelland-tube heat exchanger. Samples for gas analysis were taken downstream the heat exchanger and fed to continuous gas analyzers: NOx chemiluminescence analyzer (Rosemount model 951A), CO and CO2 infrared analyzers (Maihak model UNOR 6N), O2 paramagnetic analyzer (Maihak model OXYGOR 6N), and HC flameionization detector (Rosemount model 402). The combustion chamber was provided with a glass peephole for the observation of the burner deck and a suction pyrometer for the measurement of the flue gas temperature. Through the peephole, the transition from a fully radiant to a fully blue-flame operating regime could be followed. When the specific heat load and the excess of air were varied, the above direct observation allowed us to determine the maps of the operating conditions belonging to the radiant, the transition, and the blueflame regimes, together with their boundaries. When Ea (0-50%) was varied at different Q values (200-2000 kW/m2), the composition of the flue gases was monitored. CO2, CO, HC, and NOx concentrations (referred to the dry gases at 0 °C, 1013 mbar, and 0% O2 equivalent concentration) could be measured through specific continuous analyzers. 3. Results and Discussion 3.1. FeCrAlloy Pretreatment. The R-alumina formation, obtained at temperatures higher than 900 °C in air, was requested to enable the unique hightemperature corrosion resistance of the FeCrAlloy material. It has been demonstrated15,16 that at temperatures higher than 900 °C the aluminum diffused preferentially, compared to the other alloy components (e.g., iron and chromium), from the bulk of the material to the surface where the alumina was formed. The formation of Al2O3 was favored by its much lower free energy of formation compared to those of iron and chromium oxides. Conversely, even short exposure periods (a few minutes) at temperatures higher than 1200 °C already induced degradation mechanisms (iron and chromium oxidation) seriously affecting the FeCrAlloy corrosion resistance. This occurs because in such conditions the aluminum content of the alloy is rapidly depleted, thereby enabling iron and chromium oxidation. The formation of the R-alumina phase was confirmed by XRD analysis of the foil samples. Figure 2, parts a
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 27
Figure 2. XRD spectra of an FeCrAlloy sheet sample. (a) as is; (b) after calcination at 1100 °C for 24 h in calm air; (c) after 12 LaAlO3-deposition cycles; (d) after 12 subsequent LaMnO3-deposition cycles. Symbols: F ) FeCrAlloy; R ) R-alumina; A ) LaAlO3, and P ) LaMnO3 perovskite.
Figure 3. Relative weight gain and its time derivative during thermal oxidation at 1100 °C for the fiber burner (20 × 20 mm2; initial weight ) 1.03 g) and sheet (15 × 20 mm2; initial weight ) 1.84 g) samples.
and b, refer to unoxidized and surface-oxidized FeCrAlloy. The growing thickness of the R-alumina layer was checked by weight increase measurements of the aged samples. At the aging temperatures of 900 and 1000 °C the alloy oxidation was rather slow. Conversely, at 1200 °C all the aluminum was rapidly driven by diffusion to form a protecting surface layer which, however, was not able to avoid further diffusion of oxygen to the metal phase after 2 h. As a consequence, rough crystals of chromium and iron oxides were then formed, causing quick corrosion and embrittlement of the alloy. On the contrary, at 1100 °C the aluminum oxidation for either burner or sheet samples was significant but did not lead to remarkable corrosion effects. For this reason, 1100 °C was elected as the optimal FeCrAlloy preoxidation temperature. With regard to this, Figure 3 shows the relative weight increase (∆W/W0), corresponding to some fiber burner and sheet samples versus the exposure time at
