Investigation Of Multiphase Flow, Heat Transfer And The Behaviors Of Molten Slag In Entrained-Flow Gasifier And Its Radiant Syngas Cooler

The current study is motivated by a desire to optimize the design of gasification system, and improve the operation stability and reliability of the entrained-flow gasifier with radiant syngas cooler (RSC). The numerical simulation and experimental method has been cooperated to investigate the multiphase flow, heat transfer and the behaviors of molten slag in entrained-flow gasifier and its radiant syngas cooler.1. The gas-particle flow field characteristics of Opposed Multi-Burner (OMB) gasifier have been investigated using numerical simulation. The effects of molten slag droplets collision and particle Stokes number on gas phase flow field have been studied. The fraction of entrained-particle escaping from slag outlet is about 8%. A stochastic Markov model has been developed for simulating the particle residence time distribution (RTD) in gasifier. The predicted residence time distribution agrees well with experimental data. And the particle mean residence time is about 6.2s in industrial-scale OMB gasifier.2. The connection of gasifier to RSC is a key technology of gasification system. Four kinds of connection designs have been studied by numerical simulation. The results indicated that the gas-particle flow velocity is increased in the straight section of connection. The water tube at the outer of refractory wall can be used to decrease the temperature of refractory brick. And the syngas temperature drop in connection is about 31.5℃.3. A large-scale cold model of RSC has been set up. The Dantec hot-wire anemometry system and pitot tube were used to measure the gas flow field in the RSC. And the particle size distributions collected from different section of RSC were analyzed by Malvern laser particle size analyzer. The results indicated that the gas flow field simulated by Realizable k-e turbulent model agrees well with the experimental data. And most particles are captured by slag pool, and only a little of small particles entrained by gas flow will flow into annular of RSC or escape from outlet. The larger and higher density particle is, the worse the following behavior is.4. A numerical program for simulating the heat transfer, multiphase flow and the behaviors of molten slag in entrained-flow coal gasifier and its syngas cooler has been designed. This program has been applied to solve problems and optimize the operation of the industrial-scale coal gasification system. And the reliability and accuracy of the numerical model has been validated by measured data. The Realizableκ-εturbulent model and two-way coupling have been used to calculate the turbulent flow and the interaction beween two phases, respectively. The discrete ordinate model (DOM) was used for solving the radiative heat transfer equation when the radiative properties were calculated by weighted sum of gray gases model (WSGGM). The Ranz-Marshall correlation for the Nusselt number was used to account for convection heat-transfer between the gas phase and the particle phase. The ash particle radiative heat transfer was also considered. The physical properties of gas mixtures were calculated by the mass-weighted-mixing law.The results of industrial-scale RSC simulation show that a troch shape injet flow was formed at the top of RSC, and the temperature of inject region is higher than its nearby zone. The injet flow field angle decreases when the fin water wall is designed in the inner cylingder of RSC. The length of inject flow also decreases as the length of fin water wall increases. A recirculation region is formed at the center of inner cylingder of RSC when the fin water wall is designed. And the main heat transfer procese is completed in the inner cylingder of RSC. Moreover, the thickness of ash deposition in RSC has large effect on heat transfer efficiency.5. The charicaterizes of four typical entrained-flow gasification syngas cooler have been compared. The results indicated that the heat transfer process was completed in dip tube of water quench chamber, and much water vapour was produced when the sensible heat of gasification products was absorbed by chilling water. The process of crude syngas preliminary purification was achieved at the bottom of quench chamber. For syngas quench cooler, the heat transfer area of cooler and the quantity of chilling gas should be plenitudinous for ensuring the temperature of syngas and ash particle low enough, and avoiding the ash deposition forming in the following equipments. The critical separation size of particle in RSC is about 400μm. A center circulation region was formed below the chilling plane when the syngas chilling was designed in the center of inner cylingder.6. A mechanism slag droplet deposition model has been developed. And the concept of "exessed rebound energy" has been introduced to divide the impact results. The effects of particle temperature, particle impact angle, particle size, particle impact velocity and wall temperature were investigated. It can be found that the particle temperature, particle impact angle, particle size and particle impact velocity have large effect on impact results. However, the wall temperature has limited effect on impact results.7. A mathematic model for simulating the processes of molten slag droplets collision deposition and slag flow along the gasfier wall has been bulit. Through comparing the results calculated by present model with two literature models, the calculation precision of present model has been improved. The slag multiphase-multilayer flow has significant difference between coal water slurry gasifier and pulverized coal gasifier. The operation temperature pulverized coal gasifier is suggested to be above the ash flow temperature 150℃for achieveing the stable slag tapping. And most time there is no solid slag layer in the coal water slurry gasifier, because the thick refractory line has higher thermal resistance. At last, a method has been recommended to calculate the critical operation temperature when the solid slag layer is just formed in the coal water slurry gasifier.