Biomass As A Renewable Reductant In Metallurgical Processes: Opportunities, Challenges, And Future Perspectives

As a result of the increasing need for metals and minerals in addition to environmental concerns, there is an urgent requirement for the development of sustainable as well as low carbon processes that could replace traditional fossil fuel based metallurgical processes. Conventional processes, such as iron production, metal production from non ferrous ores, and mineral beneficiation processes, make extensive use of fossil fuels like coal and coke as reducing and heating agents [1]. Biomass has been identified as an important renewable source that can partially replace fossil carbon in several metallurgical applications. Biomass refers to materials created by biological processes carried out by living things like plants and animals including waste from agriculture and forestry operations as well as saw dust and bagasse. Unlike fossil fuels, biomass is a carbon neutral process because the amount of carbon dioxide released after burning biomass is neutralized by the carbon dioxide fixed through photosynthesis in its growing phase [2]. It should be noted that biomass had previously played an important role in metallurgical processes.For example, the use of wood charcoal was quite widespread for the production of iron until the introduction of an improved blast furnace technique using coke. However, massive deforestation in consequence of producing charcoal necessitated a change in fuels to fossils. Recently, biomass gained popularity due to the necessity of implementing laws, developing technologies, and reducing carbon emissions while conducting industrial activities [3]. Biomass is distinguished for its flexibility since it can be used in its raw form or transformed into substances such as biochar, syngas, and bio-oil.Of all three listed substances, biochar stands out due to high carbon content and increased reactivity relative to regular coke. Moreover, biomass-derived charcoal also features a highly porous structure which makes it preferable for metallurgical purposes [4].

Metallurgy, especially iron and steel production, constitutes one of the most pollutant industrial processes. In case of blast furnaces, traditional coal-based reduction requires the employment of coke as an essential support and a source of carbon. Thus, substitution of coal with biomass-derived carbonaceous substances represents an efficient way to minimize carbon emissions. Nevertheless, there exist several issues that need to be overcome such as the reduced mechanical strength, increased reactivity, and variations in biomass composition as opposed to conventional fuels [5]. The composition generally, biomass is made up of cellulose, hemicellulose, lignin, and small amounts of inorganic materials [6]. The amount biochar obtained from biomass that has high amounts of lignin has relatively high fixed carbon and strength levels [7].Not only the chemical composition but also the physical properties like particle size, density, and moisture content of biomass may affect their behavior in metallurgy operations. The high level of moisture may decrease thermal efficiency, and low bulk density makes biomass handling and transportation complicated. Therefore, it is important to carry out the preprocessing process of biomass such as drying, milling, and densification [8].

Biomass transformation into biochar usually takes place under pyrolysis conditions a thermochemical reaction performed without oxygen. Pyrolysis can be slow, fast, and intermediate. For biochar generation, slow pyrolysis is more popular because of higher carbon yield. It is possible to control the characteristics of biochar using process variables like temperature, heating rate, and feedstock [9].Biochar derived from biomass demonstrates specific features, which facilitate its use in metallurgy. Firstly the porous nature of biochar creates conditions for the easier diffusion and reduction reactions. Secondly, biochar has a smaller amount of ash than coal. This property decreases slag formation [10].

However, despite the above-stated benefits, there are also several obstacles when it comes to replacing fossil fuel-based energy with biofuel energy in metallurgy. First, the lower mechanical strength of biochar compared to coke may be a problem in the load-bearing applications like the blast furnace process. Besides, variations in the quality of biomass can cause problems for the product. Hence, the quality control system needs to be established [11]. The low energy density of biomass poses another problem since it causes additional cost for transporting and storing this type of energy. Therefore, there are various types of densification such as pelletization and briquetting to increase the energy density of biomass [12]. Currently, much attention is paid to composite material production using biomass or biochar. Composite pellets of iron ore fines and biomass reductants have proved effective in the direct reduction process, showing better reduction kinetics and energy savings [13].

Moreover, the potential use of biomass as an alternative to pulverized coal has been investigated for use in injection systems in blast furnaces. In addition to providing supplemental reducing agents and thermal energy, biomass can replace some of the pulverized coal used in these furnaces [14].

Biomass use not only reduces carbon emissions but also other air pollutants. Biomass is an alternative to coal that could minimize the presence of sulfur dioxide and nitrogen oxides since there is less sulfur and nitrogen present in biomass. Also, biomass can be used in agricultural and forest activities to handle waste management [15]. Incorporating biomass into metallurgical processes not only reduces carbon emissions but also contributes to sustainable development goals. The world has set many policies to curb greenhouse gas emissions, and governments around the globe have taken measures to promote the use of renewable energy in industries. This has increased interest in developing biomass-based technologies in metallurgy [16].The use of biomass in the metallurgical industry is faced with many challenges. These include the need to guarantee an adequate supply of biomass, its efficient transformation, and optimal process parameters for good performance. Collaboration between academia, industries, and policymakers is required to resolve these problems towards sustainable metallurgy [17].

