Rapid Evaluation of VOCs Treatment by Non-thermal Plasma Using Proton Transfer Reaction Mass Spectrometry

Author:Guo Teng

Supervisor:cheng ping


Degree Year:2019





In recent years,with the advantages of simple system structure,diversified treatment components and high removal efficiency,non-thermal plasma(NTP)has become an emerging technology in the field of organic waste gas treatment,and has been highly sought after by experts and scholars.As the "fourth state" of matter,the plasma is rich in a large number of electrons,ions,radicals and neutral particles,which will cause a series of chemical reactions with VOCs molecule,such as excitation,dissociation,ionization and oxidation.The reaction channels are complex,and the products are not controlled.In addition to the ideal degradation products CO2 and H2O,there may be more toxic and harmful traces of organic by-products in the tail gas.However,conventional VOCs detection methods such as gas chromatography(GC),gas chromatography-mass spectrometry(GC-MS)or Fourier transform infrared spectroscopy(FTIR)are difficult to achieve real-time,online and highly sensitive quantitative analysis of these organic by-products.Proton Transfer Reaction Mass Spectrometry(PTR-MS)is a trace VOCs monitoring method that was developed in the late 1990s.It has a short response time(on the order of seconds),a low detection limit(<lpptv)and direct quantification without calibration,has been widely used in the detection of trace VOCs in the field of the environment.This paper reports on the structure and performance of the first PTR-TOF-MS device developed in China.And for the first time,use PTR-MS to complete the systematic detection of trace organic by-products produced during the removal of VOCs by NTP.The main contents include:1.The development and performance study of PTR-TOF-MS.First,water vapor passes through the hollow cathode discharge zone to generate reagent ions.Second,the reagentions react with the VOCs in the drift tube to force ionization of the VOCs molecule.The remaining reagent ions and ionized VOCs are then separated by a time-of-flight mass analyzer,and eventually reach the detector for ion current intensity detection.The results of mass spectrometry indicate that there are a few NO+,O2+and H3O+(H2O)n ions in the reagent ion stream besides H3O+ ions,the purity of H3O can be increased to over 99%in the reagent ion stream by optimizing the amount of water vapor and synthetic air,discharge parameters and drift tube electric field strength.At this time,the ion molecular reaction that occurs after the VOCs enter the drift tube is a single proton transfer reaction,and other ion molecular reactions are well suppressed.By measuring the known concentrations of formaldehyde,methanol,acetonitrile,formic acid,acetone,benzene,toluene and xylene,it has been confirmed that our self-developed PTR-TOF-MS has good linearity and quantitative accuracy in the range of 1 ppbv-lppmv,and the sensitivity is better than lcps/ppbv.2.PTR-TOF-MS is used for the evaluation of non-thermal plasma VOCs treatment technology.The NTP is generated by means of dielectric barrier discharge,and the discharge performance and power consumption of the NTP generator are determined by diagnosing the voltage-current waveform diagram and voltage-charge Lissajous figure,respectively.Taking toluene as the degradation target,and PTR-TOF-MS instrument as the detection method,investigate the real-time dynamic change of toluene content in the period before and after the NTP is turned on,by adjusting the initial concentration of toluene,the reaction gas flow rate,the composition of background gas and the dielectric barrier discharge power,and the key factors affecting toluene removal rate,CO2 selectivity and discharge by-product production are systematically explored.3.Determination of trace organic by-products resulting from the removal of toluene by NTP using PTR-TOF-MS.Based on the progress of mass spectra analysis in VOCs detection by PTR-MS technology in the past 20 years,we have completed the qualitative analysis of each organic by-product comprehensively according to the mass-to-charge ratio information detected and combined with some scattered GC-MS or FTIR detection results of predecessors,and the degradation mechanism of toluene was discussed.NTP technology is adopted to remove VOCs,high concentration of nitrogen oxides and ozone will be produced in the tail gas inevitably.Because nitrogen oxides and ozone are different from air(nitrogen,oxygen,etc.),they will react with H3O+,disturbing the ion molecular reaction model established by PTR-TOF-MS,thus affecting the quantification of trace organic by-products in the tail gas.Therefore,under the background of high nitrogen oxides and ozone,we have modified and tested the original quantitative formula,and the results showed that the quantitative error of VOCs was reduced from the original 35%to 5%,which was within the acceptable range.Finally,the modified quantitative formula was used to realize the quantitative analysis of trace organic by-products in the tail gas,and the risk assessment of non-thermal plasma VOCs treatment technology was completed through the inquiry of relevant health information of each organic by-products emission.In summary,this paper provides a theoretical basis and research method for inhibiting the formation of organic by-products in the degradation of VOCs by NTP,and provides some technical reserves for the large-scale industrial application of this technology.