Economizer water recirculation system for boiler exit gas temperature control in supercritical pressure boilers

An economizer inlet mixing device located upstream of an economizer in a supercritical pressure boiler having at least one waterwall disposed within a furnace to produce a recirculation fluid stream, the economizer inlet mixing device comprising:

• a housing to mix a feedwater stream and the recirculation fluid stream, including steam, to provide a mixture water stream;
• a first inlet to receive the recirculation fluid stream from the waterwall of the furnace;
• a second inlet to receive the feedwater stream and offset by less than ninety degrees from a central axis that extends vertically through the housing, defining an offset angle of the inlet;
• an outlet to provide the mixed water stream to an economizer;
• a sparger assembly disposed in the housing through which at least a portion of the recirculation fluid stream from the waterwall passes through a plurality of holes to mix the recirculation fluid stream with the feedwater stream;
• and
• a wave breaker assembly disposed at the outlet and comprising a plurality of baffles through which is received the mixed water stream, wherein the plurality of baffles minimize fluid vibration and the mixed water stream comprises a combination of the flow of the recirculation fluid stream through the sparger assembly and the flow of the feedwater stream, with the mixed water stream being of a first temperature sufficient to raise a second temperature of a flue gas exiting the economizer to an optimum value for less than full load operation of a selective catalyst reduction (SCR) system located downstream of and in communication with the economizer
• wherein the offset angle of the inlet, the sparger assembly and the wave breaker assembly allow for optimum mixing of the recirculation fluid stream and the feedwater stream so that the first temperature of the mixed water stream entering the economizer can be precisely controlled to thereby control the second temperature of the flue gas exiting the economizer to the optimum value to ensure catalyst reactiveness in the SCR system.