Partial Catalytic Hybrid Combustor

The peculiar features of the PdO-Pd catalyst will be discussed in a following section. 

Heat generated at the catalytic wall of the active channels is efficiently transmitted by conduction through the thin metal foil and is dissipated in the gas flow on both the catalytic and the non-catalytic side, allowing the wall temperature to be kept well below the adiabatic reaction temperature. The structure can be adjusted in order to tune the fraction of active channels in the monolith cross-section. 

The deposition of an inert porous diflfusion barrier on top of the catalyst layer can significantly hinder the rate of mass transfer of reactants to the catalyst surface, at the same time affecting only negligibly the rate of heat transfer to the gas phase. The effect is equivalent to that observed with fuels, like higher hydrocarbons, whose mass diffusi vity in air is considerably lower than thermal diffusivity of the fuel-air mixture (Lewis number 1). Such unbalancing of heat and mass transfer rates results in a significant decrease in the catalyst wall temperature. 

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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. 

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. 

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. 

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