Based on the NOx emission information of 671 coal-fired power units, the NOx emission characteristics of non-ultra-low-emission (non-ULE) and ultra-low-emission (ULE) coal-fired power units during the start-up process were compared and analyzed. The results showed that in the start-up stage of non-ULE units, the exceeding standard rate (ESR) of NOx emission concentration was 81.53%, the average exceeding standard hours (ESHs) was 2.64 h, and the average maximum emission hourly mean concentration (MEHC) was 284.06 mg·m-3. The NOx ESR of ULE units was 79.86%, the average ESHs was 2.52 h, and the average MEHC was 231.61 mg·m-3. There was no significant difference in the ESR and the average ESHs between non-ULE and ULE units, but the difference in the average MEHC was significant. There was no significant difference in the ESR, average ESHs, and average MEHC between different grades of non-ULE units, except that the average MEHC of 300 MW units was significantly lower than that of 200 MW units. There was statistical difference in the ESR, average ESHs, and average MEHC among different grades of ULE units, and 600 MW units performed best in the ESHs control, whereas 1000 MW units performed best in the emission concentration control.
Keywords: NOx; coal-fired power unit; emission characteristics; start-up process; ultra-low emission.