Spent liquor, or so-called black lye, is generated particularly in some manufacturing processes of the paper industry, such as pulping. The black lye is burnt in recovery boilers made for the processing of black lye, i.e. so-called soda recovery boilers, wherein energy, flue gases and chemicals used during the pulping are released from the black lye. The feeding of combustion air affects particularly the usability of the apparatus and the releases.
It is known to feed combustion air to different air feeding levels of the soda recovery boiler, i.e. so-called primary, secondary, tertiary and quaternary levels, wherein it is possible to control the combustion in the different parts of the boiler and thereby to achieve as complete a combustion as possible. In the feeding of combustion air, it has turned out to be problematic that the combustion air is typically not divided evenly on the whole air feeding level but various turbulences are developed in the boiler which prevent the appropriate combustion of the black lye.
To eliminate the problem, various systems have been developed for feeding combustion air. U.S. Pat. No. 5,305,698 discloses an arrangement, in which the combustion air is introduced into the furnace of the boiler from opposite edges in such a way that the air jets emitted from the opposite edges of the furnace are interlaced. A problem in such an arrangement is that the air jets either impinge on the opposite wall of the furnace or the air jets remain short. In the impact between the air jet and the wall, a turbulence area is developed which is harmful for the combustion process. If the air jets remain short, the combustion air is not mixed in a desired way with the substance to be combusted.
U.S. Pat. No. 5,305,698 also discloses an arrangement in which combustion air is introduced from opposite edges of the furnace of the boiler in such a way that short and long air jets are introduced alternately from the edges of the furnace, and the air jets emitted from the opposite edges of the furnace meet in such a way that a short air jet meets a long air jet coming from the opposite side (in other words, so-called partial interlacing of the air jets takes place). Typically, in such an arrangement, the air jets hit each other and a so-called droplet lift is developed in the impact area where the droplets of black lye are entrained in a strong upwards flue gas flow carrying said droplets to the upper part of the furnace. In this case, the combustion process in the boiler is distorted as the combustion is effected “in the wrong place” wherein, for example, the efficiency of the boiler is changed and the releases are increased.