The Study Of Cr₂O₃-Al₂O₃-ZrO₂ Refractories And Chromia-free Refractories For Coal Water Slurry Gasifier

In order to meet the demand of chromia-bearing refractory reduction and chromia free refractories for coal water slurry gasifier,the service failure mechanism of Cr₂O₃-Al₂O₃-Zr O₂ brick in coal water slurry gasifier was systematically studied,and the effect of Cr₂O₃ aggregates coated by Zr O₂ layers structure on its thermal shock resistance was studied.The proven mechanism of the thermal shock resistance and corrosion resistance point the way to effectively increasing the service life span of Cr₂O₃-Al₂O₃-Zr O₂ brick.Thermodynamic equilibrium state of reaction between typical refractory compositions（Cr₂O₃,Zr O₂,Al₂O₃,Mg O,Mg Al₂O₄,Mg Cr₂O₄,Si O₂,Si C and Si₃N₄）and coal slag are analyzed by software Factsage.Moreover,reaction mechanism between coal slag and high-zirconia bricks,fused AZS bricks,magnesia chrome brick,Si C bricks etc.various refractory productsare analyzed by combining thermodynamic calculations and static crucible slag test method,which provides a theoretical basis for the research of chromia free refractories.The conclusions based on the above researchs is following:The change of phase and chmical composition of Cr₂O₃-Al₂O₃-Zr O₂ brick before and after used for coal water slurry gasifiers wasanalysed by x-ray diffraction（XRD）and chemical analysis,and the microstructure evolution of Cr₂O₃-Al₂O₃-Zr O₂ brick before and after use was studied by SEM and EDS.The results show that the liquid slag intrudes into Cr₂O₃-Al₂O₃-Zr O₂ brick along the pores,and the zirconia in Cr₂O₃-Al₂O₃-Zr O₂ brick melts into slag to form areaction dezirconia layer and melting holes,which leads to reduction or even disappearance of Zr O₂,increased porosity and bigger pore size,thus making slag penetration more serious.The reaction dezirconia layer consists of Cr₂O₃ and composite spinel composition.Meanwhile,the Fe O_n and Mg O in slag react with Cr₂O₃ and Al₂O₃ in Cr₂O₃-Al₂O₃-Zr O₂ bricks to form a（Mg,Fe）（Al,Cr）₂O₄composite spinel erosion reaction layer.Due to the difference of thermal and physical properties between the corrosion reaction layer and the original brick layer,the stress concentration and cracks occur under the temperature fluctuation,resulting in structural spalling and damage.Cr₂O₃ with Zr O₂ coating aggregates were synthesized by mixing Zr O₂ powders and Cr₂O₃ aggregates with a phenolic resin binder followed by thermosetting treatment were employed as modified Cr₂O₃ aggregates to obtain Cr₂O₃-Al₂O₃-Zr O₂ bricks.The effect of zirconia-coated Cr₂O₃ aggregates on thermal shock resistance of Cr₂O₃-Al₂O₃-Zr O₂ brick is investigated.Because porous Zr O₂ coating layer wraps modified Cr₂O₃aggregate,forming more thermal stress crack deflection and longer free path of crack propagation would result in increased crack propagation energy consumption.Therefore,the crack propagation is effectively prevented and the thermal shock resistance of Cr₂O₃-Al₂O₃-Zr O₂ brick is improved.According to the measurements,the variation trend of stability parameter of thermal stress crack R_st is consistent with trend of residual of cold modulus of rupture（CMOR）and residual ofelastic modulus（E）.Because thermal stress crack propagation is the main factor for the failure of Cr₂O₃-Al₂O₃-Zr O₂ brickunder thermal stress,the effective methods to improve the thermal shock resistance of Cr₂O₃-Al₂O₃-Zr O₂ brick are increasing the fracture work and reducing the elastic modulus.The standard Gibbs free energy of the reaction between refractory compositions and slag is calculated by software Factsage.Thermodynamic equilibrium state phase of the reaction between refractory compositions and slag at 1300℃and 1500℃is analyzed.The reaction results between typical refractory composition and coal slag are researched.The results are as following:（1）due to its excellent chemical stability,chromia（Cr₂O₃）has better slag corrosion resistance;（2）zirconia（Zr O₂）also has good chemical stability,but a small amount of zirconia（Zr O₂）melt loss into the liquid slag;（3）because spinel materials（Mg Al₂O₄ and Mg Cr₂O₄）have poor chemical stability,they could react with coal slag to form feldspar,pyroxene and other low melting phases,which is melted loss into liquid slag;（4）Mg O,Al₂O₃ and Si O₂ have the worst chemical stability,so they are easy to react with compositions in coal slag to form low melting phase;（5）Silicon carbide（Si C）and silicon nitride（Si₃N₄）only react with Fe O_n in coal slag at high temperature to form a certain amount of Si O₂ and Fe-Si alloy,which has excellent chemical stability to low iron contained coal slag in reducing atmosphere.Thermodynamic equilibrium state of coal slag and refractories is calculated by software Factsage.Compared with Cr₂O₃-Al₂O₃-Zr O₂ brick,the slag corrosion resistance of four typical refractories is investigated using static crucible method.The results are as following:（1）Cao,Si O₂ and Fe O in coal slag accelerate the melting loss of Zr O₂ grains in high zirconia brick,so the key to improve the slag corrosion resistance of high zirconia brick is to reduce the porosity and slag penetration;（2）the fused AZS brick reacts with slag to form a reaction layer of（Mg,Fe）Al₂O₃ composite spinel and feldspars,and its dense structure can effectively resist the penetration of coal slag;（3）Mg O in magnesia chrome brick reacts with coal slag to form low melting phases,which results in poor slag corrosion resistance of magnesia chrome brick;（4）coal slag has poor wettability to silicon carbide（Si C）brick and Si O₂ formed by the reaction of Fe O_nand Si C increases the viscosity of the slag,as well as Si O₂ dense protective layer is formed by bricks surface oxidation,which make Si₃N₄ bonded Si C brick have excellent slag corrosion resistance.