In operation of coalfired boilers

the ash fusion point

as well as gravity and thermal stress of the slag layer can act as restraining the slag layer to grow and selfclearing of slag in the furnace. Taking theoretic study and numeric calculation as main methods

the effect of three selfclearing mechanisms of slag

namely ash fusion point

gravity and thermal stress of slag crust

upon droppingoff of slag crust with different size has been studied. In the case of ash fusion point

i.e. fluidity temperature

is lower than the flue gas temperature in the furnace

the ash fusion point can restrain the slag infinitively to grow; and the gravity of slag can make huge and thick slag to dropoff

but this is easy to result in accident of boiler

and being not expected to happen during boiler operation; thermal stress occurred in loadreducing operation of the boiler can break slag layer with thickness from several milimeters to several centimiters

and make the slag layer to dropoff from the walltubes

being helpful to clear the small slag crust and the outer part of big slag crust

playing a main role of selfclearing of slag during safe operation of the boiler.