Red Mud Packed Surface Discharge Reactor for Nox/THC Removal: Exploring Plasma Catalysis of Diesel Exhaust

For the past three decades, electrical discharge-based non-thermal plasma treatment, with or without additional treatment techniques, is being used at the laboratory level for the abatement of various pollutant gases present in diesel exhaust. In the current work, a novel red mud packed surface discharge reactor has been proposed for exploring the possibility of treating the oxides of nitrogen (NO_X) and total hydrocarbons (THC) present in diesel exhaust using plasma catalysis. The inexpensive nature and abundant availability of industrial wastes like red mud makes them a feasible alternative to commercially available, expensive catalysts and adsorbents used in conventional exhaust treatment methods. In the current plasma catalysis study, a maximum NO_X removal of 96% and THC removal of 43% have been observed. NO_X removal studies using plasma-only and plasma-red mud cascade configurations were conducted, and the results were compared with those obtained using plasma catalysis to ascertain the increased contribution of plasma-activated red mud towards NO_X removal. The possible reaction mechanisms for NO_X/THC removal during plasma catalysis have been identified, and the results have been discussed at length for different engine loads emphasizing the role of plasma activation of red mud towards NO_X/THC removal.

Keywords: Plasma Catalysis; Surface discharge; Red mud; Diesel exhaust; NOX; THC; Pulsed electrical discharge; Industrial waste

The diesel engine continues to be used as a major source of power in automobiles and industries across the world. This has led to increasing concerns over the adverse effects on human health and the environment caused by the high pollutant emissions produced from diesel fuel combustion in stationary and mobile sources. The major pollutants of concern present in the diesel exhaust are the oxides of nitrogen (NOX) and a mixture of hydrocarbons, referred to as total hydrocarbons (THC). Undesirable environmental effects such as acid rain and greenhouse effect are known to be increased due to presence of NOX, while adverse atmospheric conditions such as ground level ozone formation and greenhouse effect have been attributed to THC. Industrial regulations to reduce NOX and THC emissions are becoming more rigorous across the world to curb such environmental and health hazards. Therefore, several research efforts involving usage of catalysts, adsorbents and non-thermal plasma are being pursued, to find economically feasible solutions for reducing these emissions [[1]–[4]].

In the past few decades several technologies for NOX/THC abatement are being researched to find economic treatment methods [[5]–[10]]. One such method which involves treatment of NOX/THC using non-thermal plasma generated from dielectric barrier discharge (DBD) has yielded promising results at the laboratory level [[11]–[15]]. Non-thermal plasma produces an oxidative environment containing several charged species, which include energetic electrons, excited species, ions and radicals, at atmospheric pressure and ambient temperature conditions. Diesel exhaust exposed to such a non-thermal plasma environment has been found to cause the formation of higher oxides of nitrogen and oxidized hydrocarbon intermediates, which necessitates exposing them further to adsorbents or thermal catalysts for effective removal of the harmful pollutants [[16]–[19]]. In recent years, a treatment technique which involves filling a plasma reactor with catalytic materials/pellets that enhance reactions in the presence of plasma, referred to as plasma catalysis, has given promising results at laboratory level in terms of pollutant removal efficiency on par with conventional thermal catalysis. The highly reactive environment produced by the interaction between reactive species in the plasma and the surface of the catalytic material can facilitate reactions that usually occur only at high temperatures in conventional (thermal) catalysis [[20]–[22]]. The literature available on plasma catalysis for several gas treatment applications reveals the utilization of expensive, commercially available catalytic materials. The expensive rare metals used in such catalysts and the need for replacement due to choking of the catalytic material, makes their usage an economically non-viable option. It is at this juncture that the utilization of freely available industrial wastes as potential catalysts or adsorbents becomes an economically feasible alternative. In recent research, using such solid wastes in cascade with DBD plasma-based exhaust treatment has yielded encouraging results [[23]–[27]]. Thus, such environmentally safe and inexpensive treatment techniques for NOX/THC abatement are a desirable and welcoming option for exhaust treatment.

