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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
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.
Centrifugal particle receiver during operation on DLR solar tower
Pile of solid sulphur particles