Overfire air admission with varying momentum air streams

Claim 1

In combination, a vertical furnace of substantially square cross section, in which pulverized coal is burned, a rear pass connected to the upper portion of the furnace through which the combustion gases are exhausted from the furnace, a heat exchanger in which the combustion gases give up heat to combustion air passing therethrough, a first duct through which air is supplied to the heat exchanger, a first fan in the first duct for supplying low pressure air to the heat exchanger, first nozzle means located in each of the four corners of the furnace, for introducing coal into the furnace tangentially of an imaginary circle located in the center of the furnace, so that a fireball is created which flows upwardly therein along a first rotational path, a second duct for conveying low pressure air from the heat exchanger to the first nozzle means, a pulverizer mill in which coal is pulverized, a third duct having an inlet connected to the second duct, and an outlet connected to the pulverizer mill, second fan means positioned in the third duct, for supplying high pressure air to the pulverizer mill, a fourth duct for supplying pulverized coal carried in a high pressure air stream from the pulverizer mill to the first nozzle means, second nozzle means located in each of the four corners of the furnace above the level of the first nozzle means, for introducing overfire air to the furnace tangentially of an imaginary circle therein, in a direction of rotation opposite to that of the fireball, so that the gases flowing to the rear pass have little or no rotational flow, a fifth duct for conveying low pressure air from the second duct to the second nozzle means, and a sixth duct for conveying high pressure air from the third duct at a point downstream of the second fan to the second nozzle means.