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

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) c an be achieved with oxygen, carbon dioxide, water vapour or nitrogen dioxid e. In this review, the experimental techniques which have been used to stud y the gasification of soot are described and the methods and results obtain ed by analysis of the data from them are considered. Firstly, the mechanism of soot formation and its structure are briefly discussed. The various sca les 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 ki netics at low temperatures are measured either in fixed beds or by thermogr avimetry. The apparatus may be operated as a thermally programmed desorptio n 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 condit ions. The oxidation pathway involves interaction between adsorption and des orption processes, which determine the primary products, the order of react ion and the activation energy. The concensus is that two types of adsorbed surface species are present in uncatalysed combustion. The combustion mecha nism of individual spherules is considered in terms of basic property chang es. During thermogravimetry, the influence of the competition between react ion and oxygen diffusion in soot beds is analysed. The reaction of catalyse d soot displays a different mechanism, as the primary products, the order o f reaction and the activation energy all change. The lower activation energ y 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 ca talyst 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, namel y H2O, CO2 and NO2 is summarised. (C) 2001 Elsevier Science Ltd. All rights reserved.