| The conventional pollutants of dust,SOx,and NOx in coal-fired power plant have been removed effectively by the ultra-low emission retrofit.However,the low-concentration organic pollutants are present in coal-fired flue gas,and the coal-fired power plants are important source of organic pollutants due to the large volume of flue gas emission.Organic pollutants in air can induce multiple environmental effects and pose a serious threat to human health.Removal of organic pollutants from coal-fired power plants is of great importance to improve environmental quality and maintain human health.According to the emission characteristics of organic pollutant in coal-fired power plants and the flue gas treatment process,activated carbon（AC）injection coupled with bag filtering（ACI+BF）was proposed innovatively to remove the organic pollutants from coal-fired flue gas.The adsorption behaviors of organic pollutants in coal-fired flue gas environment were systematically studied,and the removal performance of organic pollutants in actual power plants was investigated further.According to coal-fired flue gas environment,the adsorption behavior of volatile organic pollutants（VOCs）and semi-volatile organic pollutants（SVOCs）on AC under complex flue gas environment was firstly investigated by a fixed bed.The adsorption mechanism of organic pollutants in coal-fired flue gas environment was investigated through experiment and density functional theory calculations.The results showed that the adsorption capacities of VOCs（toluene and chlorobenzene）and SVOCs（tetrachlorobenzene（TCB）and phenol）on commercial AC were 13.9～80.3 mg/g and24.0～324.0 mg/g,respectively,at medium-high temperatures and low concentrations.Increasing the temperature inhibited the VOCs/SVOCs adsorption.The higher temperature increases the difficulty of organic pollutant adsorption,especially for VOCs,leading to a decrease in effective adsorption sites on AC.The organic molecules adsorbed on AC at medium-high temperature had a high desorption temperature,and the desorption curve of SVOCs did not show symmetry,the desorption temperature was higher than 780°C.The adsorption energies of toluene,TCB,and phenol on graphite layer were-53.97 k J/mol,-75.59 k J/mol,and-55.00 k J/mol,respectively.The interaction between SVOCs and graphite layer was stronger than that of VOCs.Organic molecules with aromatic structure tend to be adsorbed on graphite layer by a parallel manner.The adsorption energy of organic pollutants at the edge of graphite layer was improved evidently by adding carboxyl group.The inhibition of SO2 on VOCs/SVOCs adsorption was weak.With increasing the water vapor content,the inhibition of organic molecule adsorption was enhanced.The high-concentration fly ash not only reduced the adsorption capacity of AC but also the adsorption rate.The adsorption capacity of organic pollutants on AC was reduced by 18.1～20.7%in the simulated flue gas environment.The competition between inorganic and organic molecules for the adsorption on oxygen-containing groups and the clogging of the macropore by fly ash are the main reasons for the decreasing of adsorption capacity.In practice,the adsorbent is injected into the flue for organic pollutant adsorption.Therefore,an ACI+BF system was built to investigate the removal behavior of different organic pollutants and the influence of inorganic components on the organic pollutant adsorption.It was found that the adsorption efficiency of six benzenes（VOCs）ranged from 21.2 to 53.8%,and their adsorption capacity on AC was 7.19～19.2 mg/g,while the removal efficiency of phenol（SVOCs）was 59.8%.The removal efficiency has a good linear relationship with the boiling point and saturation vapor pressure of organic molecules.Adsorption efficiency was reduced with increasing the temperature and concentration,while that was improved with increasing the AC injection amount and residence time.However,increasing the AC injection amount resulted in some adsorption sites not being utilized.When the mass ratio of the spent AC was about 25%,the adsorption efficiency was closed to that of new AC.In the ACI+BF system,the adsorption on the adsorption layer of filter bag contributed more than 50%of the removal efficiency.The adsorption of organic pollutants in complex coal-fired flue gas environment could be divided into three stages,and the adsorbents were fly ash,fly ash and AC,and AC,respectively.The H2O and SO2 influenced the organic pollutant adsorption in the pipeline and adsorption layer of the filter bag.However,the fly ash had a weak influence on AC adsorption in pipeline,while that was strong in the adsorption layer of the filter bag.The removal efficiency of organic pollutants by commercial AC is lower.Based on the organic pollutant adsorption mechanism,the modified commercial AC was prepared by acid/base chemical modification.Meanwhile,the wine lees-based biochar was prepared using the industrial waste wine lees.Their adsorption performance for organic pollutants was studied.The results showed that the SBET and Vt of commercial AC were decreased after modification with nitric acid or ammonia,but the microporosity was increased.The oxygen-containing groups on AC was decreased after the alkali modification,while that was opposite with acid modification.The adsorption performance of acid-modified AC for organic pollutants was better than that of alkali-modified AC.The interaction force between modified AC and organic molecules was stronger.The adsorption capacity of acid-modified AC for chlorobenzene was increased by 2.2%under simulated flue gas environment.Wine lees-based biochar was rich in microporous structure,and there were some large pores on their surface.The micropore sizes of AC-（KOH-1:1）-750 and AC-（KOH-2:1）-700 presented bimodal distribution,which had a better adsorption performance for organic pollutants.The adsorption capacity of biochar is twice that of commercial AC,and the adsorption rate is faster.Biochar is more adaptable to coal-fired flue gas environment than commercial AC.On the ACI+BF system,the removal efficiencies of chlorobenzene by the acid-modified AC and wine lees-based biochar were 50.1%and 66.2%,respectively,with lower injection amount.Based on the above study about the organic pollutant adsorption in the simulated experimental setups,the removal performance of organic pollutant from the actual coal-fired flue gas was investigated.The adsorption performance of organic pollutant was studied on a 1200 Nm3/h scale coal-fired flue gas pilot platform and 660 MW/300 MW coal-fired units.The dust removal methods for 660 MW and 300 MW coal-fired units are bag filter and electrical precipitator,respectively.The study showed that methane was present in the coal-fired flue gas generated from the pilot plant,with the non-methane total hydrocarbon concentration of 1.90 mg/m3.The removal efficiency of non-methane total hydrocarbons was 41.1%with the AC injection,and that was increased to 58.4%with the acid-modified AC injection;the removal efficiency of oxygen-containing VOCs was improved significantly.There is almost no methane in the flue gas of coal-fired power plants,and the total hydrocarbon concentration is 0.512～0.998 mg/m3.The concentration of organic pollutants was reduced as the flue gas pass through the bag filter,while some VOCs were released as the flue gas pass through the electrical precipitator.AC with large pore volume and specific surface area had higher adsorption efficiency;the presence of mesopores could promote organic pollutant removal,especially for the SVOCs.In the actual coal-fired flue gas,the AC injection amount should be maintained at 100～150 mg/m3.The removal efficiency of VOCs in flue gas is generally in the range of 15～50%,and that was 45～95%for SVOCs.The removal efficiencies of halogenated aromatic and aliphatic hydrocarbons are higher,and the presence of inorganic components leads to a lower removal efficiency for oxygenated VOCs.AC injection has a synergistic effect on the removal of mercury and condensable particulate matter in the flue gas. |
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