Waste energy recovery and energy efficiency improvement in China’s iron and steel industry
Introduction
Recently, there has been global concern on the climate change and energy resource depletion. As one of the most significant contributors to fossil fuel consumption and related severe environmental pollution, iron and steel industry in differing countries around the world faced the common challenge that is energy-saving and emission reduction. This can be achieved by improving energy efficiency, as well as by efficient recovery of waste energy. Improving energy efficiency covers numerous energy reducing options including fuel switching, improved process control, and increasing the thermodynamic efficiency of specific production processes [1]. Enhancing waste energy recovery and utilization (WERU) contributes not only to the reduction of primary energy consumption but also to the mitigation of pollutant emissions and the achievement of associated economic benefits [2].
Many industrial processes generate waste energy during manufacturing production processes. The United States Department of Energy (US DOE) [2] stated that as much as 20–50% of primary energy inputs in industry sector is ultimately discharged as waste heat in some forms of heat energy at the temperature range from 1500 °C to near ambient temperature, but the exact quantity of industrial waste heat is poorly quantified. Meanwhile, the lack of standardized methodology and full understanding of technical benefits and cost effectiveness have created barriers against clearly identifying WERU potential. Therefore, the current quantification of waste energy resource and its effects on energy efficiency improvement need to be evaluated.
China’s iron and steel industry (CISI) is an important basic industry of the national economy and is characterized by energy-intensive manufacturing processes. In 2014, China’s crude steel production reached 822 million tonnes, accounting for nearly 49.5% of the world’s crude steel production [3]. The rapid growth of crude steel production has resulted in excess capacity, high energy consumption and severe environmental pollution. The energy consumption of CISI accounts for approximately 15% of the total domestic energy consumption in recent years [4], [5]. However, if small- and medium-sized steel enterprises1 are taken into consideration, then a 10–20% gap of specific energy consumption and a 25–30% higher steel production cost exist compared with international advanced level [6], [7].
To enhance energy conservation and emission reduction, and promote construction of energy-efficient systems, a series of measures and multiple binding targets have been identified in China’s national development strategy plan [6], [8], [9], [10], [11]. For example, the Ministry of Industry and Information Technology (MIIT) issued the 12th Five-Year Development Plan for CISI [8] and the Guidebook of Advanced and Applicable Energy Savings and Emission Reduction Technologies for CISI [9] (hereinafter referred to as ‘the Plan’ and ‘the Guidebook’). The Guidebook provided comprehensive data, such as energy savings, capital costs, and current technology implementation situation, which are crucial in evaluating energy saving potential and cost effectiveness of the selected technologies. The Plan specified the development objectives and the main tasks to reduce comprehensive energy consumption per tonne of crude steel from 605 kg of coal equivalent (kgce)/t in 2010 to 580 kgce/t in 2015; it highlighted the strategic significance of popularizing waste recycling technology, strengthening management and improving efficiency, and supporting development and technological innovation.
Numerous studies evaluated the WERU potential for different sectors in different countries. Through a bottom-up approach, the US DOE [2] evaluated waste heat quantity, quality, current recovery practices, research, development and demonstration (RD&D) needs and barriers for implementing waste energy recovery options in US manufacturing. They found that the work potential of investigated waste heat is approximately 600 TBtu/year, but roughly 60% of unrecovered waste heat is of low quality and has poor thermal and economic efficiency. Lu et al. [12] presented that the practical waste heat potential in CISI is 1 GW. However, these studies focused solely on exhaust gas without considering large amounts of sensible heat (SH) from product and slag. In accordance with the study scale, data collection, and chosen approach, Brückner et al. [13], [14] categorized different methods to estimate the waste heat potential of regions and further investigated the potential of industrial waste heat for heating and cooling applications, as well as the technical and economic potentials of heat transformation technologies. Miró et al. [15] proposed a methodology to confirm the reliability and feasibility of data in reference to industrial waste heat quantities from different countries. The results indicated that without a transparent or standardized methodology, China’s technical or practical potential for WERU is unclear. However, these studies did not estimate industry-specific potential, as well as not address any discussion of the potential in China. In this paper, waste energy is considered as all forms of waste heat (SH of industrial exhaust gas, product, melting slag and cooling water) as well as pressure energy, and chemical energy that released from system due to residues.
