Hydrodynamic properties(axial pressure proiles, axial soild holdup and the radial gas mixing characteristics have been determined in a downer reactor (0.1 m-I.D. X 3.6 m-high) of a circulating fluidized bed loop (0.1 m-I.D. X 7.6 m-high). The solid particles used in this study was silica sand particles of four different sizes (d_p = 84, 104, 163, 236 ㎛) with a density of 3120 kg/㎥.
The effects of gas velocity(0.5-4.5 m/s), solid circulation rate(0-40 kg/㎡s), and particle size on the axial pressure profiles and pressure gradients in the downer reactor have been determined. The axal pressure gradients and pressure variation increase with increasing solid circulation rate but they decrease with increasing gas velocity. However, the effect of particle size on the pressure gradients and cariation is insignificant at the given gas velocities and solid circulation rates.
In the fully developed flow region (H > 2.0 m), pressure gradients estimated from a single particle motion are higher than that of the experimental value at lower gas velocities (< 3.0 m/s). whereas, they are similar to the experimental value at higher gas velocities (> 3.0 m.s). These behavior are caused by particle agglomeration or cluster formation that has been confirmed by the comparison of slip and erminal velocities of a single and cluster particles.
The effect of gas velocity, solid circulation rate and particle size on the radial gas dispersion coefficient, (D_r) in the downer reactor have been determined. In the reactor with gas flow alone,D_r increases with increasing gas velocity whereas, D_r Decreases with increasing gas velocity at a given solid circulation rate. From these data, D_r is higher than that of gas flow alone at lower gas velocities (< 3.0 m/s) but a reverse trend was observed at higher gas velocities (< 3.0 m/s). With increasing gas velocity, cariation of D_r is more pronounced with smaller particle than that of larger ones. At lower gas velocities (< 3.0 m/s), D_r increase with increasing solid circulation rate but it decreases with solid circulation rate at higher gas velocities (> 3.0 m/s) due to the particle agglomeration or cluster formation that enhances turbulence intensity in the reactor.
The radial dispersion coefficients in terms of Peclet number has been correlated with the dimensionless numbers with the data of the present and previous studies as:
Based on the D_r values in the downer and riser reactors, it can be claimed that gas-solid contacting in the downer is superior to that in the riser reactor.
The effects of superficial gas velocity, solid circulating rate, suspension density (0-19 kg/㎥)and partivle sizes on the bed-to-wall heat transfer coefficient have beendetermined in a downer reactor. The bed-to-wall heat transfer coefficient increases with increasing thecross-sectional averaged particle suspension density. While the heat transfer coefficient by gas convection increases with increasing gas velicity, the heat transfer cofficient exhibits defferent behavior with solid circulation rate. The heat transfer coefficient by gas convection played a dominant role, especially at lower solid circulation rates or suspension densities and larger particle sizes. At a given particle suspension density in the downer reactor the heat transfer coefficient increases with decreasing particle size.
A model is proposed to predict the bed-to-well heat transfer coefficient in a downer reactor and the predicted value from the model is in good agreement with the experimental data.
To investigate the effect of devolatilization yields on the product gas yield in the gasification region of downer reactor, the effects of reaction temperature and gas velocity on the gas product yeild from pyrolysis in a fluidized bed reactor were determined. The yields (g-gas/kg-coal) of all product gas increase with increasing reaction temperature due to the increase of oil and tar decomposition. Product gas yields from pyrolysis have been correlated with the reaction temperature at an average fluidizing velocity as
where y_i (g-gas/kg-coal feed) and T (℃) are product gas yield from pyrolysis and pyrolysis temperature, respectively.
From coal gasification in a downer reactor(0.1 m I.D.\times5,0 m-high), the effect of reaction temperature (750-850 ℃), steam/coal ratio (0.23-0.86), O_2/H_2O ratio (0-1.81) and coal feeding rate (5.3-9.0 kg/h) on composition of product gas, carbon conversion, cold gas efficiency, gas yield and calorific valye have been determined.
In case of air injection into the loop-seal, compositions of the product gas with reaction temperature in the gasification region are H_2 (23.3-30.8%), CH_4 (16.7-8.9%), CO (21.2-26.1%), CO_2 (33.5-26.8%), C_2H_6 (1.8-5.4%), C_2H_6 (0.8-2.5%), C_3H_6 (0.8-2.8%) and C_3H_8 (0.1-1.6%). Since combustion of volatiles in this gasifier was lower than those in the other gasifiers due to the lower oxygen/coal ratio in the gasification region, higher concentrations of hydrocarbon were obtained. Due to the higher concentrations of hydrocarbon in the product gas, calorific value of the product gas obtained from the gasification region is 6.3-10.6 MJ/㎥.
In case of steam injection into the loop-seal, compositions of the product gas are H_2 (39.4-35.8%), CH_4 (15.5-6.8%), CO (15.9-14.9%), CO_2 (20.7-29.5%), C_2H_4 (2.8-4.9%), C_2H_6 (2.5-3.5%), C_3H_6 (1.6-3.3%) and C-3H_8 (1.7-2.5%). Gas yield, carbon conversion and cold gas efficiency of the product gas increase with reaction temperature regardless of the loo-seal condition due to an increase of pyrolysis yields. By changing the reactant gas supplied into the loop-seal for solid circulation from air to steam, the yields of H_2, C_2H_4, C_2H_6, and C_3H_8 increase whereas, total product gas yield and carbon conversion decrease with reaction temperature. COmpared to the air injection, calorific value of the product gas with steam injaction increases from 6.3-10.6 MJ/M-3 to 13.0-15.2 MJ/m-3 which is much higher than those in the annulus region of an ICFB with arifice type draft tube. Cold gas efficiency of the product gas with steam injection is higher than that with air injection at temperature below 830 ℃ since the effect of pyrolysis yields on the product gas yield with steam injection is higher than tha with air injection with increasing reaction temperature due to the increase of char combustion and gasification reaction.