Development of an FDM-TDLAS Sensor for Long-Term Online Monitoring of H2S and CO to Predict Water Wall Corrosion Trends

A laser sensor system was developed to be employed in power plants and specifically engineered for high-particulate and high-temperature environments to be applicable for the online monitoring of hydrogen sulfide (H₂S) and carbon monoxide (CO) near the furnace water wall. The system was designed based on frequency division multiplexing (FDM) and tunable diode laser absorption spectroscopy (TDLAS). A corrosion-resistant Herriott multipass cell was designed to withstand temperatures of up to 120 °C while providing an 18 m optical path length. The sensor system exhibits measurement uncertainties below 1.5% and 2% under mechanical vibration testing and thermal cycling testing. Allan variance analysis revealed minimum detection limits of 111 ppb for H₂S and 116 ppb for CO, achievable at integration times of 178 and 144 s, respectively. Long-term field validation in a coal-fired boiler included multipoint sampling to assess the sensor system performance, simultaneously analyzing water-wall combustion dynamics and evaluating corrosion propensity.