Energy Fuels 2011, 25, 136–143 Published on Web 12/16/2010
: DOI:10.1021/ef100656y
Lean Catalytic Combustion for Ultra-low Emissions at High Temperature in Gas-Turbine Burners Fabrizio D’Alessandro,† Giovanna Pacchiarotta,† Alberto Rubino,† Mauro Sperandio,† Pierluigi Villa,*,† Arturo Manrique Carrera,‡ Reza Fakhrai,‡ Gianluigi Marra,§ and Annalisa Congiu§ †
Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universit a di L’Aquila, Via Campo di Pile, Zona industriale di Pile, 67100 L’Aquila, Italy, ‡Department of Energy Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and §Istituto Ente Nazionale Idrocarburi (ENI) Donegani, Via Fauser 4, 28100 Novara, Italy Received March 26, 2010. Revised Manuscript Received November 24, 2010
Catalytic systems for methane combustion, with Rh and Pt in a BaZrO3-based perovskite, were synthesized at the University of L’Aquila and tested at close to industrial conditions at the KTH Energy Centre in Stockholm. Because of the resistance to high temperature of BaZrO3 (up to ∼2600 °C), such systems are suitable for resolving stability problems frequently encountered with high-temperature operations. Furthermore, these perovskites contain the noble metal in a high oxidation state, giving rise to very active compounds. They also result in ultra-low emissions, compatible with legislation in such places as southern California and Japan.
methods; both have been described in recent reviews.2-4 Secondary methods operate downstream of the turbine to bring down NOx in the exhaust gases formed during combustion. Primary methods function by reducing NOx formation by taking part directly in the combustion process, controlling the critical parameters that favor their formation: flame temperature, air/fuel ratio, and residence time of the gas in the combustor. Traditional methods of reducing NOx emissions from gas turbines (water and steam injection) are limited in their ability to reach the limits at present required in many localities.5 Dry low NOx (DLN) is the emerging technology that allows for lower emissions; a premixing of air/fuel is performed followed by a lean combustion (to reduce the flame temperature and thermal NOx emissions) without a diffusive flame. Low emissions of NOx can thus be achieved with a thorough fuel/air mixing and control of the adiabatic flame temperature.6 Unfortunately, at fuel/air ratios low enough to achieve such low NOx concentrations, flames are unstable and susceptible to flame-out or fluctuations; combustion-driven oscillations occur, which result in reduced reliability. A number of gas turbine sites have encountered such problems, resulting in premature failure of the combustion components.7 The operational window is rather narrow. The residence time in the flame zone should be short enough to achieve low NOx but with acceptable levels of combustion noise, stability at partial load operation conditions, and sufficient residence time for CO burn-out.5 Significant progress in the DLN
Introduction Thermoelectric Power Generation: The Problem of NOx. Combined cycles have come to represent the reference technology for combustion-driven electric power generation for the following reasons: much higher efficiency, lower specific investment, shorter construction times, lower pollutant and greenhouse gas emissions, lower manpower costs, smaller plant dimensions, absence of solid and liquid wastes, absence of chemical exhaust-gas cleaning costs, lower cooling water requirements, and enhanced operative flexibility (e.g., short startup times). They combine the positive aspects of high-temperature gas-turbine operation with those of vapor closed cycles at low (close to ambient) temperature. The progress of combined cycles in recent decades is linked to the increase of the turbine inlet temperature, which, in recent years, has increased by some 12.5 °C/year, arriving at 1430 °C for the last generation of turbines (H system of GE1). As a result, the most recent generation of combined cycle engines has surpassed the 60% efficiency barrier. Combined cycles are characterized not only for their high efficiency but also because they are environmentally clean. Using natural gas, they produce less CO2 for the same power output, with methane being the hydrocarbon with the highest hydrogen/carbon ratio, but most of all, for the composition of the combustion products, CO, unburned hydrocarbons, and soot are quite negligible and NOx concentrations are lower than about 25 ppm as a result of premixed combustion conditions. The current technology for the reduction of NOx emissions allows for a choice between secondary and primary
(4) Beer, J. M. Low NOx burners for boilers, furnaces and gas turbines; Drive towards the lower bounds of NOx emissions. Combust. Sci. Technol. 1996, 121, 169–191. (5) Davis, L. B.; Black, S. H. Dry low NOx combustion systems for GE heavy-duty gas turbines, GER-3568G. Electr. Power Syst. Res. 2000, No. October, 1–22. (6) Peltier, R. Gas turbine combustors drive emissions toward nil. Power 2003, No. March, 23–34. (7) Rea, S.; James, S.; Goy, C.; Colechin, M. J. F. On line combustion monitoring on dry low NOx industrial gas turbines. Meas. Sci. Technol. 2003, 14, 1123–1130.