In the present work, a solid waste from the aluminum industry i.e., red mud, has been tested as a possible catalytic material in plasma catalysis, using a surface discharge plasma reactor. Such a combination of surface discharge reactor and plasma catalysis is being attempted for the first time. Red mud has previously shown good results as a thermal catalyst and an adsorbent for NOX abatement in diesel exhaust [[28]–[30]]. The aim of the current work is to achieve similar NOX removal using pulse-energized plasma activation of red mud at room temperature. A comparative study with plasma-only and plasma-cascaded configuration was conducted to highlight the increased benefits of using plasma catalysis for NOX (= NO + NO2) removal. In addition, THC removal using plasma-activated red mud has also been explored. Studies were conducted at laboratory level at different engine loads and a discussion of the results has been provided, with an emphasis on plasma catalysis and the possible reaction pathways involved in NOX/THC removal.

The experiments in the present work were carried out at a controlled flow rate and ambient temperature conditions. The exhaust piping was carefully designed to allow most of the exhaust to the atmosphere, with only a sample to be drawn into the laboratory for the plasma treatment. In the current study main emphasis is laid on the qualitative assessment of the principle of plasma catalysis for NOX/THC removal, using red mud industrial waste and surface discharge plasma reactor. Figure 1(a) shows the schematic of the experimental setup used for studies involving plasma catalysis. Figure 1(b) shows the setup layout, for plasma-only and plasma-red mud cascade treatment of the exhaust, wherein the red mud pellets are placed downstream of the plasma reactor to check for the removal of NO2 by adsorption. This check was conducted because the critical temperature of NO2 is close to the ambient temperature, which can cause capillary condensation of NO2 in the porous structure of the red mud pellets [[31]]. A possible avenue for reusing the red mud is by desorbing the adsorbed NO2. This can be realized in an industrial setup by using a sealed heating chamber and a gas extraction/storing mechanism to prevent leakage of gases to the atmosphere. The NO2 thus stored can be better utilized in nitric acid plants and fertilizer industries. The details of the various components shown in the layouts of Fig. 1 are discussed below.

Graph: Fig. 1 Schematic of experimental setup for a plasma catalysis b plasma-only and plasma-red mud cascade treatment

In the current work, a stationary diesel generator (5-kW DG: PV-4 model, class B1, Prakash Marketing India Pvt. Ltd., India) was chosen as the source for NOX and THC. A sample of the exhaust was allowed to flow into the laboratory at a flow rate of 2 lpm using a suction pump and a flow controller. The exhaust sample was then passed through a dual stainless-steel filtering unit (5-μm/1-μm) to remove any soot and solid particulate from the gas stream. Studies were conducted at four different values of electrical loading of the diesel generator. The composition of the exhaust gas at each loading condition is shown in Table 1.

Table 1 Composition of Diesel Exhaust

Two types of reactors were used for the exhaust treatment studies conducted in the present work. For the plasma catalysis and plasma-only treatment of the exhaust, the surface discharge reactor shown in Fig. 2(a) was used, which consists of two square acrylic sheets of 21 cm length and 2 mm thickness, with a 4 mm gap between the sheets. It is worth mentioning here that the surface discharge reactor designed by the second author in previous research work [[27]] has yielded good THC and NOX removal. This was the main reason for selecting the surface discharge reactor for testing its efficacy in plasma catalysis of diesel exhaust. Further, the geometry of this reactor accommodates pellets of varying sizes (as is the case with red mud pellets), and hence provides an additional merit while filling the reactor. The inner surfaces of the sheets were provided with conducting aluminum strips which served as the high voltage electrode, whereas the aluminum sheet on the outer surface served as the ground electrode. The reactor was sealed on all sides with silicone sealant and the necessary exhaust inlet and outlet pipes were provided. During the plasma catalysis studies, this reactor was filled with red mud pellets, as shown in Fig. 2(b). For the plasma-cascaded studies, a cylindrical glass tube filled with red mud, shown in Fig. 2(c), was connected downstream of the plasma-only reactor.

