Combustion and hydrodynamic characteristics of high ash contents anthracite coal in a cold model fluidized bed (0.38 m-ID × 9.6 m-high) and two fluidized bed combustors (0.3 × 0.3 m ×4.7 m-high, 1.01 × 0.83 m $\times$ 4.2 m-high) have been studied.
The effects of fluidizing gas velocity, bed temperature, static bed height, air-fuel ratio, and solids recycle rate on the combustion characteristics such as the axial temperature profile, carbon conversion in each particle size, overall carbon combustion efficiency, and particle entrainment rate have been determined in two combustors.
A theoretical model for a mean bubble size and its frequency based on the collision theory with the random spatial bubble distribution in freely bubbling gas-fluidized beds has been developed. A hemispherical bubble velocity diagram about the time-averaged instantaneous bubble motion is constructed in a fluidized bed to determine the average bubble collision frequency.
The proposed theoretical model equation for predicting bubble size is
$(U-U_{mf}) (D_b-D_{bo}) + 0.474 g^\frac{1}{2} (D_b^\frac{3}{2}-D_{bo}^\frac{3}{2}) = 1.132 (U-U_{mf})h$
As can be seen in the above model equation, the gradient of bubble size increases linearly with bubble voidage. Also, the bubble Froude number increases along the bed height with bubble voidage. The bubble Froude number represents approximately a linear relationship with the average fractional change of square root of the static energy of bubble rise along the bed height. The present model of bubble size is found to represent well the data in literature.
The overall particle entrainment was measured by collecting the entrained particles in dust collectors and it is analyzed by the one-dimensional particle motion in the freeboard, the entrainment rate at the bed surface, the distribution of particle rising velocity from the bed surface, and the particle attrition in the bed.
The overall entrainment rate of particles at the bed surface in the cold model fluidized bed and combustors is represented by
$E_0=9.6 (U-U_{mf})^{2.5}A D_{bs}[\frac{298}{T_b+273}]^{3.5}$
The rising velocity distribution of particles from the bed surface in the cold model fluidized bed and combustors is represented by
${V_{po}/V_{pomax}} = exp (-2.44X)$
$V_{pomax}/U_{bs}=0.71 d_p^{-1.65}[\frac{T_b+273}{298}]$
The overall entrainment rate increases with an increase in gas velocity and coal feed rate. However, the overall entrainment rate decreases somewhat with an increase of bed temperature, but it is independent of the static bed height.
The entrainment of coarse particles having terminal velocities greater than the superficial velocity at the top of the freeboard is more pronounced in the larger combustor than that in the smaller combustor which has an expanded freeboard.
The entrainment rate with the variation of gas velocity is less pronounced in the combustor than that in the cold model fluidized bed. However, the rising velocity of entrained particle at the bed surface of combustors is higher than that of the cold model fluidized bed.
From the particle population balance which includes the carbon conversion, particle entrainment rate and particle attribution rate, the solid flow rate of a given particle size-cut is derived.
The combustion rate of a char particle in the combustor is analyzed with the intrinsic kinetic rate and the mass transfer rate between the gas and particles.
Carbon conversion increases with an increase in residence time of particle in the combustor. However, carbon conversion decreases with an increase in particle size. The present combustion reaction kinetics agree qualitatively to the data obtained from a thermobalance reactor. The following correlation is derived for the carbon conversion of coal particles (1 mm to 12.7 mm in diameter) from the present experimental investigation:
$1-X_b=[\frac{R_c}{R_p}]^3 = 0.647[\frac{R_p}{K'}]^{1.09}$
where
$X_b = 1 if X_b > 1$,
$K'=k_0e^{-E/RT_b}[\frac{C_{ao}+C_{ae}}{2}]^{0.7}\bar{t}$
The gas composition along the combustor height has been determined by assuming that the gas flow in the emulsion phase is in a well-mixed state, but the gas flows in the bubble phase and in the freeboard are in the plug-flow. Also, the char combustion and the homogeneous combustion of CO have been considered as the main reaction terms.
In combustors, the effluent solids are a sum of the bed drain and the entrained solids. The bed-drained particle size is larger than 0.25 mm, whereas, the size of entrained particle is smaller than 0.5 mm in diameter. Therefore, coal particles larger than 1 mm can be regarded as the main combustible fuel in the bed.
The axial temperature profile of the combustor is found to be very uniform in the bed. However, the temperature in the freeboard increases with increasing the gas velocity, bed temperature, and recycle rate of cyclone-collected particles.
Without the recycle of cyclone-collected solids, the overall carbon combustion efficiency increases with an increase in bed temperature, but it decreases with an increase in gas velocity.
The effects of excess air ratio and static bed height on the overall carbon combustion efficiency are found to be insignificant in the combustors.
The total amount and carbon concentration of entrained particles decrease with an increase in recycle rate of cyclone-collected solids. In addition, the overall carbon combustion efficiency increases with an increase in recycle rate.
The minimum fluidizing gas velocity, solid flow rate and size distribution, carbon conversion of each particle size-cut, and overall carbon combustion efficiency derived from the proposed model have been compared with the present experimental data and found that the agreement is fairly good.
