**1. Introduction**

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140 Biofuels - Status and Perspective

The depletion of fossil fuels which meet most of our energy requirements in near future and the pollutants from fossil fuels necessitates the usage of alternative renewable energy sources extensively. In this context, biomass is considered as an important alternative energy source to fossil fuels. Biodiesel and bioethanol produced from biomass sources are one of the best alternatives for petroleum-based fuels and recently, they are commonly used for transporta‐ tion in many countries. Bioethanol is the most produced biofuel in the world and especially in Brazil and the United States two main producing countries with 62% of the world produc‐ tion. Large scale manufacture of ethanol as fuel is performed from sugar cane in Brazil, while it is produced from corn as a raw material in the United States [1]. Bioethanol production of 2013 in the countries is given in Table 1 [2].


**Table 1.** Bioethanol production amounts of countries in 2013 (million liter) [2]

Bioethanol is basically produced from first or second generation feedstocks. First generation bioethanol is produced from some cereals and lugumes such as corn, sugar beet, wheat and barley used for also food sources. Sugars which are obtained from first generation feedstock

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such as sugar cane, molasses, sugar beet and fruits can be fermented via yeast directly. Advantages of these raw materials are high sugar yields and low conversion cost. Their disadvantage is their production in just certain periods of the year. While 25 gallons of ethanol produced from an average of 1 ton sugar beet, 20 gallons of ethanol is produced from 1 ton of sweet sorghum stalk yearly. However their production is more expensive than that produced from sugar cane due to its energy and chemical inputs [3].

Usage of this first generation feedstock for bioethanol production leads to various discussions about increasing food prices and occupation of agricultural land. These problems are solved partially by using second generation feedstocks lignocellulosic materials such as waste or forest residues. Second generation feedstocks have some advantages over first generation feedstocks due to not being used as food source and less land requirement. However their harvesting, purification and various pre-treatment needs made their production quite challenging and not economical. Algae which are the third generation feedstock for biofuels are an alternative for the first and second generation feedstocks due to their productivity, easily cultivation and convenient harvesting time [4-6]. Recently, they are mostly utilized for biodiesel production because of their high lipid content. On the other hand, they have cellulosic structure and large amounts of carbohydrate embedded in, so they can be also utilized for bioethanol production directly or with the remains which is obtained after oil extraction. Since bioethanol production from conventional feedstock is considered for emitting more green‐ house gases than fossil fuels in consequence of the production steps and applications during the process, algal bioethanol production can overcome these problems. In comparison with conventional feedstocks, algal production areas don't occupy agricultural lands and they needn't any fertilizer for cultivation. With these advantages and significant carbohydrate content, higher ethanol yields are obtained from algae. In table 2, ethanol yield values from different feedstocks including first and second generations are given [7].


**Table 2.** Ethanol yield values from different feedstocks [7]

Although it depends on the raw material which is used, ethanol production have three main steps: to obtain fermentable sugars, conversion of sugars to ethanol via fermentation process and distillation and purification of produced ethanol. In this chapter, these steps are presented in detail with their alternatives. All literature studies on the subject are reviewed, discussed and also new approach to pre-treatment methods of raw materials to produce bioethanol is presented.
