A comparison between hexaaluminates and perovskites for catalytic combustion applications

Hexaaluminates and perovskites are two promising candidates for use in catalytic combustion applications. In the present study two hexaaluminates, LaMnAl₁₁O₁₉ and LaCoAl₁₁O₁₉, were compared with two perovskites, LaMnO₃ and LaCoO₃, with respect to their thermal stability and catalytic activity for combustion of methane and gasified biomass. The results showed that the hexaaluminates retained a much higher surface area even after calcination at 1200 °C compared to the perovskites. LaMnAl₁₁O₁₉ showed the highest catalytic activity of all catalysts. LaCoAl₁₁O₁₉ generally showed low activity. Of the two perovskites, LaCoO₃ was the most active, and the initial test run the activity for biomass combustion were close to that one of LaMnAl₁₁O₁₉ even though its surface area was only one tenth of the hexaaluminate's. However, it was severely deactivated in the second test run. Similar deactivation but less severe was also found for the other catalyst.     

Catalytic combustion system for a 10 MW class power generation gas turbine

In early 2000, GE Energy launched a program to develop a catalytic combustion system for one of its small power generation gas turbines, the GE10-1 engine. The target was to release to the market a new combustor able to guarantee NO_x emissions lower than 2.5 ppmvd (referred to 15 vol.% O₂). Today, a full-scale engine test campaign has been completed, during which measured NO_x emissions were as low as 1 ppmvd in the 90–100% load range.

The article is aimed to illustrate the developed technology and the results obtained. The combustion system's configuration is briefly described, focusing on the XONON^® catalyst module installed. Reported data show combustion system's performances, mainly in terms of pollutant emissions and operability. Perspectives for future development of such combustion system are outlined.     

Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts

An aqueous (NH₄)₂CO₃ coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr_1−xLa_xMnAl₁₁O₁₉₋ hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH₄)₂CO₃ coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH₄ oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl₁₁O₁₉₋, shows high surface area with 19.3 m²/g after calcination at 1400°C for 2 h. It is active for CH₄ oxidation with T_10% at 450°C using 1% CH₄ in air and 70 000 cm³/h g space velocity. The stability and activity of LaMnAl₁₁O₁₉₋ prepared by (NH₄)₂CO₃ coprecipitation method is a simple and important step forward for the application of CH₄ catalytic combustion for gas turbines.     

Catalytic combustion concept for gas turbines

Catalytic combustion for gas turbines was investigated, based on a partial catalytic combustion section followed by a homogeneous combustion zone. A pressurized test rig (<25 bar) was built to test the influence of various parameters on this concept using Pd and Pt catalysts.

The pressure influence on the apparent catalytic reaction rate was of the order 0.4, assuming that the reaction kinetics could be described by a power rate function which was of first order with respect to methane. Pd catalysts showed a pressure-dependent temperature for the transition of the active PdO to the much less active Pd. Combining Pd and Pt within one catalyst resulted in a considerably lower transition temperature.

Homogeneous combustion reactions set on from 650°C, depending on the methane concentration, pressure and flow. With inlet temperatures above 800°C the homogeneous combustion always started. At outlet temperatures below 1050°C high CO concentrations could be measured. At higher temperatures the CO, CH₄ and NO_x concentrations were lower than 5 ppm. During several experiments total conversion of CH₄ and CO was observed.     

Circumventing fuel-NOx formation in catalytic combustion of gasified biomass

It has been shown that it is possible to decrease fuel-NO_x produced from NH₃ using catalytic combustion of a synthetic gasified biomass at fuel-lean conditions. In a certain temperature regime where the conversion of fuel components, such as CO, H₂ and CH₄, is low and conversion of NH₃ is high, it is suggested that the formed NO_x is reduced by the remaining fuel components, mainly hydrocarbons. With oxide catalysts only ca. 10% of the NH₃ was converted to NO_x, the rest to N₂. It has also been shown that the ignition sequence of CO, H₂ and CH₄ varied for different catalysts and different experimental conditions, and that methane coupling and methanation reactions occurred before ignition of CH₄.     

