Conversion of Algal Biomass to Biofuels

Introduction

Fossil fuels and their derived products have played a pivotal role in advancements relating to automobiles, power generating sectors, infrastructure, urban development and human lifestyle since the industrial revolution. Nonetheless, the exploiting combustion of fossil fuels has led to the emission of greenhouse gases, particularly C0₂, to an alarming level (Rana et al. 2018; Rana et al. 2019; Rana et al. 2020). This increase in the concentration of CO, and other greenhouse gases has disturbed the balance between the solar radiation obtained by the earth and its reflection, thereby enhancing the heat retention ability of earth that results in global warming (Nanda et al. 2016e; Parakh et al. 2020). Besides global warming, other environmental concerns associated with fossil fuel usage are pollution, rising fuel prices and geopolitical issues between crude oil importing countries and exporting countries (Nanda et al. 2015a; Okolie et al. 2020b). To address these problems, research in the field of bioenergy and other renewable energy sources has been intensifying (Sarangi and Nanda 2018; Sarangi and Nanda 2019a; Sarangi and Nanda 2019b; Sarangi and Nanda 2020; Sarangi et al. 2020).

Several waste residues such as lignocellulosic biomass (e.g. agricultural crop residues, forestry refuse, energy crops and invasive crops), microalgae, food waste, municipal solid waste, sewage sludge, industrial effluents, livestock manure and other waste organic matter have the potential to be converted to biofuels, biochemicals and bioproducts with a low-carbon footprint or with carbon neutrality (Nanda et al. 2015b; Nanda et al. 2015c; Reddy et al. 2016; Nanda et al. 2016b; Nanda et al. 2016c; Nanda et al. 2016d; Gong et al. 2017a; Gong et al. 2017b; Nanda et al. 2017d; Nanda et al. 2018b; Nanda et al. 2019a; Nanda et al. 2019b; Singh et al. 2020; Okolie et al. 2020a; Okolie et al. 2020c; Okolie et al. 2020d). The biofuel products resulting from the thermochemical and biological conversion of the above-mentioned waste residues are bio-oil, biodiesel, bioethanol, biobutanol, biomethanol, biogas (biomethane), biohydrogen, synthesis gas and biochar (Nanda et al. 2014b; Nanda et al. 2016a; Nanda et al. 2017e; Okolie et al. 2019).

Microalgae has many promising potentials to produce biofuels, biochemicals and bioproducts (e.g. nutraceuticals, pharmaceuticals and cosmeceuticals) besides carbon sequestration and phycoremediation of wastewater (Reddy et al. 2018; Yadav et al. 2019). Microalgae are single phototrophic organisms in freshwater and marine environments. By utilizing sunlight, C0₂, water, organic matter and dissolved nutrients, algae can synthesize lipids, proteins, carbohydrates and pigments in their cells that can be extracted to produce various useful bioproducts (Roller et al. 2014). Microalgae find their potential in human and animal nutrition, whereas their extractives can also be used as a starting material in textile, pharmaceutical, cosmetics and food industries (Roller et al. 2014). Besides, algal biomass has gained promising applications in biofuel sectors for solving future energy problems acting as the third-generation biofuel feedstock to produce biofuels such as biodiesel, algal oil and bio-jet fuel (Reddy et al. 2018; Yadav et al. 2019). This chapter discusses the potential of algae for the next-generation biorefineries.

Bioprospecting and Cultivation of Algae

Figure 7.1 shows the conversion of algal biomass to several value-added products. Algae biorefinery depends on the compositions of algal species such as lipids, carbohydrates and proteins, which serve as the precursors for biofuels and biochemicals (Laurens et al. 2017). Furthermore, the production of nutraceuticals, pharmaceuticals, pigments, vitamins, antioxidants can be obtained from algal biomass. Microalgae are also the important reservoirs of high-value nutrients, pigments, proteins, carbohydrates and lipid molecules (Ghosh et al. 2016). Algae are the known producers of various value-added compounds including extracellular products like exopolysaccharides, exoenzymes, etc. (Pierre et al. 2019). Besides biofuel production, algae can also be used for the phycoremediation of wastewater and industrial effluents in reducing the levels of hazardous chemicals and organic matter (Rumar et al. 2018). The promising option for the utilization of algae is for capturing the runoff fertilizers from different farms to lakes and water reservoirs.

Chlamydomonas reinhardtii and Dunaliella salina are some fast-growing species of algae that have been extensively explored along with various Chlorella sp. Botryococcus braunii can accumulate enormous quantities of lipids (Scott et al. 2010; Dragone et al. 2011). The lipid content of Chlorella sp. is very high (approximately 60-70%), making it quite popular among algal varieties. There are some geographical and cultural variations associated with algal biomass, which determine their lipid composition.

The growth of microalgae is dependent upon the type of cultivation system employed, availability of light, levels of C0₂ and 0₂, temperature and availability of nutrients like nitrogen and phosphorus (Abdollahi and Dubljevic 2012; Li and Yang 2013). The three basic modes of algal cultivation are photoautotrophic, heterotrophic

FIGURE 7.1 Biorefinery of algal biomass towards biofuels and platform chemicals

and mixotrophic. In photoautotrophic cultivation, algal growth using C0₂ is beneficial as an inexpensive process. On the other hand, the heterotrophic growth of algae consumes sugars and organic acids as the carbon sources in the absence of light. However, the mixotrophic mode uses both organic matter and C0₂ along with light for cultivating algae. The process of photosynthesis and carbon fixation by algae is much more efficient and faster than many terrestrial plants (Sayre 20Ю). Various harvesting processes such as centrifugation, filtration, flotation, sedimentation and flocculation along other downstream process technologies are used for the separation of algae and other bioproducts. Several flocculation techniques such as biological, magnetic, chemical, electro and auto-flocculation are used in this process.

