Practical implications of HC storage and release on SCR catalysts for HDD applications
Manasa Sridhar, CNH Industrial, FPT Industrial
Manasa Sridhara, Reto Schlegela, Evangelos Katsaounisa, Davide Rodonob
a FPT Motorenforschung, Arbon, Switzerland, b FPT Industrial, Turin, Italy
Selective catalytic reduction (SCR) is a widely implemented technology for NOx reduction in Diesel engine applications to levels demanded by emission regulations applied worldwide. Besides performance, long term durability of SCR catalysts is of critical importance in order to remain emission compliant over lifetime of vehicle usage. Among the reported causes of catalyst deactivation, thermal aging is an important pathway that deteriorates the DeNOx activity over time. Thermal aging occurs during extended operation at high loads, especially during regeneration events. Additionally, severe thermal aging might occur due to uncontrolled burning of hydrocarbons (HCs) following ignition of HCs accumulated in the SCR catalysts after prolonged vehicle idling . Hence, it is of immense interests for the automotive industry and academia to understand the phenomenon of HC storage and release with the ultimate aim of developing robust catalysts as well as ECU predictive models that can prevent or at best minimize the occurrence and impact of such exothermic events.
In this work, we adopted an engine bench approach to investigate the impact of idling length under varying load conditions to obtain a correlation between stored amount of HCs and peak temperatures in SCR obtained as a result of exothermic events triggered by drop-to-idle maneuvers. Two different FPT production engines, namely, Cursor 13 and NEF 6 fitted with full ATS were used for this study. It was found that Vanadium-based SCR (V-SCR) catalysts exhibit a markedly different behavior in comparison to Cu-zeolite SCR (Cu-SCR) catalysts. While, the former was highly prone to exothermic events, the latter was largely resistant which can be attributed to the small-pore nature of the zeolite that blocks entry of large HC molecules, leading to lower HC storage levels and consequently exhibit less severe exothermic HC-burn off behavior. The occurrence of local hotspots during exothermic events was distinguished by instrumenting the SCR bricks with several thermocouples along the axial and radial directions. Over a realistic period of idling, the highest temperature during the exothermic HC-burn off is reached close to the inlet of V-SCR catalyst, and towards the exit of Cu-SCR catalyst. This difference originates from different HC saturation levels in the two SCR technologies. Impact of temperature, space velocity, engine out HC concentration and soot loading (in the case of SCR-coated filter) on HC storage and release are other aspects that will be discussed in this work. These findings are important for understanding the consequences of specific field operations as well as for the development of more robust ECU strategies that aim at releasing the accumulated HCs at lower temperatures to prevent exothermic events.
 Girard, J., Snow, R., Cavataio, G., and Lambert, C., “Influence of Hydrocarbon Storage on the Durability of SCR Catalysts,” SAE Technical Paper 2008-01-0767, 200