Removal Of Volatile Organic Compounds By Non-thermal Plasma Technology

China’s VOC emissions are closely related to the development of local economy. In 2008, the total amount of national emissions is 20,000 kilotonnes, where 7 provinces including Zhejiang emit more than 1,000 kilotonnes. In the next few years, the government aims to maintain 7-9 percent GDP growth rate. It is more urgent for demanding organic chemical products, and it may cause more serious VOCs emissions. The conventional treatment methods costs high for low concentration and high flow rate VOCs emission control. Non-thermal plasma techniques have been considered as promising because of simultaneous treatments of multi-types of VOCs.For deep cleaning, however, it is still facing two technical problems:low efficiency and by-products emission. This work aims to study some critical subjects, such as the mechanism of plasma induced VOCs removal, the collection of by-products and VOCs removal process. All experiments are carried out in a novel closed-loop flow system.At the first part of investigation, styrene emission control is conducted negative and positive DC power source energization. The results show that for corona of negative polarity, the styrene and humidity has no significant effect on the current/voltage characteristic. In contrast, a pronounced decrease in current intensity is detected in the presence of styrene and high humidity under positive DC energization. In order to produce 80 ppm of ozone, the energy densities for positive and negative corona are 200 J/L and 38 J/L, respectively. When changing relative humidity from 20 to 72%, the ozone concentration various from 121 to 50 ppm at an energy density of 280 J/L for negative corona air discharges. With regard to positive corona discharges, however, the ozone concentration various from 106 to 12 ppm at an energy density of 40 J/L. The by-products phenylacetaldehyde has very high chemical activity. It begins to influence the processing efficiency when styrene removal efficiency is around 67%. FTIR spectrum and GC signal show that phenylacetaldehyde, N2O, CO, CO2 and benzaldehyde are main products.At the second part of investigation, we study plasma induced benzene derivatives removal characteristics in air with a home-made AC/DC plasma system. The removal efficiency and reaction kinetics are investigated in terms of source specifications, relative humidity, initial concentrations, by-products and specific plasma energy density. The result shows that more than 50 kW of peak power and 85% of energy transfer efficiency can easily be achieved by present AC/DC power source. At an energy density of 37 J/L, the styrene removal efficiency is about 95% without using the Ca and at Cp=0.70μF. However, it drops to about 74% with Ca=3.86 nF and at Cp=8.25μF. When changing the pulse repetition rates from 120 to 30 pps, the efficiency slightly rises from about 83 to 93% at the energy density of 42 J/L. The humidity has negative effects on benzene derivatives removal. Both benzene and toluene show first-order removal kinetic at various humidity atmospheres. For styrene, however, increasing the humidity leads to the global reaction kinetics changes from non-significant to significant linear radical termination processes. At a certain level of humidity and energy density, the ozone yields decreases in the order of benzene> toluene>> styrene (0) when processing benzene derivatives. For styrene processing, the removal process is mainly initialed from ring-retaining C=C oxidation. For toluene processing, both ring-opening reaction and ring-retaining oxidation (-CH3 oxidation) are important. For benzene, however, only ring-opening reaction is important. The ozone is used as recycled O radicals for styrene removal if the treatment duration lasts several seconds. With regard to benzene and toluene removal, the reaction with ozone is not a significant way.The plasma induced aerosols are mainly with a diameter of 28-384 nm. At an energy density of 27-32 J/L, the number concentration of the produced aerosols from styrene is about 9×105#/cm3, which is a factor of 3-4 times more than that observed when processing benzene or toluene. Moreover, the AC/DC corona system can be used as a precipitator for the aerosol collection. Fourier transform infrared spectroscopy indicates that the non-volatile aerosol deposits contain the -COH and -COOH groups. When processing styrene, the aerosol yield with diameter of 28 nm,55 nm,93 nm and 157 nm are 4×105 #/(cm3·ppm), 1.1×105#/(cm3·ppm),5×104#/(cm3·ppm) and 1×104#/(cm3·ppm), respectively. For two stage plasma system, the grade collection efficiency is dependent on the performance of ESP. The maximum collection efficiency is 75%, and it is found in the diameter of 157 nm. The use of pre-charger is helpful for ESP upgrading. At an optimal condition, the grade collection efficiency is enhanced by 16% and 21% for aerosol diameter of 28 nm and 264 nm.