| Coal gasification has been introduced,absorbed and innovated as a key technology for clean,high value and efficient coal utilization in China based on its own energy structure characteristics.Cr2O3-Al2O3 refractories have been used as the lining of coal-water slurry gasifier for over 40 years.There is still no new material system invented to challenge Cr2O3-Al2O3 refractories.Under the financial support from NSFC,SiC-oxide composite type have been recently developed as promising chromia-free refractories.To further improve key performances,i.e.,the strength at elevated temperatures,slag corrosion resistance and oxidation resistance,of the SiC-oxide composites,Al powder was added to adjust the phase composition and micro structure of SiC-MgAl2O4-Al composites in this work.The influence of temperature and atmosphere during heat-treatment on the phase composition and microstructure,and thus on the key performances of composites was investigated.The degradation mechanism of the prepared SiC-MgAl2O4-Al composites was studied via dynamic slag corrosion testing under mimic gasification conditions.The effect of phase evolution of added Al on the microstructure of SiCMgAl2O4 spinel composites was verified.The XRD analysis and SEM observation indicated that Al reacted with N2 forming AlN,Al ON and(Al2OC)x·AlN(1-x)solod solution whiskers,locating in pores in the composites.The strengthening mechanism of Al in SiC-MgAl2O4-Al composites was studied.It can be deduced that AlN formed from interactions between Al and N2,reacted with MgA2O4 spinel forming MgAlON,and with 2H SiC generating solid solution by SEM,EDS,XRD and thermodynamic analysis.The in situ formed whiskers bonded with both SiC aggregates and MgA2O4 spinel particles,thereby increasing the strength of the composites.The underlying mechanism of improved oxidation resistance of SiCMgAl2O4-Al composites was explored.The results from oxidation tests of SiCMgAl2O4 revealed that AlN preferentially reacted with oxygen compared with SiC,thereby suppressing the oxidation of SiC aggregates.Additionally,the in situ generated whiskers formed micro-sized pores in the composite matrix,retarding the impregnation of oxidizing gases into bulk composites,and thus enhancing the oxidation resistance.The formation mechanism of dense MgAl2O4 spinel layer at the slag/composite interface and the influence of this MgAl2O4 spinel layer on the corrosion resistance of SiC-MgAl2O4-Al composites were clarified.Microstructure analyses of specimens from cup slag tests and thermodynamic calculations indicated that,Al-based non-oxide constitute reduced the pO2 in the composite,increasing the pMg and thus favoring the mass transportation of Mg vapor to the slag/composite interface.The Mg gas was oxidized at the slag/composite interface and interacted with Al2O3,forming a dense layer consisting of MgAl2O4 spinel.As a result,the slag infiltration resistance of composite was improved.The microstructure development and its influence on the strength of SiCMgAl2O4-Al during heat-treatment was studied.The phase composition,microstructure and properties of samples heat-treated at 200～1 600℃ under N2 were attained.Results indicated that the strength of heat-treated samples augmented with the increased amount and grain size of Al-O-C-N whiskers.When the heattreatment temperature was lower than 1000℃,no Al-O-C-N whiskers were generated,because oxide layer on Al particle surface hindered the interaction of Al with N2,resulting in the lower strength.When composites were heated at 1300～1550℃,Al-O-C-N whiskers grew up,increasing the bonding level between composite matrix and SiC aggregates,and favoring the formation of solid solution with MgAl2O4 spinel.The composite strength was,therefore,enhanced.Once the firing temperature reached at 1600℃,the vapor pressure of Mg in composite increased.Mg gas was involved in secondary MgAl2O4 spinel formation,and enhanced the conversion of whiskers into plate-like MgAlON,thereby decreasing the composite strength.Hence,the adjustment of phase composition and microstructure of SiC-MgAl2O4-Al composites can be attained by modifying the phase composition,morphology and amounts of Al-O-C-N whiskers via controlling the heat-treatment temperature.The improvement in strength of SiC-MgAl2O4-Al composites can be,therefore,achieved.The correlation between atmosphere during heat-treatment,phase composition and microstructure of phases in Al-O-C-N quaternary system,and strength of composites was identified.The size effect of SiC-MgAl2O4-Al composites during heat-treatment under N2 was investigated.Results illustrated that Al would be partially consumed to form a thicker oxide layer by the relatively high pO2 at low heat-treatment temperature in coke bed,whereas more Al could diffuse easily to react with N2 forming Al-O-C-N whiskers since the pO2 is low and constant under N2 atmosphere.A large number of Al-O-C-N whiskers with larger grain size was,consequently,generated in composites heat-treated under N2 atmosphere.As a result,the strength of composites prepared under N2 atmosphere was twice higher than that obtained in coke bed.When composites were prepared with larger size,a dense layer was generated at the surface of the sample because of the presence of Al,blocking the inward penetration of N2.Al in the bulk composites would react with residual carbon formed from decomposed resin to generate Al4C3,leading to the generation cracks of the samples because of Al4C3 hydration.The phase composition,morphology and amounts of Al-O-C-N whiskers could be modified via adjusting the atmosphere during heat-treatment.The hydration of Al4C3 in bulk composite can be,therefore,avoided and the strength of composite at high temperature can be enhanced.The underlying degradation mechanism of SiC-MgAl2O4-Al composites in coal-based slag was identified.The mechanism of improvement in slag corrosion resistance via Al addition was verified.The results from rotary slag resistance test under mimic gasification conditions demonstrated that,interfacial reactions between SiC-MgAl2O4-Al composites and coal-based slag occurred at the slag/composite interface.SiC in the composite was oxidized by FeOx from the slag and MgAl2O4 spinel dissolved into the bulk slag at the slag/composite interface.AlN and SiC at the composite surface were oxidized and then combined with MgAl2O4 spinel,forming viscous Al2O3-MgO-SiO2 glass phase.Simultaneously,Mg vapor transferred from bulk composite to the slag/composite interface,forming a dense layer consisting of MgAl2O4 spinel.This newly-generated dense MgAl2O4 spinel layer and viscous Al2O3-MgO-SiO2 glass phase could form a "self-healing’physical barrier at the slag/composite interface.As a result,the penetration of molten slag and gas into bulk composite was retarded,resulting in the improvement of slag corrosion resistance of SiC-MgAl2O4-Al composites.Based on the above results and properties comparison with Cr2O3-based refractories,it can be summarized that SiC-MgAl2O4-Al composites possess better comprehensive performance,and are environment-friendly.SiC-MgAl2O4-Al composites are,therefore,believed to the be most promising Cr2O3-free material candidates used in lining gasifiers.Industrial trials are,however,deemed necessary to investigate the oxidation resistance of SiC-MgAl2O4-Al composites during gasification. |
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