Fireside Corrosion

Importance of maintaining an Oxidizing atmosphere at the Furnace Exit

Furnace slagging propensity is profoundly influenced by excess oxygen levels in the furnace.
To maintain an oxidizing environment, and prevent slagging, furnace oxygen levels should be uniform at 3.0% with no single point below 2.0% at the furnace exit. A reducing atmosphere (absence of oxygen), is conducive to slagging of furnace heat transfer surfaces because iron compounds in the coal undergo chemical changes which allow for a lower
melting temperature of the coal ash. In areas of the furnace where there is adequate oxygen, pyrite (FeS2 ), is converted to Fe2O3 ( melting temperature of ~2900°F) and SO2. In reducing atmosphere, FeS2 can be converted to FeS ( that has a lower melting temperature of 2179°F-2190°F) and other ferric compounds of lesser oxidation that have lower
melting temperatures. Ensuring the furnace exit is an Oxidizing atmosphere is also of critical importance in reducing Carbon in flyash, reducing formation of CO and prevention of coal-ash corrosion.

With a reducing atmosphere (0% oxygen) slag is forming on the nose arch because the melting (fusion) temperature of the ash is lower in the absence of oxygen.

With 3% oxygen and the same temperature as the photograph to the left, slag is not forming because the melting (fusion) temperature of the ash is higher in an oxidizing environment.