*To whom correspondence should be addressed. E-mail: pierluigi.
[email protected]. (1) Valenti, M. Reaching for 60%, the General Electric H turbine system taking shape in Wales is making a bid for a new record in thermal efficiency. Mech. Eng. 2002, No. April, 35–39. (2) McGowan, P. E. Charting a path for cost-effective NOx control. Chem. Eng. 2004, No. October, 36–41. (3) Ganapathy, V. How emissions affect HRSGs. Hydrocarbon Process., Int. Ed. 2003, 82 (7), 45–49. r 2010 American Chemical Society
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technology has been achieved in the past decade; 3 ppm NOx is now possible.8 In the reaction zone, there are some spots of higher temperature, where almost all of the NOx formation occurs. Reducing the temperature within these hot spots, which result from poor mixing, can achieve the above values. On the other hand, to keep the CO emissions low, it is necessary to avoid cold spots in the reaction zone, such as those as a result of the introduction of cold cooling air downstream of the reaction zone. Further improvements have been obtained with the invention of a pilot for a gas turbine combustion, which uses the heat of combustion within the pilot to increase the reactivity of a portion of fuel/air mixture used by the pilot,9 by the introduction of oscillation suppressors (referred to as screech suppressors10) and by the introduction of surface-stabilized burners constructed of small metal fibers, which are compressed and sintered to result in a seamless and porous all-metal structure in the shape of a tube with a domed end.11 Improvements in the NOx emissions can be obtained by exhaust gas recirculation, which is mainly aimed at reducing the cost of CO2 capture,12 and by inlet air cooling.13 Catalytic combustion is an alternative technology that has the potential for achieving NOx emissions lower than 3 ppm and, at the same time, avoiding the pressure oscillation typical of DLN systems. The combustion occurs mainly at the catalyst surface, following a flameless path at lower temperatures, and provides for greater flexibility with respect to the employed fuel. The operative windows for catalytic and DLN combustors are given by Fant et al.14 The adiabatic flame temperature required for catalytic combustion is lower than that for DLN, and as a result, the associated thermal NOx emissions are lower. The current upper catalyst temperature limit, however, is not adequate. To fully realize the potential of catalytic combustion, further work into both high-temperature material development and reactor design are required. Recent reviews of catalysts for combustion have been provided by Choudary et al.,15 Forzatti,16 and Vatcha.17 The thermal resistance of a catalyst is limited by the vaporization temperature of the catalyst itself and the sinter-
Figure 1. (Top) Hybrid combustor and (bottom) XONON combustor configuration.
ing temperature of the support material.18,19 The most catalytically active material appears to be palladium, which is recommended for long-term use in high-temperature combustors. Pd, however, is only able to withstand temperatures up to about 900 °C, whereas current turbine inlet temperatures are much higher. Furthermore, PdO, the most active form, is reduced to metallic Pd at high temperature and only reoxidized to PdO at temperatures below 500 °C, with a complex hysteresis cycle20-22 that gives rise to additional oscillations. To overcome these problems, the hybrid combustor and the partial catalytic combustor have been proposed. In the hybrid combustor16 (top of Figure 1), there are two points for the introduction of fuel: one before the catalyst (which is supported on a monolith and fed with a fuel/air ratio low enough to obtain a temperature that the catalyst can withstand) and the other between the catalytic segment and the homogeneous combustion segment, which increases the energy intensity of the feed to the turbine. Some thermal NOx is formed, however, in the homogeneous phase, and the two points of fuel injection involve dynamics that make them difficult to control. In the XONON configuration6,16,17,19,23 (bottom of Figure 1), all of the fuel is injected into the catalytic segment but the contact time is sufficiently short to obtain only partial combustion (∼50% fuel conversion) and, therefore, to avoid excessive thermal stress on the catalytic material; also, in this
(8) Peltier, R. Three-ppm NOx now possible. Power 2005, No. March, 54–60. (9) Etemad, S.; Ul Karim, Pfefferle, W. C. Dry, low NOx pilot. U.S. Patent 6,270,337, B1, Aug 7, 2001. (10) Gulati, A.; Warren, R. E.; Voorhees, E. B. Screech suppressor for advanced low emissions gas turbine combustor. U.S. Patent 5,487,274, Jan 30, 1996. (11) De Biase, V. Surface-stabilized burner limits NOx to 3 ppm and CO to 10 ppm. J. Eng. Gas Turbines Power 2004, No. June-July, 14–16. (12) Evulet, A. T.; ELKady, A. M.; Brand, A. R.; Chinn, D. On the performance and operability of GE’s dry low NOx combustors utilizing exhaust gas recirculation for post-combustion carbon capture, GHGT9. Phys. Procedia 2008, 1–8. (13) Abdul-Wahab, S. A.; Zurigat, Y. H.; Bortmany, J. N. Gas turbine emissions and environmental impact of efficiency boosting techniques. Int. J. Environ. Pollut. 2005, 23 (3), 273–288. (14) Fant, D. B.; Jackson, G. S.; Karim, H; Newburry, D. M.; Dutta, P.; Smith, K. O.; Dibble, R. W. Status of catalytic combustion R&D for the Department of Energy Advanced Turbine Systems Program. J. Eng. Gas Turbines Power 2000, 122 (April), 293–300. (15) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Catalysts for combustion of methane and lower alkanes. Appl. Catal., A 2002, 234, 1–23. (16) Forzatti, P. Status and perspectives of catalytic combustion for gas turbines. Catal. Today 2003, 83, 3–18. (17) Vatcha, S. R. Low-emission gas turbines using catalytic combustion. Energy Convers. Manage. 1997, 38 (10-13), 1327–1334. (18) McCarty, J. G.; Gusman, M.; Lowe, D. M.; Hildenbrand, D. L.; Lau, K. N. Stability of supported metal and supported metal oxide combustion catalysts. Catal. Today 1999, 47, 5–17.
