Environmental Engineering Reference
In-Depth Information
Most research on torrefaction of various biomasses has resulted in the same
conclusions. Mild torrefaction temperatures (240
C) produce biomass that is
hydrophobic in nature (Arias et al.
2007
), while the resulting heating value yield
remains relatively high. Residence temperature has been shown to have a much
greater effect upon the chemical changes than residence time. Previous work (Arias
et al.
2007
) has indicated that residence times beyond 1 h have no signi
250
°
-
cant impact
upon the fuel properties of biomass. The weight loss that occurs from mild torre-
faction is highly dependent upon the hemicellulose fraction of the biomass being
torre
C.
Limited studies were done on the torrefaction capability of CO
2
. Considering the
temperature limits for Boudouard reaction, the effect of using CO
2
under the pre-
treatment conditions (200
ed due to the thermal degradation of hemicellulose occurring below 280
°
C) should be studied further (Thanapal et al.
2014
).
Thermogravimetric studies have been done to extract the kinetic constants:
activation energy and pre-exponential factor from the biomass pyrolysis data. A
number of studies have focused on determining the effect of heating rate on
pyrolysis of biomass (Biagini et al.
2006
; Vamvuka et al.
2003
). Different methods
have been used to determine the kinetic constants to predict the release of volatile
matter from biomass. Some of the common methods which are used to determine
the reaction kinetics includes Broido
300
°
-
zadeh model for the pyrolysis of cel-
lulose (Bradbury et al.
1979
), Ozawa (Ozawa
1965
), single reaction conventional
Arrhenius (Chen
2012
), independent parallel reactions (Hu et al.
2007
; Wang et al.
2012b
;M
-
Sha
ros et al.
2004
; Manya et al.
2003
), successive reactions (Varhegyi
et al.
1989
), and distributed activation energy method (Anthony et al.
1976
; Sonobe
and Worasuwannarak
2008
). The kinetic parameters have been determined for
individual components of biomass materials: hemicellulose (Peng and Wu
2010
),
cellulose (Antal and Varhegyi
1995
), lignin (Faix et al.
1988
; Ferdous et al.
2002
),
and extractives using the above-mentioned methods. A comprehensive review by
Di Blasi (
2008
) gives detailed information on studies done on the pyrolysis of
biomass including different models for pyrolysis process.
Kinetics of pyrolysis of biomass and other fuels are useful for modeling the
combustion reactions occurring within a burner. Also such kinetics can be used to
determine the amount of mass loss which occurs during thermal pretreatment
processes such as torrefaction. Limited studies have focused on utilizing the
kinetics extracted from the pyrolysis of biomass constituents on the modeling of
torrefaction. Prins et al. (
2006c
) used a two-step reaction mechanism to model the
torrefaction of willow in the temperature range of 200
é
sz
á
C. Repellin et al.
(
2010
) used three models to predict the mass loss during the torrefaction process. A
simple model based on global weight loss kinetics, Di-blazi Lanzetta two-step
reaction model, and Rousset model to study the torrefaction process.
In addition to reduced transportation costs, the improved quality results in lower
CO
2
emission per unit amount of energy released. While biomass is considered as a
renewable fuel and as such CO
2
emitted during combustion is not considered as
emission, the production of such biomass fuel (e.g., ethanol) still involves use of
fossil energy which releases CO
2
. Thus, it is of interest to formulate a rating system
for all fuels including renewable biomass fuels which will directly yield CO
2
emitted
300
°
-
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