Influence of carbonation under oxy-fuel combustion flue gas on the leachability of heavy metals in MSWI fly ash

Abstract

Due to the high cost of pure CO₂, carbonation of MSWI fly ash has not been fully developed. It is essential to select a kind of reaction gas with rich CO₂ instead of pure CO₂. The CO₂ uptake and leaching toxicity of heavy metals in three typical types of municipal solid waste incinerator (MSWI) fly ash were investigated with simulated oxy-fuel combustion flue gas under different reaction temperatures, which was compared with both pure CO₂ and simulated air combustion flue gas. The CO₂ uptake under simulated oxy-fuel combustion flue gas were similar to that of pure CO₂. The leaching concentration of heavy metals in all MSWI fly ash samples, especially in ash from Changzhou, China (CZ), decreased after carbonation. Specifically, the leached Pb concentration of the CZ MSWI fly ash decreased 92% under oxy-fuel combustion flue gas, 95% under pure CO₂ atmosphere and 84% under the air combustion flue gas. After carbonation, the leaching concentration of Pb was below the Chinese legal limit. The leaching concentration of Zn from CZ sample decreased 69% under oxy-fuel combustion flue gas, which of Cu, As, Cr and Hg decreased 25%, 33%, 11% and 21%, respectively. In the other two samples of Xuzhou, China (XZ) and Wuhan, China (WH), the leaching characteristics of heavy metals were similar to the CZ sample. The speciation of heavy metals was largely changed from the exchangeable to carbonated fraction because of the carbonation reaction under simulated oxy-fuel combustion flue gas. After carbonation reaction, most of heavy metals bound in carbonates became more stable and leached less. Therefore, oxy-fuel combustion flue gas could be a low-cost source for carbonation of MSWI fly ash.

Keywords: Carbonation; Heavy metals; Leaching; MSWI fly ash; Oxy-fuel combustion.