Design and Durability of Vanadium-SCR Catalyst Systems in Mobile Off-Road Applications 2011-01-1316

The emission regulations for mobile off-road applications are following on-road trends by a short delay. The latest Stage 3B and 4 emission limits mean a gradual implementation of oxidation and SCR catalysts as well as particulate filters with off-road machines/vehicles in the 2010s. The driving conditions and test cycles differ from on-road truck applications which have been the first design base for off-road aftertreatment technologies. Aftertreatment systems for Stage 4 were first analyzed and they will include oxidation catalysts, a NO_x reduction catalyst (SCR or LNT), a particulate filter and possibly units for urea hydrolysis and ammonia slip removal. The design and durability of V₂O₅/TiO₂-WO₃ catalysts based on metallic substrates were investigated by engine bench and field experiments. NO_x emissions were measured with 6.6 and 8.4 liters engines designed for agricultural and industrial machinery. The criteria NO_x conversions with NH₃ slip below 20 ppm and varied catalyst volumes were used as a design base for dosing strategies over the lifetime of the system. The target NO_x conversion over ISO 8178 cycle was about 50% for Stage 3A with first SCR engines and will be 80-95% for Stage 4, which high conversion target has a crucial effect on the required catalyst amount and dosing strategy margins, particularly after ageing. NO_x conversions were stable in the designed urea dosing values after engine bench ageing for 3000 hours and field ageing for 8000 hours. Durability and reaction studies were applied to the SCR catalyst design for Stage 3B and 4.

In thermogravimetric and mass spectrometric (TGA-MS) analysis, no vanadium evaporation was detected below 1000°C but near to the melting point (690°C) of V₂O₅, the catalytic activity of vanadium-SCR catalyst was dropped due to sintering of active sites. The commercial vanadium-SCR systems were designed to the maximum temperatures of 600÷C. The characterization of 3000 and 8000 hours aged catalysts revealed the axial accumulation of elements (P, Zn, Ca, Na, K, S, Si, Fe) originating from lubrication oil and fuel. However, only the short front part of the catalyst had a higher concentration of deactivating compounds correlating to a decreased NO_x performance by laboratory experiments. This long-term deactivation has also been included into the SCR catalyst design for off-road applications by the target NO_x conversions.