RU2797838C1 - Method for utilization of carbon dioxide using microalgae chlorella - Google Patents

Abstract

FIELD: bioengineering.

SUBSTANCE: methods for biological purification of exhaust gases from power plants. The method allows to utilize CO₂ bymicroalgae Chlorella kessleri. The method includes the following steps: supplying CO₂ to a photobioreactor-biofilter containing Chlorella kessleri microalgae; photosynthesis, during which the absorption of CO₂ by the cells of the microalgae Chlorella kessleri occurs, then the biomass of the microalgae Chlorella kessleri is obtained. An air mixture or exhaust gases are supplied from power plants with a high content of CO₂ in the range of 15-39%. The photosynthesis stage is carried out in a temperature condition of 25-30°C. Illumination is set in the range from 2500 to 3000 lux.

EFFECT: absorption of gas emissions with high content of CO₂ in the range of 15-39% by Chlorella kessleri microalgae by means of directional cultivation, which is carried out in a matched mode.

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Description

The invention relates to the field of biotechnology, in particular to methods for biological purification of exhaust gases from power plants (for example, CHP, State District Power Plant) with a high content of CO ₂ (15-39%) with further use of the residual biomass of Chlorella microalgae in biotechnological processes for the purpose of waste-free removal of carbon dioxide and obtaining value-added products.

The problem of elevated levels of carbon dioxide in the atmosphere today is a serious threat to the environment. Recently, the introduction of technologies for capturing, utilizing and storing carbon ( Carbon Capture, Utilization and Storage - CCUS ) has become more and more active. They involve the use of adsorption, absorption, membrane separation of CO ₂ methods. Hybrid technologies based on a combination, a combination of different approaches are also being developed.

The fixation of CO ₂ by microalgae has a number of advantages, which are due to the high efficiency of photosynthesis, the growth rate of microalgae, and their adaptability to environmental conditions. In addition, these organisms are able to transform the absorbed carbon dioxide into lipids, proteins, pigments, carbohydrates.

According to approximate data, 100 tons of microalgae fix 183 tons of carbon dioxide during the period of maximum growth of biomass when cultivated for 8-10 days. This value will vary depending on many factors (temperature, cultivator design, CO ₂ supply method, microalgae type, etc.).

Microalgae biomass absorbs CO ₂ through photosynthesis. This releases oxygen and increases the biomass of microalgae. The photosynthesis reaction can be represented as:

 C(H₂O) + O₂ + 120 ккал/моль. CO ₂ + H ₂ O

C (H ₂ O) + O ₂ + 120 kcal / mol.

As a result of photosynthesis, biomass (plants, algae, etc.) annually forms more than 100 billion tons of organic matter, absorbs about 200 billion tons of CO ₂ and releases about 145 billion tons of oxygen into the environment.

Absorption of CO ₂ with the help of a biofilter, which uses a suspension of microalgae as a bioload, is a cheap and affordable way to reduce CO ₂ emissions, as well as a way to provide high-speed biomass synthesis of microalgae of the genus Chlorella . Microalgae are able to use inorganic carbon CO ₂ for the synthesis of organic carbon - biomass. The biomass formed during the purification process can be used to produce value-added products such as biodiesel, fertilizers, feed, etc. At present, a large number of CO ₂ absorption technologies have been developed and are being used.

The main ways of utilizing carbon dioxide are absorption and adsorption. As a result of cleaning by these methods, wastes such as sludge and spent adsorbents are generated, which need to be disposed of. It is not possible to create a waste-free carbon dioxide removal system using adsorbents and absorbents.

At present, the question of finding the most effective, cheap, and available ways of absorbing CO ₂ without the formation of by-products that require additional processing remains topical. When using microalgae to absorb CO ₂ , a value-added product is obtained, which is a raw material for the production of biodiesel, fertilizer, feed, etc.

• There is a way to absorb greenhouse gases containing CO ₂ (patent No. RU2552172C2, IPC C12P 5/00; C12P 39/00; C10L 1/00; publication of the application PCT: WO 2011/161273 (29.12.2011)), the authors of which propose to obtain artificial oil from a gas containing CO ₂ by thermochemical decomposition of fermented biomass obtained in the process of photosynthesis carried out by a certain type of microalgae based on CO ₂ received for the production of biomass. Thermochemical decomposition is proposed to be carried out at a pressure of 0<P<20 MPa and at a temperature between 200 and 420°C. The above method does not provide information on the composition of the nutrient medium required for the cultivation of microalgae. The lack of information about the composition of the nutrient medium suggests the introduction of a separate stage of technology, at which it is obtained using chemical reagents. This increases the cost of obtaining artificial oil and complicates the process of implementing this technology.

