Development of the Flash Ironmaking Technology (FIT) for Green Ironmaking with Low Energy Consumption

This article describes the development of a novel ironmaking technology based on flash reduction. The development started with the proof of the kinetic feasibility, considering that a typical flash reactor provides only a few seconds of residence time. This was followed by tests in a laboratory flash reactor and finally a pilot plant operation. The rate equations formulated in this work were developed considering the optimum combination of temperature, residence time, and reducing gas partial pressure to achieve > 95% reduction degree. Experiments in the intermediate-scale laboratory flash reactor indicated that more than 90% reduction degree could be obtained in a few second residence time at temperature as low as 1175 °C. A pilot reactor operating at 1200–1550 °C was installed and run to collect data necessary for scaling up the process. The tests in this large reactor validated the design concept in terms of heat supply and residence time, and identified technical hurdles. This investigation proved the technical feasibility of the flash ironmaking technology. The results of this work will facilitate the design for the industrial flash ironmaking reactor. The novel technology is expected to decrease the energy consumption in ironmaking by up to 44% compared with the average blast furnace process, and will reduce CO2 emissions by up to 51%. When hydrogen is used, the proposed process would use up to 60% less energy with little carbon dioxide emissions. However, it is noted that the energy requirements and CO2 emissions during the production of natural gas, hydrogen or coal must be added for a comprehensive comparison.


Introduction
This paper describes the development of the novel Flash Ironmaking Technology (FIT) [1], which is based on the reduction of iron oxide concentrates by gas in a flash reactor. The development of the novel process also includes two of the most serious aspects in steelmaking, i.e. carbon dioxide emissions and energy consumption. The steel industry contributes around 6 ~ 7 % of the total anthropogenic emissions of carbon dioxide [2].
Worldwide, the blast furnace process currently produces more than 90% of iron, while the rest is produced by the Direct Reduction processes [3]. The blast furnace process needs the solid charge to be sinters or pellets and coke. The production of these two solid charges, which are eliminated in the proposed process described in this paper, requires much energy and is pollution-prone.
Two general groups of gas-based alternate technologies for making iron have been developed: shaft furnace processes [4] and fluidized-bed processes [5][6][7]. These technologies, however, are not sufficiently intensive to compete with the blast furnace, for they cannot be operated at high temperatures because of the sticking problems.
Therefore, a novel Flash Ironmaking Technology (FIT) by which iron is produced directly from concentrates by a flash reduction process has been developed. This new technology utilizes hydrogen or natural gas as a fuel as well as reducing agent. It does not require pellets, sinters or coke [1,[8][9][10]. Furthermore, iron ore concentrate that would be used in a FIT reactor is currently produced in large quantities from taconite ore in the U.S. [11].
In this article, the development of the novel ironmaking process is described starting from the kinetic feasibility determination to the laboratory flash furnace work and finally the tests in a pilot plant. Process simulation and economic analysis on the new process [12][13][14][15] is also summarized.

Description of the Flash Ironmaking Technology (FIT)
A sketch of the Flash Ironmaking process is shown in Figure 1. A gaseous fuel is partially oxidized with industrial oxygen to generate a reducing gas at 1600 -1900 K. Iron ore concentrate is fed from the top, and the reduced iron product can be collected as a solid powder or as a molten bath for direct steelmaking.
Natural gas is also plentiful in the U.S. and could easily supply potential ironmaking based on the FIT [16]. Hydrogen would be cleaner once the hydrogen economy is developed [17,18]. On the other hand, the application of hydrogen to ironmaking would provide a large market for hydrogen, thus facilitating its development. The development of the FIT started with the establishment of sufficient kinetic feasibility, considering the fact that there are only a few seconds of residence time available in a typical flash reactor. Upon the establishment of the kinetic feasibility, a laboratory flash reactor was tested, which was followed by a pilot-scale reactor test program.

