Volatile organic compounds (VOCs) are typical pollutants that affect air quality. Discharge plasma is thought to be a potential method that can remove VOCs from flue gas. In this experiment, pulsed corona discharge plasma combined with a biological tower was carried out to remove the benzene series, and toluene was selected as the typical VOC. The results indicated that the removal efficiency of toluene by pulsed corona plasma was slightly higher than that of direct current (DC) corona plasma, while its energy efficiency was much higher than DC corona plasma. Under the optimal experimental conditions of pulse voltage 8.5 kV, initial toluene concentration 1400 mg m-3, and toluene flow rate of 12 l h-1, the toluene removal efficiency reached 77.11% by the single method of pulsed corona discharge plasma, and the energy efficiency was up to 1.515 g/(kW·h) under the pulse voltage of 4.0 kV. The trickling biofilter was constructed by using the screened and domesticated Acinetobacter baumannii, and the highest toluene removal efficiency by the pulsed corona discharge plasma combined with the trickling biofilter rose up to 97.84%. Part of the toluene was degraded into CO2, H2O, and some intermediate products such as o-diphenol under the influence of Acinetobacter baumannii. When the remaining waste gas passed through the discharge plasma reactor, the benzene ring structure could be directly destroyed by the collision between toluene and plasma. Meanwhile, O·, OH·, and some other oxidizing radicals generated by the discharge also join into the oxidative decomposition of toluene and its intermediate products, thereby further improving the removal efficiency of toluene. Therefore, the two-stage plasma-biofilter system not only showed a high toluene removal efficiency, but also had a good energy efficiency. The results of this study will provide theoretical support and technical reference for industrial VOC treatment.
As typical air pollutants, volatile organic compounds (VOCs) were found to be one of the main precursors for the formation of photochemical smog, which not only helped the synthesis of the ozone (O3), but also directly affected the contents of organic carbon in fine particular matters (PM2.5) [1, 2]. The control of VOC emissions could effectively improve the air quality in cities. Low-temperature plasma technology showed many advantages in the treatment of VOCs, such as easy access, simple equipment, easy operation, high efficiency and so on [3–5]. Such advantages made this method to be a research hotspot in the field of waste gas treatment in recent years [6, 7]. According to the characteristics of different discharge forms, these plasma technologies contained pulsed corona discharge plasma, direct current (DC) corona discharge plasma, arc discharge plasma, glow discharge plasma, and dielectric barrier discharge plasma. The key point was to find a suitable gas discharge form and a plasma generator for the special waste gas that contained various VOCs [8–10]. When compared with traditional technologies in air pollution control, plasma technology showed its great potential in the treatment of organic waste gas under low-mass concentration, and low energy consumption was another advantage of plasma technology when used in industrial applications [11–13].
Various plasma technologies were used in the field of VOC removal, and different kinds of VOCs from various industrial processes could be treated by discharge plasma methods. A coaxial cylindrical dielectric barrier discharge (DBD) reactor was used for the degradation of benzene, and its benzene degradation rate reached 50.41% when packed with porous particles in size of 75 μm pore former [14], while an individual method could hardly reach a high degradation rate to industrial application. Negative DC corona plasma enhanced by micro discharge was selected to oxidize and degrade toluene, and the degradation efficiency of toluene was limited to 54.2% due to the strong π bond in toluene molecules [15]. Glow discharge was applied to remove trichloroethylene under a DC-excited atmospheric pressure in a multi-pin-to-plate electrode configuration, and the removal fraction of 47% was obtained at an energy density of 35 J l-1 [16]. Pulsed corona discharge plasma was also a good choice to degrade VOCs, such as DDTs and methyl isobutyl ketone [17, 18].
