Reduced syngas-based chemical kinetics mechanisms for dual fuel engine combustion applications

Stylianidis, Nearchos (2019) Reduced syngas-based chemical kinetics mechanisms for dual fuel engine combustion applications. Doctoral thesis, Northumbria University.

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Abstract

The interest in sustainable and environmental friendly fuels such as syngas and their use in dual fuel engine applications, has intensified the research for an accurate and reduced chemical kinetics mechanism. The chemical kinetics mechanism should be applicable to simulate not only multicomponent syngas combustion but also NOx formation and the co-oxidation between the primary fuel (premixed syngas) and the pilot injected diesel based fuel. For the diesel based fuel n-heptane was used as a surrogate due to the fact that it has similar physical and chemical characteristics with the diesel and identical rate of heat release (ROHR). Despite the development of various chemical kinetics mechanisms for the simulation of syngas combustion and n-heptane oxidation, a robust and reduced chemical kinetics mechanism that includes full syngas and NOx chemistry and n-heptane chemistry remains elusive. Therefore, this thesis aimed to develop a reduced and robust chemical kinetics mechanism for multicomponent syngas combustion, NOx formation and syngas/n-heptane co-oxidation.

This study is separated into three main sections: a) The development of a reduced syngas mechanism, b) development of a reduced syngas/NOx mechanism and c) development of a reduced n-heptane/syngas/NOx mechanism.

The first section is the construction of a robust reduced chemical kinetics mechanism for multicomponent syngas combustion. Important chemical reactions were investigated by using sensitivity analysis and their rate constants were updated. By using sensitivity analysis, it was shown that the reactivity of syngas mixtures is governed by H2 and CO chemistry for H2 concentrations lower than 50% vol and mostly by H2 chemistry for H2 concentrations higher than 50% vol. Reactions responsible for the decomposition of H2O2 and the formation of high reactive OH species, found to play a key role in the combustion process during high pressure conditions and therefore their rate constants were updated. The constructed mechanism was validated against experimental results and simulated data obtained by using other well-validated chemical kinetics mechanisms, in terms of ignition delay and LFS. Finally, the new mechanism was implemented in a multidimensional CFD simulation for the prediction of syngas combustion in a micro-pilot-ignited supercharged dual-fuel engine. Results from the CFD were compared against experiments. However, while mixtures with H2 concentration > 50% vol used, the reactivity of the mixture increased due to the faster formation of OH and therefore some modification were adopted in the new mechanism in order to improve its accuracy. Modification such as the adaptation of new rate constants on important hydrogen reactions and the removal of reactions with very low sensitivity factor. At the end, a two-part mechanism was constructed for low and high H2 concentrations.

The second section of this thesis was the optimization of the reduced mechanism for low H2 content proposed in Part 1, by updating the rate constants of important hydrogen reactions that were found to be very sensitive during high pressure conditions (10, 20 and 30 atm) and by incorporating a 12 reaction NOx pathway. The NOx sub-mechanism was selected after different NOx models available in the literature were tested and validated. The new reduced syngas/NOx mechanism was validated against experimental data as well as the simulated results by using other chemical kinetics mechanisms from the literature, in terms of LFS, ignition delay time, and NO concentration profiles, and showed very low error in all of the conditions. For LFS simulations the calculated absolute grand mean error for the developed mechanism was lower than 2%, for ignition delay times lower than 5% and for NOx formation profiles lower than 5%. Finally, similar to the first part of this study, the new mechanism was used in a multidimensional CFD simulation to predict the combustion of syngas in a micro-pilot-ignited supercharged dual-fuel engine.

The final section of this research was the construction of a reduced n-heptane/syngas/NOx mechanism for modelling n-heptane/syngas co-oxidation, syngas combustion and NOx formation in a micro pilot-ignited dual fuel engine. For the construction of the reduced chemical kinetics mechanism, a comprehensive mechanism for n-heptane oxidation was reduced by using necessity analysis and was coupled with the reduced syngas/NOx mechanism developed in Part 2. The reduced mechanism consists of 276 reactions and was validated against experimental measurements for different fuel types obtained from the literature and numerical results by using other well validated mechanisms in terms of ignition delay time, LFS and NO concentration profiles. Moreover, a multidimensional CFD analysis was conducted for the prediction of syngas combustion in a micro-pilot-ignited supercharged dual-fuel engine. The reduced mechanism simulates accurately the experimental in-cylinder pressure and ROHR for all conditions except from the cases where 100% hydrogen was used.

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