Application and Development of Biochar in Ironmaking Production

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1		Shijie Wang	--	2131	2023-11-09 09:32:13	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	2131	2023-11-12 13:42:30	\|

This entry is adapted from the peer-reviewed paper 10.3390/met13111844

The concept of green, low-carbon and clean energy consumption has been deeply rooted in the hearts of the people, and countries have actively advocated the use of new energy. In the face of problems such as resource shortage and environmental pollution, scholars began to explore the use of new fuels instead of coal for production. Biomass resources have the characteristics of being renewable and carbon neutral and having large output. As an energy utilization, it is helpful to promote the transformation of the energy structure in various countries. Applying it to ironmaking production is not only conducive to energy conservation and emission reduction in the ironmaking process but also can achieve efficient utilization of crop waste.

biochar ironmaking low-carbon metallurgy energy saving and emission reduction

1. Introduction

Climate change is a major global challenge facing human society today. The emission of carbon dioxide and other polluting gases has a huge impact on the world environment ^[1]. The iron and steel industry is an important basic industry to promote the development of the national economy, an important support for building a modern power, and also a large energy consumer and contributor to CO₂ emissions. According to statistics, in 2021, China’s annual energy consumption reached 5.24 billion tons of standard coal, of which coal resource consumption accounts for 56.0% of total energy consumption ^[2]^[3]. The ironmaking process is the highest part of CO₂ emissions in the steel production sector, which is due to the extensive use of fossil fuels to heat, melt, and reduce iron ore ^[4]. At present, China’s steel production is still dominated by a long process, high carbon emission intensity, large energy consumption, and serious environmental pollution. According to the EU’s goal, fossil carbon dioxide emissions should be reduced by 80% by 2050 ^[5]. Japan’s COURSE50 project reduces CO₂ emissions by 10% with hydrogen reduction and separates and recovers CO₂ from blast furnace gas to reduce carbon by 20% ^[6]. The COOLSTAR project in South Korea reduces CO₂ emissions by 15% via modifying the by-product gas of the steel plant to prepare ‘gray hydrogen’ and injecting it into the blast furnace as a reducing agent ^[7]. ThyssenKrupp Group in Germany has made a breakthrough in the ‘hydrogen instead of coal’ blast furnace, achieving a 16% reduction in CO₂ emissions ^[8]. Brazil has partially replaced pulverized coal with charcoal powder for blast furnace injection, achieving a carbon reduction of 30% ^[9]. The Swedish iron and steel industry (ISI) sector, which is heavily dependent on fossil fuels and reducing agents, together with the mining industry, accounts for 63% of Sweden’s industrial fossil energy use and 46% of greenhouse gas emissions, so it plans to reduce its fossil carbon dioxide emissions in the short to medium term ^[10]. The European Union (EU) has set climate targets to gradually reduce greenhouse gas emissions by 80% via increasing the share of renewable energy in the energy structure and improving energy efficiency ^[11]. Therefore, the steel industry is considered to be an energy-intensive industry in all countries, especially since energy conservation and climate change issues (including polluting gas emissions, dust generation, etc.) have driven energy and ecological transformation ^[12]. The CO₂ footprint of a direct reduction plant fed with biomass-based reducing gas is more than 80% lower compared with the conventional blast furnace route. The biomass-based production of reducing gas could definitely make a reasonable contribution to a reduction in fossil CO₂ emissions within the iron and steel sector in Austria ^[13]. It is particularly important for countries to seek a green and low-carbon production method, use clean fuel for production, and fundamentally solve a series of problems brought on by production. As the iron ore reduction process in the blast furnace is fully dependent on carbon, mainly supplied by coal and coke, bioenergy is the only renewable energy that presents a possibility for their partial substitution ^[11]. As a renewable energy, biomass is a globally recognized clean and low-carbon fuel, which has great advantages compared with traditional fossil energy. The content of N and S in biomass is low, which can reduce the emission of SO₂ and other pollutants in production. Because of its carbon neutral characteristics, it can partially realize the carbon neutral cycle of the ironmaking process in iron and steel production, thereby reducing CO₂ emissions and reducing environmental pollution by the greatest extent. The use of renewable biomass in the industry is likely to reduce greenhouse gas emissions by 10% in 2050, which is equivalent to a 25% reduction in expected emissions from the industrial sector, equivalent to the current total carbon dioxide emissions in Germany, France, Italy, and Spain ^[14]. With the proposal of the ‘double carbon’ target ^[15]^[16], the steel industry is facing the transformation and upgrading of energy saving and carbon reduction, and the application of biomass energy in steel production has also received extensive attention. From the perspective of the ironmaking process application technology, biomass generally has the disadvantages of high moisture and alkali metal content, low fixed carbon content and calorific value, and low energy density.