The lipids composition of microalgal cells differs based on the algal variety and species, ingredients of the culture medium and environmental conditions along with growth factors such as temperature, complementation, luminous power and photoperiodicity (Brown 1991; Khoeyi et al. 2012; Prochazkova et al. 2014). At the optimal conditions, microalgae rapidly multiply but the accumulation of reserve substances like carbohydrates and lipids may not take place. During adverse conditions, it tends to rouse the gathering of pigments. Thus, regulations in experimental conditions are very crucial in the production of metabolites as the main products or by-products.

As mentioned earlier, the composition of the culture medium has a great role to play in the growth of algal cells and their proliferation. Diverse microalgal species differ in their nutritional necessities, although they can adapt to various supplementation conditions. Industrial and domestic wastewaters are also suitable as a culture medium

(Lv et al. 2017: Reyimu and Oz^imen 2017; Wu et al. 2017). Synthetic culture media with a well-defined composition can assess the response of algal cells to the alteration in concentration and modification of components. In certain situations, the product of interest can be obtained if the organism’s metabolic pathway is stimulated after the supplementation of particular nutrients. The usage of cost-effective nutrient sources such as urea, human urine as well as glycerol have also been reported (Campos et al. 2014; Sengmee et al. 2017; Wu et al. 2017). The stirring process in microalgal cultures is important to reduce the sedimentation and homogenization of cell suspension. The stirrer is an essential part of a photobioreactor to enhance the cells to receive an equal quantity of light.

Microalgal growth is facilitated in either the existence or deficiency of light depending on the variety and strain. Algae can utilize both organic and inorganic carbon as the energy source. Photoautotrophic condition is w'idely employed for the cultivation of algae, which occurs in the presence of light. According to Kim et al. (2013), the intensity of light along w'ith its color regulates the biomolecules developed and accumulated by the algal cells. Light is not mandatory for the occurrence of biochemical reactions in heterotrophic culture as organic carbon acts as the energy source. Light along w'ith organic carbon sources are prerequisites for culturing microalgae in mixotrophic and photoheterotrophic growth conditions. During mixotrophic culture, photosynthesis is performed by cells by using both organic as well as inorganic carbon. On the other hand, during the photoheterotrophic condition, light is needed by cells to use organic compounds (Chen et al. 2013). Certain photobioreactors can be employed using indoor artificial lighting for the growth of microalgal cells on a laboratory scale. Outdoor lighting is suitable for simulation studies either on a small or pilot scale to enhance productivity (Lu et al. 2015). These factors regulate the production and accumulation of lipids and carbohydrates in microalgae to estimate the yield of bio-crude oil, biohydrogen, biodiesel, bioethanol, biobutanol and other biofuels, biochemicals and bioproducts.

Biodiesel From Algae

The lipid content of algae decides its potential to generate biofuels. Algae have huge potential as a feedstock for biodiesel production because of the following attributes (Chisti 2008; Mata et al. 2010):

i. Enormous lipid content (20-50%).

ii. Fast grow'th rates.

iii. No competition to cultivable or fertile lands.

iv. Capability to develop in rigorous conditions.

v. Growth on low-cost substrates and wastewater effluents.

vi. Ability to capture and fix C0₂ from flue gases.

vii. Cost-effective and eco-friendly resource.

The emission of C0₂ from fossil fuels (e.g. diesel and gasoline) is a major environmental concern for which the focus has now gradually shifted towards renewable biofuels (e.g. biodiesel) (Reddy et al. 2018; Nayak et al. 2019; Nayak et al. 2020). Widjaja et al. (2009) stated that the accumulation of lipids as triacylglycerides takes place in microalgal cells when the environmental conditions become unfavorable (stress conditions) either in the form of nutrient deficit or the availability and intensity of light. The deficiency of nitrogen may significantly reduce the cell division as protein, pivotal for the formation of the cell wall, is produced in lesser amounts (Aremu et al. 2015). The production of biomass is also negatively affected when algae are deprived of phosphate. Similarly, significant reduction in the lipid concentration with the significant hike in unsaturated fatty acids concentration is seen (Praveenkumar et al. 2012). The cell development and lipid accumulation of lipids in microalgae are stimulated when an organic carbon source is supplied to the growth media.

The transesterification process or hydrogenolysis helps in the extraction of lipids from algae to yield biodiesel and aviation fuel (a derivative of kerosene grade alkane) (Bwapwa et al. 2017). By successive steps of methanolysis, triacylglycerols found in the algal oil can be broken down into diglycerides and monoglycerides. Using several acids, bases, metal catalysts, biocatalysts (e.g. enzymes) and supercritical fluids, the efficiency of transesterification can be enhanced (Reddy et al. 2018). Finally, three chief products such as fatty acid methyl esters (FAME), fatty acid ethyl esters (FAEE) and glycerol are generated from transesterification (Gong and Jiang 2011). If complete solubilization of triacylglycerols does not take place by the solvents, the extractive process is considered incompetent resulting in lower oil yields (Velasquez-Orta et al. 2013). Another aspect i.e. biomass drying temperature can also considerably affect the recovery of oil since the fatty acids are oxidized at high temperatures (Widjaja et al. 2009). Consequently, the oil extraction processes and techniques from microalgal cells can influence the yields of biodiesel. Lipid extraction becomes difficult when the water content of algal biomass is high. Therefore, it is necessary to dewater it by either centrifugation or filtration. The processes of culturing and dewatering are energy-intensive, which can add significantly to the overall process expenditures (Ri'os et al. 2013).