Sustainability and climate neutrality are the buzzwords of the moment, and they are now inseparable from society in general and from corporate action in particular. More than 50 years ago, the “Club of Rome” already highlighted both the necessity and the principles of a “sustainable” way of life for societies, in order to protect ecosystems and, above all, to point out the limits of economic and industrial growth — yet it has taken this long for concrete, legally binding measures to be put in place across the board.

The heat-treatment and hardening industry must therefore now swiftly implement appropriate measures for sustainability and climate neutrality in the operation of industrial furnace systems. The key measures on the “furnace side” are thermal insulation and heating technology.

Sustainability and/or decarbonisation is understood as the transformation of the energy sector with the aim of minimising carbon turnover. The long-term goal is carbon-free business and production as part of the energy transition. The priority is to change or replace the processes that release carbon dioxide. Corresponding emission reductions are intended to meet legal requirements and targets as measures for climate protection and the energy transition — that is, CO₂ neutrality in processes, production, and the economy.

The decarbonisation measures currently being discussed, and in some cases already implemented, mainly concern the shift away from fossil fuels toward renewable energy. In the transport sector, decarbonisation is currently taking place primarily through the use of renewably generated electricity and corresponding electric drive technology. For the energy-intensive processes of steel — but also cement and glass production — a 2019 study indicates that it is possible to cut greenhouse gas emissions by 80 to 95 percent by 2050, compared with 1990 levels [1]. According to statements by policymakers and accompanying scientific assessments, Germany as a business location ranks among the leading nations in decarbonisation measures, with the goal of reducing global greenhouse gas emissions by 40% to 70% by 2050 compared with 2010, and fully decarbonising the world economy by 2100 [2] — all with the aim of reducing CO₂ emissions enough to limit global warming to 1.5°C.

The steel industry, and with it the heat-treatment and hardening sector, plays a key role on the path to a climate-neutral future, since a large share of industrial CO₂ emissions arises from the resource- and energy-intensive process chains involved in steel production and finishing, such as heat treatment and hardening.

So-called “green production methods” entail enormous process conversions and high investments for heat-treatment businesses. On top of that come significantly increased costs for climate-friendly processes and, more generally, the problem of rising energy costs — especially at sites in Germany.

The authors of this article have already pointed out, in an earlier publication, the costs associated with converting to climate-neutral processes and the corresponding funding opportunities: the energy transition requires comprehensive support for the additional investment and operating costs involved. European and national funding programmes exist for this purpose, for which heat-treatment and hardening businesses can apply. The authors have already described this in detail in other technical articles and are happy to provide support in this area [3, 4].

This article presents the key considerations for improving energy efficiency in heat treatment and hardening, along with the corresponding funding opportunities for significantly improving the cost structure.

Aspects of increasing energy efficiency as a basis for decarbonisation

Heat-treatment processes are subject to a greater or lesser degree of high energy consumption, with correspondingly large amounts of CO₂. About 40% of industrially used energy is consumed by thermal processes in industrial furnaces. In addition, such systems are typically used for decades, so modernisation measures aimed at climate neutrality — or simply at improving the energy efficiency of these furnace systems generally — make economic sense. As already mentioned, federal, state, and EU authorities have made corresponding funding pools available, which make investments very attractive for operators from an economic standpoint and which are used as part of the general modernisation drive toward decarbonisation within companies.

Industrial furnace builders have always aimed to improve the energy efficiency of their systems in the areas of furnace insulation, heating systems, waste-heat utilisation, and power consumption, as well as the integrated use of waste heat for the thermal process — though operators of these systems have not always taken up and implemented these developments accordingly. Until a few years ago, what was mainly missing was the economic viability that the industry’s high expectations demanded of these modernisation measures. In the past, the return on investment was simply too long, ranging from 3 to 10 years depending on the measure. In the meantime, this situation has changed completely. “Galloping” energy prices — gas prices above all — now make economical production in heat treatment and hardening possible only with the greatest effort. Often, the only “help” then is to tighten or limit process control, for example in emissions from heat-treatment processes, such as lowering the nitrogen oxide (NOx) limit from 500 to 100 mg/m³!

“Climate neutrality” is also the mandate of the hour. It follows naturally that the efficiency improvements “also” mandated by national and European legislation — particularly for energy-intensive processes — should be pushed further. For the future, the EU has set out its goals with the EU Energy and Climate Package, namely reducing greenhouse gas emissions and supporting this through funding for renewable energy. Under the EU’s New Approach, only products that comply with this directive may henceforth be placed on the market. In this sense, there are now two “driving forces”: (1) improving energy efficiency in industrial furnaces, and (2) improving climate neutrality in heat-treatment processes — both of which are now technically “quite achievable” and, moreover, economically “imperative.” The following report presents approaches and measures that address the goal of improved efficiency alongside the requirement of climate neutrality.

