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Microalgae can generate meaningful quantities of polysaccharides (sugars) and triacylglycerides. They have piqued attention as a possible feedstock for biofuel generation (fats). These are the raw ingredients used to make bioethanol and biodiesel, both of which are used as transportation fuels. Let us read further to know how algae can be used as an energy source.
The majority of microalgae are exclusively photosynthetic, which means they rely on light and carbon dioxide for energy and carbon. Photoautotrophic Culture is the term used to describe this type of Culture.
On the other hand, some algae species can thrive in the dark and use organic carbons like glucose or acetate for energy and carbon. This type of Culture is termed Heterotrophic Culture. Heterotrophic algal Culture is difficult to justify for biodiesel production due to high capital and operating expenses. Algal biofuel production often relies on photoautotrophic growth, which uses sunshine as a free light source to reduce expenses.
There are several photoautotrophic-based microalgae biomass culture systems on the market. For example, algae can be cultivated in suspension or adhered to a solid surface. Each cultivation system has its advantages and drawbacks. Suspension-based open ponds and enclosed photobioreactors now dominate algal biofuel production. A photobioreactor is a complex reactor design that may be housed indoors in a greenhouse or outside. An open pond is just a set of raceways outside.
Photobioreactors have reduced pollution and evaporation problems associated with open ponds. These gadgets are frequently made of transparent materials and placed outside to receive natural light. The culture containers have a high surface-to-volume ratio.
A tubular photobioreactor is the most common type, having several clear translucent tubes that are normally aligned with the sun’s rays. The tubes are typically fewer than 10 centimeters in diameter to optimize sunlight penetration. The medium broth is pumped to the tubes, where it would be exposed to light for photosynthesis and then returned to the reservoir.
We can use either a mechanical or an airlift pump to keep the algal biomass from settling. These pumps maintain a very turbulent flow within the reactor. After the sun collects tubes, a part of the algae is normally taken. Continuous algal cultivation is therefore conceivable.
The oxygen levels in the photobioreactor will rise until they impede and poison the algae. The Culture must be reintroduced to a degassing zone regularly, which is a zone where the algae broth is bubbled using air to remove excess oxygen. Algae also consume carbon dioxide, resulting in carbon deficiency and a rise in pH. Carbon dioxide must be delivered into the system to successfully develop microalgae on a big scale.
The benefits of enclosed photobioreactors are self-evident. They can overcome the pollution and evaporation issues that plague open ponds. On average, the biomass productivity of photobioreactors is 13 times that of a typical raceway pond.
Wastewater treatment (WWT) is a global issue that a single technology cannot handle due to the large range of sizes, types of contaminants, and geographical situations involved. Traditional WWT plants employ activated sludge to remove suspended particles (mostly mechanically) and minimize biological oxygen demand.
According to life-cycle assessments (LCAs), several algal biofuel processes have net energy ratios of about 1 and are only just approaching carbon neutrality. Other environmental and economic benefits, such as enhanced WWT effluent quality, increased downstream fisheries health, and fertilizer output, support algae biofuels as a more sustainable technology than currently used fuel sources.
In wastewater treatment, algae might be included in the secondary treatment process or incorporated as a tertiary polishing stage. Although ideal sun irradiation may be difficult to attain given the turbid circumstances usual at this stage, oxygen-producing algae might lessen the requirement for aeration in secondary treatment.
The addition of a tertiary treatment process would raise operational expenses, but this might be mitigated by producing biofuels or other valuable products. When algae are used for tertiary treatment, it has more access to light, which improves their capacity to remove nutrients that remain after secondary treatment.
Secondary-treatment algae would settle out with the other biosolids, but algae generated in a tertiary-treatment stage would require a separate settling/harvesting procedure. For this technology to become widely adopted, problems with harvesting efficiency and maintaining target algal species in the availability of various other microbial species and under changing wastewater conditions must be addressed. In contrast, ideal species for nutrient removal and the generation of useful products must be identified.
The current status of seaweeds and microalgae as environmentally benign sources of bio alcohols (bioethanol and biobutanol). Nonfeed resources such as lignocellulosic wastes and algae have been used to manufacture biofuels from second-and third-generation feedstocks to overcome these challenges. Despite this, algae are thought to be a great feedstock for biofuel generation due to their abundance.
A typical open pond may produce 5 to 10 grams of biomass per m2 of surface area each day, equating to 7.4 to 14.8 tonnes of dry biomass per acre per year.
Sugars, proteins, and lipids/natural oils are the three primary components of algal biomass. Microalgae are the only emphasis in the algae-to-biodiesel arena because most of the natural oil produced by microalgae is in the form of triacylglycerol, which is also the proper kind of oil for creating biodiesel.
Algae have two major advantages: their extraordinarily quick growth rate and their ability to sequester carbon dioxide through high carbohydrate content, which can be readily turned into bioethanol or other fuel components such as butanol (C4H9OH).
Bioethanol is gaining popularity worldwide due to its usage as a liquid fuel and the prospect of a safe, biodegradable substitute to diesel. Furthermore, as a by-product of algal biomass processing for bioethanol following the extraction of lipids, C4H9OH has become an intriguing organic fuel due to its enhanced energy content and lower vapor pressure than ethanol (C2H5OH).
CO2 conversion using algae is a low-cost approach for lowering carbon emissions. Furthermore, an algae-based CO2 mitigation technique may yield valuable goods after the process.
Several problems face the long-term generation of advanced biofuels from algal biomass. Algal productivity, carbon dioxide (CO2) supply sustainability, and the utilization of fossil-based energy in algal processing methods are among them. Among them, the long-term supply of CO2 is sometimes disregarded, or it is expected that CO2 will be easily accessible at the algal biorefinery site without causing significant emissions.
Emissions from biorefineries can include up to 80% CO2, which might be highly appealing to algae farmers. Pipes and blowers for pure CO2 gas would be smaller than for more dilute streams. Higher CO2 concentrations also provide more driving power for mass transfer requirements. Capture, storage, and transport solutions must meet the algae farm’s CO2 purity requirements.
Increased marketing of the value proposition of algal CO2 use to unconventional bioeconomy stakeholders would help the algae sector.
To use algae as an emissions mitigation approach, the field must explain what resources CO2 emission point sources require. These resources reflect that algae cultivation is an agronomic practice and include abundant and relatively flat land, a favorable climate, infrastructure to distribute algae products, and water resources of sufficient quantity and quality to ensure that algae cultivation does not harm the environment.