ABSTRACT

Due to the increasing climate change concerns, biofuels have attracted more  attention in the energy field as potential alternative energy sources. Particularly,  microalgal biofuel has stood out because of its higher fuel yield potential and lower water  and land demand than terrestrial biomass. Because of its outstanding photosynthetic  efficiency, the microalgal technology is also investigated by researchers around the world  as a potential biological solution for carbon capturing in the industrial sector. To explore  the prospects of microalgal technology in a local context, this research lays it focus on  investigating the potentiality of microalgal biofuel in the cement industry on Gotland,  which is the largest emitter of greenhouse gases on the island. For this purpose, the thesis  implements a series of estimations based on the emission data of Cementa AB, Slite, a  picture of the potential production of algal biomass and biofuel was created, followed by  comparisons to the energy situation on Gotland. 

While practical data of the selected microalgae species are presented, the results  indicate a high potential of microalgae in the production of algal biofuel and the possibility  for algal biofuel to power the industrial sector of Gotland, or even the island entirely.  Although the estimations are made based on an assumption where all controlling  parameters are assumed to be perfectly manipulated, the results still indicate the  significance of microalgal technologies in the near-future bioeconomy and global energy  system. 

ACKNOWLEDGEMENTS

This thesis becomes a reality with the kind support and help of many individuals  in my life. I would like to extend my sincere thanks to all of them. 

First of all, I would like to thank my program director, course lecturers and friends  for making my time at Uppsala University Campus Gotland an exceptional and  unforgettable experience in the past three years.  

Thanks also go to my supervisor, Dr. Simon Davidsson Kurland, for imparting his  knowledge and expertise in this study, without whom, this project could not have been  possible. And my thesis group peers, for providing guidance and support throughout the  course of this research. 

Personally, I would like to thank my parents for their constant source of  encouragement and understanding, and their accompany through every step of this  incredible journey.  

Last but not least, thanks go to Johnny the cat, for accompanying me through the  most frustrating times and filling my life with happiness and goofiness.

NOMENCLATURE

CO2 Carbon Dioxide  

CCS Carbon Capture and Storage 

GHG Green House Gas 

HCI Hydrochloric Acid 

LCA Life Cycle Assessment  

NOx Nitrogen Oxide 

SOx Sulfur Oxide

CHAPTER 1.INTRODUCTION

Climate change has remained a heatedly debated subject for decades. Scientists’  warnings on climate change has now been proved farsighted and the current warming  trend is of particular significance. The discussion on global warming can be dated back  to 1988, when James Hansen, the climate scientist of the National Aeronautics and  Space Administration with three other senior researchers suggested that the  accumulation of carbon dioxide and other greenhouse gases produced from human  activities was causing the warming trend in Earth’s temperature (Butler, 2018). The  heat-trapping nature of carbon dioxide and other greenhouse gases is affecting the  transfer of infrared energy through the atmosphere and causing global warming (NASA,  2021). The threats from climate change are not a bitter pill only for human society to  swallow, the ecosystems are also under huge pressure. The role of fossil fuels is  undeniably significant in the phenomenon of global warming: human societies have been  heavily relying on the burning of fossil fuels over the past century. Even today, fossil  fuels including oil, coal, and gas still contribute to 84 percent of the world’s energy  needs (BP, 2020).

The enormous toll on the environment and humanity urges all governments and  individuals to seek more sustainable alternatives and approaches to mitigate and adapt to  global warming. As one of the renewable fuel alternatives, biofuel is considered an  environmentally friendly benign alternative to fossil fuels(Rodionova et al, 2017). The  development of biofuel is mainly driven by the depletion of fossil fuels and the  mitigation of climate change. Being a green substitute to petroleum resources, the  production of biofuel aims to generate energy fuel through biological processes or derive  from biomass to ensure energy security and minimize the impact on the environment  (Rodionova et al, 2017). Biomass products are famous for their ability to create energy dense, liquid-storage fuels that are compatible with petroleum-based energy  infrastructure (Sayre, 2010). This type of energy source is expected to contribute to the welfare and sustainability of the energy system through its environmental, economic,  and energy security effects (Lundgren and Marklund, 2013).

However, recent research has raised doubts about the carbon neutrality of traditional  biofuel. Criticism towards biofuels mainly lies in the production of first and second generation biofuel, where fuels are derived from food-grade agricultural products and  other non-food biomass like wood, plant, and animal material (Itskos et al, 2016). The  debate around greenhouse gas emission (GHG) from biofuel intensifies as the  agricultural areas continue to expand to boost biofuel production. Meanwhile, given that  water is consumed at all stages throughout biofuel production, the large consumption of  water for irrigation and cooling becomes a threat to the ecological environment and local  communities (Bergtokl et al, 2017). The limitations from available arable land, the  demand for food and water quality lead to the development of third and fourth generation biofuel, known as the algal biofuel.

1.1 Microalgae in sustainable development

As an attempt to promote the development of biofuel and minimize its environmental  impact, algae was introduced as an alternative feedstock for biofuel generation. The  process of photosynthesis has been proven to be one of the most important links in the  global carbon cycle. Meanwhile, the nature of microalgae made it a unique and fastest growing photosynthetic organism, which distinguishes it from other energy feedstock  (Chisti, 2007). Statistically, microalgae species (58,700 to 136,900 L/ha) have shown  much higher production of oil per yield if compared to terrestrial energy feedstock like  corn (172 L/ha), soybean (446 L/ha), and canola (1190 L/ha). Thanks to its outstanding  photosynthetic efficiency, microalgae species have much higher efficiency in the  conversion of water, light and CO! into biomass and oxygen (Chisti, 2007).

