Characterization of trace element emissions from coal combustion by equilibrium calculations

Abstract

A computer model based on equilibrium thermodynamics was used to characterize emissions of trace elements, such as arsenic (As), lead (Pb), cadmium (Cd), selenium (Se) and mercury (Hg), from pulverized coal combustion (PCC) and fluidized bed combustion (FBC). For PCC, most of the As, Pb and Cd entered fly ash, whereas most of the Se and Hg were in the vapor phase. For an air/coal ratio=1.1, all Cd was in the solid phase, whereas for 1.0 all Cd was in the vapor phase. The same change in the air/coal ratio had a much less pronounced effect on partitioning of other trace elements, however, it shifted the condensation of the trace elements compounds to lower temperatures. The air/coal ratio influenced the type of trace element-containing compounds in the vapor phase. Chlorine (Cl) in coal had a pronounced effect on the emissions of Hg and Pb. For FBC, more of the trace elements entered bottom ash rather than fly ash. Also, partitioning of the trace elements was influenced by the distribution of limestone and/or its products between fly ash and bottom ash. Limestone had a diluting effect on the content of trace elements in the ashes. The effect of the Ca/S ratio on partitioning of trace elements was less pronounced than that of the air/coal ratio. Thus, sulfides of calcium and iron were present in the ashes when the air/coal ratio was lower than 1.1. A low air/coal ratio had an adverse effect on gaseous emissions as well.