**1. Introduction**

The choice for the most suitable energy carrier to be produced from algae is a promising option. Algae have been explored for their unique potential to yield a variety of biofuels concomitantly with generation of value-added products and phycoremediation of wastewater (**Figure 1**). Many algal strains like *Chlamydomonas, Chlorella*, *Scenedesmus*, *Botryococcus braunii*, and so on have been reported to produce biofuels (**Table 1**). The selection of algal strains is exclusively dependent on various factors like oil content, production yield and downstream processing and also on adaptability of microalgae toward high oxygen concentration, temperature variations and water chemistry [1].

The algal metabolism consisting of photosynthetic potential, which makes it unique in comparison to other microorganisms when it comes to processing sugars from cellulosic sources

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

**Figure 1.** Potentials of microalgae.

such as grass and wood chips. After algal biomass degradation into sugar, there are substances like lignin associated with it, which are toxic to microorganisms. Removal of lignin is, thus, essential to promote further microbial growth leading to processing of sugar. Algae are tolerant to the presence of lignin, which makes the processing convenient coupled with reduction in the economic cost. In addition to this, there are other applications of algae like aquaculture, high-value products and nutraceuticals, which can be extracted from algae [2]. The microalgae require minimal inputs for metabolic processes—namely sunlight, CO<sup>2</sup> and water, with few required mineral nutrients. Sunlight is the most readily available and inexpensive source of energy on earth. The efficiency of microalgae in converting captured solar energy into biomass exceeds the potential of terrestrial plants. Microalgae do not compete with terrestrial plants for land or water supply as they can be grown in wastewater, leading to their remediation coupled with biomass production. The acumen of microalgae to inhabit diverse habitats could be exploited to allow for the production of compounds near the site of use, which could reduce the transportation costs [3].

#### **1.1. Advantages of microalgae as source of biofuels**

Microalgae are one of the most promising candidates for plethora of biofuels owing to their easy, inexpensive and simple cultivation system. They grow easily with basic nutritional requirements like air, water and mineral salts with light as the only energy source. They grow on liquid media, so diverse wastewater can also be utilized, which can be efficiently remediated by algae coupled with biofuel production.

The optimal use of light energy through photosynthesis is very efficiently executed by microalgae. They possess higher photosynthetic levels and growth rates and can be used for the production of desired biofuels. They can contain considerable amounts of lipids that are mainly

**Table 1.** Biofuel yields from microalgae [27].

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**Table 1.** Biofuel yields from microalgae [27].

such as grass and wood chips. After algal biomass degradation into sugar, there are substances like lignin associated with it, which are toxic to microorganisms. Removal of lignin is, thus, essential to promote further microbial growth leading to processing of sugar. Algae are tolerant to the presence of lignin, which makes the processing convenient coupled with reduction in the economic cost. In addition to this, there are other applications of algae like aquaculture, high-value products and nutraceuticals, which can be extracted from algae [2]. The microalgae require minimal inputs for metabolic processes—namely sunlight, CO<sup>2</sup>

water, with few required mineral nutrients. Sunlight is the most readily available and inexpensive source of energy on earth. The efficiency of microalgae in converting captured solar energy into biomass exceeds the potential of terrestrial plants. Microalgae do not compete with terrestrial plants for land or water supply as they can be grown in wastewater, leading to their remediation coupled with biomass production. The acumen of microalgae to inhabit diverse habitats could be exploited to allow for the production of compounds near the site of

Microalgae are one of the most promising candidates for plethora of biofuels owing to their easy, inexpensive and simple cultivation system. They grow easily with basic nutritional requirements like air, water and mineral salts with light as the only energy source. They grow on liquid media, so diverse wastewater can also be utilized, which can be efficiently remedi-

The optimal use of light energy through photosynthesis is very efficiently executed by microalgae. They possess higher photosynthetic levels and growth rates and can be used for the production of desired biofuels. They can contain considerable amounts of lipids that are mainly

use, which could reduce the transportation costs [3].

