carbohydrates, proteins, and lipids in the microalgal cells (Mathimani et al. 2018; Li
et al. 2008). Various advantages are offered by the thermochemical conversion as it
is environment friendly, achieves better recovery of nutrition, and involves shorter
processing time. The emission of fugitive gases is eliminated and is applicable for
both wet and dry biomass. Various feedstocks and blends can be handled and owing
to the higher temperatures employed and a small amount of residue are obtained after
the conversion process, and efficient elimination of pathogens and pharmaceutically
active compounds is observed (Razzak et al. 2013; Milano et al. 2016).
8.5.1.1 Gasification
A large array of feedstock can be processed by the method of gasification and is
specifically suitable for feedstocks with low moisture content such as lignocellulosic
biomass. In the microalgal biofuel production process, two types of gasification
methods are used: conventional and supercritical. In conventional method, the
biomass undergoes partial oxidation in the air, oxygen, or steam that acts as a
gasification medium. Temperatures between 700 and 1000
C are employed, and
the following steps occur in a gasifier (Basu 2010; Mathimani et al. 2018): the
residual moisture is first removed through proper drying, and the solid structure of
the biomass is broken down to yield less complex molecules through the process of
pyrolysis. This step is followed by the process of oxidation which involves burning
some of the incoming biomass and production of heat in order to sustain further
endothermic processes. Lastly, the pyrolysis products are then converted to highenergy-containing smaller molecules through the process of gasification. In case of
supercritical method, high water content of the microalgal feedstock demands
extensive drying when processed by the conventional method. Hence, the supercritical method of gasification provides a more suitable alternative for the conversion of
algal biomass into biofuels. The temperature and pressure conditions employed are
in the supercritical range for water as 400–500
C and 24–36 MPa. Similar to the
hydrothermal liquefaction (HTL), this process is also performed in an aqueous
media, but employs supercritical conditions, whereas HTL involves subcritical
conditions. The microalgae structures are decomposed into smaller molecules by
the supercritical water and involve C–C bond breakage (Mathimani et al. 2018).
8.5.1.2 HTL
In case of HTL, temperatures from 250 to 380
C and 5–20 MPa of pressure are
employed to obtain biofuel. These subcritical conditions are provided in an aqueous
medium and result in the breakdown of algal structures to simpler and smaller
molecules. As compared to pyrolysis, a more deoxygenated and vicious liquid
blend (or bio-oil) is produced from HTL process. Nitrogen, hydrogen, carbon
dioxide, carbon monoxide, and light hydrocarbons constitute the bio-oil or the
bio-crude (Brown et al. 2010), while the secondary products are characterized as
being ashes and char. The secondary products are obtained as gases or as solid
particles. HTL carbonization takes place at temperature < 250
C and results in
production of hydro-char as the primary product. The carbohydrate and protein
portions of the microalgal cells are often involved in hydro-char production, thereby
8 Algal Biomass: Potential Renewable Feedstock for Biofuels Production – Part I
219
et al. 2008). Various advantages are offered by the thermochemical conversion as it
is environment friendly, achieves better recovery of nutrition, and involves shorter
processing time. The emission of fugitive gases is eliminated and is applicable for
both wet and dry biomass. Various feedstocks and blends can be handled and owing
to the higher temperatures employed and a small amount of residue are obtained after
the conversion process, and efficient elimination of pathogens and pharmaceutically
active compounds is observed (Razzak et al. 2013; Milano et al. 2016).
8.5.1.1 Gasification
A large array of feedstock can be processed by the method of gasification and is
specifically suitable for feedstocks with low moisture content such as lignocellulosic
biomass. In the microalgal biofuel production process, two types of gasification
methods are used: conventional and supercritical. In conventional method, the
biomass undergoes partial oxidation in the air, oxygen, or steam that acts as a
gasification medium. Temperatures between 700 and 1000
C are employed, and
the following steps occur in a gasifier (Basu 2010; Mathimani et al. 2018): the
residual moisture is first removed through proper drying, and the solid structure of
the biomass is broken down to yield less complex molecules through the process of
pyrolysis. This step is followed by the process of oxidation which involves burning
some of the incoming biomass and production of heat in order to sustain further
endothermic processes. Lastly, the pyrolysis products are then converted to highenergy-containing smaller molecules through the process of gasification. In case of
supercritical method, high water content of the microalgal feedstock demands
extensive drying when processed by the conventional method. Hence, the supercritical method of gasification provides a more suitable alternative for the conversion of
algal biomass into biofuels. The temperature and pressure conditions employed are
in the supercritical range for water as 400–500
C and 24–36 MPa. Similar to the
hydrothermal liquefaction (HTL), this process is also performed in an aqueous
media, but employs supercritical conditions, whereas HTL involves subcritical
conditions. The microalgae structures are decomposed into smaller molecules by
the supercritical water and involve C–C bond breakage (Mathimani et al. 2018).
8.5.1.2 HTL
In case of HTL, temperatures from 250 to 380
C and 5–20 MPa of pressure are
employed to obtain biofuel. These subcritical conditions are provided in an aqueous
medium and result in the breakdown of algal structures to simpler and smaller
molecules. As compared to pyrolysis, a more deoxygenated and vicious liquid
blend (or bio-oil) is produced from HTL process. Nitrogen, hydrogen, carbon
dioxide, carbon monoxide, and light hydrocarbons constitute the bio-oil or the
bio-crude (Brown et al. 2010), while the secondary products are characterized as
being ashes and char. The secondary products are obtained as gases or as solid
particles. HTL carbonization takes place at temperature < 250
C and results in
production of hydro-char as the primary product. The carbohydrate and protein
portions of the microalgal cells are often involved in hydro-char production, thereby
8 Algal Biomass: Potential Renewable Feedstock for Biofuels Production – Part I
219