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 CO
2 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][2][3]. The ironmaking process is the highest part of CO
2 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 CO
2 emissions by 10% with hydrogen reduction and separates and recovers CO
2 from blast furnace gas to reduce carbon by 20%
[6]. The COOLSTAR project in South Korea reduces CO
2 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 CO
2 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 CO
2 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 CO
2 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 SO
2 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 CO
2 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[15][16],
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
[29][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
[30][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.