Graph: Fig. 2 Reactors used a surface discharge reactor b open-view of surface discharge reactor with red mud pellets c reactor filled with red mud (to be cascaded with plasma)

High-voltage repetitive pulses of positive polarity were used to energize the surface discharge reactor in the present study. The pulses were produced by repetitive discharge of a high voltage capacitor (0.5 µF, 50 kV DC) using a rotary spark gap switch. The frequency of the pulses was kept at a constant 80 Hz (pulses per second) for the duration of the experiments, and the pulse rise time was found to be in the range of 20 ns. A transformer-rectifier unit (50 Hz, 25 kV) was used to charge the high voltage capacitor. In the current work, the output voltage was varied from 10 to 16 kV.

The solid industrial waste used in the present work, known commonly as red mud, is the bauxite residue obtained from the aluminum industry. The red mud was sourced from National Aluminum Company (NALCO), India. The major oxides present in the bauxite ore are Al2O3 (30%–60%), Fe2O3 (5%–25%), SiO2 (1%–9%), and TiO2 (1%–7%) [[30]]. After the Bayer's process extracts most of the Aluminum present in the bauxite ore, the residue (red mud) still contains some amount of alumina and other major oxides, which can serve as potential catalysts under suitable experimental conditions. Red mud has been studied in the literature as a catalyst for reduction of NO at different temperatures [[30]]. Table 2 lists the constituents of the red mud used in the present work, obtained from electron dispersive X-ray analysis (EDAX) using the FEI Quanta 250 SEG scanning electron microscope (SEM) [[25]].

Table 2 Constituents of red mud used in the present work

Figure 3 shows the red mud pellets prepared in a pellet mill (EIRICH) at Central Power Research Institute, India. The red mud was crushed and grinded into a fine powder as the first step of the pellet making process. A binding solution of Na2SiO3 and water (100 g:1 L) was mixed with the red mud and the mixture was dried in a furnace at 200 °C. The last stage involves pelletization to get hard and porous red mud pellets. The surface area and pore volume of the red mud pellets was found to be 26.21 m2/g and 0.08 cm3/g, respectively.

Graph: Fig. 3 Pellets made from red mud

The magnitude of the high voltage pulses energizing the plasma reactor and their frequency/rise-time were measured using a voltage divider (ratio 2000:1, 50 kV p-p, AC, EP-50 K, PEEC, Japan) connected to a digital storage oscilloscope (DL 1540, 200 MS/s, Yokogawa, Japan). The current pulse was measured using a current probe (Tektronix P6021, 10 mA/mV). Figure 4 shows the voltage and current waveforms (pertaining to one of the pulses) of the surface discharge reactor, with and without the red mud pellets. It can be observed from the waveforms that the voltage and current are a function of the discharge geometry only and are unaffected by the presence of the pellets. The power input to the plasma reactor involves differential power measurement, given by the difference of input powers measured at the wall-point with and without the plasma reactor. For ease of understanding the results obtained in the present work, the power input to the reactor was converted to the specific energy consumed by the plasma reactor, using the following relation: Specific Energy (J/l) = Power consumed by reactor (watt) ÷ Gas flow rate (l/sec).

Graph: Fig. 4 Waveforms of voltage and current pulse in the surface discharge reactor for a common input voltage a without pellets b with pellets

A flue gas analyzer (Testo-350, Germany) was used to measure the major pollutants present in the exhaust i.e., NO and NO2. The flue gas analyzer measures NO and NO2 concentrations and internally adds the two values providing the NOX value. The total hydrocarbons (THC) were measured by flame ionization detector (FID) based THC analyzer (Mexa 1170HFID, Horiba Ltd., Japan). The sample input to both the analyzers was taken from a gas sampling bottle as shown in Fig. 1, so as to maintain the appropriate pressure and flow rate. Both the flue gas and THC analyzer used in the study provide the value of gas concentration in ppm, and the percentage removal of the pollutants was defined as the difference of initial to final ppm with reference to the base initial ppm. It should be noted that the initial composition of the exhaust for different engine loads shown in Table 1, refers to untreated exhaust.

