05 August 2009
As exhaust emissions regulations become increasingly stringent,many engine and vehicle manufacturers are looking at selective catalytic reduction (SCR) with urea injection as the most viable option for NO, reduction. With the European market readily adopting this technology and Mack and Volvo announcing SCR as its strategy going forward in North America,manufacturers such as ACS Industries Inc. are introducing SCR-compatible products. The latest introduction from the Lincoln, Rhode Island, U.S.A.-based company capitalizes on its wire mesh portfolio, offering a noncatalyzed in-line knitted wire mesh mixer specifically aimed at SCR.
While the wire mesh mixer is targeted toward dosing applications such as SCR aftertreatment systems, which use urea to reduce NOx, it is also designed for use in fuel reformers used along with lean NOx traps and retrofit systems. Using it in an SCR system, Sivanandi Rajadurai, ACS vice president of engineering - exhaust products, said it provides a more uniform distribution of
urea. "The flow uniformity is maintained because of the wire mesh. If you look at regular flow, it is going in a longitudinal way - the gas is - but with the wire mesh, you have longitudinal and radial. The gas can go anywhere. It has a larger spread and a torturous path because the gas molecules go around and around in a zigzag pattern.
In a conventional urea SCR system,urea solution is injected through the exhaust pipe and into the catalyst. "A gas molecule going into a substrate channel can only go straight and is not able to redistribute," said Rajadurai."The molecules are in the center only.They cannot go anywhere else."
The construction of the wire mesh allows the gas molecules to go throughout the body of the substrate, said Rajadurai. "When they get in here (the wire mesh mixer) they can go any way because the wire mesh is like a sponge.It provides a lot of contact time, lots of distribution and the efficiency of the flow is better."
The optimized flow-through of the mixer also allows the thermal load to remain uniform throughout the body of the mixer, as no heat degradation occurs as exhaust passes through the mixer. This, ACS said, prevents urea crystallization and allows installation at the
cone of the catalyst. Because the exhaust temperature does not reduce,a shorter mixing distance is achievable with the wire mesh mixer.
"Knitted wire mesh mixer has efficient mixing properties with low back pressure," said Rajadurai. "Its flexible construction gives it optimal flow distri bution and minimizes the space needed in the underbody layout. In the exhaust system, they don't have that space - so this mixer is able to provide mixing in a short distance."
According to ACS, the distance between the injector and the front face of the SCR catalyst can be reduced by 75%. The wire mesh mixer is also designed with rapid warmup and easy installation, requiring only a retainer ring to keep it in place.
To date, ACS is in the prototype stages of the mixer, working with OEMs, engine manufacturers and Tier 1 exhaust suppliers. "We will not only provide samples to the European market and Japanese market, we will do the North American market," said Rajadurai. ACS will produce the in-line wire mesh mixer at its Monterrey, Mexico,facility, with production scheduled to ramp up for 2010. The company also recently opened a production facility in Shanghai, China, which ACS said has the production capabilities to produce the mixers if required.
Besides the wire mesh mixer, ACS also produces a range of knitted wire mesh, rolled expanded metal/woven wire elements and wound wire exhaust products.
Diesel Progress