Emission reduction of NOx and CO by optimization of the automatic control system in a coal-fired stoker boiler

To date research on NO, and CO emission reduction in stoker-fired boilers has been devoted to combustion modification to the overfire air, diverting air to a selected set of burners, using modified low-NOx, burners, using flue gas recirculation or flue gas treatment with specially controlled catalyst and additives. This study introduces a concept that focuses on the dynamics of the boiler and the automatic control system. The objective of this study was to reduce the NO and CO emissions by restructuring the automatic control system and then tuning the control system with parameters that have been optimized with emission reduction as the objective. Dynamic data were obtained from a step-input test of either the underfire air or the overfire air. These data were used to model the boiler with a transfer function describing the emissions. The analyzer dynamic response was included in the overall model. The control parameters were determined from this overall emissions transfer function by mathematical optimization. These control parameters constituted the initial values in the automatic control system used for the final tests in the boiler. Additional adjustments to reduce the emissions were carried out during boiler operation. A low controller gain and a fast reset timemore » were found to be the most suitable setting for the control system. The NO emissions controlled by the overfire air and CO emissions controlled by the underfire air produced the best results.« less