1100 °C. In the same figure, the corresponding time derivative, δ(∆W/W0)/δt, is also drawn. As expected, at the same operating conditions (oxidation time and temperature) the fiber burner exhibited a higher oxidation rate than the foil, because of the wider surface area exposed per unit mass. For both configurations the weight profiles were similar: the weight increase, due to oxygen uptake for oxide formation, initially appeared rather quick (in the first hour), soon coming to be less evident and nearly constant, until a new comparatively fast growth regime was noticed. These particular profiles are easily explained by taking into account that in the beginning the alloy surface was not covered with any oxide layer and, consequently, the oxidation rate was the highest. However, as the thickness of the alumina coating increased, diffusion phenomena (oxygen diffusion toward the alloy or aluminum diffusion from the bulk toward the alloy-oxide interface) became rate-limiting. At the gradual decrease of the bulk aluminum content (down to about 1.3% if the current literature has to be taken as an indication18) iron and chromium oxidation became competitive versus aluminum oxidation. This is confirmed by the quicker weight increase of the samples, more remarkable and anticipated for the fiber (occurring after about 10 h) than for the foil (just perceivable after about 30 h). An exposure time of 3-5 and 24 h at 1100 °C was thus fixed for the fiber burner and sheet specimens, respectively, so as to allow the buildup of a strong R-Al2O3 protecting layer but preventing any risk of incipient corrosion due to iron or chromium oxide formation. Besides, it was verified that the 4-5-h treatment on the fiber specimens allowed the formation of a very compact alumina layer, providing a suitable support for the perovskite materials. Figure 4, parts a and b, show respectively the SEM micrographs of a cross section and the surface of an FeCrAlloy foil calcined at 1100 °C for 24 h. It can be seen that the alumina layer is wellanchored to the basic alloy in a tight structure: the alumina crystals are so close to one another as to form a protective layer. According to the literature in the field (e.g., ref 15), such a layer should hinder oxygen diffusion, occurring along the Al2O3 grain boundaries from the air toward the basic alloy. 3.2. The Catalyst: LaMnO3. The catalyst to be deposited on the FeCrAlloy burner has to possess, besides a good catalytic activity, scarce tendency to interact with the support or to sinterize in the whole range of operating temperatures of the burner. Highly recommendable catalyst properties should also be good resistance to sulfur poisoning and high emissivity, for the above-mentioned reasons. LaMnO3 was elected as the most promising catalyst among the various catalysts tested in earlier papers of ours,13,14 mostly owing to its prevalent emissivity and catalytic activity. To these last merits, Figure 5 shows the T50 values obtained through TPO runs on pure lanthanum manganate samples calcined at progressively higher temperature. These data show a progressive, unavoidable loss of catalytic activity versus the calcination temperature mainly due to specific surface area reduction owing to sinterization. The tendency to react with the alumina support to form lanthanum aluminates was unfortunately shared with all the most active lanthanum-based perovskites presented in the literature (e.g.m LaCoO3 and LaFeO319). However, this drawback will be overcome, as described later on, by
28
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 Table 1. T50 Values of Various Nonstoichiometric LaMnO3 Perovskites Calcined at 900 °C for 24 h catalyst
T50 (°C)
catalyst
T50 (°C)
La0.8MnO3-δ La0.9MnO3-δ LaMnO3
555 570 500
LaMn0.9O3-δ LaMn0.8O3-δ
455 501
Figure 6. XRD patterns of the LaMnO3 samples directly supported on alumina powders calcined for 24 h in still air at different temperatures: (a) 900 °C; (b) 1100 °C. Symbols: R ) R-alumina, P ) LaMnO3 perovskite, 1 ) LaAl11O18, and 2 ) LaAlO3.
Figure 4. SEM micrographs of an FeCrAlloy sheet after thermal oxidation at 1100 °C for 24 h: (a) cross section; (b) surface view.
Figure 5. Comparison of T50 values for the LaMnO3 catalyst prepared pure and in different supported forms calcined for 8 h at different temperatures. T50 value for the homogeneous, noncatalytic combustion: 770 °C.
