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

Detailed kinetics of pyrolysis and combustion of catechol and guaiacol, as reference components of bio-Oil from biomass

Fast biomass pyrolysis is an effective process to produce bio-oils thus allowing to partially replace nonrenewable fossil fuels. Bio-oils are complex mixtures with a great amount of large oxygenated organic species, such as substituted phenolic components. Although experimental and kinetic modeling studies of phenol and anisole pyrolysis and combustion are available in the literature, only a minor attention has been devoted to kinetic mechanisms of substituted phenolic species, such as catechol and guaiacol. Multiple substitutions on aromatic ring can originate proximity effects and thus significantly modify bond energies, consequently affecting reaction pathways. Careful evaluations of bond dissociation energies and reference kinetic parameters, based on theoretical computations, are first performed. Guaiacol and catechol pyrolysis and combustion reactions are then compared with the corresponding phenol and anisole mechanisms. This kinetic study allows to identify some preliminary rate rules useful to validate a detailed kinetic mechanism of bio-oil pyrolysis and combustion

Evaluation of Polycyclic Aromatic Hydrocarbon Formation in Counterflow Diffusion Flames

Polycyclic aromatic hydrocarbons (PAHs) have been heralded as mutagenic and carcinogenic substances and currently, their emissions are subject to regulatory control due to recently imposed stricter environmental regulations. Hence, it has become necessary to have a detailed understanding of their chemistry. In this work, a short review of the available PAH relevant counterflow diffusion flame datasets is presented. Following that, the reliability of four widely used PAH mechanisms and the revised PAH mechanism, within the scope of this work, is assessed by validating them against these collected experimental datasets. The formation of the first aromatic ring is investigated based on the performed reaction path analyses. The results show that the dominant reaction pathways for the formation of benzene are “even carbon atom” pathways (H-abstraction acetylene addition) and “odd carbon atom” pathways (recombination of propargyl radicals). The dominance of one pathway over the other was found to be strongly dependent on the fuel structure and its doping with other components

Analysis of acetic acid gas phase reactivity: Rate constant estimation and kinetic simulations

The gas phase reactivity of acetic acid was investigated combining first principle calculations with kinetic simulations. Rate constants for the unimolecular decomposition of acetic acid were determined integrating the 1D master equation over a Potential Energy Surface (PES) investigated at the M06-2X/aug-cc-pVTZ level. Energies were computed at the CCSD(T)/aug-cc-pVTZ level using a basis set size correction factor determined at the DF-MP2/aug-cc-pVQZ level. Three decomposition channels were considered: CO2+ CH4, CH2CO + H2O, and CH3+ COOH. Rate constants were computed in the 700-2100 K and 0.1-100 atm temperature and pressure ranges. The simulations show that the reaction is in fall off above 1200 K at pressures smaller than 10 atm. Successively, the PESs for acetic acid H-abstraction by H, OH, OOH, O2, and CH3were investigated at the same level of theory. Rate constants were computed accounting explicitly for the formation of entrance and exit van der Waals wells and their collisional stabilization. Energy barriers were determined at the CASPT2 level for H-abstraction by OH of the acidic H, since it has a strong multireference character. The calculated rate constant is in good agreement with experiments and supports the experimental finding that at low temperatures it is pressure dependent. The calculated rate constants were used to update the POLIMI kinetic model and to simulate the pyrolysis and combustion of acetic acid. It was found that acetic acid decomposition and the formation of its direct decomposition products can be reasonably predicted. The formation of secondary products, such as H2and C2hydrocarbons, is underpredicted. This suggests that reaction routes not incorporated in the model may be active. Some hypotheses are formulated on which these may be

