13
hydrophobic features make this solid residue more resistant in terms of water
absorption [69, 70]. Torrefaction is also considered as a biomass pre-treatment for
pelletization, combustion, pyrolysis and gasification, originating a syngas with
lower tar content and making these thermochemical processes more efficient and
more comfortable to operate [71].
Intermediate pyrolysis works with the modest heating rates (100–500 °C/min),
gentle temperatures (about 400 °C), the vapour residence time in seconds, the solid
residence time in minutes and effective cooling system for the organic gases to
reduce thermal post-degradation. Moderately, intermediate pyrolysis produces
equal yields of gas, solids and liquids. Intermediate pyrolysis can produce energy
vectors with enhanced physicochemical properties which depend on process parameters and feedstock [72, 73]. In comparison with other pyrolysis technologies, this
procedure is capable of pyrolysing fine particles (from chopping, shredding and
grinding) and also bulky feedstocks (briquettes, pellets and chips) [73]. Furthermore,
intermediate pyrolysis can convert a diverse range of feedstocks (such as sewage
sludge, algae, grass, food and market waste, agricultural waste, industrial and forest
residues, de-inking sludge, digestate and others) into valuable products [54, 74].
Fast pyrolysis is a continuous process, and it requires a short vapour residence
time and high heating rate (up to 1000 °C/s), which produces a high amount of liquids (on a dry basis, 75 wt%) [73, 75]. However, woody biomass fast pyrolysis liquid is a mixture of bio-oil and aqueous phase, which cannot be separated easily. Fast
pyrolysis bio-oil has a low calorific value (17 MJ/kg) and high water (25 wt%),
oxygen (38 wt%) and viscosity (40–100 mm
2
/s) [57]. During fast pyrolysis, the
organic material degrades very fast, forming char and pyrolysis gases, which are
condensed, generating the bio-oil and non-condensable gaseous compounds considered as remaining products [76]. The aim of fast pyrolysis is to avoid extra breaking
of the pyrolysis products into non-condensable elements to maximize the liquid
yields (the desirable product) controlling the following process conditions [57,
77, 78]:
• Very high heating rate is up to 1000 °C to raise the heat transfer on the feedstock
particles.
• The reaction temperature is about 500 °C and vapour temperature between 400
and 450 °C. These conditions maximize the yield of bio-oil at the expense of
permanent gases and char.
• Short solid and vapour residence times are usually less than 2 s to reduce secondary reactions.
• Fine biomass particle size is normally less than 3 mm to guarantee quick reactions, higher thermal conductivity and heat transfer in fluidized bed reactors.
• Feed moisture content is below 10 wt% to decrease the water in the bio-oil.
• High condensation efficiency is needed for organic gases to reduce thermal postdegradation and increase bio-oil formation and collection.
• Quick extraction of the biochar is required to reduce vapour cracking.
Theoretically, fast pyrolysis is able to convert any kind of biomass. However, this
process has only shown consistent and reasonable results using woody feedstocks
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…
hydrophobic features make this solid residue more resistant in terms of water
absorption [69, 70]. Torrefaction is also considered as a biomass pre-treatment for
pelletization, combustion, pyrolysis and gasification, originating a syngas with
lower tar content and making these thermochemical processes more efficient and
more comfortable to operate [71].
Intermediate pyrolysis works with the modest heating rates (100–500 °C/min),
gentle temperatures (about 400 °C), the vapour residence time in seconds, the solid
residence time in minutes and effective cooling system for the organic gases to
reduce thermal post-degradation. Moderately, intermediate pyrolysis produces
equal yields of gas, solids and liquids. Intermediate pyrolysis can produce energy
vectors with enhanced physicochemical properties which depend on process parameters and feedstock [72, 73]. In comparison with other pyrolysis technologies, this
procedure is capable of pyrolysing fine particles (from chopping, shredding and
grinding) and also bulky feedstocks (briquettes, pellets and chips) [73]. Furthermore,
intermediate pyrolysis can convert a diverse range of feedstocks (such as sewage
sludge, algae, grass, food and market waste, agricultural waste, industrial and forest
residues, de-inking sludge, digestate and others) into valuable products [54, 74].
Fast pyrolysis is a continuous process, and it requires a short vapour residence
time and high heating rate (up to 1000 °C/s), which produces a high amount of liquids (on a dry basis, 75 wt%) [73, 75]. However, woody biomass fast pyrolysis liquid is a mixture of bio-oil and aqueous phase, which cannot be separated easily. Fast
pyrolysis bio-oil has a low calorific value (17 MJ/kg) and high water (25 wt%),
oxygen (38 wt%) and viscosity (40–100 mm
2
/s) [57]. During fast pyrolysis, the
organic material degrades very fast, forming char and pyrolysis gases, which are
condensed, generating the bio-oil and non-condensable gaseous compounds considered as remaining products [76]. The aim of fast pyrolysis is to avoid extra breaking
of the pyrolysis products into non-condensable elements to maximize the liquid
yields (the desirable product) controlling the following process conditions [57,
77, 78]:
• Very high heating rate is up to 1000 °C to raise the heat transfer on the feedstock
particles.
• The reaction temperature is about 500 °C and vapour temperature between 400
and 450 °C. These conditions maximize the yield of bio-oil at the expense of
permanent gases and char.
• Short solid and vapour residence times are usually less than 2 s to reduce secondary reactions.
• Fine biomass particle size is normally less than 3 mm to guarantee quick reactions, higher thermal conductivity and heat transfer in fluidized bed reactors.
• Feed moisture content is below 10 wt% to decrease the water in the bio-oil.
• High condensation efficiency is needed for organic gases to reduce thermal postdegradation and increase bio-oil formation and collection.
• Quick extraction of the biochar is required to reduce vapour cracking.
Theoretically, fast pyrolysis is able to convert any kind of biomass. However, this
process has only shown consistent and reasonable results using woody feedstocks
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…