Comparison of Bio-Coke and Traditional Coke Production with Regard to the Technological Aspects and Carbon Footprint Considerations

Abstract

In a world facing the challenges of climate change, it is imperative to prioritize the search for sustainable technical solutions. This study focuses on evaluating the environmental impact of using bio-coke compared to traditional metallurgical coke, employing Life Cycle Assessment (LCA) as the evaluation tool. Bio-coke, produced from a blend of coking coals enriched with biomass, offers greater environmental potential than traditional coke due to a reduced share of non-renewable raw materials. The steel and coking industries are significant contributors to carbon dioxide emissions. LCA provides a comprehensive assessment of the environmental impact of bio-based additives, considering raw material deliveries, the coking process, application in metallurgy, and product end-of-life disposal. The analysis results indicate that the use of biomass additives leads to lower greenhouse gas emissions compared to coke production without bio-additives. Given the urgency of addressing global warming and the increasing demand for sustainable energy sources, this study’s findings can advocate for bio-coke as a more environmentally friendly alternative to traditional coke in the steel industry.

1. Introduction

One of the current challenges is the ongoing climate change and environmental degradation. The search for increasingly efficient energy sources is crucial not only due to resource availability, but also for sustainable and environmentally friendly development. The rising demand for energy correlates with an increasing need for high-quality steel, and metal products, which form the foundation of contemporary industries: construction, building, automotive, and machinery manufacturing. Currently, alongside iron ore, coke is the primary raw material used in metallurgical processes. It is estimated that to produce one ton of raw steel, approximately 0.6 Mg of coke is consumed, which demands to about 0.75 kg of coking coal blend used in its production [1]. In addition to the metallurgical and steel industry, coke is used in the production of refractory materials, metal alloys, and as a fuel for heating purposes. Due to the progressing global warming resulting from climate change, an increasing number of countries and international organizations are implementing regulations aimed at reducing greenhouse gas emissions, such as CO₂, into the environment. One such initiative is the “Fit for 55” package introduced by the European Union, which aims to reduce greenhouse gas emissions by 55% for member states by 2030 [2]. Already in 2005, a mechanism was introduced to determine the total emissions limit of certain greenhouse gases, known as the European Union Emissions Trading System (EU ETS) [3]. In 2023, within the global economic structure, the metallurgical industry was responsible for emitting 2.19 billion tons of carbon dioxide [4]. The production of coke played a significant role in this aspect. Additionally, coke production generates additional pollutants at a rate of approximately 2.5 kg of pollutants per ton of coke. These pollutants include “Dust,” main gaseous pollutants such as NO_x, SO_x, CH₄, CO, H₂S, as well as “Micro” pollutants such as PAHs, heavy metals, and NMVOCs, which exhibit the highest environmental harmfulness. Therefore, seeking solutions to improve environmental effects is necessary [5]. These actions are necessary from an ecological and an economic standpoint. Given the costs associated with carbon dioxide emissions, minimizing them becomes essential. In the coming years, the European Union is expected to implement increasingly stringent measures aimed at reducing greenhouse gas emissions under the European Green Deal. It is anticipated, that analogous charges for carbon dioxide emissions will be introduced for the construction and road transport sectors within the new ETS (Emissions Trading System) [6]. Consequently, various solutions are increasingly being proposed to reduce CO₂ emissions, from production and economic activities. However, to obtain comprehensive technical and technological knowledge regarding the effectiveness of existing solutions, it is essential to employ simulation tools that illustrate the broader environmental context. For this purpose, the so-called Life Cycle Assessment (LCA) tool is used. The Life Cycle Assessment of a product is a fundamental tool for evaluating the environmental impact of producing and using coke in metallurgical processes. Using LCA allows for understanding the benefits of using bio-coke within the context of the product’s full life cycle.

