Model for No Formation in Recovery Boilers, 1998 International Chemical Recovery Conference Proceedings

This paper presents a model for NO formation and reduction in recovery boilers. The model consists of the following parts: N release during black liquor pyrolysis, NO formation during char combustion, volatile NO formation, NO destruction, and NO formation from combustion air. Nitrogen release during pyrolysis is divided into two parts: devolatilization and extended pyrolysis. During devolatilization the fractional nitrogen release is directly proportional to the tractional carbon release. The formation of NO from char is modeled to depend on whether O2 reaches the char surface or not. For the oxidation of the reactive volatile N intermediate NH,, global gas phase reactions are used. NH3 destruction and oxidation is catalyzed by Na2 CO3 and kinetic expression for the reactions in the absence and presence of O2 are given based on fixed bed reactor experiments. For NO reduction besides homogeneous reactions, two heterogeneous processes for NO reduction are included: reduction by black liquor char and fume particles. Laboratory experiments suggest that part of the NO may be prompt NOx.