Submicrometer Particle Formation and Mercury Speciation Under O₂−CO₂ Coal Combustion

TLDR

The study compared submicrometer ash and mercury speciation in coal combustion under O₂–CO₂ versus conventional air. Experiments varied O₂:CO₂ and O₂:N₂:CO₂ ratios. Replacing nitrogen with CO₂ reduced submicrometer ash mass and size by ~28 %, while higher O₂:CO₂ or N₂:CO₂ ratios increased particle size due to faster vaporization; primary particles were spherical and the elemental‑to‑oxidized mercury ratio remained ~4:1 in both combustion modes.

Abstract

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 different 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 (dpg) 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 pathway was prevalent. The ratios of elemental to oxidized mercury (approximately 4:1) were similar for O2−CO2 and air combustion.