Algae As A Source Of Biofuel Environmental Sciences Essay

Published: 2021-07-06 10:15:05
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As the price of crude oil skyrockets and the world's reserves are steadily depleted, biofuels are on the horizon as a possible solution to a looming energy crisis. Currently research into biofuel technology is spread across a broad spectrum of possible sources and refinement methods. One of the most promising biofuel sources currently being explored extracts biofuel from algae.
The idea of extracting biofuels from the single celled photosynthetic organisms known as algae has been around for a number of years. In fact NASA began researching the uses of algae as biofuel as early as the 1970s [1]. Algae have long taken a backseat to more traditional land based biofuel crops such as corn or palm. These traditional crops however require high quality agricultural land to grow which inevitably will cause problems as the demand for biofuel increases and they begin to compete with food crops [2][3]. Luckily this fact has sparked new interest in algae which does not have special land requirements [2].
Algae can produce a number of different possible biofuels depending on the process and species of algae used [5]. These various biofuels include syngas, biodiesel, ethanol, and hydrogen [5]. Figure shows a number of these biofuels and their place within a tree of production processes. As the figure shows the extraction portion of the algae to biofuel conversion process is dependent on the desired fuel type. The overall process however can be laid out as a number of steps common to the production of algae.
To begin with a specific species of algae is chosen based on a number of factors. These factors include the timing of its growth cycle, its nutrient requirements, the rate at which the cells produce lipids and carbohydrates and the efficiency of its photosynthetic process [8] [7]. The choice of algae species can greatly influence the efficiency and productivity of a biofuel operation. This is so much the case that research is currently being done on genetically engineering strains of algae optimized for biofuel applications [7].
After selection, algae is grown in a water based medium where it rapidly multiplies. Depending on the desired fuel type, the conditions in which the Algae is grown are varied to influence its metabolic processes [2]. The degree to which these conditions can be altered is depended on the facility used to grow the algae. Two main facility types are currently considered for growing algae. They are photo reactors and open pond systems [4].
Figure : Conversion process tree for various biofuels [5].
Open pond systems are simply large shallow open air ponds which are used to grow the algae. They typically take the form of long narrow "raceways" that circulate the growth medium along their length [4]. Open pond systems are far cheaper to setup than other systems, they also consume less energy than bioreactors during the growing process [4] [2]. They are however not without fault. Their open nature leaves them prone to contamination by other species of algae and water born organism that compete for nutrients [2]. They also produce less biomass than bioreactors requiring greater space to achieve the same output as the bioreactor system [4].
Bioreactors on the other hand, are closed systems where algae is grown in clear pipes, trays, or bags [2][4]. They allow the operators to control light levels, light and dark cycles, and even CO2 levels. Bioreactors are capable of growing higher densities of algae by carefully mixing the growth medium to allow even light exposure to more algae cells than could be exposed at the surface of the reactor alone [4]. Bioreactors also allow highly productive and sensitive species to grow in specialized environments. Their downside is the expense and complexity of setup, and the higher energy cost to operate functions such as temperature control and mixing [2].
Regardless of the facility type used for growing, a harvesting process is required to remove the algae from their water based growth medium and condense them into a more concentrated biomass. While the best method of condensing algae into higher density biomass is as of yet not know, there are again a number of technologies currently being explored.
Technologies for harvesting the algae from its growth medium depend somewhat on the species of algae grown. Algae with larger cell sizes can be separated from water using filtration systems [5]. Similarly species that tend to float on the surface of liquid can be skimmed from the top, this works especially well when combined with open pool systems [5]. Another common method of removing algae from the water in which it grows is centrifuges [5]. Centrifuges use centripetal force to force the more massive algae cells out of suspension in water. Centrifuges however use significantly more energy than skimming or filtration [5].
After the biomass has been condensed it must be dried before biofuel can be extracted. The main drying techniques are spray drying, drum drying and sun drying [4]. Spray drying is a common technique however the more widely used and lowest cost option for drying tends to be sun drying [4].