Reduction Kinetics of Magnetite Concentrate Particles
Magnetite concentrate from a taconite ore of the Mesabi Range was used in this study, the chemical composition of which is presented in Table 1. Sohn and coworkers [19][20][21][22][23][24] have studied the reduction rates of magnetite concentrate under the conditions of the FIT. The results were expressed by the following equation for component gases H2 or CO: where kj is the reaction rate constant for gas , = , exp (− ); pj is the partial pressure of gas j; Kj is the equilibrium constant for the reduction of FeO by gas j; is the reaction order with respect to gas j; nj is the Avrami parameter; − is the particle size dependence; X is the fraction of the total removable oxygen in the concentrate particles removed by the reaction. The relevant kinetic parameters are given in Table 2. The reader is referred to the original papers [19,20,23,24] for other details of the rate measurements and data analyses. When magnetite concentrate is reduced by a mixture of H2 + CO, the CO enhances the rate of reaction between H2 and iron oxide. This is most likely due to the effect of CO on the morphology of the reduced iron by forming whiskers, which was observed in a separate study [25]. Taking this into consideration, Fan et al. [20] developed the following rate expression: The complete rate equations for magnetite concentrate reduction by a H2+CO mixture at 1423 K (1150 ˚C) -1623 K (1350 ˚C) and 1623 K (1350 °C) -1873 K (1600 °C) are given, respectively, as: where | 2 and | represent the rates of reduction individually by H2 and CO, respectively, obtained from Eq. (1) with the parameters listed in Table 2. These kinetics measurements confirmed the fact that a few seconds of residence time in a flash reactor at temperatures of 1473 K or higher are sufficient to reduce iron ore concentrate.

Tests in a Laboratory Flash Reactor
After verifying the sufficiently rapid kinetics of the reduction, experiments on flash ironmaking were conducted in a laboratory flash reactor shown in Figure 2 [8,26,27]. The reactor was made of steel. A uniform temperature zone was maintained at 1175 ± 25°C by the partial oxidation of methane and/or hydrogen with oxygen. Methane was used here to represent natural gas that is likely to be used in industry, which is largely made up of methane. The heating was augmented by electrical heating. The partial oxidation of methane generated H2 + CO [26][27][28][29].

Flame Configuration
The burner consisted of a nozzle, shown in Figure 4, made of Inconel. Two flame configurations were tested by exchanging the oxygen and fuel feeding ports. In the F-O-F configuration (H-O-H when hydrogen was the fuel), the fuel was fed through Slot 1 and surrounded the oxygen fed through Slot 2. In the O-F-O configuration, oxygen was fed through Slot 1 surrounding the fuel fed through Slot 2. These changes in the burner configuration affected the temperature profile in the reactor. Figure 5 shows the different temperature distributions in the cases of hydrogen as the fuel in (a, b) and methane as the fuel in (c, d) simulated using CFD analysis [27].

Experiments with Hydrogen
A high-temperature mixture of hydrogen and water vapor, with a sufficient reducing power for iron oxide, was generated from the partial oxidation of hydrogen. A reduction degree greater than 90% was achieved with some excess hydrogen in a few seconds of residence time at a temperature as low as 1175 °C. The reactor was made of a steel tube and thus this temperature was the maximum wall temperature that could be reached in this laboratory flash furnace. The experimental data are presented in Table 3. Figure 6 shows the extent of reduction as a function of reaction time and the Excess Driving Force (EDF). Here, EDF denotes the excess H2 present in the gas above the equilibrium level, defined as follows:   The temperature in the center of the flame with the O-H-O flame configuration was lower than that in the case of H-O-H flame, 1234 °C and 2624 °C, respectively. Therefore, particles retained their irregular shape and were reduced more rapidly. Figure 8 shows the particle shapes in the cases of different flame configurations.

Experiments with Methane
The laboratory flash reactor was also operated by partially combusting methane gas with pure oxygen, which produced heat and a gas mixture of H2+ CO+H2O+CO2 [28]. Hydrogen was fed with methane gas to help stabilize the flame. When only methane was injected, the flame was unstable under the partial oxidation conditions and soot was formed at the burner nozzle and the top part of the reactor. Further, in an industrial flash furnace, the injected fuel and reductant gas should be natural gas mixed with the recycled hydrogen recovered from the off-gas, considering the fact that the off-gas must contain a significant amount of hydrogen even at the equilibrium condition. The tested flowrate of hydrogen was varied from 0.1 to 2 L/min.
The temperature in the flame in the methane experiments was lower than in the hydrogen experiments, as shown in Figure 5, and this also contributes to the lower reduction degree with methane.
The residence time was calculated based on the measured temperature profile. Table 4 lists the experimental conditions. A reduction degree of 80 ± 5% was obtained at EDF =1 based on hydrogen content and particle residence time of 8 seconds even at the low temperature of 1175 °C. Based on the results from the experiments with hydrogen, the O-F-O configuration was used when the solid was fed through the burner to avoid melting the particles. Hydrogen is the main reductant at 1175 °C temperature, and CO contributes much less to the reduction especially in the presence of CO2. The hydrogen EDF values tested were 0.5 and 1. This configuration yielded reduction degrees higher than with the F-O-F configuration, as shown in Figure 10.
The samples collected from these experiments showed no melting.

