Search

In a circular economy perspective, solid plastic wastes (SPW) can become a valuable source of chemicals, energy vectors and fuels through pyrolysis, gasification, and partial oxidation technologies, but their modelling requires first the definition of suitable condensed phase pyrolysis mechanisms for each constituent. This work proposes a semi-detailed kinetic model for polyethylene (PE) and polypropylene (PP) pyrolysis based on the functional group approach implemented for polyvinylchloride (PVC) and biomass pyrolysis to consistently address mixture modelling. This approach distinguishes polymeric chains in High Molecular Weight species, represented through their chemical functionalities, and Low Molecular Weight species, described with accuracy comparable to literature detailed models, employing the reaction classes proposed in the scientific literature. Several validated lumping techniques are introduced to reduce the model computational cost, and the resulting liquid-phase model accounts for 74 species for PE and 126 species for PP. Model validation is carried out by an extensive comparison with experimental data proving the soundness of the approach and the model capability of predicting mass-loss and product distribution profiles with similar accuracy to more expensive detailed models from the literature. The proposed condensed phase approach can be extended to other polymers and coupled with other existing subsets in the CRECK kinetic framework (e.g., biomass, PVC) paving the way for unravelling mixture interactions and secondary cracking and/or gasification reactions. The model here proposed is a powerful tool to support design and optimization of SPW thermochemical recycling technologies.

This work presents a comprehensive kinetic modeling study of the pyrolysis and oxidation of chloromethane. Theoretical calculations were performed for the decomposition reaction CH3Cl + M = CH3 + Cl + M and for a set of relevant H‐abstraction reactions (CH3Cl + R, R = Cl, Ḣ, ȮH, HȮ2, O2, Ö, ĊH3, Ċ2H3, Ċ2H5; CH4 + Cl, C2H6 + Cl, C2H4 + Cl, C2H2 + Cl). Comparison with previous experimental or theoretical determinations, when available, proved the appropriateness of the adopted protocols and the accuracy of the calculated rate constants. Previously developed subsets for CH3Cl and HCl/Cl2 were updated with the theoretical rate constants and combined with the CRECK C1–C3 mechanism, yielding a kinetic mechanism consisting of 143 species and 2347 reactions. The validation was performed through comparison with all the pyrolysis and oxidation measurements available for pure CH3Cl and its mixtures with CH4, H2, and CO/H2O. The kinetic discussion highlights the role of key reaction steps that have been theoretically investigated in this work and in previous efforts and sheds light on additional reaction pathways needing better assessments.

The Power Sector is undergoing a rapid technological change with respect to implementation of low carbon technologies. The IEA Energy Outlook 2017 shows that the investments in Renewables for the first time are equal to those on the fossil sources. It is likely that the conventional gas turbines and internal combustion engines will need to be integrated in systems employing biofuels and/or CCUS (Carbon Capture Usage and Storage). Also, the European Union is moving rapidly towards low carbon technologies (i.e. Energy Efficiency, Smart Grids, Renewables and CCUS), see the Energy Union Strategy. This paper presents the basic for the design of CLC combustors to be coupled with gas turbines. Based on CFD modeling and detailed kinetics models.

