Combustors catalytic combustion

Once an undesirable material is created, the most widely used approach to exhaust emission control is the appHcation of add-on control devices (6). Eor organic vapors, these devices can be one of two types, combustion or capture. AppHcable combustion devices include thermal iaciaerators (qv), ie, rotary kilns, Hquid injection combusters, fixed hearths, and uidi2ed-bed combustors catalytic oxidi2ation devices flares or boilers/process heaters. Primary appHcable capture devices include condensers, adsorbers, and absorbers, although such techniques as precipitation and membrane filtration ate finding increased appHcation. A comparison of the primary control alternatives is shown in Table 1 (see also Absorption Adsorption Membrane technology). 

New research in combustors such as catalytic combustion have great promise, and values of as low as 2 ppm can be attainable in the future. Catalytic combustors are already being used in some engines under the U.S. Department of Energy s (DOE), Advanced Gas Turbine Program, and have obtained very encouraging results. 

In this paper we attempt a preliminary investigation on the feasibility of catalytic combustion of CO/ H2 mixtures over mixed oxide catalysts and a comparison in this respect of perovskite and hexaaluminate type catalysts The catalysts have been characterized and tested in the combustion of CO, H2 and CH4 (as reference fuel). The catalytic tests have been carried out on powder materials and the results have been scaled up by means of a mathematical model of the catalyst section of the Hybrid Combustor. 

Catalyst monoliths may laos be employed as catalytic combustion chambers preceding aircraft and stationary gas turbines. As shown diagramatically in Fig. 16, a catalytic combustor comprises a preheat region, a catalyst monolith unit and a thermal region. In the preheat region, a small fuel-rich flame burner is employed to preheat the fuel-air mixture before the hot gases reach the monolith unit. Additional fuel is then injected into the hot gas stream prior to entry to the monolith where... 

Catalytic combustion has been commercially demonstrated to reduce NO.. emissions to below 3 ppm while keeping CO and UHC emissions below 10 ppm without the need for expensive exhaust clean-up systems. In addition, a catalytic combustor reduces typical DLN problems such as risk of blow-out and flame instability. Also, the economic advantage of primary methods including catalytic combustion as opposed to secondary clean-up measures (SCR and SCONOx) has recently been assessed [1]. 

J2.2 Lean Catalytic Combustion for Gas Turbines 365 Table 12.1 Design criteria and operating conditions of GT combustors. 

Different design concepts have been proposed to match the severe requirements of catalytic combustors. A main classification criterion is based on fuel/air stoichiometry in the catalyst section, which has a dominant effect on the selection of catalytic materials and on the operating characteristics of the combustor. In this section, only configurations based on lean catalytic combustion will be described. The peculiar characteristics of rich catalytic combustion will be described in a separate section. 

Catalytic combustion for gas turbines has received much attention in recent years in view of its unique capability of simultaneous control of NOX) CO, and unbumed hydrocarbon emissions.1 One of the major challenges to be faced in the development of industrial devices is associated with the severe requirements on catalytic materials posed by extreme operating conditions of gas turbine combustors. The catalytic combustor has to ignite the mixture of fuel (typically natural gas) and air at low temperature, preferably at the compressor outlet temperature (about 350 °C), guarantee complete combustion in few milliseconds, and withstand strong thermal stresses arising from long-term operation at temperatures above 1000°C and rapid temperature transients. 

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. 

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.). 

Modeling of catalytic combustors has been the subject of a number of studies. The models used varied in degree of complexity and could therefore answer various types of questions. General issues of modeling monolith catalytic reactors are discussed in Chapter 8 of this book and in the reviews of Irandoust and Andersson [57] and Cybulski and Moulijn [58]. Hence, only topics that are specific to the modeling of catalytic combustion in monolith catalysts are considered here. A description of some important aspects of different types of models are as follows. 

An LHV gas may contain up to a few thousand ppm of ammonia, produced from fuel-bound nitrogen during the gasification of a solid fuel. One of the major challenges in the catalytic combustion of LHV gases is to circumvent the formation of NO from this ammonia. The selectivity for this reaction is strongly dependent on the air-fuel ratio in the catalytic combustor and on the catalyst type [102,105], Clark et al. [102] and Tucci... 