1.1. Industry Application of Biochar

Biochar, also known as carbonized biomass, has found increasing applications across various industries due to its versatility. The primary production process of biochar, relies on pyrolysis, a controlled thermal process of transforming biomass under limited oxygen conditions. The quality of the resulting char is largely determined by several key factors. Firstly, the temperature of the process significantly influences the physical and chemical properties of biochar, affecting its structure and overall quality. The heating rate also plays a crucial role in product quality, impacting the preservation of the original properties of the biomass. Particle size and shape, or the granulation of the biomass, also play an important role in ensuring even heating, and determining the quality of the ultimate product. Lastly, the design and parameters of the reactor in which the pyrolysis process takes place shapes the quality of the biochar [7,8,9]. In the energy sector, biochar is primarily used in combustion processes for energy production. Thanks to its biomass origin, it serves as an eco-friendly alternative to conventional fossil fuels. Its low-emission nature makes it an increasingly attractive choice in terms of environmental protection and CO₂ emission reduction [10,11,12]. Biochar’s applications are not limited to the energy sector. It is also gaining traction in metallurgy and steel production. The strategy of decarbonization, or reducing carbon emissions, is becoming a pivotal element in planning the future of these industries. Using of biochar in these sectors can contribute not only to environmental influence reduction but also to improving process efficiency and optimizing production costs [13,14]. Transitioning to sustainable production in metallurgy is a multifaceted challenge. It requires not only a revolution in technological approaches and production processes but also the development of individual, tailored technical and technological solutions. A pivotal aspect of this transition is conducting rigorous industrial research and development work. These activities should encompass pilot-scale tests, which are subsequently verified through full-scale industrial testing on a broad scale. Comprehensive research and testing are essential to ensure that new technologies and solutions are not only effective but also economically viable and compliant with environmental requirements. This approach allows the introduction of innovations, in a manner that is environmentally and economically beneficial [15,16]. It is possible to implement sustainable solutions through partial or complete substitution of fossil coal with biochar, a product of biomass carbonization. However, the specific nature of the steel sector and particular technological requirements, eliminating fossil coal from production processes is challenging and problematic. Faced with these constraints, it becomes essential to seek compromise solutions. One approach may involve the partial replacement of hard coal with biochar or limiting its quantity through integration with biochar. This approach allows for a reduction in CO₂ emissions and enhances the sustainability of production processes while maintaining the technological efficiency and performance of the steel sector [17,18]. Biochar in such scope is a neutral and environmentally friendly material, whose application allows for greenhouse gas emission reduction [18]. As previously mentioned, biochar, a reductant produced from a coal blend enriched with biomass material, will exhibit different physicochemical properties compared to conventional coke. These differences may impact its applicability in metallurgy, requiring adjustments to production processes and quality control. Introducing biochar into the coking blend can affect key properties of the coal blend, such as its plasticity, fluidity, and volatile content. The final properties of coke influence in the efficiency of metallurgical processes and the quality of produced steel. Therefore, understanding and controlling these properties are crucial to ensure that the coke meets the required technical parameters and maintains high-quality metallurgical products [19,20,21,22]. As the authors indicate, a high proportion of biomass-derived additives can adversely affect key quality parameters used to evaluate the suitability of coke in metallurgy, such as the Coke Reactivity Index (CRI) and the Coke Strength after Reaction (CSR). These alterations directly impact the final quality of coke, thereby affecting the efficiency of metallurgical processes and the quality of the produced steel. Furthermore, non-carbonaceous additives can also degrade the bulk density of the coal blend, which is crucial for the coking process. However, it is worth noting that compared to unprocessed biomass, charcoal has a lesser impact on the quality of the final produced bio-coke. According to the authors [23], optimizing the process conditions in bio-coke production can make it a better substitute for coal. In this context, it is crucial to conduct optimization regarding strength and thermal properties, while also considering the environmental costs associated with the entire process. Therefore, controlling the composition and properties of the coal blend is essential to ensure optimal technical parameters of coke and maintain high standards in metallurgical products [24]. The introduction of bio-components into the coke production process represents a significant step towards sustainable and eco-friendly practices in metallurgy. The key challenge is in finding a compromise between increasing the share of bio-additives to enhance ecological aspects and maintaining the necessary technical properties of coke essential for efficient metallurgical processes. To precisely assess the impact of these changes on both the environment and the technical properties of coke, a comprehensive assessment is required using environmental assessment tools such as Life Cycle Assessment (LCA). LCA offers a holistic view of production processes, considering all stages from raw material extraction to the final application of the product, allowing for a full understanding of its environmental footprint. In the scope of this study, a detailed LCA will be conducted to evaluate the application of bio-coke compared to the traditional use of coke as a reductant in metallurgy. Due to the numerous potential emission sources into the environment in the coking process, even a minor process change can cause significant alterations in emission indicators such as process gas, coke yields as the final product, and by-products of the pyrolysis process. Some analysis shows that the presence of biomass in general for metallurgical purposes causes sustainable effects [25]. On the other hand, LCA requires additional analysis to understand the impact of the existing solution on the environment fully [26]. This analysis will encompass the assessment of both the conventional coking process, based solely on fossil carbons, and a process involving a blend containing bio-origin additives. The objective is to identify potential ecological benefits and any technical compromises associated with the integration of bio-coke into the metallurgical industry.

