2020 Volume 60 Issue 5 Pages 856-864
Blast parameters are easy to be changed and are often applied to control the gas distribution in blast furnace. However, the effects of blast parameters on the gas and solid distribution characteristics were seldom investigated. Therefore, a 3D model considering gas and solid fluid phases is developed and used to analyze effects of blast parameters on the gas and solid distributions in the blast furnace B of Baiyi Steel. The results show that the gas temperature increases a little in the lower part while decreases in the middle and top parts of blast furnace when the oxygen enrichment ratio and blast temperature increase. The CO utilization ratio increases with the increase of oxygen enrichment ratio, humidified blast amount and blast temperature, and it increases most for the case of blast temperature. The metallization ratio at the bottom of the blast furnace increases with the increase of oxygen enrichment ratio and humidified blast amount while it decreases with the increase of gas temperature.
An ironmaking blast furnace is a huge chemical reactor involving counter-current flows of gas and solid.1,2,3) In this process, sinter, pellet, lump ore and coke are charged at the top of the furnace. Hot air, enriched oxygen and pulverized coal are blown into the furnace through the tuyeres. The reducing gas are generated from the combustion of coke and coal in the cavity around the exit of a tuyere called the raceway.2,3,4) The hot reducing gas then flows upward, heats up and reacts with the iron-bearing materials, and escapes from the top. Ironmaking blast furnace are predicted to dominate global hot metal production capacity in the foreseeable future.1,5) Therefore, it is important to analyze the gas and solid characteristics inside blast furnace since the smooth and stable operation of gas-solid flow plays a significant role in achieving process high performance.6)
Due to the harsh environment (high temperature, high pressure, molten materials and so on), it is very difficult to directly measure the internal flows in a blast furnace.7) Therefore, numerical modelling, often coupled with physical modelling, has become an attractive alternative to study not only blast furnace but also oxygen blast furnace, COREX, MIDREX and so on.8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33) For example, Tang et al. investigated the application of high-carbon metallic briquette in the ironmaking blast furnace based on gas-solid continuum model.9) Dong et al. studied the shaft injected gas penetration behavior in the oxygen blast furnace based on computational fluid dynamic-discrete element method (CFD-DEM).11) Zhang et al. evaluated the performance of central gas distribution device in COREX shaft furnace with top gas recycling based on gas-solid continuum method.13) Zhou et al. studied the effect of center gas supply device on the solid flow of a large-scale reduction shaft furnace based on discrete element method (DEM).14) Ghadi et al. studied the effect of dual gas injection system on the distribution of process variables and energy consumption in the MIDREX shaft furnace based on gas-solid phase model.15) The existing approaches can be discrete-based (e.g. DEM, CFD-DEM) or continuum-based (e.g. two fluid model, three fluid model, four fluid model) with respect to the solid phase.34) In 1993, Yagi reviewed the continuous flow behavior of four phases, extended from the models for single-, two-, and three-phase flows for the whole blast furnace.35) Dong et al. reviewed the model developments of both continuum and discrete studies for gas-solid, gas-liquid, gas-powder, and multiphase flows inside blast furnaces from 1993 to 2007.7) Kuang et al. reviewed the mathematical models for different blast furnace regions from the top to the bottom in respects of both continuum and discrete studies mainly from 2007.8) Ueda et al. and Ariyama et al. reviewed the some progresses on application of DEM to blast furnace in 2010 and 2014, respectively.36,37)
As for the blast furnace operation, the whole blast furnace can be divided into four main regions: burden charging system in the top, hearth, raceway, and main body including lumpy zone, cohesive zone, dripping zone and deadman.8,38) There have been lots of literature concerning on the details of the multi-scale in-furnace states based on the numerical simulation.39,40,41,42,43,44,45,46,47,48,49,50,51,52) The main body is very important for the operation and optimization of a blast furnace. Ueda et al. investigated the influence of shape of cohesive zone on the gas flow inside main body of blast furnace based on DEM-CFD model.45) Yang et al. established an axi-symmetric 2D steady CFD model considering two phases of gas and solid to investigate the effect of burden distribution on the internal state of a blast furnace.47) Guo et al. analyzed the in-furnace status of blast furnace operation with hot burden charging by means of multi-fluid model.48) Castro et al. simulated the blast furnace operation under simultaneous injection of pulverized coal and charcoal into the blast furnace by developing a 3D CFD model considering six phases.49) They then investigated the effects of injection of pulverized coal and natural gas on blast furnace production efficiency, coke rate and slag rate with a 2D CFD model considering five phases.50) Shen et al. developed a 3D CFX-based on model to describe the internal state of a blast furnace in terms of multiphase flow, the thermochemical behavior and process indicators.51) However, these studies concentrated more on the basic characteristics of the main body of the blast furnace. Numerical simulations of the effects of blast parameters on the gas and solid distribution characteristics inside the blast furnace were seldom performed. Chu et al. investigated the effects of humidified blasting, natural gas injection and waste plastics injection, on the operation of all-coke blast furnace by means of multi-fluid model.52) However, most blast furnaces are not all-coke operation but with pulverized coal injection. Therefore, it is necessary to investigate the effects of blast parameters on the gas and solid characteristics inside the blast furnace with coal injection.
