Search2022 Vol. 29, No. 10
Japan started the national project “COURSE 50” for CO2 reduction in the 2000s. This project aimed to establish novel technologies to reduce CO2 emissions with partially utilization of hydrogen in blast furnace-based ironmaking by 30% by around 2030 and use it for practical applications by 2050. The idea is that instead of coke, hydrogen is used as the reducing agent, leading to lower fossil fuel consumption in the process. It has been reported that the reduction behavior of hematite, magnetite, calcium ferrite, and slag in the sinter is different, and it is also considerably influenced by the sinter morphology. This study aimed to investigate the reduction behavior of sinters in hydrogen enriched blast furnace with different mineral morphologies in CO–CO2–H2 mixed gas. As an experimental sample, two sinter samples with significantly different hematite and magnetite ratios were prepared to compare their reduction behaviors. The reduction of wustite to iron was carried out at 1000, 900, and 800°C in a CO–CO2–H2 atmosphere for the mineral morphology-controlled sinter, and the following findings were obtained. The reduction rate of smaller amount of FeO led to faster increase of the reduction rate curve at the initial stage of reduction. Macro-observations of reduced samples showed that the reaction proceeded from the outer periphery of the sample toward the inside, and a reaction interface was observed where reduced iron and wustite coexisted. Micro-observations revealed three layers, namely, wustite single phase in the center zone of the sample, iron single phase in the outer periphery zone of the sample, and iron oxide-derived wustite FeO and iron, or calcium ferrite-derived wustite 'FeO' and iron in the reaction interface zone. A two-interface unreacted core model was successfully applied for the kinetic analysis of the reduction reaction, and obtained temperature dependent expressions of the chemical reaction coefficients from each mineral phases.
Isothermal thermogravimetric analysis was used to study the reduction process of solid/liquid wustite by hydrogen. Results show that wustite in both states can be reduced entirely at all temperatures. The thermal and kinetic conditions for the hydrogen reduction of molten phases are better than those when the reactants and products are in the solid state, with a higher reaction rate. The hydrogen reduction of different wustite phases fits the Mampel Power model (power exponent n = 1/2) well, and this model is independent of the phase state. The average apparent activation energies of the reduction process calculated by the iso-conversional method are 5.85 kJ·mol−1 and 104.74 kJ·mol−1, when both reactants and products are in the solid state and the molten state, respectively. These values generally agree with those calculated by the model fitting method.
To explore the iron coke application in hydrogen-rich blast furnace, which is an effective method to achieve the purpose of low carbon emissions, the initial gasification temperature of iron coke in CO2 and H2O atmosphere and its cogasification reaction mechanism with coke were systematically studied. Iron coke was prepared under laboratory conditions, with a 0–7wt% iron ore powder addition. The properties of iron cokes were tested by coke reactivity index (CRI) and coke strength after reaction (CSR), and their phases and morphology were evolution discussed by scanning electron microscopy and X-ray diffraction analysis. The results indicated that the initial gasification temperature of iron coke decreased with the increase in the iron ore powder content under the CO2 and H2O(g) atmosphere. In the 40vol% H2O + 60vol% CO2 atmosphere, CRI of iron coke with the addition of 3wt% iron ore powder reached 58.7%, and its CSR reached 56.5%. Because of the catalytic action of iron, the reaction capacity of iron coke was greater than that of coke. As iron coke was preferentially gasified, the CRI and CSR of coke were reduced and increased, respectively, when iron coke and coke were cogasified. The results showed that the skeleton function of the coke can be protected by iron coke.
The effect of hydrogen injection on blast furnace operation and carbon dioxide emissions was simulated using a 1D steady-state zonal model. The maximum hydrogen injection rate was evaluated on the basis of the simulation of the vertical temperature pattern in the blast furnace with a focus on the thermal reserve zone. The effects of blast temperature and oxygen enrichment were also examined to estimate coke replacement ratio, productivity, hydrogen utilization efficiency, and carbon dioxide emission reduction. For blast temperature of 1200°C, the maximum hydrogen injection rate was 19.0 and 28.3 kg of H2/t of hot metal (HM) for oxygen enrichment of 2vol% and 12vol%, respectively. Results showed a coke replacement ratio of 3–4 kg of coke/kg of H2, direct CO2 emission reduction of 10.2%–17.8%, and increased productivity by up to 13.7% depending on oxygen enrichment level. Increasing blast temperature further reduced the direct CO2 emissions. Hydrogen utilization degree reached the maximum of 0.52–0.54 H2O/(H2O + H2). The decarbonization potential of hydrogen injection was estimated in the range from 9.4 t of CO2/t of H2 to 9.7 t of CO2/t of H2. For economic feasibility, hydrogen injection requires revolutionary progress in terms of low-cost H2 generation unless the technological change is motivated by the carbon emission cost. Hydrogen injection may unfavorably affect the radial temperature pattern of the raceway, which could be addressed by adopting appropriate injection techniques.
