Innovative analyzer supports low-emissions measurement

  • 12-Dec-2008 10:59 EST
Picarro AE fig1.jpg

The basic principle of Picarro’s WS-CRDS involves light from a tunable laser that is trapped in a three-mirror cavity. When the laser is turned off, the ring-down time for the light to decay is shortened by the presence of any species that absorbs at the laser wavelength.

To enable the automotive industry to meet and exceed increasingly tough emissions standards, several technologies are now at various stages of development. These include lean NOx traps, selective catalytic reduction (SCR), and “zero-emissions” fuel-cell technologies. Successful development requires real-time measurement of specific gaseous compounds (e.g., H2S, NH3) with extreme sensitivity, absolute precision, and no crosstalk from changes in other gas components.

Wavelength-scanned cavity ring down spectroscopy (WS-CRDS) is a recently commercialized technology from Picarro Inc. that enables small molecules to be quantitatively detected using their unique near IR absorption lines. In a WS-CRDS instrument, light from a wavelength-tunable laser diode enters the sampling cavity that contains three mirrors with exceptionally high (>99.999%) reflectivity. When the laser is switched off, the light inside the cavity slowly leaks out because the mirrors do not have 100% reflectivity. This “ring down” (decay) is followed in real time by a photodetector.

If the cavity contains a gas species that absorbs even weakly, it introduces a second light-loss mechanism. This shortens the decay time, from which the instrument calculates the sample absorbence and, hence, concentration. Even with a cavity of only 25 cm (9.8 in) in length, the average distance that any photon effectively travels within the cavity can be more than 20 cm (7.9 in), thus giving rise to sensitivity in the parts per billion range and even parts per trillion for some gases. And, because the laser provides high-wavelength resolution, several trace gas species can be simultaneously and independently monitored with no data crosstalk. Just as important, because these compact, rugged instruments contain no moving parts, they can go months between calibrations.

The LNT (lean-NOx trap) is a dual-mode catalytic approach in which NO is oxidized to NO2 by a platinum catalyst and temporarily trapped (stored) as nitrates by alkaline earth metals such as barium. (In a second stage, the nitrates are ultimately reduced to N2.) Unfortunately, LNT efficiency can be poisoned by the reaction of sulfur compounds with the alkali material to form sulfates. Consequently, LNT systems must be periodically desulfated at higher temperature, a process that needs to be characterized and optimized.

Desulfation can be monitored by measuring the H2S it releases—at concentrations in the tens to a few hundred parts-per-million range. Most real-time H2S detectors are plagued by crosstalk. Not so with the WS-CRDS, which provides linear detection over the entire 50 PPBv to 500 PPMv range.

In a test in which the H2O concentration was varied from 2% to 5% while the CO2 was varied from 0% to 7%, there was no detectable change in the H2S signal during this test. Similar tests were performed using varying levels of propane, SO2, CO, and NO. Again, there was no interference at the 50 PPBv lower detectable limit of the H2S analyzer. So raw undiluted exhaust streams can be directly analyzed for H2S over a full range of duty cycles.

In SCR, a reductant such as ammonia or urea is added to the exhaust stream before passing through a catalytic chamber. With ammonia, both NO and NO2 are ultimately reduced to H2O and N2. With urea, the end products are H2O, N2, and CO2.

SCR is already used in several niche applications with large stationary engines. But successful implementation of SCR in heavy-duty and perhaps light-duty automotive applications requires dynamic optimization of the reductant flow rate. For example, a common problem with SCR systems is the release of unreacted ammonia referred to as “ammonia slip.”

Engineers investigating and optimizing SCR systems thus need a high-performance, real-time ammonia analyzer. The latest WS-CRDS-based NH3 analyzers are ideal for this development work, providing a precision of 350 PPTv (over 10 s) or 70 PPTv (with 5 min of averaging) and upper range of 25 PPMv. Again, no crosstalk from other gas species allows direct sampling of both raw and diluted exhaust streams. And the high sampling speed enables direct correlation of the NH3 signatures to the dynamics of the vehicle engine and exhaust manifold in real time.

Fuel-cell technology continues to advance at a fast pace. But some of these fuel cells (e.g., based on methanol or ethanol) can produce trace amounts of harmful gases, such as carbon monoxide and formaldehyde. Since WS-CRDS can be easily configured to monitor any of these potential harmful species at PPBv level or better, WS-CRDS instruments are expected to find additional niche applications in fuel-cell development and optimization.

Aaron Van Pelt and Eric Crosson of Picarro Inc. wrote this article for Automotive Engineering.

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