1.2. Various Possible Applications of Coke Valorized with Biomaterials Application

Due to specific technological parameters, besides the traditional direction of using blast furnace coke, the appropriate direction is to attempt to utilize bio-coke as needle coke. This bio-coke is produced from tar and petrochemical products and its utilization can be beneficial both in the production of graphite electrodes for steelmaking and in steel production using arc furnaces [27,28].

For this purpose, Super High Power and Ultra High Power electrodes are utilized, characterized by high thermal resistance and, due to the graphitization of the raw material, high thermal durability. Additionally, as stated by the authors [29,30,31], consideration can be given to their application in the production of anode coke. According to one study, in the case of producing such bio-coke for anodes, the introduction of bio-origin additives, such as bio-oil obtained from biomass pyrolysis, can result in bio-coke with good conducting properties [29].

However, other studies suggest that the ecological benefits of reducing greenhouse gas emissions from anode coke production for aluminum production are achievable only with increased thermal efficiency of the process [30]. Nevertheless, caution should be exercised when introducing any bio components, as their excess may deteriorate coke quality. Producing bio-coke with a bio-component content of 3% in the mixture does not deteriorate its physicochemical properties compared to coke without additives [31].

Therefore, it should be noted that any component not being carbon material may worsen the plastic properties of coke during the transition from the coking blend to semi-coke and coke [32]. Consequently, its utility properties may worsen, and the share of bio-additives should not exceed about 5% [33]. Among various directions to make bio-coke production more ecological, introducing components of petroleum origin to reduce the energy demand of the coke production process is suggested [34].

On the other hand, such actions may entail the risk of other technological problems, such as increasing expansion pressure during the coking process, which may lead to damaging the ceramic masses of coke oven batteries. Therefore, each such action should be preceded by technological tests, including tests on ovens with movable walls, to assess the impact of the mixture on the ceramic inside the coking chamber The potential of utilizing bio components seems promising; however, further research considering the environmental context is necessary. Initial raw material preparation, such as their pyrolysis into carbonates or oils, may provide opportunities to make the resulting bio-coke more environmentally friendly. Nevertheless, considering the life cycle assessment of products, this increases the energy demand to produce raw materials with specific properties, which may increase the carbon footprint and extend the potential supply chain, generating additional environmental costs.

2. Methodology

Life Cycle Assessment (LCA) is a comprehensive tool that allows for a detailed analysis of the entire coke production process, taking into account all intermediary stages related to sourcing and processing materials and raw materials necessary for producing high-quality bio-coke and conventional coke [35,36]. The environmental analysis was conducted based on the LCA methodology described in ISO standards [35]. Data were input into SimaPro software version 9.05, and the main processes were sourced from the ecoinvent database. Mass and process balances were provided by referring to conventional coking technology modelled in the program. The concept of the coking process involves high-temperature pyrolysis (coking) of coal blends and coal blends enriched with a bio component. The gas, which is a by-product of the coking process, is used to fire the coke oven batteries in an amount of approximately 40-45%. The average quantity of 1 metric ton of coking coal blend ranges from 310 to 350 Nm³ [37]. The LCA process typically consists of three main stages: defining the goal and scope of the assessment, collecting inventory data, and conducting a full life cycle assessment to interpret the gathered detailed data. The aim of this study is to investigate and compare the climate impact associated with sourcing biomass and coal, their transportation, preparation, and processing for the coking process. The analysis includes the coking process of a conventional coal blend and a coal blend enriched with additives. Life Cycle Assessment (LCA), as one of the actions associated with evaluation and optimization, incorporates the environmental cost of producing a specific technological solution. The topic of optimization is significant from technical, economic, and ecological standpoints, and has been addressed in numerous studies related to the use of bio-based additives in various branches of industry [16,38,39,40]. Additionally, the environmental aspect aligns with issues related to the circular economy and sustainable development, as well as the quest for alternative environmentally friendly solutions. This encompasses a holistic approach to resource utilization, waste reduction, and the promotion of practices that minimize environmental impact, thereby fostering long-term ecological balance and resilience in industrial processes and product lifecycles [41]. Additionally, the study focuses on evaluating the efficiency of this process and its impact on producing a valuable metallurgical reductant—coke. The diagram illustrates the scheme and boundaries of the systems covered by the study. The Life Cycle Assessment focuses on identifying the potential reduction in greenhouse gas emissions when using bio components and improving the environmental aspects of the entire coking process. The long-term goal is to make coke production more environmentally friendly.

