Design and thermodynamic analysis of a flash power system driven by process heat of continuous casting grade steel billet

Significance Statement

Integrated steel plants are characterized by substantial amounts of waste heat such as in the cooling down of steel billets. Metallurgical experts have therefore devised techniques that are sustainable and energy-efficient for the recovery of waste heat in the steel production lines. Hot charging has emerged as an effective use of the process heat in continuous casting-reheating furnace route. However, if hot charging is used for some continuous casting grade steel billet such as C-Mn steel and C-Cr-Mo steel, it tends to result in hot brittleness, and therefore, the standard practice is cooling down the billet prior to rolling in order to attain considerable heat loss.

In a recent research published in Energy, Wenqiang Sun at Northeastern University (China) and Fengyuan Zhang at Shanghai Jiao Tong University designed and optimized a flash power system referred to as process heat flash power system that is driven by the continuous casting grade steel billet process heat, and have demonstrated the viability of the system.

The system is composed of a flash chamber, heat recovery boiler, as well as a steam-replenishing turbine. A feed-water pump conveys water into an economizer, that then conveys the water into a flash chamber and a boiler drum. From the drum, the water returns to the boiler to be superheated, after which it flows into the steam turbine through a high-pressure inlet. From the flash chamber, water also flows into the steam turbine through a low-pressure inlet. A cycle is completed by piping the exhaust steam and saturated water from the flash chamber into the heat recovery boiler. The system’s exergy analysis was used to check inefficiencies and both flash pressure and the shunt temperature were selected as the key factors of study.

The research team observed from analysis that there was an initial increase in exergy recovery rate which then decreases as the flash pressure increases. The increase in flash pressure resulted in an increase in steam pressure, which however caused a decrease of flash steam. There was an increase in the temperature of shunted water as a result of an increase in water temperature that enters the flash chamber, which in turn increases the rate of exergy recovery. At 0.6MPa flash pressure and a rise in shunt temperature from 170°C to 220°C, the rate of exergy recovery increases to 33.34% from 31.22%.

Further analysis showed that the rate of flow of shunted water is a function of shunt temperature and the flash pressure whereby, lower flash pressures result in lower rate of flow of shunted water, while an increase in shunt temperature reduces the rate of flow of shunted water. It was inferred that there exists an optimum flash pressure, which is determined through approach point temperature. Through trade-off of both investment cost and safety, a 9°C approach point temperature was selected.

Sun and Zhang determined that the maximum exergy rate of recovery was 33.41% with a 0.89MPa optimal flash pressure. From this, the net outlet power was determined to be about 6361kW which accounts for about 34.9% of net output of the system power. As compared with other traditional systems of waste heat recovery without a flash evaporation system, the proposed system generates about 53.7% of the power, which reduces the outsourcing of electricity in a steel plant, and therefore resulting in a substantial economic benefit.

Design and thermodynamic analysis of a flash power system driven by process heat of continuous casting grade steel billet. - Advances in Engineering

Reference

Wenqiang Sun, Fengyuan Zhang. Design and thermodynamic analysis of a flash power system driven by process heat of continuous casting grade steel billet. Energy 116 (2016) 94-101.

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