Numerical Simulation And Experimental Study On Thermal Plasma Melting Furnace For Municipal Solid Waste Incineration Fly Ash

In recent years,as China’s urbanization rate continues to rise,the total amount of urban household waste is huge,reaching 235 million tons in 2020.Incineration technology has gradually replaced sanitary landfill as the main treatment technology for municipal solid waste.However,waste incineration produces a large amount of fly ash,which contains heavy metals and dioxins that pose potential risks to the environment and human health.The thermal plasma melting waste incineration fly ash technology is considered as an environmentally friendly and efficient treatment technology due to its high volume and weight reduction rate,complete harmlessness,and generation of resource-usable vitreous slag.However,there are some problems in the treatment of municipal solid waste incineration fly ash by thermal plasma technology,such as difficulty in monitoring,high energy consumption and high requirements for refractory materials.With the rapid development of numerical simulation and the improvement of finite element analysis software,the method of numerical simulation is used to optimize the design of plasma melting furnace,which can achieve the goal of cost saving and optimization of the solution.In this study,based on the thermal plasma melting municipal solid waste incineration fly ash process with a treatment capacity of 5 t/d,a three-dimensional numerical simulation of the gas-solid two-phase flow of waste incineration fly ash melting in a thermal plasma furnace was carried out using ANSYS Fluent software.The thermal plasma furnace structure and operating conditions were optimized to improve fly ash melting efficiency and reduce fly ash erosion on resistant materials,and the simulation results were experimentally verified.By introducing the transport equation,water evaporation boiling model,devolatilization model,heterogeneous phase interaction model,and gas-phase reaction model,a numerical model of plasma-fly ash-pulverized coal multiphase flow is established,and the melting of pulverized coal with fly ash in the plasma furnace was simulated numerically.The effects of coal mixing content and pulverized coal particle size on the temperature field,velocity field,concentration field and fly ash melting effect in the thermal plasma furnace were investigated to optimize the coal blending conditions and improve the comprehensive utilization of resources.The physicochemical properties of municipal solid waste incineration fly ash and slag generated after thermal plasma treatment were studied by XRF,SEM,XRD and other characterization means.The leaching characteristics of heavy metals and the distribution characteristics of dioxins were detected,and the toxic degradation effect and mechanism of thermal plasma on heavy metals and dioxins in fly ash were analyzed.Combined with thermodynamic simulation software Fact Sage,the migration and transformation law of heavy metals in fly ash during the melting process was revealed.It lays the foundation for the industrial application of thermal plasma treatment of waste incineration fly ash technology.The main research conclusions are as follows:（1）Simulation results of the optimization of the furnace structure and operating conditions of the thermal plasma melting furnace show that,when the horizontal angle between the fly ash inlet and the plasma torch is 60°,the angle between the flue gas outlet and the fly ash inlet is 145°,and the height of gas outlet is 3.385 m,the fly ash stays in the furnace for a short time,the exit escape rate is low,the trapping rate by the bottom of the furnace is high,and the plasma furnace melts the fly ash with high efficiency.When the molten liquid level of fly ash is below 200 mm,it can reduce the erosion of fly ash on the refractory layer in the plasma furnace and increase the service life of refractory materials.When the inlet velocity of fly ash is 3 m/s,the fly ash reaches the bottom in a short time,the escape rate is low,less work is done on the fly ash,and the power consumption is small.When the fly ash particle size is 160μm,it can not only ensure the effective heating of fly ash in plasma furnace,but also reduce the cost of post-treatment.Therefore,the fly ash particle size should be appropriately increased by pre-treatment methods such as granulated and bonded.The experimental validation study shows that,the temperature inside the plasma furnace is above 1800 K before the addition of waste incineration fly ash,and after the addition of fly ash,the temperature inside the furnace decreases by 43～786 K.The velocity of the gas phase inside the furnace overall increase.The simulation results are basically consistent with the experimental measurement data,and the established numerical model of gas-solid two-phase flow can simulate the thermal plasma melting waste incineration fly ash process well.