To conduct a life cycle assessment of bio-coke, a comprehensive block diagram was developed encompassing elements that consider the procurement of biomass and coal, as well as their processing stages leading to the production of valuable products such as coke and bio-coke (Figure 1). This intricate schematic delineates each stage of biomass and coal acquisition, processing, and transformation into the final products. It allows for a holistic understanding of the environmental impacts associated with the entire life cycle of bio-coke, from resource extraction to end-product utilization. By analyzing each component within this framework, researchers can identify potential environmental hotspots, evaluate the efficiency of different processing methods, and ultimately optimize the production process to minimize ecological footprint while maximizing product value.

To facilitate a comparison between the amount of produced bio-coke and conventional coke, a uniform functional unit will be used. In industrial practice, it is assumed that one coke oven chamber, for example at the Czestochowa Coke Plant, can accommodate a carbon charge with a total mass of approximately 18 tons [42]. According to previous technological data [1] it allows for the production of around 14 tons of coke (yield of 75%). For coal-biomass blends, due to their higher reactivity and susceptibility to thermal degradation, this yield will be lower, around 55–60%. For a biomass-coal blend with an assumed 5% share, the composition of this blend consists of 17.1 tons of coal and c.a. 0.9 tons of biomass, prepared with homogenization exceeding 95%. In this analysis, we will conduct an assessment of two operational variants differing in raw material composition, while maintaining consistent technological and process conditions:

• Coking of a conventional coal blend composed of coking coals used in production.
• Coking of a coal blend enriched with biomass-derived additives.

The coking process involves the pyrolysis (dry distillation) of the coal blend in coke ovens, carried out at a temperature of approximately 1000 °C. During this process, water and volatile components are degassed, and the blend transitions through the so-called “plastic state,” transforming into semi-coke and coke with altered structure. Biomass is characterized by greater porosity and a lower reaction temperature with CO₂ [35]. The introduction of biomass can also adversely affect coke-forming properties, such as caking ability, plasticity, and dilation [43]. The addition of non-coal components may also influence the change in the pressure of expansion, which, during oven operation, can lead to damage or uncontrolled emissions [44]. In summary, the life cycle assessment (LCA) of the coke production process encompasses the following stages:

