NO Removal by Plasma-Enhanced NH₃-SCR Using Methane as an Assistant Reduction Agent at Low Temperature

Abstract

The effects of using CH₄ as an assistant reduction agent in plasma-assisted NH₃–SCR were investigated. The new hybrid reaction system performed better than DBD–NH₃–SCR when the O₂ concentration varied from 2% to 12%. Compared with DBD–NH₃–SCR, DBD–NH₃–CH₄–SCR (NH₃:CH₄ = 1:1) showed a more significant promotion effect on the performance and N₂ selectivity for NO_X abatement. When the O₂ concentration was 6% and the SIE was 512 J/L, the NO removal efficiency of the new hybrid system reached 84.5%. The outlet gas components were observed via FTIR to reveal the decomposition process and its mechanism. This work indicated that CH₄, as an assistant agent, enhances DBD–NH₃–SCR in excess oxygen to achieve a new process with significantly higher activity at a low temperature (≤348 K) for NO_X removal.

1. Introduction

Nitrogen oxide (NO_X) emissions from fossil fuel combustion are one of the primary air pollutants, inducing various environmental problems, such as secondary aerosols and tropospheric ozone [1,2,3,4]. Hence, the abatement of NO_X is one of the most extensively studied fields in the history of environmental science. Selective catalytic reduction (SCR) is regarded as a practical method for the removal of NOx, which is highly efficient and environmentally friendly compared to other denitration methods. However, promising catalysts for NO_X reduction, such as Ag/Al₂O₃ [5,6,7,8,9,10,11,12,13] and Ce/Fe–ZSM-5 [14,15,16,17,18], have sufficient activity when the temperature exceeds 673 K. The V₂O₅–WO₃/TiO₂ [19,20,21,22,23,24,25,26,27] catalyst can achieve superior activity only when the temperature is between 573 K and 673 K. A high reaction temperature leads to expensive energy consumption and other problems [28,29]. Therefore, low temperature SCR has attracted much attention [30,31,32]. Many studies have demonstrated that low temperature (423–673 K) SCR could be achieved by changing the composition and support of the catalyst [33,34,35,36]. However, these modifications still have limitations, and finding a solution that can improve the low temperature activity of the SCR catalyst is necessary. Dielectric barrier discharge (DBD), one of the most promising normal temperature plasma (NTP) generation technologies, has also been proven to improve the activity of diverse SCR catalysts at low temperatures [37,38,39,40,41]. The technology of SCR assisted by DBD has received much attention in the past decade because it offers the advantages of being highly efficient and eco-friendly with a distinguished performance on the decomposition of NOx [42,43,44,45,46].

SCR assisted by DBD provides synthesis effects to reach significantly higher activity than SCR when the temperature is lower than 673 K. However, there are some drawbacks to using NH₃ as a reducing agent in the DBD–SCR process. On the one hand, the negative effect of ammonia escape is unavoidable as a result of excess NH₃ in SCR. On the other hand, the excited oxygen atom activated by DBD in an excess of O₂ can react with NH₃ in the feed gas to produce secondary products and then weaken the reduction of NO_X in DBD–SCR [47,48,49,50]. Some studies have shown that the use of hydrocarbons as reducing agents in DBD–SCR could be of particular interest [51,52], not only for avoiding the production of ABS (ammonium bisulfate), but also for controlling the by-product of NO abatement. However, some limitations [53] are unavoidable in HC–SCR assisted by a DBD system, such as the production of coking.

Based on the superiority of hydrocarbons in DBD–SCR, using CH₄ as an assistant reducing agent in DBD–NH₃–SCR probably overcomes the negative effect of NH₃, which conduces the good performance of NO abatement for DBD–NH₃–SCR with exceeded O₂. Nevertheless, there are few reports concerning the effects of and mechanisms involved in plasma enhanced NH₃–SCR using methane as the assistant reducing agent in such a hybrid reaction system.

In this work, a new hybrid reaction system using CH₄ as an assistant reducing agent in DBD–NH₃–SCR has been studied in detail. The reaction products of NO abatement in DBD–SCR with different reducing agents were detected by Fourier transform infrared spectroscopy (FTIR). The selectivity of NO abatement for two hybrid systems has been researched and the involved chemical mechanism was also revealed.

