Alkali Metal Partitioning in Ash from Pulverized Coal Combustion

Abstract

The relationship between the original form of sodium and potassium in coal and the ultimate fate of these metals was explored through experimentation in a 17kw laboratory downflow combustor. Composition analyses of size segregated samples of airbone ash at exhaust conditions for six different coals are reported. In all cases sodium was enriched in the small particle size range, and was shown both to form a sodium rich fume as well as an enriched surface layer around existing particles. The fraction of sodium appearing in the small particle size range, and therefore previously vaporized, could be correlated with the fraction of sodium that was acid soluble and inversely with the amount of silicon present in the parent coal. Increased temperature led to a decrease in the amount of sodium vaporized.

For the coals examined, potassium was present primarily in the mineral form. Although the fraction of potassium vaporized could be positively correlated neither with total nor with acid soluble forms of the metal, it did correlate with the amount of sodium vaporized. These data, together with equilibrium predictions, suggest that potassium normally tied up in minerals can be displaced by “mobile” sodium, and that, therefore, the partitioning of potassium is directly dependent on that of sodium.