ELECTRIC STEELMAKING WITH ORC HEAT RECOVERY IN EUROPE

AND BEYOND

Alessandro Foresti - Turboden S.r.l., Italy

Daniele Archetti, Turboden S.r.l., Italy

Abstract

ORC – Organic Rankine Cycle - is a special kind of Rankine system using in a closed loop an organic fluid to convert heat to power. ORC modules are today the preferred solution in medium and small size power plants where ease of operation, reliability and minimum operating costs are the key factors. The record of accomplishment in hundreds of systems producing renewable energy from the combustion of biomass or from geothermal water has facilitated the introduction of ORC technology for waste heat recovery in energy intensive industries to enhance efficiency and reduce carbon footprint. A growing number of ORC systems in Oil & Gas, Cement or Glass testify the inbuilt advantages of this technology due to the working fluid peculiar properties. These plants are continuously running, automatically adapting to the variable process conditions with no dedicated personnel. The first ORC equipped heat recovery system in Electric Steelmaking started up in December 2013 at Elbe-Stahlwerke Feralpi (ESF) plant in Riesa Germany. The innovative system is operating commercially since then. The efficiency gains demonstrated at ESF led ORI Martin to install a new ORC based system at its EAF shop located in Brescia. Turboden will supply in 2017 two more ORC system for EAF off-gas heat to important steel companies in Europe and Asia.

KEYWORDS: ORC - WASTE HEAT – RECOVERY – EAF – OFF-GAS – WHR – STEAM - THERMAL OIL -

DISTRICT HEATING – EXHAUST - EMISSIONS

INTRODUCTION

European Steelmakers are strongly engaged to reduce the energy intensity of their existing production plants.

Their efforts contribute to lower the steel industry emissions but they are not sufficient to achieve the drastic

reduction targets set by the European Union for 2030. These targets could be within reach only replacing in

addition some part of the present coal/coke production with lower emission processes. In the next few years,

while new alternative lower carbon intensity production processes will not be ready for wide implementation in

Europe, the best available option seems to increase the share of electric steelmaking using as feed material

not only scrap but also DRI produced with gas based reduction processes.

The DRI, supplementing the recycled scrap expected to become scarcer in the future, would come from

overseas where natural gas is available (a project of this type is being realized by a major EU steel company)

but could be also produced in Europe. This alternative is under study in a couple of cases.

In this scenario, with a further increase of EAFs in Europe, recovering the waste heat generated in Electric Arc

Furnaces seems a priority goal for steelmakers in order to improve the energy efficiency and sustainability of

their primary production processes.

The EAF off-gas related losses, typically almost 30% of the energy balance in scrap-based steelmaking, are

the first target to address for an effective heat recovery system [1].

The four Turboden ORC (Organic Rankine Cycle) systems now operating or being installed in EAF steel plants

in Europe and Asia demonstrate that ORC is an effective, commercially viable way to recover and valorize the

off-gas waste heat meeting at the same time the challenging conditions of EAF steelmaking.

WASTE HEAT VALORIZATION

The first option in waste heat valorization is to use the process residual energy directly for heating. This is often

inadequate or impractical, due to the large amount of heat (bigger than any local user), its relative low

temperature, the possible mismatch with the user demand when it is variable, the distance or layout constraints

to reach a thermal user.

When direct use of heat is limited or impossible, the best valorization option is to convert it to

mechanical/electrical power usually with a Rankine Cycle system.

The power produced is only a fraction of the heat available, due to the intrinsic efficiency of the conversion.

This efficiency is a function of the temperatures of the heat source and of the heat sink.

ORC VS STEAM TURBINE

Traditional Rankine Cycle systems use water and steam as working fluid. They are the common solution for

power plants above 10 MW, with efficiencies above 30% when using superheated steam at high

pressure/temperature. Steam turbine Rankine Cycle systems are preferred in large utility size power plants

where fuel is the most important cost factor outweighing the higher O&M costs due to steam at high

temperature/pressure.

ORC technology, employing high molecular weight working fluids (siloxanes, hydrocarbons and refrigerants)

that guarantee dry vapor expansion in every operating condition, is typically preferred for smaller scale power

systems up to 10 MW, due to good efficiency, high flexibility and minimum running costs (no dedicated

personnel necessary).

