allowing the extraction of lipids prior to the carbonization process (Heilmann et al.
2010; Mathimani et al. 2018).
8.5.1.3 Pyrolysis
The algal biomass is subjected to thermal decomposition in an atmospheric pressure
inert environment. It involves temperatures as high as 400–600
C and oxygen-free
environment. In slow pyrolysis, the biomass is exposed to a lower heating rate of
5–10
C/minute and for longer residence times of 1 h, while in fast pyrolysis, it
consists of shorter residence time of a few seconds. The method of fast pyrolysis
employing heating rates as high as 600
C/min is more suitable with respect to the
process of slow pyrolysis and allows for operation of a continuous process. Syngas,
also called pyrogas, is the principle product obtained from the process of pyrolysis
and comprises of non-condensable gases, solid char, and bio-oil (Mathimani et al.
2018). Flash pyrolysis is a promising substitute for biofuel production and future
replacement of fossil fuels by the produced biofuels. It employs temperature of
500.8
C and a short residence time of about 1 s. Approximately 95.5% biomass-toliquid conversion ratios can be achieved through this method (Brennan and Owende
2010; Demirbas 2006; Clark and Deswarte 2008).
8.5.1.4 Direct Combustion
In this method, microalgae are burned in an oxygen-rich environment in a boiler or
furnace at around 850
C. The biomass having a moisture content greater than 50%
is loaded in the boilers, and 10% excess of air (relative to the feedstock) is pumped in
the combustor. This scenario favors the release of heat and further allows reaction to
achieve completion. During the process of combustion, the photosynthetically
obtained chemical energy in microalgae is transformed to hot gases. Combustion
results in the production of a large amount of heat which cannot be stored feasibly
and is thus further converted into other valuable products, e.g., generation of
electricity in a turbine (Mathimani et al. 2018). In addition, combustion can be
employed from very small-scale utilities (as for the domestic purposes) to large-scale
industrial processes (McKendry 2002b; Goyal et al. 2008; Brennan and Owende
2010). Various pretreatments of biomass, such as drying, grinding, or chopping, are
needed for this conversion process. This process leads to additional energy and cost,
thereby delimiting the use of this technique.
8.5.1.5 Carbonization
Carbonization involves the production of carbon or carbon-rich residues from the
organics in an exothermic process. This process further results in the release of a
large amount of heat out of the system. Although this technique demands lesser
energy in comparison to other conversion methods (Benavente et al. 2017;
Mathimani et al. 2018), its use remains restricted owing to the high costs and high
nitrogen contents involved in cultivation of microalgae as a feedstock for the
carbonization process. When the operating parameters are 150–250
C and the
pressure is less than 100 bars, the carbonization process is designated as hydrothermal carbonization (HTC). The residence time of this mild-treatment method is
220
K. Agrawal et al.
2010; Mathimani et al. 2018).
8.5.1.3 Pyrolysis
The algal biomass is subjected to thermal decomposition in an atmospheric pressure
inert environment. It involves temperatures as high as 400–600
C and oxygen-free
environment. In slow pyrolysis, the biomass is exposed to a lower heating rate of
5–10
C/minute and for longer residence times of 1 h, while in fast pyrolysis, it
consists of shorter residence time of a few seconds. The method of fast pyrolysis
employing heating rates as high as 600
C/min is more suitable with respect to the
process of slow pyrolysis and allows for operation of a continuous process. Syngas,
also called pyrogas, is the principle product obtained from the process of pyrolysis
and comprises of non-condensable gases, solid char, and bio-oil (Mathimani et al.
2018). Flash pyrolysis is a promising substitute for biofuel production and future
replacement of fossil fuels by the produced biofuels. It employs temperature of
500.8
C and a short residence time of about 1 s. Approximately 95.5% biomass-toliquid conversion ratios can be achieved through this method (Brennan and Owende
2010; Demirbas 2006; Clark and Deswarte 2008).
8.5.1.4 Direct Combustion
In this method, microalgae are burned in an oxygen-rich environment in a boiler or
furnace at around 850
C. The biomass having a moisture content greater than 50%
is loaded in the boilers, and 10% excess of air (relative to the feedstock) is pumped in
the combustor. This scenario favors the release of heat and further allows reaction to
achieve completion. During the process of combustion, the photosynthetically
obtained chemical energy in microalgae is transformed to hot gases. Combustion
results in the production of a large amount of heat which cannot be stored feasibly
and is thus further converted into other valuable products, e.g., generation of
electricity in a turbine (Mathimani et al. 2018). In addition, combustion can be
employed from very small-scale utilities (as for the domestic purposes) to large-scale
industrial processes (McKendry 2002b; Goyal et al. 2008; Brennan and Owende
2010). Various pretreatments of biomass, such as drying, grinding, or chopping, are
needed for this conversion process. This process leads to additional energy and cost,
thereby delimiting the use of this technique.
8.5.1.5 Carbonization
Carbonization involves the production of carbon or carbon-rich residues from the
organics in an exothermic process. This process further results in the release of a
large amount of heat out of the system. Although this technique demands lesser
energy in comparison to other conversion methods (Benavente et al. 2017;
Mathimani et al. 2018), its use remains restricted owing to the high costs and high
nitrogen contents involved in cultivation of microalgae as a feedstock for the
carbonization process. When the operating parameters are 150–250
C and the
pressure is less than 100 bars, the carbonization process is designated as hydrothermal carbonization (HTC). The residence time of this mild-treatment method is
220
K. Agrawal et al.