Dimethyl ether (DME), a widely studied alternative fuel, is known to exhibit complex low- and high-temperature oxidation chemistry. It is also the smallest molecule in the families of symmetric ethers and oxymethylene ethers that receive attention as renewable fuels. Thanks to several studies performed in facilities such as shock tubes, jet-stirred reactors and flames, it can be assumed that the DME oxidation is well understood. However, DME oxidation in flow reactors has been addressed comparatively rarely, although this configuration presents an interesting system with influences of both kinetics and fluid dynamics on the reaction behavior. To examine the interplay of both influences and potential uncertainties resulting from such effects, DME oxidation was experimentally investigated over an extended range of conditions in a flow reactor equipped with mass-spectrometric analysis. Quantitative species profiles were obtained at near-atmospheric pressure in a temperature range of 40 0-110 0 K for three equivalence ratios phi (0.8, 1.0 and 1.2), and three flow rates at each stoichiometry. These nine different cases were first analyzed using a detailed chemical reaction mechanism with a Plug-Flow Reactor (PFR) model. In-depth examination of experimental and reaction model uncertainties led to updates in the reaction mechanism that were performed on the basis of most recent, reliable kinetic information. In spite of the good agreement of the PFR model with the experimental data at selected conditions, especially in the low temperature regime, substantial deviations in the reactivity and associated species profiles were noted in several cases, particularly for lean conditions at low flow rates and intermediate temperatures around and above 700 K. A two-dimensional (2D) computational fluid dynamics (CFD) model was therefore employed to characterize the reactive flow conditions more accurately. Significant contributions of fluid dynamics effects were observed in the cases that presented the most severe deviations, and overall good agreement within experimental uncertainty was obtained for the nine cases with the 2D simulations. With the aid of a mathematical curve matching procedure using a variety of recent, established kinetic mechanisms, it could be convincingly demonstrated under the current conditions that improvements in predictive modeling capability in the sensitive test cases were not a question of improved kinetics but were mainly achieved by considering the two-dimensional reactive flow. As a consequence, the present investigation can serve to alert the community to the potential major influences that might be neglected if standard PFR models are used to predict fuel oxidation without detailed analysis whether the conditions are suited to that approach. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The development of processes with near-zero emissions such as MILD, flameless, and OXY-fuel combustion are of great interest in various energy scenarios. The assessment and design of new burners to meet energy demands and pollution reduction strongly depends on an accurate description of the chemistry involved in the combustion process. The main outcome of this study is the collection and review of a vast amount of experimental data on MILD and OXY-fuel combustion of methane that have been reported in recent years, together with a thorough kinetic analysis to identify aspects of the mechanism that requires further revision. The CRECK core model presented here is developed upon the Aramco 2.0 mechanism [1], and further extends the validation objectives to MILD and OXY fuel combustion conditions. The aim of this work is not only the mechanism validation but also a better understanding of the combustion characteristics and critical reaction pathways in MILD, flameless, and OXY-fuel combustion., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; Benzaldehyde is an aromatic aldehyde commonly considered in bio-oil surrogate formulation, and an important intermediate in the oxidation of other aromatic reference fuels such as toluene. However, its oxidation has never been previously investigated experimentally and no product formation profiles were reported in the very limited pyrolysis studies available in the literature. In this study, the gas-phase oxidation of benzaldehyde was investigated in a jet-stirred reactor. 48 species were detected using gas chromatography, mainly CO, CO2 and phenol. The important formation of CO and phenol indicates 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 the reactions of phenyl radicals and oxygenated aromatic compounds have been updated.

The recent interest in bio-oils combustion and the key role of mono-aromatic hydrocarbons (MAHs) in existing kinetic frameworks, both in terms of poly-aromatic hydrocarbons growth and surrogate fuels formulation, motivates the current systematic theoretical investigation of one of the relevant reaction classes in MAHs pyrolysis and oxidation: ipso substitution by hydrogen. State-of-the-art theoretical methods and protocols implemented in automatized computational routines allowed to investigate 14 different potential energy surfaces involving MAHs with hydroxy and methyl single (phenol and toluene) and double (o-,m-,p-C6H4(OH)2, o-,m-,p-CH3C6H4OH, and o-,m-,p-C6H4(CH3)2) substituents, providing rate constants for direct implementation in existing kinetic models. The accuracy of the adopted theoretical method was validated by comparison of the computed rate constants with the available literature data. Systematic trends in energy barriers, pre-exponential factors, and temperature dependence of the Arrhenius parameters were found, encouraging the formulation of rate rules for Ḣ ipso substitutions on MAHs. The rules here proposed allow to extrapolate from a reference system the necessary activation energy and pre-exponential factor corrections for a large number of reactions from a limited set of electronic structure calculations. We were able to estimate rate constants for other 63 Ḣ ipso addition–elimination reactions on di-substituted MAHs, reporting in total 75 rate constants for Ḣ ipso substitution reactions o-,m-,p-R′C6H4R + Ḣ → C6H5R + Ṙ′, with R,R′ = OH/CH3/OCH3/CHO/C2H5, in the 300–2000 K range. Additional calculations performed for validation showed that the proposed rate rules are in excellent agreement with the rate constants calculated using the full computational protocol in the 500–2000 K range, generally with errors below 20%, increasing up to 40% in a few cases. The main results of this work are the successful application of automatized electronic structure calculations for the derivation of accurate rate constants for Ḣ ipso substitution reactions on MAHs, and an efficient and innovative approach for rate rules formulation for this reaction class.

