碳移除 CDR－淨零路上最後一哩（下）｜方格子 vocus “Vocus / Fang ge zhi”
2023/09/03 Reading time about 7 minutes
Brief Discussion on Carbon Removal Series 2: Introduction to Existing Carbon Removal Technologies
Carbon removal methods, often called “pathways”, are mainly divided into two types: biological (nature-based) and technological (technology or engineering-based). Nature-Based Solutions (NbS) increase carbon sinks in forests, soils or ocean systems through ecosystem management practices; Engineered Solutions (Engineered Solutions) chemically separate, compress, and remove carbon dioxide directly from the atmosphere. Transported and stored in deep geology beneath the earth’s surface. New planting and reforestation, soil carbon sequestration and coastal blue carbon are recognized as nature-based approaches. Other approaches, such as direct air capture and biomass carbon capture and storage, are typical engineering approaches. But in fact, most carbon removal approaches should be hybrid solutions of nature and technology.
Nature-based carbon removal pathways
The nature-based approach uses ecosystems to absorb carbon dioxide from the atmosphere through natural processes, which is a sustainable and environmentally friendly approach. Harnessing the natural ability of ecosystems and biospheres to absorb and sequester carbon to mitigate climate change.
forest carbon removal
Forests are valuable natural resources and carbon removal machines. Through protection, restoration and management, forests provide important social, economic and climate benefits. In addition to absorbing carbon dioxide, forests can also improve local air and water quality, promote community health, and protect against extreme weather and rising temperatures. Trees naturally absorb carbon dioxide from the ambient atmosphere through photosynthesis and store the carbon in their biomass to form trunks, branches, leaves and roots, with some of the carbon transferred to the soil. Forest ecosystems are therefore a carbon sink, storing carbon in soil organic matter, roots and decaying plants for decades to centuries.
Methods such as new afforestation (Afforestation), reforestation (Reforestation), and forest restoration (Forest Restoration) mainly increase forest carbon removal potential by maximizing tree growth and practicing forest management. Reforestation is the establishment of forests in areas that were previously treeless; reforestation is the restoration of forests in areas that were previously logged. In addition to achieving carbon removal, afforestation and reforestation projects must also ensure the purpose of promoting biodiversity, providing ecosystem services, and strengthening the climate adaptability of forests.
soil carbon removal method
Soil has the ability to store large amounts of carbon, which is naturally stored in the soil over time to nourish plants and crops. Agricultural soil carbon storage is one of the land-based carbon removal solutions, which can increase soil organic carbon and inorganic carbon content through a variety of land management practices. Conservation tillage, growing perennial crops, cover crops and agricultural management all work to capture carbon dioxide from the atmosphere and store the carbon in the soil. Generally the most recommended ones are:
- Cover planting: planting vegetation during the fallow period to prevent soil erosion, improve soil fertility, and increase carbon input.
- Conservation tillage: Reduce soil disturbance from farming activities to preserve soil structure and organic matter and improve carbon storage capacity.
- Rotational grazing: Grazing in rotating areas gives pastures the opportunity to restore and accumulate organic matter, thereby increasing soil carbon storage.
Increasing soil carbon is a win-win for farmers and the climate. U.S. agricultural soils alone may store up to 10% of U.S. greenhouse gas emissions each year. Agricultural soils rich in carbon can enhance the resilience of crops to climate impacts and provide synergistic economic and ecological benefits.
Ocean Carbon Removal Method
The ocean plays a vital role in the global carbon cycle. The complex root systems of mangroves can effectively sequester carbon; seagrasses, as marine plants, can accumulate carbon in their leaves and roots; and salt marshes in coastal wetlands have extraordinary soil carbon sequestration capabilities. Coastal blue carbon ecosystems such as mangroves, seagrasses and salt marshes serve as biological habitats and can also store plant biomass and sediments. They are natural carbon removal methods with high carbon sequestration potential. Other ocean-related carbon removal pathways include:
- Biopumps: Use the process by which phytoplankton incorporates carbon into their biomass through photosynthesis to remove carbon dioxide from the surface, allowing the organic matter to sink into deeper ocean layers after the phytoplankton dies, thus storing the carbon in the deep ocean.
- Physical pump (solubility pump): By driving the difference in carbon dioxide concentration in seawater, more carbon dioxide can be absorbed by the ocean surface.
- Artificial upwelling: Using artificial methods to raise deep seawater to the surface, thereby stimulating the growth of phytoplankton and macroalgae to enhance the “biological pump” effect.
