A comparison of pulsed and continuous lasers for high‐temperature Raman measurements of anhydrite

Abstract

AbstractOxygen carrier particles (OCPs) serve as fuel oxidant in emerging chemical looping combustion systems. However, chemical looping combustion process optimization is hindered by the lack of online sensors for measurements of OCP oxidation states at temperatures up to 1,000 °C and pressures up to 10 atm. We are investigating Raman spectroscopy as a potential solution, as this technique is known for its ability to provide noninvasive, molecularly specific information in real time in a wide variety of applications. As a case study, Raman spectra from high‐temperature (>1,000 °C) anhydrite, a potential OCP, are presented using pulsed and continuous wave excitation at 532 nm. When pulsed excitation is coupled with time‐gated detection, background signal from thermal radiation can be significantly reduced from single pulse spectra, outperforming results using an ungated detector. Also, laser heating of the sample is not observed but laser‐induced breakdown spectra increasingly appear with increasing temperature and pulse intensity. Compared with pulsed excitation, continuous wave excitation with longer acquisition time, offers higher signal to noise and avoids the risk for laser‐induced breakdown spectra but demonstrates minor sample heating. Finally, the data demonstrate practical utility by (a) providing a calibration of anhydrite's ν1 (1,017 cm−1) Raman temperature‐dependent band position and by (b) providing estimates of temperature using the ratios of the Stokes/anti‐Stokes ν1 integrated intensities. These results demonstrate the first high‐temperature, pulsed Raman spectra from anhydrite while evaluating challenges associated with high‐temperature, Raman measurements of OCP materials in general.