Combustion chemistry of methoxymethanol. Part Ⅱ: Laminar flames of methanol + formaldehyde fuel mixtures

摘要

In the present study, the laminar burning velocities of mixtures of up to 16.4 (mol) formaldehyde inmethanol, burning with air, were determined at atmospheric pressure using the heat flux method coveringlean, stoichiometric and rich flames at initial gas mixture temperatures of 298, 318 and 338 K.Results published in the literature indicate that evaporation of CH_2O + CH_3OH fuel blends should leadto a gaseous mixture of formaldehyde, methanol and methoxymethanol, although the composition ofthese components in the gas phase was not well defined. To interpret the measurements performed inthe present study, the detailed kinetic model developed by the group of Konnov was used. The recentlyupdated mechanism was further extended by the reactions of methoxymethanol with the rate constantscalculated in Part I of the present study. A comparison of the predictions of this mechanism with the newexperimental data indicated that between 40 and 60 of CH_2O present in the investigated CH_2O + CH_3OHmixtures were at 473 K evaporated as gaseous formaldehyde monomer, while the rest was releasedwithin CH_3OCH_2OH. Laminar burning velocity results suggest partial condensation of methoxymethanolin the CH_3OH + CH_2O fuel mixture with 5.84 formaldehyde at rich conditions and 298 K. These observationsallowed evaluation of the partial pressure of CH_3OCH_2OH which was found to be between 0.35 and0.52 kPa. The sensitivity and rate-of-production analyses revealed that the reduced reactivity with the increasedamount of methoxymethanol in the fuel mixtures is explained by the conversion of CH_3OCH_2 OHto CH_3OCHOH radicals due to favored H-abstraction from the secondary hydrogen atoms predicted byab initio calculations compared to other sites of methoxymethanol. Hydroxyl-methoxyl-methyl radicalsfurther decompose forming slowly reacting formic acid and methyl radicals.