Advanced technology catalytic combustor for high temperature ground power gas turbine applications

We report results from a lean burn ultra-low emission catalytic combustor. In a sub-scale rig, atmospheric testing with methane demonstrated NO_x<3, CO<5, and UHC<1 ppm, with stable combustion at inlet temperatures of 400–500°C (750–1020°F) and combustor discharge temperatures of 1150–1540°C (2100–2800°F). Catalyst temperatures were held well below metal substrate material limits, while combustor discharge temperatures of up to 1540°C (2800°F) were achieved.     

Catalytic combustion of gasified biomass: poisoning by sulphur in the feed

The influence of sulphur on the catalytic combustion of gasified biomass for gas turbine applications has been studied over precious metal and metal oxide based catalysts, namely Pd/LaAl₁₁O₁₈, Pt/LaAl₁₁O₁₈, Pt/La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉, La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉ and LaMnAl₁₁O₁₉. The samples were washcoated on cordierite monoliths and tested in a bench-scale reactor with a synthetic low-heating value fuel mixed with air. The fuel gas, that resembles the gas from air-blown fluidised bed gasification of wood, was composed of hydrogen, carbon monoxide, methane, ethene, carbon dioxide, water and nitrogen. Different concentrations of hydrogen sulphide as well as sulphur dioxide were added to the fuel gas. The results show that all samples were deactivated to some extent by addition of sulphur, although poisoning of the catalytic combustion for each fuel component varied depending both on the active phase and on the support and generally was reversible. The palladium catalyst was severely deactivated for combustion of methane, although activity for carbon monoxide and hydrogen was almost maintained. Platinum catalysts were more severely poisoned for carbon monoxide and hydrogen, but not for methane. Metal oxide catalysts were severely deactivated for all fuel components, especially for carbon monoxide, and the La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉-sample was irreversibly poisoned. The samples were also characterised by BET, XRD, ICP, SEM–EDX, XPS and SIMS.     

Catalytic combustion over platinum group catalysts: fuel-lean versus fuel-rich operation

Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800 °C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9 MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications.

Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650 °C, with reactivation upon cooling. Rh catalyst also deactivated above 750 °C, but did not reactivate. Pt catalyst was active above 600 °C. Fuel-lean reaction products were CO₂ and H₂O for all three catalysts.

The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600 °C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H₂) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior.

High-pressure (0.9 MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion.     

Application of catalytic combustion to a 1.5 MW industrial gas turbine

A catalytic combustor is described for a 1.5 MW gas turbine engine. The catalyst temperature is limited and the high combustor outlet temperatures required by the turbine are generated downstream of the catalyst. The combustor design places a low NO_x preburner upstream of the catalyst and uses this preburner to achieve optimum catalyst operation by providing the desired catalyst inlet temperature. The combustor system employs the catalyst during engine acceleration and loading. The catalyst design has been tested on a sub-scale rig under full pressure and flow conditions simulating turbine operation over the entire operating range including acceleration and loading. The design should achieve emissions at full load operation of <3 ppm NO_x and <10 ppm CO and UHC. Low emissions operation is expected over the 75–100% load range. In addition, long-term sub-scale rig test results are reported at simulated full load operating conditions including cyclic operation and full load trips.     

Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO_x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation or coking.

Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h⁻¹ in lab test and 500 000 h⁻¹ in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS.

In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another.

Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium.     

Catalytic combustion of methane over palladium exchanged zeolites

Palladium cation exchanged zeolites (ZSM-5, mordenite and ferrierite) were studied as catalysts for methane combustion. Pd-zeolites showed much higher activities than PdO/Al₂O₃. For comparable palladium loadings, PdO/Al₂O₃ requires a reaction temperature of ca. 70–80°C higher than Pd-ZSM-5 for conversions between 50–100%. The catalytic activity of Pd-ZSM-5 seems to be related to its reducibility. Temperature-programmed reduction experiments with carbon monoxide showed a lower reduction temperature (ca. 157°C) for Pd-ZSM-5 than for PdO/Al₂O₃ (225°C). Further, the positioning of the palladium by ion exchange offers a highly dispersed form of Pd^II supported on the high surface area zeolite.     

