On the relevance of axial and transversal fuel segregation during the FB combustion of a biomass

The fluidized bed combustion of a typical Mediterranean biomass (pine-seed shells) was investigated in a pilot-scale bubbling FB combustor (200 kWth) at different operating conditions. Both over- and under-bed fuel feeding options were considered. The focus of the experiments was to study the extent of volatile matter mixing/segregation in the bed and the subsequent postcombustion in the splashing zone and freeboard. To this end, temperature profiles along the combustor axis as well as transversal gas concentration profiles above the bed surface were measured. Experiments highlighted that complete conversion of the fuel within the combustor was obtained, even if a significant volatiles postcombustion above the bed occurred. This results in an appreciable overheating of the freeboard. Both the fuel feeding option and the excess air affect the extent of volatiles postcombustion. Gas concentration profiles were not uniform along the combustor diameter because of the single-point fuel feeding and the incomplete fuel mixing in the bed. The measured carbon-containing gaseous species profiles along the bed diameter were used to estimate the fuel lateral dispersion coefficient in the fluidized bed, by means of a simplified dispersion/pyrolysis model. Comparison of experimental profiles with model results at over- and under-bed feeding conditions allowed us to evaluate both the fuel lateral dispersion coefficient inside the bed and on the bed surface, which were determined to be of the order of 0.01 m2/s and 0.1 m2/s, respectively.