Reactions relevant to methane combustion : fundamental kinetic study and modelling
The pyrolysis of dimethyl ether in the presence of a methane bath, over the temperature range 790-850K, follows the simplified reaction scheme numbered 1-5: CH3OCH3 CH3O + CH3 (l) CH3O + CH4 CH3OH + CH3 (2), CH3O + M CH2O + H + M (3),H + CH4H2 + CH3 (4),CH3 + CH3 C2H6 (5) Thus monitoring the level of ethane, or ethylene derived from ethane, yields a measure of the rate of dimethyl ether decomposition, (l) Results obtained from the current work, together with those of other workers obtained over a wider temperature range, yields the rate constant for reaction (l): k1 = 10l6.3 1 .3 exp-81.4 0.3 k cal mol-1. Hence a value for the heat of formation of the methoxy radical is calculated to be 4.1 kcal mol-1. However, both computer modelling of the full reaction scheme and further experimental work has highlighted the importance of sensitised decomposition reactions within this system, reactions (6), (7): CH3 + CH3OCH3 CH4 + CH2OCH3 (6), CH2OCH3 + M CH2O + CH3 (7) For the case where neat dimethyl ether is pyrolysed the reaction is dominated by sensitised decomposition. Computer modeling successfully resolved the mechanism of reaction under the experimental conditions. Attempts to obtain an experimental rate constant for the decomposition of the methoxy radical have proved unsuccessful due to the complex nature of this reaction system. This also proved to be the case where dimethyl ether-d6 acted as the precursor of methoxy-d3. The decomposition of the trifluoromethoxy radical was examined using an RRKM computer model, reconciling the calculated parameters for this radical with the pressure dependent data of Descamps and Forst. This study of the trifluoromethoxy radical yielded data which enabled application of Unimolecular Gas Kinetic Theory to the similar methoxy radical. With so few experimental data on the decomposition of the methoxy radical it was essential to reduce the number of variables prior to applying the RRKM model to this reaction (3). Formaldehyde pyrolysis in a methane bath, over the temperature range 740-910K, was examined to resolve controversy over the mode of formaldehyde decomposition: whether the reaction proceeds by a molecular elimination path, A, or via a radical chain mechanism, B. Although ethane was not observed as a principle product, computer modelling of this reaction indicated that the combination of methyl radicals was the principle termination step. To test for the participation of the molecular elimination pathway (15), the ratio CD2O + M CO + D2 + M (15) RD2/RHD was followed as a fuction of both the formaldehyde-d2 concentration and temperature. As (D) is a function of the concentration of formaldehyde-d2, an intercept will be obtained upon plotting RD2/RHD versus (CD2O)/(CH4) where the molecular elimination path participates. No evidence of the molecular elimination route was obtained. Reaction data for steps (10) and (11) were obtained together with evidence of heterogeneous reaction contributing to the rate of formaldehyde decay.