Introduction

Steel stands as the most widely utilized and important metallic material, both in terms of production volume and its vast array of versatile applications. Despite its unrivaled significance, the iron and steelmaking process heavily relies on fossil fuels to meet energy demands and facilitates the primary extraction and refinement of ores [1, 2]. Globally, the iron and steel industry represents one of the largest contributors to greenhouse gas (GHG) emissions, annually releasing 3.7 Gt of CO2 [3]. This level of emissions averages around 2 tons of CO2 for every ton of steel produced, constituting approximately 7% of global CO2 emissions [4]. Global steel production has increased dramatically in recent decades, a trend that is projected to continue given the strong correlation between steel market demand, population growth, and economic expansion [5]. Therefore, to address the growing issue of GHG emissions, steelmakers currently face the challenge of reducing their CO2 emissions by developing new technologies for sustainable iron and steelmaking.

The two well-established methods predominantly used in steelmaking are the blast furnace-basic oxygen furnace (BF-BOF) and the electric arc furnace (EAF), with the former currently accounting for the majority (70.7%) of the global steel production [6]. However, the decarbonization potential of EAF steelmaking has garnered interest in recent years, aligning with the growing focus on green growth within the steel industry [1]. An EAF functions by passing a current through graphite electrodes to create arcs that act as the main heat source. EAF is primarily used for recycling scrap steel, as well as for melting of other materials such as direct reduced iron, hot briquetted iron, or pig iron. Additionally, carbon material is included in the charge materials to carburize the molten metal which supplies supplementary chemical heat during subsequent carbon oxidation. Carbon material is also injected into the molten slag during EAF operation to generate a slag foam layer, which increases energy efficiency, protects the furnace lining, and decreases noise and electrode consumption [7]. These two carbon applications are named as ‘charge carbon’ and ‘injection carbon,’ respectively. Another application of carbon is recarburizing in the steel ladle to meet chemistry specifications, though this topic will not be explored in detail in this review. Presently, fossil sources of carbon are used as charge carbon and injection carbon in the EAF process. This includes materials such as coke, petroleum coke, or anthracite. Typically, ~ 12 kg of fossil carbon is used as injection or charge carbon per ton of steel produced, which accounts for 40–70% of direct emissions from an EAF [8]. This presents an opportunity to further decarbonize the steelmaking process by substituting fossil carbon with biocarbon derived from biomass. Since biocarbon is taken from the natural carbon cycle, it can be considered as a carbon neutral material [9].

Typically, waste biomass is used as a feedstock to make biocarbon. This creates a potential waste-to-resource supply in addition to using a carbon neutral material to lower CO2 emissions of the EAF process. However, it is important to recognize that the feedstock for biocarbon production depends heavily on local conditions and the maturity of the industry. For example, Brazil, the world’s largest producer of biocarbon (charcoal), utilizes its mature biocarbon industry to support pig iron production. Brazilian industry was the first to carefully optimize the biomass conversion process and produce high-quality biocarbon. Given the significant potential benefits, research on the substitution of fossil carbon with biocarbon in EAF steelmaking has received considerable attention in the past decade, with research accelerating in the past 5 years.

Review Objectives

Several reviews have been conducted on the use of biomass in ironmaking processes [10,11,12,13,14]. Mathieson et al. [15] reviewed the utilization of biomass as a fuel in ironmaking, noting its potential for significant CO2 emissions reductions when used as a substitute for fossil fuels in the BF. Suopajarvi et al. [12] reviewed the use of biomass in integrated BF-BOF steelmaking and found small-to-moderate opportunities to utilize biomass-based fuels. Similarly, Mousa et al. [13] found that partial substitution of fossil carbon with biochar in the BF is a possible pathway. A review by Khasraw et al. [14] found that if 20% of coke was substituted by biomass, there would be a 15% reduction of CO2 emissions per ton of pig iron. Each of these reviews also pointed out the economic challenges of using biomass in ironmaking. Additionally, broad scope reviews have been completed on biomass utilization in all metallurgical processes [16,17,18,19]. In the context of EAF steelmaking, Echterhof [20] provided a broad overview of alternative carbon sources and Kieush and Schenk [21] reviewed the relationship between carbon properties and slag foaming dynamics. Mathieson et al. [22] and Jahanshahi et al. [23] both reviewed the CO2 savings from biomass substitution in EAF steelmaking, but did not detail the biocarbon functionality. To facilitate further progress and develop new perspectives on biocarbon as a carbon material for EAF steelmaking, it is essential to collectively analyze the performance and evaluation methods presented in recent literature.

This review aims to provide a state-of-the-art analysis on the recent advances in biocarbon utilization, specifically in EAF steelmaking. By collecting insights from various studies, the objective is to comprehensively analyze the results from multiple scales of testing and provide mechanistic explanations for the suitability of biocarbon as a potential alternative carbon source in the EAF process. Furthermore, the review seeks to identify the current limitations and key areas for future research and development, recognizing that biocarbon integration in EAF steelmaking is an emerging field with significant potential for innovation and optimization. Through this exploration, discussion is aimed at contributing to the advancement of biocarbon integration into EAF steelmaking practices, while addressing the critical technical gaps in understanding and implementation.

Carbon in the Electric Arc Furnace Process

Charge Carbon

As the name suggests, charge carbon is charged with the metallic feedstock. Typical fossil charge carbons include nut coke or anthracite coal. Charge carbon serves two primary purposes, to consume excess oxygen during melting and to carburize the molten metal. Carburizing the molten metal allows for excessive carbon to be oxidized (through oxygen blowing) which provides chemical heating. The relevant oxidation reactions are shown in Eqs. 1–3. The reaction enthalpies are taken from previous work by Lei Deng [24]. The heating provided by these reactions is critical to efficient EAF steelmaking. In fact, chemical energy accounts for 15–25% of the total EAF process energy [25, 26].

$$ {\text{2C}}_{{\left( {\text{s}} \right)}} + {\text{O}}_{{{2}\left( {\text{g}} \right)}} \to {\text{2CO}}_{{\left( {\text{g}} \right)}} \Delta {\text{H}} = - {1}0{4}.{\text{29 kJ}}/{\text{mol}} $$
(1)
$$ {\text{C}}_{{\left( {\text{s}} \right)}} + {\text{O}}_{{{2}\left( {\text{g}} \right)}} \to {\text{CO}}_{{{2}\left( {\text{g}} \right)}} \Delta {\text{H}} = - {368}.{9}0{\text{ kJ}}/{\text{mol}} $$
(2)
$$ {\text{2CO}}_{{\left( {\text{g}} \right)}} + {\text{O}}_{{{2}\left( {\text{g}} \right)}} \to {\text{2CO}}_{{{2}\left( {\text{g}} \right)}} \Delta {\text{H}} = - {278}.{\text{61 kJ}}/{\text{mol}} $$
(3)

Given the functionality and role of charge carbon in the EAF process, the critical characteristics for effective charge carbon are dissolution into molten steel, reactivity, and calorific value. The dissolution of charge carbon influences its availability for reaction with oxygen and subsequent carbon removal from the metal bath. Reactivity determines the speed and efficiency of carbon oxidation reactions during steelmaking, impacting process kinetics and energy consumption. Additionally, the calorific value of charge carbon directly influences the heat balance within the furnace, affecting temperature profiles and overall process stability. These key characteristics must be conserved when considering a shift from fossil carbon to biomass (in the form of biocarbon) for charge carbon.