Oluleye et al. [16] developed a mathematical method to identify WERU potential by considering the temperature and quantity in process sites using a case study of a petroleum refinery. Findings indicated that site energy efficiency increased by 10% when the demand for recovered energy is taken into account; combining technologies into the system increased the waste heat potential. However, the methodology of the applicability for other industrial sectors, regional, or countries has not to be verified. Through a hybrid material and energy flow analysis approach at plant level, Zhang et al. [17] quantified waste heat recovery and carbon emission mitigation potential in CISI and found that the case plant has 4.87 GJ/t crude steel waste heat potential, equal to 26.08% of the total energy consumption; reducing metallurgy gas dissipation and recycling waste heat are the main ways of improving system energy efficiency. However, this paper did not consider the economic conditions or environment constraints. Chen et al. [18] presented that recycling waste energy plays an important role in energy savings and carbon emission reduction, and highlighted that a critical issue for the low efficiency of WERU is the lack of thermodynamic optimization, while this paper’s intention was not to quantify waste heat potential.
Other studies investigated WERU potential from the standpoint of thermodynamic assessment. Utlu [19] created a consistent basis for determining system efficiency and reliability at low-, medium- and high-temperature on the basis of real data from 1990 to 2011 for the Turkish industrial sector. Results indicated that the iron and steel subsector has the highest technical potential for WERU because of the use of high-quality energy resources for high-temperature applications. Ammar et al. [20] addressed the potential for low-grade heat recovery with regard to new incentives and technological advances and found that the benefit of recycling and utilizing low-grade thermal energy is highly dependent on the qualities and properties of the waste streams heat. Shigaki et al. [21] developed a simulation model and applied exergy analysis to optimize steel production and recycling system from various viewpoints. Results indicated that sensible heat (SH) recovery is effective for the improvement of overall energy efficiency in the steelmaking process, and the electric arc furnace (EAF) process is more effective in exergy utilization than the blast furnace and basic oxygen furnace (BF-BOF) process in terms of exergy efficiency, although they have almost the same exergy loss. Cai et al. [22] analyzed WERU potential based on real production data for CISI and found that the current energy consumption of per tonne steel for the investigated large- and medium-sized enterprises is 9.9–17.2% higher than the international advanced level, and the quantity of waste energy resources is 455.1 kgce/t, equal to 13.32 GJ/t crude steel. Therefore, reducing specific energy consumption, improving energy efficiency, and strengthening the waste heat recovery are the main orientation of future energy conservation in CISI.
The aforementioned studies applied the thermodynamics method to analyze WERU rationality and potential, and have increased widespread public awareness about the usefulness and desirability of recycling waste energy. However, little attention has been devoted to evaluating industry-specific potentials or considering the economic conditions and environment constraints, especially the financial parameters of energy-saving technologies (ESTs).
Numerous studies presented quantitative analyses of ESTs for various industrial sectors worldwide. Napp et al. [23] provided a comprehensive overview of the process improvements, technologies and economics for achieving energy saving and reducing emissions in the industrial sector, and addressed the effectiveness of policy instruments to promote the implementation of those technologies and discussed other cost-effective ESTs to achieve further decarbonization. Viklund et al. [24] reviewed different measures for the recovery and utilization of industrial waste heat, and further applied energy systems modeling tool reMIND to quantify the potential for waste energy recovery and to investigate the effect of related technologies on carbon emission mitigation in Gävleborg County in Sweden [25]. Chen et al. [26] applied the TIMES model to analyze future steel demand, scrap consumption, energy consumption, and carbon emissions from 2010 to 2050. Wen et al. [27] constructed an AIM model and Lin et al. [28] used a multivariate regression model combined with risk analysis to evaluate the future energy saving potential and carbon emission reduction in CISI. With the use of a bottom-up model, Fleiter et al. [29] assessed 17 ESTs to improve energy efficiency in the German pulp and paper industry up to 2035. Moya et al. [30] investigated the cost effectiveness of the best available technologies and innovative technologies in the EU27 iron and steel industry up to 2030 under different payback periods. Flues et al. [31] analyzed the effects of EST attributes, energy prices, policy factors, and total steel productions on the reduction in specific energy consumption in the iron and steel industry.
