Combustors catalytic combustor design

This chapter deals with the specific research and development issues related to the use of a catalyst at temperatures up to 1500 C in a catalytic combustor. A short survey of emission problems related to combustion and strategies for emission reduction is followed by a discussion of catalysis in combustion. Specific aspects of monolith combustion catalysts, such as material problems and combustor design, are then treated briefly. 

It can be concluded from the previous sections that the p>erfect combustion catalyst has not been found yet, and it is most likely very hard to develop. Hence, reaction engineering must help to circumvent the inherent compromise between activity and stability and the limitations of material science as of today. In this section, the principles of the most promising approaches are outlined. Figure 9 shows schematically the three currently most promising approaches in catalytic combustor design. 

The second issue is the improvement of the low-temperature performance of combustion catalysts, i.e., the activity at combustor inlet conditions. All the proposed catalytic combustor designs available today need a pilot flame, or a heat exchanger in the case of recuperative gas turbines, to heat the compressed combustion air to a temperature sufficient for ignition of the catalyst. The possibility of avoiding this pilot flame is considered very important, since it would further reduce NO emissions. The catalyst surface area and washcoat loading are very important for the low-tempcraturc activity. 

Another important feature of gas turbine operation is the very high velocity of the fuel/air mixture. For example, in a full scale catalytic combustor designed by General Electric [8], the air flow through the 51 cm diameter unit was 23 kg s . This corresponds to a gas hourly space velocity (GHSV) of 300 000 h" at 12 bar pressure and 450 °C. The catalyst must obviously be able to operate effectively at these very high gas velocities. 

I. Stambler, Cool catalytic combustor design limits NO to less than 0.5 ppm. Gas Turbine World, 1993, 32-44, May-June. 

During the last five years, several successful pilot- and full-scale demonstrations of catalytic combustors for gas turbine applications have been presented. Here, we have divided these systems into five different classes the large- and small-size fully catalytic combustor (designs la and b) and the hybrid designs with partially inactive catalyst, with secondary fuel and with secondary air (designs Ila, b and c). The first part of this section is devoted to fundamental gas turbine considerations, which will be followed by a summary of demonstrations of catalytic combustors. 

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

Question 5 ("Is combustion with air the only chemistry intended at your facility ") can be answered YES in this case, assuming the "facility" being addressed is limited to the incinerator system. Due to the great number of combustion systems in operation, many other resources are available for ensuring safe design and operation of the combustion part of the incinerator facility. However, it should be noted that many combustors now have effluent treatment systems, such as selective catalytic reduction (SCR) systems, that involve intentional chemistry beyond the combustion reaction. 

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. 

Two configurations of the RCL burn technology have been designed a catalytic pilot burner, which replaces the existing diffusion flame or partially premixed pilot of the DLN combustor [26], and a full catalytic burner [25]. 

Different design concepts of catalytic combustors were proposed in order to meet the stringent requirements on emission control and on durability under the severe... 

For an efficient absorption tower design, the plant should emit tail gases at less than 1000 ppm of nitrogen oxides. This level is about half the current emissions limit in Western Australia. Should emissions exceed this figure, then consideration must be given to the installation of a catalytic tail-gas combustor which enables emission levels to be lowered below 400 ppm. The plant does not normally produce any liquid waste streams (see Section 5.4.7 Environmental Impact Analysis). 

The coupling of heterogeneous reactions on the catalyst surface and homogeneous gas-phase reactions, as discussed in the previous section, is important for the design and operation of a catalytic combustor with maximum temperatures over 900 C, which is the case for gas turbine combustors. It is worth pointing out that the ignition of the fuel-air mixture over the catalyst at much lower temperatures than possible for homogeneous gas-phase combustion is the reason why catalytic combustors can operate at flame temperatures as low as 1100 C [53]. Hence, the formation of thermal NO, which is the most important type of NOx for gas turbine combustors, is practically avoided. 

The contradictory demands of high activity and high stability impose the need to apply reaction engineering to the design of catalytic combustors Promising approaches are presented in the next section. 

The preceding sections show that catalytic fuel combustion is a process in which complex kinetics for heterogeneous and homogeneous reactions are combined with mass and heat transfer effects. This leads to difficulties in predicting the behavior of combustion catalysts under real conditions. Therefore, mathematical modeling is a powerful tool to assist experimental work, to interpret results, and to aid in the design of catalytic combustors. 

The third outstanding issue in the development of catalytic gas turbine combustors is the development of a complete system, i.e., monolith and washcoat design, fuel inlets, and... 

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. 

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