Microalgae Carbon Capture

Introduction

Fig. 1: Microalgae farm on the Kona Coast of Hawai'i. (Source: Wikimedia Commons)

For many, microalgae are simply something to be avoided when swimming outdoors. However, compounds originating from algal biomass have had vast applications in human nutrition, pharmaceuticals, and cosmetics. Microalgae have also emerged as a promising microorganism for use in carbon sequestration and fixation. Algae photosynthesis accounts for almost 50% of the Earth's oxygen while composing only 1% of Earth's plant biomass. Additionally, microalgae are able to convert solar energy into chemical energy at a rate much higher than that of even the most efficient plants (C₄ plants). [1] Let us now explore the use of microalgae in carbon capture, as well as a subsequent use as biofuels.

Carbon Capture via Microalgae

Carbon capture and storage (CCS) is an important area of research as CO₂ concentration in the atmosphere, widely regarded as the most formidable greenhouse gas, continues to increase. CCS is characterized by 3 major steps: CO₂ capture, CO₂ transportation, and CO₂ storage. Current CCS methods are currently quite limited, with many being able to only capture CO₂ from sources producing high concentrations of CO₂ - diffuse emissions and low concentrations of CO₂ cannot be captured. [2] Capture methods also come at a high energy cost, such as chemical absorption and cryogenic separation. Additionally, CO₂ storage often involves storing the CO₂ in reservoirs, such as saline formations, aquifers, and depleted oil/gas wells. A major drawback of this type of storage is the environmental threat of long-term CO₂ leakage. It is thus desirable to find a method of CCS that does not have the environmental threats and economic drawbacks that current methods do.

Microalgae constitute one such promising alternative to current CCS technologies. The potential for CCS via microalgae in part stems from the fact that one can immediately use the captured CO₂ for energy reproduction in the form of biofuels. [3] The photosynthetic process of microalgae makes use of solar energy to convert CO₂ and water into lipids, carbohydrates, and oxygen. Not only do microalgae go through this process at a higher rate compared to most plants, they can also achieve a doubling of biomass per day with a lipid yield ten times higher than that of terrestrial crops, making microalgae one of the fastest growing plants on the planet. [3] Microalgae are capable of using 1.47 tonnes of CO₂ per tonne of dehydrated algal biomass (calculation below). The resulting biomass can be converted into a multitude of commercially valuable products and biofuels, sidestepping the glaring storage problem of CCS. [2] Were the biomass not to be converted to biofuels, the biomass would have to be buried in order to sequester the carbon, reintroducing the issues previously mentioned with CO₂ storage. One may worry that using the resulting biomass as fuel would simply reintroduce the captured carbon into the atmosphere. However, since this process recycles atmospheric CO₂ from the beginning, it still represents a decrease in CO₂ through the avoidance of burning fossil fuels.

Microalgae Cultivation

Microalgae are most often cultivated in open ponds, but can also be cultivated in closed systems, called bioreactors. Open systems have large areal requirements and an increased risk of contamination compared to closed systems, but are most often used to cultivate microalgae on an industrial scale. One of the largest open pond microalgae farms in the United States is shown in Fig. 1, owned by Nutrex Hawai'i. It should be noted that there are no current large-scale microalgae farms dedicated to producing biofuels. The first attempt at such a project was carried out by Sapphire Energy with Green Crude Farms. The green-crude oil produced by Sapphire Energy was used in many demonstrations since it opened in 2009. For example, Sapphire Energy provided fifty gallons of their green crude oil to be used by Algaeus, a Toyota Prius that took the world's first journey across the United States using a blend of algae-based renewable gasoline. Despite its success in producing green crude oil, Sapphire Energy sold Green Crude Farms to Green Stream Farms in 2019, who currently use the algae to produce nutritional products and livestock feed. While the exact reasons for this decision are unclear, one may consider the fact that the cultivation of microalgae for biofuel production is unable to compete economically with fossil fuels. [4]

Can Microalgae Offset Excess Carbon in the Atmosphere?

One interesting question to explore is whether microalgae cultivation could offset all of the excess carbon dioxide in the atmosphere. In the spirit of optimism, we will assume that 2 tonnes of CO₂ is used per tonne of dry algal biomass. The current global average of atmospheric CO₂ was 417.06 parts per million (ppm) in 2022. Currently, the target for atmospheric carbon is 350 ppm. [5] Then microalgae farms would have to offset 417.06 - 350 = 67.06 ppm of CO₂. Using the conversion factor for gigatonnes to ppm of CO₂, we see that this is equivalent to

We now wish to know how much dry algal biomass (COH₂) is required to use this amount of CO₂. In atomic mass units, we have (44 amu/30 amu) = 1.47 units of CO₂ per unit of dry algal biomass. Then,

The maximum potential yield of algae biomass is 30-40 g/m²/day. [6] We note that the potential yield depends on many factors, include microalgae species and growing conditions. Accounting for all of these factors is out of the scope of this report, and thus we will use this value in the following calculations. In the spirit of optimism, we will take this to be 40 g/m²/day. If we restrict our time for the algae to use the excess CO₂ to a year, we have a yield of 40 g/m²/day × 365 days = 14600 g/m² in one year. Then the microalgae farms would need to have a total area of

This is larger than the total area of land in the United States. From this calculation, we can conclude that it is currently unfeasible to consider using microalgae cultivation to offset excess carbon in the atmosphere in one calendar year. Even if we allow for 10 years, the area needed is still larger than the total area of land in the United States, assuming that no more carbon enters the atmosphere over that 10 years. Additionally, this calculation assumes that the carbon captured by the algae is not reintroduced into the atmosphere, meaning that the algae could not be used to make biofuels and would instead need to be buried to sequester the captured carbon. We thus conclude that microalgae cultivation alone is currently not sufficient to offset excess carbon in the atmosphere.

© Brianna Cantrall. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] J. Kurano et al., "Fixation and Utilization of Carbon Dioxide By Microalgal Photosynthesis," Energy Convers. Manag. 36,689 (1995).

[2] J. Singh and D. W. Dhar, "Overview of Carbon Capture Technology: Microalgal Biorefinery Concept and State-of-the-Art," Front. Mar. Sci. 6, 29 (2019).

[3] T. Iglinda, P. Iglin, and D. Pashchenko, "Industrial CO₂ Capture by Algae: A Review and Recent Advancesd," Sustainability 14, 3801 (2022).

[4] K. W. Chew et al., "Microalgae Biorefinery: High Value Products Perspectives," Bioresour. Technol. 229,53 (2017).

[5] C. Azar, H. Rodhe, "Targets for Stabilization of Atmospheric CO₂," Science 276, 1818 (1997).

[6] J. C. Goldman, "Outdoor Algal Mass Cultures - II. Photosynthetic Yield Limitations," Water Res. 13, 119 (1979).