This study investigates the use of sustainable hydrochar-mill-scale briquettes for promoting slag foaming in the electric arc furnace (EAF) process. Two types of biochar are tested, namely, a pristine green waste hydrochar (GWH) and its pyrolyzed char (PGWH) produced at 873 K. Each briquette weighs ≈20 g and has Cfix/OFeOx molar ratios ranging from 0.07 up to 0.90. Briquettes are charged into a molten EAF slag (600 g) at 1923 K. Successful foaming is achieved for all seven briquette recipes developed. The maximum slag foaming height is 1.6–2.5 times of the initial slag height, and the slag foaming duration is in the range of 1.5–3.4 min. GWH briquettes promote rapid slag foaming through the abrupt release of volatile matter, while PGWH briquettes promote a more gradual and long-lasting foaming process through gas production (CO and CO2) from carbothermic reduction. It is estimated that 1 kg of anthracite applied for slag foaming should be replaced by 2.4 kg of GWH or 1.9 kg of PGWH added in composite briquette form. At the given briquette addition rate tested in this study (30 g per kg of slag), no appreciable impurity (sulfur, phosphorous) transfer from hydrochar to the slag is observed.

This study systematically investigates the impact of externally added waste wood on the reduction behavior of biocarbon-containing pellets during roasting in a rotary kiln. Experimental analyses focused on understanding the effects of key parameters such as carbon addition amount, roasting temperature, and roasting duration on pellet compressive strength, metallization rate, and kiln ring formation. The findings indicate that biochar generated by the pyrolysis of externally added waste wood significantly enhances the reduction reactions in pellets and effectively reduces ring formation tendencies. Industrial computed tomography (CT) was employed to precisely characterize the three-dimensional pore structures of roasted pellets, while scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses provided insights into the microstructural characteristics and crystal structures of pellets, elucidating the microscopic mechanisms by which external carbon addition improves pellet properties and inhibits ring formation. Based on these experimental outcomes, the study proposes optimized process parameters suitable for low-carbon ironmaking technology, offering theoretical and technical support for achieving low-carbon and sustainable transformation in the steel industry.

Hydrothermal carbonization (HTC) technology converts biomass into a carbon-rich, oxygen-containing solid fuel. Most studies have focused on hydrochar produced under laboratory conditions, leaving a gap in understanding the performance of industrially produced hydrochar. This study comprehensively analyzes three types of industrially produced hydrochar for blast furnace (BF) injection. The results indicate that hydrochar has a higher volatile and lower fixed carbon content. It has a lower high heating value (HHV) than coal and contains more alkali matter. Nevertheless, hydrochar exhibits a better grindability and combustion performance than coal. Blending hydrochar with anthracite significantly enhances the combustion reactivity of the mixture. The theoretical conversion rate calculations reveal a synergistic effect between hydrochar and anthracite during co-combustion. Environmental benefit calculations show that replacing 40% of bituminous coal with hydrochar can reduce CO2 emissions by approximately 145 kg/tHM, which is equivalent to an annual reduction of 528 kton of CO2 and 208 kton of coal in BF operations. While industrially produced hydrochar meets BF injection requirements, its low ignition point and high explosivity necessitate the careful control of the blending ratio.

Against the backdrop of escalating global carbon emissions, the steel industry urgently requires a transition toward green and low-carbon practices. As a conditionally carbon-neutral renewable energy source, biochar holds potential for replacing traditional fossil-based reducing agents. This study aims to investigate the mechanism and performance differences between biochar (wood char, bamboo char) and conventional reducing agents (semi-coke, coke powder, anthracite) in the direct reduction process of carbon-bearing briquettes. Through reduction experiments simulating rotary kiln conditions, combined with analysis of reducing agent gasification characteristics, carbon-to-oxygen (C/O) molar ratio control, X-ray diffraction (XRD), and microstructural examination, the high-temperature behavior of different reducing agents was systematically evaluated. Results indicate that biochar exhibits superior gasification reactivity due to its high specific surface area and developed pore structure: wood char and bamboo char show significantly enhanced reaction rates above 1073 K, approaching complete conversion at 1173 K. In contrast, anthracite and coke powder, characterized by dense structures and low specific surface areas, failed to achieve complete gasification even at 1273 K. Pellets containing bamboo char achieved the highest metallization rate (90.16%) after calcination at 1373 K. The compressive strength of the pellets first decreased and then increased with rising temperature, consistent with the trend in metallization rate. The mechanism analysis indicates that the high reactivity and porous structure of biochar promote rapid CO diffusion and synergistic gas–solid reactions, significantly accelerating the reduction of iron oxides and the formation of metallic iron.

