NOx reduction with dimethyl ether in lean exhaust

Licentiatavhandling, 2009

There is a growing concern worldwide about the increase in anthropogenic greenhouse gas emissions, particularly carbon dioxide (CO2) from the combustion of fossil fuels. Vehicles with internal combustion engines contribute significantly to these emissions, which has led to an increased demand for engines running on alternative (non-fossil) fuels. For diesel engines, dimethyl ether (DME) is a promising alternative fuel with high energy efficiency and large potential for low CO2 emissions. Although it is possible to reach very low engine out emissions with DME, aftertreatment of nitrogen oxides (NOx) is likely needed. Two catalyst technologies for NOx reduction under lean conditions utilizing the engine fuel as reducing agent are lean NOx catalyst (LNC) and lean NOx adsorber (LNA). The main purpose of this thesis was to investigate NOx reduction with DME in lean exhausts. DME was evaluated as reducing agent for NOx over two commercial catalysts in a synthetic exhaust flow reactor. DME was found to be a very efficient reducing agent for NOx over the BaO-based LNA, but a very poor reducing agent over the Cu-ZSM-5-based LNC, although the oxidation of DME occurs at similar temperatures over both catalysts. The influence of the reducing agent on lean NOx reduction over Cu-ZSM-5 was studied. A screening of nine reducing agents (alkanes, alcohols, and ethers) showed that DME and methanol did not reduce NOx over Cu-ZSM-5, whereas higher ethers and alcohols did. Underlying reasons for the differences in NOx activity were investigated with methanol, DME, ethanol, and ethylene glycol. The chemical structure of the reducing agents was found to be particularly important. Despite being more oxidized than DME, ethylene glycol was active in NOx reduction. The most likely explanation is that a C-C bond is needed in the reducing agent for lean NOx reduction over Cu-ZSM-5. The absence of this bond in DME and methanol would explain their low activity for NOx reduction over this catalyst.