According to researcher Rodionova et al (2017), algae contains approximately 50  percent of lipids for the production of biodiesel, and the remaining components like  sugar and proteins can be used for bioethanol production. Besides its ability to produce several bio-oils, the advantages of microalgae-based biofuel can be seen in its greater  production yields, available land area, and its ability to capture CO! through its  biofixation process. In comparison to land plants, algae have higher photosynthetic  efficiency, and up to 50 percent of algae’s weight is composed of oil (Sayre, 2010). A  variety of industrial sources CO! are utilized in the cultivation of microalgae, including  industrial wastewater and flue gas. Additionally, the injection of industrial flue gases  into algae ponds are proven to be effective in elevating algal biomass yields (Sayre,  2010).

Meanwhile, the potentiality of microalgae biofuel has become a popular research topic in  the academic world. Researches on microalgae biofuel commonly focus on technological  outbreaks and their outstanding lipid content. Researches Ghasemi et al (2011) point out  the attractive characteristics of microalgae biofuel: algal biodiesel has been proven to be  

a non-toxic and sulfur-free energy source, which is highly biodegradable and has way  less environmental impact than conventional fuels (Ghasemi et al, 2011).

1.2 Research on the application of microalgae technology

The application of microalgae technology and its feasibility have been investigated by  many researchers. Medeiros et al (2014) conducted research to get an insight into the  application of the technology through previously done life cycle assessment (LCA)  microalgae studies. The research compares microalgae biofuel with fossil fuel options  and the integration of algae biofuel in a local electricity grid. The result shows the high  efficiency of microalgae as an energy feedstock and its huge potential in reducing the  dependence on fossil fuels (Medeiros et al, 2014). The carbon fixation ability of  microalgae is another attractive topic in the field of biotechnology. The research done by  Yahya et al (2020) is an example: the research looks into the use of native algal species  for carbon fixation at coal-fired power plants. Eduardo et al (2019) also wrote a chapter  in the literature “Biofuels from Algae”, dedicated to the use of microalgae as both  energy feedstock and biological carbon capture solution.

1.3 Microalgae research at Cementa Öland

In Öland Cementa, researchers Olofsson et al (2015) looked into the transformation of  cement flue gas into valuable biomass in the cement factory, where industrial flue gas  was used as CO! sources for microalgal cultivation. Olofsson et al (2015) examined the  impact of cement flue gas toxicity on algal biomass production, the compositions such as  lipids, proteins, and carbohydrates are examined to assess the feasibility of using cement  flue gas in algal biomass production. The algae cultivated with cement flue gas was  compared to the natural microalgal community and the results show that high quality and  high production of microalgal biomass can be achieved with the integration of industrial  flue gas. The research results concluded that microalgae cultivation is a feasible  biological solution to convert industrial waste into renewable energy sources (Olofsson  et al, 2015). The research is particularly referential for other Cementa power plants that  hope to explore more alternatives in their sustainable development program.

1.4 Scope and Aim

To explore the potentiality of microalgae technology in a local context, an investigation  is made in this research to examine the potential of microalgae biofuel as a climate  mitigation strategy in the cement industry on Gotland. 

As a pilot area for sustainable development, Gotland has a long record in successful  local initiatives and coping with international climate goals. In 2019, the Swedish  Energy Agency (Länsstyrelserna) published a Sankey diagram that shows the energy  flow of Gotland. The result indicates that 62 percent of used energy came from the  industrial sector on Gotland (Länsstyrelserna, 2017). This large consumer of energy is  supplied primarily by non-renewable waste and fossil fuels, making it a huge challenge  for Gotland to reach 100 percent renewable or even move further in its sustainable  development. The Cement industry in Slite is the dominant contributor, 45 percent of its  energy consumption came from fossil fuels in 2018 (Ahlvin et al, 2018). As the major  contributor to Gotland’s energy consumption, Slite Cementa faces pressure from both local and national environmental policies. The industry is one of the biggest obstacles in  the move towards a carbon-neutral energy system on Gotland.

The biological nature of microalgae makes it a win-win solution for Slite Cementa.  While microalgae are produced in the power plant as a renewable energy source, the  cement flue gas is decarbonized during microalgae cultivation. The potential of  microalgae biofuel in Slite Cementa is investigated through several calculations. The  research begins with simplified estimations to show the amount of algal biofuel that can  be produced with the current carbon emission at the cement production site. The  estimations further allow the exploration into the efficiency of different algal species and  the heat production of the produced algal biofuel. The estimations are expected to start a  discussion around the opportunities and challenges of the algal biofuel technology in a  local energy system. With the purpose in mind, a research question is formed:

How much microalgal biofuel can be produced therotically with the carbon emission from  Slite Cementa and what is its potential in the sustainable development of Slite Cementa?

