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7/23/2019 A Model for Primary and Heterogeneous Second
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272
Chem. Eng. Technol. 19 (1996) 272-282
A Mo del for Prima ry a nd Heterogeneous Secondary Reactions
of
Wood Pyrolysis
Pradeep Ahuja, Surendra Kumar and Prem Chand Singh*
A ch ain growth model for heterogeneous second ary reactions is developed for the pyrolysis
of la rge wood par t ic les and the parameters de termined by nonl inear op t imizat ion . The
model takes both the volati le retention time and cracking and repolymerization reactions
of
the vapours w ith the decomposing solid as well as autocatalysis into consideration. Th e
extent of the secondary reactions is s trongly influenced by the t ime and the ratio of the
autocatalytic (propagation) reaction rate to noncatalytic ( init iation) reaction rate. The
woo d which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exo-
thermic heat of reaction and yields a higher amount of f inal char residue. T his fact con-
firms that the heterogeneous secondary reactions lead to carbon enrichment of the f inal
residue and are accompanied with an exothermic heat
of
reaction. The lower activation
energies of the in i t ia t ion and p ropagat ion reactions as compared to pr imary reactions (com-
petitive reaction model consisting of weight loss and char forming reactions ) confi rm a uto -
catalysis in large particles . The sealed reactor s tudies of small quantit ies of f ine wood
samples show that heterogeneous secondary reactions and not lower heating rates in large
particles are the main source of char formed dur ing the thermal decompos i t ion of la rge
woo d particles. The m odel predictions are in agreement with the weight loss an d tempera-
ture versus t ime curves over a wide range of particle s ize and furnace temperatures .
1 Introduction
Secondary vapour-solid interactions are the main source of
char form ed durin g biomass pyrolysis. I t has even been sug-
gested that char formation is not a primary s tep but is a
result of repolymerization of volati le m atter [I ] . Th e
vapo ur-solid in teraction s play a key role in the pyrolysis be-
haviour of macroscopic particles of biomass . The tempera-
ture profiles within
a
pyrolyzing solid particle are influenc-
ed not only by heat transfer phenomena but also by hetero-
geneous chemical reactions.
The least understood aspect of pyrolysis is the inte racti on of
the nascent, hot pyrolysis vapours with the decomposing
solid, which the va pours m ust transverse during their escape
to the environment [2] . Moreover, solid phase decomposi-
t ions are a challenging and poorly understood interdisci-
plinary field of inquiry. A ntal an d Varhegyi [2] conclude
that vapour-solid interactions (secondary reactions) are ef-
fectively the only source of char formed during the pyroly-
sis
of
pure cellulose. These reactions ar e key to the eff icient
(high yield) manufacture
of
charcoal f rom biomass . Mok
an d Antal [3] attr ibuted the increase in char yield a nd exo-
thermic heat
of
reaction at low purge gas f low rate to the
format ion
of
a volatile intermediate, which reacted by a
s t rong exotherm when i t was no t removed f rom the
DSC
cell by a high f low of purge gas .
*
P.
Ahuja, Prof. Dr. S. Kumar (to whom correspondence should be ad-
dressed), and Prof.
Dr.
P.C. Singh, Department of Chemical
Engineering and Technology, Institute
of
Technology, Banaras Hindu
University, Varanasi-221005, India.
Yields as high as 47% have been achieved by control
of
those conditions which affect the vapours born during py-
rolysis of the solid substrate [4] . High charcoal yields are
only ob ta ined when the vapours a re he ld in contac t wi th the
hot solid residue. Mok et al . [4] bserved that higher sam ple
loading increased charcoal yield and the associated exo-
thermic heat release an d lowered th e reaction onset temper-
a ture . The lower ing
of
reaction onset tempera ture may be
due t o the au tocata ly t ic ef fec t of the secondary reactions.
I t has been suggested by Akita [5] that th e primary decom -
position of wood has a very low heat of reaction an d tha t
the main cause of heat generation in decomposing wood is
secondary decomposition
of
volatile matter, possibly
catalysed by the solid residue. Th us, the heat of reaction in-
creases with increasing residence time of volati le matter in
the solid prior to evolution. Thus, under gentler heating
condition s as in the case of large particles , the expulsion of
volati le matter once formed would be less rapid and more
time would be available for secondary decomposition [6] .
Bradbury et al . [I] found that small sample s izes lead to
smaller char fractions. This indicated that the residence
time
of
volatiles in the cellulose during the pyrolysis reac-
tion largely influences the extent of char form at ion . Pyro ly-
sis
of
levoglucosan is known to give some residual char, an d
it has been suggested by Lewellen et al. [7] that char forma-
tion is not a primary s tep but is a result of repolymerization
of volati le matter . Arseneau [8] report that the depolymer-
ization reaction consists of two reactions contr ibu t ing to the
endothe rm, f irs t , the depolymerization of th e residual cellu-
lose followed by the volatilization
of
mo s t of the product ,
levoglucosan, thereafter . Thicker samples trap the levoglu-
CH Verlagsgesellschaft mbH, D-69451 Weinheim, 1996
0930-7516/96/0306-0272 $10.00+ .25/0
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Chem . Eng . Technol. 19 (1996) 272-282
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cosan, thereby eliminating the latter portion of the endo-
therm. The exotherm is not the result of the decompos i tion
of the anhydrocellulose but rather is the breakdown of
the product of the depolymerization reaction, presumed to
be levoglucosan. This was confirmed with the thermogram s
of the prepared levoglucosan. The f inal reaction varies
depending upon the conditions of th e experiment. I t may be
endothe rmic or exothermic. The levoglucosan formed m ay
be trapped by the overlay of cellulose an d decom posed ex-
othermally. Th e above results suggest that a crit ical param -
eter in cellulose pyrolysis is the residence time
of
volatiles
in the pyrolyzing cellulose matrix. The existence of an opt i -
mum residence t ime for obtaining a maximum rate of vola-
tilization is indicated. It is suggested that cracking, cross-
linking, and repolymerization reactions occur if this t ime is
substantially exceeded. Low temperatures or low heating
rate conditions, particularly involving large samples , wo uld
promote longer residence times of all pr imary products
within th e pyrolyzing ma trix and tha t repolymerization and
cross-linking leads to higher charcoal yields.
