Environmental Engineering Reference
In-Depth Information
Hemicellulose, cellulose, and lignin are the main constituents that comprise the
cell structure of lignocellulosic biomass. Changes in the percent of each constituent
cause variation in the results obtained from the torrefaction of different biomasses.
Recently, there has been a signi
cant amount of research done on how the com-
position of lignocellulosic biomass in
uences the thermal degradation. Chen and
Kuo (
2010a
,
b
) and Chen et al. (
2011
) have completed studies on the effect of
torrefaction temperature on the lignocellulosic structure of several different bio-
masses. The effect torrefaction has upon the three main constituents is of particular
interest due to the chemical makeup of hemicellulose, cellulose, and lignin.
Understanding the chemical behavior of these components is integral in optimizing
the torrefaction process. Demirba
fl
(
2005
) investigated the percent of each of the
three main constituents of lignocellulosic biomass and their contribution to the
HHV of the fuel. From the analysis of the structural composition, it was determined
that, in general, an increase in the amount of lignin in a lignocellulosic biomass
results in an increase in HHV. Elemental composition analysis of the lignin portion
of each biomass indicated that lignin has an increased carbon content and decreased
oxygen content when compared to hemicellulose and cellulose. This decrease in the
O/C ratio between lignin and the other two constituents (hemicellulose and cellu-
lose) is what causes lignin to have a higher HHV. Based upon the data obtained, it
was determined that there is no direct relation between the hemicellulose and
cellulose content of a biomass and its HHV. However, there is a good relation
between the amount of lignin within a biomass and the HHV of the biomass. The
importance of lignin becomes magni
ş
ed biomass due to most research
(Arias et al.
2007
; Chen and Kuo
2010a
; Prins et al.
2006b
), suggesting a mild
torrefaction process (240
ed in torre
250
°
C) for the pretreatment of biomass. Mild torrefac-
-
tion, which causes a signi
cant breakdown in hemicellulose, a moderate breakdown
in cellulose, leaves lignin as the main contributor to the HHV of the resulting
biomass.
The thermal breakdown of the lignocellulosic components of biomass was
further studied by Wu et al. (
2009
) using a TGA. Three different compounds were
used to model the three major components of biomass: xylan (a major component
of hemicellulose), avicel (cellulose), and alkali lignin. Ultimate analysis of the three
substances showed similar results compared to other literature (Demirba
ş
2005
),
with lignin having a lower oxygen
carbon ratio than cellulose and hemicellulose.
Weight trace curves and DTG analysis of the data clearly showed the thermal
breakdown of each component. The majority of hemicellulose breakdown occurred
between 210 and 320
-
°
C with minimal weight loss occurring beyond 320
°
C.
Cellulose weight loss occurred predominantly between 310 and 390
C with almost
no weight loss occurring outside that temperature range. Lignin showed the widest
range of thermal degradation with weight loss occurring between 200 and 550
°
°
C.
Related research done by Yang et al. (
2007
) on the pyrolytic behavior of hemi-
cellulose, cellulose, and lignin had similar results, with the exception of lignin
continuing to breakdown up to 900
°
C. Since torrefaction occurs within the tem-
perature range of 200
C, thermal degradation of hemicellulose, cellulose, and
lignin will occur during torrefaction, as previously shown (Chen and Kuo
2010b
).
300
°
-
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