Understand Carbon Management

Welcome to Energy Spotlight, our newly rebranded monthly publication for Stanford University’s Understand Energy Learning Hub! In this issue, we are focusing on carbon management. If you like what you see, please share with your friends and family and encourage them to subscribe.

What you need to know

Significance: Carbon management strategies will play an essential role in meeting net-zero goals and limiting global warming to 1.5°C or 2°C above pre-industrial levels according to both the International Energy Agency and the Intergovernmental Panel on Climate Change. Carbon management can help address difficult to decarbonize economic sectors and remove legacy carbon dioxide (CO₂) already in the atmosphere, but it must scale up. By 2050, the annual amount of carbon removed and sequestered with carbon management activities needs to be at 7.75 billion tons. That’s more than 150 times the ~50 million tons of annual carbon removal being achieved today.

Although carbon management is crucial for a net-zero system, we can’t scale it enough to keep up with the amount of CO₂ our fossil fuel use is pumping into the atmosphere. We currently emit over 35 billion tons of CO₂ each year, and that is expected to keep growing. We need to partner carbon management with other greenhouse gas mitigation strategies like dramatically reducing our fossil fuel usage, increasing our use of zero-carbon energy resources, and improving energy efficiency.

What is carbon management? Carbon management includes natural and technological solutions that remove ambient CO₂ from the air or capture CO₂ emissions from industrial processes and power plants, and then use the CO₂ to make products or sequester it so that it doesn't contribute to climate change. In some processes, we capture and sequester solid carbon or biomass rather than CO₂, which is a gaseous form of carbon.

Diagram showing carbon dioxide removal, carbon capture, utilization, and storage

How do we get the CO₂? We can either remove CO₂ that is already in the atmosphere or capture CO₂ before it is released into the atmosphere.

Carbon dioxide removal (CDR): CDR refers to methods that remove CO₂ already in the atmosphere. Oceans, forests, soils, and wetlands naturally remove CO₂ from the air through physical and biological processes like photosynthesis. CDR enhances these natural processes to remove more CO₂ or uses technology to separate out and remove CO₂ from ambient air.
Carbon capture: Technology is used to capture CO₂ before it’s emitted into the atmosphere from fossil fuel or biomass power plants or industrial facilities like cement and steel plants.

The higher the concentration of CO₂, the easier and cheaper it is to remove or capture. That’s why CDR can be harder than industrial carbon capture: CO₂ concentration levels are much higher in industrial emissions than in ambient air. CDR is like trying to find and remove 400 red jelly beans from a bag of one million—an extremely time and energy intensive process!

colorful jelly beans being split into two groups: red jelly beans and all the rest.

What do we do with the CO₂? Removed or captured CO₂ must be transported and then either permanently stored to prevent its release into the atmosphere or utilized to make products.

Storage: In geologic storage, CO₂ is injected into deep underground geological formations for permanent/durable storage. Other forms of storage or sequestration include deep ocean biomass sinking, enhanced mineralization, reforestation, and soil-based sequestration. CCS refers to carbon capture paired with storage.
Utilization: CO₂ is converted into useful products that either store or re-release the carbon. CCU refers to carbon capture paired with utilization.

Carbon dioxide removal

CDR encompasses a wide array of nature-based and technological project types.

CDR approaches include: enhanced weathering, reforestation/afforestation, direct air capture, direct ocean removal, ocean fertilization, artificial downwelling and upwelling, intentional biomass sinking, macroalgal cultivation, ocean alkalinity enhancement, coastal blue carbon, and biochar. — Source: NOAA.

Here are more details about a few of the CDR approaches shown in the image above (click on each image to view larger):

Enhanced weathering (nature-based)

CO₂ from the atmosphere naturally reacts with minerals like magnesium or calcium in rocks to create new rocks that store the CO₂ safely for thousands of years. These minerals, which must be mined, can be ground up so they react more easily with CO₂ in the air, and then spread across agricultural lands, forests, or oceans to accelerate the uptake of CO₂.

Minerals are mined, crushed, transported, and dispersed on land or in the ocean for increased CO2 absorption from the atmosphere.

