Coal Particle Ignition Temperature And Effect Of O₂/CO₂Atmosphere On It In Dense Phase Of Circulating Fluidized Bed

Circulating fluidized bed has natural superiority for coal combustion underO2/CO2atmosphere. Investigation of the coal ignition process in the dense phase offluidized beds would make it much easier to get a comprehensive understanding on thecoal combustion behavior in fluidized beds.In the dense phase of fluidized beds, the inert particles inhibit the oxidation of COgenerated on the surface of graphite spheres used in this work. At the meantime, thechemical reaction on the surface of the graphite spheres is kinetics controlled under700℃and is diffusion controlled within800～850℃. According to these features, afluidized bed experimental system was built up to investigate the chemical reaction,species diffusion and heat transfer; Correlations were derived with theoretical analyses,experiments and data processing; A coal ignition model was established based on theSemenov ignition theory with the correlations obtained in this work and otherliteratures. The impact of the O2/CO2atmosphere on the coal ignition temperature wasdiscussed with the help of the model. The main conclusions are as follows:1) The same as under O2/N2atmosphere, the main reaction on the carbon particle iscarbon oxidation under O2/CO2atmosphere. The ratio between CO and CO2in theproduct, η, increases as the temperature rises, and decreases exponentially with theincreasing O2concentration: η=4.6×104exp (84200/RT) c0.395O2.2) The nonequimolar counter diffusion at the active particle surface contributesadditional resistance on the mass transfer process. A correlation was proposed as:She m=2ε mf+0.743(Re/ε05281/3mfmf).Scwith data processing and theoretical analysisfor predicting the Sherwood number at equimolar counter diffusion situation. Theexpression of thickness of the effective mass transfer boundary layer was obtained,with the help of which it was revealed that the relative motion between the activeand inert particles enhances the mass transfer process at the active particle surface.3) It was indicated by both the mass transfer and heat transfer experiment results thatthe active particle resides in the emulsion phase as it moving in the dense phase offluidized beds. The surface contact resistance model is able to describe the solidparticle convection at the surface of the active particle freely moving in thefluidized bed. 4) The surface contact resistance constant,φ, is actually not invariant for an active particle freely moving in fluidized beds. It is a function of the ratio between the sizes of the active particle and the inert particle, da/d1:φ=2.5·1n(da/di)+2. The mean residence time of the emulsion at the active particle surface,θ, increases with the increasing da and di, and declines as the fluidization velocity: θ=0.318[(2.00×105di+24.6)da-93.3di+0.154]·Uex-0.610.5) A fluidized bed dense phase coal ignition model was established by combining the experimental results obtained in this work on the chemical reaction, mass transfer and heat transfer processes. According to the model computation results, the ignition temperature of a coal particle in a fluidized bed under O2/CO2atmosphere is higher than that under O2/N2atmosphere. With a low fluidization number, the difference between the coal ignition temperatures under O2/CO2and O2/N2atmosphere is controlled by the variations of heat and mass transfer coefficients; While, it is caused mainly by the change of the mass transfer resistance when the fluidization number is much higher.