Plasma Exhaust Treatment

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Abstract: Non-thermal plasma technologies are being developed to reduce NOx emissions from gasoline and diesel exhaust. Since oxidation reactions dominate during plasma discharges in lean exhaust, the plasma alone is ineffective in reducing NOx. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NOx removal efficiency. Non-thermal plasma reactors can be also designed as diesel particulate matter reducing devices. Plasma technologies still require a significant improvement in their consumption of electrical energy and in other areas.

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 as well as NOx and cold start hydrocarbons in lean gasoline exhaust. 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 the 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, there is now a wide consensus that plasma alone, due to its oxidizing character, is not a viable NOx control method. However, combinations of plasma with catalysts, referred to as “plasma-assisted catalysts” or simply “plasma catalysts”, have been suggested for NOx reduction. The plasma is believed to show potential to improve catalyst selectivity and removal efficiency. Current “state-of-the art” plasma catalysts have efficiencies comparable to those of active DeNOx systems, removing about 50% of NOx at a fuel economy penalty of less than 5% [415]. While such performance would not be sufficient to reduce NOx in heavy-duty engines from the US2004 level of about 2 g/bhp-hr to the 2010 level of 0.20 g/bhp-hr, it could be sufficient to reach NOx levels of about 1 g/bhp-hr, as will be needed during the 2007-2009 phase-in period of the NOx standard.

In the case of diesel exhaust a removal of particulate matter emissions would be also a valuable benefit of plasma systems. Plasma systems have been shown to be capable of reducing of diesel particulate matter by low temperature oxidation. It is not currently clear whether the NOx and PM control functions by plasma can be combined in one device, i.e., if NTP reactors can be designed for the simultaneous control of NOx and PM.

An increasing number of research reports are published in the literature increasing our understanding of plasma chemistry in the engine exhaust gases. However, this technology has still a novel character and published results need to be evaluated with caution. Since many studies are conducted in small scale laboratory experiment, as opposed to a full-flow engine experiment, erratic interpretation of data is frequently suspected. It is very easy to overlook a formation of unidentified chemical compounds in the plasma or to confuse adsorption and storage of material in the test equipment with its steady-state removal. For vehicle plasma applications, it is very important to make a distinction between NO removal by chemical oxidation and NO removal by chemical reduction. The desired overall process is chemical reduction to benign products, such as nitrogen and oxygen. In the plasma processing literature many authors use the term “NO reduction” even when the NO removal is accomplished by oxidation to NO₂ and nitric acid. It is not sufficient for a plasma experimental work to record a decrease of the NO or NOx concentration. Possible reaction products may include many other nitrogen species which may be not acceptable. Several of these by-products may be also difficult to detect in the laboratory setup. Besides nitrous and nitric acids these products may include nitrates, nitrites or organonitrites which can be deposited on reactor walls, on particulates, or on the pellets material if a packed bed reactor is used. Even if chemical reduction of NO dominates, the products may include nitrous oxide N₂O which, although not a regulated emission, is not an acceptable product. Commercialization of non-thermal plasma technology for emission control from mobile sources requires significant advancements and much more development work. The plasma may or may not become a viable choice for lean NOx or PM removal systems.