Algae carbon capture (ACC) is a biological process that uses microscopic and macroscopic aquatic organisms to remove carbon dioxide ($\text{CO}_2$) from the atmosphere or industrial emissions. This method harnesses the algae’s natural ability to absorb the greenhouse gas and convert it into organic matter through photosynthesis, effectively sequestering the carbon. Exploring algae offers a way to mitigate rising atmospheric $\text{CO}_2$ levels by creating a biological carbon sink integrated with existing industrial infrastructure. This approach transforms industrial emissions into valuable biomass.
The Biological Mechanism of Carbon Sequestration
The foundation of algae carbon capture lies in the photosynthetic machinery of the algal cell. Algae absorb $\text{CO}_2$ and water to synthesize carbohydrates, lipids, and proteins, releasing oxygen as a byproduct. The absorbed carbon atom is chemically fixed and incorporated into the algal biomass.
Algae demonstrate significant advantages over terrestrial plants due to their rapid growth rates and high photosynthetic efficiency. Algae can double their biomass in hours or days, allowing for a quicker turnover of carbon fixation. This rapid growth is supported by a high affinity for $\text{CO}_2$, allowing microalgae to thrive in concentrations significantly higher than atmospheric levels.
For industrial purposes, microalgae are often preferred for their high productivity and ease of cultivation in controlled systems. Species like Chlorella and Scenedesmus can fix up to 1.83 grams of $\text{CO}_2$ per gram of biomass produced. Macroalgae also sequester carbon but are typically cultivated in open ocean or coastal environments, which presents different engineering and harvesting challenges.
Engineered Systems for Industrial Deployment
Scaling up algae cultivation requires sophisticated engineered systems designed to maximize growth conditions. These systems are broadly categorized into two main types: open ponds and closed photobioreactors (PBRs).
Open Systems (Ponds)
Open systems, like raceway ponds, are shallow, circulation-driven channels that are simpler and cheaper to construct and operate. They are suitable for large-scale, low-cost biomass production. However, open ponds are subject to environmental variables such as temperature fluctuations, high water evaporation rates, and contamination. Direct exposure to the air can also lead to $\text{CO}_2$ stripping, where the gas bubbles back out of the water before absorption.
Closed Systems (PBRs)
Closed photobioreactors (PBRs), which include transparent tubes or flat-plate containers, offer a more controlled environment for optimal algal growth. PBRs minimize water loss and contamination risk while allowing for precise regulation of temperature, pH, and nutrient delivery. The closed nature of these systems prevents $\text{CO}_2$ from escaping, leading to a higher carbon capture efficiency and biomass density compared to open ponds.
These engineered systems are integrated with industrial emissions sources, such as the flue gas stacks of power plants or cement factories. $\text{CO}_2$-rich flue gas is captured, cleaned of pollutants, and then bubbled directly into the algae culture medium. This provides the algae with a concentrated stream of inorganic carbon necessary for accelerated growth and industrial-scale carbon mitigation.
Utilization of Algal Biomass Products
The success of algae carbon capture is linked to the economic viability of the resulting algal biomass, which provides the commercial incentive for the operation. Once harvested, the carbon-rich biomass is processed into a wide array of commercially valuable products.
One major pathway is the production of biofuels, including bio-oil, biodiesel, and bioethanol. However, the high cost of production often makes algae-derived fuels less competitive than petroleum-based alternatives, prompting the industry to focus on high-value applications.
The industry utilizes algal biomass for several high-value products:
- Nutraceuticals, such as omega-3 fatty acids and pigments like astaxanthin, used in human food supplements and cosmetics.
- Sustainable animal feed formulated from the protein-rich residue.
- Feedstock to synthesize specialty chemicals for bioplastics production.
By transforming the captured carbon into commercially desirable goods, algae-based systems create a circular economy model. This self-sustaining approach utilizes industrial $\text{CO}_2$ to generate revenue and offsets the operational costs of the technology.