Tak amo to an d Pe t r i ch [9] in their s tudy on the effect of
heterogeneous secondary pyrolysis reactions on the therm al
decomposition of polyure thane scrap , concluded tha t b o th
act iva ted carbon and polyure thane char improve the l iqu id
product by promoting heterogeneous secondary pyrolysis
reactions. The polyurethane char provides the additional
benefit of increasing the amount of char produced . Char
yields as high as 40 were obtained when polyurethane
char was used as the carbon bed material , as compared to
15 t o 25 with other bed m aterial . They pyrolysed po lyure-
thane scrap in a batch laboratory-scale reactor and used a
packed bed t o influence the com position an d yield of pyrol-
ysis prod ucts . G lass beads and glass wool were also used as
inert packing materials . Cha r deposits on these m aterials in-
dicated that volati les a dsor b on the packing. T he reactions
of wood pyrolysis volati les with wood char and activated
carbon [ l o ] have also sho wn similar results . Crac king reac-
tions, leading t o lower molecular weight prod ucts , an d de-
position reactions, leading to increased char yields , have
been observed. T he pyrolysis volati les c an ad sorb , deposit ,
and react in the presence of a second carbon bed, altering
the composition and yields of the pyrolysis products .
Mazum dar and Chat terjee [ I I ] found th e d imens ions of the
coke charge
to
be quite significant in determining volatile
yields, while heating rate was
of
secondary impor tance .
Similar f indings are reported by Anth ony e t al . [12]. The ef -
fect of sample s ize is sometimes misconstructed a s a he ating
rate effect, s ince the t ime required to attain the f inal tem-
pera ture usually decreases with increasing sam ple size.
There is a broad support that these reactions are primarily
concerned with the ta r fraction of the volati les .
The ex ten t of the secondary reactions is s trongly influenced
by the residence t ime and concentration of reactive species
in contact with hot surfaces is also proved from the facts
that th e large beds accompany ing s low rate of heating yield
min imu m amo u n t s of volati les and devolati l ization at
higher pressures increases
the
secondary
decomposition
[12]. An th o n y an d Ho war d [I31 in fer tha t som e of the frag-
ments a re highly reactive free radicals subject to a variety
of secondary reactions such as cracking and repolymeriza-
t ion . Shaf izadeh and Bradbury [I41 also predict free radi-
cals in their work. The extent
of
secondary reactions can be
reduced by enhancing the transport of volatile fragments
away from the reactive environment, such as by operating
at redu ced pressures w ith smaller and m ore widely dispersed
particles . Th e free radical species would polymerize to coke
unless they could escape as tar at some intermediate s tage
of
polymerization and that the escaping tar might also
undergo cracking reactions producing coke and gas. Pre-
sumably, both cracking and polymerization are involved,
but their relative extents are une stablished.
Chen et al. [15] formula te a model for coal pyrolysis in
which the volati le tar can transport out of the char matrix
and u ndergoes cracking to yield m olecules with lower mo-
lecular weights. After the release of primary tar , active
char, which contains solid char and condensed tar , under-
goes relatively slow chemical decomposition and evapora-
tion, thereby generating additional tar molecules of dif-
ferent molecular weights. After leaving the char matrix, tar
molecules can recondense on the char surface followed by
interphase reactions. Moreover, the tar molecules with
lower molecular weights yield gaseous produc ts throug h py-
rolysis . Production of gaseous species can also take place
directly from char, for example, hydrogen evolves from
aromat iza t ion of char , and carbon d ioxide format ion
precedes tar release. Chan et al. [16] have also shown in-
crease char and gases yields and lower tar yields when the
particle sizes were increased.
Ram an e t a l .
[17]
report that th e f irs t s tep during pyrolysis
is devolati lization, which produce s volati le matter and char.
The second s tep consis ts of secondary reactions involving
the evolved volati les and the solid char. The f inal product
dis tr ibution for gasif ication is dictated by bo th th e devola-
ti lization an d the secondary reactions. T hus, the engineer-
ing design and de velopment of a process for the gasif ication
will require
a
basic knowledge
of
kinetic da ta o n the devola-
ti l ization as well as secondary reactions involved. This
becomes important considering the fact that in some reac-
tors (moving bed, rotary furnace) used in gasif ication and
pyrolysis processes, large particle sizes are processed. In
these cases , heat an d mass tra nsfer resistances m ay appe ar.
Signif icant temp erature profiles may be produced inside the
solid which influence the solid conversion and the amount
and compos i tion of the products ob ta ined .
Since the f irs t work by B amfo rd et al . [18], several models
have been proposed for the thermal decomposition of
wo o d . Mo s t of them [6, 19
-
31 was mod ified by Kung [24]
and Bilbao et al. [25] to incorpora te the ef fec ts of internal
convection; Kansa et
al.
[26] subsequently included the
momentum equation for the motion of the pyrolysis gases
within the solid and the influence of physical s tructure on
these phenom ena. Models which recognize therm al decom-
position as rate controlling appear mo re realis tic an d should
prove more f ru i t fu l in expIoring wider ranges of condi t ions .
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Chem. Eng. Technol.
