(33a) Solar Steam Reforming for Hydrogen Production Using Solar Salts As Heat Transfer Fluid

Introduction

The application of solar-thermal power to directly drive heat demanding thermochemical conversion is one of the most rational ways to exploit solar energy. This approach enhances the reduction of carbon footprint of chemical conversion, and allows the chemical storage of solar energy. Particularly, in fuel refinery it is possible to improve the heating value and the environmental impact (i.e. the overall “quality”) of the primary feedstock by the aid of solar energy: in this case the final product, often called “solar fuel”, will partially or totally drive solar energy in its chemical energy.

Additionally, the worldwide hydrogen demand is increasing, and its use in Fuel Cells (FCs) is expected to make hydrogen one of the foremost fuels in a future more sustainable energy system, mainly thanks to its low environmental impact when used in FC systems.

On the other hand, the extensive introduction of hydrogen as energy vector is limited by the actual lack (and costs) of a reliable hydrogen distribution infrastructure. This barrier can be surmounted by the development of systems for decentralized hydrogen production (i.e. close to the end-user). Fuel-flexibility and the possibility to power the process with renewable energy sources (solar and/or biomass) are two additional preconditions for sustainable hydrogen production.

The above considerations are on the basis of the project CoMETHy (Compact Multifuel-Energy to Hydrogen converter) co-funded by the European Commission under the Fuel Cells and Hydrogen Joint Undertaking (FCH JU). Indeed, CoMETHy’s general objective is to support the intensification of hydrogen production processes, by developing an innovative compact solar steam reformer to convert reformable fuels (methane, ethanol, etc.) to pure hydrogen, according to the following general reaction scheme:

where C_nH_2n+2 is the hydrocarbon feedstock that can be even replaced by an oxygenated chemical like ethanol or glycerol.

In case of unavailability of the solar radiation, the reformer should be adaptable to other back up fuels (e.g. natural gas, biomass), depending on the locally available energy mix, in order to keep the desired hydrogen production rate.

Process concept and results

In principle, the process under development is based on low-T steam reforming. Process heat is supplied by means of a low-cost and environmentally friendly liquid heat transfer fluid, i.e. the binary Molten Salts (MS) mixture NaNO₃/KNO₃ (60/40 w/w), often referred as “solar salt”. This allows coupling with Concentrating Solar Power (CSP) plants using this molten salts mixture as thermal storage system at temperatures up to 550°C. Accordingly, this fluid collects heat from the different available heat sources and transfers it to the thermochemical plant. A suitable heat storage system, based on the use of the solar salt mixture, allows mismatch between the fluctuating solar source and the often steady running chemical plant: in principle, this makes possible to drive the thermochemical plant at steady state, regardless of the effective instantaneous availability of the solar radiation, even overnight and during cloudy periods of time. Clearly, for a given location of the plant (i.e. given yearly solar radiation characteristics), there is an interplay between the nominal power of the CSP plant (i.e. the solar field area), the heat storage capacity and the amount of back up fuel to balance the solar radiation lacks: this is on the bases of CSP plant hybridization, which can be either achieved by a biomass (or RDF, Refuse Derived Fuel) or fossil fuel (e.g. off-gas) combustor and a best compromise between the CSP and the back-up fuel duties should be identified for the technical-economical optimization of the process [1,2]. Selective membranes allow recovery of high-grade hydrogen and increase conversion despite the relatively low operating temperatures.

If compared to a typical steam reforming process, this steam reforming technology operates at lower temperatures, from typical 850-950°C down to 400-550°C, with consequent significant gain in material costs since no special steel alloys for high-temperature operation are required. The high-temperature furnace is then replaced by a flameless heat exchanger heated by a liquid molten salts stream, making the whole reactor envelope more compact. Additionally, by operating at lower temperatures, it is possible to combine steam reforming and water-gas-shift (WGS) reactions into a single stage at 400-550°C, resulting into a low outlet CO content (<10 %vol) and a reduction of the reformer heat duty. The integration with membranes avoids dedicated hydrogen separation and purification units, to further improve the compactness and enhance the conversion despite the thermodynamic limitations of low-temperature reforming.

When the thermochemical plant is powered by solar heat, in principle, there will be no combustion in the whole process, and no combustion generated CO₂-containing flue gases emitted to the atmosphere: this will result in reduction of fuel consumption and greenhouse gas (GHG) emissions being in the order of 40% to more than 50%  compared to the conventional route [3].

The reduction of fossil fuel consumption will make the hydrogen production cost less sensitive to the fossil (e.g. NG) price in the solar steam reforming: therefore a breakeven point for the hydrogen production cost is foreseen, resulting in a solar route economically more convenient than the fossil based one [4]. The same solar steam reforming process applied to biomass derived fuels (i.e. bio-fuels like biogas, bioethanol, etc.) allows totally renewable hydrogen production.

An additional advantage of the use of molten salts as heat transfer fluid for steam reforming is when some stand-by periods of the plant are foreseen, as it is the case of small-medium scale reformers for hydrogen refueling stations. In this case, the continuous molten salts recirculation eases the overall process management by maintaining all plant components (e.g. catalyst, membrane) at working temperature (400-550°C) also during stand-by periods using: this minimized start-up periods and ageing due to thermal cycling.

The development of this technology, however, involves a number of interconnected R&D challenges to be faced. Three reference feedstocks have been chosen in the perspective of the multi-fuel application: methane-rich gas (e.g. natural gas), CH₄/CO₂ mixtures (up to 50%vol. CO₂, representing biogas), and bioethanol (i.e. diluted ethanol). First, it is necessary to identify and develop suitable catalysts and membranes for this application. Second, the catalyst and the membrane will be integrated in the design of a molten salts heated steam reformer. Finally, the best strategies to couple this reformer with a CSP plant will be studied.

Extensive catalyst studies led to the identification of promising catalyst materials suitable for the low-temperature steam reforming of methane (biogas) and ethanol. Open foam ceramic silicon carbide foams have been successfully developed and characterized as catalyst supports to enhance the heat transfer and to minimize pressure drops. In parallel, relevant effort has been addressed to the development of low-cost and suitable hydrogen selective membranes for CoMETHy application, basically composite membranes consisting of a thin Pd (alloy) layer (2-5 µm) coated on a porous ceramic or stainless steel support. Different membrane reactor configurations have been conceived and the performances evaluated experimentally.

Summary and conclusions

An innovative solar reformer is being developed in the CoMETHy project. Besides the interesting potentials, its development involves different research topics: selection of advanced catalyst systems with enhanced heat transfer capability and selective composite membranes, and the identification of the best options for catalyst/membrane assembly, and reformer coupling with the CSP plants.

Acknowledgements

CoMETHy project has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n. 279075.

References

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[4] Moeller S, Kaucic D, Sattler C. Hydrogen production by solar reforming of natural gas: a comparison study of two possible process configurations. ASME J Sol Energy Eng 2006;128:16–23.