Real-Time, In Situ Sensor for Control of Ammonia Slip in SCR Installations

In this program, we proposed to demonstrate a compact, rugged, low-cost ammonia sensor with better than {+-}0.2 ppmv lower detection limit and resolution. Extensive design and trade studies indicated that this measurement could be made by exploiting the infrared absorbance of ammonia at 1527 nm to determine concentration. Key features of the design were (1) a electronically-tunable, wavelength-agile diode laser-pumped fiber laser, (2) a unique, low-cost folded path optical cell, and (3) an InGaAs photodiode (light detector). The sensor would be designed to make in situ measurements of ammonia in flue gas just downstream from the catalyst. The design proved robust and was able to operate at pressures in the range 1-3 atm and temperatures up to 500 C. With extensive signal processing, the sensor was able to make unambiguous ammonia measurements. But the signal processing degraded response time to almost 3 seconds and the measurement ambient conditions limited achievable resolution to about 20 ppmv ammonia. Extensive re-design, including a much longer optical path length and more robust real-time signal processing would be required to make sub-ppm measurements using this approach. During the performance period of this project, conventional in-situ laser based monitors and thick film ceramic ammonia sensors have been introduced and have gained acceptance in real-time feed-forward control for this application