Biodiesel production by microalgal biotechnology
Introduction
The production of biodiesel has recently received much attention worldwide. Because of the world energy crisis [1], many countries have started to take a series of measures to resolve this problem [2]. Finding alternative energy resources is a pressing mission for many countries, especially for those countries lacking conventional fuel resources. In the 1930s and 1940s, vegetable oils has been used as diesel fuels in the emergency situation. With the rapid development of the modern industry, the demand for energy has been greatly increased in recent years, and therefore alternative energy sources are being explored. Thus, the term “biodiesel” has appeared very frequently in many recent reports [3].
The world total biodiesel production was estimated to be around 1.8 billion liters in 2003 [4]. Although there was no increase in biodiesel production between 1996 and 1998, a sharp increase in biodiesel production was observed in the past several years. It is speculated that the production of biodiesel will be further tremendously increased because of increasing demand for fuels and “cleaner” energy globally.
Biodiesel is made from biomass oils, mostly from vegetable oils. Biodiesel appears to be an attractive energy resource for several reasons. First, biodiesel is a renewable resource of energy that could be sustainably supplied. It is understood that the petroleum reserves are to be depleted in less than 50 years at the present rate of consumption [5]. Second, biodiesel appears to have several favorable environmental properties resulting in no net increased release of carbon dioxide and very low sulfur content [6], [7]. The release of sulfur content and carbon monoxide would be cut down by 30% and 10%, respectively, by using biodiesel as energy source. Using biodiesel as energy source, the gas generated during combustion could be reduced, and the decrease in carbon monoxide is owing to the relatively high oxygen content in biodiesel. Moreover, biodiesel contains no aromatic compounds and other chemical substances which are harmful to the environment. Recent investigation has indicated that the use of biodiesel can decrease 90% of air toxicity and 95% of cancers compared to common diesel source [64]. Third, biodiesel appears to have significant economic potential because as a non-renewable fuel that fossil fuel prices will increase inescapability further in the future [8]. Finally, biodiesel is better than diesel fuel in terms of flash point and biodegradability [9].
Conventional biodiesel mainly comes from soybean and vegetable oils [10], palm oil [11], sunflower oil [6], rapeseed oil [12] as well as restaurant waste oil [13]. The number of carbon in the carbon chain of the diesel oil molecular is about 15, which is similar to that of the plant oil with 14–18 carbons. The structural characteristic of biodiesel determines that biodiesel is a feasible substitute for conventional energy. Nevertheless, the production cost is generally high for biodiesel. The price of biodiesel is approximately twofold that of the conventional diesel at present. The production cost of biodiesel consists of two main components, namely, the cost of raw materials (fats and oil) and the cost of processing. The cost of raw materials accounts for 60% to 75% of the total cost of the biodiesel fuel [14]. Though there might be large amounts of low-cost oil and fats available such as restaurant waste and animal fats [15], the major problem of using these low-cost oils and fats is that they often contain large amounts of free fatty acids (FFA) which is difficult to convert to biodiesel through transesterification [16]. Raw materials that contain large proportions of fatty acid triglycerides are preferred. For example, plant oil is found to contain more fatty acid triglycerides and therefore has been used in the production of biodiesel for some years [17].
Earlier studies on liquid fuel from microalgae had begun in mid-1980s. During the world war II, although some German scientists attempted to extract lipids from diatom in order to resolve energy crisis [18], and soon later in the USA, research was conducted by a group of scientists at the Carnegie Institution of Washington, and their experiences had been summarized in a book [65] entitled “Algal Culture from Laboratory to Pilot Plant”, but the technologies of making microalgae as fuels had not been fully exploited. The reasons could be as follows. First, as a source of lipids, microalgae are less known than plants and animals. Second, the prices for most plant oils are relatively low and animal fats are even cheaper; therefore, processes for the microbial oils production have mainly focused on high-valued products that cannot be produced by plants, such as omega-3 polyunsaturated fatty acids, especially EPA and DHA [19].
In order to resolve the worldwide energy shortage crisis, seeking for lipid-rich biological materials to produce biodiesel effectively has attracted much renewed interest. Oleaginous microorganisms are favorably considered for their short growth cycles, high lipid contents and ease of being modified by biotechnological means (see Table 1). Some microalgae appear to be suitable group of oleaginous microorganism for lipids production [20]. Microalgae have been suggested as potential candidates for fuel production because of a number of advantages including higher photosynthetic efficiency, higher biomass production and higher growth rate compared to other energy crops [21], [22], [23]. Moreover, according to biodiesel standard published by the American Society for Testing Materials (ASTM), biodiesel from microalgal oil is similar in properties to the standard biodiesel, and is also more stable according to their flash point values (Table 2).
Section snippets
Microalgal biotechnology for lipids production
Microalgae have high potentials in biodiesel production compared to other oil crops. First, the cultivation of microalgae dose not need much land as compared to that of terraneous plants [20]. Biodiesel produced from microalgae will not compromise the production of food and other products derived from crops. Second, microalgae grow extremely rapidly and many algal species are rich in oils. For instance, heterotrophic growth of Chlorella protothecoides can accumulate lipids as high as 55% of the
Pyrolysis technologies
Although oils extracted from microalgal cells have been investigated for fuel production of internal-combustion engines by transesterification of fatty acids [48], industrial biodiesel production from microalgal oils is still not well developed. At least, high content of microalgal oils is required for this method to realize economic benefits. Since it is difficult to obtain microalgae with high fatty acid content conventionally, it has been considered that only crude fat (lipid) is used for
Transesterification technologies in the production of biodiesel
The viscosities of vegetable oils and microalgal oils are usually higher than that of diesel oils [55]. Hence, they cannot be applied to engines directly. The transesterification of microalgal oils will greatly reduce the original viscosity and increase the fluidity. Few reports on the production of biodiesel from microalgal oils are available [20]. Nevertheless the technologies of the biodiesel production for vegetable oils can be applied to the biodiesel production of microalgal oils because
Conclusion
Since oil crisis in the mid 1970s, finding new energy resources to replace petroleum has been a hot topic worldwide. Because of the many advantages over the conventional energy resources, the production of biodiesel has attracted much attention in recent years. There have been numerous publications on the production of biodiesel made from vegetable oils and other oil-plants. At present, the high cost of oleaginous materials is the main problem hindering commercial production of biodiesel.
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