Accelerated Carbonation Technology Plus Direct Air Capture: A New Approach to Capturing and Storing CO₂

By Prof. Don MacElroy, Chairman of Advisory Board at NEG8 Carbon, and Dr David Mulrooney, Business Development Manager at NEG8 Carbon

As momentum builds behind Direct Air Capture (DAC), it’s worth considering Accelerated Carbonation Technology (ACT) – also known as ex situ Mineral Carbonation (MC) – as a viable option for storing DAC-captured carbon dioxide.

Exploring ACT for CO₂ Sequestration in Ireland

Considering the momentous task ahead of us, a multitude of approaches is needed to meet Ireland’s net-zero target and, optimistically, net negative targets in future years. In this article, we aim to explore a theoretical solution relevant to Ireland that looks into how we can optimise the Enhanced Rock Weathering (ERW) process by accelerating the carbonation of basalt type rocks, which are in abundance in Ireland.

Carbon Dioxide Removal (CDR) refers to actively removing carbon dioxide that has already been emitted into the atmosphere. If consistently employed at a global scale, it could reinvigorate the planet, reverse the effects of climate change, and over time address the impact of the global climate emergency.

CDR methods involve two key steps:

1. Capturing carbon dioxide, either directly from ambient air using Direct Air Capture or from point sources such as power stations.
2. Permanently sequestering/storing or utilizing the captured CO₂ to prevent it from re-entering the atmosphere.

The Need for Permanent Storage of CO₂ 

In a net negative scenario, captured CO₂ must be removed from the atmosphere and permanently stored. This usually involves processing, transport and sub-surface injection, but challenges such as cost, long permitting timelines, public acceptance, and the need for suitable geology can complicate matters.

One promising approach is Enhanced Rock Weathering. This method captures CO₂ by promoting natural chemical reactions between carbon dioxide and silicate minerals, particularly basalt, to form stable carbonates. In essence, this locks away the CO₂ for thousands of years. Companies such as Silicate Carbon in Ireland and Heirloom are already active in this field, offering carbon credits and validating CO₂ capture results.

However, verifying long-term sequestration and accelerating the process remain hurdles. Currently, ERW takes decades to capture CO₂ in large enough quantities, although this is down from the millennia is takes for the natural process to happen without grinding and spreading the rock. This prompts us to look into ways to accelerate this process even more.

Why Basalt?

Basalt is a volcanic rock rich in calcium and magnesium which are used in forming carbonates during carbonation reactions. Ireland has substantial basalt deposits, making it a suitable candidate for ERW and ACT (ex situ mineral carbonation).

Speeding Up the Carbonation Process with Accelerated Carbonation Technology 

ACT builds on the same principles as ERW but operates in an engineered system that drastically reduces reaction times from years to hours.

Recent research has identified several ways to speed up this carbonation process in basalt. These include optimising the crushed rock’s grain size, increasing temperature and pressure, and adjusting the reaction pH.

The aim is to reduce the time required for the carbonation of CO₂.

Grain Size

The carbonation reaction occurs at the surface of the rock particles, so smaller particles present more surface area for CO₂ to react.

• Rinder and von Hagke¹ found that 1 tonne of basalt (<100 μm) can absorb 45 kg of CO₂ over 10 years.
• Reducing the grain size to <10 μm increases uptake to 153 kg, a 240% increase, though at the reaction conditions it still takes 134 years for full reaction.
• Further reduction to <1 μm could theoretically bring this down to 10.7 years, but such fine particles are challenging to produce with commercial milling equipment and may pose health risks².

Temperature and pH 

Adding heat speeds up the reaction:

• Snaebjornsdottir et al.³ varied the drill depth of the sub surface well to offer “free” heat to drive reactions.
• Gislason and Oelkers⁴ demonstrated that raising the temperature from 0°C to 100°C under acidic conditions (pH 3.5, similar to orange juice) boosts dissolution rates by a factor of 60.
• The same temperature change under basic conditions (pH 9, similar to baking soda) gives a 4.5-fold increase, highlighting the importance of pH.

