| Adsorption desulfurization by activated carbon is a resource utilization technology among a variety of flue gas desulfurization technologies. This technology can also remove other pollutants such as NOx, HF, HCl, dioxins, and heavy metals and so on. Therefore, there is a very promising future for application. However the moving-bed adsorption and heating regeneration technology has disadvantage of lager size of reactor and high investment. This technology uses formed activated coke of high mechanical strength as adsorbent. The activated coke of5-9mm diameter has drawbacks of severe internal diffusion, low utilization of inner surface, mechanical loss due to fragmentation and abrasion during the transportation process, complex production process, and high cost and operating expense. Based on this background, a new type of adsorption device, circulating fluidized bed adsorption reactor, was proposed. The circulating fluidized bed has the advantage of excellent heat and mass transfer, high performance of gas-solid mixing and low pressure drop. The circulating fluidized bed technology can use powder activated carbon to avoid abrasion of adsorbent and simply the production process of activated carbon. The adsorption equilibrium, adsorption thermodynamics and apparent adsorption kinetics of SO2onto powder activated carbon were investigated. The physicochemical property of activated carbon and its effect on SO2adsorption were analyzed in order to provide a theoretical basis for the preparation of activated carbon. The SO2adsorption performance along the way was studied in a low temperature drop tube furnace experimental system. The effects of operating parameters on the desulfurization performance of fluidized activated carbon were investigated in the lab circulating fluidized bed experimental system in order to provide a foundation support for industrialization.The effects of activated carbon particle size, SO2volume fraction and adsorption temperature on SO2adsorption were investigated based on a thermostat fixed-bed reactor. The adsorption equilibrium, adsorption thermodynamics and apparent adsorption kinetics of SO2onto powder activated carbon were analyzed. The results show that there is a rapid SO2adsorption rate on activated carbon in the initial stage, which is determined by the surface adsorption, after that the adsorption rate drops sharply due to the effect of intraparticle diffusion, and then the adsorption rate drops slowly and the amount of SO2adsorbed increases slowly until the adsorption equilibrium, which is determined by the desorption rate of H2SO4. The Bangham kinetic model can be used to predict the kinetics of SO2adsorption on powder activated carbon. The powder activated carbon of0.075mm diameter shows a larger SO2adsorption rate and the amount of SO2adsorbed at equilibrium compared with granular activated carbon of2mm diameter. The initial SO2adsorption rate and the equilibrium adsorption increase with increasing SO2inlet concentration. Langmuir and Freundlich adsorption isotherm model present better fitted results for SO2adsorption equilibrium on powder activated carbon. The adsorption thermodynamics results indicate that the adsorption enthalpy, the Gibbs free energy, and the entropy are negative, so the adsorption is a spontaneous, exothermic, and entropy decreasing process. The SO2adsorption on powder activated carbon is unfavorable at higher temperature.Five kinds of powder activated carbons were investigated to SO2removal in a fixed bed reactor. The pore texture characteristics of the adsorbents were characterized by analyzing N2adsorption isotherm through several approaches, namely DR, as-plot, HK, BJH, DFT and micropore fractal dimension. The surface chemical properties were characterized by elemental analysis, Boehm titration, thermogravimetric analysis and FT-IR. The results show that the powder activated carbons have different pore structure and surface chemical properties because of the different precursor and activation method. The activated carbon YAC and CAC1present narrow micropore size distribution and has a large amount of small micropores, while WAC, CAC2and CAC3have relatively wide micropore size distribution. The activated carbon WAC, CAC1and CAC3have a certain amount of mesopores and macropores, while YAC and CAC2have few of mesopores and macropores. The powder activated carbon WAC has more element of oxygen, mainly in the form of acidic oxygen functional groups such as carboxyl, lactone, phenol and so on, while YAC and CAC1have more basic functional groups. Five kinds of powder activated carbon present different SO2adsorption rate and equilibrium adsorption capacity due to different physicochemical properties. The desulphurization activity of powder activated carbon is independent of specific surface area and pore volume. Micropore size distribution has a certain influence on SO2removal. The type and quantity of surface functional groups have a great effect on the desulphurization performance. There is a good correlation between the amount of basic functional groups and SO2adsorption.Based on the results of fixed-bed studies, a low temperature drop tube furnace experimental system was built up. SO2adsorption over powder activated carbon along the way was investigated, and an apparent adsorption kinetics model was proposed. The results show that SO2is quickly adsorbed by powder activated carbon in the initial stage, and then the desulfurization efficiency increases slowly along the way. The carbon-sulfur molar ratio has a great effect on SO2adsorption, and the desulfurization efficiency gradually increase with carbon-sulfur ratio increasing due to the increase in the number of active sites. However, the initial adsorption rate and adsorption capacity of SO2gradually decrease with carbon-sulfur ratio increasing due to competitive adsorption between powder activated carbons. The desulfurization efficiency decreases with increasing adsorption temperature and SO2volume fraction. The initial adsorption rate and adsorption capacity increase with increasing SO2volume fraction at the same feed rate of powder activated carbon. O2and H2O from flue gas promote SO2adsorption by activated carbon. Circulating of powder activated carbon plays a very important role to enhance desulfurization efficiency, reduce the amount of adsorbents and improve the utilization of activated carbon. The desulfurization efficiency increases with increasing circulating ratio of activated carbon, and circulating of powder activated carbon can achieve efficient removal of SO2from flue gas. The Bangham kinetics model can be used to predict the kinetics of SO2adsorption on powder activated carbon in the low temperature drop tube furnace experimental system.A circulating fluidized bed adsorption system was built up in which SO2removal performance of powder activated carbon was investigated. The effects of operating parameters such as carbon-sulfur molar ratio, adsorption temperature and H2O volume fraction on SO2removal efficiency were investigated. The cyclic adsorption characteristics and adsorption kinetics of powder activated carbon were analyzed. The results show that the carbon-sulfur molar ratio has a great effect on SO2removal. The desulfurization gradually increases with increasing carbon-sulfur ratio, however the adsorption capacity of SO2gradually decreases. Different kinds of activated carbon present different desulfurization efficiency and change trends due to different pore texture and surface chemistry. SO2removal efficiency increases with H2O concentration increasing. The desulfurization rate decreases with temperature increasing in the range of above60℃, and changes little below60℃.In the cycling of powder activated carbon, activated carbon shows a higher initial adsorption rate, and then the adsorption rate decreases rapidly with adsorption time increasing The Bangham kinetics model can be used to predict the kinetics of SO2adsorption on powder activated carbon in the circulating fluidized bed adsorption system. |
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