The oxidation of soot: a review of experiments, mechanisms and models Review Article

Soot may be formed when carbonaceous fuels are burned under local reducing conditions. Its subsequent oxidation is of great significance for pollution control in industrial flames, auto engines etc. Oxidation (gasification) can be achieved with oxygen, carbon dioxide, water vapour or nitrogen dioxide. In this review, the experimental techniques which have been used to study the gasification of soot are described and the methods and results obtained by analysis of the data from them are considered. Firstly, the mechanism of soot formation and its structure are briefly discussed. The various scales of particulate which comprise it, i.e. spherule, particle and aggregate, influence its properties and behaviour. Next, the experimental equipment used in the study of its gasification is briefly described. Gasification kinetics at low temperatures are measured either in fixed beds or by thermogravimetry. The apparatus may be operated as a thermally programmed desorption system to identify the species involved. High temperature investigations have been carried out in entrainment burners and shock tubes. The chemistry of soot oxidation is discussed for both non-catalytic and catalytic conditions. The oxidation pathway involves interaction between adsorption and desorption processes, which determine the primary products, the order of reaction and the activation energy. The concensus is that two types of adsorbed surface species are present in uncatalysed combustion. The combustion mechanism of individual spherules is considered in terms of basic property changes. During thermogravimetry, the influence of the competition between reaction and oxygen diffusion in soot beds is analysed. The reaction of catalysed soot displays a different mechanism, as the primary products, the order of reaction and the activation energy all change. The lower activation energy and higher reactivity lead to lower ignition temperatures. Catalysts may be incorporated into the soot spherules by addition to the fuel, or may be added after formation. Two types of contact between the carbon and added catalyst have been identified, ‘loose’ and ‘tight’. Tight catalyst, which has been mechanically ground with the soot, produces more pronounced effects. Finally, the behaviour of soot during gasification by other oxidants, namely H2O, CO2 and NO2 is summarised.