Uncertainties in CO₂ Impurity Measurements Using FTIR

As carbon capture, utilization, and storage (CCUS) rapidly expands, reliable CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> impurity monitoring is essential for verifying compliance with purity specifications. Although these specifications define which impurities must be controlled and at what levels, only a few CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> purity requirements are typically determined on a project-by-project basis, and only a limited number are publicly available. Moreover, most specifications focus solely on specifying acceptable concentration limits, without guidance on how impurities should be measured and monitored. FTIR is a promising technique for online CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monitoring, but variations in calibration procedures can introduce significant uncertainty into the measurements. This study addresses the lack of calibration standardization in FTIR-based CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> impurity monitoring by evaluating how operator-dependent selection of absorption regions affects the quantification of SO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CO. Seven operators independently calibrated the same FTIR system, each selecting their own absorption region based on their judgment and experience with spectral interpretation. This approach was used to assess the uncertainty introduced by operator-dependent calibration choices in the absence of standardized guidelines. The results show that these subjective decisions can lead to substantial variability in the predicted concentrations, exceeding 92.6% at low impurity levels, even though all models achieved detection limits well below the relevant specification thresholds. This highlights the risk of inconsistent impurity reporting in the absence of standardized procedures. In conclusion, this study not only demonstrates the magnitude of uncertainty introduced by operator-dependent FTIR calibration but also highlights the need for a standardized calibration procedure. By doing so, the study provides a practical approach to minimizing variability in impurity measurements and enhancing the reliability of compliance verification across CCUS projects.