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New scrap-based steelmaking process predominantly using primary energyWith the objective of enhancing the productivity of electric arc furnaces (EAFs), the share of fossil energy (primary energy) used to complement electricity has grown continuously over the past few decades. The timeframe available for using primary energy efficiently in the EAF is, however, very small, being limited to the melting phase of the scrap. A new scrap-based steelmaking process predominantly using primary energy as fuel is an interesting alternative to the EAF as it reduces energy costs and CO2 emissions.

With open hearth technology having been largely superseded by the end of the last century, the electric

arc furnace (EAF) has prevailed as the number one process for scrap-based steelmaking. In 2006, roughly 40% of the world’s steel was produced by the EAF process.

In the past few decades, to boost productivity, fossil energy in the form of burners installed in the furnace sidewall panels, has been used to supplement electrical energy. However, the timeframe for the effective use of primary energy in the EAF is limited to a short period in which the scrap column has not yet become fully molten. During this time, the specific energy input is high, the energy is efficiently used through the transfer of heat across the surface of the scrap, and hence tap-to-tap times are reduced. This fact leads to the question of how the

Authors: Jens Kempgen, Jochen Schlüter, Udo Falkenreck and Walter Weischedel SMS Demag GmbH

total energy used in the chain of production in scrap-based steelmaking could be better utilised than what is currently possible in the EAF.

OPTIMAL USE OF VARIOUS ENERGIES IN THE EAFFigure 1 shows the theoretical energy requirement in scrap-based steelmaking in the three consecutive steps: heating, melting and superheating. About 71% of the energy is needed in the first step where the scrap is still solid and which offers a very large surface for heat transfer. This is the most important precondition for the use of primary energy. On this premise, fossil energy for melting can be transferred to the material better than the electrical energy of an electric arc. A further 19% is used in the melting phase.

It is only in the last superheating step that the use of fossil energy becomes inefficient. The specific surface of the charged material, which is now molten, is now very small and hence the utilisation of primary energy is poor. This was one of the reasons why the open hearth process became obsolete. In contrast, the use of an electric arc in this step does indeed make sense.

With these boundary conditions the question now is how a reactor must be designed in which the first two steps can be performed with the use of primary energy and the third step with electrical energy.

ELECTRICAL ENERGY GENERATIONThe electrical energy used in the EAF today still predominantly comes from primary energy. For example, in Germany, electrical energy is produced via a number of routes (see Figure 2), of which fossil sources are ~60%.

r Fig 1 Energy requirement for melting scrap

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SPECIFIC ENERGY CONSUMPTIONTo compare the specific energy consumption of the PEM route versus the EAF route, characteristic values of existing plants have been compiled. These are:

` Data from the International Iron and Steel Institute (IISI) on approximately 95 EAFs` The EAF at Stahlwerk Thüringen (Alfonso Gallardo Group), SWT` The EAF at Peiner Träger (Salzgitter Group), PT

Conservative values were used for the PEM. The corresponding figures for the heating energy and electrode consumption in the LF were obtained from the relevant technical literature. To allow a direct comparison to be made between the various energy sources, electricity and oxygen were converted to the primary energy required for their generation. For the generation of 1kWh of electrical energy from fossil energy, the mean German power station efficiency of 36%, was used as a base. Also included as a dotted red line is the theoretical minimal energy for loss-free melting of 1t of scrap. At a steel temperature of 1,600°C for a low-alloy steel grade this amounts to approximately 368kWh/t. Although a future improvement in the

Figure 3 shows the route from the source of energy generation to the EAF. The energy source is first converted to heat and then into electrical energy in a power station. These two conversion processes, like all conversions, entail losses which are determined by the efficiency of the power station. In state-of-the-art power stations this efficiency is no higher than 42%. In Germany, the average is 36%. In the melt shop further electrical losses occur before the energy is converted back to heat for steelmaking, resulting in almost two-thirds of the original primary energy being lost on the conversion route from source to melt shop. Direct use of the primary energy source would significantly improve efficiency.

CONCEPT OF THE PRIMARY ENERGY MELTER (PEM)The efficient use of primary energy in the heating and melting steps should be possible in a counter-current reactor. In such a reactor the scrap is continuously charged at the top, is heated by the combustion of fossil fuels with oxygen, becomes liquid and then is tapped at just above liquidus temperature.

Since it is not possible to significantly superheat the melt in the presence of solid material, superheating occurs in a separate EAF with a power requirement comparable to that of a ladle furnace. Figure 4 is a schematic of the process units and Figure 5 shows the arrangement for a 145t furnace. Unlike in the conventional EAF which separates heating and melting in terms of time, these steps are separated spatially in the primary energy melter (PEM) process. The concept is characterised by:

` Power consumption in melting vessel <530kWh/tls and 245kWh/tls in EAF` Ferrous yield > 90%` Taphole in the melting vessel linearly arranged with the tilting axis of the superheating vessel

This concept has, in part, been tested as long ago as in the 1970s.

r Fig 2 Energy production in Germany in 2006

r Fig 3 Power conversions during generation and use of electrical energy in the melt shop

r Fig 4 Schematic of the primary energy melter

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efficiency of power stations will slightly reduce the benefits of the PEM process, it will remain financially attractive.

Current worldwide discussions form another important aspect, namely the assessment of the PEM process concerning the emission of the various lines in terms of CO2. Data on this topic are provided in Figure 6. Here, the mean CO2 emissions for 1kWh of electricity are 0.212kg CO2/kWh electrical energy. On this basis the specific CO2 emissions of the PEM process are around 30% lower than those of the conventional production routes.

OPERATING EXPERIENCE SO FAR The strategy was proven in several tests in a pilot unit of approximately 1t/hr capacity at Clausthal University of Technology, where the operational conditions were varied to verify the process reliability.

COST COMPARISON OF EAF VERSUS PEM In addition to the basic considerations presented here, regional conditions such as cost of energy, availability of

r Fig 5 Comparison of specific energy requirement of various process lines

r Fig 6 Comparison of specific CO2 emissions

r Fig 7 Energy costs of primary energy melting

fossil fuels and the stability of the electrical power supply, must of course be taken into account. For the comparison of energy costs the diagram shown in Figure 7 has been developed. The relationship between EAF energy costs and the price of electricity is represented by the blue lines. The dotted line indicates the average price for the IISI study, whereas the continuous line allows a comparison to be made with best practice. The red lines show the relationship between total energy costs and electricity costs for the primary melting process with the price of primary energy.

In Germany in 2005, at an electricity price of 0.0775 Euros per kWh, the cost for melting and superheating is approximately 47 Euros/tls. Best practice EAF at the same conditions has energy costs close to 40 Euros/tls. For primary energy costs of approximately 0.028 Euros kWh, a PEM process of equivalent performance can be expected to cost only approximately 32.50 Euros/tls. The reduced CO2 emissions of the total energy generation are not considered here.

SUMMARYThe results show that scrap-based steelmaking using predominantly primary energy is an interesting alternative to the EAF, as it leads to a considerable reduction in CO2 emissions and a significant reduction in energy costs. After a successful period of intensive tests on a pilot plant SMS Demag is now ready to implement the process industrially. MS

Jens Kempgen, Jochen Schlüter, Udo Falkenreck and Walter Weischedel are with SMS Demag GmbH, Düsseldorf, Germany.

CONTACT: fudo@sms-demag.com

The technology described in this article has been developed in cooperation with the Institute of Energy Process Engineering and Fuel Technology (IEVB), Clausthal University of Technology, Germany.