A pair of contradictory problems of unstable combustion at low load and serious slagging near the burners, appear on a 330MW boiler made by Babocock. By diagnosis from aspects of the distributed control system (DCS) control logics and the boiler operation modes, it is found that the root cause is the coarse controlling of primary air(PA) flow which is caused by improper measure or calculation logical blocks of DCS. These improper DCS control logics are corrected. The PA flow rates are calibrated by tests in order to get precise coefficients for the new logic. The combustion mode, the control curves of the primary air pressure and the secondary air pressure are optimally adjusted at last. As a result, the boiler efficiency increases by 0.75%, the power consumption rate decreases by 0.2%, the NOx emission concentration reduce to half.
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