Exhaust after treatment systems and emission control
The state of the art in NG catalysis at the start of the project was high precious metal loaded methane oxidation catalyst, which are highly susceptible to sulphur poisoning are highly efficient at temperatures > 400˚C. Efficiencies at lower temperatures are a concern for emissions standards of Euro VI and beyond, however.
Developing an understanding of NG catalyst deactivation mechanisms and in parallel the development of optimized methane control catalyst formulations allowed easier manufacturability of NG applications. Combining these methane catalysts with NOx control required an integrated approach to the system specification and design. HDGAS looked at enhanced NG catalyst performance via fundamental studies and system integration.
Case study of a modern lean-burn methane combustion catalyst for automotive applications: What are the deactivation and regeneration mechanisms? by Niko M. Kinnunena, Janne T. Hirvi, Kauko Kallinen, Teuvo Maunula, Matthew Keenan, Mika Suvanto
One way to lower CO2 and other harmful emissions of the transportation sector is the development of natural gas fueled vehicles. Availability of natural gas is good, and it is easy to apply to stoichiometric and lean-burn engines, which makes it ready-to-use technology. The main concern in the field is a sulfur poisoning of the exhaust gas after treatment system. HDGAS aimed to clarify mechanisms of sulfur poisoning and regeneration of a lean-burn methane oxidation catalyst. Overall, it is concluded that sulfur itself is not the only reason for the deactivation of methane oxidation catalyst, but it is a joint effect of water vapor and sulfur species. The irreversible sulfur poisoning deteriorates oxygen mobility and hinders water desorption, which inhibits low temperature methane oxidation activity. The regeneration of sulfur poisoned catalyst takes place stepwise: PdSO4 →PdSO3 + 0.5O2 →Pd + SO2 + 0.5O2. The formation of metallic palladium makes the catalyst vulnerable for sintering, which leads to deactivation during long-term regeneration. Read more..
The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles by Roberta Villamaina · Isabella Nova · Enrico Tronconi · Teuvo Maunula · Matthew Keenan
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. Read more…
Formation of NH3 and N2O in a modern natural gas three-way catalyst designed for heavy-duty vehicles: the effects of simulated exhaust gas composition and ageing by Pauliina Nevalainena, Niko M. Kinnunen, Anna Kirveslahti, Kauko Kallinen, Teuvo Maunula, Matthew Keenan, Mika Suvanto
An increasing number of heavy-duty vehicles are using liquefied natural gas (LNG) as a fuel due to the expanding refuelling station network for LNG and lower overall emissions compared to diesel vehicles. The latest EURO VI regulation or natural gas fuelled vehicles set a limit for NH3 of 10 ppm, and N2O exhaust is expected to be restricted in Europe in the near future. Poisonous and corrosive NH3 and the greenhouse gas N2O are formed as by-products in a three-way catalyst used to minimize the emissions of stoichiometric heavy-duty engines. In this work, we studied how high temperature NH3 and N2O formed in modern, fresh and aged bimetallic Pd/Rh threeway catalysts in simulated exhaust gas. More precisely, the exhaust gas composition and temperature were examined. Decreases in NO concentration and increases in temperature lowered the formation of NH3 and N2O, whereas a decrease in CH4 concentration reduced only NH3 formation. According to Raman and powder X-ray diffraction experiments, the structure of the catalyst changed during the ageing, and this reputedly affected the function of cerium-zirconium mixed oxides and thus the formation of NH3 and N2O. Temperature programmed reduction (H2-TPR) measurements showed changes in cerium-zirconium mixed oxide performance after ageing supporting Raman spectroscopy findings. Catalyst ageing in oxidizing conditions increased the formation of N2O. This study showed that exhaust gas composition plays an important role in the formation of undesired NH3 and N2O emissions. Read more…