2.1 Hydrothermal Carbonization
This is a process in which the biomass is kept suspended in water at elevated
operating temperature and pressure range of 180–350
C and 2–10 MPa, where
high-potential solid chars of moderate calorific value are obtained. Generally, the
process is exothermic since it utilizes the oxygen and hydrogen present in the raw
material [15]. The reaction pressure is not actually controlled; rather presence of
liquid in biomass helps to promote the pressure up to a maximum of 10 MPa
[16]. Though the process is carried over at mild operating conditions, care should
be taken in keeping the water under subcritical state. Sometimes the medium
becomes alkaline due to the drop in pH, and this is because of the formation of
by-products inside the reactor. The main advantage of HTC is handling of high
moisture content feedstock; the solid char is highly sterilized (biologically) due to
the thermal conversion, and energy incentive product is derived [17]. But still HTC
was found to gain less interest among the researchers because of the datum that
hydrochar possesses less energy-carrying space for solid products than liquefaction
and gasification [18].
2.2 Hydrothermal Liquefaction
Biomass with higher percentage of moisture can be utilized for the liquefaction
process. Unlike hydrothermal carbonization, liquefaction is carried out under lower
temperature (250–450
C) and high pressure (4–20 MPa) in presence of water or
Fig. 1 Phase diagram for water
206
C. D. Venkatachalam et al.
This is a process in which the biomass is kept suspended in water at elevated
operating temperature and pressure range of 180–350
C and 2–10 MPa, where
high-potential solid chars of moderate calorific value are obtained. Generally, the
process is exothermic since it utilizes the oxygen and hydrogen present in the raw
material [15]. The reaction pressure is not actually controlled; rather presence of
liquid in biomass helps to promote the pressure up to a maximum of 10 MPa
[16]. Though the process is carried over at mild operating conditions, care should
be taken in keeping the water under subcritical state. Sometimes the medium
becomes alkaline due to the drop in pH, and this is because of the formation of
by-products inside the reactor. The main advantage of HTC is handling of high
moisture content feedstock; the solid char is highly sterilized (biologically) due to
the thermal conversion, and energy incentive product is derived [17]. But still HTC
was found to gain less interest among the researchers because of the datum that
hydrochar possesses less energy-carrying space for solid products than liquefaction
and gasification [18].
2.2 Hydrothermal Liquefaction
Biomass with higher percentage of moisture can be utilized for the liquefaction
process. Unlike hydrothermal carbonization, liquefaction is carried out under lower
temperature (250–450
C) and high pressure (4–20 MPa) in presence of water or
Fig. 1 Phase diagram for water
206
C. D. Venkatachalam et al.