Abstract:

A Japan-based research team led by Kanazawa University have used ozone from an atmospheric-pressure non-equilibrium plasma together with the desulfurization catalyst MnO 2 to almost completely eliminate NO x and SO x from diesel exhaust gas at a low temperature of 473 K. This research shows that ozone can be used to remove not only SO x but also NO x from fossil fuel combustion exhaust streams.

Diesel engines are widely used in agricultural machinery, vehicles, and ships because of their high thermal efficiency. The sulfur contained in the diesel fuel is oxidized to sulfur dioxide by combustion. This sulfur dioxide not only harms human health but also causes deactivation of the catalysts used to treat NO x in the exhaust stream.

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 MnO 2 filter that produces exhaust free of NO x and SO x . This technology augments the desulfurization properties of MnO 2 with the activity of ozone from an atmospheric-pressure non-equilibrium plasma (Figure 1). Activated chemical species (O 3 , OH radicals, etc.) present in the plasma promote desulfurization and denitration reactions.

[Results]

MnO 2 reacts with sulfur and nitrogen oxides to produce sulfates and nitrates, respectively. The interaction between SO 2 and NO 2 degrades the performance of MnO 2 catalysts in eliminating both species. Prof Huang of the Guangzhou Institute of Energy Conversion analyzed the MnO 2 catalyst material after exposure to simulated exhaust gas containing both SO 2 and NO 2 and found that both manganese nitrate and manganese sulfate were produced.

We evaluated the impact of ozone on the performance of the catalyst for SO 2 and NO 2 removal (Figure 2). An atmospheric-pressure non-equilibrium plasma was generated by the dielectric barrier discharge method. The performance of the catalyst in eliminating both SO 2 and NO 2 was improved by the introduction of ozone at a low concentration of about 50 ppm. The enhancement in NO 2 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 SO 2 and NO 2 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 MnO 2 filter. The plasma-assisted filter seems to augment the elimination of SO 2 because of SO 3 generation and also reduce nitrogen oxides to nitrogen.

[Future prospects]

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 MnO 2 filter. We hope to spur further development of plasma-assisted MnO 2 filters and thus allow for a greater diversity of fuels to be used without adversely impacting air quality.

Figure 1. Conceptual scheme of plasma-assisted MnO 2 filter

Activated chemical species (O 3 , OH radicals etc.) are generated by inducing an atmospheric pressure non-equilibrium plasma. These species promote desulfurization and denitration reactions with MnO 2 . In this paper, we evaluated the influence of ozone on the desulfurization and denitrification performance of an MnO 2 filter

Figure 2. The effect of ozone induction on SO 2 and NO 2 elimination

Ozone generated in an atmospheric-pressure non-equilibrium plasma was passed through the MnO 2 filter together with simulated exhaust gas. The simulated exhaust gas consisted of 500 ppm SO 2 , 500 ppm NO 2 ,10wt% O 2 , 6wt% CO 2 , an N 2 base, and 50 ppm O 3 (when plasma is induced). The MnO 2 was supported on an alumina honeycomb filter and the flow conditions (space velocity of 104 h−1) mimicked typical vehicle exhaust streams and filter dimensions.

Figure 3.

TEM images (a, b) of HSSA MnO 2 (MnO 2 having a high specific surface area of about 300 m2/g) and photographs (c, d) of the HSSA MnO 2 filter supported on alumina honeycomb used in these experiments. MnO 2 was laminated onto the alumina honeycomb substrate by the dip coating method. The packing density of MnO 2 was 50 g/L of filter

Article

Basic study on exhaust gas purification by utilizing plasma assisted MnO 2 filter for zero-emission diesel

Journal: Separation and Purification Technology

Authors: Yugo Osaka, Kentaro Iwai, Takuya Tsujiguchi, Akio Kodama, Xing Li, Hongyu Huang

DOI: 10.1016/j.seppur.2018.12.077

Funder

This work was supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research (C)) Grant Number 17K00595.