Carburization Behavior of Hydrogen-Reduced DRI Using Synthetic Bio-syngas Mixtures as Fossil-Free Carbon Sources

摘要

The introduction of carbon (in the form of cementite) into directly reduced iron (DRI) is vital or desired even in the wake of the transition toward fossil-free iron production technology, where iron ore pellets are reduced only by hydrogen. It is essential for the smooth and efficient melting of DRI in steelmaking operations. One way to carburize DRI in a fossil-free manner is to use bio-syngas generated from the gasification of biomass as the carburizing media. Therefore, it is necessary to understand the carburization behavior of hydrogen-reduced DRI when it is exposed to various gas compositions. Bio-syngas comprises several species, such as CH4, CO, H-2, CO2 and N-2, in varying proportions and can be further altered by lowering the % CO2 by gas cleaning and the % N-2 by enrichment with O-2, which is often available in excess in the process chain as a by-product from the electrolysis of water. Therefore, in this study, to identify the optimum thermal and gaseous atmospheres, the carburization of hydrogen-reduced DRI exposed to bio-syngas mixtures, which was varied by lowering the N-2 and CO2 contents to three levels between 600 and 800 degrees C, was investigated. The isothermal experiments were designed with FactSage to determine the temperatures and gas compositions favorable for the formation of cementite (Fe3C) and were performed in a thermogravimetric analyser (TGA), and the samples were in the form of a single pellet. The carburized DRI was examined microscopically to determine their cementite content and distribution using an image analysis method developed in-house that was embedded with a deep learning module. It was found that eradicating N-2 from the gas mix to the maximum possible extent increased the carburizing extent and rate. Although it is preferred to have CO2 in the gas mix, as it helps stabilize the cementite formed by slowing its decomposition into iron and free carbon, a CO2 quantity > approx. 7-8% significantly decreased the rate of cementite formation. Higher amounts of CO (> 35-49%) accompanied by 20-30% H-2 and 14-15% CH4 in the gas mix enhanced the rate and extent of cementite formation. Additionally, it was observed that DRI exposed to temperatures of 700 degrees C had a uniform distribution of cementite and were relatively more stable against decomposition.