Research on degradation mechanism of trichlorobenzene and Hg0 by nonthermal plasma catalysis

Wenjing BIAN(邊文璟) ,Tao ZHU(竹濤) ,Fan ZHU(朱繁) ,Yan CUI(崔巖) ,Jianhua WANG (王建華) ,Guang SHI (史光) ,Jiawang LI (李加旺) and Jingzhang HAO (郝景章)

1 Institute of Environmental Protection and HVAC Engineering Technology,MCC Capital Engineering &Research Incorporation Limited (CERI),Beijing 100176,People’s Republic of China

2 Institute of Atmospheric Environmental Management and Pollution Control,China University of Mining&Technology,Beijing 100083,People’s Republic of China

Abstract Aiming at mercury and dioxin in fire coal gas as research objects,nonthermal plasma (NTP)catalytic technology was used to investigate the degradation effect of operating condition parameters on mixed pollutants in mixed flue gas condition,and to explore the synergistic degradation of Hg0 and TCB (1,2,3-trichlorobenzene,TCB) under mixed flue gas conditions.The research results showed that the conversion efficiency of mercury and TCB increased with the additional output of voltage,and decreased with the increase of the gas flow rate.Under optimal reaction conditions: voltage=17 kV,frequency=300 Hz,gas flow rate=2 l min-1,the conversion efficiency of Hg0 and TCB reached the highest 91.4% and 84.98%,respectively.In the NTP catalytic system,active free radicals played an important role in the synergistic conversion of mercury and TCB,which have a competitive effect,to make the conversion efficiency of mixed pollutants lower than a single substance.In the mixed flue gas condition,the mixed gas has an inhibitory effect on the synergistic conversion of mercury and TCB.Kinetic modeling of NTP catalytic synergistic reaction was established.Under three conditions of TCB,mercury and TCB,mixed simulated flue gas,the NTP catalytic technology showed a quasi-firstorder kinetic reaction for the degradation of TCB.According to the synergistic effect of NTP and composites,the transformation and degradation of TCB mainly included two processes: TCB and ring opening,and Hg0 was finally oxidized to Hg2+.

Keywords: fire coal,Hg0 and TCB,non-thermal plasma catalytic technology,radical,kinetic modeling

1.Introduction

Coal contains large amounts of heavy metals and in coal-fired boilers,heavy metals and chemicals are released during the combustion process[1,2].Relevant data show that the input of mercury in coking coal accounts for 35%-46% of the national total.At the same time,some polychlorodibenzo-p-dioxins(PCDDs)are synthesized by chlorine and benzene series in the boiler and are emitted with the flue gas [3].Current technologies such as carbon-based material adsorption technology,thermal catalytic oxidation technology,photocatalytic oxidation,wet scrubbing and nonthermal plasma (NTP) enable the effective removal of multiple pollutants[4-7].Compared with the above technologies,NTP is considered to be one of the most effective methods to remove multiple pollutants simultaneously.In the previous study by the research group,NTP had a stable removal efficiency for Hg0and trichlorobenzene(TCB) in the mixed gas of SOx,NOxand HCl [8].Currently,Mn-based catalytic materials are widely used in the environmental area,mainly due to their structural stability,high redox capacity,relatively inexpensive cost,commonly used for NH3-SCR (selective catalytic reduction,SCR),catalytic oxidation of VOCs (volatile organic compounds,VOCs),Hg0adsorption oxidation,etc [9-12].Related studies have been carried out to improve the catalytic performance by modifying the chemical properties and crystalline structure of Mn oxides,as Mn4+has higher catalytic oxidation properties than Mn2+and Mn3+[13].The rare Earth element Ce combined with Mn to form a composite oxide enhances the oxygen storage properties of the catalyst[14,15].Under the conditions of SOx,NOxand HCl mixed gas.The spinel-structured Mn-based catalysts synergistically degrading chlorobenzene with NTP exhibit strong conversion rates and stability to Cl and resistance to sulfur.In summary,in this work,Co1Mn2Ce3Ox/13X catalysts were prepared by co-precipitation method,and Hg0and TCB were used as study pollutants to explore the effect of nonthermal plasma catalytic coupling technology on the degradation of mixed gases under mixed gas atmosphere,and to reveal the multi-pollution degradation mechanism and pathway under the action of metal double cycle.

