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Viessmann Wall-Hung Boiler Repair Guide: How does the furnace slag?
The temperature at the combustion center inside the boiler furnace can reach as high as 1500～1600℃, at which time the coal ash is mostly in a molten state. A properly designed furnace has the necessary cooling capacity to reduce the temperature of the flue gas near the furnace outlet or around the water-cooled wall to below the softening temperature of the ash. At this point, the molten ash in the combustion center solidifies into solid ash as it approaches the water-cooled wall or the furnace outlet, and will not adhere to the heat-absorbing surface to form slag.
However, if the cooling capacity of the furnace design is insufficient, or if improper operation causes the combustion center to skew or overloads the system, the flue gas temperature near the water-cooled wall may become too high. Under such excessively high flue gas temperatures, the molten ash cannot solidify and will adhere to the water-cooled wall, forming slag. Once a layer of slag forms on the water-cooled wall, it reduces the wall's ability to cool the flue gas, making it easier for subsequent molten slag to adhere, thus creating a vicious cycle that rapidly thickens the slag layer while simultaneously causing the flue gas temperature outside the slag layer to rise sharply.
When this flue gas temperature rises to the ash melting temperature (t3) or higher, the incoming molten slag no longer solidifies on the slag layer but instead flows downward along the surface of the slag layer, quickly expanding the area of slagging downward. It is clear that the process of slagging is an automatically exacerbated vicious cycle.