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Lean-burn engines, such as diesel engines, are widely used in mobile and stationary applications. Operation of lean-burn engines leads to formation of distinct amounts of nitrogen oxides (NO and NO2). Efficient aftertreatment is mandatory to meet legal requirements, especially at low exhaust temperatures, as for the future a decline of the exhaust temperature level can be predicted due to improved engine efficiencies. Within this review, potential technical solutions to enhance the DeNO(x)-aftertreatment efficiency at low exhaust temperatures are presented.
Ammonia-based selective catalytic reduction (SCR) systems are the most widely used technology for reduction of nitrogen oxide emissions from lean-burn engines such as diesel engines. However, at low exhaust temperatures, the SCR process is limited by difficulties in the decomposition of the ammonia precursor urea, which is carried on-board using an aqueous solution "AdBlue". In this study, the decomposition of AdBlue urea induced by electrical current and the resulting associated pH shifts were investigated in a divided cell configuration in the liquid phase. The decomposition was found to be favored in both electrochemical compartments, anodic and cathodic, at temperatures of 60-80 degrees C compared to a reference without electrochemical treatment. In addition to the determination of ammonia contents using an ammonia sensor, IC/HPLC analyses were carried out for each sample. Different side products such as biuret, nitrate, cyanuric acid, ammelide, and others were formed. In the anodic compartment, nitrate formation could be observed due to oxidation of ammonia at the electrode surface.
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