Method of firing a pulverized fuel-fired steam generator

In a steam generator for burning pulverized fuel having a mill for pulverizing the fuel, a furnace wherein the fuel is burned thereby generating hot flue gas, steam heating surface disposed downstream of the furnace in the flow path of the hot flue gas leaving the furnace whereby heat is transferred from the hot flue gas thereby cooling the flue gas traversing said steam heating surface, a particle collector disposed downstream of the furnace wherein particles entrained in the flue gas are removed therefrom, and a fan disposed downstream of said heating surface and said particle collector for boosting the pressure of the cooled clean flue gas before it is vented to the atmosphere, a method of firing said steam generator comprising:

• a. feeding solid fuel to the mill for pulverizing therein at a controlled rate in response to load demand on the steam generator;
• b. recirculating a portion of the cooled clean flue gas leaving said booster fan to the mill;
• c. mixing ambient air into the recirculated flue gas before introducing the gaseous mixture thereof into the mill;
• d. conveying the fuel pulverized in the mill to the furnace entrained in said gaseous mixture of recirculated flue gas and air;
• e. controlling the volume flow rate of said gaseous mixture to the mill in response to the feed rate of fuel to the mill;
• f. measuring the oxygen content of said gaseous mixture of recirculated flue gas and air entering the mill;
• and
• g. controlling the volume flow rate of recirculated flue gas and the volume flow rate of air with respect to each other in response to said oxygen measurement so as to maintain the oxygen level in said gaseous mixture entering the mill at a level of at least 12% by volume.