In situ, multiparameter optical sensor for monitoring the selective catalytic reduction process of diesel engines

Selective catalytic reduction (SCR) is an effective technique to reduce nitrogen oxide (NOX) emission in high-temperature combustion processes. SCR requires strict reaction conditions, including proper temperature and ammonia (NH3) injection volume. To satisfy SCR conditions, we present a low-cost, in situ optical sensor for multiparameter measurement during SCR processes. The sensor simultaneously detected NH3 concentration, water (H2O) concentration, temperature, and pressure by using a single near-infrared diode laser. To ensure the high-quality multiparameter measurement, we realized two novel ideas: (1) wavelength modulation spectroscopy (WMS) with varied modulation amplitude and (2) an optimized multispectral fitting algorithm. We demonstrated the method by theory analysis, model simulation, laboratory verification, and field-testing with a diesel engine. The relative measurement uncertainty of NH3 concentration, H2O concentration, temperature, and pressure were 8.4%, 8.0%, 5.3% and 7.0%, respectively. The results of the field test were consistent with those obtained by thermocouples and electrochemical sensors. The presented sensor is quite suitable for SCR processes and other harsh-environment applications because of its low cost and robustness. The proposed multispectral detection method also can be used for other WMS applications.