2. Experimental Section

2.1. Materials

A V₂O₅–WO₃/TiO₂ catalyst, with a vanadium pentoxide content of 2% and a tungsten trioxide content of 9%, which had been prepared by a sol-gel method, was purchased from XinRui Co. (Hunan, CN) and was used without further purification.

2.2. Experimental Setup and Procedure

This experiment includes two type reactors of DBD–NH₃–SCR (DBD enhanced SCR with NH₃ as the reduction agent) and DBD–NH₃–CH₄–SCR (DBD enhanced SCR with NH₃ as the reduction agent and CH₄ as the assistant reduction agent). The feeding gas composition was 400 ppm of NO, 0–12% O₂, 0–90 ppm NH₃, 0–90 ppm CH₄, and N₂ as the balance gas to simulate the actual working conditions. The total flow rate and space velocity of the gas mixture were 15 L∙min⁻¹ and 66,000 h⁻¹, respectively. The mixed gases passed through a buffer chamber and then were led into the DBD reactor. The flow rate of each feeding gas was controlled independently by a mass flow controller (Dandong, Horiba Stec-4400, JPN). The reactor consisted of an inner high-voltage electrode (graphite), two quartz tubes (outer tube with 30 mm inner diameter and 200 mm length; inner tube with 6 mm outer diameter and 300 mm length), and an outer electrode (aluminum foil, thickness 0.2 mm). The quartz tubes were in the shape of coaxial cylinders with 12 mm gap. The length of the discharge area was 10 mm and the volume of the discharge area was 27 cm³. The temperature of the discharge area was measured by infrared thermometer (Omega, OS423, GA, USA). The concentration of NO₃ was measured by ion chromatography (Thermo Fisher, DX-120, MA, USA).

The V₂O₅-WO₃/TiO₂ catalyst was performed in a fixed-bed (outer diameter, 30 mm), operating at atmospheric pressure. The fixed-bed was loaded onto a quartz wool at the center of the DBD reactor.

2.3. Analytical Measurements

The DBD power supply could provide a sinusoidal alternating voltage varying from 1 kV~4 kV at frequencies of 10 kHz~20 kHz. The voltage and power applied was measured via a 200 MHz digital phosphor oscilloscope (Tektronix, TDS2024B, Shanghai, China) connected to a 1000:1 HV probe (Tektronix, P6015A, OR, USA).

The concentration of all the gaseous components was continuously quantified using a Fourier transform infrared absorption spectrometer (FTIR 850, Gangdong Co., Tianjin, China, 0.5 cm⁻¹). The NO removal efficiency (η_NO) and N₂ selectivity (NO to N₂) were defined as follows:

$${\mathsf{\eta }}_{\mathrm{NO}}=\frac{\left({\mathrm{NO}}_{\mathrm{inlet}}-{\mathrm{NO}}_{\mathrm{outlet}}\right)}{{\mathrm{NO}}_{\mathrm{inlet}}}\times 100\%;$$

$${\mathrm{N}}_2\mathrm{selectivity}\%=\frac{{\mathrm{NO}}_{\mathrm{inlet}}+{\mathrm{NH}}_{3\mathrm{inlet}}-{\mathrm{NO}}_{\mathrm{outlet}}-2\times {\mathrm{N}}_2{\mathrm{O}}_{\mathrm{outlet}}-{\mathrm{NO}}_{2\mathrm{outlet}}-{\mathrm{NO}}_{3\mathrm{outlet}}-{\mathrm{NH}}_{3\mathrm{outlet}}}{{\mathrm{NO}}_{\mathrm{inlet}}+{\mathrm{NH}}_{3\mathrm{inlet}}}\times 100\%,$$

where NO_outlet, N₂O_outlet, NO_2outlet, and NO_3outlet are the concentrations (ppm) of NO, N₂O, NO₂, and NO₃ at the outlet of the reactor, respectively.

3. Results and Discussion

3.1. Performances of the DBD–NH₃–CH₄–SCR and DBD–NH₃–SCR Hybrid System

The effect of SIE (specific input energy) on the NO removal efficiency for the DBD–NH₃–CH₄–SCR and DBD–NH₃–SCR hybrid system is shown in Figure 1. A maximum NO conversion of approximately 70.5% was achieved at 512 J/L for the DBD–NH₃–SCR hybrid system, with 0% O₂ in the feed. The NO conversions ranged from 0% to 38.1% for the DBD–NH₃–CH₄–SCR hybrid systems.