Polycyclic aromatic hydrocarbons (PAHs) are known as soot precursors, but their formation/consumption is not fully understood. A recent comprehensive experimental study of premixed laminar ethylene flame [23] investigated the transition from gas-phase to soot particles. The complex fluid dynamics of this system is taken into account to compare model predictions with experimental measurements and thus further validate a detailed kinetic mechanism of soot formation. The relatively low inlet velocity and the large distance between the burner and the stagnation plate lead to significant influence of buoyancy, which requires a 2-D simulation. The observed constricted (necking) flame structure can be reproduced only using a comprehensive 2-D simulation, which includes buoyancy effects, radiative heat losses, and thermal diffusion. Predicted axial gaseous and PAH species profiles obtained from the CRECK mechanism are in good agreement with the measurements, especially even-carbon-number aromatics. Reasonable agreement of the predicted soot volume fraction profiles is also observed. Additionally, simulation results from different literature kinetic mechanisms are also discussed to highlight similarities and differences. The largest discrepancies among the predictions of the mechanisms are observed for phenylacetylene, a key-species representing the first building block of PAHs synthesis in flames. A comprehensive analysis of relevant physical sub-models is also carried out in 2-D simulations. Additionally, predicted profiles from 2-D and 1-D simulations are compared. Following the literature, a 1-D simulation with imposed mass flux from the 2-D model was carried out to account for buoyancy effects. This approach provides an axial predicted flame temperature profile similar to the 2-D case. However, the predicted mole fraction profiles are quite different, especially for hydrogen and aromatics species because of the failure in accounting for the interplay of enhanced diffusion due to Soret effect, flame stretch, and large radial velocities in the proximity of the stagnation plane., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; Fast-pyrolysis bio-oils (FPBOs) obtained from lignocellulosic biomass are gaining attention as sustainable fuels for various applications, including the transport sector and power production. A significant fraction of bio-oils is constituted by nitrogen-containing compounds (N fuels) that should be considered when developing surrogate models for FPBOs. Moreover, the content of N fuels in FPBOs is expected to strongly contribute to the production of nitrogen oxides (NOx) directly from fuel-bound nitrogen (fuel NOx), in addition to the thermal NOx formation pathways typical of high-temperature combustion conditions. This work investigates the pyrolysis and combustion chemistry of pyrrole (C4H5N), a candidate reference fuel component for FPBO surrogate models. Speciation measurements in an atmospheric pressure jet-stirred reactor have been performed for both pyrolysis and oxidation conditions. Pyrolysis experiments have been performed for 1% pyrrole/helium mixtures over the temperature range T = 925-1200 K. Oxidation experiments were carried out for 1% pyrrole/oxygen/helium mixtures at three equivalence ratios (φ = 0.5, 1.0, and 2.0) over the temperature range T = 700-1200 K. These new data significantly extend the number of experimental targets for kinetic model validation available at present for pyrrole combustion. After a thorough revision of previous theoretical and kinetic modeling studies, a preliminary kinetic model is developed and validated by means of comparison to new experimental data and those previously reported in the literature. The rate of production and sensitivity analyses highlight important pathways deserving further investigations for a better understanding of pyrrole and, more in general, N fuel combustion chemistry. A critical discussion on experimental challenges to be faced when dealing with pyrrole is also reported, encouraging further experimental investigation with advanced diagnostics.

