Three-Dimensional Simulation Of Gas-solid Mixing Behavior In Thermal Plasma Reactor For Coal Pyrolysis

Coal pyrolysis in plasma for acetylene is efficient,clean and short in process.It is a new way for clean utilization of coal.The well performance of pilot project by Tianye shows that it is technically and financially feasible.In the process of reactor scale-up from 2MW to 5MW,we find that both the conversion of coal and concentration of acetylene are reduced.Meanwhile it cokes heavily in the mixing section,which stops the continuously working of reaction.Based on the analysis of thermodynamics,the initial mixing of gas and solid has a great effect on the process of coal pyrolysis and releasing volatile matter,then the conversion of coal and concentration of acetylene.As to this problem,firstly we built the cold state mathematical model of the gas-solid mixing process in the plasma reactor,and then we described gas-solid mixing behavior in the reactor by numerical simulation.The result shows that in the process of reactor scale-up,as the increase of scale and gas flow rate,the gas-solid mixing efficiency is quite low in the 5MW reactor compared with 2MW reactor.Coal failed to be well mixed the plasma gas in a short time,which led to the poor efficiency of reaction and seriously coked in the 5MW reactor.In order to find the factors to improve the gas-solid mixing behavior in the 5MW reactor,firstly we confirmed three ways to solve the problem:(1)Increasing the particle injecting depth in the reactor.(2)Increasing the contact aero of gas and solid.(3)Increasing the residence time of coal in the vertical gas.Then we made numerical simulations of different operating conditions.The results are listed below.1.Compared with circular nozzle,plat nozzle can provide higher penetrating power and more contact aero.2.Appropriate increase of injecting velocity can make particles inject deeper in the reactor and prolong residence time.When the velocity increased to 60m/s,particles penetrate through vertical gas and fail to be well mixed with gas.3.Increase of feeding rate may decrease gas-solid mixing efficiency.The result of 2MW experiment shows that concentration of acetylene increases firstly and reduces afterward with the increase of feeding rate.4.Particle size affects both the gas-solid mixing behavior and thermal conduction between the two phases in the plasma reactor.In the 5M W process,great particle size will strength mixing efficiency of gas and solid in the reactor.The structure of reactor also has a significant effect on the gas-solid mixing behavior.We made a preliminary numerical simulation to describe the particle movement in a venturi reactor,and then compared with the tubular reactor.We find that venture reactor consists of two parts.One is for mixing and the other is for reacting.Its special structure can increase particles injecting depth,prolong residence time and increase contact aero for the two phases.The result of 50kW experiment confirmed our simulation.At last we discussed the effect of reaction section scale.It shows appropriate scale can provide the two phases with optimal contacting time.