2. Preparation of Biochar

The use of biomass carbon must meet the requirements of iron and steel production. For example, biomass as a fuel for blast furnace injection must meet the physical, chemical, and technological properties of the fuel in the furnace. From a physical point of view, biomass must be crushed and screened to a suitable particle size for injection. From a chemical point of view, biomass must have similar chemical composition and combustion reactivity to coal for injection. The composition and structure of raw biomass are very different from those of pulverized coal. Compared with pulverized coal, the calorific value, fixed carbon content, grindability, and energy density of biomass are lower, the volume is larger, and the moisture and volatile content are higher. These differences greatly affect the direct and effective utilization of biomass in the steel production process. Over the past decade, antipyretics have developed into a promising thermochemical technology that can modify microorganisms under inert gas or anoxic conditions. Like the original microorganisms, the solid material obtained by biomass pyrolysis is called biomass semi-coke, which has a lower moisture concentration, greater carbon concentration, and greater calorific value and energy density. Under certain pyrolysis conditions, it can achieve the need for high-temperature jet fuel.

Pyrolysis is a thermal decomposition process that occurs in the absence of oxygen. Pyrolysis converts biomass rich in lignocellulose into gases, liquids, and solids rich in carbon. The main components of lignocellulosic biomass are cellulose, hemicellulose, and lignin, which are high molecular polymers. In the temperature range of 300~500 °C, these polymers are converted into combustible gases, bio-oil, and biochar to varying degrees through decomposition and polycondensation. Pyrolysis technology can be divided into slow, fast, and flash pyrolysis ^[17]; the most commonly used are slow and fast pyrolysis processes. Different heating rates have an important influence on the composition of pyrolysis products.

3. Application of Biochar in the Ironmaking Process

In recent years, biochar has been widely used in the ironmaking process ^[18]. In addition to the mixed injection of the blast furnace, researchers have also tried to add biochar to sintering, coking, iron ore reduction, and other processes (as shown in Figure 1) to replace some pulverized coal to reduce fossil energy consumption.

Figure 1. Partial application of biochar in blast furnace ironmaking process.

3.1. Application of Biochar in Coking Process

Coke is an indispensable material in the blast furnace, providing a permeable matrix for slag and metal to pass through during the falling process and hot gas to penetrate during the rising process, while generating the required gas to reduce iron oxide (CO). Coke reactivity is a very important parameter to determine the quality of coke. Applying biomass to the coking process helps to reduce the temperature of the reserve area of the blast furnace and reduce the energy required for production. Seo, M et al. ^[19] used boxwood as a raw material to add to coking coal in different proportions (0, 10, 15, 20, and 30 wt%) and carbonized it at different final temperatures (500–800 °C) to generate biochar. The calorific value of biochar is between 7200~7560 Kcal/kg, which is higher than the standard value (7000 Kcal/kg), indicating that biochar can be a suitable substitute for traditional fossil fuels and has the potential to reduce carbon dioxide emissions in the ironmaking industry. Yuuki, M et al. ^[20] used woody biomass to produce coke and used vapor deposition (VD) and other methods to explore the properties of coke and the study of composite/coal mixtures for gasification reactions; they determined that the optimal coal blending amount depends on the type of caking coal. It was found that 3–15 wt% of biomass can be used as coke addition for coke production.

