As fossil fuel reserves decline and atmospheric CO2 concentration increases, the need to find renewable sustainable energy sources has become pressing. The current generation of biofuels, derived from food crops and oil seeds, has relatively low negative environmental effects from its consumption, but it is limited in its economic feasibility because it relies on drawing from food supplies. Microalgae is becoming increasingly viewed as an alternative source of biomass for biodiesel creation, as it has many biological and economic advantages. In their 2010 review article, Mata et al. outline the viability of microalgae as a biofuel source, discussing areas that need to be considered and further substantiated before microalgae can be used for wide scale fuel production. Additionally, they highlight other ways that microalgae may be used to ameliorate various climate and health problems, including flue gas CO2 emission mitigation, wastewater purification, and medicinal applications. They conclude that, while having significant potential as an economically and environmentally feasible source of biodiesel, there remains a substantial amount of scientific research and technological advancement that must be completed before microalgae can be cultivated and processed on a large-scale.—Karen de Wolski
Mata T., Martins A., Caetano N. 2010. Microalgae for biodiesel production and other applications: a review. Renewable and Sustainable Energy Reviews 14, 217–232.
Mata et al. emphasize the dire need to find renewable and sustainable energy alternatives, listing the negative effects of global warming and high atmospheric CO2 concentration and implicating the burning of fossil fuels as the main source of greenhouse gas (GHG) emissions. Biofuels are illustrated as a more environmentally friendly alternative, as they have lower combustion emissions per unit than diesel and gasoline. However, current biofuels are limited because they draw from foodstuffs, and they are therefore not only expensive but also lead to an increase in food prices while lowering food availability worldwide. Biodiesel, comprising 82% of total biofuels production and derived from vegetable oils and animal fats, cannot meet the current market demands, as both land availability and feed stocks are insufficient. In order to meet demand and ameliorate global warming, it is therefore necessary to utilize an alternative biomass source that is less costly to cultivate and has lower land requirements. Microalgae is currently under investigation as such a source, as it can provide feedstock for multiple biofuels, including biodiesel, ethanol, and methane, while having relatively low cultivation requirements. Mata et al. focus primarily on microalgae for biodiesel production, outlining what is already known about microalgae for this use, and what still needs to be elucidated in order to utilize it on a large-scale.
It is estimated that 50,000 species of microalgae exist, and these prokaryotic and eukaryotic organisms are able to thrive in a variety of harsh conditions with high growth and reproduction rates. Extensive collections of algal species have been gathered at many research institutions, the largest being 4000 strains of 1000 species at the University of Coimbra. Microalgae are relatively easy to grow, requiring little attention, able to utilize non potable water, and needing 49–132 times less land than rapeseed or soy. They reproduce rapidly, completing a growth cycle every few days, and different species are adapted to live in different conditions, making them a versatile organism for cultivation. Microalgae as a source of biodiesel are additionally advantageous in that they can be simultaneously used to remove CO2 from industrial flue gases by bio-fixation, remove contaminants from wastewater, and provide other useful compounds for various industrial and pharmaceutical purposes. The large-scale production of biodiesel from microalgae could therefore serve multiple positive functions while meeting energy demands.
While the large-scale culture of microalgae began in the 1960s in Japan, study of them as a renewable energy source has been limited by insufficient funding. An R&D program in the United States, carried out between 1978 and 1996, concluded that microalgae as a low-cost biodiesel source was technically feasible, but would require long term investigation and development to achieve. This project began to identify algal strains with particularly high lipid content for oil extraction and to screen for genetic variability between algal isolates, but the research was curtailed in 1995 by a funding cut. Microalgae research has been brought back to the forefront recently by high crude oil prices and the prevalent need to find less environmentally devastating sources of energy. Most contemporary studies focus on the genetic engineering of microalgae to optimize cultivation success and oil production, with an especial concentration on maximizing lipid content.
