Catalytic effect of iron on decomposition of carbon monoxide: I. carbon deposition in H2-CO Mixtures

Hematite ore reduced with hydrogen was used as a catalyst in the present investigation of the rate of decomposition of carbon monoxide in H₂-CO mixtures. It was found that the amount of carbon deposited from purified carbon monoxide was directly proportional to the amount of porous iron catalyst present in the system. However, this simple relation did not hold for H₂-CO mixtures. During carbon deposition the porous iron granules dis-integrated and were dispersed evenly in the carbon deposit. The deposit consisted of graphite, cementite and iron, with cementite/iron ratio increasing as more soot accumulated. When most of the iron was converted to cementite, carbon deposition ceased. A small amount of hydrogen enhanced markedly the rate of decomposition of carbon monoxide. Indications are that hydrogen adsorbed on iron catalyzes the decomposition of carbon monoxide, 2CO → C + CO₂, in addition to the occurrence of the second reaction H₂ + CO → C + H₂O.     

Gaseous reduction of iron oxides: Part III. Reduction-oxidation of porous and dense iron oxides and iron

The internal reduction of high-grade granular hematite ore in hydrogen and carbon monoxide, and also the internal oxidation of porous iron granules in CO₂-CO mixtures have been investigated. To assist the interpretation of the rate data for porous iron and iron oxides, rate measurements have been made also with dense wustite, previously grown on iron by oxidation. The iron formed by reduction of dense wustite is porous, similar to that observed when porous hematite is reduced. It is found that the rate of dissociation or formation of water vapor or carbon dioxide on the iron surface is about an order of magnitude greater than that on the surface of wustite. The results of the previous investigations using dense iron and wustite are in general accord with the present findings. The rate of reduction of hematite increases with increasing pore surface area of the reduced oxide. The results indicate that the rate of reduction of granules is controlled primarily by the formation of H₂O or CO₂ on the pore walls of wustite. The specific rate constants evaluated from internal reduction, using the total pore surface area, are about 1/50 to 1/100 of those for dense wustite. These findings indicate that with porous wustite or iron, the effective pore surface area utilized is about 1 to 2 pct of the total pore surface area. The rate of reduction in H₂-CO mixtures is in accord with that derived from the rate constants for reduction in H₂ and CO.     

The influence of carbon deposition on the reduction kinetics of commercial grade hematite pellets with CO,H2, and N2

Experimental measurements are reported on the rate at which commercial grade, low silica hematite pellets react with a gas mixture consisting of CO, H₂, and N₂ over the temperature range 500 °C to 1200 °C. Systems of this type are of considerable practical interest, both regarding the operation of direct reduction processes and ironmaking in the blast furnace. The results of the work may be summarized as follows: No carbon deposition was found when operating the system above 900 °C and in the absence of CO gas. When operating the system below 900 °C carbon deposition occurred, which in effect prevented the normal conversion from reaching completion. The maximum rate of carbon deposition was found to occur between 500 °C and 600 °C. In general hydrogen (in the presence of CO) tended to promote carbon deposition, while the presence of nitrogen appeared to retard the deposition process. When the reaction process was being carried out below 900 °C with CO + H₂ gas mixtures, the reduction process occurred simultaneously with carbon deposition. At lower temperatures, say around 500° to 600 °C, the deposition process dominated, while at the higher temperatures, and particularly at a high hydrogen content of the reactant gas, the reduction process was dominant. The structural examination of the partially reacted specimens has shown that the carbon deposited was found primarily in the form of elemental carbon rather than cementite. Furthermore, X-ray analysis of the free pellet surface has indicated that iron was present in the carbon deposit phase. The practical industrial implications of these findings are discussed in the paper.     

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part I. The role and kinetics of volatile reduction

With iron ore reduction processes using coal-ore pellets or mixtures, it is possible that volatiles can contribute to reduction. By simulating the constituents of the individual reducing species in the volatiles, the rates for H₂ and CO were investigated in the temperature and reduction range of interest; hydrogen is the major reductant and was studied in detail. The kinetics of the reduction by H₂ has been found to be a complex mechanism with, initially, nucleation and growth controlling the rate. There is a catalytic effect by the existing iron nuclei, followed by a mixed control of chemical kinetics and pore diffusion. This results in a topochemical reduction of these iron oxide particles. Up to 1173 K, reduction by H₂ is considerably faster than by carbon in the pellet/mixture or by CO. It was also found that H₂S, which is involved with the volatiles, does not affect the rate at the reduction range of interest.     