(19) Dalla Betta, R. A. Catalytic combustion gas turbine systems: The preferred technology for low emissions electric power production and co-generation. Catal. Today 1997, 35, 129–135. (20) Su, S. C.; Carstens, J. N.; Bell, A. T. A study of the dynamics of Pd oxidation and PdO reduction by H2 and CH4. J. Catal. 1998, 176, 125–135. (21) Carstens, J. N.; Su, S. C.; Bell, A. T. Factors affecting the catalytic activity of Pd/ZrO2 for the combustion of methane. J. Catal. 1998, 176, 136–142. (22) Datye, A. K.; Bravo, J.; Nelson, T. R.; Atanasova, P.; Lyubosky, M.; Pfefferle, L. Catalyst microstructure and methane oxidation reactivity during the Pd-PdO transformation on alumina supports. Appl. Catal., A 2000, 198, 179–196. (23) Kajita, S.; Dalla Betta, R. Achieving ultra low emissions in a commercial 1.4 MW gas turbine utilizing catalytic combustion. Catal. Today 2003, 83, 279–288.
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case, a significant quantity of NOx is formed in the subsequent homogeneous combustion segment. In May 2003, a test campaign was undertaken to evaluate the XONON catalytic combustion technology on a General Electric Nuovo Pignone engine of 10 MWe, GE PGT 10B. This showed that, with small modifications, it is possible to obtain a NOx emission lower than 5 ppm and CO and unburnt hydrocarbon emissions lower than 10 ppm at 90% of the maximum power. Tests on a gas turbine of greater size, MS9001E of GE (100 MWe), gave less satisfactory results; in particular, to maintain CO values below 10 ppm, it was necessary to introduce a postcombustion chamber significantly larger than that expected for the compact geometry of the turbine. The XONON technology used involves starting the combustion reactions on catalytic sections at temperatures compatible with such materials and then completing the combustion in a large homogeneous combustion chamber at a much higher temperature. This illustrates the weak point of current catalytic combustion technology: the unavailability of catalytic systems stable at the temperature of the gas turbine inlet temperature. No further improvements or long-term durability tests and demonstrations have been performed in recent years on a fullscale plant. In this paper, we report the results of tests of catalytic combustion methane, using new high-temperature refractory ceramics containing noble metals in a high oxidation state in the active phase. The catalysts, supported on zirconia monoliths, were tested at near industrial conditions.
Figure 2. Granulometric distribution of a wet ground sample.