In the process of DBD–SCR, the temperature of the plasma reaction zone was determined by the SIE. In two cases, with the rise of SIE, the reaction temperature increased from 293 to 348 K. At 348 K, the experiment results indicated that no obvious NO conversion was found over the V₂O₅–WO₃ catalyst, only by thermal activation without DBD. Therefore, the heat effect of DBD on NO conversion can be neglected in this work.

The enhancement in SCR activation by plasma processes is generally attributed to the direct interaction of the radicals, electrons, and UV photons created by the plasma, with the catalyst and molecules adsorbed on its surface. With the condition of O₂ absence, the reactions of NO + NO Ā→ N₂ + O₂ and NO + NH₂ → N₂ + H₂O cannot be ignored. Because NH₃ is more likely to be converted to NH₂ by DBD, as shown in Figure 2, the increasing SIE promoted NO conversion, and the performance of the DBD–NH₃–SCR hybrid system was better with O₂ free in the feed.

The overall conversion of NO in the presence of O₂ was presented in Figure 3. For the DBD–NH₃–SCR process, the NO conversion was reduced from 70.5% to 43.3%, with an increase in O₂ concentrations from 0% to 12%. Following the presence of methane in the reaction system, the results became different. For DBD–NH₃–CH₄–SCR, the NO conversion increased rapidly from 38.1% to 77% as the O₂ concentration reached 2%, and the NO conversion reached 85.2% when the O₂ concentration was 6%. In conclusion, the NO removal in the DBD–NH₃–SCR reactor was better than that in the DBD–NH₃–CH₄–SCR system with O₂ free; however, the opposite result was observed when the O₂ concentration was higher than 2%. This result indicates that O₂ has a negative effect on NO abatement in DBD–NH₃–SCR.

N₂O and NO₂ are known to be critical in the formation of N₂ in SCR reactions, and thus, the production of N₂O and NO₂ in the DBD–NH₃–SCR and DBD–NH₃–CH₄–SCR processes is also examined. The effect of SIE on N₂O and NO₂ concentration in the reaction system is shown in Figure 4a,b. For the DBD–NH₃–SCR process, the N₂O and NO₂ concentration is obviously higher than that in the DBD–NH₃–CH₄–SCR processes under the condition of 12% O₂. Therefore, these results indicate that DBD–NH₃–CH₄–SCR had a lower production of N₂O and NO₂ and achieved better selectivity than DBD–NH₃–SCR in the condition of exceeded O₂.

3.2. Effect of CH₄ and NH₃ on the Product Selectivity of the DBD–SCR Hybrid System

To explore the mechanisms involved, the products observed by FTIR in DBD–SCR are shown in Figure 5. Figure 5a shows the effect of SIE on NO abatement with 12% O₂ in the DBD–NH₃–SCR process. The NO concentration decreased gradually, however, the concentrations of N₂O and NO₂ increased obviously with increasing SIE. In addition, as shown in Figure 5b, the peak intensities of NO, NO₂, and N₂O, with peaks at 1900 cm^–1, 1600 cm^–1, and 2200 cm^–1, respectively, increased with increasing O₂ concentration.

Products detected in the DBD–NH₃–SCR process with the assistant agent CH₄ at different SIE and O₂ concentrations are shown in Figure 5c,d. These results show that there was less N₂O and NO₂ in the process compared with the results shown in Figure 5a,b under excess O₂. This result indicates that DBD–NH₃–SCR with CH₄ as an assistant reduction agent achieved a preferable performance and product selectivity for NO_X abatement.

To further verify and explore the above experimental results, the final products for the two DBD-catalyst systems with only NH₃/O₂ or N₂/O₂ in the feed are shown in Figure 6a,b, respectively. According to Figure 6a, N₂O (16.76 ppm) and NO₂ (10.97 ppm) were observed in the final products when the supplied gases only included NH₃, O₂, and N₂ balance gas. According to a previous research report [47], O₂ not only competitively shares the input power but also contributes to the oxidation of NO. NO, N₂, and NH₃ were dissociated by electron impact dissociation reactions in the discharge area, namely, $\mathrm{e}+\mathrm{NO}\to \mathrm{e}+{\mathrm{NO}}^{*}$, e+N₂→e+2N and e+NH₃→e+$·$NH₂+$·$H. Excited N, ${\mathrm{NO}}^{*}$, NH₃, and $·$NH₂ would recombine with O and O₂ to produce new NO_X. However, according to Figure 6b, there was only O₃ (98.16 ppm) in the reaction system of N₂/O₂. This indicates that the electrons generated from the DBD–SCR reactors could not decompose N₂ in this reaction system, similar conclusions are studied in another paper [54].

