**2. Algal-based biorefinery**

Algal-based biorefinery is a cost-effective approach to producing biofuels, bioenergy, and other value-added products by integrating algal biomass conversion processes and equipment [10]. It adds to the concept of converting algal biomass into useful, commercially important products and energy. The major stages in algal biorefinery include upstream and downstream processing, such as cultivation, harvesting, drying, and conversion processes to produce biofuel and other valueadded products.

Algal cultivation becomes economically feasible since algae can be grown in wastewater as a culture medium to cultivate algae. The importance and necessity of aquaculture wastewater for the purpose of cultivating algae and even highly flourished growth of microalgae in fertilizer wastewater leads to the production of biodiesel from algal biomass in a cost-effective way [11, 12]. Open raceway ponds and closed photo-bioreactors comprise the principal method for algal cultivation. Compared with other algal culture systems, open culture systems are cost-effective and easy to install and maintain, and their energy consumption is preferably lower. The negative impact of this system is a lack of control over water temperature, light intensity, and evaporation [13, 14]. Whereas in the case of a closed culture system, photobioreactors can produce 3–5 times more biomass. It can cultivate single species of microalgae in a considerable quantity. Tubular, flat plate, column, and membrane photobioreactors are different types of closed systems [14]. A novel, cost-effective algal cultivation strategy, mixotrophic microalgae biofilm, was introduced to improve productivity [15].

The size of algae is relatively minute in particular, and its negative surface charge makes the separation process difficult, making it challenging for harvesting. Several techniques are applied to neutralize these negative charges [16]. Algae harvesting from the aqueous suspension can be done mechanically, chemically, biologically, or using electrical-based methods. A combination of two or more of these methods is also used [17]. Different technologies are used to harvest algal biomass, including centrifugation, flocculation, bio-flocculation, flotation, filtration, gravity sedimentation and electrocoagulation. Another cost effective method of easiest harvesting is combination of flocculation-sedimentation cum centrifugation [15, 16]. Partial harvesting of algal biomass with vacuum gas lift prior to the complete harvesting (centrifugation) proved efficient and cost-effective [18–20]. In another harvesting experiment, auto flocculation uses appropriate flocculants like poly aluminum chloride, aluminum sulfate, and pH adjusted chitosan is the best and economical way to harvest the microalgae. Harvesting efficiency can also be enhanced by adding auto flocculating microalgae, which can induce faster sedimentation of non-flocculating microalgae [21].

Drying can be done to protect the algal biomass from spoilage. For the hydrothermal process, the algal biomass need not be dried because the process is carried out in the water and requires 95% moisture content. The other thermochemical processes like pyrolysis, gasification, and combustion needs to be dried algal biomass to produce biofuel and high value products [17]. The significant algae drying process comprises rotary dryer, solar heat drying, spray drying, cross flow, and vacuum shelf drying [22]. Among that, solar heat drying or sun drying is the most basic drying with a low cost of budget but requires more duration time to dry. Algal biomass is disrupted in order to release intracellular biomolecules. Nowadays, mechanical and non-mechanical cell disruption methods are used to disrupt the algal cell wall. Nonmechanical methods comprise a chemical method, osmotic shock, and treatment using enzymes and detergents. Osmotic shock involves applying a high concentration of a solute, such as a dextran, salts, or polyethylene glycol, around a cell to lower its osmotic pressure. These cause disruption of the algal cell wall and the release of intracellular molecules. Moreover, hypotonic osmotic shock can damage the membrane of all algal species but not the cell wall [23]. Chaetoceros mueller algae produced 35% methane and 72% algal lipid in an osmotic shock experiment [24]. Cell disruption can also occur using various chemicals such as organic solvents, surfactants, hypochlorite, and chelating agents. Acids and alkali treatments are also used for the algal cell disruption. Several parameters were studied and optimized in order to increase lipid extraction potency from Scenedesmus sp. (cellulase, pectinase, xylanase, protein concentration, pH, temperature, and incubation time) [25]. In the case of the enzymatic cell disruption method, enzymes are used to recover intracellular components. It can degrade cell wall components such as cellulose, hemicellulose, alginates, and glycoproteins. Mechanical methods in the form of liquid and solid shearing (bead milling, high-speed homogenizer, and high-pressure homogenizer), energy transfer (ultrasonication, microwave, and laser), and heat (thermolysis and autoclaving) and as a current (pulsed electric field) are considered as an alternative method to disrupt the cell wall of algae [26]. The bead milling method induces direct mechanical damage to the algal cell. These cells are damaged by applying forces from collisions between cells and beads. The collision is propped up with the help of a rotating shaft in the grinding chamber [27]. Another technique method is ultrasonication which uses ultrasound waves to disrupt algal cells.

Similarly, the pulsed electric field technique uses an external electric field, creating a critical electric potential across the algal cell wall, thereby causing disruption of the cell wall. Heat treatment methods such as autoclaving and thermolysis are also effective for cell disruption [28]. Many valuable biomolecules can be extracted from

#### *Thermochemical Conversion of Algal Based Biorefinery for Biofuel DOI: http://dx.doi.org/10.5772/intechopen.106357*

algae by cell disruption methods. After the cell disruption process, the extraction process begins. Supercritical fluids and deep eutectic solvents are used in solvent extraction. The organic solvent extraction technique is a well-known method for the extraction of algal biomolecules. This technique enhances the extraction yield by facilitating the access of solvents to inner cellular molecules. In addition to terpenes, liquid polymers, ionic liquids, and deep eutectic solvents, bio-based solvents are used for solvent extraction. In the case food and pharmaceutical industries supercritical fluid extraction technique is primarily employed as it is a contamination-free method of extraction. Separation and purification methods are done to separate the impurities and the molecules of least interest. Separation methods to purify the extracted components include electrophoresis, membrane separation, ultracentrifugation, etc. [26].

Various conversion technologies are employed to convert algal biomass into valueadded products, including biochemical, chemical, and thermochemical technologies. Biochemical conversion of algal biomass is achieved through biological treatments to produce biofuels. These conversion methods include fermentation, anaerobic digestion, and transesterification. Anaerobic digestion converts algal biomass to hydrogen and methane, while fermentation produces ethanol, acetone, and butanol; transesterification produces biodiesel.
