Submicrometer particle formation and mercury speciation under O₂-CO₂ coal combustion

The characteristics of the submicrometer mode of ash and mercury speciation on combustion of coal in an oxygen-carbon dioxide and air (conventional) system were compared. The experiments were conducted at digerent O₂:CO₂ and O₂:N₂: CO₂ mixing ratios. On replacing the nitrogen in air with carbon dioxide, the total mass of the particles in the submicrometer mode (<0.5 μm) of ash was smaller. Correspondingly, the geometric mean size (d_pg) of the submicrometer aerosol was smaller by approximately 28%. When the O₂:CO₂ ratio was increased from 1:4 to 4:4, the geometric mean size of the submicrometer mode increased from 29 to 54 nm because of a faster vaporization rate as a result of a higher coal particle temperature. An increase in the geometric mean size was observed on increasing the N₂:CO₂ ratio at a fixed O₂ concentration because of the same reasons. The shape of the primary particles was spherical in all the tests, indicating that a vapor to particle transformation pathhway was prevalent. The ratios of elemental to oxidized mercury (approximately 4:1) were similar for O₂-CO₂ and air combustion.