The Building Blocks of CDR: A Primer on Carbon Dioxide Removal Technologies

A lot of buzz has been going around carbon dioxide removal (CDR) lately. With climate change accelerating, it's clear we need every tool in our toolbox to reduce greenhouse gas emissions and remove excess CO2 from the atmosphere. But when I started my deep dive in CDR, it felt like a maze as there are so many different approaches and technologies being proposed and developed.

So I decided to write on CDR technologies so it becomes a bit easier to consume information and eventually one can build know-how on top of it. Consider this your crash course on the building blocks of carbon dioxide removal systems.

The Big Picture on CDR

Before we get into the specific technologies, it's worth taking a step back to look at the big picture. CDR refers to a range of approaches that aim to remove CO2 directly from the atmosphere and store it long-term.

The goal is to achieve "negative emissions" - actively reducing the amount of CO2 in the air rather than just reducing new emissions.

Most climate models now show that we'll need large-scale CDR, on the order of billions of tons of CO2 removed per year, to have a chance of limiting warming to 1.5°C or 2°C. That's a massive undertaking, considering total global emissions are currently around 51 billion tons of CO2 equivalent (GHG) per year.

The good news is that we have a diverse portfolio of CDR approaches in development:

  • Biological approaches that leverage photosynthesis, like reforestation and soil carbon sequestration
  • Engineered systems like direct air capture
  • Hybrid approaches that combine natural and technological elements
  • Geological storage methods to permanently sequester captured CO2 underground

Each of these has different pros, cons, costs, and scaling potential. Let's break them down one by one.

Mineralization: Turning CO2 to Stone

One of the more intriguing CDR approaches is CO2 mineralization - essentially, turning carbon dioxide into rock. This mimics and accelerates natural weathering processes where CO2 reacts with certain minerals to form stable carbonate compounds.

There are a few different flavours of mineralization being explored:

  • Ex-situ mineralization involves extracting reactive minerals, grinding them up, and exposing them to CO2 in industrial reactors. This allows for precise control but is energy intensive.

  • Surficial mineralization spreads ground-up minerals on the land or ocean surface to naturally react with atmospheric CO2. This is simpler but slower.

  • In-situ mineralization injects CO2 underground into formations with suitable minerals for reactions to occur. The CarbFix project in Iceland is pioneering this approach.

The big advantage of mineralization is that it provides extremely stable, permanent CO2 storage. We're talking timescales of thousands to millions of years. The challenge is that it requires massive quantities of suitable minerals and can be quite slow and energy intensive.

Current estimates suggest mineralization could potentially sequester up to 10 billion tons of CO2 per year. But costs are still high - anywhere from $20 to $200 per ton of CO2. More research and pilot projects are needed to bring costs down and scale up.

Ocean Alkalinity Enhancement: Harnessing the Seas

The oceans already absorb huge amounts of CO2 from the atmosphere. Ocean alkalinity enhancement aims to boost this natural carbon sink by increasing the ocean's capacity to take up and store carbon.

The basic idea is to add alkaline materials like ground up limestone to seawater. This shifts the ocean's chemistry, allowing it to absorb and hold more CO2 as dissolved bicarbonate.

In theory, this approach has enormous potential - the oceans could potentially store trillions of tons of additional CO2. And some initial estimates suggest it could be fairly low cost, potentially under $100 per ton.

But there are major uncertainties around the environmental impacts. Dramatically altering ocean chemistry could have all kinds of unintended consequences for marine ecosystems. We'd need extensive research and small-scale trials before even considering large-scale deployment.

For now, ocean alkalinity enhancement remains largely theoretical. But it's an intriguing option that merits further study as we search for ways to drawdown massive amounts of CO2.

Soil Carbon Sequestration

Now we're getting into some approaches that leverage nature's own carbon cycle.

Soil carbon sequestration aims to increase the amount of carbon stored in agricultural soils through improved land management practices.

Some key soil carbon building techniques include:

  • No-till farming to reduce soil disturbance
  • Planting cover cropsto increase organic inputs
  • Adding compost and biochar as soil amendments
  • Improved grazing management on grasslands

The potential here is significant - estimates suggest soils could sequester 3-5 billion tons of CO2 per year globally. And it comes with major co-benefits for soil health, agricultural productivity, and climate resilience.

The challenges are around measurement, verification, and long-term maintenance of soil carbon stocks. We need better ways to accurately quantify soil carbon changes over time. And practices need to be maintained for decades to achieve lasting storage.

Costs are relatively low - potentially even negative when accounting for agricultural benefits. But we need stronger incentives and support systems to drive widespread adoption by farmers.

Forest Carbon: Growing Our Way to Negative Emissions

Forests are the original carbon removal technology. Through photosynthesis, trees pull CO2 from the air and store it in biomass and soils. Forest-based CDR aims to enhance this natural process through improved forest management, reforestation, and afforestation (planting new forests).

The CDR potential here is large - up to 10 billion tons of CO2 per year according to some estimates. Costs are also relatively low, potentially under $50 per ton. And forest projects can provide major co-benefits for biodiversity, water resources, and local communities.