CHAPTER 2. CEMENT INDUSTRY ON GOTLAND, SWEDEN

The Swedish cement industry is dominated by Cementa AB, which owns three cement  plants across Sweden. The cement industry in Sweden contributes to 19 percent of total  industry emissions, posing a challenge for the decarbonisation of Swedish industries.  Although energy efficiency strategies and fuel switching has contributed to a lower  emission level in Sweden, the reduction of carbon emission per ton of cement remains  concerningly high: the current CO! emission is around 722 to 701 kg CO! -eq/ton  cement, while the historical cement emission is approximately 1 tonne CO! -eq/ton  cement. The main reason for this improvement is the increased share of biofuels (21  percent) in the system as a more sustainable energy alternative (Klugman et al, 2019).

2.1 Sustainable development at Cementa

According to Cementa, approximately 60 percent of carbon emission comes from the  calcining process during the cement production, while 40 percent of emissions are  released during the combustion of fuels used to provide heating during the production  process (Cementa, n.d). To cope with the global responsibility to keep the rise of  worldwide temperature below 2 degree celsius, the Cementa group has set out objectives  to reduce its CO! emission per tonne of cement by 30 percent compared with the 1990  level by the end of 2030 (HeidlbergCement, 2019).

The Heidelberg group has successfully achieved approximately 22 percent reduction of carbon emission across all its production site by the end of 2019, and the group  continued to work on intensifying the use of alternative raw materials  (HeidelbergCement, 2019). Specifically, Slite Cementa has adopted the carbon capture  and storage (CCS) technology as an approach to separate carbon dioxide from flue gases  to enable further storage and recycling (Cementa, n.d). Although algae cultivation has  not been adopted in Slite Cementa, the technology has been investigated by the group in  a demonstration project in the Safi cement plant in Morocco. The project investigates the possibility to use CO! from the cement kiln to cultivate microalgae and examine the  efficiency of different algae species (HeidlbergCement, 2019).

2.2 Composition of cement flue gas

Being an important sector for climate change mitigation, the cement industry contributes  to 5% of global carbon dioxide emissions. The CO! emission in the cement industry is  created in multiple processes, including the calcination of limestone, combustion of fuel,  and power generation (Worrell et al, 2001). As an important part of the Cementa’s  group, Cementa Slite serves as a strong cornerstone for sustainable community building  in Sweden. Due to the requirement for high temperature during limestone burning, the  production of cement has high energy demand and thus a large amount of combustion related CO! emission. The CO! emitted from the production is mainly concentrated in  the cement flue gas, which requires additional emission and pollution control (HeidlbergCement, 2019).

Cement kilns typically compose of NOx, SOx, CO!, HCI, and dust. According to the  World Business Council for Sustainable Development the range of emissions from  European cement kilns contains 71-995 ppm NOx, 0-1,693 oom SOx, and 0.3-227 mg  Nm-3 of dust (WBCSD, 2012). A higher concentration of water and carbon dioxide can  be observed in cement flue gas, while a reduction of the levels of oxygen and HCI is  shown. The carbon dioxide rate in flue gas varied based on the type of raw material,  combustion process, cleaning techniques, and fuel in the industry. Typically, the burning  of coal contributes to 10 to 15 percent of the carbon emission during the combustion  process, while the use of natural gas takes a share of 5 to 6% (Naggapan et al, 2019)

The components of cement flue gas include carbon dioxide, nitrogen, and sulfur oxides,  which can be used as nutrients for microalgae cultivation. Microalgae can also  sequestrate heavy metals contained in cement kiln dust, which adds benefits to the  pollution reduction of the cement industry. Thus, the inclusion of microalgae biofuel production in the cement industry has drawn more and more attention (Naggapan et al,  2019).

CHAPTER 3.MICROALGAE TECHNOLOGY AND PRODUCTION

The initial studies about microalgae cultivation took place in the 40’s, where microalgae  was investigated for its potential as a source of food. Researches Sydney et al (2019)  suggested that the interest in the use of microalgae in wastewater treatment grew with  the increase of awareness on climate change in the 60s and eventually moved its focus to  renewable fuel production in the 70s. The popularity of microalgal biofuel grew with the  increasing price of oils in the 00’s, attracting the attention of the industrial sector and its  investors (Sydney et al, 2019).

3.1 Microalgae as a tool for carbon sequestration

Microalgae is a group that covers both prokaryotes and eukaryotes. While a  cyanobacteria (blue-green algae) facilitates the process of photosynthesis through the  photosynthetic membranes in its internal organization, the eukaryotic autotrophic  microorganism rely on their light-harvesting photosynthetic pigments to generate energy  for its growth (Sydney et al, 2019). The photosynthetic microorganisms convert carbon  dioxide into organic compounds and release molecular oxygen with the help of solar  energy.

The reaction shown above describes the photosynthesis process of microalgae: algae  transform light, carbon dioxide and water into biomass and oxygen through the catabolic  process and the storage of energy in organic form (Eloka-Eboka and Lnambao, 2017). In  other words, the cultivation of microalgae achieves the decarbonisation of CO! in air  and the production of energy feedstock at the same time.

Researcher Li et al (2013) in the literature of Bioresource Technology indicated the  carbon content of microalgae is approximately 50 percent. This means that roughly 1.83  ton of CO! is needed to produce 1 ton of microalgae. The research result from Li et al  (2013) is endorsed by Yahya et al (2020): as seen in equation 2, the balanced  photosynthesis formula on the ratio between carbon dioxide moles and molecular  formula of biomass indicates that approximately 1.8 gram of CO! can be fixed by every  gram of microalgae produced.

