Abstract: In order to keep the NO_x emission concentration and ammonia escape volume within a controllable and stable range after the transformation of ultra-low emission of the SCR reactor, by taking the SCR denitrification system of a 600 MW coal-fired unit as the research object in the case studies, the inconsistency between the data of on-line monitoring (CEMS) measurement acquired from SCR reactor outlet and total discharge outlet is analyzed and investigated in this paper. Through the measurement and testing on the flow distribution at both the entrance and exit of SCR reactor, as well as the concentration field of NO_x and NH₃ with the structural characteristics of SCR denitrification system taken into account, the causes of the problems are explored, then the corresponding optimization and tuning scheme are formulated and the effectiveness of the scheme is verified. The results show that the heterogeneity of the molar ratio of ammonia to nitrogen at the inlet of SCR reactor is the main reason for the uneven distribution of NO_x concentration at the outlet of SCR reactor. Through the optimization and tuning processes, the homogeneity of NO_x concentration distribution at the outlet of SCR reactor is effectively improved, and the ammonia escape volume is reduced as well. The measurement data of the NO_x emission concentration acquired by CEMS from SCR reactor outlet are basically consistent with the total discharge from the outlet of chimney. Meanwhile, based on the curve representing the relationship between NH₃ and NO_x concentration at the outlet of reactor SCR obtained by repeated experiments, this paper proposes both the minimum concentration for safety control and the optimal concentration of NO_x safety-control, so as to facilitate the operation and control of SCR denitrification system in the later stage.

Key words: coal-fired unit, ultra-low emission, SCR, denitrification, ammonia spray grid, optimization and adjustment