Plasma Exhaust Treatment

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Abstract: Non-thermal plasma technologies have been considered for emission reduction from diesel and lean burn gasoline engines. Since oxidation reactions dominate during plasma discharges in lean exhaust, plasma alone is ineffective in reducing NOx. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NOx removal efficiency. Plasma-catalytic reactors have also been studied to control diesel PM emissions and to reduce methane and NOx emissions from marine engines.

Introduction

Non-thermal plasma (NTP) discharges in exhaust gas have been studied as a potential method to reduce NOx and PM emissions in diesel exhaust, NOx and cold start hydrocarbons in lean gasoline exhaust, as well as methane and NOx emissions from LNG marine engines. Vehicle exhaust gases, both diesel and gasoline, undergo chemical changes when exposed to plasma. Logically, oxidation processes dominate in the presence of oxygen. These reactions include oxidation of hydrocarbons, carbon monoxide, and, to a degree, diesel particulate matter. Nitric oxide (NO) can be oxidized by plasma to NO₂.

The oxidation properties of plasma have been utilized in the treatment of flue gases from power plants [118]. In power plant flue gas treatment, the purpose of the plasma is to oxidize NO to NO₂ and subsequently to nitric acid. The desired products, in the form of ammonium salts, are then obtained by reacting the formed acid with ammonia. Industrial plasma systems have also been demonstrated for VOC removal.

Obviously, this method of NOx removal is not applicable for trucks or cars. The objective in the plasma treatment of exhaust gases from internal combustion engines is the reduction, as opposed to oxidation, of NOx. Contrary to some earlier literature reports, plasma alone is not a viable NOx control method due to its oxidizing character. However, combinations of plasma with catalysts, referred to as “plasma-assisted catalysts” or simply “plasma catalysts”, have been suggested for NOx reduction. Plasma catalysts were demonstrated to have efficiencies comparable to those of active HC-SCR systems, removing about 50% of NOx at a fuel economy penalty of less than 5% [415]. Unfortunately, this level of performance was not sufficient to meet the regulatory NOx reduction targets that became effective in the United States and the EU around 2005-2010.

Plasma systems have also been studied for the reduction of diesel particulate matter by low temperature oxidation. It was attempted without success to combine the NOx and PM control functions by plasma in one device—an NTP reactor for the simultaneous control of NOx and PM. More recent research has focused on the use of plasma-catalytic reactors for the reduction of methane and NOx emissions from natural gas (LNG) marine engines [6600][6601].

In spite of the large body of research conducted in the late 1990s and early 2000s, plasma-based technologies could not be developed to meet the regulatory emission reduction targets and have not been commercialized for internal combustion engines. However, there is some continued interest in this technology. In the future, plasma could become a viable choice for exhaust emission abatement systems for some niche applications.