Large drop in the furnace draught pressure is a good indicator of clinker formation. Clinker formed inside the boiler reduces the heat transfer area for the flue gases and increases the electrical consumption of the ID & FD fans.
Clinker also known as slag, consist of the noncombustible elements and minerals found in fuel that melt and fuse together as lumps. Clinker either sticks to the walls of the boiler or falls to the floor where it is covered by another boiler operation by-product, ash (in case of solid fired boilers).
The most common cause of clinker in a boiler is inferior fuel sources. Sometimes poorly maintained or operated equipment is also the cause of clinker formation. If the boiler's air-to-fuel ratio is not correct, the fuel does not burn completely and the un-burnt particles fuse in the high temperature zones of the boiler combine to form clinker.
Large clinkers accumulation in the path of the flue gas hinders air passage. The ID and FD fans work to push the air through the furnace to the chimney. Clinker accumulation in the flue gas path restricts air flow and creates a backpressure. Thus the draught pressure will drop and now the ID & FD fans will need to work harder to maintain the required draught. If the ID & FD fans now need to be run at much higher rpm than normal, it is a good indication that clinker is creating obstacles in the flue gas path. Clinker only increases the electrical consumption of the ID & FD fans but also reduces heat transfer area when it settles on the boiler tubes.
Thus it is recommended to check the furnace surfaces regularly for clinker. Additionally ensuring right air to fuel ratio and ensuring right fuel quality as part of the operating practice will also minimize its occurrence.