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

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

Evaluation of devolatilization behaviour of different carbonaceous materials under rapid heating for the novel HIsarna ironmaking process

A drop-tube furnace coupled with quadrupole mass spectrometer (DTF-QMS) was employed to simulate rapid heating conditions that carbonaceous materials experience during HIsarna injection with the measurement of gas composition change as a result. A devolatilization study for thermal coal (TC) and charcoal (CC) samples was carried out at three temperatures of 1000, 1250 and 1500 °C under an initial high purity Ar gas environment. The volatiles released were measured online by QMS, while the char yield was determined directly by the weight of particles collected and the deficit was calculated by subtracting the gas yielded from the total weight loss. The study reveals that working temperature has a strong impact on the devolatilization rate, the maximum weight loss and the variation in gas species produced. Due to intensification in the carbon oxidation and secondary reactions at higher temperatures, there was an increase in the weight loss, which led to a greater yield of H2 and CO but less yield of hydrocarbons, CO2 and H2O. Despite lower volatile matter content in charcoal, the weight loss for charcoal (29%) was higher than that for thermal coal (23%) at 1500 °C. Although the amount of H2 produced for both materials is similar, the amount of CO produced by charcoal is twice of that by thermal coal, and accounts for 79% of the total gas weight formed by charcoal. This suggests that a higher rate of carbon oxidation takes place through O2 containing groups within the charcoal, which results in lower char efficiency. It was found that thermal coal produces a significant amount of tar, while a large number of particles in the form of soot/dust escaped from the bulk material during charcoal injection but no tar formation was observed

Devolatilisation characteristics of coal and biomass with respect to temperature and heating rate for HIsarna alternative ironmaking process

HIsarna process offers a novel low CO2 emission alternative to the blast furnace for primary iron production. This new smelting ironmaking technology is flexible in raw material usage such as the substitution of biomass for coal as a reductant. Reduction is conducted through multiple mechanisms within the smelting vessel including gaseous reaction products from thermal decomposition of volatile matters reacting directly with iron oxide containing slags and injected iron ore. In this study, four coals with notable differences in volatile matter content along with two biomass samples sourced from wood and grass origins were investigated for the selection of suitable fuel mix. Thermogravimetric analysis (TGA) was used to measure the weight loss of the carbonaceous materials and a vertical tube furnace coupled with a quadrupole mass spectrometer (VTF-QMS) was employed for off-gas analysis during the devolatilisation. During TGA tests the samples were heated under a 99.9999 % argon atmosphere to 1500 °C at three different heating rates to investigate the kinetics of thermal decomposition for these materials. Through use of the Kissinger– Akahira–Sonuse model an average activation energy was determined as a function of the conversion degree. The furnace experiments were carried out under a 99.999% Ar atmosphere to a peak temperature of 1500 °C, at a heating rate of 10 °C/min. The wt% of reducing gases e.g. H2, CO, and hydrocarbons, and the temperature required for these gases to evolve was notably different for each materials, but the respective maximum peaks of evolution of these gases corresponded well to the maximum rate of mass loss. Furthermore, the off-gas analysis reveals torrefied grass contains large amount of water and carbon dioxide which will be released at very low temperature, therefore pre-treatment to the temperature of ~400 °C is necessary to produce chars with similar properties to coal injected in HIsarna

Dissolution behavior of different inclusions in high Al steel reacted with refining slags

Al2O3, Al2O3·TiN, Al2O3·MgO and CaO·2Al2O3 are four different types of inclusions in high Al steels. To improve the steel cleanness level, an effective removal of such inclusions during secondary refin-ing is very important, so these inclusions should be removed effectively via inclusion dissolution in the slag. The dissolution behavior of Al2O3, Al2O3·TiN, Al2O3·MgO and CaO·2Al2O3 in CaO-SiO2-Al2O3-MgO slags as well as the steel-slag reaction was investigated using laser scanning confocal mi-croscopy (LSCM) and high-temperature furnace experiments, and thermodynamic calculations for in-clusions in steel were carried out by FactSage. The results showed that Al2O3·TiN was observed to be completely different from the other oxides. The composite oxides dissolved quickly in slags, and the dissolution time of inclusions increased as their melting point increased. SiO2 and B2O3 in the slag were almost completely reacted with [Al] in steel, so the slags without SiO2 content showed a positive effect on avoiding formation of Al2O3 system inclusions and promoting inclusions dissolution as com-pared to SiO2-rich slags. The steel-slag reaction was also found to influence the inclusion types in steel significantly. Due to the rapid absorption of different inclusions in the slag, it was found that the dissolution time of inclusions mainly depends on the diffusion in the molten slag

A comprehensive literature review of biomass characterisation and application for iron and steelmaking processes