Colleague Egger recently reported, in this same publication, on ways to decarbonise heat-treatment processes using protective-gas atmospheres [5]. He compared the CO₂ footprint of various gas carburising processes and accordingly proposed an environmental classification system for the individual processes, relating the amount of carbon mass absorbed by the components during the carburising process to the amount of carbon that must be supplied to the furnace systems for the corresponding processes. As a vacuum process with minimal consumption of reaction gases, and using acetylene (C₂H₂) — which is harmless for CO₂ accounting purposes — low-pressure carburising “as expected” received an “A” classification. By contrast, the classic gas carburising process was rated “E” in the environmental-impact classification.

Authors Hiller, Bertoni, Irretier, and Jost already reported in heat processing in 2020 on the ecological advantages of low-pressure carburising, particularly in optimised, modular plant technology [6].

Measures for improving energy efficiency in heat treatment and industrial furnace construction

Industrial furnace builders began, some years ago, to significantly improve the energy efficiency of their systems in the areas of furnace insulation, heating systems, waste-heat utilisation, and power consumption, as well as the integrated use of waste heat for the thermal process. Modern furnace systems, in energy-efficient designs, show 20 to 30% lower energy losses through wall insulation compared with older systems. Through exhaust-gas measures such as heat recovery, up to 75% of energy can be saved in some cases. Implementing energy-efficient measures is both practical and technically feasible, whether by retrofitting existing systems or in new installations.

“Climate neutrality” is the mandate of the hour. It follows naturally that national and European legislation will continue to act to further increase efficiency, particularly for energy-intensive processes. For the future, the EU has set out its goals with the EU Energy and Climate Package, namely reducing greenhouse gas emissions and supporting this through funding for renewable energy. Under the EU’s New Approach, only products that comply with this directive may henceforth be placed on the market [7].

Industrial furnace construction, as a supplier, and the heat-treatment industry, as an operator, began some years ago to jointly improve the energy efficiency of their systems in the areas of furnace insulation, heating systems, waste-heat utilisation, and power consumption, as well as the integrated use of waste heat for the thermal process. Implementing energy-efficient measures is very much achievable, whether through retrofitting existing systems or in new installations. Assessing the economics of a given modernisation measure, and the associated more efficient use of energy in heat-treatment and furnace technology, is always tied to the fundamental question of heat and mass transfer — that is, how the available heat (the energy content of a component, an atmosphere, or a substance) can be transferred to another medium or to the surrounding environment via a temperature gradient. The challenge in this balancing act is that the available quantity of heat can arise discontinuously, depending on the process, and may also depend on the time of day or season, while the waste heat or energy must be supplied on demand. During heat treatment, components are heated to high temperatures, held there, and then cooled again after an appropriate holding time. Heat transfer to the component occurs mainly by (forced) convection at temperatures up to 700 °C, while at higher temperatures thermal radiation becomes increasingly responsible for heating. It follows that, particularly at lower temperatures up to around 700 °C, forced circulation is needed for faster and more uniform heating of the load. Alongside conventional circulation via hot-gas fans, heating by high-speed convection has become established in recent years for some system types. This technology allows furnaces to be built more compactly, with very good temperature uniformity and high energy efficiency.

Optimising and increasing circulation and flow within the industrial furnace is therefore a key aspect of increasing energy efficiency. High-speed convection, for example, can significantly shorten the heated length of continuous furnaces — in some cases by up to 70%. As a result, for example, the overall length of such a continuous furnace, with short holding times of around 5 minutes for thick-walled sheet metal, is only about 30% of that of a pure radiant furnace, or only about 50% of a classic convection furnace. Overall, by optimising circulation and flow, furnace systems can be made smaller, since the thermal processes run correspondingly faster, thereby increasing throughput.

Optimising the insulation structure

The choice and combination of thermal insulation materials significantly affects the furnace’s characteristics in terms of energy consumption, heating and cooling rates, energy losses, stored heat, and therefore energy efficiency. Fibrous insulation materials, for example, have low mechanical strength but, by contrast, high insulating capacity and low heat-storage capacity. Heavy insulation materials such as refractory concrete or lightweight refractory bricks are mechanically highly resilient, have a large heat-storage capacity, and lower insulating effect. Only the optimal combination of different insulation materials (making the best use of insulating capacity, storage capacity, mechanical strength, and maximum application temperature), adapted to the specific application, enables energy-efficient furnace operation. Microporous thermal insulation materials, for instance, can reduce energy losses through the furnace walls by around 20% compared with conventional insulation structures, which typically also achieves a roughly 10 °C reduction in the outer furnace wall temperature.

Optimising burner technology

The economic viability and efficiency of a heat-treatment process depend on energy consumption per component or weight. Modern industrial furnaces, particularly for high operating temperatures, are generally equipped with recuperative or regenerative compact pulse-flame burners, which achieve a practical efficiency of around 75%. Burners with integrated regenerators even achieve efficiencies of more than 85%. Both burner types also allow furnace operation with very low CO₂ emissions while simultaneously minimising NOx emissions.

