Theoretical and kinetic modelling study of phenol and phenoxy radical decomposition to CO and C5H6/C5H5 in pyrolysis conditions

Bio-oils from biomass fast-pyrolysis are an economically viable solution to reduce carbon footprint [1]. Lignin-derived bio-oils are a complex mixture of oxygenated species, including phenolic compounds such as phenol, anisole, guaiacol, catechol and vanillin (20-30% in weight) [2]. Hence, an accurate characterization of the pyrolysis and combustion kinetics of phenolic species, starting from phenol, is essential to assess the technical viability of these biooils. Furthermore, phenol plays a key role in the mechanism of oxidation of benzene, a building block of PAHs chemistry, precursors of soot and PM [3]. Finally, substituted phenolic species have recently gained attention for their antiknock properties and are being considered as possible octane boosters [4]. Nevertheless, the kinetics of phenol has not been systematically addressed yet, and the available experimental data are limited. Therefore, a theoretical approach for the prediction of accurate kinetics provides a major contribution to improve the current knowledge. This work investigates with ab initio methods the two main decomposition pathways of phenol: 1) the molecular pathway forming C5H6+CO, and 2) the radical pathway forming C6H5O+H. This latter pathway justifies the additional investigation of the decomposition of phenoxy radical (C6H5O) to CO and cyclopentadienyl (C5H5). For a consistent investigation of phenol kinetics, also the H-abstraction reactions from cyclopentadiene are included. The kinetic constants thus obtained are included in the CRECK kinetic model and validated with experimental data

Coupling chemical lumping to data-driven optimization for the kinetic modeling of dimethoxymethane (DMM) combustion

The kinetic mechanisms describing the combustion of longer-chain fuels often have limited applicability due to the high number of species involved in their oxidation and decomposition paths. This work proposes a combined methodology for developing compact but accurate kinetic mechanisms of these fuels and applies it to dimethoxymethane (DMM), or oxymethylene ether 1 (OME1). An automatic chemical lumping procedure, performed by grouping structural isomers into pseudospecies, was proposed and applied to a detailed kinetic model of DMM pyrolysis and oxidation, built from state-of-the-art kinetic sub-models. Such a methodology proved particularly efficient in delivering a compact kinetic mechanism, requiring only 11 species instead of 35 to describe DMM sub-chemistry. The obtained lumped kinetic model was then improved through a data-driven optimization procedure, targeting data artificially generated by the reference detailed mechanism. The optimization was performed on the physically-constrained parameters of the modified-Arrhenius rate constants of the controlling reaction steps, identified via local sensitivity analyses. The dissimilarities between the predictions of the detailed and lumped models were minimized using a Curve Matching objective function for a comprehensive and quantitative characterization. Above all, the optimized mechanism was found to behave comparably to the starting detailed one, throughout most of the operating space and target properties (ignition delay times in shock tubes, laminar flame speeds, and speciations in stirred and flow reactors). The successful application of the proposed methodology to the DMM chemistry paves the way for its extensive use in the kinetic modeling of longer OMEs as well as heavier fuels, for which the computational advantages are expected to be even higher

Towards a better understanding of the combustion of oxygenated aromatic hydrocarbons. Comparing benzene, toluene, phenol and anisole with ignition delay times in a rapid compression machine

Ignition delay times (IDTs) of the oxygenated aromatic hydrocarbons (OAHCs) anisole (C6H5OCH3) and phenol (C6H5OH) and the analogues non-oxygenated aromatic hydrocarbons (AHCs) toluene (C6H5CH3) and benzene (C6H6) have been measured in the PCFC rapid compression machine (RCM) at stoichiometric, fuel-in-air conditions. With the two targeted compression pressures () of 1 and 2 MPa a temperature range of 870 to 1100 K was covered. The IDTs of all four molecules revealed an Arrhenius behavior. The different reactivity can be ranked as the following, starting with the lowest reactivity: benzene < toluene < phenol < anisole. Literature available models containing anisole and phenol have been used to simulate the IDTs of this study highlighting discrepancies in both, model to experiment and model to model accordance. Finally, the CRECK mechanism was used to conduct rate-of-production (ROP) and sensitivity analysis to gain insight into the combustion of OAHCs and highlight interconnections and shortcomings of OAHCs