in 1050 mL of water was prepared, and BaO2 was added to it under stirring. The formation of small bubbles of oxygen was observed. After about 0.5 h, the platinum solution and zirconium solution described above were added. Finally, the ammonium hydroxide solution, cooled in an ice bath to limit the loss of ammonia during the mixing stage, was added. For the synthesis of 200 g of BaZrO3 perovskite containing 5% Rh b.w., the reagents were 121.19 g of BaO2 (Acros, 80.36% Ba b.w.) 271.44 g of zirconium propoxide solution (Aldrich, 20.9% Zr), 25.47 g of rhodium acetate (Chempur, 39.25% Rh b. w.), 627 g of citric acid monohydrate (Carlo Erba), 2500 mL of ammonium hydroxide (Sigma Aldrich, 25% NH3), and 2500 mL of water. A solution of 349 g of citric acid was dissolved in 1400 mL of water and poured in a beaker containing the zirconium propoxide, which was dissolved under heating and stirring in 7 h. In another beaker, a solution of 278 g of citric acid in 110 mL of water was prepared, and BaO2 was added to it under stirring. After about 0.5 h following the BaO2 dissolution, rhodium acetate and the zirconium propoxide solution described above were added. Finally, the ammonium hydroxide solution, cooled in an ice bath to limit the loss of ammonia during the mixing stage, was added. Both solutions precursors of the Pt- and Rh-containing perovskite were concentrated in a 20 L volume Rotavapor (Laborota 20 Control of Heidolph). To avoid sudden boiling with the possible loss of solution, a gradual increase of the temperature of the heating bath together with gradual decreases of pressure were imposed. A first value of 50 °C and 350 mbar of pressure was rapidly reached, and then increases of 5 °C and 50 mbar pressure decreases were alternatively imposed, following a 10 min pause after reaching the set value. Final values of 85 °C and 100 mbar were obtained. An expanded meringue-like solid was obtained, which was further dried under vacuum at a heating rate from 90 to 230 °C in 50 h and then held at this temperature for a further 10 h. The meringue thus obtained was ground to 70 mesh (0.25 mm) and placed in a quartz reactor. It contained at this stage a relevant amount of organic substance, which is decomposed under controlled conditions in a flow of a N2-O2 mixture (5.5 L/min, 1.5% O2). The decomposition was started at 330 °C and completed in about 60 h. The temperature was then increased to 600 °C, and the oxygen percentage in the gas flow was gradually increased to 20%. Finally, the powder was brought to 800 °C in a 2 L/min air flow with a heating rate of 1 °C/min, and the final value of the temperature was held for 10 h. During the calcination step, a full crystallization of the powder occurs, in which the ions are finely interspersed in a monophasic system, perovskite in our case. The solid thus synthesized has to be attached to a structured support designed to ensure low pressure drops coupled with high mass-transfer rates between the solid and the gas. This requires a wet grinding of the powder
Experimental Section Manufacture of Catalysts Active and Stable at High Temperature. The synthesis of these catalysts has involved a modification of the citrate route.24 This is a wet method for synthesizing mixed oxide catalysts with a wide range of composition and with excellent interspersion of the elements in the final product. The method is characterized by a decomposition of the organic part under mild conditions, avoiding the use of nitrates as starting salts. The process involves five separate steps: the first step consists of the preparation of a solution containing all of the required elements; the solution is then concentrated in a rotavapor to produce a very viscous product, which is dried in the second step in a vacuum oven. The solid thus obtained is ground and sieved in the third step to obtain a powder. The organic part of the ground solid is decomposed by oxidation in a fluidized bed, operating with a nitrogen/air flow. The final step calcines the decomposed powder in a fluidized bed with an air flow. The new synthesized catalysts have a general formula of BaZr(1-x)MexO3 . For the work reported here, rhodium and platinum were introduced within the crystal structure to obtain BaZr(1-x)PtxO3 (2% platinum b.w.) and BaZr(1-x)RhxO3 (5% rhodium b.w.). For the synthesis of 200 g of BaZrO3 perovskite containing 2% Pt, the reagents were 120.59 g of BaO2 (Acros, 80.36% Ba b. w.), 310.8 g of zirconium propoxide solution (Aldrich, 20.4% Zr), 37.66 mL of tetrammine platinum hydroxide solution (Heraeus, 9.61% Pt b.w.), 646 g of citric acid monohydrate (Carlo Erba), 2700 mL of ammonium hydroxide (Sigma Aldrich, 25% NH3), and 2600 mL of water. A solution of 390 g of citric acid in 1550 mL of water was prepared and poured in a beaker containing the zirconium propoxide. A white precipitate of hydrolysis is immediately formed, which is dissolved by heating and stirring under reflux for about 7 h to obtain a full dissolution. In another beaker, a solution of 256 g of citric acid (24) Villa, P. L. Solid solutions, applicable as catalysts, with a perovskite structure comprising noble metals. U.S. Patent 7,166,267, B2 Jan 23, 2007; European Patent Application 02764701.5, July 16, 2004.
