Boiler Air and Flue Gas System


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The boiler air and flue gas system consists of combustion air system, gas recirculation system and flue gas system.

1) Combustion air system

The combustion air system consists of two forced draft fans, two steam coil air heaters and two air heaters. Two 60% capacity centrifugal forced draft fans are provided to supply the combustion air to the windbox and after air port with adequate flow rate and pressure for combustion.

The forced draft fan draws air from the atmosphere through the inlet silencer, and discharges air to the steam coil air heater, the air heater and the windbox. Modulating inlet vane of the centrifugal forced draft fan controls the combustion air flowing to the boiler wind box and after air port. Air heater inlet air damper creates the suitable backpressure for seal air supply through the forced draft fan outlet connecting air duct.

Combustion air flow is measured by flow element located at the outlet of air heater. Two 50% capacity, bi-sector, vertical shaft regenerative type air heaters are provided to heat combustion air using flue gas from the economizer. To achieve in-furnace NOx reduction, two-stage combustion system using the after air port dampers is applied. Part of the combustion air is supplied to the after air ports for two-stage combustion. The after air port damper (4 dampers) control shall be modulated to maintain the optimum two-stage combustion at each load.

2) Gas Recirculation system

The gas recirculation system consists of two gas recirculation fans. Two 60% capacity centrifugal gas recirculation fans are provided to supply gas recirculation flow to the furnace for reheat steam temperature control. The gas recirculation fan draws flue gas from the economizer outlet flue gas duct and discharge gas to the furnace. Modulating inlet damper controls gas recirculation flow rate. The gas recirculation flow set point is derived from the reheat steam temperature control.

3) Flue Gas system

The flue gas system consists of two air heaters. The combustion gas transfers its own heat to the water wall, the superheaters, the reheater and the economizer, then leaves the boiler to the flue gas ducts. The flue gas goes to the stack through the AH and is exhausted to the atmosphere.

Steam Coil Air Heater

Two 50% capacity steam coil air heaters are installed in the combustion air duct upstream of the air heaters to heat combustion air before air flows into the air heaters. The steam coil air heater raises the air heater cold end temperature to predetermined set point over the entire load range when fuel (Mazout) oil is burnt. The steam coil air heater is a finned, steel tube heat exchanger. Steam Used for heating flows through the tube side, Combustion air is heated as it flows across the outside of the finned tubes.

Steam coil air heaters are installed vertically at the air heater inlet duct. The steam coil air heater casing and coil section covers the full dimension of the duct. Heating steam is supplied for the steam coil air heater system from auxiliary steam system.

“This description is about HITACHI Steam Generator “