The production of cement ranks as one of the world’s largest industrial sources of CO2, but now a new study shows how in the future it could be used to remove, rather than produce the greenhouse gas.
Every year, around 4 billion metric tons of CO2 are produced through an energy-intensive process, and particularly during the thermal decomposition of limestone, the most important natural raw material for cement.
In a new study, researchers at ETH Zurich, one of the top engineering schools worldwide, have shown that these emissions could be significantly reduced if cement production were combined with a technology known as Direct Air Capture (DAC).
For the study, the researchers collaborated with the US company Heirloom Carbon Technologies which utilizes a DAC method called calcium looping.
Calcium looping and cement production are both very similar, as they rely on the heating of limestone to separate CO2 and quicklime, a process known as calcination. ETH Zurich’s research team, led by PhD student Vittoria Bolognaro, determined that 78% of a cement kiln’s carbon emissions could be cancelled out if the kiln ran electrically rather than on fossil fuels.
Once water has been added to quicklime, the slaked lime absorbs further CO2 from the atmosphere and is converted back into limestone, which can then be reused as a raw material for cement production. Burning oil, coal, or gas to power a cement kiln, however, combines deleterious and heavier elements into the emissions that prevent them from being absorbed back into the limestone.
The more often the calcium undergoes this cycle of contact with the air before being processed into cement, the more CO2 the plant removes from the atmosphere. The CO2 captured from the atmosphere is not bound in the cement. Instead, it is compressed and transported to underground storage sites.
“From a climate perspective, the combination of DAC and cement production is very promising,” stated Bolongaro, the lead author of the publication.
“Heirloom was an ideal partner for us because the company is already operating the first calcium looping DAC systems on a commercial scale,” she added.
In California, the company has been operating a plant since 2023 with an annual nominal capacity of 1,000 tons of CO2. The current study is the first prospective life-cycle analysis for DAC using calcium looping on an industrial scale.
Initially, Bolongaro and her team wanted to find out whether larger, commercial plants remove more CO2 over their entire life cycle than they produce. In a second step, the researchers examined whether the reduction in CO2 emissions comes at the expense of other environmental factors, such as water or land use.
Their findings showed that by far the largest share of the environmental footprint is attributable to the energy required for the process, as the capture of CO2 from the air is very energy-intensive.
This also applies to other DAC processes. As part of the study, various energy scenarios were therefore tested: operating the plant using the current US electricity mix, drawing on a heavily decarbonized electricity mix comprising wind and solar energy, and employing a fully autonomous system with photovoltaics and battery storage.
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“We were able to show that the technology has a net-negative carbon footprint; in other words, commercial calcium looping DAC plants with CO2 storage remove more CO2 than they generate over their entire lifecycle,” says Bolongaro. “Depending on the energy mix used in our projections, the efficiency of CO2 removal by 2050 ranges between 85 and 96%.”
Caveats abound. Key calculations for the study are based on future scenarios up to the year 2050 and assume significant progress in the decarbonization of the electricity supply, which may not come to pass for any number of reasons.
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Furthermore, some of the plant components, most notably the electric calcining kilns, are not yet in large-scale industrial use. It therefore remains to be seen whether the system can be operated economically; a detailed cost analysis was not part of the study to date.
Consequently, for Bolongaro, the results represent an important starting point that highlights the climate potential of the approach.
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