Study On The Collection Efficiency And Heat Transfer Characteristics Of Hot Dusty Gas Flowing Through The Packed Granular Bed

In the industrial processes of metallurgy,chemical industry and energy engineering,the temperature of hot exhaust gas is about 800~1200?.The hot exhaust gas has high grade waste heat energy,and the potential of waste heat recovery is great.However,due to high dust content in the waste hot gas,especially for coagulative particles(tar,low melting point molten metals)contained in the waste hot gas,it is difficult to remove them at high temperature.Heat exchange surface will be clogged because of the accumulation of dust,if the waste heat of hot gas is utilized directly.Particularly it is difficult to clean the congealed sediments.Pore blockage,difficulty in granules regeneration,low waste heat recovery and low collection efficiency of the granular bed are the essential issues to be solved.Researches show that the fixed granular bed has prominent advantages in waste heat recovery of high temperature flue gas and removal of coagulative particles.However,few studies have been reported on the computational fluid dynamics(CFD)simulation of collection and heat transfer characteristics of hot dusty gas flowing through a packed granular bed or a dual-layer granular bed filter.Besides,there has been no report on the heat transfer characteristics of hot gas with coagulative particles flowing through a packed granular bed.Moreover,few studies have been carried out on granular bed in the dust removal of hot dusty gas and waste heat recovery simultaneously.Based on the above problems,the coupling relationship between the collection and heat transfer characteristics of hot dusty gas flowing through the granular bed is the key scientific problem.Thus,the numerical study of the collection efficiency and heat transfer characteristics of hot dusty gas flowing through the packed granular filter and dual-layer granular bed is carried out,qualitative and quantitative analyses of influence factor on collection efficiency and heat transfer characteristics are presented.The experimental study on the heat transfer of gas with coagulative particles flowing through a packed granular bed filter is conducted.The Nusselt number(Nu)relations under different conditions are obtained.An integrated device of dust removal and waste heat recovery is proposed,the experimental study and optimized analysis of this device are performed.The influences rules of various factors on collection efficiency,dust capacity and waste heat recovery efficiency are obtained.It provides technical support for further engineering application.Firstly,the CFD and discrete element method(DEM)were used to generate a three-dimensional(3D)randomly packed granular filter model.The collection mechanism and heat transfer characteristics of hot dusty gas flowing through a packed granular filter were investigated.The bridging method was introduced to optimize the grids of contact points between granules.In the simulation process,particles were considered to be trapped when they collided with the particle surface.The effects of granular bed depth,gas velocity,particles diameter,gas temperature and flow direction on the grade collection efficiency were numerically studied.The results indicated that the increase of bed depth and gas velocity can promote collection efficiency when the particle diameter was greater than 5?m.However,the increase of temperature would reduce collection efficiency under constant wall-heat-flux condition.The collection efficiency can be improved when the flow direction and gravity direction were the same.The relationship between Stokes number(St)and collection efficiency was fitted.The grade collection efficiency maintained at a stable value when the St was less than 0.009,but increased linearly with lg(St)when St?0.009.The logarithmic mean temperature difference(LMTD)and heat balance were used to obtain overall heat transfer coefficients of gas-solid two-phase flowing through the packed granular filter.The results showed that heat transfer was enhanced due to the introduction of solid particles in the bed.The overall heat transfer coefficient increased approximately linearly with an increase in particle loading ratio.The Nu was related to the Archimedes number(Ar),the Reynolds number(Re),and the particle loading ratio.Then,based on the CFD and DEM methods,the model of 3D dual-layer granular bed filter packed with randomly arranged granules was built.The effects of bed depth of the lower layer of fine granules,gas velocity and particles diameter on the collection mechanism were studied numerically.The computational results showed that the total pressure drop increment of the dual-layer granular bed filter increased approximately linearly with the increase of bed depth of the lower layer.The grade collection efficiency was remarkably higher for the dual-layer granular bed filter than that for a single-layer granular bed filter.The grade collection efficiency increased with the increasing of bed depth of the lower layer.When the particle diameter was greater than 15?m,the increasing amplitude in the grade collection efficiency slowed down.While the particle diameter was 1~10?m,the dual-layer granular bed showed good performance on grade collection