Experiments were planned at laboratory level with an exhaust flow rate of 2 lpm and ambient temperature conditions. The aim of the experiments conducted in the present work is to understand the possibility of NOX/THC removal by plasma-activation of red mud pellets in the presence of non-thermal plasma. The diesel engine was operated at four different loads, namely 0%, 10%, 20% and 30% of the engine power rating, and plasma catalysis studies were conducted at each of these loads. For reference, the NOX and THC variation observed in plasma-only treatment at 0% load is shown in Fig. 5.

Graph: Fig. 5 NOX and THC variation under plasma-only treatment of exhaust at 0% engine load

As seen from the graph, the NOX levels show only a slight decrease as most of the NO is converted to NO2. The NOx and THC reduction were around 18% in the plasma-only treatment. With this as the reference, further treatment studies were carried out exposing the exhaust to plasma catalysis, and with plasma-red mud cascading wherever necessary. In order to protect the sensitivity of NOX sensors present in the flue gas analyzer, moisture traps were used during the NOX treatment studies. Figure 6 shows the comparative NOX treatment results at different diesel engine loads, and Fig. 7(a) shows the bar graph comparison of THC removal when subjected to plasma catalysis at different loads. A discussion on the results shown in these graphs and the possible reaction pathways for NOX/THC removal, will be taken up in the subsequent paragraphs.

Graph: Fig. 6 NOX removal in plasma catalysis and plasma-red mud cascade studies at engine loads of a 0%, b 10%, c 20%, d 30%

Graph: Fig. 7 a Maximum THC removal in plasma catalysis studies at different engine loads b Comparison of THC removal using different treatment methods at 0% load

As shown in Fig. 7(a), a moderate THC removal was obtained through plasma catalysis at the different engine loading conditions, ranging between 43% (at 0% load) to 31% (at 30% load). Figure 7(b) compares the THC removal at 0% load for different treatment methods. The hydrocarbons present in the diesel exhaust have very low critical temperature, and hence they do not get adsorbed by red mud pellets at ambient temperature. As seen in Fig. 7(b), the same amount of THC removal was obtained in plasma-only and plasma-cascaded methods. If adsorption of THC had occurred on red mud, the plasma-cascaded approach would have yielded a higher THC removal than the plasma-only method. Since there was no change in the THC removal obtained in plasma-only and plasma-cascaded methods, this can be seen as evidence that there was no THC removal by adsorption. In comparison, the NOX removal achieved through plasma catalysis was significant, as shown in Fig. 6. The exhaust treatment by plasma-activation of red mud yielded NOX removal results ranging from 96% at 0% load to 94% at 30% load. From the comparison of NOX removal between plasma catalysis and plasma-cascaded studies provided in Fig. 6, it is clear that although the cascaded configuration was able to perform on par with plasma catalysis at 0% load, at higher loads plasma catalysis performed significantly better in terms of NOX removal. Moreover, unlike plasma-cascaded studies, plasma catalysis yielded consistently good NOX removal results at increased engine loadings, that produced exhaust with higher pollutant concentrations. Hence, it can be inferred from these results that certain mechanisms that are unique to the plasma catalysis environment may have contributed to the higher NOX removal observed during plasma activation of red mud. The various mechanisms and reaction pathways that may have contributed to the NOX/THC removal will now be discussed.