Experimental and Modeling Study of the Oxidation of Benzaldehyde

The gas-phase oxidation of benzaldehyde has been investigated in a jet-stirred reactor. Benzaldehyde is an aromatic aldehyde commonly considered in bio-oils surrogates or in the oxidation of fuels such as toluene. However, its oxidation has never been previously investigated experimentally and no product formation profiles were reported in the few pyrolysis studies. In this study 48 species, mainly CO, CO2 and phenol were detected using gas chromatography, which indicate a rapid formation of phenyl radicals. This was confirmed by a kinetic analysis performed using the current version of the CRECK kinetic model, in which reactions have been updated

Master equation lumping for multi-well potential energy surfaces: A bridge between ab initio based rate constant calculations and large kinetic mechanisms

Ab initio transition state theory-based master equation methodologies for the calculation of rate constants have gained enormous popularity in the past decades. Nevertheless, introducing these rate constants into large kinetic schemes is a non-trivial task when large potential energy surfaces (PESs) are investigated. To determine proper phenomenological rate constants it is in fact necessary to account for the formation of all the thermodynamically stable wells considered in the master equation (ME), even if most wells do not exhibit significant secondary reactivity. Moreover, reactions involving intermediates with lifetimes comparable to the rovibrational relaxation timescale can exhibit discontinuities both in the rate constants and in the number of thermodynamically stable wells across the investigated temperature and pressure ranges. In this work, we address these problems with a “master equation-based lumping” (MEL) approach specifically designed to process the output of ME calculations of multi-well PESs. Simple kinetic simulations allow identifying both intermediate wells with limited lifetime and isomers with similar reactivity. Then, equivalent rate constants for a smaller set of pseudospecies are derived so as to reproduce the kinetics of the detailed mechanism. Our methodology is independent of any experimental data or experience-based assumptions. The power of MEL is demonstrated with three case studies of increasing complexity, namely the PES for CH3COOH decomposition, and the portions of the C5H5OH and C10H10/C10H9 PESs accessed from C5H5 + OH and C5H5 + C5H5 recombination. This work constitutes the first systematic step addressing the robust integration of rate constants derived from ME simulations into global kinetic schemes and provides a useful approach for the entire chemical kinetics community filling the gap between detailed theoretical investigations of complex PESs and the development of detailed kinetic models.SCOPUS: ar.jDecretOANoAutActifinfo:eu-repo/semantics/publishe

An experimental, theoretical and kinetic-modeling study of hydrogen sulfide pyrolysis and oxidation

International audienceHydrogen sulfide chemistry has recently undergone a renewed interest due to the current energy transition, requiring a proper treatment of such impurities in the sources like shale gas or biogas. Moreover, the lower-temperature, diluted conditions considered nowadays for reducing pollutant emissions require a wider-range development and validation of the pyrolysis and oxidation mechanisms. In this work, this was addressed through an experimental campaign carried out in three reactor facilities, namely a jet-stirred reactor and two flow reactors. A wide range of operating conditions could thus be covered, in terms of equivalence ratios under lean conditions (0.018 ≤ Φ ≤ 0.5), temperatures (400 K ≤ T ≤ 2000 K) and residence times (0.1 s ≤ τ ≤ 2 s). The mole fractions of reactants (H2S, O2), products (SO2, H2O) and intermediates (H2) were measured. In parallel, a kinetic mechanism of H2S pyrolysis and oxidation was developed by including the latest available kinetic rates on sulfur pyrolysis and oxidation chemistry, which were added to a core H2/O2 module, previously validated. Such a mechanism included a re-evaluation of selected key reaction steps, identified via sensitivity analysis. Results showed a general agreement of the experimental measurements with predictions: in the case of pyrolysis, the thermal decomposition reaction (H2S+M=H2+S+M) was identified as the sole controlling step: a critical choice of the kinetic rate had to be made, due to the significant disagreement among the literature rates. Concerning oxidation, the H-abstraction from H2S by O2 was found to be the major bottleneck at the lowest temperatures, with HO2 becoming a key abstractor, too, under very lean conditions. At higher temperatures, a key role was played instead by the H-abstraction of H2S with S (H2S+S=SH+SH), acting in the reverse direction and providing S radicals, boosting the oxidation process