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The study of low temperature/long residence time hemicellulose pyrolysis is complicated when
cellulose is present. Under such conditions the hemicellulose content of wood may appear to remain
constant. This apparent contradiction arises because cellulose is gradually degraded into hexosan units,
which are then associated with the hemicellulose fraction upon analysis 91 . Despite such difficulties, a
number of reaction pathways have been proposed for the pyrolysis of xylan. In summary, such
processes, which are dependent on reaction conditions, include dehydration, disproportionation,
random condensation and direct fragmentation of the glycosyl unit derived from the thermal cleavage
of the glycosidic bond 89,92,93 .
The pyrolysis of hemicellulose involves random dehydration reactions, and cleavage of glycosidic
bonds via a transglycosylation reaction, to yield monomeric glycosyl type compounds in a manner
analogous to that of cellulose pyrolysis 11,9 . Condensation of the glycosyl residues may then occur,
yielding oligosaccharidic material (tar), or decompose further yielding low molecular weight volatile
compounds 9,93,94 . Such volatile compounds include furan derivatives, such as furfural, which are
generally associated with dehydration of carbohydrates and therefore may be obtained in significant
quantities from both cellulose and hemicellulose 95,96 . Evidence generated from model compound
studies has also revealed that furfural, and numerous other compounds, may be produced from
decomposition of oligomeric material (tar) obtained from condensation of glycosyl units from both
cellulose and hemicellulose pyrolysis 89,97 . Thus, furfural may be derived from the pyrolysis of wood
carbohydrates via at least two reaction pathways:
1. Glycosidic cleavage followed by dehydration
2. Decomposition of condensation products derived from transglycosylation reactions.
A mechanism for the formation of furfural has been described 92 . The mechanism involves formation of
3-deoxy-pentosone, a stable intermediate, which then undergoes dehydration to yield furfural, or 5-
hydroxy-2-furfuraldehyde if the starting material was a hexose 98 .
Another type of reaction involved in hemicellulose pyrolysis is fragmentation of glycosyl units. It has
been reported that the rate of such fragmentation reactions increase more rapidly with increasing
temperature compared to the rate of dehydration reactions, leading to the expectation that
fragmentation predominates at higher temperatures 68 . Thus, it would be expected that the yield of
furfural would be maximal at relatively low temperatures. Moreover, as the rate of cellulose pyrolysis
is not significant below 300 o C, the yield of furfural from cellulose decomposition will be low at all
temperatures. This is because at higher temperatures, where the rate of cellulose pyrolysis is
significant, the rate of glycosyl fragmentation is high compared to its rate of dehydration.
Under fast pyrolysis conditions, hemicellulose may be pyrolysed at temperatures exceeding 500 o C.
However, at lower heating rates, the reaction temperature is considerably less, regardless of the applied
temperature 68 . Moreover, high rates of heating result in lower yields of char, probably due to the
predominance of transglycosylation and fragmentation reactions 99 .
The pyrolysis of hemicellulose under nitrogen and oxygen enriched atmospheres has been investigated
by DTA. The presence of oxygen does not seem to alter the temperature at which exothermic events
occur, although it does result in an additional exothermic process 21,49 . Overall, the influence of
atmosphere composition on the pyrolysis of hemicelluloses has received little attention.
1.1.4 Pyrolysis research in Australia
In Australia, very little research has been performed on the derivation of chemicals from the pyrolysis
of native species. In fact, there is relatively little interest in biomass pyrolysis research within
Australia. This is probably due to lack of incentive caused by low petrochemical commodity prices
combined with Australia’s relatively lax greenhouse gas and renewable energies policies.
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