EXHAUST GAS BOILER

PURPOSE:To prevent precipitation of acid ammonium sulfate in a low temperature heat transfer tube and to obtain high heat recovery rate by almost completely decomposing excess ammonia at a downstream of a denitrifying unit even when sulfur oxide is contained in exhaust gas in an exhaust gas boiler assembled with the unit. CONSTITUTION:A high-pressure evaporator is divided, a residual ammonia decomposing unit 14 is installed between divided evaporators 3a and 3b, and excess ammonia not consumed in a denitrifying unit 13 is decomposed to nitrogen gas and steam. A temperature region having high ammonia decomposing rate is provided between the evaporators 3a and 3b. Accordingly, even if sulfur oxide is contained in the gas, acid ammonium sulfate is not precipitated on surfaces of a low temperature evaporator 6, a low temperature economizer 7, etc., disposed at a downstream, and heat recovery up to a low temperature area of the gas is conducted, and hence a plant efficiency is improved. Since an ammonia injection nozzle 15 at an upstream of the unit 13 can supply a sufficient amount of ammonia with safety, a high denitrification rate is obtained.