This study provides a fundamental literature review of biomass characterisation and application for utilisation as a reducing agent and fuel in an integrated steelmaking process. This review contains and evaluates the existing literature concerning the properties of different biomass sources and available pre-treatment technologies for upgrading biomasses to have similar characteristics to fossil coal. The suitability of biofuels in different operating units including Brazilian mini blast furnaces, cokemaking, larger blast furnace injection, sintering and production of carbon composite agglomerates are analysed. There is a considerable opportunity to reduce CO2 emissions in the integrated steelmaking process by using biomass-based reducing agents. The literature reports that charcoal optimisation in the integrated steel plant can achieve reduction in net CO2 emissions of 31-57%, depending on the substitution rate. Although, some level of pre-treatment is also necessary for efficient use of biomass, the pre-treatment requirement will depend on the unit operation where the biomass material is added. Solid biofuels are considered the most suitable form of biomass-based reducing agents due to similarities in the physical properties with the injected coals; this will make coal replacement more straightforward and lower the costs required for plant modifications

Reduction of FeO in molten slag by solid carbonaceous materials for HIsarna alternative ironmaking process

To investigate the use of biomass in the novel HIsarna technology, the reduction of FeO in the slag by chars produced from thermal coal (TC), charcoal (CC), and Bana grass char (BGC) was studied. A drop tube furnace coupled with a quadrupole mass spectrometer (DTF‐QMS) was employed to study the injection of chars into pre‐melted slag in the temperature range between 1450 °C and 1525 °C. The reduction rate was calculated from evolved gases and the extent of FeO reduction was confirmed by Wavelength Dispersive X‐Ray Fluorescence (WDXRF). The FeO reduction proceeds through two stages, starting with a rapid reduction, which is dependent on the carbon type, and followed by gradual levelling off. The reduction rate with the charcoal char (CC) was highest, over 60% reduction was achieved in the first 500s at 1500 °C, while ~50% and 40% achieved with TC and BGC chars respectively for the same reaction time. The kinetic analysis suggests that the first and second stages of the reaction can be described by the second order (F2) and three‐dimensional diffusion (D3) models respectively. The apparent activation energy values for the first stage were 290, 229 and 267 kJ/mol for reactions with TC, CC and BGC chars, while 265, 369, and 282 kJ/mol were obtained for the second stage. Based on the experimental data and kinetic results, it can be concluded that the first stage is controlled by chemical reactions on the carbon surface, and the second stage is influenced by a mixed controlling mechanism

Kinetic study on reduction of FeO in a molten HIsarna slag by various solid carbon sources

To investigate the reduction behaviour of different reductants such as charcoal (CC), thermal coal (TC), and carbon black (CB) with HIsarna slag, a series of isothermal reduction experiments were performed in a vertical tube resistance furnace (VTF), coupled with a Quadrupole mass spectrometer (QMS) at 1450, 1475 and 1500 ˚C. The results confirm that the highest overall reduction rate was achieved by CC, followed by TC and CB. The reduction mechanism between FeO containing molten slag and the selected carbonaceous materials is determined by studying the morphology of the water quenched samples at the intervals of 1.5, 3 and 5 minutes, using optical and scanning electron microscopes. The results reveal that the overall reaction is controlled by two main mechanisms: (1) nucleation and growth of CO bubbles, proceeded by the gaseous intermediates CO and CO2; and (2) diffusion of FeO in the molten slag. The initial reduction period in which chemical reaction control is dominant, can be described by the Avrami-Erofeev model, whereas the final period is described by the three-dimensional diffusion model

Gasification and structural behaviour of different carbon sources and resultant chars from rapid devolatilization for HIsarna alternative ironmaking process

To evaluate the potential of using renewable biomass in the novel HIsarna technology, reactivity of thermal coal (TC), charcoal (CC), Bana grass char (BGC) before and after rapid devolatilization at 1500 °C in a drop tube furnace (DTF) was investigated. Thermogravimetric (TG) was used for CO2 gasification study, and high temperature confocal scanning laser microscope (HT-CSLM), Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) were used to characterise the morphology of all three raw carbonaceous materials and their chars produced by rapid devolatilization. CC has fastest gasification reaction before and after rapid heat treatment, and BGC raw is more reactive than TC raw but BGC 1500 °C and TC 1500 °C have very similar gasification behaviour. The reactivity index of the rapidly devolatilized char is reduced to 84.21% (BGC), 92.11% (TC) and 94.23% (CC) compared to their raw materials. This shows that BGC is more severely affected by the rapid devolatilization, and this behaviour is likely to be governed by the high ash content which will melt and cause pore blockage during heat treatment. According to HT-CSLM results the average particle sizes decreased by 28%, 24% and 20 % for TC, CC and BGC respectively. While the SEM images shown that TC has gone through significant structural changes during the rapid devolatilization, but CC and BGC maintained their parent structural shapes. The BET results indicate that TC is non-porous, but both CC and BGC contain a large number of constricted micropores with significantly larger surface area