On the pyrolysis and combustion of furans: quantum chemical, statistical rate theory, and chemical kinetic modelling studies
Somers, Kieran Patrick
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The development of synthetic methods which can convert inedible waste biomass into liquid transportation fuels is an exciting breakthrough which may reduce our dependence on fossil fuels for energy production, and contribute to the development of environmentally, socially, and economically sustainable technologies. Biomass-derived furan derivatives have emerged as promising candidates in the search for sustainable next-generation biofuels and platform chemicals. This work focuses on two such promising compounds, 2-methylfuran (2MF) and 2,5-dimethylfuran (25DMF), with the aim of developing detailed computation models which can numerically describe their combustion properties. The thermochemistry of a range of polyoxygenated furan platform chemicals has also been investigated. A general overview of Rice-Ramsperger-Kassel-Marcus (RRKM) theory with energy-grain Master Equation (ME) analysis, and the less advanced Quantum-Rice-Ramsperger-Kassel (QRRK) theory with a Modified Strong Collision (MSC) approximation for collisional energy transfer, is provided. The two approaches are compared for some model systems to assess the accuracy of the latter in computing pressure-dependent kinetic parameters to serve as input for kinetic mechanisms. A suite of codes have been developed based on the Perl programming language to organise and expedite the procedure of carrying out quantum chemical, Transition State Theory (TST), RRKM/ME and QRRK/MSC computations, and their use is described in detail. Quantum chemistry coupled with the isodesmic reaction and atomisation methods has been applied to determine the gas-phase formation entalpies ([delta] f H [theta])of a range of substituted ( -OH, -OCH(3), -CH=O, -C(=O)CH(3), -CH(2)OH,-C(=O)OH) furans. The computed ([delta] f H [theta]) are compared with experimental data where they exist. Group additivity rules and values were subsequently developed, which allow for the rapid estimation of thermochemical properties of alkyl- and poly-oxygenated furans and their radicals. Quantum chemistry, TST and RRKM/ME methods were applied to derive thermodynamic and kinetic functions to describe the reactions of importance in the high-temperature pyrolysis and oxidation of 2MF. The potential energy surfaces show a complex multi-step reaction mechanism for opening and fragmentation of the stable furan ring. These computations form the basis of a detailed chemical kinetic mechanism which was validated against shock-tube pyrolysis and ignition delay time data, atmospheric pressure laminar burning velocities, and species concentration measurements in a low-pressure laminar flame. A combination of quantum chemistry and TST, and literature data were used to construct and validate a detailed chemical mechanism to describe the pyrolysis and oxidation of 25DMF. The mechanism is validated against shock-tube and flow-reactor pyrolysis data, shock-tube ignition delay time measurements at high temperatures and low pressures, and at intermediatetemperatures and high pressure, speciation measurements in a jet-stirred reactor, laminar burning velocity data from combustion bombs and burners, and flame speciation data, thus offering a comprehensive assessment of 25DMF oxidation and pyrolysis. The reactions of importance in the high-temperature pyrolysis and oxidation of 2MF and 25DMF are shown to be quite similar at temperature over 1000 K. 25DMF exhibits an interesting intermediate-temperature (800-1200 K) oxidation mechanism. The rate constant for abstraction of a hydrogen atom at the alkyl side-chain by O2 was shown to be particularly important in predicting experiments in this regime. The reactions of .OH radical with 25DMF, and H.O2, CH3 .O2 and .CH3 radicals with 5-methyl-2-furanylmethyl radical, were also highlighted as important in predicting ignition delay times and speciation profiles.