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To solve the unreasonable energy distribution and excessive energy waste in the steam system of iron and steel (I&S) industry, a mixed integer linear programming model based on the theory of exergy and energy level is developed in this study to optimize the steam system. After optimization, the utilization potential of waste energy is found. Then, several waste energy recovery technologies are compared according to their application results. The results show that the exergy efficiency of steam system is 74.065%. After optimization, the exergy efficiency of steam system is increased by 10.568%, the energy input of the I&S enterprise is reduced by 84.30 MJ t−1‐CS−1, and waste energy potential is found at hot rolling, sintering, and converter. Herein, flue gas waste heat from hot rolling and sintering is recovered using series two‐stage Organic Rankine Cycle with cyclopentane as the working fluid, and steam waste energy from converter is recovered using saturated steam power generation. These waste energy technologies increase the electricity generation of I&S enterprise by 15.25 kWh t−1‐CS−1. Overall, combining the optimization model with the use of waste energy recovery technologies is necessary and effective for I&S enterprise to increase their energy efficiency. [ABSTRACT FROM AUTHOR]

[Display omitted] • Exergy loss of enterprise is optimized by building mathematical programming model. • Influences of 9 key factors on system's total exergy loss and cost are discussed. • Recovery and utilization of waste heat are optimized by detailed technical models. • Waste heat potential and heating capacity of enterprise are evaluated. • Interface loss between upstream & downstream processes is quantified by exergy. A substantial quantity of waste heat resources generated in iron and steel manufacturing process (ISMP) can fulfill certain demand for urban heating. However, the waste heat utilization level of different iron and steel enterprises varies a lot and few studies have combined the upstream enterprise's production with downstream waste heat utilization for discussion. By establishing an optimization model based on material balance, thermodynamic laws and reaction mechanism of the ISMP, this study firstly obtain the minimum exergy loss and the corresponding production cost ＆ exergy efficiency. The total exergy loss of ISMP is reduced from 8413.19 MJ/t-MS to 7669.27 MJ/t-MS. The exergy efficiency of the whole process is increased from 63.75 % to 65.46 % and the total waste heat potential reaches 1471.25 MJ/t-MS. It is found that coke ratio, air leakage rate, scrap steel ratio, etc. have great impact on exergy loss of ISMP. Then, the optimization and evaluation of the waste heat utilization are carried out by establishing models with taking urban heating into consideration. The available external heat supply is 813.15 to 853.55 MJ/t-MS and the enterprise's heating load can reach 229.23–251.47 MW, which allow to supply heat for an area of 5.46–5.93 million m2. Finally, the problem of waste heat instability in the heating process is briefly discussed and some solution strategies are proposed to overcome this defect. [ABSTRACT FROM AUTHOR]

The traditional iron and steel industry (ISI) began to implement technological innovations aimed at lowering carbon footprint and emissions, resulting in hydrogen metallurgy gradually becoming a new trend of green development in ISI. However, there is a lack of comprehensive and in-depth analysis of energy-exergy based on the second law of thermodynamics in the current research of hydrogen metallurgy. To address these issues, a numerical calculation model of hydrogen metallurgy based on material-energy-exergy is established. Two novel evaluation indexes are introduced to better evaluate the impact of various parameters on hydrogen metallurgy. It was found that the exergy intensity is mainly affected by the amount of reducing gas, and increasing the preheating temperature of the ore cannot always reduce the exergy intensity due to the influence of element conservation. And when the ore preheating temperature is higher than 623 K, the exergy utilization efficiency decreases. After calculation, the lowest exergy intensity is 1.23 × 107 kJ/t-DRI when the H 2 content is 41.37% and the ore temperature is 773 K. Furthermore, a simple process is designed to recover a substantial volume of unreacted gas from the furnace top gas. Overall, this study can serve as a reference for actual hydrogen metallurgy production. [Display omitted] • A material-energy-exergy model based on the second law of thermodynamics is created. • In addition to the traditional evaluation indexes, two novel ones are also introduced. • Preheating ore temperature above 623 K reduces the exergy utilization efficiency. • Rising ore preheating temperature does not decrease exergy intensity continuously. • A simple waste energy recovery process is proposed. [ABSTRACT FROM AUTHOR]