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論文題目「Investigation on decomposition mechanism of N-containing non-biodegradable liquid waste by DC water thermal plasma at atmospheric pressure」
Duan Chengyuan
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論文題目「Investigation on decomposition mechanism of N-containing non-biodegradable liquid waste by DC water thermal plasma at atmospheric pressure」
Duan Chengyuan
Introduction
The treatment of non-biodegradable contaminants was a big challenge for conventional wastewater treatment processes in past years attracting wide attention. Many efforts have been made to remove such contaminants from wastewater. Physical techniques like separation and adsorption were developed, however, the high cost and low capacity still limit their application. Catalytic and photocatalytic oxidation were also introduced for their degradation, however, the long-term operation was still not avoidable.
Waste treatment process with thermal plasma has a fascination, as a green technology which produces valuable syngas from organic waste with no require for additional reagent. A DC water plasma torch was developed by Watanabe, for the treatment of non-degradable liquid and gas organic waste, and the production of syngas. Recently, a mist-type new torch, which introduces the solution into the electrodes directly using ultrasonic atomizing technology, was developed by Munekata et al. in 2019. Treatment of aqueous organic contaminants like phenol and acetone by water plasma has been demonstrated. However, the behavior of nitrogen during the process was still unclear.
The purpose of this study is to investigate the decomposition mechanism of nitrogen-containing non-biodegradable liquid contaminants in the water plasma process.
Experiment
The main part of the process consisted of a non-transferred nozzle-type plasma torch, equipped with a reaction tube. A DC power supply was used to generate plasma. The torch consists of a hafnium cathode, which was embedded into a copper rod, and a copper anode.
Waste solution was fed into the system by a pump with a stable feed rate, and then be introduced into the electrode region as mist generated by ultrasonic atomizing. The mist feed rate can be controlled by changing the power supply of the mist generator.
Pyridine and N, N-Dimethylformamide (DMF), two typical non-biodegradable contaminants widely found in industrial production were introduced for the investigation. The concentration of DMF solution used was 0.1~5.0mol%, while that of pyridine was 0.1~1.0mol%.
A gas chromatography equipped with a thermal conductivity detector (GC-TCD, Shimadzu GC-14B) was used for quantitative analysis of gas, while the qualitative analysis was carried out by a quadrupole mass spectrometer (QMS, Ametek Dycor Proline). The flow rate of gas effluent was analyzed by a soap flowmeter.
A high-performance liquid chromatography (HPLC, Jassco) equipped with a UV detector (UV-975) was used for quantitative analysis, for the determination of decomposition rate and the formation of by-products in the liquid effluent. The qualitative analysis of liquid effluent was performed with LC-MS (LCMS-IT-TOF, Shimadzu), using the same solvent as that in HPLC. PH value and the concentration of CN- ion in liquid effluent were measured by a benchtop water quality meter (HORIBA LAQUA F-72), with a standard pH electrode (9615S-10D), and a cyanide ion electrode set (8001-10C and 2060A-10T), respectively. Concentrations of nitrogen with three forms in liquid effluent were determined to investigate the reaction pathway of nitrogen in water plasma. Ammonia nitrogen (NH3-N) and nitrite nitrogen (NO2-N) were determined by UV-vis with Nessler's reagent and Griess reagent respectively, nitrate nitrogen (NO3-N) was measured using HPLC.
The decomposition rate of DMF was calculated from the difference of concentration measured by HPLC. For the experiment with pyridine, given that the soot formation during the experiment is negligible, and the main gas products containing carbon were CO2, CO, and CH4, the real concentration of the fed solution and decomposition rate was estimated based on the carbon balance.
The optical emission spectroscopy (OES) of nozzle exit of water plasma torch was carried out using an imaging spectrometer (Ihr-550, HORIBA Jobin Yvon) with a charge-coupled device detector (CCD, Sygnature from HORIBA Jobin Yvon). The distance between lens and nozzle exit was set to be 0.40 m, and the observation point was calculated about 0.1 mm from the nozzle exit. The temperature of nozzle exit was calculated by the means of Boltzmann plot method, from the emission intensity of Balmer-series H-I atoms (Hα at 656.3 nm and Hβ at 486.1 nm).
Decomposition of DMF
The average decomposition rates of more than 94% was achieved in all conditions, and decrease slowly with the increase of concentration. The energy efficiency, which evaluates the power input per gram of DMF, increases with the concentration non-linearly and reached about 42 g/kWh at the condition of 5mol%.