LaAlO3 interposition between the perovskite and the R-Al2O3 layer. 3.2.1. Effect of La or Mn Substoichiometry on LaMnO3 Catalytic Activity. The reaction of part of the lanthanum of the basic perovskite LaMnO3 could either lead to destruction of the perovskite structure or to a simple reduction of the catalytic activity. This last
occurrence seems to be more likely. In fact, considering the La1-Mn1-′O3 catalyst series prepared, XRD analysis detected for each sample the presence of the rhombohedral phase alone (the same crystal structure of the LaMnO3), independently of the or ′ values, thereby confirming an extended existence field of the mother phase LaMnO3. On the basis of the data in Table 1, it can be concluded that catalyst-support interactions can actually lead to deactivation, even if the perovskite structure is preserved. In fact, according to the T50 values of nonstoichiometric La1-Mn1-′O3 catalysts listed in Table 1, it has to be noted that lanthanum substoichiometry in the La1-Mn1-′O3 catalyst entails, contrary to manganese substoichiometry, an important decrease in the catalytic activity. 3.2.2. LaMnO3 Compatibility with the Support. The expected loss of activity due to interaction between LaMnO3 and R-Al2O3 was confirmed by TPO runs performed, according to the procedures described in section 2.2, on samples of the LaMnO3/Al2O3 powders calcined at different temperatures. The activity of these samples was in any case much lower than that of the pure LaMnO3, as outlined in Figure 5, where the T50 values of the prevalent catalysts studied is plotted as a function of the calcination temperature. A peculiar decrease of activity can be noticed when passing from a calcination temperature of 900-1000 °C. The different powder samples were analyzed by XRD analysis to better clarify this point. In particular, Figure 6 shows XRD spectra of LaMnO3/Al2O3 powders calcined for 24 h at 900 °C (pattern “a”) and 1100 °C (pattern “b”). In pattern “a” both the separated phases, the R-alumina (JCPDS card 46-1212) and the rhombohedral LaMnO3 (JCPDS card 32-0484), are detected, while in pattern “b” also the diffraction peaks of LaAlO3 (JCPDS card 31-0022) and LaAl11O18 (JCPDS card 33-0699) are clearly evident. It can be deduced that at 1100 °C (Figure 6b) alumina strongly reacted with lanthanum. The solid reaction led to the formation of considerable
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 29
amounts of the LaAlO3 and LaAl11O18 side products. On the contrary, at 900 °C the absence of those side products is presumably to be ascribed to the very low kinetics of the Al2O3/LaMnO3 reaction. Even if small amounts of lanthanum aluminates were formed at 900 °C, XRD spectra could not detect them because of their rather high sensitivity limit (5 wt %) for the detection of new phases. 3.2.3. LaAlO3 Precoating. A possible way to prevent reaction between the perovskite and the R-alumina protecting layer, which would invariantly lead to catalyst deactivation, could lie in precoating the R-Al2O3coated fibers with LaAlO3 prior to LaMnO3 deposition. Lanthanum aluminate indeed shows a scarce tendency to react with the alumina as well as with the perovskite catalyst and keeps these two last counterparts separated from each other. Some activity tests (TPO runs) were performed on different samples to investigate the potential of this procedure. The T50 values of LaMnO3 powders prepared in different forms (pure, deposited on alumina, and deposited on LaAlO3/alumina) are reported in Figure 5, together with those of pure alumina and LaAlO3/alumina (30 wt %). The role of LaAlO3 preliminary deposition on R-alumina powders in reducing the LaMnO3 deactivation is quite evident. It has to be outlined that, in any case, the typically low BET areas limited, to some extent, the overall catalytic activity of any sample. The BET surface areas of all the powder catalysts prepared were in the rather limited range of 18-22 m2/g for a calcination temperature of 900 °C. As expected, these values dropped to 2-4 m2/g, owing to grain sintering, when a calcination temperature of 1200 °C was used. The LaMnO3/LaAlO3/Al2O3 powders (Figure 5) showed catalytic activity (T50 ) 533 and 605 °C for the samples calcined at 900 and 1100 °C, respectively) even better than that of the LaMnO3/Al2O3 