Numerical study of pulverized coal-fired utility boiler over a wide range of operating conditions for in-furnace SO2/NOx reduction

Abstract

Important tasks during pulverized coal-fired utility boiler exploitation are efficient utilization of variable quality fuels, operation in a wide range of loads and emission reduction of pollutants, like oxides of nitrogen and sulfur. Combustion process modifications for NOx control and the furnace sorbent injection for SO2 control are cost-effective clean coal technologies. For optimization of boiler operation mathematical prediction is regularly used and the need for modeling is most apparent in complex flows, such as turbulent reactive flows in coal-fired furnaces. Simulation of processes in a utility boiler pulverized lignite-fired furnace was performed by an in-house developed numerical code. The code is a promising numerical tool to be used also by engineering staff dealing with the process analysis in boiler units. A broad range of operating conditions was examined, such as different boiler loads, fuel and preheated air distribution over the burners and the burner tiers, grinding fineness of coal, cold air ingress and recirculation of flue gases from the boiler exit. Ash deposit on the screen walls, affecting the heat exchange inside the furnace, was considered as well. Simulations suggested optimal combustion modifications providing NOx emission reduction, with the flame geometry improvement, as well. SO2 reduction by injection of pulverized Ca-based sorbents into the furnace was also analyzed. Models of the sorbent particle calcination, sintering and sulfation reactions were optimized and implemented within the numerical code. Numerical experiments considered different operation parameters, such as Ca/S molar ratio, sorbent particle size and dispersion, local gas temperature in different injection zones and the particle residence time. A proper distribution of finely ground sorbent particles could be expected to provide an efficient absorption of SO2. With respect to the boiler thermal calculations, the facility should be controlled within narrow limits of operation parameters due to often contradictory requirements with respect to emission reduction and the boiler unit efficiency with safe operation of superheaters. A number of influencing parameters require such a complex approach to evaluate alternative solutions and enable efficient, low emission and flexible operation of power plant boiler units.