According to Mining News Pro - As the second largest energy user in the global industrial sectors [1], the iron and steel industry is highly dependent on fossil fuels [2] and releases massive amounts of environmentally harmful substances [3]. With rapid urbanization and industrialization, the demand for steel has increased over the last several decades [4]. Crude steel production reached 1870 Mt globally for the year of 2019, with energy intensity of 20 GJ per tonne of crude steel [5], fresh water intensity of 3.3 m3 per tonne of crude steel [6], and a CO2 emission intensity of 1.9 t per tonne of crude steel [7]. Previous studies have concluded that the increasing output of crude steel is the most important factor leading to the remarkable increase in the total energy consumption and environmental emissions of the iron and steel industry. By contrast, decreasing the energy intensity (i.e. specific energy consumption [8]) is the most important factor that reduces gross energy consumption and emissions [9], [10]. Energy efficiency improvement or energy conservation are the most controllable factors that influence the energy consumption and emissions of the iron and steel industry, and climate change and rising energy prices further increase their importance [11]. However, the opportunity to achieve energy savings becomes narrower and narrower after decades of hard work by the iron and steel community [12].With the rapid development of global informatization, modern steelworks are equipped with the enterprise resource planning (ERP) systems and manufacturing execution systems (MESs), meeting the basic hardware requirements for fast information transmission of material flow and networked energy flow management [13]. The current computer integrated manufacturing systems (CIMSs) have great field data acquisition and monitoring capability, which enables the integration of material and energy flows. However, the efficient, optimal, and smart scheduling and the synergistic interaction of material flow and energy flow have not been fully implemented yet. Thus, it is essential to figure out the operating rules of material and energy flows and achieve the dynamic optimization and synergistic operation of them.
There have been many studies conducted on integrating material and energy flows in the iron and steel industry in recent years. In order to identify challenges and solutions to reduce energy intensity and cost of the iron and steel industry, it is necessary to conduct a systematic literature review of material and energy flows in steelworks. The literature review in this paper aims to compile the relevant contributions from previous publications. The review covers journal articles and conference papers from the publication databases of ScienceDirect, Springer, Taylor & Francis, CNKI, Wiley Online and IEEE Xplore, as well as white papers and industrial reports. This paper attempts to provide a timely and comprehensive review of the material and energy flows of the iron and steel industry with no limitation on the publication years. More specifically, the contributions of this review paper are as follows:
First, an overview of the steel production routes is provided by summarizing different route architectures. In addition, the material and energy flows and the dynamic operation of the steel production process are briefly introduced. These contents are presented in Section 2 to provide readers with an in-depth understanding of steel production processes.
Next, the status quo of the practice and research on the material and energy flows of the iron and steel industry is presented. Many journal articles, conference papers, white papers and industrial reports have been reviewed here. The selected publications are divided into three categories, namely, (1) material flow and material flow scheduling, (2) energy flow and the energy flow network, and (3) the interrelation between material and energy flows. For each of these categories, we discuss the principles, technologies, mathematical models, as well as the potential problems that must be overcome for future development. The present review differs substantially from the existing ones, classifying the initiatives according to their specific models/algorithms and technical characteristics. To our knowledge, this paper is the first systematic review of material and energy flows in the iron and steel sector, based on the state-of-the-art studies that have been conducted so far in this area.
Finally, an in-depth discussion of the challenges of material and energy flows is conducted. Insights about where integrated material and energy flow research is heading are provided, with the energy-related issues for the iron and steel industry discussed.
2. Steel production routes
Classification of steel production routes
Steel is produced via the following two main routes, which are characterized by the type of raw material and energy consumed.
