A systematic comparison of calcium carbonate quantification techniques for the monitoring of carbon dioxide removal via lime carbonation direct air capture

Lime carbonation direct air capture (DAC) systems remove atmospheric carbon dioxide (CO₂) by carbonating calcium hydroxide (Ca(OH)₂) to produce calcium carbonate (CaCO₃), which can release CO₂ for durable storage. Accurate and precise measurement of generated CaCO₃ is essential in quantifying CO₂ removed from the atmosphere, and for optimizing the carbonation process. Methods for measurement of carbonate content are well established, but have yet to be applied to materials produced by this system (i.e., almost solely Ca(OH)₂ and CaCO₃). Five carbonate content analysis techniques (loss on ignition, LOI; thermogravimetric analysis, TGA; combustion analysis of carbon via infrared absorption, CAC-IR; volumetric calcimetry; and quantitative Fourier transform infrared spectroscopy, FTIR) were investigated for their measurement accuracy and precision over a range of carbonate contents. Sample throughput and levelized cost of analysis were considered in addition to accuracy and precision. LOI and CAC-IR proved favorable against equal consideration of the four factors. Weighting for accuracy and precision, LOI was favorable. Standard operating procedures, including established accuracy and precision levels, for viable carbonate content quantification techniques should be developed, tested, and presented to assure carbon credit buyers, the scientific community, and the public on the validity of carbon credits generated by lime carbonation DAC.