powders (T50 ) 602 and 651 °C, respectively), despite the lower specific amount of active species (LaMnO3). On one hand, the LaAlO3 increased the area on which the catalyst was dispersed; on the other hand, it hampered the deactivating reaction between alumina and the catalyst. In fact, as previously explained, the LaMnO3/Al2O3 catalyst calcined at 900 °C was probably characterized by the formation of LaAlO3 and LaAl11O18 in so little amounts that XRD analysis could not detect them. These side products could, though, lead to a partial deactivation of the LaMnO3 catalyst as a consequence of induced La substoichiometry (see section 3.2.2). On the basis of these positive results, the interposition of the LaAlO3 between the LaMnO3 catalyst and the alumina coating of the fibers appeared to be highly desirable to possibly ensure a good long-term stability, further to optimal catalytic activity. 3.3. Optimization of the Catalyst-Deposition Technique. As seen in section 2.3, the first step of the proposed catalyst-deposition procedure consisted of preheating of the precalcined FeCrAlloy samples in an oven, prior to sudden dipping of such hot samples in the precursor solution. To find the proper preheating temperature (Tp), some preliminary experiments were carried out on various sheet specimens, by keeping the same LaMnO3 precursor solution (0.67 M) and varying the Tp value from 300 to 1000 °C. Figure 7 shows the weight increase resulting from various Tp values. For Tp values < 700 °C XRD analyses detected the presence of an amorphous phase, lining the sample surface.
Figure 7. Tp influence on the relative weight gain after one LaMnO3-deposition cycle for the precalcined sheet samples (precursor salts concentration 0.67 M).
Figure 8. Influence of precursor salts concentration on the relative weight gain after a single LaMnO3- or LaAlO3-deposition cycle for precalcined sheet samples (Tp ) 900 °C).
Generally, for Tp < 900 °C the catalyst turned out to be not uniformly dispersed (as evaluated by SEM observations), notwithstanding the weight increase (hence the catalyst coating) was higher than that for Tp ) 900 °C. Conversely, for Tp > 900 °C the evaporation of the water was so intense that it led to the formation of a poorly adhered layer of perovskite crystals all over the samples. 900 °C was thus chosen as the suitable preheating temperature since it allowed a good and even bonding of the perovskite crystals of the specimen, despite the relative amount of catalyst deposited per single-deposition cycle resulted in the lowest among the different Tp values tested. After the deposition step, a 15-min posttreatment at 900 °C was performed so as to attain a good crystallization of the perovskite phase. The same technique could then be tested, not only for LaMnO3 deposition but also for the LaAlO3 precoating, with the only difference being that the crystallization treatment after the deposition procedure had to be performed, to be effective, at 1100 °C for 15 min. Some experiments were thus performed on the sheet samples by varying the concentrations of the precursor salts in the range of 0.33-1.2 and 0.2-1 M, for the LaMnO3 and LaAlO3 phases, respectively (Figure 8). The more concentrated the precursor solution, the higher, in both cases, the material load obtained after each deposition cycle. As far as the catalyst dispersion over the specimens was concerned (evaluated by SEM observations), some remarkable differences were noticed between the two systems. The distribution of the LaMnO3 always resulted, even for all the precursor concentration levels, leading to the formation of layers well stuck onto the support. Conversely, concerning the LaAlO3 phase, concentration values >0.3 M were responsible for the formation of several defects (cracks and uneven thickness) in the aluminate layer obtained. For these reasons, the optimal precursor concentration values for the
30
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
Figure 10. SEM micrograph of a FeCrAlloy fiber after three LaMnO3-deposition cycles.
Figure 9. Influence of the number of deposition cycles of either LaAlO3 (graph a, precursor salts concentration ) 0.3 M) and LaMnO3 (graph b, precursor salts concentration ) 1 M) on the relative weight gain of both the sheet and the burner samples.
