Catalytic combustion applications

Most of the work on catalytic combustion applications has dealt with the use of natural gas, since this is a clean gaseous fuel with low nitrogen content. However, a number of other hydrocarbon fuels have been investigated as well [99-101], such as propane, gasoline, diesel fuel, and kerosenes. 

Catalytic combustion applications can be classified as either primary or secondary pollution control, that is, emissions prevention or emissions clean-up. The most common example of catalytic combustion for emissions clean-up is the catalytic converter in the exhaust system of automobiles. Catalytic combustion is also increasingly used for the removal of volatile organic compounds (VOCs) from industrial exhaust streams. The use of catalytic combustion in exhaust gas clean-up is discussed in other sections of this Handbook this section deals only with primary control applications. 

The use of noble metals in catalytic combustion applications allows for fuel lean feeds and lower gas temperatures, thus avoiding the formation of thermal NOx... 

Industrial catalysts require supports with desirable properties such as resistance to thermal shocks, mechanical strength and chemical stability. For catalytic combustion applications they must be adequately shaped to achieve low pressure drop. The monolithic honeycomb type is the most technologically advanced substrate and successfully satisfies these criteria. 

Pyrochlores - The pyrochlores are a group of materials with the general formula A2B2O7. They have been mentioned as a material for catalytic combustion. The structure allows vacancy at the A site and the O sites to some extend. The A position can be a rare earth metal or an element with lone pair of electrons and the B position can be a transition metal or a post-transition metal. This make the structure rather flexible as the oxidation state of the transition metal B can be varied as well as the nature of the A and B metal ions. Subramanian and Castro et al. have prepared several pyrochlores. When studying the thermal stability of different complex oxides, Zwinkels et al. have shown that La2Zr207 pyrochlores have a surface area lower than 5 m g , already after calcination at 1000 °C. Hence, such materials are probably not suitable for high temperature applications unless the preparation method is improved. However, pyrochlore compounds have been patented for catalytic combustion applications, see Section 5.5. 

Palladium is the most often used PGM in catalytic combustion applications for natural gas. Palladium exists in the form PdO at low temperatures, probably at least on the surface of the particle. As the temperature increases there is a reduction of PdOx to metallic Pd, around ca. 800 The reaction is reversible... 

Heinzel et al. [77] compared the performance of a natural gas autothermal reformer with that of a steam reformer. The ATR reactor was loaded with a Pt catalyst on a metallic substrate followed by a fixed bed of Pt catalyst. In the start-up phase, the metallic substrate was electrically heated until the catalytic combustion of a stoichiometric methane-air mixture occurred. The reactor temperature was increased by the heat of the combustion reaction and later water was added to limit the temperature rise in the catalyst, while the air flow was reduced to sub-stoichiometric settings. With respect to the steam reformer, the behavior of the ATR reactor was more flexible regarding the start-up time and the load change, thus being more suitable for small-scale stationary applications. 

In the following, a review of the traditional and novel concepts of catalytic combustion for GTs is addressed, with emphasis on the requirements and challenges that the different applications open to catalysis. The most relevant characteristics of PdO-supported catalysts and of transition metal-substituted hexaaluminates (which have been most extensively considered for lean combustion applications) are described, along with those of noble metal catalysts adopted in rich combustion systems. 

Further, the development of miniaturized devices for the generation of power and/ or heat is discussed here as it represents an emerging field of application of catalytic combustion. Due to the presence of the catalytic phase, the microcombustors have the potential to operate at significantly lower temperatures and higher surface-to-volume ratios than non-catalytic microcombustors. This makes them a viable solution for the development of miniaturized power devices as an alternative to batteries. 

Numerous studies have been published on catalyst material directly related to rich catalytic combustion for GTapplications [73]. However, most data are available on the catalytic partial oxidation of methane and light paraffins, which has been widely investigated as a novel route to H2 production for chemical and, mainly, energy-related applications (e.g. fuel cells). Two main types of catalysts have been studied and are reviewed below supported nickel, cobalt and iron catalysts and supported noble metal catalysts. 

Rich catalytic combustion will offer wide opportunities with respect to most of the above issues, including flexible integration in different machines, low-temperature ignition ability, tolerance to fuel concentration and temperature non-uniformities and fuel flexibility. Further, the production of syngas in short contact time catalytic reactors could be exploited in several energy-related applications such as fuel cell and oxy-fuel combustion. 

