A scheme for a low-emission, electrochemically based cement plant.
MIT points way to emissions-free cement
01 December 2019
A team of researchers at the Massachusetts Institute of Technology (MIT) has found a new way of manufacturing cement that eliminates the emission of greenhouse gases from the material altogether. In fact, some other useful products could be made in the process.
It’s well known that the production of cement is a major source of greenhouse gas emissions, accounting for about eight per cent of all such releases. If cement production were a country, it would be the world’s third-largest emitter.
The findings, published in the PNAS journal, were presented by Yet-Ming Chiang, the Kyocera professor of Materials Science and Engineering at MIT, with postdoc Leah Ellis, graduate student Andres Badel, and others. The research was partly supported by the Skolkovo Institute of Science and Technology.
According to Chiang, about 1 kg of carbon dioxide (CO2) is released for every kg of cement made today, which adds up to 3 to 4 gigatonnes (billions of tonnes) of cement, and of carbon dioxide emissions, produced annually today.
And that amount is projected to grow since the number of buildings worldwide is expected to double by 2060. To top it all, the commodity is now very cheap to produce, costing only about 13 cents per kg – making it cheaper than bottled water, Chiang adds.
So it’s a real challenge to find ways of reducing the material’s carbon emissions without making it too expensive. Chiang and his team have spent the last year searching for alternative approaches, and hit on the idea of using an electrochemical process to replace the current fossil-fuel-dependent system.
Ordinary Portland cement, the most widely used standard variety, is made by grinding up limestone and then cooking it with sand and clay at high heat, which is produced by burning coal.
The process produces carbon dioxide in two different ways: from the burning of the coal, and from gases released from the limestone during the heating. Each of these produces roughly equal contributions to the total emissions.
Chiang says the new process would eliminate or drastically reduce both sources. But although they have demonstrated the basic electrochemical process in the lab, the process will require more work to scale up to industrial scale.
“First of all, the new approach could eliminate the use of fossil fuels for the heating process, substituting electricity generated from clean, renewable sources. In many geographies, renewable electricity is the lowest-cost electricity we have today, and its cost is still dropping,” says Chiang.
In addition, he says, the new process produces the same cement product. The team realised that trying to gain acceptance for a new type of cement – something that many research groups have pursued in different ways – would be an uphill battle, considering how widely used the material is around the world and how reluctant builders can be to try new, relatively untested materials.
The new process centres on the use of an electrolyser, where a battery is hooked up to two electrodes in a glass of water, producing bubbles of oxygen from one electrode and bubbles of hydrogen from the other as the electricity splits the water molecules into their constituent atoms. Importantly, the electrolyser’s oxygen-evolving electrode produces acid, while the hydrogen-evolving electrode produces a base.
In the new process, the pulverised limestone is dissolved in the acid at one electrode and high-purity carbon dioxide is released, while calcium hydroxide, generally known as lime, precipitates out as a solid at the other. The calcium hydroxide can then be processed in another step to produce the cement, which is mostly calcium silicate.
The carbon dioxide, in the form of a pure, concentrated stream, can then be easily sequestered, harnessed to produce value-added products such as a liquid fuel to replace gasoline, or used for applications such as oil recovery or even in carbonated beverages and dry ice.
The result, Chiang says, is that no carbon dioxide is released to the environment from the entire process. By contrast, the carbon dioxide emitted from conventional cement plants is highly contaminated with nitrogen oxides, sulphur oxides, carbon monoxide and other materials that make it impractical to “scrub” to make the carbon dioxide usable.
Calculations show that the hydrogen and oxygen also emitted in the process could be recombined, for example in a fuel cell, or burned to produce enough energy to fuel the rest of the process, producing nothing but water vapour, Ellis says.
In their laboratory demonstration, the team carried out the key electrochemical steps required, producing lime from the calcium carbonate, but on a small scale. The process looks a bit like shaking a snow-globe, as it produces a flurry of suspended white particles inside the glass container as the lime precipitates out of the solution.
While the technology is simple and could, in principle, be easily scaled up, a typical cement plant today produces about 700,000 tonnes of the material per year.
“How do you penetrate an industry like that and get a foot in the door?” asks Ellis, the paper’s lead author.
One approach, she says, is to try to replace just one part of the process at a time, rather than the whole system at once, and “in a stepwise fashion” gradually add other parts.
Chiang says: “The initial proposed system the team came up with is not because we necessarily think we have the exact strategy for the best possible approach, but to get people in the electrochemical sector to start thinking more about this and come up with new ideas. It’s an important first step, but not yet a fully developed solution.”