DIFFERENT TYPES OF BIOMASS

For assessing the suitability of biomass as an alternative fuel source for metallurgy and mineral processing operations, the need for classifying biomass arises. There exist great differences in the physical and chemical characteristics of various types of biomass that include structure, composition, ash content, and reactivity among others. The major types of biomass include agricultural residues, forestry biomass, industrial biomass waste, aquatic biomass, and energy crops. Agricultural wastes represent one of the largest types of biomass sources, including biomass like rice husks, wheat straws, corn stalks, sugar cane bagasse, and coconut shells. Such wastes are found abundantly in agro-based economies and are regarded as wastes, rendering them cost-effective for industrial use. For example, rice husks have a high percentage of silicon dioxide, thus affecting slag composition during metallurgical processing, whereas bagasse has high amounts of volatile matter in the form of biochar, which is highly reactive [18]. However, the low bulk density of agricultural residues makes it difficult to transport and store them, hence necessitating densification in the form of pelletizing. Forestry biomass consists of wood chips, saw dust, bark, and logging wastes. Compared with the agricultural biomass, increased lignin content in forestry biomass makes them less susceptible and gives rise to biochars having higher percentages of fixed carbon. They are highly suitable in areas that require mechanical strength like blast furnaces and in composite pellets [19]. Besides, forestry biomass is homogenous in nature and possesses less ash content, making it favorable for metallurgical uses. Industrial biomass wastes refer to wastes produced by wood based industries, paper mills, and food processing factories. Biomass types in this category include black liquor, sawmill wastes, and nutshell wastes. This kind of biomass is usually concentrated in industries, thus making it easy for integration into existing processes. The application of these types of waste reduces environmental pressure through waste management practices within a circular economy approach [20]. Another type of biomass being used in industry includes aquatic biomass such as algae and seaweeds. Even though not widely applied in metal extraction due to the limitations involved in their direct use, the advantages of rapid growth rate and ability to fix carbon effectively make them an ideal choice [21].Industrial use of energy crops involves the use of plants grown solely for biomass production purposes. Energy crops such as willows, poplars, and switchgrasses are highly controlled with regards to quality and quantity. The use of energy crops is faced with the disadvantage of competing with other agricultural products for land use [22].Various factors play a role in determining the biomass type to use for metallurgy, including accessibility, cost, carbon, ash composition, and reactivity. Fixed carbon content and low ash are desirable characteristics of the reductant, while high volatile matter is preferable when gases are produced [22].

(a) Commercial Applications: Overall Applications in Mineral and Metallurgical Industries

There has been great progress made in the applications of biomass in the mineral and metallurgical industries owing to increasing demands to reduce the levels of greenhouse gases and decrease the usage of fossil fuels. Biomass is currently incorporated at different stages of metallurgical processes, such as iron making, extraction of non ferrous metals, and mineral processing. Charcoal obtained from biomass, as well as biochar, is currently widely used as an alternative source of energy in iron and steel manufacturing in place of coke. Biofuel charcoal iron and steel industries have been developed in some areas around the world due to their abundance of forested land. For instance, Brazil has developed its own charcoal-based iron and steel industry, where biomass fuel charcoal serves as a fuel and reduction agent [23]. The use of charcoal in iron-making process is associated with lower emission of sulphur gas and minimizes the amount of carbon emission.

Biomass fuel is also used as a reducing agent during the direct reduction processes of iron, where the process is enhanced by high reactivity of biochar and lower temperature required in the reaction. Iron ore fines together with biomass fuel have produced effective composite pellets in laboratory and pilot plant studies [24].Moreover, biomass has been explored in the field of non ferrous metallurgy in the form of metals extraction, specifically in relation to the removal of copper, nickel, and zinc. In such cases, biomass based carbon may be employed as either a reducing agent or as a means of providing energy. Moreover, biochar has also been applied as an absorbent in recovering metal ions from solutions [25].

Another area where biomass has found use is the field of mineral processing, particularly in the process of froth flotation. Biochar and lignin have been used as reagents in flotation processes and have shown significant potential due to the natural nature of biomass. In such cases, both of the mentioned substances were used as collectors and depressants [26]. However, despite the mentioned areas of biomass application, there are certain obstacles that restrict the widespread use of biomass within the metallurgical industry. These obstacles include the inconsistency in quality of biomass, problems with biomass supply chain management, and required modification of existing procedures [27].

