Study On The Effect Of Coal Gasification Slag Composition On High-temperature Permeability Behavior Of High-chromium Bricks

There are many kinds of coal in China,which has an irreplaceable position in the economic energy system.As an important clean coal gasification technology,coal-water slurry gasification technology has been widely used in China.Because of being short of high-quality coal,the development of low rank coal for coal gasification technology and the exploration of the high temperature effect of different components of coal slag on refractory materials at high temperatures are of great significance.Based on the statistics and analysis of coal components in China,this work designs different Na₂O content and high alkali coal slag.Under the influence of complex oxidation-reduction atmosphere and scouring airflow in coal gasifier,the interaction between refractory materials and slag at high temperatures was investigated in order to pave the way for use of high Na₂O content and high alkali coal for coal gasification technology.In this experiment,different Na₂O contents?1%,2%,4%,6%,8%?and high alkali slag were prepared,while three experimental atmospheres were designed?oxidized atmosphere,strong reducing atmosphere,weak reducing atmosphere?.The high temperature effect of different components of coal slag on high chromia materials was studied.The results were compared with the results by thermodynamic software simulation.The results show that the melting temperature of coal slag decreases first and then increases with the increase of Na₂O content.This is because when the Na₂O content is low,it would ionize O2-,which destroys the network structure in the coal slag.When the content of Na₂O is too high,Na₂O would form NaAlO2 with Al2O3,the accumulation of NaAlO2 causes an increase in the melting temperature of the slag.Because the contents of CaO and Fe2O3 is too high,the high alkali slag would form calcium orthosilicate,which could increase the melting temperature of the coal slag.The viscosity of coal slag decreases with the increase of Na₂O content.And the viscosity of high alkali residue is the lowest.This is because the viscosity of the slag is mainly provided by the network structure formed by SiO2 and Al2O3.Na₂O could form Na+and O2-'.Na+,Ca²⁺and Fe²⁺would destroy the network structure,resulting in a decrease in viscosity.The results of corrosion experiments in oxidizing atmosphere show that the corrosion of coal slag on high-chromia bricks is not strong,but the slag penetration phenomenon is obvious.The penetration effect of coal slag with different Na₂O content on high chromia bricks is not obviously different.High alkali slag has the deepest penetration depth into high chrome bricks.The depth of penetration is affected by the viscosity of the coal slag and the rate of formation of the spinel.According to SEM-EDS analysis,the surface of the high chromia brick was dissolved in corrosion test.The MgO and Fe2O3 in the slag react with Cr2O3 to form spinel,which destroys alumina-chromia solid solution structure.Then the AI2O3 in surface layer dissolves into the slag.The degree of reaction increases with increasing Na₂O content.According to the thermodynamic simulation,the formation of zirconia-removed layer in the slag-high chromia brick system is found.The low Na₂O slag does not react with the high chromia brick to form the spinel phase,which mainly exhibits penetration in the test.In high Na₂O slag corrosion test,it founds that the slag reacts with the brick to form spinel phase in which MgCr2O4 and FeCr2O4 account for the majority.The spinel phase effectively hinders the penetration of the slag,resulting in a poor correlation between the penetration distance of the high Na₂O slag and its viscosity.Spinel layer is also formed when high alkali slag erodes bricks.Due to the lower viscosity,high alkali slag can penetrate into the brick before the formation of the spinel layer.The results of corrosion experiments under strong reducing atmosphere show that the slag has obvious corrosion on the high chromia brick,while corrosion and penetration are more severe as Na₂O content increases.High alkali slag has the longest penetration distance in the brick.Under strong reducing atmosphere,the penetration depth of cinder is mainly related to the viscosity of coal slag.According to SEM-EDS analysis,the contents of CaO and Fe2O3 remained basically unchanged in the working surface with the increase of Na₂O content in the slag.The contents of Al2O3 and SiO2 decreased,but the contents of Cr2O3 and Na₂O increased at the same time.After the high alkali slag erodes the high chrome brick,the content of Fe2O3 on the working surface increases significantly.This is because slag reacts with the high chromia brick to form a spinel layer so that alumina-chromia solid solution in the reaction layer is destroyed under strong reducing atmosphere.Na₂O would increase the degree of reaction and reduce the viscosity of the slag,enhancing slag penetration.According to the results of thermodynamic simulation,coal slag can react with high-chromia brick to form spinel,such as MgCr2O4 and Cr3O4.The Fe2O3 in the slag is reduced to iron.Cr3+1 in bricks is reduced to Cr²⁺ For high Na₂O slag and high alkali slag,the formation of Cr²⁺ and spinel is more than in low Na₂O slag.Cr²⁺ can only exist under strong reducing atmosphere.In the process of cooling with the furnace,Cr²⁺ would be oxidized to Cr3+ which reacts with FeO to form FeCr2O4 in the reaction layer.This layer would not hinder the penetration of slag.The results of corrosion experiments under weak reducing atmosphere show that the permeability of coal slag is greatly enhanced by the combination of weak reducing atmosphere and impinging gas flow.The depth of slag penetration increases with the increase of Na₂O content.The high alkali slag has the deepest penetration depth.Under weak reducing atmosphere,the penetration depth is related to the viscosity of slag and the rate of formation of spinel.The viscosity of coal slag is the main influencing factor.According to the SEM-EDS analysis,Fe2O3 is converted into FeO in the slag.FeO would reacts with Cr2O3 to form FeCr2O4,which destroys the alumina-chromia solid solution in the reaction layer.The content of spinel increases with the increase of Na₂O content.According to the thermodynamic simulation results,all three slags react with high-chromia bricks to form a spinel phase.The reaction-generated spinel is mainly MgCr2O4 and FeCr2O4.The dense spinel layer forms at the interface of the slag and the high chrome brick.