Advanced Utilization Technologies of Secondary Energy and Resources from Energy-Intensive Industries

Energy-intensive industries (EIIs) refer to industries that are highly dependent on energy (fuel), which are primarily responsible for industrial energy consumption. The typical sectors include iron and steel metallurgy, ceramics, glass, cement, coal chemicals, and electric power. These EIIs are of great significance to economic development and are closely related to people’s daily life. However, because of high levels of energy consumption and CO₂ emissions, EIIs have become the main focus of energy conservation, emission reduction, and green sustainable development strategies. Especially, it is generally believed that a huge mass of high-temperature substances produced in the industrial production process has great potential utilization value. These substances include some high-temperature waste liquids; waste solids; and exhaust gases, such as molten or hot slags, hot flue gases and effluents, and coal fly ash, which are also considered important secondary energy or resources. A fair number of advanced technologies have been developed for the utilization of energy and resources. These technologies improve cleanliness and the energy and resource efficiency of EIIs, contributing greatly to the global climate and environmental strategies. In this Special Issue entitled “Advanced utilization technologies of secondary energy and resources from energy-intensive industries”, we provide a comprehensive view of these utilization technologies in terms of scheme, method, and system, including the conceptual design and specific pivotal details.

This Special Issue on “Advanced utilization technologies of secondary energy and resources from energy-intensive industries” includes nine articles whose themes cover secondary energy and resources in a new type of steelmaking slag-cement composite cementing material [1], a green steel slag-based castable [2], simulation and characteristics of fly ash and slag in a waste incinerator [3], assessment of the synergy between recycling and thermal treatments of solid waste [4], a CO₂-derived working fluid [5] and design of novel supercritical CO₂ power cycles [6] for high-temperature heat recovery, optimal design of a novel two-tank latent and metal hydrides-based thermal energy storage system [7] and the impact of active and passive thermal management on this type of system [8], and finally, the development of decision support system of latent heat storage [9].

The following is a brief summary of the content of each paper selected for this Special Issue:

Weng et al. [1] focused on a type of slag-cement composite cementitious material (SCCCM) and built a mathematical model of hydration expansion. The slag used came from the process of converter steelmaking. The authors established the quantitative relationship between expansion properties and stability of SCCCM and hence established the model. The model can be used to calculate the maximum slag addition in SCCCM, which is of great significance to the large-scale use of metallurgical slag in cement.

Bharati et al. [2] developed a sustainable product of castable for refractory applications from steelmaking slag. This product features low cost and low CO₂ emissions because of the 100% replacement of limestone by slag. This paper also presented technology for using 100% of the process waste of the iron and steel metallurgical industry to prepare the castable batch, which showed excellent economic and environmental benefits.

Luo et al. [3] adopted computational fluid dynamics methods to investigate the combustion process of municipal solid waste in a “V-type” incinerator. Furthermore, the authors analyzed the characteristics of the main secondary products, namely, the waste incineration fly ash and slag. They focused on the macroscopic and microscopic morphology, chemical composition, and environment-related characteristics of these secondary resources. The work provided a theoretical basis for utilizing fly ash and slag from municipal solid waste incineration plants. As the author mentioned, the research data also provided a certain reference for the comprehensive utilization of municipal solid waste (MSW).

Furthermore, Abis et al. [4] assessed the synergy between recycling and thermal treatment technologies of MSW based on the data of management in EU-28 in 2018, which were the most recently available in Eurostat. The authors proposed the necessity of optimization of recovery of secondary resources, including the incineration bottom ash, metals, and nonmetallic slag components. They emphasized that the evolutionary direction of the whole MSW sector should be an equilibrium between recycling operations and thermal treatments. Additionally, the sustainable MSW management system can be applied to different types of secondary raw materials from industrial production.

Some novel technologies for waste heat recovery have been developed for higher energy efficiency.

Ayub et al. [5] analyzed the possibility of adopting CO₂-derived mixtures as a working fluid to recover the waste heat of flue gases. The authors conducted a performance comparison of supercritical CO₂ and organic Rankine cycles at the thermodynamic level based on a simple configuration of a power plant. Eight working fluids of molecular complexity were used to evaluate the cycle expansion ratios, heat transfer, and thermal efficiency.

The research of Manente et al. [6] focused on the design of a supercritical CO₂ power cycle to utilize medium and high-temperature waste heat sources. The novel cycle was proposed to ensure high heat extraction and thermal efficiency. The basis of design is the superimposition of elementary thermodynamic cycles, and through systematic evaluation of thermal efficiency and heat recovery, the novel layouts showed high performance, and the related mechanism was revealed. This work provided critical guidance for the design of efficient CO₂ power cycles for the recovery of secondary energy.

In the work of Nyamsi et al. [7], a two-tank latent heat storage system for solar power plants or industrial waste heat recovery was investigated through a two-dimensional mathematical model. The system was characterized by a phase change material used to store and restore the high-temperature waste heat. Furthermore, this study adopted a multi-objective optimization method based on a genetic algorithm to optimize the thermal energy storage systems, and the system obtained an excellent performance with large power density, energy density, and storage efficiency. This study contributes to the development of phase change materials for heat storage.

Then, Nyamsi et al. [8] investigated the effects of thermal management techniques on the performance of the heat storage system. In detail, the management techniques can be divided into two categories, namely, active and passive heat-transfer enhancements. The performance studied included energy storage density, storage efficiency, and specific power density. Based on the established two-dimensional mathematical model, the authors studied the impact of these variables of convective heat transfer, thermal conductivity improvement, and phase change material. This work filled the gap in the relationship between thermal management techniques and the system’s performance.

Finally, Royo et al. [9] developed a MATLAB model to select the phase change material for latent heat thermal energy storage systems. The selection depended on the most relevant system parameters. The model database was built using the production data of an industrial plant. The main inputs of the model consisted of the inlet temperature and flow of waste heat and combustion air. The outputs covered the technical, economic, and environmental information of the thermal energy storage system. This tool can be flexibly applied at the industrial scale with high accuracy and reliability.

The papers mentioned are only some typical examples related to this Special Issue. Many relevant achievements have been made in the fields of energy, resources, economy, environment, etc. Nonetheless, it is significant to develop more efficient, cleaner, and low-cost technologies. Because the secondary products (hot liquids, solids, and gases) from EIIs can often be regarded as both energy and resources, we encourage the development of combined technologies that can simultaneously recover waste heat and components. Such technologies should focus on the balance between the two to achieve optimal overall benefit. For example, for high-temperature inorganic, nonmetallic liquids and solids (molten slag and hot solid slag), suitable recovery routes should be designed to ensure that the cold slag can be recycled into the construction materials while heat extraction and thermal efficiency are as high as possible. For hot exhaust gases, heat recovery can be coupled with the utilization of gases in chemical processes. We believe that these technologies can greatly contribute to energy conservation and emission reduction in EIIs.