While microalgae has been successfully used for fixation of atmospheric CO!, it is  important to consider the difference of CO! concentration between atmospheric air  (0.038 percent) and industrial flue gas (4-14 percent). The endurance of flue gas  components varied between different microalgae species, therefore it should be  considered while implementing a microalgal biofuel project. As seen in table 1, the  species Chlorella sp shows that the CO! fixation performance and biomass productivity  change with the temperature CO! concentration of NOx/SOx concentration (Zhang,  2015). The CO! fixation rate can be stabilized at 50 percent while the nutrient,  temperature, light and CO! concentration are controlled in a reasonable manner. Thus,  the average proportion of carbohydrates in microalgal biomass can be estimated as 55.5  percent as suggested by Yahya et al (2020) and Li et al (2013).

Table 1. Growth characteristics of Chlorella sp. Under different CO2 concentration,  temperature and NOx/SOx contents. Adapted from (Zhang, 2015).

3.2 Flue gas toxicity in algae cultivation

While carbon capture by microalgae has been considered an attractive mitigation  strategy in the cement industry, researchers still remain doubtful about the influence of  NOx and SOx components on microalgae. The research done by Lara-Gil et al (2014)  investigates the toxicity of cement flue gas in microalgae CO! mitigation systems, and  the growth of certain algae species in a controlled environment with tested concentration  of NOx and SOx components. The typical flue gas conditions in modern cement plants  contain high concentration of CO!, ranging from 14 to 33 percent v/v. The exhaust gas  also contains significant amounts of nitrogen oxides (NOx), sulfur oxides (SOx) and  dust. These components have been proven to have an impact on microalgae growth at  different levels in the several researches (Kurano et al, 1995).

The toxicity of cement flue gas is further investigated in other microalgae biofuel  projects, where the growth of microalgae is observed under the increase of NOx and  SOx components. In the research by Yoshihara et al (1996), the elimination of nitric  oxide and carbon dioxide was reported in the cultivation of marine microalgae, strain  NOA-113. In a long tubular photobioreactor, about 40 mg of nitric oxide and 3.5 g of  carbon dioxide were eliminated per day in a controlled reactor column. Researchers  Yang et al (2004) demonstrated that high SO4 2− and HSO3 − concentration do not have  a negative impact on the growth of Botryococcus. However, an opposite result was  achieved by Lar-Gil et al, where the species D.abundans experienced a drastic decline in  its growth under a high SO4 2− concentration (Lara-Gil, 2014). This result indicates that  the NOx and SOx tolerance of microalgae largely depend on the species .

3.3 Lipid Content of Microalgae Species

Based on a calculated demand for algal biofuel, the lipid content of microalgae species is  investigated to select a suitable microalgae species for the algal biofuel production.  Researchers Sun et al (2018) in the literature of Biotechnol Biofuels indicated the  benefits of microalgae as a promising sustainable source of lipids and carotenoids. The  lipid and carotenoids content of microalgae is determined not only by its species, but  also the stress conditions such as nutrient limitation, exposure to physical factors and  temperature. Sun et al suggests that the oil levels of 20 to 50 percent are common among  the normal microalgae species, oil content by weight of dry algae can reached up to 80  percent by some outstanding species. In most cases, microalgae with high oil  productivity are desired for the production of microalgae biofuel (Sun et al, 2018).

To determine the influence of microalgae species in the production of algae biofuel in  Gotland’s context, the research result by Chisti (2007) in the literature Biotechnology  Advances is adopted. As seen in table 2, six species are selected among the 14 species  

that were investigated and compared in Chisti’s publication. The selected six species are  some of the most common microalgae species used in algal biomass production (Chisti,  2007).

3.4 Open pond system and photobioreactor

The open pond production system has been used for microalgae cultivation since the  1950s. Covering different sizes and shapes of open systems, both natural waters (lagoons, lakes, ponds) and artificial ponds or containers like tanks and circular ponds  can be utilized in an open pond production system (figure 2). Compared to a closed  system, open pond systems require smaller investment and less maintenance. However,  the productivity of an open pond system is highly dependent on the location and  weather. At the same time, temperature, natural competitors and predators can make a  significant impact on the growth of microalgae (Zhang, 2015). In the context of Slite  Cementa, surrounding natural waters are not available. Yet, there are possibilities to  construct safer and cost efficient open systems like tanks and raceway ponds.

In comparison to an open pond system, a closed system allows higher biomass  productivity because of a higher mass gas transfer rate. The design of photobioreactors  (PBR) block the cultivation system from the outside environment by obstructing any  direct exchange of gases and contaminants and turn the system into a greenhouse. The  technology allows axenic algal cultivations of monocultures, better control over the  controlling factors like water pH, temperature, light and CO! concentration and prevent  water evaporation. The loss of gas is typically much lower in a closed system than in an  open pond system. Yet, the high equipment and maintenance cost are unignorable  drawbacks to be considered while using a closed system method (Zhang, 2015). 

3.5 Microalgal biofuel systems in cement industry

As explained in 3.1, microalgal technology uses solar energy to facilitate photosynthesis  for its growth. The cultivation of microalgae can take place in both closed  photobioreactor and open pond systems. Thus, the selection of the system can be made  based on the geographical and financial situation of the industrial site. Typically, the  system operator regulates the controlled nutrients, conditions (light, temperature) and the  selection of species to achieve a desired CO! fixation performance and fuel production.  As can be shown in figure 3, the cultivation stage of microalgae biofuel involves the  insert of cement flue gas in the system as a nutrient source. The contribution of CO! in  algal cultivation is significant, which takes approximately 60 percent of nutrient cost. 