The results discussed so far provide a qualitative indication that the industrial waste red mud does exhibit some catalytic activity towards NOX/THC removal under the influence of non-thermal plasma, particularly at room temperature. The plasma activation of catalytic materials placed in a plasma environment leads to several synergistic effects. The effects observed include hot-spot formation, microdischarges in pores, reduction of metallic oxides to metallic form, change in oxidation state of the catalyst, activation of lattice oxygen, interaction of gas-phase species with adsorbed pollutants, interaction of gas phase pollutants with adsorbed species etc. Such interactions can affect the chemical activity in the plasma reactor as well, by changing the physical characteristics of the electric discharge. Moreover, the high concentrations of reactive species such as radicals, atoms and excited molecules present at room temperature in the non-thermal plasma, can facilitate in bringing about a reduced operating temperature for the catalytic processes that are usually associated with high temperature thermal catalysis [[32]–[34]]. In previous research on NOX abatement in diesel exhaust, red mud has been tested as a thermal catalyst (200–400 °C) and has yielded promising results, attributed to the presence of various metallic oxides in red mud [[23]]. This opens the possibility that such chemical activity may also be observed during plasma activation of red mud, which may even be enhanced further due to the highly reactive environment and synergistic effects produced during plasma catalysis, as mentioned earlier. Given below are the possible reaction pathways that may have contributed to NOX and THC removal during plasma catalysis with red mud pellets.

The treatment of diesel exhaust in a plasma environment brings out several oxidizing reactions owing to the presence of active species, which leads to the formation of higher oxides of nitrogen as well as many oxidized hydrocarbon intermediates. This can lead to further reduction reactions leading to the formation of N2 and CO2. Such oxidation and reduction reactions (reactions 1 – 41), which are known to occur in a plasma environment, may occur with an increased probability during plasma catalysis, when the charged species and pollutants adsorb on the surface of the catalytic material. In addition to the reactions listed below that play a role in hydrocarbon removal, a detailed discussion on the removal mechanisms of individual hydrocarbons during plasma treatment of exhaust has been discussed in a previous research work of the second author [[27]]. The reactions involving O, N radicals and molecular O3 formation when diesel exhaust is exposed to discharge plasma are as follows:

Graph

$${\text{O}}_2+\text{O}\to {\text{O}}_3$$

Graph

3 $${\text{N}}_2+\text{e}\to \text{N}+\text{N}+\text{e}$$

Graph

4 $${\text{H}}_2\text{O}+\text{e}\to \text{OH}+\text{H}+2e$$

Graph

5 $$\text{O}+{\text{H}}_2\text{O}\to 2\text{OH}$$

Graph

6 $$\text{N}+{\text{O}}_2\to \text{NO}+\text{O}$$

Graph

7 $$\text{N}+{\text{O}}_3\to \text{NO}+{\text{O}}_2$$

Graph

8 $$\text{N}+{\text{O}}_3\to \text{NO}+{\text{O}}_2$$

Graph

9 $$\text{NO}+\text{O}\to {\text{NO}}_2$$

Graph

10 $$\text{NO}+{\text{O}}_3\to {\text{NO}}_2+{\text{O}}_2$$

Graph

11 $$\text{NO}+{\text{NO}}_3\to {\text{NO}}_2+{\text{NO}}_2$$

Graph

12 $$\text{NO}+\text{N}\to {\text{N}}_2+\text{O}$$

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13 $${\text{NO}}_2+\text{O}\to \text{NO}+{\text{O}}_2$$

Graph

14 $${\text{NO}}_2+{\text{O}}_3\to {\text{NO}}_3+{\text{O}}_2$$

Graph

15 $${\text{NO}}_2+{\text{NO}}_3\to {\text{N}}_2{\text{O}}_5$$

Graph

16 $${\text{NO}}_2+\text{N}\to {\text{N}}_2\text{O}+\text{O}$$

Graph

Additionally, there exists a possibility of hydrocarbons reacting with NOX as well as active O and O3 species present in the plasma environment, as shown below:

17 $${\text{C}}_3{\text{H}}_6+\text{O}\to {\text{CH}}_2\text{CHO}+{\text{CH}}_3$$

Graph

18 $${\text{CH}}_3+{\text{O}}_2\to {\text{CH}}_3{\text{O}}_2$$

Graph

19 $${\text{CH}}_3{\text{O}}_2+\text{NO}\to {\text{NO}}_2+{\text{CH}}_3\text{O}$$

