Zero-emission diesel combustion using a non-equilibrium-plasma-assisted MnO₂ filter

This problem can be overcome by using sulfur-free fuels based on biomass or clean coal technology or by installing a desulfurizing filter to remove sulfur oxides upstream of the NO_x catalyst. Researchers at Kanazawa university have developed a plasma-assisted MnO₂ filter that produces exhaust free of NO_x and SO_x. This technology augments the desulfurization properties of MnO₂ with the activity of ozone from an atmospheric-pressure non-equilibrium plasma. Activated chemical species (O₃, OH radicals, etc.) present in the plasma promote desulfurization and denitration reactions.

MnO₂ reacts with sulfur and nitrogen oxides to produce sulfates and nitrates, respectively. The interaction between SO₂ and NO₂ degrades the performance of MnO₂ catalysts in eliminating both species. Prof Huang of the Guangzhou Institute of Energy Conversion analyzed the MnO2 catalyst material after exposure to simulated exhaust gas containing both SO₂ and NO₂ and found that both manganese nitrate and manganese sulfate were produced.

We evaluated the impact of ozone on the performance of the catalyst for SO₂ and NO₂ removal. An atmospheric-pressure non-equilibrium plasma was generated by the dielectric barrier discharge method. The performance of the catalyst in eliminating both SO₂ and NO₂ was improved by the introduction of ozone at a low concentration of about 50 ppm. The enhancement in NO₂ elimination was particularly notable. The introduction of ozone seems to give a reaction to reduce nitrogen oxides to nitrogen. At the initial stage of the reaction, over 99% of SO₂ and NO₂ were removed from the exhaust stream. The Kanazawa University researchers, led by Yugo Osaka, demonstrated for the first time that zero emissions of NO_x can be achieved even in the presence of sulfur oxides by using a plasma-assisted MnO₂ filter. The plasma-assisted filter seems to augment the elimination of SO₂ because of SO₃ generation and also reduce nitrogen oxides to nitrogen.

These findings are expected to be widely applicable in the purification of exhaust from diesel engines using sulfur-containing fuels. We have clarified the mechanism by which the induction of the non-equilibrium plasma augments the performance of the MnO₂ filter. We hope to spur further development of plasma-assisted MnO₂ filters and thus allow for a greater diversity of fuels to be used without adversely impacting air quality.