The blast furnace–basic oxygen furnace (BF–BOF) route. About 75% of steel in the world is produced by using the BF–BOF route [14], in which iron ores are reduced to iron, also called pig iron or hot metal, in the BFs. Then, the iron is converted to steel in the BOFs. For the BF–BOF route as shown in the left-hand side of Fig. 1, the material inputs are predominantly iron ores and the energy inputs are coal and electricity [15]. The steel is produced with several processing steps, including coking, sintering, pelletizing, ironmaking, primary and secondary steelmaking, casting, and hot rolling [16]. These processes are generally followed by various fabrication processes, including cold rolling, forming, forging, joining, machining, coating, and heat treatment [17]. Finally, the steel is delivered as coils, plates, sections, or bars.
Another steelmaking technology using hot metal from BFs as the main material is open hearth furnaces (OHFs). The OHF process is highly energy intensive. Owing to its environmental and economic disadvantages, the OHF process makes up only about 0.4% of global steel production and is still in decline. Therefore, the OHF process is not discussed in this paper.
The electric arc furnace (EAF) route. About 25% of steel in the world is produced via the EAF route [18]. The EAF route produces steel using recycled steel scrap as the major raw material and electricity as the major form of energy. Additives, such as alloys, are used to adjust to the desired chemical composition. Depending on the availability of recycled steel and the plant configuration [19], other sources of metallic iron, such as direct reduced iron (DRI) or hot metal, can also be used in the EAF route [20], as shown in the right-hand side of Fig. 1. Downstream processes, such as casting, reheating and rolling, are similar to those in the BF–BOF route.
Variations and combinations of production routes also exist. Casting iron is sometimes produced in the BFs without being sent to the BOFs [21]. In addition, most steel products will remain in service for decades before they are recycled, resulting in the fact that current recycled steel scrap is not enough to meet the growing demand for steel production if the EAF route is used alone. Therefore, a combination of the BF–BOF and EAF routes is usually used [22]. For example, hot metal from BFs can also serve as an input of EAFs.
The iron and steel industry is facing challenges because it would like to achieve multiple objectives at the same time. The objectives include maintaining high product quality, boosting productivity, reducing business costs, reducing energy consumption, and mitigating environmental emissions. To recognize and overcome these challenges, the integration of material and energy flows should be put forward as a crucial concern [18], [23], rather than treating the material flow and energy flow separately.
Material and energy flows in steelworks
In this paper, the term flow refers to any dynamic variation of material and energy with time. As shown in Fig. 2, material flows present the dynamic movement and transformation of iron-bearing materials [23], including iron ores, steel scrap, hot metal, liquid steel, cast slabs, finished steel, etc. Energy flows include coke, coal, blast furnace gas (BFG), coke oven gas (COG), Linz–Donawitz converter gas (LDG) or BOF gas, power, water, steam, waste heat, compressed air, etc. [18], [24]. In the iron and steel production processes, energy flows serve as drivers, reaction agents, and thermal media to process material flows efficiently, economically, and sustainably.
Dynamic operation of the steel production process
Complicated iron and steel production processes can be simplified to processes of the input-output of material flows, the input-output of energy flows, and the interaction of material and energy flows [23]. The essence of the iron and steel production processes has been revealed as dynamically ordered displacement and conversion of material and energy flows under designed process networks. These designed networks involve the material flow network and energy flow network in the steel production process, respectively [26].
The concept of an energy flow network in the steel production process has been widely accepted and actively promoted. An energy system is a complex system which contains the conversion and transfer of various energy forms [27]. To achieve the networked management of different forms of energy, an IDDD + N principle (i.e., Integration of the processes, Differentiation of the demand, Diversification of the supply, Decentralization of the grid, and Network of multi-energy flows) was proposed for achieving systemic energy conservation and a synergistic energy system [28].
Two major international conferences (‘The 2nd European Steel Technology and Application Days (ESTAD)’ held in 2015 in Düsseldorf, Germany [29] and ‘The 148th TMS Annual Meeting’ held in 2019 in Texas, USA [30]) indicated that the dynamic optimization and synergistic operation of material and energy flows would give birth to a new round of energy-saving technology innovations in the iron and steel industry.
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