The following table shows the main differences between traditional Rankine Cycle with Steam Turbine and

ORC (Figure 1).

Fig.1 - ORC vs Steam turbine

ORC units, after being widely used in distributed generation system with renewables sources (Biomass,

Geothermal, Solar), proved to be as well effective tools for waste heat recovery in energy intensive industries

like Oil & Gas, Cement, Glass and more recently Steel.

The ORC simple, automatic, modulating, fail-safe operation can match the actual primary process regime in

all conditions, maintaining the overall system reliability with minimum O&M cost and no additional personnel.

These advantages make the ORC the preferred option for many waste heat valorization systems and probably

the only possibility when the primary process is highly variable and discontinuous as in scrap based Electric

Steelmaking.

ORC TYPICAL SIZE & APPLICATIONS

ORC modules are used for different applications, most commonly in sizes up to 10 MW. In case of low

temperature sources (around 150°C) such as geothermal applications, ad hoc ORC configurations can extend

to 20 MW and above (Figure 2).

Fig.2 - Turboden ORC: size and applications

ORC LINK WITH HEAT SOURCE

When the main heat source is in the gaseous phase (e.g. streams of dust laden EAF exhaust gases), a Heat

Carrier loop is typically interposed between the hot source and the ORC (Figure 3).

Fig.3 - Heat recovery scheme with heat carrier

The heat carrier loop allows flexibility in layout and easier decoupling from the primary heat source (e.g.

biomass combustion system, main industrial process). In the case of highly variable discontinuous heat loads

as in EAF Steelmaking, the heat carrier allows to insert a thermal buffer between the primary heat exchangers

and the ORC.

In some cases when these issues are not critical, the heat carrier may be avoided.

The most common heat carriers used in ORC systems are Thermal Oil, Saturated Steam and Pressurized

Water. The choice depends on the application and site considering efficiency, capital cost and ease of

operation (Figure 4).

Fig.4 - Heat carrier alternatives

Thermal oil is widely used in small biomass power plants and for waste heat recovery in Oi l& Gas, Cement,

Glass, Non Ferrous Metals. Saturated steam is well known in the steel industry being used since a long time

in large integrated plants with Blast Furnace and BOF converters. In fact, the use of steam for EAF heat

recovery originates in a plant where an EAF was installed where a BOF had been operating previously [2].

ORC COOLING

The ORC, like any other thermodynamic cycle, needs a cooling system (heat sink) to dispose the lower

temperature heat remaining after converting to power a portion of the high temperature heat. Typically, small

ORCs use water-cooling. The water that condenses the ORC working fluid is recirculated and cooled with

radiators or with evaporative cooling towers. Large ORC, typically above 3 MW, often employ air cooled

condensers (no water). Air cooled condensers are more expensive, require more space but can be convenient

avoiding water issues and corresponding costs in operation.

The choice depends also on the site and climatic conditions.

When the ORC is a CHP (Combined Heat & Power) system, the heat client defines the temperature of the

cooling water (e.g. 85 °C water in case of district heating). On the contrary, when the ORC is for power only,

the cooling water temperature must be as low as possible.

The efficiency of the ORC is highly dependent on the temperature of the cooling system.

This is an important issue often overlooked by plant operators accustomed to cooling water systems (to

dissipate heat). These systems, when employed to cool EAF off-gas ducts, would work at temperature rise

unsuited for high efficiency in power conversion.

Water quality is also important to insure efficiency and maximum power production continuously.

TURBODEN ORC PLANTS

The following table summarizes the current fleet of Turboden ORCs, indicating for each type of application the

number of units, size range and the heat carrier employed (Figure 5).

Fig.5 - Turboden ORC plants (update February 2016)

The number of ORCs installed increased greatly in the last 10 years with many systems, used in different

applications, joining the original fleet mostly concentrated in biomass power plants. The size of the ORCs also

increased greatly in the same years due to the new applications and the clients demand in countries where

incentives on renewables are absent or smaller.

