Water vapor quantification in raw product gas by THz quantum cascade laser

Abstract

Online quantification of water vapor in hot and complex gases, like raw product gas from biomass gasification, is essential for process understanding and control. The complex nature of these gases presents many challenges, e.g., band overlap or dust and tar deposits on equipment. Offline measurement by condensing water is labor-intensive and does not provide continuous real-time data. This study introduces a spectroscopic setup consisting of a quantum cascade laser emitting in the far-infrared range, a gas cell heated to around 250 °C, and a pyroelectric detector to quantify water vapor content in real-time. A 1^st-order distributed feedback grating ensures single-mode operation of the laser at the desired water absorption line (2.294 THz). This setup was successfully tested for online analysis of raw product gas from steam gasification of waste wood. The average result from the new spectroscopic setup was 45.8 vol-% water vapor content, compared to the condensation measurement, which showed 46.7 vol-% water vapor content. Uncertainty was determined as −0.7 to +1.1 vol-% H₂O. New data from the QCL-based measurement were available every 1 to 5 s, allowing for a better understanding of the process while operating the gasifier. The permanent gas species detected in the raw gas included CO, H₂, CO₂, CH₄, NH₃, and H₂S. Additionally, 4.16 g/Nm³_dry of tar was detected gravimetrically and 31.21 g/Nm³_dry by gas chromatography-mass spectrometry. Measurement continued without issue in this raw, hot product gas from biomass steam gasification for two hours. This work showcases quantum cascade lasers’ strong potential for spectroscopy applications in hot and complex gases.