• Resources—Biomass Collection—Biomass, characterized by high fragmentation and low bulk density, typically results from wood processing by-products such as cutting, trimming, or shredding. Once collected, biomass undergoes sorting and packaging into bags, big bags, or containers for transport preparation. Due to its relatively low market price and functionality, it is economically viable to use biomass in coking processes, provided the source is within a distance of up to several dozen kilometer’s from the coking plant. Repackaging biomass generates emissions of dust and lightweight particles during both packaging and reloading. Biomass is transported by road, rail, or sea for larger quantities.
• Resources—Coking Coal Extraction—Coking coal is extracted from depths of several hundred meters from layers of transformed organic matter. Unlike thermal coal, coking coal, when subjected to high temperatures, becomes plasticized, forming coke. Coking plants often operate as production departments of steelworks, and transportation from mining to processing facilities is largely automated through conveyors and chutes. For coals with exceptional coking properties, overseas imports from countries such as Australia, Canada, or the United States are necessary. For analytical purposes, the delivery distance of coal to the facility is assumed to be similar to that of biomass.
• Preparation for Coking (2 stages)—Raw materials, namely coal and biomass, are prepared for coking through grinding, drying, and milling. From a technological standpoint, the blend should have a grain size below 3.15 mm in 95% of cases. To ensure blend uniformity, materials are blended in specialized blenders. In cases of excessive drying, a small amount of water (approximately 9–10%) is added, which also acts as a binder. Homogenization increases with subsequent technological stages.
• Coking Process—The coal or coal-biomass blend is introduced into the coking chamber and heated by indirect heating through heating channels where coke oven gas is burned. The process lasts from 18 to 33 h, depending on the type of blend and the desired coke. During the process, coke oven gas is recovered, purified, and burned in heating channels, which heat the chamber. After completion, the glowing coke is pushed out and directed to a quenching car.
• Preparation 3—coke cooling—During the coke-pushing process, unorganized gas emissions occur. Subsequently, after being pushed out and placed in a quenching car, the coke is directed under a quenching tower. An automated wet quenching system is activated, where water spraying is conducted for approximately 1–2 min, followed by 1 min for water dripping off. For the discussed case, “wet quenching” using water is applied. The amount of water used for quenching varies, ranging from 1.5 to 3.5 m³ (average of 2.5 m³) per one Mg of coke.
• Preparation 4—coke sorting—After cooling, the coke consists of a blend of grains of different sizes, which, due to the requirements of end-users, needs to be sorted. Sorting is carried out in a sorting facility. A commonly used system involves sorting the coke into grain sizes above 40, 30, and 25 mm. For sorting, roller and shaking sifters are utilized. The classifying element is a sieve with specified mesh dimensions. Only after this sorting process are the categorized fractions considered commercial grades suitable for market trading.

For the conducted Life Cycle Assessment (LCA) analysis, SimaPro software and the Ecoinvent database were used. The analysis included an assessment of the life cycle related to the production of conventional coke and coke enriched with biomass, focusing on the impact of these actions on the carbon footprint. The analysis considered two levels of bio-additive inclusion in the coal blend: 5% and 10%, assuming that such levels would not significantly deteriorate the quality of the final coke product. Generally, the presence of biomass can deteriorate coke quality by lowering its coking properties, such as caking ability and plastometric properties.

Simulations were carried out based on database models selected for the European and global coking industries. The purpose of this approach was to determine whether the existing trends and legal frameworks related to environmental protection and sustainable development, which have been in place in Europe for a longer time, could influence the environmental context of further pro-environmental actions on a global scale.

3. Results

The results of the life cycle analysis (LCA) of coke produced using a conventional coking blend and a coking blend enriched with biomass are presented in Figure 2 and Figure 3.

The carbon footprint associated with coke production ranges from 0.021 to 0.03 kg CO₂ equivalent per MJ of energy for global coking. The difference exceeding 10% in the carbon footprint of conventional coking technologies, especially within the European context, can be attributed to the necessity of importing coking coal from countries such as the United States, Australia, and South Africa. The significant distances involved in transportation increase the overall carbon footprint. In Europe, numerous emission regulations impose additional requirements on producers and technology providers to use low-emission technologies in compliance with European Union regulations. These regulations, along with the associated costs within the CO₂ emissions trading system, result in European technologies being characterized by higher technological maturity and greater governmental awareness. However, the importation of raw materials remains necessary. Conversely, the lower carbon footprint observed outside of Europe, despite many countries still in the process of creating regulations and building awareness about sustainable development, highlights the significant impact of delivery chain length and complexity on carbon footprint determination. Implementing such regulations is a long and complex process, with measurable effects only achievable after many years. A sustainable approach in the context of a full life cycle assessment involves seeking local suppliers that can meet raw material requirements. Introducing easily available bio-based additives appears justified. Data presented in Figure 2 show that biomass exhibits a relatively high CO₂ absorption capacity, approximately 0.11 kg CO₂ equivalent per MJ of energy produced. This is because trees absorb carbon dioxide from the atmosphere during photosynthesis, which can reduce the ultimate carbon footprint of a given technology. Therefore, based on the conducted analyses, a variant with a small percentage of biomass added to the coking blend has been evaluated.

