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4.1 Overview of Thermal Conversion Processes
Biomass conversion by thermal processes (i.e., direct combustion, gasification,
pyrolysis) involves the decomposition of biomass into various solid, liquid, and
gaseous components by exposure to heat (normally above 300 °C). Different process conditions, such as temperature, reaction time, heating rate, absence or presence of oxygen, use of catalysts, and pressure, affect the distribution and quality of
the products generated (Capareda 2014; Maguyon and Capareda 2013; MaguyonDetras and Capareda 2017). Table 4.1 shows the typical process conditions and
products for thermal conversion processes.
Pyrolysis is the thermal conversion of biomass at temperatures typically below
600
°
C in the complete absence of oxygen to produce higher-energy density materials including char, bio-oil, and gaseous products. Gasification usually occurs at temperatures greater than 600 °C with an oxygen supply that is only a fraction of what
is theoretically required for complete combustion. The resulting gaseous product,
which typically consists of hydrogen, carbon monoxide, methane, carbon dioxide
and nitrogen, is said to be more versatile than the original feedstock and can be
burned to produce process heat and steam or be used in gas turbines to produce
electricity (Demirbas 2001). Combustion is the oldest method of thermal conversion of biomass, which accounts for almost 97% of the world’s bioenergy production (Demirbas et al. 2009). In this process, the feedstock is subjected to high
temperatures (typically above 700 °C) with an excess amount of air to produce gaseous products consisting mainly of CO 2 , H 2 O, N 2 , and heat.
Pyrolysis serves as the core reaction of thermal conversion processes since the
irreversible degradation of the biomass starts at temperatures of about 150–200 °C
with the absence of oxygen. According to Pollex et al. (2011), even if the feedstock
is surrounded by oxygen from the air, it does not participate in the reaction since the
pyrolysis gases produced during this process flow out from the biomass particles and
prevent oxygen from reaching it. As the temperature goes up and some oxygen (less
Table 4.1 Overview of thermal conversion processes, their process conditions and products
Pyrolysis
Gasification
Combustion
Process conditions
Temperature
(°C)
300–600
>600
>700
Reaction time 1 s (fast pyrolysis) days (slow
pyrolysis)
Several seconds to
minutes
–
Air supply, λ
a
λ = 0
λ = 0.2–0.5
λ ≥ 1
Products
Gaseous
product
CO, CH 4 , C X H Y , CO 2 , H 2 O,
pyrolysis oils, N- and Scontaining compounds
CO, H 2 , CO 2 , H 2 O, CH 4,
C X H Y, tars, NH y, NO x ,
H 2 S, COS
CO 2 , H 2 O, CO,
C X H Y, NO X, SO X
Solid
C m H n O k, (N, S), ash
C, (N, S), ash
Ash, (N,S)
Source: Adapted from Capareda (2014) and Lohri et al. (2015)
a
λ = O 2 supplied/O 2 theoretical
M. C. Maguyon-Detras et al.
4.1 Overview of Thermal Conversion Processes
Biomass conversion by thermal processes (i.e., direct combustion, gasification,
pyrolysis) involves the decomposition of biomass into various solid, liquid, and
gaseous components by exposure to heat (normally above 300 °C). Different process conditions, such as temperature, reaction time, heating rate, absence or presence of oxygen, use of catalysts, and pressure, affect the distribution and quality of
the products generated (Capareda 2014; Maguyon and Capareda 2013; MaguyonDetras and Capareda 2017). Table 4.1 shows the typical process conditions and
products for thermal conversion processes.
Pyrolysis is the thermal conversion of biomass at temperatures typically below
600
°
C in the complete absence of oxygen to produce higher-energy density materials including char, bio-oil, and gaseous products. Gasification usually occurs at temperatures greater than 600 °C with an oxygen supply that is only a fraction of what
is theoretically required for complete combustion. The resulting gaseous product,
which typically consists of hydrogen, carbon monoxide, methane, carbon dioxide
and nitrogen, is said to be more versatile than the original feedstock and can be
burned to produce process heat and steam or be used in gas turbines to produce
electricity (Demirbas 2001). Combustion is the oldest method of thermal conversion of biomass, which accounts for almost 97% of the world’s bioenergy production (Demirbas et al. 2009). In this process, the feedstock is subjected to high
temperatures (typically above 700 °C) with an excess amount of air to produce gaseous products consisting mainly of CO 2 , H 2 O, N 2 , and heat.
Pyrolysis serves as the core reaction of thermal conversion processes since the
irreversible degradation of the biomass starts at temperatures of about 150–200 °C
with the absence of oxygen. According to Pollex et al. (2011), even if the feedstock
is surrounded by oxygen from the air, it does not participate in the reaction since the
pyrolysis gases produced during this process flow out from the biomass particles and
prevent oxygen from reaching it. As the temperature goes up and some oxygen (less
Table 4.1 Overview of thermal conversion processes, their process conditions and products
Pyrolysis
Gasification
Combustion
Process conditions
Temperature
(°C)
300–600
>600
>700
Reaction time 1 s (fast pyrolysis) days (slow
pyrolysis)
Several seconds to
minutes
–
Air supply, λ
a
λ = 0
λ = 0.2–0.5
λ ≥ 1
Products
Gaseous
product
CO, CH 4 , C X H Y , CO 2 , H 2 O,
pyrolysis oils, N- and Scontaining compounds
CO, H 2 , CO 2 , H 2 O, CH 4,
C X H Y, tars, NH y, NO x ,
H 2 S, COS
CO 2 , H 2 O, CO,
C X H Y, NO X, SO X
Solid
C m H n O k, (N, S), ash
C, (N, S), ash
Ash, (N,S)
Source: Adapted from Capareda (2014) and Lohri et al. (2015)
a
λ = O 2 supplied/O 2 theoretical
M. C. Maguyon-Detras et al.