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calcined at 800 °C, to obtain a slurry of fine particles. A zirconia bowl was used, which was equipped with zirconia spheres and 2-methyl-1-butanol as liquid with a ratio of 1 g of powder/10 mL of liquid. The planetary Pulverisette was used at 600 rpm speed until the desired grain distribution shown in Figure 2 was obtained. The monoliths were dipped in the slurry for 5 min, then slowly pulled out over another 5 min, and dried in a well-ventilated oven by increasing the temperature from ambient to 80 °C over 2 h, and this final value was held until a constant weight was obtained. The monoliths were then cooled to room temperature over 2 h, and the dip-coating and drying procedures were repeated from 5 to 10 times until the desired amount of coating had been achieved. The Rh and Pt perovskite had a final coating of rhodium of 0.286% b.w. and platinum of 0.179% b.w. measured with respect to the total weight of the monolith. Calcination was then performed, increasing the temperature from ambient to 950 °C with a heating rate of 120 °C/h and keeping this final value constant for 10 h, followed by a cooling back to ambient temperature at a rate of 120 °C/h. The used monoliths are of zirconia-mullite and can withstand temperatures of up to about 2000 °C. Physicochemical Characterization. Samples were characterized by X-ray diffraction (XRD) using a PANalytical X’Pert Pro powder diffractometer in Bragg-Brentano geometry using Cu KR radiation (λ = 1.5416 A˚), with the X-ray tube set to 40 V and 40 mA. The spectra were collected in the range of 5-90° (2θ) with a step size of 0.02° and time acquisition set to 25 s/step. Qualitative phase analysis of powder samples was carried out by means of the Hanawalt search method using PDF-2 [Powder Diffraction File, International Centre for Diffraction Data (ICDD)] data set. Structure refinement and quantitative phase analysis were performed by application of the full profile fitting the Rietveld method included in the GSAS software package. The XRD goal is to verify the amount of precious metal (platinum or rhodium) in the crystalline structure. The X-ray fluorescence (XRF) measurements were performed by means of a PANalytical Axios Advanced WD-XRF spectrometer, equipped with a 4 kW Rh X-ray tube. The standardless quantitative analysis IQþ was performed measuring samples as received. IQþ makes it possible to derive concentrations in unknown samples, using the fundamental parameter calculation, based on a single-point calibration line for each element in F and U. Surface area measurements were performed using the Micromeritics ASAP 2000 V2 instrument. The scanning electron microscopy (SEM) images were obtained with a Micromeritics ASAP 2000 V2.05 instrument. Particle size distribution was measured with a Cilas Instrument model 1180 equipped with three laser beams. Catalytic Combustion Tests. The experimental tests were performed on the high-pressure plant of the KTH Energy Center within the SUSPOWER project. The combustion reactor is shown in Figure 3. The purpose of the high-pressure catalytic plant is to study innovative catalytic systems under industrial conditions for low NOx emission processes. The possible feeds are methane, gaseous fuels, and gasified biomasses. The plant consists of three sections: air compression, humidificaton, and combustion; every section contains substructures, such as preheaters and measurement systems, for the process parameters through computerized data acquisition. A two-stage compressor supplies the plant with the necessary air feed (up to 100 g/s) at the desired pressure (up to 40 bar). The combustion unit is represented by a 3 m long pressurized chamber of 300 mm in diameter. The preheaters, consisting of controlled electrical resistances, are positioned in the upper part of the unit over a length of 2 m to create a uniform temperature profile for the air feed. After the preheaters is a “can type” combustor, 500 mm long and 35 mm in diameter, insulated to maintain the system close to adiabatic conditions. Indeed, the adiabatic flame temperature was calculated using a program provided by
Figure 3. Combustion reactor.
Norwegian Technical University (NTNU) called FUELSIM Average_v2.1. It was found that the heat losses under the most severe conditions amounted to about 0.3%. Therefore, the heat losses may be reasonably neglected for the data collected in this experimental rig. In the combustion section, 200 mm of the length is dedicated to the feed mixing and the remaining 350 mm is dedicated to the combustion reactions. The fuel injection is positioned at the beginning of the combustor unit; the injector has a cylindrical geometry with eight holes and is fixed on the central axis of the inlet pipe. The mixing pipe has a “Venturi” shape with injection holes positioned at the minimum section; this geometry, with its high turbulence, guarantees good mixing of the feed. The catalytic combustor is a simple pipe consisting of three insulated ceramic segments inside, within which up to three catalytic monoliths can be fitted. This solution allows for a rapid and easy access to make possible adjustments or changes of configuration. The ceramic material is such to avoid thermal shocks and provides for efficient insulation of the system. The exhausted gases are discharged after pressure reduction to minimize the noise. The combustion capacity of the system depends upon the operative pressure; in fact, to maintain the residence time at a constant value for each experiment, it is necessary to increase the mass of the air feed and, consequently, the fuel feed, to keep the λ ratio constant. The measurement system supplies values of pressure, flow rate, temperature, and emission concentrations. The data are acquired through software at the frequency of 1 Hz. Perovskite catalysts, containing Rh and Pt, were designed to operate at high temperatures. Indeed, preliminary combustion tests at atmospheric pressure showed that the light-off temperature of these catalysts is rather high, above about 550 °C.25 For this reason, it was important to develop methods that allow for the charge to reach the operating temperatures of the catalysts to be tested. After some preliminary tests, experiments were planned for two different configurations of combustion (Figure 4). The first involved the insertion of two ignition catalytic segments (consisting of a catalyst based on Pd and Pt supported on alumina), followed by two monoliths with perovskite containing Pt. Upon opening the reactor after the tests with the Pt perovskite, the ignition catalytic segments were found to be damaged, and therefore, a different configuration was used for the following tests performed on Rh perovskite. This second configuration incorporated an auxiliary hydrogen feed, which (25) Villa, P. L.; et al. Manuscript to be published.
