Mechanism Study Of NOx Reduction By Char In Flue Gas

Nitrogen oxides(NOx)is one of the major air pollutants.Reduction of NOx by char has attracted extensive attention in flue gas denitrification with high NO conversion and relatively low cost.The presence of Oz in flue gas helps to increase NO conversion,while the C-O2 competition reaction also causes a large amount of carbon consumption.The C(O)complexes on char surface significantly promote the C-NO reaction.Therefore,the thesis investigated the influence of char physicochemical features,including the distributions of surface C(O)complexes and their roles distinction in the reaction between NO and carbon in the presence of O2.The main results are as follows:Chars with high denitrification efficiency(type ? adsorption isotherms,obvious hysteresis loop)have more micropores(<2nm)compared with chars showing low denitrification efficiency(type ? adsorption isotherms,no hysteresis loop).TEM show that the layer structure of char with high denitrification efficiency is more ordered than the other composed of small platelets,randomly arranged and not well aligned.The relatively ordered layer structure may result in the formation of micro-sized slit pores,which cause the low pressure hysteresis of desorption in the isotherms of Group ? chars.Research has shown that the char with microporous structure usually has a high NO adsorbability and adsorption of NO on char surface contributes to NO-char gasification.With surface carbon-oxygen complexes on char surface divided into C-O,R2C=O and O-C=O by X-ray photoelectron spectroscopy(XPS),the distributions and roles distinguish of different surface carbon-oxygen complexes on char surface were evaluated in the reduction of NO by char in the presence of oxygen It is found that thermally relatively stable C-O species play a more important role compared with R2C=O and O-C=O.Considering the textural and structural properties of chars,and product distribution affected by surface C-O species,two different reaction mechanisms,i.e.,surface carbon-oxygen complexes involved reactions and directly burn-off unoccupied surface,are put forward to explain different performances exhibited by the samples examined in this work Thermally relatively stable C-O species gives the highest correlation coefficient for both NO reduction and the selectivity.Enrichment of surface C-O species on char surface can enhance NO reduction and simultaneously control CO formation.These are helpful to solve the problem of big carbon consumption in NO reduction by char.Finally,the stability of C(O)complexes on char surface was investigated by temperature programmed desorption(TPD)analysis of the non-impregnated and potassium-impregnated char.With the content of C-O species on char surface obviously improved,potassium-impregnated char increased the denitrification efficiency and reduced the CO production,That is,the presence of potassium in chars would increase the content of C-O species on char surface.The mechanism of the thermodynamically stable C-O species mentioned before was discussed.The interaction of potassium and surface C-O species may be important in the process of NO reduction by char.