Injection Carbon

Unlike charge carbon, injection carbon is added the EAF during the process. This is done using specialized injection systems, such as lances or tuyeres, typically using air as a carrier gas. Injection carbon is typically anthracite coal, coke, or petroleum coke, in granular form with a size of ~ 150 μm. This method allows for precise control over the amount of carbon that is injected, to get the desired effect and benefits. The primary purpose of injecting carbon into the slag layer is to create a ‘slag foam.’ Slag foaming provides several benefits and is critical to modern EAF steelmaking. Having a foamy slag protects the refractory lining, water-cooled sidewalls and roof of the furnace from the arc radiation, increases energy efficiency by thermally insulating the melt, decreases N introduction into the molten metal, and decreases noise from operation. The injection carbon initiates slag foaming by reducing the FeO in the slag to create CO bubbles, which are trapped in the slag and increase the overall volume of the slag layer. Although not the focus of this review, it is important to note that the slag must have specific properties that lend itself to slag foaming, most importantly, surface tension, basicity, and viscosity [27,28,29]. These parameters are generally the result of the slag composition, which needs to be carefully controlled [27].

FeO enters the slag either as rust from the charge materials or by the oxidation of Fe in the melt from oxygen blowing (Eq. 4). As mentioned in the previous section, oxygen is blown into the EAF to generate chemical heat and to provide stirring. Although it is thermodynamically preferable for the oxygen to react with the carbon, some of the oxygen inevitably react with the Fe due its abundance in the melt compared to carbon. This kinetic effect leads to FeO in the slag. Typically, the slag needs to be 15–40% FeO to facilitate the reduction reaction and sufficient CO bubble generation [29, 30]. Matsuura and Fruehan [31] found that CO bubbles produced by FeO reduction were smaller than CO bubbles produced from decarburization (Eq. 1). They concluded that the smaller bubbles led to a more stable foam, since large bubbles may rise rapidly through the slag. However, Pretorius and Carlise [27] stated that CO bubbles from decarburization are more uniformly distributed and smaller in size. Although there is disagreement about which reaction produces finer bubbles, it is generally accepted that small bubbles are beneficial for a stable foamy slag [32, 33]. The injection carbon process and resultant foamy slag are illustrated in Fig. 1.

Fig. 1
figure 1

Formation of foamy slag by carbon injection a initial state with FeO present in the slag b carbon injection into the slag layer c CO bubbles form from FeO reduction and create a foamy slag with increased height

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The chemical reaction governing slag foaming by FeO reduction is given by Eq. 5, which is the direct reduction of FeO by solid C. However, the specific kinetics of an indirect reduction reaction are more complex. Once the reaction in Eq. 5 occurs, a gas film of CO/CO2 is developed on the solid carbon particle. From there, FeO reduction proceeds in three stages. First is the mass transfer of FeO to the slag/gas interface. The FeO is reduced by the CO contained in the gas film (Eq. 6). The reaction product CO2 gas then travels to the solid carbon particle/gas interface and is reduced to CO gas (Eq. 7). The CO gas moves to the slag/gas interface and the process is repeated to reduce FeO. These process steps are illustrated in Fig. 2. This reaction sequence was first observed by Min and Fruehan [34] and was further studied by Sarma et al. [35] using X-ray fluoroscopy to directly observe the reaction. The rate determining step during slag foaming remains unclear [36] but it is influenced by several factors such as FeO content or slag basicity which control the various reactions [35, 37,38,39].

$$ {\text{2Fe}}_{{\left( {\text{l}} \right)}} + {\text{O}}_{{{2}\left( {\text{g}} \right)}} \to {2}\left( {{\text{FeO}}} \right) $$
(4)
$$ \left( {{\text{FeO}}} \right) + {\text{C}}_{{\left( {\text{s}} \right)}} \to {\text{Fe}}_{{\left( {\text{l}} \right)}} + {\text{CO}}_{{\left( {\text{g}} \right)}} $$
(5)
$$ \left( {{\text{FeO}}} \right) + {\text{CO}}_{{\left( {\text{g}} \right)}} \to {\text{Fe}}_{{\left( {\text{l}} \right)}} + {\text{CO}}_{{{2}\left( {\text{g}} \right)}} $$
(6)
$$ {\text{CO}}_{{{2}\left( {\text{g}} \right)}} + {\text{C}}_{{\left( {\text{s}} \right)}} \to {\text{2CO}}_{{\left( {\text{g}} \right)}} $$
(7)
Fig. 2
figure 2

Reaction sequence for the indirect reduction of FeO in slag showing the gas film around the solid carbon particle and intermediate reactions

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Given the complex and dynamic process of slag foaming, there are specific requirements of injection carbon that must be preserved when transitioning to biocarbon. The reactivity with FeO must be such that it facilitates the consistent generation of fine CO bubbles, essential for promoting effective slag foaming [33, 40]. The carbon particles must exhibit good wettability with the slag to enable proper interfacial contact [41, 42]. Also, it is crucial any differences in composition (either in the ash layer or particle itself) do not negatively affect the slag or steel product composition. Although biocarbon offers potential environmental and economic advantages over traditional carbon sources, such as reduced carbon footprint, its suitability for slag foaming hinges on meeting these critical performance criteria. Thus, careful evaluation and optimization of biocarbon properties are essential to ensure its successful integration into EAF steelmaking processes.

Biocarbon and Fossil Carbon

The properties of injection and charge carbon play a pivotal role in the efficiency of EAF steelmaking. For slag foaming, carbon must exhibit adequate reactivity with FeO to generate fine carbon monoxide bubbles, alongside good wettability with slag to facilitate the reduction reaction. Charge carbon, utilized for heating, requires properties conducive to efficient carbon dissolution into the molten metal and heat generation during oxidation. It is essential to note that biocarbon derived from biomass differs from fossil carbon in terms of reactivity, purity, and morphology. Due to its higher moisture absorption and dependence on humidity compared to fossil carbon, biocarbon analyses must be reported on a dry basis to ensure consistency. While biocarbon offers potential sustainability benefits, its unique properties necessitate careful consideration and development to ensure successful integration into the EAF steelmaking process.

