Cement production is one of the most carbon-intensive industrial processes in our economy today. It is a source of about 8% of the world’s annual carbon dioxide emissions. That is why starting decarbonization with cement production is a leading strategy of governments and green technology companies.
The so-called “hot solar” startup Synhelion has partnered with global cement producer CEMEX back in 2020 to collaborate on creating alternatives to fossil fuels in both solar fuels and in cement production.
Now Synhelion and CEMEX announced they have built a pilot plant, on the Very High Concentration Solar Tower of IMDEA Energy in Spain to produce zero emissions cement, by substituting the fossil-fired heat in cement production with heat produced from solar energy. The companies also announced they succeeded in producing the first solar clinker in the world.
By concentrating and reflecting the sun’s rays up to a solar receiver to generate heat, Synhelion has achieved the highest temperature possible today in this solar technology – 1500°C. In cement production, there are two steps that require extremely high temperatures – calcination and clinkerization, normally achieved by burning fossil fuels. Around 95% of the CO2 emissions in cement production come just from these two steps.
Calcination requires heat at about 1000°C for several minutes and clinkerization happens at around 1400 to 1450°C. The companies started testing the new methodology with the calcination process. They managed to supply heat just for this calcination process entirely from solar energy, while also siphoning off the CO2 emissions that happen naturally due to the chemical reaction, for a 100% emissions-free calcination process.
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The team then decided to go further and experiment with solar heat even for the clinkerization. This step is considered a great challenge for solar as clinkerization has higher heat requirements and the residence time is much longer – up to 30 minutes for each cohort of material rotating through the almost 90 meters-long kiln, operating continuously.
According to experts that participated in the trials, the residence time was a more critical issue in the process rather than the heat requirement but they managed to pass that step.
“What we achieved is a real milestone in the sense that we managed to go beyond – to also do the clinkerization… What Synhelion is doing, to be able to reach the 1500 degrees, is essential for us because now we can go beyond calcination. This is a key element. It allows us to completely eliminate fossil fuels from the production of clinker in cement,” said Davide Zampini, CEMEX Head of Global R&D.
The calcination step, using the traditional fossil fuel method, generates CO2 in two ways – 60% from the chemical reaction itself, and just 40% is from burning the fossil fuel to heat the reaction. Solar power is also used in separating CO2 from the calcination chemical reaction. The sunlight is reflected from a solar field of mirrors that concentrate highly focused solar flux up to a receiver on a tower. Then, the created heat is carried in a heat transfer fluid, piped down to the kiln in a stream of very high-temperature water vapor and CO2.
As this heat transfer fluid is in a closed loop with no air, it is easy to separate out and siphon off the CO2 from the calcination chemical reaction in a pure stream, by condensing out the water which leaves just CO2 behind.
Synhelion then uses this CO2 to produce solar synfuels, so the whole process also has a utilization side to it. Making solar synfuels is also the main business of Synhelion.
Now that CEMEX and Synhelion already succeeded in producing a few kilograms of clinker, they plan to develop the system further to have kilns that can produce clinker in a continuous fashion and perform the process continuously. They want to achieve that goal this year.
According to Synhelion, for a 1,000 tons of clinker per day setup, they would need about 150 megawatts thermal solar input to the receiver which is around 300,000 square meters of heliostats.
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To be able to produce cement at scale using this innovative process, the system would also have to include thermal energy storage like in a solar thermal energy plant to maintain the supplied heat continuously. Now that they solved the critical issues with heat requirements and residence time, incorporating storage is the next step for CEMEX and Synhelion to produce clinker in larger quantities to reach an industrial scale.
This new innovative process is revolutionizing cement production as it eliminates entirely its heavy carbon footprint. It is groundbreaking and could make a big difference in climate change mitigation efforts going forward.
