Fate of Boron and Selenium during Pulverized Coal Combustion

To investigate the fate of boron and selenium in low-rank coals during coal combustion, the effects of the dust collector and wet-flue gas desulfurization (FGD) conditions on the distributions of B and Se have been studied with a lab-scale pulverized combustor equipped with an FGD unit. Although the residence time of fly ash (FA) from the furnace inlet to the dust filter does not affect B distribution, the proportion of Se condensed onto the FA increases with increase in the residence time. Further, the proportion of B transferred into the bottom ash (BA) during the combustion of high ash-containing coal is greater than that during the combustion of middle-level ash-containing coal. On the other hand, the amounts of B condensed onto the FA are equal for the two types of coals examined. This result shows that the B distribution during coal combustion strongly depends on the coal type. No such coal type dependence is observed in the case of Se, because almost all of the Se in raw coal transfers into the gas phase during coal combustion, resulting in a very low amount of Se in BA. Examination of the effect of flue gas temperature on B distribution reveals that the gaseous B produced during coal combustion does not condense onto the FA as FA-B in the temperature range of 90-400 degrees C. On the other hand, the amount of gaseous Se decreases with decrease in the flue gas temperature, and gaseous Se condenses onto the FA. We hypothesize that the ash composition in the FA and/or the sulfur content in the flue gas (Ca/S or Fe/S) may affect Se distribution, because the condensation behavior of Se onto the FA is quite different between for high-ash and high-sulfur-containing coals. In addition, it is found that over 90% of the gaseous Se can be removed at a flue gas temperature of 90 degrees C. In the FGD test, almost all of the gaseous B or Se passing through the dust filter is trapped in the FGD solution, and the residual gaseous B or Se transfers into gypsum. The pH or temperature of the FGD solution does not affect B and Se distributions in the FGD unit. It is found that the gaseous B or Se formed during the combustion of pulverized coal can be removed via the flue gas and FGD unit.