Towards a lumped approach for solid plastic waste gasification: Polyethylene and polypropylene pyrolysis.

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.

Data Ecosystems for Scientific Experiments: Managing Combustion Experiments and Simulation Analyses in Chemical Engineering.

The development of scientific predictive models has been of great interest over the decades. A scientific model is capable of forecasting domain outcomes without the necessity of performing expensive experiments. In particular, in combustion kinetics, the model can help improving the combustion facilities and the fuel efficiency reducing the pollutants. At the same time, the amount of available scientific data has increased and helped speeding up the continuous cycle of model improvement and validation. This has also opened new opportunities for leveraging a large amount of data to support knowledge extraction. However, experiments are affected by several data quality problems since they are a collection of information over several decades of research, each characterized by different representation formats and reasons of uncertainty. In this context, it is necessary to develop an automatic data ecosystem capable of integrating heterogeneous information sources while maintaining a quality repository. We present an innovative approach to data quality management from the chemical engineering domain, based on an available prototype of a scientific framework, SciExpeM, which has been significantly extended. We identified a new methodology from the model development research process that systematically extracts knowledge from the experimental data and the predictive model. In the paper, we show how our general framework could support the model development process, and save precious research time also in other experimental domains with similar characteristics, i.e., managing numerical data from experiments.

Pyrolysis and Combustion Chemistry of Pyrrole, a Reference Component for Bio-oil Surrogates: Jet-Stirred Reactor Experiments and Kinetic Modeling.

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 (NO x ) directly from fuel-bound nitrogen (fuel NO x ), in addition to the thermal NO x 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.

Kinetic Modeling of the Ignition of Droplets of Fast Pyrolysis Bio-oil: Effect of Initial Diameter and Fuel Composition.

Fast biomass pyrolysis is an effective and promising process for high bio-oil yields, and represents one of the front-end technologies to provide alternative, sustainable fuels as a replacement of conventional, fossil-based ones. In this work, the effect of droplet initial diameter on the evaporation and ignition of droplets of crude fast pyrolysis bio-oil (FPBO) and FPBO/ethanol blend (50% vol) at ambient pressure is discussed. The experimental tests were carried out in a closed single droplet combustion chamber equipped with optical accesses, using droplets with a diameter in the range of 0.9-1.4 mm. The collected experimental data show a significant effect of droplet diameter and initial fuel composition on the evaporation and combustion of the droplets. At the same time, 1-dimensional modeling of the evaporation and ignition of different droplets of crude FPBO and its blend with ethanol is performed to understand the complex physical and chemical effects. To this purpose, an 8-component surrogate was adopted, and a skeletal mechanism (170 species and 2659 reactions) was obtained through an established methodology. The comparison of numerical and experimental results shows that the model is able to capture the main features related to the heating phase of the droplet and the effect of fuel composition on droplet temperature and evaporation, particularly the increased reactivity following ethanol addition and the variation of diameter with time. Also, a sensitivity analysis highlighted the reactions controlling the autoignition of the droplets in the different conditions. It was found that the autoignition of pure FPBO droplets is governed by dimethyl furane (DMF), because of its high volatility and in spite of not being the most abundant species. On the other side, ethanol chemistry drives the gas-phase ignition in the case of the blended (50/50 v/v) mixtures, due to its higher volatility and reactivity.

Can Small Polyaromatics Describe Their Larger Counterparts for Local Reactions? A Computational Study on the H-Abstraction Reaction by an H-Atom from Polyaromatics.

Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system.

Electronic structure-based rate rules for ipso addition-elimination reactions on mono-aromatic hydrocarbons with single and double OH/CH3/OCH3/CHO/C2H5 substituents: a systematic theoretical investigation.

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.

Ab initio calculations and kinetic modeling of thermal conversion of methyl chloride: implications for gasification of biomass.

Limitations in current hot gas cleaning methods for chlorine species from biomass gasification may be a challenge for end use such as gas turbines, engines, and fuel cells, all requiring very low levels of chlorine. During devolatilization of biomass, chlorine is released partly as methyl chloride. In the present work, the thermal conversion of CH3Cl under gasification conditions was investigated. A detailed chemical kinetic model for pyrolysis and oxidation of methyl chloride was developed and validated against selected experimental data from the literature. Key reactions of CH2Cl with O2 and C2H4 for which data are scarce were studied by ab initio methods. The model was used to analyze the fate of methyl chloride in gasification processes. The results indicate that CH3Cl emissions will be negligible for most gasification technologies, but could be a concern for fluidized bed gasifiers, in particular in low-temperature gasification. The present work illustrates how ab initio theory and chemical kinetic modeling can help to resolve emission issues for thermal processes in industrial scale.