3.2. Influence of Coke Reactivity
In reality, many kinetic parameters, such as porosity, reaction rate constant and effective diffusivity and so on, can impact the iron-ore reducibility and coke reactivity. Therefore, these parameters should be simplified through a modified reaction rate expression to reflect the reducibility of iron ore and reactivity of coke for simulation. In this paper, the simplification has been conducted to set a series of reaction rate factors to represent the changes of the reactivity of coke and reducibility of iron ore. They are known as the reaction rate factors α and β to modify the overall reaction rates (R’ = αR and R’ = βR). For the baseline case, α = 1 and β = 1 are assumed for coke reactions and reduction reactions of iron ore.
The relationship between the gas utilization efficiency and coke reactivity is shown in
Figure 3. Because that the thermodynamic force of the reduction of FeO with CO is improved with a higher coke reactivity, the gas utilization efficiency should have been increased from the thermodynamic aspect. However, as can be seen in the figure, the utilization efficiency of gas increases at the beginning, but decreases later when the reactivity of coke increases continuously. The reason is that a higher decrease in the temperature of TRZ deteriorates the environment of reduction kinetics, which in turn, reduces the utilization efficiency of gas. To be specific, due to the decrease in TRZ temperature, the reduction rate slows down and the lower edge of the TRZ moves up, which decreases the height of indirect reduction region and the time of indirect reduction. As a result, the gas utilization efficiency increases first and then decreases.
In order to further analyze the reasons for the change of relationship between gas utilization efficiency and coke reactivity, the chemical energy utilization of gas for different coke reactivity and the relationship between driving force control of reduction reaction and coke reactivity in the indirect region are discussed through the concept of surplus gas reduction potential. The surplus gas reduction potential are calculated by difference between the reduction potential and the equilibrium reduction potential at corresponding temperature. The reduction of Fe2O3 is ignored in the discussion because Fe2O3 is quickly reduced to Fe3O4 and the temperature has little influence on the surplus reduction potential of this reaction. Since both the reduction of Fe3O4 and reduction of FeO with H2 are endothermic reactions, the decrease in temperature increases equilibrium volume fraction of reducing gas and decreases reduction potential of surplus reducing gas. However, for the reduction of FeO with CO, the influence of the temperature decrease on the equilibrium volume fraction and the reduction potential of surplus gas is opposite because this reaction is exothermic.
In order to correspond with the discussion on thermodynamics and kinetics of indirect reduction in the above analysis, the coke reaction rate factors,
α = 0.1, 2 and 10, are selected. The distributions of the volume fraction of CO and H
2, as well as solid temperature and indirect reduction degree in the indirect reduction region are described in
Figure 4, respectively. The results show that as the coke reactivity increases, the starting temperature of coke solution loss reaction decreases from 1258 K to 1002 K and the height of indirect reduction region decreases from 21.3 m to 15.2 m. Moreover, increasing the coke reactivity decreases the volume fraction of H
2 and the solid temperature as shown in
Figure 4b,c. For the third reaction rate factor, the
YCO and
fP are least, meanwhile, the
YCO and
fP for the first reaction rate factor are greater compared with those for the second one until the reduction of FeO begins, and then the comparison results are opposite, as shown in
Figure 4a,d. The change of slope of CO volume fraction curves and indirect reduction degree curves is caused by FeO reduction. The reason has been explained in
Section 3.1.
Figure 5 shows the reduction potential of surplus reducing gas for different coke reactivity. It can be seen from this figure that the reduction potential of surplus reducing gas decreases during the transformation from Fe
3O
4 to FeO with the increment of coke reactivity. This result implies that during this reduction stage, the surplus thermodynamic condition of reducing gas is weakened and the chemical energy utilization is increased, which indicate kinetics controls the reduction of Fe
3O
4. As shown in
Figure 5a, as the coke reactivity increases, the reduction potential of surplus CO in FeO reduction increases first and then decreases. In other words, the order of the reduction potential of surplus CO under three coke reaction rate factors is as follows: Δ
φ2 > Δ
φ0.1 > Δ
φ10, Δ
φ represents the reduction potential of surplus CO, the number of subscript is the coke reaction rate factor. As shown in
Figure 4a, the volume fraction of CO during FeO reduction increases when
α changes from 0.1 to 2, which induces higher Δ
φ. It illustrates that the surplus thermodynamic condition of reducing gas is enhanced and the utilization of chemical energy is reduced with the increase of coke reactivity, and the reduction is controlled by thermodynamics. When
α increases from 2 to 10, the equilibrium volume fraction of CO decreases and the volume fraction of CO decreases in the stage of FeO reduction, as shown in
Figure 4a, finally resulting in that Δ
φ2 > Δ
φ10. It indicates the chemical energy utilization of gas increases, but also implies the kinetics becomes a dominant role in the reduction. According to
Figure 5b, as the coke becomes higher reactive, the reduction potential of surplus H
2 in FeO reduction decreases, which means the chemical energy utilization of gas increases and kinetics dominates the reduction.