3.2. Application of Biochar in Sintering Operation

Traditional iron ore sintering is dominated by coke powder, and the CO₂ emissions generated during the sintering process account for 11% of the steel process ^[18], second only to the blast furnace. The application of biomass to iron ore sintering reduces the use of coke powder from the source and reduces the emission of polluting gases, as shown in Figure 2. To this end, scholars at home and abroad have conducted a series of studies.

Figure 2. Application of biochar in sintering process.

Gan, M et al. ^[21] used fruit core biochar instead of coke powder to conduct sintering experiments at different ratios to study the effect of adding fruit core charcoal on sintering. The results showed that the appropriate ratio of fruit stone charcoal to replace coke powder was 40%, which had little impact on the production and quality of sintered ore. However, the emissions of CO₂, NO_X, and SO_X were reduced by 23.05%, 30.99%, and 42.77%, respectively, achieving a good emission reduction effect.

3.3. Application of Biochar in Iron Ore Reduction

Biomass carbon is low-carbon and environmentally friendly. The use of biomass for iron ore reduction can achieve the green development of iron and steel enterprises, and the application of some biomass carbon can also effectively improve the reduction capacity of iron ore. Biomass carbon can not only reduce iron oxides by its own carbon but also pyrolysis. The reduction gas reacts with iron oxides to improve the reduction efficiency, and the gas–solid synergistic effect of biomass carbon is used to reduce iron ore, as shown in Figure 3. The use of biochar to reduce iron ore not only improves the reduction efficiency but also contributes to the protection of the environment.

Figure 3. Application of biochar in reducing iron ore.

Huang, Z.C et al. ^[22] carried out experiments on the temperature reduction of iron concentrate by pine wood. The results showed that biomass could be pyrolyzed to generate CO, CO₂, CH₄, C₂H₄, H₂, and other gases. Due to the increase in pyrolysis temperature, the proportion of CO and H₂ in the reduced iron oxide was significantly increased. When the molar ratio of carbon to iron was 0.4 and the reduction temperature was 1050 °C, the metallization rate of the reduced material was 82.97%, which could efficiently reduce the iron concentrate at low temperatures.

3.4. Biochar Used in Blast Furnace Injection

The co-combustion of biomass and pulverized coal is a very advantageous measure for using biomass to replace fossil fuels. Moreover, biomass has good flammability, and co-combustion with pulverized coal can promote the combustion reaction of pulverized coal. Therefore, many scholars at home and abroad have carried out a lot of research on the co-combustion characteristics and co-combustion products of biomass and pulverized coal and are committed to promoting the utilization of biomass resources. Chen W H et al. ^[23] studied the feasibility of injecting different types of biomass products such as charcoal, baking materials, and sawdust particles into blast furnaces to replace coal injection. The results show that charcoal has a significant effect on the operation of the blast furnace. Pulverized coal can be completely replaced by charcoal, while baking materials and sawdust particles can only be added in a small amount to blast furnace smelting. Through research, it is found that after the injection of charcoal, roasting materials, and sawdust particles, the annual CO₂ emission reduction potential of coal injection smelting is about 1140 kton, 260 kton, and 230 kton, respectively, which can effectively reduce CO₂ emissions. In addition, for the addition of biomass, the recycling of resources is also realized.

Li J et al. ^[24] systematically studied the physical and chemical properties of biomass residue char and anthracite by means of X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The combustion characteristics and mechanism of biomass residue, biomass residue char, anthracite and biomass residue char, and anthracite mixture were studied by thermogravimetric analysis. The addition of biomass residue biochar to anthracite can improve the combustion performance of anthracite. The combustion process of the mixed samples has an obvious synergistic effect. When the ratio of biomass residue to hydrochar is 60%, the activation energy of the mixed sample is 38.5 kJ/mol, which has good combustion characteristics in blast furnace smelting. Zheng, W.C et al. ^[25] showed that biomass had good combustion performance with different combustion temperatures of biomass and temperatures lower than the combustion temperature of coal.

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Shijie Wang

Yifan Chai

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Guoping Luo

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Update Date: 12 Nov 2023

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