The use of algae as carbon dioxide absorber in heat production industry

: There are approximately 15 million users of system heat in Poland, but unfortunately nearly 70% of the fuel used in heat production is fossil fuel. Therefore, the CO 2 emission reduction in the heat production industry is becoming one of the key challenges. City Heat Distribution Enterprise Ltd. in Nowy Sącz (Miejskie Przedsiębiorstwo Energetyki Cieplnej sp. z o.o.) has been conducting a self-financed research and development project entitled The use of algae as carbon dioxide absorbers at MPEC Nowy Sącz . The project deals with post-combustion CO 2 capture using Chlorella


Introduction
CO 2 emissions continue to increase annually.Therefore, to reach the goals set in the Paris Climate Agreement, it is necessary to reduce emissions and to implement methods of CO 2 capture (Kammerer et al., 2023).The necessity to reduce CO 2 emissions is required by a number of international laws, including the Fit For 55 package (Brożyna et al. 2023) and the revision of the Emissions Trading System (EU ETS) (Bordignon andGamannossi degl'Innocenti, 2023, Rogulj et al. 2023).
In 2022, in the global emission of CO 2 by sectors, the largest emissions were observed in the electric energy and heat generation sectors, amounting to 39.7% of the total emissions (International Energy Agency, 2023).In Poland, system heat is used by approximately 15 million people, and regulated heat accounts for 42% of the market in households (Izba Gospodarcza Ciepłownictwo Polskie 2023).The diversification of fuels used in heat production is progressing slowly.The Polish market is still dominated by fossil fuels, which in 2021 accounted for 69.5% of all fuels used in heat sources (in 2020 -68.9%, in 2019 -71%, in 2018 -72.5%, and in 2017 -74.0%).In 2021, 14,089 thousand tons of this raw material were used for licensed heat engineering needs (Urząd Regulacji Energetyki 2022).It must be pointed out that apart from the combustion process, the excavation of coal poses a serious burden on the environment (Chłopek et al. 2021).
The above data show that the reduction of CO 2 emissions constitutes a serious challenge.Four ways of decreasing the carbon dioxide emission level can be distinguished: reducing the use of fossil fuels (including the use of renewable energy sources and green hydrogen, reducing energy demand, improving the efficiency of energy conversion processes), replacing coal-based technologies, reducing deforestation processes and, as a result, storing larger amounts of CO 2 in biomass, capturing CO 2 from burning fuels, and storing it in appropriate geological structures (Erdiwansyah et al. 2023, Nord andBolland 2020).These methods of capture of carbon dioxide in power stations and heat and power stations are vital due to the scale of the phenomenon and the slowly progressing process of decarbonization of Polish heat engineering.Potential methods and techniques of CO 2 capture in power stations and heat and power stations include: -CO 2 capture before the carbon combustion process: in this method, the fuel undergoes oxidation in the gasification process, resulting in the production of synthetic gas composed of hydrogen and carbon monoxide (Faizal et al. 2021, Madejski et al. 2022).-CO 2 capture during the combustion process in an oxygen atmosphere involves separating the CO 2 generated during the combustion process in oxygen.This method includes capturing and storing CO 2 in geological structures deep below the earth's surface (Madejski et al. 2022, Sifat andHaseli,2019) The problem of reducing CO 2 emission also affects MPEC Nowy Sącz.The company, which produces heat, has an installed capacity of 80 MW.In 2019, in the main heatgenerating plants, which accounted for 95% of heat production, namely Millenium (the largest boiler plant in MPEC, consisting of 7 coal-fired boilers with a total capacity of 70 MW) and Sikorskiego (MPEC's second largest boiler plant, which includes 4 coal-fired boilers with a total capacity of 13 MW), produced 100% of heat from coal.Despite significant expenditure and diversification of heat sources, approximately 40-49% of heat is still generated from coal (Kupczak 2021(Kupczak , 2022)).Consequently, the company has undertaken a selffinanced research and development project entitled The use of algae as carbon dioxide absorbers at MPEC Nowy Sącz.The main objective of the project was to assess the carbon dioxide absorption capacity of the installation in question under industrial conditions.The project focuses on postcombustion CO 2 capture using Chlorella vulgaris algae.The primary advantage of algae is their capability to assimilate large quantities of CO 2 , ranging between 1.65 kg and 1.83 kg of CO 2 per kilo of generated biomass (Skompski et al. 2023).
The carbon content of algae dry matter was estimated based on information from the work of W. Kozieł and T. Włodarczyk (2011).The literature highlights numerous benefits resulting from using algae in CO 2 capture to reduce carbon dioxide emissions (Iglina et al. 2022).The reduction of CO 2 emissions will contribute to improving air quality in the city of Nowy Sącz and also the Kotlina Sądecka region, which has a significant impact on the health of residents and tourists.Recently, there has been a growing recognition of challenges related to algae and carbon sequestration.Algae have the potential to offer long-term solutions to critical global issues such as the energy crisis and climate change.It is worth noting that carbon dioxide contributes to global warming, accounting for 68% of total greenhouse gas emissions (Sarwer et al. 2022).Algae, being photosynthetic organisms, utilize their photosynthetic machinery to sequester carbon dioxide from the environment, exhibiting an increased photosynthetic efficiency 10 to 15 times higher than that of traditional plants (Zhou et al. 2017).