Because NH₃ could recombine with excited O to produce new NO_X, it is an inevitable fact that O₂ has a negative effect on NO removal in DBD–NH₃–SCR, which is consistent with the results shown in Figure 3. On the contrary, the recombination of NH₃ and O₂ could be controlled in DBD–NH₃–CH₄–SCR. During plasma discharge, hydrocarbon could be decomposed and further generate useful intermediates, such as methyl ($·{\mathrm{CH}}_3$) and methyldioxy (${\mathrm{CH}}_3{\mathrm{O}}_2$), which could react with NO molecules [55,56]. The enhancement in hydrocarbon activation by plasma processes is attributed to those active particles, and the entire activation reaction is given by:

$${\mathrm{CH}}_4+2\mathrm{NO}+{\mathrm{O}}_2\to {\mathrm{N}}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2.$$

For this reason, CH₄ enhanced the removal efficiency of NO for DBD–SCR with excess O₂ and DBD–NH₃–CH₄–SCR achieves the best selectivity for NO abatement.

Table 1. The N₂ selectivity and concentration of feed gas as well as products at outlet.

	Feed Gas Concentration of Inlet (ppm)		Product Concentration of Outlet (ppm)						N2 Selectivity (%)
	NO	CH4	NH3	NO	NO2	N2O	NO3	NH3
DBD–NH3–SCR	400	0	90	220	32.5	19.8	37.2	7.7	31.2
DBD–NH3–CH4–SCR	400	45	45	80	15.4	4.71	25.5	3.8	69.9

Experiment condition: 12% O₂, SIE 512 J/L, the NO₃ was collected from outlet.

shows the concentration of feed gas and product at outlet for DBD–NH₃–SCR and DBD–NH₃–CH₄-systems. According to this table, there were fewer N₂O, NO₂, and NO₃ in the outlet production of DBD–NH₃–CH₄–SCR. Furthermore, because the synthesis effects of DBD–SCR and assistant reductant agent, the N₂ selectivity of DBD–NH₃–CH₄–SCR reached 69.9%, while the N₂ selectivity of DBD–NH₃–SCR was only 31.2%.

3.3. Effect of Different Ratios of CH₄ as An Assistant Agent on Final Products

The FTIR spectra for the reducing agent with different ratios of NH₃ and CH₄ under 2% O₂ are shown in Figure 7. They show that with the increase in the CH₄ proportion, N₂O decreased rapidly from 13.97 ppm to 0.80 ppm and there were few N₂O (0.80 ppm) products when the ratio of NH₃ and CH₄ was 1:1. This result indicates that the production of new NO_X could be controlled in DBD–NH₃–CH₄–SCR (NH₃:CH₄ = 1:1) with the favorable product selectivity.

4. Reaction Mechanism

For NH₃–SCR, the reaction of NH₃ with NO occupied the main position in the reaction system. NH₃ could convert NO to N₂ by the reaction, $4{\mathrm{NH}}_3+4\mathrm{NO}+{\mathrm{O}}_2\to 4{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$ [57]. However, the NO removal efficiency was unsatisfactory in DBD–NH₃–SCR with excess O₂, because high-energy electrons generated from DBD could excite NH₃, NO, and O₂ in the feed gas and produced high-energy particles, namely,$\mathrm{e}+\mathrm{NO}\to \mathrm{e}+{\mathrm{NO}}^{*}$, $\mathrm{e}+{\mathrm{O}}_2\to \mathrm{e}+2\mathrm{O}$, $\mathrm{e}+{\mathrm{NH}}_3\to \mathrm{e}+·{\mathrm{NH}}_2+·\mathrm{H}$. Furthermore, excited ${\mathrm{NO}}^{*}$, NH₃, and $·{\mathrm{NH}}_2$ recombined with O₂ as well as these high-energy particles on the catalyst surface to generate new NO_X through the reactions [54,57,58,59]:

$${\mathrm{NO}}^{*}+\mathrm{O}\to {\mathrm{NO}}_2;$$

$${\mathrm{NO}}_2+\mathrm{O}\to {\mathrm{NO}}_3;$$

$$2{\mathrm{NH}}_3+2{\mathrm{O}}_2\to {\mathrm{N}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O};$$

