A Study of fume particle deposition

by Goerg, Kristin A.

Mechanisms responsible for fume deposition on kraft recovery furnaces were studied. The main reason for determining the mechanisms is to understand the controlling parameters, thereby obtaining the ability to affect the deposition rate by changing these parameters. The following mechanisms were reviewed and examined as possible fume deposition mechanisms: molecular diffusion, Brownian motion, turbulent diffusion, particle impaction, thermophoresis, and vapor diffusion/crystallization. Thermophoresis was found to be the main deposition mechanism for fume particles under the following experimental conditions: 1. Fume particle sizes from 0.1 gm to 1 am in diameter. 2. Flue gas temperatures from 2500C to 5800C. 3. Reynolds numbers less than 3 (based on cooled tube diameter). The following equation was derived: Dep. rate (g/min/cm2) = 0.036 aT dp C T where: (aT= thermal diffusion factor, dimensionless dp = particle diameter, gm C = fume concentration in flue gas, g/L Te = bulk flue gas temperature, absolute Tw = tube surface temperature, absolute - vi - This equation applies to Na2CO3, Na2 SO4, NaC1, and simultaneous Na2SO 4/NaCl fume deposition and is similar in form to thermophoretic equations derived by other authors. These results are directly applicable to fume deposition in the generating bank and the economizer section in recovery boilers, where flue gas temperatures range from 2000C to 7000C. The Reynolds number in a boiler bank is between 3000 and 5000; the heat transfer coefficient is a function of Reynolds number and the deposition rate is proportional to the heat transfer coefficient. If the difference in flow rate between the experimental system and the recovery boiler is taken into account, the experimental results show close agreement with actual recovery boiler data. The presence of water vapor in the incoming gas streams to the reactor affected the fume composition; fume contained less Na2CO3 and more Na2SO4 when water vapor was present during fume formation. -1-