Comparative evaluation of ammonium carbamate and UWS based SCR system for diesel engine NOX reduction under WHTC
Hassan Raza, Korea Institute of machinery and materials (KIMM), South Korea
Hassan Raza1,2, Woo Sanghee2, Hongsuk Kim1,2
1 Environment & Energy Mechanical Engineering, University of Science and Technology, Republic of Korea
2 Engine Research Lab, Korea Institute of Machinery & Materials, Republic of Korea
After-treatment technologies are being developed to achieve the upcoming stringent emissions legislation and 2027 target for ultra-low NOX emissions set by California Air Resouces Board (CARB). The currently commercialized UWS technology has limitations in injecting the reductant at exhaust gas temperature below 200⁰C and deposition issues. This study uses ammonium carbamate (AC) as an alternative gaseous ammonia source due to its low decomposition temperature and opportunity to inject at low exhaust gas temperature (<200⁰C) without deposit formation. The world harmonized transient cycle (WHTC) tests were conducted using a 3.9 L diesel engine over Cu zeolite-based catalyst to evaluate the NOX reduction performance of the system. The ammonium carbamate-based SCR system’s capability to meet NOX emissions regulations, the effect of NH3/NOX (α) ratio, injection threshold temperature (Tinj), and 0D ammonia adsorption-desorption model on the NOX conversion efficiency was investigated and compared with UWS. AC-based SCR system showed superior deNOX performance in both phases of WHTC; especially, the difference was significant during the hot phase. The gaseous ammonia injection at low exhaust gas temperature is the reason for good deNOX performance; decreasing Tinj leads to an increase in the NOX conversion efficiency. However, the N2O emissions were slightly increased for AC than UWS due to the formation of ammonium nitrate by injecting gaseous ammonia at low temperatures. Increasing the NH3/NOX (α) ratio is effective for increasing the NOX conversion efficiency. Still, robust injection leads to ammonia slip, which can be converted to NOX when passed over the ammonia oxidation catalyst. The model-based control using NH3 adsorption and desorption leads to improvement in the deNOX performance of the AC-based SCR system, especially during the cold phase of WHTC. The further development in the ammonia injection model-based control can be considered one of the possible options to reach the ultra-low NOx levels.