Large-scale pulverized coal-fired power plants in China are frequently operated at partial loads,where NO_x emission from the furnace is generally high.The present work aimed at controlling NO_x emissions from a 1000 MW ultra supercritical boiler at partial load operation through combustion optimization.In-situ tests were conducted to optimize the combustion through evaluating the impacts of varying major operation parameters on NO_xemission and boiler efficiency and comparing testing results with historic operational data extracted from the unit digital control system.High combustion excess O₂ level and blending lean coal with bituminous coal were identified to be the major factors leading to high NO_x emissions of usual operation particularly at low loads.The optimization successfully reduced NO_x emissions from the furnace co-firing lean coal with bituminous coal at both high and low loads,significantly lower than the averages of usual operation.It demonstrated the effectiveness of combustion optimization in controlling NO_x emissions for low load operation.Additionally,it was found that,for 700 MW load operation,applying the mode of mid four mills (B,C,D and E) in service to replace the often used mode of five mills in service achieved not only low NO_x emissions but also good boiler performance.