A Study on N
O Formation Mechanism and Its Reduction in a Urea SCR System Employed in a DI Diesel Engine
N₂O is known to have a significantly high global warming potential. We measured N₂O emissions in engine-bench tests by changing the NO/NH₃ ratio and exhaust gas temperature at the oxidation catalyst inlet in a heavy-duty diesel engine equipped with a urea SCR (selective catalytic reduction) system. The results showed that the peak N₂O production ratio occurred at an exhaust gas temperature of around 200°C and the maximum value was 84%. Moreover, the N₂O production ratio increased with increasing NO/NH₃. Thus, we concluded that N₂O is produced via the NO branching reaction. Based on our results, two methods were proposed to decrease N₂O formation. At low temperatures ~200°C, NO should be reduced by controlling diesel combustion to lower the contribution of NO to N₂O production. This is essential because the SCR system cannot reduce NOx at low temperatures. At temperatures higher than 200°C, it is necessary to reduce NH₃ slip because N₂O is produced via reactions involving NO generated by NH₃ oxidation and NH₃ slip. To investigate methods to reduce NH₃ slip from the SCR catalyst, we conducted chemical kinetics simulations using Boost v5.1 by AVL. The main parameters governing NH₃ slip were the urea equivalence ratio (UER) and SCR catalyst volume. It is recommended that NH₃ emitted from the tailpipe should be lower than 10 ppm. We identified the optimal UER and volume ratio (defined as SCR catalyst volume to engine displacement ratio) at different temperatures to ensure that catalyst outlet NH₃ emissions were less than 10 ppm. At 200°C, the results showed that decreasing the UER was sufficient to reduce NH₃ slip. At 400°C, we also considered a 90% NOx reduction efficiency. We demonstrated that an increase in the volume ratio was required to sufficiently reduce NH₃ slip at high temperatures.