Hybrid catalytic combustion

T. Furuya, S. Yamanaka, T. Hayata, J. Koezuka, T. Yoshine, and A. Ohkoshi, Hybrid catalytic combustion for stationary gas turbine—Concept and small scale test results, ASME Paper 87-GT-99 (1987). 

Toshiba, in collaboration with Tokyo Electric Power Company, has developed a hybrid catalytic combustion. Here only a part of the fuel is converted heterogeneously on the catalyst. The system consists of a pre-combustion mixing zone, a low-temperature catalyst zone, and a gas-phase combustion zone. The fuel-air mixture is controlled to maintain the temperature of the catalyst below 800 C, because the catalyst is not stable above the temperature. More fuel is added downstream to attain the final combustion temperature. The function of the catalyst is to be a source of additional "pre-heat" to support the lean, homogeneous down-stream combustion. 

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. 

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.)...

The effect of catalytic combustion on NO.v formation is shown very clearly in hybrid systems in which part of the fuel is burned by catalytic combustion and the... 

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. 

The introduction of solid catalysts into a traditionally non-catalytic free-radical process like combustion occurred in recent years under the influence of two pressures, the energy crisis and the increased awareness of atmospheric emissions. The major applications of catalytic combustion are twofold at low temperatures to eliminate VOC s and at high temperatures (>1000 C) to reduce NOx emission from gas turbines, jet motors, etc. Both these applications are briefly reviewed here. Some recent developments in high-temperature catalytic combustion are trend-setters in catalysis and hence of particular interest. For instance, novel materials are being developed for catalytic applications above 1000 C for sustained operation for over one year. Where material/catalyst developments are still inadequate, systems engineering is coming to the rescue by developing multiple-monolith catalyst systems and the so-called hybrid reactors. 

Fig. 7. Three systems engineering solutions for high-temperature catalytic combustion. A multiple monolith catalyst design B partial catalytic combustion C hybrid (catalytic + thermal) combustion LGC Lean gas-phase combustion.

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... 

A comparison between lumped and distributed models for hybrid reactors for catalytic combustion of methane was more recently investigated by Groppi et a/. The validation of one-dimensional models was made on the basis for different Nusselt correlations and geometrical properties of the monolith channel. In general, the simulations showed a mismatch of the wall temperature profiles predicted with the one-dimensional model. One-dimensional models could show good agreement of the gas exit temperature, which is an important parameter for hybrid reactors as described in the previous section. The catalyst has to ignite in... 

Carroni R, GrifFin T Catalytic hybrid lean combustion for gas turbines, Catal Today 155 2-12, 2010. 

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. 

Precision Combustion Catalytica, TKK KK and Ib, fully catalytic Ila, hybrid, partial inactive catalyst Recuperative small-scale Gasoline or JET Natural gas Advanced Technology Surface Reactor Detailed composition is proprietary, 2-3 stages FeCrAl-metal monolith washcoated with metal oxide, first stage Pd Atm + Ultra-short channels yield low emissions, low pressure drop and good fuel efficiency + Prevent catalyst overtemperature, no second mixing —Control of post-catalytic autoignition delay time 19 15, 176, 177... 

Allison Ila, hybrid and lie, second fuel and air 4.5-15 MW Natural gas As above NR + Lean premixed post-catalytic homogeneous combustion zone 138... 

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. 

Catalytic method US 5720605, 5720609 W.C. Pfefferle Lean hybrid combustion with very short channels (e.g. Microlith )... 

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