LaMnO3 and the LaAlO3 phases were set equal to 1 M (rather close to the solubility limit of about 1.2 M) and 0.3 M (the best compromise between the amount deposited per deposition cycle and layer quality), respectively. The formation of the LaAlO3 and LaMnO3 phases over an FeCrAlloy foil was confirmed by EDAX and XRD analyses. Figure 2 shows the evolution of the surface layer chemical composition (XRD pattern) of FeCrAlloy sheet samples after each step of the LaMnO3 deposition: pattern “a” refers to the FeCrAlloy foil, pattern “b” to the precalcined one (mainly R-alumina forming the protective layer), pattern “c” to the same sample after 12 LaAlO3-deposition cycles, and finally pattern “d” to the specimen itself after 12 LaMnO3-deposition cycles. The deposition of each phase was repeated several times and, as expected, the sample weight increased with the number of deposition cycles, Nc (Figure 9, parts a and b). Quite reasonably, the relative weight increase turned out to be higher for the burner samples, owing to their larger specific surface area compared with that exposed by the FeCrAlloy sheet. Besides, at the same Nc value, the LaMnO3 amount resulted in a higher amount of LaAlO3, owing to the higher concentration of the precursor solution (1 M versus 0.3 M, respectively). As a general issue, on the basis of the experimental data, the described procedure was rather well reproducible in the case of the LaMnO3 (Figure 9b), but slightly less for the LaAlO3, in accordance with the worst dispersion obtainable for this last phase. The selection of the proper Nc value was based on the TPO tests on burner samples. Initially, the catalytic activity rapidly increased with the catalyst content (hence, Nc), then gradually approached a nearly constant value according to an asymptotic trend. The enhancement of the catalytic activity was obtained during the first four to five deposition cycles; also, the catalytic area was accordingly enhanced. Further depo-
sition treatments did not lead to significant activity improvements because the catalyst covered and hid itself. This fact was confirmed by the SEM micrograph shown in Figure 10 which referred to a FeCrAlloy fiber after three catalyst-deposition cycles: the LaMnO3 coating resulted from the formation of thin films stacked onto one another. Such multilayer structures seemed to be very attractive for a potential stronger mechanical stability: the spalling of the outer film did not immediately compromise the catalytic performance, thanks to the presence of the inner layers. The multiphase structure of a FeCrAlloy sheet deposited with the LaMnO3 catalyst (LaAlO3 about 2 wt % and LaMnO3 about 27 wt %) can be seen in Figure 11a, where the LaMnO3 seems to cover the alumina FeCrAlloy coating. In fact, the LaAlO3 layer could not be detected with confidence by SEM-EDAX analysis, owing to the very low amount of lanthanum aluminate with respect to the other phases. Moreover, the morphology of the catalyst coating (Figure 11b) appeared as a very porous structure to the benefit of a wider surface area made available to the reacting gases. In fact, the tumultuous gases generation (mainly NOx, CO2, and H2O) occurring during the precursors’ pyrolysis treatment caused the particularly attractive feature of the catalyst layer. On the basis of the above results, a catalytic burner was prepared according to the optimized preparation route described in Table 2. 3.4. Performance of the Catalytic Burner. As outlined above, the effect of the presence of catalyst over the burner performance was investigated in a boiler test rig, mainly by checking its impact on pollutant emissions (NOx, CO, and HCs). The obtained results are shown in Figures 12-14 where, for the sake of clarity, only the pollutant concentrations measured for three Q values are reported. With regard to the NOx emissions (Figure 12), at constant Ea values, the higher NOx concentrations formed at the higher Q values; in fact, because of the establishment of the blue-flame regime, the flame temperatures rose, favoring the NOx generation. In a comparison between the burners, significant differences were not appreciable, notwithstanding it was expected that the increased emissivity of the darker LaMnO3 coating would lead to lower flame temperatures. Anyway, in both the burners the NOx concentrations re-
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 31 Table 2. Optimized Preparation Route for the Catalytic Fiber Mat Burner step
operating conditions
cleaning preoxidation LaAlO3 deposition
ultrasounds in acetone 3 h in air at 1100 °C five deposition cycles consisting of spray pyrolysis with 0.3 M precursors solution, drying, and 15-min calcination at 1100 °C five deposition cycles consisting of spray pyrolysis with 1 M precursors solution, drying, and 15-min calcination at 900 °C
LaMnO3 deposition
notes R-Al2O3 protecting layer formed LaAlO3 deposition up to 2 wt % obtained LaMnO3 deposition up to 27 wt % obtained
Figure 12. NOx emissions as a function of Ea for some Q values for the two burners tested. Transition regime boundary lines: - ‚ -, noncatalytic burner; s s s, catalytic burner.
