Impact of Gas-Phase Chemistry

Gas-phase reactions are usually neglected in numerical investigations of catalytic microreactors. However, recent studies have pointed out to the importance of homogeneous chemistry in enhancing steady-state combustion stability against external heat losses, especially at elevated pressures p — 5 bar) [17]. The enhanced combustion stabihty is not always a result of the total oxidation of methane via gas-phase reactions an important coupled hetero-Zhomogeneous reaction route is the incomplete oxidation of methane to CO via gas-phase reactions, followed by the main exothermic oxidation of the formed CO to CO2 not via homogeneous but via catalytic reactions. 

In this work, a selected number of simulations were performed, wherein detailed gas-phase chemistry was included in the model. Cases 1, 5, 6 and 10-12 were recomputed with full hetero-Zhomogeneous chemistry the fjg and tsi for these simulations are reported in Table 8.2 as Cases 20-25, respectively. In Fig. 8.17, 2-D distributions are given for methane mass fraction and temperature for Case 5, and methaneZOH mass fractions and temperature for Case 21. 

Gas-phase chemistry affects ignition and steady-state times by contributing to the heat generation inside the channel and altering the spatial extent of the reaction zone over which fuel is consumed. At both investigated pressures of p = 1 and 5 bar and for both cordierite and FeCr alloy materials, gas-phase reactions only have a small impact on fjg. In Cases 20 and 21 the ignition time is slightly reduced compared to Cases 1 and 5 by 0.3 and 0.2 s, respectively similarly, for Cases 22 and 23 fjg is reduced by 0.5 and 0.2 s compared to Cases 6 and 10, respectively. 

Following ignition and during the heat-up phase, the contribution of the homogeneous reaction pathway in fuel conversion increases substantially (see Fig. 8.18), with flames sustained in the channel by the exothermicity of the 

With a fraction of the heat release zone now shifted from the surface to the gas, the solid substrate is partly heated via convection and partly via direct heat generation on the surface. At a time instance 17.5 s when the maximum value of gas-phase fuel conversion rate surpasses the corresponding catalytic one (see Fig. 8.18), the locus of maximum temperatures inside the mieroreactor shifts from the channel surface to the gas phase. 

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Liao H, Seinfeld JH (2005) Global impacts of gas-phase chemistry-aerosol interactions on direct radiative forcing by anthropogenic aerosols and ozone. J Geophys Res 110(D1) 8208. doi 10.1029/2005JD005907... 

Gas-phase chemistry played a minor role in the reactor ignition time, but had an appreciable effect in increasing the steady-state time, especially for high-thermal-conductivity microreactors, due to a less effective heat transfer from the reacting gas to the microreactor wall during the heat-up phase. Moreover, an increase in pressure from 1 to 5 bar augmented the impact of gas-phase chemistry on t t, predominantly for the FeCr alloy microreactors. 

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