Biomass Processing

Biomass is typically taken from a variety of sources including, agricultural crop wastes, forestry residues, planted wood species, or dedicated energy crops. Typically, biomass or biocarbon material composition is evaluated and reported, which includes ash content, volatile matter (VM) content, and fixed carbon (FC) content [43]. VM signifies the proportion of combustible gases emitted during heating, impacting energy consumption and emissions. FC indicates the stable carbon residue remaining after volatile components are driven off, demonstrating the carbon availability for steelmaking reactions. However, raw biomass typically does not have suitable properties for use. Biomass has high VM content, high moisture content, low density, and low calorific value, all of which would be problematic for use in an EAF. Furthermore, the carbon content is typically around 50% and the FC content is less than 20% [44]. Therefore, additional processing is required to make biomass suitable for the partial or complete replacement of fossil carbon in EAF steelmaking. Thermal processing converts the biomass into a usable solid biocarbon, which can be achieved by pyrolysis [45, 46], torrefaction [47], hydrothermal processing [48], or gasification [49]. These thermal treatments reduce the moisture content, increase the carbon content, and increase the calorific value.

Pyrolysis and Torrefaction

Pyrolysis, conducted in an oxygen-free environment, stands as the predominant method in biochar production. Essentially, it is the breaking down of complex organic substances into simpler molecules [50]. This thermal decomposition process yields three primary products: solid biochar, pyroligneous acid, liquid bio-oil, and pyrolysis gas comprising carbon monoxide, carbon dioxide, hydrogen, methane, and higher hydrocarbons [51]. Operating temperatures for pyrolysis typically range from 300 to 900 °C, with variations in parameters, such as heating rate and residence time, influencing product proportions. Fast pyrolysis, characterized by higher temperatures (surpassing 400 °C), rapid heating rates, and brief residence times lasting only seconds, primarily serves bio-oil production [45]. However, it may be an interesting economic option to use the solid by-product as biocarbon. In contrast, slow pyrolysis, favored for solid biocarbon production, occurs at lower temperatures, below 500 °C, with extended residence times on the scale of hours to days [51]. Torrefaction, akin to an incomplete slow pyrolysis, operates at temperatures below 300 °C for typically less than 2 h [52, 53]. The process initiates with the removal of surface moisture within a temperature range of 25–105 °C, followed by the dehydration of organic molecules and the evolution of light organic volatiles during the post-drying stages [54]. While not fully realizing thermochemical conversion and evaporation as pyrolysis does, torrefaction aims to enhance biomass’s mechanical properties, like grindability, while preserving its energy content.

Hydrothermal Carbonization and Gasification

Hydrothermal carbonization (HTC) and gasification represent alternative pathways for biochar production. In HTC, biomass undergoes immersion in water, subject to temperatures typically ranging from 180 to 250 °C, with residence times spanning from a few hours to a full day, within an autogenous pressure environment ranging between 2 and 6 MPa. The process is mainly driven by hydrolysis [55]. During hydrolysis, hemicellulose, cellulose, and lignin break down into smaller fragments, enabling subsequent reactions like dehydration and decarboxylation. For a detailed examination of the chemical reactions involved, refer to Pauline and Joseph [55]. Gasification, on the other hand, aims to generate bio-gas at temperatures exceeding 750 °C. Similar to fast pyrolysis, the solid by-product is an interesting biocarbon option. While these methods exist, they are less commonly employed for primary biochar production [56].

Biocarbon Properties

After processing, the biocarbon is characterized by the ash, VM, and FC content on a dry basis. These three properties are generally considered to be the most important for solid biocarbon utilization in EAF steelmaking [57] and can be measured by proximate analysis [56]. The ash content refers to the residue in the biocarbon following thermal processing and is typically composed of a variety of oxides. Clean biomass typically has ash contents of < 5% meaning ash contents above 5% represent contamination of the feedstock by soil or other foreign materials. It is important to be mindful of the ash content as it may have implications on the slag or steel chemistry.

There have been a variety of biocarbon studied for applications in EAF steelmaking. Table 1 summarizes the literature in this area and identifies the biocarbon source, the thermal process, and the resultant biocarbon composition (ash, VM, and FC content). HTC appears to produce biocarbon with relatively low FC and high VM, which is not ideal for steelmaking. The low FC content indicates that much of the biocarbon will be lost as VM. However, the most recent study on HTC-derived biocarbon by Wei et al. [58] showed improved FC content compared to the older work. HTC is observed to have a range of ash contents from the literature presented in Table 1. Two cases of high ash content were reported by Han et al. [59] and Echterhof et al. [60] (20.8 and 51.3%, respectively). However, in the study by Han et al. [59], the biomass also gave a relatively high ash content using pyrolysis methods, meaning the feedstock may exhibit a high ash content regardless of processing. Additionally, the large variations for ash content reported in Table 1 also suggest the feedstock is the dominating factor controlling ash content and not the HTC process. Interestingly, research on biocarbon produced by torrefaction is very limited, as it has only been reported in the GREENEAF2 [61] project.

Table 1 Summary of biocarbon studied for EAF steelmaking applications
Full size table

From Table 1, it is clear that pyrolysis is the most common process to produce biocarbon for EAF-related applications. This is likely due to the improved biocarbon properties, which is characterized by high FC content, low ash content, and low VM content [81]. The FC content generally exceeds 70% and, in several cases, approaches 90%. The surveyed FC and VM values were plotted against pyrolysis temperature in Fig. 3. In the figure, a positive trend can be observed between pyrolysis temperature and FC content and a negative trend for VM content can be seen. However, in both cases there are several outliers which may be due to differences in feedstock, which is known to have an effect on carbon composition [82] or the (typically unreported) pyrolysis time. It is also important to note that most points in the cluster of low VM values (< 500 °C) are biocarbon with high ash content. Huang et al. [70, 71] tested a fast pyrolysis (indicated as (FP) in Table 1) process but, given the biocarbon is merely a solid by-product of the production of bio-oil, its properties are not optimized. It is possible that the high FC content outliers in the 400–500 °C range used a specifically designed slow pyrolysis. Ash content was not found to have any trend with pyrolysis temperature, further affirming it is mostly governed by feedstock. Although metrics for biocarbon properties have been collected and discussed in this section, they must be put into the proper perspective. To determine how biocarbon properties may perform in EAF steelmaking, a comparison to typical EAF fossil carbon is needed.

