Experimental And Mechanism Studies On Combined Removal Of HCI/NO/SO₂ Using Activated Carbon Fibers

Activated carbon fibers(ACF)can efficiently remove the gaseous contaminants emitted from coal-fired and incineration power plants,i.e.sulfur oxides(SOx),nitrogen oxides(NOx)and hydrogen chloride(HCl),and readily recycle products due to their huge specific surface areas,developed microporous structure and high catalytic activities.In current work,the effects of surface oxygen/nitrogen groups on HCl removal efficiency were discussed,and the mechanism of HCl removal over the ACF surface was investigated.With experiments and quantum calculation,the mechanism of NO oxidation on the ACF surface was subsequently proposed accordingly.Based on the information obtained,a laboratory-scale bench was built to study the competitive adsorption between HCl,NO and SO2.Subsequently,HNO3 modified ACF placed in two-stage reactors was designed to remove SO2,NO and HCI in the flow gas.First,the distribution of surface oxygen/nitrogen groups,surface morphology,surface elemental composition was accurately analyzed.The effects of various modification methods on the distribution of surface oxygen/nitrogen groups were compared.The analysis results indicated that thermal treatment could decompose surface oxygen groups(i.e.C-O,C=O,COO,COOH)and generate C-C groups;H2O2 and HNO3 impregnation could increase the relative ratios of surface oxygen groups by oxidizing C-C bond;HNO3 and Cu(NO3)2 impregnation could change the distribution of surface alkaline nitrogen groups,and simultaneously introduce-NO2 and NO3-onto the carbon surface.Then,the mechanism of NO and HCl removal over the ACF surface was investigated based on the review of SO2 removal.HCl was removed by ACF after directional modification on the surface oxygen/nitrogen groups,and catalytic removal mechanism of HCl was comprehensively illustrated.Meanwhile,the catalytic oxidation pathway of NO and detailed mechanism were obtained by means of experimental methods with quantum calculation.The results indicated that in the absence of O2,the adsorptions of NO and HCI onto the carbon surface can be attributed to the adsorption of gas molecules into the micropores,and the reactions between the acidic gases and alkaline surface functionities.HCl could react with surface alkaline groups(i.e.pyridine and pyrrole),and-NO2 groups could promote the adsorption of HCl since Cl atoms in HCl molecules could react with the-NO2 groups and form chlorides.On the other hand,NO molecules can be bound onto the C-C bonds and surface alkaline groups,and thus the retention abilities of NO over the ACF surface are significantly restricted by the amount of surface functional groups especially when in the absence of O2.In the presence of O2,O2 molecules can be adsorbed onto the ACF surface,interact with the ACF surface and generate semi-quinones and active oxygen atoms,which could oxidize NO to NO2.Subsequently,the combined removal of NO/HCl,SO2/HCl and SO2/NO were studied under N2 and N2/O2 atmospheres.The competitive adsorption in N2 indicates that the binding abilities between gas molecules and adsorption sites can be sequenced as:HCl>SO2>NO.In the presence of O2,HCl can hinder the adsorption of SO2 and NO,while NO can hardly inhibit the removal of HCl,and only SO2 of higher concentration can restrain the removal of HCl.Furthermore,SO2 could restrain the removal of NO while NO show weak inhibition effect on SO2 removal.Simultaneously,the intermediates-NO2 groups,generated during NO oxidation,could pronote the removal of SO2 and HCl.Based on the information obtained,a new method was proposed and a platform with two-stage reactors was designed to remove HCl,NO and SO2.As the experimental results,the increase of O2 concentration could promote the combined removal of HCl,NO and SO2.The primary reactor can capture all HCl and most SO2,and promote the subsequent adsorption of SO2 and NO in the secondary reactor.The adsorption amount of HCl,NO and SO2 were respectively increased from 58.8,6.5,17.3 mg/g to 84,8.4,23.4 mg/g,by margins of 40%,30%and 35%.Moreover,HNO3 modification can introduce-NO2 onto the carbon surface and enhance the removal of HCl,NO and SO2.The adsorption results suggested that two-stage reactors using HNO3 modified ACF significantly improved the adsorption amounts of SO2 and NO,from 23.4,8.4 mg/g to 67.2,21.7 mg/g.