Ocean fertilization (nature-based)

Oceans naturally absorb CO₂. Ocean fertilization adds nutrients like iron, nitrogen, and phosphorus to the ocean’s surface to stimulate the growth of phytoplankton, which absorb CO₂ through photosynthesis. When the phytoplankton is eaten by other marine life or dies, the carbon it has absorbed sinks to the deep ocean where it can stay for hundreds of years and may eventually become sediment at the bottom of the ocean where it will be permanently stored (~1 million years).

Nutrient enrichment is added to the ocean, stimulating phytoplankton growth to absorb more CO₂ from the atmosphere and leading to more biological productivity, resulting in increased carbon storage at the bottom of the ocean.

Bioenergy with carbon capture and storage (BECCS)

BECCS combines nature-based CDR with CCS. It uses biomass sources, which have naturally removed and stored CO₂ from the atmosphere, to produce electricity in a thermal power plant. CO₂ from the power plant is captured before it is emitted to the atmosphere, and then permanently stored or utilized.

Trees in a replanted forest absorb atmospheric CO₂. Woody biomass from the trees fuels a bioenergy conversion plant to provide heat, power, hydrogen, etc. CO₂ emissions from the power plant are captured, transported and sequestered underground in depleted petroleum reservoirs, saline aquifers, etc. or into the soil as "bio-char."

Direct air capture (DAC)

Large fans draw in ambient air which passes through a material that absorbs and captures CO₂. The CO₂ is then either used to make products or permanently stored. DAC projects in particular require significant amounts of energy because they extract CO₂ from the air, where concentration levels are extremely dilute (~0.04%). Capturing even a small amount of carbon requires processing a lot of air.

Ambient air is drawn into filters that use heat to extract CO2. The CO2 is captured and stored underground or contained and transported to market.

Carbon capture

Carbon capture uses technologies to capture CO₂ from higher concentration sources like fossil fuel or biomass power plants and industrial facilities like cement and steel plants. CO₂ is captured at the point source (where it is produced) before it is emitted into the atmosphere, and is then either used to make products or permanently stored. Example carbon capture methods include (click on each image to view larger):

Post-combustion capture

After the fuel has been burned to generate energy, CO₂ is separated out from the flue gas exhaust by a chemical system.

Air and fuel enter a boiler. Steam from the boiler turns a turbine to generate electricity. Additional steam, along with a mixture of CO₂, nitrogen, and water, goes into a chemical wash that separates out the CO₂. The CO₂ is compressed and dehydrated and then transported and stored.

Pre-combustion capture

Carbon is separated from the fuel before it is burned, sometimes through a gasification process that produces a mixture of CO₂ and hydrogen, and sometimes through a pyrolysis process that produces solid carbon and hydrogen. The CO₂ or solid carbon can be stored or utilized, and the hydrogen can be used for carbon-free energy production.

Oxygen is separated from air and gasified with a fossil fuel, creating syngas. A carbon capture unit separates the CO2, which can be utilized or stored, and the hydrogen, which can be used for generating electricity.

Oxy-combustion

Fuel is burned with pure oxygen instead of air, creating a flue gas stream that is primarily CO₂ and water vapor. When it’s cooled down, the CO₂ remains so it can be used as the working fluid or captured and stored or utilized. Oxy-combustion is currently much less commonly used than post- and pre-combustion methods.

Oxygen is separated from air and combusted with natural gas resulting in CO₂ and water vapor. The CO₂ mixture expands and turns a turbine to generate electricity and then goes into a heat exchanger to cool. Water is removed from the CO₂ and then repressurized, captured, and exported for sequestration or commercial use. Recirculated CO₂ is reheated to be used again

Geologic storage

Where do we store CO₂? We know how to durably store CO₂ underground. CO₂ can be injected into depleted oil and gas reservoirs and deep saline formations. CO₂ is typically injected at depths greater than 800 meters. Suitable storage locations have rocks with good porosity (that make up the reservoir), which are overlain by rocks with low porosity and permeability to trap the CO₂ and keep it from escaping.

Many potential locations for geologic storage exist around the world. For example, we’ve been storing about 1 million metric tons of CO₂ annually underground in the North Sea since 1996. For any potential CO₂ storage project, extensive reservoir characterization is performed to confirm that the host site is well understood. Additionally, the storage site must be continuously monitored for CO₂ leakage.

Utilization

What can we make with captured CO₂? Captured CO₂ can be used to make products that are currently made with fossil fuels. Some products, like plastics and construction materials, sequester the CO₂ and can be carbon negative. Others, like liquid fuels, re-release the CO₂ back into the atmosphere when they are burned, making them carbon neutral at best.

