Abstract
We present a systematic investigation of the deNOx activity of two commercial metal exchanged zeolite NH3-SCR catalysts, a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehicles. The catalytic activity data collected under realistic operating conditions, representative of the after-treatment system of lean-burn vehicles, were compared to those obtained adding methane to the gas feed stream in order to assess the impact of this hydrocarbon, which is usually emitted from natural gas engines, on the NH3-SCR catalytic chemistry. Our results indicate a negligible impact of methane on the SCR activity at all conditions, but in the presence of a large excess of NO2 at T > 400 °C due to methane oxidation by NO2. The data collected over the two individual metal-promoted zeolites were also compared with those obtained combining both catalysts in sequential arrangements, in order to take advantage of their complementary high activities in different temperature ranges. The Fe-zeolite + Cu-zeolite sequence outperformed the two individual components in terms of both overall deNOx efficiency and N2O selectivity, and was equally insensitive to methane.
Highlights
The abatement of gaseous polluting emissions from combustion processes has become one of the main challenges in the automotive field
Lean operating natural gas fuelled engines have the potential to deliver low CO2 transportation solutions compared to diesel and dual fuel applications
In order to purify the exhaust gases emitted by lean-burn natural gas vehicles and meet the current emission limitations, the typical after-treatment system has to comprise two main sections: the first one is dedicated to the oxidation of unburned methane over a dedicated catalyst (MOC— methane oxidation catalyst)
Summary
The abatement of gaseous polluting emissions from combustion processes has become one of the main challenges in the automotive field. The use of natural gas (NG), in engines operating in lean conditions, or its blend with diesel can be a good way to produce lower emission compared to those produced by using regular. Lean operating natural gas fuelled engines have the potential to deliver low CO2 transportation solutions compared to diesel and dual fuel applications. In order to purify the exhaust gases emitted by lean-burn natural gas vehicles and meet the current emission limitations, the typical after-treatment system has to comprise two main sections: the first one is dedicated to the oxidation of unburned methane over a dedicated catalyst (MOC— methane oxidation catalyst). The second one is devoted to the abatement of N Ox emissions (SCR—selective catalytic reduction) [1] followed by an ammonia slip catalyst (ASC) which prevents the NH3 release caused by the limited SCR activity at low temperatures and during rapid changes in engine operation. Due to the nature of methane combustion and the fuel itself
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