Abstract

Improving the overall energy efficiency of processes is necessary to reduce costs, lower the specific energy consumption and thereby reduce the direct or indirect emission of gases that contribute to climate change. In many metallurgical processes, a large amount of energy is lost with the off-gas. In metallurgical recycling processes, off-gas often can be used to preheat the to-be-recycled metal scrap, leading to significantly higher energy efficiency. However, the application of preheating has the disadvantage that it often requires more precise planning in the design and better control of the process. In this paper, a simplified look at a continuously charged scrap preheating aggregate for the widely used electric arc furnace (EAF) in the steel processing industry is used as illustration. Continuous scrap charging in EAF-type furnaces in general has much higher demands on process control and general process knowledge, which is why they are found only very rarely. General issues and basic modeling approaches to mitigate such issues allowing a better process control will be described. In particular, a fast, one-dimensional modeling approach for the determination of the temperature distribution inside a constantly moving scrap bulk, with hot air (or exhaust gases) flowing through it, will be described. Possible modeling applications, assumptions, possible enhancements and limitations are shown. The first results indicate that this approach can be used as a solid basis for the modeling of scrap bulks with thermally thin parts, consisting of materials with similar thermodynamic material properties. Therefore, as a basis, this approach may help improve the design and control of future or existing preheating devices in metal recycling processes.

Highlights

  • Making better use of waste energy, usually in the form of heat, is necessary to increase the energy efficiency of nearly every process

  • CO2 per ton of steel, saving around 945 million tons of CO2 emissions per year [1], not to mention the significant damages to the environment induced by primary iron ore or coal mining

  • With CO2 -neutral primary iron production processes such as direct reduced iron (DRI), the importance of the electric arc furnace will continue to increase, as it is required in this process chain

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Summary

Introduction

Making better use of waste energy, usually in the form of heat, is necessary to increase the energy efficiency of nearly every process. Thereby, the enthalpy of the charged material is increased, and the heat required for melting the scrap inside the metal bath is reduced. Why is this not the common practice ? CFD models and process models regarding the topic of scrap preheating can be found [11,12,13,14,15], but they either lack a detailed description of the model, the simplification level is too high (for example lacking a prediction of the temperature distribution in the bulk) or these approaches are too computationally intensive to be used to build online control models.

Materials
Thermophysical
Off-Gas
Modeling
Illustration first discretization
Modeling Simplifications
Heat Conduction over Characteristic Scrap Thickness
Heat Conduction in the Off-Gas
Convective Heat Exchange between Scrap and Off-Gas
Balancing
Radiation Modeling—Bulk Incident Radiation
Radiation Modeling—Inside of the Scrap Bulk
Exemplary
Calculated
Results
Influence of Convective
Influence
Verification of the Model’s Heat Balance
17. Energy
Conclusions
Summary
Full Text
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