Abstract: The anticipated target of NO_x emission control have not been reached in some power plants after the ultra-low emission retrofit project. To solve this problem, taking a 330 MW ultra-low emission unit as an example, onsite investigation was conducted and the operation data of a 15 500 h flue gas denitrification unit was collected. The operating catalyst was also sampled and tested. Besides, the causes of excessive NO_x emission were investigated individually. The research results show that the decline of denitrification efficiency is attributed to low design margin of SCR denitrification, low content of main active component V₂O₅ as well as the decrease of catalyst micropore number and active point number. In addition, the high NO_x concentration at the inlet of the reactor, the frequent load changes, and untimely ammonia spray response cause the NO_x emission at the outlet to exceed 50 mg/m³. In order to ensure that stable emission standards are met, it is suggested that the optimal operation of the boiler should be strengthened to avoid the large fluctuation of NO_x concentration and improve the response speed of ammonia injection. One extra layer of catalysts is also advised and the existing two layers of catalyst should be regenerated.

Key words: thermal power plant, flue gas denitration, SCR deNO_x system, honeycomb catalysts, operation diagnosis