Optimized fluegas denitrification control strategy to enhance SCR performance during load-cycling transient processes in coal-fired power plants

Abstract

Selective catalytic reduction (SCR) is a common method for controlling NOx emission via NH3 injection. Coal-fired power plants are taking load-cycling processes with increasing frequency. During load-cycling processes, NOx emissions fluctuate and NH3 escape aggravates, which increases environmental pollution and degrades equipment safety. Therefore, the NH3 injection control strategy must be optimized to enhance denitrification performance during transient processes. In this study, an integrated NOx emission model of coal-fired units is developed and verified. The model includes the thermal system simulation, NOx production, and SCR system models. On the basis of the model, load-cycling processes are simulated under the traditional denitrification control strategy, wherein excessive NH3 injection and increased NOx and NH3 emissions are observed. The typical transient processes of the SCR system and boiler are simulated with various initial operating conditions and input parameter disturbances to explore the main factors affecting denitrification performance during load-cycling processes. The analysis of dynamic characteristics reveals that the inertia of NH3 storage is an important factor of unstable denitrification performance, and the temperature and fluegas flow rate influence the delay caused by this inertia, therefore the denitrification performance shows significant difference in various load-cycling processes. Consequently, an optimized NH3 control strategy is proposed. In this strategy, the change rate of NOx concentration is predicted according to the boiler load command, and the fluegas temperature and flow rate are also taken to adjust the additional NH3 injection. The simulation of typical load-cycling processes demonstrates that the optimized control strategy comprehensively enhances denitrification performance, where the NH3 injection, NOx emission and NH3 emission are all reduced. In particular, for the low- and medium-load range, the total amount of NH3 emission can be reduced by 5.69%, and the maximum values of NOx and NH3 emissions are reduced by 7.46% and 19.26%, respectively, which benefits both air pollutants emission reduction and equipment safety.