Computational Fluid Dynamics (CFD) Simulation
The laboratory flash reactor runs were simulated by a three-dimensional computational fluid dynamics (CFD) model. The governing equations for the gas phase in the Eulerian viewpoint and for the particles tracked in the Lagrangian framework using the stochastic trajectory model were solved using the commercial CFD software package ANSYS FLUENT 15.0 [27,28]. Figure 11 shows the shapes of the injection ports used in the CFD simulations. The calculated % reduction and the associated experimental results are compared in Figure 12. There is a satisfactory agreement, except when the % reduction was low. The ability to predict higher % reduction is more important in the design of industrial reactors, because the new technology should produce iron with a reduction degree greater than 90 %. Table 5 shows the test conditions, experimental and calculated degrees of reduction for the experiments selected for simulation from those listed in Table 3.  The same CFD program used for hydrogen reduction in the Utah Flash Reactor was also used to simulate the experiments with methane-oxygen feed, as reported elsewhere [27].

Operation of a Pilot-Plant Scale Flash Reactor
Here, we will describe a Pilot Flash Reactor operated at 1200 -1550 °C with a solid feed rate of 1 -7 kg/h, shown in Figure 13 [28] together with its operation and the operational difficulties. In this larger-scale reactor, natural gas was used as the fuel/reducing gas instead of methane, as would be in an industrial flash ironmaking process.

Facility
The Pilot Flash Reactor consisted of a reactor vessel, burners, a quench tank for product collection, a stack, a powder feeding system, and human-machine interface. Figure 14 shows a sketch of the main parts of the reactor vessel.
Reactor Body and Roof: The carbon steel vessel was lined with 0.3 cm of high quality aluminasilica fiber blanket, 8 cm of insulating layer of high fired, lightweight crystalline silica, and 18 cm of castable 99.8% alumina refractory layer. The outer shell temperature was about 220 °C when the temperature of the inside alumina surface was 1450 °C. The inner diameter of the vessel was 80 cm and the height 200 cm, as shown in Figure 15. The roof had an opening for burners as well as an emergency off-gas conduit equipped with a rupture disk, which was designed to break when the pressure inside the reactor exceeded 1.5 atm above the external pressure.
B-type thermocouples were inserted such that the junctions protruded 2.5 cm into the reactor from the inside surface of the wall. These thermocouples were capable of measuring temperatures up to 1860 °C. The thermocouple signals were used to control the temperature inside the reactor vessel.
Burners: The Pilot Flash Reactor had a burner for preheating the reactor and a separate burner for the process gases. Figure 16 shows a sketch for the cross section of the preheat and the main burner. The preheat burner used a natural gas-oxygen flame. The main burner fed natural gas and industrial oxygen to produce a high-temperature reducing gas mixture. The maximum flow rates that the main burner could operate at were 954 SLPM of oxygen and 1116 SLPM of natural gas. The standard conditions were pre-defined in the instruments as 1 atm and 25 °C. The burner nozzle was made of Inconel alloy and a zirconia refractory block surrounded the burner tip. This burner was designed to have a flame configuration of O-F-O, as determined from the results of the laboratory flash reactor.
Off-gas Analyzer: The composition of the off-gas was analyzed before it flowed into the flare stack, using a NOVA gas analyzer Model 875A Steel Making Analyzer supplied by Tenova, ON, Canada. An NDIR (infrared) detector was used to measure the contents of CO, CO2 and CH4, a thermal conductivity cell measured the H2 content, and an electrochemical sensor measured the O2 content. This gas analyzer had a resolution of ± 0.1%. The gas analyzer was calibrated before each run with a calibration gas mixture that contained a fixed composition of H2, CO, CO2 and CH4.
Concentrate Feeding System: An HA5171P-D pneumatic powder feeder supplied by HAI, Placentia, CA, U.S. fed the concentrate into the reactor at a rate of 1-7 kg/h. Nitrogen gas at a flow rate of 11 SLPM was used as the carrier gas. The powder feeder had weighing cells that recorded the average feeding rate during the operation.

