**2. Algae as bioethanol feedstock**

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

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

from sugar cane due to its energy and chemical inputs [3].

142 Biofuels - Status and Perspective

different feedstocks including first and second generations are given [7].

**Corn stover** 112–150 1,050–1,400 **Wheat** 277 2,590 **Cassava** 354 3,310 **Sweet sorghum** 326–435 3,050–4,070 **Corn** 370–430 3,460–4,020 **Sugar beet** 536–714 5,010–6,680 **Switch grass** 1,150 10,760 **Algae** 5,000–15,000 46,760–140,290

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

**(gal/acre) Ethanol yield (L/ha)**

**Feedstock Ethanol yield**

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

presented.

Algae are simple organisms containing chlorophyll and they use light for photosynthesis. Algae can grow phototrophically or heterotrophically. Phototrophic algae convert carbondi‐ oxide in atmosphere to nutrients such as carbohydrate. Conversely, heterotrophic algae continue their development by utilizing organic carbon sources [8]. Algae can grow in every season and everywhere such as salty waters, fresh waters, lakes, deserts and marginal fields etc. However for their cultivation, generally open systems like ponds and photobioreactors as closed systems are used. Open ponds are the most used cultivation systems in industry. They are more preferable than other systems due to having low investment and operation costs. On the other hand difficult control of cultivation conditions and contamination risk are the main disadvantages of the open systems. Besides being cheap and low energy need, their cleaning also can be done easily. Although, open tanks have low cost and easy operation, parameters like light intensity, temperature, pH and dissolved oxygen concentration cannot be controlled easily. Most produced algae species in open systems are *Spirulina, Chlorella* and *Dunaliella* [9].In comparison with open systems, photobioreactors have very high photosynthetic efficiency. Thus, photobioreactors ensures high biomass yield. Though they are expensive, they are preferred for specific algal production. Algal production which is controlled in terms of parameters like light, pH, carbon dioxide etc., can be achieved and also contamination risk is not seen mostly in photobioreactors. Since they are closed systems, evaporation doesn't occurred and they enable production of special biochemical materials. Although there are many types of photobioreactors, most commonly systems are vertical and horizontal tubular columns and flat-type photobioreactors [10]. These photobioreactors which are made of glass or plastic, can be designed in shapes of horizontal vertical, conical or curved etc [11,12].

Algae are classified as microalgae and macroalgae. Microalgae as their name implies, are prokaryotic or eukaryotic photosynthetic microorganisms. They can survive in hard condi‐ tions with their unicellular or simple colony structures [13]. Because of being photosynthetic organism, they can produce high amount of lipid, protein and carbohydrate in a short time. Besides biodiesel and bioethanol there are lots of high value products and sub-products produced from microalgae such as biogas [14, 15], biobutanol, acetone [16], Omega 3 oil [17], eicosapentaenoic acid [18], livestock feed [19], pharmaceuticals and cosmetics [20, 21]. Especially sub-products are preferred for economic support of main process [22]. Chemical composition of microalgae can change according to the cultivation type and cultivation conditions. They can have rich or balanced composition of protein, lipid and carbohydrate amounts. Microalgae especially get attention due to have high lipid content [23]. Many species of microalgae accumulate a significant amount of lipids in their structure and can provide high oil yield. Their average lipid content can change between 1-70%, but this ratio can reach up to 90% of dry weight under certain conditions [13]. Macroalgae or seaweed are plants which are adapted to the marine life, often located in coastal areas. They are classified as brown seaweeds, red seaweeds and green seaweeds according to their pigments [24]. Due to have high photo‐ synthesis capability, they have sufficient carbon source for usage in biorefinery. On the contrary of their appearances, their features of morphologic and physiological and chemical compositions are different from terrestrial plants [24]. Unlike the structure of the lignocellu‐ losic biomass of microalgae, they comprise substances such as carrageenan, laminaran, mannitol, alginate which are used in various sectors [25]. They are separated from microalgae with having low lipid content and different from lignocellulosic material with having less or no lignin in their structure [6].

Microalgae stand out as biodiesel feedstock with the ability of lipid production and high photosynthetic efficiency. As for macroalgae, they are utilized for biogas or bioethanol production with their carbohydrates [26]. First studies as algal biofuels are focused on biodiesel production. However, there is a potential for carbohydrates in the structure of algae which can be utilized for ethanol production after various hydrolysis processes. Algal cells in the water don't need structural biopolymers such as hemicellulose and lignin which are necessary for terrestrial plants [4]. This simplifies the process of bioethanol production. Marine algae can produce high amount of carbohydrate every year. Also it is expected that algae will meet the demand of biofuel feedstock due to harvest in a short time than other biofuel raw materials [27]. Microalgae which have high amount of starch such as *Chlorella, Dunaliella, Chlamydomonas, Scenedesmus* are very useful for bioethanol production. In addition to that, microalgae don't need energy consumption for distribution and transportation of molecules like starch. Like microalgae, macroalgae are also raw materials that can be used in ethanol production. Absence of lignin or having less lignin in the structure, simplifies the hydrolysis stages [4,28]. Although it changes with the algal species, they have various amounts of heteropolysaccharides in their structures. Whereas red algae contain carrageenan and agar, brown algae have laminaran and mannitol in their structure [6].
