Exploring the chemical kinetics of partially oxidized intermediates by combining experiments, theory, and kinetic modeling

Abstract

Partially oxidized intermediates play a central role in combustion and atmospheric chemistry. In this perspective, we focus on the chemical kinetics of alkoxy radicals, peroxy radicals, and Criegee intermediates, which are key species in both combustion and atmospheric environments. These reactive intermediates feature a broad spectrum of chemical diversity. Their reactivity is central to our understanding of how volatile organic compounds are degraded in the atmosphere and converted into secondary organic aerosol. Moreover, they sensitively determine ignition timing in internal combustion engines. The intention of this perspective article is to provide the reader with information about the general mechanisms of reactions initiated by addition of atomic and molecular oxygen to alkyl radicals and ozone to alkenes. We will focus on critical branching points in the subsequent reaction mechanisms and discuss them from a consistent point of view. As a first example of our integrated approach, we will show how experiment, theory, and kinetic modeling have been successfully combined in the first infrared detection of Criegee intermediates during the gas phase ozonolysis. As a second example, we will examine the ignition timing of n-heptane/air mixtures at low and intermediate temperatures. Here, we present a reduced, fuel size independent kinetic model of the complex chemistry initiated by peroxy radicals that has been successfully applied to simulate standard n-heptane combustion experiments.

Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides

“…The compilation strategy for our new "nested" mechanism is along the lines we have established for C1-C8 hydrocarbon fuels in the past fifteen years (e.g. [22][23][24][25][26][27] ). The idea is to merge existing, well tested models and adapt the resulting mechanism to simulations comprising a much broader target range than all previous studies.…”

Combustion in the future: The importance of chemistry

“…Mechanism development can also become difficult regarding abundant structural diversity, as, e.g., in low-temperature oxidation of larger alkanes [343] . On the one hand, lumping all structurally similar isomers can lead to oversimplification while on the other, including all isomers generates very large mechanisms that may become impractical regarding computational and memory resources [343] . For example, internal H-atom abstraction produces 18 QOOH isomers (see also Section 2.2 ) for n -heptane, and 60 QOOH isomers for n -hexadecane [343] .…”

“…On the one hand, lumping all structurally similar isomers can lead to oversimplification while on the other, including all isomers generates very large mechanisms that may become impractical regarding computational and memory resources [343] . For example, internal H-atom abstraction produces 18 QOOH isomers (see also Section 2.2 ) for n -heptane, and 60 QOOH isomers for n -hexadecane [343] . To reduce the number of species for subsequent reactions, structural isomers were assigned to different lumped groups, e.g.…”

“…To reduce the number of species for subsequent reactions, structural isomers were assigned to different lumped groups, e.g. , by similar geometrical features or functional groups at primary or secondary sites [343] . From consistent, detailed mechanisms, using predictive chemical kinetics approaches [344] and systematic reduction and optimization procedures [345] , [346] , [347] , [348] , kinetic schemes may be derived for fuel design and engine applications [349] , [350] , [351] , [352] , considering also uncertainties of the involved kinetics expressions [348 , [353] , [354] , [355] .…”

Insights into nitromethane combustion from detailed kinetic modeling – Pyrolysis experiments in jet-stirred and flow reactors

“…The present study is conducted in a similar manner. This makes the kinetic model more robust and reliable for combustion modeling under engine conditions (see section on n-heptane spray ignition in [33]).…”