Patent ID: 8521333

Claim:
In an air quality control system comprising a boiler having a flue gas exhaust, a scrubber having an inlet duct coupled in fluid communication with the flue gas exhaust of said boiler, an induced draft (ID) fan having an inlet connected in fluid communication downstream of said scrubber and an outlet, and an exhaust chimney coupled by an exhaust duct to the outlet of said ID fan for exhausting treated flue gas, an improvement comprising an apparatus for recirculating flue gas from the exhaust duct downstream of said ID fan to upstream of said scrubber, the flue gas recirculation apparatus further comprising: a recirculating duct connected at one end at a junction to said exhaust duct, and connected at another end to said scrubber inlet duct; a low load flue gas recirculation device restricting said exhaust duct downstream of said junction, said low load flue gas recirculation device further comprising a variable flow restrictor located inside said exhaust duct to selectively induce backpressure in the exhaust duct forcing flue gas into said recirculation duct, a first actuator engaged to said variable flow restrictor for varying the position thereof between fully open and partially restricted, and a first position controller connected to said first actuator for electronic positioning of said actuator and flow restrictor anywhere between said fully open and partially restricted positions: a flue gas recirculation flow control damper located in said recirculation duct between said junction and said scrubber inlet duct, said flue gas recirculation damper further comprising a variable position damper located inside said recirculation duct to selectively vary flue gas recirculation to said air quality control system input, a second actuator engaged to said variable position damper for varying a position thereof between fully open and fully closed, and a second position controller connected to said second actuator for electronic positioning of said second actuator and variable position damper anywhere between said fully open and fully closed positions; a computer controller programmed with control software comprising computer instructions stored on non-transitory computer memory for controlling said first and second position controllers, and in communication with said master boiler controller for deriving there from two control signals, for controlling both of said first and second position controllers to meet the minimum flue gas velocity requirement of the air quality control system.