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Development of a hierarchically detailed chemical reaction mechanism from C₃ to C₅ hydrocarbons
(2022)
The oxidation of fuel molecules can be described by using a reaction mechanism, a tool that combines thermodynamic and transport properties with reaction rates to predict the behavior and sub-products at different temperatures, pressures and equivalence ratios. A detailed reaction mechanism helps to understand the fuel-specific pollutant formation process. The aim of this doctoral thesis is to generate a hierarchically-detailed chemical reaction mechanism from C3 to C5 hydrocarbons that can be used to understand the reaction decomposition pathways for different fuels at high temperature regime, e.g. propene, propane, butane isomers, butene isomers and pentene isomers. A new nomenclature based in the IUPAC rules, has been developed and implemented as part of this work. The naming follows the order of priority for choosing a principal characteristic group. These naming rules and some examples are explained here. As starting point for this investigation, the chemical model presented in Schenk et al. (2013) has been used. Thermodynamic data for sensitive species from C3 chemistry were revised and updated. Updates in reaction rates for n-butane (C4H10) and iso-butane (C4H10-Me2) are shown. The chemistry of the butene (C4H8) isomers have been revised and a correction taking into account the H-atom allyl abstraction is implemented. Laminar flame speeds and ignition delay times for the different isomers are presented and discussed together with experiments in similar conditions for burner-stabilized flame for the three butene and butane isomers.
The high-temperature chemistry for branched and linear C5H10 species is implemented in the model. 2-Methyl-2-butene (C5H10-D2Me2) is the most interesting isomer because 9 of its 10 C-H atoms are in allylic position and it is compared to n-Pentane as an example of a linear molecule. The validation of a burner-stabilized flame, ignition delay time, and laminar flame speed experiments for these fuels are presented and discussed. The compilation strategy was used and it aims to continuously increase the number and type of targets for mechanism validation.
Research on energy saving technologies surged in the last decades. One especially relevant technology regards thermal energy storage via phase change materials, or PCM. These materials function as regenerative thermal batteries that can absorb and release thermal energy via the latent heat associated with a phase change, while temperature is kept constant. The advantage of this technology is that due to the latent heat effect the energy density is very high, which reduces the required size of the medium and makes it easier to be coupled with heat loss sources, both in industrial and household applications. The challenge lies, however, in identifying correct PCMs for specific operation temperatures. The goal of the thesis is, then, to develop a novel thermodynamic database that describes the thermodynamic properties of salt mixtures with potential as phase change materials, both for high (up to 800 ℃) and low temperature (up to 100 ℃) applications; and, then, to perform a screening to identify potential PCM compositions in the database.
The database is created with FactSage, a Calphad software, and the systems covered are the anhydrous CaCl2-Ca(NO3)2-KCl-KNO3-NaCl-NaNO3, for high temperature PCMs, and the hydrated Mn(NO3)2-H2O, Zn(NO3)2-H2O, MgSO4-H2O and ZnSO4-H2O, for low temperature PCMs. The liquid solution in all systems is modelled with the non-ideal associates model and, therefore, no aqueous solution model is required. The experimental data used for the assessments come from the literature and from new measurements performed by the partners of the PCM-Screening project (FKZ 03ET1441).
A new program called DataOptimizer has been developed to assist with the optimisation of thermodynamic databases. Relying on the ChemApp software and the NOMAD optimizer, DataOptimizer overcomes many shortcomings of similar database optimisation programs. A graphical user interface featuring a real-time plotting output is also implemented, which allows for a much easier and user-friendly experience. Details about the implementation and features of the program are given.
Finally, the identification of PCM candidates is performed using both phase diagrams calculated with FactSage and a new numerical screening algorithm, which relies on ChemApp. The screening algorithm proves to be capable of identifying eutectics in multicomponent systems automatically without the need for phase diagrams. As a result, twenty-two PCM candidates are identified for high temperature applications within the anhydrous system and, fourteen candidates, for low temperature applications within the hydrated systems.