Therefore, a three dimensional model of gas-solid two phase numerical simulation is established based on the actual production data of blast furnace B in Bayi Steel of Baowu Group. It should be mentioned that the blast furnace B is operated with high reactivity coke, which is different from other blast furnaces in China. Therefore, the present study is interesting and meaningful for the numerical simulations of blast furnaces. The established model in this work is then used to analyze the effects of oxygen enrichment ratio, humidified blast amount, blast temperature on the gas and solid distributions in the blast furnace.
A one-sixth (60 degree section) sector of a blast furnace was established based on the blast furnace B in Bayi Steel of Baowu Group in China, as shown in Fig. 1(a), where the part below the taphole in the blast furnace hearth is not included in the model for the sake of simplification. Though 2D models may reveal the change of furnace indicators under different conditions to a certain degree, 3D model and analysis are naturally important for practical blast furnace problems and recommended for simulating the multiphase flow in blast furnace, especially in the vicinity of the raceway.35) The characteristics of raceway and the part between two near raceways are different and may have effects on the gas and burden distribution in blast furnace, therefore, it is better and more accurate to carry out the simulation in 3D model.
Geometric and mesh models and volume fraction distribution of solid phase of No. B blast furnace in Bayi Steel of Baowu Group (a) geometric model, unit: mm; (b) mesh model; (c) volume fraction distribution of solid phase. (Online version in color.)
The present model considered both gas and solid phases. The gas phase was the same as that in practice. Gas phase components at different positions were calculated based on the material balance of the chemical reaction at the corresponding positions. The ideal gas law was applied to calculate the density of gas phase. The solid phase was simplified to include Fe2O3, Fe3O4, FeO, Fe and C. The density of solid phase was 4190 kg·m−3, the same as the apparent density of the original solid material. Both gas phase and solid phase were considered as continuous phases using the Eulerian method. The mass, energy and species transfer can be described by Eq. (1) under steady state.35,53)
Rn refers to different reactions.
The chemical reactions between gas phase and solid phase are as follows.
The chemical reaction rates were calculated by the three-interface unreacted core model based on physical chemistry data from Perry et al.’s book.58) The reaction rate constants of the indirect reduction of iron ore by CO or H2, carbon solution loss reaction, water gas reaction, combustion of carbon and water gas shift reaction were taken from other work.58,59,60,61,62) The combustion rate of H2 with O2 was taken from Kuwabara et al.’s work.63) The effective diffusion coefficients were taken from other work.58,59,60,61,62) The viscosity and thermal conductivity of gas were obtained from the literature.19,64,65,66)
In the numerical solution of the arising differential equations, a structured grid was applied. The average mesh size is 40 mm, and this mesh model has been proved to be mesh independent.
The assumptions for this model were as follows: (1) the powder phase was not considered; (2) other chemical reactions such as flux decomposition and reduction of non-ferrous compounds were ignored. (3) the melting of solid was ignored.
The total Fe content was 56.20% in the iron bearing materials, and the C contents in coke and coal were 87.38% and 62.46%. Based on the material balance, Fe in the iron bearing materials was converted to and considered as Fe2O3 and only C in the coke was taken into account. Therefore, the mass fraction of Fe2O3 and C were 77.05% and 22.95% in the blast furnace top, respectively. The volume fraction of the solid phase was fixed, as shown in Fig. 1(c).67,68,69,70,71) The burden velocity distribution was calculated firstly without considering the heat and energy transfer. The results of burden velocity were then loaded in the simulation with consideration of all the transfers and reactions. The coal injection and the oxygen of blast at the tuyere were converted to CO. Therefore, the gas compositions at the tuyere were N2-74.92 vol%, O2-15.34 vol%, CO-9.15 vol%, H2O-0.59 vol%. The conservation equations were solved numerically by the finite volume method with commercial software ANSYS FLUENT (release 17.0).72) The first order upwind scheme was used for discretization and then the coupled SIMPLE method was applied.53,62) The simulation was considered to have converged when the residuals for each variable are less than 10−5.