(b) Recent Trends

Current trends in biomass application to metallurgical industry include improvements in properties, increases in efficiency, and integration into existing industrial facilities. The main development in the field of thermochemical biomass transformation techniques is associated with the creation of biochar with high energy density and hydrophobicity [27]. Thermochemical methods allow increasing the storage properties of biomass, making it easier to use it in industrial processes. The development of composite biomass materials, including carbonized pellets, is another recent trend in the field of biomass applications to metallurgy. Composite biomass materials are intended to improve certain properties necessary for the functioning of metals, including their mechanical properties, activity, and thermal resistance. In particular, composite pellets made from biomass and binders show high reduction efficiency and durability in direct reduction procedures [28].

Pulverized biomass injection technology represents another interesting area that may be relevant to metallurgy. Pulverized biomass can be injected into a blast furnace to replace part of the fossil fuel [29]. Nevertheless, some questions relating to combustion behavior and ash properties must be investigated. Models and simulations are being used in predicting the performance of biomass based materials under different process conditions [30]. Life cycle assessments of biomass based systems are also another recent research trend. Life cycle assessments of biomass based systems help in assessing the environmental effects caused by using biomass. This includes the amount of carbon emitted from these processes, energy use, and consumption of resources [31]. Use of hybrid energy systems that utilize biomass along with other renewable energies is an emerging trend. The goal of such hybrid systems is reducing the amount of carbon emitted and increasing the process efficiency. One such system is hydrogen enrichment in reduction processes that use biomass-based carbon [32].

BIOMASS AND BIOCHAR

Biochar, a carbon-based material obtained by subjecting biomass to thermal breakdown under anaerobic conditions, has increasingly been seen as one of the alternative sources of carbon for metallurgy. Biochar has various physical and chemical attributes that allow for its application in reduction reactions, absorption, and as fuel replacement at high temperatures in metallurgical processes. Converting biomass into biochar leads to improved energy density of the biomass as well as making it easier to handle and use in metallurgical processes [33]. Properties of biochar are highly affected by the source biomass feedstock and conversion conditions used. An increase in the temperature of conversion produces higher yields of biochar with higher fixed carbon content and lower volatile matter. For instance, while coke is made from coal through carbonization at high temperatures and characterized by high strength, biochar is created from biomass and generally more reactive yet less strong than coke. This increase in reactivity will be beneficial for the reduction reaction due to improved kinetics [34].Pore size is another attribute that makes biochar effective in different applications. The existence of micro and mesopores increases the effectiveness of the gas phase diffusion and the presence of active sites that facilitate chemical reactions. It is highly desirable in metallurgical processes such as reduction of iron ore where gas-solid reactions are involved [35]. The composition of the ash in biochar represents another essential feature. Nevertheless, specific biomass like rice husks could contain considerable amounts of silica, affecting slag formation processes [36].

(a) Biomass to Biochar Conversion Process

Conversion of biomass to biochar can be accomplished through several methods. Among those methods, thermochemical treatments prevail, especially the technique called pyrolysis. In this process, the thermal decomposition of an organic compound takes place under the condition of an absence of oxygen to produce solid char, liquid bio oil and gases. There exist three types of pyrolysis namely slow, intermediate, and fast.Slow pyrolysis with its low rates of heating and long residence times produces more biochar than fast pyrolysis does. Slow pyrolysis is often applied in the field of metallurgy because the produced biochar tends to have a higher proportion of fixed carbon. On the other hand, fast pyrolysis, involving rapid heating and short residence times, produces more bio oil than biochar [37]. Biomass Gasification is a thermochemical technique through which biomass is partially oxidized to yield syngas consisting mostly of carbon monoxide and hydrogen gas. Gasification leads to the formation of both gaseous and solid products. The gaseous product, i.e., the syngas, constitutes the main product. However, a residue resembling biochar can also be recovered and used in metallurgy [38]. Torrefaction refers to mild thermal processing performed under oxygen-free conditions at a temperature of 200-300 degrees Celsius. As a result, the moisture and volatile matter are removed to produce material that has increased energy density and hydrophobicity. The torrified biomass. The technique is ideal for treating wet biomass and results in hydrochar production, which is structurally and chemically different from biochar [40].

Figure 1: Process Flow Diagram for Biomass to Biochar Conversion

In Figure 1 below, the process flow diagram for the thermochemical conversion of biomass into useful products is shown. Firstly, the raw this is followed by treatment of the dried biomass using thermochemical processes like pyrolysis, torrefaction, and gasification. In this case, the main products produced include biochar (solid product), gaseous products (e.g., CO, H2, and CH4), and bio-oil (liquid).