High-phosphorus iron ore resource is considered a refractory iron ore because of its high-phosphorus content and complex ore phase structure. Therefore, the development of innovative technology to realize the efficient utilization of high-phosphorus iron ore resources is of theoretical and practical significance. Thus, a method for phosphorus removal by gasification in the hydrogen-rich sintering process was proposed. In this study, the reduction mechanism of phosphorus in hydrogen-rich sintering, as well as the reduction kinetics of apatite based on the non-isothermal kinetic method, was investigated. Results showed that, by increasing the reduction time from 20 to 60 min, the dephosphorization rate increased from 10.93% to 29.51%. With apatite reduction, the metal iron accumulates, and part of the reduced phosphorus gas is absorbed by the metal iron to form stable iron–phosphorus compounds, resulting in a significant reduction of the dephosphorization rate. Apatite reduction is mainly concentrated in the sintering and burning zones, and the reduced phosphorus gas moves downward along with flue gas under suction pressure and is condensed and adsorbed partly by the sintering bed when passing through the drying zone and over the wet zone. As a result, the dephosphorization rate is considerably reduced. Based on the Ozawa formula of the iso-conversion rate, the activation energy of apatite reduction is 80.42 kJ/mol. The mechanism function of apatite reduction is determined by a differential method (i.e., the Freeman–Carroll method) and an integral method (i.e., the Coats–Redfern method). The differential form of the equation is f(α) = 2(1 − α)1/2, and the integral form of the equation is G(α) = 1 − (1 − α)1/2.
The influence of different pre-oxidation temperatures and pre-oxidation degrees on the reduction and fluidization behaviors of magnetite-based iron ore was investigated in a hydrogen-induced fluidized bed. The raw magnetite-based iron ore was pre-oxidized at 800 and 1000°C for a certain time to reach a partly oxidation and deeply oxidation state. The structure and morphology of the reduced particles were analyzed via optical microscope and scanning electron microscopy (SEM). The reaction kinetic mechanism was determined based on the double-logarithm analysis. The results indicate that the materials with higher oxidation temperature and wider particle size range show better fluidization behaviors. The lower oxidation temperature is more beneficial for the reduction rate, especially in the later reduction stage. The pre-oxidation degree shows no obvious influence on the fluidization and reduction behaviors. Based on the kinetic analysis, the reduction progress can be divided into three stages. The reduction mechanism was discussed combing the surface morphology and phase structure.
Iron ore powder was isothermally reduced at 1023–1373 K with hydrogen/carbon monoxide gas mixture (from 0vol%H2/100vol%CO to 100vol%H2/0vol%CO). Results indicated that the whole reduction process could be divided into two parts that proceed in series. The first part represents a double-step reduction (Fe2O3→Fe3O4→FeO), in which the kinetic condition is more feasible compared with that in the second part representing a single-step reduction (FeO→Fe). The influence of hydrogen partial pressure on the reduction rate gradually increases as the reaction proceeds. The average reduction rate of hematite ore with pure hydrogen is about three and four times higher than that with pure carbon monoxide at 1173 and 1373 K, respectively. In addition, the logarithm of the average rate is linear to the composition of the gas mixture. Hydrogen can prominently promote carbon deposition to about 30% at 1023 K. The apparent activation energy of the reduction stage increases from about 35.0 to 45.4 kJ/mol with the increase in hydrogen content from 20vol% to 100vol%. This finding reveals that the possible rate-controlling step at this stage is the combined gas diffusion and interfacial chemical reaction.
Hydrogen-based shaft furnace process is gaining more and more attention due to its low carbon emission, and the reduction behavior of iron bearing burdens significantly affects its operation. In this work, the effects of reduction degree, temperature, and atmosphere on the swelling behavior of pellet has been studied thoroughly under typical hydrogen metallurgy conditions. The results show that the pellets swelled rapidly in the early reduction stage, then reached a maximum reduction swelling index (RSI) at approximately 40% reduction degree. The crystalline transformation of the iron oxides during the reduction process was the main reason of pellets swelling. The RSI increased significantly with increasing temperature in the range of 850–1050°C, the maximum RSI increased from 6.66% to 25.0% in the gas composition of 100% H2. With the temperature increased, the pellets suffered more thermal stress resulting in an increase of the volume. The maximum RSI decreased from 19.78% to 17.35% with the volume proportion of H2 in the atmosphere increased from 55% to 100% at the temperature of 950°C. The metallic iron tended to precipitate in a lamellar structure rather than whiskers. Consequently, the inside of the pellets became regular, so the RSI decreased. Overall, controlling a reasonable temperature and increasing the H2 proportion is an effective way to decrease the RSI of pellets.