Structured reactors for kinetic measurements under severe conditions in catalytic combustion over palladium supported systems

The collection of chemical kinetics data in catalytic combustion over very active palladium catalysts under conditions relevant to practical applications (e.g. gas turbine combustors) is extremely difficult, mainly due to strong exothermicity and very fast rate of combustion reactions. Within this purpose in this paper two types of laboratory structured reactors, which closely resemble industrial monolith catalysts, are investigated: (a) the annular reactor, consisting of a catalyst coated ceramic tube, co-axially placed in a quartz tube; (b) the metallic plate-type reactor, consisting of an assembled packet of metallic slabs coated with a ceramic catalytic layer.

The design of the annular reactor configurations for kinetic investigations is first addressed by mathematical modeling. The resulting advantages, including: (i) negligible pressure drops; (ii) minimal impact of diffusional limitations in high temperature–high GHSV experiments; (iii) effective dissipation of reaction heat are then experimentally demonstrated for the case of CH₄ combustion over a PdO/γ-Al₂O₃ catalyst with high noble metal loading (10% (w/w) of Pd).

The feasibility of a near-isothermal operation with the metallic plate-type reactor by an extremely effective dissipation of reaction heat through proper selection of highly conductive support material and of the geometry of the metallic slabs is finally discussed and experimentally demonstrated for the case of combustion of CO at high concentrations over a PdO/γ-Al₂O₃ (3% (w/w) of Pd) catalyst.     

Catalytic combustion of methane over supported bimetallic Pd catalysts: Effects of Ru or Rh addition

A series of Pd/γ-Al₂O₃ catalysts with various amounts of Ru or Rh with, and/or without, BaO were prepared by successive incipient wetness impregnation. The catalysts were investigated for the catalytic methane combustion before, and after, H₂S poisoning in an oxygen-rich atmosphere. The addition of ruthenium enhanced the catalytic activity for methane oxidation even after H₂S poisoning while maintaining the initial catalytic activity of the fresh catalyst. These results are explained in terms of dispersion of palladium by ruthenium and poisoning resistance of ruthenium. The addition of rhodium did not improve the overall activity in methane oxidation.     

Catalytic combustion of VOCs on non-noble metal catalysts

Volatile organic compounds (VOCs) are toxic and contribute significantly to the formation of the photochemical smog, which has remarkable impact to the air quality; therefore, the research on the removal of VOCs has attracted increasing interests during the last decade. This review covers the recent developments in catalytic combustion of VOCs over non-noble metal catalysts including mixed metal oxide catalysts, perovskite catalysts and Au-containing catalysts. The effect of water vapor, coke formation and the effect of supports on the catalytic combustion process will be discussed. The concept of an adsorption/catalytic combustion dual functional system is introduced and several examples of such systems are evaluated. To develop efficient and cost effective VOC removal technologies, further research in catalytic combustion needs to develop novel non-noble metal catalysts and adsorbents, and improve the understanding of catalytic mechanisms involved.     

Development of a novel catalytic burner for natural gas combustion for gas stove and cooking plate applications

On the request of Gaz de France Research Department, Catator AB has designed, constructed and evaluated a catalytic burner, based on Catator's patented wire mesh catalysts, for natural gas combustion in gas stoves or cooking plates. The results have shown that burner operation results in extremely low NO_x emissions (1–3 mg NO_x/kWh), acceptable CO-levels (0–15 mg CO/kWh), relatively high thermal efficiencies over a broad range of power inputs (40–50% for 1–4 kW) and a long catalyst life-time (>10 000 h). Other advantages of this burner design are its compactness and ease of cleaning. The critical concern is the high emissions of unburned hydrocarbons measured at slow cooking mode (<1 kW), which is believed to be overcome by developing and implementing an appropriate heat-exchanger with the burner.     

Partial oxidation of methane over rhodium catalysts for power generation applications

The partial oxidation of methane (POM) to syngas, i.e. H₂ and CO, over supported Rh catalysts was investigated at atmospheric pressure. The influence of support material, Rh loading and the presence of water vapor on the methane conversion efficiency and the product gas composition was studied. The catalysts containing ceria in the support material showed the highest activity and formation of H₂ and CO. By increasing the Rh loading, a decrease of the ignition temperature was obtained. The addition of water vapor to the reactant gas mixture was found to increase the ignition temperature and the formation of hydrogen, which is favorable for combustion applications where the catalytic POM stage is followed by H₂-stabilized homogeneous combustion.     

