20.2 Cultivation and Processing of Microalgae

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●Acidogenesis and acetogenesis: the molecules which are hydrolyzed are con-

verted to acetate, CO2, and H2.

●Methanogenesis: production of methane from acetate, carbon dioxide, and

hydrogen.

Efforts are currently being undertaken to develop inexpensive biomass feedstock,

maximize energy production, and minimize associated environmental risks [14].

20.2.7.2

Liquid Oil Production by Thermal Liquefaction Process

Thermal liquefaction (HTL) or hydrous pyrolysis is the process of transformation of

algal biomass into liquid fuels in the presence of water. In this process, in a closed

reactor, the wet biomass is converted into biofuel. The change in reaction conditions

affects the reaction progress. Since subcritical water has good heat transfer and mass

transferability, the HTL process is independent of the heating rates and biomass

particles size. Depending on the type of feedstock, processing conditions, and

presence of a catalyst, the product yield is obtained using relevant physiochemical

properties. HTL of the feedstock containing high protein content results in the

formation of the product with high nitrogen content and aromatic content. It is

essential to upgrade the HTL biocrude for it to be necessary for transportation

grade. HTL also produces gaseous products which contain malodours. They have to

be reduced before being discharged into the environment [15].

HTL is highly expensive than pyrolysis and gasification. It has also been observed

that the energy balance of hydrothermal liquefaction is unfavorable in case of water

content, thus exceeding 90% of the biomass. Based on the type of microalgae, around

23–49% of the original dry mass is recovered as bio-oil. The bio-oil recovered contains

up to 75% of energy of the initial biomass feedstock. The inorganic nutrients obtained

in the hydrothermal liquefaction process can be used as crop fertilizer [16].

20.2.7.3

Transesterification Process

This is the most common method of production of biodiesel. In the presence of

a catalyst, oil is converted into methyl ester in the consecutive reactions between

alcohol and vegetable oils. In this process, the oil which is extracted from the algae

is preheated to 60 ^∘C. It is then added to a mixture of methanol and sodium hydroxide

in suitable proportions. The transesterification reaction occurs in the reactor. Once

the products are formed, the products are allowed to settle. Initially, the glycerine

will be removed; the desired product biodiesel is then filtered and washed [17].

20.2.7.4

Nano-Catalyzed Transesterification Process

Initially, for the production of biodiesel from microalgae, algal strains are selected

because each strain has varying lipid composition. In conventional methods,

biodiesel is produced using homogeneous and heterogeneous, also enzymatic

catalysts such as potassium hydroxide, sodium hydroxide, zeolites, and lipase.

Recent studies involve the use of nanocatalysts for the generation of biodiesel using

microalgae by extracting algae oil and converting into biofuel. The process involves

a biocompatible, mesoporous nanoparticle that absorbs hydrophobic molecules.