Since it was difficult to achieve good VOC removal efficiency through only one method [19, 20], many combined technologies based on discharge plasma have been developed, among which the low-temperature plasma synergistic technology was a popular trend [21, 22]. Nonthermal plasma was used to remove VOCs with a biotrickling filter, because it could avoid the limitations of an individual biotrickling filter for treating slowly biodegradable VOC gases with high concentrations [23]. The nonthermal plasma removed ethylene more effectively than p-xylene and toluene, due to the fact that ethylene has a weak π bond. Plasma-catalytic destruction of xylene over Ag-Mn mixed oxides was carried out in a pulsed sliding discharge reactor, and the Ag-Mn/γ-Al2O3 (1:2) presented the best performance in plasma-catalysis process, with 91.5% of degradation efficiency [24]. While, DBD coupled with CeO2-MnOx catalysts was used in the removal of toluene, and the highest removal efficiency was 95.94% [25]. In a similar study, DBD coupled with MnOx/γ-Al2O3 catalysts was also used for the degradation of chlorobenzene, and the degradation rate of chlorobenzene could reach up to 96.3% [26]. The mixtures of BTEX (benzene, toluene, ethylbenzene, and xylene) treated with biological applications of microplasmas, and maximum VOCs destruction efficiencies between 90% and 100% were obtained for all four compounds [27].
There were many ways to combine low-temperature plasma to remove VOCs, some representative ones were low-temperature plasma combined with catalysis technology [28–30] and adsorption technology [31]. Most studies focused on the experimental influence factors on VOCs removal efficiencies; however, the energy efficiency and VOCs removal load were less discussed. This study combined pulsed corona discharge plasma with trickling biofilter to carry out research on synergistic decomposition and removal of VOCs, and toluene was selected as the typical benzene VOC. The characteristics of energy efficiency and toluene removal load were analyzed during the VOCs removal process.
2.
Experimental setups
2.1
Experimental device
A self-made pulsed corona discharge plasma reactor was used to study the toluene removal by the two-stage plasma-biofilter system, and the experimental device is shown in figure 1. This experimental device consisted of a toluene gas distribution system, a high-voltage discharge system, and a plasma reactor. Line-cylinder reactor was selected for this experiment. The access voltage of the transformer was 220 V, the type of high-voltage silicon stack was 2CL1A-100 kV, the withstand voltage of the discharge ball gap was 20 kV, the water resistance was about 2.3 MΩ, and the capacitance was 0.0005 μF. Special stubs were used for the ground contract of the experimental device. When a voltage was input across the discharge plasma reactor, the ball gap discharge could be realized. Meanwhile, a pulsed corona discharge was formed in the plasma reactor to generate corona plasma. The discharge form was a typical negative pulsed corona discharge. The U-tube containing toluene was placed in a thermostat water bath to evenly release toluene gas, thereby ensuring the stability of the initial toluene concentration during the experiment. The mixed gas with toluene was degraded in a discharge plasma reactor, and some of the gas was sent to a gas chromatograph for analysis after the treatment.
Figure
1.
Experimental setups for the removal of toluene by discharge plasma. (1. Air compressor; 2. Mass flow meter; 3. U-tube; 4. Headspace bottle with toluene; 5. Glass beads; 6. Air mixing chamber; 7. Thermostat water bath; 8. Trickling biofilter; 9. Line-cylinder reactor; 10. Exhaust gas absorption device; 11. Discharge ball gap; 12. Water resistance; 13. High-voltage silicon stack; 14. Oil-immersed transformer; 15. High-voltage console; 16. Grounding resistance; 17. Capacitor components; 18. Gas chromatography; 19. Computer).
A line-cylinder plasma reactor was used in this experiment, and the discharge form for generating plasma was pulsed corona discharge. The shell of the line-cylinder reactor was made of stainless steel, and the corona line was made of a 3.0 mm nickel-chromium wire. The outer diameter of the cylinder was 20.0 mm, the inner diameter was 16.0 mm, the distance between the corona wire and the inner wall of the cylinder was 6.5 mm, and both sides of the cylinder were sealed with rubber stoppers. By changing two different plasma discharge circuits, which were the pulsed corona plasma discharge circuit and the DC corona plasma discharge circuit, two different types of plasmas could be generated in the line-cylinder plasma reactor.
