8Carbon Mineralization

Carbon mineralization is the process in which CO₂ chemically reacts with minerals and becomes permanently stored in rock. This occurs naturally over thousands of years; however, new approaches can significantly accelerate the process. For example, CO₂ can be stored in concrete, known as ex-situ mineralization, in a matter of hours or injected underground for geologic carbon storage, known as in-situ mineralization, where the process occurs within a few years.

The global climate community has started to recognize the need for climate restoration, which involves carbon dioxide removal (CDR) as a complement to carbon emission reduction (mitigation). However, skeptics have pointed out that most CDR methods may not be able to scale to meet the scope of our carbon challenge. Yet, carbon mineralization taps into the natural geologic carbon cycle and has a far greater capacity to store carbon for longer periods than other CDR methods on our planet. The challenge is to accelerate timelines, reduce costs, minimize risks, and maximize equity in adopting and scaling these methods.

Today, there is scientific consensus that carbon mineralization is essential for limiting global warming and fulfilling the Paris Agreement on climate change. To achieve this, carbon mineralization approaches are being developed and refined to accelerate these processes, below and above ground, described as follows:

Below Ground

In-situ refers to the process of carbon mineralization that takes place deep underground. This method involves injecting concentrated CO₂ into underground rock formations, where it chemically reacts with basaltic rock and gradually mineralizes, remaining there for tens of thousands of years. This process is also known as geologic carbon storage.

Above Ground

Ex-situ refers to the process of creating (carbonated) aggregates, e.g. for use in low-embodied carbon concrete. This involves a chemical reaction where CO₂ is combined with an alkaline feedstock (such as mine tailings, industrial by-products, or specific types of mined rocks) under high pressure and high temperature.

Surficial methods refer to the process of carbon mineralization that occurs on land, coastlines, or oceans. This mineralization happens when CO₂ combines with an alkaline feedstock, which is a basic, water-soluble material. The chemical reaction can be accelerated by increasing the surface area of the mineral. Acceleration methods may involve grinding rock into dust, commonly done in mining operations, or stirring the material to enhance its contact with atmospheric CO₂. For instance, crushed basalt, a widely available mineral feedstock, can be spread on agricultural soil, within fields or forests, or along coastlines or oceans. There, it reacts with and removes CO₂ and stores the carbon as carbonates in rock formations.

Carbon Mineralization Approaches

Source: World Resources Institute

Given the differences between below- and above-ground carbon mineralization approaches, we consider how they contribute to restoring our climate to pre-industrial conditions by applying these guiding principles: durability, financeability, scalability, and equity.

Durability / guiding principle

Geologic carbon reservoirs hold carbon for tens of thousands to hundreds of millions of years. Once CO₂ has chemically reacted with rock and mineralized, it is not rereleased unless it is heated to temperatures above 1,000°C (e.g., in a kiln during cement production or as magma in a volcano), which breaks its chemical bonds. Due to these chemical reactions, carbon mineralization is one of the most permanent, and therefore most promising, CDR approaches.

Financeability / guiding principle

Carbon mineralization must have a low enough cost per ton to be financed, and the funds needed must be available either through the voluntary carbon market, government tax credits and subsidies, or the sale of commercial products with embedded carbon.

Tax Credits & Subsidies

In-situ: In the U.S., tax credits encourage the capture and storage of CO₂ underground or in durable products like concrete

Ex-situ: Tax credits and subsidies available for in-situ mineralization methods are typically also available for ex-situ methods. This can further offset the cost of mineralization approaches.

Surficial: While there are tax incentives for adopting “carbon farming” approaches in some areas, these are not yet widespread and are hard to monitor over time.

Commercial Products 

In-situ: The process of injecting CO₂ into underground rock formations does not yield commercial products that can offset its cost.

Ex-situ: These methods can generate valuable commercial byproducts, such as concrete and aggregates for construction, which can be sold to cover costs. 

Surficial: These methods can produce co-benefits in enhanced soil health that improve the productivity of agricultural land and/or reduce ocean acidity, increasing the abundance of marine life. These co-benefits can yield tangible financial benefits that offset the cost of the interventions.

