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

Abstract

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 α-pinene, β-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomerresolved 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 β-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 βscission 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 β-pinene flame are closer to the myrcene flame than to the α-pinene flame.These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during β-pinene combustion indicate that β-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 αpinene flame. Conclusively, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes α-pinene, β-pinene, and myrcene give insights into their combustion kinetics.

Sooting tendencies of terpenes and hydrogenated terpenes as sustainable transportation biofuels

“…This hypothesis is also consistent with our hexadiene study [48], where 1,5-hexadiene produced relatively low concentrations of aromatics but high concentrations of C3 species. It is also consistent with the flame-flame molecular-beam MS (MBMS) study of Bierkandt et al [49], where MC produced relatively large amounts of C5 species.…”

Fundamental Insights into the Effect of Blending Hydrogen Flames with Sooting Biofuels

“…The CH 2 O ROP is an order of magnitude greater than the A 2 ROP, which further inhibits the PAH formation in these flames. The computational analysis also reveals that the reaction pathways of naphthalene formation in the vapourised toluene and anisole blended H which is lower than 438 kJ•mol −1 for the primary C-H bond [68]. Therefore, hydrogen abstraction is more likely to take place at the allylic carbon atom level in D-limonene, which subsequently promotes the growth of PAH via the HACA mechanism.…”

Photoelectron Photoion Coincidence Spectroscopy Provides Mechanistic Insights in Fuel Synthesis and Conversion

“…The first step of bimolecular decomposition and combustion processes is mostly the abstraction of atomic hydrogen by small radicals, such as O, OH, or H, forming a fuel radical. In some fuels, namely, 1-hexene 137 or the acyclic monoterpene myrcene (7-methyl-3-methylideneocta-1,6diene), 138 the initial decomposition is initiated by a unimolecular dissociation of a C−C bond. While in a hightemperature environment, H-abstraction is followed by βscission to form smaller species, for example, alkenes and alkyl radicals in the case of alkane fuels; O 2 is added to the fuel radical at low temperatures forming the alkylperoxy radical as shown in Figure 13.…”