• A known method for the disposal of combustion products of power plants using natural gas (patent No. RU2608495C1 IPC A01G 7/02; A01G 33/00; C12N 1/12; C12M 1/04 published: 01/18/2017). The method comprises pumping out part of the flue gases from the chimney of a power plant, directing the gases through atomizers into microalgae biomass production tanks; pumping water with microalgae from tanks through a reverse osmosis filter concentrator to separate the liquid into water and microalgae concentrate; feeding the concentrate into the bioreactor, enriching the concentrate with carbon dioxide from flue gases; plasma treatment of the algae concentrate by using non-combustible electrodes under high pressure; separation of the algae concentrate as a result of plasma treatment into components with the release of biofuel; feeding the treated algae concentrate to a distillation column, where free ethanol is released. This method does not imply high efficiency of cleaning flue gases from CO ₂ . The use of reverse osmosis for biomass dehydration and the use of plasma treatment for biofuel extraction significantly increase the cost of CO ₂ absorption technology, which makes this technology uneconomical.

The closest analogue is the method of artificial cultivation of microalgae and installation for its implementation according to patent No. CN104524964A, IPC: B01 D53/62; B01D53/84; B01D53/96; published on 2015-04-22. A method is proposed for cleaning industrial waste gases from CO ₂ using microalgae. The method includes a microalgae carbon dioxide purification system used for simultaneous cultivation of microalgae and absorption of carbon dioxide in industrial waste gases, an energy collection and production system that communicates with the microalgae carbon dioxide purification system and is used to collect mature microalgae, and a circulating culture solution. According to the invention _{, CO 2} in industrial waste gases is purified and utilized by microalgae, and the waste water that must be discharged in the industrial production process is effectively disposed of by the culture solution circulation system, so as to effectively avoid the loss of water and nutrients during the industrial process. This method does not involve the purification of gases with a high content of CO ₂ (15-39%), and also does not disclose the possibility of using biomass to further obtain valuable components.

The technical problem to be solved by the present invention is to create a method for absorbing gas emissions with a high content of CO ₂ (15-39%) by Chlorella microalgae with the formation of biomass, which is a product for obtaining valuable components (biofuel, fertilizers, dietary supplements), and the absence by-products formed during the purification process that require subsequent disposal. This will reduce the cost of air purification technology from CO ₂ and obtain products with high added value (biofuel, fertilizers, dietary supplements).

The technical result is achieved by the absorption of CO ₂ by Chlorella microalgae during their cultivation in a nutrient medium, including on the basis of wastewater generated during the production process, and (as a result of which) with the further production of high quality biomass, which is proposed to be used as a raw material for obtaining value-added products (biodiesel, fertilizers, feed, etc.). This method is environmentally sound and cost effective.

The technical result of the claimed invention is the absorption of gas emissions with a high content of CO ₂ (15-39%) by microalgae Chlorella by directional cultivation, which is carried out in a selected mode.

Brief description of the figures

On the figures attached to the description it is given:

Figure 1 - installation for purification of emissions from carbon dioxide by Chlorella microalgae, where 1 is a source of gases with a high content of CO ₂ , 2 is an aeration pipe with perforation for uniform distribution of gases, 3 is a photobioreactor-biofilter, 4 is a valve for draining microalgae biomass.

Figure 2 is a diagram of a method for cleaning emissions from carbon dioxide by microalgaeChlorella, where 1 is a source of gases with a high content of CO₂, 3 - photobioreactor-biofilter, 5 - mother culture supply unit, 6 - generated oxygen output unit, 7 - sewage supply unit, 8 - nutrient medium and wastewater mixing unit, 9 - nutrient medium, 10 - microalgae biomass mechanical processing unit, 11 - block for obtaining biomass, 12 - block for extracting valuable components from the obtained biomass.

Figure 3 is a plot of CO ₂ concentration versus time after CO ₂ was introduced into the photobioreactor-biofilter for Chlorella Vulgaris

Figure 4 - Comparative analysis of CO ₂ uptake by two species of microalgae Chlorella Kessleri and Chlorella Vulgaris.

Air purification from CO ₂ and cultivation of biomass is carried out with continuous air purge using a bubbling device at surface illumination in the range from 2500 to 3000 Lx in laboratory sealed photobioreactors-biofilters with a capacity of 100 l, equipped with aeration system, lighting, temperature sensors, carbon dioxide supply, microalgae biomass suspension removal system. Compared to natural light sources, artificial sources can generate more irradiance, which increases biomass growth.

