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.

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