The feed rates decrease because of the increase of viscosity of the solution, which led to a lower mist generation rate from the ultrasonic disc. The gas generation rate was almost 0 in the condition of pure water, however, more gas generated when increasing the organic compounds, because the combination of carbon with the active radicals like O will reduce the recombination of O and H radicals. The liquid generation rate was mainly affected by the feed rate and gas generation rate. gas effluent produced from the process mainly consists of H2, CO, and CO2, with percentages of about 60%, 28%, and 9% respectively.
The arc current have a strong effect on the temperature, a maximum of about 7,400 K was achieved at the condition of 5mol% DMF decomposed with the current of 9.5A.
Decomposition of pyridine
Mean decomposition rate and TOC removal rate of more than 94% were achieved in all conditions. The difference between decomposition rate and TOC removal rate becomes greater with increasing concentration, indicates that due to insufficient decomposition, more by-products were found in the liquid effluent. The organic component in the exhaust liquid mainly consists of undecomposed pyridine and byproducts with little molecular weight, such as HCOOH and CH3COOH. Energy efficiency increases rapidly with increased concentration and achieved a maximum of 23 g/kWh during the decomposition of 2.2mol% pyridine
With the increase of pyridine concentration, the vapor/mist ratio also increases, leading to a higher fed concentration. The trend of flow rates was the same as that observed in the treatment of DMF.
A high temperature was observed during the decomposition of 2.2mol% pyridine, which decreases with further increases of concentration. The decrease in temperature refers to the higher electrical conductivity or the decreased arc length in the torch. Current showed a significant effect in the temperature, which reached a maximum of 7,300 K in the condition of 9.5A.
Optical emission spectra
The peaks of C (247.85 nm), OH (strongest heads at 306.76 and 309.04 nm), NH (336.0 and 337.0 nm), CN (358.39~359.04 and 385.10~388.34 nm), CH (431.42 and 432.30 nm), and H (434.05, and 486.14 nm) were observed during the process.
Higher NH/Hβ and CN/Hβ ratios can be observed during the experiments with higher concentrations, indicating they are important intermedia in the formation of nitrogen-containing byproducts.
Discussion
The plasma torch can be divided into two parts by its temperature and density, based on the result of temperature calculation and our previous observation of the torch, named high-temperature region and downstream region.
In the high temperature region, with a mean temperature of about 6,000 K, the main reactions there were considered as thermal decomposition and radical reactions. Contaminants fed into this region were soon decomposed into radicals like C, CHx, N, and fragments from insufficient decomposition. Due to the existence of a large amount of H, OH radicals, intermedia like CN and NH was produced by radical reactions, and then further converted into CO, N2, NO, producing H2 simultaneously.
In downstream region, the temperature decreases drastically to a low level of about 1,000 K, recombination of radicals and the formation of by-products was considered mainly in this region. NH radical in the downstream region will be promoted to convert into ammonia nitrogen, due to the existence of a large amount of H and OH radicals. NO was considered the main precursor of nitrite and nitrate nitrogen in the liquid effluent. Further oxidation of CO occurs in both high-temperature and downstream region, which was considered the main source of CO2, while a small fraction of CO can be oxidized into liquid-phase byproducts such as HCOOH.
Conclusion
The application of water plasma in the treatment of high-concentration N-containing non-biodegradable wastewater was demonstrated as an alternative technology for industrial wastewater treatment. DMF and pyridine solution with max concentrations of 1.75 x 105 and 9 x 104 mg/L was treated successfully, the decomposition mechanism and the formation of by-products in different regions were discussed based on results.
The treatment of non-biodegradable contaminants was a big challenge for conventional wastewater treatment processes in past years attracting wide attention. Many efforts have been made to remove such contaminants from wastewater. Physical techniques like separation and adsorption were developed, however, the high cost and low capacity still limit their application. Catalytic and photocatalytic oxidation were also introduced for their degradation, however, the long-term operation was still not avoidable.
Waste treatment process with thermal plasma has a fascination, as a green technology which produces valuable syngas from organic waste with no require for additional reagent. A DC water plasma torch was developed by Watanabe, for the treatment of non-degradable liquid and gas organic waste, and the production of syngas. Recently, a mist-type new torch, which introduces the solution into the electrodes directly using ultrasonic atomizing technology, was developed by Munekata et al. in 2019. Treatment of aqueous organic contaminants like phenol and acetone by water plasma has been demonstrated. However, the behavior of nitrogen during the process was still unclear.
The purpose of this study is to investigate the decomposition mechanism of nitrogen-containing non-biodegradable liquid contaminants in the water plasma process.
