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Renewable Power Generation by Solar Particle Receiver Driven Sulphur Storage Cycle

Periodic Reporting for period 2 - PEGASUS (Renewable Power Generation by Solar Particle Receiver Driven Sulphur Storage Cycle)

Reporting period: 2018-05-01 to 2019-10-31

The overall objective of PEGASUS is the development and demonstration of an innovative solar tower receiver based on solid particles combined with a novel thermochemical solar energy storage technology based on elemental sulphur, to achieve dispatchable and firm renewable electricity generation with a significant cost reduction vs. current state-of-the-art molten salt concepts. In this perspective, the project’s specific technical objectives are to:
• Synthesize catalytically active proppants that can demonstrate in combination: conversion of SO3 to SO2 and O2 close to respective thermodynamic value, low catalytic deactivation in long-term exposure to reaction conditions, high solar absorptivity and low losses due to abrasion.
• Synthesize the necessary large-scale quantities (>3 tons) of such particles and demonstrate on-sun the capability of a prototype 500 kWth centrifugal particle solar receiver for heating them to temperatures >900 °C.
• Design a high-temperature particle storage system that can maintain particle temperatures >900 °C for at least 6 hours.
• Design, build and operate a laboratory prototype sulphuric acid decomposition cascade comprised of a sulphuric acid evaporator and a SO3 decomposer heat exchanger, employing moving heated particles as the heat source required for both these processes.
• Develop and realize a novel lab-scale sulphur burner in the 10 kW range able to modulate up to 15 bar outlet pressure for gas turbine applications.
• Demonstrate the feasibility of the overall process, draft the complete flowsheet and techno-economic analysis of an optimized integrated process scaled-up to 5 MWth, assess the technology and evaluate its potential for realizing a sulphur storage cycle enabling solar power production at levelized electricity costs (LCOE) of 0.08 €/kWh.
The work carried out until project month 36 comprises 6 tasks:

1) Development, characterization and shortlisting of catalytically active particles for solar sulphuric acid splitting.
• Evaluation of oxide proppants modified to include SO3 dissociation catalytically active materials for use as direct solar irradiation absorbing media.
• Abrasiveness in the range of 5 % measured for selected unspent proppants.
• More than 80 catalytic activity screening experiments performed and 7 materials found to demonstrate promising compromise among parameters.
• Copper oxide-containing compositions selected as best catalytic material; their durability testing for >1000 hours demonstrated conversion of >60 % with no catalytic activity decrease and no degradation in crushing strength.
• Absorptance measurements of selected modified proppants demonstrated:
- Very high pristine-state absorptance, >94 %.
- Absorptance >90 % after thermal-only treatment, thus minor thermal aging effect.
- Absorptance reduction to 84 % after on-SO3/steam-stream exposure due to color alteration; considered non-critical in cavity receivers where reflection losses are minute.
• Copper oxide-containing proppants less flowable and of lower mechanical strength than commercial sintered bauxite ones; flow restrictions/dust discharge during operation possible.
• Commercial sintered bauxite proppants demonstrated non-negligible catalytic activity.
• Iron-oxide-based particle/honeycomb/foams also showed notable catalytic activity.

2) Operation of particle receiver demonstrator on solar tower.
• Prototype 500 kWth centrifugal receiver operated with inert bauxite particles on solar tower Juelich achieving particle outlet temperature 925 °C.

3) Centrifugal particle solar receiver optimization
• Receiver efficiency calculations showed that efficiencies >90 % can be achieved at high incident flux densities (>1700 kw/m²); very promising since until now the receiver has been experimentally tested under solar loads way below its foreseen operation (at ~200 kW/m², having been designed for 2-2.5 MW/m²).
• By minimizing the cavity depth/diameter ratio, reduction of receiver cost by 60% can be achieved with consequent LCOH reduction of 12%.

4) Design of laboratory particle reactor prototype for sulphuric acid splitting
• Moving heat transfer-only bed and non-moving catalytic bed configuration with indirect heat exchange between particles and fluid selected as best solution.
• For sizing, 1-D model created including correlations of heat transfer coefficients for particle and gas side.
• Design parametric studies with a sulphuric acid flow rate of 1.9 kg/h (75 wt%) at a particle mass flow of 10 kg/h resulted in a total heat duty of ~2 kW.
• Complete design of particle reactor for sulphuric acid splitting developed.
• 50 iron-oxide-catalyst-coated ceramic foams prepared.

5) Development of sulphur combustor prototype.
• Suitable burner concepts for high power density sulphur combustion elaborated, identifying two different kinds of air flow fields as most promising to be tested experimentally.
• Scheme of laboratory test rig for liquid sulphur combustion elaborated.
• Pre-filming airblast atomizers identified as advantageous for specified operating conditions for application in gas turbine.
• Test rig for sulphur atomization under construction and most parts completed.
• CFD isothermal calculations of flow in nozzle performed; first results of spray dispersion on isothermal flow field available.
• Simplified kinetics mechanism for sulphur combustion integrated in CFD model to determine impact of different resulting flame velocities on combustion process.
• Design of laboratory-scale combustor completed.

6) Flowsheet development of overall process and solar field design.
• First version of overall conceptual plant design and relevant preliminary flowsheets developed.
• Solar field optimization performed for plant capacities of 12 and 96 MWth at Noor site in Morocco as selected reference plant location.
Progress beyond state of the art:
• High-strength, hybrid catalytic/solar-irradiation-absorbing particles, with SO3 conversion >60 % over >1000 h on stream.
• Prototype 500 kWth centrifugal receiver operated with inert particles on solar tower achieving particle outlet temperature of 925 °C.
• New, improved, quantum mechanics-based chemical kinetics mechanisms for sulphur combustion.
• Development, construction and experimental demonstration of first-of-its-kind, high power density sulphur burner compatible with gas turbine operation.

Expected results
• Construction and lab testing of sulphuric acid splitting particle reactor; relevant design model validation.
• Experimental operation of prototype sulphur burner; relevant design model validation.
• Over-all plant process simulation and techno-economic analysis.

Potential impacts
• Higher process efficiency of solar thermal power plants operating with particle streams as heat transfer fluids due to higher operating temperatures.
• Significant costs reduction due to use of cheap solid sulphur as storage material, bypassing need of loss-intensive heat exchangers.
• Practically indefinite seasonal storage of solar thermal energy.
• Integration of power generation via “solar” sulphur into “traditional” chemical industries (e.g. sulphuric acid production, refineries), reduction of their dependency on fossil fuels.