Juho Haapakangas

The worldwide commitment to large-scale CO ₂ reductions is forcing steel producers to find improvements to reheating processes for steel slabs. In this study, three practices with potential to lower the carbon footprint of reheating process for steel slabs were studied: use of hydrogen as fuel gas, replacement of air with pure oxygen (oxy–fuel), and electrical heating. The oxidation behavior of steel in these conditions was simulated using a thermogravimetric analyzer (TGA) at high temperatures (up to 1250 °C). Four carbon steel grades, seven different gas atmospheres, and two dynamic heating profiles were included in the test matrix. When compared to the standard reheating practice using natural gas–air, scale formation increased moderately in hydrogen–air and significantly in oxy–fuel gas atmospheres but decreased in the case of simulated electrical heating. With simulated oxy-fuel combustion, no meaningful difference was found between H ₂ and a 50:50 CH ₄ –H ₂ mix in terms of oxidation kinetics. A reduction of the free oxygen content from 2.5 to 1.0 pct was more effective at reducing overall oxidation in the case of CH ₄ –air compared to H ₂ –oxy–fuel.

The mitigation of CO₂ emissions is one of the major areas of research in iron ore-based steelmaking. In this study, four simulated current and potential future reheating scenarios with different fuel and oxidizer gases were studied regarding the amount of oxide formation and the adhesion of the steel–oxide interface: (1) methane–air; (2) coke oven gas–air; (3) hydrogen–air; (4) and an oxyfuel scenario with 50:50 methane/hydrogen as fuel gases. Isothermal oxidation tests were conducted at temperatures of 1150, 1230 and 1300 °C. Four low-carbon steel grades were tested in the previously mentioned gas atmospheres. The structure and composition of the formed oxide scales was analyzed with FESEM-EDS microscopy. The amount of oxide formation correlated with the water vapor content of the gas atmosphere for all four steel grades; however, notable differences were found between individual steel grades regarding the degree of oxidation increase. No clear evidence was found of the gas atmospheres affecting the adhesion of oxide scales to the steel substrate. The adhesion of the interface was mainly determined by the content of silicon in the steel grade and the test temperature.

The strength of coke at high temperatures is of major importance for efficient blast furnace operation. Despite this, there is little information regarding the hot strength of coke. In this research, the hot strength of three industrial European coke grades was studied using a Gleeble thermomechanical simulator. The hot strength was analyzed at three different temperatures: room temperature, 1600 and 1750 °C by measuring the compressive strength of roughly 50 coke samples at each temperature. A significant decrease in strength was observed for all three coke grades at high temperatures. Notable differences between the coke grades in compressive strength were observed at room temperature. At high temperatures, differences were observed in strength and deformation behavior; however, the order of magnitude of strength remained the same. The deformation behavior at high temperatures was also studied and discussed based on stress–strain curves. Several structural properties of coke such as total porosity, pore size distribution, pore shape factor, amount of inerts, and degree of graphitization were determined in order to explain the obtained strength results. Both hot and room temperature compressive strength values were compared to industrial strength tests and discussed. The reliability of the results was evaluated with statistical analyses.

The strength of coke is of major importance for efficient blast furnace operation. There are only a few studies related to compressive coke hot strength, possibly due to experimental difficulties. In this work, it has been demonstrated that the Gleeble thermomechanical simulator is suitable for evaluating coke hot strength and offers many benefits compared to purpose‐built devices reported in the literature. This finding could open coke hot strength research to a wider number of scientists. The compressive hot strength of coke was evaluated at 1000 and 1600°C by testing 50 samples at both temperatures. The yield strength and ultimate strength of the coke were, respectively, 46 and 20% lower at 1600°C than 1000°C. Stress–strain curves showed that the coke was brittle at 1000°C but partially plastic at 1600°C. The significantly lower coke strength at 1600°C could help explain the smaller coke lump size found near the tuyere level in quenched blast furnaces.