The fate of char nitrogen in black liquor combustion—cyanate formation and decomposition

The main gaseous nitrogen emissions from a Kraft pulp mill are in the form of NOx and NH3. In black liquor combustion roughly 60% of the black liquor nitrogen is released during the devolatilization phase and finally forms N2 or NO. The remaining 40% is retained in the char and forms sodium cyanate during char conversion. A part of the sodium cyanate decomposes in the lower furnace, while the rest exits the recovery boiler with the smelt. Sodium cyanate decomposes to NH3 in green and white liquor, which can contribute to the overall nitrogen emissions of a pulp mill. The purpose of this work is to better understand the formation and decomposition of cyanate during thermal conversion of black liquor. The formation and stability of cyanate was studied during thermal conversion of black liquor droplets in different gas compositions, and the formation was also studied by controlled char gasification in CO2 atmosphere. The decomposition was studied by exposing laboratory made black liquor smelt to various gas atmospheres at different temperatures. The results show that gas atmosphere and temperature play a role in how much cyanate is formed as well as the rate of decomposition. The share of black liquor nitrogen converted to cyanate is clearly higher if the gas composition contains CO2 or O2 mixed with N2. Less cyanate is formed if the surrounding atmosphere is pure N2 gas or contains water vapor. The formed smelt cyanate is stable at 800 °C in atmospheres containing only CO2 and N2, whereas it decomposes slowly at 900 °C in atmospheres containing only CO2 and N2. The cyanate in smelt decomposes quickly in oxygen and water vapor containing atmosphere compared to pure N2 atmosphere. Also, importantly, cyanate leads to very little NO formation (less than 5% of the original black liquor N) in all of the conditions tested. This information will be utilized in the future to develop a simplified char-N model for implementation in computational fluid dynamics (CFD) with the ultimate goal of finding new ways of further lowering NO emissions from recovery boilers without downstream NOx reduction technologies.