Methane fuel-lean

The reaction diagram of Fig. 14.1 applies to methane oxidation under both flame [423] and flow reactor [146] conditions. At high temperatures and fuel-lean to stoichiometric conditions, the conversion of methane proceeds primarily through the sequence CH4 -> CH3 -> CH2O -> HCO -> CO -> CO2. At lower temperatures or under fuel-rich conditions the reactions of CH3 with O or O2 are less competitive. Under these conditions two CH3 radicals may recombine and feed into the C2 hydrocarbon pool,... 

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

Because of the large amount of heat release from combustion, gas explosions always involve high temperature rise. For example, the maximum flame temperatures for hydrogen and methane are 2045°C and 1875°C, respectively.f l Even for weak deflagrations in fuel-lean mixtures near the LFL, the flame temperatures of hydrocarbons are in the range of 1300-1350°C (p. 330 in Ref9 (). This is why even weak deflagrations such as flash fires can cause severe burn injuries. 

Methane oxidation was the first reaction studied using the AIMS system. The goal was to operate the microreactor under fuel-lean conditions with millisecond contact times (Williams et al, 2005) while achieving high degrees of conversion for O2, which was the limiting reactant. 

Figure 3 Model fit (dotted line) for the. simplified, analytical model and experimental data (.symbols) for fuel lean methane/air mixtures. The diagramm depicts the (conventional) equivalence ratio r.s foil temperature.

An NG vehicle can run eitlier near tlie stoichiometric point, or under fuel-lean conditions, due to its wide ignition range. If the vehicle is nm near the stoichiometric point, optimum conversion of methane is obtained mider slightly rich conditions. Rumiing the engine under fiiel-lean conditions provides better fuel economy and lower CO and NOx emissions [7]. Adequate NOx reduction luider fliel lean conditions is not yet practical, so tailpipe NOx emissions are likely to be lower with operation near the stoichiometric point. This study primarily focused on controlling emissions from vehicles running near the stoichiometric point, which is where conventional tliree-way catalysts will be the most useflil. 

Deshmukh SR, Vlachos DG (2007) A reduced mechanism for methane and one-step rate expressions for fuel-lean catalytic combustion of small alkanes on noble metals. Combust Flame 149 366-383... 

The ROD is similar to a cold feed stabilizing tower for the rich oil. Heat is added at the bottom to drive off almost all the methane (and most likely ethane) from the bottoms product by exchanging heat with the hot lean oil coming from the still. A reflux is provided by a small stream of cold lean oil injected at the top of the ROD. Gas off the tower overhead is used as plant fuel and/or is compressed. The amount of intermediate components flashed with this gas can be controlled by adjusting the cold loan oil retlux rate. 

Hydrocarbons heavier than methane that are present in natural gases are valuable raw materials and important fuels. They can be recovered by lean oil extraction. The first step in this scheme is to cool the treated gas by exchange with liquid propane. The cooled gas is then washed with a cold hydrocarbon liquid, which dissolves most of the condensable hydrocarbons. The uncondensed gas is dry natural gas and is composed mainly of methane with small amounts of ethane and heavier hydrocarbons. The condensed hydrocarbons or natural gas liquids (NGL) are stripped from the rich solvent, which is recycled. Table 1-2 compares the analysis of natural gas before and after treatment. Dry natural gas may then be used either as a fuel or as a chemical feedstock. 

Variation of the normalized remaining percentage of CH4 fuel (c/Cj) after a run, measured by the gas chromatography, plotted over a very wide range of normalized turbulent intensities (u /Sl 10 100), where the subscript "i" refers to the initial condition. Both very rich (0 = 1.45 Cj = 13.2%) and very lean = 0.6 q = 5.92%) pure methane/air mixtures are investigated, showing critical values of Ka for the transition across which global quench occurs. 

It is presumed that the global-quenching criteria of premixed flames can be characterized by turbulent shaining (effect of Ka), equivalence ratio (effect of 4>), and heat-loss effects. Based on these aforemenhoned data, it is obvious that the lean methane flames (Le < 1) are much more difficult to be quenched globally by turbulence than the rich methane flames (Le > 1). This may be explained by the premixed flame shucture proposed by Peters [13], for which the premixed flame consisted of a chemically inert preheat zone, a chemically reacting inner layer, and an oxidation layer. Rich methane flames have only the inert preheat layer and the inner layer without the oxidation layers, while the lean methane flames have all the three layers. Since the behavior of the inner layer is responsible for the fuel consumption that... 

Of course, all the appropriate higher-temperature reaction paths for H2 and CO discussed in the previous sections must be included. Again, note that when X is an H atom or OH radical, molecular hydrogen (H2) or water forms from reaction (3.84). As previously stated, the system is not complete because sufficient ethane forms so that its oxidation path must be a consideration. For example, in atmospheric-pressure methane-air flames, Wamatz [24, 25] has estimated that for lean stoichiometric systems about 30% of methyl radicals recombine to form ethane, and for fuel-rich systems the percentage can rise as high as 80%. Essentially, then, there are two parallel oxidation paths in the methane system one via the oxidation of methyl radicals and the other via the oxidation of ethane. Again, it is worthy of note that reaction (3.84) with hydroxyl is faster than reaction (3.44), so that early in the methane system CO accumulates later, when the CO concentration rises, it effectively competes with methane for hydroxyl radicals and the fuel consumption rate is slowed. 

From other more recent studies of NO formation in the combustion of lean and slightly rich methane-oxygen-nitrogen mixtures as well as lean and very rich hydrocarbon-oxygen-nitrogen mixtures, it must be concluded that some of the prompt NO is due to the overshoot of O and OH radicals above their equilibrium values, as the Bowman and Seery results suggested. But even though O radical overshoot is found on the fuel-rich side of stoichiometric, this overshoot cannot explain the prompt NO formation in fuel-rich systems. It would appear that both the Zeldovich and Fenimore mechanisms are feasible. 

Use laminar premixed free-flame calculations with a detailed reaction mechanism for hydrocarbon oxidation (e.g., GRI-Mech (GRIM30. mec)) to estimate the lean flammability limit for this gas composition in air, assuming that the mixture is flammable if the predicted flame speed is equal to or above 5 cm/s. For comparison, the lean flammability limits for methane and ethane are fuel-air equivalence ratios of 0.46 and 0.50, respectively. 

Several hydrocarbons, including benzene, diisobutylene, and methane, do not form cool flames in engines (26, 37, 105). The absence of cool flame radiation does not indicate the absence of preflame reaction, as oxidation products have been isolated from a diisobutylene-air mixture in a motored engine (105). At lean air-fuel ratios, benzene, diisobutylene, and methane have been observed to form blue flames (36). 

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