Experiments On The Transformation Characteristic Of Nitrogen During Pyrolysis Of Coal In The Fluidized Bed

Nitrogen is one of the main pollutants in coal.In the process of coal pyrolysis or combustion,nitrogen will be converted into NH₃?HCN?nitric oxide and other pollutants.The research of nitrogen transfer during pyrolysis of coal is of great significance to the clean utilization of coal.Circulating Fluidized Bed Thermoelectric polygeneration technology based on fluidized bed pyrolysis proposed by Zhejiang University is a clean and efficient technology for clean coal utilization.In this technology,coal and coal ash are thoroughly mixed in a pyrolysis furnace,circulating coal ash supplies the heat for pyrolysis of coal,and in addition to using gas as a fluidizing gas.There are differences between the pyrolysis of this conditions and traditional coal.Therefore,this paper explored the effect of coal ash,the reaction atmosphere of coal to nitrogen transfer during pyrolysis process in a fluidized bed reactor,in order to provide a reference for the subsequent research on nitrogen transfer from pyrolysis and provide a reference for the process of nitrogen transfer in coal fractional conversion utilization.Firstly,the influence of coal ash on nitrogen migration and conversion in pyrolysis process was investigated in fluidized bed.Experiments using two kinds of coal and two coal ash cross-mixed experiments.The pyrolysis temperature is 500??800?Experiments were carried out to investigate the effects of coal types?ZGE coal and XLT coal?,types of coal ash?ZGE coal ash and XLT coal ash?,and the ratio of ash to ash?1:1,1:3?.The results show that both coal ash can reduce the nitrogen content of pyrolysis products tar and increase the yield of NH₃ and HCN.After adding 300%ash during pyrolysis of ZGE coal at 700 ?,the yield of tar-N decreased by 16.7%while the yield of NH₃-N increased from 4.0%to 5.6%and the yield of HCN-N increased from 1.5%to 5.5%.The mechanism of ash action in this study is that the ash catalyzes the secondary decomposition of the tar in the fluidized bed dilute phase with tar.However,the influence of coal ash on the char-N of the two kinds of coal is just the opposite.Coal ash increases the char-N proportion of ZGE coal and reduces the char-N proportion ofXLT coal.Because Ca,Fe and other mineral components in XLT coal ash are much higher than ZGE coal ash,so the catalytic effect of XLT coal ash slightly better than ZGE coal ash.Increasing the proportion of adding ash can improve the catalytic effect of coal ash.In addition,in order to explore the specific role of minerals in coal ash,the effects of CaO,Fe2O3 and CaSO₄ on pyrolysis of ZGE coal were studied at 700 ?.The results showed that the addition of minerals reduced the tar-N and increased the yield of HCN and NH₃.The effect of coal ash on the char-N proportion of the two types of coal is different because the catalytic effect of CaSO₄ in the coal ash on the char-N proportion is coal-selective.Then the influence of the atmosphere on the nitrogen transfer and conversion characteristics during pyrolysis process was carried out on the fluidized bed.Two kinds of coal were used in the experiment.The pyrolysis temperature was 500??800 ?.The reaction atmospheres were Ar,Ar + H2,Ar + CH4 and Ar + CO respectively.The results show that all three atmospheres can significantly reduce the char-N proportion and increase the tar-N proportion.The principle of H2 is to stabilize the hydrogen radical fragments produced in the pyrolysis process,which is conducive to the fracture of the nitrogen-containing aromatic ring,thereby reducing the char-N proportion,and the larger nitrogen-containing aromatic ring breaks to form volatiles into the tar.The H2 atmosphere also catalyzed the partial nitrogen of coke and tar freed into the gas phase,increasing the yield of HCN and NH₃.The principle of CH4 is to generate methyl and hydrogen radicals by free radicals produced by pyrolysis.,which in turn promote the pyrolysis and conversion of char-N.The CH4 atmosphere reduces the yield of HCN and increases the yield of NH₃ at high temperature.The methyl group can react with the intermediate product of NH₃ and HCN generation process.The increase of the methyl group promotes the polycondensation of the nitrogen-containing carbon chain or the nitrogen-containing aromatic ring,thereby reducing the precipitation of HCN and NH₃.CO,a polar molecule with arc-pair electrons,can generate free radicals.Therefore,CO can accelerate the breaking of nitrogen-containing aliphatic chains and ether bonds in coal and the opening of benzene rings,thus reducing the char-N proportion.CO can increase the yield of HCN and NH₃ during low temperature,and inhibit the precipitation of HCN and NH₃ during high temperature.Disproportionation of CO occurs at higher temperatures produce CO2 which inhibits the precipitation of HCN and NH₃.