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Figure 6. SEM pictures of the Rh- and Pt-based perovskite catalysts.
conversion data being used as a control of the results obtained from the thermal data.
Results The top and bottom of Figure 5 show the XRD of BaZrand BaZr(1-x)PtxO3 calcined at 800 °C, respectively. Peaks as a result of perovskite and BaCO3 are marked differently. The Rietveld analysis showed that the catalyst with a Rh nominal content of 5% b.w. contains a perovskite phase with a cell parameter a = 4.1898(1) A˚ with an impurity of BaCO3, which resulted in 7.7% b.w. After calcination at 900 °C, a decreased to 4.1736(1) A˚ and no barium carbonate was detected. The catalyst with a Pt nominal content of 2% b.w. is formed mainly by a perovskite phase with small amounts of barium carbonate (7.7% after calcination at 800 °C for 10 h in a fluidized bed and 3.6% at 900 °C). The value of crystal lattice parameter a was found to be 4.1802(1) at 800 °C and 4.1831(1) at 900 °C. These values are very close to those of pure barium zirconate and suggest that, during calcination, a relevant amount of platinum is lost in the gas phase or that Pt is segregated at the grain border of perovskite. No metallic platinum was found by Rietveld analysis. Indeed, the XRF analysis showed that the sample contained 2.17% b.w. Pt (this feature was also confirmed for samples with different Pt values; nominal Pt contents of 1 and 4% were found by XRF to contain 1.04 and 4.3%, respectively). The crystal size (Scherrer) was 14.6 and 35 nm for the Rh and Pt catalysts calcined at 800 °C, respectively. The surface area values found were 19.3 and 8.7 m2/g for Rh- and Pt-containing perovskite calcined at 800 °C, respectively. The pore mean radii were 12.9 and 14.6 nm, respectively, thus close to crystal size values. The SEM of the catalysts calcined at 800 °C (before the wet grinding) with a resolution at 100 provides an overview of the particles size; it shows smaller particles embedded in a homogeneous context of larger particles. The black background is the biadesivi coal that has been placed under the sample (Figure 6). The resolution at 500 highlights the individual particle size of about 50 μm; the porosity of the surface of the particle of 50 μm and smaller particles of about 5 μm can be seen, stuck on the particle size. Therefore, it can be inferred that the size distribution of particles is homogeneous with an average size of 50 μm, with the presence of dust and smaller particles of about 1-5 μm. The surface appears porous and uniform among the particles. The dimensions of these particles is 3 orders of magnitude greater than the crystals and the pore dimensions; this is not unexpected because the particles are formed by several domains of crystals agglomerated together. Furthermore, these dimensions are smaller after the wet grinding step.
Figure 4. Used configurations.
(1-x)RhxO3
Figure 5. Diffractograms of (top) BaZr(1-x)RhxO3 and (bottom) BaZr(1-x)PtxO3 calcined at 800 °C. (Black line) 01-074-1299 (A), barium zirconium oxide, BaZrO3. (Blue line) 01-071-2394 (/), witherite, BaCO3.
can give, by burning, the necessary heat to bring the temperature of the gas mixture up to the desired values. Thermocouples inserted between the monoliths enabled the various temperatures and, hence, the temperature rise, ΔT, because of the combustion reaction to be measured. In the second configuration, only two Rh-based monoliths were inserted, with light-off being assured by the additional feed of hydrogen. The tests were carried out in cycles operating at constant pressure, where the flow rate gradually increases and then decreases to return to the initial value. The measured values were the methane conversion and the ΔT generated on the various segments; this value, as compared to the adiabatic theoretical value of ΔT, provides an estimate of the performance of the various segments, with the 140
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Table 1. λ Is the Equivalence Ratio, i.e., the Ratio of the Oxidizer/ Fuel Ratio to the Stoichiometric Oxidizer/Fuel Ratio λ
CH4 (kW)
air flow (g/s)
GHSV (h-1)
velocity (m/s)
τ (s)
5 4 3 2
5.65 7.06 9.41 14.12
10 10 10 10
137719 138399 139531 141744
3.137 3.152 3.178 3.229
0.026 0.026 0.026 0.025
Figure 8. Evolution of measured ΔT.
Figure 7. Temperature profile along the combustor.
