1. Introduction
Steel is the most critical material for engineering and construction, playing a key role in global economic development. In 2022, global crude steel production reached a record high of 1.878 billion tons [
1], and the steel demand is expected to increase by over 33% by 2050 [
2].
A total of around 70% of steel is produced via the blast furnace–basic oxygen furnace (BF-BOF) route [
3], where iron ores are reduced to iron in the BF first and then converted into steel in the BOF [
4]. Because a huge amount of coal is used as the reduced agent and heat source in BFs, the steelmaking industry is energy-intensive and ranks as the industrial sector with the highest CO
2 emissions. According to the statistics published by the International Energy Agency (IEA), the steel industry produces approximately 2.6 billion tons of CO
2 annually, accounting for 6.7% of global direct CO
2 emissions [
5]. The decarbonization of the steelmaking industry is, thus, vital to support and maintain the sustainable development of the future society [
6].
Two main technical pathways have been applied to reduce the CO
2 emissions of the steelmaking industry. The first involves the efficient utilization of waste resources [
7]. For BF–BOF-based steelmaking, blast furnace gas (BFG) is the main waste gas that arises as a byproduct, and it is mainly composed of nitrogen, carbon monoxide, and small amounts of carbon dioxide and hydrogen [
8]. Proper utilization of BFG can significantly improve the efficiency of a whole steelmaking plant and, thus, reduce CO
2 emissions. At present, steel plants are usually equipped with BFG-fired power plants, which can alleviate BFG dissipation and generate energy to compensate for the energy consumption in them [
9]. However, conventional BFG-fired boiler–turbine units are low in efficiency. Therefore, blast-furnace-gas-fired combined-cycle power plant (BFGCC) technology has been promoted in recent years for heat and power generation [
10]. BFGCCs can achieve a high thermal efficiency of 40–45% through the cascaded utilization of energy, nearly doubling the efficiency of conventional BFG-fired boilers [
11].
Many studies have been conducted to improve the operating performance of BFGCCs. Campos et al. [
12] compared the operating performance of a BFGCC and the Rankine cycle. The results showed that the combined-cycle plant exhibited higher efficiency, rated power, and flexibility but with a higher cost and operational complexity. Yao et al. [
13] carried out an economic analysis of a BFGCC with a dual-pressure waste-reheating boiler. The design parameters, such as the pressure ratio, isentropic efficiency, and temperature of the combustion chamber, were optimized to reduce the operating cost. Kirin and Gubarev [
14] optimized the design of a compressor and combustion chamber to enhance the BFG combustion efficiency. A software product that calculates the cycle efficiency of BFGCCs was developed. Zhang et al. [
15] developed a dynamic matrix control to regulate the total power generation and flue gas temperature in the gas turbine of a BFGCC. The calorific value of BFG is used as a feedforward in the system so that the impact of its fluctuation can be quickly rejected. Wu et al. [
11] proposed a coordinated control strategy for an integrated BFGCC–carbon capture process. The re-boiler steam flow rate was flexibly adjusted to enhance the power-load-tracking ability of the plant.
The other pathway to reduce CO
2 emissions is the deployment of renewable energy sources, such as wind turbines and solar panels, to replace conventional fossil-fuel-based power generation. This trend is particularly evident in steelmaking plants that utilize the electric arc furnace (EAF) route for steel production or are considering adopting H
2 direct reduced iron (H
2-DRI) technology. This is because a significant amount of additional electricity is required to melt recycled steel or produce hydrogen [
16].
However, wind and solar sources exhibit intermittent and uncertain characteristics, which have strong impacts on the stable operation of steelmaking plants. The coordination between renewable energy and other adjustable sources is the key to solving this issue. Zhao et al. [
17] proposed installing rooftop photovoltaic (PV) power generation systems in steelmaking plants, evaluating both the capital costs and operating revenue. The results showed that a rooftop PV system can provide 5–10% of the total power consumption in a steelmaking plant, with an investment payback period of 7 years. Xi et al. [
18] developed an intelligent scheduling approach for an energy supply system in a steelmaking plant, which included conventional gas-fired power generation, renewable generation, energy storage, and carbon capture and utilization (CCUS) technologies. Wu et al. [
19] proposed decarbonizing a steelmaking plant by deploying multiple low-carbon technologies. A two-stage low-carbon robust planning approach was proposed to find the appropriate development time and installed capacities of renewable energy, energy storage, hydrogen energy, and CCUS technologies. The results of these studies also pointed out that in the presence of renewable energy, BFGCCs have to change load frequently over a wide range to ease the accommodation of renewable energy or compensate for their insufficiency. Although the relevant control studies indicated that BFGCCs have satisfactory flexibility in load following [
15], the transformation from a base-load energy source to a flexible energy source upgrades their operating challenges [
20], especially in cases where both heat and power demands are required to be met simultaneously.
The uncertainties of renewable generation and load demands should also be considered in the capacity optimization of energy systems. Aliari et al. [
21] proposed a stochastic optimization model with a combination of resource-and-chance-constrained approaches to determine the capacities of several devices, including wind turbines. Li et al. [
22] established a hybrid energy system that included wind turbines, concentrated solar plants, and electric heaters. Two-stage stochastic capacity optimization was carried out to minimize the life cycle cost and the loss of power supply probability of the integrated system. The case study results demonstrated the reliability of the stochastic optimization method. Compared to robust optimization methods that focus on the worst-case condition and may lead to more conservative results, stochastic programming is relatively balanced in most operating conditions and is commonly used in the configuration of the capacity of energy systems.
