Carbon Storage in Coal Fly Ash by Reaction with Oxalic Acid

Coal combustion is one of the leading sources of CO₂ emissions, and it is predicted to remain so for the foreseeable future. The environmental effects of coal burning can be partially offset by utilizing fly ash, which is a combustion byproduct, to mineralize and store carbon. Our study tests a novel method for carbon storage through the reaction of fly ash with oxalic acid (H₂C₂O₄), creating durable solid oxalate phases. Our results show that whewellite (CaC₂O₄·H₂O) and weddellite (CaC₂O₄·(2 + x)H₂O, x ≤ 0.5) are formed when fly ash reacts with H₂C₂O₄ at ambient temperature and pressure. We examined 2 types of ash and found that the reaction occurs relatively rapidly, reaching completion within 4 days. Moreover, the reacted material comprised ∼18% Ca oxalate. During the reaction, portlandite, the primary calcium-bearing mineral in the ash, was dissolved entirely, although mass balance calculations indicate that amorphous phases also serve as an important source of Ca for the oxalate minerals. Reaction modeling suggests that Ca is released by two phases that dissolve at different rates, with the rapidly dissolving phase releasing Ca at a rate 40 times faster than that of the slow phase. Based on our calculations, 1 tonne of reacted coal fly ash could store over 34 kg of carbon, and the method has the potential to store more than 35 Mt of carbon per year on a global scale. Thus, our findings indicate that reacting fly ash with oxalic acid could reduce the environmental impact of coal burning, and adapting the technique for use with other alkaline solid wastes may represent a critical green technology.