Investigation Of Gas-solids Flow In A Cold Model Of A Dual Fluidized Beds Gasifiers By Electrical Capacitance Tomography Measurement And CPFD Simulation

Fluidized beds(FB)coal gasification process plays an important role in the filed of energy due to its high efficiency and good mixing between gas and solids phase.FB,as one of the gasification devices,have been successfully applied in the energy-related industrial sector due to their simple structure,easy operation and flexible fuel types.Dual fluidized beds gasifier(DFBG)is the extension and improvement of the FB,which gasification and combustion process are in two different reactors,i.e.gasification and combustion reactors,to better utilize of the fuel and improve the quality of syngas.In terms of reactor,there are different types of DFBG.Among them,the DFBG reactor with a riser as the combustion reactor and a bubbling fluidized bed(BFB)as the gasifier,is one of the common used reactors due to its excellent heat,mass transfer and high gasification efficiency.Compared with single reactor fluidized beds,the gas-solids flow characteristics in a DFBG are complex.In order to ensure the stable operation and better control,it is necessary to carry out experimental research and CFD simulation for the complex gas-solids flow characteristics in a DFBG system.Due to the non-invasive and non-intrusive natural,process tomography technology has many advantages compared with singe point based measurement.In this research,electrical capacitance tomography(ECT)combined with pressure transducers are used to investigate the gas-solids flow characteristics in a lab-scale DFBG system.A lab-scale DFBG facility with upper and lower U valves was built.Three ECT sensors were installed outside the wall of the bottom of the riser,the bottom of the BFB and the lower U-valve.In addition,twelve pressure transducers were installed on the DFBG to monitor the pressure drop in different positions in the DFGB system.Different operation conditions,namely the gas velocity in the riser,the gas velocity in the BFB,the aeration velocity in the lower U valve and the inventory and type of the bed material,were conducted to investigate the gas-solids flow behavior in the DFBG.Time and frequency domain analyses were employed to the ECT signals.The result indicates that ECT is a good option to monitor the gas shortcut and defluidization behavior in the DFBG system.The main flow patterns in the riser of the DFBG and the independence of the riser and BFB were verified.The results also indicate that the pressure balance of the DFBG was mainly affected by the gas velocity in the BFB.Based on the experimental measurement,the computational particle flow hydrodynamics(CPFD)accompanied based on a commercial platform Barracuda~TMM was used to investigate the detailed flow characteristics in the lab-scale DFBG system.In the CPFD model,the Wen-Yu/Ergun drag model was employed to study the gas-solids behavior in the system.The CPFD simulation results indicate that the particle distribution in the DFBG has high concentration in the lower section and low concentration in the upper section.The main pressure loss is in the bottom of the riser and BFB.In addition,the gas-solids flow in the riser,the BFB and the recycle chamber of the lower U-valve was studied and the bubble behavior and particle internal circulation was summarized.The results indicate that the operation of the lower U-valve is affected by the gas velocity in the riser and this may lead to the gas shortcut.The CPFD simulation results has a good agreement with ECT measurement.In terms of process monitor,ECT is more suitable for long-term monitoring and CPFD can provide detailed information for the gas-solids flows in a DFBG system.