The comprehensive establishment of a circular economy is essential for a sustainable industry. In this context, the DLR Institute of Future Fuels is looking in particular at carbon and sulphur cycles and the recycling of metals, and is developing processes to realise suitable industrial procedures in an efficient, resource-saving and economically viable manner.
Many people have long recognised that it makes ecological and economic sense to recycle and reuse materials and products instead of just using them once and then throwing them away. This behaviour is a necessary part of a sustainable lifestyle and will be unavoidable in the future as global demand for raw materials increases and resources become limited. The topic of the circular economy has therefore become increasingly important in recent years.
The circular economy initially describes a regenerative system in which the use of resources, waste, emissions and energy losses are minimised. This is achieved by slowing down, closing and shortening material and energy cycles. This can be achieved by reusing and reprocessing, refurbishing and recycling raw materials and products.
In 2020, the European Commission published an action plan as part of the EU Green Deal, which describes the expansion of the circular economy as a key contribution to achieving climate neutrality, decoupling economic growth from resource use and securing the EU's long-term competitiveness. The circular economy is a key research topic for the Institute of Future Fuels. In particular, it analyses the cycles of carbon, sulphur and metals and develops sustainable technologies.
Carbon cycles
The natural carbon cycle is essential for life on Earth. However, the absorption of CO2 from the atmosphere through photosynthesis, the formation of biomass and the formation of natural gas, crude oil and coal in the ground are comparatively slow processes. As a result, the concentration of CO2 in the atmosphere has been rising continuously since the beginning of industrialisation due to the burning of fossil fuels, leading to the anthropogenic greenhouse effect.
In order to limit global warming to 1.5 to a maximum of 2 degrees Celsius, greenhouse gas emissions must be drastically reduced in all sectors by 2030, according to the Intergovernmental Panel on Climate Change. The measures required to achieve this include reducing energy consumption through behavioural changes and more efficient appliances and processes, expanding electrification, better coupling of sectors, replacing fossil fuels with synthetic fuels and expanding energy storage systems. However, studies show that it will also be necessary to capture CO2 from industrial processes and CO2 already released into the atmosphere and store it in the long term (example: Mengis et al. 2022 "Net-Zero CO2 Germany - A Retrospect From the Year 2050"). In addition to conventional measures such as the reforestation of forests or the renaturalisation of moors, technical solutions are needed for this.
In this context, the Institute of Future Fuels is looking at processes such as direct air capture (DAC). The CO2 obtained in this process can either be used as a chemical feedstock, for example for the production of synthetic fuels - known as CCU (Carbon Capture and Utilisation) - or it can be stored permanently. In this carbon storage process, the captured pure CO2 is compressed, transported and pumped into a geological repository. This process is known as CCS (Carbon Capture and Storage).
New carbon cycles
Illustration of historical linear carbon economy burning fossil carbon (yellow arrows), and novel approaches allowing for a more circular carbon economy (light blue arrows) and carbon dioxide removal measures (dark blue arrows).
Credit:
Helmholtz-Klimainitiative / Tanja Hildebrandt
Sulphur cycles
Elemental sulphur (S) is a starting material for sulphuric acid (H2SO4), one of the world's most important and most produced basic chemicals. The chemical industry either mines sulphur from geological deposits, extracts it from sulphur-containing compounds of the fossil energy sources crude oil, natural gas and coal or from sulphide ores. Due to the limited geological deposits and the decreasing production volumes of fossil fuels, it will become increasingly important in the future to manage sulphur sustainably in recycling processes. The Institute of Future Fuels is developing such processes in which the sulphur is not consumed but recovered again after use.
A solar thermal system uses mirrors to concentrate sunlight and convert it into high-temperature heat energy. This high-temperature heat can be used to split sulphuric acid. The resulting fission products sulphur dioxide (SO2) and water (H2O) are the starting products for producing fresh sulphur. This can either be stored or burnt in a gas turbine to generate electricity. This in turn produces sulphur dioxide (SO2) as a fuel gas, which is fed into conventional sulphuric acid plants. This produces fresh sulphuric acid and a large amount of waste heat. The waste heat drives a steam turbine, which generates additional electricity. The fresh sulphuric acid is then available again for the splitting of sulphuric acid.
In another process, the SO2 produced during sulphuric acid decomposition can be used to produce hydrogen in so-called SO2-depolarised electrolysis (SDE). This produces fresh sulphuric acid as an additional product. The cycle process developed by the Institute of Future Fuels in this context is the so-called sulphuric acid hybrid process (HyS).
Simplified representation of the sulphur storage cycle
Solar thermal plants based on sulphur production and combustion can be operated effectively, particularly in sunny regions. The excess sulphur produced in this process can then be easily transported to regions with less sunlight.
Metals such as steel, aluminium, copper and lithium are core materials of our industrial world and often invisible, central building blocks of the everyday things we surround ourselves with. The use of metals has been part of everyday human life for thousands of years, but the range and scope of metals used has increased significantly in recent decades and has been expanded to include a large number of substances, such as rare earths, which include cerium or yttrium, or transition metals such as palladium and platinum. Electronic devices sometimes contain more than fifty metals.
The production and processing of metals are among the most energy-intensive processes in industry. The processes are often complex, consist of several steps and often require temperatures of over 1,000 degrees Celius to melt and process the metal. As they are usually carried out by burning fossil fuels, these processes have very high CO2 emissions. The production of steel is therefore one of the largest CO2 emitters in the industry. The scarcity of raw materials and unsustainable mining methods are one reason why the recovery and recycling of metals will be essential in the future. Recycling metals is also often less energy-intensive than producing them from primary raw materials.
Melting aluminium scrap
The High-Flux Solar Furnace in Cologne-Porz can be used to conduct high-temperature experiments with molten metals.
Converting the production, processing and recycling of metals to a sustainable basis are among the other circular economy processes being developed at the Institute of Future Fuels. The Institute is researching ways of integrating renewable energy sources. Process heat, for example, can be provided by concentrating solar systems. This technology can generate temperatures of over 1,000 degrees Celsius, which is why it is also interesting for melting metals or metal waste. The utilisation of green hydrogen is also an issue. For example, it can be used as a reducing agent in the production and processing of steel, thereby reducing CO2 emissions.