Fig. 3
figure 3

Plots of a FC content with pyrolysis temperature b VM content with pyrolysis temperature from the relevant EAF studies collected in Table 1

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Comparison to Fossil Carbon Properties

Fossil carbons such as anthracite, coke, and petroleum coke (petcoke) are typically used in EAF steelmaking for charge and injection carbon. Presumably, steelmaking processes have been designed to incorporate fossil carbon and their associated characteristics. Due to the robust manufacturing methods and infrastructure, the properties are more consistent compared to biocarbon. Table 2 compares the properties of interest for fossil carbon and biocarbon, according to the data collected in Table 1 and other literature sources [83,84,85,86,87]. Typically, fossil carbon is high in FC and low in VM and ash. These characteristics facilitate the efficient carburization of the molten metal (charge carbon) and make it a suitable reductant for FeO in the slag (injection carbon). In contrast, biocarbon displays a broad range of values. In some cases, a similar composition to fossil carbon has been observed [57, 68, 70] which means it can possibly be a suitable replacement. Caloric value of biocarbon is comparable to fossil carbon, which supports its application as a charge carbon replacement. Additionally, sulfur levels are typically lower in biocarbon due to the difference in feedstocks, but phosphorus levels are higher. Sulfur and phosphorus are a known harmful impurities in steelmaking [88]. The use of biocarbon may introduce less sulfur into the steel, taking stress off downstream desulfurization processes. In contrast, the higher phosphorus content may require further dephosphorization, which is an additional burden on the process. Lastly, there is a stark difference in density between fossil carbon and biocarbon. Since EAF off-gas collection systems are designed with denser fossil carbon in mind, it is questionable whether the less dense biocarbon will penetrate the slag layer or be captured with the off-gas. Additionally, injection parameters (feed rates, pressure, particle size) may need to be adjusted or biocarbon may need to be processed into agglomerates to accommodate the lower density. While biocarbon and fossil carbon may differ in their physiochemical properties, the overarching benefit of utilizing a carbon–neutral material should be kept in focus. Despite the potential challenges of biocarbon substitution arising from physiochemical differences with fossil carbon, it is a worthy pursuit to research and develop pathways for its implementation into EAF steelmaking.

Table 2 Comparison of chemical and physical properties for fossil carbon and biocarbon on a dry basis
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Multi-Scale Testing of Biocarbon for Electric Arc Furnace Steelmaking Applications

With the appropriate background information on the role of carbon in the EAF process and the properties of biocarbon discussed, the status and recent progress for its use in EAF steelmaking can be assessed. Research studies have been conducted at several scales including, sessile drop testing, laboratory furnace slag foaming experiments, and pilot-scale or industrial-scale EAF testing. Each of these scales can provide valuable insights into the effectiveness and usage potential of biocarbon as a charge or injection carbon.

Sessile Drop Testing

The sessile drop test is a method used to measure the surface energy of a solid material by observing the shape of a liquid droplet placed on its surface. Typically, a drop of liquid EAF slag is placed on the solid biocarbon material. By analyzing the contact angle between the droplet and the material, the surface properties such as wettability and adhesion can be determined. A schematic of the test setup is shown in Fig. 4. As described in Sect. “Injection Carbon,” wettability is a crucial factor in slag foaming. Injection carbon must have sufficient wettability to facilitate the reduction reactions with FeO in the liquid slag. Also, this method can examine the gas generation and slag droplet volume evolution in detail. Therefore, sessile drop testing has been commonly used to study the suitability of biocarbon as an injection carbon material.

Fig. 4
figure 4

Schematic of typical sessile drop testing of biocarbon and slag with the contact angle labeled

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Yunos et al. [77] were the first to take this approach using a palm shell char biocarbon and had several key findings. They detected CO, CO2, and H2 in the off-gas measurements, indicating the reduction reaction occurred between biocarbon and FeO. More interestingly, the concentration of CO in the off-gas for biocarbon exceeded the reference coke. Furthermore, greater fluctuations in the slag droplet volume were observed when using biocarbon. Generally, these observations show the potential use of biocarbon as an effective slag foaming agent. However, other studies have shown contrasting results. Mayyas et al. [89] found that the lignin biomass was “slag-phobic” as its contact angle exceeded 90°, whereas the reference coke consistently showed a low (< 90°) contact angle. Similarly, Huang et al. [70, 71] also found the wettability of biocarbon material to be poor. Their studies showed that a variety of biocarbon materials were the least interactive with EAF slag, particularly when compared to coke. It was postulated that the difference in wettability was due to the low surface roughness of the biocarbon, which was partially solved by modifying the surface morphology using densification. Also, it is noteworthy that a 50%/50% blend of biocarbon and coke showed a wettability comparable to 100% coke. Finally, Wei et al. [58] used sessile drop testing to monitor the volume expansion of slag droplets on HTC biocarbon. The HTC process was used to produce an optimized biocarbon product, which outperformed the commercial foaming agent in terms of slag droplet growth. Overall, these studies provide contrasting results in terms of biocarbon’s suitability for use as an injection carbon material. Generally, it seems there are challenges with the wettability of biocarbon but there are other indications such as CO gas generation and slag droplet response that indicate its potential. It is possible the issues around wettability can be mitigated by choosing favorable feedstock, by optimizing the biomass thermal processing, or by further processing of the biocarbon, e.g., by densification.

Other small-scale studies have been carried out to chemically characterize biocarbon or to study the reaction kinetics of FeO reduction by biocarbon. Fidalgo et al. [73] characterized the thermochemical properties and reactivities of two industrially sourced biocarbon (pumpkin seed and grape seed) using proximate analysis, thermogravimetric analysis (TGA), and a wire mesh reactor. It was found that the biocarbon derived from pumpkin seed was well suited to be used as a charge carbon as its composition and reactivity aligned with an industrially sourced charge carbon. The grape seed char did not align as closely with either charge carbon or injection carbon based on this thermochemical behavior criterion. However, it was supposed that grape seed char may be a suitable injection carbon due to its high VM content, which can aid in slag foaming [90]. Yunos et al. [69] studied the reaction kinetics of palm kernel shell derived biocarbon and found the reactivity with slag to be greater compared to coke. The authors stated that the high reactivity was due to the amorphous carbon structure and larger specific surface area of the biocarbon. Similarly, Kieush et al. [64] also observed an inferior microstructural order by Raman spectroscopy and XRD in their chemical characterization of wood pellet biocarbon. Most recently, Han et al. [59] compared the reaction kinetics of corn straw pellets thermally processed by pyrolysis, pyrolysis in a superheated steam atmosphere, and by HTC. The samples prepared by pyrolysis showed favorable CO/CO2 gas production for use as an injection carbon, but without a reference fossil carbon sample, their performance is difficult to measure.