Gas Leak Detectors:
Industrial gas leak detectors supplied by Honeywell, Lincolnshire, IL, U.S. were placed in different places around the reactor and inside the building to monitor any H2, CO, and CH4 gas leak. All the detectors were connected to the main PLC to shut down all the gas flows in the case of a major leak that required evacuation of the building and purging of the vessel.
Human Machine Interface: The Human Machine Interface (HMI) consisted of the main PLC and a PC. The operator monitored all parts of the facility and ran the reactor through the computer to which the main PLC continuously fed information. All the safety and emergency procedures relied on the PLC. The main PLC was supplied by ACS Company, Boise, ID, U.S. A screen shot of the computer monitor that displayed and controlled all the parameters of the operation is shown in Figure 17. Other Details of Pilot Plant: Other details of the operation of the Pilot Reactor are described elsewhere [28].

Operation of the Pilot Flash Reactor
The solid fed to this reactor was as-received magnetite concentrate with a particle size range of < 90 µm by sieving the concentrate with a 90 µm sieve and using the concentrate that has passed through it. For the experiment that was performed with particle size range 32-90 µm, the concentrate was sieved and collected between 90 and 32 µm sieves. All the components of the reactor were installed and a leak test was performed by capping the off-gas pipes and pressurizing the vessel to 2.0 atm for 45 minutes to make sure that there were no leaks from any components. The system was preheated to the target temperature at a rate of 90-95 °C/h, which was the highest heating rate that could be applied to avoid cracking the refractories. The heating cycle was automatically controlled by the HMI, and the flow rates of natural gas and oxygen were varied based on the measured temperature of the reactor vessel.
Once the system reached the target temperature, the oxygen and natural gas were switched to the main burner. However, the pilot burner was kept ignited all the time with small flows of the fuel and oxygen. The system then sent a signal to the flare stack to turn on a pilot flame and burn the off-gas. Once the system received a signal that the flare stack was operating, the operator changed the flow rates of the oxygen and natural gas through the main burner to the target experimental values.
The off-gas composition was measured during the run and the hydrogen reducing power was calculated on the HMI. The flow rates of oxygen and natural gas were adjusted to achieve the target reducing power. After feeding a predetermined quantity of the concentrate, the system was shut down by switching all the gas streams to nitrogen. The flare stack was kept running for 5 minutes longer to burn any residual combustible gases flowing out of the reactor. The reactor was left to cool to a temperature below 400 °C in the main vessel and 25 °C in the quench tank. This typically took 4 days. The collected sample was analyzed by ICP to determine its reduction degree. Table 6 shows the results from the Pilot Flash Reactor runs. These results were used to develop a CFD model that would be used to optimize the operating conditions and reactor sizes to be used in an industrial reactor [30]. Different experimental runs were designed and made in this reactor to yield a range of reduction degrees, deliberately at less than complete reduction, to better examine the effects of the operating conditions and validate the CFD model.

Results from Pilot Flash Reactor Runs
In this reactor, no soot was formed during the experiments. LECO analysis was performed on some of the samples collected from this work. Only one sample contained a small amount of carbon, less than 0.24% C, while all other samples contained no detectable amount of carbon.
The experiments in the pilot plant were performed at different temperatures and reducing powers of the gas with the aim of obtaining enough data for designing the industrial flash reactor. A nominal residence time for the particles was calculated, for a general reference, based on the total volumetric flow rate of the gases at the experimental temperature and the total volume of the reactor vessel which was 1.01 m 3 . When the velocities of the gases and the particles at the injection points are rigorously applied, the actual residence time would be shorter. More accurate residence time distribution was obtained by the CFD simulation of the operation.
The results showed good reproducibility within ±5% of the average reduction degree by repeating the same experiment at least 3 times. This represents a very high degree of reproducibility, considering the complexity of the operation and design of this large unit.

