Research On The Reduction Of N₂O By Char Based On Structural Evolution During Pyrolysis

N2O is a strong greenhouse gas that will deteriorate the ozone layer and affect the ecological environment.The lower combustion temperature conditions of fluidized bed boilers lead to high N2O emission concentrations,and the resulting N2O emission control needs to be solved urgently.In this paper,two kinds of bituminous coals were selected and chars at different temperatures were prepared.The carbon structure,crystallite structure and surface functional groups of the char samples were characterized by Raman spectroscopy,XRD and XPS techniques.Na and Fe catalysts were supported on the char surface,and the thermogravimetric experiment of char(catalytic)reduction of N2O was carried out.In-situ diffuse reflectance infrared spectroscopy(In-situ DRIFTS)was used to analyze the changes of char surface functional groups during the reaction of char reduction of N2O.The experiment of reducing N2O with char was carried out by using a fixed bed reactor system.The NOx concentration at the reactor outlet was measured,the effects of pyrolysis temperature and char chemical structure changes on N2O conversion were studied,and the structure changes of residual char after the reaction were analyzed.The results of Raman spectroscopy show that the structural order of char A decreases in the range of 500～800℃,and increases after 800℃,and the order of char B decreases gradually at 500～900℃.XRD results show that the microcrystalline structure of A and B chars grows more developed in the lateral direction with the increase of temperature,and the degree of graphitization gradually increases.The results of thermogravimetric analysis showed that the char adsorption and reduction of N2O mainly occurred at 98～680℃.After loading NaCl and FeCl3,the weight loss rate of char decreased,indicating that Na and Fe had catalytic effect on char-N2O reaction,but the weight loss rate of char after adding Fe was lower than that after adding Na.Both Na and Fe catalysts can reduce the activation energy of the A char reduction N2O reaction,and the activation energy does not change much at the heating rate of 10℃/min and 30℃/min.The catalytic effect of Na and Fe catalysts on the reduction of B char to N2O is different.Under the heating rates of 10℃/min,20℃/min and 30℃/min,the reaction activation energy is increased after adding Na,and adding Fe.The activation energy of the post-reaction decreases but does not change much.In situ infrared spectroscopy(DRIFTs)of char reduction of N2O showed that the main functional groups on the surface of char A and char B before and after catalyst loading included kaolinite,aromatic C=C,COOH,phenolic C-OH,aromatic CH,aliphatic CH,NH3 and N2O.After the addition of Fe and Na catalysts,the adsorption capacity of char for N2O is enhanced.Fe has a stronger catalytic activity in the range of 400-650℃,and Na has a stronger catalytic activity in the low temperature range(40～400℃).The fixed-bed experimental results of the char-N2O reaction show that the char is gasified with CO and CO2 in the reaction of char reduction of N2O.The evolution of char structure will affect the char’s ability to reduce N2O.Although the high-temperature pyrolysis char has a low degree of order,its reactivity is reduced,and its ability to reduce N2O is also reduced.XRD spectrum and particle size distribution analysis of residual char after reduction in fixed bed experiments were carried out.The XRD spectrum shows that the main minerals in the residual A char after the reduction of N2O are quartz,troilite,magnetite,calcite,hematite,anatase,anhydrite,albite,and mafic spinel,wollastonite,mayorite and other minerals,the main minerals in the residual B char after the reduction of N2O are quartz,magnetite,calcite,hematite,anhydrite,albite,mayorite and trothion calcium minerals such as stone.The particle size distribution shows that after the reduction reaction,the average particle size of A800 in the residual A char after reduction is the smallest,and the average particle size of A500 is the largest,while the average particle size of B500 in the residual B char after reduction is the smallest,and the average particle size of B800 is the largest.