17
5.2.3 Biogas
Biogas and/or syngas produced in pyrolysis is the consequence of the degradation
and cracking of big particles from the original biomass. It is mostly composed of
CO, CO 2 , H 2 , CH 4 and other light hydrocarbons such as C 2 H 6 , C 3 H 8 and C 3 H 6 [109].
If the condensation process is not efficient, the pyrolytic gas can contain some volatiles such as xylenes, benzene, acetaldehyde, pentane and toluene [76].
The composition of the pyrolysis gas is influenced by the reactor parameters, the
chemical structure and the particle size of the materials. Cellulose and hemicellulose decomposition enhances the production of CO and CO 2 because of thermal
cracking of carboxyl and carbonyl groups [110]. The reforming of aromatics and
methoxy groups present in the lignin produces a gas rich in CH 4 and H 2 [111]. Light
hydrocarbons result in the degradation of methylene and ethylene bonds [112]. The
water content in the biomass is another factor affecting the syngas yield [113]. The
particle size of the material also influences the composition and yield of the gaseous
product. Bigger particle size inhibits the cracking reactions of hydrocarbons and
volatiles, producing less CO and H 2 and subsequently more CO 2 and light hydrocarbons [114]. Additionally, larger particles have a negative impact on the heating rate
of biomass, decreasing the gas yield for higher biochar production [89].
In terms of temperature, when there is an increment, the thermal cracking, devolatilization and degradation of biomass are stimulated. Concurrently, secondary
reactions such as dehydrogenation, decarboxylation, deoxygenation and decarbonylation favour the production of volatiles [115]. As a consequence, the pyrolysis
vapour is richer in H 2 and CO, but it contains less CO 2 and hydrocarbon gases. The
combustible gases of the pyrolytic gas (CO, H 2 , CH 4 and other hydrocarbon gases)
can be combusted as a fuel in industrial facilities, generating heat and power [94]. If
the syngas contains a high level of H 2 , it can be used for fuel cell applications.
However, high H 2 yields are not normally obtained via fast or slow pyrolysis, unlike
the intermediate and TCR technologies [116, 117].
5.3 Biomass Thermal Decomposition
Pyrolysis thermal mechanisms for the conversion of biomass can be designated in a
simultaneous combination of fragmentation, isomerization, condensation, dehydration, rearrangement, aromatization, cracking, depolymerization and char formation
[78, 118]. The components of biomass (lignin, hemicellulose, cellulose, water, inorganics and extractives) and the reactor factors have a significant influence in the
pyrolysis reactions. Figure 6 shows the chemical composition of biomass.
The pyrolysis reactions can be classified as primary and secondary.
Depolymerization, fragmentation and charring (char formation) are part of the primary reactions (Fig. 7) [55]. Depolymerization is the prevailing mechanism of
pyrolysis, and it implies the breaking of the monomer’s bonds, producing volatiles
and gases [98, 102]. Fragmentation is responsible for the formation of
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…
5.2.3 Biogas
Biogas and/or syngas produced in pyrolysis is the consequence of the degradation
and cracking of big particles from the original biomass. It is mostly composed of
CO, CO 2 , H 2 , CH 4 and other light hydrocarbons such as C 2 H 6 , C 3 H 8 and C 3 H 6 [109].
If the condensation process is not efficient, the pyrolytic gas can contain some volatiles such as xylenes, benzene, acetaldehyde, pentane and toluene [76].
The composition of the pyrolysis gas is influenced by the reactor parameters, the
chemical structure and the particle size of the materials. Cellulose and hemicellulose decomposition enhances the production of CO and CO 2 because of thermal
cracking of carboxyl and carbonyl groups [110]. The reforming of aromatics and
methoxy groups present in the lignin produces a gas rich in CH 4 and H 2 [111]. Light
hydrocarbons result in the degradation of methylene and ethylene bonds [112]. The
water content in the biomass is another factor affecting the syngas yield [113]. The
particle size of the material also influences the composition and yield of the gaseous
product. Bigger particle size inhibits the cracking reactions of hydrocarbons and
volatiles, producing less CO and H 2 and subsequently more CO 2 and light hydrocarbons [114]. Additionally, larger particles have a negative impact on the heating rate
of biomass, decreasing the gas yield for higher biochar production [89].
In terms of temperature, when there is an increment, the thermal cracking, devolatilization and degradation of biomass are stimulated. Concurrently, secondary
reactions such as dehydrogenation, decarboxylation, deoxygenation and decarbonylation favour the production of volatiles [115]. As a consequence, the pyrolysis
vapour is richer in H 2 and CO, but it contains less CO 2 and hydrocarbon gases. The
combustible gases of the pyrolytic gas (CO, H 2 , CH 4 and other hydrocarbon gases)
can be combusted as a fuel in industrial facilities, generating heat and power [94]. If
the syngas contains a high level of H 2 , it can be used for fuel cell applications.
However, high H 2 yields are not normally obtained via fast or slow pyrolysis, unlike
the intermediate and TCR technologies [116, 117].
5.3 Biomass Thermal Decomposition
Pyrolysis thermal mechanisms for the conversion of biomass can be designated in a
simultaneous combination of fragmentation, isomerization, condensation, dehydration, rearrangement, aromatization, cracking, depolymerization and char formation
[78, 118]. The components of biomass (lignin, hemicellulose, cellulose, water, inorganics and extractives) and the reactor factors have a significant influence in the
pyrolysis reactions. Figure 6 shows the chemical composition of biomass.
The pyrolysis reactions can be classified as primary and secondary.
Depolymerization, fragmentation and charring (char formation) are part of the primary reactions (Fig. 7) [55]. Depolymerization is the prevailing mechanism of
pyrolysis, and it implies the breaking of the monomer’s bonds, producing volatiles
and gases [98, 102]. Fragmentation is responsible for the formation of
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…