Catalytic effect of iron on decomposition of carbon monoxide: II. Effect of additions of H2, H2O,CO2, SO2 and H2S

In this work the effect of additions of H₂, H₂O, CO₂, SO₂, and H₂S on the catalytic decomposition of CO by iron has been investigated at 400, 600, and 800°C and atmospheric pressure. The catalyst was porous iron formed by the reduction of hematite ore granules with hydrogen. The relative importance of carbon deposition by the reactions 2CO → C + CO₂ and H₂ + CO → C + H₂O was determined as a function of hydrogen concentration. It was found that even low concentrations of hydrogen greatly enhanced the rate of decomposition of CO, presumably by a catalytic action of adsorbed hydrogen on iron. The presence of water vapor had a dual effect. At low concentrations of hydrogen the rate of decomposition of CO increased with the addition of H₂O, apparently by a catalytic effect. At high concentrations of hydrogen, however, carbon deposition was retarded because of the effect of the reverse reaction H₂O + C → H₂ + CO. In CO-CO₂ mixtures the rate of carbon deposition decreased with increasing CO₂ content, because of the effect of the reverse reaction CO₂ + C → 2CO. The presence of traces of sulfur-bearing gaseous species, such as SO₂ and H₂S, retarded the decomposition of CO on iron and brought about the early cessation of carbon deposition. This strong effect may be due to the retardation of the decomposition of the intermediate product, cementite, and the formation of pyrrhotite on the surface of the iron catalyst.     

The rate of reduction of iron oxides by carbon

The rate of reduction of Fe₂O₃ and FeO by coconut charcoal, coal char and coke, in an inert atmosphere within the temperature range 900 to 1200°C was investigated. The effects of pressure, particle size, and the amount of carbon were determined. The results indicate that the reaction takes place by means of the gaseous intermediates CO and CO₂, and that the overall rate is controlled by the oxidation of the carbon by CO₂. The rates of reduction of FeO and Fe₂O₃ by CO are relatively fast, and the CO₂/CO ratio for the oxidation of carbon is determined by their equilibria. The reduction of Fe₂O₃ by carbon is accomplished in two stages, with FeO forming first. The reduction of Fe₂O₃ to FeO is faster than that of FeO to Fe because its CO₂/CO equilibrium ratio is higher and hence the rate of oxidation of carbon is faster. A direct comparison was made between the rate constants for the reduction of FeO by carbon and those for the oxidation of carbon in the appropriate CO₂-CO gas mixtures, and they are in good agreement. Apparently the iron formed by the reduction does not significantly catalyze the oxidation of carbon; whereas for the reduction of NiO by carbon, the Ni formed does catalyze the oxidation of carbon.     

Blast Furnace Operating Conditions Manipulation for Reducing Coke Consumption and CO2 Emission

A comparative reduction behavior of wüstite samples prepared from iron ore sinter was investigated to find the optimum way for reducing coke consumption and CO₂ emission in blast furnace technology. A series of wüstite reduction experiments was carried out using different gas mixtures. A correlation between the experimental results and real data of blast furnaces at Egyptian Iron and Steel Company (EISCO) was demonstrated in order to optimize the coke consumption inside blast furnaces. Different theoretical models were applied on real data of blast furnaces to calculate the effect of operation parameters on the coke consumption. It was found that the wüstite reducibility can be controlled and enhanced using certain ratio of H₂ and CO gases. Such kind of enhancement decreases the remaining quantity of unreduced wüstite which descends to the high temperature region of blast furnace. The theoretical analysis of real data using certain values of H₂ and CO shows that increasing the amount of natural gas injection in blast furnace of EISCO will decrease the degree of direct reduction of iron oxide and consequently will decrease the amount of coke consumption.     

Mathematical Modeling and Exergy Analysis of Blast Furnace Operation With Natural Gas Injection

Blast furnace operation with natural gas (NG) injection is one of the effective measures to save energy, reduce CO₂ emission, and decrease environmental load for iron and steel industry. Numerical simulations on blast furnace operation with NG injection through tuyeres are performed in this paper by raceway mathematical model, multi‐fluid blast furnace model, and exergy analytical model. With increasing NG injection volume, the simulation results are shown as follows: (1) the theoretical flame temperature and bosh gas volume can be constant by decreasing blast volume and increasing oxygen enrichment. (2) The utilization rate of CO enhances while that of H₂ decreases. The proportion of H₂ in indirect reduction tends to be increased, which accelerates the reduction of burdens. The pressure drop shows that the permeability of blast furnace gets better. The blast furnace productivity is increased from 2.07 to 3.08 t · m^?3 · day^?1. The silicon content in hot metal is decreased from 0.26% to 0.05%. When BF operation with 125.4 kg · tHM^?1 NG injection, coke rate and carbon emission rate are decreased by 27.2% and 32.2%, respectively. (3) The thermodynamic perfection degree is increased from 88.40% to 90.50%, the exergy efficiency is decreased from 51.94% to 49.02% and the chemical exergy of top gas is increased from 4.69 to 6.22 GJ · tHM^?1. It is important to strengthen the recycling of top gas.     