Current microalgae to biodiesel production processes consist of cell growth in a production unit, cell separation from the growth media, and lipid extraction. From these lipids, biodiesels can be created by similar methods to those for other biofuel feedstocks. A key step in cultivation is site selection, which must take into account carbon and light availability, and the metabolic requirements of the algal species of interest. Some species have proven difficult to cultivate at high volume, and light and temperature are the most limiting factors for successful growth. Additionally, salinity, turbulence, and contaminants must be controlled as required by each individual species. The authors list a series of studies by different researchers investigating the effects of various growth conditions, including pH, and the concentrations of CO2, iron, and nitrogen. It is emphasized that different species have relatively individualized requirements, and these must be understood if microalgae cultivation is to be maximized.
Biomass recovery, or “harvesting,” can make up to 30% of biomass production cost, but no single method has proven to be optimal. The development of such a method does, however, remain an important area of interest. Harvesting can include sedimentation, centrifugation, filtration, flocculation, and flotation. The two main criteria for harvest method selection are identified as desired product quality and acceptable moisture level. Sedimentation can be used for lower quality products, while the more costly centrifugation produces higher value products. Processing is also an expensive production cost, and it consists of dehydration and cell disruption for metabolite release. Several extraction methods, including solvents, ultrasound, and microwave are currently used or under investigation. Biodiesels are produced from the lipids by a transesterifaction reaction, in which tricglycerides are converted to esters (biodiesel) and glycerol (by-product) through a multiple step chemical reaction. Current industrial transesterification processes are carried out in a stirred reactor in batch mode, although improvements have been proposed that would enable continuous mode production, allowing for decreased reaction time.
Other considerations for microalgae cultivation include culture system type and operation mode. Open-culture systems, such as lakes, are easier and cheaper to build and operate. They are, however, more difficult to regulate and therefore could be limited in large-scale microalgae cultivation abilities. Closed-culture systems, or photo-bioreactors (PBR), while much more expensive, are more flexible, allowing for optimization of pH, temperature, evaporation levels, and CO2 loss. It has been shown that PBRs have higher volumetric productivity and cell concentrations than open-culture systems, but congruent areal productivity, and the competition between the two technologies is therefore not necessarily as important as the genetic engineering of microalgae to appropriately fit each system type. The second aforementioned cultivation consideration, operation mode, centers on batch versus continuous PBR operation. Continuous mode offer higher degree of control over conditions, and hence can produce more reliable and higher quality results. However, different bio-reaction types are better suited than others to this operation type, and this therefore needs to be understood when designing a cultivation process for any given microalgae species.
The review ends with a brief discussion of other potential microalgae applications. Flue gases from power plants account for over 7% of world CO2 emissions, and microalgae could be used to diminish these emissions through natural bio-fixation processes. Microalgae could also be used for waste water treatment, as they require many common contaminants (nitrogen and phosphorous) as nutrient sources. Additionally, many species contain chemical compounds that can be used for both industrial and health applications. These include pigments, antioxidants, vitamins, and food additives. These compounds could be extracted from the microalgae processed for biodiesel production, and could therefore potentially offer multiple simultaneous benefits. Several nutritional supplements have also been found in microalgae, including sterols and carotenoids, which could prove highly beneficial for human health. There is also promising evidence to show that microalgae culture could be a significant source of food for aquatic animal rearing. Microalgae can be utilized to culture zooplankton, the food source of farmed crustaceans and finfish. Further understanding of the nutritional value of microalgae could hence be widely applied to aquaculture.
The authors conclude that microalgae as a biodiesel source has great promise as a sustainable and environmentally friendly alternative to the current food crop and seed oil derived biodiesels. While much is already understood about the processes required for mass cultivation, much remains unknown. A significant amount of investment and research will therefore be necessary to develop this technology. Given the importance of reducing GHG emissions and supplementing fossil fuels, this capital investment into microalgae development could prove vital for human energy sustainability.