2.2
Reactor performance testing
The voltage-ampere characteristic test was an important test for the discharge process. The voltage-ampere characteristics of the reactors in the pulsed corona plasma and DC corona plasma experimental devices were tested respectively, and the results are shown in figure 2(a). Combined with the quantitative relationship between current and voltage, the relationship between discharge power and voltage are shown in figure 2(b). The voltage waveforms of pulsed corona discharge and DC corona discharge could be obtained by using a high-voltage probe and an oscilloscope, and results were shown in figures 2(c) and (d).
Figure
2.
(a) Relationship between current and voltage, (b) relationship between discharge power and voltage, (c) voltage waveform of pulsed corona discharge, (d) voltage waveform of DC corona discharge.
It can be seen from figure 2(a) that the current was proportional to the applied voltage when flowing through the reactors of both the pulsed corona plasma and DC corona plasma experimental devices. From the voltage-power curve of figure 2(b), the instantaneous power injected into the reactor could be calculated. Therefore, the energy efficiency of toluene removal could be analyzed and discussed. Figure 2(c) shows that the output pulse from the high-voltage power supply system was a negative pulse. The average peak voltage of the pulse discharge was -6.7 kV, the pulse width was about 20 ms, and the discharge frequency was 50 Hz. In addition, the pulse voltage waveform in the whole discharge was regular and complete, which indicated that the circuit designed in this experiment was suitable and could meet the requirements of pulse discharge plasma for toluene removal. Figure 2(d) shows that the voltage of DC corona discharge was about -16.6 kV, and the discharge waveform was complete, which also met the requirements of DC discharge plasma for toluene removal.
2.3
Method for evaluating experimental results
The two parameters of toluene removal efficiency and energy efficiency were used for evaluation in this experiment.
(1) The toluene removal efficiency could be calculated as shown in equation (1).
η=C0-C1C0×100%
(1)
In this equation, η was toluene removal efficiency (%); C0 was the initial concentration of toluene (mg m-3); C1 was the concentration of toluene in the exhaust gas (mg m-3).
(2) The energy efficiency ζ (g/(kW·h)) of toluene removal by discharge plasma reflected the energy loss of the toluene degradation device, and it could be calculated as shown in equation (2).
ζ=C0×ηED×10-3
(2)
In the equation, C0 was the intake concentration of toluene, mg m-3; η was the toluene removal efficiency, %; ED was the injection energy density of the plasma reactor, kJ l-1.
The injection energy density ED (kJ l-1) of the plasma reactor reflected the amount of energy that was injected into the reactor per unit volume, and its calculation formula is shown in equation (3).
ED=PaveQ×60
(3)
In the equation, Pave was the injection power of the plasma reactor, kW; Q was the intake flow rate of toluene exhaust gas, l min-1.
The formula for calculating the average power (Pave) is shown in equation (4).
Pave=f×∫t0V(t)×I(t)dt
(4)
whereV(t) was the instantaneous voltage, kV; I(t) was the instantaneous current, A; t was the duration of a single pulse, s; f was the pulse repetition frequency, Hz.
3.
Results and discussion
3.1
Experimental study on toluene removal by plasma
3.1.1
Effect of applied voltage on toluene removal
The effect of the applied voltage on the toluene removal efficiency was investigated under the conditions of the initial toluene concentration 1400.0 mg m-3 and the gas flow rate 48 l h-1. The applied voltage gradients were set to be 4.0, 5.5, 7.0, 8.5, 10.0, and 11.5 kV. Results of the toluene removal efficiencies are shown in figure 3(a), and the energy efficiency and energy density of toluene removal during the reaction are shown in figure 3(b).
Figure
3.
Effect of applied voltage on the toluene removal process.
