Reduction of self-reducing pellets for the use in shaft furnace by involving hydrogen reduction
Wang, Rongrong; Senk, Dieter (Thesis advisor); Springer, Hauke Joachim (Thesis advisor)
Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2023
The iron and steel industry, particularly the blast furnace ironmaking process, takes the highest share of CO2 emissions from the manufacturing sector of the total global greenhouse gas emissions in the world, which makes it essential to develop carbon-free ironmaking technologies to achieve decarbonization. One of the strategies is to replace fossil energy by using green H2 in the direct reduction process, that is to develop H2-based direct reduction technology (H-DR) and to produce carbon-free DRI. However, since carbon in the DRI is necessary to achieve smooth and efficient smelting operations, how to produce carbon-containing DRI became the problem. Application of biomass embedded self-reducing pellets (SRP) in the H-DR process is proposed and expected to solve the problem of low/zero carbon content in the DRI product for H-DR. In the scope of current thesis, a 3-stage study: TG/DTA → isothermal→ non-isothermal study, on the H2 reduction of biomass embedded SRP was conducted to investigate the reduction performance of SRP and therefore to demonstrate the feasibility of using biomass embedded SRP in the H-DR process. Torrefied biomass and charcoal were used as embedded biomass reducing agents. The reduction experiments were conducted under isothermal and non-isothermal conditions by using a 3-zone tube furnace and Thermogravimetric/Differential Thermal Analyzer (TG/DTA). Scanning electron microscopy (SEM) was used to characterize the microstructure and porosity of pellets. Evaluation indexes such as relative mass loss ratio of pellets during reduction, reduction degree, metallization degree, carbon utilization rate of embedded biomass, residual carbon content in the product, and swelling behavior were examined. The reduction experiments conducted in the TG/DTA analyzer under N2/H2/CO atmospheres at different temperatures up to 1200 °C show that the reduction of biomass embedded SRP by H2 is a combination of solid phase reduction and gaseous phase reduction; reduction of iron ore by solid torrefied biomass starts at about 400 °C; residual carbon that can be found in the reduction product initially corroborated the feasibility of using SRP in H-DR to produce carbon-containing DRI. The isothermal reduction of biomass embedded SRP was investigated under various atmospheres and in the temperature range of 750-950 °C. From the perspective of relative mass loss ratio during reduction, reduction degree, and metallization degree, 40% H2-60% N2 atmosphere and 950 °C is selected as the optimal reduction atmosphere and optimal reducing temperature in this study; The use of torrefied biomass could enhance the reduction efficiency more significantly than charcoal, SRP embedded with 12%-18% torrefied biomass or 6% charcoal have good reduction performance in the current study. From the perspective of carbon utilization rate and residual carbon content of reduction products, torrefied biomass shows a higher carbon utilization rate and the residual carbon content of the product is more in line with the actual demand; Carbon in charcoal is more stable and Cfix content of charcoal is larger, results in a high residual carbon content of the product. Therefore, 18% torrefied biomass or 6% charcoal is a suitable addition amount to the pellets in the current study. From the perspective of reduction swelling, all pellets show a tendency to shrink without catastrophic expansion and are able to meet the actual production requirements. The non-isothermal reduction of biomass embedded SRP was investigated under simulated Midrex direct reduction condition and H2-based direct reduction condition. The swelling index of biomass embedded SRP could meet the requirements under experimental conditions, while only carbon content in the reduction product of 18% torrefied biomass embedded SRP and 6% charcoal embedded SRP could meet the requirement for the subsequent EAF smelting operation.
- Division of Materials Science and Engineering 
- Chair of Metallurgy of Iron and Steel