An improved method and system for flue gas recirculation (FGR) from the ID
Fan located downstream of a scrubber (or other Air Quality Control
Equipment including cyclones, baghouses, etc., hereafter "AQC equipment")
back to the scrubber (or other AQC Equipment) inlet to maintain the
minimum required flue gas throughput during low-load conditions, thereby
eliminating the need for a separate booster fan. A flue gas recirculation
duct is connected from downstream of the ID Fan to the inlet of the
scrubber (or other AQC Equipment). A variable flow restrictor is placed
in the duct downstream of the ID Fan for controlling pressure of the
recirculated flue gas. The flow restrictor is controlled in accordance to
a function of boiler load or flue gas flow to produce the required
backpressure that enables the flue gas to flow through the recirculation
duct back to the inlet of the scrubber (or other AQC Equipment) at low
loads. This side stream of treated gas is recirculated from the discharge
of the ID Fan to the inlet of the AQC Equipment to maintain the
appropriate gas velocity regardless of boiler load conditions. The
pressure differential between fan discharge and the AQC Equipment inlet
enables recirculation of a sufficient amount of treated flue gas using
only a small duct and simple flow control damper, without using a booster
fan.