This study carried out a detailed investigation into the potential application of hydrothermally treated bituminous coal (hydrochar) as an injectant in blast furnace (BF) ironmaking. A tuyere model was constructed through simulation methods, and the influence of hydrochar injection on the thermal conditions within the BF hearth was also thoroughly analyzed. The results show that the gas flow velocity at the lower part of the tuyere of hydrochar injection increases, and the residual carbon mass fraction of the tuyere decreases. As the oxygen-enriched concentration increases, the CO concentration decreases. The CO concentration in the swirl zone after hydrochar injection is the highest, reaching 43.93%. The distributions of CO and CO2 exhibit opposite tendencies. Following hydrochar injection, a marked rise in temperature is observed. At an oxygen enrichment level of 30%, the tuyere zone temperature associated with hydrochar injection peaks, surpassing 2700 K. The corresponding pulverized coal burnout rate is also the highest. Thus, the injection of hydrochar has a positive impact on the air flow and temperature field, which can effectively maintain the heat balance and is conducive to strengthening BF smelting.

The substitution of fossil coal with biocarbon in the metallurgical processes can help to decrease fossil CO₂ emissions. Biocarbon’s characteristics, such as high volatile matter contents and high reactivities with CO₂, are beneficial for increasing the reduction degrees and reduction rates of iron oxides in carbon composite agglomerates (CCA). This study compared the reduction of hematite by of two types of carbonaceous materials (CM): hydrochar (high-volatile biocarbon) and anthracite (a low-volatile coal) in the form of CCA. CM, hematite, and binder (starch) were mixed together to obtain mixtures with C/O molar ratios equal to 0.4–1.2. The mixtures were reduced non-isothermally in nitrogen atmosphere up to 1003 K or 1373 K. Up to 1003 K, the volatiles released from CMs and starch reduced hematite by 18–35%. Between 1003 K and 1373 K, both hydrochars (produced from lemon peels and rice husks) reacted with iron oxides more rapidly than anthracite below 1360 K, when the samples had C/O ratios in the range of 1.0–1.2. In this temperature range, rice husk hydrochar promoted a slower reaction with iron oxides than lemon peel hydrochar, which was possibly influenced by its higher ash content which decreased the rate of Boudouard reaction. Samples with C/O ≥ 1.0 achieved complete reduction at 1373 K, regardless of the type of CM used, whereas samples with C/O equal to 0.4–0.5 achieved 63–86% reduction. It can be concluded from this study that hydrochar can fully substitute anthracite for direct reduction of iron oxide to decrease fossil CO2 emissions during ironmaking processes.

Gas-based direct reduction in a shaft furnace is the dominant process in the world for production of direct reduced iron. As fresh reducing gas passes through the iron ore burden, it is diluted by the gas emitted from the reacted iron ores which decreases the reduction potential of the reducing gas. Previous reduction experiments mostly used single pellet which could not examine this phenomenon. In this study, hematite pellets arranged in multiple layers inside a molybdenum basket are reduced isothermally at 1173–1273 K using 50% CO + 50% CO₂% and 100% CO gases under flow rates of 0.2–5.0 NL min⁻¹ to simulate the dilution of CO by CO₂ in the shaft. It is discovered that the reduction of pellets in the basket is highly uneven even in pure CO atmosphere. Pellets in the middle layer are reduced ≈2 times less than the pellets in the top and bottom layers. The top side of a pellet is also less reduced than the bottom side facing the gas inlet. During melting of incompletely reduced pellets at 1873 K, intensive interaction between the unreduced iron oxides and the alumina crucible was observed. Thus, smelting of incompletely reduced iron could potentially shorten the refractory lifetime.

