Selective catalytic reduction (SCR) technology uses diesel exhaust fluid (DEF), a solution that is comprised of urea and purified water.
Urea is a nitrogen compound that occurs naturally or is synthesized from natural gas and transforms into ammonia when heated. It’s used in a variety of applications, often as a “scrubber” agent to reduce pollution from smoke-stack industries and as a fertilizer in agriculture. It’s also naturally occurring in urine, but not in concentrate levels or volume sufficient for use as DEF.
When urea is combined with purified water, it creates a nontoxic, colorless and odorless agent. DEF is mixed one-third urea to two-thirds water.
DEF consumption, according to the engine-maker Cummins, is expected to be around one gallon for every 50 gallons of diesel fuel used, or approximately two percent of total fuel consumption depending on vehicle type, use and geography. Cummins says DEF has a shelf life of up to two years, and all major truck stops are expected to sell it.
The cost of urea-based DEF is somewhat volatile and can range from $19 per gallon to $2 per gallon. The American Trucking Association Technology & Maintenance Council reports DEF will likely cost $5 to $6 per gallon, depending on the quantity purchased.
Detroit Diesel says truck tank sizes for DEF will probably range from six gallons to 23 gallons, depending on the type of truck and its application.
As DEF becomes more widely used in the United States, the solution is expected to be sold and dispensed in bulk and to be available on store shelves.
Christy Nycz, the market communications manager for Cummins, which is using SCR technology, said filling the DEF tank will be no more challenging than filling a windshield washing fluid tank or a fuel tank.
Storage, particularly on-board apparatus, may be challenging, as DEF is mostly purified water and subject to freezing, as well as evaporation at temperatures greater than 120 degrees.
To combat freezing, truck manufacturers are expected to use in-tank heaters to keep the fluid liquid and able to be injected into the exhaust stream.
Bob Neitzel, the emergency vehicle marketing manager with Navistar International, which decided against using SCR technology, said DEF freezing is a big problem from an emissions perspective. The EPA, he said, allows SCR-equipped apparatus to run for up to a half hour without the system working to help thaw frozen tanks.
People Are Concerned
Dave Siler, the director of marketing for Detroit Diesel, which is using SCR, said he understands people are concerned about the weight and space the SCR system will take up. But he said improved fuel efficiency and engine performance will more than offset other issues.
The North American SCR Stakeholders Group, an ad-hoc industry alliance of engine, truck and commercial vehicle industry representatives, as well as DEF distributors and suppliers, said SCR systems allow the engines to be tuned to deliver three to five percent fuel savings while keeping the environment clean.
The stakeholders group said there’s a commitment to have 1,800 or more DEF distribution sites in place by 2010 nationwide.
“Nitrogen oxides (NOx) contribute to smog, and smog contributes to more than 400,000 hospital visits each year for conditions related to asthma and respiratory and heart diseases – all of which have been linked to diesel exhaust,” the SCR stakeholders group said. “Commitment to reducing total emissions from our engines today to the lowest possible levels is our responsibility as executives, as parents and as human beings.”