UV-Vis unit and DEF Testing
by Patrick Kilbane, Northeast Business Manager, Tribology

Reduction of NOx emissions from diesel engines has been mandated since the 1980s. The use of exhaust gas recirculation (EGR) technology to reduce NOx emissions was a significant equipment design implementation for diesel engines to meet 2002 and 2004 EPA mandated requirements for engine manufacturers. This was part of an overall effort to reduce a variety of pollutants from diesel engines such as hydrocarbon pollutants, particulate matter, carbon monoxide, and nitrogen oxides (NOx). The reduction of NOx emissions will see tighter controls for new over the road diesel engines in 2010 and will further help to reduce smog and acid rain caused by the NOx emissions. One approach to meet these new emission standards is the use of after treatment technology in the form of selective catalyst reduction (SCR) which converts harmful NOx compounds into harmless compounds.

The last phase of the SCR catalyst takes any remaining, unreacted ammonia in the exhaust stream and reacts it with oxygen to form additional nitrogen gas and water. In the United States the urea solution is commonly referred to as Diesel Exhaust Fluid (DEF).

Purity of both the urea and water for blending the aqueous solution is very important for proper performance. The first Standard, ISO 22241-1, addresses the quality characteristics or purity, the chemical characteristics, and the precision of the test methods used for analyzing the urea solutions.

Aldehyde – Aldehyde is considered an undesirable by product that is found in many urea sources such as agricultural grade fertilizers and industrial urea formulations used in resin manufacturing. Only very pure grades of urea can be used to ensure effectiveness of the SCR. This is measured by specialized equipment (UV/Vis spectrophotometer, see Image 1).

Alkalinity – The alkalinity is measured as ammonia in the solution and may be present as either or both a residual production component or as a urea break down product. The amount of ammonia present in the urea solution needs to be controlled to prevent noxious odors from the solution and to minimize competing reactions within the SCR.

Biuret – Biuret is a residual byproduct of urea production and is also a break down product of urea. The amount of biuret present in the urea solution needs to be controlled to optimize the reactions within the SCR. This is measured by specialized equipment (UV/Vis spectrophotometer).

Phosphate – Phosphorus, often in the form of phosphates, are commonly found in agricultural fertilizers. The urea solution needed for the 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 excessive back pressure may build up in the exhaust stream.

Metals and Elements – The presence of metals may interfere with the performance of the aqueous urea solution and potentially cause fouling of the catalyst material. This test is also an indicator of overall purity of the urea and water. Calcium, Iron, Copper, Zinc, Chromium, Nickel, Aluminum, Magnesium, Sodium, and Potassium elements are tested by ICP, see Image 2.

Urea Concentration – The urea concentration in the aqueous solution should be at 32.5%. This concentration provides optimal efficiency for the reactions taking place in the SCR. Urea solutions varying from this concentration by more than 0.7% may result in reduced NOx removal or residual ammonia in the exhaust gases. The urea is measured indirectly 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 actually measure the urea directly, an additional test is available for the qualitative confirmation of the presence of urea. This test simply indicates that the compound urea is actually present in the solution and confirmed by identifying peaks using an FTIR.

Proper maintenance and monitoring of your SCR system, including use of only the specified DEF, can help ensure a long and efficient life of your system and minimize the need for replacement.