Clearing the Air

by Joann Muller | Jun 21 '04

Hydrogen-powered cars are a long way off. But one auto supplier is already using hydrogen to clean up diesel exhaust fumes.
The plasmatron is nothing like the orgasmatron in Woody Allen's Sleeper , but it does have engineers like William Taylor breathing heavily. Taylor and his colleagues at ArvinMeritor, the $8 billion (sales) auto supplier, think their funny-sounding invention may solve one of the auto industry's most intractable problems: how to rid the air of those noxious diesel fumes from trucks and buses. If they're right, ArvinMeritor could grab a big slice of a $10 billion market.

The plasmatron, whose official name is the plasma fuel reformer, is an aluminum canister about the size of a Big Gulp drink. It acts like a giant spark plug, busting apart a tiny portion of the diesel fuel with an electric charge so that the molecules rearrange themselves into carbon monoxide and hydrogen. These gases are then used to clean up smog-producing nitrogen oxides, or NOx.

For now ArvinMeritor is marketing its plasma fuel reformer to the commercial trucking industry, which is scrambling to comply with tough new emission laws that are looming in Europe and the U.S. By 2007 the U.S. Environmental Protection Agency will require a 90% reduction in the amount of soot (unburned carbon). Between 2007 and 2010 the agency also mandates a phased 92% reduction in the amount of NOx emitted from a truck's tailpipe. New particulate traps are fairly effective at filtering out most of the soot. "It's the NOx where they're really struggling," says Taylor. "Hydrogen is the golden key. It gets them where they need to go."

Diesel engine makers have been trying to block NOx where it originates, in the engine cylinders. Increasing fuel pressure has worked so far by lowering combustion temperatures. The cooler the engine runs, the less NOx emitted.

But when the new emissions laws take effect, merely tweaking the engine won't suffice. Michael Cross, who heads up emissions solutions at diesel engine maker Cummins, says they will need help from an exhaust treatment system.

In Europe the trucking industry has settled on a technology called selective catalytic reduction, in which urea is periodically injected into the exhaust system to break down the NOx, converting nitrogen oxides into (somewhat) harmless ammonia. But the U.S. EPA has rejected that idea, calling it impractical and difficult to en-force. Truckers would have to be trusted to refill their urea tanks regularly and gas stations would have to install urea pumps.

Instead the EPA is pushing the use of NOx "adsorption" systems, which act like giant sponges, essentially trapping the NOx chemically in the exhaust system. Just like the filter in a vacuum cleaner, however, the trap must be cleaned periodically. In most systems that's done when a bit of diesel fuel is injected into the exhaust system, where it mixes with the stored NOx. The resulting chemical reaction converts the nasty NOx fumes into nonpolluting nitrogen, water and carbon dioxide.

The problem is that both NOx and diesel fuel are hard to break down. A chemical reaction between the two will occur only at temperatures that are hard to control in a truck that's rolling down the highway or stopping frequently in city traffic. As a result, adsorption systems require running the engine hotter. They can gulp down up to 10% more diesel than a normal truck.

To make adsorption more feasible, ArvinMeritor engineers went looking for a substance other than diesel fuel to attack the NOx. Hydrogen seemed ideal. It's willing to react with just about any compound, even dirty exhaust. "Hydrogen is outgoing," says Taylor. "Diesel fuel wants to stay to itself."

The challenge was finding a way to produce hydrogen inside the vehicle. Working with scientists at the Massachusetts Institute of Technology, ArvinMeritor researchers found an answer: the plasma fuel reformer, or plasmatron.

Here's how it works: A small amount of diesel fuel is injected into the canister, where it is mixed with a tiny bit of air and ignited by an electrical spark. The result is a highly charged, ionized gas called plasma that burns with a bluish glow. The combustion is slow and rich, using just enough oxygen to bust apart the diesel molecules into hydrogen and carbon monoxide without burning off the diesel entirely. That hydrogen and carbon monoxide mixture is then pumped into one of two NOx traps, where it strips the oxygen from the NOx, forming nitrogen, carbon dioxide and water.

Because hydrogen can clean the exhaust right from a cold engine start, the plasma reformer method is 20% to 50% more efficient at reducing NOx than other clean-diesel technologies, allowing less unburned fuel to pass through in the process. ArvinMeritor says the plasmatron creates less sulphur buildup, too. The plasmatron doesn't deal with soot emissions, but ArvinMeritor engineers are hoping to develop by the end of the decade future variants that would.

The new technology could provide an important new source of revenue for ArvinMeritor, which already has a $2.4 billion light vehicle exhaust system business. The company believes there's a $3 billion worldwide market for diesel after-treatment systems for big rigs, with another $7 billion for cars, most of them in Europe.

Enginemakers Cummins and Detroit Diesel have already settled on other technologies to meet the 2007 standards. One is exhaust gas recirculation, which pumps cooled exhaust back into the engine air intake, lowering the combustion temperature and reducing NOx formation.ArvinMeritor's plasmatron will help trucks get to 2010 and beyond.

The technology behind the plasmatron could produce benefits far into the future. Engineers believe it could be used to boost the performance of small gasoline engines. Even more important, the plasmatron could speed the long- awaited transition to hydrogen-powered fuel-cell vehicles.

"Once you have hydrogen [produced] on board, you can do a lot of things," says Pedro Ferro, vice president and general manager of ArvinMeritor's commercial vehicle emissions business. The plasmatron probably wouldn't be able to power a fuel-cell vehicle by itself or eliminate the need for hydrogen-fueling stations but, say the engineers, it could be a bridge to hydrogen technology. "It'll take us to whatever comes next," says Taylor.

Copyright 2004