The final step of the biofuel production process is extraction. The algae cells within the dried biomass contain the valuable components of biofuel. To extract these chemical components the cell walls of the algae must be broken down. There are a number of ways this can be accomplished depending on the type of fuel being extracted. Figure presented earlier in this paper shows the relation of biofuels to extraction process.
One of the most common methods of extraction uses solvents to break down the cell walls of the algae [5]. This method is somewhat energy intensive as it requires processed chemicals. It is typically used for extraction of lipids for biodiesel [5]. Another well researched method for extracting biodiesel is called pyrolysis. Pyrolysis uses high temperatures to extract bio-oil and syngas from dried biomass [5]. It also produces the useful byproduct of charcoal [5].
While biodiesel is one of the most useful biofuels that can be extracted from algae ethanol is arguably just as useful to the automotive industry [5]. The process to extract ethanol from algae is very similar to the process of extracting ethanol from corn. The biomass is fermented to convert sugars to ethanol then distilled to remove impurities [5]. The waste from this process can be used to feed cattle [5].
Biomass can also be directly burnt or co-fired with coal to power stream turbines [5]. In this way biomass can be converted fairly directly into electricity. This makes algae an ideal way to generate clean electricity for the emerging class of purely electric vehicles. Such a direct conversion also makes use of existing generation facilities.
Finally it is worth mentioning that researchers are currently working on a new method of converting algae biomass to bio-oil. This method called thermochemical liquefaction. Compared to other methods the equipment required for thermochemical liquefaction is very expensive and complex [5]. The advantage is that it takes wet biomass as input eliminating the need for an energy or time intensive drying stage in the production of algae based biofuel. Currently studies into the efficiency of thermochemical liquefaction are showing promising results [5].
Though there is much research left to do on optimizing the algae to biofuel process, algae based biofuels seem to be a promising answer to some of the growing energy concerns, both in America and around the world. While other biofuel sources have already gained a footing in the market, the benefits of algae as a fuel source, such as reduced land footprint, high yield and quick turnover, may give algae production a leg up on its competition. If all goes well on the path to clean renewable energy, algae biofuel plants may become a common sight in the coming decades.
[1] Danielo, O. (2005). An algae-based fuel. Biofutur, 255, 1-4.
[2] Stuart A Scott, Matthew P Davey, John S Dennis, Irmtraud Horst, Christopher J Howe, David J Lea-Smith, Alison G Smith, Biodiesel from algae: challenges and prospects, Current Opinion in Biotechnology, Volume 21, Issue 3, June 2010, Pages 277-286, ISSN 0958-1669, 10.1016/j.copbio.2010.03.005.
[3] G Charles Dismukes, Damian Carrieri, Nicholas Bennette, Gennady M Ananyev, Matthew C Posewitz, Aquatic phototrophs: efficient alternatives to land-based crops for biofuels, Current Opinion in Biotechnology, Volume 19, Issue 3, June 2008, Pages 235-240, ISSN 0958-1669, 10.1016/j.copbio.2008.05.007.
[4] Neltner, B. (2008). Algae Based Biodiesel. Algae, 2(5).
[5] Liam Brennan, Philip Owende, Biofuels from microalgae—A review of technologies for production, processing, and extractions of biofuels and co-products, Renewable and Sustainable Energy Reviews, Volume 14, Issue 2, February 2010, Pages 557-577, ISSN 1364-0321, 10.1016/j.rser.2009.10.009.
[6] Life-Cycle Assessment of Biodiesel Production from Microalgae
Laurent Lardon, Arnaud Hélias, Bruno Sialve, Jean-Philippe Steyer, and Olivier Bernard
Environmental Science & Technology200943 (17), 6475-6481
[7] Laura L Beer, Eric S Boyd, John W Peters, Matthew C Posewitz, Engineering algae for biohydrogen and biofuel production, Current Opinion in Biotechnology, Volume 20, Issue 3, June 2009, Pages 264-271, ISSN 0958-1669, 10.1016/j.copbio.2009.06.002.
[8] Ayhan Demirbas, M. Fatih Demirbas, Importance of algae oil as a source of biodiesel, Energy Conversion and Management, Volume 52, Issue 1, January 2011, Pages 163-170, ISSN 0196-8904, 10.1016/j.enconman.2010.06.055.

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