Experiment
The main part of the process consisted of a non-transferred nozzle-type plasma torch, equipped with a reaction tube. A DC power supply was used to generate plasma. The torch consists of a hafnium cathode, which was embedded into a copper rod, and a copper anode.
Waste solution was fed into the system by a pump with a stable feed rate, and then be introduced into the electrode region as mist generated by ultrasonic atomizing. The mist feed rate can be controlled by changing the power supply of the mist generator.
Pyridine and N, N-Dimethylformamide (DMF), two typical non-biodegradable contaminants widely found in industrial production were introduced for the investigation. The concentration of DMF solution used was 0.1~5.0mol%, while that of pyridine was 0.1~1.0mol%.
A gas chromatography equipped with a thermal conductivity detector (GC-TCD, Shimadzu GC-14B) was used for quantitative analysis of gas, while the qualitative analysis was carried out by a quadrupole mass spectrometer (QMS, Ametek Dycor Proline). The flow rate of gas effluent was analyzed by a soap flowmeter.
A high-performance liquid chromatography (HPLC, Jassco) equipped with a UV detector (UV-975) was used for quantitative analysis, for the determination of decomposition rate and the formation of by-products in the liquid effluent. The qualitative analysis of liquid effluent was performed with LC-MS (LCMS-IT-TOF, Shimadzu), using the same solvent as that in HPLC. PH value and the concentration of CN- ion in liquid effluent were measured by a benchtop water quality meter (HORIBA LAQUA F-72), with a standard pH electrode (9615S-10D), and a cyanide ion electrode set (8001-10C and 2060A-10T), respectively. Concentrations of nitrogen with three forms in liquid effluent were determined to investigate the reaction pathway of nitrogen in water plasma. Ammonia nitrogen (NH3-N) and nitrite nitrogen (NO2-N) were determined by UV-vis with Nessler's reagent and Griess reagent respectively, nitrate nitrogen (NO3-N) was measured using HPLC.
The decomposition rate of DMF was calculated from the difference of concentration measured by HPLC. For the experiment with pyridine, given that the soot formation during the experiment is negligible, and the main gas products containing carbon were CO2, CO, and CH4, the real concentration of the fed solution and decomposition rate was estimated based on the carbon balance.
The optical emission spectroscopy (OES) of nozzle exit of water plasma torch was carried out using an imaging spectrometer (Ihr-550, HORIBA Jobin Yvon) with a charge-coupled device detector (CCD, Sygnature from HORIBA Jobin Yvon). The distance between lens and nozzle exit was set to be 0.40 m, and the observation point was calculated about 0.1 mm from the nozzle exit. The temperature of nozzle exit was calculated by the means of Boltzmann plot method, from the emission intensity of Balmer-series H-I atoms (Hα at 656.3 nm and Hβ at 486.1 nm).
Decomposition of DMF
The average decomposition rates of more than 94% was achieved in all conditions, and decrease slowly with the increase of concentration. The energy efficiency, which evaluates the power input per gram of DMF, increases with the concentration non-linearly and reached about 42 g/kWh at the condition of 5mol%.
The feed rates decrease because of the increase of viscosity of the solution, which led to a lower mist generation rate from the ultrasonic disc. The gas generation rate was almost 0 in the condition of pure water, however, more gas generated when increasing the organic compounds, because the combination of carbon with the active radicals like O will reduce the recombination of O and H radicals. The liquid generation rate was mainly affected by the feed rate and gas generation rate. gas effluent produced from the process mainly consists of H2, CO, and CO2, with percentages of about 60%, 28%, and 9% respectively.
The arc current have a strong effect on the temperature, a maximum of about 7,400 K was achieved at the condition of 5mol% DMF decomposed with the current of 9.5A.
Decomposition of pyridine
Mean decomposition rate and TOC removal rate of more than 94% were achieved in all conditions. The difference between decomposition rate and TOC removal rate becomes greater with increasing concentration, indicates that due to insufficient decomposition, more by-products were found in the liquid effluent. The organic component in the exhaust liquid mainly consists of undecomposed pyridine and byproducts with little molecular weight, such as HCOOH and CH3COOH. Energy efficiency increases rapidly with increased concentration and achieved a maximum of 23 g/kWh during the decomposition of 2.2mol% pyridine
With the increase of pyridine concentration, the vapor/mist ratio also increases, leading to a higher fed concentration. The trend of flow rates was the same as that observed in the treatment of DMF.
