The use of waste-heat recovery to generate high quality energy

Energy efficiency, reduction of greenhouse gas emission and landfill consumption, and higher quality products will be the drivers of innovation in the steel industry over the next few years. The increasing demand for energy and its consequent rise in prices pushes steel producers to apply new solutions. The efficient use of resources is an integral part of Tenova’s selling proposition.

#5_May 2022

Tenova is a worldwide partner for sustainable, innovative, and reliable solutions in the metals and the mining industries. The company has a wide range of environmental technologies designed to assist metal makers in achieving internal goals and regulatory mandates. Monitoring and reducing emissions, particulate pollution reduction, and energy savings are some of the areas in which the company has specialized systems.

From DRI to EAF, reheating furnaces to heat treatment furnaces, cold mills to processing lines and roll grinders, Tenova’s technologies minimize the environmental impact of industrial activities, operations, and final products.

iRecovery®: The technology for recovering the thermal power

Year after year, process optimization and energy efficiency to reduce emissions are more important for all steel producers. In the past decade, Tenova developed the iRecovery® system for recovering the thermal power available in the Electric Arc Furnace (EAF) waste gas.

The high amount of latent and sensible enthalpy (heat content) discharged from the melting process in an EAF through the off-gas offers a high potential for waste heat recovery. Through iRecovery®, saturated steam is generated by Evaporative Cooling Systems (ECS) technology applied to the fume treatment plant in service to the EAFs. The generated steam can be utilized for further applications such as power generation, house heating through the district networks, or directly for vacuum generation in steel production.

According to Enzo Chiarullo, Tenova’s Product Line Director, heat-recovery technologies’ efficacy “is always well proven because it recovers 20-25% of the overall energy input in the EAF process fully charged by scrap, or even more efficient – up to 30% – if partly charged by hot-metal or DRI”.

iRecovery® technology results in EAF off-gas thermal energy. This energy is the most significant fraction of the primary energy input in the EAF process typically wasted away. The iRecovery® system is based on the Heat Recovery for Steam Generation (HRSG) technology, with a radiant Evaporative Cooled System (ECS) and convective section, the Waste Heat Boiler (WHB), which completely processes the waste gas from approximately 1,700°C to 200°C. Pressurized water at boiling point feeds the ECS + WHB, where it is partly converted into steam through the heat exchanged by the waste gas.

iRecovery® was first developed for a standard 100% scrap EAF basket charging process, where the energy in the off-gas is almost 25% of the primary input in a standard process using about 650-750kWh/tls (i.e. tls: ton of liquid steel) of both electrical and chemical energy. iRecovery® performance is based on a saturated steam production of about 150-220kg/tls.

“The use of the recovered energy drives a reduction of CO2 emission in the steel production. If the produced steam is directly used for HVAC applications or directly in the steel process for vacuum generation, about 5% of CO2 direct emission in the steel production by EAF route can be avoided. If the steam is applied to power generation through turbine technology, the reduction in CO2 direct emission is approx. 1.5% – based on the electrical energy carbon intensity in the EU” explained Chiarullo.

“Even if the electricity production by iRecovery® technology leads to the lower environmental benefit, it will permit to transform the steelmakers from typically energy consumers into energy prosumers, which can play a role in the energy market, balancing the costs of the bought energy and the sold energy, particularly for network regulation. The general working hours per year of steel shops in the EU is about 7,000–7,200h/year. In terms of fossil-free electricity production – like the power generated by steam from iRecovery® – this amount is about threefold the 2,050h/years of working hours for a wind farm (in 2019 about 417TWh produced over 205GW installed), and more than six times more of the 1,035hh/year for the solar field (in 2019 about 139TWh produced over 133GW installed). Therefore, we can contribute to creating legislation as the waste-heat recovery offers carbon-free energy”, Chiarullo stated.

The well-known Tenova EAF Consteel® process for continuous charging and charge preheating is one of the main technologies for directly recovering and re-using the waste gas thermal power in the melting process. Tenova coupled the Consteel® and iRecovery® to improve the effectiveness of its technologies: while the high-temperature thermal power contained in the waste gases – up to 800°C – is first used in scrap preheating by Consteel®, the WHB is recovering the residual waste gas energy. This combination can produce about 85-140kg/tls of steam.

The Green Revolution Starts at Home

 The green revolution starts at home with families rejecting heat and electricity delivered to buildings generated from obsolete, inefficient, and polluting boilers and long-standing systems alike.

In Brescia, Italy, hundreds of families rely on clean energy thanks to Tenova iRecovery® Technology – the energy recovery system for EAF / BOF that can produce electric power.

The thermal heat supplied to 2,000 households in the wintertime is recovered from the waste gas of the ORI-Martin steel plant by Tenova iRecovery® Technology. Thanks to the same technology, 700 families rely on clean electricity during summer. Overall, the community benefits from reducing approx. 10,000 tons of CO2 per year.

Compared to conventional cold-water cooling, an iRecovery® waste gas duct is a pipe-web-pipe instead of pipe-to-pipe construction with the same look and working principle. The main difference is the pressure and temperature level inside. While cold water cooling typically uses 20-50°C, an iRecovery® system works with water at 180-250°C through the ducting (high-temperature section), and it decouples the off-gas energy through evaporation. Besides the energy recovery, the iRecovery® produces other operational benefits. Because ECS technology exploits the latent heat (approx. 2350kJ/kg of water) instead of the heat capacity (approx. 45kJ/kg of water over a temperature difference of 15°C standards in the cooling system), the amount of water flow needed to cool down the waste gas is approximately three times less than standard cold water-cooling technology. This benefit has an impact not only on water consumption but even on the energy consumption for water pumping into the fume treatment system.

Advantages

• As far as hall layout/mechanical restrictions allow such adjustment, shorter waste gas ducts can be designed. The reason is simply economic, as the same amount of heat transfer capacity is typically cheaper in a waste heat boiler than in a cooling duct.
• The horizontal boiler design with pneumatic rapping devices for boiler cleaning delivers reliable operation conditions.
• The direct utilization of steam on the plant infrastructure or nearby industries significantly reduces the pay-back time with respect to power generation.
• End of dew point problems – cooling system elements always stay above the dew point of sulphuric or hydrochloric acid formation according to the standard EAF waste gas chemical composition.
• iRecovery® means less thermal stress to the ducting – the system works with a constant temperature during all different energy input phases of the melt.
• Drastically reduced cooling water consumption – iRecovery® is a closed-loop; cooling towers consume 3-6% of the water in every circuit.

As for the future, Chiarullo underscored: “There is a growing interest in viewing emissions for their energy intensity, for decreasing the carbon footprint of the industrial process, and environmental quality (dust, CO content, heavy metals, dioxin), and, as time goes by, these regulations are likely to become more stringent, which allows us to treat emissions differently by incorporating new, disruptive technologies. Besides Europe, there’s interest in Asia, particularly Japan and China, for example. Waste heat recovery should become an essential pillar for sustainability, which I would call a waste heat economy, mainly for steel industries, but this requires a slight change in the current mindset: the steel industry could become an energy hub, not just a consumer.”