Experimental Investigation on Optimal Carbon/Hydrogen Ratio for Developing Iron Bath Reactor with H2–C Mixture Reduction

The basic idea of H₂–C mixture reduction reflects advantages of hydrogen for fast reaction and low heat absorption in smelting reduction reactor, where hydrogen is used as main reducing agent and carbon as main heat generator on purpose to cut down total energy consumption and CO₂ emission. This article aimed at the experimental investigation of optimal carbon/hydrogen ratio, a key parameter of iron oxide reduction with mixture reductive agents of carbon and hydrogen. Experiments were carried out using a pure Al₂O₃ crucible which was placed in a tubular furnace for high temperature. Two investigation methods were adopted, one was injecting acetylene/hydrogen mixture reducing gas into molten iron oxides and another was blowing hydrogen into iron bath during continuous feeding fine ore mixing solid carbon dust. Parameters such as apparent de‐oxidation rate and utilization ratio of reductive agents were calculated from content analysis of exhaust gas after dust removing and drying. In experiments highest total de‐oxidation rate and satisfied apparent utilization ratio of hydrogen were obtained under conditions with temperatures of 1823 K and carbon/hydrogen ratio in region from 0.5:1 to 2:1.     

Reduction of hematite compacts by H2-CO gas mixtures

The reduction behaviour of hematite compacts by H₂-CO gas mixtures was investigated at 1073-1223 K. The total porosity, pore size distribution and surface area of the compact was measured using mercury pressure porosimeter. The reduction tests were carried out using Cahn balance. The reduction behaviour could not be described in terms of a single rate-determining step; the reduction process was initially controlled by the chemical reaction at the oxide/iron interface, controlled by the intraparticle diffusion through the reduced layer towards the end of reduction, and the mixed control, in between. Over the whole range, the reduction rate decreased with CO content in the gas mixture. The chemical reaction rate constants were two to three times higher for H2 reduction than those of CO reduction, and the effective diffusivities of H2 reduction were three to four times higher than those of CO reduction. Values of activation energy for chemical reaction were found to be 19.8-42.1 kJ/mol depending on the gas compositions; 100% CO showing the lowest.     

Swelling Behavior of Iron Ore Pellets during Reduction in H2 and N2/H2 Atmospheres at Different Temperatures

The need to develop green steelmaking techniques has led to the replacement of reducing agents such as CO with H₂. H₂ and N₂/H₂ mixtures can be used for the carbothermal reduction of iron ore. Herein, the reduction swelling index (RSI) of iron ore pellets in a forming gas (N₂/H₂) atmosphere at temperatures of 700–1000 °C is investigated and it is compared with that in pure H₂. It is showed in the experimental results that the RSI increases with increasing temperature for both the H₂ and N₂/H₂ atmospheres. The maximum swelling is reached approximately 5 min into the H₂ reduction process, while in the N₂/H₂ atmosphere, it is reached after 25–45 min of reduction, depending on the temperature. When the reduction temperature exceeds 900 °C, the RSI is greater than 20%. Scanning electron microscopy/energy-dispersive X-ray spectroscopy is performed to detect the changes in the microstructure and chemical composition of the samples. The nonreduced areas in the reduced pellets during the N₂/H₂ reduction process are analyzed using light optical microscopy.     