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(1996) 272-282
T he a im of the present work is to provide a rational model
for the purpose of describing the pyrolysis of a cylindrical
wood particle. The reaction at any point in the particle is
governed by the temperature at that point and consists
of
primary and heterogeneous secondary reactions. The pri-
mary reactions in the case of small particles are studied
through TG A an d isothermal heating. A model is described
which is found t o be applicable fo r both the modes of he at-
ing for small particles. The heating ra te used in TG A is 10,
20, and 40 K/min. The isothermal temperatures of the fur-
nace are 573, 623, 673, an d 723 K. The primary reaction
mod el consists of a weight loss and char forming reactions
which are compe titive in nature .
The propose d heterogeneous secondary reaction model con-
sists of chain growth mechanism including initiation and
propagat ion s teps and the rate parameters (act ivat ion
energy, preexponential factor, and deposition coefficient)
are determined by sim ulating the complete model (primary
and heterogeneous secondary) with the experimental residu-
al weight fraction and temperature versus time curves
of
large particles. The primary reaction p aramete rs were deter-
mined by simulating the primary reaction model with the
experimental weight fraction curves of small particles. The
secondary reactions also depend on the contact time be-
tween the volatiles an d the hot char a nd this factor is also
taken up in secondary react ion model . Apa r t f rom the ad -
sorpt ion of the volatiles o n the prim ary char, the chemical
reaction between the primary char and the volatiles takes
place and leads to the secondary char formation.
To
show that vapour-sol id in teractions are mo re impo rtant
than the lower heating rates which accompany large par-
ticles, stud ies
of
small and fine samples
of
the woods were
carried out in
a
specially designed sealed reactor in which
the volatiles a nd gases remain in contact with the dec ompos-
ing solid. The autocatalytic effects are described and the
heat of reaction, ratio of autocatalytic/noncatalytic reac-
tion rates, deposition coefficient, and the final ch ar yields
of large particles and those of small particles in sealed reac-
tor are determined to compare the physical and chemical
phenomena during pyrolysis of a large cylindrical wood
particle.
2
Model Description
2. Primary Reaction Kinetic Model
The primary reactions are described by the following
scheme:
ases +Volatiles
Biomass (B)
------()
Prim ary C h a r (C,)
It is assumed that the two reactions follow the Arrhenius
law. The mo del can predict the fin al char yield in different
heating conditions. Koufopanos et al. [27] and Dumpel-
mann et
al.
[28]
also attempted simplified m odels but with
added complications of intermediatehntermediates forma-
tion. Th e ordina ry differential equations which describe the
above kinetic schem e are:
_B
-
- A l e x p ) B n l - A 2 e x p s ) B n 2 (1)
d t
with the initial conditions: B =
1 ,
C,
=
0 at t = 0. B is the
educt wood f ract ion and is the f ract ion of virgin wood re-
maining in the solid residue, and
C1
is the primary char
weight frac tion. T he residual weight frac tion is given by the
sum:
W =
B + C In dynamic thermogravimetric analysis ,
the temperature may be a l inear function of time:
T=@T+TO.
The characteristics of a good model are that it should re-
quire the least number of parameters for the engineering
design
of
the reactor an d that it should be mechanistic. The
model which predicts the weight loss and char form ation a t
low temperatures and heating rates should be applicable at
high heating rates, because even at high heating rates and
tempera tures the sample has to pass through th e low heating
rates, where most of the devolatilization occurs. Our model
is able to f i t the data under dynamic a nd isothermal heat ing
conditions.
Under t he heating rates which are considered for the engi-
neering design of the equipments , the in termediate forma-
t ion does not occur as taken up in the model by B radbury
et al. [I]. Even for low heating rates, Varahegyi et al. [29]
has shown that in termediates formation does not take
place.
The two- or three-s tep model proposed by Lipska and
Park er [30] in the case
of
isothermal condit ions an d Tang
and Neil1 [31] for dy namic T G cond itions require a lot of
param eters, an d the lower steps govern the pyrolysis at very
low heating rates which are n ot required for the engineering
design of the gasifiers an d com bustors o r pyrolysis equip-
ments in which most of the tars or char formation takes
place. Therefore, a s tep model was not taken into con-
sideration to fit the experimental data for the various
heating rates and under isothermal conditions. Moreover,
the appearance of the single smooth curve during
nonisothermal
TG
study supports the fact that the transi-
t ion f rom char dom inat ing to tar dominat ing reactions is
gradual.
The model is consistent with a pyrolysis scheme in which
two competing sequences of cellulose pyrolysis reactions a re
initiated by (1) an intermolecular dehydration leading to
char form ation and (2)
a
depolymerization reaction.
Non-linear optimization procedure utilizing Marquardt
algorithm
[32]
an d Press et al.
[33]
were used for param eter
1
List of
symbols
at
the end
of
the
paper.
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275
estimation (activation energy, pre-exponential factor , and
order of reaction). The best fitting was achieved by mini-
mizing the sum of the squares funct ion :
constants could not be used to give the rates as a funct ion
of temperature for the various isothermal conditions. So,
regression was used to f i t the rate constan ts as a function of
tempera ture .
2.2 Heterogeneous Secondary React ion Kinet ic Mo del
For s implicity, the o rder
of
the reaction 1 ) was set equal to
that of reaction (2) . In order to avoid poor parameter es ti-
mat ion due to improper sca l ing or parameter in terac t ion ,
the reparameterization recomm ended by Chen an d Aris [34]
was adopted:
In the large particles , along with the primary reactions,
heterogeneous secondary reactions also take place and the
model was expanded as:
Volatiles
+
Gases
ai
=
l n A i
ej =
Ei /
1200
R E )
W o o d ( B )
6
p
= T/1200
(3) (4)
Pr imary
-
Secondary - econdary + Gases
Therefore , tak ing the reparameter iza t ion and n , = n2= n ,
the ordinary differential equations become:
Reaction (3) is the init iation reaction a nd includes th e coef-
ficient of deposition 6, which depends o n the volati le con-
tact time inside the pyrolyzing particle. Reaction (4) is the
propagation reaction.
