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Caverns, Concrete and Carbonate: The Future of Storing Captured Carbon

Carbon capture has a critical role to play in offsetting the hardest-to-abate emissions and helping limit global warming. This calls for a huge scaling up in carbon storage capacity.

Carbon sequestration is the process of storing atmospheric CO2. It is part of Earth's natural carbon cycle — with trees, peat bogs and oceans all playing important roles in storing CO2, mainly through biological processes. There are also growing efforts to develop technologies that can store captured CO2.

These technologies could store carbon more reliably, efficiently and flexibly than nature allows. As is the case with carbon capture, utilization and storage (CCUS) more broadly, deployment of carbon sequestration technologies has been limited to relatively small-scale projects. However, the recent surge in interest and investment in CCUS could be a sign that the time has come for these technologies to be deployed at scale.

Underground caverns 

For almost fifty years, gases have been injected and stored in the ground. Captured CO2 can be compressed until it reaches a fluid state suitable for transporting in pipelines and injecting into underground systems like saline formations or former oil and gas reservoirs.

Under suitable conditions, it can be kept stable beneath the ground for up to millions of years. However, there remain considerable challenges around identifying ideal storage sites and – once CO2 is sequestered – monitoring these sites for leakage, contamination of drinking water sources and pressure build-up, which can lead to fracturing.

A much-hyped upcoming project is the USD$165m Moomba project, led by oil and gas producer Santos, to store CO2 in former oil and gas reserves in the South Australian outback. This project aims to be one of the largest and most cost-effective of its kind with annual capacity of 1.7 MtCO2 per year. First injection is targeted for 2024.

Other major projects in the pipeline include a Petronas- and Shell-led project to store captured CO2 in a depleted Malaysian gas field, and a Santos- and Eni-led project, which could see captured CO2 injected at the Bayu Undan field off the coast of East Timor.

Carbon mineralization

An alternative approach to geological carbon storage is carbon mineralization — a chemical process that's triggered when certain rocks are exposed to CO2, turning the gas into a solid mineral. It occurs naturally, but the process can be accelerated.

While storing CO2 in underground sedimentary reservoirs either leaves it compressed or dissolved in groundwater, carbon mineralization forms a solid mineral within rocks, preventing possible escape and eliminating the need for further monitoring.

The world's largest carbon mineralization company is Iceland-based Carbfix. The company manages a major carbon storage hub, the Coda Terminal, to which captured CO2 is transported from all over Europe for pumping and mineralization.

Electricity pylon over a boggy field

One of the challenges with carbon mineralization is the need for specific types of rock formations. So far, all large-scale carbon mineralization projects have been located in areas with a specific type of volcanic geology. But new projects could expand that range. Mining giant Rio Tinto, a partner on the Coda Terminal, recently received US$2.2m funding from the US Department of Energy to explore carbon storage potential at a site in Minnesota. This project, the Tamarack Nickel Project, could open up new avenues for carbon mineralization, allowing carbon to be stored in a much wider range of rock types.

Carbon-negative concrete

Alongside storage, utilizing captured CO2 could open up new business models to support carbon capture. Injecting CO2 into concrete mix sequesters CO2 permanently while helping reduce the environmental impact of a carbon-intensive industry. The latter is a major pillar of the transition to net-zero, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC), as cement and concrete production accounts for eight percent of global CO2 emissions.

Using captured carbon in this way has been limited by the cost and energy intensity of the process, which often requires extra steps like purifying CO2 before injection. However, a new approach for utilizing CO2 in construction cuts out these extra steps, allowing CO2 to be converted at the source (e.g. factory exhaust). The process, developed by engineers at UCLA, produces portlandite – a replacement for limestone and cement – using only minerals, water and electricity. Research suggests it could cut CO2 emissions associated with cement production by 65 percent while producing concrete just as strong and durable as conventional concrete.

A project to demonstrate this technology is among a handful at UCLA's Institute for Carbon Management supported with a $21m pledge from the Chan Zuckerberg Initiative.

With the pressure on to halve CO2 emissions by 2030, there is an urgent need for scaling up of carbon capture, utilization and storage as a complement to decarbonization. A mixture of approaches – both nature-based and technological – will be required to meet the need for carbon sequestration around the world. 

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