Gas Recirculation Fan
- Gas Recirculation Fan (GRF) draw gas from a point between the economizer outlet and the air heater inlet and discharge it into the bottom of the furnace outlet.
- Recirculation gas introduced in the vicinity of the initial burning zone of the furnace is used for steam temperature control, while re circulated gas introduced near the furnace outlet is used for control of gas temperature.
-RH outlet steam temperature is normally controlled by regulating the gas recirculation flow. Increased flue gas flow over Reheater of the convection heating surfaces increases the heat absorption and RH temperature is increased.
-The following picture will show the full specification of the GRF ( motor and impeller ).
-Due to large GRF duty , Bearings are equipped with water cooling flexible line , the bearing at the motor side is fixed to the shaft, while at the counter to the motor side is set free for the shaft expansion caused by the heat of high temperature gas in the casing during the continuous operation.
-Because the fan treats the high temperature gas, it is adopted with a turning device, If the fan stopping remain the high temperature gas in the casing, the impeller has the unbalance by heat effect. So the purpose of the no unbalance of the heat effect drive the turning device regular time at about 50 rpm until the economizer heat decreased to a safety limit about 100 centigrade.
Gas Recirculation control
-RH outlet steam temperature is normally controlled by regulating the gas recirculation flow. Increased flue gas flow over Reheater of the convection heating surfaces increases the heat absorption and RH temperature is increased. The amount of gas recirculation flow is controlled by positioning the inlet dampers on the two Gas Recirculation Fans (GRF).
(1)During start -up (approximately < 25% boiler load), GRF inlet damper position is control led by the function generator from Boiler Master. Because RH temperature feedback control is difficult during the start-up due to the slow response.
(2) Dynamic feed forward signal (BIR) is added to GRF inlet damper to improve RH steam temperature control during load change.
(3)High and low limit from Boiler Master for GRF inlet damper are provided. High limit is to avoid the unstable combustion and the over current of GRF. And low limit is to protect the water wall and avoid the high NOx.
