Cold section corrosion is corrosion of sections operating at less than 1000°F. These sections include the air preheater, the economizer, and the stack.

The primary reason for cold section corrosion 1s the condensation of sulfuric acid on the metal surface. Fuel oil contains sulfur either

in Its elemental form or as hydrogen sulfide, mercaptan, sulfides, disulfides, thiophenes, or polysulfides. During fuel oil combustion the sulfur reacts with oxygen forming sulfur dioxide:

A variable fraction of the sulfur dioxide reacts with oxygen forming

Sulfur trioxide:

        catalyst

S0₂ + 1/2 0₂ ----------> S0₃

In the presence of moisture the sulfur trioxide will condense as sulfuric acid.

The graph shows the relationship of dewpoint to the concentration of S03 in the flue gas. It should be noted that a high dew point is undesirable because it increases the chance for sulfuric acid condensation before the flue gas leaves the stack.

The catalyst which drives the formation of SO3 may be vanadium

deposits or the metal surfaces of the boiler. When cold section corrosion is a problem the two possible means of solution are to use combustion catalysts or slag modifiers.

The use of WSF to catalyze combustion will reduce the amount of excess air required for combustion and thus reduce the amount of oxygen available to oxidize sulfur dioxide to the trioxide form.

The use of WSF, as slag modifiers will distort the vanadium deposits which act as catalysts toward sulfur trioxide formation, thus poisoning their catalytic activity.

The slag modifiers will also act as neutralizing agents toward sulfur trioxide.