Nitrogen oxide formation in black liquor single droplet combustion

Nitrogen oxide formation in single droplet combustion of black liquors was measured under different combustion conditions (droplet size 5-45 mg, temperature 800-900degreesC, O-2-concentration 3-21%). The NO formation in relation to the carbon conversion is presented. The amounts of NO formed during pyrolysis (pyrolysis-NO), i.e. from the burning of volatile nitrogen species, and during char burning stage (char-NO) are reported. Under all conditions studied, the NO formation followed the carbon conversion up to about 40-50%, after which the NO formation stopped, and began again only when about 90% of the carbon had been converted. The relative amount of pyrolysis-NO varied with droplet size at 900degreesC and with oxygen concentration at 800degreesC. Possible mechanisms for these effects are suggested. The char-NO originated mostly in the oxidation of the inorganic residue left after the burnout of the char. The amount of char-NO depended on temperature: more NO was formed at 800degrees than at 900degreesC. The implications of the findings for NO, emissions in black liquor burning in recovery boilers are discussed.