Purification of landfill gas by extracted calcium ions from municipal solid waste incineration fly ash

This study proposes the utilization of CO₂ based on the purification of landfill gas (LFG). The process included absorption of CO₂ from LFG using monoethanolamine (MEA) absorbent, extraction of calcium ions from municipal solid waste incineration (MSWI) fly ash using various acids, and formation of calcium carbonate using the extracted calcium ions. During LFG purification, the concentration of CH₄ in the gas after absorption was time dependent. The pH swing method was used for the extraction of calcium ions and comprised three phases: calcium ion leaching from MSWI fly ash phase, removal of cations from the supernatant, and calcium ion recovery. Hydrochloric and nitric acids, known as strong acids, and citric, acetic, and formic acids, which are weak acids, were used as extraction agents. Hydrochloric acid, nitric acid, acetic acid, and formic acid showed significant calcium ion recovery rates of 99.32%, 99.18%, 98.35%, and 97.99%, respectively, whereas citric acid showed a relatively low recovery rate of 82.82%. The extracted calcium ions were converted into calcium carbonate by reacting with ionic CO₂ in the saturated MEA. The calcium carbonate formed showed different crystal structures based on the extraction agent used: aragonite for hydrochloric acid and nitric acid, amorphous CaCO₃ for citric acid, vaterite for acetic acid, and calcite for formic acid. The results of this study can be applied to various CO₂ utilization processes based on LFG and MSWI fly ash.