Effect Of Iron Addtive On NO_x Control During Pyrolysis And Combustion Of Long-flame Coal

The nitrogen oxides(NO_X)emitted by coal combustion cause serious pollution to the ecological environment.Among them,the pollutants emitted from the direct combustion of low-quality household coal are 10?20 times that of the large boilers with ultra-clean emissions after equivalent combustion.The coal pyrolysis process is the initial process of coal combustion.In the coal pyrolysis process,a large number of NO_X precursor(HCN?NH₃)will be generated,which will be converted into NO_X in the subsequent combustion process.Therefore,it is necessary to simultaneously considering the two stages of coal pyrolysis and coal combustion to control NO_X emission.Based on this,the idea of reducing NO_X emission through two-step coal pyrolysis and combustion process is put forward.The nitrogen reduction idea of this subject is:in the coal pyrolysis,the metal additives introduced in advance can make the nitrogen and nitrogen in the coal migrate to the volatile matter as N₂ as much as possible,and at the same time,reduce the nitrogen content in the solid fuel of the pyrolysis product.The idea of denitration is:in the combustion of coal-based clean fuel,the metal additives in the fuel make the coke nitrogen in the fuel release mainly in the form of N₂,reducing the production of NO_X.In this paper,HF/HCl pickling was first used to treat long-flame coal to produce demineralized coal,and then mechanical mixing method was used to prepare experimental samples of iron addtive precursor with raw coal or demineralized coal.The influence of iron additives on nitrogen migration and transformation in the pyrolysis process of experimental samples was carried out in a tubular furnace at a pyrolysis temperature of 800?1100?,and the influence factors such as pyrolysis temperature and load ratio of iron additives on nitrogen content in gas,solid and liquid phases was investigated.In the combustion temperature range of 800??1100?,the experiment on the effect of iron additives on the denitration performance of coke combustion process was carried out,and the influence of combustion temperature on NO_X emission characteristics was investigated.XPS and XRD were used to characterize the contents of elements and crystal phases on the surface of the samples.The content of nitride in gas,solid and liquid phases during pyrolysis and combustion was analyzed and detected by elemental analyzer,GC,flue gas analyzer,spectrophotometer and chemiluminescent nitrogen analyzer.Based on the analysis of tar nitrogen content by chemiluminescent nitrogen analyzer and the nitrogen balance in the whole pyrolysis and combustion process,the mechanism of NO_X control by iron auxiliaries in the pyrolysis and combustion process of coal was preliminarily determined.The main research results are as follows:(1)With the increase of pyrolysis temperature,the NH₃ and HCN yields increased,and the Tar-N and the Char-N decreased.Iron promoted the conversion of nitrogen to N₂ in coal during pyrolysis.(2)The optimal load ratio of iron additives was 0.5 wt.%,and the optimal pyrolysis temperature of nitrogen removal rate was 1000?.The proportions of NH₃,HCN,Tar-N and Char-N decreased after iron loading,while the proportions of nitrogen increased greatly at 1000?,and the nitrogen removal ratio increased from 66.6%to 93.6%,achie Ving the purpose of reducing nitrogen during pyrolysis.(3)High temperature and load of iron additive are beneficial to NO_Xremoval during combustion.As the combustion temperature increases,the NO_Xemission decreases gradually,and at each temperature point,the amount of coke burning NO_X after loading iron is lower than that of coke.When the combustion temperature is 1100?,the NO_X emissions reach the lowest,and the emission is reduced by 46.7%.(4)Through the analysis of nitrogen balance during pyrolysis and combustion,it was found that no matter during pyrolysis or combustion,iron promoted the conversion of nitrogen to N₂,while the content of other nitrides significantly decreased.(5)It is feasible to reduce NO_X emission by reducing nitrogen and denitration in the process of two-step coal pyrolysis and combustion.Iron in coke after high temperature pyrolysis mainly exists in the forms of Fe₂O₃,Fe₃O₄and?-Fe,catalyzing the conversion of more nitrogenous compounds into N₂.Meanwhile,Fe₂O₃ formed by combustion has a certain catalytic effect on the reduction reaction of NO_X,CO and C.Through the one-step addition of iron additives,the effects of two-step pyrolysis and nitrogen reduction and combustion denitrification were achieved.