ORC IN IRON & STEEL – FROM CUPOLA TO EAF

Back in 1996, Turboden started up an ORC based waste heat valorization system at Fonderia di Torbole, a

leading European iron foundry supplying the automotive industry. The prototype installation, using the residual

heat of the foundry Cupola Furnace exhaust gas conveyed by thermal oil, consisted in fact of two cascaded

ORC units. The system operated with one ORC for nearly 10 years producing electricity for the local utility

Cogeme. Recently Fonderia di Torbole decided to install an up to date Turboden ORC unit for waste heat

valorization at the upgraded Cupola Furnace using again thermal oil as heat carrier.

After that, a new ORC application reported in steel was in 2011 at the Slab Reheating Furnace of Toscelik Hot

Strip Mill in Turkey where multiple small ORCs use hot water as heat carrier [3].

In February 2013, Turboden started up an ORC based waste heat recovery system at NatSteel in Singapore.

This is a small direct heat exchange unit (no heat carrier) using the exhaust of the billet reheating furnace at

the merchant bar mill of NatSteel, a Tata Steel Enterprise.

Finally and most important, on December 18th 2013 Elbe-Stahlwerke Feralpi GmbH (ESF) started up the world

first ORC based waste heat valorization system in its Electric Arc Furnace steelmaking plant in Riesa

(Germany) [4].

This is a more challenging but also more rewarding application due to the nature of the process and the amount

of energy involved. Since the EAF off-gas stream is discontinuous and highly variable, heat recovery and

power production required ad hoc solutions in equipment and controls.

ORC OPERATION AT ELBE - STAHLWERKE FERALPI EAF

At ESF the EAF off-gas iRecovery®1 system produces nominally 30 t/h of saturated steam with up to 20 t/h

conveyed to the 2.7 MW ORC while the rest is delivered to the local utility (Stadtwerke Riesa) for use in the

nearby Goodyear Dunlop tire plant.

The ORC reached its nominal power on December 19, 2013, the second day after the start up.

More time was then necessary in commissioning with the joint work of ESF, Tenova and Turboden to solve

the teething problems of the innovative system and to get fully satisfactory performance in continuous

operation.

During that time, the ORC demonstrated to well exceed the contractually guaranteed power output.

The measuring campaign done at the end of commissioning demonstrated that the overall efficiency of the

EAF with the new energy recovery system had increased by 4.5% [5].

Once completed commissioning, the new EAF off-gas valorization system has been running continuously

following the normal steel shop operating routine practiced at ESF.

The account of the operating experience, from the original startup in December 2013 to date, testifies the

achievement of this innovative ORC application and offers considerations for new projects in EAF and

elsewhere [6].

In the last 365 days recorded (20.1.2015 to 20.2.2016), steam was available for power production 6,959

hours, the ORC was in parallel 5,783 hours (83.1% availability) producing 12,086,622 kWh (average power

2,090 kW).

The relatively low availability is the consequence of a prolonged shutdown caused by an accident in August

2015 at the medium voltage switchgear.

Aside from that, the ORC availability was in line with typical ORC values. In fact, in the last four months

(1.10.2015 to 2.2.2016) the ORC had steam for 2138 hours, was in parallel 2,045 hours (95.7% availability)

producing 4,478,863 kWh (average power 2,190 kW).

Three screenshots of the supervisory PC show the actual operation of the ORC at ESF.

1 iRecovery® is the Tenova technology for evaporative cooling and heat recovery in EAF, SAF, BOF and reheating furnaces..

Fig.6 – Screenshots of Turboden ORC at Elbe-Stahlwerke Feralpi EAF

The commercial operation of the ORC based energy recovery system at ESF electric steel plant in Riesa, well

demonstrates the ability of the ORC to operate continuously, automatically adjusting to the highly variable EAF

working conditions

The European Union recognized the results obtained and the practical experience gained at ESF through the

Sustainable Industry Low Carbon scheme. Under this scheme, the project WHAVES (Waste Heat Valorization

for more Sustainable Energy Intensive IndustrieS) was launched to disseminate the results obtained and to

promote further similar applications [7].

Thanks to ESF hospitality, the innovative EAF waste heat recovery plant in Riesa received visits by many steel

mill operators interested to evaluate directly the system features and the actual performance.

These visits convinced other steelmakers to follow the route pioneered by ESF installing ORC based systems

at their existing EAF steel plants.

NEW ORC AT ORI MARTIN EAF READY TO START UP

A new ORC equipped EAF waste heat valorization system is now in the commissioning phase at ORI Martin

steel making plant in Brescia (Figure 7).