2.Materials and methods

2.1.Experimental materials

2.1.1.Catalyst preparation.The zeolite was prepared by hydrothermal synthesis and the catalysts were prepared by coprecipitation method[16].A certain mass of Co(NO3)2·6H2O,Cu(NO3)2·3H2O,Ce(NO3)3·6H2O,and Mn(CHCOO)2were weighed separately and dissolved in a certain amount of anhydrous ethanol.A certain amount of zeolite13X was added to the mixture,and 0.5 mol l-1Na2CO3was slowly added dropwise,controlling the conditions pH=10,and age at 75°C for 24 h.Deionized water and anhydrous ethanol were repeatedly rinsed until neutral and then filtered.It was dried at 120°C in an oven for 12 h.The sample was then heated in a muffle furnace at a rate of 5°C min-1to 150°C to remove residual water from the sample,heated to 500°C at a rate of 5°C min-1,and continued for 4 h.

2.1.2.Adsorption materials.For Hg2+adsorption,an oxidation catalyst material was used,placed at the back end of the discharge area,and ground to a particle size of D=2 mm sieved and dried [17].

2.2.Experimental equipment

Figure 1.Diagram of NTP reactor structure.

2.2.1.NTP reactor device.The DBD discharge was used in the experimental process: the reactor was composed of a quartz glass tube,acting as a medium,with an inner diameter Φ=22 mm and a thickness of D=2 mm,and the high voltage electrode used a high purity tungsten wire with a diameter Φ=0.8 mm,as shown in figure 1.The driving power supply of the NTP reactor used a variable frequency high voltage AC power supply developed by Wuhan Sanxin Huatai.The power supply power system was composed of two parts: the variable voltage controller and the power supply.The voltage range was adjustable in the range of 0-100 kV,and the frequency was adjustable in the range of 0-1000 Hz.

2.2.2.Experimental systems.The experiments on the degradation of Hg0and TCB by the NTP reactor system in this work were completed on the experimental system,as shown in figure 2.The experimental system had four main components,namely (a) the mixed pollutant generation system,(b) the NTP power supply system,(c) the NTP catalytic system,(d) the exhaust gas detection system.Also,to prevent liquefaction of Hg0and TCB in the gas path,the experimental gas path was insulated throughout by an insulation device.The in-plasma catalysis (IPC) coupling method was used,i.e.the catalyst was placed directly in the discharge zone of the reactor and then fixed by quartz wool.The exhaust gas was fed into the laboratory exhaust gas ventilation ducting system and was connected to the GC,GCMS,flue gas analyzer,ozone analyzer,RA-915+and VM3000 mercury analyzer using a three-way valve for exhaust gas detection.The Hg0concentration was achieved by temperature (T) change.The main method was to control the carrier air to carry the mercury-containing vapor out into the gas pipeline system through the U-shaped tube(containing mercury permeation tube) heated by the oil bath,and to keep the gas circuit heated throughout to avoid liquefaction of the Hg0.

2.3.Evaluation indicators

TCB and Hg0conversion efficiency η was used as the main parameter indicators in this paper,while Specific Input Energy (SIE)was the electrical parameter indicator,and NOxconcentration and O3concentration were the auxiliary parameter indicators.The main calculations were communicated as follows

Figure 2. Diagram of the experimental systems.

SIE is an important indicator parameter for the NTP energy consumption for the degradation pollutants,i.e.the energy consumed by the NTP per unit gas flow rate,which is mathematically calculated as follows:

where SIE,J l-1;Pwis the NTP discharge power,W;f is the discharge frequency,Hz;A is the integrated area of the Lissajous oscilloscope graph;Q is the gas flow rate,l min-1.