Graph

20 $${\text{CH}}_3{\text{O}}_2+\text{NO}\to {\text{HNO}}_2+{\text{CH}}_2\text{O}$$

Graph

21 $${\text{CH}}_3\text{O}+{\text{NO}}_2\to {\text{CH}}_3{\text{ONO}}_2$$

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22 $${\text{CH}}_3\text{O}+\text{NO}\to {\text{CH}}_3\text{ONO}$$

Graph

23 $${\text{C}}_2{\text{H}}_4+{\text{O}}_3\to {\text{CH}}_2\text{O}+{\text{CH}}_2\text{OO}$$

Graph

24 $${\text{C}}_3{\text{H}}_6+{\text{O}}_3\to {\text{C}}_2{\text{H}}_4\text{O}+{\text{CH}}_2\text{OO}$$

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25 $${\text{CH}}_2\text{OO}+\text{NO}\to \text{HCHO}+{\text{NO}}_2$$

Graph

26 $${\text{CH}}_2\text{O}+{\text{NO}}_2\to \text{CHO}+{\text{HNO}}_2$$

Graph

The possible reaction pathways involving atomic oxygen species and OH radicals are given below:

27 $${\text{O}}_3\to {\text{O}}_2+\text{O}$$

Graph

28 $${\text{CH}}_4+\text{O}\to {\text{CH}}_3+\text{OH}$$

Graph

29 $${\text{C}}_2{\text{H}}_4+\text{O}\to {\text{C}}_2{\text{H}}_3+\text{OH}$$

Graph

30 $${\text{C}}_3{\text{H}}_6+\text{O}\to {\text{CH}}_3+\text{H}+\text{CHCHO}$$

Graph

31 $${\text{C}}_2{\text{H}}_4+\text{OH}\to {\text{CH}}_3+\text{HCHO}$$

Graph

32 $$\text{HCHO}+\text{O}\to \text{HCO}+\text{OH}$$

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33 $${\text{CH}}_{\text{3}}+\text{OH}\to \text{HCHO}+{\text{H}}_{\text{2}}\text{O}$$

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34 $$\text{HCHO}+\text{OH}\to \text{HCO}+{\text{H}}_2\text{O}$$

Graph

35 $$\text{HCHO}+\text{OH}\to \text{HCOOH}+\text{H}$$

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36 $$\text{HCO}+\text{O}\to \text{CO}+\text{OH}$$

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37 $$\text{HCO}+\text{O}\to {\text{CO}}_2+\text{H}$$

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38 $$\text{HCO}+\text{OH}\to {\text{H}}_2\text{O}+\text{CO}$$

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39 $$\text{HCO}+{\text{HO}}_2\to \text{OH}+\text{H}+{\text{CO}}_{\text{2}}$$

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40 $$\text{HCOOH}+\text{OH}\to {\text{H}}_2\text{O}+{\text{CO}}_2+\text{H}$$

Graph

41 $$\text{HCOOH}\to {\text{H}}_2\text{O}+\text{CO}$$

Graph

In case of plasma catalysis, the unique reaction environment facilitates certain surface mediated reactions owing to the adsorption of active species and pollutants on the surface of the catalyst. Moreover, the presence of significant amounts of alumina and silica in the red mud samples used in the present work and the preparation method followed in making the pellets can lead to the formation of active reaction sites observed in aluminosilicate catalysts (zeolites). Such reactive structures have also been observed in pellets made from fly ash [[35]], which has high amounts of alumina and silica, by following a similar pellet making procedure as used in the present work. The mechanisms that may have contributed to such surface mediated catalytic activity in the plasma- activated red mud pellets can be summarized as below [[37]]:

Langmuir–Hinshelwood (L–H) mechanism:

42 $${\text{O}}_3+\ast \to {\text{O}}_2+\ast {\text{O}}_{\text{ads}}$$

Graph

43 $$\ast {\text{O}}_{\text{ads}}+\ast {\text{HC}}_{\text{ads}}\to {\text{CO}}_{\text{X}}+{\text{H}}_2\text{O}+\ast$$

