FreeWord – Microalgae: the future of biofuels?
Nearly 15 years ago, microalgae were introduced to the world as the solution for the green technology space due to its high lipid content, ease of cultivation, and rapid growth rate. Everybody was hyped about the huge potential of microalgae. Hundreds of millions of dollars were raised in the private sector on the promise that microalgae could scale up to produce millions of liters of fuel in a matter of years.
Ever since then, we have witnessed multiple algae-based biofuel companies being driven out of business or shifted their business models to algae-based production of higher-value products such as cosmetics, dietary supplements, etcetera. While the industry hasn’t been able to fulfill its production goals or cost-competitiveness with fossil fuels, algae breakthrough has remained a distant dream until today.
Microalgae is the most photosynthetically efficient plant on earth. The cells grow in aqueous suspensions such as fresh and marine water, municipal wastewaters, industrial wastewaters, and animal wastewaters as long as there are sufficient amounts of the right chemical compounds present. Microalgae have more efficient access to water, CO2, and other nutrients than any plants on the surface. For most microalgae growth, the temperature should remain within 20°C to 30°C and relies on available sunlight.
There are different ways microalgae can be cultivated: open-culture systems such as lakes or (raceway) ponds, and closed-culture systems called photobioreactors (PBRs).
Open-culture systems are relatively inexpensive to build and operate but often suffer from low productivity for various reasons. Open ponds are more predisposed to weather changes such as temperature, evaporation, and light intensity. Other factors causing the open systems to be more energy-expensive include mass transfer difficulties and cultivation depth limits. Moreover, these open systems require more land area than PBRs and are more susceptible to contamination, exposure to bacteria, and other external harms. Intractable problems have been encountered in terms of the energy balance of lipid extraction, maintaining suitable growth, and the immense volumes of water, CO₂ and fertilizer required to allow the algae to photosynthesize fast enough at large scales.
Closed-culture system environments can be manipulated according to the species requirements. Every parameter, including carbon dioxide level, water supply, temperature, light intensity, culture density, pH level, aeration rate, and mixing pattern, can be controlled. Since PBRs thrive in controlled circumstances, higher and continuous productivity can be expected. However, the capital cost and technical aspects in sterilizing the system prevent the use of photobioreactors on a large scale.
PBRs system photoreactors vary by its used material, shape, system, operating platform, etc. The tubular photoreactor is considered the easiest to scale up production and is the only type of closed system used in semi large scale production of microalgae. Compared to open ponds, tubular photobioreactors can offer better pH and temperature control, better protection against culture contamination, better mixing, less evaporative loss, and higher cell densities.
Overall each system has relative advantages and disadvantages. One of the significant challenges of using raceways and tubular photobioreactors is biomass recovery. This challenge has been mitigated to an extent by immobilized cultures or attached algal processes. Algal biofilms could play a significant role in overcoming the major challenges to the production and harvesting of microalgae. If enough surface areas are provided, algae biofilm growth can be more than suspended growth.
Various methods are used for harvesting algae, such as chemical-, mechanical-, biological-, and infrequently electrical-based operations. Also, diverse combinations or sequences of these methods are common. Since the cell size of algae is very small, chemical flocculation is often performed as a pre-treatment to increase the particle size of algae before using another method, such as flotation. The mechanical based process combined with centrifugation is the most reliable and rapid method used for recovering suspended algae.
Biofuel Production from Algae
There are several ways the microalgae biomass can be converted into energy sources, which include: biochemical conversion, chemical reaction, thermochemical conversion, and direct combustion. In addition to biofuel production, other compounds may also be extracted with valuable applications to different industrial sectors. Overall, the creation of biofuels is a complex, technologically challenging, and economically expensive process.
A diverse list of possible fuel types that can be produced from microalgae includes biodiesel, butanol, ethanol, and even jet fuel. Certain seaweed species that are rich in hydrocarbons hold incredibly high yield, even superior to any land-based feedstock. Some species of microalgae grow incredibly fast, and the shortest recorded time to double their biomasses has been around 3.5 hours.
Fuel conversion from algae is broadly based on the feedstock’s high concentrations of lipids: fatty, oil containing acid molecules, that can be extracted to create biofuels. The oil content in microalgae varies remarkably. Several recently discovered species of microalgae can have an oil content of up to 80% of their dry body weight, although not all species are suitable for biodiesel production oils.
Biomass-based biofuels have several problems that vary from the optimization of high density and large surface units of production to the location of the microalgae production unit. In any case, the initial decision is the adoption of open or closed systems and the election of batch or continuous operation mode. As I mentioned before, there are numerous advantages and disadvantages, depending on the method and way of operation.
The algae biofuel industry has been facing numerous barriers, and while many companies have deserted this environmentally friendly fuel, scientists believe that the recent research results and discoveries could offer a breakthrough for the necessary steps to reach commercial viability. Along with this, algae still has the potential to produce a diverse range of products as excellent sources of vitamins, minerals, proteins, and can be processed into antioxidant capsules and other prebiotic food products. Diversifying lines of algae products could be the key element to boost the algae fuel industry strive to success.
Algae could be one of the solutions to reduce our dependence on fossil fuels and assist in maintaining a healthy global environment. The Energy Bit is excited about the future and eagerly following the progress!
Picture credit: www.nhm.ac.ukour-sciencedataalgaevision.html