CFD Simulation of the Pilot Reactor Operation
The same CFD model for the laboratory flash reactor discussed above was used for the pilot reactor runs [30]. 480 particle streams were released from the particle injection ports. During a run the wall temperature somewhat varied with time, Since our model was a steady-state model, the wall boundary condition was represented by an average tempeature that gives a rate constant which represents the average rate constant over the period of change, computed using a separate MATLAB program. This averaging method was thought to be somewhat better than the arithmetic average of the varying temperature. These inner wall temperatures used in the simulation are shown in Table 7.
The particle size distribution of the concentrate measured by the supplier was < 90 µm and the concentrate was used as-received in Runs 2, 3, 5, and 6. For Runs 1 and 4, the concentrate was screened to 32 -90 µm. The rates of a particle assemblage can be represented by that of the uniform particles of the mass average size [31,32] and the particles in this size range move with a negligible slip velocity in the flash reactor [21]. Considering these factors, the CFD simulation used the mass average particle sizes listed in Table 7, which simplified the computation greatly, without much loss of accuracy of the computed results. The CFD model predicted the H2 and CO concentrations within 93% accuracy for most runs, as shown in Figure 18. The experimental values of % reduction are compared with the CFD results in Table 7. The reduction degrees agree well for the first three runs. The agreement is not as good for the last three runs. The reason for this is likely to be because of the neglect of particle interactions for Runs 4 -6. The temperature of the particle-gas stream in the main reaction zone was largely uniform, and this value is used to represent the reactor temperature. These runs had higher ratios of oxygen to natural gas and thus higher temperatures than the other runs, above 1577 °C, which is higher than the melting point of iron at 1538 °C. Particle agglomerate together more readily at these high temperatures, as shown previously during flash smelting of copper [33][34][35]. This might have caused lower reduction rates in the actual cases than in the simulation. This points to the need for improving the CFD model to account for particle coalescence at high temperatures. The temperature of the particle-gas strean in Run 1 was also above 1577 °C, but the solid feed rate in this run was only about one-half of the values in Runs 4 -6. The lower solid feed rate in Run 1 together with the fact that portions of the particles usually get stuck on the wall significantly lowered the possibility of particles in the gas stream to agglomerate in Run 1. The scanning electron microscope (SEM) micrographs of the samples from Runs 1 -3 in Figure 19 indicate that particle did not fuse in these runs, yielding the satisfactory agreements between the experimental and CFD results. The micrographs of the sample collected from Run 4 shown in Figure 19 indicates a larger amount of big particles, pointing to greater extent of particle agglomeration.

Process and Economic Analysis
Based on the potential advantages pf the new technology and the results of the process feasibility studies, process and economic analyses have been performed [12][13][14][15]. The results of these analyses indicated that the new ironmaking technology will consume up to 44% less energy than the blast furnace when the former is run in the reformerless mode, i.e. direct partial combustion in the reactor, and it will emit up to 51% less carbon dioxide. When hydrogen is used, the proposed process would consume up to 60% less energy with little CO2 emissions. However, it is noted that a more accurate comparison must include the energy consumption and CO2 emissions for the production of natural gas, hydrogen or coal.
An economic feasibility analysis [15] indicated that the new technology using natural gas with insitu partial combustion would be economical at this time, owing to the small capital and operating costs as well as the low price of natural gas. The sensitivity analysis performed on the estimated NPV indicated that the price of natural gas affects the NPV most strongly. These economic analyses point to the fact that the proposed ironmaking technology would be economically feasible at this time if it is operated using natural gas.

Summary
The overall process of developing a novel Flash Ironmaking Technology (FIT) have been described in this article.
Rate equations for the reduction of iron ore concentrate by hydrogen, carbon monoxide, and a mixture of the two formulated in this work established the fundamental feasibility of the concept of the flash ironmaking and form the basis of the design of a reactor to realize the process. Scale-up experiments were performed in a laboratory flash reactor that operated at conditions similar to those of the industrial flash ironmaking reactor, and > 90% reduction degree was obtained at temperature as low as 1175 °C.
A pilot reactor that operated in the temperature range 1200-1550 °C was installed and operated to collect the data necessary for scaling up the process to an industrial scale. This reactor was used to validate the design concept of the Flash Ironmaking in terms of heat supply, residence time, reduction degree, and the determination of optimum operating conditions. These tests also identified a number of technical hurdles. Full reduction of the magnetite concentrate was achieved at 1350 °C and H2 EDF = 1.5 in the pilot reactor. This investigation proved the technical feasibility of the flash ironmaking technology for large-scale iron production. The results of this work will facilitate the complete design for the industrial flash ironmaking reactor.
The new technology does not require pellets, sinters and coke. Instead, it would produce iron directly from concentrates using natural gas or hydrogen. As a result, the energy consumption is expected to be up to 44% less than that for the average blast furnace process when the Flash Ironmaking Technology (FIT) is operated with natural gas in the reformerless mode, i.e. in situ partial combustion, and it will emit up to 51% less carbon dioxide. When hydrogen is used, the proposed process would consume up to 60% less energy with little carbon dioxide emissions.
It is hoped that the planning of the overall development of the flash ironmaking technology, the experiences obtained from the testing steps, and results from them would be a significant source of information for future researchers and engineers who may either work on this technology or develop similar or related other technologies.