Solar-heated cement calcining: “This is the key point: not only are we addressing carbon emissions from the heat input, which is traditionally supplied by fossil fuels, we’re also addressing the CO₂ naturally liberated from calcining the calcium carbonate.”
Researchers at Sandia National Laboratories are advancing a new way to cut CO2 emissions from calcining cement using heat produced from concentrated sunlight.
The concept was laid out in this SolarPACES interview with Davide Zampini from CEMEX, the cement company partnering to develop the technology, and Gianluca Ambrosetti from the concentrated solar fuels developer Synhelion. The two partners have been exploring this path since 2019 with theoretical works as well as experimental projects.
To make their drop-in aviation fuel, Synhelion developed a high-temperature solar receiver able to achieve a record-setting 1,500°C maximum temperature with a unique heat transfer fluid.
“Then Synhelion saw an opportunity with the novel receiver design,” said Sandia R&D Engineer Nathan Schroeder.
“They said, ‘We have this high-temperature heat transfer fluid. What else can we do with it?’ And at these temperatures, you can calcine cement, which is particularly hard to decarbonize due to the high temperatures required.”
This heat transfer fluid of steam and carbon dioxide is why Synhelion’s solar heat-trapping receiver is so efficient. It leverages the greenhouse gas effect: By injecting H2O and CO2 into the solar receiver, the sunlight collected by the receiver cannot escape back out through the aperture; the heat is trapped in a greenhouse gas blanket. Synhelion developed it to use the trapped heat in an adjacent reactor for making solar fuels.
However, for calcining cement, the heat transfer fluid would instead be flowed through a calciner, directly heating and decomposing calcium carbonate from CaCO3 into the CaO used to make cement; which releases the excess CO2 as a natural byproduct.
“That heat transfer fluid is going to pick up additional CO2 as it is liberated from the calcium carbonate, and so now you have a concentrated stream of CO2 coming off of the calcination process that you can sequester or turn into solar aviation fuels, Synhelion’s primary focus,” Schroeder said.
“This is the key point: not only are we addressing carbon emissions from the heat input, which is traditionally supplied by fossil fuels, we’re also addressing the CO2 naturally liberated from calcining the calcium carbonate.”
At the 2023 SolarPACES conference, Schroeder’s Characterization of Calcium Carbonate Decomposition Thermodynamics in a Steam and Carbon Dioxide Suspension and the SolarMEADPresentation described this work. The project is funded by the United States Department of Energy and will be completed by December 2025.
International research has been performed on the use of concentrated solar thermal energy to cut CO2 in cement processing, heating the container, including at PROMES in France. However, this new approach will characterize the advantages of directly heating the material itself with the heat transfer fluid.
“What’s new about our project’s approach is that we have a heat transfer fluid interacting directly with the material,” Shroeder said. “When you have a wall or a barrier between the particles and the solar flux, transferring all the heat into the material isn’t easy. Flowing the heat transfer fluid directly through the particles makes getting that material to temperature much easier without worrying about material limits.”
Over the next year and a half, the Sandia team will advance this technology by validating computational fluid dynamics models to inform scale up performance, developing balance of plant models to investigate future integration, and characterizing the kinetics of this reaction, in collaboration with the project partners, CEMEX and Synhelion.
“It turns out that when you calcine that material, having H2O in the system improves the kinetics,” Shroeder explained.
“It can reduce the time it takes to react the material and lower the temperature at which that reaction occurs. Some research teams have looked into calcium carbonate calcination in a H2O atmosphere and characterized the reaction rates. We are excited about the potential to heat H2O with concentrated solar power to reduce the carbon emissions of this industry while improving the efficiency of the process.”
The way cement calcining has been done till now, fossil fuels are burned in air to drive calcining processes, but the high proportion of nitrogen in the air means the concentration of CO2 is relatively low simply due to dilution. So, it’s been too costly to separate a stream of CO2 that is concentrated enough to be worth capturing.
This solar-heated process is entirely different: Because the solar heat is delivered to the calciner as a heat transfer fluid that is almost totally carbon dioxide and water vapor, simply condensing out the steam will leave a concentrated stream of carbon dioxide that is easily siphoned off.
Although the process is in a closed loop, excess CO2 is generated during the natural chemical reaction, which will be extracted for use by Synhelion for making drop-in solar aviation fuels. (This carbon dioxide naturally liberated from calcium carbonate is still categorized as non-renewable under current EU regulation.)
“Our project is focused on what happens when you move that heat transfer fluid into the calciner where you would interact the H2O and CO2 with the cement raw material to calcine it and remove the CO2 from that mineral.” said Schroeder.
Sandia will test a 10 kW thermal scale reactor in New Mexico to demonstrate the process. The particles are exposed to the heated stream of water vapor and carbon dioxide for between two and three seconds.
Since the test is primarily geared toward model validation, the solar heat is simulated using an oxy-propane torch. Next year, Synhelion’s solar receiver will heat the H2O/CO2 that is delivered to the calciner within the German Aerospace Center (DLR)’s Synlight facility in Germany.
Sandia’s results should advance solar-heated cement calcining from today’s early Technological Readiness Level of 2 to a TRL of 4. The Synhelion solar receiver, which will be paired with this calciner, has a TRL of 7 and is now in the demonstration stage.
The next step would be to develop a small-scale solar-heated calcining demonstration plant or retrofit a current plant, working with their partners at CEMEX and Synhelion. Schroeder noted that the EU-based CEMEX research division they are working with is very invested in industrial decarbonization.
“They see the writing on the wall, especially in Europe. With the increases in carbon pricing, there’s increased pressure to mitigate the carbon emissions from cement plants. They see this as imperative to keep their business moving,” he said.