Figure 3(a) shows that the toluene removal efficiencies of both discharge forms increased with the rising applied voltage, and the toluene removal efficiency of the pulsed discharge plasma was higher than that of the DC discharge plasma. When the applied voltage rose up to 11.5 kV, the removal efficiency of toluene by pulsed corona discharge plasma reached 66.63%, and DC corona discharge plasma was only 54.56%. While the applied voltage varied from 4.0 to 11.5 kV, toluene removal efficiencies of both discharge forms did not exceed 70%. It can be seen from figure 3(b) that the energy density of both the two discharge forms increased with the rising applied voltage, while the energy efficiency of the reactors for toluene removal decreased. The rising applied voltage meant that the external energy input to the plasma reactor increased, which would lead to a rising number of high-energy electrons and strong oxidizing radicals generated by the discharge. The energy density of the reactor in the case of DC corona discharge was much higher than that of pulsed corona discharge, but its energy efficiency was lower than that of pulsed corona discharge. Thus, it could not find that the higher the energy density in the reactor, the higher the energy efficiency of toluene removal. The energy efficiency of toluene removal had a great relationship with the discharge form.
3.1.2
Effect of initial toluene concentrations on its removal
The effect of initial toluene concentrations on its removal efficiency was investigated under the conditions of pulse voltage 8.5 kV and gas flow rate 48 l h-1. The initial concentration gradients of toluene were set to be 0.5, 0.8, 1.1, 1.4, 1.7, and 2.0 g m-3. The results of the changing toluene removal efficiency and energy efficiency with the initial concentrations of toluene are shown in figure 4.
Figure
4.
Relationship between toluene removal efficiency, energy efficiency and initial toluene concentration.
Figure 4(a) shows that toluene removal efficiencies under the two different discharge forms both decreased with the rising initial toluene concentrations, and the removal efficiency of toluene treated by pulsed corona discharge plasma was slightly higher than that of DC corona discharge plasma. When the initial concentration of toluene was 500.0 mg m-3, the removal efficiency of toluene by pulse discharge plasma was 75.34%, while that of the DC discharge plasma was 72.62%. With the increase of the initial toluene concentration, the toluene removal efficiency gradually decreased. The greater the initial toluene concentration, the more obviously the toluene removal efficiency decreased. There were two main reasons for the decrease in toluene removal efficiency. First, when the voltage was constant, the input discharge energy would be constant, which would also lead to a constant density of high-energy electrons generated by the discharge. In the same volume of a plasma reactor, the number of toluene molecules increased rapidly with the rising toluene concentrations, which reduced the probability of effective collisions between a single toluene molecule and high-energy electrons. Then, it would result in a decrease in the degradation efficiency of toluene by discharge plasma. On the other hand, the increase of toluene content in a fixed volume of plasma reactor would reduce the contents of O2 and other molecules in the exhaust gas, which leads to a decrease in the strong oxidizing substances such as O3, O·, and HO· produced by the discharge plasma. Thereby, it would result in a decrease in the toluene removal efficiency.
It can be seen from figure 4(b) that the energy efficiency of the reactor for toluene removal in the form of pulsed corona discharge was much higher than that of DC corona discharge, which verified the previous results. Different from the relationship between the energy efficiency and the applied voltage, the energy efficiencies of the two discharge forms showed a trend of rising first and then falling when the initial concentration constantly increased. That meant there would be an optimal value for the initial concentrations on the relationship between the energy efficiency of toluene removal and initial concentration, and the optimal value was 1.7 g m-3 for both the pulsed corona discharge plasma and DC corona discharge plasma.
3.1.3
Effect of air flow rate on toluene removal
The effect of the air flow rate on the toluene removal process was investigated under the conditions of pulse voltage 8.5 kV and initial toluene concentration 14000.0 mg m-3. The gradients of the air flow rate were set to be 12, 24, 36, 48, 60, and 72 l h-1. Results of the changing toluene removal efficiency and energy efficiency with the gas flow rate are shown in figure 5.
Figure
5.
Relationship between toluene removal efficiency, energy efficiency and air flow rate.
