EXERGY-BASED ENERGY EFFICIENCY ASSESSMENT AND OPTIMIZATION OF INDUSTRIAL VACUUM DEGASSING OF LOW-CARBON STEEL

УДК 669.046.554

Abstract

The steel industry accounts for 7–9% of global anthropogenic CO₂ emissions, which makes the energy efficiency of individual process stages a central concern of modern metallurgy. This study presents an exergy-based assessment and optimization of industrial vacuum degassing (VD) of low-carbon steel performed on a 60-tonne ladle degasser operating in the electric arc furnace – ladle furnace – vacuum degasser route at Baku Steel Company (Azerbaijan). A combined methodology was applied: first-order kinetic modeling of nitrogen removal, heat-balance calculations and exergy-balance analysis of the treatment cycle, verified against plant measurements of chamber pressure, melt temperature, argon flow and electric energy consumption. The results show that deepening the vacuum from 10 mbar to 1 mbar at 1600 °C raises the hydrogen removal efficiency from 82 to 94% and the nitrogen removal efficiency from 73 to 84%, while the specific energy consumption decreases from 63 to 55 kWh per tonne (about 14%). The exergy efficiency of the VD stage increases from 72 to 82%, and the combined optimization of vacuum depth, temperature regime and argon circulation yields a 12–18% reduction in total energy demand. For an annual program of 950 heats this corresponds to about 475 MWh of saved electricity and roughly 238 tonnes of avoided CO₂ emissions. Recommendations for energy-efficient VD control are formulated.

Аннотация

Черная металлургия обеспечивает 7–9 % мировых антропогенных выбросов CO₂, что делает энергоэффективность отдельных технологических стадий центральной задачей современной металлургии. В работе представлены эксергетическая оценка и оптимизация промышленной вакуумной дегазации (ВД) низкоуглеродистой стали, выполненные на 60-тонном ковшовом вакууматоре, работающем по маршруту «дуговая сталеплавильная печь – печь-ковш – вакууматор» на предприятии Baku Steel Company (Азербайджан). Применена комбинированная методика: кинетическое моделирование удаления азота (модель первого порядка), расчеты теплового баланса и эксергетический анализ цикла обработки, верифицированные по заводским измерениям давления в камере, температуры расплава, расхода аргона и потребления электроэнергии. Показано, что углубление вакуума с 10 до 1 мбар при 1600 °C повышает эффективность удаления водорода с 82 до 94 %, азота — с 73 до 84 % при снижении удельного энергопотребления с 63 до 55 кВт·ч/т (около 14 %). Эксергетический КПД стадии ВД возрастает с 72 до 82 %, а совместная оптимизация глубины вакуума, температурного режима и циркуляции аргона обеспечивает снижение общего энергопотребления на 12–18 %. Для годовой программы 950 плавок это соответствует экономии около 475 МВт·ч электроэнергии и предотвращению около 238 т выбросов CO₂. Сформулированы рекомендации по энергоэффективному управлению ВД.

Keywords: vacuum degassing; low-carbon steel; exergy analysis; energy efficiency; nitrogen removal; secondary metallurgy; green metallurgy.

Ключевые слова: вакуумная дегазация; низкоуглеродистая сталь; эксергетический анализ; энергоэффективность; удаление азота; внепечная обработка; зеленая металлургия.

Introduction

Steel remains the basic structural material of the world economy, and its production is responsible for roughly 7–9% of global anthropogenic CO₂ emissions, with an average intensity of about 1.9 t CO₂ per tonne of crude steel [17]. The technology roadmap of the International Energy Agency therefore identifies the improvement of process-stage energy efficiency as one of the fastest and least capital-intensive decarbonization levers available to steelmakers [8]. Within the electric arc furnace (EAF) route, secondary metallurgy — ladle furnace (LF) refining and vacuum degassing (VD) — consumes a considerable share of the electric energy used downstream of melting, so its optimization directly affects both product quality and plant economics [3; 4].

Vacuum degassing removes dissolved hydrogen and nitrogen from liquid steel and thereby prevents hydrogen-induced cracking, flakes and nitride embrittlement in the final product [6; 7]. The metallurgical fundamentals of the process — Sieverts' equilibrium, bubble nucleation and growth, and circulation-driven mass transfer — have been studied in detail [10; 12; 20], and recent plant-scale studies confirm the decisive influence of vacuum depth and stirring intensity on degassing efficiency [13; 14; 18; 19]. Most published work, however, concentrates on the metallurgical outcome (final gas contents, steel cleanliness), while energy consumption is treated only as a secondary constraint. Exergy analysis, which quantifies the thermodynamically usable part of energy flows and locates irreversibilities, is well established for EAF melting [3] and for steel plants as a whole [4; 5; 15], but a systematic coupling of reaction kinetics with an exergy balance at the level of an individual industrial VD unit has received little attention.

