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Renewable Power Generation by Solar Particle Receiver Driven Sulphur Storage Cycle

Periodic Reporting for period 3 - PEGASUS (Renewable Power Generation by Solar Particle Receiver Driven Sulphur Storage Cycle)

Reporting period: 2019-11-01 to 2021-06-30

The overall objective of PEGASUS is the development and demonstration of an innovative solar tower receiver utilizing solid particles (proppants), 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 with respect to current state-of-the-art molten salt concepts. The specific technical objectives are to:
• Devise an effective concept for employing the proppants-harvested solar irradiation heat directly or indirectly in the implementation of the sulphuric acid evaporation/SO3-splitting chemistry endothermal reactions, capable of 24/7 operation.
• Develop catalytically active systems 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 or chemical inactivity.
• Synthesize/procure the necessary large-scale quantities (>3 tons) of proppants and demonstrate on-sun the capability of a prototype 500 kWth centrifugal particle solar receiver for heating them to temperatures > 900 °C.
• Design, build and operate a laboratory prototype sulphuric acid decomposition cascade consisting 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, assess the technology and evaluate its potential.
The work carried out comprised 5 tasks:

1) Development, characterization and shortlisting of catalytically active systems for solar sulphuric acid decomposition.
Evaluation of modified oxide proppants with respect to their potential simultaneous operation as SO3 splitting catalysts and direct solar irradiation absorbing media:
• Catalytic & thermomechanical evaluation of > 50 samples, identified Cu-Mn-containing bauxite compositions as best, after:
- durability testing for >1000 hours demonstrating conversion of > 60 % at 850 oC with no catalytic activity decrease and no degradation in crushing strength.
- measurements demonstrating very high pristine-state absorptance > 94 % and minor reduction after on-SO3/steam-stream exposure.
• Cu-Mn oxide-containing proppants less flowable and of lower mechanical strength than commercial sintered bauxite ones; flow restrictions/dust discharge during operation possible.
Shift of concept to non-moving catalytic bed systems heated by solar-irradiated non-catalytic moving commercial particle streams.
• Preparation of 49 iron oxide-coated silicon carbide foams with:
- High catalyst loading with minute catalyst-induced pressure drop.
- Near-equilibrium 89 % SO3 conversion at 850 oC, reproducible in on-stream exposure > 360 hrs.

2) Operation of centrifugal particle receiver demonstrator and receiver optimization.
• Prototype 500 kWth centrifugal receiver operated with inert bauxite particles on solar tower under solar load up to 20 % (way below its foreseen operation at 200 kW/m², designed for 2-2.5 MW/m²) achieving particle outlet temperature 925 °C; calculated efficiencies can be > 90 % at high incident flux densities (> 1700 kw/m²).
• Second prototype 300 kWth centrifugal receiver manufactured, installed and tested in solar simulator under higher solar load of 40 %, achieving efficiency of 81 ± 20 % at 757 kW/m² incident radiation.
• Reduction of receiver cost by 60% achievable with consequent LCOH reduction of 12%, by minimizing the cavity depth/diameter ratio.

3) Design and operation 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.
• Complete design and sizing of 2 kW particle reactor for sulphuric acid splitting.
• Thermal pre-tests of reactor without sulphuric acid achieving 890 °C particle bed temperature at heater outlet.
• Chemical test demonstrating sulphuric acid vaporisation and SO3 splitting.
• Coupling of 300 kWth centrifugal receiver with moving bed heat-exchanger prototypes successfully demonstrated in solar simulator with water/steam; thermal efficiency up to 70 %.

4) Development of sulphur combustor prototype.
Sulphur combustion test rig successfully commissioned:
• Novel lab-scale sulphur burner prototype operated at power densities > 5 MW/m³ (target> 1.5 MW/m³) at atmospheric pressure.
• Experimental confirmation of very broad power modulation capability in terms of relative pressure drop and equivalence ratio at atmospheric pressure, allowing all operating conditions relevant for targeted gas turbine application.
• No degradation of burner/combustor materials after operation for several hours.
• Validation of burner capability to operate well at elevated pressure via numerical simulations of pilot-scale-combustor.

5) Flowsheet development of overall process and solar field design.
• Overall conceptual plant design and relevant flowsheets developed.
• Solar field optimization performed for plant capacities of 12 and 96 MWth at 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.
• High catalyst-content, low pressure-drop, iron oxide catalytic foams with near-equilibrium SO3 conversion 89 % over 360 h on-stream.
• Prototype 500 kWth centrifugal receiver operated with inert particles on solar tower achieving particle outlet temperature of 925 °C.
• Development, construction and experimental demonstration of first-of-its-kind, particles-heated sulphuric acid splitting reactor.
• Prototype 300 kWth centrifugal receiver in solar simulator successfully demonstrating coupled operation potential with particles moving bed heat exchanger new prototype for thermal operation with water/steam through transportation of 500 kg of hot particles.
• New, improved, quantum mechanics-based chemical kinetics mechanisms for sulphur combustion, operated at power densities > 5 MW/m³
• 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 particles-heated reactor and demonstration of its potential for coupled operation with centrifugal receiver.
• 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.
• Technology transfer to solar energy-driven implementation of other endothermal reactions.
Centrifugal particle receiver during operation in DLR solar simulator
Sulphur burner for gas turbine application at KIT
Pile of solid sulphur particles
Particle reactor for sulphuric acid splitting in DLR lab
Centrifugal particle receiver during operation on DLR solar tower