Lab-scale alloying experiments were carried out by first adding commercial low-carbon ferrochrome (LCFeCr) alloys and then adding ferroniobium (FeNb) alloys in 316-grade austenitic stainless steel in this study. The inclusion and precipitation characteristics in LCFeCr and FeNb were evaluated as well as in a 316 austenitic stainless steel after the alloy additions by using two- and three-dimensional characterization methods in combination with thermodynamic calculations. The results showed that MnCr2O4 spinels and pure Al2O3 were the main types of inclusions in LCFeCr alloys, while pure TiOx, Al2O3 inclusions and complex TiOx-Al2O3 aggregates were mainly found in FeNb alloys. After the addition of LCFeCr alloy to the steel, the SiO2 contents in liquid inclusions decreased to some extent, while more inclusions containing higher MnO contents were observed. Some MnCr2O4 spinel inclusions can be reduced by Si in steel and form liquid inclusions. Some MnCr2O4 spinel and Al2O3 inclusions from LCFeCr alloy can remain in the steel melt, which decreased the steel cleanliness. After the addition of FeNb alloy, pure TiOx inclusions present in this alloy can hardly be found in the steel melt. The inclusion types in steel were not changed so much but high Nb-containing phases were found around the inclusions and coarse Laves phases were formed in the matrix. Overall, this work aims to understand the impurity particle behavior during the alloying process when using ferroalloys to produce high-performance stainless steels.

The iron and steelmaking industry faces the dilemma of the need to decrease their greenhouse gas emissions to align with decarbonization goals, while at the same time fulfill the increasing steel demand from the growing population. Replacing fossil coal and coke with biomass-based carbon materials reduces the net carbon dioxide emissions. However, there is currently a shortage of charcoal to fully cover the demand from the iron and steelmaking industry to achieve the emission-reduction goals. Moreover, the transportation and energy sectors can compete for biofuel usage in the next few decades. Simultaneously, our society faces challenges of accumulation of wastes, especially wet organic wastes that are currently not reused and recycled to their full potentials. Here, hydrothermal carbonization is a technology which can convert organic feedstocks with high moisture contents to solid fuels (hydrochar, one type of biochar) as an alternative renewable carbon material. This work studied the differences between a hydrochar, produced from lemon peels (Lemon Hydrochar), and two types of charcoals (with and without densification) and an Anthracite coal. Characterizations such as chemical and ash compositions, thermogravimetric analyses in nitrogen and carbon dioxide atmospheres, scanning electron microscope analyses of carbon surface morphologies, and pyrolysis up to 1200 °C were performed. The main conclusions from this study are the following: (1) hydrochar has a lower thermal stability and a higher reactivity compared to charcoal and Anthracite; (2) densification resulted in a reduction of the moisture pickup and CO₂ reactivity of charcoal; (3) pyrolysis of Lemon Hydrochar resulted in the formation of a large amount of tar (17 wt%) and gas (39 wt%), leading to its low fixed carbon content (27 wt%); (4) a pyrolyzed hydrochar (up to 1200 °C) has a comparable higher heating value to those of charcoal and Anthracite, but its phosphorous, ash, and alkalis contents increased significantly; (5) based on the preliminary assessment, hydrochar should be blended with charcoal or Anthracite, or be upgraded through slow pyrolysis to fulfill the basic functions of carbon in the high-temperature metallurgical processes.

Hydrochar (a solid product from hydrothermal carbonization of organic feedstock) and charcoal have the potential to substitute coke and coal consumption in the iron and steelmaking processes for reduction of greenhouse gas (GHG) emissions. Among steelmaking processes, melt carburization is an important but less-studied application. In this study, briquettes produced with mixture a of iron powder, hydrochar or charcoal powder, and binder were tested as iron melt recarburizers. It was found that the hydrochar briquettes have good mechanical properties, whereas those of charcoal briquettes were poor. Melt carburization with briquettes was performed in a lab induction furnace (10 kg) in two steps: firstly, by heating up some briquettes with charged electrolytic iron from room temperature up to 1600 °C, followed by the addition of some briquettes into the melt. Recarburization efficiency (RE) during the first step of carburization was found to be controlled by the amount of carbon content bound in the solid phase (fixed carbon) determined at 1200 °C. Thus, the REs of charcoal briquettes (70–72%) were higher than those of hydrochar (43–58%) due to the higher fixed carbon contents in charcoal. REs obtained from the second step were strongly affected by the amount of briquette losses during their addition into the iron melt, which correlate with the mechanical strengths of the briquettes. Thus, the REs for hydrochar briquettes (48–54%) were higher than those of charcoal (26–39%). This study proves the feasibility of using hydrochar and charcoal as liquid steel recarburizers.