Measuring the benefits

To benefit the climate, carbon management projects must remove and sequester more CO₂ than they emit across their entire lifecycle. Measuring the carbon benefits of projects can be complicated:

One challenge is making sure that the carbon removed is additional, real, and verifiable, especially if it’s being used for a carbon offset or credit. What would have happened if the project had not been done? How much additional carbon is really sequestered and for how long? Sequestration must be long term to mitigate climate change. For example, if a wildfire burns down trees planted in a reforestation project, that carbon is re-emitted to the atmosphere, reducing the duration of the carbon storage.
Accounting for the energy inputs and associated emissions through all stages of a project’s life cycle is complex but necessary to determine the net amount of carbon sequestered. For example, if wind energy is used to power a DAC project, and that wind energy could have instead been used to offset electricity generation from a coal-fired power plant, should the electricity powering the DAC be considered carbon free?
In order to ensure the integrity of carbon benefit measurements, systems must be continuously monitored for leakage at all steps along the way, including transportation and storage or utilization.

Environmental impacts

Is carbon management bad for the environment? Carbon management projects can have both positive and negative impacts on the environment. On the plus side, for example, some methods can help restore ecosystems and habitats, boost soil health and stabilization, and counteract ocean acidification. But projects can also use a lot of energy, create competition for water and land resources, induce seismicity, contaminate soil and water, and negatively impact ecosystems and habitats. Some of these risks, like induced seismicity, can be avoided with good project management. The impacts of some carbon management methods, like enhanced mineralization, ocean alkalinity enhancement, and intentional biomass sinking in the deep ocean, are still not fully understood.

Current and future trends

Annual durable CDR purchases from 2020 to 2024. Growth has been exponential from 18.5 thousand metric tons in 2020 to 8 million in 2024.

CDR is in the early stages of development, with very few commercial operations. CCS has been ongoing since the 1970s; however, only ~50 million tons of CO₂ is being captured and stored annually. Massive growth in both CDR and CCS will be necessary to reach the 7.75 billion tons per year required by 2050 to meet climate goals. Carbon regulations or a price on carbon can help drive growth in carbon management solutions.

Advance market commitments like Frontier and self-imposed carbon reduction goals for businesses (voluntary carbon market) are also vehicles for increasing investment in carbon management solutions. For example, these vehicles are driving growth in durable (permanently stored) CDR purchases.

In the news

News: California has approved its first carbon storage project. The project is expected to be capable of injecting over 1 million metric tons of CO₂ annually into a depleted oil reservoir at the Elk Hills Oil Field in Kern County, and have a total estimated storage capacity of up to 38 million metric tons. It is the first project in California to receive U.S. Environmental Protection Agency Class VI well permits for long-term underground CO₂ storage. Learn more about the Elk Hills project.

Oil well pumping at Elk Hills oil field. — Elk Hills oil field, the site of the first proposed carbon storage project in California. Photo by Larry Valenzuela, CalMatters/CatchLight Local

Context: Carbon management remains a small portion of Califonia's decarbonization strategies. Supporters of this project say it can reduce emissions from difficult to decarbonize sectors, preserve jobs, and help transition oil-producing regions toward lower-carbon economies. The project faces pushback from environmental advocates and Kern County residents, who cite risks of CO₂ leaks, increased air pollution, and the potential to prolong fossil fuel infrastructure. Several organizations have filed a lawsuit, arguing the county failed to fully evaluate the health and environmental impacts, especially for nearby low-income communities of color.

Fun Fact

Underground fungal networks are one of Earth’s largest carbon sinks. Mycorrhizal fungi (which grow in partnership with plant roots) store up to 13 billion tons of CO₂, equivalent to about 36% of annual global fossil fuel emissions.

A girl looks at a mushroom with a magnifying glass as it shares resources with a tree through underground connections. Arrows show nutrients moving to the tree, carbohydrates moving to the fungus, and carbon being stored in the soil — Image from Frontiers for Young Minds

Plants transfer carbon to the fungi, and in return, the fungi deliver vital nutrients that support plant growth. Because fungi cannot photosynthesize, they rely entirely on this relationship to obtain the carbon they need to grow and build biomass. This underground carbon trading system is a major reason why soils store three times more carbon than the atmosphere. Read more about underground fungal networks.