A comprehensive number of CFD simulations were carried out at Albon- air to analyze the behavior of spray droplets in the exhaust gas flow and showed a very evident advantage of small droplet sizes. The larger droplet sizes need a much longer evaporation length, which can lead to a poor distri- bution of ammonia across the catalyst — resulting in insufficient NOx conver- sion rates and/or ammonia slip. A laser diffraction analyzer was also used to determine the particle size dis- tribution of airless dosing systems and air-assisted dosing systems. The re- sults of the analysis showed that air- assisted dosing system are capable of generating a much finer spray than their airless counterpart. Finally, an SCR system was exam- ined on the engine test bench with both an airless and an air-assisted urea dos- ing system. Four different loads with the corresponding urea dosing rates were chosen and the NOx conversion rates were determined. A considerably better NOx conver- sion rate was registered at the low mass flow rate combined with a low temperature. The other three points showed less remarkable differences. However, the air-assisted system showed an improved NOx conversion at all four loads achieved with low air consumption. It is clear then that a homogeneous mixture of the reducing agent in the exhaust gas is necessary for high SCR catalyst performance. A well-dispersed and fine spray of the urea solution is key to achieve this even distribution, especially at low exhaust gas flow rates and temperatures. Air-assisted dosing systems offer the optimum method of controlling the spray quality and deliv- ering the desired distribution. dp An Albonair selec- tive catalyst reduc- tion (SCR) system with an air-assisted urea dosing system. How Urea Droplet Size Affects SCR Efficiency
Dr. Georg Hüthwohl is managing director of Albonair, Dortmund, Germany. Albonair is a supplier of complete SCR systems, diesel particulate filter systems and re- lated components for the global commer- cial vehicle markets. The company’s North American operations are in Troy, Mich.
BY DR. GEORG HÜTHWOHL The most commonly used NOx reduction system in exhaust emissions of commercial vehi- cles operating in many parts of the world is the selective catalytic reduc- tion (SCR) catalyst. The conversion of nitrogen oxides into harmless nitrogen in SCR catalysts needs a reducing agent, ammonia, on the surface of the catalyst. To generate gaseous am- monia in the exhaust gas, a spray of urea solution is injected, which is con- verted by hydrolysis reaction. The performance of the catalyst depends very much on an even supply of the ammonia on the catalyst. This uniform ammonia spread can only be accomplished when the urea solution spray is distributed uniformly and evaporated as well as decomposed. The quality of the spray — that is the particle size distribution — has a signif- icant influence on these properties. The droplet sizes of the spray along its path in the exhaust gas are influ- enced by a number of processes,
February 2010 DIESEL PROGRESS NORTH AMERICAN EDI TION 23
namely the primary breakup, the sec- ondary breakup and the evaporation and hydrolysis.Th e primary breakup is the forma- tion of the first droplets as the liquid exits the injector nozzle in a thin jet. Spray generation by air-assisted dos- ing systems support this process and leads to improved primary breakup.In the secondary breakup, the motion of the droplets relative to the gas phase — the different velocities of the two phases — induce forces that can result in an unstable large droplet breaking up into a number of smaller droplets.In the evaporation and hydrolysis phase, heat is transferred from the hot exhaust gas to the relatively cold droplet by convection and causes an increase in temperature of the drop- let. Once the boiling point of the liquid is reached, the droplet will start to evaporate significantly — with the water portion of the urea solution evaporating first. While this happens, the urea concentration increases and with it the current boiling point. When all the water is evaporated, the liquid urea will be decomposed and subse- quently converted to ammonia in a hydrolysis reaction. Small droplet sizes and high exhaust temperatures will increase the evaporation rate and reduce the hydrolysis length.at
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