The aim of the present work is to close this gap: to quantify the exergy efficiency of an industrial 60-tonne VD unit, to identify the main sources of exergy losses and destruction, and to determine the operating parameters (vacuum depth, temperature regime, argon circulation) that minimize specific energy consumption without sacrificing degassing quality. The kinetic basis of the analysis relies on the rate constants determined earlier by the authors for the same unit [1; 2]; the present article extends that work with a complete exergy balance, a heat-balance sensitivity analysis and industrial validation of the resulting recommendations.

Materials and Methods

Industrial trials were carried out at Baku Steel Company (Azerbaijan) on a 60-tonne tank-type ladle degasser operating in the EAF–LF–VD sequence during the production of low-carbon structural steel. The melt temperature was maintained within 1580–1620 °C, the chamber pressure was varied from 10 mbar (baseline practice) down to 1 mbar (deep-vacuum practice), argon was injected through porous plugs at 8–14 m³/h, and the total treatment time was 30–45 min. Steel samples for gas analysis were taken before treatment and after 10, 20, 30, 40 and 45 min of processing. Hydrogen and nitrogen contents were determined on a LECO TCH-600 gas analyzer; the temperature was monitored with calibrated immersion thermocouples (±5 °C), the chamber pressure with digital vacuum sensors, and the electric energy consumption with the plant power meter. Classical descriptions of vacuum treatment practice were used as the methodological baseline [9; 11].

The equilibrium solubility of diatomic gases in liquid steel obeys Sieverts' law:

C = K_s·√P_gas                                           (1)

where C is the equilibrium concentration of the dissolved gas, P_gas is its partial pressure above the melt, and K_s is the temperature-dependent solubility constant [7; 12]. The removal kinetics were described by a first-order mass-transfer model:

dC/dt = −k_m(A/V)(C − C*)                       (2)

where k_m is the mass-transfer coefficient, A/V is the specific reaction surface, and C* is the equilibrium concentration corresponding to the instantaneous chamber pressure. For engineering analysis, the combined pressure–temperature dependence of the process was approximated as

C = k·Pⁿ·exp(−E/RT)                              (3)

with an apparent activation energy of E ≈ 45 kJ/mol determined for the studied unit [2]. The rate constants were obtained by Levenberg–Marquardt regression of triplicate measurement series.

The heat balance of the treatment cycle was written as

Q_input = m·c_p·ΔT + Q_loss                           (4)

where m is the heat mass, c_p is the specific heat of liquid steel, ΔT is the temperature change during treatment, and Q_loss combines radiation and convection losses and heat transfer into the ladle lining. The exergy balance of the VD stage was formulated following the classical methodology [5; 15]:

Ex_in = Ex_out + Ex_loss + Ex_dest                      (5)

and the exergy efficiency was calculated as

η_ex = (Ex_out/Ex_in)·100%                            (6)

To compare operating regimes, an integral process efficiency indicator was used:

η_proc = (k_N·Δ[N])/(E_sp·t)                           (7)

which relates the achieved nitrogen removal Δ[N] and the rate constant k_N to the specific energy consumption E_sp and the treatment time t.

The kinetic rate constants used as inputs to the energy model (Table 1) were determined in the authors' previous study of the same unit [2]; the exergy balance, the sensitivity analysis and the industrial energy validation presented below constitute the new contribution of this article.

Results and Discussion

Table 1 summarizes the nitrogen removal kinetics of the studied degasser for three vacuum stability regimes [2]. The differences between the regimes are statistically significant (F = 12.4; p < 0.01), with a large effect size (Cohen's d = 1.12 between the stable and unstable regimes). A stable deep vacuum (below 5 mbar with pressure fluctuations Δp/Δt ≤ 0.5 mbar/s) accelerates nitrogen removal by a factor of 1.8 and lowers the final nitrogen content by about 60% compared with unstable operation. Alternative kinetic descriptions (diffusion-limited and mixed-order models) gave R² = 0.86–0.91 and were inferior to the first-order fit; this agrees with the surface-reaction-controlled mechanism of nitrogen removal reported for industrial sulfur and oxygen levels [10; 19; 20]. Figure 1 visualizes the rate constants and the final nitrogen contents.

Table 1. Nitrogen removal kinetics of the 60-t VD unit for different vacuum stability regimes [2]

Figure 1. Nitrogen removal rate constant and final nitrogen content for three vacuum stability regimes of the 60-t VD unit (error bars — 95% confidence intervals)

The heat-balance sensitivity analysis shows that the two dominant control variables act on different parts of the energy budget. A ±20% change in chamber pressure alters the energy demand of the vacuum pumping system by about 10%, whereas a ±50 °C change in melt temperature changes the degassing efficiency by about 6% [1]. Excess superheat is particularly costly: raising the temperature above 1650 °C increases the removal rate by less than 3%, while the heat demand grows by about 10%. As an illustrative magnitude, lowering the superheat of a 60-t heat from 1670 to 1600 °C (c_p = 0.75 kJ/(kg·K)) is equivalent to ΔQ ≈ 3.15 GJ (about 0.88 MWh) per heat. Table 2 summarizes the contributions of the individual optimization components; their combined effect reaches 12–18% of the total energy demand of the treatment cycle.