According to the rate formulas introduced in
Section 2.4, the reduction rate distributions under different coke reactivity in the indirect reduction region are obtained as shown in
Figure 6. The average reduction rates of Fe
3O
4 are calculated for a better comparison. The results show that with the increase of coke reactivity, the average CO reduction rate decreases from 57.3 mol/s to 39.3 mol/s and H
2 reduction rate decreases from 14.4 mol/s to 8.1 mol/s. It indicates that the temperature plays a great influence on the reaction rate and kinetics controls the reduction. As for the CO reduction of FeO in
Figure 6a, when
α increases from 0.1 to 2, the reaction rate increases at the same height because of Δ
φ2 > Δ
φ0.1, which indicates that the reduction potential has a greater impact on the reaction rate. It means the thermodynamics plays the leading role in the reduction. When
α raises from 2 to 10, the leading role becomes kinetics because the reaction rate decreases at the same height which illustrates the temperature has a great effect on the reaction rate. It can be seen from
Figure 6b, the reduction of FeO with H
2 is controlled by kinetics because the reaction rate decreases at the same height as the coke reactivity increases, which illustrates temperature acts a key role in reduction.
It is also concluded from the above analysis that the gas utilization efficiency of BF may not increase with the increase of coke reactivity if H2 content increases. Therefore, the improvement of gas utilization efficiency cannot be achieved by applying high-reactivity coke in the scenario that volume fraction of H2 is high.
An indirect reduction is mainly affected by CO reduction as mentioned, so the relationship between reduction degree and coke reactivity shown in
Figure 4d is similar to that between CO reduction rate and coke reactivity. The reason lies in the fact that volumetric rate of solids descent is fixed in this model, which indicates that the reaction time is the same within the same height. This leads to a conclusion that the greater the reaction rate, the greater the degree of reduction in the same height. Therefore, it can be inferred from
Figure 4d that the reduction process is controlled by thermodynamics first, and then by kinetics.
3.3. Influence of Iron Ore Reducibility
As mentioned above, iron ore reducibility plays an important role in the gas utilization efficiency. Therefore, it is necessary to study the effect of iron ore reducibility on the gas utilization efficiency. In
Figure 7, it can be seen that the effect of iron ore reducibility on the gas utilization efficiency is consistent with common sense, namely, it is a positive effect. When the reactivity of coke remains constant, i.e.,
α = 1, with the improvement of iron ore reducibility, the gas utilization efficiency increases. However, it can also be observed that the increment of the gas utilization efficiency is limited when
β is larger than 1. The reason lies in the fact that the gas utilization efficiency is also affected by other factors, such as the height of indirect reduction region, which remains unchanged when the coke reactivity remains stable.
The effects of coke reactivity and iron ore reducibility on the gas utilization efficiency at top of the BF were discussed above. The results demonstrate that simply increasing coke reactivity does not necessarily improve the gas utilization efficiency of the BF, so it is necessary to discuss the combination of coke and iron ore in order to obtain the maximum gas utilization efficiency. The results are shown in
Figure 8, taking
β = 0.1, 1, 10 as an example.
In
Figure 8, as a whole, the gas utilization efficiency decreases after increasing with the increase of coke reactivity. The reason has been illustrated above and is not repeated here. Moreover, overall, the curves of the gas utilization efficiency are convex and they become more gentle with the increment of reducibility of iron ore under the current operating conditions. This can be explained by the reason that when the iron ore reducibility increases to a certain extent, the deterioration of kinetic reduction condition, caused by the decrease in the TRZ temperature and height of indirect reduction region will be alleviated. In addition, it can be observed that although the combination of highly reducible iron ore and both highly and lowly reactive coke can get high gas utilization efficiency [
3,
10,
23], the lowly reducible iron ore cannot be ignored, either from the actual energy situation or from the economic point of view. For example, as shown in
Figure 8, lowly reducible iron ore, i.e.,
β = 0.1, combining with coke with reactivity of 2, i.e.,
α = 2, can obtain optimal gas utilization efficiency.