Sessile drop testing and other small-scale tests can provide insightful details on the properties and reactivity of biocarbon. To further understand the suitability of biocarbon as an injection carbon material, actual slag foaming must be observed and studied. Given the complex nature of slag foaming, it is not certain the performance implied by property characterization will occur in the dynamic process, which will combine several key components (biocarbon reactivity, slag properties, foamy slag evolution). Laboratory furnaces enable direct observation of slag foaming dynamics, providing crucial insights into behavior of biocarbon under conditions better resembling EAF steelmaking.

Laboratory Furnaces

To scale up testing, specifically for injection carbon, laboratory furnaces capable of housing slag foaming experiments have been developed. Echterhof and Pfeifer [60] were the first to conduct slag foaming experiments at this scale. Industrially sourced EAF slag and injection carbon candidates were charged into a crucible and put into natural gas/oxygen burner furnace at 1600 °C. Although slag foaming could not be measured in-situ, marks left on the crucible walls were used to observe the slag foaming response of four biocarbon candidates. Generally, it was found that the biocarbon candidates reacted faster and more intense than the fossil carbon. This aligns with the previous sessile drop results by Yunos et al. [69, 77]. However, it should be noted that the fossil carbon reacted over a longer period. Huang et al. [70] also included slag foaming experiments in their study using an induction furnace. Unlike the setup used by Echterhof and Pfeifer [60], the injection carbon was not charged in the crucible but was stirred in once the slag was molten. Additionally, steel was melted with the slag, which acted as the heat source given this work was completed with an induction furnace. The authors found that the biocarbon candidate did not cause any meaningful slag foaming, which aligns with their sessile drop testing. Similar to the wettability results, slag foaming (as indicated by maximum slag foaming height) increased when coke was blended with the biocarbon. No results on slag foaming duration were reported. Hoikkaniemi [57, 67] studied the slag foaming of a spruce wood biocarbon using a chamber furnace. Similar to Huang et al. [70], the injection carbon was added after the slag was molten, however, a screw feeder was used rather than dropping and stirring. The biocarbon performed comparable to the reference coke, both in terms of maximum slag height and foaming duration. In fact, the biocarbon took 44% less time to reach the maximum slag foam height, which is an indication of its reactivity with slag and gas generation. However, it should be noted that the feed rate and overall mass of biocarbon used were unintentionally greater than the reference coke. DiGiovanni et al. [66] ranked various injection carbon candidates using the same experimental setup as in [70]. It was found that loose biochar performed the worst among all candidates but a biocarbon briquette exceeded the industrial injection carbon. It is important to note that this study did not measure slag foaming duration and only maximum growth was assessed.

Kieush et al. [30, 65, 91, 92] have conducted several slag foaming experiments in recent years. Each study has used a similar induction furnace setup which is presented in [93]. An alumina crucible was used to contain the charge materials (slag and pure iron) and an outer graphite crucible was used as a heating susceptor. The injection carbon material was added and stirred in once the slag was molten and the foaming response was monitored. Kieush et al. [65] studied a variety of fossil carbon materials (anthracite, petcoke, coke) and a wood pellet biocarbon candidate. Biocarbon showed the most inferior slag foaming characteristics for a variety of reasons including low wettability, excessive reactivity, and low density (many particles covered only the surface of the slag). Additionally, a relationship was noted between the structure of the carbon materials and slag foaming. An increase in the structural ordering of the carbon material correlated to better slag foaming. By using a 50%/50% blend of coke and biocarbon, similar slag height and volume as 100% coke was observed, which aligns with the observations of Huang et al. [70]. In a follow-up study, Kieush and Schenk [30] compared coke, biocarbon, and a 50%/50% blend in a variety of slag compositions. Generally, the biocarbon did not perform as well as coke but the optimal ranges of slag basicity and FeO content were similar for both carbon materials. However, the blend provided stable foaming across all studied slag compositions and basicity. It behaved similar to the reference coke and exceeded the slag foaming duration of the biocarbon. These results indicate the potential of using biocarbon as a partial replacement for coke. Kieush et al. [91, 92] also conducted research on biocoke as an injection carbon. Biocoke was synthesized at laboratory scale using either 95% coal and 5% wood pellets or 90% coal and 10% wood pellets and using an appropriate coking process. The 95%/5% case was observed to have increased slag growth and stable foaming, but the slag foaming time was reduced due to an increased reaction rate by incorporating the wood pellets in the coke. The 10% addition of wood pellets further decreased foaming time, which limited the process.

Several insights on injection biocarbon have been uncovered and unified by laboratory scale furnace studies from explicitly observing slag foaming, such as the differences in reactivity or issues with low density. As biocarbon (either as injection or charge carbon) scales up to larger volumes, such as in pilot-scale or industrial trials, the need for validation becomes apparent. Large-scale tests provide a real-world assessment of biocarbon’s feasibility in EAF steelmaking systems and instruments, ensuring its practicality and feasibility on an industrial scale.

Pilot-Scale and Industrial Trials

Pilot-scale testing provides a more accurate indication of industrial performance than laboratory furnaces given its larger size and volume of material. Here, the scientific mechanisms uncovered at smaller scale can be observed in a more relevant setting. However, the final evaluation must be done by industrial trials at EAF melt shops.

Charge Carbon

Unlike laboratory furnace testing which almost exclusively focused on injection carbon, much of the pilot-scale testing in literature has focused on biocarbon as a charge carbon. This is likely due to charge carbon’s simpler application in the EAF process and biocarbon’s inherently suitable properties, which ease the transition from fossil carbon. Demus et al. [74] studied two biocarbon candidates as a charge carbon in a pilot-scale EAF furnace. The results indicated there was no negative effect on the steel or slag chemistry from the substitution of fossil carbon with biocarbon. In fact, sulfur content was observed to decrease when the biocarbon candidates were used and phosphorus levels were the same as when using fossil carbon. However, the carburization of the molten metal was found to not be as efficient compared to the reference fossil carbon, due to the biocarbon’s high reactivity. It was proposed this could be remedied by the briquetting of biocarbon. In a follow-up study, Demus et al. [8] agglomerated biocarbon into 20 mm briquettes using an optimal ratio of molasses and water in the binder. The resultant briquettes matched the abrasion resistance of the reference fossil carbon, indicating its physical robustness. Pilot-scale charge carbon testing showed the combustion behavior of the briquettes better matched the reference fossil carbon. Similarly, Reichel et al. [80] observed no negative impact on steel quality, while using biocarbon as a charge carbon in pilot-scale testing. Furthermore, due to the higher reactivity of biocarbon, it was shown that melting time could be shortened when using it as a charge carbon. Recently, Lu [79] trialed a hydrochar biocarbon in a pilot-scale EAF and studied the carburization yield, which is the ratio of carbon dissolved into the molten metal to carbon input. The hydrochar did not match the reference anthracite carburization yield (56 vs 26%), which was attributed to the stark difference in FC content (81 vs 32%). These results indicate FC is a key element contributing to the carburization of liquid steel and high-quality biocarbon is needed for a 1:1 mass replacement of fossil carbon.

