10
• Oxidation: In this step, the reaction takes place between organic vapours, solid
carbonized material and oxidizing agent, producing CO 2 . The H 2 is oxidized to
form water. Furthermore, partial carbon oxidation might happen if O 2 is in substoichiometric quantities and produces CO.
• Reduction: In this stage, the reduction process happens to remove oxygen from
the produced gases to form combustible gases at a high temperature
(800–1200 °C). The main reactions are water gas reaction, Boudouard reaction,
water gas shift reaction and methanation reaction.
4.3 Pyrolysis
Biomass pyrolysis is biomass decomposition through heating in the absence of
oxygen. Pyrolysis is an endothermic process. It occurs in a temperature range from
200 °C to 800 °C in comparison with gasification, which is from 800 °C to 1200 °C
[54]. Pyrolysis is the first phase of both combustion and gasification. Pyrolysis is
divided into different categories which are fast, intermediate, slow and torrefaction
[55, 56]. Residence time is an essential factor in biomass pyrolysis.
Torrefaction and slow pyrolysis have hours and days solid residence times.
Intermediate pyrolysis has minutes solid residence time, while biomass fast pyrolysis has a very short solid/vapour residence time, which is around 2 s [57]. The characteristics of the main three groups are presented in Table 2. Figure 4 shows the
different products of biomass pyrolysis with different methods (mass balance). This
figure shows that longer residence time reactions (slow pyrolysis and torrefaction)
favour charcoal production, while shorter residence time reactions (intermediate
and fast) favour liquid and gas production. Fast pyrolysis reaction yields the maximum amount of liquid (75%) and the least char content (12%) in comparison with
other biomass pyrolysis reactions [57, 58].
Common pyrolysis reactors are bubbling fluidized bed (BFB), circulating fluidized bed (CFB), cyclone (vortex), rotating cone, ablative, screw and auger or kiln,
which are shown in Fig. 5. The fluidized bed pyrolysers deliver high heat transfer
rates by applying solid-gas heat transfer mediums. However, the maximum particle
size for fluidized beds should be less than 6 mm [52]. In a cyclone pyrolyser, the
material particles are entrained in a hot inert vapour (gas or steam) flow and then
move into the reactor to be melted on the heated reactor wall. One of the issues with
cyclone pyrolyser is scale-up [59].
The rotating cone reactor pyrolyses the material on a high-speed rotating cone
with the presence of hot sand (capable of having fast heating rates). The ablative
pyrolyser presses the biomass against a hot surface, making fast biomass disintegration. The ablative system can process small-size materials, and the heat supply is
problematic [52, 57]. The auger (kiln) or screw reactor transfers the biomass particles using a conveyer or screw system. The screw system can pyrolyse different
sizes of materials. Furthermore, it is reported that the vacuum reactor can be used as
a pyrolyser but has slower heating rates in comparison with other reactors, and the
process is costly and complicated [52, 57].
H. Jahangiri et al.
• Oxidation: In this step, the reaction takes place between organic vapours, solid
carbonized material and oxidizing agent, producing CO 2 . The H 2 is oxidized to
form water. Furthermore, partial carbon oxidation might happen if O 2 is in substoichiometric quantities and produces CO.
• Reduction: In this stage, the reduction process happens to remove oxygen from
the produced gases to form combustible gases at a high temperature
(800–1200 °C). The main reactions are water gas reaction, Boudouard reaction,
water gas shift reaction and methanation reaction.
4.3 Pyrolysis
Biomass pyrolysis is biomass decomposition through heating in the absence of
oxygen. Pyrolysis is an endothermic process. It occurs in a temperature range from
200 °C to 800 °C in comparison with gasification, which is from 800 °C to 1200 °C
[54]. Pyrolysis is the first phase of both combustion and gasification. Pyrolysis is
divided into different categories which are fast, intermediate, slow and torrefaction
[55, 56]. Residence time is an essential factor in biomass pyrolysis.
Torrefaction and slow pyrolysis have hours and days solid residence times.
Intermediate pyrolysis has minutes solid residence time, while biomass fast pyrolysis has a very short solid/vapour residence time, which is around 2 s [57]. The characteristics of the main three groups are presented in Table 2. Figure 4 shows the
different products of biomass pyrolysis with different methods (mass balance). This
figure shows that longer residence time reactions (slow pyrolysis and torrefaction)
favour charcoal production, while shorter residence time reactions (intermediate
and fast) favour liquid and gas production. Fast pyrolysis reaction yields the maximum amount of liquid (75%) and the least char content (12%) in comparison with
other biomass pyrolysis reactions [57, 58].
Common pyrolysis reactors are bubbling fluidized bed (BFB), circulating fluidized bed (CFB), cyclone (vortex), rotating cone, ablative, screw and auger or kiln,
which are shown in Fig. 5. The fluidized bed pyrolysers deliver high heat transfer
rates by applying solid-gas heat transfer mediums. However, the maximum particle
size for fluidized beds should be less than 6 mm [52]. In a cyclone pyrolyser, the
material particles are entrained in a hot inert vapour (gas or steam) flow and then
move into the reactor to be melted on the heated reactor wall. One of the issues with
cyclone pyrolyser is scale-up [59].
The rotating cone reactor pyrolyses the material on a high-speed rotating cone
with the presence of hot sand (capable of having fast heating rates). The ablative
pyrolyser presses the biomass against a hot surface, making fast biomass disintegration. The ablative system can process small-size materials, and the heat supply is
problematic [52, 57]. The auger (kiln) or screw reactor transfers the biomass particles using a conveyer or screw system. The screw system can pyrolyse different
sizes of materials. Furthermore, it is reported that the vacuum reactor can be used as
a pyrolyser but has slower heating rates in comparison with other reactors, and the
process is costly and complicated [52, 57].
H. Jahangiri et al.