Research On Characteristics Of Gas-Solid Flow In Air Nozzle Area In The Furnace And U-Valve In CFB Boiler

Material backflow into air box from air nozzle on air distributor plate in CFB boiler and material return interruption, accumulation, over-temperature coking in return valve system are currently two prevalent major issues in CFB boiler operation. the first study in this article is on theoretical and experimental research of mechanism of material backflow into air box from air nozzle on air distributor plate in CFB boiler. a theoretical criteria for critical condition of backflow in air nozzle nozzle in CFB boiler has been built, also test-bed has been put up to carry out test. critical condition of backflow in air nozzle is: momentum flow rate of the particles flow into the nozzle is equal to exhaust gas momentum flow rate of nozzle.Г-type air nozzle and arrow-headed nozzle are used in the experiment. critical points in test are obtained through regulation of fluidized air volume in the main bed and the air flow in the lateral test nozzle.test measurement is static pressure difference between the bed and inside the nozzle, which is used to calculate the particle velocity and momentum flow rate, at the same time , measurement of gas flow rate in external nozzle is used to calculate momentum flow rate of exhaust gas in the nozzle.in the calculation of particle flow rate backward the nozzle, based on the assumption that the particles are similar to fluid, introduction of formula of nozzle outflow for single-phase fluid is used to caculate speed of bed material backflowing into the nozzle in this article.comparing the size of two momentum flow rates of various test groups in critical backflow state, the result shows that when the test is in a critical state of material backflow, both of the momentum flow rates are indeed close to the same. from the mechanics equation of critical backflow in nozzle correlation between critical backflow air speed and operating bed pressure has been obtained. critical backflow air speed calculated is in line with the existing design experience. theoretical criteria for critical condition of backflow and correlation of critical backflow air speed obtained can not only give reasonable theory to explain the existing phenomenonthe of material leakage from air nozzle, but also can effectively guide correct design for the air nozzle in CFB boiler.The second study in this article is on theoretical and experimental research of gas-solid flow characteristics in U-vavle return system. According to gas-solid flow characteristics of various components in U-valve return system commonly used in large CFB boilers, selecting control volume of both sides of the return U-valve system from solid level in standpipe to the upward side of transport chamber as research object, energy and quality conservation equation of inlet and outlet of control volume under a stable flow state is established. Among them, the flow resistance through control volume– calculation of pressure drop in standpipe and resistance of particulate material flows through the horizental passage between two chambers in U-vavle is by citing recommended formula from predecessors, accordingly the integrity of theoretical model for gas-solid flow characteristics in U-valve return system is obtained, by further theoretial derivation, the most important feature in U- valve return system:explicit formula between solids height in standpipe and volume of fluidized air in transport chamber which is control parameter of U-vavle, under steady state, the relationship between reasonable solids height in standpipe and fluidized air volume is the most important feature of U-valve ensuring reliable operation, therefore, this result of great significance to the gas-solid flow study on U-vavle return system. in the fourth chapter of this article, a U-valve open-system test bed is established according to the theoretical model of the third chapter, also material recycling in the experiment is realized.Various test conditions and parameter measurement are carried out in the experiment including particle mass flow, air volume in transport chamber, the height of transport chamber as well as the height of horizental passage between material supply chamber and transport chamber in U-valve return system, On this basis,the comparison results between experimental data and theoretical model show that the theory and experiment matches well, which verifies the correctness of gas-solid flow theoretical model for U-valve return system in CFB boiler, also has laid a good foundation for engineering application of this theoretical model.