By taking the selective catalytic reduction(SCR) denitration system of an ultra-supercritical 660 MW coal-fired unit as the research object

this article systematically diagnoses the problems occurred during the unit ultra-low emission operation. Moreover

a series of measures have been taken to improve the denitrification performance and optimize the SCR operation. Finally

the fluctuation range of the NOx concentration at outlet of the denitration system was controlled within ±10 mg/m3

and the relative standard deviation of the average NOx distribution on the outlet cross section was reduced to less than 20%. By regeneration of the initially installed catalysts

the average ammonia escape concentration on site was controlled below 0.5 μL/L

which effectively eliminated the phenomenon of locally high ammonia escape concentration on the cross section of the denitration outlet

and created conditions for safe operation of the unit and further deep reduction of NOx.