| Biomass pyrolysis liquefaction technology is acknowledged to be the most potential biomass energy development method.Three pyrolysis products,bio-oil,gas,char,have been identified as the high added-value products.At present,the related studies on biomass pyrolysis have achieved great progress,but most of them are confined to micro-reactor or bench-scale device.The development and optimization of fluidized bed pyrolysis reactor with the prospect of industrial application are mainly faced with two problems.One is the realization of efficient and stable fluidization of biomass particles.The fluidization characteristics of biomass particles are more complex than those of quartz sand and pulverized coal due to their irregular shape,more burrs on surface and higher water content.The existing fluidization results cannot be directly referred to.There are momentum,energy and mass exchanges between gas and solid phases in biomass pyrolysis fluidized bed.The flow state and heat and mass transfer process are extremely complex,and the related studies are not yet mature.Another problem is the parameters coupling relationship in fluidized bed pyrolysis.Difficulties arise in the accurate operation and stability control of fluidized bed pyrolysis reactor.Based on this,the present investigation aims to study the fluidization and pyrolysis characteristics of biomass particles.Firstly,a visualized fluidized bed test platform was established,and its gas distribution system was optimized by numerical simulation.Then,the main influencing factors of biomass particle fluidization were analyzed on the platform,and the improvements were suggested,and the particle separation behaviours in biomass fluidized bed pyrolysis process was also simulated.Finally,the characteristics of volatilization analysis,distribution and enrichment regularities of products during biomass fluidized pyrolysis were studied in a pilot plant of fluidized bed pyrolysis coupled with fractional condensation.The specific research of this paper is as follows:(1)Numerical simulation on gas distribution system of fluidized bed pyrolysis reactor.FLUENT was used to simulate the gas distribution system of fluidized bed pyrolysis reactor within the range of gas velocity of common biomass fluidized.Results indicated that the static pressure in all parts of gas chamber was basically equal when the gas flow in gas supply duct was adjusted by the diffuser,whereas the middle gas velocity at the bottom of gas chamber was inconsistent with the both sides.The gas velocity can be evenly distributed only at the position height above the critical wind chamber height,and the larger the inlet flow rate,the higher the critical wind chamber height.The resistance characteristics of gas cap were influenced by internal structure and fluizing medium characteristics.The internal structure of mushroom-shaped gas cap was relatively simple,and the local resistance mainly concentrated on the inlet and outlet parts.The resistance of the whole gas caps could be controlled easily by adjusting the size of the inlet and outlet of gas cap.The opening rate and inlet size of gas cap were negatively correlated with its resistance.The outlet holes arrangement of gas cap did not affect its resistance characteristics,but had a larger influence on flow field of the fluidied bed.Numerical simulations were performed on four air distribution panels equipped with mushroom-type caps(8 air outlets with two layers of misalignment).These results showed that the less air caps of distributor and the lower the opening rate,the greater the resistance of the distributor,and the resistance increased rapidly with the flow rate increasing.However,the resistance of gas distributor had no effect by the arrangement of gas caps on the plate.From the relationship between resistance and flow rate,it could be considered that the gas caps were equivalent to parallel to each other,but the arrangement of gas caps had a considerable influence on the flow field.Under the same opening rate,the triangular arrangement of the caps on gas distribution plate was more conducive to the uniform distribution of the flow field.Through the numerical simulation of gas distribution system,it was determined that the installation height Hg of gas distribution plate should be higher than 0.4 m.The gas distributor installed 37 gas cap(8 outlets with two layers of misalignment)in a triangle with the opening rate of 6.04%was used as the gas distribution system of biomass pyrolysis fluidized bed device.(2)Cold fluidization characteristics of biomassSelecting rice husk and walnut shell as biomass representatives,the fluidization characteristics of biomass were further studied from three aspects:particle size and distribution,initial bed height and biomass moisture content.Results indicated that the particle sizes of rice husk and walnut shell had a significant effect on their fluidization characteristics.The Gardert A and D of rice husk and walnut shell could not be fluidized.The former was related to van der Waals force between particles,and the latter was related to large bubbles in the bed.The fluidization behavior of Geldart B rice husk particles was similar to that of Geldart D particles,while that of Geldaty B walnut shell particles was similar to that of Gelart A particles.For the bed materials with wide particle size distribution,removal of fine powders belong to Geldart C could significantly improve fluidization quality.The initial bed height had negligible effect on critical fluidization velocity of