ORI Martin, leading Italian supplier of engineering steel in bars for the automotive industry and for other

mechanical applications is implementing a multimillion Euro energy saving investment in its facilities in Brescia

[8].

Fig.7 – ORI Martin steel shop

The projects involves a major revamping of the electric steel melt shop with a new improved Consteel®2, an

iRecovery® system producing steam partly exported to local utility A2A for the citywide district heating network

and the 1.9 MW Turboden ORC fed with the rest of the steam (Figure 8).

Fig.8 - EAF Waste Heat valorization scheme at ORI Martin, Brescia, Italy

Like at ESF Riesa, the heat recovery system uses evaporative cooling and saturated steam, but the pressure

and temperature are lower.

2 Consteel® is the registered trademark of the Tenova system to continuously pre-heat and feed metallic charge to the EAF.

Differently from ESF there is no radiant heat exchange section close to the EAF. The convection heat

exchanger is a horizontal waste heat boiler placed downstream of the underground, refractory lined, off-gas

duct. The existing quench tower is bypassed (Figure 9).

Fig.9 - EAF Waste Heat valorization layout at ORI Martin, Brescia, Italy

The new Consteel® EAF was started up in August 2015, the iRecovery® system with convection waste heat

boiler was commissioned in January 2106 conveying steam to the A2A district heating heat exchangers. The

ORC installed in 2015 will start up beginning in March when the utility A2A will reduce the heat demand near

the end of the winter heating season.

Fig.10 - Turboden ORC unit at ORI Martin, Brescia, Italy

The following table summarizes the main features of the ORC based Waste Heat Valorization system at ORI

Martin Brescia compared to the system at ESF Riesa (Figure 11).

Fig.11 - ESF and ORI Martin waste heat recovery parameters

The waste heat valorization system installed by ORI Martin will improve significantly the energy efficiency of

the steelmaking operation while maintaining a competitive industrial activity within the city of Brescia.

For these reasons the project obtained the support of the European Union Smart Cities program called

PITAGORAS, acronym for Promoting Sustainable urban Planning with Innovative and low energy Thermal

And power Generation frOm Residual And renewable Sources [9].

NEXT ORC PROJECTS IN STEELMAKING

In 2015, two prestigious steel companies decided to follow the example of ESF and ORI Martin installing new

EAF heat recovery projects in their existing plants. As a result, Turboden will deliver in 2016/2017 two new

ORC units for two electric steel plants, one in Europe and one in Asia. The name of the customers will be

disclosed after receiving their authorization.

Compared to the systems at ESF Riesa and at ORI Martin, the two new plants now under construction have

similarities but also significant differences.

In both cases a new iRecovery® system with waste heat boiler retrofitted to the existing EAF will produce saturated steam feeding the ORC unit. In one case the steam generated by the basket charged EAF off-gas will go predominantly to the ORC to

produce 2.5 MW with a small fraction used in steam ejectors for steel degassing.

In the other case, the heat recovered from a large Consteel®, EAF will go entirely to the ORC producing initially 7.5 MW and 10 MW in the future. In no case, steam will be exported and used as a heat source. This means that in both new projects the ORC

power will generate the payback for the investment.

Counting these two important projects under way, in 2017 six Turboden ORCs will be operating in waste heat

valorization systems at different Iron & Steel plants in Europe and Asia. Four of them will be in EAF steelmaking

applications.

The following table summarize these plants and their main features (Figure 12).

Fig.12 – Six Turboden ORC installations in Iron & Steel

CONCLUSION

The experience cumulated in various Iron & Steel plants confirms that ORCs well respond to the steelmakers

requirements to have waste heat recovery systems that are dependable, flexible, easy to run, guaranteeing

significant energy savings with no prejudice to the overall process reliability and no additional personnel.

Two years operation at Elbe-Stahlwerke Feralpi GmbH electric steel plant in Riesa prove that ORC is a best

available technology in EAF energy recovery.

The new ORC systems in EAF off-gas heat valorization, ready to start up or under construction, benefit from

the feedback of the first project at ESF.

Further optimized solutions reduce the specific investment cost of the whole heat recovery system with ORC.

The lower installed cost per kW together with the ORC minimal O&M costs improve the economic viability of

the next projects in EAF waste heat valorization.

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