Eveleigh, A; (2015) The influence of fuel molecular structure on particulate emission investigated with isotope tracing. Doctoral thesis , UCL (University College London).

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Abstract

This thesis is concerned with the formation of particulate matter, a topic of scientific and practical importance due to the toxicity of particulate emissions from automotive and other combustion sources. At present, fuels are predominantly derived from fossil sources, but as production technology improves, biofuels and synthetic fuels are expected to emerge as scalable long-term sources of liquid fuels. Efforts are being made to ensure that this next-generation of fuels is cleaner burning than the last. In order to inform the production and processing of cleaner burning fuels, more needs to be known about how molecular structure influences the formation of pollutant emissions. This thesis presents research that has been carried out in order to better understand the role of functional group chemistry on the conversion of carbon atoms in the fuel to the particulate matter (PM). In particular, the propensity of individual molecules or carbon atoms within molecules to form PM is reported quantitatively. To this end, a technique using carbon-13 (13C) labelled fuel molecules was used so to track the labelled carbon atoms in the fuel to PM. The technique required only very low levels of 13C enrichment, and isotope ratio mass spectrometry equipment (IRMS) was used as a means of 13C detection. Samples of particulate matter were formed using a tube reactor, and also in a compression ignition diesel engine. The tube reactor was designed and commissioned in order to study the pyrolysis of various fuel molecules under well-controlled, homogenous conditions. The contribution to PM of a number of molecules containing various functional groups was assessed, including: alcohols, esters, aromatics, double bonded carbon atoms, a ketone, and a carboxylic acid. Tests were conducted using single-component fuels, and blended in a binary mixture with n−heptane. The results show that the contribution of carbon atoms within molecules to PM, is not equal, but depends on the local molecular structure. For example, oxygenated molecules significantly reduced the contribution to PM of the carbon atoms directly attached to oxygen. The thesis presents one of only a handful of investigations that have been published on the conversion of specific carbon atoms of various molecules to soot and particulate. It advances the field of study by providing data for validation, at the sub-molecular level, for chemical kinetic models of soot formation, and advances fundamental understanding of how fuels convert to soot and particulates.

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