3.Results and discussions

3.1.Experimental study on the effect of operating conditions on the conversion of Hg0 and TCB

3.1.1.Effect of discharge voltage on the conversion efficiency of Hg0and TCB.The initial concentrations of the pollutants were Ctcb=10 mg m-3for TCB and CHg=325 μg m-3for Hg0.The control initial conditions were gas flow rate Q=2 l min-1,frequency 300 Hz,and gas mixture with the concentration of 240 ppm NO,140 ppm SO2,and 100 ppm HCl.When the discharge voltage exceeded 18 kV,the Hg0degradation efficiency was unstable.As a result,the discharge voltage was adjusted to range from 11 to 17 kV.The change of TCB and Hg0conversion with voltage was shown in figure 3.

Figure 3.Diagram of the effect of discharge voltage on the degradation of the mixture.

Under the condition of the reactor filled with catalyst,the discharge mode changes from filamentary discharge to surface discharge.Non-uniform electric field is generated near the contact point between the high dielectric constant particles and the walls of the granular medium,which corresponds to enhance the electric field strength,leading to an accumulation of charge and polarization effects,and more energy is stored in one cycle of the sine wave.It can be seen in figure 3,the conversion efficiency of the mixture increases as the voltage increases.This is because as the voltage increases,more active material is input into the reactor during the discharge cycle at a constant discharge power,increasing the chance of collision with the pollutants and thus enhancing the conversion efficiency of the pollutants.Shortlived active species are generated within the porous pore channels of zeolite13X in the surface discharge mode,and they use the catalyst surface as a reaction site to create new active reactants,thereby facilitating the NTP-catalyzed reaction [18].The highest mixed pollutant conversion efficiency is achieved when the discharge voltage reaches 17 kV,with 91.4%conversion of Hg0and 84.98%conversion of TCB.The conversion efficiency is greatly improved compared with the NTP discharge mode.

Figure 4. Diagram of the effect of frequency on the degradation of the mixture.

3.1.2.Effect of discharge frequency on the conversion efficiency of Hg0and TCB.In order to investigate the effect of discharge frequency on the conversion efficiency of mixed pollutants,the initial concentrations of the pollutants were Ctcb=10 mg m-3for TCB and CHg=325 μg m-3for Hg0.The control initial conditions were gas flow rate Q=2 l min-1,discharge voltage 17 kV,and gas mixture with the concentration of 240 ppm NO,140 ppm SO2,and 100 ppm HCl.The frequency was adjusted to range from 50 to 500 Hz.The variation of TCB and Hg0conversions with frequency is shown in figure 4.

As seen in figure 4,as the discharge frequency increases,the overall conversion efficiency of the mixed pollutants increases and then decreases.The highest conversion efficiency for mixed pollutants is achieved when the discharge frequency is in the range of f=250-350 Hz.The built-in catalyst can significantly increase the efficacy of mixed pollutants.The movement amplitude of the activated factors in the electric field is inversely proportional to the discharge frequency.Increasing the discharge frequency means increasing the number of reactor discharges per unit time,generating more energetic ions and activated factors and thus increasing the conversion efficiency of the mixed pollutants.However,the impedance in equivalent capacitors dramatically increases significantly reverting to an insulated state,when the frequency exceeds the frequency of the corresponding series resonant circuit,at which point most of the power input energy is converted to reactor heat,resulting in a reduction in conversion efficiency [19].The resonant frequency f0is between 300 and 350 Hz,and the corresponding η(TCB),η(Hg0) can reach 85.0%,91.2%,respectively.

Figure 5. Effect of gas flow on the degradation of the mixture.

3.1.3.Effect of gas flow rate on the conversion efficiency of Hg0and TCB.In order to investigate the effect of gas flow rate on the conversion efficiency of mixed pollutants,the initial concentrations of the pollutants were Ctcb=10 mg m-3for TCB and CHg0=325 μg m-3for Hg0,and gas mixture with the concentrations of 240 ppm NO,140 ppm SO2,and 100 ppm HCl.The control initial conditions were discharge voltage 17 kV,and the discharge frequency 300 Hz.Gas Hourly Space Velocity was controlled to set the gas flow rate Q=2-8 l min-1.The variation of TCB and Hg0conversion with gas flow rate is shown in figure 5.

