Clean Ironmaking and Steelmaking Processes

Steel demand is going to grow in the next future. With the present production routes, greenhouse gas emissions are destined to double by 2050 with a very remarkable effect on global warming. To face these problems, energy-efficient new ironmaking technologies must be employed and developed. Meeting the targets will require rapid and comprehensive implementation of mitigation technologies and measures that are commercially available today and emerging technologies that are still in the early phases of development. Among several greenhouse gases with different impacts on the air quality, CO₂ is the main contributor accounting for about 60% of the greenhouse effect because of its huge and broad emission levels. Among all the industrial sectors, ironmaking and steelmaking one is calculated and measured to be the largest emitter of carbon dioxide and one of the users with largest energy demand. Obviously, the different adopted or to be developed solutions vary in different countries and regions because of fuels and materials supplies and prices. In addition, different government decisions and low limits drive the different choices. This chapter describes the main available technologies employed in the traditional or innovative routes capable of reducing the energy consumption and the dangerous greenhouse emissions. Being the limits of existing production techniques (mainly coal-based) reached, the development and implementation of new breakthrough technologies together with supportive energy infrastructure and services are required. Obviously the energy topic will be described taking into account the direct and indirect energy consumption per each analyzed technology. The methods to improve the energy efficiency are the energy consumption optimization, the online monitoring, and the energy audits.

The traditional integrated ironmaking route is based on coke making. Coke provides the support for the materials in the BF as well as acts as reducing agent for the iron oxides increasing temperature through its thermal energy. Coke making is responsible for the 10% of the energy consumption of the integrated ironmaking route; it is a material-intensive process, and it consumes enormous volumes of water. Water is considered as fundamental in the design of sustainable steelmaking routes. In the present chapter, the water treatment solutions, devoted to dangerous compound elimination, are described. Coke making faced important issues related to the increasing environmental pressure, the reduction of the availability of good coking coals, and the need to renew old coke making facilities. Many and different technologies have been developed for integrating or substituting the existing ones in order to reduce the coke needing in the traditional integrated steelmaking plant. CDQ and CSCB are described as efficient solutions. These are examples of how the old coke ovens must be substituted in order to meet the climate protection objectives. The coal moisture control and other plant solutions are analyzed. The energy balance, the plant costs, and the efficiency in the greenhouse emissions abatement per each described solutions are underlined. CSQ (Coke Stabilization Quenching) as an advanced wet quenching system with low environmental impact was underlined. Other emerging technologies that could become important alternatives in the near future are described. For example, the chemical-looping combustion (CLC) of COG, with the objective to improve combustion efficiency and facilitating the capture of the CO₂ produced in the system, has been proposed.

The sintering process is finalized to transform the small grained raw material into larger grained iron ore sinter of the right dimensions to be used in the blast furnace. Achieving an adequate sintered product depends on the adequate raw materials supply and the previous stage to the sintering process, granulation. The air emissions comprise of various pollutants such as dust, SO₂, NOx, CO, organochlorine compounds, heavy metals, etc. Atmospheric emissions also include volatile organic compounds (VOCs) formed from volatile material in the coke breeze, oily mill scale, etc. Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), commonly known as dioxins and furans, are also formed. Emission limits of the sinter waste gas have been continually and strictly tightened in the last years, so new solutions are continually needed. Multiple methods should be collectively considered to ensure that the final emissions are below the limiting values. To face these issues, waste heat recovery systems are employed. Exhaust gases can be processed, adsorbed, decomposed, and/or collected as nontoxic by-products to increase the quantity and improve the quality of steam recovery, reaching high fuel savings. Among all the technologies, the largely used methods are denitrification equipment, desulfurization equipment, and activated coke packed bed adsorption. EOS takes advantage of the fact that only a part of the oxygen in the air is consumed for coke combustion. Optimized exhaust recirculation in the sinter bed allows for the improvement of energy efficiency and emissions control. Charcoal and biomass utilization find optimal results in the emissions abatement. Optimization of raw material quality and processing conditions contribute to the overall efficiency of the process.

