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Hydrogen from Solar Thermal Energy: High Temperature Solar Chemical Reactor for Co-production of hydrogen and carbon black from natural gas cracking

Final Report Summary - SOLHYCARB (Hydrogen from Solar Thermal Energy: High Temperature Solar Chemical Reactor for Co-production of hydrogen and carbon black from natural gas cracking)

SOLHYCARB project developed a nonconventional route for simultaneous production of a rich hydrogen gas and a high-value nano-material, named carbon black (CB), by natural gas (NG) decomposition by concentrated solar energy. In that way both H2 and a marketable material are produced using renewable energy. The main scientific and technical challenges addressed were the design and operation of high temperature solar chemical reactors containing nano-sized particulates, the production of two commercially beneficent products with desirable properties in the same reactor and the proposal of a methodology for solar reactors scaling-up based on modelling and experimental validation. The reactors operating temperature range of 1500 K to 2300 K was also a significant issue, since material limitations arose.

The project aimed at designing, constructing and testing solar reactors (SR) of different scales, and thus included the manufacture of three different SRs, a 10 kW direct heating tornado reactor (SR10W), a 10 kW indirect heating tubular reactor (SR10C) and a 50 kW indirect heating tubular reactor (SR50) at a pilot scale. As part of the reactor modelling works the following main points were covered:
1. methane decomposition kinetics;
2. comparison of a kinetic model with experimental results obtained from SR10C;
3. model development for carbon particles formation and growth;
4. complete model development including radiation heat transfer, heat and fluid flow and chemical reaction in solar reaction prototypes and
5. comparison of the complete model with experimental results obtained from the 10 kW scale.

From the SRs testing and qualification it occurred that the SR10W tornado solar reactor operated successfully. The influence of the operating temperature was significant for CH4 conversion in the SR10C reactor while the methane conversion was total for temperatures higher than 1823 K and space timed higher than 25 ms. For SR50 pilot scale reactor nine experiments were carried out, however the carbon yield never exceeded 63 %, while representative quantities of CB were obtained for further characterisation with respect to the industrial product.

Bag filter was designed and tested successfully for product separation during the experimental campaign, and PSA was the selected process for pure hydrogen acquisition. It was shown, both experimentally and through simulation, that a residence time of 10 ms was necessary in order to avoid hazardous molecule (PAH) formation, and gloves and respiratory protection were employed for handling nanoparticles throughout the project.

The produced CB had BET surface area corresponding very well to the commercial product, while the higher operating temperatures resulted in higher crystalline level of carbon particles. Obtained CB has lower electrical conductivity, charge carrier concentration and mobility compared to the industrial conductive CB; however these properties were seriously approaching the industrial material ones. As such significant progress in CB production was obtained by the solar reactor up-scaling and the reaction conditions improvement, and the obtained CB was close to the qualitative requirements for use in polymers, primary and secondary batteries.

A 10 MWth solar reactor was designed based on the validated simulation model. The construction of smaller scale reactors was proven non beneficial through preliminary evaluation. The heliostat field, in consideration of all losses, was of 20 000 m2 area with the acceptance angle of the CPCs no more than 30 degrees. Three absorber cavities on a tower receiver were planned, with assumed efficiency of 90 %. A flow sheet of the complete solar process was prepared, including the SR, a heat exchanger, a filter for particle separation, a compressor and a PSA separator. The products of the solar process were either hydrogen and carbon or electricity and carbon. The energy payback time was equal to one year. The process profitability was proved depending on the SR performances, the produced CB quality and the cost estimation hypothesis. For future industrial use of SOLHYCARB results a 400 MWth SR should be demonstrated, since it is the mean power delivered by one CPC of the 10 MWth SR, as well as a high temperature heat exchanger aiming at heat recovery at the SR outlet.