Role of thermophoresis in the deposition of fume particles resulting from the combustion of high inorganic containing fuels with reference to kraft black liquor

Fume generation and deposition are key processes in many manufacturing oper ations. These include the manufacture of carbon black and silica fume. In a ddition to its role in these processes, fume deposition is an important phe nomenon in the combustion of fuels containing inorganic compounds. One comb ustion process where fume deposition presents special problems is the burni ng of kraft black liquor. This paper presents a laboratory study of fume de position with emphasis on the kraft recovery boiler. A unique aspect of thi s study was the construction of an experimental system capable of generatin g sodium sulfate, sodium carbonate, and sodium chloride fume particles. Wit h this experimental system, variables such as particle size, particle gas-p hase concentration, particle composition, temperatures and gas flow rates c ould be independently controlled. The major objectives of this study are to : (1) determine the mechanisms responsible for particle deposition within t he kraft recovery furnace, (2) provide experimental data to support the hyp othesis that thermophoresis is the major mechanism responsible for depositi on of fume type particles, and (3) correlate the experimental results obtai ned in the study to deposition rates observed in operating recovery furnace s. The experimental results obtained in this study showed that thermophores is is the dominant mechanism for fume deposition under conditions similar t o those existing in the kraft furnace. This result is well supported by the experimental data. For example, thermophoresis theories accurately predict effects of the temperature, particle concentration, and particle size that were observed in this study. The experimental results obtained in this stu dy were extrapolated to deposition in an operating kraft furnace and found to provide a reasonable prediction of boiler bank deposition. Although the study was targeted at the kraft recovery boiler, the results should be appl icable to the combustion of any fuel containing a high level of inorganic m aterial. (C) 1999 Published by Elsevier Science B.V. All rights reserved.