Steel production causes a third of all industrial CO2 emissions due to the use of carbon-based substances as reductants for iron ores, making it a key driver of global warming. Therefore, research efforts aim to replace these reductants with sustainably produced hydrogen. Hydrogen-based direct reduction (HyDR) is an attractive processing technology, given that direct reduction (DR) furnaces are routinely operated in the steel industry but with CH4 or CO as reductants. Hydrogen diffuses considerably faster through shaft-furnace pellet agglomerates than carbon-based reductants. However, the net reduction kinetics in HyDR remains extremely sluggish for high-quantity steel production, and the hydrogen consumption exceeds the stoichiometrically required amount substantially. Thus, the present study focused on the improved understanding of the influence of spatial gradients, morphology, and internal microstructures of ore pellets on reduction efficiency and metallization during HyDR. For this purpose, commercial DR pellets were investigated using synchrotron high-energy X-ray diffraction and electron microscopy in conjunction with electron backscatter diffraction and chemical probing. Revealing the interplay of different phases with internal interfaces, free surfaces, and associated nucleation and growth mechanisms provides a basis for developing tailored ore pellets that are highly suited for a fast and efficient HyDR.
As part of efforts to reduce anthropogenic CO2 emissions by the steelmaking industry, this study investigated the direct reduction of industrially produced hematite pellets with H2 using the Doehlert experimental design to evaluate the effect of pellet diameter (10.5–16.5 mm), porosity (0.36–0.44), and temperature (600–1200°C). A strong interactive effect between temperature and pellet size was observed, indicating that these variables cannot be considered independently. The increase in temperature and decrease in pellet size considerably favor the reduction rate, while porosity did not show a relevant effect. The change in pellet size during the reduction was negligible, except at elevated temperatures due to crack formation. A considerable decrease in mechanical strength at high temperatures suggests a maximum process operating temperature of 900°C. Good predictive capacity was achieved using the modified grain model to simulate the three consecutive non-catalytic gas–solid reactions, considering different pellet sizes and porosities, changes during the reaction from 800 to 900°C. However, for other temperatures, different mechanisms of structural modifications must be considered in the modeling. These results represent significant contributions to the development of ore pellets for CO2-free steelmaking technology.
The industrial application prospect and key issues in basic theory and application are discussed by the methods of theoretical analysis and calculation to promote the development of the pure-hydrogen reduction process. According to the discussion of thermodynamics and kinetics of pure-hydrogen reduction reaction, the reduction reaction of iron oxide by pure hydrogen is an endothermic reaction, and the reaction rate of hydrogen reduction is significantly faster than that of carbon reduction. To explore the feasibility of the industrial applications of pure-hydrogen reduction, we design the hydrogen reduction reactor and process with reference to the industrialized hydrogen-rich reduction process and put forward the methods of appropriately increasing the reduction temperature, pressure, and temperature of iron ore into the furnace to accelerate the reaction rate and promote the reduction of iron oxide. The key technical parameters in engineering applications, such as hydrogen consumption, circulating gas volume, and heat balance, are discussed by theoretical calculations, and the optimized parameter values are proposed. The process parameters, cost, advantages, and disadvantages of various current hydrogen production methods are compared, and the results show that hydrogen production by natural gas reforming has a good development prospect. Through the discussion of the corrosion mechanism of high-temperature and high-pressure hydrogen on heat-resistant steel materials and the corrosion mechanism of H2S in the hydrogen gas on steel, the technical ideas of developing new metal temperature-resistant materials, metal coating materials, and controlling gas composition are put forward to provide guidance for the selection of heater and reactor materials. Finally, the key factors affecting the smooth operation of the hydrogen reduction process in engineering applications are analyzed, offering a reference for the industrial application of the pure-hydrogen reduction process.
Currently, iron is extracted from ores such as hematite by carbothermic reduction. The extraction process includes several unit steps/processes that require large-scale equipment and significant financial investments. Additionally, the extraction process produces a substantial amount of harmful carbon dioxide (CO2). Alternative to carbothermic reduction is the reduction by hydrogen plasma (HP). HP is mainly composed of exciting species that facilitate hematite reduction by providing thermodynamic and kinetic advantages, even at low temperatures. In addition to these advantages, hematite reduction by HP produces water, which is environmentally beneficial. This report reviews the theory and practice of hematite reduction by HP. Also, the present state of the art in solid-state and liquid-state hematite reduction by HP has been examined. The in-flight hematite reduction by HP has been identified as a potentially promising alternative to carbothermic reduction. However, the in-flight reduction is still plagued with problems such as excessively high temperatures in thermal HP and considerable vacuum costs in non-thermal HP. These problems can be overcome by using non-thermal atmospheric HP that deviates significantly from local thermodynamic equilibrium.
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