Temperature control system for an air preheater installation

A method of retrofitting an air preheater installation with a temperature control, wherein the air preheater is a trisector air preheater, the method comprising:

• providing the trisector air preheater comprising:
• a flue gas inlet duct configured for feeding flue gas from a boiler to the trisector air preheater,
• a flue gas outlet duct configured for discharging flue gas from the trisector air preheater,
• a primary air inlet duct configured for providing primary air to the trisector air preheater,
• a primary air outlet duct configured for feeding a first amount of primary air to the boiler via at least one pulverizing mill,
• a secondary air inlet duct configured for feeding a secondary air to the trisector air preheater,
• a secondary air outlet duct configured for feeding a second amount of secondary air direct to the boiler,
• a heat exchange rotor having a hot end proximate the gas inlet duct and a cold end proximate the secondary air inlet duct,
• a secondary air bypass duct in communication with the secondary air inlet duct and the secondary air outlet duct, the secondary air bypass duct configured to bleed a portion of the secondary air as secondary air bypass from the secondary air inlet duct upstream of the trisector air preheater for reintroduction downstream into the secondary air outlet duct, and
• a flow control device for controlling volumetric flow of the secondary air bypass and tempering primary air flow, the primary air flow configured to maintain the flue gas outlet temperature at or above a second minimum temperature for the flue gas volumetric flow exiting the trisector air preheater to maintain primary air outlet temperature within a safety range of a minimum temperature required to facilitate coal drying in the pulverizing mill to a maximum temperature that would lead to auto-ignition of coal in the pulverizing mill;
• and
• utilizing a pre-existing primary air bypass duct throttled with a pre-existing damper valve;
• and
• installing an appropriately sized secondary air bypass duct closable with the damper valve,
• wherein during low load operation, upon opening of the damper valve and full opening of the damper valve the primary air outlet temperature is increased by a predetermined amount sufficient to raise primary air outlet temperature to at least a minimum primary air outlet temperature.