【Haichuan Chemical Technology News】Microalgae can be used to convert carbon dioxide into triglycerides with high energy density

Tiny microalgae can convert “carbon” into “oil.” The ability of these tiny microalgae to do this is related to the components found within them. “Esters and sugars abundant in microalgae are excellent raw materials for producing liquid fuels. ”Liao Qiang, Dean of the School of Energy and Power Engineering at Chongqing University and Director of the Key Laboratory of Low-grade Energy Utilization Technology and Systems under the Ministry of Education, explained that driven by solar energy, microalgae are able to convert carbon dioxide into triglycerides with high energy density. These lipid molecules can not only be used to produce biodiesel but also serve as important raw materials for extracting unsaturated fatty acids such as EPA and DHA, which are highly nutritious compounds. “The photosynthetic efficiency of microalgae is the highest among all organisms on Earth, being 10 to 50 times that of terrestrial plants. ”Liao Qiang said that it is estimated that microalgae on Earth fix about 90 billion tons of carbon through photosynthesis each year, with an energy output of 1380 trillion terajoules. The energy that can be extracted from them is equivalent to 4–5 times the world’s annual energy consumption, indicating that these resources are extremely abundant.

It is understood that China emits about 11 billion tons of carbon dioxide each year, with more than half of that coming from carbon dioxide in coal-fired smoke. Using microalgae for photosynthetic carbon sequestration in coal-fired industrial enterprises can **reduce carbon dioxide emissions. Moreover, compared to traditional flue gas emission reduction technologies in coal-fired power plants, microalgae-based carbon sequestration techniques offer advantages such as simpler process equipment, easier operation, and environmental friendliness. In addition, microalgae have advantages such as forming large populations, being easy to cultivate, and being able to grow in oceans, lakes, saline-alkali lands, swamps, and other places. Liao Qiang said that microalgae have attracted widespread attention both domestically and internationally, as they are capable of reducing carbon dioxide emissions and producing clean energy. However, turning microalgae that grow freely in nature into effective carbon-sequestering \"workers\" in industrial processes is no easy task. How to cultivate algae artificially? Which microalgae has a better carbon sequestration effect? How to improve the carbon sequestration efficiency of microalgae? These are all difficult problems that scientists need to solve.

Last December, in the Hainan Power Plant of China Resources Group located in the Shenshan Special Cooperation Zone, China’s first vertical microalgae photosynthetic reactor system for reducing emissions by converting carbon dioxide from coal-fired power plant flue gas was completed. This project has overcome the key cutting-edge technologies for reducing carbon dioxide emissions from flue gas in coal-fired power plants through microalgae, breaking through technical limitations such as the large land area required by traditional raceway ponds and low efficiency in utilizing carbon dioxide, thereby enabling a scalable, efficient, and cost-effective process for carbon sequestration using microalgae. It is estimated that, compared to traditional raceway microalgae reactors, vertical microalgae photosynthesis reactors increase the microalgae yield and carbon dioxide fixation per mu by 5 times. Moreover, due to their compact structure and superior lighting conditions, these vertical reactors **reduce the space required for microalgae carbon fixation equipment.

Industrialization provides economically viable technical route options. ”As the leader of the project \"Carbon Dioxide Reduction Technology Using Microalgae\" under the **Key R&D Program ‘Clean and Efficient Utilization of Coal and New Energy-Saving Technologies’\", Cheng Jun, a professor at the School of Energy and Power Engineering at Chongqing University, did some calculations. The cost of capturing and liquefying carbon dioxide at a concentration of 99% in the flue gases from coal chemical plants is less than 100 yuan per ton, while the cost of purifying, capturing, and compressing carbon dioxide at a concentration of 15% in the flue gases from coal-fired power plants is around 250 yuan per ton. If this carbon dioxide cannot be utilized, additional costs are required for its storage. For companies, this represents a loss-making deal. However, by using microalgae to sequester carbon, it is possible to utilize that carbon and generate economic value. Currently, on the market, 1 ton of food-grade algae powder can be sold for 40,000 yuan, while the price for feed-grade algae powder is between 10,000 and 20,000 yuan per ton. With our technology, if microalgae production plants are established near coal processing enterprises, the microalgae that have absorbed carbon dioxide can be turned into valuable algae powder, which can then be used in health products, cosmetics, and animal feed.

