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Comparable to lignin, the pyrolysis of cellulose also yields four product phases, although the relative
yields and composition differ significantly. Much of the interest in cellulose pyrolysis has involved
optimisation of the yield of certain decomposition products, such as levoglucosan and other
hydrolysable sugars 59,57,58 , as well as in the area of flame retardant development as much of the
flammable gases associated with wood pyrolysis are associated with cellulose.
CO, CO2, H2O, C
O2
Glowing ignition
Cellulose
Levoglucosan
Polymers
Combustable volatiles
O2
Flaming combustion
Pyrolysis
Combustion
Figure 1.1.2.1. Schematic description of the pyrolysis and combustion of cellulose
At temperatures below 300 o C, the overall rate of cellulose pyrolysis under an inert atmosphere is quite
low 9 . For example, based on the formation of carbon dioxide and hydrogen, cellulose pyrolysis may
commence at temperatures as low as 70 o C, albeit at an extremely low rate 60 .
Differential Thermal Analysis (DTA) has revealed that when cellulose is heated, the first processes to
occur are loss of bound moisture, followed by dehydration reactions 9,43,49 . The loss of bound water
occurs at 100 o C whereas dehydration reactions occur between 170 and 220 o C depending on the nature
of the cellulose 49,61 . The initial degradation processes to occur in cellulose pyrolysis are reduction in
the degree of polymerisation, formation of free radicals, and dehydration and elimination reactions that
yield water, carbon monoxide, carbon dioxide and 1,6-anhydro-3,4-dideoxy-β-D-glycero-hex-3-
enopyranos-2-ulose (levoglucosenone) 43,61 as well as other furan derivatives 9,56,62,63 . Based on such
phenomena, the following reaction sequence has been proposed by Chatterjee and Conrad 64 for the
pyrolysis of cellulose below 300 o C:
1. Initiation: formation of free radicals
2. Propagation
3. Product formation
X-ray diffraction and viscosity measurements have revealed that a substantial reduction in cellulose
depolymerisation may occur without significant mass loss 61 . That is, in the early stages of pyrolysis,
the degree of polymerisation decreases from about 2,000 to approximately 200 with less than 15%
mass loss. The mass loss is associated with cross-linking condensation reactions within the amorphous
regions of the cellulose. A number of theories have been proposed to describe cellulose
depolymerisation. One theory suggests that bond scission occurs at “strain points” located at the
crystalline-amorphous interfaces 61,65,66 . A second theory suggests that the depolymerisation is
random 63 , whereas a third theory suggests that depolymerisation occurs along the cellulose chain in a
manner analogous to “unzipping” 67 , or a chain reaction process.
For reaction temperatures between 310 and 375 o C, the rate of mass loss from the pyrolysis of cellulose
is quite high 6,9,68 and proceeds exothermically 49 . Thermogravimetric analysis has indicated that this
process results is 26-38% conversion of the original cellulose into volatile compounds 6 . The main
processes occurring within this temperature range are rapid cleavage of the glycosidic bond
(depolymerisation), followed by various rearrangement reactions of the subsequent volatile products,
to yield 1,6-anhydro-β-D-glucopyranose (levoglucosan), 1,6-anhydro-β-D-glocofuranose, water,
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