H2-CO还原钛精粉的热重法试验

 采用热重分析法研究了H2-CO还原钛精粉的特征。热重、相组成和形貌分析表明：在纯H2基础上,配加CO还原钛精粉的热重曲线具有相似性,即质量分数首先随时间的延长而下降,达到平衡后维持一定时间,随后逐渐增加;在质量分数降低的过程中,失重率随H2含量的增加而增加;相反,在质量分数增加的过程中,增重率首先随CO含量的减小而增加,至80%CO时达到最大值,而后逐渐减小。要保证还原产物中铁元素主要以金属相、非渗碳体相形式存在的合理温度是1150℃;开始阶段由于氢气原子半径小、扩散能力强,H2含量高时,产物表面孔隙较多,利于气体传输,还原能力强,随后由于渗碳体参与还原反应,产生大量气体,产物破碎程度增强,增加了其比表面积。     

Redox Properties of Gas Phases

In the H₂–O₂–C system, in the general case, two reversible reactions of carbon gasification and the water gas reaction, the gas mixture H₂–H₂O–CO–CO₂ is formed at high temperatures. In this mixture, the very low content of oxygen formed by the dissociation of H₂O and CO₂ is represented by the oxygen potential log (\({p_{{O_2}}}\), atm). Thus, the redox properties may be assessed in terms of the oxygen potential. In any gas mixture containing H₂O and/or CO₂, it may be calculated from the equations

$${\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{{H_2}O}}}}{{{x_{{H_2}}}}}) - \frac{{25708}}{T} + 5.563}$$

;

$$\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{C{O_2}}}}}{{{x_{CO}}}}) - \frac{{29529}}{T} + 9.149$$

.In the present work, possible compositions of the H₂–O₂–C system at 700–1500 K and a total pressure of 1 atm are considered: H₂–H₂O, CO–CO₂, CO–CO₂–C, H₂O–CO₂–O2, H₂–CO–C, H₂–H₂O–CO–CO₂, and H₂–H₂O–CO–CO₂–C. Analysis yields two nomograms in the following coordinates: log(\({x_{{H_2}O}}\)/\({x_{{H_2}}}\))–log\({p_{{O_2}}}\)–T and log(\({x_{C{O_2}}}\)/xCO)–log\({p_{{O_2}}}\)–T. Using the nomograms and reference information regarding the dissociation pressure of metal oxides, the redox properties of the gas mixtures with respect to those oxides may be assessed. In CO–CO₂ systems without hydrogen that are obtained in the combustion of CO, carbon may be formed as soot. This explains the existence of a limited region of gas-phase compositions and log\({p_{{O_2}}}\) in the corresponding nomogram and hence the limited potential for the reduction of some metal oxides in CO–CO₂–C systems. The introduction of hydrogen permits the creation of gas mixtures with extremely low oxygen pressure and hence increases the thermodynamic probability of reduction for any metal oxide. Hydrogen may be introduced in the system by methods that differ in economic expediency: from the use of pure hydrogen to the production of gas mixtures as a result of the reaction between water vapor and carbon. In the first case, the reduction of the oxide by hydrogen in the MeO–C–H₂ system activates the gasification of carbon by water vapor, the water gas reaction, the reduction of carbon monoxide, and the gasification of carbon dioxide. In the second case, practically pure H₂–CO mixture may be obtained above 1300 K. The utility of representing the results on a three-dimensional diagram based on the H₂–O₂–C concentration triangle is analyzed. If methane formation is taken into account, the equilibrium parameters of gas mixtures are changed markedly only at temperatures below about 900 K.

     

Carburization of iron by CO-based mixtures in nitrogen at 925 °C

The rates of carburization by mixtures of CO, CO₂, H₂ and H₂O in N₂ have been measured gravimetrically at 925 °. The rate equation which best describes the experimental data is based on a mechanism which involves a rapid surface dissociation of CO into carbon and oxygen atoms, and a subsequent rate determining step between this atomic oxygen and either CO or H₂. The CO-H₂ system carburizes much faster than CO alone, because H₂ combines faster with atomic oxygen than does CO. The carburizing rate constant for CO-H₂ is 44 times that for CO alone. The mechanism is confirmed by the additivity of the separate rates for the CO-H₂ mixtures and for CO alone.     

Reforming of Blast Furnace Gas with Methane,Steam, and Lime for Syngas Production and CO2 Capture: A Thermodynamic Study

The production of iron and steel by the blast furnace process is a major source of CO₂ release, with blast furnace gas contributing about 5% to the anthropogenic greenhouse gas emission. Its main components are CO₂, CO, N₂, and H₂. Chemical equilibrium calculations are made to determine the thermodynamic constraints for converting these components into valuable syngas for producing hydrogen, methanol, Fischer–Tropsch hydrocarbons, or ammonia, by either reforming with CH₄, water-gas shift reaction, partial oxidation, or CaO carbonation—while achieving partial or complete CO₂ capture. By a two-step thermochemical cycle, the CaCO₃ formed by lime carbonation could be calcined back to CaO, while releasing relatively pure CO₂ for utilization. The implications of such reactions with respect to hydrogen production, CO₂ emission avoidance, and process efficiency are examined.     

