Application of a non-thermal surface plasma discharge in wet condition for gas exhaust treatment: NOx removal
Highlights
► Non-thermal surface plasma discharge in wet conditions for gas exhaust treatment is investigated. ► Several parameters are tested such as voltage-frequency, coil and catalyst. ► The removal of NOx is higher with the variation of waveform frequency rather than the applied voltage. ► Inductance enhances the NOx remediation efficiency and reduces the power consumed in the case of the voltage variation. ► γ-Al2O3 enables a synergetic effect with plasma discharge mainly on the NO2 oxidation.
Introduction
The control of air pollution represents a capital issue for the industry, and especially for the transport industry. Indeed, the increasing concern over the emission of air pollutants on the environment and human health has motivated the research to remove these harmful gases. To address this problem, several technologies to pollution control, environmentally acceptable and energy efficient, have emerged in recent years. Among these, the non-thermal plasma techniques offer an innovative approach to solve the problems of air pollution. The promising possibilities of non-thermal plasma processes (also called plasma reactors) for the elimination of pollutants and toxic molecules have already demonstrated, such as the removal of volatile organic compounds [1], [2], [3], sulphur oxides [4], [5] and also nitrogen oxides [5], [6], [7], [8].
In a non-thermal plasma, the mean energy of electrons is considerably higher than that of the other gas species. The highly energetic electrons produced by an electrical discharge, such as corona discharge or DBD, have a much higher probability to collide with neutral molecules of the surrounding gas (O2 and N2) than with the pollutant molecules because the content of pollutants is generally low. The collisions between neutral molecules and electrons produce active N and O species and OH radicals able to react with the polluted gases and to ensure their oxidations. In the case of NOx removal, the decomposition of NOx (the sum of nitric oxide (NO) and nitrogen oxide (NO2)) by the non-thermal plasma process consists of the oxidation of NO by O to NO2.
So, the NO is efficiently converted to NO2, while the NO2 can not be reduced effectively by N2. Consequently, the non-thermal plasma technology presents limitations. To overcome this limitations, several researchers have used a wet-type plasma reactor [9], [10], [11], [12], [13], [14]. The underlying process is the dissolution of NO2 into water and its conversion to nitrate (NO3−) and nitrite (NO2−) ions.
In addition, most investigations on pollution control using a non-thermal plasma reactor are based on volume discharge. Alternatively to the volume discharge, a way to treat the exhaust gases consists of using a surface discharge. Still, this type of discharge is under employed in the field of the pollution control [15], [16], while it has known an important development over the past fifteen years in the aerodynamic field [17], [18].
This study focuses on the removal of NOx using a wet-type reactor combined to a non-thermal surface plasma discharge. The first part of this paper discusses the ability of the present system (i.e. reactor using a surface discharge) to clean the gas exhaust. In a second part, we show the experimental results about the effect of a coil inserted in the electric circuit on the treatment process. In a third part, we highlight a synergestic effect between the plasma and a catalyst (here γ-Al2O3). And in a last part, a comparison between the present results and plasma reactors from the literature is realized.
The three contributions listed above are analyzed by measuring the power injected into the gas, and comparing the variation in concentrations of NO and NOx through FTIR measurements. In addition, all experiments are conducted at low and constant flow rate (1 L/min), with an initial content of 100 ppm NO.
Section snippets
Non-thermal surface plasma reactor
A schematic illustration of the non-thermal surface plasma discharge setup is shown in Fig. 1. The plasma device consists of two electrodes (electrodes #1 and #2) flush mounted on each side of a dielectric barrier, plus two counter-electrodes (electrodes #3 and #4) placed on the top side of the insulating wall. These counter-electrodes are separated relatively to the electrode #1 by an air gap of 25 mm, named the SD gap. Each electrode is made of 50-μm thick aluminium strip tape whose ends are
NOx removal test
In this section, we focus on the ability of the non-thermal surface plasma discharge to clean the gas exhaust, in particular to remove nitrogen oxides (i.e. NO, NO2 and NOx). To achieve this, the experiment was performed with a gas flow rate equal to 1 L/min and 150 mL of Na2SO3 (1 mol/L). The initial content of pollutant (NO) is set at 100 ppm. Only the input electrical parameters are modified. First, the effect of peak voltage is tested. Then, we investigated the influence of the frequency.
Conclusion
In this paper, we investigated the NOx removal with the help of a non-thermal surface plasma discharge in wet conditions, low flow rate (1 L/min) and 100 ppm of NO. The ability of the proposed system for the cleaning of gas exhaust was studied. Then, to improve the chemical conversion, a coil was inserted in the electric circuit, and a catalyst (here, γ-Al2O3) was placed in the plasma area, respectively. The main results are as follows:
- (1)
The ability of the proposed system to remove nitrogen
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