In recent years, the idea of integrating molten salt heat storage (MSHS) into large-scale combined heat and power (CHP) plants has received much attention. Molten salts have the advantages of strong heat storage capacity, low viscosity, strong fluidity, and stable chemical properties [
23]. By storing a portion of the generated heat in MSHS during valley hours and releasing this heat during peak hours for heat supply or power generation, a CHP plant can effectively achieve heat–power decoupling and, thus, enhance the flexibility of operating performance [
24]. Luo [
25] proposed a molten salt heat storage system for coal-fired cogeneration power plants and analyzed the performance of the system based on the first and second laws of thermodynamics. The results show that the temperature difference between two molten salt tanks has a significant effect on the system design. Li [
26] proposed the extraction of flue gas from a CHP plant for heat storage to improve the load-peaking performance when renewable energy is connected to the grid. The results showed that the coordination between molten salt heat storage and the original boiler–turbine unit upgraded the peak-shaving capabilities of the thermal power unit and improved the response speed during load tracking. Fu et al. [
27] optimized the integration of molten salt energy storage in coal-fired power generation units. Multi-stage heat exchangers and buffer tanks were applied to improve the storage efficiency and reduce the minimum operating load of the plant.
Although the integration schemes and optimization of the operation of MSHS in CHP plants have been extensively studied, the literature has mostly focused on the optimization of the systems themselves. There is still limited research on how to determine the optimal capacity of MSHS and the way it cooperates with renewable sources and other equipment to effectively maximize its value. A comprehensive consideration of external renewable generation and load disturbances, internal operation constraints, and MSHS investment and operation costs is required to carry out a system-level optimization study [
28].
In addition, there have been no studies conducted on the integration of MSHS in BFGCCs. Unlike the steam heat storage used in conventional coal-fired power plants, in a combined-cycle plant, it is easier to apply a direct high-temperature flue gas heat storage system, which is more efficient [
29] than a steam heat storage system. Moreover, BFG holders and MSHS can be jointly deployed to better enhance system flexibility and to reduce BFG dissipation and renewable curtailments [
30].
To this end, this paper proposes the deployment of MSHS in a BFGCC to store heat in the flue gas of a gas turbine so that the coupling between the power and heat supply can be unlocked to enhance the flexibility of the BFGCC. On this basis, a capacity configuration optimization approach is proposed to determine the best installed capacities for a BFG holder, MSHS, wind turbines, and PV panels. The capacity optimization approach aims to minimize an overall objective function that includes the investment cost, operation and maintenance cost, energy purchase cost, and penalty cost for fluctuation in the BFG holder’s position. The power and heat balance constraints, the constraints of the steam turbine’s power–heat characteristics, the operating constraints of other equipment, and initial–final state constraints are fully considered. Moreover, uncertainties in load demand and renewable generation are taken into account to improve the reliability of the capacity configuration optimization.
The innovations of this article are as follows:
- (1)
The integration of a molten salt heat storage system is proposed to improve the flexibility of BFGCCs.
- (2)
Capacity configuration optimization is performed for an integrated BFGCC–MSHS–wind–solar energy system for the decarbonization of steelmaking plants.
- (3)
Uncertainties in renewable generation and load demand are considered to enhance the reliability of the results.
2. System Introduction and Problem Description
A schematic diagram of a BFGCC is illustrated in
Figure 1; it mainly consists of a Mitsubishi M251S single-shaft heavy-duty gas turbine, a natural-circulation dual-pressure waste heat boiler, and a sliding-pressure-operated condensing steam turbine. The BFG generated in the blast furnace is first stored in the BFG holder and then supplied to different production sectors in steelmaking plants, including the BFGCCs. In a BFGCC, the BFG and air are compressed separately and mixed in a combustion chamber for combustion. The high-temperature flue gas that is generated expands in the gas turbine, spinning the turbine blades and generating electricity. The exhaust gas is subsequently used in the waste heat boiler for the generation of steam, which is then sent to a steam turbine to produce additional electricity. Part of the steam is extracted from the outlet of a high-pressure cylinder for heat supply. A detailed plant-wide dynamic model of this BFGCC was developed and validated by Ren et al. [
31]. The key operating parameters under the rated operating conditions are listed in
Table 1.
To meet the increasing energy demand while reducing the CO
2 emissions caused by fossil-fuel-based energy supply, wind turbines and PV panels are deployed near the steelmaking plant. Moreover, an MSHS system and additional BFG holder are integrated into the BFGCC to improve its flexibility, allowing for the better utilization of intermittent and uncertain renewable power. Ternary salt (53% KNO
3, 40% NaNO
2, 7% NaNO
3) is considered the heat storage medium in the MSHS system, and it has the advantages of stable chemical properties, low cost, and high safety. Its melting point temperature is 142 °C, with an upper temperature limit of 535 °C. The specific heat capacity at constant pressure and the density parameters are shown in Equation (1) [
32]:
When the energy generation is in excess, part of the flue gas emitted from the gas turbine flows through the heat exchanger of the heat storage to transfer the heat to the molten salt; in the case of insufficient energy supply, the high-temperature molten salt is used to heat the exhaust steam of the low-pressure cylinder to the inlet temperature, enabling it to be used for heat supply.
The resulting integrated BFGCC–MSHS–wind–solar (BMWS) energy system includes many types of devices with different behaviors. Moreover, the deployment of renewable energy introduces significant uncertainty on the sides of both the source and the load. Therefore, it is essential to determine the optimal capacity configuration of the wind turbine, PV panel, MSHS, and BFG holder according to the weather and load conditions of the plant. This lays a foundation for a stable and economic energy supply.