Study On Flow, Separation And Reaction Process In Gasification Unit Of Triple-bed Circulating Combined Fluidized Bed

The circulating fluidized bed(CFB)has been widely used in chemical industrial processes such as combustion,gasification,and fluid catalytic cracking because of its higher gas-solids contact efficiency,enhanced mass,and heat transfer characteristics.However,in the traditional CFB gasifier,when pyrolysis and gasification occur in the same reactor,the tar,light hydrocarbon gas,and hydrogen produced by coal pyrolysis seriously hinder the gasification of char.To maintain the activity of the catalyst and improve the gasification efficiency of char,the pyrolysis process and gasification process should be separate.A new concept of triple-bed circulating combined fluidized bed(TBCFB)was proposed by Tsutsumi et al.A TBCFB is mainly composed of a downer(pyrolysis reactor),a bubbling fluidized bed(BFB)(gasifier),and a riser(combustor).A typical TBCFB first pyrolyzes the coal quickly in the downer pyrolyzer,and then,the produced gases and tar are separated from the char by a gas-solid separator.The produced char enters the BFB gasifier for steam gasification.The non-reacting char flows out of the BFB and moves into the riser combustor to be combusted by oxygen.Many heat carrier particles circulate in the system to transfer heat from the riser combustor to the downer pyrolyzer for coal pyrolysis and to the BFB gasifier for char gasification.In the entire cycle,the system must be operated under the condition of high solid particle circulating flux to efficiently utilize the heat produced by char combustion in the riser.In this study,the gas-solid flow behavior of a BFB with high solids continuous feeding/discharging mass flux was simulated by an Eulerian–Eulerian model incorporating the Lagrangian(discrete phase model)properties of particles as a tracer to analyze particle motion and determine the optimal feeding tube location and diameter.It was observed that a U-shaped track of particles from the inlet to the outlet and double vortexes resulted in particle back-mixing and extended particle residence time.According to the results,some feeding-tube design principles were proposed to achieve smooth solids flow and improve the particle residence time in the BFB in solids mass circulating fluxes ranging from 800 to 1200 kg/m~2s.The particles fall rapidly under the influence of gravity and drag force in downer,and residence time is very short,which leads to the presence of some unreacted particles.When these particles enter the bubbling bed,they impede the gasification reaction in the bubbling bed.Therefore,the outlet position of the downer is set to change diameter structure.By reducing the diameter to increase the solid holdup and form moving layer,it can make the unreactive coal particles continue to be pyrolyzed.By gas-solid flow simulation of moving layer and gas-solid separator which connects bubbling fluidized bed and downer,we explored the best structure of these parts,to ensure the coal particles be pyrolyzed completely.Coke obtained by high sulfur coal and Ca-based can transfer most sulfur elements to ash,reducing the emission of sulphide.Using the special coke as raw material for gasification experiment with steam as gasification agent can obtain high hetaing value syngas.In this study,using a visual fluidized bed reactor,a detailed basic study of coke obtained by high sulfur coal and Ca-based gasified with steam was carried out,and the residence time was investigated by using Eulerian–Eulerian–Eulerian model incorporating the Lagrangian(discrete phase model)properties.