Study On Ash Slag Flow Behavior And Mechanism During The Co-gasification Of Coal And Coal Indirect Liquefaction Residue

Entrained-flow gasification technology is considered as the most promising gasification technology because of its clean and efficient utilization characteristics.The stable fluidity of slag at high temperature is the key to affect the long-period operation of gasifier.The diversity and difference of coal ash-slag systems are difficult to meet the requirements for stable slagging in entrained-flow gasifiers.Therefore,coal blending or adding fluxes have been used in industry for a long time to improve the flow behavior of coal ash.In essence,the coal ash flow behavior is controlled by changing the chemical composition of feedstock coal ash.As a by-product of indirect coal liquefaction process,indirect coal liquefaction residue（ICLR）is a kind of industrial solid waste,which is usually disposed of by stacking or landfilling.However,this disposal method will not only occupy land resources,but also cause serious pollution to soil and water bodies by leachate containing toxic and harmful metals.As a carbon containing matrix,ICLR is not only a kind of waste,but also a kind of energy substance.Utilizing existing industrial gasifier to blend combustion ICLR can not only transform industrial solid waste into fuel,but also save coal resources.Moreover,the ICLR usually contains unique ash chemical composition different from coal,which will affect the slagging performance of the gasifier.Therefore,exploring the effect of the ash addition in ICLR on the coal ash flow behavior can not only improve the slagging performance and gasification efficiency of entrained-flow gasifier by using the unique ash chemical composition of ICLR,but also provide some theoretical support for the large-scale consumption of industrial solid waste.In this work,the effects of ICLR addition on the coal ash melting characteristics and viscositytemperature characteristics were investigated by using ash melting point tester,high-temperature rotational viscometer off-line technique and high-temperature stage coupled with optical microscope system（HTSOM）on-line technique.X-ray diffraction（XRD）experimental characterization combined with FactSage thermodynamic software calculation method was employed to explore the influence of mineral evolution on the ash melting process during the heating process of ash slag.The surface morphology and elemental distribution of slag were characterized using scanning electron microscopyenergy dispersion spectrum（SEM-EDS）.The influence mechanism of slag structure on the coal ash slag fluidity was expounded from multiple angles and scales by the combination of molecular dynamics simulation and Raman spectroscopy experimental characterization,and the relationship between the evolution law of slag short/medium range microstructure and macroscopic flow behavior was discussed.The main research contents and conclusions are as follows:（1）Based on ash melting point tester,HTSOM,XRD,SEM-EDS,and FactSage thermodynamics software,the effect of the addition of iron-rich liquefaction residue on the coal ash slag melting and viscosity-temperature characteristics was investigated.The formation of mullite phase with skeleton structure at high temperature was the main reason for the high melting temperature of coal ash.According to the analysis of mineral evolution,it was found that the introduction of iron could inhibit the formation of refractory mullite phase in high silicon aluminum coal ash and promote the formation of lowtemperature eutectic substances（such as hercynite and fayalite）.thus leading to the continuous reduction of ash melting temperature.According to the analysis of ash melting mechanism.iron will transform ash melting behavior change from "softening-melting" mechanism to "melting-dissolution" mechanism.T_mullite（the temperature when mullite was the only solid phase in the molten slag）was defined based on the phase diagram calculated by FactSage thermodynamic software,and the correlation analysis between T_mullite and flow temperature（FT）found that there was a good linear relationship,which could be expressed as FT=1.35*T_mullite-336.94.Based on mineral transformation,interface crystallization and slag structure,the effects of iron on the crystallization behavior and viscosity-temperature characteristics of coal ash slag were systematically investigated.The increase of iron content could destroy the network structure and reduce the polymerization degree of coal ash slag.The decrease in the polymerization degree of slag results in lower viscosity and stronger ion transport ability,which will promote the formation of crystal nucleus and crystal growth.The calculation of mineral evolution by FactSage thermodynamics software and the calculation of mineral molecular orbital by quantum chemistry theory indicated that the crystals precipitated from molten slag during the cooling process were anorthite crystals.（2）The influence of silicon-rich liquefaction residue addition on the crystallization behavior and viscosity-temperature characteristics of coal ash slag was investigated by high-temperature rotary viscometer and HTSOM,and the influence of abnormal crystallization phenomenon of slag on viscosity fluctuation behavior was revealed.The formation of a large number of "barbed-shape" needle-like crystals during the cooling process was the main reason for the viscosity fluctuation of coal ash slag.When the addition amount of silicon-rich liquefaction residue was 1 wt%.the decrease of length-diameter ratio and the volume fraction of "barbed-shape" needle-like crystals reduced the coal ash slag viscosity fluctuation behavior.When the addition amount of silicon-rich liquefaction residue was 3 wt%.the "barbed-shape"needle-like crystals no longer precipitated and the viscosity no longer fluctuated during the entire crystallization process.As the mass fraction of silicon-rich liquefaction residue increased to 5 wt%,no crystals precipitation was observed in the cooling process of coal ash slag,and it indicated the glass slag characteristics,which met the liquid slagging requirement of gasifier.Based on mineral analysis,it was found that these "barbed-shape" needle-like crystals belong to the anorthite crystals.（3）The simulation calculation method combining molecular dynamics（MD）simulation and ring statistics was employed to investigate the influence of iron element on the local structure of[SiO4]4tetrahedron and[AlO₄]^5-tetrahedron atomic groups,as well as the ring-forming structures between[SiO₄]^4-tetrahedron and[AlO₄]^5-tetrahedron structure in coal ash slag.Meanwhile,the relationship between the distribution of Qn structural units and ring structures in slag and its viscosity-temperature characteristics was elucidated.The high silicon and aluminum content in coal ash led to the formation of complex[SiO₄]^4-tetrahedron and[AlO₄]^5-tetrahedron structures at high temperature,and the[SiO₄]^4- tetrahedron and[AlO₄]^5-tetrahedron were connected by oxygen atoms to form a multi-membered ring structure with high polymerization degree.Due to the action of multi-membered ring structure,the internal friction in the slag increased,and the viscosity value was higher on the macro-level.As a network modified ion,Fe²⁺could replace Si⁴⁺and Al³⁺ in the multi-membered ring structure and combine with O^2-,which will destroy the bridge oxygen（BO）structure and transform more complex tricluster oxygen（TO）and bridge oxygen（BO）into simple non-bridge oxygen（NBO）structure.As a result,a large number of multi-membered rings with high polymerization degree were depolymerized into low-membered rings with low polymerization degree.The evolution of oxygen types and ring structures in slag reduced the structure complexity and polymerization degree of coal ash slag,resulting in a decrease in the viscosity of coal ash slag.By further analyzing the correlation between the Qn structural unit and viscosity,it was found that there was a quadratic function relationship between Q⁴ proportion and viscosity,which could be expressed as μ=0.025（Q⁴）²-Q⁴+12.03.In addition,the ring distribution coefficient（Rdc）was further defined,and it was found that there was a linear relationship between Rdc and viscosity,which could be expressed as η=8.98R_dc-22.63.