Fundamental understanding on the use of different carbon sources in the HIsarna alternative ironmaking process

Abstract

Environmental problems such as air pollution and global warming have resulted in more stringent environmental legislations which challenges major industries to reduce carbon dioxide emissions. The most recent approach by the steel industries to address the climate challenges is the Net-Zero Steel strategy which has been initiated as a roadmap to eliminate the emissions of greenhouse gases by 2050. In 2004, the (Ultra- Low CO2 Steelmaking) ULCOS research program lunched by major European steelmakers shortlisted HIsarna process as one of the most promising technologies to reduce CO2 emissions from steel industry. This research conducted with the aims to provide fundamental understanding on the behaviours of renewable biomass in the HIsarna SRV and support HIsarna development in optimising carbon source selection. Initially the slow devolatilization experiments were performed to compare coals (with low to high volatile matter content) with two biomass samples sourced from wood and grass. The results show that similar types of volatiles components were produced for all the carbonaceous materials, however the wt% of reducing gases e.g., H2, CO, and hydrocarbons, and the temperature required for these gases to evolve were notably different. Furthermore, the off-gas analysis reveals that torrefied grass contains large amount of H2O and CO2 which are released at low temperatures, therefore pretreatment to the temperature of ~ 400 °C is necessary for this material to be utilised effectively. The study then progresses into the thermal conditions similar to HIsarna SRV using drop-tube furnace with quadrupole mass spectrometer (DTF-QMS). It was found that the gas species detected were similar for coal and biomass samples but char oxidation for charcoal (CC) was significantly faster. Despite high fixed carbon and low VM content, the weight loss (under rapid devolatilization) for charcoal (29%) was higher than that for thermal coal (23%) and Bana grass char (22%) at 1500 °C, which could lead to low solid char yield during CC injection. Furthermore, the CC char has the fastest CO2 gasification reaction, this behaviour is likely to be governed by combination of low ash content, ash composition and char morphology in the CC material compared to thermal coal (TC) and Bana grass char (BGC). Reactions between carbonaceous materials and molten slag under simulated HIsarna thermal conditions were carried out by injecting different carbonaceous materials (CC, TC and BGC) into molten synthetic HIsarna slag in laboratory. The results show that the reduction process proceeds through two stages, starting with an initial rapid reduction and followed by gradual levelling off until the end of the process. The reaction rate and reduction degree of FeO in molten slag were the highest with CC chars, achieving over 60 % reduction at 1500 °C in the first 500s, compared to only ~50 % and just over 40 % with TC and BGC chars respectively for the same reaction time. The kinetic analysis suggests that the first stage reaction is controlled by chemical reactions at the carbon-slag interface, and the apparent activation energy values were 290, 229 and 267 kJ/mol for reactions with TC, CC and BGC chars respectively. On the other hand, the second stage can be described by threedimensional diffusion model (D3) and mixed influence from gas diffusion, liquid phase mass transfer, chemical reaction and carbon diffusion is likely to control the reduction. The results show that there are some common characteristics between coals and biomass materials selected, but the overall behaviour was different. Charcoal showed to have much higher combustibility and reactivity among the tested materials. The higher reactivity for charcoal may result in some of the solid chars to burn prematurely during HIsarna injection and this could lead to generation of higher amount of CO for CCF section on the expense of the solid chars required for SRV. Therefore, to maintain the process efficiency during CC injection it is necessary to increase the CCF productivity to utilise the extra reductive gas proportion produced to improve the balance between devolatilization/gasification and solid char yield. To build on the current findings and for efficient use of biomass or other alternative fuels, further research is suggested to consider biomass/coal blending, continuation of slag/carbon reaction (e.g., quenching), molten metal carburisation, slag chemical composition (e.g., different FeO content), effect of impurities in the raw materials and the ash content and ash chemistry