The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully in...The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer(φ0.1 m×6.5 m).Since binary particles are used in this system,a reasonably validated 3 D,transient,multi-fluid model,in which three heat transfer modes relating to the convection,conduction and radiation are considered,is adopted to simulate the flow behavior,temperature profiles as well as volatile contents.The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region.In the further downward section of the downer,the particle flow disperses near the right wall and develops uniformly.Meanwhile,the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode.To explore the heat transfer mechanism more quantitatively,two indexes(mixing index and contacting index)are proposed,and it is found that the mixing index initially increased fast and later remained at a relatively flat state.For the contact index,it shows a trend with a first rising and then falling,finally rising continuously.Also,it is found that the convection heat transfer is closely correlated to the contacting status of gas-coal which indicates that the improving of the gas-coal contacting efficiency should be an effective way to strengthen the coal particle heating process.展开更多
Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua Unive...Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua University,and high-severity FCC.Moreover,aiming to promote industrial application,the fundamental studies are comprehensively described,particularly focusing on high-density downer reactors,clusters,and up-scaling.Furthermore,from the perspective of industrial application,some research directions toward further developments are suggested.展开更多
The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the sys...The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the system. This simulation achieved an averaged solid fraction in the bed as high as 18% in this operating regime. The flow development in high-density downer consists of 3 regions, which are first acceleration, second acceleration, and fully developed regions. In the fully developed region, the lateral distribution of the solid volume fraction is low and almost uniform in the center region with a high density peak near the wall region. Gas and solid velocities gradually increase toward the wall and form a peak near the wall region. In addition, the solid volume fraction, gas and solid velocities increase with solid circulation rate.展开更多
The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The...The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand,but also is affected by the inner structural design of the baffle internals in the downer reactor.As presented in the previous publication of the author,the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier,and increase the mean residence time of the coal and heat-carrier particles in the downer reactor.The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length,a 30°baffle-slope angle and at least four baffles designed in the downer reactor,which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.展开更多
Riser has the advantage of high gas-solids contact efficiency,high gas/solids flux and so on.But there is relatively significant gas and solids backmixing.On the ether hand,downer has the great advantage of uniform ga...Riser has the advantage of high gas-solids contact efficiency,high gas/solids flux and so on.But there is relatively significant gas and solids backmixing.On the ether hand,downer has the great advantage of uniform gas and solids residence time, but the entrance structure has great influence on its performance and the solid concentration is much lower than that in riser.A new type of Riser-Downer-Coupling Circulating Fluidized Bed (RDCCFB) is devised in this research, which is a close combination of riser and downer.This new type of CFB takes advantage of both riser and downer.Phosphor particles were used as tracers to study the solid mixing behavior in a cold-model RDCCFB.The results show that the overall Peclet Number is greater than that in a single riser.And the average residence time and the residence time distribution of the particles can be changed according to the requirement.These characteristics make this coupling reactor attractive in many areas.展开更多
基金supported by the National Natural Science Foundation of China(U1710101)。
文摘The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer(φ0.1 m×6.5 m).Since binary particles are used in this system,a reasonably validated 3 D,transient,multi-fluid model,in which three heat transfer modes relating to the convection,conduction and radiation are considered,is adopted to simulate the flow behavior,temperature profiles as well as volatile contents.The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region.In the further downward section of the downer,the particle flow disperses near the right wall and develops uniformly.Meanwhile,the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode.To explore the heat transfer mechanism more quantitatively,two indexes(mixing index and contacting index)are proposed,and it is found that the mixing index initially increased fast and later remained at a relatively flat state.For the contact index,it shows a trend with a first rising and then falling,finally rising continuously.Also,it is found that the convection heat transfer is closely correlated to the contacting status of gas-coal which indicates that the improving of the gas-coal contacting efficiency should be an effective way to strengthen the coal particle heating process.
基金the funding of the project by SINOPEC (No. 120009)
文摘Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua University,and high-severity FCC.Moreover,aiming to promote industrial application,the fundamental studies are comprehensively described,particularly focusing on high-density downer reactors,clusters,and up-scaling.Furthermore,from the perspective of industrial application,some research directions toward further developments are suggested.
文摘The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the system. This simulation achieved an averaged solid fraction in the bed as high as 18% in this operating regime. The flow development in high-density downer consists of 3 regions, which are first acceleration, second acceleration, and fully developed regions. In the fully developed region, the lateral distribution of the solid volume fraction is low and almost uniform in the center region with a high density peak near the wall region. Gas and solid velocities gradually increase toward the wall and form a peak near the wall region. In addition, the solid volume fraction, gas and solid velocities increase with solid circulation rate.
文摘The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand,but also is affected by the inner structural design of the baffle internals in the downer reactor.As presented in the previous publication of the author,the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier,and increase the mean residence time of the coal and heat-carrier particles in the downer reactor.The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length,a 30°baffle-slope angle and at least four baffles designed in the downer reactor,which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.
文摘Riser has the advantage of high gas-solids contact efficiency,high gas/solids flux and so on.But there is relatively significant gas and solids backmixing.On the ether hand,downer has the great advantage of uniform gas and solids residence time, but the entrance structure has great influence on its performance and the solid concentration is much lower than that in riser.A new type of Riser-Downer-Coupling Circulating Fluidized Bed (RDCCFB) is devised in this research, which is a close combination of riser and downer.This new type of CFB takes advantage of both riser and downer.Phosphor particles were used as tracers to study the solid mixing behavior in a cold-model RDCCFB.The results show that the overall Peclet Number is greater than that in a single riser.And the average residence time and the residence time distribution of the particles can be changed according to the requirement.These characteristics make this coupling reactor attractive in many areas.