In the industrial park of Etoke Banner, Ordos City, Inner Mongolia, there is the largest demonstration project in China for using microalgae to capture carbon dioxide; this facility is capable of capturing 10,000 tons per year of food-grade carbon dioxide obtained from flue gas purification (that is, carbon dioxide suitable for use in the food industry). The economic value generated by these microalgae for carbon capture amounts to over 200 million yuan per year. The spirulina from Etoke Banner has been applied for recognition as a **geographical indication agricultural product. At present, the achievements related to carbon sequestration using this microalgae have been put into industrial use in regions such as Shandong, Jiangsu, Guangxi, and Hainan.

Accelerated technological development is still needed to enable microalgae to produce oil. \"Although the use of microalgae for carbon sequestration has been industrialized, there is still a long way to go before microalgae can be used to produce oil.\" ”Liao Qiang said that the main methods for producing biofuels from microalgae include direct transesterification, microbial fermentation, pyrolysis, gasification, and hydrothermal liquefaction. When methods such as pyrolysis or gasification are used, it is necessary to dry it thoroughly, which results in high energy consumption ； The transesterification method that uses wet algae as a raw material requires the addition of large amounts of organic solvents such as methanol and ethanol to extract the oils, due to the presence of large quantities of water. This leads to secondary environmental pollution. Moreover, subsequent separation and oil purification are necessary, making the process technically complex and costly, with poor safety aspects, which hinders its industrial application. However, research on microalgae biomass energy in our country has never ceased. At present, our country possesses strong R&D capabilities in the basic research on microalgae biomass energy, and has carried out numerous innovative studies in areas such as high-density cultivation of microalgae, dehydration and concentration, as well as energy conversion technologies.

In the experimental platform for microalgae photosynthetic bio-carbon sequestration and the production of multi-energy complementary biofuels, established at the Key Laboratory of Low-grade Energy Utilization Technologies and Systems of the School of Energy and Power Engineering at Chongqing University, microalgae can use photosynthesis to convert carbon dioxide in exhaust gases, as well as nitrogen and phosphorus in wastewater, into biomass. Researchers break down microalgae cells to extract organic components such as oils and sugars from within them, which can then be used to produce clean fuels like biofuels and biogas. The microalgae industry also has some unique advantages: large algae must be cultivated in the sea, but microalgae can be grown anywhere; one only needs to place seawater in glass tanks or reactors, and as long as there is sunlight, carbon dioxide, and nitrogen, phosphorus, and potassium, they can keep growing ； After the mature microalgae are separated using a centrifuge, the water can be reused. Deserts have plenty of sunlight, which is an excellent condition.

If microalgae could be cultivated using 5% of China’s ocean area, or all of its deserts, semi-deserts, and saline-alkali lands, it would theoretically be possible to sequester over 7 billion tons of carbon dioxide emitted from coal combustion in the country, and to produce 3.8 billion tons of biomass. ”Liao Qiang said that microalgae represent a highly promising source of bioenergy, but scale and cost are the two major challenges preventing the industrialization of microalgae for oil production. To truly make microalgae a source of bioenergy, it is necessary to increase investment in research and development in order to achieve breakthroughs in the key technologies for producing biofuels. On the other hand, microalgae energy development and its resource utilization technologies should be regarded as a long-term endeavor; it is necessary to attach great importance to cutting-edge basic research while also making concerted efforts to overcome key technical bottlenecks. To make full use of the high-value components in microalgae, such as DHA, EPA, and lutein, it is necessary first to extract these valuable components for use in the production of health products, cosmetics, and feed, and then to utilize them for energy production in order to increase the value of these products.

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