Heat Transfer Characteristics Of Horizontal Tubes In The Pressurized Fluidized Bed Furnace

Pressurized fluidized bed combustion is one of the important clean coal technology which can achieve clean and efficient coal utilization.In order to solve the problem that the heat exchanger layout is restricted due to the reduction of the furnace size for the pressurized fluidized bed,basic research on the bed-to-surface heat transfer characteristics of horizontal tubes at different locations in the pressurized fluidized will be significant.However,the heat transfer experiment for horizontal tubes requires that the cross-sectional area of the furnace cannot be too small and the design and operation of the pressurized fluidized bed reactor under high-temperature and high-pressure conditions will be difficult.Therefore,relevant experiments were mainly carried out with cold pressurized fluidized beds at present.By contrast,the key point of this study is to study the heat transfer characteristics of horizontal tubes in pressurized fluidized bed at high temperature.A hot pressurized fluidized bed heat transfer experimental platform adopting the overall jacketed container design scheme was developed,equipped with gas supply system,water cooling system,electric heating system,sealing system and data acquisition system to ensure the safe operation and the heat transfer coefficients determination.Based on the experimental platform,the heat transfer coefficients of horizontal tubes at different positions in the furnace were measured.The effects of different experimental conditions such as pressure,temperature,particle size and fluidization velocity on the local and average heat transfer coefficients of horizontal tubes were discussed.The accuracy and applicability of different well-known heat transfer models used for immersed horizontal tubes in pressurized fluidized bed have been detailed analyzed with the data of pressurized experiments.A new semi-empirical emulsion renewal heat transfer model was proposed to predict averaged heat transfer coefficients of horizontal tubes in the pressurized fluidized bed.The experimental results of the immersed horizontal tube heat transfer characteristics in dense phase indicated that the heat transfer will be enhanced with the increase of system pressure.When the system pressure increased from 0.1MPa to 0.7MPa,the average heat transfer coefficient is increased by 11.4%-22.5% for different particle sizes,and it is more significant for fine bed materials.The bed-to-surface heat transfer coefficients increase with the increase of bed temperature.When the bed temperature increased from 400? to 800?,the radiant heat transfer rate increased from 5.1% to 12.5% under normal pressure and this rate would decrease slightly as the system pressure increases.The average heat transfer coefficients of the immersed tube increased rapidly as the fluidization number increased and then decreased slowly as the fluidization number continued to increase.There was a maximum heat transfer coefficient in the process and the experimental maximum heat transfer coefficients fit well with the prediction results of Shah model.At low fluidization numbers,the distribution of circumferential local heat transfer coefficients indicated that the heat transfer coefficients at windward surface was higher than the leeward surface,and the horizontal sides had the highest values.At high fluidization numbers,the distribution of circumferential local heat transfer coefficients would tend to be uniform.The dilute phase heat transfer experiment indicated that the increase of system pressure strengthened the heat transfer of the horizontal tubes in the hot pressurized bubbling fluidized bed.The heat transfer tube with a lower height position had higher heat transfer coefficients because of the particle concentration effect.With the increase of fluidization number,the heat transfer coefficients of horizontal tubes at different heights showed the same increasing trend.Under the same system pressure,the circumferential heat transfer coefficients of the horizontal tube were slightly higher at 0° and 180° circumferential positions,but the overall distribution tended to be uniform.Based on the experimental results,an empirical model for the horizontal tubes heat transfer coefficient prediction in the dilute phase was established.The model calculation results can predict 97.4% of the experimental data form this study and other different sources within 25% error with the absolute average deviation of 8.18%.The accuracy of different kinds of heat transfer models used for immersed horizontal tubes in the pressurized fluidized bed were analyzed with the data of pressurized experiments,which indicated the limitation of these models in the application of pressurized conditions.A new heat transfer model based on the semi-empirical emulsion renewal theory was proposed.The model took the advantages of thermal contact resistance model and heat penetration model into account and modified the correlation for gas convection heat transfer calculation at elevated pressure.156 experimental data points from this study and other different sources were predicted and 94.9% of the model calculation results can predict the experimental heat transfer coefficients within 20% error with the absolute average deviation of 11.86%.The results in this study are expect to provide reference for the arrangement and optimization of heat exchanger in the pressurized fluidized bed furnace.