It can be seen from figure 5(a) that toluene removal efficiencies of the two discharge forms both showed a downward trend with the rising air flow rate. Under the same toluene flow rate, toluene removal efficiency treated by pulsed corona discharge plasma was slightly higher than that of DC corona discharge plasma. When the air flow rate was 12 l h-1, the removal efficiency of toluene by pulsed discharge plasma reached 77.11%, which was higher than that of DC discharge plasma 71.53%. When compared with the effect of the initial toluene concentration on toluene removal, the effect of air flow rate was more obvious. Since the size of the plasma reactor was fixed, the rising air flow rate shortened the residence time of the mixed gas in the reactor. Some toluene molecules even had no time to collide with high-energy electrons or react with oxidative free radicals. The part of toluene molecules escaped from the plasma reactor, which reduced the removal efficiency of toluene by the discharge plasma. Figure 5(b) shows that the energy efficiency of toluene removal in the form of pulsed corona discharge was much higher than that of DC corona discharge. The energy efficiency curves of the two discharge forms both showed a trend of first increasing and then decreasing when the gas flow rate increased, which indicated that there would be an optimal flow rate for the energy efficiency of the toluene removal. When the rate value of toluene removal decrease was bigger than the rate value of intake flow increase, the energy efficiency would increase, otherwise it would decrease. The best flow rate for both pulsed corona discharge and DC corona discharge was 60 l h-1.
3.2
Experimental study on toluene removal by trickling biofilter
Since it was difficult to achieve a good VOC removal efficiency only through a single removal method, this study tried to combine pulsed corona discharge with a trickling biofilter to remove toluene from mixed flue gas. Suitable microbial strains were obtained by screening and domesticating, and a trickling biofilter system was constructed to remove toluene. The evaluation index of toluene biological removal included not only the removal efficiency, but also the toluene removal load. The calculation formula of the toluene removal load is shown in equation (5).
EC=Q×(C0-C1)1000V
(5)
whereEC was toluene removal load (g/(m3·h)); Q was air flow rate (l h-1); C0 was the initial toluene concentration (mg m-3); C1 was the outlet concentration of toluene (mg m-3); V was the volume of the reactor (m3).
The effect of air flow rate on toluene removal by trickling biofilter towers was studied under the conditions of the initial concentration of toluene 1000 mg m-3 and the flow rate of spray circulating liquid 6.0 l h-1. The gradients of the air flow rate were set to be 40, 60, 80, 100, and 120 l h-1. The results of the changing toluene removal efficiency and removal load with the gas flow rate are shown in figure 6(a). The effect of initial toluene concentrations on toluene removal was studied under the conditions of the air flow rate of 40 l h-1 and the flow rate of spray circulating liquid 6.0 l h-1. The initial toluene concentration gradients were set to be 1000, 1200, 1500, 2000, and 25000 mg m-3. The results of the changing toluene removal efficiency and removal load with the initial toluene concentrations are shown in figure 6(b).
Figure
6.
Effect of air flow rate and initial toluene concentrations on its removal efficiency and removal load by trickling biofilter.
Figure 6(a) shows that the toluene removal efficiency decreased with the rising air flow rate. When the air flow rate of toluene-containing waste gas was 40 l h-1 and the initial concentration of toluene was 1000 mg m-3, the removal efficiency of toluene could be higher than 80%. As the air flow rate increased, the residence time of mixed gas in the tower decreased, and the contact time between the toluene and the biofilm also decreased. Part of the toluene was emitted out of the trickling biofilter tower without being degraded. The removal load of toluene increased firstly with the rising air flow, and then tended to be flat after the air flow reached 100 l h-1. At this time, the removal load was up to 73.6 g/(m3·h). Since the biomass was limited in the trickling biofilter tower, the amount of waste gas that could be treated was also limited. When the air flow rate exceeded this limit, the removal load would not increase anymore, but only remained in the limit value. Meanwhile, excessive air load and flow rate could even lead to microbial poisoning. Therefore, proper control of the air flow could maintain not only a high toluene removal efficiency, but also a high activity of the microorganisms in the biological trickling filter tower.
