NOx-reduction in diesel exhaust gas with urea and selective catalytic reduction

High values of NOx reduction may be obtained with urea as a reducing agent and a standard SCR catalyst based on TiO2-WO3-V2O5. The process was carefully investigated for possible secondary emissions and it could be shown that urea-SCR does not lead to relevant emissions of nitrogen dioxide, nitrous oxide, hydrogen cyanide and isocyanic acid. Further investigations using a HPLC method have also proved that addition compounds of higher molecular mass than urea (and urea itself) are not emitted in appreciable amounts as long as the process is properly managed, i.e., as long as the emission of ammonia is kept low. The limiting secondary emission is ammonia slip, the major problem when ammonia is used directly as a reducing agent.