Hybrid catalytic combustors

Figure 11 Toshiba hybrid catalytic combustor. A1--A3 air inlets FI-F3 fuel inlets Zl precom-buslion zone Z2 premixing zone Z3 catalyst zone Z4 gas-phase combustion zone. (From Ref. 97.)...

To attain high combustion efficiency, a rapid increase in conversion by the homogeneous reaction is desirable. One typical construction of hybrid catalytic combustor is shown in Figure 14. This system consists of four reaction zones, i.e., a precombustion zone, a premixing zone, a catalyst zone, and a gas-phase combustion zone. The catalyst... 

Figure 14 Schematic Illustration of hybrid catalytic combustor. ...

On the other hand, following the development of hybrid combustor configurations that prevent operation of the catalyst module at temperatures above 900-1000 °C, the major drawback of metallic monoliths, namely the limited maximum operating temperature, has been overcome. Accordingly, honeycombs made of metal foils have been adopted in GT catalytic combustors in view of their excellent thermal shock resistance and thermal conductivity properties [9]. In addition, metallic substrates are a promising option for the fabrication of microcombustors. 

Figure 9 Simplified representation of different catalytic combustor concepts. A Multimonolith combustor B partial catalytic combustor C hybrid combustor. LPF = lean premixed flame.

The third approach is the hybrid combustor, which was developed at Toshiba and presented in a series of publications [95-97]. The difference with the partial catalytic combustor is that only part of the fuel is added upstream of the catalyst. This fuel is nearly completely combusted over the catalyst, bringing the temperature up to approximately 8(X)-900 C. At this point the rest of the fuel is added and then combusted homogeneously. The advantages of this approach are the same as for the partial catalytic combustor. However, the problem with catalyst overheating is less pronounced here, since complete conversion of all the fuel added to the catalyst is allowed. On the other hand, the additional fuel injection downstream of the catalyst renders the system much more complex and harder... 

It needs to be mentioned here that there is no clear dividing line between any two of the three alternatives. The partial combustor and the hybrid combustor may both be equipped with a multimonolith catalyst zone. Furthermore, the temperature in the hot segments of a multimonolith combustor will be so high that homogeneous combustion takes place in the monolith channels. It is not clear what the importance of the catalytic activity and catalyst surface area is under such conditions. There is still much ambiguity about this aspect of high-temperature catalytic combustors. 

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. 

However, the fully catalytic design (la and b) is different compared to a second class of catalytic combustors, denoted hybrid designs (IIa,b and c), where a post-catalytic homogeneous combustion zone is used to complete the combustion and increase the temperature further. Hybrid designs are discussed in the following section. 

Currently, two approaches for the design of catalytic combustors are being tested. The first approach, the multi-monolith catalytic combustor, is based on a very active catalyst at the combustor inlet, followed by less active but more thermostable catalyst segments [4). Complete combustion is to be achieved within the monolithic catalyst in this case. TTie second approach, a hybrid combustor, is based on a partial combustion of the fuel in the catalyst, while the remainder of the fuel is converted in a homogeneous combustion zone downstream of the catalyst [5,6]. The advantage of the multi-monolith is its simplicity whereas the hybrid combustor provides a way to limit the temperature of the catalyst, thereby decreasing the demands placed on the catalyst materials. 

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. 

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. 

Hybrid combustor with catalytic part consisting of three catalyst segments using Pd and Pt based catalysts... 

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