What is Chemical Deaeration in Boilers ?

Chemical deaeration is the use of Chemicals to remove the dissolved gases, usually oxygen.  Chemical deaeration is usually used after Mechanical deaeration.  Even after mechanical deaeration, all the oxygen will not be removed.  An chemical known as an oxygen scavenger is used.  This ensures that all the oxygen has been removed. 

A common oxygen scavenger is Sodium Sulphite.  Sodium sulphite reacts with the trace amounts of oxygen.  Sodium sulphite, however, cannot be used at high pressure as it can decompose to acidic gases which can increase corrosion. 

Another oxygen scavenger is hydrazine.  Hydrazine reacts with oxygen and produces volatile compounds which do not dissolve.  Hydrazine also does not cause corrosion.  However, the downside is that it is a carcinogen (cancer causing substance) and thus has to be used very carefully.  It may be banned in the future.