Performance Evaluation And Experimental Study On The Adsorption Of Gas-phase PCDD/Fs By Activated Carbon

Incineration is the mainstream method for harmless disposal for municipal solid waste in China,and the emission requirements for PCDD/Fs in the flue gas from incineration are also becoming increasingly stringent.Activated carbon adsorption technology,with its simple process,low cost,and effective removal,is the most widely applied method for the removal of PCDD/Fs from flue gas.However,there is a rich variety of activated carbon with significant differences in performance,leading to uneven quality of activated carbon in the market.Thus,there is a lack of a comprehensive performance evaluation method for activated carbon used in waste incineration plants.In addition,due to the difficulty of laboratory research on PCDD/Fs,there is limited research on the factors affecting the adsorption of PCDD/Fs by activated carbon.Most studies are either field experiments or use PCDD/Fs model compounds.There is a significant shortage of in-depth research with high accuracy.To grasp the adsorption mechanism of activated carbon and increase the PCDD/Fs removal efficiency,this paper studied the adsorption mechanism of gas-phase PCDD/Fs by activated carbon,focusing on three aspects:performance evaluation of activated carbon,adsorption conditions,and the impact of flue gas components.The research results provided theoretical guidance for activated carbon selection and operation under complex conditions.The main research contents and conclusions are as follows:（1）This study focused on comprehensive performance evaluation for activated carbon in adsorbing gas-phase PCDD/Fs.Theoretical calculations and experimental results both indicated that the abundance of mesoporous structures in activated carbon is crucial for effective PCDD/F adsorption,with both mesopore volume and surface area showing strong positive correlations（R²=0.99 and R²=0.98）with PCDD/Fs removal efficiency（RE）.The iodine value is not suitable for evaluating the PCDD/F adsorption capacity of activated carbon,while the molasses coefficient shows a strong negative correlation with PCDD/Fs RE.Due to the fact that high-chlorinated PCDD/Fs homologues have lower volatility compared to low-chlorinated ones,activated carbon was less effective at removing low-chlorinated PCDD/Fs.The study revealed that while principal component analysis was reasonable for a general assessment of activated carbon,however,it fell short in evaluation scenarios under complex working conditions.Based on the entropy method,enhanced with subsystem coefficient assignment,this study established an accurate and specific performance evaluation method for activated carbon,aligning closely with laboratory results on the adsorption efficiency（AE）of activated carbon for gas-phase PCDD/Fs.（2）The study explored the influence of typical operating parameters,temperature and dosage,on the adsorption of gas-phase PCDD/Fs by activated carbon through laboratory research and field investigations.Laboratory findings revealed that as temperature increased,the amount of PCDD/Fs adsorbed per gram of activated carbon decreased exponentially.Specifically,the TEQ value fell from 131.3 ng TEQ/m3at 150℃to 55.9 ng TEQ/m3at 180°C.The increase in temperature enhanced the proportion of polar functional groups like carboxyl and anhydrides on the activated carbon surface and weakens theπ-πcovalent bond interactions,partially reducing the adsorption capacity for gas-phase PCDD/Fs.There is an exponential increase in the PCDD/F RE with the amount of activated carbon used,rising from 65.8%with 0.10 g to 93.0%with 0.20g.In a 700 t/d waste incineration furnace,a positive linear correlation（R²=0.98）was observed between the injection rate of activated carbon and the removal efficiency of PCDD/F.When the injection rate of activated carbon exceeded 50 mg/Nm3,the incinerator could ensure that PCDD/F emissions met the standard limit（0.1 ng TEQ/m3）.（3）The study further examined the impact of flue gas components（SO₂,CaO and Model fly ash）on the adsorption of gas-phase PCDD/Fs.The presence of sulfur to a certain extent inhibited the adsorption capacity of activated carbon for gas-phase PCDD/Fs.The inhibitory effect of SO₂on PCDD/F adsorption mainly stemmed from occupying the active adsorption sites within the carbon pores,predominantly micropores,and weakening theπ-πcovalent interaction between activated carbon and PCDD/Fs.Blending CaO with activated carbon enhanced the PCDD/F removal,increasing the RE from 58.5%to 92.5%.The addition of CaO reduced the entire polarity of the mixtures,somewhat enhancing its surface adsorption capacity for PCDD/F molecules.Model fly ash was capable of removing a modest quantity of gas-phase PCDD/Fs,achieving the same level of PCDD/F RE as activated carbon when used in quantities five times larger.Mixing activated carbon with model fly ash increased the RE from 47.2%to 69.7%.The increase in contact area between activated carbon and PCDD/Fs and the adsorption of model fly ash for PCDD/Fs play an equally important role in this process.（4）This study conducted simulation research on the adsorption behavior of PCDD/F molecules on activated carbon by Density Functional Theory（DFT）calculation.A single-layer graphene structure was adopted to represent the activated carbon layer model,and the 2,3,7,8-TCDD molecule was used to indicate the PCDD/F molecules.The adsorption energy of 2,3,7,8-TCDD on a 23-ring activated carbon layer in a horizontal position was-1.02 e V,and its primary force stems from theπ-πEDA（Electron Donor-Acceptor）interaction between their largeπbonds,with carbon atoms acting as strongπelectron acceptors during adsorption.The adsorption capacity of PCDD/F molecules on activated carbon increases with the level of chlorination,with the OCDD molecule having an adsorption energy of-1.24 e V under the same condition.Theπ-πEDA effect in horizontal adsorption has a greater impact distance than the van der Waals forces happened in tilted and vertical adsorption,but it forms a stronger bond with higher adsorption energy.In edge functional group adsorption,the adsorption energies of various functional groups are ranked from highest to lowest as follows:O=C-O-C=O（-0.35 e V）>C=O（-0.33 e V）=O-C=O（-0.33 e V）>C-OH（-0.29 e V）>C-O-C（-0.23 e V）=COOH（-0.23 e V）.The adsorption energy of the 2,3,7,8-TCDD molecule on the activated carbon layer decreases with increasing temperature;as the temperature increased from 120°C to 210°C,the Gibbs free energy of the adsorption system drops from-0.32 e V to-0.09 e V.Pure SO₂ molecules underwent physical adsorption through van der Waals forces at the center of the activated carbon layer,with an adsorption energy（-0.26 e V）weaker than theπ-πEDA interaction between the 2,3,7,8-TCDD molecule and the activated carbon layer.Compared to 2,3,7,8-TCDD molecules,SO₂ molecules have a prominent adsorption advantage on C-OH and COOH functional groups,while O-C=O and O=C-O-C=O have stronger adsorption forces on 2,3,7,8-TCDD molecules.In the aforementioned simulation research,the impact patterns of chlorine levels,functional group adsorption,temperature,and SO₂ on the adsorption of PCDD/Fs by activated carbon are consistent with the experimental results presented in this paper.