But there are some key challenges:

  • Land competition with agriculture and other uses
  • Vulnerability to disturbances like fire and drought
  • Long timeframes - it takes decades for forests to reach full carbon storage potential
  • Difficulties in accurately measuring and verifying carbon stocks

We also need to be thoughtful about where we implement forest projects. Afforestation in some regions could actually warm the climate by reducing surface reflectivity.

Despite the challenges, forest-based approaches will likely play a major role in near-term CDR efforts given their readiness and co-benefits. But we need to pair them with more permanent storage solutions.

Coastal Blue Carbon: Leveraging Wetland Ecosystems

Coastal and marine ecosystems like mangroves, salt marshes, and seagrass meadows are incredibly efficient at capturing and storing carbon. "Blue carbon" approaches aim to protect and restore these ecosystems to enhance natural carbon sequestration.

While the total carbon removal potential is relatively modest - maybe 2-4 billion tons CO2 per year globally - blue carbon projects can be highly cost-effective. Estimates suggest costs under $100 per ton when accounting for co-benefits like coastal protection and fisheries support.

The big challenge is that many of these ecosystems are under threat from coastal development and sea level rise. We need to create stronger incentives and policies to protect existing blue carbon sinks and support restoration efforts.

Biochar: Locking Away Plant Carbon

Biochar is a carbon-rich material produced by burning biomass in a low oxygen environment (pyrolysis). When added to soils, it can lock away carbon for hundreds to thousands of years while improving soil health.

Global biochar potential is estimated at 1-3 billion tons of CO2 removal per year. Costs are moderate - likely in the $30-100 per ton range depending on feedstocks and production methods.

The main barriers are around scaling up production and distribution networks. We also need more long-term field trials to better understand biochar's impacts on different soil types and crops.

BECCS: Combining Bioenergy and Carbon Capture

Bioenergy with carbon capture and storage (BECCS) is a hybrid approach that combines biomass energy production with CO2 capture technology. The basic idea is to grow plants that absorb CO2, burn them for energy, then capture and store the emissions.

In theory, this creates a carbon-negative energy source. BECCS features prominently in many climate models, with removal potentials of 5+ billion tons CO2 per year often assumed.

But there are major questions around the true carbon balance and sustainability of large-scale BECCS. Growing all that biomass would require vast amounts of land, potentially competing with food production and threatening biodiversity.

BECCS may have a role to play, but likely a more limited one than many models assume. We need to be realistic about constraints and focus on sustainable implementations.

Direct Air Capture: Pulling CO2 Straight from the Sky

Direct air capture (DAC) uses engineered systems to pull CO2 directly out of ambient air. There are a few different technological approaches, but they generally involve passing large volumes of air over CO2-absorbing materials.

DAC is very flexible in terms of location and scalability. But it's also energy intensive and currently quite expensive - $200-600 per ton of CO2 at present.

With continued R&D and deployment, costs are expected to fall substantially. Some analysts project costs could drop below $100/ton by 2030. But that will require massive investments in the coming years.

The total removal potential for DAC is essentially unlimited. But realistically, energy requirements and costs will constrain deployment. Most projections show DAC scaling to maybe 5-10 billion tons CO2 removal per year by mid-century.

Geological Sequestration: Putting Carbon Back Where It Came From

Geological sequestration isn't a removal method on its own, but it's a critical piece of the CDR puzzle. It involves injecting captured CO2 deep underground into suitable rock formations for permanent storage.

This leverages the same mechanisms that have kept oil and gas trapped underground for millions of years. When done properly, geologic storage can securely lock away CO2 for millennia.

Global storage capacity in deep saline aquifers and depleted oil/gas reservoirs is massive - likely over 5 trillion tons CO2. Costs are relatively low, around $10-20 per ton for the storage component.

The main challenges are around site selection, long-term monitoring, and scaling up the necessary infrastructure. We'll need thousands of injection wells and an extensive CO2 transport network to achieve gigaton-scale storage.

Putting It All Together

So there you have it - a whirlwind tour of the major carbon dioxide removal approaches. A few key takeaways:

  1. We have a diverse portfolio of CDR options, each with different strengths and limitations. We'll likely need to deploy a mix of approaches to achieve the scale of removal needed.

  2. Many CDR methods are still in early stages of development and deployment. We need major investments in R&D and pilot projects to drive down costs and better understand real-world impacts.

  3. There's no silver bullet. All CDR approaches face challenges around cost, scalability, measurement, and environmental impacts. We need to be clear-eyed about the tradeoffs.

  4. CDR is not a replacement for aggressive emissions reductions. It's a complement to help us deal with residual emissions and eventually drawdown atmospheric CO2.

  5. We need to start scaling up CDR efforts now. Given the long lead times involved, waiting until 2050 to remove billions of tons of CO2 per year is not a viable strategy.

Understanding the CDR maze, the pros and cons of each and creating a CDR solution is critical to reach net zero targets. I am absolutely optimistic about CDR technologies and the potential of these technologies to reach net zero. Looking forward to more technologies and innovation in the CDR family.