Driving Diesel

How nickel alloy foam could reduce emissions and fuel consumption
by Carroll McCormick

Nickel Magazine, June 2007 -- Known for their fuel efficiency, diesel engine vehicles are attracting greater and greater attention at a time when the global transport sector accounts for about 20% of carbon dioxide emissions.

In Europe diesel fuels a large proportion of private vehicles, as well as most transport trucks. In North America the main users of diesel are small passenger vans, buses and transport trucks. Modern diesel exhaust treatment systems, though designed to meet emission standards, increase fuel consumption by 5-10% owing to back pressure caused by accumulated soot and the periodic injection of diesel into the exhaust systems to burn this soot. However, a recent innovation could reduce that penalty by using a nickel alloy foam to improve the efficiency of diesel exhaust systems.

The diesel exhaust treatment technology that enables cars and trucks to meet current and near-term emission regulations relies on a three-part process: diesel oxidation catalysts (DOCs) convert carbon monoxide and unburnt hydrocarbons to carbon dioxide and water vapour, diesel particulate filters trap soot particles, and so-called DeNOx units reduce oxides of nitrogen to their gaseous oxygen and nitrogen compounds.

Inco Special Products (ISP), a business unit of CVRD Inco Limited, decided it could improve on the filter and catalyst-carrying materials (substrates) in the exhaust treatment systems, which consist of ceramics, metal foils and fibers. The business unit developed a high-temperature corrosion-resistant alloy foam substrate based on pure nickel foam it manufactures in China for sale to makers of rechargeable nickel batteries worldwide.

ISP creates specialty nickel products for advanced applications. It developed its first high-temperature nickel alloy foams in 2003. The foams enable manufacturers to design diesel exhaust treatment systems with several advantages over conventional systems. For example, they can be built in a wide range of shapes beyond the usual cylindrical form; they also will be lighter and smaller, require less platinum catalyst to achieve the same catalytic effect, and potentially reduce fuel consumption.

Testing is well-advanced and selected car manufacturers will be outfitting some models with the new systems as early as the end of 2007. ISP is also targeting the North American truck market, where new diesel emission guidelines will require truck manufacturers to install treatment systems on all 2007-model vehicles. Recently the door opened to another market for diesel exhaust treatment systems when the U.S. Environmental Protection Agency proposed setting new emission standards for locomotives starting in 2009.

Pure nickel foam is made by electroplating nickel on polyurethane foam that is up to four millimeters (mm) thick and one metre wide. A special process then burns off the polyurethane, leaving behind pure nickel foam. “This nickel material is a precursor for numerous other products, such as alloy foam, says Dr. Dirk Naumann, director of new technology and market development for ISP. “We can add other metallic elements to the foam to provide attributes such as resistance to high temperatures.”

The alloy foam developed by ISP consists of 50% nickel, 22% iron, 22% chromium and 6% aluminum, with good oxidation resistance at temperatures as high as 1,100°C. A powder metallurgical process is used to coat the nickel foam with a binder solution followed by an alloy powder. A sintering step, during which the alloy powder and nickel foam diffuse together, yields a homogeneous alloy of the desired composition and the required physical properties. These include mechanical strength, chemical resistivity, and a pore sizes that can range from 450 microns to 2 mm, depending on the nickel foam precursor.

“Alloy foam is an excellent medium for trapping soot, and works differently than conventional wall-flow filter technologies,” says Dr. Alexander Boehm, who invented the nickel-to-alloy transformation process and is overseeing development of nickel alloy foam products at ISP. “This is a deep bed filter. We're relying on a combination of diffusion, inertial impaction, and flow-line interception to collect soot from the gas. These mechanisms target different particle sizes, including those in the critical nanometre range.”

The alloy foam also serves as a substrate on which a catalyst can be applied to carry out DOC and DeNOx functions.

Catalytic action is much more effective in alloy foam than in conventional flow-through structures, owing to the mixing effect of the foam’s structure. As well, its higher surface area exposes much more diesel exhaust to the catalyst on its longer trip through the bed.

The result is a treatment system that needs to be only about half the size of a conventional one, with half the expensive catalyst (usually platinum) required to cleanse the diesel exhaust to the extent required by emission regulations.

For filters to continue working properly they have to be regenerated periodically by burning off accumulated soot. Passive regeneration occurs at temperatures above 250°C by the reaction of soot with nitrogen dioxide to form carbon dioxide. But in the case of most systems, when a vehicle’s operating cycle does not raise the exhaust system temperature high enough for passive regeneration, diesel fuel is added at intervals to burn off the soot at temperatures that can run as high as 1,000°C. This is called active regeneration.

Since the smaller filters have a lower thermal mass, they heat up to the active regeneration temperature more quickly, reducing emissions during cold starts. And since the alloy foam can tolerate higher soot loading than can ceramic filters, they require less frequent active regeneration. This means less fuel is needed to achieve the required temperature for active regeneration, resulting in potential fuel savings. Depending on the complex relationship with backpressure, the result could be a reduction in overall vehicle fuel consumption and fewer emissions.

This is good news for the transport sector which is such a large contributor to carbon dioxide emissions to the atmosphere and is being watched very closely by regulators.

Carroll McCormick is a Montreal-based freelance writer..

Photos: CVRD Inco and iStock

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