Industrial trials have been carried out on biocarbon as a charge carbon material. Echterhof et al. [72] used palm kernel shells as a charge carbon in a 140 ton EAF industrial trials. No negative impact on the quality of the slag or the steel produced was detected, however, the off-gas behavior was different, and temperature was higher compared to the reference fossil carbon. The authors stated this was likely due to the larger amount of VM but noted that there was a 6% reduction in electrical energy input when using the palm kernel shells. In a separate study by Robinson et al. [68], two biocarbon candidates were trialed as charge carbon in a 50 ton EAF. Prior to the industrial trial, this study showed that ash content inhibited the dissolution kinetics of biocarbon. Therefore, a 33% substitution of the standard fossil carbon with a low ash biocarbon briquette showed no deviation from normal operating conditions across three heats.

Clearly, there has been noteworthy progress and success for biocarbon as a charge carbon in EAF steelmaking. This has been demonstrated at pilot-scale and during industrial trials. Although some modifications, such as briquetting, had to be made to accommodate the biocarbon, the literature collectively identifies the potential of biocarbon for this application in EAF steelmaking.

Injection Carbon

Studies on biocarbon as an injection carbon material at pilot-scale or in industrial trials have sparsely been reported. This is possibly due to the challenges that were identified in “Laboratory Furnaces” Sect. and are still be researched using laboratory furnaces. As part of the charge carbon study by Demus et al. [74], biocarbon was also injected into the pilot-scale EAF. It was stated there was no negative effect on slag foaming behavior, but no foaming metrics were reported. Wibberley et al. [78] conducted one of the first industrial trials on biocarbon injection. The observations state that slag foaming was at least as good as when using coke. However, it should be noted that high-quality biocarbon was being used and it is also not clear if a larger mass of biocarbon was required to sustain the foamy slag. An industrial trial study by Echterhof et al. [72] used palm kernel shells as an injection carbon material and CO gas generation was observed to exceed the reference fossil carbon, specifically when foaming occurred. The authors state that there was no substantial change in slag foaming detected visually, but also state that more trial heats are required to validate these results. Biocarbon was also industrially trialed as an injection carbon during the GREENEAF2 project [61]. Poor foaming behavior was observed due to two identified factors. First, the lower density of the material, which decreased the penetration into the slag layer and most of the biocarbon was carried away by the off-gas stream. The second factor was a possibly low reactivity of biocarbon with FeO in the slag.

When looking at the laboratory furnace studies on biocarbon as an injection carbon, numerous technical challenges were discussed, which were thought to limit its progression to larger scale testing. After reviewing the literature on pilot-scale and industrial trials there may also be a limitation on monitoring systems. The studies discussed in this section did not have quantitative data to study the slag foaming behavior of the biocarbon candidates. Without measurement data it is difficult to study the slag foaming effectiveness of biocarbon, which may make testing futile unless pilot-scale systems are upgraded.

Summary of Multi-Scale Testing

Biocarbon has been studied on several scales for use in EAF steelmaking. By surveying the available literature, each scale provides specific insights, which are needed to provide a holistic perspective on biocarbon’s suitability for use in EAF steelmaking. Figure 5 provides the key insights and uses for each scale of biocarbon testing outlined in this review. Sessile drop testing and chemical characterization have typically been done to study the interaction between molten slag and biocarbon at a detailed level. In the literature, this method was commonly applied to determine the wettability of biocarbon, while observing changes in slag droplet shape. Off-gas was also monitored to understand the reaction kinetics between the carbon and FeO in the slag. Each of these details provided indications of biocarbon’s performance in EAF steelmaking but bulk material interaction is eventually needed to determine its suitability. Laboratory furnace testing allowed the study of bulk material interactions, typically on injection biocarbon for slag foaming applications. In this way, the slag foaming process would be explicitly observed and measured. Insights into the chemical nature of biocarbon and its interaction with slag from sessile drop testing were used to explain the results in the laboratory furnace. This feed of information demonstrates the usefulness of multi-scale testing since specific material interactions or reaction mechanisms cannot be studied often at bulk scale.

Fig. 5
figure 5

Key insights and uses of each scale for testing biocarbon’s suitability for EAF steelmaking applications

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Laboratory furnace results on slag foaming can be used to interpret results at pilot-scale or in industrial trials, again showing the benefit of multi-scale testing. However, industrial trials provide an additional evaluation that cannot be done at any other scale, such as how well biocarbon integrates into EAF systems. In the case of injection carbon, the low density raises questions about slag layer penetration, which has not been directly studied in literature. There is a possibility the industrial off-gas handling system may remove the biocarbon before it reaches the slag layer. Additionally, given the high reactivity of biocarbon, there is a possibility it may react with the atmosphere before it reaches the slag layer. Industrial trials or pilot-scale EAFs with similar systems are well suited to carry out studies on these subjects. In contrast, the issues surrounding biocarbon as a charge carbon at pilot scale have been studied and adaptions have been made for their successful use in industrial trials. However, charge carbon effectiveness depends much less on EAF instruments compared to injection carbon, which eases the industrial integration process.

Each study reviewed, regardless of scale, provided a specific insight which when viewed collectively gives perspective on the current utilization status. Table 3 shows a summary of the available literature on biocarbon utilization in EAF steelmaking (organized by experimental scale, then publication year) along with their key findings. Pilot-scale or industrial trials studies have mostly focused on charge carbon and most of the work was completed relatively early. This is a testament to the quick progression and ease of implementation of biocarbon as a charge carbon material. In terms of injection carbon, studies are still ongoing at several scales to understand the foaming behavior when using biocarbon materials.

Table 3 Summary of available literature on biocarbon utilization for applications in EAF steelmaking organized by experimental scale and publication year
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When looking at the laboratory furnace studies, it is clear the slag foaming from biocarbon is the current attention of research. Studies have focused on how to quantitatively measure slag foaming to capture biocarbon’s effectiveness. Generally, it has been observed that biocarbon reacts too rapidly to cause effective slag foaming or poor wettability/low density leads to negligible slag foaming. It is interesting to note that biocarbon/coke blends have been observed to be effective for slag foaming in multiple studies. It is possible a partial replacement of fossil carbon for injection carbon is more feasible in the interim than a full replacement.