Pt Catalyst. For the Pt perovskite catalyst, four tests were performed at four different pressures, in which the methane flow was gradually increased and then lowered to the initial value. As already mentioned, these catalysts are designed for high-temperature operation and need to be brought to a high-temperature condition before working efficiently. The experimental procedure was as follows: (a) We explored in which conditions the catalyst had a good response at 5 bar, with a 3 kW methane feed. This condition was taken as the initial condition. (b) We increased the pressure to 10 bar, and to keep the same residence time, the fuel flow was doubled (i.e., 6 kW). (c) When the pressure was scaled to 15 bar, the fuel flow was tripled (i.e., 9 kW). The fuel limitation was determined only by the operation on the catalyst at the lower pressure tested and not by a limit of temperature resistance of the catalyst. This was decided for safety reasons because it is standard to check the rig status before increasing the pressure. This procedure allowed us to save experimental time as well as operate under safe conditions. Having reached the maximum pressure and fuel flow, we also tried to obtain data at the same gas hourly space velocity (GHSV) but with lower methane values in the feed. Table 1 gives the results at a pressure of about 13 bar, with maximum methane flow equal to 14 kW. Increasing the methane and keeping the air flow constant, the λ values change. The operating conditions of the test are reported. The gas velocity inside the channels and the residence times are comparable to the conditions of an industrial combustor of small to medium power. The temperature profile (Figure 7) along the combustor shows that, increasing the methane flow and, therefore, decreasing the λ values and increasing the pressure, the temperature increases, mainly after the second and third thermocouples; there is a substantial contribution of Pd/Pt on Al2O3 segments. The final temperature is around 1000 °C. Figure 8 shows the ΔT measured between the outlet and inlet of the first and second Pt-perovskite catalytic segments. The observed ΔT is an estimate of the progress of the combustion reaction of methane; the first catalytic section is responsible for the largest percentage of methane conversion. The lower ΔT of the last segment is due to the absence of methane, which is totally converted in the previous segments. It is
Figure 9. Conversion trend of the perovskite sections.
noteworthy that, decreasing the methane flow, the ΔT observed on the first Pt-perovskite segment is higher. Therefore, it appears that, after having been ignited, the segment is performing even better, also compensating for a possible deactivation of the first two Pd/Pt on Al2O3 segments. This trend is confirmed by the conversion dependence upon the amount of methane feed shown in Figure 9. It appears that the platinum monoliths are able to maintain full conversion once they have been brought to a high-temperature regime and also slightly decreasing the methane feed. It is noteworthy that, in this respect, they do not behave like the PdO catalyst, which deactivates at temperatures higher than 900 °C. Moreover, the hysteresis found in this Pt catalyst is favorable, because the catalyst keeps working also for sudden and possibly unwanted decreases of methane in the feed. On the contrary, the hysteresis found in the PdO catalyst may lead to oscillations in working conditions. At partial load conditions, however, the performance of the first-stage monolith depends upon the hysteresis cycle. If high-temperature values have already been reached, the monolith performs well; on the contrary, Figure 9 shows that conversion does not reach the 100% value if the monolith has not reached this high-temperature range. Figure 10 gives the conversion measured for different values of pressure and methane in the feed, keeping the GHSV constant. It is noteworthy that conversion increases with methane; this is an interesting result, because increasing the pressure and the methane flow at the same time, the increased temperature gives rise to higher conversions. We have no explanation for this behavior at present. 141
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: DOI:10.1021/ef100656y
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Figure 10. Conversion trend with pressure.
Figure 12. ΔT measured for the two Rh-perovskite monoliths.
