CFD Modeling Of Biomass Decoupling Combustion In Dual Fluidized Bed System

Dual fluidized bed decoupling combustion is a technology of efficiently utilizing biomass wastes with high contents of moisture and nitrogen(spirits lees in this study).The system contains a bubbling fluidized bed pyrolyzer and a riser combustor.The raw spirits lees are continuously fed into the pyrolyzer through a screw feeder for drying and pyrolysis.The produced biomass char is then transported into the riser combustor for combustion.The pyrolysis gas is fed into the riser so as to reduce the generated NOx during combustion and decrease the emissions of NOx.In this technology,it is critical to guarantee that in the pyrolyzer the raw spirits lees and heat carrier ash are well mixed.Meanwhile,the residence time of spirits lees is long enough to ensure the sufficient progresses of drying and pyrolysis.At the same time,the combustion efficiency of biomass char and the reduction of NOx are decided by the fluidization characteristics of gas and solids in the riser.With regard to these requirements,in this work Computational Fluid Dynamics(CFD)simulations are conducted to investigate the hydrodynamic characteristics of the combustor and riser so as to guide the future scale-up and further optimization of this technology.The main contents and conclusions of this work are as follow:(1)Mixing/segregation behavior of binary particle mixture in bubbling fluidized bed.In Chapter 2,the influences of modeling parameters settings on the CFD simulation results are systematically investigated and discussed through comparing the simulation and experimental data.It is found that the predicted mixing/segregation behavior is closely related to the boundary wall condition,particle-particle restitution coefficient,frictional viscosity,as well as the transport equation for granular temperature.The analysis of flow structure shows that the size of gas bubble and particle vertical velocity play critical roles on the mixing extent of the binary mixture.For small-scale bubbling fluidized bed,the side walls suppress the size increase of gas bubble and hinder the vertical movement of particles.Such wall boundary effects lead to the results that mixing behavior of particle mixture is clearly different from that in large-scale bubbling fluidized bed.The simulation results suggest that there exist a critical bed depth(around 40 particle diameter)larger than which the predicted mixing behavior is independent from wall boundary conditions.Such results provide theoretical guidance for future experimental design on investigating the scale-up effect on particle mixing behavior in rectangular bubbling fluidized bed.(2)Solids residence time distribution(RTD)in rectangular cross-flow bubbling fluidized bed.In Chapter 3,the influences of bed height,solids flux and superficial gas velocity on solids residence characteristics have been studied.The simulation results show that,in the investigated rectangular cross-flow bubbling fluidized bed,solids residence time is closely related to solids inventory and solids flux.Further data analysis indicate that,through proper data processing,the descending parts of solids RTD profiles obtained from different operation conditions can be uniquely fitted by an empirical exponential function with only one parameter.And the parameter value is independent from operation conditions.Based on such result,a semi-empirical approach for predicting solids RTD is then proposed for the first time in the literature.In this approach,the ascending part of solids RTD profile is obtained through CFD simulation,and the descending part is given by the fitted empirical exponential function.This semi-empirical approach is then used to investigate solids RTDs in rectangular cross-flow bubbling fluidized beds with different sizes and validated by experimental data.Further investigation shows that this semi-empirical approach can also be applied to binary particle mixture system.It is worth noting that,due to the long tail nature of solids RTD in cross-flow bubbling fluidized bed,predicting solids RTD through CFD simulation is extremely time-consuming.The development of the above mentioned semi-empirical approach thus makes it be possible to predict the RTDs of solids in industrial-scale bubbling fluidized beds with a continuous particle flow and provides a feasible approach to design the pyrolyzer of the dual fluidized bed decoupling combustion system.(3)CFD modeling of the bubbling fluidized bed pyrolyzer.For further optimization of pyrolyzer,in Chapter 4 the influences of geometrical structure of pyrolyzer and the gas in the gas inlet conditions on the hydrodynamic and thermal-transfer characteristics are investigated.Simulation results show that increases the slope of pyrolyzer side wall leads to the strengthening of mixing between heat carrier ash and spirits lees,and the decreasing of average residence times of both solids.When the strategy of non-uniform gas distribution at gas inlet is adopted,increasing the gas velocity at the side close to spirits lees inlet is in favor of the mixing of heat carrier ash and spirits lees.It is found that,though the two solids are nearly well mixed,their residence time distributions present clear difference.And the average residence time of heat carrier ash is larger than that of spirits lees.The analysis of the heat transfer properties indicates that the thermal energy is mainly transferred through gas-solid inter-phase heat transfer.Meanwhile,the heat transfer rate between gas and ash is far more than that between gas and spirits lees.(4)CFD modeling of the riser combustor.In Chapter 5,the influences of pyrolysis gas inlet conditions,solids flux and the size of ash particle on gas mixing and particle flow characteristics are systematically investigated so as to guide the future optimization of the operation conditions in the riser.Simulation results demonstrate that in the riser the solids flow is in a very sparse pneumatic conveying state with the average solid volume fraction less than 0.002.Due to the rectangular cross-section characteristics,there exist obvious boundary wall effects and a layer of down-flow particles is formed at the cross-section corners.It is found that the distribution of pyrolysis gas tends to be uniform with the increase of bed height.For given total gas flux,increasing the pyrolysis gas flux leads to fast evening of cross-section distribution of pyrolysis gas.When the ratio of of pyrolysis gas flux to primary gas flux is larger than 0.144,the solids vertical velocity distribution is similar to that of pyrolysis gas.Whereas,if the ratio is smaller,the solids flow behavior is dominated by the primary gas.The simulation results also show that the solids residence time increases with the increases of solids fluxes and solids diameter.