Graduate Thesis Or Dissertation
 

Sintering of fume deposits in kraft recovery boilers

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  • Fume, sub-micron aerosal particles in the combustion gases, have been identified as a cause of extensive plugging and fouling in the heat transfer section of a kraft recovery boiler in kraft pulp mills. After the deposition on the heat transfer surface, fume deposits will undergo the sintering process which causes densification and hardening, making them hard to remove. There has been relatively little information available on the kinetics of the sintering of fume particles, and no information on the modeling of the sintering process of this particular material. No conclusion about the effect of chloride species on the sintering of fume particles has been made. A series of sintering tests in air at various temperatures were conducted using dense pellets made from a specific fume dust which contains a low amount of chlorides. Each pellet was heated at a constant temperature, ranging from 300°C to 550°C, for a different time interval. By the study of the microstructural change of each sintered specimen, the initial stage, intermediate stage, and final stage of sintering were identified. By the presence of an empirical constant K₁, the relative linear shrinkage data in the initial stage sintering fit well with the model proposed by Kingery and Berg⁷ when volume diffusion is the controlling mechanism. By the presence of an empirical constant K2 and the application of grain growth data, the porosity-time data in the intermediate stage sintering fit well with the model proposed by Coble¹⁶ when volume diffusion is the controlling mechanism. The apparent activation energy obtained from the initial stage sintering is equal to 54.5 ± 27.7 kcal/mol which is in good agreement with the apparent activation energy obtained from the intermediate stage sintering which is equal to 56.0 ± 31.3 kcal/mol. A semi-empirical model for the sintering process of this particular well-packed fume dust was formulated. The model does not apply well to the loosely packed fume dust due to some unidentified factors The same experiment was carried out for the other fume dust which contains a high amount of chloride. By the use of the fractional density, the qualitative comparison of the sintering of the low chloride and high chloride dusts was made. At low temperatures, the sintering rate of the high chloride fume dust is lower than that of the low chloride fume dust. At high temperatures, both dusts have the same sintering rate and can sinter close to the theoretical density. It is postulated here that the retarded grain growth rate for the high chloride dust can improve the densification process in the intermediate stage sintering.
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