The advantage of nature-based carbon removal is that it can sequester carbon while taking into account biodiversity protection and habitat restoration, and enhance ecosystem services and resilience. However, nature-based carbon removal methods also face challenges such as limited land resources for afforestation, land conflicts and competition among agriculture, fishery, animal husbandry, and forestry, and the difficulty of actually implementing measurement, reporting, and verification (MRV).
Various pathways for carbon removal. (Image source: MCC - mcc-berlin.net/)
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Various pathways for carbon removal. (Image source: MCC - mcc-berlin.net/)
Introduction to engineering-based carbon removal methods
Engineering-based carbon removal technology uses advanced engineering and chemical processes to capture and store atmospheric carbon dioxide to achieve negative emissions and mitigate climate change.
Direct Air Carbon Capture and Storage (DACCS)
Direct air capture is a carbon removal approach that uses chemical sorbents or solvents to capture carbon dioxide directly from ambient air, then separates and purifies it and then stores it in geological rocks or storage tanks. Modular DAC has the potential for large-scale deployment, but the current technology requires a large amount of energy to separate carbon dioxide, so the application of renewable energy is more likely to achieve large-scale operation.
Carbon Mineralization
Carbon mineralization is a carbon removal path that makes good use of the carbonation reaction to allow carbon dioxide to react with certain rock minerals to produce solid minerals (carbonates); two of them are represented by enhanced rock weathering and geological storage.
The technology of enhanced rock weathering involves spreading finely ground alkaline minerals such as olivine and basalt on land to accelerate the natural weathering process and quickly convert carbon dioxide into stable carbonate minerals, thereby achieving permanent The goal of atmospheric removal is a method that can be implemented on land or ocean; the geological storage method is to inject the captured carbon dioxide into a formation rich in alkaline minerals to allow the carbon dioxide to react with the minerals. Form solid carbonates to achieve the goal of permanently sequestering carbon dioxide.
Biomass Carbon Removal and Storage (BiCRS)
Biomass carbon removal and storage is a series of ways to use biomass such as plants or algae to remove and store carbon dioxide for a long time. Since biomass only removes and stores carbon before it dies and decomposes, the main purpose of the BiCRS pathway is to extend the carbon storage capacity of biomass beyond its natural life cycle. There are many different methods of BiCRS, the most common of which are biochar and biomass carbon capture and storage.
Biochar is generally made from agricultural and forestry surplus resources such as rice husks or sawdust and other organic matter, which are heated in a low-oxygen environment through thermal cracking. Biochar promotes carbon storage when added to soil and prevents organic matter from decomposing and releasing carbon by promoting microbial activity and aggregation. Bioenergy with Carbon Capture and Storage (BECCS) first converts biomass into energy or fuel, and then captures the carbon dioxide produced during combustion and stores it in geological structures or products. In addition, another way is to thermally crack biomass and convert it into bio-oil (Bio-oil) and then store it in the geology.
Ocean Fertilization and Alkalization (OF & OAE)
Ocean fertilization and alkalization are ways to engineer ways to enhance the ocean’s natural ability to absorb carbon dioxide. Ocean Fertilization mainly involves adding nutrients such as iron or nitrogen in suitable sea areas to stimulate the growth of phytoplankton to enhance the absorption benefits of biological pumps. Ocean Alkalinity Enhancement (OAE) is the addition of alkaline substances to suitable sea areas to increase the alkalinity of seawater by enhancing weathering, prompting the ocean to absorb more carbon dioxide from the atmosphere.
These carbon removal methods that require engineering intervention in natural processes provide additional flexibility to achieve negative carbon emissions, and have considerable capacity for large-scale deployment and expansion; similarly, the energy demand in the process and the energy generated by the process Carbon emissions also affect cost-effectiveness and economic feasibility. In addition, long-term monitoring of carbon dioxide removal efficiency and storage integrity is also a challenge.
Carbon removal methods, whether natural-based, engineering-based or more realistic hybrid solutions, each have their own advantages and disadvantages, and the actual implementation also varies from place to place due to different environmental conditions. It is worth noting that any method of carbon removal will not be a “panacea”. Therefore, in the face of the immediate threat of extreme climate and the urgent time pressure of net zero, investment from both policy and capital aspects is needed. Resources for research, development and deployment. Of course, the most important thing in this climate war is to substantially reduce carbon emissions, and only then will carbon removal be meaningful.