However, despite the benefits of high temperatures to reaction rates, an upper limit is approached at 300°C. Carbonation becomes less effective at temperatures above 290°C, where calcite formation is inhibited³. 

Pressure 

Gysi and Steffanson⁵ showed that higher pressures improve the formation of carbonates, especially in the presence of water. The graph below illustrates mineral formation as CO₂ pressure increases from 2 to 30 bar.

(Image source: ResearchGate)

In summary there is good evidence to support the theory of acceleration of ex-situ mineral carbonation, specifically of basalt, by modifying reaction conditions (temperature, pH, pressure and grind size). Water plays an important role in this process, aiding both pressure control and pH adjustment. However, water usage must be carefully managed due to potential environmental concerns. The table below combines the identified optimal reaction conditions. 

Suggested Optimal Conditions for Accelerated Carbonation 

Grain Size	Temperature (°C)	Pressure (bar)	Acidity (pH)
<1 μm	100–300	30	3.5

DAC + ACT

By combining DAC with ACT, you could capture and store carbon locally and more efficiently.

This method uses a closed reaction vessel, applying heat, pressure, agitation, and acidic conditions to promote carbonation.

• O’Connor et al.⁶ recommend a temperature of 185°C, pressure of 115 bar, and a grain size of 37 microns.
• Basalt remains the ideal candidate rock type, given its high reactivity, its high quantities of magnesium and calcium, and its abundance in Ireland. Different types of rock will have different capacities for absorbing carbon dioxide.

Changing the pH can be compared to the effect of brining meat to tenderise it. A diagram of a typical reaction vessel that could be used for this reaction is shown below:

Continuous stirring reactor with external heating element, impeller for stirring and feed shoot for addition of rock and carbon dioxide. (Image source: Wikipedia)

The Key to Success: Co-location of DAC and ACT Processes

Furthermore, the integration of DAC and ACT avoids the extensive monitoring, permitting and policy challenges of sub-surface injection. Instead of transporting CO₂ long distances for storage, it can be processed on-site and either returned to the quarry or used in construction materials such as concrete aggregates.

Ireland presents an ideal setting to pilot DAC + ACT projects. The location of such a site is the most important, having the rock source, the carbon dioxide source, and the processing facility all located at the same site is the most economically favourable scenario. This would:

• Eliminate nearly all transport emissions and costs.
• Simplify permitting and community acceptance.
• Use Ireland’s native basalt resources.

Saleh⁷ estimates transport accounts for 15% of operational costs when materials are moved 150 km. A co-located site would remove this burden.

Map of volcanic and intrusive rock in Ireland, volcanic and basalt rock is largely suitable for carbon storage. (Image source: Trinity Geological Museums, TCD)

Furthermore, DAC – such as NEG8 Carbon’s DAC technology – could be integrated with the rock processing facility, specifically the crusher. NEG8 Carbon’s system is designed to leverage low grade waste heat and only requires low temperatures to operate.

Is DAC + ACT Viable? 

For DAC plus ACT to be viable, it needs to be conducted at a sufficiently large scale with capture rates of 50,000 tonnes CO₂/year or more, while co-located smaller-scale projects at a basalt quarry may be feasible with current technology. The crushed and ground CO₂- rich rock could be re-introduced into the quarry from which it was originally removed thus permanently storing the carbon dioxide.

Conclusion

Ultimately, large-scale deployment of CDR technologies like DAC will be needed to meet Ireland’s emissions targets and this approach to permanent carbon removal warrants further consideration. With basalt resources already available and low-temperature DAC systems in development, this approach could offer a route for meaningful carbon dioxide removal without the need for sub-surface injection or long-distance transport.

 

For more:

• Enhanced Rock Weathering vs Direct Air capture
• Point Source vs Direct Air Capture
• Forestation vs Direct Air Capture
• Biochar Carbon Sequestration vs Direct Air Capture

 

 

Interested in NEG8 Carbon’s CO₂ capture technology?

Contact the NEG8 Carbon Team