The bottom plane of the blast furnace is defined as 0 m height position. Figure 2 presents the gas and burden temperatures along the height of blast furnace under the condition of practical operational data. The horizontal positions are the sections parallel to the bottom plane of the blast furnace by the center. It should be noticed that the value of temperature at different height is computed by dividing the summation of the facet values of temperature by the total number of the facets. The temperature distributions of the gas and the burden are almost the same except that the burden average temperature is slightly lower than the gas temperature at the same position. The difference between gas and burden is within 40°C in the middle and lower part of blast furnace. It can be seen that the temperature increased at first and then decreased later. There is a peak for the temperature curve. The reason is that the injected gas brings in lots of physical heat and the combustion of C, CO and H2 also bring in lots of heat. It should be noticed that the peak position is 6 m, which is 3.5 m higher above the tuyeres. The reason is that the temperature is the section average temperature at each height. Actually, the position of the highest temperature is about 1.5 m higher above tuyeres, which is close to the operating blast furnace. The trend of the temperature distribution along blast furnace height is similar with those in previous research, which also proves the accuracy of the present simulation.38,49,50,73) The highest temperature of burden is 2616 K in the model. In practice, only coke and iron exist at this high temperature. The present model does not consider other components like CaO, SiO2, Al2O3, MgO, et al. The fusion of these components and the forming of the slag will absorb lots of heat. This may be the reason why the temperature of burden reaches 2616 K.
Gas and burden temperatures along the height of blast furnace under the condition of practical operational data.
Figure 3 shows the burden material distributions in the blast furnace under the condition of practical operational data. Iron oxides are reduced more readily near the center of blast furnace. Fe2O3 are reduced to Fe3O4 quickly when the burden is discharged into the blast furnace. Fe3O4 is then reduced and disappears at the height of 12.5 m. FeO is formed and reduced at the same time and the mass fraction of FeO reaches maximum at the height of 11.4 m. Fe is formed at the height of 22 m and reaches maximum 89.55% at the bottom of the model.
Distributions of mass fractions of burden phase in the blast furnace under the condition of practical operational data/–. (Online version in color.)
Figure 4 shows the volume fractions of different gas compositions along the height of blast furnace under the condition of practical operational data. The value of volume fraction at different height is computed by dividing the summation of the facet values of volume fraction by the total number of the facets. It can be seen from Fig. 4 that the volume fractions of CO and H2 decreases gradually along the height of blast furnace while those of CO2 and H2O increases. The reason is that the reduction of iron oxides consumes CO and H2 and then produces CO2 and H2O. The results are similar with those from previous works.38,51,62,68) The volume fractions of CO, H2, CO2 and H2O at the top of the blast furnace model are 24.02%, 2.43%, 18.87%, 2.98%, respectively. The model is verified against some measured results in the practical production as shown in Table 4, where H2O is not listed since it is not measured in the practice. The maximum relative error between the measured and calculated results is 6.3%, so the present model is considered to be applicable to predict the characteristics inside the furnace. The CO utilization ratio is calculated as
Average volume fractions of gas phase along the height of blast furnace. (Online version in color.)
The metallization ratio is calculated as
The oxygen enrichment mode is at fixed blast volume. This means that the flowrate of the hot blast air is kept constant. Oxygen is added to the blast to form a mixture of air and oxygen, which leads to the change of gas compositions in the tuyeres. The amount of coal injection changes corresponding to the oxygen enrichment, based on the C and O element balance calculation. Based on the practical experience, the production, coke rate and coal rate change with the oxygen enrichment ratio varying. The coke rate decreases by 5 kg·t−1 and the production increases by 3.3% when the oxygen enrichment ratio increases by 1%. At the same time, the mass and heat balance should be also recalculated when the oxygen enrichment ratio changes. In this way, the amounts of injected gas, coal, coke, pig iron are calculated and determined, then the compositions of injected gas, the amount of ores, the top gas volume can be calculated based on the balances of Fe, C, H and O. After oxygen enrichment, the typical parameters when the oxygen enrichment ratio changes are shown in Table 5. The oxygen enrichment ratio was from 1% to 5% at a step of 1%. Figure 5 shows the effect of different oxygen enrichment ratios on the gas temperature along the height direction in blast furnace.
