*1.2.2. Biogas*

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. 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

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,

A plethora of biofuels are derived from microalgae by virtue of their unique potential (**Figure 2**). The biofuels include alcohols, which are produced through fermentation, processing of algal biomass through dual approach of hydrolysis and fermentation, traditional

Biodiesel has comparable engine performance to petroleum diesel fuel, while reducing sulfur and particulate matter emissions [5, 6]. Biodiesel is a biodegradable alternative fuel derived from renewable sources and is nontoxic in nature [7]. During the manufacturing process, triacylglycerols (TAGs) are transesterified with an acid or alkali catalyst to produce biodiesel

method of transesterification, gasification of biomass or Fischer-Tropsch synthesis [4].

systems are readily adaptable.

242 Advances in Biofuels and Bioenergy

fertilizers, animal feed and aquaculture.

**1.2. Biofuels derived from microalgae**

*1.2.1. Biodiesel*

and glycerol [8].

**Figure 2.** Biofuels derived from microalgae.

The anaerobic digestion of organic matter leads to formation of fuel called biogas or biomethane. Biogas is mainly formed by methane (55–75%) and CO<sup>2</sup> (25–45%). There are four stages of anaerobic digestion [12], which are described as follows:


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 to raw biomass [13].
