The method includes, providing a series of lower compartments (208) for introducing therethrough air and fuel into the combustion chamber. At least one upper compartment (210) is disposed above the topmost compartment (208TM) at a relative disposition to the topmost compartment (208TM) in a spacing range between a contiguous disposition to a more spaced disposition which is no more than twice the average spacing between any given compartment and an adjacent compartment. Air is injected from the at least one upper compartment (210) generally in opposition to the swirling fireball (RB) along a 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 in the top half of the furnace (204) characterized by portions thereof flowing upward at differing vertical velocities.