The fate of nitrogen in the chemical recovery process in a Kraft pulp mill. Part IV: Smelt nitrogen and its formation in black liquor combustion

During black liquor combustion, some of the liquor nitrogen can end up in the smelt. In previous studies in this series, this "smelt nitrogen "was found to convert gradually to ammonia (NH3) during subsequent green and white liquor processing, and cyanate (ONC)ions were identified as the NH3-forming nitrogen species, In this work, it is shown that cyanate, presumably in the form of NaOCN, is formed from char nitrogen during char conversion under recovery boiler conditions in the presence of the alkali metals (Na). Under oxidizing conditions, the cyanate forms NO, the amount of which is clearly higher at 800degreesC than at 900degreesC. NO formation from smelt oxidation is expected to be small in an actual recovery boiler.