Smaller scale testing (sessile drop or chemical characterization) has provided critical information on the physiochemical nature of biocarbon materials. Biocarbon is ubiquitously observed to be highly reactive and have a low density. However, from the collective literature in Table 3, it is clear that small-scale tests alone cannot predict effectiveness for use at larger scale. For example, the high reactivity of biocarbon was initially thought to be an advantage [69, 73, 77] but when trialed at larger scale it was found to be a disadvantage [30, 60, 66, 74]. Biocarbon had to be agglomerated to be an effective charge carbon [8] and research is ongoing to determine how to accommodate the high reactivity as an injection carbon. Bulk material or large-scale testing is needed to understand the application context for these carbon properties and its interaction with slag.

Pathways and Limitations of Biocarbon Use in Electric Arc Furnace Steelmaking

There has been substantial progress on the understanding and evaluation of biocarbon for EAF steelmaking in the past decade and a half. This section will use the findings from the surveyed literature to identify pathways for biocarbon usage and accommodations for its limitations. Given decarbonization is a critical topic for steel research and the use of biocarbon is an attractive approach, areas for future research are also recommended to ensure that impending work is targeted toward key issues or incomplete understanding.

Key Biocarbon Characteristics

Several characteristic differences were identified for biocarbon in comparison to fossil carbon. The most prominent among these was the clear difference observed in reactivity. This was universally agreed upon among the literature that examined biocarbon reactivity compared to fossil carbons. A high reactivity for biocarbon was observed in CO2 atmosphere and directly with slag [62, 66, 95] which will affect the indirect and direct reduction of FeO, respectively. Furthermore, high reactivity was consistently observed in pilot-scale charge carbon testing, given by the intense CO gas generation compared to fossil carbon [8, 72, 74, 80]. To address this key difference, there are likely two approaches, either modify the process to accommodate the high reactivity or modify the biocarbon to lower its reactivity.

Another characteristic of biocarbon is its relatively low density. This has been observed in a plethora of studies in biocarbon in various applications [15, 96,97,98,99]. The low density is likely due to the commonly observed porous and cellular structure of biocarbon. Given the biomass is typically derived from wood products or agricultural wastes (Table 1), it is likely this characteristic is inherited from the feedstock. Like the commentary on higher reactivity, the low density can either be accommodated by changes in the injection process or additional densification processes for biocarbon.

To optimize the slag foaming performance of biocarbon, concerns regarding wettability will need to be addressed. It should be noted that wettability is not controlled by reactivity, but rather by the physiochemical properties of the interfaces [100]. Theoretically, it is possible to have a highly reactive biocarbon with poor wettability. However, the chemical reactions between the biocarbon and slag are facilitated by the formed interface, making wettability a critical factor controlling slag foaming [101, 102]. There were mixed findings on the wettability of biocarbon by liquid slag. This indicates that the impact of poor wettability on slag foaming is not as clear as high reactivity or low density. It possibly depends on feedstock or thermal processing routes, but remains a characteristic of biocarbon, nonetheless.

Proposed Strategies for Biocarbon Use

The biocarbon characteristics discussed in the previous section all pose limitations on its use in EAF steelmaking. High reactivity, low density, and, in some cases, low wettability has all contributed to the issues with biocarbon identified from literature. A summary of biocarbon characteristics, pathways to accommodate them, and the associated solution is shown in Fig. 6. As a charge carbon, the high reactivity of biocarbon resulted in an undesirable rapid combustion early in the process, which led to poor carburization of the molten metal. However, the technical limitation was addressed by densification or agglomeration of biocarbon particles, which led to a lower specific surface area and thus reduced reactivity. This is a well-defined pathway for biocarbon use as a charge carbon, as it has already been demonstrated at pilot scale and would benefit from further industrial trials. However, issues pertaining to high reactivity persist for injection carbon applications. The high reactivity was observed to limit the slag foaming duration. Although in some cases biocarbon was observed to provide sufficient slag foaming intensity (as indicated by maximum slag foaming height), it was likely consumed too quickly to contribute to meaningful slag foaming duration. There is no record in literature of an equivalent mass of biocarbon matching the slag foaming duration of fossil carbons. Both foaming intensity and duration are required for effective slag foaming. However, modifications to the injection process could accommodate the high reactivity but they have not been trialed. One possible approach to remedy this issue would be to reduce the injection rate and increase the injection period. In this way, the supply of biocarbon reactant is constantly replenished as it is quickly consumed. Ideally, this could possibly result in an improved slag foaming duration without excessive slag foaming intensity. However, to test this approach, a replicate injection system would need to be implemented on a laboratory furnace or pilot-scale system, which has not yet been reported in the literature.

Fig. 6
figure 6

Identified and potential pathways for biocarbon use in EAF steelmaking with their associated mechanistic solution

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The low density of biocarbon also poses a limitation on the use of biocarbon, particularly as an injection carbon. Observations of biocarbon particles floating on the slag layer and not reducing the FeO are reported in literature. As previously mentioned, the off-gas handling system also has the potential to remove the biocarbon during injection. Specialized injection nozzles have been developed to accommodate the low density of biocarbon. Using supersonic air as a shroud during injection, biocarbon can penetrate the slag layer and cause slag foaming. This approach has been developed at industrial scale by Tallman Technologies [103, 104], although its widespread use is still being adopted. Alternatively, agglomeration is another pathway to overcome low density. The agglomeration process for biocarbon has been studied and can be optimized to replace coke for a variety of applications [105,106,107,108]. The density is increased by effectively decreasing the porosity of biocarbon, which could potentially allow for the use of conventional injection processes and prevents floating on the slag layer. However, if the particle size becomes too large, it will limit the biocarbon reactivity and it will burn rather than react with the slag. Similar to agglomeration, Somerville et al. [76] increased the apparent density of biocarbon by 80% using biomass (hardwoods) in compressed pellet form called ‘dense biomass fuel.’ These results indicate that issues around density can also be solved by densification of the feedstock. Agglomeration and specialized injection nozzles both offer sound solutions to low density, however, agglomeration has the additional advantage of reducing dust generation and easing transportation due to increased mechanical strength of the pellets. Furthermore, agglomeration decreases the risk of combustion of fine biochar during transportation, increasing overall safety of handling.

The low wettability of biocarbon limits the interfacial contact that is required for the slag foaming reactions. However, this characteristic is not as commonly reported as high reactivity and low density. It is possible the wettability is the result of the feedstock or thermal process used to synthesize biocarbon, which may affect surface properties. However, this concept needs to be validated with systematic testing and remains only a proposed pathway. Alternatively, increasing the surface roughness of the biocarbon particles has been observed to be successful. This is another observed benefit of biocarbon agglomeration, as it was shown to increase surface roughness. Interestingly, agglomeration appears to be a pathway to accommodate all the identified biocarbon characteristics.