C1
s the pr imary char , C2 s the sec-
ondary char formed by the init iation reaction, and C22 s
the secondary char formed by the propagation reaction. C2
is the sum
of
C2, an d C2, and is the total secondary char.
%=t - [exp
(al
-:) + e x p
(a2-:)] B
(3)
(4)
To determine the parameters a , , e l , a2 , and e2 , the par t ia l
differential of the above two ordinary differential equations
with respect to these paramete rs is needful. A total of eight
more equations were obtained. The above two differential
equations along with the eight equations form a system
of
non-linear differential equations . The solution of the init ial
value problem can be obtained numerically by an explicit
algorithm. The ordinary differential equations were solved
by using a 4th-order Runge-Kutta algorithm. The optimum
step s ize was fo und t o be 1.5 s and the r ise in temperature
in each s tep was adjusted according to the heating rate.
Reaction ord ers tr ied were
.O
1.1,
1.3,
and 1 .5 . The order
of reactions of th e propos ed m odel, c onsidering the experi-
mental da ta an d residual curves, were established as being
equal to
1
. l .
The concepts
of
adsorption, deposition and reaction of tar
with char leading to cracking and repolymerization [I
-
,
8
I], residence time of volatiles
[6],
and presence of free
radica ls [12- 141 sup port o ur mod el and give a mechanistic
view
of
the same.
The ordinary differential equations of the overall kinetic
model become:
( K , + K 2 ) B n
B
d t
(7)
(9)
he same primary m odel was fou nd also applicable for iso-
thermal heating of small particle, at various furnace tem-
peratures . The primary reactions are:
d B
- =
- ( K I + K 2 ) B n
d t
with the init ial condition B = 1 , C1= 0, C2= 0 a t t = 0.
Th e residual weight fraction in the case of large particle is
given by the sum: W = B + C1+ C2.
Th e ordinary differential equations were again solved using
the R unge-Kutta 4th-ord er method as described previously
for the case of primary reactions. Also, in order to avoid
poor parameter es t imat ion due t o improper sca l ing or pa-
rameter interaction, the reparameterization recommended
by Chen and Aris [34] was adopted.
with s imilar initial conditions as above. Th e parame ters K ,
and K2 were determined fo r various isothermal conditions
by non-linear optimization as described above. T he plot
of
In K, versus
1
/ T an d In K2 versus 1/T did n ot give s traight
l ines and , therefore , the Arrhenius form for the ra te
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282
The rate cons tants K3 a n d K4 are assumed to be in Ar-
rhenius form . The temperature- t ime data of large samples
at various radii are needed to determine the kinetic p arame -
ters
of
the secondary reactions. The parameters A3 E3,A4,
E4 along with
6
were determined by simulating the model
with th e experimental weight loss curves of wood cylinders
by nonl ine ar opt imizat ion using the Marquardt method by
minimizing the sum
of
the squares funct ion
F .
2 . 3 Heat Transfer Model
Taking a differential control volume
of
a solid particle dur-
ing heating and neglecting the convective hea t transfer a nd
assuming the solid properties
e,
C p, ) inside the control
volume to remain constant, i t can be shown that the equa-
ae
ax
r>O
,
x = l
,
- = - H e
The weight loss kinetics a re used to solve the abov e partial
differential equation (PDE). The radius is divided into 14
s teps and temperature a t 15 points is determined a t increas-
ing values of t ime. The A t / A x 2 value is taken to be
1.0.
The Crank and Nicolson method was used for solving the
above PDE in to
15
algebraic equation s which were solved
using the initial condition as above. The heat
of
reaction
was determined by hit and trial method by simulating the
model with the experimental curve.
t ion for a cylind rical pellet is:
Rober ts [35] pointed out the tendency of volatiles to travel
along approximately isotherm al planes parallel to the wood
grains and int o cracks. The convective heat transfer c oeffi-
cient between the pyrolysis gas and the solid is unlikely to
be significant under these conditions. Pyle and Zaror [21]
also pointed o nto the unimportance of in ternal convect ion
in the model. Moreover, the migration of the volatiles
towards the cooler parts of the virgin wood may occur,
but the resistance to flow will invariably be greater in the
virgin wood tha n in the charcoa l region a nd , therefore, the
outward f low of volatiles is likely to be mo re sign ificant. In
the work of Bilbao et al.
[25],
when convective heat transfer
in the solid due to the volatiles is used, the temperatures
predicted show lower values tha n those ob tained experimen-
1
1 )
at
Initial condition:
t = 0, T(r,O)= To
Boundary condition:
t>O, r=O, -= 0
12)
(13)
a T
ar
14)
aT
ar
t>O, r
=
R , - k - = h ( T - T f ) + m ( T 4 -T:)
In the above formulation, external heat transfer is con-
sidered to occur by a combina tion of convective and radia -
tive m echanism.
Introducing the following groups:
k
r at
a=-
,
x = -
,
7 = -
ec,
R R 2
R
k
H
= -
h+ o ( T 3 T 2T f +TT:+ T:)]
- A H
QCp(To- T f )
Q =
The energy balance equation becomes:
_ = _e
i
- + - +Q
6 a20
ar x a x ax 2
- )
r
= 0 , 6 x , O )= 1
tally, especially when a significant weight loss is pioduced.
