Optimization of limestone feed size of a pressurized fluidized bed combustor

Limestone attrition is a major cause of loss of limestone during pressurized fluidized bed combustion. In the authors'previous works, the analysis of published results of solid attrition and desulfurization was conducted to determine the attrition rate expression. The specific attrition rate (rate of decrease in diameter) was estimated to be second order with respect to particle diameter in the previous work. This rate expression implies that reduction of feed size of limestone is effective for suppression of loss of limestone by attrition. However, too much grinding of raw limestone will increase the content of fine particles that are readily elutriated by gas stream and do not contribute to the sulfur capture. In this work, modeling works are conducted for particle attrition and desulfurization in order to predict the effect of feed size of limestone on total consumption of limestone and desulfurization is discussed. Optimum particle size to suppress limestone consumption was approximately 0.7 mm (as D_p50). However, the control of solid drain rate from the bottom was found to have more influence on total limestone consumption rate. Emissions of SO₂ from low sulfur coal (S=0.33%) could be sufficiently low irrespective of limestone feed size but SO₂ emissions from coals with higher sulfur content than 0.5% were anticipated to increase drastically. Such drastic change in SO₂ emissions with the change in sulfur content is attributable to non-linear nature of reaction rate for attrition-enhanced desulfurization by limestone.