Recent advancements in solar-thermal technology have demonstrated the great potentials of the molten nitrate mixture NaNO₃/KNO₃ (60/40 w/w) as solar heat carrier and storage medium at temperatures up to 565°C. This “high” temperature storage concept enables us to drive highly energy demanding chemical processes like the steam methane reforming (SMR) for hydrogen production.
The use of solar energy as process heat in SMR allows at least 34 % fossil fuel saving, and an equivalent CO2 release reduction.
The basic reforming furnace of conventional SMR plants is replaced by compact tubular heat exchanger reactors, counter-currently flowed by the molten salts.
Several chemical process configurations and operating conditions have been studied using an one-dimensional mathematical model for the molten salt heated SMR tubular reactors, and the AspenPlus simulator used for flowsheet analysis.
In the figure below a general process schemes are represented, where methane conversion at 530-550°C is improved by different reaction stages in series (R1, R2, ...), with hydrogen removal by membranes (M1, M2, ...) set between catalyst beds. Otherwise a single tubular reactor (R1) can be used, with partial product mixture recirculation after hydrogen removal in the permeator (M1).
Results demonstrate that the application of hydrogen permselective membranes allows us to sensibly reduce the solar plant size by maximizing the solar-to-hydrogen conversion efficiency.
Technical-economical optimization is also discussed.
Interesting impacts in the current energy context can be identified like partial decarbonation of the fossil fuel with the possibility to “carry” solar energy as hydrogen in the actual natural gas grid.