Catalytic combustion features

Materials for high-temperature catalytic combustion should possess both elevated activity and high thermal stability. Generally these are opposite features,32-33 so... 

Heterogeneous reaction mechanisms range from the very simple to the complex. Many features of the formalism presented in this chapter are illustrated by the catalytic combustion reaction mechanism given in Table 11.1. The surface-reaction mechanism is due to Sidwell et al. [361], which in turn had its origins with the work of Schmidt [173,174] and Deutschmann [96,97,101],... 

A catalytic combustor is basically a lean-prenux combustor, in which the combustion is stabilized by a catalytic surface Hence, the expression catalytically ignited thermal combustion or catathermal combustion is also used [15] The catalyst stabilizes the combustion at low temperatures, which broadens the window in which both CO and NO are sufficiently low cf. Fig. 3. The next section briefly discusses the prominent features of catalytic combustion. 

Of the two curves in Figure 5 one was attained with catalytic filters one was attained by regenerating a CB-330 covered catalytic filter and the other by regenerating a BS covered catalytic filter. In spite of the change of carbon material the combustion features do not vary significantly. In both cases the carbon material starts to bum at about 500 K and the complete regeneration is achieved at about 800 K. In addition, the regeneration rate curves show a dominant peak at about 660 K in the case of CB-330 and at about 690 K in the case of BS. 

Today, combustion catalysts that can operate up to 900-1000 °C have been developed and studied in both laboratory- and pilot-scales. Still, two catalyst features have not been fully developed. To begin with, a catalyst system that can operate above 1000 °C for one year of operation or more. Secondly, a catalyst system that can ignite natural gas at compressor outlet temperatures of approximately 200-400 °C. However, several combustion chamber designs have been proposed that utilize the features of catalytic combustion, but which operate the catalyst module at approximately 500-1000 °C. Here, a homogeneous zone is used to increase the temperature of the gas to the final maximum temperature. These designs are described in detail in Section 5 of this review. 

While these features greatly facilitate monitoring worker exposures, most instruments suffer from limitations associated with the types of sensing conponents enployed. For exeunple, the majority of the truly portable Instruments used for monitoring organic gases and vapors detect contaminants by catalytic combustion, semiconductor charge-transfer, or photo-ionization. 

These features were first applied in the development of molten metal filters [8,9], and later to catalytic combustion devices [11-13]. Catalytic applications are now appearing as the combined advantages of these structures become apparent. Specifically, these advantages are ... 

Comparison of the soot catalytic combustion results obtained at different laboratories is not easy because of the particular features of the soot oxidation reactions, and these aspects must be taken into account while reading this chapter and also the specific literature about this topic ... 

Chemical looping combustion (CLC) was first introduced as a means of increasing the efficiency of catalytic combustion, and in recent years, also as a means of CO2 capture. CLC is typically carried out in a dual fluid-bed reactor system, in which a metal, typically dispersed on an inert support material, is oxidized by air in the first reactor (Eq. 8.14), and then transferred to the second reactor where it makes contact with the hydrocarbon to produce CO2 and H2O (Eq. 8.15). An additional feature of CLC is that air is separated from the oxidation zone, thereby avoiding the possibility of NOx formation. 

Processes 7 to 9 in Table III have the common feature of producing sulphur as the end product. They are therefore overall reduction processes. The Cat-Redox and CO/SO2 systems are both direct conversions to elemental sulphur. The latter has a particular advantage of being able to accept effluent gas streams at temperatures of 750 F or higher thus avoiding the need for any cooling of combustion flue gases before clean-up. The Alkalized Alumina Process, however, involves concentration and catalytic... 

The present authors and coworkers observed intense CTL emission during the catalytic oxidation of ethanol or acetone vapor on a heated aluminum oxide powder [8]. This phenomenon was applied to the consumption-free CTL-based sensor for detecting combustible vapors. The CTL response was fast and reproducible for a change in concentration of a sample vapor in air. CTL emission has three distinct features ... 

The concepts discussed so far indicate that the major challenge in asymmetric operation is correct adjustment of the loci of heat release and heat consumption. A reactor concept aiming at an optimum distribution of the process heat has been proposed [25, 26] for coupling methane steam reforming and methane combustion. The primary task in this context is to define a favorable initial state and to assess the distribution of heat extraction from the fixed bed during the endothermic semicycle. An optimal initial state features cold ends and an extended temperature plateau in the catalytic part of the fixed bed. The downstream heat transfer zone is inert, in order to avoid any back-reaction (Fig. 1.13). 

We shall develop next a single-channel model that captures the key features of a catalytic combustor. The catalytic materials are deposited on the walls of a monolithic structure comprising a bundle of identical parallel tubes. The combustor includes a fuel distributor providing a uniform fuel/air composition and temperature over the cross section of the combustor. Natural gas, typically >98% methane, is the fuel of choice for gas turbines. Therefore, we will neglect reactions of minor components and treat the system as a methane combustion reactor. The fuel/air mixture is lean, typically 1/25 molar, which corresponds to an adiabatic temperature rise of about 950°C and to a maximum outlet temperature of 1300°C for typical compressor discharge temperatures ( 350°C). Oxygen is present in large stoichiometric excess and thus only methane mass balances are needed to solve this problem. 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

Water is non-combustible and nonflammable, odorless and colorless, and is universally available important prerequisites for the solvent of choice in catalytic processes. The dielectric constant or the refi"active index can be important in particular reactions and in analyzing them. The favorable thermal properties of water make it highly suitable for its simultaneous double function as a mobile support and heat transfer fluid, a feature that is utilized in the RCH/RP process (see below). 

---