The nitrogen oxides emitted from coal-fired power plant boilers are an important air pollution source. Controlling NO

x

emission has become an important work of power plants at present. The Shanghai Caojing power plant carried out the ultra-low emission transformation project in 2014 and 2015 respectively. In the long run after retrofitting

it was found that the NO

x

data of the selective catalytic reduction (SCR) outlet were large different from that of chimney outlet in unit one. Through the numerical simulation of the flow field in the flue where SCR is located

it is found that the original measuring points at the inlet and outlet of SCR can not accurately measure the concentration value of NO

x

when the concentration distribution of NO

x

at the inlet of the flue is not uniform. And so

the technical transformation was made that the original two-point average NO

x

measurement at the SCR outlet was changed into the multi-point measurement of the grid. Finally

it is found that the change trend of the new NO

x

data coincides with the NO

x

data change trend of the chimney outlet

which provides the basis for the further injection of ammonia.