An innovative thermochemical cycle based on solid sulphur for integrated long-term storage of solar thermal energy

Project Information

SULPHURREAL

Grant agreement ID: 101115538

DOI

10.3030/101115538

EC signature date 20 June 2023

Start date 1 October 2023

End date 30 September 2026

Funded under

• The European Innovation Council (EIC)

Total cost

€ 3 982 133,00

EU contribution

€ 3 982 133,00

Coordinated by

DEUTSCHES ZENTRUM FUR LUFT - UND RAUMFAHRT EV
Germany

Periodic Reporting for period 1 - SULPHURREAL (An innovative thermochemical cycle based on solid sulphur for integrated long-term storage of solar thermal energy)

Reporting period: 2023-10-01 to 2024-09-30

Summary of the context and overall objectives of the project

SULPHURREAL sets forth an innovative approach for thermochemical direct storage of concentrated solar irradiation-harvested energy to solid elemental sulphur, a very energy-rich chemical. The basic idea is to use Concentrated Solar Energy (CSE) systems to cyclically drive a series of three chemical re-actions that interconvert sulphuric acid and sulphur, namely sulphuric acid (H2SO4) decomposition, sulphur dioxide (SO2) disproportionation and sulphur (S) combustion. In the first step, sulphuric acid is first vaporised and then thermally decomposed into sulphur trioxide (SO3) and steam (H2O) at temperatures between 450-500oC; subsequently sulphur trioxide is split to sulphur dioxide (SO2) and oxygen (O2) in a catalytic reactor at temperatures in the range 600-950oC; this is the highest-temperature endothermic reaction step. The heat required can be provided by CSE instead of combustion of fossil fuels, since these temperatures are within the capabilities of state-of-the-art CSE tower plants. In the next step, SO2 disproportionation by reaction with water with the aid of a catalyst or electrical power, a part of the sulphur contained in the SO2 produced in the first step, is converted to solid sulphur whereas the other part is converted to sulphuric acid that can be fed back into the first step. Hence, elemental sulphur - that can be stored for long time in stockpiles in open air and easily transported - stores a significant proportion of the solar energy used to decompose the sulphuric acid. In the third, on-demand, step, sulphur is combusted in air. The resulting high-temperature - in excess of 1200 °C - combustion gas product is suitable for driving a gas turbine which can produce electricity supplied to the grid. The SO2 produced from combustion can be converted back into S and H2SO4 to close the cycle. The overall objective is to identify, develop and test novel catalysts and reactor designs under operating conditions so that the sulphur trioxide splitting and the sulphur dioxide disproportionation steps can be performed in sequence with maximum compatibility in a first-of-its kind integrated approach. In parallel, SULPHURREAL will further develop and upscale a first-of-its-kind sulphur burner capable of operating at 10 times higher power density than that of conventional sulphur combustion.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

• Computational materials screening was performed via first-principles calculations taking into consideration environmental and cost aspects of known metal oxide catalysts for sulphuric acid splitting. Plausible pathways for this reaction were identified, concluding that good metal oxide catalysts predominantly act via a surface oxygen vacancy-mediated mechanism.
• More than 80 different oxide-based sulphuric acid splitting catalytic compositions were synthesized, distinguished in Medium temperature – MT (≤ 650oC) Vanadium-based catalysts and High temperature – HT (> 800oC) Iron- and Copper-based catalysts. Their performance evaluation has so far resulted 7 MT and 4 HT compositions around or above the target of 75% of the thermodynamic SO3 conversion.
• Rigid and thermally stable porous structured ceramic honeycombs made of commercial iron oxide, were successfully prepared and are currently tested as sulphuric acid splitting HT catalytic structures. The targeted porous structure specifications will then be implemented directly from industrial by products (“wasterials”).
• Catalytic and alternative, non-catalytic SO2 disproportionation routes were explored and comparatively assessed with the aim to design and construct a laboratory scale set-up.
• The benchmark process of iodide-catalyzed homogeneous SO2 disproportionation was studied in a neoteric in situ Raman spectroscopy-monitored batch reactor and the mechanistic pathway of the reaction under mild conditions was elucidated.
• Sulphur combustion possible reaction paths were investigated via ab initio quantum chemistry calculations and an extended mechanism with new reactions has been developed. Auto ignition delay time was calculated and a burner configuration for its experimental determination was designed and manufactured. Suitable coatings and substrate materials for turbine blades and combustor components were selected and the coating processes are ongoing.
• A preliminary version of the process flowsheet was already drafted, along with input for training the machine learning model. A project’s partnership workshop resulted in a foundational environmental impact analysis to guide the selection of materials and processes.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

• First principle calculations demonstrated that viable STS catalysts require sufficiently low surface oxygen vacancy formation energies as well as a low affinity for the adsorption of SO3 to prevent catalyst poisoning.
• An innovative protocol and a neoteric methodology for the deep understanding of the mechanistic routes involved in the benchmark homogeneous iodide-based disproportionation route under mild conditions - temperature T=120oC and SO2 partial pressure of 9.6 bars - via advanced in situ molecular spectroscopy techniques has been established. The introduced new concept of an in-situ Raman batch reactor enables the study of any SO2 disproportionation route. The mechanistic route of the homogeneous iodide-catalysed disproportionation is completely understood at the molecular level. The SO2 reacting pressure, reported at ca 40 bar in the state-of-the-art was kept herein to a value lower than 10 bars.
• An electrochemical SO2 disproportionation reactor that represents a novelty with respect to the current state of the art was developed. The advantages are the possibility to operate directly with gaseous SO2 at 1 bar, to work at low temperatures (40°C), to easily retrieve sulfur and to apply relative low voltages, below 1V.