Sooting Behavior of Hydrocarbons and Fuels

摘要

Soot is a major byproduct of the combustion of hydrocarbon fuels, with severe implications for human health and the climate. Reducing soot emissions is a major objective of combustion research today. The nature and composition of fuels has an enormous impact on the amount of soot produced in a combustion process. Understanding the relationship between the structure of fuels and their tendency to form soot therefore provides an avenue for controlling soot production and emissions.;In this thesis, optical and laser-based diagnostic techniques are employed to measure the sooting tendencies of various fuels and pure compounds. Sooting tendencies are quantified by doping these fuels and compounds in very low concentrations into a coflowing methane/air flame, and measuring the volume fraction of soot produced in these flames. These sooting tendencies, which are expressed as Yield Sooting Indices (YSIs), are then related to the structure of the fuel or pure compound doped into the flame.;Biodiesel fuels form an important constituent of the renewable energy resources landscape. Biodiesel are composed of esters, which are oxygenated hydrocarbons, frequently with one or more C=C double bonds. While the presence of oxygen in hydrocarbons can reduce sooting tendency, unsaturation of C=C bonds can increase it. To better understand the effects of these double bonds on the sooting properties of esters, the YSIs of a wide variety of esters were measured. The C=C double bond was found to have a significant effect on the sooting tendency, frequently completely negating the presence of the oxygen atom. Other structural features of the esters, such as the location of the C=C double bond along the carbon backbone, and distribution of carbon atoms on either side of the carbonyl group, also impacted sooting behavior. A group-additivity model for sooting behavior was also developed to systematize the effects of these structural features.;Unlike biodiesels, conventional petroleum derived fuels are complex mixtures of thousands of hydrocarbons. Surrogates, which are mixtures of a small number of pure compounds designed to mimic important properties of these fuels, are frequently used to facilitate detailed experimental and computational studies of conventional fuels. Developing surrogates which mimic the sooting behavior of their target fuels is an important requirement. Towards this end the sooting behavior of several real diesel and jet fuels, literature surrogates of these fuels, and pure compound components of these surrogates, was investigated. In some cases, the surrogates were found to closely replicate the sooting behavior of their target fuel. The degree of correspondence between the distribution of different types of carbon atoms in the fuel and surrogate, as quantified through NMR spectroscopy, was found to be the most important parameter in controlling how closely the surrogate replicated the target fuel's sooting behavior. Based on this information, and the measured sooting behavior of pure compounds, mixing rules were proposed to formulate newer surrogates which would better match the sooting behavior of their target fuels.;Finally, employing optical digital camera based diagnostics, the sooting tendencies of a large number of pure compound hydrocarbons (≥ 400) was reconciled into a single unified, internally consistent, sooting tendency database for the first time. This database contains compounds from all categories of hydrocarbons important in modern fuels, and establishes the sooting tendencies of several aromatic and oxygenated hydrocarbons on the same numeric scale for the first time. A predictive model for sooting behavior of hydrocarbons, based on decomposing compounds into their constituent carbon-atom fragments, was developed using this unified database. The model was found to be accurate in predicting the YSIs of compounds across three orders of magnitude and provided insights into the effects of chemical structure on soot formation. Prediction failures of the model were useful in revealing the presence of more complex kinetic sooting mechanisms. This work is expected to enable the rational design of low-sooting fuel blends from a wide range of feedstocks.