Thermodynamic Analysis And Heat Transfer Numerical Simulation Of Waste Plastics Gasification By Terminal Waste Heat In Steelmaking Plant

The high-temperature waste heat in the process of iron and steel industry mainly exists in steel,waste gas and molten slag.However,0.75billiontonne metallurgical slag with nearly1500℃is produced annually,corresponding to 40milliontonne coal theoretically calculated by 100%heat recovery.At present,the heat recovery rate of slag treatment process is relatively low,resulting in waste of heat resources.Therefore,it is of great significance to establish an economical and environmental slag waste heat efficient recovery network system for energy conservation and consumption reduction in the iron and steel industry.This paper analyzed the current situation of physical and chemical methods for slag waste heat recovery at home and abroad through key parameters such as heat recovery rate and granulation performance.Based on the shortcomings of existing processes,a set of waste plastic-flue gas-slag waste heat recovery system was proposed.Combined with the practice of steelmaking,the thermodynamic mechanism of waste plastic gasification with steel slag waste heat to prepare hydrogen-rich syngas and reduce carbon dioxide in converter flue gas was studied,and the numerical simulation of fluidization heat transfer of the system in bubbling fluidized bed reactor was carried out.The main research contents and results were as follows:（1）Gibbs free energy minimization method was used to study the effects of process parameters such as temperature,pressure,H₂O/Plastic（P）,CO₂/P and Steel Slag（SS）/P on syngas preparation efficiency and carbon dioxide emission reduction depth.Moreover,the effects of waste plastics and gasification agents amount on heat balance of the reaction system when preparing methane or methanol by terminated product quality regulation were discussed.The results showed that:（1）H₂,CO yield and CO₂conversion rate increased as temperature increased but decreased as pressure increased,H₂yield was promoted by H₂O/P ratio instead of CO yield and CO₂conversion rate,and CO₂/P ratio had a positive effect on CO yield except for H₂yield and CO₂conversion rate.（2）Chlorine-free plastics consumption increased as temperature increased,and less gasification agent was need.（3）This system had the maximum syngas yield consisting of 44.82-64.13%H₂and 88.88-135.69%CO when temperature,pressure,H₂O/P ratio,CO₂/P ratio and SS/P ratio were 800-1000℃,1.0atm,1-2.0,1.2-3.0 and 1.0,respectively,and as the aspect of CO₂reduction efficiency,the capacity of 75.17-90%can be obtained when 800-1000℃,1.0atm,0-0.8,0.4-1.2 and 1.0 were adopted,respectively.（2）The fluidization heat transfer characteristics of the system in a bubbling fluidized bed reactor were studied by numerical simulation.The volume fraction of steel slag,the velocity vector distribution of flue gas-steam,the variation laws of bubbles and pressure under different flue gas-steam velocity were investigated.The temperature distribution of the system was further investigated under the optimal flue gas-steam velocity.The results showed that:（1）The bubbling fluidized bed reactor was in the state of scattered fluidization,bubbling fluidization,surge saving fluidization and turbulent fluidization when the flue gas-steam velocity was 0.4m/s,0.6m/s,0.8m/s and 1.0m/s,respectively.（2）The velocity vector distribution of flue gas-steam at the bubble was dense.The larger the velocity of flue gas-steam,the more disordered the velocity vector distribution and pressure distribution.（3）The flue gas-steam temperature increased as the steel slag initial temperature,and it can reach the peak at 0.16m.（4）The heat exchange in the steel slag accumulation area at the bottom of the bubbling fluidized bed was strong,the temperature of flue gas-steam and waste plastic rose rapidly,and the waste plastic particles can reach the required temperature for gasification at the bottom of the fluidized bed.Therefore,the bubbling fluidized bed is suitable as a gasification reactor for the waste plastic-converter flue gas-steel slag waste heat recovery process.