The effect of hydrogen addition on the carbon-deposition behaviour during the reduction of pellets for the blast furnace process

With the application of large amount of pulverised coal injection into the blast furnace, the hydrogen content in the gas will increase, which accelerates the reduction of iron ore in lump zone of the blast furnace as well as carbon-deposition reaction. This study has investigated the effect of hydrogen addition on carbon-deposition reaction during the reduction of pellets through thermodynamic calculation and experiment. The results show that H₂ can promote the carbon-deposition reaction, while the increase of temperature and CO₂ can significantly inhibit it. The preference region of temperature for C formation is about 600°C. Moreover, the promotion effect of H₂ on the carbon-deposition reaction at 700°C is better than that at 600°C. The SEM observation results show that the generated carbon is mainly distributed on the surface of the pellet, and only a little carbon is located inside the pellet. The agglomerated carbon could be more easily formed due to the dramatic carbon-deposition reaction caused by the lower temperature or higher H₂ content. But, most of the carbon just exists as an individual particle at the lower carbon-deposition reaction rate. The results of SEM–EDS reveal that carbon deposited is primarily in the form of elemental carbon rather than in the form of cementite. The study also shows that with increasing reduction time, the rate of carbon-deposition increases, mainly due to the promotion effect of reduced iron during the reduction process of pellets.     

Low Temperature Formation of Iron Carbide from Iron Oxide with CO‐H2 Gas Mixtures

Commercially pure iron oxide in the form of fine particles were reacted with CO‐H₂ gas mixtures at ambient pressure (? 0.8 atm) in the temperature range 573 to 1073K. Reduction to metallic iron was carried out with pure hydrogen. The rate of formation of iron carbide was measured by recording the weight change with a thermogravimetric apparatus. The results obtained indicate that for each gas phase composition a maximum rate was observed, at apparently the same temperature. These maxima in the rate occur at a lower temperature than those reported in previous investigations. A dissociative adsorption model, based on the reduction of CO with hydrogen was developed which qualitatively describes the observed results.     

Influence of several factors on synthesis of iron carbide from iron ore

Abstract

Iron carbides as alternative pure iron sources were synthesised by thermochemical reactions of iron ores with H₂–CO gas mixtures having low sulphur pressures incapable of forming FeS. It was proved that high quality iron carbides without free carbon can be obtained more effectively at relatively high reaction rates. Some pressurisation was effective in eliminating the detrimental influence of oxidant components in the inlet gas mixtures. Ore type, reaction time, H₂ /CO ratio, sulphur potential, and total gas pressure dependencies on the iron carburisation rates were examined and the rates were analysed using a first order rate model. Comparison shows that the productivity of the proposed process should be almost one order of magnitude greater than that of a conventional Iron Carbide process.     

In Situ observations of the gaseous reduction of magnetite

The microstructural changes occurring on the surfaces of magnetite samples during the reduction in H₂/N₂ gas mixtures have been directly recorded using an optical heating stage technique. At low temperatures, below approximately 910 K, the iron forms a porous morphology and grows in a continuous manner at a rate dependent on the reaction temperature and hydrogen partial pressure. At high temperatures, iron growth occurs through an unusual discontinuous mechanism, and at any given hydrogen partial pressure, the growth exhibits a rate minimum with increasing reaction temperature. Formerly with the University of Queensland.     

Iron ore reduction/cementation: experimental results and kinetic modelling

Abstract

Iron ore reduction and iron cementation by H₂-CH₄-Ar gas mixtures were investigated in a laboratory isothermal fixed bed reactor in the temperature range 600-925°C. Iron ore was first reduced to metallic iron by hydrogen, then metallic iron was carburised to cementite by methane. Increasing temperature and hydrogen content accelerated the reduction process. However, for >55 vol.-%, the effect of H₂ content was not significant. Methane had almost no effect on the reduction process. Increasing temperature increased the rate of iron cementation and also the rate of free carbon deposition. Optimum conditions for cementite formation were: temperature 750°C and reducing/carburising gas contents of 40-55 vol.-%H₂ and 35 vol.-%CH₄. Under these conditions, reduction of iron ore to cementite was completed in ~15 min. A two interface grain model and a volume reaction model were used to simulate the process of iron ore reduction and iron cementation. The simulated results for both reduction and cementation were consistent with the experimental data.