This delay in the temperature could be due to the assump-
tion that the volatiles formed leave the particle instan-
taneously, which overestimates the heat loss. On th e other
hand, the consideration that volatiles only flow radially
toward the solid surface an d without d iffusional resistance
is also questionable. Due to this anisotropy, preferential
paths for volatiles t o escape may exist in the woo d. There-
fore, it is difficult to establish the true p aths th at generated
volatiles follow. Taking into account these difficulties, it
has no t been cons idered appropr iate to include the assump-
tion of the convective heat transfer of
the volatiles.
With respect to the particle volume, previous experiments
were carried ou t with th e differen t particle sizes and were
stopped a t different temperatures an d solid conversions. An
appreciable particle shrinkage was only observed at high
solid conversions. In add ition, in oth er studies including the
particle shrinkage [21], a variation
of
the overall numerical
solution by less than
3
was obtained with this modifica-
tion. So,
a
constant pa rticle volume was used in our calcula-
tions.
3
Experimental
Section
Pyrolysis of acacia an d eucalyptus woods were investigated
in flowing nitrogen atmosphere in the thermogravimetric
analyser, gas-purged isothermal reactor, and a specially
(15)
16)
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Chem . E ng. Technol. 19 (1996) 272-282
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designed sealed reactor . The samples of various sizes were
stored in the laboratory under dry conditions.
A gas-purged isothermal reactor was employed to measure
the pyrolysis rate of large cylindrical an d small particles .
The tem perature variations inside a pyrolyzing woo d parti-
cle were also measured. A pipe of diameter 51 mm and
length 2.0 m was taken and kept in a n iner t a tmosphere by
passing nitrogen gas f rom below. T he lower half of the re-
actor was kept in
a
tubular electr ical furna ce to which
a
con-
troller was attached to main tain isothermal conditions. The
upper portion of the reactor was used for quenching and
rapid cooling of the hot sample. This was essential because
the hot sample had
to
be cooled in neutral atmosphere as
oxidation occurs in air leading to erroneou s weight loss
of
the samples .
The sample material (cylindrical pellets of diameter
15
m m
a n d l e ng th 9 0 m m , d i a 2 0 m m a n d l e ng th 1 2 0 m m , di a
25
mm and length 150 mm to s tudy the secondary reaction
kinetics , a nd f ine samples
of
0.3 to 0 .5 mm to s tudy the pr i-
mary reaction kinetics) of acacia a nd eucalyptus wood was
placed in a stainless steel wire mesh basket hu ng o n a metal-
lic rod , which c ould easily move vertically inside the reacto r
tube.
A
f low rate
of
2700 cm3 /min of nitrogen (a t 293 K
and 101.3 kPa) was chosen. I t was sufficiently high to ma in-
tain a uniform temperature zone, displace the oxygen from
the reactor , an d carry away the volati les a nd gaseous prod-
ucts. H eat was transmitted to the sample by means of con-
vection fro m the carrier gas an d radiation. Before entering
the reaction zone, the nitrogen s tream was passed through
a preheating coil.
Smal l samples
of
size 0.3 to
0.5
mm (0.2 g spread uniformly
over an area
of
14c m2 ) were kept between the folds of a
fine stainless steel wire mesh and the weight loss with time
was recorded at various temperatures . Similar weight loss
curves with 0.15 t o 0.3 mm particles were obtained to con-
firm that heat an d mass transfer resis tances d o not exist in
the study of small particles. For large particles, experiments
with different lengthldiam eter ratio of wood cylinders were
car ried out , and i t was found tha t a ra t io of 6 was safe for
the samples to act as of infinite length. Tem peratures
of
the
axis , at half radius from the axis , and just below the surfac e
of
the pellet were record ed w ith
a
f ine chromel/alumel ther-
mocouple . In some exper iments the ax is tempera ture a t d i f -
ferent lengths from the end
of
the pellet were recorded to
conf i rm that the heat ing was only rad ia l and tha t no tem-
perature variation exis ts along the length of the pellet.
To check the applicabili ty of the primary reaction model
under various heating conditions, dynam ic TG studies were
car r ied out on a Stan ton Redcrof t 780 STA. Small (6 to
7 mg) quantit ies of samples were investigated at heating
rates of 10, 20 , and 40K/min in n i t rogen f low ra tes of
50cm3/min . High f low ra te of purge gas were adopted to
remove the gaseous an d volati le pyrolysis pro ducts evolved
and, thus, minimize any secondary interactions between
them a nd the ho t so l id . Before carry ing out a run, th e varia-
tion in the base l ine were measured by performing a blank
run with an empty crucible. The samples were spread
uniformly o n the sam ple pan. S mall samples was a necessity
because the reaction was found endothermic, and large
mass loadings caused heat and mass transfer l imitations.
To s tudy the ef fec t of vapour-solid interactions on the py-
rolysis of wood, a specially designed cylindrical stainless
steel reactor
of
inside diameter 5
1
mm and height 124 mm
was used. T he lid of the reactor consisted of a circular plate
with a hole to which a pressure gauge was attached . A tef lon
ring and a stainless steel cover were used to seal the upper
part with the reactor . A fine chromel/alumel thermocouple
was inserted into the reactor to come in contact with the
sample. Small samples of size 0.3 to 0.5 mm (0.3 g spread
uniformly over a n area
of
20 cm2) were placed in th e reac-
tor a nd f lushed with nitrogen for 8 times under higher than
atmospheric pressure and thereafter the reactor was in-
serted in to the to p of the tubular furnace . The tempera ture
of the sam ple was recorded and came t o the furnace temper-
ature within 2 min. The pressure s tarted r is ing as gases and
volatiles were formed from the pyrolysis
of
the subs t ra te .
