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

Experimental and modeling study of single coal particle combustion in O2/N2 and Oxy-fuel (O2/CO2) atmospheres

Coal particle combustion experiments were performed in a drop tube furnace (DTF) with oxygen concentration from 21% to 100%, in N2 and CO2 mixtures, under quiescent flow conditions. Small particles (75–90 micron) of a high-volatile bituminous coal (PSOC-1451) and a lignite coal (DECS-11) are analyzed with particular attention to the particle burnout times and the particle surface temperatures. These experimental measurements are compared with the predictions of a comprehensive model of coal combustion. Combustion of coal particles is a multi-scale process where both chemical and physical phenomena are involved, thus it requires a coupled and accurate description of the kinetics as well as of the heat and mass transport phenomena. Important features of the model are a multistep kinetic scheme of coal volatilization and detailed kinetics of the successive gas-phase reactions and of the heterogeneous reactions of both char oxidation and gasification. The achieved overall agreement between the experimental data and the numerical predictions, in terms of particle temperature and burnout times, highlights the capability of the model to simulate the effect of different operating conditions in the coal combustion processes

Chemical Composition of Submicrometer Particulate Matter (PM1) Emitted from Combustion of Coals of Various Ranks in O-2/N-2 and O-2/CO2 Environments

A laboratory-scale investigation has been conducted on the physical and chemical characteristics of particulate matter emissions (ashes) from pulverized coals burning in the air or in simulated oxy-fuel environments. Oxy-fuel combustion is a process that takes place in O-2/CO2 gases, using an air separation unit (ASU) to supply the oxygen and a flue-gas recirculation (FGR) stream to supply the carbon dioxide to the boiler. In order to investigate the effects of the background gas on the particulate matter generated by the combustion of coals of different ranks, a bituminous, a sub-bituminous, and a lignite coal were burned in an electrically heated laminar-flow drop-tube furnace (DTF) in both O-2/N-2 and O-2/CO2 environments (21% < O-2 < 6096). A recent publication by the authors reports on the physical characteristics of the particulate matter; hence, this work focuses on the chemical composition, specifically targeting the difficult-to-capture submicrometer size (PM1) ashes. Particulate matter was collected by a low-pressure multistage cascade impactor and was analyzed for chemical composition by Scanning Electron Microscopy Energy Dispersive X-ray Spectroscopy (SEM-EDS). Selected samples were also examined by Electron Microprobe Analysis (EMA). Results showed that submicrometer (PM1) ashes of the bituminous, the sub-bituminous, and the lignite coals contained mostly Si, Al, Fe, Mg, Ca, K, Na, and S. Prominent components of large submicrometer particle (PM0.56-1) compositions were Si and Al (Ca in sub-bituminous), whereas small submicrometer particles (PM0.1-0.18) were markedly enriched in S. The mass yields of elemental species found in the submicrometer-size particles from all three coals were lower when combustion occurred in CO2, instead of N-2 background gases. The chemical composition of the PM0.56-1 subcategory was not affected by the background gas. To the contrary, the composition of the PM0.1-0.18 subcategory was affected by replacing N-2 with CO2, and mass fractions of Si, Ca, and Al decreased whereas Na, K, and S increased. Furthermore, in PM0.1-0.18, when the O-2 mole fraction increased in either N-2 or CO2, the mass fractions of Si, Ca, and Al increased at the expense mostly of Na, K, and S, but also Fe in the case of the sub-bituminous coal. Experimentally derived partial pressures of the volatile suboxide SiO (P-SiO) at the char surface were compared with the predictions of an ash vaporization model without and with coupling with a particle combustion model; they were found to be in the range of the model predictions

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

CFD Analysis of the Channel Shape Effect in Monolith Catalysts for the CH4 Partial Oxidation on Rh

Catalysts monoliths with circular and square ducts are theoretically analyzed in detail as reactor configurations for the adiabatic CH4 partial oxidation on Rh for short-contact-times hydrogen production. By the means of CFD coupled with a detailed microkinetic description of the surface reactivity, it was found that the different transport properties of the investigated configurations primarily affect the thermal behavior of the reactor. O2 consumption is fully external mass transfer limited, and thus, local variations in mass transport properties are responsible of the differences in surface temperature