Sodium Distribution When Burning Zhundong Coal

Fossil fuels have long been the basis of industrial production.China's oil and natural gas reserves are relatively small and coal reserves are relatively large.Coal is of great significance to China's industrial production,social and economic development.the proven Zhundong coalfield that has a rich reserves?reserves of nearly 400 billion tons?is an excellent industrial coal having been making an important contribution for the coal supply our country.However,Zhundong coal has high sodium content.On the one hand,Zhundong coal volatilizes large amounts of sodium-containing substances in the combustion process,the gaseous sodium-containing substances react with other substances in the flue gas to generate new alkali metal salts,with the decline of flue gas temperature,eventually forming the deposition of sodium chloride,sodium sulfate and other sodium-containing in the boiler heating surface,further capturing fly ash and resulting in a a serious stain.On the other hand,excessive sodium content in Zhundong coal will cause more sodium-containing minerals to be formed in the coal ash.Some of the sodium-containing minerals have lower melting points,and some will lead to the occurrence of low-temperature eutectic in the coal ash system,reducing the melting point of ash.Therefore,it is of certain reference significance to solve the problem of serious fouling and slagging in the combustion process of Zhundong coal by studying the distribution law of sodium-containing substances in the process of burning high-alkali coal.In this paper,the distribution regularities of the gas-phase sodium-containing material in flue gas and solid-phase sodium-containing material in ash,during the combustion of high alkali coal,are studied through the thermodynamic equilibrium calculation,chemical reaction kinetics calculation and experiment,The thermodynamic equilibrium calculation was used to study the distribution and influencing factors of gas-phase sodium-containing substances in flue gas at different working conditions.The results show that the main existing forms of sodium-containing material in mid-temperature region are NaCl,Na₂S0₄ and NaHSO₄,and the main exists form,in high temperature region,is NaCl.The sodium chloride,a water-soluble sodium compound,precipitated during the combustion of high-alkali coal,can be sulfated to form sodium sulfates under the action of sulfurous substances.In the middle temperature region,the sulfation rate is the highest,with the increase of temperature,the sulfation rate gradually decreased.The increase of flue gas excess air ratio,the increase of SO₂ concentration and the increase of H₂O content all promoted the sulfation ratio to a certain extent.The kinetics of chemical reaction was used to calculate the sulfation process after NaCl precipitation in flue gas under different working conditions.The reaction of each elementary unit in the system was analyzed by ROP method.The results show that the gas phase NaCl precipitated during the combustion of high-alkali coal is mainly sulfated by the formation of two precursors,NaHSO₄ and NaSO₄,and the two precursors of sodium sulfate can be generated through multiple pathways.Increasing the excess air coefficient,coal sulfur content,coal moisture content can promote the sulfation process of NaCl,while increasing the flue gas temperature would greatly reduce the sulfation ratio of NaCl,which was mainly due to the increase of the decomposition rate of sodium-containing intermediate in high temperature region and the reduction of NaCl sulfation pathway.In order to study the distribution of sodium compounds in high-alkali coal ash.high-alkali coal ash with different Na₂O content was prepared and heated in a tube furnace.The X-Ray Diffraction?XRD?and Scanning Electron Microscopy?SEM?are used to tested and observed the mineral composition and microstructure of synthetic ash.The results show that,with the increase of Na₂O mass fraction,the sodium content in the ash is gradually changed from albite to nepheline,the calcium content is gradually changed from anorthire to wollastonite and gehlenite.The low temperature eutectic system formed by mineral such as nepheline,wollastonite,gehlenite will produce a large number of liquid phase to reduce the ash melting point;SEM results show that,with the increase of sodium mass fraction,the pores between the particles continue to decrease,and the surface change from coarse to smooth,which indicate that the increase of Na₂O can promote the generation of the melt phase in the system and then increasing the slagging of coal ash.