Material and methods
In industry tests, a preparation containing live Chlorella vulgaris algae culture was used.In laboratory scale, a 10 dm 3 bioreactor with constant mixing at 250 rpm was used.A synthetic nutrient medium was used, the composition of which is presented in Table 1 below.
The culture developed under the following conditions: an airflow of 2.5 dm 3 /minute and an ambient temperature of 23°C (±1°C).The tests were conducted in natural light.The concentration of algae in the reaction mixture was determined using the weight method, separating the biomass on the micro-filter.Samples of 150 cm 3 were taken for analysis.The first measurement was taken on the 4 th day of algae growth, followed by further measurements every two days.The culture grew for 30 days.Due to the sedimentation of algae layers on the vessel walls, mechanical cleaning with a brush made from non-reactive plastic was performed every 3 days, starting from the 7 th day of the process.
The experimental development work was conducted in a sealed 1000 l container, with the volume of the mixture being 800 l.To determine the concentration of biomass, samples with a volume of 0.5 l were taken.Instead of air, as was the case in the laboratory test, combustion gases taken from the channel leading to the chimney were used.Gases were administered from the bottom using the compressor with the flow rate of 10 l per minute in 30 minutes/2 hour cycles.Gases from above the surface of the mixture were returned to the combustion gas channel.Because the bioreactor had walls that did not transmit sunlight, an external source of artificial light located in the upper part of the container was used to provide light to the culture.
This LED lamp (AQUALED, model 5050-1P31-70) had a power of 9W and was designed for effective plant cultivation.The application of properly selected wavelengths (around 450÷470 nm and around 630÷660 nm) corresponded to the demand for chlorophyll present in Chlorella vulgaris (Matejczyk et al. 2023).The lamp consisted of red and blue light diodes in a repeated combination of colors 3R:1B.The algae proliferation process was conducted under conditions of periodic lighting, alternating between 18 h of light phase and 6  h of dark phase.The installation diagram is presented in Figure 1 below.
In the subject installation, the reactive liquid was mixed by bubbling, ensuring the inflow of combustion gases rich in carbon dioxide.The cultivation period was 45 days.Gases were extracted from the channel leading the combustion gases to the chimney via a 1/8'' diameter gas pipe.The gas temperature was approximately 100°C, necessitating the use of a cooling and cleaning washer placed before the compressor.A sodium carbonate solution was employed in the cleaning washer to remove sulfur oxides and nitrogen oxides while preserving carbon dioxide.The average CO 2 concentration in the entering gases was 13.3%, assessed using a "MADUR GA-21 plus" flue gas analyzer during a 3-hour measuring session with 10-second counting of the measured values.The CO 2 concentration in the exiting gases was not measured due to varying measurement error limits.The reaction of carbon dioxide fixation by algae was too slow to detect changes in CO 2 concentration under process conditions.Initially, the solution absorbed CO 2 until saturation, but in later phases, this saturation did not result in carbon dioxide absorption while neutralizing sulfur and nitrogen oxides.Periodic pH testing of the solution was conducted to monitor exhaustion, as it is known that H 2 SO 3 inhibits the process of CO 2 absorption by algae (Dyachok et al. 2020).
The liquid in the washer had a pH in the range of 8-10.The absorbing liquid from the washer, containing nitrogen and sulfur from combustion gases, served as a carrier of mineral elements for the algae culture.The bioreactor was placed near the boiler house building, ensuring proximity to relevant water and electricity connections.The research post's location facilitated research continuity during winter months by relocating the bioreactor to the boiler hall, where the heat production installation using forest biomass (wood chips) was situated.
Ultimately, during the stable operation phase, half of the volume of the reaction fluid would be removed from the reactor for biomass recovery, after which it would be returned to the bioreactor and supplemented with minerals.The recovered biomass in the form of Chlorella vulgaris algae can be used as fertilizer, biomass for energy purposes, or further processed, e.g., torrefied for longer storage, for energy production.The use of a 1000-liter container is practical because of its easy availability, resistance to process conditions, and adaptability to different configurations -including additional lighting, mixing, or connection to other containers.This makes it possible to expand the system and make it practical on a larger scale.

