Experimental And Numerical Study On Characteristics Of Coal Pyrolysis-semicoke Combustion Based Pressurized Grade Conversion In A Dual Fluidized Bed

Pressurized coal grade conversion technology in dual fluidized bed has the characteristics of simple process equipment,mild reaction conditions,high energy conversion rate and environmental friendliness.It can be used for efficient and clean utilization of coal resources.However,a comprehensive understanding of the complex characteristics of pyrolysis or combustion,such as product yields and pollutants emission varied with pressure is not available so far,which causes problem in system design,process optimization and scale up.These problems make it more difficult to design,amplify and optimize the reactor for pressurized double fluidized bed coal classification conversion,as well as to select and adjust the operation parameters.Firstly,three different types of coal were selected,and the basic physical properties of coal and semi-coke were compared,and the experimental raw materials were analyzed and evaluated.According to the evaluation indexes of ignition point,cohesive index,tar content,coal rock composition and ash characteristics,Heilonggou coal was selected as the raw coal of this subject.On this basis,the processes of coal pyrolysis and semi-coke combustion were analyzed by thermogravimetric mass spectrometry?TG-MS?and pressurized thermogravimetric analysis.The releasing characteristics of pyrolysis gas during coal pyrolysis and the combustion characteristics of semi-coke were studied.Furthermore,the reaction kinetics of coal pyrolysis and semi-coke combustion under pressure ranging from 0.1 MPa to 0.5 MPa was studied.To study the fundamental properties of coal and its semi-coke produced under different conditions?e.g.pyrolysis temperature and pressure?,a bench-scale experimental system of coal pressurized grade conversion was set up to investigate the influence of pyrolysis temperature?500,550,600,650 and 700??,pressure?0.1,0.2,0.3,0.4 and 0.5MPa?,atmosphere?N₂ and coal gas?and particle size?0-3mm,0-6mm?on the pollutant emissions,pyrolysis production yield and ash properties.Coal gas analyzer,flue gas analyzer,gas chromatography analyzer,gas chromatography-mass spectrometer?GC-MS?,N₂ adsorption,scanning electron microscopy?SEM?and X-ray diffraction analyzer were used to study the effects of these parameters on the oil and gas characteristics of pressurized coal grade conversion process,the emission of pollutants from semi-coke combustion and the surface morphology and composition of related solid products such as semi-coke and ash.It was found that the pyrolysis tar had the highest production yield at the temperature of 600?,and under the gas atmosphere,pressurization is helpful to increase the production of tar and semi-coke,as well as the quality of tar and coal gas.With the increase of pressure,the emission of CO and CO₂ increases,while the emission of NO and SO₂ decreases.Based on the fundamental research,a pilot-scale dual fluidized bed platform with a capacity of50kg/h was built and used to study the fluidization,transportation,pyrolysis and combustion of coal and semi-coke under different conditions,in which spout-fluid bed was used as pyrolysis reactor while fast fluidized bed was used as semi-coke combustor.The yield and composition of coal pyrolysis products?tar,gas and semi-coke?at different temperatures,pressures and particle sizes were studied.The characteristics of semi-coke at different temperatures,pressures,primary and secondary air ratios and pollutant emissions at different particle sizes were investigated.Based on the pilot-scale dual fluidized bed platform,a three-dimensional Eulerian-Lagrangian mathematical model of coal pyrolysis in a pressurized spouted fluidized bed and semi-coke combustion in a pressurized fluidized bed was established by using multiphase particle-in-cell?MP-PIC?method.Based on the analysis of pilot-scale,the industrial scale reactor with the capacity of 500,000tons per year was designed.The final structure sizes of the two reactors were determined by studying the effects of the structure parameters.On this basis,the effects of operating pressure,mass ratio of semi-coke to coal,initial bed height and particle size distribution on the operation characteristics of the reactor were investigated.Guiding suggestions were given for the industrial application of pressurized coal grade conversion technology.