Lipomas of the gastrointestinal (GI) tract are common, benign and usually present as innocuous findings. Larger ones (>2 cm in diameter), particularly those involving the ileum, may present with clinical symptoms such as abdominal pain (caused by intussusception) and iron deficiency anaemia (IDA) or obscure GI bleeding (OGIB) (caused by overlying mucosal ulceration); these cases warrant intervention and often end up being referred for surgery. We describe a minimally invasive endoscopic alternative to surgical resection for the management of these lesions. A 60-year-old man presented with recurrent, cramping abdominal pain and OGIB. A magnetic resonance enterography (MRE) revealed a 2.5 cm submucosal lesion in the distal ileum, in keeping with a large lipoma. In light of these findings and the clinical presentation, we perform a saline-immersion retrograde DBE under conscious sedation, for further evaluation and minimally invasive, definitive endotherapy. At DBE the lesion was identified at around 40 cm proximal to the ileocaecal valve. The endoscopic appearances revealed a 2.5 cm sessile submucosal, lumen-filling lesion with a positive ‘pillow sign’. Although the overlying mucosa appeared mostly unremarkable, a small, healed ulcer (which would account for the patient’s IDA and OGIB) was identified on the medial surface of the lesion. Endotherapy was deemed feasible and this was facilitated by the buoyancy properties provided by SITE. In order to reduce the risk of perforation and bleeding, an endoscopic loop ligating device was first deployed tightly at the base of the lesion. A ball-tip, needle-type endoscopic submucosal dissection (ESD) knife was then used to incise and unroof the lesion. This allowed for exposure and spontaneous extrusion of the lipomatous tissue (already under pressure from the loop-lighting device). Saline-immersion allowed for maintenance of a clear visual field, through avoidance of clouding of the endoscopic lens and flotation of extruded micelles of fatty tissue. A submucosal tattoo and a clip were placed as endoscopic and radiological markers, respectively. The procedure was performed under antibiotic cover. No significant immediate, early or late adverse events were encountered. Histopathological examination of retrieved tissue showed mature adipocytes with fibrofatty submucosal changes, in keeping with a submucosal lipoma. No dysplasia or sarcomatous transformation was identified. The patient’s symptoms have resolved completely post-endotherapy. Our case demonstrates the safety and usefulness of minimally invasive endotherapy of symptomatic large ileal lipomas. The combination of DBE with SITE-facilitated unroofing, after securing the lipoma’s base with a loop-ligation device, allows for safe, spontaneous extrusion of the benign lipomatous tissue and avoids the need for operative surgery in symptomatic patients.

