Biohydrogen From Algae
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Biohydrogen is the most potent fuel that can substitute conventional fuels in the intermediate and extended terms (Reddy et al. 2014a; Reddy et al. 2014b; Nanda et al. 2017c; Reddy et al. 2020). Hydrogen is probably the cleanest fuel, energy vector and energy carrier because its combustion releases a significant amount of heat and w'ater. The major limitations associated with the efficient usage of hydrogen lies in the fact that its production cost is high along with the difficulties associated with its safe storage and transportation (Khetkorn et al. 2017).
The main sources for hydrogen are fossil fuels, natural gas, organic biomass and water (Sarangi and Nanda 2020). Catalytic reforming, electrolysis, photolysis, thermochemical decomposition (e.g. gasification and pyrolysis), photoelectrochemical and biological systems (e.g. dark fermentation, photo-fermentation and microbial electrolytic cell) are the main routes for hydrogen production (Holladay et al. 2009; Nanda et al. 2017e; Singh et al. 2018; Shafiqah et al. 2020). Biohydrogen can be produced biologically involving certain microorganisms’ photosynthetic biomachinery or by nonphotosynthetic processes (Khetkorn et al. 2017). Metabolic reactions of microalgal cells also generate hydrogen. Water biophotolysis (direct or indirect) during photosynthesis generates hydrogen by green microalgae (Nanda et al. 2017c).
The photosystems I and II in algae capture sunlight during oxygenic photosynthesis, thereby mediating direct biophotolysis. During direct biophotolysis, breaking down of water molecules takes place, thus producing hydrogen with subsequent release of oxygen. The carbohydrate (starch) produced during the dark reaction generates hydrogen through the indirect biophotolysis process. The carbohydrate is produced biologically in the existence of water and adsorbed C02. Hence, H2 and C02 are generated by the breakdown of carbohydrates. Due to high sensitivity to oxygen, hydrogenase enzyme works under the anaerobic condition to produce hydrogen, whereas oxygenic photosynthesis generates oxygen (Khetkorn et al. 2017). The conditions considered favorable for algae to undergo photoproduction of hydrogen are when the freshwater algae are deprived of sulfur and phosphorus and when seawater algae are deprived of phosphorus (Sengmee et al. 2017).
Bioethanol and Biobutanol From Algae
Bioethanol and biobutanol are the products of fermentation of sugars obtained from lignocellulosic materials and organic wastes (Nanda et al. 2014a; Nanda et al. 2017b). The production of bioethanol and biobutanol is treated as a green technology due to its energy efficiency and ecologically benign nature for its renewable precursors. The combustion of alcohol-based biofuels is also cleaner owing to negligible emissions of CO, hydrocarbons and particulate matter (Balat et al. 2008). While the production of bioethanol is mediated by Saccharomyces cerevisiae, biobutanol production is performed through Clostridium-iaciWOA&A acetone-butanol-ethanol (ABE) fermentation (Nanda et al. 2017a; Nanda et al. 2020). Both bioethanol and biobutanol can be blended with gasoline in flexible ratios for use in vehicles.
Microalgae cell composition is affluent in lipids along with proteins, whereas its carbohydrate amount is relatively low (Hernandez et al. 2015). Hence, the production of biodiesel from algae is extensively studied when compared to that of bioethanol and biobutanol. However, there are some notable studies reported on bioethanol and biobutanol production from the hydrolysis, saccharification and fermentation of algal biomass using several bacterial and fungal species (Hirano et al. 1997; Ueno et al. 1998; Kim et al. 2011; Ellis et al. 2012). For bioethanol and biobutanol production from microalgae, it is vital to explore the strain that possesses a significant amount of polysaccharides in its cell wall along with the ability to accumulate starch (Dragone et al. 2011).
Thermochemical and Hydrothermal Conversion of Algal Biomass
The residual algal biomass after the lipid extraction can also either be saccharified to recover fermentable monosaccharides for the production of alcohol-based biofuels, biogas and biohydrogen or can be used for thermochemical conversion (e.g. gasification. pyrolysis, liquefaction and carbonization). The advancement of third-generation biofuels depends on the production of microalgal biomass. Microalgae biomass can be transformed into valuable products by thermochemical conversion (e.g. gasification, pyrolysis and liquefaction) and biochemical conversion (e.g. fermentation and anaerobic digestion). Through hydrothermal processing (e.g. gasification, liquefaction and carbonization) can result in hydrogen-rich syngas, bio-crude oil and hydrochar (Yadav et al. 2019). Moreover, hydrothermal processing technologies are found to be suitable for high-moisture containing algal biomass because it reduces the overall cost of biomass drying, as the reaction medium is water. Hydrothermal gasification and liquefaction involve the use of subcritical and supercritical water as the reaction medium, which act as green solvents to crack algal biomass to fuel products. Subcritical water is a fluid phase of water occurring below its critical points, whereas subcritical water occurs beyond its critical points (Reddy et al. 2014b). The critical temperature and critical pressure of water are 375°C and 22.1 MPa, respectively (Nanda et al. 2018c).
During pyrolysis, biomass undergoes thermal depolymerization at moderate to high temperatures under an inert atmosphere to produce bio-oil, biochar and gases (Azargohar et al. 2013; Mohanty et al. 2013; Nanda et al. 2013; Azargohar et al. 2014; Nanda et al. 2014d). The bio-oil can be catalytically upgraded to produce synthetic transportation fuels or serve as a precursor for numerous value-added platform chemicals (Nanda et al. 2014c). There is a wide array of value-added products such as Omega-3 fatty acids that can be obtained from microalgal oil, thus making it more economically justified for a sustainable bioresource (Gutierrez et al. 2017). The hydrochar or biochar obtained from hydrothermal and thermochemical processing of algae can be used for several applications in agronomy, biomedicine, carbon sequestration, adsorption of environmental pollutants, support of metal catalysts, production for activated carbon and other specialty materials (Nanda et al. 2016a; Nanda et al. 2018a).
Microalgae are considered as a sustainable and economical source for the production of biofuels, biochemicals and bioproducts (e.g. food supplements, pharmaceuticals, nutra- ceuticals and cosmeceuticals) having industry-wide importance. Having potential in biorefineries, extensive researches are being conducted for the recovery of value-added products. Algae are potential sources for carbon sequestration as they consume CO: for photosynthesis and release oxygen, while at the same time, fixing the carbon as lipids and polysaccharides within its cells. Moreover, algae can also be cultivated on diverse streams of wastewater and industrial effluents, thus leading to their phycoremediation and environmental scrubbing. A wide array of co-products can be generated along with algal biofuels to support biorefineries and making them profitable and environmentally sustainable. The optimization of the cultivation process for maximum lipid and biofuel recovery from algae is highly essential. The implementation of genetic engineering strategies and biotechnological tools can produce high-yielding varieties of algae.