Results and discussion
At the stage of industrial research, the optimal concentration of biomass was obtained between the 22 nd and 26 th day, which is presented in Figure 2 below.The constant supply of air which contains carbon dioxide allowed for intense growth of the biomass.
The greatest growth of biomass, averaging 100 mg/l per day, was observed between the 22 nd and 26 th day of culture growth.At this stage, it was concluded that biomass separation might pose challenges on an industrial scale.Various separation methods were explored, with coagulation using Fe 2 SO 4 •7H 2 O followed by flocculation and filtration proving most efficient.SUPERFLOC 492 HMW was utilized as the flocculant, but other positive ion polyelectrolytes could also be considered, necessitating testing the whole range of preparations available on the market.Depending on the initial concentration of biomass in the culture and the conditions of the process, the time needed to reach the maximum growth of biomass and its optimal concentration may vary.This has already been proven, as can be seen from the results of available research (Dziosa andMakowska 2015, Szatyłowicz et al. 2017).
In their papers, researchers indicated the influence of temperature, mineral elements or light on the speed of biomass growth (Metsoviti et al. 2019).An important issue affecting the algae growth speed is the presence of mineral elements, such as compounds of nitrogen and phosphorus, whose sources may also be found in sewage (Tleukeyeva et al. 2021;Urbina-Suarez et al. 2021, Yu et al., 2023) without atrazine and heavy metals, which may be absorbed by algae (Liu et al., 2021, Matejczyk et al. 2023).The rate of algae growth is also influenced by the by-products of the reactions, as well as waste, which additionally affects the pro-ecological nature of such installations (Urbina-Suarez et al. 2022).The use of gases with increased CO 2 content was in line with the direction adopted by other authors (Dyachok et al. 2019), who described the possibility of absorbing carbon dioxide by microalgae synthesizing chlorophyll.It was demonstrated that both CO 2 and sulfur oxides, nitrogen oxides and phosphorus oxides are absorbed by the stems of microalgae.A control experiment conducted in an open tank under similar light and temperature conditions showed 5.3 times slower biomass growth.
In the experimental development work, reactions were conducted in a pilot installation presented in Figure 3, resulting in similar biomass concentrations.The bioreactor, constructed from high-density polyethylene, offered the advantage of translucency, enabling observation of the fluid in the tank.A compressor, visible on the tank, injected exhaust gases through a pipe located on the left side of the green cap, facilitating mixing of the reaction liquid.Inside the tank, a light source was attached to its upper wall.Excess gases were discharged through a pipe attached to the green cap on the right side.The green hue of the liquid in the tank was attributed to the presence of Chlorella vulgaris algae.
In the initial period of culture growth, before the 12 th day, a gradual intensification of the green color of the reactive liquid was observed, indicative of algae proliferation.Subsequently, after the 12 th day, we noted the sedimentation of algae on the container walls, primarily at the liquid level.Due to the substantial flow rate of combusted gases relative to the rate of CO 2 absorption by the algae, there were differences in the concentration of CO 2 in the gases at the inlet and outlet of the reactor within the measurement uncertainty limits.As a result, we determined the amount of CO 2 absorbed by examining changes in the concentration of algae in the solution, as depicted in Figure 4 illustrating the changes in biomass concentration over time.
After reaching an algae concentration of 1200 mg/l, half of the 800 l of the reactive mixture underwent filtration, yielding 480 grams of biomass.The experimental setup utilized a container with a liquid height of approximately 80 cm, a specificity noted in the experimental design.According to available literature, the optimal depth or height of the liquid in a photobioreactor for the cultivation of Chlorella vulgaris algae ranges from 15 to 50 cm (Daliry et al. 2017).Despite this, vigorous mixing of the container's contents aimed to enhance light accessibility for the algal cells within.Consequently, the fluctuating biomass growth could be attributed to variable conditions such as lighting and temperature, especially as the process was conducted outside the boiler house.Unlike other researchers (Yerizam et al. 2023), who relied more on natural light in their investigations, our approach employed a closed container.
It was determined that under optimal temperature and light conditions, the recovery of biomass can be conducted in weekly cycles, yielding around 25 kilograms of biomass per year.Assuming that half of the dry mass of algae is carbon, it can be assumed that in one cycle 240 grams of carbon is bound, which corresponds to 880 grams of CO 2 .Assuming weekly biomass recovery, this yields 52 cycles per year of 0.24 kg, or 12.48 kg of bound carbon.Thus, it is possible to absorb around 45.8 kilograms of CO 2 per year.The cost of such an installation amount to around USD 650.The installation consumes approximately 0.5 kWh/day, and this energy can be obtained from renewable sources.The total energy consumed by the installation is 943 kJ/day, while the energy stored in the carbon absorbed in the biomass is approximately 1695 kJ.Therefore, the energy balance is positive, totaling approximately 752 kJ/day.
Biomass recovery from CO 2 -absorbing algae colonies represents an innovative approach to efficiently treat the carbon dioxide contained in flue gases.This process facilitates the conversion of CO 2 gas into solid and dry organic biomass, thereby contributing significantly to the mitigation of greenhouse gas emissions.Algae colonies, serving as natural filters, not only absorb pollutants but also enable the recovery of a valuable raw material in the form of biomass.
The method described differs from others in the literature (Fawzy et al. 2022) in several ways.Firstly, biomass growth is induced by providing specific conditions, primarily aimed at absorbing carbon dioxide.Additionally, it serves as the raw material for the biocarbon production process outlined in the description.
To implement the described method of CO 2 sequestration, only minor modifications were required, such as the construction of an intake point and flue gas discharge, as well as the purchase of individual components of the installation.The advantage of the described solution lies in the simplicity of implementation and the relative ease of the process.However, achieving significant capacities exceeding 50 kg of absorbed CO 2 per year in an installation with the given parameters requires careful control of the conditions and operation of the system.Based on the results obtained, scalability can be achieved by multiplying the number of individual tanks (reactors).Increasing the volume could result in zones of negligible light penetration in a larger volume of reaction liquid (Skawińska et al. 2014).The main advantage is speed, as optimal conditions for the process are reached in just a few days, while trees need many years to do so.In terms of the number of trees, the project's 1000-liter container can absorb about 45.8 kg of CO 2 per year, which is equivalent to the amount absorbed by 8 trees.
To assess the feasibility of implementing the proposed solution on a larger scale in thermal power plants, the application for 100% CO 2 absorption during year-round   operation of a 5 MW coal-fired boiler will be analyzed.The calculations consider the combustion of 650 kg of coal dust per hour with a calorific value of 23 MJ/kg, an ash content of 15.5% and a total moisture content of 9.8%.With these parameters, the CO 2 capture installation would need to have a volume of 340,000 m 3 , equivalent to a cube with a side of 70 meters.The estimated cost of such an installation would be USD 221 million, which is not economically justified.In addition, there is uncertainty about the linearity of the results as the volume increases.Additionally, heating costs would need to be considered for such an installation.In the current project, excess heat from the plant is wasted, but with full utilization, there would be additional heating costs during the periods of temperatures below +25 0 C.

