High-sensitivity in situ QCLAS-based ammonia concentration sensor for high-temperature applications

Abstract

Haber-Bosch process, the organic synthesis of numerous compounds, and the selective catalytic reduction (SCR) of nitric oxide (NO x ) emissions in combustion-based energy systems (e.g., gas turbines and diesel engines) [1]. In SCR, ammonia is typically injected into the combustion exhaust and allowed to react with the NO x species to form water vapor and nitrogen. NO x and NH 3 concentrations are directly correlated in such applications. If NH 3 is injected at a sub-optimal temperature or flow rate, the optimal NO x abatement is not attained; if too much NH 3 is added, unreacted NH 3 will remain in the exhaust and escape into the environment, known as ammonia slip [2,3]. This poses a significant risk to the public as both NO x and NH 3 are toxic to public health and to the environment. To prevent sub-optimal operation of SCR systems, it is desirable to implement closed-loop control systems that can quickly respond to changing conditions within the SCR. A key element in such closed-loop systems is a real-time sensor for ammonia concentration. Unfortunately, common samplingbased speciation techniques either require high sampling flow rates to enable temporally resolved NH 3 concentration measurements (as in gas chromatography) or are slow by nature (as in Fourier transform infrared spectroscopy). Additionally, NH 3 adsorption onto the sampling manifolds can result in significant discrepancies between the measured and actual concentrations. Therefore, a high-sensitivity in situ sensor for trace quantities of NH 3 is needed to provide real-time monitoring of these industrial processes. To achieve this capability, laser absorption spectroscopy (LAS) is often used because of its significant advantages for in situ monitoring of gas properties associated with high temporal resolution, measurement sensitivity, and robustness against various forms of noise [4].Several studies have demonstrated in situ LAS-based NH 3 sensors in high-temperature environments. Webber Abstract A novel quantum cascade laser (QCL) absorption sensor is presented for high-sensitivity in situ measurements of ammonia (NH 3 ) in high-temperature environments, using scanned wavelength modulation spectroscopy (WMS) with first-harmonic-normalized second-harmonic detection (scanned WMS-2f/1f) to neutralize the effect of non-absorption losses in the harsh environment. The sensor utilized the sQ(9,9) transition of the fundamental symmetric stretch band of NH 3 at 10.39 µm and was sinusoidally modulated at 10 kHz and scanned across the peak of the absorption feature at 50 Hz, leading to a detection bandwidth of 100 Hz. A novel technique was used to select an optimal WMS modulation depth parameter that reduced the sensor's sensitivity to spectral interference from H 2 O and CO 2 without significantly sacrificing signal-to-noise ratio. The sensor performance was validated by measuring known concentrations of NH 3 in a flowing gas cell. The sensor was then demonstrated in a laboratory-scale methane-air burner seeded with NH 3 , achieving a demonstrated detecti...

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

“…In such cases, the optimal modulation depth am may largely deviate from the theoretical value of ~1.1 ν. Peng et al proposed a novel procedure for choosing an optimal WMS modulation depth in the presence of spectral interference by defining a new figure of merit to account for the spectral interference [96,97].…”

“…Inverse WMS-2f signal strength (F, blue line), NH 3 interference sensitivity (σ, red line), and cost function (C, red line) vs modulation depth for a 3 ppm NH 3 mixture in CH 4 − air φ = 0.6 combustion exhaust at T = 600 K, P = 1 atm. The optimal modulation depth a m, opt is shown in the black/dashed line.Figure adapted from[96].…”

Dynamic ammonia measurement based on quantum cascade laser absorption spectroscopy

“…However, the absorption spectrum of NH3 at 9.06 μm is tolerant to interferences from temperature and other absorption lines, which limits its industrial application 16 . Peng et al 21 . developed an NH3 sensor based on quantum cascade absorption spectroscopy for concentration detection at high temperature.…”

Ammonia Sensing Performance of Polyaniline-Coated Polyamide 6 Nanofibers

“…Thus, a variety of ammonia sensors based on organic materials have been developed because some general ammonia detectors or sensors always require high work temperature (such as γ-Fe 2 O 3 and γ-Fe 2 O 3 –TiO 2 , 5 WO 3 , 6 SnO 2 , 7 etc. 8 − 11 ) or are time-consuming by nature (for example, Fourier transform infrared spectroscopy detectors 12 ). Among these organic materials, polyaniline (PANI), a classical conducting polymer 13 − 15 with low work temperature, good environmental stability, 16 low cost, and high thermal stability, has been demonstrated to be a promising candidate in gas sensors.…”

“…It is also a kind of important chemical resource being employed in the cooling systems, production of fertilizer, and the reduction of NO x gases in diesel vehicles. − However, it is also a toxic and flammable gas and a kind of signal of some disease, so it is essential to monitor the concentration level of ammonia in the places where one may release it. Thus, a variety of ammonia sensors based on organic materials have been developed because some general ammonia detectors or sensors always require high work temperature (such as γ-Fe 2 O 3 and γ-Fe 2 O 3 –TiO 2 , WO 3 , SnO 2 , etc. − ) or are time-consuming by nature (for example, Fourier transform infrared spectroscopy detectors). Among these organic materials, polyaniline (PANI), a classical conducting polymer − with low work temperature, good environmental stability, low cost, and high thermal stability, has been demonstrated to be a promising candidate in gas sensors. − However, PANI is infusible, almost insoluble and non-processable, and its physical and mechanical properties are not satisfactory for some applications.…”