When the pressure reached a constant value, the reaction
was completed and the appara tus was removed f rom the
furnace and purged wi th n i t rogen to cool the sam ple and the
reactor . Precautions were taken that everytime a posit ive
pressure was maintained in the reactor so tha t the ou ts ide
air does not enter . The f inal weight of the char was re-
corded .
Elemental analysis
of
acacia and euc alyptus wood were de-
termined using Perkin Elmer 2400 CHN analyser . The
elemental analysis of acacia used is C = 45.13%, H
=
6.14070, ash = 0.8% and by difference = 47.93%. Euca-
lyptus contains C
=
46.21%, H
=
6.08%, ash
=
0 .7 % an d
by difference = 47.01 070 (dry weight basis).
4 Results and Discussion
Figs.
1
and 2 show the experimental points and theoretical
curves of the residual weight fractio n
of
acacia an d eucalyp-
tu s wo o d a t a heating rate of 20K/m in . Alongwith are
shown the theoretical char and virgin wood fractions ob-
tained by the primary reaction model. The residual weight
reported was norm alized as (residual - ash weight)/(initial
ash weight). T he activation energies , preexponential fac-
tors , and order
of
the reaction determined by nonlinear op-
t imization are repor ted in T ab . 1 . The isothermal residual
weight frac tion curves were obtain ed a t 573, 623, 673, an d
Table
1.
Primary reaction kinetic parameters for pyrolysis of acacia
and eucalyptus woods by TGA in nitrogen atmosphere.
~ ~~
Sample To Tf n A El A2
E2
[K] [K] [g-' '/s] [kJ/mol]
[g-'
'Is] [kJ/mol]
Acacia 413
833
1.1 5.87104 91.5 6.5710 3 85.9
wood
Eucalyptus 413
833
1 . 1 1.2810 5 93.1 1.16 lo4 86.7
wood
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278
0.0.
:
. .
8
Educ t
residue ... .
I
I I
....__.
I
....-.-
.
C,:
Ch o r
/ ...c,
._....
I
l O F - y - - - - -.9
0 8
d
P
0.4
0.3
0.2
0.1
0 0
1.0
_ .
- Exp. Theo. Heot
rote
0
2OKlnnn
B E d u c t residue f r ac t i o n
C,- Char fraction
-
-
/...*.:
I_- .
__.
__.-
_..-
I I
I I
-4
_ - -
_ -
0.9
t
d
P
0.4
g
0.3
0.2
0.1
0 0
0 . 8L
-
-
-
__.
__.-
_..-
I I
I I
-4
_ - -
_ -
Exp. Theo. Heot rote
0
2OKlnnn
B E d u c t residue f r ac t i o n
C,- Char fraction
+ 0.6
3
0 . 5
I
w
TEMPERATURE
(CI
Figure
2. Thermogravimetry of eucalyptus wood. Experimental points
in comparison with the residual weight fraction, educt residue B and
char C curves predicted by the primary kinetic model for heating rate
of 20 K/min.
723K for acacia and eucalyptus wood and the rate
constants for the weight
loss
and char forming reactions are
reported in Tab.
2.
The same model
is
found applicable
under dynamic TG and isothermal studies. The order of the
reactions in the case of TG studies was found to be larger
than one but in isothermal studies the model was found fit
for an order equal to one.
Chem. Eng. Technol. 19
(1996)
272-282
The primary reaction kinetics of small particles is used in
the overall reaction model of pyrolysis of large particles to
determine the secondary reaction kinetic parameters.
Figs. 3 and 4 show the simulated and experimental residual
weight fraction curves and normalized temperatures of the
Table2. Values of rate constants of weight
loss
and char formation
reactions during isothermal pyrolysis
of
acacia and eucalyptus woods.
Sample Temperature [K]
K , [s -]
K2 [s-I
Acacia 573
wood 623
673
123
Eucalyptus 573
wood 623
673
723
1.31 1 0 - ~
7.74 1 0 - ~
1.36 I O - ~
2.04
6.07
1.07
2.52 lo-
6.85
lo-
1.36
1.47 I O - ~
4.73 1 0 - ~
1.21 1 0 - 2
5.20
3.49
4.84 1 0 - ~
1.05 lo-*
w 0.8
-
a
2
a
0.4
N
r
a
0.6-
I
w
W
-
2 0.2 -
0.0-
0 3 6
9
12
15
18 21
24
T IME (rnin.)
Figure 3. Experimental points and theoretical curves
of
residual weight
fraction and centre temperature during isothermal pyrolysis of acacia
wood cylinder (diameter 15mm and length 90 mm) at 623 K.
- 0 . 8
g
-
I
a
-0.6
I
I
-
0.4
?
-0 .2
2
- 0 . 0 L
I
e
ul
0 3 6
9
12 15
18
21 24
TIME min.1
Figure
4. Experimental points and theoretical curves of residual weight
fraction and centre temperature during isothermal pyrolysis of eucalyp-
tus wood cylinder (diameter 15 mm and length 90 mm) at 623 K.
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Chem. Eng. Technol. 19 (1996) 272-282
279
axis of the 15 mm cylindrical pellet and length 90 mm at fur-
nace temperature of 623
K
for acacia and eucalyptus wood
respectively. The data for 20 mm dia and length 120 mm
wood cylinders are shown in Figs. 5 and 6 at furnace tem-
peratures of 673 and 723
K.
The temperature was normal-
ized as (temperature final temperature)/(initial
-
final
temperature).
The optimized parameters (deposition coefficient, preexpo-
nential factor, and activation energy) for the initiation and
propagation reactions of the chain growth mechanism,
alongwith the values of
autocatalytic/noncatalytic
reaction
rates within the temperature range which accompany the
pyrolysis of larger particles are given in Tab.
