MIT scientists have developed a process to produce cement via electrolysis, which would lower the emissions associated with cement production as well as produce gas streams that can be used in other processes.
Cement production is responsible for around 8% of global emissions, equating to around 2.8bn t/y of CO2. The emissions come partially from the use of fossil fuels, which could possibly be reduced with carbon capture and storage. However, around half of CO2 emissions come from calcinating limestone to produce the clinker needed for cement, according to Carbon Brief. When limestone (CaCO3) and clay are heated in a kiln at around 1,400oC, lime (CaO) and CO2 are produced.
Portland cement is the industry standard and is used in 98% of concrete. It is cheap to produce, so trying to find an alternative method that competes on cost is challenging. There are various possibilities to reduce the emissions in cement production, such as using alternative binders compared to conventional clinker. However new types of cement can have limited applications.
Researchers from MIT’s Department of Material Science and Engineering have developed a new route to producing clinker that can be used in Portland cement. Electrolysis of near-neutral water produces a pH gradient with an acid at the anode and a base at the cathode. When ground CaCO3 is added to the acid, solid calcium hydroxide (Ca(OH)2) is precipitated out. When this is heated with silicon dioxide (SiO2), the Ca(OH)2 decomposes to CaO, and then combines with the SiO2 to form alite, the most abundant mineral in Portland cement.
The electrochemical decarbonation reactor simultaneously functions as an electrolyser and a chemical reactor, as it also produces a stream of O2/CO2 at the anode and H2 at the cathode. The CO2 can be captured directly without the need for expensive CCS processes like amine scrubbing. The O2/CO2 stream could also be used as oxy-fuel in the kiln. Oxygen-enhanced combustion is more efficient than burning fossil fuels in air as the nitrogen doesn’t have to be heated and there are also no nitrous oxide emissions. The flue gas from oxy-fuel combustion has a higher concentration of CO2, which would make carbon capture more efficient.
The hydrogen produced could be used as fuel to power the reactor or other operations at the cement plant. The H2 or CO2 could also be used as feedstocks in other processes, for example H2 can be used in ammonia and fertiliser production, and CO2 can be used to make syngas.
The process can use renewable electricity instead of fossil fuels to generate heat. The team’s analysis suggests that the process would be cost-competitive compared to conventional clinker production based on energy costs, although a full analysis was not possible due to unknown factors such as the lifetime cost of the electrochemical decarbonation reactor. However, carbon capture would cost less from the electrolyser.
The proof-of-concept experiment produced lime from calcium carbonate on a small scale, but according to the researchers it could be scaled up after further development. However, the challenge will be how to penetrate the industry. A typical cement plant produces 700,000 t/y and lead author of the research paper Leah Ellis said that trying to replace one part of the process at a time would be easier than trying to replace the whole system at once.
If all of the energy is supplied by renewable electricity, then a scaled-up version would lead to emissionless cement production that would still be in line with industry standards.
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