Fundamental Research Of Dioxins Like Organic Pollutants Degradation By Gliding Arc Plasma

In recent years, municipal solid waste incineration were rapidly developed and its emissions of dioxin, chlorobenzenes, PCBs and some chlorinated organic pollutants from incineration process gradually increased. The emission of chlorinated organic pollutants was contrary to the national environmental protection requirement. A novel gliding arc plasma method was used to destroy dioxin like organic pollutants in flue gas and fly ash based on a comprehensive understanding of the characteristics of the gliding arc plasma. Several parameters of destroying pollutants by gliding arc plasma were investigated. By the analyzing the active components of plasma and destruction products, the destruction model and mechanism were obtained, including the following conclusions:(1) For the novel rotation gliding arc plasma, the effects of input voltage, gas content and gas flow on its discharge shape, arc motion characteristics, current-voltage characteristic, the electron density and temperature, and the gas rotational and vibrational temperature were studied. a. AC plasma was applied in pollutants treatment because it could operate for a long time and owned a lower energy consumption. b. Lower input voltage (10.0 kV) was beneficial for better ionization. c. In 11% O2 atmosphere condition, more active species could be generated because of greater ionization state. d. the choice of gas flow should simultaneously consider the ionization state and energy consumption for a plasma reactor. The above mentioned discussions were helpful in parameter optimization of pollutants treatment by gliding arc plasma. Increase input voltage or add some water in gas flow were not beneficial for the destruction of PCDD/Fs, but increase O2 concentration performed the opposite result.(2) Based on the experimental platform of only rotation gliding arc plasma, gaseous PCDD/Fs were degraded, and the highest mass destruction efficiency could reach 91.2% with TEQ destruction efficiency of 88.4% in the plasma-only treatment mode. However, destruction efficiencies of low chlorinated PCDD/Fs were low, and T4CDF even increased after destruction which was not beneficial for TEQ reduction. For solving the problem, plasma coupled with catalyst was used for gaseous PCDD/Fs destruction. By experiments, the coupling of catalyst plays a promoting effect on PCDD/Fs destruction, especially on avoiding the regeneration of T4CDF.200 degrees of catalyst temperature and relatively low gas hourly space velocity(13200/h) were both beneficial for the destruction of gaseous PCDD/Fs, but the addition of SO2 could poison the SCR catalyst for PCDD/Fs destruction. Under this condition, the highest destruction efficiency of TEQ was 92.6%, with destruction efficiency of mass of 94.8%.(3) Based on the experimental platform of rotation gliding arc plasma coupled with SCR catalyst, destruction of gaseous of PCBzs(PeCBz and HxCBz) were studied. The input voltage of 15.0 kV, catalyst temperature of 220 degree, higher O2 content(21% O2) and lower gas hourly space velocity(20500/h) were proved advantageous for PCBzs destruction. The highest destruction efficiencies could reach 92.8%, with 97.2% of PeCBz and 86.2% of HxCBz. PeCBz was more sensitive to plasma with catalyst method, and the optimized catalyst temperature of PeCBz was 200 degrees with 250 degrees of HxCBz. Destrucion products of PCBz contained some low chlorinated PCBz, such as 1,2,4-TrCBz, but they only occupied a low proportion(4.3%) of the total residual PCBz.(4) The harmfulness of fly ash and bottom ash was investigated by measuring the particle characteristics of residual ash, the content of PCDD/Fs, PCBzs and PCBs, as well as heavy metals. The concentration of chlorinated organic pollutants followed the order of PCDD/Fs>PCBs>PCBzs with the mass concentration of 114.1,74.62 and 75.44 ng/g and TEQ concentration of 1.34 and 0.02 ng I-TEQ/g. Based on the experimental platform of vortex gliding arc plasma, the PCDD/Fs and PCBs in fly ash were directly destroyed by plasma methods. Bigger surface area and higher original concentration of PCDD/Fs were beneficial for the mass destruction of PCDD/Fs, and the decrease of fly ash particle diameter and increase of micropores volume in fly ash was beneficial for TEQ destruction of PCDD/Fs. Moreover, there were only little gaseous PCDD/Fs residual in flue gas after plasma treatment without second pollution. PCBs destruction efficiency increase with the increase of O2 content, and the highest destruction efficiency was occurred under pure O2 condition with destruction efficiency of mass of 81.6% and destruction efficiency of TEQ of 74.3%, different from the destruction mechanism of PCDD/Fs by gliding arc plasma.(5) By several diffcrent measurement methods, the plasma region were divided into 3 regions, including arc core region, arc expanding region and plasma-catalyst interreaction region, where existed different active components including excited state of N2 and N2+, OH radical, active O, NO3* radical and UV irradiation. The plasma could directly promote the O content on the surface of the catalyst, and the active species could couple well with the active sites of cataslyt for pollutants destruction. In the plasma core and expanding region, there simultaneously existed dechlorination and oxidation effect. Dechlorination played important roles in plasma core region, but oxidation effect was stronger in plasma expanding region. In plasma coupling with catalyst region, the main effect on pollutants treatment was oxidation damagement.(6) By different detection methods, the destruction products of PCDD/Fs and POPS PCBz were recognized. Based on the investigation of products and active species, the total destruction process was divided into primary, secondary and end destruction stages. In primary destruction stages, the chlorinated organic pollutants were dechlorinated and became single benzene-ring structure. In secondary destruction stages, the products with benzene-ring structure and-NO3 structure were destroyed to compounds with short carbon chain, such as propionate and methanol. In the end destruction stages, chlorinated organic pollutants were finally composed to small organic compounds and inorganic products.