Plasma Modification Of Activated Carbon Fiber For The Adsorption Of SO₂and NO

Coal-fired power is still the main part in our power structure and this will not be changed in a very long time. Pollutions such as SO2and NOx that generated by the combustion of fuel are harmful to environment and human. Wet flue gas desulfurization technology (WFGD) such as CaO/CaCO3which is the most widely used and the denitration technology such as selective catalytic reduction (SCR) have many disadvantages such as the complex systems and only the removal of individual pollutant, large area occupied for WFGD, and high cost in investment and operation, high quality inlet flue gas and catalyst poisoning for SCR. Compared to WFGD, dry flue gas deSO2and deNOx shows lots of advantages in many aspects. For example, the high efficiency, low invest and cost, arrange easily and no secondary pollutions. Activated carbon fibers (ACF) as the latest generation of carbonaceous material make the dry technology more useful in many aspects, because its adsorption and desorption abilities are better than the traditional carbonaceous material, such as granular activated carbon (GAC) and power activated carbon (PAC).The researches on the directly use of ACF as the catalyzer for deSO2and deNO are little, and deSO2and deNO by ACF are a series of complex process of adsorption and catalytic oxidation which are closely related to ACF surface area and the surface chemical characters. Contrast to GAC and PAC, the surface area changed greatly, but the change in surface chemical characters is not obvious. In order to make ACF more suit for deSO2and deNOx, ACF should be modified and strengthen the surface chemical characters.The traditional modification of ACF is chemical treatment and heat treatment, but the effect of these treatments is not obvious, and the cost is high, besides these, heat treatment also changes ACF microstructure and sometime destroys ACF. Low temperature plasma surface treatment technology is new surface modification method to material surface which grows rapid in recent years. It has many advantages compared to other methods, such as simple technique, convenient operation, low cost, short treatment time, high efficiency and the modification only occurs on the surface layer and don’t change the inherent properties of ACF.In this work, ACF is used directly as the catalyzer for deSO2and deNO, and plasma is applied to modify ACF for deSO2and deNOx. Nitrogen-containing functional groups are introduced to ACF surface and the relative contents of oxygen-containing functional groups are improved in the modification. The adsorption and oxidation ability are strengthened in the modification. Corona discharge and dielectric barrier discharge (DBD) which are the two easily achieved discharge are used in the experiment, and discharge form, gas type, discharge condition etc. are discussed. SEM, BET, FTIR, XPS etc. are implied to analyze the physicochemical characteristics of the ACF before and after treatment.The experiments indicate that both corona discharge and DBD modification have effect on ACF, and the modification effects are different. The modification effect is not obvious and the physicochemical characteristic changes little when discharge voltage is low or treatment time is short, bur too high discharge voltage or long time treatment makes the ACF surface destroyed. When air corona discharge is applied to modify VACF, the optimum conditions are discharge vltage of12kV and treatmemt for5min. The specific surface area and pore volume increased a little and the average pore size decreased a little. The active material on ACF surface formed in modification increased after treatment and nitrogen atom is introduced to ACF surface in air corona discharge, and the content of C-O functional group decresed. The content of C=O fuctional group that is basic and benifical to absorb SO2and NO is increased. Nitrogen-containing functional groups are generated in nitrogen and NH3corona discharge, and oxygen-containing functional groups relative content increases in oxygen corona discharge, especially the C=O group. When ACF is treated by DBD, the different features are shown on the specific surface area and pore volume comparing to corona discharge. In nitrogen and air DBD, the surface area and pore volume decreased after modification. However, the surface area and pore volume decreased first then increased and decreased again with the treatment time at discharge voltage of8kV. When treated by DBD, more nitrogen functional groups or basic functional groups that are benifical to absorb and oxidize SO2and NO are introduced to ACF surface. In nitrogen DBD, nitrogen functional groups are formed on surface and the content of C=O is also increased; more nitrogen and oxygen atoms are introduced to ACF surface in air plasma; in oxygen plasma, the content of oxygen atom is increased greatly and the content of basic functional groups are also increased; more nitrogen functional groups are form in NH3plasma. The optimum treatment parameters of different conditions are as follow:discharge voltage of8kV and treatment time of20min for nitrogen plasma; discharge voltage of8kV and treatment time of8min for oxygen plasma; discharge voltage of8kV and treatment time of5min for NH3plasma.After treatment, the modified ACFs are used for the adsorption of SO2and NOx, and the influence factors and the mechanism are also discussed. The adsorption experiments indicate that the NH3DBD plasma treated ACF shows the best deSO2and deNOx effect. The time that keeps the desulfurization rate over95%lasts from14min of original to24min, and the SO2adsorption capacity increases from120mg/g to173mg/g and that of NO increases from2.8mg/g to10mg/g. Oxygen plasma treatment ACF has the secondary adsorption efficiency, and the nitrogen and air plasma treatment have the same effect.The efficiency of desulfurization and denitrification of ACF are higher when O2and H2O exist in gas, but the adsorption and oxidation capacity will be affected if the content of O2and H2O are too high. NO in gas is beneficial to deSO2by ACF. However, on the contrary the effect is negative. The active sites on ACF surface are occupied by H2SO4and HNO3after adsorption, which block the process of adsorption and oxidation. XPS results indicate that the content of pyridine-like functional groups decreased a lot after deSO2and deNO, which indicate that pyridine-like functional groups play an important role in removal of SO2and NO and the effect on SO2is more. It also shows that amides and quaternary nitrogen groups disappeared after adsorb of SO2and NO, respectively. That is to say these functional groups also have benifical effects on removal of SO2and NO. The oxidation process of SO2maybe the reaction control step of deSO2, and the hydration process maybe the reaction control step of deNO. The functional groups recovered after hot air desorption and the relative contents are nearly the same as the treated ACF.