TDLAS-based NH3 mole fraction measurement for exhaust diagnostics during selective catalytic reduction using a fiber-coupled 2.2-µm DFB diode laser

Abstract

hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO x ) and particulate matter (PM) or number (PN) [1,2]. To meet upcoming European regulation standards for diesel engines and comparable regulations worldwide, like US Environmental Protection Agency's Tier 3 [3] for example, an improved exhaust aftertreatment approach is necessary. The combination of diesel oxidation catalysts (DOC), diesel particulate filters (DPF) and selective catalytic reduction (SCR) systems is a state of the art and promising exhaust gas treatment concept already in use on heavy-duty vehicles as well as passenger cars. The SCR technology uses ammonia (NH 3 ) as the reactant with NO x directly in the exhaust. Due to the risks and restrictions when handling gaseous ammonia, a non-toxic aqueous urea solution consisting of 32.5 % urea (AUS32/AdBlue) [4] is commonly used to provide a liquid pre-stage of ammonia.A modern diesel exhaust system is quite complex and equipped with several sensors for air-fuel ratio λ, NO x , temperature and pressure drop. For SCR, AUS32 is injected downstream of the DOC and DPF, forming H 2 O, CO 2 and NH 3 . The latter reduces the NO x to water vapor and molecular nitrogen in a subsequent catalyst [5]. With urea-SCR and an optimal NO/NO 2 ratio of 1, NO x conversion rates up to 100 % can be achieved [6].The drawbacks of a SCR system are evidently the increased complexity, higher weight and the need to refill an extra operating fluid. However, unlike the concurrent denox technology lean NO x trap (LNT), the SCR technology does not impede engine optimization concerning performance and efficiency. Thus, up to 10 % lower fuel consumption compared to vehicles using LNT can be achieved and even operational costs decrease [7], which is why SCR is the only exhaust aftertreatment system used by European heavy-duty vehicle manufacturers and also applied in midsize and bigger passenger cars [5].Abstract A new developed tunable diode laser spectrometer for the measurement of ammonia (NH 3 ) mole fractions in exhaust gas matrices with strong CO 2 and H 2 O background at temperatures up to 800 K is presented. In situ diagnostics in harsh exhaust environments during SCR after treatment are enabled by the use of ammonia transitions in the ν 2 + ν 3 near-infrared band around 2300 nm. Therefore, three lines have been selected, coinciding near 2200.5 nm (4544.5 cm −1 ) with rather weak temperature dependency and minimal interference with CO 2 and H 2 O. A fiber-coupled 2.2-μm distributed feedback laser diode was used and attached to the hot gas flow utilizing adjustable gas tight high-temperature fiber ports. The spectrometer spans four coplanar optical channels across the measurement plane and simultaneously detects the direct absorption signal via a fiber-coupled detector unit. An exhaust simulation test rig was used to characterize the spectrometer's performance in ammonia-doped hot gas environments. We achieved a temporal resolution of 13 Hz and temperature-dependent precisions of NH 3 mole fraction ranging from 50 to 70 ...

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

“…A reference temperature of 296 K was used. Reproduced from Sur et al [15] Transition Quantity (4) 0.044 (2) 0.202 (8) 0.033 (1) 0.443 (14) 0.263 (11) n i 0.66 (5) 0.69 (5) 1.14 (5) 0.59 (5) -0.86 (10) sQ(10,6) γ i,0 (cm −1 /atm) 0.104 (5) 0.034 (2) 0.216 (20) 0.033 (1) 0.423 (26) 0.257 (8) n i 0.98 (10) 0.41 (10) 1.09 (10) 0.48 (10) -0.86 (20) sQ (9,9) γ i,0 (cm −1 /atm) 0.109 (4) 0.051 (2) 0.214 (7) 0.042 (2) 0.57 (17) 0.307 (9) n i 0.81 (5) 0.76 (5) 0.74 (5) 0.69 (5) -0.98 (10) sQ (9,8) γ i,0 (cm −1 /atm) 0.098 (3) 0.048 (2) 0.187 (10) 0.039 (1) 0.511 (16) 0.316 (13) n i 0.70 (5) 0.67 (5) 0.69 (5) 0.64 (5) -0.90 (10) at linecenter, we also have to consider how sensitive the resulting WMS-2f/1f signal will be to the presence of interfering species. Here, we present a novel procedure for choosing an optimal WMS modulation depth in the presence of spectral in...…”

“…For these situations, in addition to maximizing the WMS-2f signal The superscripts denote the uncertainties in the measurements with respect to the last decimal digit (e.g., for sQ(10,7): S(296 K) = 0.2195 ± 0.055 cm −2 /atm). Reproduced from Sur et al [15] Transition Linecenter (cm −1 ) E ′′ (cm −1 ) S @ 296 K (cm −2 /atm) sQ ( The superscripts denote the uncertainties in the measurements with respect to the last decimal digit (e.g., for sQ (10,7): γ N 2 ,0 = 0.087 ± 0.004 cm −1 /atm). A reference temperature of 296 K was used.…”

“…At higher hydrocarbon concentrations on the order of 100 ppm expected in practical environments, spectral interference from the strong hydrocarbon C-H symmetric stretch bands at 2.25 µm is expected to significantly degrade sensor performance. More recently, Stritzke et al [7] demonstrated detection limits of 50-70 ppm also using DFB diode lasers at 2.20 µm in 800 K exhaust with high H 2 O and CO 2 concentrations (15 % each). For NH 3 concentrations below this detection limit, however, this sensor fails because of strongly interfering H 2 O and CO 2 features.…”

“…The novelty of the current detection scheme lies in: (1) the selection of a previously unused NH 3 transition near 10.39 µm that is strong and relatively free of spectral interference from H 2 O and CO 2 transitions at high temperature compared to transitions previously showcased in the literature [5][6][7]14], and (2) the use of a novel procedure for selecting scanned-WMS tuning parameters that minimizes sensitivity to fluctuations in the concentration of spectral interferers without significantly sacrificing signalto-noise ratio (SNR) of the WMS lineshapes. Experimentally derived spectroscopic parameters of the transitions used for this sensor were reported by Sur et al [15], thus enabling higher-accuracy NH 3 spectral models than current databases (e.g., HITRAN).…”

Simultaneous two cross-sectional measurements of NH₃ concentration in bent pipe flow using CT-tunable diode laser absorption spectroscopy

“…The absorption signal intensity becomes an integrated value of the optical path. In this study, several optical paths are intersected to each other to form the analysis grid, reconstructing 2D temperature distribution by a CT method (Ma et al, 2008;Wang et al, 2010;Kasyutich et al, 2011;An et al, 2011;Deguchi et al, 2012;Deguchi et al, 2015;An et al, 2015;Cai, et al, 2015;Jatana, et al, 2015;McCann, et al, 2015;Seidel, et al, 2015;Stritzke, et al, 2015;Kamimoto et al, 2016). Figure 4 shows concept of analysis grids and laser beam paths.…”