Effect of different oxygen enrichment ratios on the gas temperature along the height direction in blast furnace. (Online version in color.)
Gas temperature below 8.4 m is higher than that of the base model and increases with the increase of oxygen enrichment ratio. The gas temperature above 8.4 m is just the opposite. When the oxygen enrichment ratio is 5%, the temperature of lower part is about 130°C higher than the base model, and the temperature of upper and middle part is about 80°C lower than the base model. The reason is that the heat in the lower part of the blast furnace increases with the oxygen enrichment, which leads the temperature to rise. However, the gas volume for one ton production decreases with the increase of oxygen enrichment, which leads to the reduction of heat in the middle and upper part of the blast furnace, and the temperature decreases as a result. The temperature of top gas decreases gradually as the oxygen enrichment ratio increases. The top temperature under 5% oxygen enrichment ratio is 44°C lower than that of base model.
Since the tendency of H2 and H2O contents is similar to that of CO and CO2 contents, only CO and CO2 contents under the effect of different gas parameters are analyzed in the following part. Figure 6 shows the effect of oxygen enrichment on the gas composition in the blast furnace. The CO and CO2 contents increase with the increase of oxygen enrichment. The reason is that more coke and pulverized coal are burned at the tuyeres with the increase of O2, which leads to the increase of the content of reducing components in the gas. Boudouard reaction also consumes CO2 to produce CO, which also contributes some more CO. However, Boudouard reaction is a reverse reaction. Therefore, a balance of the amounts of CO and CO2 involved in the iron oxides reduction and Boudouard reaction occurs.
Effect of oxygen enrichment ratios on the gas compositions in blast furnace. (Online version in color.)
Table 6 shows the effects of oxygen enrichment on CO utilization ratio and metallization ratio. It can be seen that the CO utilization ratio and metallization ratio increase with the increase of oxygen enrichment. On the one hand, the increase of CO content in the gas makes the indirect reduction easier, and the lower temperature in the upper part of the blast furnace is also beneficial to the indirect reduction of CO. However, due to the increase of production, the residence time of the burden in the blast furnace becomes shorter, which is bad for the indirect reduction. Therefore, the CO utilization ratio of gas slightly increases under oxygen enrichment condition. Due to the increase of the reducing components in the gas, the reduction rate is accelerated. Therefore, the metallization ratio is higher than that of base model.
The boundary conditions at different humidified blast amounts are also calculated based on material balance. Humidified blast is always performed with oxygen enrichment in order to keep a relative constant theoretical combustion temperature. Coke rate also increases after humidified blast. According to the practical operation and previous work, the coke rate decreases by 1 kg·t−1 when the humidified blast amount increases 1 g·m−3. The humidified blast amount is from 10 g·m−3 to 30 g·m−3 at a step of 5 g·m−3. For every 5 g·m−3 increase in blast humidity, the oxygen enrichment rate increases by 0.45% and the production increases by 132 t while the coal rate does not change. The typical parameters when the humidified blast amount changes are shown in Table 7. It can be seen that both gas volume and production increase but the increase degree of gas volume is lower than that of production, therefore, the gas volume for per ton production decreases. Figure 7 shows the effect of different humidified blast amounts on the gas temperature along the height direction in blast furnace.
Effect of different humidified blast amounts on the gas temperature along the height direction in blast furnace. (Online version in color.)
It can be seen that the gas temperature decreases with the increase of humidified blast amount. The reason is that the gas volume for per ton production decreases though the total gas volume increases. And the reduction reaction with H2O as reactant is endothermic. It should be noticed that the temperature decreases more in the range of 1000–1500°C, and the gas temperature is 145°C lower in the case of 30 g·m−3 than the base model. This is because this temperature range is the main area where direct reduction occurs. The H2O and H2 contents increases under the humidified blast condition. Therefore, the rate of water gas reaction increases and absorbs more heat to lower the temperature. The temperature of the top gas decreases by 44°C when the blast humidity is 30 g·m−3.
Figure 8 shows the effect of humidified blast on the gas composition in the blast furnace. Under humidified blast conditions, the CO and CO2 contents in the gas increase a little compared with the base model because oxygen enrichment is required to maintain the temperature of lower part of blast furnace. The H2 and H2O contents increase more than CO and CO2 contents because the H2 content in the tuyeres increases.
Effect of humidified blast amounts on the gas compositions in blast furnace. (Online version in color.)
