A population balance model on sorbent in CFB combustors: The influence of particle attrition

A population balance model on sorbent particles in an atmospheric circulating fluidized bed combustor fueled with sulfur-bearing solid fuel is developed. The model aims at the prediction of the following quantities establishing at the steady state in the combustor: sorbent inventory and particle size distribution, partitioning of the sorbent between fly and bottom ash, desulfurization efficiency, and the mass flow rate of the sorbent circulating around the loop of the combustor. The core of the model is represented by the population balance equations on sorbent particles, which embody terms expressing the rate of sorbent attrition/fragmentation. The effect of the progress of sulfation on attrition is taken into account by the selection of appropriate constitutive equations. Model results are presented and discussed with the aim of clarifying the influence of particle attrition. In particular, the effect of attrition on bed sorbent partitioning between lime and sulfated lime and on SO2 capture efficiency is highlighted. The model enables one to assess the balance between opposed effects of attrition on desulfurization: on one hand, attrited fines are characterized by a better reactivity with respect to SO2, when compared with the mother particles; on the other hand, attrition is responsible for larger amounts of unsulfated material reporting to the fly ash. A sensitivity analysis is also carried out with reference to relevant operational parameters of the combustor in order to correlate changes in ash partitioning and desulfurization efficiency with the extent of sorbent attrition and solids circulation.