Table 1: Comparison of Biomass Conversion Processes

Type of conversion technology depends on the required properties of the final product and its purpose. In metallurgy, for instance, slow pyrolysis is normally recommended since it enables production of biochar containing higher proportions of fixed carbon and favorable reactivity.

(b) Treatment of Biomass and Biochar

Pre-treatment and post treatment activities are key elements in improving the performance of biomass and biochar as metallurgical products. Generally, raw biomass material tends to be wet, and of various sizes and qualities that may negatively affect its behavior during thermo-chemical reactions.Drying is among the basic pre-treatment technologies, whose main objective is to lower moisture levels [41].The densification process through methods such as pelletization and briquetting is yet another pre-treatment process. Chemical pre-treatments can be carried out on both the biomass and the biochar Acid washes remove inorganic compounds, whereas alkali treatment changes the structure of functional groups. Such procedures are very important especially when the highest purity of the obtained carbonaceous materials is required [42].

Biomass undergoes activation processes that include either physical or chemical activation to maximize its surface area and porous structure. Physical activation includes steam gasification, and carbon dioxide gasification conducted under high temperatures. Chemical activation involves using chemicals such as potassium hydroxide, phosphoric acid, etc. Activated biochar shows higher adsorption abilities and reactivity due to a more complex structure [43]. Biochar may go through a thermal treatment process by applying high temperature treatment. This procedure leads to reducing the share of volatile substances and increasing stability and the carbon content in the material. This way, biochar becomes much more appropriate for conducting high-temperature metallurgical processes such as ironmaking [44]. Another aspect to consider about biochar treatment lies in the management of ash composition since it significantly influences slag formation and efficiency of the whole process. Leaching and blending of different biomass types are some of the methods for optimization of ash composition [45]. Recently, there has been a lot of interest shown in composites of biochar mixed with binders and other additives. The purpose of these materials is to improve the mechanical strength and durability of biochar, which was the main drawback of biochar when compared to coke. This type of material is particularly suitable in cases where mechanical strength is critical, like in blast furnaces and pellet-based reduction [46].

BIOMASS AS A REDUCTANT IN METALLURGICAL OPERATIONS

The use Reduction reaction is a vital part of metal production from ore materials. Carbonaceous material acts as reductants in iron making processes. Besides, they improve the reduction kinetics and help reduce the required temperatures [47]. Another notable aspect about biochar is its structure. While coke has an ordered and dense carbon structure, biochar usually presents amorphous and porous structures. This makes it more reactive, as it contains more reactive sites, and enhances gas diffusion [48]. The major drawback of using biochar is the low mechanical strength.

(a) Biomass as a Reductant

Biomass may be employed as a reducing agent both in its natural state and after carbonization. When subjected to high temperatures, biomass will decompose through devolatilization to yield reactive gases and carbonaceous solids. The general reduction process may occur via two distinct routes, namely direct and indirect reductions. In case of direct reduction process, carbonaceous materials interact with oxides of metals producing carbon monoxide gases and metals [49]. In case of indirect reduction, gases react with oxides of metals producing metal compounds. The reactivity of carbon obtained from biomass is greater than the reactivity of coke, therefore, the rate of reduction increases. This is especially beneficial during reduction roasting and direct reduction of iron ores. Nevertheless, higher reactivity tends to increase the consumption rates, which needs to be optimized properly [50]. The ash contents present in biomass are important in determining the performance as a reductant. For instance, some elements like potassium and calcium tend to act as catalysts for the reaction. However, if there is too much of silica in the biomass ash, slagging might occur [51].

(b) Reduction Roasting of Low Grade Iron Ore Using Biomass Reductant

There are many ways of upgrading low-grade iron ores to make their extraction efficient. One such method is through reduction roasting of iron oxides to magnetite forms. In this regard, there have been several studies on using biomass as the reductant for the purpose of lowering energy consumption. Reduction roasting involves heating the iron ore together with a reductant in controlled conditions. Carbon produced by biomass interacts with oxygen present in the iron ore and results in the conversion of hematite to magnetite. Such a phase transition enables effective magnetic separation, thus making it possible to recover iron from low-grade iron ores [52].The efficiency of biomass in reduction roasting is influenced by various parameters such as temperature, retention time, and ratio of biomass and ore. Research indicates that biomass is capable of providing similar or even better results than coal-based reductants used in traditional reduction processes. The high amount of volatile matter in biomass promotes the production of reducing gases [53].

