Study On Sodium Captured During Sodium-rich Coal Combustion

Sodium-rich coal distributed all over the world,such as China,USA,Australia,et al.In particular,it is abundant in China.Unfortunately,due to the high sodium content of the coal,a lot of the vaporized sodium compounds exists in the flue gas during the sodium-rich coal combustion.The vaporized sodium compounds will condense to form the initial sticky layer on the heating surface of the boiler.The initial sticky layer leads to the heavy slagging and fouling in boiler during the sodium-rich coal combustion.Therefore,the utilization of the sodium-rich coal has been greatly limited.In order to promote the clean and efficient utilization of the sodium-rich coal,it is imperative to reduce the content of the vaporized sodium compounds in flue gas.Thus,firstly the study is to investigate the sodium emission from the sodium-rich coal to prove that the sodium compounds of the coal was vaporized into flue gas during the sodium-rich coal combustion.Then,the study on the sodium capture capacity of the sodium-rich coal ash,sodium-low coal ash and mineral sorbents was conducted and their sodium capture mechanism was explore.The research can be useful for the utilization of the sodium-rich coal by providing theoretical support.The main work includes the following three parts.Two sodium-rich coals from Xinjiang,Wucaiwan coal?WCW?and Hami coal?HM?were chosen as the raw material.A sodium-low coal,Pingshuo coal?PS?was used for contrast experiments.The combustion experiment was carried out in the muffle furnace.The sodium content of the coal ash prepared at different combustion temperature was determined by inductively coupled plasma atomic emission spectroscopy?ICP-OES?and the main crystalline compounds in the resulted ash samples were identified by X-ray diffraction?XRD?.The results showed that the softening temperature of HM coal ash?1100?1200^oC?is the lowest and that of PS coal ash?>1500^oC?is the highest.The sodium compounds were vaporized during the sodium-rich coal combustion.With the increase of the combustion temperature,the sodium emission form the coal increased.The sodium emission mainly took place below 815^oC.In addition,the sodium of the coal was transformed into aluminosilicates during the sodium-rich coal combustion.The two sodium-rich coal ashs,Wucaiwan coal ash?WCW-900?and Hami coal ash?HM-900?prepared at 900^oC was chosen as the raw material,repectively.The sodium-low coal ash?PS-900?,prepared from Pingshuo coal at 900^oC was used for the contrast experiments.The sodium capture experiment was carried out using the sodium capture experiment device.The sodium content of the samples was determined by ICP-OES and the main crystalline compounds in samples were identified by XRD.The results showed that the sodium content of WCW-900 is saturated,WCW-900 did not capture sodium in the flue gas,while HM-900 and PS-900 could capture sodium effectively.With the sodium capture temperature increasing,the amount of the sodium captured by HM-900decreased,but that of PS-900 increased and the growth rate of the amount of the soiudm captured of PS-900 increased.In the sodium capture temperature range of 810?1000^oC,the amount of sodium captured by HM-900 is higher than that of PS-900,which is higher than WCW-900.The sodium capture and sodium release existed in the sodium capture process simultaneously.The sodium capture was caused by reaction between the vaporized NaCl and the minerals in the coal ash and the physical adsorption.In addition,with the sodium capture temperature increasing,the soluble sodium in the sodium-rich coal ash vaporized.The three mineral sorbents?kaolinite,SiO₂ and bentonite?were selected as raw material for the sodium capture experiment.The thermal stability of the sorbents was conducted by thermal gravimetric analyses?TG?.The sodium content of the samples was determined by ICP-OES and the main crystalline compounds in samples were identified by XRD.The results showed that kaolinite,SiO₂ and bentonite can capture sodium effectively.In the sodium capture temperature range from 810^oC to 1000^oC,the amount of the sodium captured of the bentonite is highter than that of kaolinite and SiO₂ and that of kaolinite,which is highter than SiO₂.The amount of the sodium captured by kaolinite at 810^oC?0.10?g/g?is equal to that of kaolinite at 900^oC.When the sodium capture temperature increased to 1000^oC,the amount of the sodium captured by kaolinite increased to 0.14?g/g.With the sodium capture temperature increasing,the amount of the sodium captured by SiO₂ first increased from 0.02?g/g to 0.05?g/g,and then decreased to 0.04?g/g.With the sodium capture temperature increasing,the amount of the sodium captured by bentonite increased from 2.16?g/g to 2.33?g/g.It is the main way of the sodium capture of kaolinite,SiO₂ and bentonite that NaCl retention was achieved through the chemical reaction between the NaCl and SiO₂,Al₂O₃ presenting in the minerals.In addition,the active site in the surface of kaolinite and SiO₂ has a certain physical adsorption capacity to capture sodium.