Fluidization characteristics of iron ore powder in fluidized bed

The fluidization characteristics of iron powder were studied on a self-made cold fluidized bed in the laboratory. The pressure distributed in the axial inside the lifting pipe of a fluidized bed gradually decreases to a constant value from underbody to roof. As the gas flow rate in the fluidized bed increases, the pressure on each axial section first increases and then decreases. When the inflation numbers are fixed, as the operating gas speed increases, the pressure on the same height segment decreases. When the operating gas speed of a fluidized bed is constant, the pressure increases with the increase of inflation numbers. The pressure at different bed heights is affected differently by the operating air speed: at the underbody of the bed, the pressure rapidly decreases with increasing operating airspeed. As the cross-sectional height increases, the influence of operating gas speed on pressure gradually decreases. As the charge increases, the pressure at the underbody of the fluidized bed rises rapidly, while the pressure at the roof changes gently. When iron ore fine powder is fluidized, distribution in axes of particle concentration shows an uneven distribution of upper dilute and lower dense. Under a certain loading number, increasing the operating airspeed will reduce the average concentration of granules in each axial section of the entire bed, and the particle concentration distribution in that section will become uniform. As the number of charges increases, the average concentration of granules in the riser increases, and the difference in particle concentration between the underbody and roof parts of the riser decreases.


Introduction
As an ironmaking reactor, a fluidized bed has relatively loose requirements for iron ore composition [1] .It can use iron ore powder with fine particle size and wide distribution of partial size and has the advantages of uniform temperature and concentration of the whole bed, high efficiency of heat, and mass transfer.In foreign countries [2] , there is much research on ironmaking technology by fluidizing gas reduction, and the development of ironmaking technology by fluidizing gas reduction is fast.In China [3][4] , there is relatively little research on the fluidized gas reduction ironmaking technology.This article systematically analyzes the impact of operating parameters and properties of iron concentrate particles on the pressure and local particle concentration distribution inside the riser through many experiments on the pressure of the gas-solid two-phase flow and local particle concentration in a fluidized bed.This will provide an experimental and theoretical basis for understanding, scaling up design, and optimizing the operation of fluidized bed ironmaking reactors [5] .The fluidized system made in the laboratory is shown in Figure 1.The gas inlet uses a conical gas distributor, and the distribution plate is fixed by a flange.Several pressure gauges are opened from the roof to the underbody of the fluidized bed to measure the bed pressure drop of iron ore powder.The inner diameter of the fluidized bed is 140 mm, the height is 1.7 m, and the inlet diameter is 40 mm.

ICAMIM-2023
During the experiment, air is transported by Roots blower, passes through a conical gas distributor, enters the fluidized bed riser from the underbody, and is discharged through a cyclone separator and bag filter.Iron ore powder is fed through the silo, and the iron ore powder and gas circulate in a fluidized bed.Fine iron powder particles and gas enter the cyclone through the tangential inlet and undergo complex rotational motion in the cyclone.The particles separated by the tangential rotational motion enter the fluidized bed riser through the reflux valve and continue to circulate for fluidization.
During the experiment, the gas flow rate is measured using an electronic flow meter, and the pressure drop is obtained using a U-shaped pressure gauge.

Pressure distribution in fluidized bed riser
The pressure in the axis of the riser gradually decreases from underbody to roof to a constant value, which means that the velocity of iron concentrate particles in the axis of the riser increases as the concentration decreases.It can be found that: (1) At the same gas speed (U g ), the less the charge is, the more obvious the acceleration of the impact of iron concentrate particles is; (2) Under certain loading conditions, the higher the gas speed (U g ) is, the greater the increase in velocity of iron powder particles will be; (3) Iron ore powder always accelerates upwards in axis of the riser to the fully developed part, without any deceleration process.Figure 2 shows the pressure distribution under different operating conditions.Figure 2 shows the axial pressure diagram in the riser.When the charge remains constant, the pressure drop at the same height decreases with the increase of gas speed (U g ).This is because as the gas speed (U g ) increases, the speed of iron ore powder increases, and the quantity of iron ore powder in the same height range decreases, the concentration of iron ore powder decreases, and the resistance required to support particles also decreases, resulting in a decrease in pressure.When the gas speed (U g ) is constant, the pressure increases with the expansion.The reason for this is that with a certain gas speed and an increase in the number of iron ore powders, the probability of particle fluidization increases, the resistance increases, and therefore, the pressure also increases.Through the analysis of a single curve, the pressure difference change rate in the lower half of the riser is extremely severe.In contrast, the upper half is relatively gentle, which is similar to the phenomenon observed in actual experiments.

The impact of operating parameters on the pressure
In order to compare the characteristics of the pressure (ΔP/ΔZ) at different height sections with the operating conditions, five representative sections of the riser are selected for analysis.

The impact t of apparent gas speed on pressure at different height sections.
Under a certain number of charges, the pressure of each axial section is (ΔP/ΔZ).Figure 3 shows that as the gas flow speed increases, the pressure on each axial section of the riser first increases and then decreases.The reason is that as the airflow speed increases, the state of the fluidized bed transitions from bubbling fluidization to turbulent fluidization.Therefore, as the number of fluidized iron ore powder particles increases and the weight of particles at the same height increases, the resistance required to maintain the particles will increase, resulting in a corresponding increase in pressure.However, as the gas speed continues to increase, the fluidized bed state changes from a turbulent fluidization state to a rapid fluidization state, the particle velocity will increase, and the particle numbers will correspondingly decrease.The particle concentration will become smaller, and the drag force required to hold the particles will also decrease, so the pressure will also decrease.However, the pressure varies with airspeed at different heights.At the underbody of the bed, the pressure rapidly decreases with the increase of gas speed, and the influence of gas speed on the pressure gradually decreases with the increase of cross-sectional height (Z).In the underbody acceleration section, gas speed has the greatest impact on pressure.At the same operating gas speed, the pressure difference between adjacent sections is very large, indicating that the iron ore powder is in the acceleration stage.
The pressure curve at section z=1.15 m and section z=1.45 m is almost the same, which shows that from the height of section z=1.15 m above, iron ore powder particles are in the fully developed section.

