Dust and flue gas chemistry during rapid changes in the operation of black liquor recovery boilers: Part 1--dust formation.

Application: The work adds insights on the topic of dust formation in kraft recovery boilers, and demonstrates the possibilities an on-line dust monitoring system can provide.

This paper is the first in a series concerning dust and flue gas chemistry in two kraft recovery boilers. The mill measurements were made during dynamic changes in boiler load. The two boilers were firing softwood liquors with similar composition; only the dry solids concentrations of the liquors differed (71%-74% versus 78%-80%). This paper describes the test procedures and presents the results of dust formation. Part II will present dust composition results, and Part III will concentrate on gaseous emissions, especially to NC formation.

We conducted these studies to shed light on the formation mechanism of fume and gaseous emissions. We paid special attention to the relative importance of the fume produced from the burning black liquor droplets versus the fume volatilized from the char bed. For the first time through this study, quantitative on-line data are available on the amount of particulate formed during char bed burning, as the liquor spraying was totally interrupted for a few minutes.

The amounts of dust in flue gases and the boiler load showed clear dependency. The spraying pressure and droplet size of the black liquor were kept constant. We altered the loading by changing the number of liquor guns operating. The more liquor droplets we introduced to the furnace, the more dust was formed. The amount of fume formed was 6-8 wt-% of the black liquor solids load of the boiler during full load operation of the two boilers.

The release of fume during the total interruption of black liquor flow was about 1 g/s per floor square meter of the two boilers. From 90%-95% of the dust in the flue gases originated in black liquor droplets and only 5%-10% was released from the surface of the char bed in these two boilers.

Tamminen and Hupa are with Abo Akademi University, Process Chemistry Group, c/o. Combustion and Materials Chemistry, Lemminkaisenkatu 14-18 B, 20520 Turku, Finland; Kiuru is with Oy Metsa-Botnia Ab, Kaskinen mill, 64260 Kaskinen, Finland; Kiuru is with Oy Metsa-Botnia Ab, Rauma mill, PL 165, 26101 Rauma, Finland; and Janka is with Kvaerner Pulping Oy, PL 109, 33101 Tampere, Finland. Email Tamminen at [e-mail] or [e-mail]