As the gas flow rate increases,the TCB and Hg0conversions decrease.When the gas flow rate increases from 2 to 8 l min-1,the TCB conversion decreases to 65.33% and 21.45% and the Hg0conversion decreases to 63.12% and 17.65%(q=2-8 l min-1corresponded to a residence time t0=3.18-0.909 s).When the discharge energy is kept constant,increasing the flow rate will result in a lower SIE value,i.e.the energy gained per unit volume of reactive gas decreases as the residence time of the gas in the reaction zone decreases,thus inhibiting the oxidation of Hg0.However,relevant research shows that excessive residence time will not improve the removal efficiency of pollutants,but lead to oversized reactor volume.Thus,a certain removal efficiency can be guaranteed by increasing the input energy[20].In the case of the NTP mode,under the action of the surface discharge,the activated substances produced by the channels within the aperture of the carrier are transient in nature,and will quickly dissipate when leaving the discharge point area,failing to react adequately with the pollutants as the residence time shortens,thus leading to a reduction in the conversion efficiency.

3.2.Study on coupling control mechanism of Hg0 and TCB

Figure 6. Effect of plasma-catalysis systems on the degradation of single substances.(a) Hg0,(b) TCB.

Table 1. Reaction rate coefficients for NO,Hg0 and various reacting species [23].

The initial conditions of the experimental simulation were Ctcb=10 mg m-3,CHg0=325 μg m-3,mixed gas: NO,SO2,HCl concentrations of 240 ppm,140 ppm,100 ppm respectively,discharge SIE=535.5 J l-1,gas flow rate Q=2 l min-1,to investigate the coupling control mechanism of the Hg0+TCB.The experimental results are shown in figures 6(a) and (b).As can be seen from the figure,within 50 min of discharge,the IPC maintains a relatively high level of conversion for Hg0and TCB alone.After 50 min,the NO,SO2,HCl gas mixture branch starts to open and the conversion of Hg0and TCB starts to decrease instantly.For the NTP discharge mode,the reduction in conversion for Hg0and TCB alone is more pronounced.Under this condition,the conversion efficiency can be seen that the mixed gas of NO,SO2and HCl plays an inhibiting role in the conversion and degradation of Hg0and TCB.After 100 min,a mixture of pollutants is introduced,at which point the conversion efficiency of both decreases again and remains at a relatively constant level until 240 min.

Under the mixed gas atmosphere of NO,SO2and HCl,the contribution of different gases to the conversion of Hg0to Hg2+is different.Previous studies by the research group showed that NO and SO2had an inhibitory effect on Hg0oxidation under the plasma discharge system [8].Related studies have shown that NO is an inhibitor of Hg0oxidation under homogeneous catalytic conditions.Sliger et al [21]investigated the oxidation efficiency of different concentrations of NO on Hg0under 170 ppm HCl gas atmosphere conditions.At lower concentrations of NO,NO promoted the oxidation of Hg0,but it took 20 ppm NO to achieve maximum oxidation.In response to the inhibitory effect of SO2on Hg0oxidation,relevant studies have also proved that different SO2concentration conditions have different effects on Hg0oxidation,with low concentrations instead promoting the oxidation of Hg0[22].The reason is that low concentrations of SO2are oxidized in a non-homogeneous catalytic system in combination with reactive oxygen species toand SO3.Thecluster nuclei,formed by the combination of SO2with reactive oxygen groups,which have good oxidation properties,can direct the oxidation of Hg0by guiding the chain oxidation process,thus promoting the oxidation reaction of Hg0.At the same time,SO3also has strong oxidation characteristics and can combine with Hg0to form stable compound HgSO4[22].HCl plays an important role in the oxidation of Hg0to HgCl2,and Galbreath et al [23] proved that HCl did not react with Hg0at 453 K.However,under the action of NTP,HCl was decomposed into Cl and Cl2species,which then promoted the oxidation of Hg0by the following reactions:

Figure 7. (a) Effect of SIE on TCB conversion efficiency,(b) kinetic fit of TCB degradation.

