Study On The Transformation Of Sodium,Sulfur And Nitrogen During Gasification Of Zhundong Coal

At present,coal is still the main source of energy in China.In the face of the prominent environmental problems,clean coal technology is extensively concerned,in which fluidized bed gasification technology is particularly concerned due to its large gasification intensity and mild gasification conditions.However the transformation characteristics of sulfur and nitrogen during coal gasification is still unclear.With great reserves,good reactivity,Zhundong coal can be efficiently used to supply electricity and chemical products.However,during the combustion and gasification process,the high sodium content in Zhundong coal could induce severe problems,such as slagging,fouling,erosion,et al.Therefore,it is of great significance to study the migration and transformation characteristics of sodium,sulfur and nitrogen during gasification of Zhundong coal for the development and application of Zhundong coal and alleviating the situations of energy shortage and environmental deterioration in China.In this dissertation,the influence of temperature and O2/C on the gasification performance,as well as the migration and transformation of sodium,sulfur and nitrogen were investigated by thermogravimetric-mass spectrometry(TG-MS)and the established vertical furnace at the condition closing to fluidized bed gasifiers.Besides,the relationship between the experimental study and the thermodynamics equilibrium calculation was constructed.The migration of sulfur and nitrogen during combustion and CO2 gasification of Zhundong coal was studied by thermogravimetric-mass spectrometry.The results show that at combustion conditions,the sulfur-containing gases released from the Zhundong coal and Shenmu coal are mainly COS and SO2,and the nitrogen-containing gases are mainly HCN,NO,NO2 and NH3.At CO2 gasification conditions,sulfur mainly released as H2S,COS and SO2,while nitrogen released almost entirely in the form of HCN.The conversion mechanisms of sulfur and nitrogen are different at combustion and CO2 gasification conditions.The fluidized bed gasification performance of Zhundong coal at air agent was investigated by the small vertical furnace.The results show that temperature and pressure in the reactor are stable,and no obvious agglomeration occurs.With the increase of the gasification temperature,the volume percentages of CO and H2 in the fuel gas increase,and the LHV of fuel gas,the gas yield,the carbon conversion and the cold gas efficiency also increase.With the rise of O2/C,the volume percentages of H2 and CO decrease,whereas the percentages of CO2 and N2 increase.The LHV of fuel gas decreases,the carbon conversion and gas yield increase,while the cold gas efficiency first increases then decreases.Compared with Shenmu coal,the gasification reactivity of Zhundong coal is higher than that of Shenmu coal.The migration characteristics of sodium,sulfur and nitrogen during the gasification of Zhundong coal were investigated using the small vertical furnace.The results show that the bottom char and fly ash particles are independent of each other,and no obvious melting and bonding phenomenon occurs.The sodium presents mainly as NaCl,NaAlSi2O6 and NaAlSiO4 in fly ash,and as NaAlSi2O6 and NaAlSiO4 in bottom char.There are more NaAlSiO4 in the fly ash and bottom char at higher temperature and higher O2/C conditions.With the rise of temperature,the sodium retention ratio decreases first and then increases,while with the increase of O2/C,the sodium retention ratio slightly decline first then increase obviously.Sulfur in Zhundong coal,bottom char and fly ash presents mainly as sulfate.During the gasification process,65%to 75%of the total sulfur releases into the gas phase mainly as H2S,COS and CS2.Nitrogen in Zhundong coal presents as quaternary,pyrrole and pyridine,but the nitrogen forms changes after gasification.There is about 60%to 75%of the total nitrogen releases into gas phase,most of which releases as N2,and a small amount nitrogen changes into HCN and NH3.The content of HCN is higher than NH3,however,with the increase of temperature and O2/C,more nitrogen changes to NH3.The chemical thermodynamics equilibration of gasification of Zhundong coal was carried out by FactSage 6.1.The effects of gasification temperature and O2/C on the migration and transformation characteristics of sodium,sulfur and nitrogen were discussed.The results show that the temperature and O2/C can significantly affect the forms of sodium,sulfur and nitrogen.According to the calculation and experimental results,enhancing mixing in the bed and inducing formation of high-melting-point sodium-containing compounds,as well as avoiding using silica as bed materials are helpful for preventing agglomeration during high sodium coal fluidized bed gasification.