Graph

$\text{Eley}-\text{Rideal}\left(\text{E}-\text{R}\right)\text{mechanism}$ :

44 $$\ast {\text{HC}}_{\text{ads}}+{\text{O}}_3\to {\text{CO}}_{\text{X}}+{\text{H}}_2\text{O}+\ast$$

Graph

where * is the active site on the catalyst and *Oads is the active oxygen species adsorbed on the catalyst and *HCads is the hydrocarbon (HC) adsorbed on the catalyst.

The plasma activation of various catalytic oxides present in red mud can also contribute towards NOX removal during plasma catalysis. Fe2O3 has been reported to play a role in reduction NO/NO2 in the reactions involving CO and metallic Fe through the following reaction pathways [[23], [30]]:

45 $$3\text{CO}+{\text{Fe}}_2{\text{O}}_3\to 3{\text{CO}}_2+2\text{Fe}$$

Graph

46 $$8\text{Fe}+12\text{NO}\to 6{\text{N}}_2+4{\text{Fe}}_2{\text{O}}_3$$

Graph

47 $$8\text{Fe}+6{\text{NO}}_2\to 3{\text{N}}_2+4{\text{Fe}}_2{\text{O}}_3$$

Graph

Similarly, Al2O3 has been shown to act as a catalyst in the reactions of NOX with methoxy radicals. The methoxy radicals form in the plasma environment when O radicals react with acetaldehyde present in the diesel exhaust. The following reaction pathways can lead to reduction of NOX to methyl nitrate and methyl nitrite in the presence of Al2O3 [[23], [27]]:

48 $${\text{CH}}_3\text{CHO}+\text{O}\to {\text{CH}}_3\text{CO}+\text{OH}$$

Graph

49 $${\text{CH}}_3\text{CO}+\text{O}\to {\text{CH}}_3\text{O}+\text{CO}$$

Graph

50 $${\text{CH}}_3\text{O}+\text{NO}\to {\text{CH}}_3{\text{NO}}_2$$

Graph

51 $${\text{CH}}_3\text{O}+{\text{NO}}_2\to {\text{CHNO}}_3$$

Graph

The reduction of NOX present in the exhaust gas can be caused due to the CO and hydrocarbons which reduce NOX into N2 and O2, while themselves oxidizing into CO2 and H2O. NOX reduction has also been attributed to the conversion of NO2 to N2 through the formation of reaction intermediates similar to those present in the oxidation of nitromethane (CH3NO2). Another metallic oxide present in red mud, TiO2, has been described to work as a photocatalyst in the plasma environment during the treatment of diesel exhaust. The photons present in the plasma can activate the TiO2 present in red mud, and it can also be activated by the adsorption of high-energy metastable N2* species present in the plasma environment [[25], [39]]. Based on these findings from previous literature, the following reaction mechanism can be proposed for NOX/THC removal [[40]–[42]]:

52 $$2\text{CO}+2\text{NO}\to 2{\text{CO}}_2+{\text{N}}_2$$

Graph

53 $${\text{NO}}_2+\text{N}\to \text{O}+{\text{N}}_2\text{O}$$

Graph

54 $${\text{N}}_2\text{O}+\text{CO}\to {\text{CO}}_2+{\text{N}}_2$$

Graph

55 $${\text{CH}}_3\text{O}+\text{NO}\to {\text{CH}}_3{\text{NO}}_2$$

Graph

56 $${\text{CH}}_3\text{O}+{\text{NO}}_2\to {\text{CH}}_3{\text{NO}}_3$$

Graph

57 $$2{\text{NO}}_2+8{\text{CH}}_3{\text{NO}}_3\to 5{\text{N}}_2+8{\text{CO}}_2+12{\text{H}}_2\text{O}$$

Graph

58 $$6{\text{NO}}_2+8{\text{CH}}_3{\text{NO}}_2\to 7{\text{N}}_2+8{\text{CO}}_2+12{\text{H}}_2\text{O}$$