Figure 2: Reduction roasting mechanism using biomass reductant

Moreover, it is worth mentioning that biomass-based reduction roasting also has an advantage of causing less pollution, especially sulfur oxides since the amount of sulfur in biomass is minimal [54].

(c) Direct Reduction of Iron Ore with Biomass as Reductive Material

As has already been mentioned above, in direct reduction iron ore is being turned into pure metallic iron without melting, at temperatures below the melting point of metallic iron. Both biomass and biochar were researched as new possible reductants in direct reduction ironmaking, especially when using rotary kilns and shaft furnaces. The following benefits of utilizing biomass in direct reduction should be highlighted. Due to its high reactivity, biochar enables quick reduction, whereas the release of hydrogen-containing gases in the biomass thermal destruction increases reducing properties of the process. As a result, the process becomes more efficient and less energy-consuming [55]. Nevertheless, there are also disadvantages to using biomass in direct reduction processes. The low mechanical strength of biochar leads to biomass breaking up into pieces and thus to the production of dust which may affect process efficiency. To prevent this effect, biomass is commonly used together with other materials or is processed into dense form like pellets and briquettes [56]. One more aspect that should be considered is carbon efficiency of the process. The carbon of biomass nature is more reactive but contains less fixed carbon than that of the coke; thus, there may appear a need for higher consumption rates to reach a desirable extent of reduction. Thus, optimization is necessary to increase the efficiency of the use of biomass [57].

(d) Biomass Composite Pellets with Biomass as Reductant

The emergence of composite pellets which utilize biomass as a reductant is a huge step in the development of metallurgy techniques. In general, these pellets are formed from iron ores, biomass/biochar, and binder. Their distinctive feature lies in the fact that they reduce themselves when heated up. There are certain advantages to using composite pellets. In this case, reductant and ore are located in close vicinity to each other inside the pellet. Thus, there is higher effectiveness of reduction reactions, fewer diffusional limitations and faster rates of reduction [58].There are many aspects of composite pellets' behavior. First of all, their composition, size, and heating are critical. Also, there can be different binders used  bentonite, organics, ligninetc. [59].

Figure 3: Schematic of biomass composite pellet reduction

The biomass composite pellet reduction schematic diagram is shown in Figure 3. At the beginning stage, iron ores, biomass, and binder are mixed to produce composite pellets. After that, the heating process takes place, and in that process, the pellets undergo self-reduction owing to the reaction between iron oxides and the gases produced from the biomass. In the end, metallic iron is formed. Through experimentation, it has been found that the biomass-based composite pellets show high metallization at low temperatures [60]. The presence of volatile matter in biomass helps in producing reducing gas in the pellet, resulting in an effective reduction process. Furthermore, using biomass in composite pellets eliminates the need for external reducing agents and process design becomes simpler. It makes it feasible for ironmaking operation at small-scale or decentralized production [61].

SINTERING APPLICATIONS OF BIOMASS AND BIOCHAR

In the iron and steel industries, sintering plays a crucial role in the agglomeration process, in which iron ore particles are melted using flux and solid fuel to form porous masses called sinter. This is considered as one of the most important raw materials for the blast furnace production processes. Traditionally, coke breeze, which is made from coal, is used in blast furnace operations as the major fuel. Due to increased environmental issues and sustainability of operations, researchers are considering the use of biomass and biochar as alternative fuels for coke breeze. Incorporation of biomass in blast furnace operations brings numerous benefits as well as possible problems. The use of biomass is known to increase the volatility and reactivity of the materials being processed during sintering. Biomass undergoes fast devolatilization in the bed leading to the liberation of combustible gases which increases the heat and the rate of combustion in the sinter bed [62].

Nevertheless, the relatively low fixed carbon and density values of biomass compared to coke breeze may affect the heat generated during the sintering process. This may result into poor sinter performance in terms of strength and efficiency. For this reason, the biomass is added to the blast furnace operation in small proportions replacing part of the coke breeze [63]. The carbonized biomass known as biochar offers better characteristics in regard to application for sintering compared to the non-carbonized biomass. One of those characteristics is a high level of fixed carbon content and low amounts of volatiles; thus, it is thermally stable and can serve as a fuel substitution product. Porosity in biochar leads to better efficiency and gas permeability during the process of combustion [64]. The presence of various elements in the ash of biomass and biochar influences their properties as sintering material. Examples of the mentioned elements are potassium and sodium, which have a beneficial effect on reducing the melting point and facilitating bonding. On the other hand, too much alkali might lead to complications such as slag formation.Thus, proper selection of biomass is critical [65].

Figure 4: Sintering process with biomass substitution