Renewable energy – for electricity – can now easily and cheaply replace fossil fuels for electricity, however producing liquid transportation fuels like gasoline and aviation fuel using solar energy remains a challenge. Sustainable drop-in fuels are synthetic liquid hydrocarbons made from the precursor syngas – a mixture of hydrogen and carbon monoxide.
Solar researchers are making headway internationally in efficiently and sustainably producing syngas using concentrated solar thermal energy. The heat drives a thermochemical process that converts water and biogas into syngas.
Start-ups like the ETH-Zurich spinoff Synhelion are leading the way to commercial production of these sustainable drop-in fuels, especially sustainable aviation fuels.
Why producing solar fuels at moderate temperature can advance commercialization
In principle, solar syngas can be produced by splitting water and CO2 via a ceria-based redox cycle that runs at 1500°C for the reduction step and 1000 °C for the oxidation step, using concentrated solar heat.
But to accelerate the technology development readiness, researchers at ETH Zurich explored an alternative cyclic process that can be driven isothermally, with both reduction and oxidation steps at around 1000 °C. The research project was funded by the Swiss Federal Office of Energy.
“The main driver was to work at lower reduction temperatures,” explained Mario Zuber, a doctoral student in Professor Steinfeld’s lab at ETH Zurich, who is lead author of the journal paper, Methane dry reforming via a ceria-based redox cycle in a concentrating solar tower.
“If you want to progress faster and transfer the technology to industry, it’s easier to employ materials that can sustain 1000°C, instead of 1500°C. You want to show the technical feasibility of the process before you go to such harsh conditions required by the water and CO₂ splitting cycle.”
How the consortium tested lower temperature solar syngas production
In a joint demonstration project by Synhelion, IMDEA Energy, and ETH Zürich, researchers demonstrated the production of syngas in the 10 kW solar reactor tested in the 500-sun concentrating solar tower at IMDEA Energy in Spain.
The solar tower of IMDEA Energy in Spain has a solar field of 169 heliostats (sun-tracking parabolic mirrors) reflecting the solar irradiance up to a solar reactor mounted atop a 15 m solar tower.
If all of the heliostats were in use, the field would deliver 250 kW of solar radiative power. But for this experiment, only 38 of these heliostats were used, delivering 10 kW of solar radiative power with a mean solar concentration ratio of 560 suns over the 160 mm-aperture of the solar reactor.
This solar reactor has a cavity-type receiver with two reaction zones: one zone inside the cavity and a second one inside a tubular section behind it.
Cavity receiver used in the on sun solar syngas production test at the IMDEA Energy test site in Spain
Here’s how the technologies work:
The splitting of water and CO₂ via a ceria-based redox cycle consists of two steps.
Water/CO₂ splitting redox cycle:
Step 1 is reduction at 1500°C:
Ceria is reduced, releasing oxygen.
Step 2 is oxidation at 1000°C:
Reduced ceria is oxidized with CO₂ and/or H2O to produce CO and/or H₂, regenerating ceria for step 1.
The research team then combined the reduction step 1 with the dry reforming of methane to produce CO and H₂ at 1000°C, in a cyclic process denoted as “dry redox reforming”.
“Theoretically you can go even to lower temperatures, for example to 700°C by introducing catalysts for speeding up the kinetics, but with the risk of coke formation,” Zuber noted. “In the absence of added catalysts, you would need to operate around 1000°C for favorable thermodynamics and kinetics.”
Dry redox reforming:
Step 1 is reduction at 1000°C
Ceria is reduced by methane (CH4), producing CO and H₂.
Step 2 is oxidation at 1000°C
Reduced ceria is oxidized with CO₂ and/or H₂ O to produce CO and/or H₂ , regenerating ceria for step 1.
“The main difference is in the reduction step,” he explained. “By feeding methane, the reduction step can proceed at the same temperature as the oxidation step, eliminating the temperature swing required in the water/CO₂ splitting cycle.”
The advantages over the water/CO₂ splitting is that it is isothermal, the reduction step is at lower reduction temperatures, and it is more energy efficient and selective.
According to the paper;
“an experimental parametric study yielded a peak CH4 molar conversion of 70% and a solar-to-fuel energy efficiency of 16%, while the tubular section increased syngas yield by 32% [5].”
“With the original ceria redox cycle you have this very cyclic nature in how you produce syngas,” Zuber explained,
“In one step, you’re just producing O₂ , and then the next step, you’re producing syngas. While in this new method, we are able to produce syngas in both steps. So it’s more continuous.”
As well as doubling the output, their new method lends itself to continuous production day and night, assuming the heat is supplied by solar thermal energy that has been stored. This is important because many industrial processes are run continuously 24/7.
“Of course, we need to make sure that our methane comes from a biogenic source for this process to be considered sustainable, so it’s biogas,” Zuber noted.
Reference:
Zuber M., Patriarca M., Ackermann S., Furler P., Conceição R., Gonzalez-Aguilar J., Romero M., Steinfeld A., “Methane dry reforming via a ceria-based redox cycle in a concentrating solar tower”, Sustainable Energy & Fuels, Vol. 7, pp. 1804–1817, 2023. https://doi.org/10.1039/d2se01726a
