Preparation And Properties Of Ti₃AlC₂ Containing Al₂O₃-C Refractories For Coal Water Slurry Gasifier

China is a country rich in coal but poor in oil and gas.Coal will remain the main source of energy in China for a long time in the future,and water gasification technology is an important way to realize the efficient and clean utilization of coal resources.The key equipment in the coal water slurry gasification process is the gasifier.Its lining refractory needs to work under complex environments such as high temperature and high pressure,reducing atmosphere and liquid acid ash.Besides,it must withstand the high-speed scouring of solid-liquid-gas flow,meaning that the service conditions are extremely harsh.High chromium refractories are regarded as the preferred materials for gasifier working lining because of their excellent resistance to gasifier slag erosion.However,there is a risk of hexavalent chromium pollution in the production and use of high chromium bricks.Therefore,it is urgent to develop chromium-free refractories for coal water slurry furnaces.Traditional Al2O3-C refractories have excellent corrosion resistance to metallurgical slag.However,the carbon in the composition and SiC formed after high-temperature heat treatment are unstable in the high-temperature water vapor environment in the coal water slurry gasifier,which is easy to be oxidized,resulting in structural degradation and performance failure of the materials.In the field of high temperature ceramics,Ti3AlC2 is known as a lamellar compound with "self-healing" oxidation repair ability.A dense Al2O3 layer will be formed on its surface during air or water vapor oxidation to prevent further oxidation of Ti3AlC2 materials;Secondly,Ti3AlC2 has a lamellar structure like graphite with high toughness,which can be used as a reinforcing and toughening phase to form composites with other materials.Therefore,the introduction of Ti3AlC2 into Al2O3-C refractories can improve the water vapor oxidation resistance and thermal shock stability of the materials and is expected to replace high chromium bricks as refractories for coal water slurry gasifiers.However,considering the complex atmosphere(CO/N2)in the preparation of Al2O3-C refractories,the introduced Ti3AlC2 phase powder will undergo oxidative alteration.Besides,it is considered that Ti3AlC2-Al2O3 multiphase material has higher mechanical properties and oxidation resistance than single-phase Ti3AlC2 material.The introduction of Ti3AlC2-Al2O3 into Al2O3-C refractories will have better chemical compatibility with the matrix.So far,Ti3AlC2-Al2O3 powder is usually prepared by thermit reduction method.The proportion of Ti3AlC2/Al2O3 in the product varies with the choice of carbon source,which will also affect the evolution of the structure of Al2O3-C refractories.However,the reaction process of thermit reduction method is complex,the formation mechanism of Ti3AlC2 has not been unified and clear,which affects the optimization of product phase composition.Moreover,whether Ti3AlC2 can play its "self-healing" oxidation and repair ability in sintered Al2O3-C refractories at high-temperature water vapor environments needs to be further verified.To solve the above problems,considering that Ti3AlC2 and typical carbon source of Al2O3-C refractories graphite have similar layered structure,and the high oxidation activity of Al atoms in Ti3AlC2 layered structure is similar to that of aluminum powder introduced into Al2O3-C refractories,the effect of Ti3AlC2 replacing metal Al powder and flake graphite on the properties of Al2O3-C refractories was investigated firstly to reveal the evolution of Ti3AlC2 in the preparation of Al2O3-C refractories.Secondly,titanium carbide and carbon black were used as carbon sources to investigate the effects of heating-up profiles and raw material composition on preparation of Ti3AlC2-Al2O3 composite powder by thermit reduction method.And reveal the reaction process of different carbon source reaction systems and the formation mechanism of Ti3AlC2 at high temperatures.On this basis,the effect of pre-synthesized Ti3AlC2-Al2O3 composite powder with different alumina content on the structure and properties of Al2O3-C refractories was studied,and its evolution and action mechanism in Al2O3-C refractories were revealed.Finally,the structural and performance changes and coal ash erosion resistance of Ti3AlC2 containing fired Al2O3-C refractories under high-temperature water vapor were systematically investigated.The interaction mechanism between Ti3AlC2 containing matrix and high temperature water vapor and coal ash was also revealed,which provided a theoretical basis for the practical application of Ti3AlC2 containing fired Al2O3-C refractories in coal water slurry gasification atmosphere.Based on the above research works,the following main conclusions can be drawn:1.In the preparation process of Al2O3-C refractories with Ti3AlC2 replacing aluminum powder,due to selective oxidation of Al atoms in Ti3AlC2 structure Ti3AlC2 was gradually oxidized to form Ti3Al1-xC2/layered TiC at 800～1200℃,and then oxidized to Al2TiO5 and other ceramic phases at 1600℃.Compared with the pore diameter coarsening during hightemperature gasification of alumina powder,a proper volume expansion during Ti3AlC2 oxidation promoted the densification of matrix structure.This inhibited the escape of Si(g)and SiO(g)at 1600℃,thereby enhancing the formation of SiC whiskers at high temperatures and significantly improving the strength and oxidation resistance of the material.2.In the preparation process of Al2O3-C refractories with Ti3AlC2 replacing flake graphite,Ti3AlC2 oxidized at 1200～1400℃ to form a core-shell structure composed of internal porous/layered TiC and external active Al2O3 layer.At 1600℃,the active Al2O3 layer reacted with TiO2 formed by oxidation of internal TiC to form Al2TiO5,while reacted with SiO2 from the matrix to form mullite,and eventually formed a TiC-Al2TiO5-Mullite multilayer core-shell structure.Compared with the increased brittleness caused by plenty of SiC ceramic phases formed by high temperature alteration of flake graphite,the above coreshell structure played a toughening role towards crack deflection in the process of material fracture and eventually increasing the thermal shock resistance.3.The thermit reduction reaction system of Ti3AlC2-Al2O3 with carbon black as carbon source shows obvious dissolution and precipitation mechanism.Compared with the diffusion reaction mechanism of TiC as carbon source system,the nucleation temperature of Ti3AlC2 is higher,but the high-temperature conversion is faster.Combining the steps heating-up profile and controlling Al-excess in the raw material composition,the formation of intermediate products is improved and the conversion of Ti3AlC2 at high temperature is further promoted.For carbon black as the carbon source system,the purity of Ti3AlC2 in the synthetic product at 1400℃ reaches the highest 91.5 wt%.4.The stability of high-alumina Ti3AlC2-Al2O3 composite powder(～60 wt%Al2O3)synthesized with carbon black as carbon source in Al2O3-C refractories was higher than lowalumina Ti3AlC2-Al2O3 composite powder(～30 wt%Al2O3)synthesized with TiC as carbon source.Therefore,a more compact multilayer core-shell structure is formed by alteration of high-alumina Ti3AlC2-Al2O3 composite powder at high temperature,which is conducive to improving the strength and thermal shock stability of Al2O3-C refractories5.For the Al2O3-C refractories with TiAlC2 powder,the matrix and decarburization layers were more compact.The diffusion of water vapor was restrained,due to the formation of TiC-Al2TiO5-Mullite core-shell structure.By adding high-alumina Ti3AlC2-Al2O3 composite,a denser core-shell structure was formed,the water vapor oxidation resistance was promoted,resulting in the highest area of non-oxidized layer at 70.1%.Compared with the decreased coal ash corrosion resistance of Al2O3-C refractories caused by decarburization under high temperature water vapor,the slag resistance of Ti3AlC2 containing Al2O3-C refractories was improved due to its better oxidation resistance on inhibiting the formation of decarburization layer.