Exploring Combustion Chemistry in Laboratory-Based Flames

As of today, most of the world’s transportation energy is supplied by gasoline and diesel-fueled internal combustion engines. This ubiquitous use of fossil fuels contributes significantly to the formation of airborne pollutants, such as particulate matter (soot), oxides of nitrogen (NOx), the greenhouse gas CO₂, and unburned volatile organic compounds (VOCs). It becomes increasingly important to understand the chemical sources of these combustion-generated pollutants because of the associated adverse health and environmental effects, and in order to meet forthcoming regulations on emissions and efficiency for combustion devices. The first part of the presentation summarizes our recent experimental combustion chemistry studies that have been focused on the formation of soot as a byproduct of incomplete combustion of petroleum-derived fuels. Because practical combustion devices are still not well suited for direct investigations of the complex chemical reaction networks occurring in flames, we employ a variety of different model flames which are designed to minimize turbulent flow-chemistry interactions. The chemical structures of such laboratory-scale flames are studied with mass spectrometry, which is the only viable technique that provides selective and sensitive detection of both radical and stable compounds found in combustion processes. Data obtained from these simple model flames revealed the chemical pathways and the fuel-structure dependence of the early stages of soot formation as well as the chemical composition of soot particles. All our experimental results serve as benchmarks for the development of a predictive detailed chemical kinetic model, and only when a step-by-step description of the overall combustion processes is achieved for such simple flames, the model can be applied with confidence to sophisticated practical devices. The second part of the talk is concerned with our works on the combustion chemistry of oxygenated, alternative fuels, specifically alcohols and esters. For many years, combustion chemistry research has focused on petroleum-derived hydrocarbon fuels, while studies of the more complex combustion chemistry of oxygenated, bio-derived fuels have only just begun. Detailed kinetic modeling and quantitative mole fraction profiles of species from within n- and iso-butanol flames are combined to investigate the combustion of these next generation biofuels. From the results at hand, it might be inferred more generally that the combustion of oxygenated, bio-derived fuels would reduce the formation of soot and its precursors compared to the combustion of fossil fuels while generating novel undesired emissions.