Optical Emission Spectroscopic Diagnosis Of Carbon Dioxide Microwave Discharge Plasma At Atmospheric Pressure

Because a large amount of carbon dioxide?CO₂?has been discharged into the air without control and more forest has been destroyed,the natural carbon cycle balance has been broken with greenhouse effect becoming more and more serious.Therefore,it is a hopeful way to deal with greenhouse effect by developing a carbon capture and utilization?CCU?technology to enhance natural carbon cycle ability.At atmospheric pressure,microwave plasma discharge does not need vacuum equipment and is suitable for industrial applications with a high power-density and high coupling-efficiency.So,using microwave plasma technology to dissociate and convert CO₂ becomes the focus of research at present.In this paper,we had conducted microwave plasma discharge experiment of CO₂ at atmospheric pressure and observed the morphology of discharge.At the same time,we got the plasma temperature by means of optical emission spectroscopy diagnosis,in order to provide some discharge information for the application of microwave plasma in conversion of CO₂ in the future.By comparing the discharge patterns of atmospheric-pressure CO₂ microwave plasma with torch,we found that surface wave effect of microwave plays an important role in discharge of CO₂ microwave plasma at atmospheric pressure.At the same time,we got the optical emission spectrum from various positions and found that the spectral composition of the excitation region and the afterglow region in the CO₂ plasma is very different from each other.We chose a series of spectral lines of C₂ Swan?0,0?spectrum with rotational quantum number J in the range of 27?J?46 and processed the data with method of Boltzmann plotting.By this means,we can get rotational temperature of C₂ molecule and gas temperature was considered to be equal to rotational temperature in our experimental condition.The results showed that the rotational temperature of C₂ molecule is about 6100 K±200 K,and the temperature does not change significantly with power,gas flow rate and position.By contrasting with other reported experimental results,we analyzed the error of experimental results and found our results of temperature were reliable.Moreover,we used different gratings to repeat experiment.The results showed that the temperature has the identical variation tendency,but the temperatures obtained with the low-resolution grating are smaller than those with high-resolution grating.We analyzed the reason leading to difference of temperature with low-resolution and high-resolution grating respectively.We found that the error is mainly caused by the overlapping of spectral lines,resulting in the experimental temperature lower than the actual temperature and temperature of low-resolution grating lower than temperature of high-resolution.Finally,we mixed 2%N₂ into the microwave plasma discharge of CO₂,and fitted the CN???=0?band spectrum that can be observed in the discharge.In the case that the rotational temperature equals to vibrational temperature,the experimental data are well fitted by the theoretically synthesized spectrum.The range of calculating temperature was close to the temperature in the pure CO₂ plasma.When a few of N atoms have been introduced in the plasma of CO₂ whether there would be a significant change in state of discharge and parameters of CO₂ that still needs to be observed and studied in the subsequent experiments.