The temperature of the suspension solution is maintained in the range from 25 to 30°C by thermostats. Intensive bubbling with gases with a high content of CO ₂ (15-39%) of the Chlorella microalgae suspension makes it possible to intensify the processes of absorption of CO ₂ by the biomass of microalgae. CO ₂ from gas emissions is a source of inorganic carbon, which microalgae transforms into biomass organic carbon, which promotes the reproduction of Chlorella microalgae cells and maintains the required pH of the solution in the range from 6.0 to 9.0.

Installation for purification of emissions from carbon dioxide by Chlorella microalgae is shown in Figure 1.

The method of purification of emissions from carbon dioxide by Chlorella microalgae is schematically shown in Figure 2.

Suspension of microalgae is a solution of microalgae in a nutrient medium (it is recommended to add wastewater from the food industry as a source of additional nutrients). The composition of the nutrient medium for the cultivation of Chlorella microalgae was selected experimentally (table 1).

Table 1

The composition of the nutrient medium

Name of substances Concentration, mg/l ZnSO4 7H2O 100 CuSO4 5H2O 10 CoSO4 7H2O 100 MnCl2 4H2O 500 H3BO3 WF 50 Na2MoO4 2H2O 100 FeCl3 6H2O 4,000 Na2EDTA 2H2O 6,000 KNO3 3.03 KH2PO4 0.32 MgSO4 7H2O 2.4

Example 1: Purification from CO ₂ is carried out in 2 sealed photobioreactor-biofilters with a volume of 100 l each, in which the suspension of microalgae is aerated for 648 hours (27 days). Purification and cultivation is carried out at a temperature of 25-30°C and artificial lighting of 2500-3000 Lx. 50 liters of Chlorella microalgae suspension with a nutrient medium are added to each photobiorector-biofilter. Chlorella species used in the experiment: Chlorella Kessleri and Chlorella Vulgaris . These strains of microalgae are capable of high-speed photosynthesis and are typical for RF.

Through aerators for 648 hours (27 days) add CO ₂ in the amount of 7.5-17.5 l (15-39%) of the unoccupied volume of the photobioreactor-biofilter). The total absorption of CO ₂ by Chlorella Vulgaris (optical density D = 3.0-3.9 at a wavelength of λ = 750 nm) after 480 hours (20 days) was more active compared to Chlorella Kessleri for the same period. Figure 3 shows the time for which Chlorella Vulgaris is able to absorb 19.5 liters of CO ₂ .

The diagram shows that the amount of carbon dioxide supplied to the photobioreactor-biofilter is absorbed by Chlorella Vulgaris in 72 hours (3 days). In this case, the optical density of the suspension increases from 0.449 to 1.49. Thus, the suspension of microalgae Chlorella is able to absorb a percentage of CO ₂ greater than the percentage of CO ₂ in emissions from power plants (CO ₂ emissions). For example, in Uganda in 2017 alone, carbon dioxide emissions from power plants with a capacity of 241,992 MW amounted to 64,612 tons (Kavuma, C., De Dieu, HKJ, & Sandoval, D. (2020). Reducing carbon dioxide emission generated by thermal power plants with solar power on the uganda's electricity grid.Paper presented at the Proceedings of the 2020 International Conference and Utility Exhibition on Energy, Environment and Climate Change, ICUE 2020, doi:10.1109/ICUE49301.2020.9307064). Using the residual biomass of Chlorella microalgae allows solving two environmental problems at once: reducing the carbon footprint and obtaining value-added products (biodiesel, fertilizers, feed, etc.).

Example 2: For the implementation of the method, 2 types of microalgae were selected: Chlorella Kessleri and Chlorella Vulgaris. The process of utilization of CO ₂ is carried out in 2 sealed photobioreactors-biofilters with a volume of 100 l each, in which the microalgae suspension is aerated for 648 hours (27 days). Carbon dioxide was supplied to the photobioreactor-biofilter using a _CO2 cylinder (imitation of gas emissions from power plants) through an aeration pipe located at the bottom of the photobioreactor-biofilter with holes. The supply of the mother culture is carried out together with the nutrient medium (in the future, it is planned to use the wastewater of food industry enterprises as a source of additional nutrient components). After 648 hours (27 days) the biomass is drained using a crane. To separate the sediment from the solution and thicken the biomass, mechanical treatment (centrifugation) is used. The resulting biomass can be used as a raw material for the production of value-added products, such as biodiesel, fertilizers, feed, etc., and the supernatant liquid is returned to the photobioreactor-biofilter and can be used as one of the components for a new nutrient medium in the equalizer.