（2）Research on the effect of coal blending shows that:when the mass ratio of pulverized coal to fly ash is 1:5,the temperature at the bottom of the plasma furnace and near the plasma torch increases,the temperature in the middle of the plasma furnace decreases,and the temperature at the top remains unchanged.It can meet the melting temperature of fly ash and reduce the burden of resistant materials.As the amount of coal blending increases,the velocity of the gas phase in the plasma furnace increases and the molar fraction of volatile fraction,CO₂,H₂ and CO increases.Pulverized coal precipitates volatile fraction at the bottom of the plasma furnace,and water gas shift reaction and methane water vapor reforming reaction occur at the top to produce H₂ and CO.With the increase of coal content,the capture of fly ash increases at the bottom of the plasma furnace,while the capture of fly ash decreases at the furnace wall,which has little influence on the escape rate,and the average running time of fly ash in the furnace decreases.The study on the effect of pulverized coal particle size shows that:the furnace temperature decreases with the increase of pulverized coal particle size,but the temperature at the bottom of the plasma furnace increases,which is conducive to the melting of fly ash at the bottom of the furnace.The change of pulverized coal particle size has little effect on gas velocity.The coal with smaller particle size is easily carried by the airflow in the furnace,which is more dispersed in the furnace,resulting in smaller concentration of volatile matter,H₂,CO,CO₂ in the furnace and larger distribution area.The running time of fly ash in the plasma furnace increases with the particle size of the pulverized coal,and the trap and escape rates remain constant.（3）Analysis of the physicochemical properties of municipal solid waste incineration fly ash and slag showed that,the particle size of fly ash was mainly distributed in the range of 83～220μm and formed a smooth and rigid slag with dense microstructure after thermal plasma treatment.The high-temperature melting by the thermal plasma caused the crystalline transformation of the fly ash and formed a glassy slag.Heavy metal leaching experimental research showed that,the leaching concentrations of Cr,Cu,Zn,Pb,Cd,Ni,As and Hg in municipal solid waste incineration fly ash was lower than the toxic leaching standard values in China,and Cr and As in fly ash were slightly soluble,while other heavy metals were insoluble.The leaching concentrations of Cr,Cu,Zn,Cd,Ni,and Hg in the slag formed after the thermal plasma melting were below the detection line,and the leaching concentration of Pb reduced by 80%,indicating that the vitreous structure in the slag has a good fixation effect on most heavy metals.And the leaching concentration of As increased by 11.6%,due to the formation of gaseous As O at high temperature,volatilized and condensed during the melting process and deposited on the surface of the glass slag,which is easier to leach under the action of leaching agent.The degradation study of dioxins showed that,the toxic equivalent concentration of dioxins in waste incineration fly ash was 1545.8 ng I-TEQ/kg,which was much higher than the limit value for Class I sites in soil（100 ng I-TEQ/kg）,and the contribution of PCDFs reached 82.93%.After plasma treatment,the dioxin toxic equivalent concentration in the slag reduced to 0.55ng I-TEQ/kg,and the contribution of PCDDs reached 56.53%,indicating that PCDFs are more susceptible to dechlorination and degradation than PCDDs under high temperature.The reduction of dioxin homolog in fly ash from municipal solid waste incineration by thermal plasma melting reaches more than 99%.Degradation through two pathways:dechlorination and decomposition.Thermodynamic simulation study showed that,heavy metals are easily formed with Cl to produce Zn Cl₂（g）,KZn Cl₃（g）,Cu Cl（g）,Cu Cl₃（g）,Pb Cl₂（g）,Hg Cl₂（g）,etc.Cl in waste incineration fly ash is an important factor affecting the precipitation of heavy metals in the gas phase,which drives the transfer of heavy metals to the flue gas.The oxides of Si,Mg,Fe,Al,Ca,etc.can serve to fix the heavy metals in the slag in the form of heavy metal salts.