Alkali metal partitioning in a pulverized coal combustion environment.

Abstract

Fouling, slagging, corrosion, and emission of submicron particulate from pulverized coal combustors have been linked to vapor alkali. Size segregated fly ash samples extracted from a 17 kW down-fired pulverized coal combustor showed strong evidence of alkali vaporization. The fraction of sodium in sizes smaller than 0.65 μm (f(8A)) showed a correlation with acid soluble sodium divided by total silicates in the parent coal. Addition of silicates to coal reduced f(8A) for sodium. Potassium existing primarily in the mineral matter, did not show a similar correlation, but f(8A) for potassium did correlate with f(8A) for sodium. Bench scale experiments indicated potassium does not vaporize in the presence of Na or Cl alone, but requires both, and was only released when sodium was captured. Additional of sodium acetate to coal increased f(8A) for potassium. Equilibrium calculations, experiment, and modelling of sodium capture by silicates during pulverized coal combustion identified several important mechanisms governing alkali behavior. The mode of occurrence of alkali in the parent coal dictates its ability to vaporize, its release kinetics, and its sate as it diffuses to the char surface. Other species such as chlorine, sulfur, moisture, and other metals influence alkali stability in the vapor, its reactivity, and its condensation characteristics. Char oxidation can influence alkali vaporization, and capture by affecting included silicate surface area. Sodium reaction with silicates captures from 70 to over 95% of total sodium for typical coals. Silicate additive appears to be a viable technique for reducing the fraction of alkali in the vapor during combustion.