The cultivation stage further involves the growth of microalgae, where nutrients, light  intensity, water condition and temperature will be scientifically manipulated to favor  outputs. Maximization and optimization of strategies like thermochemical processes can  be employed to improve performance. The harvesting stage requires extra attention in  reducing the water content of algae suspension. This step involves the separation of  microalgae from bulk suspension (bulk harvesting) and the concentration of slurry  through centrifugation, filtration and ultrasonic aggregation. The thickening step  typically has a higher energy consumption than the bulk harvesting (Zhang, 2015)). The

drying step is grouped into the cultivation stage by Culaba et al (2020), where the drying  process is considered the last preparation stage before further processing. This definition  is not endorsed by Zhang, he describes drying as the most costly processing stage, which  can constitute 70 to 75 percent of the processing cost. Zhang suggests that drying  

methods such as drum drying, spray drying, solar drying can be considered based on the  condition of the production site. (Zhang, 2015). 

The drying process typically involves the conversion of dry algal biomass into powder or  a compressed form. As explained in 3.3, approximately 20 to 50 percent of the dry algal  biomass can be converted into liquid depending on the lipid content of microalgae  species. While the solvent extraction is typically used for extracting oil from dry  microalgae biomass, the supercritical fluid extraction can be used to process wet paste  microalgae biomass (Zhang, 2015).

In the processing stage, the oil extraction step leads to the production of different types  of sustainable energy carriers, including biodiesel, bioethanol, biogas (methane) and  biohydrogen (Zhang, 2015). At the same time, the surplus unextracted product can be  used as fertilizer or animal feed for agricultural activities, which can make extra profits  for the cement company. The production of biodiesel is specifically investigated in this  research as a renewable alternative to the fossil fuels used in the energy system of Slite  Cementa. 

CHAPTER 4. METHODOLOGY

Based on the scope and aim illustrated in 1.4, methodologies are selected to create a  feasible research design with the highest accuracy possible. The research design consists  of a series of calculations that are made based on research results and formulas sourced  in various publication regarding microalgae cultivation and microalgal biofuel. In  Section 3.1, the chemical reaction occurred in the biological process of microalgae  cultivation lays out the theoritical fundation for this research: the amount of algal biofuel  that can be produced can be estimated based on the current carbon emission from fuel  combustion at Slite Cementa. The results were expected to help start a discussion around  the selection of microalgae species and the heat production with algal biofuel in chapter  6. 

The emission data from Slite Cementa plays an important role in the research, which  again highlights the need for decarbonsiation in the cement industry. The carbon  emission of Cementa AB, Slitefabriken in 2020 is published through Naturvårdsverket,  showing the emission from the production of cement, clinker or lime. Following the  environmental management system ISO 14001, the data is provided under the  supervision of the county Administrative Board of Gotland County (Naturvårdsverket,  2020). As seen in figure 4, the total CO! emission and the emission from the burning of  fossil fuel and biomass at Cementa AB, Slite in 2020 are provided. Due to the lack of  information regarding the share of cement flue gas in the total emission, the carbon  emission to air from fossil fuel combustion is used in the thesis as the estimated amount  of CO! in cement flue gas at Cementa AB, Slite in 2020.

As indicated in 2.7, the open pond system has been a popular and cost efficient method  in microalgae cultivation, while photobioreactors are experiencing lower popularity due  to its high investment cost. The context-based comparison between the open pond and  closed pond methods seems to be understudied in the academic world. Researchers tend  to focus on the differences theoretically, rather than on a particular location or site.  Therefore, this research adviced from several researches that use the open pond method  to avoid too idealistic and unfeasible results that require high investment in  tehcnological innovation. Considering the nature of this research, the contribution from  the ongoing CCS project is excluded from the estimation to allow investigation into the  maximum potentiality of microalgae biofuel technology. The environmental impact of  microalage technology is not investigated in this research, yet the discussion around the  environmental concern and possible solutions for the integration of microalgae  technology is included in Section 6.3.

4.1 Microalgal biofuel production with cement flue gas

Previously mentioned in 3.3, approximately 1.8 gram of carbon dioxide is consumed to  produce 1 gram of microalgal biomass (dry weight) (Li et al, 2004). With this in mind, a  simplified equation is formed:

The Biomass from the given equation above refers to the dry weight of algal biomass  that can be produced with the given amount of carbon dioxide. WCO2 refers to the  amount of CO2 used as the carbon source of algae cultivation. In the context of Slite  Cementa, WCO2 is the amount of carbon emission to air through fossil fuel combustion.

The production of algal biofuel through the cultivated dry algal biomass is calculated  based on the lipid content illustrated in table 1. Based on the range of lipid content of  different species from figure 1, a simplified equation is generated:

The equation 4 calculates the amount of biofuel (Wbiofuel) that can be produced with the  given amount of biomass (kg). L refers to the lipid content of a microalgae species,  which can be found in figure 1 in 3.3. The result further allows the comparison of  microalgae species and the discussion around the relevant controlling factors that  contribute to the efficiency of biofuel production.

