The role of peripheral percolation in detachment of fines from burning carbon particles

Abstract

Batch combustion of graphite spheres and char particles made by devolatilization of a South African coal has been carried out on a grate and in a fluidized bed. 0.04 m ID combustors equipped with a two-exit head have been used. Overall amount of carbon fines formed during a combustion test as well as time-resolved curves of carbon fines generation rates were obtained with this apparatus. Temperatures were varied within ranges 660 to 820°C and 400 to 550°C for graphite and South African char, respectively. Optical microscopy and computer aided image analysis were used to measure sizes of carbon fine particles (up to 250μm) collected during combustion. A 2-D percolation model, based on Monte Carlo simulations for a square lattice, has been developed to obtain qualitative trends on the influence of peripheral percolation in actual combustion of carbon particles. The main feature, of the model is that non-zero gradients of porosity are considered across the particle. Increasing such gradients can be interpreted as the change of carbon combustion rate from chemical kinetics towards diffusional control due to increases of operating temperature. Along this line, theoretical numbers and sizes of percolated clusters and experimental losses and sizes of carbon fines correspond to each other. All these variables decrease as porosity gradients increase. Percolation is the only source of fines generation in combustion on grate and, as a consequence, when conversion rate is controlled by external diffusion, fractional carbon loss due to fine generation is very small, namely, less than 1%. Experimental results are in agreement with these expectations. Conversion rate control in practical fluidized combustion is also dominated by external diffusion, so that, even in this case, the loss of carbon should be that small. Actual loss, more than one order of magnitude larger, is due to attrition by abrasion of particles which by far overtakes peripheral percolation in producing carbon fines.