Study On Nitrogen Transformation Mechanism Of High Alkali Coal Pyrolysis Under Slag-tap Condition

Sodium species volatilization during thermal utilization of Xinjiang high alkali coal can bring a series of serious problems,such as slagging,fouling and corrosion,which have been a barrier for further utilization of high alkali coal.The clean and efficient utilization of high alkali coal not only solves the boiler problems resulted from sodium release,but also reduces NOx emission.Therefore,it is of great significance for clarifying the influence of sodium form and content on nitrogen transformation mechanism during high temperature pyrolysis to develop low-NOx combustion control technology.In this study,model coal was combined with raw coal,and the production distribution and yield change of gas-N,char-N and tar-N were qualitative and quantitative analyzed to investigate the effect mechanism of water soluble sodium?Na Cl?and organic sodium?CH₃COONa?on nitrogen conversion pathway of high alkali coal pyrolysis under slag-tap conditions.Firstly,DSC-FTIR-MS,XPS and Py-GC/MS were employed to qualitative analyze the effect of sodium form and content on the conversion mechanism of three-phase N-containing products during high alkali coal pyrolysis.It was found that Na Cl and CH₃COONa increased gas-N yield due to the reduction of tar-N was greater than the increase of char-N,and the yield of NH₃ and HCN increased by promoting the secondary cracking of amion-N and nitrile-N.Secondly,the model coals were used to further investigate the transformation mechanism of N-5 which is the main N-containing function group in coal.The result show that N-5 mainly converted to N-6 and N-Q which have higher thermal stability in char.The model N transformed to tar-N in the form of 4-hydroxycarbazole and quinoline at low temperature and medium temperature,respectively.The heterocyclic nitrogen compounds cleaved to ammonia-N and nitrile-N at medium and high temperature.The yields of NH₃ and HCN increased for the reason of Na Cl promoted the cracking of heterocyclic nitrogen,but CH₃COONa was contrary.This phenomenon could be attributed to the difference that anions in Na Cl and CH₃COONa had opposite effects on the cracking of heterocyclic nitrogen.Finally,the high temperature fixed experiments were conducted to verify the conclusions of nitrogen conversion obtained from the above research.The results indicated that the yield of NH₃ was higher than that of HCN at any temperature,both of Na Cl and CH₃COONa promoted the production of NH₃ and HCN.Na Cl and CH₃COONa respectively increased the yield of NH₃by 207%and 200%at 1100°C,which was consistent with the experimental results of qualitative analysis.The N-containing gas with the highest yield was N₂,which mainly derived from the direct cracking of char-N,but the presence of sodium inhibited the conversion of char-N to N₂.More than 50%of N elements converted to N₂ when the final pyrolysis temperature was 1500°C,and Na Cl and CH₃COONa reduced the yield of N₂ by 7.13%and 15.22%,respectively.