High set seperated overfire air system for pulverized coal fired boilers

A furnace (10) includes a series of lower compartments (18) extending in a vertical arrangement, with at least one of the lower compartments (18) having one or more fuel nozzles (20) which tangentially fires fuel into the combustion chamber at an offset from a diagonal (DD) passing through opposed corners of the combustion chamber. At least one of the lower compartments (18) has one or more air nozzles (22) which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal (DD) to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create a swirling fireball (RB) in the combustion chamber. At least one overfire compartment (24) has at least one air nozzle (40) for injecting air in opposition to the swirling fireball (RB), along an opposition offset direction which is offset to the other side of the diagonal (DD), in a manner such that the injected air promotes the evolution of the swirling fireball (RB) into an upward flow.