However,the concentrations of mixed SO2and NO flue gases in this study were higher than the promoting Hg0oxidation concentration.In NTP non-homogeneous catalytic system,the synergistic interaction between Co-Mn-Ce-O results in a catalyst with large oxygen storage capacity and high mobility of reactive oxygen,which promotes the oxidation of NO and SO2.The reaction rates of NO,Hg0with free radicals are shown in table 1.The GC-MS examination of the by-products shows that the dechlorinated benzene ring appears to contain oxygen functional group substituents,which proves that OH,O andplay a dominant role in the oxidation of TCB.As a result,the presence of a mixture of NO,SO2and HCl competes for the action of the oxidation radicals,resulting a reduction in TCB conversion.

3.3.Kinetic analysis of TCB conversion

The variation of TCB conversion efficiency with SIE for the three reaction conditions is shown in figure 7(a).As can be seen from the figure,the SIE increases and the conversion efficiency of TCB increases.The reason is that the SIE increases,the more energy input into the reactor increases,and more activated substances are produced,which increases the collision probability with TCB.The reason is that the higher the SIE,the more energy is fed into the reactor,producing more activated material and increasing the chances of collision with the TCB.Although the gas mixture inhibits the conversion of TCB,it is not directly involved in the TCB degradation reaction.Ideally the final products are HCl,CO2and H2O and the degradation equation can be achieved by deforming equation (8) as follows:

where a is the total reaction apparent constant.

Based on the above NTP catalysis model,the SIE in the IPC reaction shows a linear relationship with -ln(1-η),indicating that the IPC conversion of TCB is a quasi-primary reaction.The slope of the fitted straight line is the total reaction constant for the conversion under this system.As can be seen from the figure 7(b),the total reaction rate constant for the degradation of TCB in the three reaction systems k02=0.003 08 l J-1＞k03=0.001 77 l J-1＞k01=0.001 39 l J-1,proving that IPC has the fastest reaction rate for the degradation of TCB,and the presence of the gas mixture with Hg0leads to a slower rate of degradation of TCB,resulting in a lower total reaction rate constant.The value of the fit coefficient R2is an important parameter in evaluating whether this degradation process conforms to the kinetic model,and＞0.99 is fully consistent with the quasi-first order kinetic model.The presence of a more complex reaction system in the mixed system results in a lower correlation of the fit,withandboth greater than 0.9,indicating that the mixed gas conversion degradation is still consistent with the quasi-primary kinetic model.

Figure 8. By-product generation.

3.4.The control of by-product generation

Under the condition of SIE=535.4 J l-1,by-products of TCB+Hg0conversion by IPC are generated as shown in figure 8.

When the gas mixture is injected,the O3concentration shows a certain degree of decrease compared without gas mixture.From the reaction equations (9)-(11),it can be concluded that O3is converted into O· at the oxygen active sites on the catalyst surface,to participate in the NO and SO2oxidation reactions,thus promoting the decomposition of O3.The mixture leads to an overall increase in NO2concentration,part of which is generated by the N2ionization by the plasma,the rest part of which is generated by the NO oxidation

Figure 9.GC-MS of TCB degradation by-products.

3.5.Analysis on degradation pathways of Hg0 and TCB by IPC

The results of the by-products by GC-MS are shown in figure 9.As can be seen,only three complex benzene ring substituents are present,and the variety of primary degradation by-products is significantly reduced after ring opening,proving that the primary by-products can be further converted by the synergistic effects of the catalyst.

