الفهرس 
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Abstract
The rapid economic growth that took place in the second half of the 20th century caused a re-orientation in the manner of utilization of energy raw materials. A new model of the world economy has developed mainly on the basis of petroleum and natural gas, with a declining importance of hard coal (Ryan et al., 2006; Mata et al., 2010). However, it appeared that the resources of those raw materials deplete fast and their use causes a number of unfavorable effects, such as acid rains or global warming with the resultant climate changes (Demirbas, 2007; Somerville, 2007). Apart from this, transport and the energy producing industry are the primary anthropogenic sources of greenhouse gas emissions in the European union that are responsible for more than 20 and 60%, respectively, of that emission (Mata et al., 2010).
It was reported that the present petroleum consumption is 105 times faster than the nature can create (Satyanarayana et al., 2011) and at this rate of consumption, the world’s fossil fuel reserves will be diminished by 2050 (Demirbas, 2009). And unfortunately, the fuel consumption is expected to rise by 60% or so in the next 25 years (Rittmann, 2008). Aiming to reduce dependency on the fossil fuel sources and imports from oil-rich countries and maintain environmental sustainability, many countries have committed to renewable energy production increases and/or greenhouse gas emission reductions at national and international levels (Fabbri et al., 2007), enhance diversity in energy supply markets, contribute to securing long term sustainable energy supplies, and provide commercially attractive options to meet specific energy service needs, particularly in developing countries and rural areas helping to create new employment opportunities there (Herzog et al., 2001).
Among many renewable energy sources solar thermal and photovoltaic collectors are still not mature and are cost-prohibitive. For instance, energy conversion efficiency of the photovoltaic modules available in the market is at the maximum of 15%. Wind and geothermal sources have limitations such as location, availability, and intensity. Since most of the transportation and industrial sectors need liquid fuels to drive the machinery and engines, more emphasis is needed on alternative fuel sources such as biofuel (Sissine, 2007).
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Biofuels are fuels obtained from biomass (organic matter such as plants, animals and microorganisms) (Frac et al., 2010), and are classified as solid, liquid and gaseous. While solid biofuels include such materials as straw (in the form of bales, pellets or briquettes), specific tree species, such as basket willow, Sida hermaphrodita, but also granulated sawdust or straw (pellets), hay or other plant species, Liquid biofuels are obtained mainly through alcohol fermentation of carbohydrates to ethanol, butyl fermentation of biomass to butyl alcohol or from vegetable oils (rapeseed oil) esterified to biodiesel and Gaseous biofuels (biogas) are formed through anaerobic fermentation of liquid and solid wastes from agricultural animal production, such as liquid manure or farmyard manure. They can also be produced in the process of biomass gasification (wood gasification), from which generator gas (so-called wood distillation gas) is obtained (Demirbas, 2007; Demirbas 2009).
Biofuels can also be classified into 1st, 2nd and 3rd generation biofuels. First generation biofuels are those produced from organic matter that can be used for the production of food or fodder. That organic matter includes primarily starch, sugars, animal fats and vegetable oils. The sources of those materials are potatoes, cereal grain, rapeseed, soybean, maize, or sugar cane. The use of these raw materials, that can also be used to produce human food or animal feed, shows that if too much fuel is produced from such materials the food prices may rise drastically, which may be a challenge for some countries (Somerville, 2007; Brennan and Owende, 2010). At present, they are still not very popular due to the high costs of production, but research in this area has permitted a notable reduction of the costs involved. It is assumed that in the future, such fuels will make first generation biofuels obsolete.
Second generation biofuels may contribute to an alleviation of the problem partially to satisfy the requirements for fuels in a sustainable, inexpensive and environment-friendly manner. The advantage of second generation biofuels is the possibility of using the whole plant (including the stem, leaves and husks) and not just a part of it (grain) as in the case with raw material for first generation biofuels. Second generation biofuels can also be produced from plants of which no part is edible, such as Jatropha curcas, cereals with very low yield of grain, wastes from the wood processing industry and fruit skins or pulp from fruit processing. Such plants can grow in marginal areas and use salt water for their growth, which is an obvious advantage (McKendry, 2002; Schenk et al., 2008).
Third generation biofuels are primarily fuel cells, using hydrogen as the primary source of energy. At present, algae are the main raw materials from which such biofuels can be produced at high
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efficiency levels and at low investment. Algae are a material that is cost- effective and provides a relatively high yield of biofuel. Undoubted advantage is the fact that, they are not a burden on the environment. The culture of such algae as Botryococcus braunii and Chlorella.