Abstract: Some major problems were encountered such as the difficulties to control NO_x emission concentration and ammonia escape concentration as well as the resistance increase of air preheater during the transformation of SCR denitrification system for ultra-low emission. In this paper, an SCR System of 300 MW unit after the ultra-low retrofit was selected for study purpose. The testing results are summarized as follows: (1) The NO_x at the outlet of unit denitrification was unevenly distributed. However, since the ammonia escape concentration limit was severely exceeded and the on-line ammonia escape monitoring meter readings were no longer eligible to represent the right values, NO_x concentration control would become more difficult and the increase of the resistance of the air preheater had been observed. (2) Excessively pursuing the ultra-low NO_x emission concentration may result in the drastic attenuation of the catalyst activities and some main chemical components. Then eventually severe deposition of ammonium bisulfate comes up on its surface. Therefore corresponding improvement plans are proposed. CEMS multi-point sampling transformation at denitrification outlet should be carried out to monitor NO_x concentration after ultra-low emission transformation. Both the ammonia injection optimization adjustment and the catalyst performance testing should be done on a regular basis to improve the effect of SCR denitrification device and the economy of denitrification operation.

Key words: ultra-low emissions, SCR system, NO_x distribution, air preheater resistance, catalyst activity, ammonia slip