4.2 Heat Production with microalgae biofuel

While the energy content of biodiesel can vary depending on its energy feedstock, the  average calorific value of biodiesel is estimated as 37.27 MJ/kg (Matthews and  Mortimer, 2003). Using the equation shown below, the energy value of the produced  biofuel was calculated:

The energy production was calculated to show the amount of energy can be produced  with the selected microalgae species cultivated in Slite Cementa. The results enable  discussion around the potential in reducing the share of fossil fuels at Slite Cementa and  the possibility to influence the sustainable development of Gotland. The comparsion  with the energy demand of Slite Gotland and the energy consumption on Gotland are  made based on the Sankey diagram published by Länsstyrelserna in 2017.

CHAPTER 5.RESULTS

This chapter presents the results of the calculations from Section 4. The estimations are  adjusted into commonly used units to enable easier discussions and comparisons in the  following chapter.

5.1 Algal biofuel cultivation at Slite Cementa

Calculated from the current carbon dioxide emission of Slite Cementa, table 1 from  Section 3.1 shows that the carbon dioxide emission in Cementa Slite primarily comes  from the combustion of fossil fuels. Result from eq 3 in Section 4.1 indicates that  approximately 2,628,171 tonne of dried algal biomass can be produced at Slite cementa  with its CO! emission to air from fossil fuel combustion. 

As illustrated in 3.3, the biofuel production largely depends on the lipid content of  microalgae species. The oil content of a species can be maximized if an ideal growing  environment is provided, and further achieve an increase in biofuel production. 

Figure 4 shows the estimtation of minimum and maximum biofuel production with the  selected six species. Schizochytrium sp as the most efficient algal energy feedstock, has  shown up to 77 percent of lipid content of its dried weight. This means that if the  growing environment is controlled in an responsible way, 2024 kilotons of biofuel can  be produced in Cementa (figure 5). The lipid content of Schizochytrium sp (50 to 77  percent) indicates a possibility of the species to be an exceptional energy feedstock.  However, international researchers typically lay their focus on the potentiality of  replacing fish oil with Schizochytrium sp in fish farming. Thus, further investigation is  required if the species is applied in energy production.

Botryococcus braunii appears to have the second highest lipid content, reaching up to 75  percent of oil content. The species has been previsouly investigated as an energy  feedstock to produce transport fuels (Hillen et al, 2004). While showing the possibility  of producing 1971 kilotons of biofuel for Slite Cementa, the species also show high  vulnerability to a poorly controlled environment: the oil content of botryococcus braunii  can drop to 25 percent under undesired growing conditions.

Nannochloropsis sp as the third most efficient microalgae species in figure 2, show  relatively higher stability than botryococcus. The lowest lipid content in the species is 35  percent, which allows a minimum of 815 kilotons of biofuel produced in Slite Cementa. 

Neochloris oleoabundans as the fourth most efficient species in the selection of six,  shows a lipid content of 35 to 54 percent. This means that the species is less vulnerable  to changing conditions compared to other selected microalgae species. Approximately  920 to 1419 kilotons of algal biofuel can be produced with Neochloris oleoabundans under different controlled environments.

Chlorella sp and Dunaliella primolecta have the lowest lipid content compared to the  others, providing the opportunity of producing 604 to 841 kilotons of biofuel. Chlorella sp has shown to be the least vulnerable species to changing conditions among the list:  only small changes of lipid content is observed compared to other listed species.  Meanwhile, information about the changes in the lipid content of Dunaliella primolecta remains understudied.

5.2 Energy production with algal biodiesel

The energy outcome estimations from section 4.2 were made under the assumption  where only one species was adopted at once from the selection of six microalgae species  (Section 3.3), in other word, the mixing of microalgae species in a project is not  considered. The results in figure 5 presents the potential energy outcome in the scenario  where the biofuel is not mixed with other types of non-renewable fuels and be controlled  under an environment where the relevant parameters (eg. light, temperature and gas  concentration) were controlled reasonably. 

As figure 6 shows, the energy potential of microalgal biofuel in Slite Cementa ranges  from 6253 to 20954 GWh depends on the weight of biodiesel produced. Dunaliella  primolecta provides the lowest biodiesel production (604 kilotons), indicating an energy outcome of 6253 GWh. Meanwhile, Schizochytrium sp gives the highest energy outcome  under outstanding growign environment (20954 GWh).

CHAPTER 6.DISCUSSION AND ANALYSIS

The following analysis considers the feasibility of microalgae technology at Cementa AB,  Slite, and the impacts of the technology on the sustainable development of the company.  The analysis discusses the efficiency of microalgae as a bio-fixation tool and its potential  as clean energy supply to the company and Gotland’s energy system. Although desirable  results were obtained in the estimations above (Section 5.1 and 5.2), parameters such as  the control of nutrients, growing environment, production methods and financial situation  can affect the final results of a microalgae project. Thus, an analysis is made in this section  to underline the major challenges to be considered if to integrate microalgae technology  in Slite Cementa. 

6.1 Potential for integrating microalgae technology at Cementa AB, Slite

The fast-growing characteristic and carbon capture ability of microalgae species provide  a sustainable mitigation method for the removal of CO! at energy intensive industry like  the Cementa factory at Slite. Table 3 in Section 4 shows that CO! emission at Cementa  AB, Slite is primarily released through cement flue gas to air, proving its suitability in  providing CO!-rich waste gas for microalgal cultivation. As mentioned in Section 3.2, the  components of average cement flue gas are satisfactory for the growth of most microalgae  species and have no stress effect on the productivity of microalgal communities. This is  endorsed by the results reported in figure 2 and 3: the lowest lipid content indicates the  productivity of microalgal cultivation in non-ideal growing environment and the lowest  expected fuel production shows a desirable energy outcome in Table 4.