Figure 11. SEM micrographs of a FeCrAlloy sheet samples after the LaMnO3 deposition following the indirect route: (a) cross section; (b) surface.
mained under 70 ppmv when operating in the Ea range usually adopted in commercial boilers (Ea ) 15-25%). The only marginal effect of the presence of the catalyst on NOx emissions might be surprising if the outstanding advantages of the application of catalytic combustion to the twin context of gas-turbine cycles are considered.20 However, catalytic gas-turbine cycles and catalytic premixed surface burners are only apparently similar to each other: (1) In the case of gas turbines the great advantage of catalytic combustion (compensated at least in part by other disadvantages) lies in the possibility of burning out methane at about 1300 °C in rather low concentrations (aroud 3 vol %). Conventional free flames cannot handle such a low methane concentration and thus noncatalytic gas-turbine cycles are forced to burn out a side stream of more concentrated methane and then dilute the effluents of such combustion with the main
Figure 13. CO emissions as a function of Ea for some Q values for the two burners tested. Transition regime boundary lines: - ‚ -, noncatalytic burner; s s s, catalytic burner.
stream of compressed air before entering the turbine at about 1300 °C or lower temperatures. High-temperature diffusive methane flames can reach temperatures as high as 1800 °C. This entails high NOx emissions that
32
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
(2) A slightly higher head loss, which however always remained below the quite tolerable value of 2 cm of H2O. On the grounds of the above encouraging results, aging runs are planned to actually check the long-term stability expected for the prepared burners. 4. Conclusions
Figure 14. HC emissions as a function of Ea for some Q values for the two burners tested. Transition regime boundary lines: - ‚ -, noncatalytic burner; s s s, catalytic burner.
can be nearly eliminated by taking advantage of catalytic combustion.21 (2) In the case of premixed fiber buners, methane is fed at high concentrations, well inside its flammability range. This allows methane to have stable flames over wide ranges of operating conditions in terms of excess air and specific heat power. Most of the potential in lowering NOx emissions is guaranteed, when operating in the radiant regime, by the flame-cooling effect of radiative heat transfer, almost independently of the presence of the catalyst. Therefore, no major advantages are actually to be expected by the presence of the catalyst unless it significantly increases the burner emissivity to the benefit of even lower flame temperatures. Concerning CO and HC emissions (Figures 13 and 14), the catalyst was invariantly capable of promoting methane combustion, independently of the Ea and/or Q values employed (the HC concentrations obtained on both the burners at Q values of 930 and 1740 kW/m2 were practically undetectable). Particularly, in the above usual Ea range, CO concentrations for the catalytic burner were lower than 30-40 ppmv, whereas for the noncatalytic one they were lower than 220 ppmv. Of course, when the excess air was very poor (below 10%) and the specific heat power was as low as 310 kW/ m2 (entailing low flame temperatures), the catalyst could not prevent the typical rapid increase of CO emissions due to imperfect mixing and local oxygen substoichiometry. Such a rapid increase was shifted to slightly lower excesses of air compared to the noncatalytic burner (see Figure 13). Concerning the HC emissions in the best-operation Ea range, the concentrations obtained for the catalytic burner were 80-90 ppmv, while those for the noncatalytic one were higher, 300-400 ppmv. This better performance was possible despite the fact that the deposition of the catalyst over the burner reduced the porosity of the original mattress (from about 85% to about 79%), which entailed the following: (1) A higher momentum of the flue gases within the porous matrix of the burner, leading to a higher tendency to blow the combustion outside the burner where catalysis cannot be exploited any longer (transition to blue-flame regime). As can be seen in Figures 12-14, the radiant-regime zone, as observed directly through the peep hole in the combustion chamber, was smaller for the catalytic burner, while the transition one was wider.
