A potentially non-contact monitor method for CO2 at the pseudo-critical region using infrared spectrometer

Abstract

Supercritical carbon dioxide (sCO2) Brayton cycle is a promising choice for thermal power generation with the advantages of high efficiency, compactness and low cost. There is a very sensitive part for monitor and control in the cycle, i.e. CO2 at the critical region, which affects the efficiency of compressor greatly and maybe brings safety problems due to the rapid and great changes in the properties of CO2 near the critical point. A non-contact measurement method is proposed to monitor the transient state of CO2 at various operating conditions using an infrared spectrometer. Experimental investigations were carried out to verify the adaptability of the method at dynamic processes in transcritical CO2 loop tests. We found the density change of CO2 fluid can directly impact the absorption spectrum near the wavelength of 3400 nm, which could serve as an efficient signal of CO2 state changes. In a steady state, a dramatic reduction of the density (− 70 %) and the absorption ratio (− 60 %) were observed between 30 and 34 ºC. In the dynamic processes, density-related scattering phenomena were observed near the pseudo-critical points, which strengthened the non-transmissive ratio (up to 0.95) of CO2 and gave a quick signal (<1 s) whenever abnormal condition occurred. Density stratification was further observed in the heating process at 32.5 ºC and 7.5 MPa, which was close to the critical point. Moreover, the gasification time was more than five times longer than the liquefaction time, which caused a stronger scattering phenomenon in depressurization processes. This work is expected to serve as the foundation work of the potentially non-contact monitor in the sCO2 Brayton cycle or other applications.