PGM catalyst technology can also be applied to the control of emissions from stationary internal combustion engines and gas turbines. Catalysts have been designed to treat carbon monoxide, unbumed hydrocarbons, and nitrogen oxides in the exhaust, which arise as a result of incomplete combustion. To reduce or prevent the formation of NO in the first place, catalytic combustion technology based on platinum or palladium has been developed, which is particularly suitable for application in gas turbines. Environmental legislation enacted in many parts of the wodd has promoted, and is expected to continue to promote, the use of PGMs in these applications. 

In the following sections, the different types of reactive filters, together with their typical applications, will be presented. The separation and catalytic combustion of diesel soot is the most important and furthest investigated application. Consequently, this type of reactive filtration, with its practical and theoretical aspects, will form the main focus of this chapter. 

Considering the case of crystalline complex salts and the amorphous precursor method, both chemical com-plexation methods have found important and innovative applications in recent years for the preparation of a wide variety of catalysts and of various perovskite-type catalysts and barium hexaaluminates, as required for high-temperature (>1500K) applications such as catalytic combustion. It is therefore worthwhile describing some applications, in the next two subsections. 

The potential of rare earth compounds as catalytically active phases and promoters in pollution control, catalytic combustion, polymer production and in the fuel and chemical manufacture and thermal stabilizers for catalyst supports (alumina, silica-alumina, titania) need to be mentioned. Application of rare earths in alternate fuels technology (Fisher-Tropsch Processes, natural gas to transport fuel pathways) is also promising. 

Applications of cation-exchanged ZSM-5 include ammoximation of alkanes with Co ZSM-5, water-tolerant oxidation of NH3 to N2 with Pd-ZSM-5, low-temperature catalytic combustion of CH4 with Pd-ZSM-5, wet N2O decomposition with Co-ZSM-5, NOv reduction with CH4 even in the presence of excess O2 with Co ZSM-5 and catalytic decomposition of NO in the absence of O2 with Cu ZSM-5. In most of these cases, the activity (rate, conversion and/or selectivity) of the redox ZSM catalysts is much higher than when the corresponding metal is supported on traditional oxides [125]. 

However, most of the following discussion will deal with catalytic combustion for gas turbine applications. Figure 4 shows a schematic view of an open-cycle gas turbine unit with a catalytic combustor (cf. a conventional unit in Fig. 2.). 

Materials such as aluminum titanate and silicon carbide appear to be promising for high-temperature catalytic combustion. However, problems such as extrudability, the application of washcoats, and reaction with deposited washcoats are not solved yet. For instance, when hexa-aluminate, presented in the introduction to this section, was applied to silicon carbide monoliths, solid-state reactions occurred at 1200-1400 C [76], causing exfoliation of the coating and the formation of new phases. The application of an intermediate mullite layer was suggested as an approach to hinder these solid-state reactions. 

G.E. Voecks and D.J. Cerini, Application of rich catalytic combustion to aircraft engines. Proceedings of Third Workshop on Catalytic Combustion, Asheville, NC, pp. 477-490 (1979). 

Another potentially interesting zeolite characteristic is the nature of gas diffiision in the intracrystalline pores. It has been suggested in the literature that certain adsorbed gas molecules close in size to the zeolite pores float within non polar zeolite crystals, instead of the standard adsorption-desorption mechanism. This concept opens the possibility that under certain circumstances, the emission of desorbed gas molecules may be directionally coherent as it emerges fi om each zeolite crystal face. Such a coherent gas emission - "a molecular laser" - may find applications in catalytic combustion or in other applications benefitting from "non thermalized" gas emissions. 

Catalytic combustion is an environmentally-driven, materials-limited technology with the potential to lower nitrogen oxide emissions from natural gas fired turbines consistently to levels well below 10 ppm. Catalytic combustion also has the potential to lower flammability at the lean limit and achieve stable combustion under conditions where lean premixed homogeneous combustion is not possible. Materials limitations [1,2] have impeded the development of commercially successful combustion catalysts, because no catalytic materials can tolerate for long the nearly adiabatic temperatures needed for gas turbine engines and most industrial heating applications. 

An incremental improvement in the path to practical applications of catalytic combustion was disclosed by Yasuyoshi et al., who recognized the important applications of manipulating the heat balance in Eq. (5). These authors coated the walls of alternate channels to have only half of the reactant flow passing through catalytic channels. For similar heat transfer coefficients in coated and uncoated channels, and assuming that there is no temperature gradient in the channel wall, the heat removal term in the heat balance is given by Eq. (7). 

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