Low-cost Interventions

In-situ: While in-situ methods make measuring the amount of CO₂ captured easier, the overall costs of in-situ geologic carbon storage are higher than ex-situ and surficial methods.

Ex-situ: These methods often have low input costs since they are designed to be integrated into existing factories or to utilize waste products as inputs. Companies developing such methods are continuously seeking lower-energy, lower-cost approaches to manufacturing. 

Surficial: These methods are also typically low-cost since they often use waste products as inputs and do not require complex processing to trigger chemical reactions. In fact, for methods like spreading crushed rock on agricultural fields, farmers often do not need to buy new equipment; the only additional cost is the acquisition and transportation of the crushed rock (which is typically cheaper than conventional soil additives).

Scalability / guiding principle

Different approaches for scaling carbon mineralization include enhanced oil recovery, carbon utilization, and rock weathering, each with its own method, process, and growth opportunity.

Enhanced Oil Recovery

In-situ: Injecting CO₂ deep underground, where it can be mineralized through in-situ processes, requires an advanced infrastructure to transport the CO₂. In many parts of the world, this already exists in the form of oil & gas infrastructure. Although convenient and cost-effective to take advantage of this existing structure, it can incentivize oil & gas companies to use geologic sequestration to conduct enhanced oil recovery (EOR), which pumps CO₂ into mostly depleted oil wells to force additional gas out. This is profitable for the companies involved, but it extends the life of the fossil fuel industry. For this reason, many climate advocates and environmental justice organizations are opposed to EOR. In addition, there remains uncertainty about whether and how CO₂ might leak from these oil wells or transportation pipelines and how this might affect local communities. Due to such complexities, more EOR research is needed.

Carbon Utilization

Ex-situ: Various ex-situ approaches can yield valuable products, such as construction materials for well-established markets. Replacing traditional materials with low-carbon alternatives could create a sizable market for these products. The rapid growth of producers and the implementation of supportive policies indicate the potential of this approach. However, quantifying the amount of CO₂ used can be challenging due to the novelty of these methods. Measuring the amount of CO₂ in a product is simple enough, but using such aggregates in a concrete mix can affect the total carbon footprint on a project-by-project basis. Consequently, ex-situ approaches can gain significant scale, yet they remain challenging to measure accurately.

Enhanced Rock Weathering

Surficial: Carbon mineralization approaches, such as spreading crushed rock over agricultural fields or coastlines, can yield the most co-benefits of any carbon mineralization approach. However, because the chemical reactions occur in open, uncontrolled environments, they can be hard to measure. New monitoring, reporting, and verification (MRV) approaches are being developed but are not sufficiently reliable yet for widespread use. Therefore, monitoring the carbon removal from these methods is currently labor-intensive and expensive due to the need for regular in-person carbon measurements. While these methods likely have a high capacity for carbon removal, that removal is less consistent across time and region due to the many variables that affect carbon mineralization in these settings (e.g., temperature, humidity, surface area of minerals).

Equity / guiding principle

Promoting equity in any large-scale intervention requires the inclusion of diverse stakeholders in all stages of project planning, design, and implementation. It also involves ensuring that the intervention’s benefits are equitably shared with local communities, especially those most affected by the negative impacts of traditional extractive industries on land, air, and water. 

Job Creation

In-situ: The growth of in-situ methods will likely create a strong demand for workers. This industry will likely offer career opportunities for existing oil and gas workers to transition into as it utilizes the existing infrastructure and tools, making the shift easier than many other renewable energy sectors.

Ex-situ: Similar to in-situ mineralization, ex-situ methods are expected to drive the growth of a new industry that will require many more workers in the coming decades. 

Surficial: Given that surficial methods typically fit into existing industries, e.g., farming, significant job creation is not expected in this area.

CO₂ Transportation

In-situ: Environmental and health concerns arising from potential CO₂ leakage at injection sites and from CO₂ transportation pipelines could counteract the proposed benefits.

Ex-situ: Unless CO₂ capture facilities are located alongside mineralization plants, many ex-situ approaches will involve transporting CO₂ via pipelines to industrial facilities, which raises health concerns similar to those of in-situ mineralization methods.

Surficial: Since CO₂ transported for surficial methods is in the form of minerals, transportation is not likely to cause significant community health impacts.