Homogeneous combustion, catalytic

Catalytic reactor. The role of the catalyst was described earlier it must burn enough of the incoming fuel to generate an outlet gas temperature high enough to initiate rapid homogeneous combustion just past the catalyst exit. 

After the ignition phase, water is fed to the reactor and the 02 CH4 and H20 CH4 ratios are varied until the desired operating conditions are reached. It must be noted that after the ignition phase, due to the lower values of the 02 CH4 ratio, the homogeneous combustion of methane is inhibited and consequently POX, SR and WGS reactions occur simultaneously in the catalytic bed, while the temperature on the SiC foam decreases to values lower than 400 °C. 

These results can be explained by considering that at this 02 CH4 ratio the homogeneous combustion reaction is not favored and consequently the feed mixture reacts in the catalytic section where the heat developed by the exothermic reactions is responsible for the remarkable temperature increase. 

The catalytic pilot burner processes only a fraction of the fuel and is targeted to retrofitting applications with minor combustor modifications. Test results indicate that to achieve effective stabilization of homogeneous combustion, 18-20% of the fuel-air must be processed in the catalytic pilot, which is a much higher fraction than the typical 2-5% processed in a conventional pilot burner. Under such conditions, test results demonstrated single-digit (<5 ppm at 15% O2) emissions of NO and CO with low acoustics at 50 and 100% load conditions. 

The technical synthesis of graphite, diamond and a variety of other forms of sp2 carbons (Fig. 3) is described in a review [39] and is not covered here. As the unintended formation of carbon in deactivation processes and the modification of primary carbon surfaces during chemical treatment (in catalytic service and during oxidative reactivation) and their chemical properties arc frequent problems encountered in catalytic carbon chemistry, it seems appropriate to discuss some general mechanistic ideas which mostly stem from the analysis of homogeneous combustion processes (flame chemistry) and from controlled-atmosphcre electron microscopy. 

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. 

The catalytic formation or destruction of gas-phase radicals is an important phenomenon. since it may have an effect on the formation of NO. This was recently demonstrated by Griffin et al. [40], who studied the formation of NO in a catalytic combustor with downstream homogeneous combustion. They found that when the adiabatic flame temperature and total residence time were kept constant, the concentration of NO decreased if the contribution of the catalytic conversion increased as compared to the homogeneous combustion. Measurements were performed at flame temperatures between 1300 and 1500 C. Their results confirmed the idea posed earlier by Markatou et al. [41] that higher hydroxyl and oxygen radical concentrations in the gas phase would cause higher NOx levels. 

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. 

For other types of systems such as highly branched reaction networks for homogeneous gas-phase combustion and combined homogeneous and catalytic partial oxidation, mechanism reduction involves pruning branches and pathways of the reaction network that do not contribute significantly to the overall reaction. This pruning is done by using sensitivity analysis. See, e.g., Bui et al., "Hierarchical Reduced Models for Catalytic Combustion H Air Mixtures near Platinum Surfaces, Combustion Sci. Technol. 129(l-6) 243-275 (1997). 

Catalytic combustion is an environmentally-driven, materials-limited technology with the potential to lower nitrogen oxide emissions from natural gas fired turbines consistently to levels well below 10 ppm. Catalytic combustion also has the potential to lower flammability at the lean limit and achieve stable combustion under conditions where lean premixed homogeneous combustion is not possible. Materials limitations [1,2] have impeded the development of commercially successful combustion catalysts, because no catalytic materials can tolerate for long the nearly adiabatic temperatures needed for gas turbine engines and most industrial heating applications. 

Methane reactants within noncatalytic channels remain unburned and they must be combusted in a fully catalytic stage or in a homogenous flame. A fully catalytic monolith would cause the catalyst to reach adiabatic temperatures and to deactivate. Thus, the only practical option is to complete the combustion in a homogenous combustion process. 

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. 

The combustor validation during steady state and transient conditions is performed in a test rig at ON ERA, France, where the high pressure/temperature conditions can be achieved. As was pointed out earlier,in order to obtain high hetero/homogenous combustion efficiency with low emission values it is important to create an even velocity profile at the catalytic section inlet. Extensive... 

Figure 10.10a shows propane conversion contours obtained from 2D CFD calculations for catalytic propane combustion in a non-adiabatic microchannel for the conditions mentioned in the caption [23]. Unlike the homogeneous combustion case, the preheating and combustion zones in catalytic microburners overlap since catalytic reactions can occur on the hot catalyst surface close to the reactor entrance. Figure 10.10b shows a discontinuity in the Nu profile, similar to the homogeneous combustion problem. In this case, it happens at the boundary between the preheat-ing/combustion zone and the post-combustion zone. At this point, the bulk gas temperature (cup-mixing average) and wall temperatures cross over and the direction... 

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