Validation of a thermal decomposition mechanism of formaldehyde by detection of CH₂O and HCO behind shock waves

Abstract

The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH 2 O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH 2 O + M → HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k 1a = 5.0 × 10 15 exp(−308 kJ mol −1 /RT) cm 3 mol −1 s −1 . The pressure dependence, the rate of the competing molecular channel (1b), CH 2 O + M → H 2 + CO + M, and the branching fraction β = k 1a /(k 1a + k 1b ) was characterized by a two-channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1-50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH 2 O, CO, and H atom measurements at temperatures of 1200-3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self-reaction of two CH 2 O molecules. At high initial CH 2 O mole fractions the reverse of reaction (6), CH 2 OH + HCO CH 2 O + CH 2 O becomes noticeable. The rate of the forward reaction was roughly measured to be k 6 = 1.5 × 10 13 cm 3 mol −1 s −1 .

A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation

“…Finally, the reaction of methoxy and hydroxyl-methyl radicals with formyl radicals forming methanol and carbon monoxide molecules has been included based on the work of Friedrichs et al [38] and the estimation of reaction of two radical species (assigning a pre-exponential Afactor of 1.0 × 10 13 cm 3 mol -1 s -1 , respectively. …”

High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane

“…It is formed during the oxidation of alkanes, natural gas, alcohols, aldehydes, ethers and others. [1][2][3] Its importance in combustion led to a number of high temperature kinetic studies for its pyrolytic dissociation [3][4][5][6] and bimolecular reactions. 1,[7][8][9][10] A substantial number of earlier studies 5,7,9,[11][12][13][14][15][16] consisted of utilizing 1,3,5-trioxane (C 3 H 6 O 3 ) as a clean thermolytic source of formaldehyde to avoid the problem of formaldehyde polymerization.…”

Direct measurements of the reaction OH + CH2O → HCO + H2O at high temperatures

“…HCO + O 2 = HO 2 + CO (4) Reactions (1) and (3) were recently measured in our laboratory [1,3,4]. Here, we describe direct high-temperature measurements of reaction (2), CH 2 O + OH = HCO + H 2 O.…”

“…1, which presents a rate of production (ROP) analysis for the formyl radical in the stoichiometric combustion of methane at 1800 K Figure 1 HCO rate of production analysis: 1% CH 4 , 4% O 2 , 1800 K, 1.2 atm. and 1.2 atm.…”