Algal biofuels: Challenges and opportunities
Highlights
► Algae are promising for biofuels production. ► Higher productivity and lipid content than plants. ► Open ponds are better than PBRs for biofuels. ► Technical hurdles include harvesting and oil extraction.
Introduction
The transportation sector plays a major role in the production of greenhouse gas (GHG) emissions, as well as being responsible for 28% of total world primary energy consumption, mainly consisting of fossil fuels, and for 71% of the total crude oil used (Energy, 2004, Pienkos and Darzins, 2009). Transportation fuels can be divided into three groups related to use: private vehicles (gasoline); commercial vehicles and stationary engines (diesel); or jet fuels (kerosene). World consumption of diesel was nearly 1460 trillion liters in 2011 (OPEC). Fuel demand in the transportation sector is projected to increase by 40% over the period 2010–2040 (ExxonMobil, 2013). Most of this demand is driven by the commercial sector with heavy duty vehicle (diesel) fuel use increasing by 65%. Although the number of light-duty vehicles (cars) could double, the increased fuel demand might be largely offset by increased fuel efficiency and the switch to hybrid technologies (ExxonMobil, 2013).
Any plan to lower GHG emissions will require the substitution of at least part of the petroleum-based fuels used for transportation. Today we “borrow land from the past” (Wackernagel and Yount, 1998), by using carbon which was fixed in another era. Even at present prices, crude oil is cheap, easily extracted and easy to use since it just needs to be taken from its natural reservoir and distilled into products. However, its use reintroduces into the atmosphere carbon trapped millions of years ago. In addition to the role of fossil fuel combustion in climate change due to the increased concentration of CO2 in the atmosphere, a well established mathematical model used to calculate crude oil field reserves and production capabilities predicts peak oil within the next few decades (Nashawi et al., 2009).
After a hundred years of intensive use, humanity has become strongly dependent on fossil fuels, we are addicted to oil. The world’s economy relies on the very efficient system of production, distribution and use that has been developed. Any transition to a new fuel will have to be “painless”, using the technology and infrastructure of the existing system as much as possible. The first generation of biofuels fit this model as bioethanol and biodiesel require minimal or no adjustment of regular internal combustion engines, and can generally be distributed, stored and pumped like conventional crude oil-derived fuels. The major drawback to the use of these alternative fuels is that arable land is used to farm the corn, sugar cane or oil seed crops needed to produce these fuels. In addition, it would be impossible to produce the quantity of biofuels that would be necessary to meet present fuel demands using first generation technology. In 2010, the US consumed nearly 220 trillion liters of diesel (Energy Information Administration, 2012). To produce this volume of fuel using soybeans for example (average yield of 600 liter per hectare), would require 367 million hectares, in contrast with the only 178 million hectares that is currently available for cropland and the 930 million hectares of total US land area (EIA, 2012). In addition, the commodities used for first generation biofuels production have other possible markets as sugar, animal feed or cooking oil. A farmer will negotiate the selling price of his product in order to profit as much as possible, enhancing even more the competition between food and fuel and creating a complex fluctuation of food prices linked to fuel demand. With actual world production of biofuels at 109 trillion liters per year (86.6 trillion liters bioethanol, 24.4 trillion liters of biodiesel) (EIA, 2012), there has been a great deal of speculation as to whether or not this is already happening. Thus it is clear that although production of first generation biofuels was an important step, it is however only a palliative solution and is untenable in the long term.
Section snippets
Microalgae
The call for advanced biofuels demands “drop in” fuels able to be used with the existing infrastructure for storage and distribution, from manufacture to the final customer, but with a production system able to be scaled up without competing with food crops for land. Microalgal biodiesel has been proposed as the most obvious choice. Microalgae are oxygen producing microorganisms containing chlorophyll “a”, mostly autotrophs, using atmospheric CO2 as primary carbon source whereas some can grow
Algal biofuels
Any organism dependent on sunlight as its primary energy source needs to store energy-rich compounds to avoid starvation when light is not available. Vascular plants synthesize a variety of energy rich molecules to save enough energy from the sunlight period for a rainy day (or night). A Canadian example would be the maple tree and the phloem with its high sugar content (Maple Syrup). Vascular plants often produce oil as a carbon reserve for germination. To increase embryo viability, some
Cultivation
Two basic alternatives for microalgae cultivation exist and their relative merits are the basis of ongoing debate. Some of the factors involved are listed in Table 2.
Harvesting
In a general sense, the production of microalgal biodiesel is very similar to the production of first generation biodiesel. The biomass is produced, harvested; lipids are extracted and then processed through transesterification into FAMES (Fatty Acid Methyl Ester), commonly called biodiesel. However, unlike oil seed plants, harvesting microalgal cells can prove to be quite challenging. The tiny cells floating in water cannot be accessed as easily as macroscopic plants, and consequently oil
Biotechnology of microalgal biofuels
Throughout this review various issues that apply to the biotechnology of microalgal biofuels have been discussed. Here we specifically highlight some specific biotechnology issues that are important in the development of large scale algal biofuels production.
Conclusion
The production of biofuels using microalgae is promising since of all photosynthetic organisms they have the highest growth rates, and they can be cultivated using non-arable land with wastewater as a source of nutrients. However, much research is still needed before the practical production of biofuels from microalgae can become a reality due to uncertainties as to cultivation strategies, the lack of effective low cost harvesting methodologies, and the need for an oil extraction and biodiesel
Acknowledgements
Algal research in the laboratory of PCH is supported by FQRNT (Fonds de recherche du Québec – Nature et technologies).
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