Another significant quantitative analysis method for ESTs is the energy conservation supply curve (ECSC) from both the technical and the economic perspectives. Meier [32] first introduced the ECSC to provide an accounting framework for the treatment of conservation potentials and guidance on predicting the effect of changes in assumptions. Moreover, ECSC has proven to be an excellent tool for establishing energy policy. The consequences of energy conservation policy are described with respect to the marginal energy savings and costs of ESTs. Subsequently, ECSC has been widely applied in energy system analysis and models, especially those by the Lawrence Berkeley National Laboratory. With the use of a bottom-up ECSC model, Worrell et al. [33], [34] assessed the energy saving potential and carbon emission reduction opportunities, as well as productivity benefits of energy efficiency investments in the US iron and steel industry based on the basis of data from numerous historical industrial case studies. Hasanbeigi et al. [35], [36] estimated the energy saving potential and cost effectiveness of different ESTs for the Thai cement and iron and steel industries. Morrow et al. [37] analyzed 22 and 25 applicable ESTs for India’s cement and iron and steel industries, respectively. Kong et al. [38] assessed the technical and economic aspects of energy conservation and analyzed future steel demand and steel scrap consumption. Li et al. [39] estimated the energy saving and cost effectiveness of 41 ESTs that are widely used or popular in CISI. Zhang et al. [40] analyzed the co-benefits of energy efficiency improvement and air pollution abatement in CISI. Ma et al. [41] developed a new evaluation framework to quantify the energy and environmental benefits, and evaluate the cost effectiveness of 36 ESTs under different scenarios.
Many studies have dealt specifically with the techno-economic analysis for different scopes. In the field of renewable energy, Davis [42] and Wright [43] examined biomass for fuel production, and Yang [44] optimized a hybrid solar-wind power generation system using techno-economic analysis. Techno-economic analysis was also conducted on a mixed biogas, solar, and (or) ground source heat pump system to investigate technical feasibility of greenhouse heating [45] or economic performance of space heating [46]. For the power plant, techno-economic analysis and optimization of the heat recovery system using boiler flue gas were carried out based on the principles of thermodynamic, heat transfer, and hydrodynamics [47]; and the economic feasibility of three different configurations of a woodchips power plant via circulating fluidized-bed gasification were performed [48]. Moving beyond equipment and to systems optimization, technically feasible energy saving potentials and associated costs of implementation of energy efficiency measure for industrial coal-fired steam systems in China were quantified [49]. Merei [50] presented techno-economic analysis and sensitivity analysis of PV-battery system to demonstrate the influence of battery storage to reduce electricity cost. From the perspective of environmental performance, Bellqvist [51] applied a process integration approach to evaluate the potential benefits of energy- and cost-saving, and CO2 mitigation, with considering the low-temperature waste heat recovery technologies of an integrated steel plant in Sweden. Through techno-economic system analysis, Fischedick [52] analyzed the technical and economical long-term potential for viable greenhouse gas emission reduction of innovative primary steel production technologies in Germany up to 2100. Up to now, the previous studies focused solely on a few sectors in China, little attention has been devoted to evaluating energy-saving potential technically and economically, especially insufficient understanding of the benefits and cost effectiveness of waste recycling technologies for CISI.
