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pyrolysis (heating rate 10–200 °C/s, residence time 0.5–10 s), and flash pyrolysis
(heating rate >1000 °C/s, residence time <0.5 s). Slow pyrolysis is traditionally used
for maximizing solid yield, whereas fast, as well as flash, pyrolysis is preferred for
bio-oil yield [1]. Pyrolysis reduces the H/C ratio of the products against
hydrogenation that improves the H/C ratio. The reactions that take place during
pyrolysis are depolymerization, dehydration, decarboxylation, esterification,
condensation, cyclization, etc. The bio-oil obtained by biomass pyrolysis is a
complex mixture of organic acids, ketones, esters, and aromatic compounds.
Pyrolysis oil is acidic in nature and contains aqueous as well as organic phases.
Hence, further treatment is necessary to get useful fuel-like products. The major
factors that affect the progress of pyrolysis are biomass heating rate, operating
temperature, residence time, and the catalyst [9]. The pyrolysis process can be
improved by proper selection of heating rate, operating temperature, catalyst, and
residence time.
Catalyst helps in enhancing the rate of cracking of higher molecules into the
smaller ones. Each catalyst exhibits its own kinetics that results in varied product
distribution under a different set of operating parameters. The catalytic pyrolysis
process can be classified into three different groups, viz., catalyst pre-added in the
feedstock, catalyst added into the primary reactor, and catalyst in the secondary
reactor located downstream to the primary reactor. The heterogeneous catalysts
used in the pyrolysis process are divided into four catalytic groups, viz., dolomite
type, Ni-type, alkali metal, and noble metal catalysts [12]. A bimetallic catalyst is a
promising option for biomass upgrading using pyrolysis. Fast pyrolysis is a proven
technology to convert renewable feedstocks to an energy-rich liquid product
(~17 MJ/kg). Fast pyrolysis utilizes heating of biomass at a high rate and rapid
cooling of vapor (residence time <2 s), thereby minimizing the secondary cracking
reactions and polymerization. Fast pyrolysis is two to three times more economical
than liquefaction and gasification but exhibits a complex reaction mechanism.
Catalytic pyrolysis is preferred as it increases the reaction rate and exhibits selectivity toward the desired product. Acid and base, in homogeneous as well as heterogeneous forms, are used as catalysts. The pretreatment of the biomass also has
positive effects on pyrolysis. The widely used catalysts for pretreatment of biomass
are H 2 SO 4 , HCl, H 3 PO 4 , and Lewis acids. For example, pinewood sawdust was
treated with phosphoric acid before being subjected to fast pyrolysis, which
increased the formation of levoglucosan content. Levoglucosan was further
converted to syngas [13]. In the case of supported heterogeneous catalyst, the
activity change with the change of support was evident from the pyrolysis of
cellulose to produce 5-methyl furfural at 773–1073 K. The catalyst SO 4
2−
supported
on TiO 2 yield furfural, while SO 4
2−
supported on ZrO 2 yield furan [9]. The solvent
also plays an important role in pyrolysis by dissolving lignocellulosic biomass or
reaction intermediates and thereby increases the yield of products. It was observed
that the yield of levoglucosenone and furfural is found to increase when pyrolysis of
cellulose is carried out at 473 K for 6 min using H 2 SO 4 as a catalyst and tetramethyl
sulfone (C 4 H 8 O 2 S) as a solvent [14]. The acid and alkaline catalysts used in pyrolysis
exhibit a corrosive effect and have its effect on the stability of the bio-oil. Hence, the
R. Bhoi et al.
pyrolysis (heating rate 10–200 °C/s, residence time 0.5–10 s), and flash pyrolysis
(heating rate >1000 °C/s, residence time <0.5 s). Slow pyrolysis is traditionally used
for maximizing solid yield, whereas fast, as well as flash, pyrolysis is preferred for
bio-oil yield [1]. Pyrolysis reduces the H/C ratio of the products against
hydrogenation that improves the H/C ratio. The reactions that take place during
pyrolysis are depolymerization, dehydration, decarboxylation, esterification,
condensation, cyclization, etc. The bio-oil obtained by biomass pyrolysis is a
complex mixture of organic acids, ketones, esters, and aromatic compounds.
Pyrolysis oil is acidic in nature and contains aqueous as well as organic phases.
Hence, further treatment is necessary to get useful fuel-like products. The major
factors that affect the progress of pyrolysis are biomass heating rate, operating
temperature, residence time, and the catalyst [9]. The pyrolysis process can be
improved by proper selection of heating rate, operating temperature, catalyst, and
residence time.
Catalyst helps in enhancing the rate of cracking of higher molecules into the
smaller ones. Each catalyst exhibits its own kinetics that results in varied product
distribution under a different set of operating parameters. The catalytic pyrolysis
process can be classified into three different groups, viz., catalyst pre-added in the
feedstock, catalyst added into the primary reactor, and catalyst in the secondary
reactor located downstream to the primary reactor. The heterogeneous catalysts
used in the pyrolysis process are divided into four catalytic groups, viz., dolomite
type, Ni-type, alkali metal, and noble metal catalysts [12]. A bimetallic catalyst is a
promising option for biomass upgrading using pyrolysis. Fast pyrolysis is a proven
technology to convert renewable feedstocks to an energy-rich liquid product
(~17 MJ/kg). Fast pyrolysis utilizes heating of biomass at a high rate and rapid
cooling of vapor (residence time <2 s), thereby minimizing the secondary cracking
reactions and polymerization. Fast pyrolysis is two to three times more economical
than liquefaction and gasification but exhibits a complex reaction mechanism.
Catalytic pyrolysis is preferred as it increases the reaction rate and exhibits selectivity toward the desired product. Acid and base, in homogeneous as well as heterogeneous forms, are used as catalysts. The pretreatment of the biomass also has
positive effects on pyrolysis. The widely used catalysts for pretreatment of biomass
are H 2 SO 4 , HCl, H 3 PO 4 , and Lewis acids. For example, pinewood sawdust was
treated with phosphoric acid before being subjected to fast pyrolysis, which
increased the formation of levoglucosan content. Levoglucosan was further
converted to syngas [13]. In the case of supported heterogeneous catalyst, the
activity change with the change of support was evident from the pyrolysis of
cellulose to produce 5-methyl furfural at 773–1073 K. The catalyst SO 4
2−
supported
on TiO 2 yield furfural, while SO 4
2−
supported on ZrO 2 yield furan [9]. The solvent
also plays an important role in pyrolysis by dissolving lignocellulosic biomass or
reaction intermediates and thereby increases the yield of products. It was observed
that the yield of levoglucosenone and furfural is found to increase when pyrolysis of
cellulose is carried out at 473 K for 6 min using H 2 SO 4 as a catalyst and tetramethyl
sulfone (C 4 H 8 O 2 S) as a solvent [14]. The acid and alkaline catalysts used in pyrolysis
exhibit a corrosive effect and have its effect on the stability of the bio-oil. Hence, the
R. Bhoi et al.