A new catalytic burner for household applications based on a perovskite catalyst and a FeCrAlloy support was developed and tested for its improved performance compared to a noncatalytic counterpart from the environmental-impact viewpoint. The study entitled different deliverables: (1) The choice of the proper catalyst (LaMnO3) for the combustion of methane in view of its final deposition on the FeCrAlloy burner. (2) The investigation of the support/catalyst interaction. (3) The development of a possible way of preventing the consequent catalyst deactivation (precoating with LaAlO3). (4) The development and optimization of a very simple catalyst-deposition technique over the burner. As far as the pollutant emissions of the catalytic and noncatalytic burners are considered, it was verified that the catalytic burner entailed a strong reduction in the CO and HC outlet concentrations, becoming respectively 3-4- and 6-8-fold lower than those of the noncatalytic burner. Conversely, NOx emissions remained nearly unaffected, but anyhow quite acceptable. It is clear that the complexity of the heterohomogeneous system of reactions underneath this quite encouraging improvement requires a deeper analysis of all the fluid-dynamic and reaction mechanisms, occurring either on the catalyst surface or in the homogeneous gaseous phase. This kind of investigation stands at the borderline of what can actually be done today with the analytical tools (temperature measurement, intermediates concentration assessment, etc.), available at such high temperatures as those typical of such processes (>700 °C). At present, our efforts are directed in working out a suitable modeling tool by combining the fluid-dynamic codes (Phoenics) and kinetics information of the homogeneous combustion (noncatalytic burner) and tentative kinetics expressions for the combined catalytic effect capable of allowing good predictions of the results obtained by the catalytic burner. From a more application-oriented viewpoint, new burner supports are being considered for catalytic activation with perovskite catalysts (i.e., mullite ceramic foams), meanwhile testing the long-term stability of the catalytic FeCrAlloy fiber mats in a commercial boiler in the presence of sulfur compounds (odorizers). Acknowledgment The financial support of Societa` Italiana per il Gas is gratefully acknowledged. Special thanks also go to Centro Ricerche Fiat for the SEM-EDAX measurements. Literature Cited (1) Sharifi, R.; Pisupati, S. V.; Scaroni, A. W. Combustion Science and Technology. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1993; Vol. 6, p 1049.
