| Polycyclic aromatic hydrocarbons(PAHs)are one of the most harmful organic pollutants in the coal combustion process and need to be effectively controlled.Among them,activated carbon adsorption technology is considered to be one of the most feasible PAHs control technologies for flue gas in coal combustion process.However,factors such as the high cost of activated carbon preparation and low adsorption efficiency of activated carbon have restricted the application of this technology in industry.Therefore,a high-efficiency and economical wheat straw-based activated carbon(WS-AC)was prepared in this paper,and the experimental research and mechanism analysis were carried out on an entrained flow type PAHs generation and adsorption test device.The molecular size of PAHs is large,and the choice of activated carbon with a wide pore size distribution is more conducive to adsorption.Therefore,in this paper,the WS-AC suitable for PAHs adsorption was prepared with wheat straw as raw material by steam high-temperature physical activation method,and its pore structure was characterized.The results showed that the wheat straw had a high volatile content and was a good raw material for the preparation of activated carbon.The optimum conditions for the preparation of WS-AC by water vapor activation were:900℃activation temperature,60 min activation time,and 40 ml/h water vapor volume.The WS-AC prepared under this condition not only had a rich microporous structure,but also had a large number of mesoporous structures,a nd its specific surface area could reach 612.56 m~2/g,which was close to the existing commercial activated carbon.In this paper,a set of entrained flow flue gas PAHs generation and adsorption test equipment was designed and built,and the reliability analysis of the device was performed.The effects of key factors such as the pore structure of WS-AC,flue gas conditions,and the amount of activated carbon injection on PAHs adsorption were investigated.The adsorption mechanism of WS-AC on PAHs under different working conditions was explored.The results showed that the entrained flo w adsorption reaction device could reflect the adsorption characteristics of WS-AC on PAHs.The experimental system was stable and the results were repeatable.At the same time,with the increase of the number of PAHs aromatic rings,the adsorption efficiency improved significantly.The pore structure of WS-AC was an important factor affecting its adsorption performance on PAHs.The adsorption capacity of WS-AC on PAHs increased with increasing specific surface area.At the same time,the adsorption capacities of low-ring PAHs and middle-ring PAHs were closely related to the micropore pore volume and mesopore pore volume,respectively.In addition,when the flue gas temperature increased from 90℃to 150℃,the total adsorption rate of PAHs decreased from 51.0%to 29.4%.At the same time,when the concentration of flue gas PAHs increased or the amount of activated carbon injected increased,the probability of effective collision of PAHs molecules with activated carbon particles per unit time increased,thereby improving the adsorption efficiency of activated carbon to PAHs.Subsequently,this paper performed PAHs adsorption experiments on a fixed-bed adsorption reaction device.The adsorption kinetic model and adsorption equilibrium equation were used to fit the experimental results.And the adsorption process,adsorption reaction kinetics and adsorption equilibrium characterist ics of PAHs were deeply studied,and the k inetic parameters and adsorption equilibrium constants of PAHs were obtained.The fitting results showed that the quasi-first-order kinetic model could be used to describe the adsorption process of PAHs,and the external membrane mass transfer process had a more significant effect on the adsorption than the internal particle diffusion.The adsorption process of PAHs on activated carbon was more suitable to be described by Langmuir equation,which showed that the surface of WS-AC had well uniformity.At the same time,the coefficient 1/n of the Freundlich equation was equal to 0.57,indicating that PAHs were more easily adsorbed by WS-AC.Based on this,a mathematical model of activated carbon injection adsorption considering only the external membrane mass transfer and surface adsorption process was established,and the model prediction results were compared with the entrained flow adsorption test results.The results showed that the model could predict the efficiency of PAHs adsorption by activated carbon pipe spraying to a certain extent.At the same time,among the parameters of the model,the influe nce of the equilibrium constant on the adsorption efficiency was far greater than the particle size and molecular diffusion coefficient of the activated carbon.The establishment of this model had a certain guiding role for the application of activated carbon injection adsorption technology in coal combustion process. |
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