3
for the
acacia and eucalyptus woods. Reaction (3) is the initiation
rate constant and reaction (4) s the chain growth (or auto-
catalytic) rate constant and the ratio
K 4 / 6 K 3
epresents the
increase i? autocatalytic over noncatalytic rates of reaction
7 3
K
a
0.0
-
- 0 . 2
-
I
I
I
I I I
0
2
4 6
8 10 12 14 1 6
TIME min.1
Figure
5. Experimental points and theoretical curves of residual weight
fraction and centre temperature during isothermal pyrolysis of acacia
wood cylinder (diameter 20
m m
and length 120 mm) at 673 and 723
K.
I
a
a
g 0.0
- 0 . 2
I I I I I I I
0 2 4 6
8
1 0 1 2 1 4
TIME min
Figure 6. Experimental points and theoretical curves of residual weight
fraction and centre temperature during isothermal pyrolysis
of
eucalyp-
tus wood cylinder (diameter 20
m m
and length 120
mm
t 673 and
723 K.
Table 3.
Secondary reaction kinetic parameters for pyrolysis of acacia
and eucalyptus wood cylinders.
Sample 6 A
E3
A4
E4 K,/6K3
[s-'] [kJ/mol] [g -l s- '] [kJ/mol]
___
_____
_ _ _ _ ~
~~ ~~
Acacia 1.48 1.045.106 80.5 1.015.106 76.0 1.39- 1.69
wood
Eucalyptus 1.40 1.329. lo6 80.3 1.264. lo6 78.3 0.95- 1.03
wood
or the relative chain growth rate. The value varies from 1.39
to 1.69 for acacia wood in the temperature range 573 to
723
K
which being greater than one clearly reflects the im-
portance of the propagation reaction in the proposed chain
growth mechanism.
For the case of eucalyptus wood the relative chain growth
rate varies between 0.95 to 1.03 which shows that in this
case the propagation reaction is not as significant as in the
case of acacia wood and the major phenomena affecting the
pyrolysis
of
eucalyptus wood are the tar adsorption and de-
composition reactions.
Heterogeneous secondary reactions play an important part
at both low and high temperatures. At low temperatures
more residence time is available for vapour-solid interac-
tions and autocatalysis take place whereas at high tempera-
tures the tar decomposition reactions take place. Fraga et
al. [36] in their investigation of biomass pyrolysis tars show
that extensive modification of primary pyrolysis tars takes
place by means of intraparticle (heterogeneous) secondary
reactions, the reactions being intensified by increasing heat-
ing rates. The chain growth model takes both the volatiles
retention time and cracking and repolymerization reactions
of the vapours with the decomposing solid as well as auto-
catalysis into consideration.
The sealed reactor studies carried at 573, 623, 673, and
723
K
of small samples yield a final char residue of 36% and
32% for acacia and eucalyptus wood respectively. It is in-
teresting that the char formed from acacia and eucalyptus
wood in sealed reactor is in similar quantities as those ob-
tained for the two woods in gas-purged reactor for large
samples, although the heating rates in the sealed reactor are
much higher than the heating rates which accompany large
particles in the gas-purged reactor. This clearly shows that
the secondary reactions and not lower heating rates are the
source of secondary char formation in large particles. Ex-
periments carried out with higher initial nitrogen pressures
resulted in the similar amounts of final char residue which
predicts that vapour-solid interactions and not higher pres-
sure which accompany the inside of the large particles [37]
are the cause of higher char yields.
During the pyrolysis process the pores of the solid are
enlarged which offer many reaction sites and more second-
ary reactions take place which lead to the enrichment of
charcoal with carbon and reform the volatiles and gaseous
products. The volatiles generated from the inner virgin
wood pass through the outer charcoal regions and the pores
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Chem. Eng. Technol.
19
(1996) 272-282
become smaller and smaller, thus, the retention time of the
tars increases, leading to again more of char, as more the
retention time of the volatiles, higher is the extent of sec-
ondary reactions. That autocatalysis observed
in
our experi-
ments is confirmed by the lower activation energies of reac-
tions (3) and (4) as compared to reactions (1) and (2). The
activation energies for acacia wood for reactions (3) and (4)
are 80.5 and 76.0 kJ/mol and are lesser than 91.5 and
85.9 kJ/mol for the primary reactions. Similar is the case
for eucalyptus wood in which the secondary reaction activa-
tion energies of 80.3 and 78.3 kJ/mol are lesser than 93.1
and 86.7 kJ/mol. Antal and Varhegyi [2] also comment that
vapour-solid interactions are accompanied with lower ac-
tivation energies. Murty and Blackshear [38] also observed
an increase in reaction rate constant at a particular tempera-
ture
on
passing from the surface to the center of the
specimen and Stamm [39] has described experiments in
which the rate of reaction was increased when products
were not free to escape, effects that could be called auto-
catalytic. Tabatabaie et al. [40] call attention to the fact that
their high temperature decomposition
of
cellulose in the
Fast-TGA sample boat may have been affected by autoca-
talysis (perhaps by the presence of organic acids such as for-
mic and acetic acids in the volatile pyrolysis products).
Autocatalysis is promoted by the presence of the lid which
hinders easy escape of the primary vapours from within the
sample boat into the inert carrier gas stream [41].
Alongwith the residual weight fraction the axis temperatures
are reported. The temperature in the pellet is simulated with
the energy balance model, and the theoretical curves and ex-
perimental points at half radius from the axis are shown in
Figs. 7 and 8 for acacia and eucalyptus wood respectively for
15
mm dia particles at 623
K.
The values of the properties
used in the model are taken as:
Wood specific heat = 2380.0 J/kg K
Char specific heat = 1600.0 J/kg
K
Wood thermal conductivity = 0.158 W/m
K
Char thermal conductivity = 0.107 W/m K
Solid emissivity = 0.95
Convective heat transfer
coefficient = 5.69+0.0098 TW/m2
K
[42].
