Characteristics Of Fine Slag In Entrained Bed Gasification And Interaction Mechanism Between Carbon And Ash In Slag

Gasification technology is a key technology in the modern coal chemical industry chain and an important approach to achieving carbon peak and carbon neutrality goals.However,gasification processes based on coal feedstocks are affected by the characteristics of carbon-containing ash,and a large amount of carbon-containing ash not only reduces carbon utilization efficiency but also affects the stable operation of gasifiers.In particular,the interaction between residual carbon and ash inside the gasifier directly leads to changes in the flow state of the ash,which in turn affects slag formation and slag discharge systems within the gasifier.However,due to the complex process of residual carbon formation,the fundamental research on the formation mechanism,morphology,and impact on the gasification processes of fine slag has been insufficient for a long time.Therefore,this paper proposes a research topic on the characteristics of fine slag in entrained bed gasification and the interaction mechanism between carbon and ash in slag.Experimental samples of industrial gasification fine slag were collected and modern analysis techniques,such as XPS,TEM,FESEM,and M（?）ssbauer spectroscopy,were used to elucidate the characteristics of fine slag and the formation path of residual carbon.Further research on melting characteristics,mineral evolution,and phase transformation processes was conducted to reveal the relationship and mechanism between high-temperature carbon and ash.The main research work and conclusions are as follows:（1）By analyzing the scale characteristics of industrial gasification fine slag in terms of particle size,composition,and structure,the physical intrinsic morphology of fine slag and the formation characteristics of carbon-containing and inorganic components were obtained.The particle size distribution of fine slag particles shows a"triple-peak distribution"characteristic,and the particle morphology is mainly composed of irregular particles and spheroidal particles.The shape of irregular particles includes accretion type,dense type,porous type,and folded type,exhibiting characteristics such as high-carbon and high-iron particles.There are a large number of spheroidal particles in the fine slag,and their formation pathway is as follows:melting of ash matrix in coal,liquid phase condensation,internal nucleation,layer-by-layer stacking,"bridging"and aggregation between small particles,and growth.Residual carbon has an obvious aromatic structure and exhibits crystal structures in the form of grains,layered and fibrous structures,reflecting relatively long and regular,regional short crystal lattice patterns,as well as complex and disordered lattice patterns,indicating that residual carbon is composed of different structures of carbon matrix.The inorganic components of fine slag are in a glassy state,mainly composed of silicon,aluminum,calcium,and iron.The iron phase mainly exists in Fe₃C and Fe²⁺-glass phases.（2）To tackle the complex problem of residual carbon formation in slag,a coal-based residual carbon tracing concept is proposed,which is based on the consistent CO₂reactivity of residual carbon and raw coal.The formation pathway of residual carbon is proposed by establishing the corresponding relationship between residual carbon and carbon structure evolution under multiple environmental conditions.Residual carbon in the slag mainly originates from the vitrinite and inertinite groups of coal,with the latter being more prone to residual carbon formation.With changes in temperature,time,and gas atmosphere,the relatively active areas of the coal coke react with CO₂,and the remaining residual carbon becomes increasingly ordered and gradually tends towards graphitization.The formation pathway of residual carbon is summarized as follows:（1）Coal particles flow with gas in the gasifier,and due to their short residence time,mainly consist of unreacted carbon structures;（2）The relatively active carbon in coal participates in the reaction,while a small amount of less active carbon is subjected to pyrolytic aggregation or is encapsulated by slag to form carbon-containing ash;（3）Carbon-containing free radicals in coal particles highly aggregate during pyrolysis and gasification,forming carbon with a higher degree of graphitization,which then remains as residual carbon in the slag.（3）The relationship between residual carbon and inorganic components at high temperatures is significant,especially in terms of mineral transformation,melt structure,and phase transition,accompanied by physical structure dissolution and collapse phenomena.The types and content of residual carbon have varying effects on the coal ash melting process,with higher residual carbon content resulting in"collapse characteristics"during the melting process and weakening of the melting coalescence characteristics.Residual carbon accelerates the process of gypsum decomposition and reacts with minerals containing iron,silicon,and other elements,reducing iron to metallic form and generating Fe₃C and Fe₃Si.The types and amounts of generated products are controlled by the concentration of free ions in the system,which includes competition between different ions.The introduction of residual carbon leads to a trend of"reduced liquid phase generation and destruction of liquid phase equilibrium"during phase transition processes.Residual carbon can strip some of the Fe and Si elements from the liquid phase,form new aggregated mineral states,destroy the co-melt structure,and exhibit a reaction history of"dissolution-melting-reaction."（4）In the Si O₂-Al₂O₃-Ca O-Fe₂O₃-C system,the mechanism of the role of residual carbon is revealed from the perspectives of minerals,solid-liquid phase,and melts.（1）Excessive residual carbon has a direct effect on the system with temperature changes;（2）Residual carbon reacts directly with silicon-containing and iron-containing minerals,changing the evolution process of minerals;（3）Residual carbon participating in the reaction also destroys the structure of the melts,disturbs the system equilibrium,changes the solid-liquid-gas phase transformation process,increases the temperature range for liquid phase generation,increases the gas phase generation,and leads to expansion of the slag under the synergistic action of gas-liquid phase.The stability of industrial processes can be evaluated based on the state of industrial ash,and the impact of residual carbon on the melting process of coal ash can be reduced by optimizing the furnace temperature or controlling the composition of coal ash.The research findings contribute to enriching the scientific theories of the intrinsic structural characteristics of fine slag,the formation of residual carbon,and the high-temperature interactions between carbon and ash.They further guide the rational disposal of fine slag and the reduction of the impact of residual carbon on gasifier operation in practical processes,thereby promoting coal utilization efficiency and facilitating the long-term stable operation of gasification equipment.Figure[109]Table[25]Reference[153]...