A high temperature was observed during the decomposition of 2.2mol% pyridine, which decreases with further increases of concentration. The decrease in temperature refers to the higher electrical conductivity or the decreased arc length in the torch. Current showed a significant effect in the temperature, which reached a maximum of 7,300 K in the condition of 9.5A.
Optical emission spectra
The peaks of C (247.85 nm), OH (strongest heads at 306.76 and 309.04 nm), NH (336.0 and 337.0 nm), CN (358.39~359.04 and 385.10~388.34 nm), CH (431.42 and 432.30 nm), and H (434.05, and 486.14 nm) were observed during the process.
Higher NH/Hβ and CN/Hβ ratios can be observed during the experiments with higher concentrations, indicating they are important intermedia in the formation of nitrogen-containing byproducts.
Discussion
The plasma torch can be divided into two parts by its temperature and density, based on the result of temperature calculation and our previous observation of the torch, named high-temperature region and downstream region.
In the high temperature region, with a mean temperature of about 6,000 K, the main reactions there were considered as thermal decomposition and radical reactions. Contaminants fed into this region were soon decomposed into radicals like C, CHx, N, and fragments from insufficient decomposition. Due to the existence of a large amount of H, OH radicals, intermedia like CN and NH was produced by radical reactions, and then further converted into CO, N2, NO, producing H2 simultaneously.
In downstream region, the temperature decreases drastically to a low level of about 1,000 K, recombination of radicals and the formation of by-products was considered mainly in this region. NH radical in the downstream region will be promoted to convert into ammonia nitrogen, due to the existence of a large amount of H and OH radicals. NO was considered the main precursor of nitrite and nitrate nitrogen in the liquid effluent. Further oxidation of CO occurs in both high-temperature and downstream region, which was considered the main source of CO2, while a small fraction of CO can be oxidized into liquid-phase byproducts such as HCOOH.
Conclusion
The application of water plasma in the treatment of high-concentration N-containing non-biodegradable wastewater was demonstrated as an alternative technology for industrial wastewater treatment. DMF and pyridine solution with max concentrations of 1.75 x 105 and 9 x 104 mg/L was treated successfully, the decomposition mechanism and the formation of by-products in different regions were discussed based on results.
研究論文
- Chengyuan Duan, Manabu Tanaka, and Takayuki Watanabe: N,N-dimethylformamide Decomposition by DC Water Plasma at Atmospheric Pressure, Journal of Chemical Engineering of Japan, 54 (9), p.486-492 (2021.9).
- Chengyuan Duan, Manabu Tanaka, and Takayuki Watanabe: Treatment of Pyridine in Industrial Liquid Waste by Atmospheric DC Arc Plasma, Journal of Hazardous Materials, 430 (15) 128381 (2022.5).
- (招待講演) Takayuki Watanabe, Soon-Ho Kim, Chengyuan Duan, Hiiro Murakami, Manabu Tanaka, and Myeong-Hoon Lee: Decomposition of Organic Waste by DC Water Thermal Plasmas, 5th Asia-Pacific Conference on Plasma Physics, A-I8 (2021.9.29 On-Line).
- Chengyuan Duan, Manabu Tanaka, and Takayuki Watanabe: Decomposition of N, N-dimethylformamide by DC Water Thermal Plasma at Atmospheric Pressure, 2nd Asian Conference on Thermal Sciences, ACTS-1061 (2021.10.6 On-Line).
- (招待講演) Takayuki Watanabe, Soon-Ho Kim, Chengyuan Duan, Manabu Tanaka, and Myeong-Hoon Lee: Characterization and Diagnostics of DC Water Thermal Plasmas for Innovative Processing, 15th Asia Pacific Physics Conference (2022.8.24 On-Line).
- Chengyuan Duan, Manabu Tanaka, and Takayuki Watanabe: Treatment of Sulfur-Containing Wastewater using DC Water Plasma, 22nd International Vacuum Congress, Mon-H2-5 (2022.9.12 Sapporo Convention Center).
- Duan Chengyuan, Manabu Tanaka, and Takayuki Watanabe: Decomposition Mechanism of Sulfur- and Nitrogen-Containing Liquid Waste in DC Water Plasma, Proceedings of 25th International Symposium on Plasma Chemistry, 1P-302 (2023.5.22 Miyako Messe, Kyoto).
- Chengyuan Duan, Manabu Tanaka, and Takayuki Watanabe: Decomposition of Organophosphate Flame Retardants by Atmospheric DC Water Plasma, The 13th Asian-European International Conference on Plasma Surface Engineering, p.133, We3D-3 (2023.11.8 Busan, Korea).
