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.

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.

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 1373K, 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.

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 C _fix /O _FeOx 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 CO ₂ ) 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.

Fluorspar serves the critical functions of facilitating lime dissolution and liquid slag formation in the argon‐oxygen‐decarburization (AOD) process. Yet, it is a material with long‐term supply risks that requires a gradual replacement. Thermodynamic calculations and industrial AOD trials were performed in this study to identify different pathways for eliminating fluorpsar addition during the production of 304 stainless steel. FactSage calculations suggest that the suitable amount of flux to replace 5 wt% CaF ₂ during the reduction phase (B4 = 1.6) is 20 wt% Al ₂ O ₃ , 5 wt% Na ₂ O, 5 wt% K ₂ O, or 3 wt% Li ₂ O. Likewise, the suitable amount of flux to replace 12 wt% CaF ₂ during the desulphurization phase (B4 = 2.1) is 20 wt% Al ₂ O ₃ , 9 wt% Na ₂ O, 12 wt% K ₂ O, or 6 wt% Li ₂ O. During one industrial AOD trial where 50 wt% of ferrosilicon reducing agent was replaced by aluminum, the overall fluorspar consumption was reduced by 75% in weight and the final sulfur content in the liquid steel at tapping was two times lower than that measured during the reference heat. In summary, the application of an aluminum reduction agent or the addition of alternative fluxes are both effective strategies for eliminating fluorspar without compromising the quality of the steel produced.