byproduct of urea production and is also a breakdown product of urea. The amount of biuret present in the urea solution needs to be controlled to opti- mize the reactions within the SCR. Metals — The presence of metals may interfere with the performance of the aqueous urea solution and poten- tially cause fouling of the catalyst material. This test is also an indicator of overall purity of the urea and water. Phosphate — Phosphorus, often in the form of phosphates, is com- monly found in agricultural fertilizers. The urea solution needed for SCR must be of a much higher purity than that used in fertilizers, and this test is used to ensure that purity. Solids — Undissolved materials or solids in the urea solution may get trapped in the catalyst material. As these solids build up on the catalyst material, the efficiency of the NOx removal will be reduced and exces- sive backpressure may build up in the exhaust stream. Urea Concentration — The urea concentration in the aqueous solution should be at 32.5%. This concentra- tion provides optimal efficiency for the reactions taking place in the SCR. Urea solutions varying from this con- centration by more than 0.7% may result in reduced NOx removal or residual ammonia in the exhaust gases. The urea is measured indirect- ly either by the refractive index of the solution or by the total concentration of nitrogen in the solution. Urea Confirmation — Since the urea concentration tests do not actu- ally measure the urea directly, an additional test is available for the qualitative confirmation of the pres- ence of urea. This test simply indi- cates that the compound urea is actu- ally present in the solution. Though ISO Standards 22241-1 and 22241-2 specify the quality con- trol parameters of the aqueous urea solution for SCR, additional testing to monitor the integrity of the urea solu- tion in use may prove necessary from time to time. Screening tests could be used for troubleshooting and active ingredient concentration confirmation. Issues seen in the field include diesel fuel contamination (spout in the wrong tank), excessive water dilution (pur- poseful dilution or a tank cap not fully secured), water evaporation (high temperatures), precipitation of urea and solution freezing. As we move closer to 2010 and the use of aqueous urea solution for re- ducing NOx emissions becomes pre- valent in diesel vehicles, it will be nec- essary to maintain the integrity of the aftertreatment and the reductant. Certain tests could be used to monitor urea concentration and water quality, since the proportion of urea to water and the water quality are important. Aqueous urea can be subject to cold-weather problems, and DEF tanks on trucks in cold-weather climates will typically be outfitted with tank heaters. Below 12°F, the urea solution will begin to crystallize and eventually freeze.
28 DIESEL PROGRESS NORTH AMERICAN EDI TION February 2010
SCR technology such as Daimler’s Blue Tec system has been widely used in Europe for several years on a range of vehicles including transit and intracity buses.
TECHNOLOGY OF CLEAN AIR
Alternatively, modifications to the DEF may ultimately be employed to lower the freezing point of the solution. Studies are under way to test additives for the solution, such as alcohols and ammonium formate. The correct concentration of urea in the mixture is critical to proper catalyt- ic reaction. Metal impurities from added water may affect the catalyst reaction. Though regular testing of in- service aqueous urea solution in SCR is not expected to be necessary, some inspection from time to time may be required in specific circumstances. Suggested in-service testing of aqueous urea solution should include examinations of urea content and contaminants. Urea content can be determined by the refractive index method. The target concentration is 31. 8 to 33.2%. There are several methods for testing contaminant lev- els. Visible observation or extraction can be used to ensure that the DEF does not contain any diesel fuel or any other organic fluids. Metals can be detected by the ICP instrument method. Metals testing should include aluminum, calcium, chromium, copper, iron, magnesium, nickel, potassium, sodium and zinc. None of the metals should be present in excess of either 0.2 or 0.5 mg/kg, depending on the specific metal. Solids can be dealt with through fil- tration at 0.8 µm. The DEF solution should not contain any more than 20 mg/kg of unfilterable material. One mechanism for preventing intentional DEF dilution with water is to outfit the DEF tank with an onboard conductivity meter wired to an engine kill switch. If the conductivity of the solution drops below a specified value, indicating excessive water dilution, the kill switch would be engaged, thus pre- venting operation of the engine. SCR aftertreatment systems in diesel vehicles are designed for long service life under a variety of condi- tions. Proper maintenance and moni- toring of SCR systems, including use of only the specified DEF, can help en- sure a long and efficient system life. dp