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7.1 Emulsification
The most straightforward pathway to upgrade bio-oil is to mix it with diesel directly
to be used as a transport fuel. Generally, fast pyrolysis bio-oils are immiscible with
hydrocarbons, but the aid of a surfactant can emulsify them. It is reported that emulsified bio-oil with different ratios of diesel (25, 50 and 75 wt%) provides a more
stable bio-oil than the original [106, 149]. The use of emulsions for a long term will
affect a stainless steel engine and its sub-assemblies because of the corrosiveness of
the emulsified bio-oil. Furthermore, other disadvantages of this method are the surfactant costs and the high energy requirement [57].
7.2 Filtration
Hot vapour filtration is reported as an appropriate method to decrease the alkali
content to less than 10 ppm and ash content of bio-oil lower than 0.01 wt%. This
filtration provides higher bio-oil quality with lower biochar. Biochar is an active
catalyst, and it cracks the vapours and decreases the yield up to 20%. There is not
much information about the performance of hot vapour filters, although it should be
similar to that of hot gas filters for a gasification reaction [150]. Furthermore, filtration of a liquid which has a very small particle size (below 5μm) is not easy because
of its physical and chemical nature [150].
7.3 Polar Solvent
Polar solvents are useful for homogenizing and reducing the bio-oil viscosity for
many years. Methanol has displayed a huge effect on bio-oil stability. Furthermore,
Diebold and Czernik [151] tested different additives, ethanol, acetone and methanol. All these additives reduced the ageing rate of bio-oil. Moreover, it is reported
that the viscosity of bio-oil was decreased significantly by adding 10 wt% of methanol to bio-oil, which was the best additive [151].
7.4 Catalytic Cracking
A catalytic pyrolysis reaction can produce fuel-ranged hydrocarbons in a single step
from the biomass [152]. Zeolites or silica can be added to the biomass fast pyrolysis
reaction to convert vapours of pyrolysis into aromatic hydrocarbons in a single step
[153–155]. Zeolite catalysts are the most popular for upgrading bio-oil through
catalytic pyrolysis, and these include HZSM-5, zeolite beta, SAPO-34, zeolite Y,
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…
7.1 Emulsification
The most straightforward pathway to upgrade bio-oil is to mix it with diesel directly
to be used as a transport fuel. Generally, fast pyrolysis bio-oils are immiscible with
hydrocarbons, but the aid of a surfactant can emulsify them. It is reported that emulsified bio-oil with different ratios of diesel (25, 50 and 75 wt%) provides a more
stable bio-oil than the original [106, 149]. The use of emulsions for a long term will
affect a stainless steel engine and its sub-assemblies because of the corrosiveness of
the emulsified bio-oil. Furthermore, other disadvantages of this method are the surfactant costs and the high energy requirement [57].
7.2 Filtration
Hot vapour filtration is reported as an appropriate method to decrease the alkali
content to less than 10 ppm and ash content of bio-oil lower than 0.01 wt%. This
filtration provides higher bio-oil quality with lower biochar. Biochar is an active
catalyst, and it cracks the vapours and decreases the yield up to 20%. There is not
much information about the performance of hot vapour filters, although it should be
similar to that of hot gas filters for a gasification reaction [150]. Furthermore, filtration of a liquid which has a very small particle size (below 5μm) is not easy because
of its physical and chemical nature [150].
7.3 Polar Solvent
Polar solvents are useful for homogenizing and reducing the bio-oil viscosity for
many years. Methanol has displayed a huge effect on bio-oil stability. Furthermore,
Diebold and Czernik [151] tested different additives, ethanol, acetone and methanol. All these additives reduced the ageing rate of bio-oil. Moreover, it is reported
that the viscosity of bio-oil was decreased significantly by adding 10 wt% of methanol to bio-oil, which was the best additive [151].
7.4 Catalytic Cracking
A catalytic pyrolysis reaction can produce fuel-ranged hydrocarbons in a single step
from the biomass [152]. Zeolites or silica can be added to the biomass fast pyrolysis
reaction to convert vapours of pyrolysis into aromatic hydrocarbons in a single step
[153–155]. Zeolite catalysts are the most popular for upgrading bio-oil through
catalytic pyrolysis, and these include HZSM-5, zeolite beta, SAPO-34, zeolite Y,
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…