Identification of principal chemical subsets of biofuel combustion : Ants walking in renewable fire

Sammanfattning: The work in this thesis was carried out to highlight important chemical pathways in skeletal mechanisms for the three smallest alcohol fuels (methanol, ethanol and npropanol), two representative fuel alkanes n-heptane and n-decane, and finally the biodiesel surrogates, methyl-decanoate, methyl-5-decenoate, and methyl-9-decenoate. This aim was set up to further the efforts of creating Computational Fluid Dynamics (CFD) suitable mechanisms, which in turn can be used by the industry to improve, for example, engines or gas turbines, or by academia to further understand turbulent combustion. The work in this thesis was divided into three parts: development of a novel reduction method, pathway analysis of biofuels, and pathway analysis of petroleum and biodiesel surrogates.Ant Colony Reduction (ACR) is a semi-stochastic reduction algorithm that has been applied successfully on kinetic mechanisms for several biofuels (methanol, ethanol, npropanol, methyl-decanoate and methyl-decenoate), alkanes, and volatile organic compound oxidation in atmospheric chemistry simulations. The method was developed within the framework of the present thesis work, as a new approach to the mechanism reduction problem, as much research has been carried out on various established reduction methods. It has been shown that in some cases the mechanisms reduced using the ACR method outperforms previously published reduced mechanisms by having a lower number of reactions or species while preserving the same accuracy compared to a chosen reference mechanism.The pathway analysis of the biofuel mechanisms was conducted in order to identify the principal reaction subsets for different combustion modes. It was then shown that similarities can be seen for each combustion mode for each of the smallest alcohols, methanol, ethanol and n-propanol. By understanding these subsets, an initial guess for the next alcohol fuel would be facilitated. Several trends were identified that was true for all the alcohol fuels, but there were also new reaction paths that was important for the new skeletal mechanism when the chain length of the alcohol was longer. Since the alcohol at some point has to decompose to two fragments, where only one can contain oxygen, alkane combustion chemistry became more important for n-propanol and ethanol than for methanol.A similar approach to the biofuel pathway analysis was conducted for n-heptane, for which separate reduced mechanisms were produced for the combustion phenomena ignition, flame propagation and extinction. The reference mechanisms for n-heptane are much larger than for the small alcohols, often resulting in larger skeletal mechanisms as well. After investigation, it was shown that for high temperature ignition and laminar burning velocity, it is not necessary to have too much detail in the chemistry. Even extinction and low temperature ignition mechanisms were below 150 reactions each, but when one mechanism for all conditions were constructed, the mechanism had 230 reactions. From the pathway analysis, the important reactions for each subset was identified and can serve as an initial guide for future reduction work on large hydrocarbon fuels.The methodology was also applied to biodiesel surrogates, methyl-decanoate, methyl-5-decenoate and methyl-9-decenoate. Once again, the mechanisms were larger in size due to the complexity of the reference mechanism. It was found that for the methyl-decenoates, the skeletal mechanisms were smaller since the number of possible, and important, intermediates are lower. Compared to other published skeletal mechanisms for the biodiesel surrogates, the sizes of these mechanisms are much smaller, between 200-861 reactions for low temperature ignition, while still retaining high predictability compared to their reference mechanism.