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 NOx 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. NOx
emissions were measured with 6.6 and 8.4 liters engines designed
for agricultural and industrial machinery. The criteria
NOx 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 NOx
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.
NOx 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
NOx performance by laboratory experiments. This
long-term deactivation has also been included into the SCR catalyst
design for off-road applications by the target NOx