Chemical recycling of solid plastic waste (SPW) is a paramount opportunity to reduce marine and land pollution and to enable the incorporation of the circular economy principle in today's society. In addition to more conscious behaviors and wiser product design (“design for recycling”), a key challenge is the identification of the leading recycling technologies, minimizing the global warming potential in an industrially relevant context. Chemical recycling technologies based on pyrolysis and gasification are leading the way because of their robustness and good economics, but an improved understanding of the chemistry and more innovative reactor designs are required to realize a potential reduction of greenhouse gas emissions of more than 100 million tonnes of CO2-eq., primarily by more efficient use of valuable natural resources. The feed flexibility of thermal processes supports the potential of pyrolysis and gasification, however, the strong variability in time and space of blending partners such as multiple and co-polymers, additives, and contaminants (such as inorganic materials) calls for accurate assessment through fundamental experiments and models. Such complex and variable mixtures are strongly sensitive to the process design and conditions: temperature, residence time, heating rates – severity, mixing level, heat and mass transfer strongly affect the thermal degradation of SPW and its selectivity to valuable products. A prerequisite in improving design and performance is the ability to model conversion profiles and product distributions based on accurate rate coefficients for the dominating reaction families established using first-principle derived transport and thermodynamic properties. These models should also help with the “design for recycling” strategy to increase recyclability, for example by identifying additives that make chemical recycling difficult. Fundamental experiments of increased quality (accuracy, integrity, validity, replicability, completeness) together with improved deterministic kinetic models, systematically developed according to the reaction classes and rate rules approach, provide insights to identify optimal process conditions. This will allow shedding some light upon the important pathways involved in the thermal degradation of the feedstock and the formation/disappearance of desired or unwanted products. In parallel, the intrinsic kinetics of the dominating elementary reaction steps should be determined with higher accuracy, moving beyond single step kinetics retrieved from thermogravimetric analysis experiments. Together with more accurate kinetic parameters, better models to account for heat and mass transfer limitations also need to be further developed, since plastic degradation involves at least three phases (solid, liquid, gas), whose interactions should be accounted for in a more rigorous way. Novel experimental approaches (e.g. detailed feedstock and product characterization using comprehensive chromatographic techniques and photoionization mass spectrometry) and available computational tools (e.g. kinetic Monte Carlo, liquid phase, and heterogeneous theoretical kinetics) are needed to tackle these problems and improve our fundamental understanding of chemical recycling of SPW.

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.

This work (and the companion paper, Part II) presents new experimental data for the combustion of n-C3–C6 alcohols (n-propanol, n-butanol, n-pentanol, n-hexanol) and a lumped kinetic model to describe their pyrolysis and oxidation. The kinetic subsets for alcohol pyrolysis and oxidation from the CRECK kinetic model have been systematically updated to describe the pyrolysis and high- and low-temperature oxidation of this series of fuels. Using the reaction class approach, the reference kinetic parameters have been determined based on experimental, theoretical, and kinetic modeling studies previously reported in the literature, providing a consistent set of rate rules that allow easy extension and good predictive capability. The modeling approach is based on the assumption of an alkane-like and alcohol-specific moiety for the alcohol fuel molecules. A thorough review and discussion of the information available in the literature supports the selection of the kinetic parameters that are then applied to the n-C3–C6 alcohol series and extended for further proof to describe n-octanol oxidation. Because of space limitations, the large amount of information, and the comprehensive character of this study, the manuscript has been divided into two parts. Part I describes the kinetic model as well as the lumping techniques and provides a synoptic synthesis of its wide range validation made possible also by newly obtained experimental data. These include speciation measurements performed in a jet-stirred reactor (p = 107 kPa, T = 550–1100 K, φ = 0.5, 1.0, 2.0) for n-butanol, n-pentanol, and n-hexanol and ignition delay times of ethanol, n-propanol, n-butanol, n-pentanol/air mixtures measured in a rapid compression machine at φ = 1.0, p = 10 and 30 bar, and T = 704–935 K. These data are presented and discussed in detail in Part II, together with detailed comparisons with model predictions and a deep kinetic discussion. This work provides new experimental targets that are useful for kinetic model development and validation (Part II), as well as an extensively validated kinetic model (Part I), which also contains subsets of other reference components for real fuels, thus allowing the assessment of combustion properties of new sustainable fuels and fuel mixtures. The authors at Politecnico di Milano and CNRS-Nancy acknowledge the financial support of IMPROOF project (H2020-IND-CE-2016-17/H2020-SPIRE-S016) European Union's Horizon 2020 research and innovation program (grant agreement no. 723706). The authors from NUI Galway acknowledge funding from Science Foundation Ireland (SFI) via project numbers 15/IA/3177 and 16/SP/3829. peer-reviewed 2021-10-27