Electric heating systems are implemented in a wide variety of materials, depending on the application and temperature. In addition, electrically resistance-heated furnaces do not require the special permits needed for installing fuel-fired furnaces. In terms of environmental and occupational safety, low noise and heat exposure at the workplace are particularly worth mentioning. There are also no fuel exhaust-gas emissions. To achieve the highest possible energy efficiency for electric heating without exceeding surface loading limits, the maximum permissible element temperature must be observed.

Heat recovery

Waste-heat quantities should fundamentally be utilised wherever possible for energy efficiency. A particular focus today is on utilising waste heat from burner exhaust gases.

Utilising the heat from otherwise unused exhaust gases should especially be considered when new production halls are being built and the heat utilisation could be credited under the Energy Saving Ordinance (EnEV), since the investment generally pays off mainly for new builds or complete renovations, and heat use during the heating season. The Energy Saving Ordinance (EnEV) was part of German economic administrative law; it was replaced on 1 November 2020 by the Building Energy Act (Gebäudeenergiegesetz).

Waste-heat quantities can only be used where there is a thermodynamic gradient — i.e., a sufficiently high temperature difference between the source and the consumer. The thermal energy in the cooling water from quenching processes during hardening, for example, can be used directly for a cleaning process via appropriate heat exchangers. Using it for hall heating has become established particularly in new installations. It should be noted here that relatively large volumes of air must be moved for space heating, which in turn requires the provision of fan energy. Underfloor heating is simpler and more energy-efficient, and also offers interesting possibilities for outdoor areas (car parks, driveways). Using the warm water for sanitary purposes is also a very economically attractive option in terms of waste-heat utilisation. In this case, an additional water-to-water heat exchanger transfers shower and heating water at a relatively high temperature to a hot-water storage tank for the stated purposes. For larger quantities of heat, feeding into public district-heating networks also makes sense. From an energy standpoint, essentially all measures operated via cooling water/heat exchangers can be improved by raising the permissible cooling-water temperature at the points of use.

Re-cooling of oil-quenching baths is generally carried out via oil-water or oil-air plate heat exchangers, which are integrated into the circuits in the same way as water cooling. Heating of cleaning systems (40–80 °C) or component drying after cleaning can likewise be achieved using waste heat from oil-quenching baths via heat exchangers. Temperature differences between the oil and cleaning bath of more than 20 °C should be present, which is generally not a problem when suitable systems are chosen. For waste-heat utilisation from oil baths for drying (with or without a vapour condenser) on continuous systems that require an annual cooling-water demand of around 20,000 m³, an energy saving of around 15 to 20 kW is possible, meaning payback periods of around three years can be expected for such measures.

References

[1] Tobias Fleiter: Industrial Innovation: Pathways to deep decarbonisation of industry. ICF Consulting Services Limited and Fraunhofer Institute for Systems and Innovation Research (ISI)

[2] Tough targets (Editorial). In: Nature 522, Issue 7555, (2015), 128

[3] Olaf Irretier, Marco Jost, Julian Irretier: Measures for energy-efficient heat treatment in industrial furnace technology — aspects and notes on funding, Prozeßwärme x/2022

[4] Olaf Irretier, Marco Jost, Julian Irretier: Energy efficiency and heat recovery in heat treatment and hardening operations (Parts 1 and 2), Prozeßwärme 1 and 2/2021

[5] Helmut Egger: Decarbonisation of protective-gas atmospheres, Prozeßwärme 5/2022

[6] Gerald Hiller, Pierre Bertoni, Olaf Irretier, Marco Jost: Modular heat treatment using nitriding and low pressure carburizing, heat processing 4/2020

[7] Irretier, O.: Resource savings and energy efficiency in heat treatment shops. heat processing 12 (2014), No. 1

[8] Matthäus, R.: Energy efficiency and heat recovery in hardening operations — savings potential and funding pools. Presentation at the HärtereiPraxis conference, 2020

[9] Hiller, G.; Bertoni, P.; Jost, M.; Irretier, O.: Modular heat treatment using nitriding and low-pressure carburising. heat processing 18 (2020), No. 4

[10] Marcus Lodde, Effizienz-Agentur NRW: Promoting and launching innovation and resource-efficiency measures with government funding. Presentation at the HärtereiPraxis conference, 30 November 2021, Neuss

Authors

Julian Irretier (Source: IBW Dr. Irretier GmbH)

(Source: IBW Dr. Irretier GmbH)

Julian Irretier
IBW Dr. Irretier GmbH

Marko Jost (Source: IBW Dr. Irretier GmbH)

(Source: IBW Dr. Irretier GmbH)

Marco Jost
IBW Dr. Irretier GmbH

Dr. Olaf Irretier (Source: IBW Dr. Irretier GmbH)

(Source: IBW Dr. Irretier GmbH)

Dr. Olaf Irretier
IBW Dr. Irretier GmbH