Research progress on the principle and industrial application of hydrogen metallurgy

ΔG0=130336-138-83T (6)

The content of H2 and CO in the equilibrium state of iron ore reduction reaction at high temperature is higher than that of the equilibrium state gas in the solid state reduction reaction at low temperature. The content of H2 and CO in the equilibrium gas phase is 45.6% and 81.8%, respectively. In summary, the calculation of the thermodynamic equilibrium components of the C-H2-O2-H2O-CO-CO2 system can be obtained [3] :

(1) When the temperature is 1500℃, carbon and oxygen are in excess, and there is no water and carbon dioxide in the equilibrium system. The proportion of oxygen in the gas phase increases, the content of carbon monoxide increases, and the content of hydrogen decreases. The amount of solid carbon is supersaturated, the content of water and carbon dioxide is reduced, and the content of hydrogen and carbon monoxide is increased; The influence of system pressure on equilibrium component content is not significant.

(2) As the temperature increases, the contents of CO and H2O in the equilibrium system increase, while the contents of H2 and CO2 decrease, and the increase of temperature is conducive to improving the hydrogen utilization rate.

(3) When carbon is in excess, the heat load of reactive carbon cannot be reduced only by spraying H2; At high temperatures, although hydrogen can react with iron oxide, carbon can also react with H2O, thus transforming H2O into H2.

According to the traditional direct reduction process practice, literature [4] proposed the carbon-hydrogen melting reduction technical route, which provides heat by oxidizing carbon to CO and uses hydrogen as reducing agent to reduce iron ore. Carbon is used as heat source and partial reducing agent, and H2 is used as main reducing agent, which solves the contradiction of high heat and strong reducing atmosphere for direct reduction of carbon in melting reduction process. The C in the raw material has participated in the reaction and produced a penjue, which provides hydrogen source for hydrogen metallurgy. The reduction of iron oxide mainly relies on hydrogen, and CO also reacts with iron oxide. At the same time, the reaction of CO2 and H2 can generate CO+H2O, so that the emission of C in the reaction process is reduced, which is conducive to the environmental protection effect of direct reduction. Therefore, ensuring a reasonable supply of raw material hydrogen can keep the reduction process normal.

1.3 Hydrogen metallurgy kinetics

The kinetic condition of hydrogen reduction of iron oxide is better than that of CO, and the mass transfer rate of hydrogen is significantly higher than that of CO [5]. Compared with CO, the reduction kinetics conditions of hydrogen-rich gas or pure hydrogen are improved. CO reduction of iron oxide is exothermic reaction, H2 reduction of iron oxide is endothermic reaction, so how to continue to supply heat to the reaction zone is the technical difficulty of hydrogen rich or pure hydrogen reduction.

1.3.1 Low temperature hydrogen reduction

The key technology of low temperature hydrogen reduction is how to strengthen the reaction rate of hydrogen and iron ore and improve the process efficiency. From the kinetic point of view, the reaction rate of hydrogen reduction iron ore at low temperature is slow, and the concentration of hydrogen in the equilibrium gas phase is high. In order to improve the rate of direct reduction reaction at low temperature, there are two technical measures that can be taken [6] : one is to reduce the activation energy of the reaction and activate H2 into H or H+ through the action of physical field; Iron ore can be reduced to metallic iron by activated hydrogen at low temperature. The second is to increase the surface area of the reactant, that is, to reduce the particle size of the iron ore; The particle size is reduced from 45μm to 5μm, and the reaction area can be increased by 9 times.

The low temperature hydrogen reduction reaction of iron oxide initially occurs at the local active point of the interface, and expands inward to form small pores. The generated active iron forms protrusions through surface diffusion and gradually develops. The porous product structure enables the gas reduction reactants and gas products to diffuse smoothly, and the interfacial reaction can proceed smoothly. With the progress of hydrogen reduction reaction, the product layer thickens, the reduction product begins to sintering and densification, and the diffusion of the reduction gas and product is affected, and gradually becomes a limiting link in the reaction process. Its macroscopic performance is that abnormal temperature effect occurs in the reduction process of iron ore, that is, at a certain reduction temperature, the reduction rate does not increase with the increase of temperature, but decreases with the increase of temperature.

At low temperature, the sintering process is slow, and the product structure does not affect the diffusion of gas, so the low temperature hydrogen reduction process is the interface reaction speed control. With the increase of temperature (higher than 700℃), the sintering process accelerates, the influence of product sintering on the reaction rate gradually increases, and the reaction rate control link gradually changes from interface reaction rate control to diffusion rate control. The problem to be solved in low temperature hydrogen reduction (below 1000℃) is to control the rapid increase of the initial chemical reaction rate before the formation of dense structures that affect gas diffusion, and to end the reduction process before the formation of dense product structures.