Catalytic combustion of toluene over Mn, Fe and Co-exchanged clinoptilolite support

Catalytic combustion of toluene over Fe, Co, and Mn transition metal oxides catalysts supported on clinoptilolite (CLT) has been investigated. The catalysts have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed reduction and oxidation (TPR/TPO) and thermogravimetric analysis (TGA). 9.5MnO₂/NaCLT catalyst exhibited the catalytic activity, over which the toluene conversion reached to 93% at a temperature of 350 °C. The increase in activity was followed by increasing metal oxide content and gave a optimum at 9.5% MnO₂ loading. Addition of metal oxide into clinoptilolite support led to a measurable decrease in the onset of combustion and an increase in toluene conversion.     

Catalytic combustion of benzene over metal oxides supported on SBA-15

The catalytic combustion of benzene over metal oxides supported on SBA-15 was investigated. The catalysts were prepared by the incipient wetness method and characterized by XRD, BET, TEM, ESR and TPR. The calcined siliceous SBA-15 and CuO/SBA-15 samples displayed well-resolved patterns with a sharp peak at about 1.0°. It is clear that the loading of CuO on the silica matrix drastically decreases the surface area and pore volume of the catalysts, as would be expected for the incorporation of CuO. Among the supported metal oxides, CuO supported on SBA-15 was found to have the highest activity for benzene oxidation. In addition, copper oxide supported on SBA-15 gives higher catalytic activity than copper oxide supported on MCM-41. From the ESR results, the CuO dispersed on the SBA-15 acts as the active site of the CuO/SBA-15 catalysts in the oxidative decomposition of benzene. The catalytic activity gradually increases with increasing CuO loading on SBA-15.     

Catalytic combustion of methane over mixed oxides derived from Co-Mg/Al ternary hydrotalcites

Co_xMg_3 − x /Al composite oxides (xCoMAO-800) were prepared by calcination of Co_xMg_3 − x/Al hydrotalcites (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively) at 800 °C. The materials were characterized using XRD, TG-DSC, N₂ adsorption-desorption and TPR. The methane catalytic combustion over the xCoMAO-800 was assessed in a fixed bed micro-reactor. The results revealed that cobalt can be homogenously dispersed into the matrices of the hydrotalcites and determines the structure, specific surface areas and porosity of the derived xCoMAO-800 oxide catalysts. The thermal stability and homogeneity of the hydrotalcites markedly depends on the cobalt concentration in the hydrotalcites. The Co-based hydrotalcite-derived oxides exhibit good activity in the catalytic combustion of methane. The catalytic activity over the xCoMAO-800 oxides enhances with increasing x up to 1.5, but subsequently decreases dramatically as cobalt loadings are further increased. The 1.5CoMAO-800 catalyst shows the best methane combustion activity, igniting methane at 450 °C and completing methane combustion around 600 °C. The catalytic combustion activity over the xCoMAO-800 oxides are closely related to the strong Co-Mg/Al interaction within the mixed oxides according to the TG-DSC, TPR and activity characteristics.     

Pd-Ba/Ce-Zr-La-Al2O3/堇青石催化剂对甲基丙烯酸尾气的催化燃烧

采用浸渍法制备Pd-Ba/Ce-Zr-La/堇青石催化剂,以甲基丙烯酸尾气为模型,考察Ba的引入对Pd催化剂催化燃烧活性的影响。并用XRD、BET、SEM和TPD等对催化剂进行了表征。研究表明,Ba的引入提高了催化剂的催化活性和热稳定性,经500 ℃焙烧的Pd-Ba/Ce-Zr-La/堇青石催化剂（Pd和Ba质量比为1∶7.5）具有很好的催化活性,起燃温度为130 ℃,完全燃烧温度仅为153 ℃,经1 000 ℃焙烧后,催化剂仍保持较高催化活性。