Finally, a partial replacement of fossil carbon with biocarbon/coke blends appear to be a viable pathway for biocarbon use in EAF steelmaking. It is possible that biocarbon can eventually replace fossil carbon as a charge material, but lab-scale trials have been successfully completed using a partial replacement blend. Given the technical challenges surrounding biocarbon as an injection carbon to induce slag foaming, a blend may be the most viable pathway in the near term. There has been limited studies on the subject, but they all agree blends are effective slag foaming agents. In fact, it has been postulated that a blend may outperform biocarbon or fossil carbon individually. The exact mechanism for the effectiveness of blends remains unclear, but DiGiovanni et al. [63] proposed a synergistic effect between the intense slag foaming of biocarbon and consistent gas generation of fossil carbon, but further validation is needed. Nevertheless, it appears that the use of biocarbon/coke blends is a viable pathway that would potentially not disrupt current EAF operations.

Areas for Future Research

This review shows that there has been considerable progress in the past decade on the use of biocarbon in EAF steelmaking, particularly as a charge carbon material. With increased interest in hydrogen-based direct reduced iron (which contains no carbon), the success of biocarbon for carburization is critical and noteworthy. However, key areas for further development, based on insights from “Proposed Strategies for Biocarbon Use” Sect., are summarized in Table 4. Agglomeration has emerged as a versatile solution to the limitations identified in the literature. Agglomeration has already been shown to remedy combustion/reactivity issues with charge carbon at pilot scale. It can also potentially increase particle density alleviating concerns on slag layer penetration and particle loss to industrial off-gas systems. Both factors are critical to successful carbon injection and slag foaming. Finally, it is possible the increased surface roughness from agglomeration will increase biocarbon wettability. Agglomeration addresses several key limitations of biocarbon use, especially as an injection carbon. The agglomeration process has also been well defined for other biocarbon applications, which possibly indicates the technology can be adapted for applications in EAF steelmaking. Likewise, dense biomass fuel provides similar benefits to agglomeration. Therefore, further research in this area should be prioritized given it has a high impact potential.

Table 4 Summary of research areas and recommended future work
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Modifications to the injection process or systems have not been widely carried out. However, biocarbon use could benefit from further exploration in this area. Quantifiable metrics and data are needed to properly discuss the potential issue of slag layer penetration, which does not currently exist. Furthermore, the highly reactive nature of biocarbon could also possibly be accommodated by adjusting the injection schedule. A reasonable approach to this area could be to combine experimental injection trials with computational fluid dynamics simulations. The experimental data could feed into the simulations to theoretically optimize the injection parameters. Alternatively, the simulations could be used to explain the observations in the experimental work by analyzing parameters that are typically difficult to measure (for example, particle depth penetration). Considering the lack of current research in this area, substantial research development will be required. However, given the potential process improvements and theoretical developments on biocarbon injection (which is fundamental to its use as slag foaming agent), it is a desirable research area.

Blends of biocarbon and coke appear to provide an immediate pathway for partial replacement of fossil carbon. The potential for blends to outperform fossil carbon is also an intriguing aspect, although it is not well understood. Additional experimental work, possibly in conjunction with kinetic modeling, is needed to fully understand the slag foaming behavior of biocarbon and coke blends. Testing also needs to be scaled up to pilot EAF systems or industrial trials to prove its validity within actual EAF systems. These two subareas of blend foaming behavior and scaled up testing can work synergistically for a robust implementation strategy. Further research in this area can drive innovation and lead to the development of optimized industrial processes, paving the way for widespread adoption of biocarbon and coke blends as a sustainable alternative in steel production.

This review has largely focused on biocarbon utilization from a steelmaking process perspective. However, given the current challenges of biocarbon use in EAF steelmaking, biocarbon engineering is likely another high value area for future research. If the identified troublesome characteristics of biocarbon can be eliminated during biocarbon synthesis (either through careful feedstock selection or thermal processing), the burden of implementation will be eased. Previous studies have indicated high biocarbon reactivity is related to the higher specific surface area or amorphous carbon structure. However, Sahajwalla et al. [109] noted high VM content in hydrocarbons (plastics, rubber, etc.) could contribute to slag foaming since the release H2 gas can reduce the FeO in the slag. Biocarbon with low VM have still shown high reactivity, but a similar analysis to determine its contribution is missing from the literature. Additionally, particle size has not been systematically studied to date, which may be a pathway to control biocarbon reactivity. To tailor biocarbon properties to match the reactivity of fossil carbon, it is imperative to better understand and therefore investigate the contribution from the respective factors (specific surface area, carbon structure, and VM content) compared to fossil carbon. Although the scientific literature on biocarbon synthesis specifically for EAF steelmaking applications is somewhat limited, it should be noted there are commercial activities in this area (CHAR Tech, Canada, Bionow, Brazil, Taaleri, Finland, etc.). Further progression in this area will require coordination between steelmakers and biocarbon produces to work together to optimize the properties for steelmaking applications. Additionally, the economic viability of carefully engineered biocarbon must be evaluated, though the commercial activities suggest it is feasible. Considering the EAF steelmaking process has become well established over the past several decades, this pathway and area for future research may offer the least modification to existing infrastructure.

Conclusions

By collectively examining the literature on biocarbon utilization in EAF steelmaking, several new insights and future directions were identified. The value of multi-scale testing was made clear by the holistic perspective it provides. The results from small-scale testing informed the interpretation of observations at larger scale. Conversely, the large-scale testing determined the impact of properties identified by small-scale testing on bulk material interactions and industrial applications. Since no testing scale can provide all the required information, multi-scale testing provides a valuable perspective for a comprehensive understanding of biocarbon behavior, effectiveness, and limitations in EAF steelmaking. Biocarbon as a charge carbon material appears to be well understood and further scientific insights into its application will only come from industrial trials. However, biocarbon as an injection carbon is still facing technical issues such as high reactivity leading to unstable slag foaming and low density limiting its penetration into the slag layer. More experimental studies and improved methodologies will be required to further advance the application of biocarbon as an injection carbon. Interestingly, agglomeration was observed to be a multi-solution pathway for biocarbon utilization in EAF steelmaking. This approach has already been proven for charge carbon applications, but injection carbon applications may also benefit in several ways from agglomeration. Another pathway for injection carbon is the use of biocarbon/coke blends in the interim, while the technical issues are being solved. Overall, there is promise for the utilization of biocarbon in EAF steelmaking and considerable progress has already been made. As research advances in the coming years, the EAF process stands to benefit from the adoption of biocarbon to reduce CO2 emissions without significant disruption of the process.