Graph

In summary, the presence of hydrocarbons in the diesel exhaust plays a crucial role during plasma catalysis in the oxidation of NO to NO2 and the formation of chemical species such as aldehydes, alcohols, and nitrogen-based organic compounds (R-NOX). These chemical species in turn get reduced to partially oxidized hydrocarbons (CXHYOZ) which are more effective in reacting with NOX, producing N2 and CO2 at the surface of the catalyst. These interactions can be summarized qualitatively through the following reaction pathways [[43]]:

59 $$2\text{NO}+{\text{O}}_2\to 2{\text{NO}}_2$$

Graph

60 $${\text{NO}}_2+\text{HC}+\text{R}-{\text{NO}}_{\text{X}}\to {\text{C}}_{\text{X}}{\text{H}}_{\text{Y}}{\text{O}}_{\text{Z}}+2\text{NO}\to {\text{N}}_2+{\text{CO}}_2+{\text{H}}_2\text{O}$$

Graph

Since significant NOX removal was achieved in the current work, it is worth comparing the current results of NOX removal with other published works that have utilized plasma catalysis for NOX treatment. While doing so, it may not be always possible to maintain a common platform for comparison. Further, the current work involving plasma catalysis using a surface discharge reactor is first of its kind in the NOX studies, thus making it difficult to find works based on similar reactors in the published literature. Given this constraint, to the extent possible, results from the published literature involving cylindrical geometry (volume discharge reactors) were collected and a comparison of the same has been presented in Table 3.

Table 3 Comparison of NOX removal results obtained in current study with other published works

NOX removal efficiency in g(NOX)/kWh was calculated as shown below:

$$\eta \left(\frac{g}{\mathit{kWh}}\right)=\frac{Amountofgascomponentremoved}{EnergyDensityinkwh}$$

Graph

$$\eta \left(\frac{g}{\mathit{kWh}}\right)=\frac{\mathrm{\Delta }gasinppm\times mol.wt/24.45}{EnergyDensity\left(J/L\right)/3.6}$$

Graph

$$\eta \left(\frac{g}{\mathit{kWh}}\right)=4.5\times \frac{NOxremovedinppm}{EnergyDensity\left(J/L\right)}$$

Graph

NOX was assumed to be majorly composed of NO for the purpose of calculation of the removal efficiency. The NOX removal efficiency in g(NOX)/kWh for the present experiment is 3.7, 5.5. 8.0 and 9.5 respectively at 0%, 10%, 20% and 30% load conditions. Further, an estimation of the scale-up requirements to achieve similar NOX removal efficiency at 100% load and a higher flow rate of 30 lpm have been mentioned in the Table 4.

Table 4 Estimation of the scale-up requirements for NOX removal at 100% load

Finally, a comparison of the NOX removal performance obtained from the various plasma-based treatment methods studied in this work, at a common specific energy input, is shown in Fig. 8. It can be seen from the graph that plasma catalysis outperformed other plasma treatment methods at larger loads with higher exhaust NOX concentrations.

Graph: Fig. 8 Comparison of NOX removal percentage in different plasma-based studies discussed in the present work, at different engine loads and a common specific energy input

Laboratory scale experiments on NOX and THC abatement from diesel exhaust were conducted in a novel way with red mud packed surface discharge reactor. The principle of plasma catalysis of diesel exhaust at room temperature was verified, and significant NOX removal was achieved with plasma-activated red mud. The plasma catalysis appeared to be the dominating mechanism in NOX removal at different engine loads when compared to plasma-alone and plasma-red mud cascade treatments. The THC removal, though moderate, has been observed to play an intermediary role in NOX removal. Through this study, it can be inferred that the industrial waste bauxite residue can be an inexpensive alternative to the commercial, expensive diesel redox catalysts. Future research efforts are being planned to study the plasma catalysis approach for the treatment of diesel engine exhaust at its full flow.

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