Solar Hybrid Fuels

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The Solar Hybrid Fuels project looked at two streams of work. The first stream aimed to make synthetic gas (syngas) from natural gas at lower temperatures compatible with conventional solar thermal storage while maintaining high efficiency, to allow for the 24/7 operation of an industrial solar fuels plant. The second stream of work produced a detailed roadmap and recommendations for solar hybrid fuel technology options and opportunities suitable for Australia.Key resultsStream 1 – Syngas from natural gas at lower temperatures compatible with conventional solar thermal storage, while maintaining high efficiencyThe membrane reactor configuration allows in-situ removal of H2 to drive the equilibrium-limited steam methane reforming (SMR) and water gas shift (WGS) are usually performed in separate stages, at different temperatures and with different catalysts, the membrane reformer requires a single catalyst with high activity to both reactions simultaneously. A bi-functional catalyst containing nickel and copper as active components was found to exhibit higher SMR activity than a commercial Ni-Al2O3 reforming catalyst, and higher WGS activity than a commercial Fe2O3-Cr2O3 WGS catalyst, at 550å¡C. This catalyst also offers enhanced resistance to metal dusting and carbon dusting.Laboratory-scale testing demonstrated that very high CH4 conversion and H2 yield can be achieved at 550 degrees celsius. It was also demonstrated that high conversion and yield can be achieved in a larger-scale reformer when using a heat transfer fluid as the only heat source. The membrane reformer configuration also had the effect of promoting the water gas shift reaction, as the in situ H2 extraction forces both the SMR and WGS reactions to the product side. The result is a product which contains significantly less CO than a typical NG-derived syngas.Appropriate temperatures were achieved through the reformer when using CO2 or air as a heat transfer fluid. A key consideration, however, is the pressure drop across the reformer and additional stages. As creating a turbulent flow is vital to the function of a heat exchanger, and as turbulence creates resistance, it is essential that the heat transfer fluid (HTF) recirculation system employed by the thermal energy storage (TES) system can tolerate significant flow resistances. The pressure drop across the reformer at the targeted CO2 HTF flow rate was 0.5 bar which was at the limit of what could be delivered by the existing blower in the TES system. To enable the trial to proceed, the CO2 HTF was vented from the system, and this would obviously be unviable in practice.The low-temperature, solar-integrated reformer can reduce natural gas consumption by as much as 25 % over conventional reforming technology. The overall cost of hydrogen produced by a solar-integrated reformer is greater than from a non-solar membrane reformer, however, because the added cost of the solar input outweighs the savings associated with the reduction in natural gas consumption, based on current prices of natural gas and solar heat.Stream 2 – Concentrating solar fuels roadmapA key learning of the concentrating solar fuels roadmap from participants at the stakeholders’ workshops, was that industry is very risk averse and needs to have good drivers to invest. The project team, mainly engineers and scientists, was reminded that addressing technical risk was only part of the issue, and that externalities such as carbon pricing, environmental credentials, incentives or mandates and public perception were all equally if not more important. Clearly engaging with industry and government on all levels is important for understanding the drivers for and barriers to action.Screening and techno-economic evaluation of a wide range of solar fuels options concluded that solar fossil hybrid systems could produce conventional liquid fuels at costs close to conventional oil derived fuels (at $100/bbl), with roughly a 30% reduction in CO2 emissions overall. It was recommended that a range of technologies be developed in parallel to provide a path to progressive decarbonisation. The study also highlighted that the Australian economy is dependent on energy exports, and that concentrating solar fuels may be a promising future export opportunity in a carbon constrained world.The analysis carried out includes an evaluation of options for large scale (~100 MWe) production of hydrogen from solar energy alone. The analysis suggests that Concentrating Solar Thermal approaches to large scale solar hydrogen production are likely to be less than half the cost of Photovoltaics plus electrolysis. The key cost driver in the electrolysis case is utilisation of the electrolyser, which is limited to the availability of renewable energy (25% for stand-alone PV). Utilisation of other sources of renewable energy, or grid sourced generation, were outside the scope of this report. The competitiveness against advanced biofuels options will very much depend on the cost of biomass available. There is only a limited supply potential for cheap biomass so both approaches have a role. Competitiveness against fossil plus Carbon Capture and Storage is hard to determine at this stage. Overall it would be suggested that major policy initiatives should be made in a technology neutral manner such that all approaches can compete in new zero emission fuel markets.

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