Catalytically stabilized thermal combustion

Figure 3.1 Hetero-Zhomogeneous combustion methodologies in power generation (A) catalytically stabilized thermal combustion (CST) and (B) fuel-rich catalytic/gaseous-lean combustion.

A catalytic combustor is basically a lean-prenux combustor, in which the combustion is stabilized by a catalytic surface Hence, the expression catalytically ignited thermal combustion or catathermal combustion is also used [15] The catalyst stabilizes the combustion at low temperatures, which broadens the window in which both CO and NO are sufficiently low cf. Fig. 3. The next section briefly discusses the prominent features of catalytic combustion. 

CPO catalytic partial oxidation CST catalyticaUy stabilized thermal combustion i-CST inverse catalyticaUy stabilized thermal combustion IGCC integrated gasification combined cycle... 

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 a catalytic burner, the combustion is ignited and stabilized under ultra-lean conditions, which results in adiabatic temperatures close to those allowed for delivering the hot compressed gas to the turbine. Hence the need for by-pass air is minimized and the formation of thermal NOj, is almost prevented due to the absence of a hot combustion zone. Reduction of N emission has been reported to be even larger than expected from the lower combustion temperature if a significant fraction of the fuel is oxidized on the catalyst surface [3]. This effect has been attributed either to the reduction in the formation of prompt NO in view of the... 

While the incorporation of transition metal oxides into complexes with materials such as alumina can lower their volatilities by factors from 10 (CuO) to 1000 (BaO) depending primarily upon the heat of reaction between the two oxides, it is also likely that formation of very stable complex metal oxides, such as aluminates, can also greatiy lower the chemical activity of the transition metal. As mentioned above, Mn, Ni, and Co may requite stabilization in complex oxides for long catalyst life, but the complex oxides generally have inferior activity. The most active transition metal oxides (Ru and Cu) may still have unacceptable volatility as relatively active complex oxides. As a consequence, there may be a technology-limiting trade-off between the catalytic activity of metals and metal oxides and their chemical and thermal stability in combustion environments. 

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. 

Oxide-supported platinum or Pd catalysts have been used for the complete combustion of hydrocarbons, mainly for the purpose of cleaning exhaust gases. However, the new application of the catalytic combustor to gas turbines or boilers is arousing research interest on catalysts with good thermal stability and combustion activity at high temperatures. 

Catalytic combustion was originally discovered in the early 1970 s [23]. The concept is based on the observation that catalysts can be used to light off combustion reactions and to sustain gas-phase oxidation reactions after the catalyst. Peak temperatures obtained via catalytic combustion are significantly lower than those achieved in admixed or premixed flames consequently, NOx emissions are lower. Catalyst requirements include low light-off temperature, low reactor backpressure at high volumetric flow rates, high temperature stability, thermal shock resistance and durability. 

Preliminary results of methane catalytic combustion indicated that Pt/H-MCM-22 sample showed a 100% conversion at 700°C with 100% selectivity toward the C02 formation. The sample showed also high thermal stability in fact, the catalytic activity was preserved after heating overnight at 800°C under air flow. (Catalytic data kindly provided by Ing. R. Pirone, Istituto di Ricerche sulla Combustione Italian CNR). 

This approach, however, requires the absence of ill-defined carbon deposits originating from defect-induced soot formation on the surface of nanocarbons during their synthesis. Pyrolytic structures often counteract the control over activity and selectivity in catalytic applications of well-defined nanocarbons by offering an abundance of highly reactive sites, however, in maximum structural diversity. Although some nanocarbons are equipped with a superior oxidation stability over disordered carbons [25], such amorphous structures can further induce the combustion of the well-ordered sp2 domains by creating local hotspots. Thermal or mild oxidative treatment,... 

Materials for high-temperature catalytic combustion should possess both elevated activity and high thermal stability. Generally these are opposite features,32-33 so... 

The potential of rare earth compounds as catalytically active phases and promoters in pollution control, catalytic combustion, polymer production and in the fuel and chemical manufacture and thermal stabilizers for catalyst supports (alumina, silica-alumina, titania) need to be mentioned. Application of rare earths in alternate fuels technology (Fisher-Tropsch Processes, natural gas to transport fuel pathways) is also promising. 

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