A first evaluation of butanoic and pentanoic acid oxidation kinetics

International audienceDespite the recent interest in large carboxylic acid oxidation due to their presence in pyrolysis bio-oils, their kinetics of pyrolysis and oxidation has not been experimentally addressed. For the first time, this paper reports a new set of experimental data for the oxidation in a jet-stirred reactor of two high molecular weight carboxylic acids: butanoic (butyric) and pentanoic (valeric) acids. This work was performed at 106.7 kPa (800 Torr) over a range of temperatures from 800 to 1100 K. The experiments were carried out under highly diluted conditions (inlet fuel mole fraction of 0.005) for three equivalence ratios: 0.5, 1 and 2. During this study a wide range of products has been identified and quantified from CO and CO2 to C5 species: 36 for pentanoic acid and 18 for butanoic acid. An interpretative kinetic model has been developed based on a recent theoretical study on the pyrolysis and oxidation of acetic acid (Cavalotti et al. PROCI, 37 (2019) 539-546) and on alkane rate rules (Ranzi et al. Combust. Flame, 162 (2015) 1679-1691). This new kinetic subset has been implemented in the CRECK kinetic framework covering the pyrolysis and oxidation of molecules from syngas up to heavy fuels, including PAHs formation. The mole fractions of fuel and product species were compared with results from model simulations over the experimental temperature range, providing reasonable agreement. A flow rate analysis allowed a better understanding of the most important degradation pathways of these acids, including a small contribution of low-temperature oxidation channels

Experimental and modelling study of the oxidation of methane doped with ammonia

The oxidation of methane doped with ammonia was experimentally and theoretically studied in order to better understand the interactions between these two molecules in combustion processes fed with biogas. Experiments were carried out in a jet-stirred reactor. Several diagnostics were used to quantify reaction products: gas chromatograph for carbon containing species, a NOx analyzer for NO and NO2, and continuous-wave cavity ring-down spectroscopy for ammonia. Experimental data were satisfactorily compared with data computed using a model developed by Politecnico di Milano

Mechanism Comparison for PAH Formation in Pyrolysis and Laminar Premixed Flames

Polycyclic aromatic hydrocarbons (PAHs) are known precursors of harmful carbonaceous particles. Accurate predictions of soot formations strongly rely on accurate predictions of PAHs chemistry. This work addresses the detailed kinetic modeling of PAH formation using two models: CRECK [8] and ITV [12], aiming to compare the model predictions with experimental data in olefin pyrolysis and laminar premixed flames. The two kinetic mechanisms are validated and compared highlighting similarities and differences in PAHs formation pathways. The validation highlights the critical role of resonance-stabilized radicals leading to the PAH formation

Experimental and kinetic modeling study of pyrolysis and combustion of anisole

open7siFast biomass pyrolysis is an effective process with high yields of bio-oil, and is a promising technology to partially replace non-renewable fossil fuels. Bio-oils are complex mixtures with a large amount of oxygenated organic species, such as esters, ethers, aldehydes, ketones, carboxylic acids, alcohols, and substituted aromatic components. Anisole is a simple surrogate of primary tar from lignin pyrolysis and it is very useful to investigate gas-phase reactions of methoxy-phenol species, expected precursors of poly-cyclic aromatic hydrocarbons (PAH) and soot during biomass pyrolysis and bio-oil combustion. This work first presents new pyrolysis data obtained in the Ghent flow reactor, and then it discusses a detailed kinetic mechanism of anisole pyrolysis and oxidation. This scheme is further validated and compared, not only with these pyrolysis data, but also with recently published data of anisole oxidation in jet stirred reactors. Ignition delay time and laminar flame speed computations complement these detailed comparisons. This kinetic mechanism is a first step and places the basis towards a successive model extension to catechol, guaiacol, and vanillin, as representative phenolic components of bio-oil from biomass.openPelucchi, Matteo*; Faravelli, Tiziano; Frassoldati, Alessio; Ranzi, Eliseo; SriBala, Gorugantu; Marin, Guy B.; Van Geem, Kevin M.Pelucchi, Matteo; Faravelli, Tiziano; Frassoldati, Alessio; Ranzi, Eliseo; Sribala, Gorugantu; Marin, Guy B.; Van Geem, Kevin M

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