Sulphur sorbent particle effects in fluidised combustion

• Main Author: Petrie, James
• Format: Doctoral Thesis
• Language: English
• Published: University of Cape Town 2014
• Subjects: Engineering
• Online Access: http://hdl.handle.net/11427/9628

Bibliography: leaves 132-137. === An overall process model is presented for the capture of sulphur dioxide by calcareous sorbents in a fluidised bed combustor for a feed of arbitrary size distribution. The description of sorbent sulphation kinetics, particle attrition and elutriation effects incorporated in this model is supported by experimental data for a wide range of South African sorbents. The sulphation of sorbent particles is described by a simple, two-parameter, kinetic model. No loss in physical relevance is incurred when the decrease in reaction rate with time is given by a negative exponential term. Both sulphation rate and capacity are shown to be functions of sorbent type. The sulphation propensity of the 16 South African samples is correlated against their geologic description. Sorbent properties such as porosity, crystallinity and topography, which affect sulphation capacity directly, are functions of geologic age. It is possible to make a first order assessment of sorbent potential simply from this geologic data. Sorbent attrition, caused by the continuous movement of particles within the combustor, is greatest for deep beds of soft friable material. Most of the attrition takes place in the distributor region where jetting action is important. The attrition model reflects an explicit dependence on fluidising velocity, bed depth, particle diameter, topography and structural strength. Measured values of attrition rate decrease with time to a steady state value, which, under normal FBC operating conditions, is attained after a time in the bed of 6 - 10 hours. The elutriation model considers the effect of fines, generated by attrition, on the carry-over of coarse particles from the bed. The overall process model confirms that choice of sulphation kinetics exerts the greatest influence on attainable sulphur capture,. although bed fluid dynamics, sorbent attrition and feed size distribution all play a role. The model is substantiated by extensive data from the performance of three dissimilar sorbents in a 10 MWth FBC. The contribution of sorbent attrition to the solids loading of downstream gas cleaning equipment is highlighted.