Catalytic combustion for gas turbines is an area that has developed rapidly during the last two decades. Several novel approaches for catalytic combustors have shown high potential often as a result of combined materials science and reaction engineering. However, a substantial effort is still needed before commercial gas turbines with catalytic combustors become available on the market. 

H. Sadamori, T. Tanioka, and T. Matsuhisa, Development of a high temperature combustion catalyst system and prototype catalytic combustor turbine test results, Proc. 2nd Int. Workshop Catalytic Combustion, 18-20 April, Tokyo (H. Arai. ed.), Catalysis Society of Japan, Tokyo, 1994, p. 158. 

The key interplay of reaction kinetics and transport phenomena in a catalytic combustor must be treated using rigorous reactor models. In the next section, we use a simple model to describe the behavior of a catalytic combustor and to interpret the technology breakthroughs that led to the successful implementation of catalytic combustion to reduce NO in power generation. The model will be kept simple, even though its additional complexities are readily incorporated, because our purpose is to show the main characteristics of a catalytic combustor rather than to provide accurate simulations of expected performance. 

There are a few drawbacks associated with catalytic combustion. First, as noted by Pfefferle, the power density is low. The volumetric heat release rates of catalytic combustors (without a homogenous flame downstream) are much lower than those found in conventional flame combustors because catalytic combustors are mass transfer limited, and mass transfer coefficients are relatively low. 

Advances in chemical reaction engineering and catalytic materials have allowed catalytic combustion for thermal energy generation to be commercialized in consumer and industrial applications. The development of catalysts coated on one side of a metal substrate, coupled with the use of diffusion barriers, has allowed controlling the combustion temperature to suit diverse applications. Catalytic materials have been developed to remain active for thousands of hours under conditions deemed too severe just a few years ago. We may expect that the need for clean distributed power increases the demand for gas turbines fitted with catalytic combustors and promotes the development of catalytic burners to be used in fuel processors for fuel cell power systems. 

Catalytic combustion is a process in which a combustible compound and oxygen react on the surface of a catalyst, leading to complete oxidation of the compound. This process takes place without a flame and at much lower temperatures than those associated with conventional flame combustion [1, 2], Due partly to the lower operating temperature, catalytic combustion produces lower emissions of nitrogen oxides (NOv) than conventional combustion. Catalytic combustion is now widely used to remove pollutants from exhaust gases, and there is growing interest in applications in power generation, particularly in gas turbine combustors. 

There is increasing interest in the use of gas turbines for power generation while at the same time more stringent regulations on NOy emissions are being implemented in many areas. This situation provides an opportunity for catalytic combustion and has stimulated much research on catalytic combustors in recent years. Because of their potential commercial importance this section will largely focus on gas turbine applications of catalytic combustion. 

In catalytic combustion of a fuel/air mixture the fuel reacts on the surface of the catalyst by a heterogeneous mechanism. The catalyst can stabilize the combustion of ultra-lean fuel/air mixtures with adiabatic combustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500°C and very little thermal NO. will be formed, as can be seen from Fig. 1. However, the observed reduction in NO. in catalytic combustors is much greater than that expected from the lower combustion temperature. The reaction on the catalytic surface apparently produces no NO. directly, although some NO.v may be produced by homogeneous reactions in the gas phase initiated by the catalyst. 

The problems encountered in developing catalysts for fully catalytic combustion have led to the development of various design approaches in which the catalyst temperature stays below the combustor outlet temperature. These approaches are described in the following sections. 

After several decades of research, catalytic combustion to eliminate emissions from gas turbines is nearing practical application. New hybrid systems in which combustion is initiated over a temperature-limiting catalyst and completed downstream in a homogeneous process hold promise for overcoming many of the problems encountered in earlier systems in which combustion occurred entirely in the catalyst. These new systems have been successfully demonstrated under turbine operating conditions at full scale in combustor test stands. The next and most important demonstration will be in an actual turbine environment, and it seems very likely that this will occur within the next few years. Indeed, the future of catalytic combustion for pollution prevention in gas turbines appears to be very bright over the next decade and beyond. 

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