New results on coke formation in the combustion of heavy-fuel droplets

Abstract

Prediction and control of particulate emissions from the combustion of residual oil in boiler systems are limited by lack of knowledge concerning the particle formation process. More information is needed concerning the dependence of the particle size and mass distribution on the spray size distribution in addition to information on the cenosphere structure and its influence on the particle oxidation process. To study these problems, the formation and early stages of the oxidation of coke cenospheres formed in the combustion of residual oil have been investigated using an isolated droplet combustion facility. The droplet oxidation process was found to undergo a transition from quiescent burning to multiple, small scale, disruptions in the late stages of the droplet lifetime. This late disruption is the likely cause of the blow holes seen in the cenospheres. Microscopic examination of the particle shells revealed a foamed structure with a porosity that varied with the fuel source. Particle surface oxidation was found to proceed nonuniformly. Qualitative elemental analysis of the particle surface suggests that variation in the presence of catalytic components is not responsible for the nonuniform oxidation. Instead, microscopic analysis suggests that local variation in shell porosity causes the nonuniform surface oxidation. Furthermore, the surface oxidation was found to involve evolution of additional surface area through the opening of internal voids rather than simple spherical surface regression. The Coke Formation Index (or fractional coke production) and the product of the cenosphere shell thickness and density were found to be insensitive to change in the droplet diameter for a given oil. Results facilitate the modeling of particulate burnout by predicting the particle size and mass from the spray size distribution.