Modeling of SO3 formation and depletion processes in coal-fired boilers

Abstract

Theoretical model developed for the SO3 formation and depletion processes in the region upstream of the economizer exit duct in coal-fired boilers. The model accounts for homogeneous conversion of SO2to SO3 as a result of gas phase reactions, SO3 catalytic formation due to Fe2O3 in the fly ash both in suspension and as deposits on boiler tubes, and SO3 depletion due to reaction with alkali compounds in the fly ash. Comparisons were made between model results and SO3 filed measurements for three boilers. There was good consistency between model results and experimental data up to the economizer section. However, within the economizers, the predicted SO3 levels were overestimated by the model. This was found to be as a result of additional SO3 depletion. Laboratory experiments were conducted to measure rates of SO3 and H2SO4 conversion and reactions with CaO, and results were used for kinetics development, especially SO3 depletion prediction at low temperatures. Further refinement was implemented to the model to consider the effect of this SO3 depletion based on experimental results from a lab scale set up. Operational recommendations are introduced to mitigate SO3 production, including reducing economizer O2, increasing the frequency of sootblowing of the high temperature heat exchangers, and fire coals with low iron content. This study provides a fundamental understanding of the mechanisms that are most important to net SO3 formation (generation minus reduction) in coal-fired boilers. Such understanding will make it possible to develop cost-effective methods for reduction of SO3 formation in coal-fired boilers.