Table 2. Contributions of the optimization components to the energy saving of the VD treatment cycle

The recommendations were validated in industrial trials in which the baseline practice (10 mbar) was compared with the optimized deep-vacuum practice (1 mbar) at the same target temperature of 1600 °C (Table 3). Deepening the vacuum raised the hydrogen removal efficiency from 82 to 94% and the nitrogen removal efficiency from 73 to 84%, while the energy consumption of the treatment cycle (LF–VD scope, including ladle reheating) decreased from 3.8 to 3.3 MWh per heat, i.e. from 63 to 55 kWh/t — a saving of about 14%, which agrees with the model prediction of about 17% within the measurement uncertainty; the deviation between the model and the plant data did not exceed 2%. It should be noted that this saving of ≈ 0.5 MWh per heat (≈ 8 kWh/t) refers to the vacuum-depth optimization alone at constant temperature, whereas the broader figure of 15–18 kWh/t characterizes the total difference between the stable and unstable operating regimes, including the shortening of the treatment time and the stirring effects. Scaled to the annual program of about 950 heats, the validated saving corresponds to approximately 475 MWh of electricity, about 38,000 USD at an industrial tariff of 0.08 USD/kWh, and roughly 238 t of avoided CO₂ emissions at a grid factor of 0.5 kg CO₂/kWh. Comparable hydrogen removal efficiencies at deep vacuum have been reported for industrial degassers of other plants [16]; the present combination of kinetic and energy data extends such observations with an explicit exergy assessment.

Table 3. Industrial validation of the optimized regime (60-t heats, 1600 °C)

In the baseline regime the exergy efficiency of the VD stage is η_ex = 72%: about 18% of the input exergy is lost with heat flows (mainly through the ladle refractory lining) and about 10% is destroyed by internal irreversibilities — friction in the pump and pipeline system, throttling, non-uniform argon dispersion and local deviations from equilibrium at the gas–metal interface, which together account for 25–30% of the destruction term. After optimization η_ex increases to 82% (losses ≈ 10%, destruction ≈ 8%), which corresponds to an exergy saving of about 0.2 GJ per heat; the energy demand of the VD stage proper is 30–40 MJ/t, i.e. 1.8–2.4 GJ per 60-t heat. The exergy analysis also identifies a sustained optimum plateau at a chamber pressure of 5–6 mbar: a deeper vacuum improves the equilibrium driving force according to Sieverts' law but increases the exergy destruction in the pumping system, so the final deepening to about 1 mbar is applied only as a short final stage to meet the hydrogen target. These values are consistent with the exergy efficiencies reported for other units of the steelmaking route [3; 4] and with the classical treatment of metallurgical exergy analysis [15].

In terms of the integral indicator (7), stable deep-vacuum operation raises the process efficiency by 22% relative to the unstable regime; the optimized argon schedule reduces argon consumption by 10–12%, and the treatment time is shortened by 20–25% [2]. A practically important perspective is adaptive closed-loop control of the pressure and argon flow based on real-time pressure and off-gas signals: according to the kinetic model, such control can additionally raise the effective nitrogen removal rate by 15–20%. This direction agrees with the current trend towards model-based and digital-twin control of secondary metallurgy units [13] and does not require capital reconstruction of the degasser — the classical vacuum practice [9; 11] is retained, and only the control level is upgraded.

Conclusion

1. The nitrogen removal kinetics of the industrial 60-t VD unit are adequately described by a first-order model; under a stable deep vacuum (below 5 mbar) the rate constant reaches k_N = 0.021 min⁻¹, which is 1.8 times higher than under unstable operation, and the final nitrogen content decreases to 18 ± 2 ppm.

2. The exergy efficiency of the VD stage increases from 72 to 82% after optimization; the main sources of irreversibility are heat transfer through the ladle lining, friction and throttling in the vacuum system and non-uniform argon dispersion. The rational operating window is a stable plateau at 5–6 mbar with a short final deepening to about 1 mbar.

3. The combined optimization of vacuum depth, temperature regime and argon circulation reduces the energy demand of the treatment cycle by 12–18%; the industrial validation confirmed a decrease from 3.8 to 3.3 MWh per heat (63 → 55 kWh/t, about 14%) with a model–plant deviation below 2%.

