16
Energy production is also one of the ways to valorize the biochar from pyrolysis.
It can be a solid fuel for combustion, fulfilling the heating necessities for pyrolysis.
Biochar can also be gasified to produce syngas/hydrogen [94, 95]. Some works
explored the biochar microscopic surface area for the elimination of contaminants
in gas and water by the adsorption of metal ions from water and the removal of SO 2
or NO x present in the gas [96, 97]. Additionally, biochar can be used as a catalyst in
relation to its inorganic compositions and surface properties [98].
5.2.2 Bio-oil
Bio-oil is a dark brown organic liquid produced from the pyrolytic conversion of
biomass, and it is the most interesting energy vector in the pyrolysis process [89].
This oil is formed through a quick condensation of the organic vapours, and it
results in the thermal breaking and depolymerization of lignin, hemicellulose and
cellulose [99]. Bio-oil is composed of water, nitrogen and hundreds of chemical
species such as esters, alkenes, aldehydes, ethers, ketones, acids, furans, alcohols,
sugars, phenols and other oxygenated compounds, making this pyrolysis product
reactive and thermodynamically unstable [100, 101]. For that reason, it is essential
to upgrade the bio-oil for fuel engine applications.
Viscosity is a relevant requirement for the pyrolysis oil, impacting the pumping
performance of the engine. According to some studies, the viscosity of the oil should
be no more than 20 mm
2
/s at 40 °C [102]. The bio-oil properties are affected by
some processing conditions such as solid residence time, heating rate, temperature,
kind of biomass and particle size of the feed. Some secondary pyrolysis reactions
produce water, which is present in the bio-oil composition, creating two fractions (a
mixture of the organic phase and aqueous phase). The aqueous phase is a combination of water-soluble compounds (mostly phenol, acetic acid and hydroxyl acetone)
and due to its limited chemical properties (such as heating value) cannot be applied
as a fuel vector [103].
It is reported that catalytic reforming implementation in intermediate pyrolysis
process can separate the organic phase from the aqueous phase and also improve the
heating value of biofuel in pyrolysis [104]. A catalytic treatment via reforming,
which promotes water gas shift reactions, helps to valorize the aqueous phase, producing green hydrogen [105]. The organic fraction can be exploited as a fuel
(directly or upgraded to improve the oil quality) or for the production of chemical
compounds. Generation of heat and power through gas turbines/engines, furnaces,
combustors and boilers is one of the applications for the organic phase of the bio-oil
without catalytic conversion which has shown success [106, 107].
For transportation purposes, the pyrolysis oil can be treated via either the upgrading processes (such as hydrodeoxygenation (HDO), emulsification, hydrocracking
and catalytic esterification) or mixing with gasoline and diesel to be used in engines
[94]. Valuable chemicals can also be extracted in the organic phase to be applied in
flavouring additives, pharmaceutical compounds, preservatives, fertilizing materials
and resin components [80, 108].
H. Jahangiri et al.
Energy production is also one of the ways to valorize the biochar from pyrolysis.
It can be a solid fuel for combustion, fulfilling the heating necessities for pyrolysis.
Biochar can also be gasified to produce syngas/hydrogen [94, 95]. Some works
explored the biochar microscopic surface area for the elimination of contaminants
in gas and water by the adsorption of metal ions from water and the removal of SO 2
or NO x present in the gas [96, 97]. Additionally, biochar can be used as a catalyst in
relation to its inorganic compositions and surface properties [98].
5.2.2 Bio-oil
Bio-oil is a dark brown organic liquid produced from the pyrolytic conversion of
biomass, and it is the most interesting energy vector in the pyrolysis process [89].
This oil is formed through a quick condensation of the organic vapours, and it
results in the thermal breaking and depolymerization of lignin, hemicellulose and
cellulose [99]. Bio-oil is composed of water, nitrogen and hundreds of chemical
species such as esters, alkenes, aldehydes, ethers, ketones, acids, furans, alcohols,
sugars, phenols and other oxygenated compounds, making this pyrolysis product
reactive and thermodynamically unstable [100, 101]. For that reason, it is essential
to upgrade the bio-oil for fuel engine applications.
Viscosity is a relevant requirement for the pyrolysis oil, impacting the pumping
performance of the engine. According to some studies, the viscosity of the oil should
be no more than 20 mm
2
/s at 40 °C [102]. The bio-oil properties are affected by
some processing conditions such as solid residence time, heating rate, temperature,
kind of biomass and particle size of the feed. Some secondary pyrolysis reactions
produce water, which is present in the bio-oil composition, creating two fractions (a
mixture of the organic phase and aqueous phase). The aqueous phase is a combination of water-soluble compounds (mostly phenol, acetic acid and hydroxyl acetone)
and due to its limited chemical properties (such as heating value) cannot be applied
as a fuel vector [103].
It is reported that catalytic reforming implementation in intermediate pyrolysis
process can separate the organic phase from the aqueous phase and also improve the
heating value of biofuel in pyrolysis [104]. A catalytic treatment via reforming,
which promotes water gas shift reactions, helps to valorize the aqueous phase, producing green hydrogen [105]. The organic fraction can be exploited as a fuel
(directly or upgraded to improve the oil quality) or for the production of chemical
compounds. Generation of heat and power through gas turbines/engines, furnaces,
combustors and boilers is one of the applications for the organic phase of the bio-oil
without catalytic conversion which has shown success [106, 107].
For transportation purposes, the pyrolysis oil can be treated via either the upgrading processes (such as hydrodeoxygenation (HDO), emulsification, hydrocracking
and catalytic esterification) or mixing with gasoline and diesel to be used in engines
[94]. Valuable chemicals can also be extracted in the organic phase to be applied in
flavouring additives, pharmaceutical compounds, preservatives, fertilizing materials
and resin components [80, 108].
H. Jahangiri et al.