Abstract: A black liquor recovery boiler furnace having quaternary air injection ports located in the furnace walls in the vicinity of, or at approximately the same elevation as, the black liquor injection guns. The quaternary air injection ports are located between the secondary air injection ports and the tertiary air injection ports. Air introduced at this level provides a much more efficient mixing and combustion without solid particle carryover. A zone of increased or intensified combustion is established around the black liquor injection guns. The injection site combustion zone is at an intermediate location to the combustion zones conventionally supplied by the secondary and tertiary air ports. In accordance with a preferred embodiment of the invention, the black liquor injection guns inject the black liquor into the furnace in a downwardly inclined direction.