High temperature oxidation of formaldehyde and formyl radical: A study of 1,3,5-trioxane laminar burning velocities

Few studies of formaldehyde flames are available, especially at pressures greater than 55 torr, due to the difficulties and hazards associated with producing formaldehyde vapor. This work experimentally and numerically investigates the flame properties of formaldehyde (CH₂O) and formyl radical (HCO) at high O₂ loadings and both atmospheric and reduced pressure by measuring and modeling the laminar burning rates of 1,3,5-trioxane/O₂/N₂ mixtures. Trioxane is shown to decompose nearly exclusively into high concentrations of formaldehyde early in the flame structure before subsequent flame chemistry reactions occur. Kinetic model predictions show that the flame properties are controlled by CH₂O and HCO kinetics. Laminar burning rate predictions of several combustion kinetic models vary significantly in comparison to experimental data and each other; however, all simulations show that the present observations are particularly sensitive to the competition between reactions HCO + M = H + CO + M (R3) and HCO + O₂ = HO₂ + CO (R4). Monte Carlo optimization of these rate coefficients allows interpretation of the measured flame speeds as indirect rate coefficient measurements at flame relevant temperatures. Although results from simple A-factor optimization agree well with the present measurements, three-parameter optimization is shown to be necessary in order to accurately model kinetics across a wide temperature range, including high temperature flames and low temperature direct rate measurements. A-factor and three-parameter optimization both show that a reduced k₃/k₄ branching ratio over the temperature range from 1100 to 1700 K improves model predictions compared to present measurements.