Study On The Migration And Transformation Of Alkali Mental During The Thermal Conversion Of High Alkali Mental Coal

Zhundong high alkali coalfield is the biggest integrated coal reserve newly found in China in recent years.With the characteristics of high volatility,low ash,good reactivity,rich reserves and low costs of mining,Zhundong high alkali coal is suitable for power fuel and liquefied coal.However,due to the high content of alkali metal in Zhundong high alkali coal,there are a series of problems during Zhundong coal direct combustion in the pulverized coal boiler of the power plant,such as severe slagging in the burner area of the furnace,the flue gas temperature of the boiler furnace outlet is high,fouling on the high temperature heat transfer surface and ash deposition on the low temperature heat transfer surface are serious,even part of the superheater occurred over-temperature burst.At present,Zhundong high alkali coal could not be used directly in the active units.In order to effectively solve the problem of slagging,fouling,ash deposition and corrosion in the utilization of high alkali coal,this paper took the advantage of circulating fluidized bed technology,studied the migration and transformation characteristics of alkali mental during the thermal conversion of high alkali coal.This study could provide new ideas and process routes for the large-scale use of high alkali coal.This study uses a combination of experimental and theoretical analysis,the chemical reaction kinetics and thermodynamics and other basic theory as a guide,the tube furnace,circulating fluidized bed combined with chemical thermodynamic calculation software as experimental platform,IC,XRD,XRF and SEM-EDX as the analytical methods,to study the migration and transformation of alkali metal during the thermochemical transformation of Zhundong high alkali coal and the condensation characteristics of alkali metal during flue gas cooling process.The main conclusions are as follows:?1?Zhundong high alkali coal is highly different with China's conventional coal in the basic physical and chemical properties.The content of Cl in coal is different depends on the mining areas.The content of Na₂O in coal ash is higher than 2%,belonging to slag coal.Zhundong high alkali has good ignition characteristics and burnout characteristics.Alkali sodium in Zhundong high alkali coal is mainly in the form of water soluble,and alkali metal potassium is different from sodium,mainly in the form of insoluble.?2?The pretreatment methods have an important effect on the accuracy of the determination of alkali metal content in high alkali coal and high alkali coal gasification ash.In order to obtain the alkali metal content of Zhundong high alkali coal and gasification fly ash accurately,it is suggested to use oxygen bomb combustion method to determine the alkali metal content in the coal.At the same time,for the determination of alkali metal sodium content,extraction method can also be relatively accurate to reflect the content of alkali sodium in coal,meanwhile the form of sodium can be obtained.?3?Because of the different alkali mental content and the different forms of alkali mental in different Zhundong high alkali coals,the operating characteristics of coal gasification process are different.The TC coal with high Na₂O content in coal ash?7.28%in the coal ash?is prone to slagging during the operation of circulating fluidized bed gasification.SEH coal with high content of Cl?1.13%in the coal?is prone to occurring corrosion on the heat transfer surface.The results of the experiments and the results of thermodynamic equilibrium show that increasing the reaction temperature and reducing the reaction pressure will promote the release of alkali metals.?4?The reaction atmosphere has an important effect on the migration and transformation of alkali metal sodium during the thermal conversion of high alkali coal.With the increase of the equivalent air ratios,the sodium in gaseous phase increases,and the sodium in the solid phase decreases.The unreacted carbon in the ash has a retention effect on the alkali metal sodium.Increasing the gasifying agency oxygen concentration and water steam concentration,the transformation of sodium from the water soluble sodium to insoluble sodium is promoted.In the reducing atmosphere of low equivalent air ratio and the oxidizing atmosphere of high equivalent air ratio,the slagging problem in the riser and the fouling on the heat transfer surface are especially serious.At the beginning of the thermal chemical conversion,part of the water soluble sodium will release as gaseous phase.Subsequently,in the oxidizing atmosphere,the gaseous phase sodium will undergo sulfating reaction to produce sodium sulfate,and in the reducing atmosphere,the gas sodium will keep the original chemical form unchanged.Another part of the water soluble sodium will retain in the solid and transfer to HCl soluble and insoluble sodium in the role of high temperature.?5?The results of chemical thermodynamic equilibrium calculation based on the Gibbs free energy minimum theory show that the effect of different reaction conditions on the equilibrium distribution of alkali metals in different coal species is different.Increasing the reaction temperature promotes the volatilization and melting of alkali metal sodium.The reaction pressure mainly affects the gas phase and solid phase equilibrium of the alkali metal.Increasing the reaction pressure will inhibit the release of gaseous sodium and increase the solid phase retention rate of the alkali metal in the solid.Compared with the oxidizing atmosphere,the content of molten sodium in the reducing atmosphere is higher,which indicates that slagging is more likely to occur under the reducing atmosphere in the actual thermal conversion of the high alkali coal.In the oxygen-enriched atmosphere with water steam,a large amount of alkali metal sodium is retained in the solid phase,the gas phase alkali is less and no molten matter is formed,the calculating results are in agreement with the experimental results basically.Oxygen-enriched gasification technology is a very promising technology to solve the problem of slagging and fouling in the actual thermal conversion of the Zhundong high alkali coal.