It is considered that the local value of the property is a func-
tion of the conversion of wood. The local value of the heat
conduction coefficient, k and of the specific heat,
Cp,
of
the pyrolyzing solid were estimated as the sum of the two
values corresponding to the virgin material and of the char.
Each of the two values contributes to the sum in a way pro-
portional to the local conversion: property = (l-conver-
sion) (property),,,,, + (conversion) (property),ha,. The heat
of
reaction is determined as equal to 23 kJ/kg for acacia
wood and - 241 kJ/kg for eucalyptus wood. The tempera-
ture history of acacia and eucalyptus wood cylinders of dia
20 mm and length 120 mm at 723
K
as predicted by the
-0.2
T I ME r n i n . 1
Figure 7. Experimental ( - - ) and theoretical ( temperature
variation at a distance
R / 2
during pyrolysis
of
acacia wood cylinder
(diameter
15 mm
and length
90mm)
at
623 K.
1 *o
\
0 3 6 9 12 15 1 8 21 24
6
9 12 15
18
21
2
TIME
(rnin.1
Figure 8. Experimental ( - and theoretical
(
temperature
variation at a distance
R / 2
during pyrolysis of eucalyptus wood cylinder
(diameter
15
mm
and length
90mm)
at
623
K.
model are shown in Figs. 9 and 10 respectively. It is interest-
ing to note that acacia wood has a higher autocatalytic/
noncatalytic ratio and also has a higher exothermic heat of
reaction as compared to the eucalyptus wood and, more-
over, they yield higher char fractions in the sealed reactor
as well as for large particles in gas-purged reactor. Mok et
al. [4] observed that high sample loadings increased char
yield and the associated exothermic heat release and
lowered the reaction onset temperature. That the secondary
reactions and heat of reaction are linked, is in support of
our chain growth model of heterogeneous reactions during
wood pyrolysis.
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Chem. Eng. Technol. 19 (1996) 272-282
550
I I
I
I
I I
0 2
6
8
10 12
T IME min . 1
Figure 9. Temperature history of acacia wood cylinder (diameter 20 mm
and length 120mm) during pyrolysis at 723 K as predicted by the model.
5
Conclusions
The chain growth model takes both the volatiles retention
time and cracking and repolymerization reactions of the
vapours with the decomposing solid as well as autocatalysis
into consideration. The secondary reaction activation ener-
gies determined are
80.5
and 76.0 kJ/mol for acacia, and
80.3 and 78.3 kJ/mol for eucalyptus wood for the initiation
and propagation reactions respectively, and deposition
coefficients larger than one are obtained for both the
woods. The activation energies of the heterogeneous reac-
tions are lesser than primary reactions which predict auto-
catalysis in large particles.
The vapour-solid interactions are an important pheno-
menon in biomass pyrolysis. Char yields of 36% for acacia
wood and 32% for eucalyptus wood have been obtained in
sealed reactor studies. Similar char yields are obtained for
pyrolysis of large particles which implies that the vapour-
solid interactions and not lower heating rates govern the
pyrolysis of large particles, since high heating rates accom-
pany sealed reactor studies.
The small samples in which the volatiles are immediately re-
moved are endothermic and those accompanied with sec-
ondary reactions in large particles are exothermic. The heat
of reaction
of
-
323 kJ/kg is determined for acacia wood,
550
I
I
I
I I
4
6 8 10 12
T I M E min.)
28
1
Figure
10.
Temperature history of eucalyptus wood cylinder (diameter
20
mm and length 120 mm) during pyrolysis at 723 K as predicted by the
model.
and of -241 kJ/kg for eucalyptus wood. Moreover, the
ratio of autocatalytic reaction to that of the noncatalytic
reaction in our chain growth model K 4 / 6 K 3 is higher for
acacia wood (1.39 to 1.69) than that of eucalyptus wood
(0.95 to 1.03) and also the deposition coefficient for acacia
wood is higher than that of eucalyptus wood, and acacia
yields more char. The positive correlation between the sec-
ondary reactions and exothermic heat of reaction proves the
applicability of the model.
Our model of secondary reactions is found to be satisfacto-
ry and can predict accurately the weight loss and char yield.
The secondary reactions are responsible for carbon enrich-
ment of the char residue and exothermic heat of reaction.
Received: July 19, 1994 [CET684]
Symbols used
preexponential factor for i
=
1,2, and 3
propagation reaction preexponential factor
transformed preexponential factor
educt (virgin) wood weight fraction
primary char weight fraction
secondary char weight fraction
specific heat capacity
activation energy
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19
1996)
272-282
ei [-I
F [-I
h [W/m2K]
A H
[kJ/kg]
H [-I
Ki
[s- 1
K4 [g-ls-']
k
[W/mK]
n j , n
[-I
P
[-I
Q
[-I
R , [J/molK]
r , R Iml
T [Kl
t [sl
w
[-I
x [-I
Greek letters
[-I
u
[W/m2K4]
Subscripts
transformed activation energy
sum of squares function
convective heat transfer coefficient
heat
of
reaction
dimensionless convective and radiative heat trans-
fer coefficient
rate constant for i = 1,2, nd
3
propagation reaction rate constant
thermal conductivity
reaction order
transformed absolute temperature
dimensionless heat of reaction
gas constant
radius of pellet
absolute temperature
time
residual weight fraction
dimensionless radius
deposition coefficient
dimensionless temperature
Prandtl number
density
dimensionless time
heating rate
solid emissivity
Stephan-Boltzmann constant
reaction number, i = 1,2,3,4
point employed in least square fitting
experimental
theoretical
initial
reactor
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