The traditional integrated ironmaking plant is based on blast furnace operations for the reduction of iron oxides to cast iron. Seventy percent of the steel produced globally is based on BF operations. The energy requirement for the blast furnace operation is in the order of 11.6 GJ/t hot metal. It is the highest energy consumer among all the phases of integrated steelmaking because of the high input quantity of reducing agents. So, the process control is crucial for energy efficiency. The main environmental problems are related obviously with CO₂ gas production in addition to dust, wastewater from gas scrubbing, slag treatment products such as SO₂ and H₂S, and sludge. The BF gases have very low calorific power, so, it is mainly employed in the BF itself or in the coke ovens. The optimization of raw materials as well as various gas injections in order to improve the energy efficiency and reduce the emissions levels is largely described in the present chapter. Various solutions for the off-gases treatment such as top gas recovery turbines are shown. Hot stove control and heat recuperation systems are analyzed. Improved recovery of BF gases and NG injection are compared to traditional BF operations. The employment of fuels alternative to coke is demonstrated to be fundamental in reducing GHGs emissions. Plastic waste injection, biomass utilization, as well as carbon composite agglomerate employment are shown basing on the last scientific evidences. Top gas recycling, hydrogen use, as well as oxygen blast furnaces are described. Slag behavior and uses solutions are analyzed. Per each described solution, the energy consumption, the plant costs, and the emissions abatement efficiency were described.

The basic process of converter is to reduce the carbon content of iron produced in the blast furnace in order to obtain low-carbon steel. This operation is universally known as basic oxygen process (BOP), basic oxygen steelmaking (BOS), and basic oxygen furnace (BOF) process. The furnace is a pear-shaped furnace where the pig iron from blast furnace, and ferrous scrap, is refined into steel by injecting a jet high-purity oxygen through the hot metal in order to reduce the carbon content. Oxidizing carbon and other metalloids are a critical part of the steel refining process. Transport phenomena in this furnace are highly complex. Scrap and direct reduced iron are employed in order to reduce the off-gases impact. The CO-rich BOFG has a very high calorific power that is mainly used to enrich the BF (with sensitive lower calorific power). The gas recovery in combustion or non-combustion processes is analyzed. Recovering energy from the BOF gas involves making efficient use of both its chemical and sensible heat. Basic oxygen furnace (BOF) steel slag is a main by-product in steelmaking, and its valorization is therefore of considerable interest, from a metal-recovery perspective and from a residue-utilization perspective. The improvement of by-products reuse contributing to reduce the environmental impact of industries and to increase their competitiveness is described. Bottom gas stirring of the iron melt in the BOF converter is important for advancing the oxidation reactions and also for achieving homogeneity for composition and temperature.

In the electric arc furnace, steel is produced only through scrap fusion. Scraps, direct reduced iron, pig iron, and additives are melted through high-power electric arcs formed between a cathode and the anodes. The emissions levels are normally mainly related to the indirect emissions due to the high energy consumption of the process. The EAF process has become increasingly cost and quality competitive to the integrated steel mills through process and technology innovations, which have significantly lowered power consumption and increased productivity while satisfying the customers’ quality needs of steels. The appropriate GHG reduction strategy is strongly influenced by the source of electricity generation (i.e., fossil fuel or nuclear). Reduction of indirect GHG emissions requires reducing electrical energy consumption by such methods as burner optimization, post-combustion, scrap preheating, and other process efficiency measures. Other dangerous emissions are due to inorganic compounds such as iron oxide dusts and heavy metal and to organic compounds such as PCB and PCDD/Fs. The current trend towards increased addition of fuel and oxygen has resulted in chemical energy sources supplying a greater proportion of the furnace’s energy inputs. Potential fuel sources include natural gas, carbon, hydrocarbons, and iron carbide. Scrap quality is fundamental for the process efficiency, energy consumption, and steel quality. Oxyfuel burners utilization and flue gas utilization are described in this chapter. Scrap preheating techniques are employed to reduce the energy consumption. Bottom stirring and heat recovery are discussed.

Smelting reduction processes have been developed in the past as an emerging alternative for hot metal production. What they have in common is that coal is directly used; thus, coke making can be avoided in comparison to the blast furnace. They have been developed to overcome the high energy and emissions levels intensive processes of coke making and sintering. Once very-low-quality raw materials are used, larger amounts of coal are needed with a consequent increase of the CO₂ emissions. For all these reasons, this kind of plants is mainly located in those regions where the raw material quality is low or where the material supply is difficult. By the type of smelting stage applied, three process groups can be distinguished: in-bed processes, in-bath processes, and electric smelting. The shaft furnace technology is applied for the reduction of pellet and lump ore, whereas fluidized bed technologies and cyclones are used for fine ore input. Coal gasification is allowed through a reaction with oxygen and iron ore in a liquid state. The heat is used to smelt iron, and the hot gas is transported to the pre-reduction unit to reduce the iron oxides that enter the process. In the present chapter, the main employed available smelting reactors are described. Those plant solutions employing pre-reduction of iron ores are described. Various plant configurations are analyzed. In particular, those solutions globally adopted for the large CO₂ abatement volumes are underlined.