Table 2 presents the parameters of the conditions for the absorption of CO ₂ during the cultivation of microalgae Chlorella , the optical density of the grown samples of biomass and their ability to absorb carbon dioxide. Sample No. 1, No. 3 - Chlorella Kessleri , Sample No. 2, No. 4 - Chlorella Vulgaris .

table 2

Values of the main factors of the conditions of absorption of CO2-cultivation

No. p / p Factors Sample No. 1 Sample #2 Sample #3 Sample No. 4 1 Temperature, °С 25 25 thirty thirty 2 Illumination (LD), Lux 2500 2500 3000 3000 3 The volume of supplied CO ₂ , l /% 15.5/31 15.5/31 19.5/39 19.5/39 4 Time of utilized CO ₂ , days 4 3 5 4 The volume of CO ₂ utilized in 1 day, 1 l of microalgae suspension * 3 3.2 2.8 3 5 CO ₂ uptake rate, g/l*day 0.11862 0.12653 0.1095 0.11965 6 Optical density on the 1st day, 0.481 0.449 0.481 0.449 7 Optical density on the 27th day, 4.225 4.631 4.061 4.369

* volume of microalgae suspension = 50 l.

The increase in biomass is estimated from the change in the optical density of the microalgae suspension. The optical density was measured using a KFK-3 spectrophotometer. The initial optical density of the suspension of microalgae Chlorella Kessleri was 0.481, Chlorella Vulgaris - 0.45 at a wavelength of 750 nm.

For 648 hours (27 days) the optical density of Chlorella Kessleri biomass increased to 4.22, and Chlorella Vulgaris - up to 4.63 at a temperature of 27°C, and at a maximum temperature of 30°C, the optical density increased in the following ratios: Chlorella Kessleri - up to 4.06, Chlorella Vulgaris - up to 4.37.

Over a period of time from 192 h to 202 h (8th day from the start of the experiment), 50 l of Chlorella Kessleri microalgae at an initial optical density of 0.95 absorbed 1.25 l of CO ₂ , i.e. 3 l CO ₂ per day. Thus, 1 liter of microalgae Chlorella Kessleri at a given initial optical density absorbs 0.025 liters of CO ₂ in 10 hours, 0.06 liters of CO ₂ per day.

Over a period of time from 456 h to 462 h (day 19 from the start of the experiment), 50 l of Chlorella Vulgaris microalgae at an initial optical density of 3.72 absorbed 0.8 l of CO ₂ , 3.2 l per day. Thus, 1 l of microalgae Chlorella Vulgaris at a given initial optical density absorbs 0.016 l of CO ₂ in 6 hours, 0.064 l of CO ₂ per day.

The Figure 4 presents a comparative analysis of the absorption of CO ₂ two species of microalgae Chlorella Kessleri and Chlorella Vulgaris. With the addition of 15.5 L of CO ₂ (at 168 hours from the start of the experiment (day 7)) in one day, Chlorella Kessleri absorbed 18.6% of the supplied CO ₂ volume, and Chlorella Vulgaris absorbed 22.3% of the supplied CO ₂ volume. Chlorella Vulgaris absorbed 1.85 liters of CO ₂ more per day than Chlorella Kessleri.

The initial optical density of Chlorella Kessleri was 0.798, Chlorella Vulgaris - 0.743. 174 hours after the start of the experiment, the optical density of Chlorella Kessleri was obtained - 0.95, Chlorella Vulgaris - 1.01. Thus, Chlorella Vulgaris grows more actively and absorbs CO ₂ .

Thus, the claimed method of absorption of carbon dioxide during the cultivation of microalgae Chlorella allows you to utilize 0.127 g of CO ₂ with one liter of suspension of microalgae Chlorella Vulgaris in one day and obtain a significant increase in biomass in 648 hours (27 days) with a change in optical density from 0.45 to ~ 4.63.

Claims (1)

A method for utilizing CO ₂ by microalgae Chlorella kessleri , comprising the steps of: supplying CO ₂ to a photobioreactor-biofilter containing microalgae Chlorella kessleri ; photosynthesis, during which CO ₂ is absorbed by the cells of microalgae Chlorella kessleri; obtaining biomass of microalgae Chlorella kessleri ; characterized in that the air mixture or exhaust gases are supplied from power plants with a high content of CO ₂ in the range of 15-39%, the photosynthesis stage is carried out in a temperature regime of 25-30 ° C, the illumination is set in the range from 2500 to 3000 lux.

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