biomass,but had a greater impact on the stability of bed.The critical fluidization velocities of rice husk and walnut shell particles both show S-shaped curves with water content,but the upper limits of those moisture content were different,16.2wt%and 26.3wt%respectively.The slow growth section on S-shaped curve was mainly related to the increase of particle density caused by moisture content increase,whereas the fast growth section was mainly affected by the liquid bridge force between particles.Biomass with high moisture content tended to agglomerate.In addition,biomass with low moisture content was easily adsorbed on bed wall due to electrostatic attraction during fluidization process.Subsequently,quartz sand with different particle sizes was selected as the fluid-assisted particles.The fluidization characteristics and mixing and separation behavior of the binary mixture of walnut shell and quartz sand were studied.Unlike single-component fluidization,there was a transition zone from fixed bed to fluidized bed of walnut shell/quartz sand two-component fluidization.In the process of walnut shell fluidization,when 60-80 mesh sand is selected as fluidizing assisted particles,the transition zone of binary mixture is the smallest and the fluidization state is more stable.Furthermore,the transition interval of binary mixtures decreased first and then increased with the sand ratio increasing,whereas the fluidization index increased first and then tended to be stable.Gas velocity has a great influence on the mixing and separation of walnut shell and quartz sand binary fluidization process.In the transition zone of fluidization,the separation degree of the two components increases with the increase of gas velocity.Howerver,the materials in the completely separated state at the end of the transition section are remixed again with gas velocity furtuering increasing within the fully fluidized section.The whole process of mixing and separation is mainly related to the growth,coalescence and rupture of bubbles.In the fluidization process,mixing and separation behaviors exist simultaneously and compete with each other.When the gas velocity is set,the two are in a state of dynamic equilibrium.(3)Study on particle separation during biomass fluidized bed pyrolysisIn order to simulate the entrainment and elutriation characteristics in a fluidized bed pyrolysis reactor under different opearting conditions,a mixed bed material model with different proportions of walnut shell,semi-char and clean char was established.Then,the experiment of the bed materials entrainment and elutriation was carried out on a cold fluidized bed.The results showed that,within the range of Ut,i-ut,f,it presents three flow patterns successively along the axial direction of the bed:dense phase bubbling zone at the bottom,intermediate separation subsidence zone and thin phase pneumatic conveying zone at the top.Depending on the gas velocity,the range of the three flow regions is different.There are two kinds of particle entrainment mechanisms in the bottom bubbling region:bubble top and bubble wake.Under the condition of high gas velocity,the ejection of particles is mainly controlled by the bubble wake,and at this time,in the freeboard,more coarse particles that originally settled at a low gas velocity and returned to the concentrated phase area will be "transformed" into elution particles to be carried out of the bed.With the increasing of gas velocity,TDH increases gradually.When the gas velocity is greater than the critical value,the whole bed enters the pneumatic conveying state,and TDH does not exist.In addition,the more light components in the bed material,the faster the increase of TDH.The elution constant K increases with the increase of gas velocity,but the purity of elution particles shows an opposite change.(4)Study on biomass pyrolysis in a pilot fluidized bed coupling with fractional condensationThe study on walnut shell pyrolysis was carried out in a pilot fluidized bed coupling with fractional condensation.Under different pyrolysis temperature and carrier gas flow,the industry analysis of the collection of char powder shows that the higher the pyrolysis temperature,the lower the volatilization content of char and the larger the pyrolysis gas yield.When the temperature reached 525 ?,the biomass volatilization can be completely precipitate.At the same time,reducing the flow rate of fluidized gas is also beneficial to increasing the yield of pyrolysis gas.Under the thermal condition,the endogenous bubbles generated by volatiles greatly promote the elution of particles,resulting in insufficient particle residence time.Then,the yield and composition of char,pyrolysis gas and bio-oil obtained by biomass pyrolysis under different gas velocity and pyrolysis temperature were analyzed.It showed that bio-oil gained the highest total yield at 500?,and the phenolics and hydrocarbons will be better enrichhed in secondary and tertiary bio-oil,respectively.With the increase of carrier gas flow,the production of pyrolysis gas firstly increases and then decreases,while the solid product presents the opposite change.Too larger carrier gas flow will increase the content of non-condensable gas in pyrolysis gas.On the one hand,it will increase the mass transfer resistance of condensable components;on the other hand,it will reduce the fractional pressure of condensable group,reduce the dew point,which resulted in decreasing of secondary bio-oil yeild and increasing of tertiary bio-oil yeild. |
PDF Full Text Request