Hybrid melting furnaces: The way to emission-free raw metal production?

Melting furnaces face new challenges due to legal requirements and growing social pressure: Heating with oil or gas is becoming increasingly unprofitable as a result of EU directives and German climate protection targets. Hybrid furnaces, which dynamically switch between different energy sources depending on the electricity mix, could be a solution. Technically, however, there are difficulties in implementing this, especially with larger plants.

First, the Paris climate protection agreement, then the climate summits in Bonn and Katowice and finally numerous public discourses make it clear: Climate and environmental protection will play an important role in business in the future. By 2020, Germany wants to emit 60 per cent less CO2 than in 1990 - and this target is not within sight yet. Also the CO2 certificate trade is controversial. Gas, coke and coal as energy sources are not permanently available, so that at least a twin-track approach makes sense.

Even now, there are numerous solutions that rely on electrical melting processes - however, for the most part because of process engineering reasons. But climate protection can also benefit from a hybrid system of gas and electric furnaces. Should the German power grid be fed in particular from renewable sources, hybrid furnaces could dynamically switch from classic combustion products to green electricity. The climate balance could thus be improved, especially during the day.

Just by taking a look into the future, one can see how important a rethink is: By 2050, according to the German government, renewable energy sources should cover 80 per cent of the total energy requirement, while only 20 per cent comes from fossil fuels. Coal, natural gas and oil will thus largely cease to be sources of heat.

It is the challenge to be prepared for the transition: Hybrid systems require redundancy, dual electronics, more frequent maintenance and trained personnel. Despite savings from more efficient raw metal production, profits are harder to generate due to higher operating and acquisition costs.

There are numerous processes that enable melting or heating processes by means of electrical energy. Direct heating processes include induction heating, which is frequently used for thin sheets and the specific heating of small areas. In addition, there are also dielectric and conductive processes.

There are many indirect heating solutions. The most classic ones are resistance heating conductors. Temperatures of up to 1,200 degrees Celsius can be achieved by using nichrome resistors. Graphite conductors are able to reach temperatures of up to 2,000 degrees Celsius. These temperatures are already sufficient for many furnaces.

Electric arc furnaces reach maximum temperatures of up to 10,000 degrees Celsius. As a result of their high active power, they can melt down, in large plants with an output of more than 100 megawatts, more than one million tonnes of raw steel a year. The furnaces, otherwise known as EAFs (Electric Arc Furnace), now account for more than a quarter of the steel produced, partly because almost no other melting technology can compete in terms of recycling. Reprocessing from steels that are no longer needed will continue to be of increasing importance in the future, because significantly less energy is required than in the extraction of raw ores.

An electric arc furnace not only requires electrical energy, but also gases that in principle damage the climate balance. Additional burners, for example, add natural gas and oxygen to the process in order to heat more cleanly. Nevertheless, the CO2 balance is better than that of a normal furnace, and the total energy requirement per tonne of produced steel is also lower.

Theoretically, larger pusher furnaces can be converted from gas to electric heating elements. NiCr heating elements reach up to 1,200 degrees and can thus be used for many metals. In a direct comparison between gas and electric furnaces measuring approximately 20 x 15 metres and with a production capacity of approximately 200 tonnes per hour, the gas furnace has an efficiency of 60-70 per cent and the electrically heated furnace is up to 85 per cent. One tonne of raw steel requires 300 kWh in the gas furnace, the melt with resistance heating elements is approximately 60 kWh less.

Hydrogen could be used as an energy carrier as well. At the same time, it can also be used as a reducing agent in production. In Germany, however, there is still no existing hydrogen network that could guarantee national distribution and reliability similar to natural gas. The success of hydrogen for furnaces depends largely on whether such a network will be built for electric mobility or whether batteries remain the number one source of electricity.

The retrofitting process is indeed a costly one. However, the use of electrically operated heating elements offers a number of advantages, not least because it is independent of gas and other fuels that emit climate-damaging by-products. The use of electric ovens will continue to increase over the next few years; perhaps one day it will even be necessary to convert to electricity.