Study On The Corrosion Mechanism Of Refractory Materials For Coal-water Slurry Gasifier

In this paper,dynamic corrosion experiments of high-chromium refractories,magnesium-aluminum-zirconium refractories and SiC-based refractories refractories interacted with the acid and basic coal slag were conducted in the improved the rotary drum furnace under simulated conditions of slagging gasifier.The corrosion mechanism was investigated by combining thermodynamic simulation and SEM analysis.The oxygen partial pressure in gasifier is analyzed according to the gas composition of typical coal gasifier.The mixture gas of C2H2 and O2 is used as fuel gas in the rotary slag resistance furnace.The flow ratio of C2H2 and O2 needed for simulating the atmosphere of coal gasifier was determined by atmosphere calibration test.The results show that when the flow ratio of C2H2/O2 is controlled to 45/55 by a high-precision flowmeter,the iron oxide in the slag is mainly FeO.The partial pressure of oxygen calculated in the furnace is also within the range of the actual partial pressure of oxygen in the gasifier so as to form an atmosphere environment consistent with the actual gasifier.Through further experimental verification,the microstructure of the corroded refractory is very similar to that of used refractories for the actual coal water slurry gasifier.Therefore,by improving the corrosion experiment method,the dynamic state,temperature and atmosphere can be controlled accurately,and the operating conditions of gasifier can be simulated more accurately.The corrosion behavior of high chromium refractories by the acid and basic coal slag was studied by using the improved corrosion test method.The results show that a continuous and dense spinel solid solution layer is formed on the surface of high chromium sample after acid slag corrosion,while the spinel solid solution layer formed from the surface of the high chromium sample to the inner gradually changed from a discrete shape to be dense after the basic slag corrosion,and the penetration depth is higher.Based on thermodynamic calculations,it was found that,when the high chromium sample is corroded by the two different characteristics of coal slag,the intrinsic formation mechanism of the spinel solid solution layer and the permeation layer is similar.FeO and MgO in the slag reacted with(Al,Cr)2O3 to form spinel solid solution on the surface.Solid-state counter-current diffusion process of M2+(Mg2+,Fe2+)from the slag and Al3+,Cr3+ from the(Al,Cr)2O3 through the spinel layer were conducting.At the interface of slag/spinel solid solution on the surface of the sample,the concentration Al2O3 and Cr2O3 are low due to the amount of slag is large,and the spinel solid solution is dissociated and dissolved into the slag.The amount of slag infiltrated into the pores is small,and the Cr2O3 and Al2O3 concentrations of the slag gradually increase.At the interface of the slag/spinel solid solution in the pores,the spinel solid solution gradually grows in the pores,and the matrix become dense.At the interface of the spinel solid solution/(Al,Cr)2O3 solid solution,Fe2+ and Mg2+ diffused from the spinel solid solution layer reacted with(Al,Cr)2O3 to form a spinel solid solution.A steady-state thickness of the spinel is achieved when the rates of growth and dissolution become equal.The formation and dissolution of the spinel layer are carried out continuously.(Al,Cr)2O3 solid solution indirectly dissolves in the slag through the spinel layer.In the permeation layer,there is no obvious reaction between the saturated slag and the matrix.This process of dissolution-precipitation mass transfer makes the grains in the matrix smooth;the porosity decreases,and the structure become denser.The dense structure makes the mechanical and thermal properties of the penetration layer different from the original brick layer.The corrosion results of magnesia-aluminum-zirconium refractories by acid coal slag and basic coal slag are obviously different.The results of microstructural analysis combining thermodynamic calculations showed that,during the process of acid coal slag and basic coal slag corrosion,Fe2+ in molten slag cannot only react with periclase and MgAl2O4 spinel at the interface like other ions,but also Fe2+ can diffuse into crystal of periclase and MgAl2O4,to form solid solution,and make it gradually produce a liquid phase and dissolve.During the acid coal slag corrosion process,the dissolution rate of periclase is higher than MgAl2O4,which results in the formation of a concave microscopic morphology in the area where periclase was located,the flow rate of slag on the surface of periclase becomes slower,and the mass fraction of MgO in the slag increases.The thermodynamic condition for the formation of Mg2SiO4 was satisfied.A denser reaction layer was formed on the surface of the sample,which was composed of(Mg,Fe)Al2O4 solid solution and Mg2SiO4 solid solution,and inhibited the infiltration of the slag into the sample,which showed that the slag was almost impermeable in the sample.Periclase is indirectly dissolved into the slag through Mg2SiO4 solid solution;MgAl2O4 dissolves directly into the slag.However,during the process of basic slag corrosion,the dissolution rate of periclase and MgAl2O4 solid solution in basic slag is very high.Periclase and MgAl2O4 dissolved directly in the slag;the structure of periclase and MgAl2O4 particle in the sample is destroyed.A discrete reaction layer formed on the surface of the sample,and the slag permeates into the sample.At the same time,it is difficult to reach the thermodynamic conditions to form a solid phase in the process of slag penetration,and the viscosity of the basic slag is very low,and the slag penetration in the sample is relatively more obvious.The corrosion process of acid slag to SiC and Si3N4-SiC refractories is similar,and there are few reactions and thin permeable layers on the surface of the samples.In the test condition,the sample undergoes active oxidation.The cristobalite formed during the reaction dissolves into the slag,and the formed gas diffused to the surrounding area at the same time.At the interface of slag/sample,SiC,Si3N4 reacts with most of FeO in the slag,and forms C,Si,Fe alloy on the surface of the sample.The activity of residual FeO in the slag is very low and no longer occurs redox reaction with the sample.In addition,the oxidation reaction changed the structure of the pore surface,but the penetration of the sample was very weak due to the poor wettability of the slag to the SiC and Si3N4-SiC samples.Moreover,the cristobalite produced by the oxidation increased the slag viscosity,which lead to very weak penetration of slag in the sample.Both magnesium-aluminum-zirconium,SiC and Si3N4-SiC refractories have shown good resistance to coal slag corrosion in this study.At the same time,the corrosion mechanism has been elucidated in combining with thermodynamic calculations.The results will provide powerful experimental data and important foundation for the selection,design and development of chrome-free refractories for coal-water slurry gasification technology.