6.1.1 On-site environmental manipulation

As previously discussed in Section 3.1, the growth of microalgae communities is highly  dependent on the conditioning of the raw cement flue gas and the input of solar energy.  Being the sunniest place across Sweden, Gotland provides an advantageous growing  environment with approximately 1900 sunshine hours per year (WACI, 2021). This indicates a possibility for open pond methods, where the solar energy can be directly input  into the container.  

As another important limiting factor for microalgae cultivation, temperature control plays  an important role in stimulating the photosynthetic ability of microalgae species. The IEA  report by Zhang indicates an optimum temperature value of 15 to 26℃ for most common  microalgae species, while most species can easily tolerance 15℃ lower than its optimal  growing temperature (Zhang, 2015). Considering the high temperature of the conditioned  cement flue gas (150 to 200 ℃), cooling methods are required to prepare the cement flue  gas for microalgae cultivation (Olofsson, 2015). In the context of Cementa AB, Slite, it  is important to consider the seasonal weather changes and provide countermeasures for  extreme weather. 

6.1.2 Algae species selection

Olofsson suggests that locally adapted species can have relatively higher performance in  outdoor biomass cultivation, due to its ability to cope with the local environmental  conditions. Depends on the seasonal adaptability of different algae species, it is possible  to integrate more than one microalgal species in the system to maximize the annual  biomass production (Olofsson et al, 2015). Previous studies have shown the success in  using multispecies to recreate an optimal growing environment in outdoor mass  cultivation of microalgae. The diversity of microalgal species have been proven to have  complementary effect to the productivity of algae species. In other words, a highly  productive monoclonal culture can be created to recreate a seasonal algal bloom in  controlled environment. The diverse microalgae communities can create a more stable and  resilient production system and decrease its vulnerability to changing factors like weather  and other controlling parameters (Olofsson et al, 2015). Additionally, the geographical  location of Gotland provides easy access to the Baltic Sea, giving the opportunity to  investigate the monoclonal culture in local environment.

As demonstrated in figure 1, the lipid content of microalgae species varied, and it is  heavily influenced by the growing conditions. While the results have shown high  productivity of biofuel with the selected microalgae species, the application of specific  algae species should consider although Schizochytrium sp has shown to be the most  productive species among all, its application in fuel production is understudied. On the  other hand, Botryococcus braunii has the history of being investigated as an alternative  feedstock to produce transport fuel, which makes it more reliable than the other poorly  studied species in the energy field (Watanabe et al, 2014). However, considering the  complexity of controlling parameters, the achieved estimation cannot be considered as a  proxy for the final biomass production. As a result, the selection of algae species for algal  biomass production at Cementa AB, Slite should be made based on the activity of algae  species in surrounding marine environment and the weather dependency of specific  species. More importantly, the avoidance of invaded species should be taken into  consideration to avoid damage to the local environment. 

6.2 Energy contribution and performance

Microalgae has been studied as an alternative feedstock for bioenergy production and its  ability of producing multiple fuel types has been proven. Figure 7 gives a summary of  the energy supply and consumption on Gotland: in 2019, according to the County  Adminsitration Boards of Sweden (Länsstyrelserna), the energy consumption on Gotland  was approximately 4114 GWh in 2017, and the industrial sector had contributed to 62  percent of the energy consumption (2546 GWh). Taking a closer look into the current  supply of biofuels on Gotland, the supplied solid biofuel, bio-oils and biofuels  contributed to 448 GWh in 2017, while non-renewable oil took a larger share (607  GWh) (Länsstyrelserna, 2017). This means that the results from Section 5.2 indicates a  high potential of microalgal biofuel: if the efficiency in growing environment and  processing technologies have been maximized, microalgae biofuel can produce up to  6253 to 20954 GWh (figure 8), making enormous contribution to the decarbonization of  Gotland’s energy system. By boosting the share of renewable fuel in Gotland’s energy system, the integration of microalgae biofuel can help phase out non-renewable fuels on  Gotland.

While the result seems to be outstanding and game-changing, it is also overly optimistic:  the estimations were made based on a flawless hypothesis where the growing  environment is assumed to be optimal for every selected species. The changes in weather  can require pauses in algal cultivation and modification of the cultivation environment  during certain seasons. As the efficiency of the transfer of cement flue gas remains unknown, the energy and nutrient loss was unable to be included. Meanwhile, the  dependency of biomass productivity on harvesting, drying and extraction technologies  should also be taken into account.  

As mentioned in 3.5, multiple fuel types can be produced with microalgal biomass via  thermochemical and biochemical conversions like direct combustion and gasification  (Zhang, 2015). This highlights the possibility to produce not only biodiesel, but also bio hydrogen, biogass, bioehanol, bio-char and jet fuel to better adapt to the energy demand  of Cementa AB, energy system’s of Gotland. However, the compaitability of algal  biomass in different sector should be further investigated. 

