Search

Towards the goal of establishing kinetic database for low-temperature combustion ( T < 1000 K) of mid-sized biodiesel surrogates, the study uses quantum chemistry and statistical kinetic methods to investigate three primary unimolecular reaction pathways of methyl octanoate peroxy radicals, including dissociation, isomerization and concerted elimination. We calculate kinetic barriers and pressure-dependent rate constants at 500–1000 K. The comparison between our computed and previously estimated rate constants offers further insight into how transition state structures and molecular mechanics are correlated with reaction kinetics. In the branching ratio analysis, we investigate the proposed unimolecular reactions and factors affecting these kinetic characteristics. For the first time, the previously measured oxidation rates of methyl octanoate under the cool flame regime (560–1000 K) are computationally verified by kinetic modeling that reflects the contribution of the present submodel to an existing detailed mechanism of methyl octanoate. Consequently, the rate-of-production analysis reveals the significance of newly proposed reaction pathways. [ABSTRACT FROM AUTHOR]

Abstract: Growth of the biodiesel industry has motivated increased study of the combustion characteristics of its constituent molecules and building combustion modeling capability. Understanding how these characteristics differ between bio-derived and conventional diesel fuels can help in evaluating biodiesel performance. A kinetic modeling comparison of methyl butanoate and n-butane, its corresponding alkane, contrasted the combustion of methyl esters and normal alkanes, towards understanding the effect of the methyl ester moiety. Utilizing a combined n-heptane and methyl butanoate kinetic mechanism in shock tube simulations, the results predicted no region of negative temperature coefficient (NTC) behavior for methyl butanoate, compared to a well defined NTC region for n-butane. We observed that oxidation pathways associated with the methyl ester moiety inhibited NTC behavior, through increased production of hydroperoxy radicals (HO2) instead of hydroxyl radicals (OH). In addition, we compared the evolution of carbon monoxide, carbon dioxide, ethylene and acetylene. The early formation of CO and CO2, directly from methyl butanoate, revealed unique reaction pathways that also influenced a reduction in soot precursor formation. Overall, these results will help to understand how combustion processes change with the inclusion of oxygenated fuels, which will inform the study and design of combustion technologies. [Copyright &y& Elsevier]

Abstract: Burgeoning global demand for energy has increased concerns about the fuel security issues and deleterious environmental impacts that result from the ubiquitous use of fossil fuels to meet these needs. This article is a review of completed work towards the goal of creating chemical kinetic mechanisms for biodiesel, which will aid in the development of clean and efficient combustors that utilize alternative fuels. As the composition of biodiesel is too complex to directly model, efforts have instead focused on the development of mechanisms for surrogates, simpler molecules that can produce the primary characteristics of biodiesel combustion. Research initially targeted smaller molecules like methyl butanoate to investigate the role of the characteristic ester group that is present in the fatty acid alkyl esters that comprise biodiesel. The study of isomers and similar unsaturated compounds elucidated the effects of molecular structure on combustion. Subsequent efforts involved the study of larger molecules that are close in scale to biodiesel molecules, such as methyl decanoate, as well as molecules that are present in biodiesel, such as methyl stearate. Applications of kinetic modeling demonstrate its utility in the study of combustion through, for example, revealing the chemistry in the early formation of CO2 in biodiesel and its soot reduction tendencies. The results of this review illustrate key limitations in kinetic modeling, namely a need for high-pressure kinetic methodology and a need for continuous improvement of kinetic mechanisms through theory and experiment. These limitations suggest direction for future research; further experimental and theoretical work will produce accurate mechanisms for appropriate biodiesel surrogates. All of these efforts represent significant advances in kinetic modeling that are important towards the goal of building a predictive capability for biodiesel combustion. Such predictive capability will aid the development of combustion technologies that will help society meet its energy needs in an environmentally conscious manner. [Copyright &y& Elsevier]