The recent interest in bio-oils combustion and the key role of mono-aromatic hydrocarbons (MAHs) in existing kinetic frameworks, both in terms of poly-aromatic hydrocarbons growth and surrogate fuels formulation, motivates the current systematic theoretical investigation of one of the relevant reaction classes in MAHs pyrolysis and oxidation: ipso substitution by hydrogen. State-of-the-art theoretical methods and protocols implemented in automatized computational routines allowed to investigate 14 different potential energy surfaces involving MAHs with hydroxy and methyl single (phenol and toluene) and double (o-,m-,p-C6H4(OH)2, o-,m-,p-CH3C6H4OH, and o-,m-,p-C6H4(CH3)2) substituents, providing rate constants for direct implementation in existing kinetic models. The accuracy of the adopted theoretical method was validated by comparison of the computed rate constants with the available literature data. Systematic trends in energy barriers, pre-exponential factors, and temperature dependence of the Arrhenius parameters were found, encouraging the formulation of rate rules for ipso substitutions on MAHs. The rules here proposed allow to extrapolate from a reference system the necessary activation energy and pre-exponential factor corrections for a large number of reactions from a limited set of electronic structure calculations. We were able to estimate rate constants for other 63 ipso addition-elimination reactions on di-substituted MAHs, reporting in total 75 rate constants for ipso substitution reactions o-,m-,p-R'C6H4R + → C6H5R + ', with R,R' = OH/CH3/OCH3/CHO/C2H5, in the 300-2000 K range. Additional calculations performed for validation showed that the proposed rate rules are in excellent agreement with the rate constants calculated using the full computational protocol in the 500-2000 K range, generally with errors below 20%, increasing up to 40% in a few cases. The main results of this work are the successful application of automatized electronic structure calculations for the derivation of accurate rate constants for ipso substitution reactions on MAHs, and an efficient and innovative approach for rate rules formulation for this reaction class.

Laminar burning velocities of methylcyclohexane and toluene blended with ammonia have been determined using the heat flux method at atmospheric pressure and initial temperature of 338 K, over equivalence ratios ranging from 0.7 to 1.3 and ammonia blending fractions in the binary fuel mixtures from 0 to 90%. It was observed that the addition of ammonia to methylcyclohexane and toluene leads to a decrease in laminar burning velocity that is not proportional to the ammonia mole fraction. Such a burning velocity reduction is due to synergistic thermal and kinetic effects. In addition, ammonia has a slightly higher impact on the burning velocities of toluene due to fuel structure effects. The CRECK detailed kinetic model has been used to interpret the experimental measurements and minor modifications on methylcyclohexane, toluene, and methyl-phenoxy radical chemistry allowed even improved agreement. New experimental results have been compared with predictions of this refined kinetic mechanism. The model provided good predictions of the measurements capturing the effect of equivalence ratio and ammonia fraction ranges investigated. Finally, a mass fraction-based mixing rule was shown to be predictive for binary blends of NH3 with methane and several hydrocarbons typically used to formulate surrogates for practical fuels.

A priori rate predictions for gas phase reactions have undergone a gradual but dramatic transformation, with current predictions often rivaling the accuracy of the best available experimental data. The utility of such kinetic predictions would be greatly magnified if they could more readily be implemented for large numbers of systems. Here, we report the development of a new computational environment, namely, EStokTP, that reduces the human effort involved in the rate prediction for single channel reactions essentially to the specification of the methodology to be employed. The code can also be used to obtain all the necessary master equation building blocks for more complex reactions. In general, the prediction of rate constants involves two steps, with the first consisting of a set of electronic structure calculations and the second in the application of some form of kinetic solver, such as a transition state theory (TST)-based master equation solver. EStokTP provides a fully integrated treatment of both steps through calls to external codes to perform first the electronic structure and then the master equation calculations. It focuses on generating, extracting, and organizing the necessary structural properties from a sequence of calls to electronic structure codes, with robust automatic failure recovery options to limit human intervention. The code implements one or multidimensional hindered rotor treatments of internal torsional modes (with automated projection from the Hessian and with optional vibrationally adiabatic corrections), Eckart and multidimensional tunneling models (such as small curvature theory), and variational treatments (based on intrinsic reaction coordinate following). This focus on a robust implementation of high-level TST methods allows the code to be used in high accuracy studies of large sets of reactions, as illustrated here through sample studies of a few hundred reactions. At present, the following reaction types are implemented in EStokTP: abstraction, addition, isomerization, and beta-decomposition. Preliminary protocols for treating barrierless reactions and multiple-well and/or multiple-channel potential energy surfaces are also illustrated.

