High combustible content of slag occurred in a supercritical 600MW unit once-through boiler running in lower five mills operating mode.Thus

the optimization combustion adjustment test was performed.Such countermeasures as increasing the operation oxygen content

decreasing the coal fineness

enlarging the mill ventilation

turning up the A1 deflectable air and turning down the surrounding air at layer A were employed

making the combustible content in slag dropped from 10.05%(under standard test condition) to 4.79%

indicating the optimization effect was dramatic.To decrease the high combustible content in slag thoroughly

it suggested that transformation on bottom-holding air

bottom secondary air and plasma burner should be conducted.