Dust and flue gas chemistry during rapid changes in the operation of black liquor recovery boilers--Part 3: gaseous emissions.

Application: The tests confirmed that Nox originate in the nitrogen of black liquor droplets burning in-flight. Nox emissions of different black liquors can thus be estimated from pyrolysis-NO in single droplet combustion tests.

This is the third of three papers concerning dust and flue gas chemistry of kraft recovery boilers during rapid changes in boiler dynamics. This paper presents the results of the formation of gaseous emissions, especially NOx, during the full-scale mill trials. The purpose of these studies was to find out how the changes in boiler load effect emission formation in the two kraft recovery boilers. In addition, special interest was paid to the share of NOx produced from black liquor droplets burning in-flight versus the NOx formed by surface reactions from the char bed.

These dynamic tests indicate that all or absolute majority of the NOx emissions originated in the black liquor droplets during in-flight burning. The surface reactions of the char bed played little or no role in NOx formation. NOx emissions increased linearly with increasing the nitrogen input to the boiler (i.e., increasing dry solids load). The black liquor nitrogen conversion to NOx in the flue gases was 25%-30% of the total nitrogen in BLS.

The single droplet combustion test with the liquors showed that the pyrolysis-NO gives a good approximation of the NOx formation tendency of the black liquor. The calculated NOx emissions (from the pyrolysis-NO) were [+ or -] 20% of the measured NOx emissions. This indicates that the char-NO in single droplet combustion test can be neglected when NOx formation tendencies of different black liquors are compared. The results confirmed that the share of black liquor nitrogen that ends up at the char bed during black liquor combustion would continue its way along with the smelt (smelt-N).

Sulfur emissions (S[O.sub.2], TRS) were low or zero during full load operation of theTse boilers due to high dry solids firing. In boiler A, sulfur emissions were sensitive to rapid changes in boiler load.

Tarja Tamminen, Mikael Forssen, and Mikko Hupa, Abo Akademi University, Process Chemistry Group, c/o Combustion and Materials Chemistry, Lemminkaisenkatu 14-18 B, 20520 Turku, Finland; email Tamminen at [e-mail] or [e-mail].