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 semiempirical 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.
Pyrolysis of solid
fuels forms a solid carbon-rich fuel, also called
char, whose physico-chemical description is rather complex. Heterogeneous
oxidation reactions take place during thermochemical conversion of
char. The present work proposes a predictive detailed kinetic model,
opening a new path for a deeper understanding of the char conversion
process. This model considers porosity, surface area, density of surface
sites, and their evolution along the conversion process. The chemical
aspects of char oxidation are modeled assuming a carbonaceous bulk
structure, surrounded by a variety of surface sites which represent
the chemical functionalities typically present in such materials.
The heterogeneous chemical reactions and their kinetic parameters
are defined based on previous studies in the literature and by analogy
to homogeneous gas-phase reactions of aromatic species. A mathematical
framework is proposed to couple physical and chemical descriptions
of the oxidation process. Although the proposed model benefits from
experimental information, it is able to comprehensively describe the
conversion rate of a broad range of carbonaceous materials such as
carbon nanotubes, graphite, and chars only on the basis of their elemental
composition. The proposed model represents a first step in exploring
the explicit and coupled treatment given to the physical and chemical
evolution of the fuel throughout its conversion, allowing us to consistently
describe the particle evolution, opening a path for reliable models
to manage the chemistry of char conversion.
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