NOx Reduction in Black Liquor Combustion - Understanding Reaction Mechanisms Reveal Novel Operations Strategy Options, 1998 International Chemical Recovery Conference Proceedings

Recovery boiler NO, emission originates mostly in the small amounts of nitrogen originally present in the black liquor. This paper summarizes the most recent research findings concerning the behavior of this black liquor nitrogen into an overall picture of the various NO formation paths in recovery furnaces. This overall description of the reactions of the fuel nitrogen is used to reveal the key operating parameters by which the NO emission can be influenced.

A major part of the recovery boiler NO is formed from the oxidation of the ammonia formed during the devolatilization of the liquor droplets. Advanced kinetic modeling clearly shows that the efficiency of this oxidation is strongly dependent on the temperature and on the number of stages by which the air is mixed’ with the devolatilized gases. Formation of NO by this route can be minimized if the oxidation of the devolatilized gases takes place in several stages, and at a relatively low temperature, preferably 850-900ºC. In practice this can be achieved by suitable adjustment of the air distribution and by introducing additional air feed levels in the upper furnace.

Another significant source for the final NO emission is the oxidation of the nitrogen remaining in the char residue after the devolatilization stage. Laboratory devolatilization and combustion tests using single black liquor droplets reveal that this char nitrogen will be readily oxidized to NO if the droplets are allowed to be completely burned out. However, typically in a recovery furnace this complete bum-out in flight happens only to the smallest droplets. Consequently, this NO from the char nitrogen can be minimized if the droplet size is large enough and spraying of the liquor is selected in such a way that most of the droplets reach the smelt surface before being fully burned out. In this case the nitrogen in the char will be converted into an inorganic nitrogen compound which leaves the furnace along with the smelt.