Alternative Solution: Some OEMs aren’t writing off SCR yet

TORONTO, (May 3, 2004) — Selective catalytic reduction (SCR) was once thought to be part of the 2007 emissions solution in North America, but its chief proponents, Detroit Diesel and Volvo, recently announced they’ll go with EGR after all.

SCR is the choice for Europe’s next round of emissions rules (called Euro 4) in October 2005, which are roughly equivalent to the 2007 North American standards. It’s a well-developed technology that reduces NOx while returning better fuel consumption than EGR. It uses ammonia produced on board the vehicle from a non-toxic, easy-to-handle aqueous urea solution.

The truck has an additional tank for the urea, with an electronic metering unit for the reducing agent, and a ceramic catalytic converter that can be accommodated in the muffler. The urea solution is added to the converter to make the required ammonia. Urea is a colorless, odourless, crystallizing liquid readily soluble in water, discovered back in 1729 and widely used in the chemical, textile, and pharmaceutical industries, among others.

The downside of SCR is that it demands a supply network for the urea, with special pumps for filling the urea tank at truckstops and fleet yards.

That’s a problem for the Environmental Protection Agency, which has also expressed concern about enforcement issues — how could it ensure that urea tanks were always filled?

The killer in North America, however, has been economics. To get to 2007 NOx levels, trucks would have to feed urea into the system at a rate of 5 per cent of fuel used. With urea and diesel priced about the same, it would get expensive. The numbers work in Europe, where fuel is by far the more expensive of the two.

But don’t count out SCR here for 2010. Given NOx reductions already made in 2007, a one per-cent concentration of urea will be all that’s necessary then, at which point the cost equation is favourable. Most engine makers represented here have extensive experience with the technology, and Cummins says it’s already using it in some off-highway applications.

Volvo says it “will continue to participate in U.S. industry efforts to develop a national distribution infrastructure for urea.” And Detroit Diesel calls it “a viable alternative for 2010.” Cat’s investigating it but it’s at “the bottom of our list,” according to on-highway engine chief John Campbell.