Combustion characteristics of carbonaceous residues from heavy oil fired boilers

Abstract

The combustion of solid carbonaceous particles generated in residual oil-fired furnaces has been investigated. These particles are large ash-rich cenospheres that are not completely burned in the furnace and end up in the effluent gases, creating problems associated with both emissions and reduced operating efficiency. To study the reactivity of these chars, single-particle combustion experiments were conducted where the particle temperature and burnout times were measured by near-infrared two-color pyrometry. The results show that the particle temperature-burn time traces exhibited higher particle-to-particle variability than those of coal-derived char particles, but the overall burnout times were of the same order of magnitude as coal chars of similar size. Taking into account random particle-to-particle property variations, and using a particle combustion model, reaction rate parameters were estimated. Two limiting forms of the model were used for this estimation. The first assumed the particles to burn as shrinking cores, with all reaction concentrated very near the external particle surface. The second assumed the particles to burn at constant radius, with increasing void fraction. Using first-order kinetics, the shrinking core model gave an average apparent activation energy of about 18 kcal/(g mol), with a corresponding average preexponential factor of approximately 37 g/(cm^2 s atm of O_2. These values are similar to the respective parameters estimated for lignite and bituminous chars burned under similar conditions, indicating that these particles can be burned at temperatures and residence times similar to those existing in coal-fired furnaces. The constant radius model gave an intrinsic activation energy of 40 kcal/(g mol), with a preexponential factor of 550 g/(cm^2 s atm of O_2). The constant radius model gave a slightly better fit to the experimental data.