The mechanism of evolution of polycyclic aromatic hydrocarbons (PAHs) into carbonaceous particles in combustion, atmosphere, and interstellar space has been the subject of intense debate. Recently, there has been emerging evidence supporting resonantly-stabilized radicals as key players in PAH growth. In this work, we build on this hypothesis and propose that, beyond a critical size, PAH reactivity can be assimilated to that of radicals. We found that odd-C-numbered PAHs embedding 5-membered rings rapidly lose a hydrogen atom to form resonantly-stabilized radicals in combustion conditions, while even-C-numbered PAHs react as open-shell rather than closed-shell molecules independently of temperature, as usually assumed. Acenes were used as molecular models of large even-C-numbered PAHs. The construction of a kinetic model including these findings allows to interpret experimental soot oxidation data otherwise irreconcilable with existing chemical kinetic mechanisms.

Alkylated aromatics constitute a significant fraction of the components commonly found in commercial fuels. Toluene is typically considered as a reference fuel. Together with n-heptane and iso-octane, it allows for realistic emulations of the behavior of real fuels by the means of surrogate mixture formulations. Moreover, it is a key precursor for the formation of poly-aromatic hydrocarbons, which are of relevance to understanding soot growth and oxidation mechanisms. In this study the POLIMI kinetic model is first updated based on the literature and on recent kinetic modelling studies of toluene pyrolysis and oxidation. Then, important reaction pathways are investigated by means of high-level theoretical methods, thereby advancing the present knowledge on toluene oxidation. H-Abstraction reactions by OH, HO2, O and O2, and the reactivity on the multi well benzyl-oxygen (C6H5CH2 + O2) potential energy surface (PES) were investigated using electronic structure calculations, transition state theory in its conventional, variational, and variable reaction coordinate forms (VRC-TST), and master equation calculations. Exploration of the effect on POLIMI model performance of literature rate constants and of the present calculations provides valuable guidelines for implementation of the new rate parameters in existing toluene kinetic models.

Pursuing a sustainable energy scenario for transportation requires the blending of renewable oxygenated fuels such as alcohols into commercial hydrocarbon fuels. From a chemical kinetic perspective, this requires the accurate description of both hydrocarbon reference fuels (n-heptane, iso-octane, toluene, etc.) and oxygenated fuels chemistry. A recent systematic investigation of linear C-2-C-5 alcohols ignition in a rapid compression machine at p = 10-30 bar and T = 650-900 K has extended the scarcity of fundamental data at such conditions, allowing for a revision of the low temperature chemistry for alcohol fuels in the POLIMI mechanism. Heavier alcohols such as n-butanol and n-pentanol present ignition characteristic of interest for application in HCCI engines, due to the presence of the hydroxyl moiety reducing their low temperature reactivity compared to the parent linear alkanes (i.e. higher octane number). The promising performances of ethanol in a HCCI engine have been recently discussed by Bissoli et al. (Energy & Fuels, 2017, Submitted), observing wider stable operability conditions in terms of fuel/air load (lambda) and exhaust gas recirculation (EGR) extent compared to PRF80 and PRF100. The aim of this study is to present briefly the reliability of the updated POLIMI mechanism for heavier alcohols and to investigate the fundamental role of chemical kinetics on the performance maps of HCCI engines fueled with n-butanol and n-pentanol, in terms of operability limits and engine efficiency. peer-reviewed

