Experimental Study On The Heat Transfer Characteristics In Process Of Waste Printed Circuit Boards Pyrolyzed By Flowing High Temperature Gas

With the acceleration of electronic product updates,the number of waste printed circuit boards?PCBs?as an important component of electronic products is increasing.Therefore,more and more attention is paid to the recycling of waste PCBs.Pyrolysis technology which has the advantages of high recovery rate of metal and nonmetal,high recovery purity and low risk of secondary pollution has become a main way to recover and process waste PCBs.However,the bottleneck of the pyrolysis technology is how to achieve high efficient heat and mass transfer,increase the effective thermal utilization and pyrolysis rate in the pyrolysis process and optimize the design and operation of pyrolysis equipment.The study on the above-mentioned problems is of great research value and practical significance for batch processing of waste PCBs.In this paper,the heat transfer characteristics of FR4 waste PCBs pyrolyzed by high temperature gas in the fixed bed was focused on research.A pyrolysis experimental device was designed,and temperature distribution of the material layer composed by different sizes particles in pyrolysis process was compared experimentally.The energy consumption of the pyrolysis system was analyzed from the viewpoint of energy balance,which can provide reference for the design and optimal operation of pyrolysis PCBs equipment with high temperature gas.It was also helpful to improve the effective thermal utilization rate of the pyrolysis process.In addition,the pyrolysis products were collected to obtain a mass distribution,and the components of the solid,liquid and gaseous products are analyzed,which provided basic data on the utilization of pyrolysis products.The results are as follows:vertical migration velocity of the pyrolysis zone in the large-size material layer moved faster than the small-size material layer.The material layers were accumulated by different particles whose side length were1.5cm,2.5cm and 3.5cm,and the pyrolysis zone migration velocities along the height of furnace in the material layer were 0.57m/h,0.61m/h and 0.68m/h,respectively.The entire material layer heated by the high temperature gas was fully pyrolyzed.The temperature difference between center and the half of radius was small in the same height in furnace,and material particles in the middle of the same horizontal plane were in the same pyrolysis state.Thanks to the heat dissipation of the furnace wall the temperature rising and pyrolysis reaction of the material particles beside the furnace wall was later than the middle in the same horizontal plane.The penetration trend of pyrolysis zone first appeared in the middle,and then appeared around in material layer,which moved upward along the height of material layer during the experiment,until the entire material layer was completely pyrolyzed.Thermal utilization rate was low through the heat balance analysis in the experiment,and the highest value of thermal utilization rate was 37.17%.The main heat losses were the heat absorbed by the pyrolysis system equipment and the heat taken away by carrier gas.The particle sizes had little effect on the mass distribution of the solid,liquid and gas produced by pyrolysis of waste PCBs.Solid products were mainly composed of glass fiber containing elements such as silicon,oxygen,calcium,carbon and copper foil.Pyrolysis oil composed of C₆^<sub>21 organics mainly contained phenols and esters substances,and the relative molecular weight of each ingredient was between 94³08,while Methyl bromide,ethylene oxide,2-methyl-1-propene and other substances that are flammable were included in the non-condensable pyrolysis gas.