The global needing for greenhouse emissions and energy consumption reduction led, in the recent past, to the increased scientific and industrial interest in the development of technologies allowing to produce direct reduced iron. DRI is produced through the removal (reduction) of oxygen from iron ore in its solid state. This technology encompasses various processes based on different feedstocks, reactors, and reducing agents. DRI processes can reduce CO₂ emissions by using natural gas instead of coal due to the replacement of carbon reductant by hydrogen from the methane. Many complementary gasification processes have been developed in order to synthesize the reducing atmospheres. Depending on the processing route, a metallization in the order of 95% with a carbon content in the range 0.5–4% is obtained. The 75% of DRI or HBI is produced with processes based on natural gas as energy resource, which has to be converted by gas reforming technology to reducing gases (CO and H₂) for the reduction of iron oxide to metallic iron. In the present chapter, the different plant layouts are described. The effect of raw materials properties on the sponge iron quality is underlined. The installation of DRI plants as a function of the natural resources availability is discussed. The DRI/scrap processing in EAFs is described in the chapter. The employment of DRI in EAF steelmaking or in BF operations is described. The most recent solutions regarding the hydrogen reduction by employing reducing agents produced via innovative routes such as water electrolysis are shown. The diffusion of such plants basing on energetic and economic issues is largely discussed in the chapter.

Steel production is a very energy-intensive process, and it requires large amounts of natural resources. In fact, energy costs account for up to 40% of the total cost in some countries. Therefore, optimizing process efficiency is one of the most effective ways to reduce energy consumption and lower costs, with the added benefit of reducing the steel industry’s impact on the environment. Iron and steel industry is the main CO₂ emitter among the most CO₂-intensive industrial sectors. The iron and steel industry accounts for about 19% of final energy use and about a quarter of direct CO₂ emissions from the industry sector. The CO₂ relevance is high due to a large share of coal in the energy mix. Unlike power plants, where CO₂ is emitted from a single source, an integrated steel mill has multiple sources of CO₂. The emissions are located at several stacks and occur from start to end of the iron and steel production. CCS is one of the most open fields for the reduction of greenhouse emissions in primary steelmaking. It is necessary for continuing to use fossil fuels. In the iron and steel industry, CCS faces many uncertainties regarding cost, efficiency, and technology choice. Obviously many solutions are under investigation to capture CO₂ and to store it avoiding its emission in the atmosphere. Selection of capture equipment will depend on factors including CO₂ capture rate, possible requirements for secondary gas treatment, energy consumption, reliability, and operational and capital costs. In the present chapter, the most innovative solutions related to energetic issues and off-gases type are described. The gas utilization depending on the plant section source and composition is underlined. The CO₂ abatement potential and the various solutions costs are indicated.

Electrolysis of iron ore has not been developed in the past because of the energetic balance and energy expenses. In addition, until now, its application in iron production has been hindered due to the difficulty in finding a suitable anode material capable of weathering the challenging conditions. The recent development of this process is motivated by the production of iron metal from an iron oxide containing electrolyte as a carbon neutral approach to replace current pyrometallurgical processes that result in copious amounts of greenhouse gas emissions. Iron ore electrolysis, as well as hydrogen direct reduction, has been recognized as the preferred future steelmaking technology across different perspectives. In the present chapter, the most innovative trends in electrolysis of iron ore such as electrowinning and molten oxide electrolysis are described. Since electrolysis produces no CO₂, it could theoretically be zero-carbon, but only if the electricity needed to power, the process is produced without generating CO₂ emissions. The energy consumption is dependent on the cell configuration, the chemistry of the electrolyte, and the process temperature. Several engineering problems still need to be solved before electrolysis becomes economically viable. This includes the development of a cheap, carbon-free inert anode that is resistant to the corrosive conditions in molten oxide electrolysis. Molten oxide electrolysis (MOE) has been identified by the American Iron and Steel Institute (AISI) as one of four possible breakthrough technologies to alleviate the environmental impact of iron and steel production.