Fluidized bed combustion of single coal char particles: an analysis of the burning rate and of the primary CO/CO2 ratio

The fluidized-bed combustion of large coal char particles was studied with a focus on the burning rate and the primary CO/CO2 ratio at the particle surface. To this end, single particle laboratory-scale experiments were carried out using a low-attrition and high-reactivity coal char. A previously proposed indirect experimental technique was applied, which was modified to cover a wider range of experimental conditions. The experiments were conducted at different bed temperatures (800-900 C), fluidization velocities (0.3 and 0.53 m/s), inlet oxygen concentrations (0.5%-8.0%), and inert bed particle size ranges (100-212, 500-600, and 900-1000 ?m). The actual sphericity and temperature of the particle were also measured during selected experiments and used in the calculations. Results showed that, under the experimental conditions investigated, carbon was completely oxidized to CO2 within the particle boundary layer. The experiments confirmed that the char particles burned under boundary layer diffusion control in the temperature range of 800-900 C. It was demonstrated that single particle burning rate experiments with a high-reactivity char can be used to estimate the particle Sherwood number in fluidized beds, but only if char attrition can be assumed to be negligible.