The reactivity of phenol is of utmost importance in combustion systems. In fact, phenol is the simplest phenolic compound, abundant in bio-oils derived from biomass fast pyrolysis and therefore included as a reference component in bio-oil surrogate mixtures. Phenol is also relevant to the mechanism of oxidation of benzene, a building block in the growth of polycyclic aromatic hydrocarbons (PAHs), precursors of soot formation. Hence, in a modular and hierarchical approach to combustion chemistry, the knowledge of the pyrolysis and combustion kinetics of phenol is essential to characterize the reactivity and the combustion properties of bio-oils and mono aromatic hydrocarbons (MAHs), as well as to improve the understanding of PAHs and soot formation. Although the reaction pathways of phenol decomposition are well defined in the literature, the rate constants still require more accurate assessment, and a validation of the reaction mechanism of phenol pyrolysis against the full set of experimental data available is still missing. In this work, we compute with the ab initio transition state theory based master equation (AI-TST-ME) method the rate constants of the main reaction pathways of phenol decomposition, also relevant to benzene oxidation. In particular, we investigate phenol molecular decomposition to C5H6 + CO and its competition with the O–H bond fission and H-atom abstraction reactions by H to form the phenoxy radical (C6H5O). We also investigate the successive decomposition of C6H5O to C5H5 + CO and the H-atom abstraction reaction on C5H6 by H, which plays a pivotal role in controlling the H concentration in phenol pyrolysis and combustion. The calculated rate constants are found to be in good agreement with experimental values. The CRECK kinetic model is updated with the new rate constants and validated against the available experimental data of phenol pyrolysis providing, to our knowledge, the first comprehensive validation of phenol decomposition kinetics. However, discrepancies are still present in the profiles of products formed from secondary reactivity. Our analysis suggests that further investigation of the reactivity of C5H6 is required, providing guidelines for a more accurate characterization of the decomposition to smaller species.

This work presents new experimental data for n-C3–C6 alcohol, combustion (n-propanol, n-butanol, n-pentanol, n-hexanol). Speciation measurements have been carried out in a jet-stirred reactor (p = 107 kPa, T = 550–1100 K, φ = 0.5, 1.0, 2.0) for n-butanol, n-pentanol, and n-hexanol. Ignition delay times of ethanol, n-propanol, n-butanol, and n-pentanol/air mixtures were measured in a rapid compression machine at φ = 1.0, p = 10 and 30 bar, and T = 704–935 K. The kinetic subsets for alcohol pyrolysis and oxidation from the CRECK kinetic model have been systematically updated to describe the pyrolysis and high- and low-temperature oxidation of this series of fuels as described in Part I of this work (Pelucchi, M.; Namysl, S.; Ranzi, E. Combustion of n-C3–C6 linear alcohol: an experimental and kinetic modeling study. Part I: reaction classes, rate rules, model lumping and validation. Submitted to Energy and Fuels, 2020). Part II describes in detail the facilities used for this systematic experimental investigation of n-C3–C6 alcohol combustion and presents a complete validation of the kinetic model by means of comparisons with the new data and measurements previously reported in the literature for both pyrolytic and oxidative conditions. Kinetic analyses such as rate of production and sensitivity analyses are used to highlight the governing reaction pathways and reasons for existing deviations, motivating possible further improvements in our chemistry mechanism. The authors at Politecnico di Milano and CNRS-Nancy acknowledge the financial support of IMPROOF project (H2020-IND-CE-2016-17/H2020-SPIRE-S016) European Union's Horizon 2020 research and innovation. program (grant agreement no. 723706). The authors at NUI Galway wish to acknowledge funding from Science Foundation Ireland (SFI) via project numbers 15/IA/3177 and 16/SP/3829. peer-reviewed 2021-10-27