Systems and methods for grinding coal with secondary air bias and bowl pressure control loops and perforation plates

In a system for grinding coal into a powder and supplying powdered coal to a burner, wherein the system comprises a plurality of mill grinding systems, each mill grinding system comprising:

• a mill housing having a coal inlet, an air inlet, and an outlet;
• a milling unit in the mill housing, the milling unit being operable to receive coal that enters the housing through the coal inlet, the milling unit comprising a moveable milling member operable to grind the coal into a powder when the milling member moves;
• a drive system for moving the milling member, the drive system including a powered drive shaft and at least one gear for drivingly connecting the milling member to the drive shaft so that rotation of the drive shaft moves the milling member;
• a gear case defining a chamber, said at least one gear being in the chamber;
• an exhauster connected to the outlet of the mill housing, the exhauster being operable to draw air into the mill housing through the air inlet and further draw a mixture of air and powdered coal out of the mill housing through the outlet for delivery of the powdered coal to the burner;
• and
• a gear case pressurization system operable to maintain a gear case pressure in the gear case at a higher pressure than an air inlet pressure at the air inlet by controlling an air inlet damper which the air supply to the gear case;
• the improvement comprising:
• a common air supply supplying air to each of the gear cases of each of the mill grinding systems;
• a controller monitoring a position of each air inlet damper providing air to the gear case of each mill grinding system and for controlling the common air supply as a function of the monitored positions.