efficiency.According to the‘turning point diameter',increasing bed depth of lower layer can improve the collection efficiency of small particles.The high collection efficiency of the dual-layer granular bed can be realized with the lower gas velocity.The dual-layer granular bed realized high collection efficiency through designing the bed depth of the fine granules and gas velocity rationally,in the case of little increase of pressure drop.The relationship between the grade collection efficiency and the St was obtained.If St was below a threshold,the grade collection efficiency remained stable;if St was above the threshold,the grade collection efficiency increased linearly with lg(St).The turning point of St appeared to advance with the increase of bed depth of the lower layer.It showed that the smaller the pore size,the more obvious the effect of inertia on collection efficiency.With increasing St,collection efficiency of the different bed structures tended to a certain value.It showed that when the inertia dominated,the influence of geometric parameters on the collection efficiency diminished.The research results can be used to guide the design of granular bed structure with high collection efficiency and low resistance.During the flow of the gas with coagulative particles in packed granular bed,the coagulative particles will be fused or congealed because of the temperature change of gas and packed granules.The different heat transfer characteristics of coagulative particles will have a great influence on the dust removal and waste heat recovery.Thus,an experimental study on the heat transfer characteristics of hot gas with coagulative particles flowing in packed granular bed filter was presented.The LMTD and heat balance were used to get the overall heat transfer coefficient.The results showed that compared with the solid particles flowing through the packed bed,the heat quantity released by the concretion of coagulative particles can greatly enhance the heat transfer.The Nu correlation was obtained,which was well related to the coagulative particles mass flowrate and the melting heat of coagulative particles in that condition.However,the heat transfer will be weakened when the coagulative particles flowing through the packed bed in liquid state.The Nu correlation was given,which was well related to the coagulative particles loading ratio in that condition.The research results revealed the different heat transfer characteristics when the phase transformation of coagulative particles.The relevant correlations can be used for the thermal calculation when the hot gas with coagulative particles flowing through the granular bed.As for the integration of dust removal and waste heat recovery of hot dusty gas,the granular bed heat exchange filter was proposed.The filter granules were supported by the compactly arranged heat exchange tubes.By adjusting the water flowrate of heat exchange tubes,the temperature distribution and waste heat recovery of the fixed bed filter can be controlled,thus well performance of particles removal and waste heat recovery can be achieved simultaneously.The results showed that coagulative particles can achieve high particles collection efficiency earlier than that of solid particles case,but the pressure drop of filter bed caused by coagulative particles was higher as compared with solid particles case.In the initial stage of filtration,the overall heat transfer coefficient flowing with coagulative particles was higher as compared with solid particles case,and increasing the inlet particles concentration can help improve the overall heat transfer coefficient.The reason was that the heat release by the concretion of coagulative particles.However,in the later stage of filtration,the overall heat transfer coefficient flowing with coagulative particles was lower than that of solid particles case,the overall heat transfer coefficient increased with the decreasing of the inlet particles concentration.Because the heat transfer deteriorated caused by the concretion of coagulative particles.The pressure drop of filter bed caused by coagulative particles increased with the decreasing of the gas velocity.By increasing the gas velocity,overall heat transfer coefficient can be enhanced.Finally,as for the waste heat recovery of the granular bed heat exchange filter,the decrease in gas velocity and inlet particles concentration resulted in an enhancement in the waste heat recovery efficiency,when hot gas flowed with the coagulative particles.The waste heat recovery efficiency can be improved by increasing the water flowrate of the second row heat exchange tubes appropriately.This filter can achieve high particles collection efficiency(>98%)and high waste heat recovery efficiency(>70%)simultaneously.Moreover,the influences of gas velocity,inlet particles concentration and water flowrate of heat exchange tubes on the waste heat recovery of granular bed heat exchange filter were discussed in the process of granules replacement.The granular bed heat exchange filter can be used as a technical prototype of the integrated structure of dust removal and waste heat recovery.It had high collection efficiency and high waste heat recovery,which laid a certain foundation for relevant demonstration projects and industrial applications.