**Figure 1.** Potentials of microalgae.

240 Advances in Biofuels and Bioenergy

**1.1. Advantages of microalgae as source of biofuels**

ated by algae coupled with biofuel production.

and

present in the thylakoid membranes. Their biofuels are nontoxic and highly biodegradable. They are essentially free-living chloroplasts and are the pinnacle of minimizing structural component. They have high carbon dioxide sequestering efficacy thereby, reducing GHG emissions.

The algal biodiesel production processes fatty acid methyl esters (FAME). The chemical composition of biodiesel is generally produced by transesterification of algal oil in the presence of acid or alkali as a catalyst [5]. The biodiesel from algae can be derived directly from transesterification of algal biomass [9]. Alternately, it can also be produced by two-step process wherein the lipids are initially extracted and later on transesterified, though either of the processes involves lipid extraction through solvents and alcohols like methanol, isopropanol and petroleum ether [8, 10]. The process of direct transesterification is fast and cost-effective technology. Biodiesel generated from microalgae can be an excellent alternative to current diesel crisis, but in order to efficiently produce biodiesel from microalgae, strains with a high

The anaerobic digestion of organic matter leads to formation of fuel called biogas or biometh-

Microalgae has been reported to produce biogas as source of fuel, although the yield of biogas formation is quite low because of the sensitivity of algal cells to bacterial degradation and low carbon and nitrogen (C:N) ratio, which leads to the formation of inhibitor (ammonia). In *Scenedesmus* spp*.,* residual biomass free from lipids and amino acids was investigated for biogas production, and results exhibited that residual biomass gives better biogas yield compared

The microalga species are capable of producing hydrocarbons, which can further be converted to diesel, kerosene and gasoline. The microalga, *Botryococcus braunii,* has been reported to produce hydrocarbons with excellent oil yield [14]. The habitat of *B. braunii* is freshwater, which can be one of the factors leading to its adaptation to varied salt concentration. In addition to this, the hydrocarbons from *B. braunii* get deposited outside the cell, thus the extraction

Microalgae can directly produce hydrogen from sunlight and water, only in the complete absence of oxygen. Hydrogen is a promising future energy source because it does not emit greenhouse gases and releases water as a by-product [18]. There are limitations existing regarding the large-scale production of hydrogen as fuel. At present, hydrogen is produced by stream reformation, photofermentation [19] and photolysis of water mediated by photosynthetic algae

• Biopolymer hydrolysis to monosaccharaides mediated by hydrolytic bacteria.

(25–45%). There are four stages

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growth rate and oil content have to be selected [11].

ane. Biogas is mainly formed by methane (55–75%) and CO<sup>2</sup>

• Conversion of monosaccharaides to acids via fermentation.

becomes relatively easier and convenient [15–17].

• Action of acetogenic bacteria leading to the formation of acetate. • Methane and carbon dioxide formation by methanogenic bacteria.

of anaerobic digestion [12], which are described as follows:

*1.2.2. Biogas*

to raw biomass [13].

*1.2.3. Hydrocarbons*

*1.2.4. Hydrogen*

They reduce nutrient load in wastewater as they can utilize nitrogen and phosphorous present in agricultural, industrial and municipal wastewater owing to their phycoremediation acumen. They can be cultivated in areas like seashore, desert, and so on, which is not suitable for agricultural plants and not competing with cultivable land. Their cultivation is independent of seasons as they can be cultivated round the year and have minimal environmental impact. The cultures can be facilitated to produce high yields through technological interventions of genetic engineering, synthetic biology, metabolic engineering, and so on as algal systems are readily adaptable.

The biofuels from algae are diverse in nature. Carbohydrate component of biomass is used for bioethanol production, while algal oil for biodiesel and the residual biomass can be utilized for methane, fuel gas or fuel oil production. The biomass after biofuel production can further be used as source of many value-added products like eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), nutraceuticals, protein supplements, therapeutics, biocontrol agents, fertilizers, animal feed and aquaculture.