In the TCB molecule,with the effect of the large π-bond and conjugation,the C-C bond energy is 5.4 eV higher than the C-Cl bond energy at 3.5 eV and the C-H bond energy at 4.3 eV.Therefore,the C-Cl bond is firstly dissociated by the effect of activated substances and free radicals produced by the IPC.Relevant studies have shown that the detection of oxygen-containing functional groups on the benzene ring proves that OH radicals play a dominant role in the dechlorination process during the degradation of chlorobenzene compounds.Secondly,the C-C and C-H bonds on the benzene ring are subjected to ring-opening reactions in the presence of OH radicals and reactive oxygen species.At the same time,the N-substituents on the benzene ring is observed in the by-products,proving that the oxidized NO-andproduced during the discharge can still be engaged in the ring opening reaction.Under the synergistic effect of the catalyst,O2and O3are continuously converted into reactive oxygen species on the catalyst surface through the conversion of Co-Mn-Ce.The reactive oxygen radicals continuously attack the carbon chain,forming a decarburization reaction.In the GCMS spectrum,only two kinds of chlorine-hydrocarbon compounds are detected,which proves that organic chlorine is converted into inorganic chlorine after dechlorination of benzene ring,which is consistent with previous research about the TCB conversion pathway.The primary conversion substances are further degraded by the synergistic effect of NTP and the catalyst to produce organic and inorganic final products with fewer carbon chains.The degradation pathway of TCB is shown in figure 10.

Halogen substances are the most important substances for Hg0oxidation,while the type of oxygen in the flue gas also plays an important role for Hg0oxidation.The reason is that activated oxygen radicals,as the strong oxidizing substances,are necessary for the conversion of HCl to Cl radicals [26].Relevant study and material balance calculations have proved that HgOH can be generated through the interaction of Hg0and oxygen-containing radicals [27],but HgOH can only exist at 298°C for 280 μs to readily decompose to HgO at room temperature.The reaction rate of NO and SO2with oxygen-containing radicals is higher than the oxidation rate of Hg0,while NO and SO2occupy for active sites on the catalyst surface.Therefore,excessive concentrations will inhibit the oxidation of Hg0.At the same time,the O3generated by the plasma discharge and the O2in the gas are converted into reactive oxygen radicals to promote the reaction under the double synergistic effect of Co-Mn-Ce-O in the catalyst[28].The conversion of Hg0in IPC reaction system is shown in the following processes

4.Conclusions

Figure 10.Analysis of degradation pathway of TCB by IPC.

In this work,NTP catalytic coupling technique was used for the degradation of Hg0and TCB under NO,SO2and HCl mixed gas condition.The effects of different operating conditions on the conversion of the mixture were investigated to reveal the synergistic degradation mechanism.The main conclusions are as follows:

(1) To investigate the effect of operating conditions on the conversion of Hg0and TCB by the IPC system.The operating working conditions for NTP coupled Co1Mn2Ce3Ox/13X catalyst for the gas mixture are:discharge voltage=17 kV,frequency=300 Hz,gas flow rate=2 l min-1,which has the highest conversion efficiency of Hg0and TCB,reaching at 91.4% and 84.98%.

(2) OH radicals and reactive oxygen radicals play an important role in the conversion of mixtures gas.The mixture gas has an inhibitory effect on the degradation of Hg0and TCB.The effect of different gas mixtures on Hg0varies,with HCl promoting the conversion of Hg0and higher concentrations of SO2and NO inhibiting the conversion.There is competition between Hg0and TCB for free radicals generated by the IPC,resulting in a lower degradation efficiency of the mixture than the single substance.

(3) Kinetic equations were established in the IPC discharge mode to simulate the kinetics of TCB degradation under the three reaction conditions.The results show that the SIE is linearly related to-ln(1-η)and that the TCB degradation by the IPC presents a quasi-first order kinetic response,where the total reaction rate constant k(single TCB) ＞k(Hg0and TCB) ＞k(mixed gas,Hg0and TCB).

(4) The degradation process of TCB mainly includes main processes,dechlorination and ring opening.Only three complex benzene ring substituents are present,and the ring opening products have oxygen-containing functional groups appearing on the benzene ring,proving that oxygen-containing radicals play a dominant role in the dechlorination process.Hg0is eventually oxidized to Hg2+under a mixed gas atmosphere.HCl promotes the oxidation of Hg0to Hg2+,while NO and SO2compete with Hg0for reactive oxygen radicals and active sites on the catalyst,thereby inhibiting the oxidation of Hg0.

Acknowledgments

This work is supported by National Natural Science Foundation of China (No.52270114).