It can be seen from figure 6(b) that the toluene removal efficiency decreased with the increase of initial toluene concentrations. When the initial concentration of toluene was 1000.0 mg m-3, the removal efficiency of toluene was greater than 80%. Since the contact area between the biofilm and toluene was limited in the trickling biofilter, the degradation ability was limited. Then, part of the toluene in the mixed flue gas would be emitted out from the reactor without reaction. The removal load of toluene increased firstly with the increase of initial toluene concentrations, and it tended to be flat after the concentration reached 2500 mg m-3. At this time, the removal load was up to 68.7 g/(m3·h). When mixed flue gas entered a limited biological filler at a high toluene concentration, the best way to keep at a high removal toluene efficiency was to set up pretreatments, such as toluene recovering by pressure swing condensation.
16S rDNA sequencing tests were conducted on the isolated single strains to determine the strains obtained from cultivation and domestication. The results of the 16S rDNA sequence on the strain were obtained through the steps of DNA extraction, PCR amplification, PCR product detection and purification, and sequencing reaction. The 16S rDNA sequence obtained by sequencing would be performed on NCBI (National center for biotechnology information) for BLAST (Basic local alignment search tool) comparison, and a high homology of the sequence would be found. The known sequence of the strain with the highest homology would show the specific information of the strain in this experiment. The sequences were searched by BLAST respectively, and the homology comparison was performed. Results are shown in table 1.
The homology analysis results of the colony samples shown in table 1 were compared with the homology analysis results of the 16S rDNA genes from other strains. Results showed that the strains screened and domesticated in this experiment were 99% similar to the Acinetobacter baumannii strain. Therefore, it could be determined that this strain was identified as Acinetobacter baumannii. This microorganism was able to be used as a microorganism for removing toluene from mixed flue gas by the biological trickling filter tower method.
3.3
Experimental study on toluene removal by the combined system
After the experiment of respectively removing toluene by discharge plasma and trickling biofilter, the study on the combined method of a two-stage plasma-biofilter system was carried out. The effect of air flow rate on toluene removal efficiency and removal load was studied under the condition of discharge voltage 11.5 kV, initial toluene concentration 1000 mg m-3, and spray flow rate of the circulating liquid 6.0 l h-1. The gradients of the air flow rate were set to be 40, 60, 80, 100, and 120 l h-1. The results of the changing toluene removal efficiency and removal load with the air flow rate are shown in figure 7(a). The effect of initial toluene concentration on toluene removal efficiency and removal load was studied under the condition of discharge voltage 11.5 kV, air flow rate 40 l h-1 and spray flow rate of the circulating liquid 6.0 l h-1. The gradients of initial toluene concentration were set to be 1000, 1500, 2000, 2500, and 3000 mg m-3. Results of the changing toluene removal efficiency and removal load with the initial toluene concentration were shown in figure 7(b).
Figure
7.
Effect of air flow rate and initial toluene concentrations on its removal efficiency and removal load by the two-stage plasma-biofilter system.
Figure 7(a) shows that the toluene removal efficiencies by the combined system and trickling biofilter both decreased with the rising air flow rate. Under the optimal condition that the pulsed corona discharge voltage was 11.5 kV, the initial concentration of toluene was 1000 mg m-3 and the air flow rate of mixed flue gas was 40 l h-1, the removal efficiency of trickling biofilter was 82.71%, while the two-stage plasma-biofilter system could reach 97.14%. Toluene removal efficiency by single trickling biofilter decreased faster than that of the combined method. That was, when the toluene removal efficiency of single trickling biofilter decreased more, the improvement of the combined method would increase more. As long as the air flow rate reached 120 l h-1, the toluene removal efficiency of a single trickling biofilter was only 52.08%, while the combined biological-discharge plasma method was still as high as 91.26%, an increase of nearly 40%.
It can be seen from figure 7(b) that the toluene removal efficiencies by the combined method and trickling biofilter both decreased with the increase of initial toluene concentration. When toluene removal efficiency by a single trickling biofilter method decreased more, the removal efficiency by the combined biological-discharge plasma method increased more, which was consistent with the trend in figure 7(a). As long as the initial toluene concentration reached 3000 mg m-3, the removal efficiency by trickling biofilter was only 56.74%, while the combined biological-discharge plasma method could be still as high as 84.54%, which is an increase of nearly 30%. Thus, the two-stage plasma-biofilter system showed a better effect on the removal of toluene in the mixed gas.