Conclusions
1.The micro-installation for CO 2 capture from coal-fired boilers in heat and power stations leads to the reduction of CO 2 emissions.2. The significant tolerance of Chlorella vulgaris algae to growth conditions enables their industrial use in CO 2 absorption plants.3.In a sealed 1000 l container, under optimal temperature and light conditions, the recovery of biomass can be conducted in weekly cycles, yielding approximately 25 kilograms of biomass per year.Assuming that half of the dry mass of algae is carbon, it can be inferred that in one cycle, 240 grams of carbon is bound, which, in terms of CO 2 , equates to 880 grams of this gas.Therefore, around 45.8 kilograms of CO 2 can be absorbed per year.4. Waste products and by-products can be used as a source of mineral elements needed for algae growth, thus reducing the burden on the natural environment.5. Eight trees can absorb the same amount of CO 2 in that time.
However, it should be noted that trees consume almost no energy during their growth.The efficiency of trees in terms of CO 2 absorption is very high.6.The solution resulting from the project The use of algae as carbon dioxide absorbers at MPEC Nowy Sącz is not suitable for application and scalability in thermal power plants due to economic reasons and a lack of confirmation of the linearity of the results.Nevertheless, given the success of the project and the promising results, cheaper solutions should be sought.7. It seems reasonable to continue the research considering two main directions -first, intensification of the process; and second, research in other systems using cascade reactors and process studies in different container dimensions.

Figure 3 .
Figure 3. Installation for absorbing carbon dioxide using Chlorella vulgaris algae in industrial conditions.

Figure 4 .
Figure 4. Changes in the concentration of alga at the stage of experimental development work.

Figure 3 .
Figure 3. Installation for absorbing carbon dioxide using Chlorella vulgaris algae in industrial conditions

Table 1 .
Ingredients of synthetic nutrient medium.