One of the drawbacks in high solids firing is the increased rate of NOx emission. Since NOx contributes to acid rain formation, a reduction in its emission is desirable. To control NOx emissions, knowledge of nitrogen chemistry in the recovery cycle is required. A study of three European mills showed that about 95% of the N2 in wood is released during pulping, with 10-15% being converted to ammonia. The remaining nitrogen is in the form of organic compounds; these compounds, and about half of the ammonia, end up in the black liquor (BL) [1]. Char gasification tests indicate that most of the nitrogen in the char is converted to cyanate during char oxidation, and that about 30% of it ends up in the smelt. Cyanate reacts to form ammonia in alkaline solutions such as green and white liquors. This, in turn, is vented from the liquor storage tanks or returned to the digester with WL. The ammonia is then vented from the digester blow tank or the early evaporator’s effects. When BL is concentrated to about 30%, some additional ammonia and a small amount of volatile organics are released into the foul condensate.

A more recent study [2] of 18 European mills over six years of operation, with recovery boilers manufactured or retrofitted by Andritz, shows that nitrogen distribution in the recovery cycle can vary widely between different mills with different recovery boilers (RBs).  The conversion to NOx was about 12-46%, while the conversion into N2 varied between 40-77%; the amount of nitrogen in the smelt was 8-28%.  For modern RBs, the conversion of nitrogen to NOx was about 20-30% of the total nitrogen content of BL. For RBs which are designed for low NOx emission, the conversion can be lower than 15-20%.  For these RB’s, the breakdown was 15% into NOx, 10% into smelt and 75% into N2 in the flue gas.  These boilers operate in steady conditions, with continuously cleaned air port openings and liquor guns, and well controlled excess air. The flue gas NOx emission rates correlated with excess air, furnace temperature, air staging and, in some cases, with the amount of carryover.

Lower NOx emissions may increase SO2 emissions if sulfidity is high and the furnace hearth temperature is low. The operating conditions for low NOx emissions may lower the melting point of carryover, resulting in fouling and corrosion in the superheaters, lower power generation capacity and increased steam consumption for soot blowing.
For some boilers, the high carryover resulted in a lower concentration of nitrogen in the smelt. This is because char particles burning in the upper furnace produce NOx, reducing the amount of nitrogen in smelt.  High liquor loading and high reduction efficiencies also decreased the nitrogen content of smelt. NOx emission did not appear to be connected to the concentration of nitrogen in smelt.

According to the results of this study, the key factors affecting the conversion of nitrogen into NOx are: furnace loading, design of the air and liquor systems, excess oxygen in firing, liquor dry solids, droplet size and design of the NCG burners.  Both the lower and upper furnace zones affect the conversion of nitrogen into NOx. A 1% unit decrease in excess oxygen in flue gas would reduce the NOx emission by 20 ppm.  The flue gas temperature in the final oxidation area has a direct effect on the conversion factor.
The nitrogen in char in the lower furnace, under reducing conditions and high temperature, forms N2 instead of NOx.

References:

1.    Distribution and Release of Nitrogen Compounds at Kraft Pulp Mills-A Survey of Three European Mills, N. DeMartini et al, Tappi 2004 Chemical Recovery Conference, June 6-10, Charleston, SC, USA.
2.    Distribution of Black Liquor Nitrogen Between Smelt, NOx and Flue Gases in Recovery Boilers, K. Saviharju et al, Tappi 2007 Chemical Recovery Conference, May 29-June 1, Quebec City, QC, Canada.