The central feature of the Combustion Chemistry project at LLNL is our development, validation, and application of detailed chemical kinetic reaction mechanisms for the combustion of hydrocarbon and other types of chemical fuels. For the past 30 years, our group has built hydrocarbon mechanisms for fuels from hydrogen and methane through much larger fuels including heptanes and octanes. Other classes of fuels for which models have been developed include flame suppressants such as halons and organophosphates, and air pollutants such as soot and oxides of nitrogen and sulfur.
Reaction mechanisms have been tested and validated extensively through comparisons between computed results and measured data from laboratory experiments (e.g., shock tubes, laminar flames, rapid compression machines, flow reactors, stirred reactors) and from practical systems (e.g., diesel engines, spark-ignition engines, homogeneous charge, compression ignition (HCCI) engines). We have used these kinetic models to examine a wide range of combustion systems.
|Chemical kinetic modeling is an essential tool in understanding and predicting performance and emissions from these three very different types of internal combustion engines.|
Science in Support of National Objectives
Combustion of hydrocarbon fuels provides the majority of the energy used in the United States for power generation, transportation and industrial processes. Dependence on imported hydrocarbon fuels is a major component of the US international balance of payments, with significant impacts on the financial and energy security of the United States. Emissions from these power systems make dominant contributions to global climate change and other environmental challenges world-wide. LLNL combustion chemistry research is directed towards understanding the details of these processes, in both qualitative and quantitative terms, leading to improved efficiency and reduction in emissions to the atmosphere. Current work is directed towards understanding and predicting the possible impacts of alternative fuels, including fuels derived from biological systems and new geological sources, such as oil shale and oil sands. The chemical kinetic reaction mechanisms developed at LLNL are used widely in the United States and many foreign groups, and dissemination of these mechanisms via this web page is a major priority of the LLNL Combustion Chemistry Group.
|Our Combustion Chemistry group studies fundamental chemistry in practical systems, like this internal combustion engine shown in cross-sectional form.|
Combustion chemistry has been pursued at LLNL since about 1975, and many significant advances can be attributed to this work. Early studies addressed major sources of unburned hydrocarbon emissions from internal combustion engines by a variety of flame quenching mechanisms. We established the role of volume bulk flame quenching in stratified charge engines. We also showed that flame quenching on cooled walls of engine combustion chambers was not the major source of hydrocarbon emissions from spark-ignition (SI) engines, overturning 50 years of engine design lore.
Later, we were able to demonstrate the major role of molecular structure of different hydrocarbon fuel molecules on their rates of ignition in SI and diesel engines, which in turn determines the octane and cetane ratings of fuels in these engines. These fuel rating systems had been determined empirically from engine experiments in the 1920s and 1930s, but no scientific basis for the ratings had been established. Our work at LLNL focused on the low temperature reaction pathways that oxidize these fuels, showing the specific chemical reactions that lead to the differences between knocking tendencies of fuels with different sizes and molecular structure. This work was recognized scientifically by being awarded the 1991 Horning Memorial Award by the Society of Automotive Engineers and the 1993 Thomas Midgley Award from the American Chemical Society.
More recently, we have given considerable attention to hydrocarbon combustion in diesel and HCCI engine systems. We have shown how diesel ignition varies with fuel molecular size and structure, and how these factors determine the subsequent production of soot and its undesirable emissions from diesel engines. This work was recognized by the 2000 Arch Colwell Award of Merit from the Society of Automotive Engineers, providing an overall understanding of the role of fuel structure on ignition and soot production in diesel engines. Our later kinetic modeling explained observations from diesel engine studies in which small amounts of additives containing oxygen atoms (e.g., alcohol, ketone or ether species) sharply reduce soot emissions, and this work was recognized by receipt of the 2003 Arch Colwell Award of Merit from the Society of Automotive Engineers.
Additional accomplishments by our group have established the role of detailed chemical kinetics in the behavior of a wide variety of other combustion systems. We provided a kinetic framework for determining the role of molecular size and structure on detonation properties via the prediction of detonation cell sizes, on stability of pulse combustion systems, on combustion in supercritical water, and in selected explosives. We also have made significant contributions to the theory of inhibition of flames and ignition via kinetic interactions with halons such as HBr and CF3Br, as well as organophosphorus compounds such as dimethyl methyl phosphonate (DMMP) and others, and we have developed the first kinetic models for selected toxic organophospates as well.
The kinetic modeling program at LLNL provides new tools for analysis of many practical and theoretical systems that were entirely unavailable only a few years ago. Part of these advances can be attributed to growth in computing capabilities in science in general, making it possible to solve the extremely large sets of coupled differential equations that occur during the combustion of the types of molecules we have been studying. However, the skills and analysis tools developed by this program offer new capabilities which, together with similar advances in experimental diagnostic tools and techniques, accelerate the rate of understanding of many technical problems that have existed for generations without scientific explanations. The example above for octane and cetane numbers in internal combustion engines demonstrates how kinetic modeling was able to provide a theoretical explanation for phenomenology that had been known in general terms for many years but was not understood previously in fundamental terms.
|Principal molecular components in biodiesel fuel derived from soybean oil.|
Current attention is being directed toward fuels that can replace or augment practical transportation fuels including gasoline, diesel fuel, and jet fuel. The two major sources with considerable promise are geologically produced fuels from oil shale and oil sands, and biologically derived fuels from oils and fats. We are working on extending current kinetic modeling capabilities to handle the very large, complex molecules found in these novel fuels. For example, soy-derived biodiesel fuel consists primarily of mixtures of the fuel molecules shown in the accompanying figure. Although these molecules are predominantly long-chain hydrocarbon species, they also show the oxygenated methyl ester group at one end of each molecule, which is a residue of the biological origins of the fuel and complicates the development of a kinetic model for their combustion.
Westbrook, C. K., W. J. Pitz, and H. J. Curran, “Chemical Kinetic Modeling of the Effects of Oxygenated Hydrocarbons on Soot Emissions from Diesel Engines,” J. Phys. Chem. A 110, 6912 (2006).
Jayaweera, T. M., C. F. Melius, W. J. Pitz, C. K. Westbrook, O. P. Korobeinichev, V. M. Shvartsberg, A. G. Shmakov, I. V. Rybitskaya, and H. J. Curran, “Flame Inhibition by Phosphorus-containing Compounds over a Range of Equivalence Ratios”, Combust. Flame 140, 103 (2005).
Pitz, W. J., C. V. Naik, T. N. Mhaolduin, C. K. Westbrook, H. J. Curran, J. P. Orme, and J. M. Simmie, “Modeling and Experimental Investigation of Methylcyclohexane Ignition in a Rapid Compression Machine,” Proc. Combust. Inst. 31, 267 (2007).
Our Combustion Chemistry group studies fundamental chemistry in practical systems, like this internal combustion engine shown in cross-sectional form.