2.1.2 Introduction

Despite the ever-escalating price for petroleum products and rapidly grow­ing concerns regarding carbon dioxide emissions, the world still remains heavily dependent upon fossil fuels. The 2011 International Energy Outlook [16] predicts by its Reference Case Scenario that the total world consumption of marketed energy will increase by roughly 42% by 2035 from 2010 with an increase in liquid fuels consumption of 30% by 2035 from 2010 in the transportation sector. This increase in demand, however, cannot be met by petroleum alone.

Many commodities, such as biodiesel and bioplastics, are made from the triglycerides found in vegetable oils, and other petrochemicals can also be derived or synthesized using processing by-products including glycerin. Algae, specifically microalgae, are a promising source of oil because, com­pared to other crops, they have fast growth rates, potential for higher yield rates, and the ability to grow in a wide range of conditions [17]. The yield of oil per unit area of algae is at least seven times greater than that of palm oil, the second highest yielding crop [18]. Another benefit of using algae is that they overcome the "food versus fuel" issue of other vegetables and grains because algae is not a universal food crop, and it does not take arable land from other crops. Algae grown on 9.5 million acres, compared to the 450 mil­lion acres used for other crops, could provide enough biodiesel to replace all petroleum transportation fuels in the United States [18]. Based on the cur­rently available technologies, harvesting of algae and the extraction of oil is technologically challenging and energy intensive [17]. There are several dif­ferent methods proposed and developed for extracting oil, an important step in making biofuels as well as bioplastics. Unlike straight vegetable oils (SVOs) and conventional biodiesels based on crop oils, algae fuels are classified as second-generation biofuels.

The lipid containing oil from algae must be separated from the proteins, carbohydrates, and nucleic acids. The steps for extracting the oil involve breaking the cell wall, separating the oil from the remaining biomass, and purifying the oil [19]. Oil can be extracted from algae by either mechanical or chemical methods. The three well-known methods for extraction of oil from algae are expeller pressing (or oil pressing), subcritical solvent extraction, and supercritical extraction [17]. Other methods include enzymatic extrac­tion, osmotic shock, and ultrasonic-assisted extraction [18]. Expeller pressing and ultrasonic-assisted extraction are mechanical processes, and subcritical and supercritical extraction methods are chemical processes. Each of the methods has its advantages and drawbacks. Extraction of oil by expeller pressing is simple and straightforward, but requires the algae to be fully dried, which is energy intensive. Furthermore, the extraction efficiency is not very high, and a substantial amount of unextracted oil is left behind. The benefit of solvent extraction is that the algae do not need to be fully dried, but common subcritical solvents, such as hexane, pose environmental, health, and safety concerns [19]. In addition, although most solvent is recovered and reused, its associated cost is also burdensome. Supercritical fluid extraction may be the most efficient method as it can extract almost all the oil and pro­vide the highest purity inasmuch as supercritical fluids are selective [17]. Furthermore, extraction with supercritical CO2 eliminates the use of harmful solvents. However, its high-pressure operation and required high-pressure equipment increase the overall process cost.

Algae oil has been proposed as both a sustainable and economically fea­sible solution to alternative liquid transportation fuels.