Consideration of the Role of Plasma in a Plasma-Coupled Selective Catalytic Reduction of Nitrogen Oxides with a Hydrocarbon Reducing Agent

Abstract

Abstract:The purpose of this study is to explain how plasma improves the performance of selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with a hydrocarbon reducing agent. In the plasma-coupled SCR process, NO x reduction was performed with n-heptane as a reducing agent over Ag/γ-Al 2 O 3 as a catalyst. We found that the plasma decomposes n-heptane into several oxygen-containing products such as acetaldehyde, propionaldehyde and butyraldehyde, which are more reactive than the parent molecule n-heptane in the SCR process. Separate sets of experiments using acetaldehyde, propionaldehyde and butyraldehyde, one by one, as a reductant in the absence of plasma, have clearly shown that the presence of these partially oxidized compounds greatly enhanced the NO x conversion. The higher the discharge voltage, the more the amounts of such partially oxidized products. The oxidative species produced by the plasma easily converted NO into NO 2 , but the increase of the NO 2 fraction was found to decrease the NO x conversion. Consequently, it can be concluded that the main role of plasma in the SCR process is to produce partially oxidized compounds (aldehydes), having better reducing power. The catalyst-alone NO x removal efficiency with n-heptane at 250 • C was measured to be less than 8%, but it increased to 99% in the presence of acetaldehyde at the same temperature. The NO x removal efficiency with the aldehyde reducing agent was higher as the number of carbons in the aldehyde was more; for example, the NO x removal efficiencies at 200 • C with butyraldehyde, propionaldehyde and acetaldehyde were measured to be 83.5%, 58.0% and 61.5%, respectively, which were far above the value (3%) obtained with n-heptane.

High-Throughput NOx Removal by Two-Stage Plasma Honeycomb Monolith Catalyst

“…14 In the plasma discharge zone, the oxidation of HCs is facilitated to form partially oxidized HCs at low temperature, which enhances the reduction of NO x . 15 Furthermore, O radicals, OH radicals, O 3 , and peroxyl radicals (RO 2 , HO 2 ), all of which are reactive chemical species, could be formed in the discharge zone. 16 In exhaust gas, most of the NO x exists in the form of NO, and the SCR of NO x would be promoted by increasing the ratio of NO 2 / NO x in the feed gas.…”

Plasma-Assisted Selective Catalytic Reduction for Low-Temperature Removal of NOx and Soot Simulant

“…Plasma coupled with a catalyst, therefore, enables the removal of both NO x and soot at low operating temperatures. Moreover, enhanced NO x removal under fluctuating temperatures was also observed in the presence of plasma [47][48][49][50].…”

“…However, the efficiency of NO x removal was not satisfactory with 48 ppm naphthalene tantamount to a C/N ratio of 1.6 (the ratio of carbon concentration in the hydrocarbon to nitrogen concentration in NO x ). Previous reports indicated that n-alkane is an effective reducing agent for the HC-SCR process [48][49][50]. In general, dodecane (C 12 H 26 ) can represent diesel fuel.…”

Application of Plasma Treatment in Preparation of Soybean Oil Factory Sludge Catalyst and Its Application in Selective Catalytic Oxidation (SCO) Denitration

“…This treatment method can not only maintain the catalyst skeleton, remove organic impurities such as template, and prevent the sintering of metal clusters from becoming large, but also has a relatively short processing time compared to conventional roasting. Many experiments have shown that this plasma roasting can replace conventional high-temperature roasting [27,28,29]. …”