Discussion
will be a tetravalent cation: Zr or, in our case, also a noble metal cation, to balance oxygen charges. This does not occur for La, which is trivalent. The important and practical effect of this is that the new perovskite systems are unusually stable at temperatures at least up to 1450 °C, with the noble metal present in the oxidation state that is more active, also under the severe conditions of industrial relevance. This may have an important impact not only in normal time-on-stream but also during the regeneration of the deactivated and fouled catalysts by controlled combustion of the carbon deposits; this is permitted by the stability of these systems at temperatures at around 1450 °C.26,27 The method of synthesis developed involves several steps and is therefore time-consuming. However, it allows for the synthesis of a reasonable amount of catalysts, of the order of 200-300 g, and it could be further scaled-up. After calcination at high temperature, the powders are well-characterized by XRD; at 800 °C in a fluidized bed, the perovskite phase is dominant, with smaller amounts of barium carbonate, which is substantially decreased by calcining at higher temperatures. No free noble metal (Rh or Pt) was found. The values of parameter a found for the powder containing Pt are very near those of pure barium zirconate, suggesting that some Pt could be lost in the gas phase during calcination. However, the XRF found a Pt content corresponding to the nominal composition. Therefore, it is possible that Pt is rather segregated at the grain border of perovskite but strongly bound to the surface to not be significantly removed after 10 h of calcination at 900 °C. The value of the surface area on the order of 10 m2/g is typical for perovskite catalysts. The powders have to be fitted on a structured support, such as a zirconia monolith, to match the pressure drop required in gas turbines. This is not a simple task because the adhesion of the powder to the support must be very strong to withstand the strong thermal and mechanical stresses, which are present in practical use. We developed a technology to obtain this result. A wet grinding of the powder calcined at 800 °C in 2-methyl-1-butanol enabled a slurry to be obtained containing a powder with dimensions less than 1 μm
The new method of the synthesis of oxides developed at L’Aquila University has resulted in catalysts with a perovskite structure (formula ABO3) containing Ba, which proved to be more refractory than the common perovskite containing La, and with noble metals incorporated within the structure in a high oxidation state. Indeed, A is a cation with a great ionic radius. Therefore, if A is a bivalent cation, such as Ba, cation B
(26) Cifa, F.; Lancione, S.; Dinka, P.; Viparelli, P.; Villa, P. L.; Benedetti, G.; Viviani, M.; Nanni, P. Catalysts based on BaZrO3 with different elements incorporated in the structure I: BaZr(1-x)PdxO3 systems for total oxidation. Appl. Catal., B 2003, 46, 463–471. (27) Viparelli, P.; Villa, P.; Basile, F.; Trifir o, F.; Vaccari, A.; Nanni, P.; Viviani, M. Catalyst based on BaZrO3 with different elements incorporated in the structure II. BaZr(1-x)RhxO3 systems for the production of syngas by partial oxidation of methane. Appl. Catal., A 2005, 280, 225–232.
Figure 11. Temperature profile along the combustor.
Rh Catalyst in the Perovskite Structure. The ignition of the combustion reaction for the configuration set for the Rh catalyst in a perovskite structure was affected by including a hydrogen flow to the methane and air mixture. Hydrogen, by burning, can bring the feed to the desired temperature of the test. Figure 11 shows the ΔT of two monolith segments on the test at a pressure of about 5 bar, with a maximum flow of gas equal to 7.25 kW. Similar to the platinum test, the temperature increases near 1000 °C upon increasing the methane flow and, therefore, increasing the equivalence ratio λ. Part of the temperature increase is due to the heat of combustion of the hydrogen in the feed. Even in this situation, the Rh catalyst in a perovskite structure can operate at virtually full conversion of methane at high temperatures. Indeed, the ΔT measured corresponds to full conversion. Despite the absence of catalytic segments of ignition, also this perovskite system has operational stability under severe pressure and temperature conditions. Rh catalysts give high conversions for high values of methane feed; the first monolith (Fig. 12) works like an ignition catalyst. Only for 7.25 kW of methane fed, the first monolith seems to convert most of the fuel and, therefore, the ΔT on the second monolith is lower.
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(top of Figure 4). A dip coating of the zirconia monoliths in the slurry containing the ground catalyst, followed by drying a few times and then a calcination step at 950 °C, allowed us to obtain the monoliths with the desired mechanical properties. It is noteworthy that the temperature of the final calcination is higher than the temperature of the first calcination of the powder. We believe that the strong adhesion to the support is also obtained through the decomposition of the residual barium carbonate and the increase of the crystal size of the perovskite phase. The aim of this work is the development of catalysts stable in the high-temperature range, although not particularly active at low temperature. Therefore, a preliminary light-off of the monolith was necessary. After some preliminary tests, two different configurations of combustion were considered as possible solutions to the light-off of perovskite monoliths. The first involved the insertion of two ignition catalytic segments (consisting of a catalyst based on Pd and Pt supported on alumina), followed by two monoliths with perovskite containing Pt. When the combustor was opened after the first test,
the ignition monoliths appeared seriously damaged because of partial melting; a different configuration for the following test was therefore adopted. The second configuration was adopted for perovskite containing rhodium and incorporated an auxiliary hydrogen feed, which can give, by burning, the necessary heat to bring the temperature of the gas mixture up to the desired value. It was not possible to further increase the temperature because of the thermal resistance limit of the plant materials. The tests indicate that these catalysts are a new class of systems that might solve the problems related to overheating of the catalyst at high temperatures. However, a test run performed in more severe conditions and durability tests including mechanical cycles should be performed to assess a real life expectancy of these catalysts. Acknowledgment. We thank Prof. Sven J€ aras of the Department of Chemical Technology (KTH) for supplying the ignition catalytic monoliths (consisting of a catalyst based on Pd and Pt supported on alumina).
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