Lean fuel catalytic combustion

Results obtained in full pressure tests demonstrated the following advantages of rich fuel over lean fuel catalytic combustion ... 

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. 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

At present, one catalytic combustion system has been implemented at a full scale the XONON Cool Combustion technology, developed by Catalytica Energy Systems 157,158). The system is operated as follows Fuel from a lean-mix prebumer and the main fuel stream together with compressed air pass through the catalyst module (palladium oxide catalyst deposited on corrugated metal foil) in which the gas reaches a temperature up to 1623 K. The UHC and CO are combusted to essentially full conversion, downstream of the catalyst in the homogenous combustion zone. The guaranteed emission levels are as follows NOj < 3 ppm. 

Lyubovsky, M., Smith, L.L., Castaldi, M., Karim, H., Nentwick, B., Etemad, S., LaPierre, R., and Pfefferle, W.C. Catalytic combustion over platinum group catalysts Fuel-lean versus fuel-rich operation. Catalysis Today, 2003, 83, 71. 

In catalytic combustion of a fuel/air mixture the fuel reacts on the surface of the catalyst by a heterogeneous mechanism. The catalyst can stabilize the combustion of ultra-lean fuel/air mixtures with adiabatic combustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500°C and very little thermal NO. will be formed, as can be seen from Fig. 1. However, the observed reduction in NO. in catalytic combustors is much greater than that expected from the lower combustion temperature. The reaction on the catalytic surface apparently produces no NO. directly, although some NO.v may be produced by homogeneous reactions in the gas phase initiated by the catalyst. 

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. 

The use of noble metals in catalytic combustion applications allows for fuel lean feeds and lower gas temperatures, thus avoiding the formation of thermal NOx... 

Monolithic catalysts (or honeycombs) have received much attention ever since they were first applied in automotive catalytic converters [1]. An increasing interest in the use of monolithic reactors for other applications has also been noticed during recent years [2]. One application which particularly profits from the opportunities offered by the honeycomb structure is catalytic combustion for use in advanced gas turbines [3]. In a catalytic combustor, a premixed lean fuel-air mixture is ignited by the catalyst which results in complete combustion at maximum temperatures far lower than possible in conventional gas-phase combustors. Hence, the thermal formation of nitrogen oxides can almost completely be circumvented. This fact has promoted large R D programs in catalytic combustion during recent years. 

In the conventional catalytically stabilized thermal (CSX) combustion approach (Beebe et al., 2000 Carroni and Griffin, 2010 Carroni et al., 2003) shown in Fig. 3.1 A, fractional fuel conversion is achieved in a catalytic honeycomb reactor operated at fuel-lean stoichiometries, while the remaining fuel is combusted in a follow-up gaseous combustion zone, again at fuel-lean stoichiometries. Nonetheless, for diffusionally imbalanced limiting reactants with Lewis numbers (Le) less than unity (such as H2 whereby Lch2 0.3 at fuel-lean stoichiometries in air), CSX is compounded by the... 

In catalytic channels, the flat plate surface temperature in Eq. (3.32) is attained at the channel entry (x O). As the catalytic channel is not amenable to analytical solutions, simulations are provided next for the channel geometry shown in Fig. 3.3. A planar channel is considered in Fig. 3.3, with a length L = 75 mm, height 21) = 1.2 mm, and a wall thickness 5s = 50 pm. A 2D steady model for the gas and solid (described in Section 3.3) is used. The sohd thermal conductivity is k = 6W/m/K referring to FeCr alloy, a common material for catalytic honeycomb reactors in power generation (Carroni et al., 2003). Surface radiation heat transfer was accounted for, with an emissivity = 0.6 for each discretized catalytic surface element, while the inlet and outlet sections were treated as black bodies ( = 1.0). To illustrate differences between the surface temperatures of fuel-lean and fuel-rich hydrogen/air catalytic combustion, computed axial temperature profiles at the gas—wall interface y=h in Fig. 3.3) are shown in Fig. 3.4 for a lean (cp = 0.3) and a rich cp = 6.9) equivalence ratio, p = 1 bar, inlet temperature, and velocity Tj = 300 K and Uin = 10 m/s, respectively. The two selected equivalence ratios have the same adiabatic equilibrium temperature, T d=1189 K. 

In fuel-lean H2/air combustion, superadiabatic surface temperatures are attained as illustrated by three computations in Fig. 3.4 (Cases 1—3). Case 1 (marked T. i) was computed using an infinitely fast catalytic step H2 +1402 —H2O (i.e., transport-limited catalytic hydrogen conversion), without gas-phase chemistry and without inclusion of heat conduction in... 

Simulations for the fuel-rich combustion concept (Fig. 3.16A) are depicted in Fig. 3.19 for a total Uin = 10 m/s, three different total fuel-lean equivalence ratios tot and five fuel-rich catalytic equivalence ratios (Pcat-Additional results for a total UiN = 20m/s are shown for cat = 3.0. 

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