The two-stage plasma-biofilter system for toluene removal showed their respective advantages of pulsed corona discharge plasma and trickling biofilter. The collection of nanoparticles was observed in the biofilter, which was similar to the conversion of benzene molecules in previous studies [32]. The mechanisms could be composed of two parts: one was the biochemical reaction, and the other was the plasma chemical reaction. Based on the experimental results, it could be figured out that the toluene degradation process by the two-stage plasma-biofilter system was as follows. First, the mixed gas containing toluene passed through the trickling biofilter under the action of the toluene-degrading bacteria, Acinetobacter baumannii. Part of the toluene molecules were degraded into CO2, H2O and some intermediate products like o-diphenol. Meanwhile, the number of free radicals in the mixed gas also increased. Then, the mixed gas containing toluene and intermediate products was directly transported to the pulsed corona discharge plasma reactor for secondary treatment. The discharge plasma could directly ionize and decompose toluene by collision and destroy the ring structure of benzene. Meanwhile, the discharge caused a collision of O2 and H2O to generate strong oxidizing free radicals such as O·, OH·, etc. These free radicals could oxidize toluene and its decomposition products into CO2 and H2O, which would lead to a further rising in toluene removal efficiency.
4.
Conclusion
The discharge plasma method and the combined biological-discharge plasma method were used in this study to treat the organic waste gas containing toluene. Toluene removal efficiency and energy efficiency by two different discharge forms were figured out, including pulsed corona discharge plasma and DC corona discharge plasma. The influencing factors such as applied voltage, initial toluene concentration, and air flow rate on the toluene removal were studied. The advantages of the two-stage plasma-biofilter system were discussed, and the main conclusions were as follows.
(1) Toluene removal efficiency increased with the rising applied voltage regardless of the discharge forms as pulsed corona discharge and DC corona discharge. The toluene removal efficiency by pulsed corona discharge plasma ranged from 32.38% to 77.11%, and energy efficiency ranged from 0.006 to 1.515 g/(kW·h) under the conditions of the discharge voltage range 4.0–11.5 kV, initial toluene concentration range 500–2000 mg m-3, air flow rate range 12–72 l h-1. While the toluene removal efficiency by DC corona discharge plasma ranged from 31.71% to 72.62%, and energy efficiency ranged from 0.012 to 0.150 g/(kW·h) under the same conditions. The removal efficiency by pulsed corona discharge was slightly higher than that of DC corona discharge, however the energy density injected into the reactor by DC corona was much higher than that of the pulsed corona, which indicated that the energy efficiency of toluene removal was strongly related to the discharge form.
(2) The combined biological-discharge plasma method showed a much higher toluene removal efficiency than that of a single removal method, including both pulsed discharge plasma and trickling biofilter. The highest toluene removal efficiency of 97.84% was realized under conditions of pulse voltage 11.5 kV, initial toluene concentration 1000 mg m-3, air flow rate 40 l h-1 and spray volume of circulating liquid 6.0 l h-1. Toluene molecules could be degraded into CO2, H2O, and some intermediate products, and the intermediate products were degraded by Acinetobacter baumannii in the trickling biofilter. When turned into a discharge plasma reactor, it would be directly destroyed by the collision of high-energy electrons, and O·, OH·, and some other free radicals generated by the discharge would also oxidize and remove toluene and its intermediate products.
Acknowledgments
This work was financially supported by the National Key Research and Development Program of China (No. 2019YFC0214303) and the Applied Basic Research Program of Wuhan, China (No. 2015060101010068). The authors are also grateful for the analytical support from the Analytical and Testing Center of Huazhong University of Science and Technology.
Zhu, J., Li, L., Tian, Y. et al. Mutual effects between a gliding arc discharge and a premixed flame. Plasma Science and Technology, 2024, 26(12): 125505.
DOI:10.1088/2058-6272/ad8120
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