Impacts and Action Mechanism of Coal Ash on CaO-Based Sorbents for CO2 Capture under an Oxy-fuel Calcination Environment

The emerging calcium looping (CaL) process, basing on the reversible reactions between CaO and CaCO3, is considered to be a potential midterm mitigation solution in capturing CO2 from coal-fired power plants. Normally, the efficient regeneration of sorbents is realized through oxygen-enriched combustion of coal above 900 °C. However, the minerals in coal will potentially affect the CO2 capture ability of sorbents and the effect could gradually intensify as the temperature increases. Therefore, in this work, effects of ash with a series of variables under a more practical oxy-fuel calcination condition are evaluated, and the action mechanism of ash on the sorption process is especially studied. Using a combination of testing approaches, both physical and chemical contributions are observed and identified for the effect of coal ash on the CO2 capturing of CaO-based sorbents. The physical influence, caused by ash deposition and following grain aggregation, on CaO-based sorbents for CO2 capture is found to be inevitable and predominated. Meanwhile, it is also suggested that solid–solid reactions involving Al and Si from coal ash and Ca from sorbents will occur as the aggregation of coal ash intensifies, which could negatively restrain the CO2 capture of CaO-based sorbents in the later stage of CaL. Furthermore, both physical and chemical mechanisms are proposed in describing and understanding the detailed interaction process between coal ash and sorbents.