Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene

Pinenes and pinene dimers have similar energy densities to petroleum-based fuels and are regarded as alternative fuels. The pyrolysis of the pinenes is well studied, but information on their combustion kinetics is limited. Three stoichiometric, flat premixed flames of the C 10 H 16 monoterpenes alpha-pinene, beta-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomer-resolved identification and quantification of the flame species formed during the combustion process. Flame-sampling molecular-beam mass spectrometry even enables the detection of very reactive radical species. Myrcene was chosen because of its known formation during beta-pinene pyrolysis. The quantitative speciation data and the discussed decomposition steps of the fuels provide important information for the development of future chemical kinetic reaction mechanisms concerning pinene combustion. The main decomposition of myrcene starts with the unimolecular cleavage of the carbon-carbon single bond between the two allylic carbon atoms. Further decompositions by beta-scission to stable combustion intermediates such as isoprene (C 5 H 8 ), 1,2,3-butatriene (C 4 H 4 ) or allene (aC 3 H 4 ) are consistent with the observed species pool. Concentrations of all detected hydrocarbons in the beta-pinene flame are closer to the myrcene flame than to the alpha-pinene flame. These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during beta-pinene combustion indicate that beta-pinene undergoes isomerization to myrcene under the studied flame conditions. Aromatic species such as phenylacetylene (C 8 H 6 ), styrene (C 8 H 8 ), xylenes (C 8 H 10 ), and indene (C 9 H 8 ) could be clearly identified and have higher concentrations in the alpha-pinene flame. Consequently, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes alpha-pinene, beta-pinene, and myrcene give insights into their combustion kinetics. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.