These studies conducted in-depth quantitative analyses by considering the economic factors of the ESTs and emphasized the importance of energy efficiency improvement for energy savings. However, other forms of waste energy, such as SH of product, melting slag, and cooling water as well as pressure energy and chemical energy, were rarely mentioned or quantified. Furthermore, little attention was paid to the influence of technology promotion on energy saving potential and energy consumption reduction. As a result of these shortcomings, fully understanding WERU and energy efficiency improvement via technology promotion is difficult for decision makers.
This paper aims to fill the research gaps by identifying WERU potential in the iron and steel industry (A case study in China) through a comprehensive and practical methodology, which involves the creation of an innovative techno-economic model that links theoretical, technical, and economic potential with the characteristics of waste energy resources and waste recycling technologies. The energy benefits and cost effectiveness of implementing energy-saving technology that can be implemented to improve energy efficiency are evaluated based on the quantitative approach of conservation supply curve and technical attributes analysis. Specifically, in addition to focusing on the common objective of industrial exhaust gas, this paper focuses on other forms of waste energy, such as sensible heat (SH), pressure energy, and chemical energy in CISI. The methodology, which considers the synergistic effect of technology promotion and structure adjustment on energy efficiency improvement as well as other forms of waste energy that are often neglected, could be further applied to other energy-intensive industries or countries.
The remainder of this paper is organized as follows: Section 2 briefly introduces iron and steelmaking processes, defines waste energy resources, and describes key waste recycling technology. Section 3 addresses the research methods used in this paper and sets four scenarios to evaluate the energy saving potential and energy consumption reduction associated with future ESTs promotion. Section 4 presents the results and key findings of the techno-economic model. An overall discussion and recommendation are presented in Section 5. Section 6 concludes this paper.
Section snippets
Brief introduction of iron and steelmaking processes
In China, two main routes are used in the production of crude steel: blast furnace and basic oxygen furnace (BF-BOF) process, which uses primarily iron ore; and electric arc furnace (EAF) process, which uses scrap as raw material. The BF-BOF route is the dominant steel production method in CISI and accounts for approximately 90% of the total crude steel production, which is higher than the worldwide level [17], [53]. EAF steel production route is also commonly used throughout the world,
Methodology
General speaking, three different kinds of potentials for WERU should be distinguished: the theoretical potential, the technical potential and the economically feasible potential [13], [57]. To quantify the energy saving potential and the energy benefits of implementing technology, as well as to evaluate the factor impacts on cost effectiveness in CISI, a novel approach was developed, which involves an techno-economic model that includes thermodynamic assessment, energy consumption, and ECSC.
Thermodynamic, technical, and economic analysis for WERU in CISI
Fig. 3 shows the results of the thermodynamics assessment of waste energy resources in CISI under theoretical and practical considerations. Approximately 62% of the total waste energy potentials come from the iron-making, rolling, and steelmaking (BOF) processes. The technical potentials account for 42.5% and 56.3% of the theoretical potential under the ambient and modified reference temperatures, respectively, and the remaining potentials are 3.76 GJ/t and 2.16 GJ/t crude steel, accordingly. The
Discussion and recommendations
This study provides a comprehensive and detailed assessment of the waste energy resource and related recovery technology from the perspectives of thermodynamic, technical and economic potentials. In this section, efficient measures to recover and utilize waste energy need to be considered, and factors, such as discount rate, energy price, capital cost, and subsidy, will be emphatically discussed based on the sensitivity analysis. Subsequently, the research limitations will be discussed, and
Conclusions
Iron and steel industry is one of the most energy intensive manufacturing industries. It consumes large amounts of primary energy as waste energy. The literature review shows that the exact quantity of waste energy is poorly quantified in practice. The Chinese government has released a series of measures and multiple binding targets to improve energy efficiency and enhance energy conservation. However, the technical potentials for waste energy recovery and utilization (WERU) have not been
Acknowledgments
This work was supported by the National Key Research and Development program (2016YFB060130503, 2016YFB060130103) and National Key Technology Research and Development Program (2015BAB18B00). The authors gratefully acknowledge the reviewers and editors for their fruitful comments.
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