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Readying for Selective Catalytic Reduction in Cold Weather

Fri August 26, 2011 - National Edition
David W.H. Fenderson

Since the early 1990s the Environmental Protection Agency has been pushing the requirements for reductions in particulate and NOx emissions for modern diesel engines. These reductions are targeted to reduce the environmental impact and the health hazards associated with diesel emissions. The 2010 requirements are .20 (g/hp-hr) an 83 percent reduction and .01 PM (g/hp-hr) a 90 percent reduction since 1994. One 1994 on-highway engine will emit the same amount of pollutants as 65 of the equivalent engines that were built in 2010.

Many changes have occurred in both engine technology and fuels in the last 15 years to meet these stringent EPA requirements. Most notably the sulfur content of diesel fuels has dropped from 500 PPM in 1993 to 15 PPM in 2006. Additionally engine manufactures have employed changes in engine timing, EGR, and diesel exhaust particulate filtration, and most recently SCR to meet these mandated emission levels.

SCR will be the most foreign and most noticeable change in truck engine design and performance, and may be the final chapter in reducing NOx emissions. Historically NOx emission requirements have been managed through a combination of engine timing changes and EGR. Although these changes have benefited the environment they have challenged both engine maintenance and fuel economy.

SCR is an exhaust after- treatment that manages NOx emissions downstream of the diesel particulate filter and uses DEF (Diesel Exhaust Fluid) to neutralize toxic NOx gasses. DEF is injected into the emissions, which react with the NOx and the catalyst creating harmless nitrogen gas and water vapor. SCR allows the engine to function at higher combustion temperatures, providing increased fuel efficiency and power. It has been reported that fuel consumption could be improved as much as 6.5 percent with trucks using SCR as their chosen method for NOx reduction.

The application of DEF in the engine will be simple as each truck will be outfitted with an outboard storage tank between 3 and 24 gal. depending on class of vehicle. The level of NOx in the emissions system will transmit a need for DEF to be injected into the system as a fine mist. This DEF mist will hydrolyze into ammonia gas (NH3) and react with the NOx in the SCR catalyst to form nitrogen and water.

DEF is a high purity mixture of 32.5 percent urea and deionized water meeting the stringent ISO 22241-1 quality standards. The amount of DEF required will vary by engine manufacturer, horsepower, and load, however, on average one can expect to use 3 to 5 percent of DEF as a relation to fuel consumed. The average truck going 100,000 miles per year at 6 miles per gallon will consume between 500 and 850 gallons of DEF. Although the 2010 engines are more costly to purchase than later model engines, this cost, as well as the cost to purchase DEF will be offset by a much cleaner and safer environment for generations to come as well as significant improvements in fuel consumption.

The critical issues surrounding DEF will be handling and storage as any contamination to the fluid will result in significant threats to the catalysts and engine performance. It also is very important to use only a certified DEF as both the urea as well as the water used to blend DEF are required to meet specifications with very close tolerances. DEF is being made available through many channels of distribution both at the pump and in packages. It will be advisable to top off DEF tanks during routine maintenance and inspections as well as carry a 2.5 gallon bottle on board the truck as an emergency dose. The average truck will go 400 miles on 2.5 gallons of DEF. DEF will be available in packages sizes of 1 Gal., 2.5 gal., drums, totes, as well as bulk deliveries.

Of potential concern to DEF users in Maine as well as other cold weather states, DEF will freeze when the temperature drops below 11F. Fortunately the SCR systems are designed for this and will not crack the DEF dispensing equipment and will quickly thaw as hot engine coolant will be circulating through heating coils in the system. Additionally during typical engine start up the need for DEF is limited as the equipment has very low NOx emissions. The recommendation will be to follow the OEM instructions for cold weather operation, which will ultimately mean increased idling to ensure the DEF is thawed. Fortunately the EU has been using SCR for more than 10 years in many of the world’s coldest environments with excellent results.

Each SCR truck will be equipped with onboard hardware to detect levels of DEF in the storage tank. If the truck runs out of DEF or if the wrong or contaminated fluid is used, the engine will derate. It is possible for the engine to derate down to a maximum speed of 5 miles per hour during these conditions. Again it cannot be emphasized enough to keep appropriate levels and quality of DEF in the truck’s reservoir to avoid these costly conditions.

For more information, call 508/450-6371.

David Fenderson is the senior vice president-marketing of Windward Petroleum.

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