A comparative analysis was conducted for the European coking variant due to the higher carbon footprint value identified in the simulation. Consequently, the priority and justified pro-environmental and sustainable action is to consider the variant characterized by greater environmental impact. The results of the conducted analysis of the carbon footprint of coke and coke produced from a coal-biomass blend are presented in Figure 3.

The analysis results indicate that incorporating a biomass component, which absorbs carbon dioxide during its life cycle, ultimately leads to a reduction in the carbon footprint from 0.031 kg CO₂-eq to 0.022 kg CO₂-eq. In the case of bio-coke containing a 5% biomass component, the resulting footprint is 0.024 kg CO₂-eq. Meanwhile, with a 10% share of this component, the footprint reduces to 0.022 kg CO₂-eq. However, it was observed that doubling the significant share increase of biomass in the coal blend does not lead to a proportionate doubling in the decrease of the carbon footprint share. This suggests that achieving environmental goals in terms of carbon footprint reduction may require only a modest share of biomass, which likely won’t compromise the quality of coke. It’s worth noting that the higher demand for biomaterial necessitates increased transportation and energy for homogenization, which might not entirely offset its carbon dioxide consumption during the lifecycle. Nevertheless, considering all ecological costs, such a direction is justified, and environmental protection is imperative.

4. Discussion

The conducted analysis has revealed that incorporating biomass additives into the coking blend can have varying environmental impacts depending on the availability of raw material resources. According to the data obtained, the introduction of biomass-origin components can significantly enhance ecological benefits by reducing the carbon footprint generated during coke production per 1 MJ of energy. Introducing bio components inherently brings about ecological benefits. In this regard, it can be clearly stated that the advantages of introducing bio-additives are more pronounced when a particular region or country has a technological advantage in terms of production practices and resources, especially if the bio components originate from the nearest surroundings. Therefore, fundamental work related to improving the efficiency of production solutions must be conducted independently of other optimization efforts concerning the research on the feasibility of using bio components as a final and sufficient element in technological activities. This illustrates the fact that the entirety of environmental actions and benefits is multidimensional and requires an interdisciplinary approach that combines various activities from the fields of science, technology, social aspects, and the environment.

The most beneficial effect is achieved by introducing the largest possible organic component. However, considering the currently functioning technical and technological solutions resulting from existing coking practices, despite the favorable environmental effect, a too high percentage may lead to adverse changes in the physicochemical properties of the obtained coke. A high percentage of biomass in the coking blend, as mentioned earlier, results in a deterioration of mechanical quality and an increase in reactivity. Consequently, the technological usability of such coke in the market is limited. Therefore, it becomes justified to seek compromise solutions that meet the technical requirements defined by the industry while considering the environmental context. The significance of sourcing regionally available biomass, which due to its nature will not find application in energy systems and its storage is impossible or heavily restricted, remains noteworthy. In this case, the high temperature of the coking process and the long duration (several hours) guarantee the neutralization of hazardous compounds (if present) and their direct elimination from the environmental cycle.

5. Conclusions

The conducted analysis has demonstrated that incorporating organic additives into coking blends is beneficial in terms of reducing the environmental impact, measured by the carbon footprint. The local and regional context remains significant in life cycle assessments (LCAs), particularly concerning raw material procurement, where technical and technological conditions significantly influence environmental considerations. Extended supply chains of raw materials have a substantial impact on the total carbon footprint value. Therefore, in addition to further refining solutions, transferring knowledge and technology, exchanging information, and sharing best practices with regions at lower technological stages are necessary to intensify comprehensive sustainable development efforts.

Furthermore, recent years have shown that apart from instability dependent on potential political and pandemic situations, the choice of local suppliers can have a significant impact on the total environmental impact. It remains crucial to find the optimal balance between achieving superior environmental outcomes, selecting local suppliers, and maintaining the highest quality parameters of produced bio-coke that meet the evolving needs of industries utilizing coke as a production component. All such actions are necessary to meet the requirements of sustainable development and ensure clean, eco-friendly production.