| OCR Text |
Show pressure build-up in the pores due to the coupled effect of rapid sulfur evolution and the resistance to its transport through the pores of the particle. The size distribution of the oxidized sample also shows that about 12 percent of the total mass is present in particles smaller than 0.1 urn. This, in conjunction with almost no mass in in the 0.1 - 0.3 urn range, suggests that a vaporization-condensation mechanism might be operating to generate the submicron fume. Vaporization of iron may be enhanced due to particle breakup, due to the associated increase in the surface area of the fragments. One consequence of mineral fragmentation is that the mineral size is no longer the lower bound on the ash particle size. The reduction in particle size due to fragmentation is also beneficial, in that the time required for oxidation, hence the slagging probability, is reduced below what would be obtained from the untransformed mineral size distribution. CONCLUSIONS A kinetic model capable of describing transformations of excluded pyrite particles in a combustion environment was developed, and used to predict the existence and duration of a melt phase in the particle. The model demonstrated that a melt phase is present for approximately 80 percent of the time required for oxidation to magnetite. The model also predicts that coarser pyrite particles spend more time in a molten state, the time being approximately proportional to square of the particle diameter. This effect increases the slagging probability of larger pyrite particles, and emphasizes the importance of pulverizer performance in controlling the top size of mi neral/coal particles in the feed to a boiler. Laboratory testing conducted with pure pyrite particles yielded results consistent with model predictions. "Stickiness" experiments demonstrated nearly 100 percent particle adhesion efficiency at residence times where molten phases were predicted to occur. Efficiencies decreased abruptly to zero under conditions where crystalline magnetite was predicted to dominate. 11 |