6.3 Environmental consideration and emission

While the CCS technology has the ability to prevent the release of large quantities of  carbon dioxide to the atmosphere (HM Government, 2018), the biological nature of algal  biofuel brings along with unavoidable GHG emission throughout the production process  (Culuba et al, 2020). Considering the carbon emission alone, it is undeniable that carbon  dioxide emitted throughout the cultivation of algae biomass and biofuel production will  add on to the current emission of Cementa AB, Slite. Table 5 shows the main co-products  produced in microalgae biomass production: both the biochemical conversion occurs in  the fermentation process and biodiesel combustion make contribution to CO! emission.  However, the emitted CO! can be collected and reused in microalgae cultivation as  CO! feedstock for microalgae cultivation. The transestertification process typically  produce co-products like glycerol and methanol, both co-products can be used in the  production of commercialied products (Culuba et al, 2020).

Meanwhile, the consideration of ecosystem conservation should also be included during  the integration of microalgae biofuel. While the algae species with higher lipid content  will be likelier to be adopted in a microalgae project, it is important to remain critical  about the impact of an imported species on the local ecosystems. Statistically speaking,  the invasive alien phytoplankton species was recorded in the Baltic Sea, while over 51  phytoplankton species are indicated as alien species in European coastal waters. The  introduction of marine alien species in aquatic ecosystems often cause a decline in  ecological quality due to changes in physical, chemical and biological properties (Olenina  et al, 2010). 

 

6.4 Scalibility and commercialization

The mass production of microalgae has been extensively exploited for its application in  food production, aquaculture and biodiesel production. Research by Wen et al (2016)  suggests that fossil fuel is still the dominent energy source on the global and its  competitive price pose a challenge for renewable fuel alternatives like microalgal biofuel.  The commercial feasibility of large-scale biofuel production remains questionable due to  its low yield of storage lipid in outdoor microalgae mass cultivation. Statistics shows that  the algal biomass cultivation in large-scale raceway reported a low solar-to-biomass conversion efficiency (3 percent) and an undesirable production rate (20 to 40 g dry weight  per square meter a day). This is caused by the low concentration of carbon dioxide in the  air and the inefficient transfer of solar energy through natural sunlight (Wen et al, 2016).  Although further research has been implemented on the isolation, screening, genetic  modification and outdoor cultivation of microalgae strain, the lipid productivity of algae  in large-scale cultivation remain unstable (Wen et al, 2016). The complex nature of  microalgae studies pose a challenge to local research, as the local environment, weather,  carbon concentration and cultivation method all more or less affect the production result. 

6.5 Future implementation and research

The complexitiy of microalgae project indicates a need for further and more context based  investigation into the technology. Table 3 has divided the pre-study of a microalgae project  into three main parts, the future project developers are encouraged to implement further  invesigation on the suggested areas to enable more feasible project implementation. The  consideration of Cementa AB, Slite as the targeted research site is included in the table. 

The optimization of microalgae cultivation requires the inclusion of a wider range of  parameters such as flow, gas transfer and cultivation culture. The optimization of  harvesting, drying and extraction technologies can improve the production performance  by reducing energy loss. It is also important to remember that the selection of algae  species determines its photosynthesis efficiency and carbon capture ability. All in all, the  interactions between algal species, controlling parameters and cultivation technologies  can be complicated and highly context based, which requires time-consuming pre-study  before actual implementation.

CHAPTER 7.CONCLUSIONS

Microalgae has made itself an outstanding candidate for biofuel production because of  its rapid growth rate and high lipid content. Due to its nature of being a biological  solution, the microalgae technolgoy has shown to be more sustainable and  environmental friendly in comparsion to current chemical or physical CO! removal  processes. The microalage technolgy can be considered a win-win solution for an energy  intensive company like Cementa AB: microalgae can also enable the production of  commercially valuable products can provide new opportunities for the company to  promote profitable by-product or sell the surplus biofuels. The combination of carbon  capture approach, biomass production can also be combined with wastewater treatment  on site, to further maximize the benefits of the technology and promote sustanable  development at Cementa AB, Slite.  

Theoretically, the microalgae technology has the ability to decarbonize cement flue gas  through bio-fixation and convert the CO! emission to microalgal biomass. This result is  reflected from the estimation on the energy outcome of algal biofuel can be produced  from Cementa: approxiamtely 604 to 2024 kiloton of algal biofuel can be produced from  the selected site, with the possbility of generating 6253 to 20954 GWh and decarbonize  1460095 tonnes of CO!. The estimations indicates a role of algal biofuel as a high potential biological solution and climate mitigation strategy. The technology has the  ability to facilitate simontaneous decarbonisation process and clean energy production,  making it an unique addition to the energy system. The integration of the algal  technology can also promote the sustainable development of Gotland in the given  aspects: algal cultivation benefits the agriculture sector by contributing to an efficient  and low-carbon food production. The production of bio-energy can be expected to  increase with the contribution from algal biomass, achieving an increase in the share of  renewable energy in the system. Most importantly, the pressure from the cultivation of  terrestrial biomass on the local ecosystems will be eased, and contribution to land and forest conservation will be made.  

The result of this study is limited to the estimation of the scenarios proposed and  investigated, and the results indicate a clear advantage of microalgae technology as a  candidate for Carbon Capture and clean energy production at Cementa AB, Slite.  Therefore, the technology can be viewed as a mitigation strategy or an addition to the  existing sustainable development as a complementary component. It is highly  recommended to make further investigation into the production environment and energy  demand at Cementa AB, Slite.

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