Table 8 shows the effects of humidified blast on CO and H2 utilization ratio and metallization ratio, where the H2 utilization ratio calculated is similar to that of CO utilization. The gas utilization ratio increases slightly as the blast humidity increases. The gas utilization ratio at the case of 30 g·m−3 blast humidity is only 0.17% higher than the base model. On one hand, CO content in the gas rises and the gas volume for per ton production decreases under humidified blast condition, which drives more indirect reduction of CO. However, on the other hand, the increase of the production increases the burden descending velocity, which limits the indirect reduction. Therefore, the gas utilization ratio changes little under the humidified blast condition. The rate reducing FeO to Fe increases since the H2 content in the gas increases under humidified blast, and the reduction ability of H2 at high temperatures region is higher than that of CO. Therefore, metallization ratio at the bottom of the model is higher than that of base model.
The boundary conditions at different blast temperatures are also calculated based on material balance. The blast temperature of the base model is 1118°C. Then the blast temperature is varied from 1150°C to 1250°C at a step of 50°C. With the increase of blast temperature, the gas volume keeps constant, and the heat of blast replaces part of the heat of coke combustion. Therefore, the coke rate decreases in practice when the blast temperature increases. At the same time, the coal amount increases to maintain a reasonable theoretical combustion temperature with the increase of blast temperature. The typical parameters when the blast temperature varies are illustrated in Table 9. Figure 9 shows the effect of different blast temperature on the gas temperature along the height direction in blast furnace.
Effect of different blast temperatures on the gas temperature along the height direction in blast furnace. (Online version in color.)
The temperature below the height of 8 m increases when the blast temperature increases. However, the temperature above 8 m decreases. The reason is that the heat brought in the lower part of blast furnace increases with the increase of blast temperature. The reason may be that the gas volume for per ton production decreases with the increase of blast temperature since the gas volume is fixed. The specific heat capacity of gas or burden changes little, and the weight of gas almost keeps constant while that of burden increases. Therefore, the water-equivalent of gas almost keeps constant while that of burden increases, which leads the water-equivalent of gas to burden decreases with the increase of blast temperature. These lead the gas temperatures of the furnace shaft and the furnace top to decrease. The temperature of top gas decreases by 40°C at 1250°C blast temperature.
Figure 10 shows the effect of blast temperature on the gas composition in the blast furnace. Under high blast temperature conditions, CO decreases while CO2 increases compared with the base model. The decrease of CO is because the fuel required in the blast furnace decreases with the increase of blast temperature. The increase of CO2 is because that total gas amount decreases due to the decrease of fuel amount.
Effect of blast temperatures on the gas compositions in blast furnace. (Online version in color.)
Table 10 shows the effects of blast temperature on CO utilization ratio and metallization ratio. The gas utilization ratio increases a little as the blast temperature increases. When the blast temperature reaches 1250°C, the gas utilization rate is increased by 1.66% than the base model. The metallization ratio decreases a little as the blast temperature increases. This is because the CO content in the gas decreases and the burden descending rate increases a little due to the increase of production, which leads to the decrease of the reduction reaction of burdens.
A three dimensional model of gas-solid two phases was developed and used for the simulation of the actual production data of blast furnace B in Bayi Steel, which is operated with high reactivity coke and with coal injection. The effects of oxygen enrichment ratio, humidified blast amounts, blast temperature on the gas and solid distributions in the blast furnace are investigated with this model, which were seldom investigated before. The conclusions can be summarized as follows.
(1) When the oxygen enrichment ratio and blast temperature increase, gas temperature increases below about 8 m height of blast furnace while decreases above about 8 m height with the increase of oxygen enrichment ratio. When the humidified blast amount increases, the gas temperature decreases in the whole blast furnace.
(2) The CO and CO2 contents increase with the increases of oxygen enrichment ratio and humidified blast amount. The CO content decreases while CO2 content increases with the increase of blast temperature.
(3) The CO utilization ratio increases when the oxygen enrichment ratio, humidified blast amount and blast temperature increase. It increases most for the case of blast temperature, followed by oxygen enrichment ratio and humidified blast amount.
(4) The metallization ratio at the bottom increases when the oxygen enrichment ratio and humidified blast amount increase while it decreases when the blast temperature increases.
This work was supported by the National Key R&D Program of China (grant number 2017YFB0603800, 2017YFB0603803), the National Natural Science Foundation of China (grant number 91634106, 51804027) and Natural Science Foundation of Chongqing, China (grant number cstc2019jcyj-msxmX0089).