Hybrid combustors

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. 

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. 

In the second configuration (hybrid combustor), only a portion of the fuel is fed to the catalyst section. The inlet air/fuel ratio is carefully controlled to limit the adiabatic reaction temperature typically below 1000 °C, and accordingly, to reduce the catalyst thermal stresses. The remaining amount of fuel is fed to a... 

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. 

The disadvantage of metallic monoliths is that their maximum operating temperatures are substantially below the 1300 °C combustor outlet temperature required for current gas turbines. They can therefore only be used in combustor designs that limit the catalyst wall temperature in some way, such as the hybrid combustors described above in Section 3.4. 

However, the maximum operating temperature is lower compared with some ceramic materials, cf. Table 1. Nevertheless, they can be used in combustor designs where the catalyst temperature is limited, like in the hybrid combustor described in Section 7.2.3. In that case, the catalyst temperature is limited to 900-1000°C. 

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

Hybrid combustor with part of the air/fuel being combusted over the catalyst... 

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. 

P-aluminates that could avoid the homogeneous postcombustion volumes typical of the hybrid combustors, thus offering solutions for the gas turbine inlet in the last turbine generation [25]. 

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