4. For an annual program of 950 heats the validated effect corresponds to about 475 MWh of saved electricity, about 38,000 USD and about 238 t of avoided CO₂ emissions, which supports the role of VD optimization in low-carbon (green) steelmaking.

5. The practical recommendations are as follows: maintain a stable vacuum below 5 mbar with the working plateau at 5–6 mbar and a short 1 mbar final stage; avoid superheat above 1650 °C; keep the argon flow at 12–14 m³/h; implement adaptive closed-loop pressure–argon control, which is expected to raise the effective nitrogen removal rate by a further 15–20%.

References:

1. Behbudlu I.A. Exergy-Based Thermodynamic Analysis and Energy Efficiency Improvement in Vacuum Treatment of Steel // Proceedings of the Republican Scientific-Practical Conference “Modern Trends in Science and Technologies”. – Baku: Azerbaijan Technical University, 2025. URL: https://atu.edu.az/storage/konfrans/2761765008891.pdf (accessed 09.07.2026).
2. Behbudlu I., Mammadov A., Bashirov M., Mirzayeva G. Industrial Kinetic Modeling and Optimization of Nitrogen Removal in Low-alloy Steels under Vacuum Degassing Conditions // International Journal on Technical and Physical Problems of Engineering (IJTPE). – 2026. – Vol. 18, No. 1. – pp. 152–160.
3. Çamdali Ü., Tunç M. Exergy analysis and efficiency in an industrial AC electric ARC furnace // Applied Thermal Engineering. – 2003. – Vol. 23, No. 17. – pp. 2255–2267.
4. Costa M.M., Schaeffer R., Worrell E. Exergy accounting of energy and materials flows in steel production systems // Energy. – 2001. – Vol. 26, No. 4. – pp. 363–384.
5. Dincer I., Rosen M.A. Exergy: Energy, Environment and Sustainable Development. 2nd ed. – Oxford: Elsevier, 2013.
6. Fruehan R.J. (ed.) The Making, Shaping and Treating of Steel: Steelmaking and Refining Volume. 11th ed. – Pittsburgh: AISE Steel Foundation, 1998.
7. Ghosh A. Secondary Steelmaking: Principles and Applications. – Boca Raton: CRC Press, 2001.
8. Iron and Steel Technology Roadmap // International Energy Agency (IEA). – Paris, 2020. URL: https://www.iea.org/reports/iron-and-steel-technology-roadmap (accessed 09.07.2026).
9. Kudrin V.A. [Theory and technology of steel production]. – Moscow: Mir, 2003. (In Russ.)
10. Mazumdar D., Guthrie R.I.L. The physical and mathematical modelling of gas stirred ladle systems // ISIJ International. – 1995. – Vol. 35, No. 1. – pp. 1–20.
11. Morozov A.N. [Hydrogen and nitrogen in steel]. – Moscow: Metallurgiya, 1968. (In Russ.)
12. Poirier D.R., Geiger G.H. Transport Phenomena in Materials Processing. – Cham: Springer, 2016.
13. Sharma M. Modeling and Optimization of the Vacuum Degassing Process in Electric Steelmaking Route // Processes. – 2025. – Vol. 13, No. 8. – Art. 2368. DOI: 10.3390/pr13082368.
14. Steneholm K., Andersson M., Tilliander A., Jönsson P.G. Removal of hydrogen, nitrogen and sulphur from tool steel during vacuum degassing // Ironmaking & Steelmaking. – 2013. – Vol. 40, No. 3. – pp. 199–205. DOI: 10.1179/1743281212Y.0000000029.
15. Szargut J., Morris D.R., Steward F.R. Exergy Analysis of Thermal, Chemical, and Metallurgical Processes. – New York: Hemisphere Publishing, 1988.
16. Tutarova V.D., Shapovalov A.N. Removal of hydrogen in the vacuum treatment of steel // Steel in Translation. – 2017. – Vol. 47, No. 3. DOI: 10.3103/S0967091217030159.
17. World Steel in Figures 2025 // World Steel Association. – Brussels, 2025. URL: https://worldsteel.org/data/world-steel-in-figures/ (accessed 09.07.2026).
18. Wu H.J., Li Q.Q., Wang Z., Jiang F.J. Vacuum denitrification and nitrogen absorption of molten steel under ultra-low nitrogen conditions // Materials Science and Technology. – 2019. – Vol. 35, No. 2. – pp. 240–246.
19. Yu S., Miettinen J., Shao L., Louhenkilpi S. Mathematical Modeling of Nitrogen Removal from the Vacuum Tank Degasser // Steel Research International. – 2015. – Vol. 86, No. 5. – pp. 466–477.
20. Zhang L., Thomas B.G. State of the art in evaluation and control of steel cleanliness // ISIJ International. – 2003. – Vol. 43, No. 3. – pp. 271–291.