Fundamental Research Of The Removal Of Volatile Organic Compounds With Enforced Discharge Based On Electrical Conductive Charcoal

The pollution of volatile organic compounds(VOCs)in China is becoming more and more obvious,thus more attention has been paid to the control of VOCs by the public and government.According to the outline of 13th Five-Year Plan for Economic and Social Development of the People's Republic of China,the reduction of overall VOCs emission in key areas and key inductries should be promoted.The overall emission of China should be reduced by more than 10%during the period of 13th Five-Year Plan.The conventioanl methods still cost high for the treatment of high flow rate and low concentration VOCs emission.Therefore,it is of practical significance to develop practicable and reliable control technology of VOCs.Bio-char with abanduant porous structure and excellent electrical conductivity is obtained from biomass by high-temperature carbonization and activation,which is also called electrical conductive charcoal(ECC).ECC has an excellent ability of absorbing and concentrating VOCs.Due to its good conductivity,discharge would be generated directly between ECC particles when subjected to high voltage.Desorption and decomposition of VOCs could be achieved simultaneously.The present work studies the approach of VOCs treatment based on ECC.Toluene and acetone,due to their high emission levels and wide range of applications,were choosen as the target VOCs.The discharge mechanism and the characteristic of adsorption and desorption of VOCs was also investigated.Firstly,the characteristic and mechanism of fluidized ECC particles discharge was experimentally investigated.The characteristics of current and voltage waveform during discharge and the influences of different factors on discharge power were studied.Tremendous of pulse waveform were generated during the discharge of fluidized bed.The maximum value of current could reach 9A.The energy conversion efficiency increased with oxygen content(when<3%)of the gas flow and the conductivity of ECC(when p>4?·cm).Using high-speed camera,photographs of fluidized discharge indicates that the discharge probability was highly related with the density(void fraction)of ECC particles.The probability of conductive status and open circuit status exhibited positive and negative correlation with particle density.The discharge arcs were mainly formed in the dense phase of bubble front and ascend with the motion of bubbles.When the gas in the bubble diffused into the dense phase,the particle density of bubble front decreased and the discharge arc disappeared.The discharge path then changed to a new bubble front.The diameter of single bubble and the frequency of bubble generation increased with void fraction,but the number of discharge path in a single discharge remained unchanged.When the void fraction wass around 0.7,the particle density of bubble front could be high enough,at the same time,fewer conductive staturs were generated.Moreover,the shape of the bubble could remain relatively longer.These factors maximized the discharge probability.The electric field distribution simulation of multiple electrical conductive particles between parallel electrodes were simulated using Comsol Multiphyics.The results showed that when the distance of particles reduced to 0.1R or when curvature radius reduced to less than 0.04mm,the electric field in the gap increased dramatically to more than 7E0.The adsorption and desorption of toluene and acetione in ECC was studied.ECC had a higher ability of absorbing toluene(211?218mg/g)than acetone(129?162mg/g).Competitive adsorption occurred when binary vapor(toluene + acetone)was adsorbed by ECC in which acetone was substituted with toluene.After absorbing toluene or acetone,ECC was conducted with TG(thermogravimetric)analysis.Based on the TG analysis,desorption activation energy of toluene and acetone in ECC were calculated to be 60.89 kJ/mol and 35.46 kJ/mol,respectively,indicating that acetone could be desorbed more easily from ECC.The fluidized ECC absorbed with toluene was conducted with desorption by discharge and external heating,respectively.The result of desorption rate indicated that fluidized discharge effectively facilitated the desorption of toluene from ECC.The decomposition characteristic of VOCs under a fixed concentration was also studied.The increase of O2 concentration facilitated the decomposition efficiency of toluene,especially in a higher discharge power.The higher the initial concentration of toluene,the higher the total amount of toluene decomposed but the lower the decomposition efficiency.The electrical conductivity of ECC had a significant influence on the decomposition efficiency of toluene.The decomposition efficiency could be higher when ECC owned a higher electrical conductivity.Under the same discharge power(e.g.32W),the decomposition rate of acetone(95%)was significantly higher than that of toluene(45%).The concentrating of toluene could effectively reduce the mass loss of ECC.The decomposition characteristic of VOCs after absorbing on ECC was conducted.The effects of adsorption amount,discharge power and external cooling on distribution of VOCs are investigated.The effect of cycle utilization on the adsorption capacity of ECC are also investigated.Under an adsorption amount of 79mg/g and an average discharge power of 26.5W,59%of the adsorbed toluene can be decomposed.The decomposition efficiency decreased with adsorption amount while increased with discharge power.Cycle utilization of ECC had a great impact on its adsorption capacity and microstructure.The first cycle reduced the adsorption capacity slightly(217mg/g to 203 mg/g)due to the loss of surface fragments.After the second cycle,the adsorption capacity gradually increased(203mg/g to 266 mg/g)due to the activation effect.The rapid increase of temperature after the ignition of discharge led to a relatively low decomposition efficiency.Proper cooling of the reactor could effectively increase the decomposition efficiency of toluene(47.8%to 73.9%),and decrease the mass loss of ECC.The optimal temperature of cooling was about 77 ?.The decomposition products contained only CO,CO2 and a small amount of CH4.Compared to toluene,acetone would be desorbed from ECC and escape from the discharge area more easily.To achieve a better decomposition efficiency of acetone,the cooling temperature should be lower.Based on the experimental results,the scheme of ECC adsorption-discharge decomposition was applied to the scenario of vehicle painting production factory.It was compared to the scheme of activated carbon adsorption-catalytic combustion.The economic analysis indicated that this method would make up for the fixed costs and gradually show economic advantages after more than 5 years of operation.Meanwhile,it occupied a smaller area of land and produced no hazardous waste,thus it would have less influence on the environment.