The impact of charge numbers on pressure at different height sections.
The pressure at different sections of the riser shows a common trend of increasing with the increase of the charge amount, as shown in Figure 2.However, the change rule of pressure at different height sections is obviously different, and the distribution of iron concentrate particles shows the upper dilute and lower concentrated.As the loading numbers increase, the pressure at the underbody of the bed layer rapidly increases while the pressure at the roof of the bed changes gently.Through the analysis, it is considered that the increase of the charge numbers in the accelerating section at the underbody of the riser will increase the concentration of iron concentrate particles and increase the collision and agglomeration behavior between iron concentrate particles.This will consume more energy, reduce the energy used to accelerate iron concentrate particles, and reduce the effective drag force on iron concentrate particles.Thus, the acceleration effect on particles is weakened, and the pressure at the underbody of the bed rises rapidly.In the dilute phase section of the upper part of the riser, the collision and agglomeration between particles are less, and the energy loss is less.The energy of the gas is mainly used to transport particles.Therefore, the change of particle conveying rate caused by changing the charge numbers is relatively stable, so the pressure and the charge numbers show a relatively stable linear relationship, as shown in the z = 0.4 m and z =1.45 m curves in Figure 4.
Figure 4 also shows that from the height of section z=1.45 m, the relationship between the pressure and the charge numbers tends to be stable; that is, the pressure is not affected by the height of the section, and at this time, the state of particles in the riser is in a fully developed state.This also fully proves the description of the axial development of two-phase circulation in the riser in Figure 2.

Study on the concentration of iron ore powder
There are many methods for measuring the concentration of iron powder particles in a fluidized bed.The simplest method is to obtain the average concentration of granules in a certain area through pressure difference measurement [6] .Due to the complexity of the two-phase circulation in a fluidized bed, many issues need to be analyzed and simplified before conducting actual experimental research.This experimental research also followed this research approach without considering the friction between air and iron ore powder particles and the bed wall.According to static analysis, the pressure drop is completely converted into the resistance of gas to iron powder particles in the bed.The formula is as follows [7] : Where ∆P is bed pressure drop, Pa; ∆Z is the bed height difference, m; A is the cross-sectional area of fluidized bed, m 2 ; ε s is the average concentration of iron ore powder; ρ s is particle density of iron ore powder, kg•m -3 ; ρ_g is gas density, kg•m -3 ; g is the acceleration of gravity, m•s -2 .
The expression of ε s can be obtained by reverse calculation: In the formula, ∆P is measured by the test, and ρ s , ρ g , and bed height difference ∆Z are known.Therefore, the average concentration of granules in the fluidized bed can be calculated.

Axial distribution of iron ore powder concentration.
Distribution in axes of the particle concentration of iron ore powder in the riser under different operating conditions, as shown in Figure 5. Distribution in axes of the particle concentration presents ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012012 the uneven distribution of the upper thin and the lower thick when the iron ore powder is fluidized in the fluidized bed.Under the experimental conditions, distribution in axes of the average concentration of particles in the cross-section is basically monotonic exponential distribution.
3.3.2.The impact of operating gas speed on particle concentration of each section.The variation of the iron ore powder particle concentration at five height sections with the operating gas speed is plotted in Figure 6.When the loading capacity remains constant, the apparent gas speed increases.For iron ore powder with the same distribution of partial size, the concentration of iron ore powder particles at any position in the riser first increases.When the apparent gas speed reaches a certain value, the particle concentration at any position in the riser begins to decrease.The reason for this is that with the increase of gas flow speed, the process of fluidized bed transition from bubbling bed to turbulent bed occurs, and the number of particles increases, resulting in an increase in particle concentration.As the gas speed increases, the particle speed also increases, so particles enter a fast fluidization state, and the particle concentration will sharply decrease.Comparing the particle concentration distribution in various axial cross-sections, it can be found that the higher the axial position is, the smaller the particle concentration is.That is to say, the particle concentration at the underbody of the bed is higher than that at the roof of the bed, indicating that the particle concentration gradually decreases in the axial overall.The higher the axial position is, the more similar and gentler the curve of particle concentration will be, changing with the operating gas speed.

The impact of charge numbers on particle concentration.
Figure 7 shows the impact of the charge numbers on the iron ore powder particle concentration at different sections.When the operating gas speed is fixed, the concentration of iron ore powder particles in each section increases with the increase of the charge numbers.According to the analysis, with the increase of the charge numbers, the probability of iron ore powder particles being fluidized increases, so the particle concentration on each section also increases.The fluctuation of particle concentration in each section is different when the charge numbers are increased.In the middle part of the riser, the particle concentration is greatly affected by changes in the number of charges.But in any case, it still conforms to the distribution characteristics of "upper thin and lower thick".
For iron powder with the same distribution of partial size, increasing the operating gas speed can reduce the average concentration of granules in each axial section of the entire bed under certain charge numbers, and the distribution of iron powder particle concentration in this section becomes uniform.As the dosage increases, the average concentration of granules in the riser increases, and the

Figure 2 .
Figure 2. The axial pressure diagram in the riser.

Figure 3 .
Figure 3. Variation of pressure with apparent gas speed at different axial sections.

Figure 4 .
Figure 4. Variation of pressure with charge numbers at different axial sections.

Table 1 .
Test results of iron powder.