9 December 2021
Researchers from the DLR institutes of Future Fuels and Solar Research have worked with European research partners in the PEGASUS research project to further develop a cyclic process that can be used to store solar energy in sulphur. They have also investigated what adjustments are needed to enable gas turbines to use sulphur instead of fossil fuels to generate CO2-free electricity.
Elemental sulphur (S) is a starting material of sulphuric acid (H2SO4), one of the world's most important and widely produced basic chemicals. The chemical industry either mines sulphur from geological deposits, extracts it from sulphur-containing compounds of the fossil fuels oil, natural gas and coal, or from sulphide ores. Due to the limited geological deposits and because of the decreasing production quantities of fossil fuels, it will become more and more important in the future to sustainably manage sulphur in recirculation processes. In such processes, the sulphur is not consumed but recovered after use.
Sulphur can store solar energy over the long term and with low energy losses
In the future, sulphur could play a role as a fuel in gas and steam turbine power plants to generate CO2-free electricity. However, the burning of sulphur only becomes truly sustainable when the emissions enter a sulphur cycle process that generates fresh sulphur again via the intermediate step of sulphuric acid and by means of concentrated solar energy. Due to its high energy density, sulphur is a promising storage medium for solar thermal power plants. Its energy density is 30 times higher than that of molten salt, which in today's solar thermal power plants absorbs, transports and stores solar energy as high-temperature heat. Another advantage of sulphur is that ships, rail cars or trucks can easily transport it as a powder or in liquid form. Storage of sulphuric acid and sulphur is also a common process for the chemical industry: special tanks hold the sulphuric acid, and sulphur is simply stored in stockpiles. Sulphur produced with renewable energy therefore seems ideally suited as a substitute for fossil fuels such as coal and gas to generate base-load electricity.
Still to be explored, however, is the recycling of sulphuric acid using concentrated solar radiation and the disproportionation process that converts the products of sulphuric acid cracking into sulphur. In addition, the further development of conventional sulphur combustion for use in gas turbines is a relevant research topic. The sulphur burners used in sulphuric acid production burn the sulphur under ambient pressure. However, if the resulting gases are to drive a turbine, the burner must operate at sufficiently high pressure. To date, no suitable burners have been available for this purpose.
A hybrid power plant with solar thermal part and connected sulphur-fired gas turbine could produce renewable electricity 24/7
In the PEGASUS project, researchers from DLR, KIT and European partner companies have investigated parts of a chemical sulphur cycle to store solar energy in sulphur (see figure).
If a solar thermal plant is integrated into the sulphur cycle, it can provide the high temperatures required for the cracking of sulphuric acid from concentrated solar radiation. In the PEGASUS project, DLR's particle technology was used for this purpose, heating ceramic, sand-shaped particles with concentrated solar energy. They store the heat and transfer it to a sulphuric acid splitting reactor. The resulting decomposition products, sulphur dioxide (SO2) and water (H2O), are the starting products in the connected sulphur dioxide disproportionation process to produce fresh sulphur. It can either be stored or burned in a suitable gas turbine to generate electricity. In turn, sulphur dioxide (SO2) is produced as the combustion product , which is fed to conventional sulphuric acid plants. There, the so-called contact process produces fresh sulphuric acid and a large amount of waste heat, which drives a steam turbine process that generates additional electricity. The fresh sulphuric acid is again available for sulphuric acid splitting. When solar radiation is available, the solar sulphur power plant can produce a surplus of sulphur. During periods of particularly high electricity demand or when the sun is not shining, for example when clouds are passing overhead or during the night, the stored sulphur allows the power plant to operate continuously, producing a surplus of sulphuric acid.
Electricity generation by means of sulphur combustion can take place continuously in 24-hour operation, depending on demand. The cycle process makes it possible to generate renewable base-load electricity at a constant production rate, while sulphuric acid and sulphur serve as energy carriers and are circulated with virtually no losses. It is even conceivable to operate the solar thermal plant for sulphur production at a solar site in the desert and transport the sulphur from there to regions without sufficient solar radiation, such as Germany, to produce electricity and sulphuric acid there.
Pilot operation of the solar prototype in DLR's Synlight high-power radiator
In the PEGASUS project, the researchers were primarily concerned with testing the sub-process of solar sulphuric acid cracking and the use of sulphur as a fuel in gas turbine power plants. The high degree of innovation of the two sub-processes was very challenging for the researchers. Never before, for example, have solar-heated particles been used to split sulphuric acid. Nor has the combustion of sulphur at elevated pressure for use in gas turbines been studied before.
To achieve the high temperatures of around 900 degrees Celsius favourable for solar sulphuric acid splitting, the researchers combined a newly developed sulphuric acid splitting reactor with a solar radiation receiver previously developed at the DLR Institute of Solar Research using ceramic particles as heat transfer and storage material.
In a particle receiver, small ceramic particles take over the absorption and transport of the irradiated thermal power to generate electricity and industrial process heat. The liquid salts currently used as heat transfer media only reaches temperatures of about 550 degrees Celsius. The hot ceramic particles enable power plant operators to operate at significantly higher process temperatures of over 900 degrees, resulting in higher efficiencies and thus lower electricity generation costs.
For the demonstration operation, DLR scientists installed a prototype of the particle receiver CentRec developed for the process in DLR's high-power emitter Synlight in Jülich. During the same period, they investigated the integration of sulphuric acid splitting into the sulphur cycle in a DLR laboratory.
As a central project result, the researchers were able to demonstrate all essential steps of the process of solar sulphuric acid splitting using solar-heated particles:
"We succeeded in developing and demonstrating all the important elements of the process chain for sulphuric acid cracking with solar particles, thus showing the way for future CO2-free sulphuric acid recycling using solar energy," summarizes Dennis Thomey from the DLR Institute of Future Fuels. "Our project partners at KIT decisively further developed the sulphur burner technology with experiments on a new prototype for pressurized operation in gas turbines."
Researchers at the DLR Institute of Future Fuels are working on the disproportionation process in the associated NRW-funded BaSiS project. The solar centrifugal receiver with its integrated thermal storage concept will then be linked to a newly developed moving bed particle reactor for sulphuric acid splitting.
The PEGASUS project received funding of 4.7 million euros from the European Commission's Horizon 2020 program. Initially, the DLR Institute of Solar Research was the coordinator of the project. When the Institute of Future Fuels was founded from the Institute of Solar Research, project management was transferred to the new institute. (→ Project-Website)
Other project partners:
The Greek research institute APTL/CERTH supplied the catalyst materials for sulphuric acid cracking. The German research university KIT developed the sulphur burner for the high pressures required. Testing of the sulphur burner took place at KIT in Karlsruhe. The Polish company Baltic Ceramics was initially responsible for the production of advanced ceramic particles, but left the project after three years. The Italian company Processi Innovativi, which has since been renamed NextChem, was responsible for process simulation and techno-economic study. The Israeli company BrightSource designed the solar field, provided input to the process simulation, and filled the role of future end user.