$$4{\mathrm{NH}}_3+4\mathrm{NO}+3{\mathrm{O}}_2\to 4{\mathrm{N}}_2\mathrm{O}+6{\mathrm{H}}_2\mathrm{O};$$

$$4{\mathrm{NH}}_3+5{\mathrm{O}}_2\to 4\mathrm{NO}+6{\mathrm{H}}_2\mathrm{O};$$

$$2·{\mathrm{NH}}_2+{\mathrm{O}}_2\to 2\mathrm{NO}+2{\mathrm{H}}_2;$$

$$·{\mathrm{NH}}_2+2\mathrm{O}\to \mathrm{NO}+{\mathrm{H}}_2\mathrm{O}.$$

In DBD–NH₃–CH₄–SCR, except for the reaction of$4{\mathrm{NH}}_3+4\mathrm{NO}+{\mathrm{O}}_2\to 4{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$, CH_4, as the assistant reduction agent, could react with NO and O₂ to convert NO to N₂, namely, ${\mathrm{CH}}_4+2\mathrm{NO}+{\mathrm{O}}_2\to {\mathrm{N}}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$. Meanwhile, CH₄ was converted to $·{\mathrm{CH}}_3$, namely, $\mathrm{e}+{\mathrm{CH}}_4\to \mathrm{e}+{\mathrm{CH}}_3+·\mathrm{H}$ [60], which further produced ${\mathrm{CH}}_3{\mathrm{O}}_2·$ as well as ${\mathrm{NO}}_2$ and CO₂ [61]. Furthermore, NO₂, generated from NO oxidation by O and CH₃O₂ could react with NO as well as NH₃ to form N₂ and H₂O through the reaction: $\mathrm{NO}+{\mathrm{NO}}_2+2{\mathrm{NH}}_3\to 2{\mathrm{N}}_2+3{\mathrm{H}}_2\mathrm{O}$. This is the fast-NH₃–SCR reaction, which has been verified to achieve effective reduction of NO_X to N₂ [62,63,64]. In addition, NO₂ could be oxidized to NO₃ in the DBD-system with excess O₂ as the reaction of ${\mathrm{NO}}_2+\mathrm{O}\to {\mathrm{NO}}_3$. Another byproduct of N₂O could react with $·{\mathrm{CH}}_3$, $\mathrm{O}$ and NO, such as the following reactions [65,66,67]:

$$·{\mathrm{CH}}_3+{\mathrm{N}}_2\mathrm{O}\to {\mathrm{CH}}_3\mathrm{O}·+{\mathrm{N}}_2;$$

$$\mathrm{O}+{\mathrm{N}}_2\mathrm{O}\to {\mathrm{O}}_2+{\mathrm{N}}_2;$$

$$\mathrm{O}+{\mathrm{N}}_2\mathrm{O}\to 2\mathrm{NO};$$

$$\mathrm{NO}+{\mathrm{N}}_2\mathrm{O}\to {\mathrm{NO}}_2+{\mathrm{N}}_2.$$

Taking all above-mentioned facts into account, the presence of NH₃/CH₄ in the feed gases significantly enhances the reduction of NO_X to N₂ and the product selectivity of NO abatement in DBD–NH₃–CH₄–SCR.

A schematic diagram of the main chemical reaction mechanism for DBD–SCR using CH₄ as the assistant agent is shown in Figure 8.

5. Conclusions

NH₃–SCR assisted by DBD can enhance the NO_X conversion when CH₄ is used as an assistant reducing agent at low temperatures (below 348 K) with O₂ concentration exceeding 2%. The new hybrid reaction system overcomes the negative effect of the NH₃–SCR process, with a higher removal efficiency of NO and N₂ selectivity. The results of FTIR spectra observed in the new hybrid systems indicate that the DBD–NH₃–CH₄–SCR (NH₃:CH₄ = 1:1) had better synthesis effects and achieved a preferable performance as well as product selectivity for NO_X abatement. In addition, the fast-NH₃–SCR reaction was verified to achieve the important contribution for the reduction of NO_X to N₂ in the DBD–NH₃–CH₄–SCR system. Using CH₄ as an assistant reduction agent in plasma-assisted NH₃–SCR may provide a new idea for the NOx removal because the new process can effectively control secondary products and achieve a feasible low-temperature NO abatement technology with excess O₂.