One of the biggest problems in dealing with carbon dioxide emissions is the making of the key component of cement: lime (calcium oxide). Now, a small company with the cute name Sublime Systems appears to have solved this issue.
The concrete industry is responsible for 8% of global carbon emissions. Ordinary Portland cement (OPC) is about 15% of concrete, and calcium oxide or lime is only 60% of OPC. Yet the making of that lime is the cause of almost all of the emissions: Lime is only 8% of concrete but creates 90% of the problem. About half comes from the “chemical fact of life”—the CO2 emitted in calcination or turning calcium carbonate into calcium oxide. The other half comes from burning coal or fossil gas to make the calcination happen.
We have described the process many times: “The key component of cement is lime, which you get by applying heat to calcium carbonate, basically limestone. CaCO3 + heat > CaO + CO2. You can’t do anything about the chemistry; it is the fundamental nature of the material that makes it emits CO2.”
It may be that you can’t do anything about the chemistry, but there are different ways of separating carbon dioxide from limestone. Sublime Systems co-founders Leah Ellis and Yet-Ming Chang described how their company made lime at room temperature using renewable electricity. They wrote:
The expanding team recently raised $40 million from investors including Siam, the largest cement producer in Southeast Asia.
“Low-carbon cement is critical to build a decarbonized economy,” said Ellis, who is also the CEO of Sublime. “We have successfully demonstrated the viability and scalability of our approach and we are able to produce cement with the same or better strength, slump, and durability than today’s portland cement. The support of our talented team and capital from our investors will enable us to operate our pilot facility, secure advance offtake agreements, and work toward producing our low-carbon cement at scale.”
After explaining what ordinary Portland cement is in some detail, Ellis gets down to the key ingredient: lime.
When it’s made in a cement kiln, the CO2 emitted in the reaction mix with the flue gases, making it almost impossible to capture. The Sublime process is based on using an electrolyzer, as is done with water, to get hydrogen and oxygen. When they stick calcium carbonate or limestone into the electrolyzer, calcium ions migrate toward the negative electrode, where they react to precipitate calcium hydroxide or quicklime, which can be reduced to calcium oxide with a bit of heat or used directly.
The CO2 that is emitted is cold, pure, and easy to collect. The process works with any source of calcium; if a non-carbonate source is used, and the electrolyzer is powered with renewable electricity, then the resulting lime is carbon-neutral. Sublime explained on its site: “Having created our process for manufacturing decarbonized lime, we had the key to creating a decarbonized cement.”
The authors wrote:
The worldwide cement industry produced 4.37 billion tons of the stuff in 2021, so it might be a while until the industry is decarbonized. But this is still a dramatic step forward.
According to a 2019 research article, it takes between 5.2 and 7.1 megajoules of energy to make a kilogram of cement. A megajoule is .277778 kilowatt hours, so it is between 1.44 and 1.97 kWh of electricity, which doesn’t seem like much. And unlike a cement kiln that has to run all the time and stay hot, this process could run when the wind blows and the sun shines. This really does change everything: a low to zero-carbon drop-in replacement for conventional high-carbon cement.
The world is changing so fast. Until recently, we saw no path to true decarbonization for steel, aluminum, or concrete. Now we have HYBRIT steel, Elysis aluminum, and Sublime cement. They are just a teensy fraction of the industry, but they are a way forward.
