Intellectual Merit: Plants are about 50 % oxygen by weight. Thus as society inevitably moves from fossil fuels towards renewable fuels, the oxygen content of combustion fuels will increase. The presence of oxygenated hydrocarbons in the fuel introduces important combustion science and public health issues that are addressed by the proposed research. First, oxygenates may reduce emissions of soot particles. Second, they can also increase emissions of other toxic combustion byproducts such as aldehydes.
In order to rationally optimize soot reductions while minimizing air toxics emissions, we need to understand the chemical mechanisms of fuel decomposition and aromatic hydrocarbon formation for oxygenates. The number of oxygenates that can be made from vegetation is large, and the sooting tendencies and fuel decomposition products vary greatly as a function of oxygenate structure; thus strictly empirical approaches for choosing among them are not reliable. A strategy that analyzes a large number of oxygenated fuels and leads to methods that can predict mechanisms and reactivity from fuel structure is needed. Although the combustion chemistry of hydrocarbons and some small oxygenates has been widely studied, little is known for most oxygenated hydrocarbons. We propose a novel approach that is based on rapid, on-line species measurements and computational simulations in co-flow flames where a small amount of the oxygenated fuel is added to a base fuel of methane. The base methane flame is has been well characterized and computations provide good agreement with experimental species measurements. Our strategy involving perturbation of a well-characterized system enables high-quality measurements and also facilitates simulations because the solutions from previously computed base methane flames can be used as a starting estimate for all of the doped flames. Importantly, our methods compare the fate of oxygenated fuel species under identical flame conditions emphasizing differences in chemical mechanisms over other factors. In earlier work we validated this methodology for regular hydrocarbons including heptanes, hexenes, hexadienes, cycloalkanes and aromatics. Here we will extend it to 100+ oxygenated hydrocarbons with up to 20 carbon atoms. Our results will provide an important comparison to other studies of oxygenates combustion chemistry, most of which use premixed flames and a limited number of oxygenated fuel structures. The analysis of a wide range of structures is required for development of structure/reactivity relationships allowing robust mechanism testing and a rational basis for oxygenated fuel selection to optimize emissions benefits.
Broader Impacts: Our work sets the stage for cleaner engine design and renewable fuels utilization by expanding the database to oxygenated hydrocarbons, and by providing rational correlation and extrapolation data for the effect of fuel structure on soot production and possible toxic oxygenated emissions. We will collaborate with local industry to facilitate this. We have provided detailed results from our previous studies to numerous groups around the world who have used them to test computational models. The databases generated here will be archived for direct access to researchers around the world. Undergraduate students have participated in our previous research and will perform laboratory modules related to this project. We will also involve students from local non-PhD granting colleges and high schools in our work and helping them develop research projects at their home institutions and to interest them in continuing study in the sciences.
Soot emissions are among the worst environmental consequences of combustion: they are the second most important source of global warming, with a radiative forcing that is two-thirds that of carbon dioxide, and they contribute to urban ambient particulate matter, which is linked to about three million annual deaths worldwide. The goal of this project was to provide fundamental knowledge that would accelerate the use of oxygen-containing fuels in engines and other combustion devices. Oxygenated fuels are a renewable resource that can be produced from vegetable oils and agricultural wastes, and they potentially emit less soot particles than conventional petroleum-derived fuels. A complication is that some oxygenated fuels actually increase soot compared to their same carbon number hydrocarbon analogs. This work uses a definition of sooting propensity (defined by our group) along with experimental measurements and fuel structure-property relationships to provide a rationale basis for choosing a particular oxygenated fuel component. This work will affect commercial fuel design. It has led to further work with industry and a national lab to design surrogate fuel formulations for diesel engines. The most important contribution of this work is the ability to predict sooting tendencies of oxygenated fuels even those that had not been previously measured. In addition, many oxygenated fuels actually increase soot compared to their hydrocarbon analog structures. These can be identified from their structures using the correlations developed in this research. In choosing fuels for new fuel formulations these are both important factors. This work has direct implications on the field of environmental engineering as it suggests how soot can be managed by fuel formulations. The major achievement of this project was to create and validate a means to relate fuel structure to its sooting propensity. An example is the sooting tendency of 20 different C4 to C7 unsaturated esters recently measured in this study. These 20 test compounds were chosen to span a wide range of carbon numbers, location of C−C double bonds, distribution of carbon atoms between the "ether" and "acid" side, and type of branching in the carbon backbone. Fuel structure, specifically the location of the double bond, significantly affected sooting propensity. Compared to the sooting tendency of alkanes (with similar structure and the same number of carbon atoms), in most cases the sooting tendency of the corresponding unsaturated ester was equal or greater. We have developed a simple structure property correlation that not only fits the 20 recently studied unsaturated esters but a large set (265 compounds) of the other oxygenates and hydrocarbons obtained from our experiments. The value of this is that using our research a good estimate of the sooting propensity of unstudied compounds can be estimated. Our research impacted many undergraduate students from Yale as well as students from local non-PhD granting institutions as well as high school students. Most of these students who were involved in this research are from under-represented groups in science and engineering. Several of these students carried out significant independent projects. For example, one of the undergraduate students studied the combustion of pure methyl butanoate in our coflow burner. She presented her work at a combustion conference showing that she understood the work and carried out the experiments herself. Thus our research resulted in important technical strategies for selecting oxygenated fuels for fuel blends and equally important exposed a wide range of undergraduate and high school students mostly from under-represented groups in science and engineering to research in an academic setting.