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 33 (2) Saracco, G.; Sicardi, S.; Specchia, V.; Accornero, R.; Guiducci, M.; Tartaglino, M. On the Potential of Fibre-Mats to Domestic Burners Applications. An Experimental Study. Gaswaerme Int. 1996, 45, 24. (3) Hargreaves, K. J. A.; Jones, H. R. N.; Smith, D. B. Developments in Burner Technology and Combustion Science. Presented at the 52nd Institution of Gas Engineers Autumn Meeting, London, 1986; Paper 1. (4) Krill, W. V.; Kesserling, J. P. Field Evaluation of the Fibre Burner in Firetube Boilers. Gaswaerme Int. 1985, 34, 162. (5) Gotterba, J. A.; Schreiber, R. J.; Blair, R. J. High Efficiency Commercial Size Warm Air Furnace Using the Alzeta Pyrocore Burner. Proceedings of the 7th Annual Energy Seminar, Erie, PA, 1985; p 27. (6) Kessler, V. C.; Schaaf, R.; Menche, O. Low NOx Ceramic Surface Burner for Process Heating. Gaswaerme Int. 1987, 36, 350. (7) Nakamachi, I.; Iseda, Y.; Kurinoto, K.; Goto, N. Status of MFB Projects at Tokyo Gas. Proceedings of the International Conference on Metal Fibre Burner Technology, Kortrijk, Belgium, 1994; Vol. 1, p 45. (8) Sullivan, J. D. Basic Research on Radiant Burners; Semiannual Report, GRI Report 91/0331; GRI: July, 1991. (9) Bos, A.; Doesburg, E. B. M.; Engelen, C. W. R. Preparation of Catalysts on a Ceramic Substrate by Sol-Gel Technology. In Eurogel’91; Vilminot, S., et al., Eds.; Elsevier: Amsterdam, 1992. (10) van Wingerden, A. J. M.; Boon, A. Q. M.; Gesus, J. W. A Method for Performing a Chemical Reaction and a Reactor for Use Therewith. European Patent Application, 90202359.7, 1990. (11) van Looij, F.; Mulder, A.; Boon, A. Q. M.; Scheepens, J. F.; Geus, S. W. Fixed-Bed Reactors Based on Sintered Metals. Presented at the 10th International Congress of Catalysis, Budapest, 1992; Paper 10. (12) Klvana, D.; Kirchnerova, J.; Gauthier, P.; Delval, J.; Chaouki, J. Preparation of Supported La0.66Sr0.34Ni0.3Co0.7O3 Per-
ovskite Catalysts and Their Performance in Methane and Odorised Natural Gas Combustion. Can. J. Chem. Eng. 1997, 75, 509. (13) Saracco, G.; Scibilia, G.; Iannibello, A.; Baldi, G. Methane Combustion on Mg-Doped LaCrO3 Perovskite Catalysts. Appl. Catal. B: Environ. 1996, 8, 229. (14) Saracco, G.; Geobaldo, F.; Baldi, G. Methane Combustion on Mg-Doped LaMnO3 Perovskite Catalysts. Appl. Catal. B: Environ. 1999, 20, 277. (15) Pint, B. A.; Garratt-Reed, A. J.; Hobbs, L. W. The Reactive Element Effect in Commercial ODS FeCrAl Alloys. Mater. High Temp. 1995, 13, 3. (16) Quadakkers, W. J.; Elschner, A.; Speier, W.; Nickel, H. Composition and Growth Mechanisms of Alumina Scales on FeCrAl-Based Alloys Determined by SNMS. Appl. Surf. Sci. 1991, 52, 271. (17) Abbattista, F.; Vallino, M.; Brisi, C. Ricerche sul sistema La2O3-Cr2O3-MgO-O. Atti Accad. Sci. Torino 1979, 113, 69. (18) Quadakkers, W. J.; Bongartz, H. The Prediction of Breakaway Oxidation for Alumina Forming ODS Alloys Using Oxidation Diagram. Werkst. Korros. 1994, 45, 232. (19) Tejuca, L. G., Fierro J. L. G., Eds. Properties and Applications of Perovskite-type Oxides; Marcel Dekker Inc.: New York, 1993. (20) Dalla Betta, R. A. Catalytic Combustion Gas Turbine System: The Preferred Technology for Low Emissions Electric Power Production and Co-generation. Catal. Today 1997, 35, 129. (21) Ismagilov, Z. R.; Kerzhentsev, M. A. Catalytic Fuel CombustionsA Way of Reducing Emission of Nitrogen Oxides. Catal. Rev.-Sci. Eng. 1990, 32, 51.
Received for review June 15, 1999 Revised manuscript received October 19, 1999 Accepted October 22, 1999 IE990425Y
