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Mathematical Modelling of Temperature Response of Low-rank Coal Particles During Pyrolysis.

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Dev. Chem. Eng. Mineral Process., 7(5/6), pp.593-610, 1999.
Mathematical Modelling of Temperature
Response of Low-rank Coal Particles
During Pyrolysis
C.A. Heidenreich*, H.M. Yan, and D.K. Zhang
CRCfor New Technologiesfor Power Generationfiom Low-rank
Coal, Department of Chemical Engineering, University of Adelaide,
Adelaide, South Australia 5005, Australia
A mathematical model has been developed to predict the temperature response of
d v low-rank coal particles during pyrolysis in an inert atmosphere. The model is
based on the unsteady-state heat conduction equation in spherical coordinates and
uses the Distributed Activation Energy Model to predict volatiles evolution.
Measurements of the temperature response at the centre of I Omm Bowmans coal
particles were conducted in a horizontal tube furnace using nitrogen as the heating
medium at furnace temperatures of I 5 0 C 350'C, 600C 700C and 800T. A
sensitivity analysis was pejormed to assess the influence of the heat of pyrolysis
and the flux of volatiles leaving the particle on the temperature response. The heat
of pyrolysis can influence the predicted temperature response, however the
uncertainty surrounding the magnitude and nature of its value remains a problem.
The gaseous flux has no signijkant effect on the model predictions. Measurements
of the temperature response of particles with va?ying moisture contents indicate
that the presence of moisture significantly influences the temperature response. The
effect of moisture is greater than that of the heat of pyrolysis and further work is
required to incorporate moisture into the current model.
* Authorfor correspondence.
593
C.A. Heidenreich, H.M. Yan and D.K. Zhang
Introduction
The abundance of low rank coals throughout South Australia and Victoria, along
with the ever increasing demand for inexpensive energy resources, has resulted in
the formation of the Commonwealth Research Centre for New Technologies for
Power Generation from Low Rank Coals which is focussed on developing
innovative techniques for power generation from low rank coals.
One such
advanced technology which has been proposed incorporates a low temperature
pyrolysis stage [l], and the development of this process relies on a thorough
understanding of the behaviour of low rank coals during low temperature pyrolysis.
It is generally acknowledged that coal pyrolysis may be controlled by three
main factors; heat transfer to and within the particle, chemical kinetics, and mass
transfer of the volatile products within the particle [2-4]. Several mathematical
models have been developed to describe the pyrolysis of large low rank coal
particles and it is accepted that heat transfer and chemical kinetics dominate the
reaction mechanism [2-51. The significance of mass transfer in low rank coal
pyrolysis has not yet been fully determined. Koch et al. [6] suggested that the char
layer which forms around the pyrolysing particle provides no resistance to the
flowing volatiles whilst the data of Anthony et al. [7] indicated that the effect of
pressure on the devolatilization behaviour of a Montana lignite was negligible.
Based on these results the effect of mass transfer has not been considered in the
present study.
Various models have been proposed to describe the kinetics of coal
pyrolysis ranging from simple fmt order models [8] to two-competing reaction
models [9], and finally to multiple parallel reaction models such as the (DAEM)
[3,5,10]. The simple first order models, whilst requiring the least number of
correlating parameters, have been found to be inadequate as the parameters must be
adjusted for each set of conditions to which the model is applied [ 111.
The
comparative effectiveness of the two-competing and multiple parallel reaction
models was investigated by Sprouse and Schuman [l 11 who concluded that the
multiple parallel reaction models could be considered more robust for predicting
594
Mathematical modelling of low-rank coal particles during pyrolysis
weight loss data over a wide range of heating rates and residence times. More
recent modelling work [3,5,10] has applied these models to the devolatilization of
large low-rank coal particles with reasonable success.
The DAEM, as described by Agarwal et al. [3], enables the fractional amount of
volatile matter remaining in the particle to be predicted based on the particles timetemperature history. Whilst not providing a mechanistic understanding of the
pyrolysis process, the model does enable certain observed trends to be predicted
such as the apparently pseudo-asymptotic volatiles yield observed at low final
temperatures [3]. Furthermore, the complex nature of the devolatilization process
does not enable individual reactions to be considered and this approach enables
these reactions to be considered using a simple holistic approach. WildeggerGaismaier [12] investigated the pyrolysis of Bowmans coal and suggested that the
kinetic parameters suitable for use with Bowmans coal are log,,&)
=
13.22, E,
=
2 10 W.mol-’, and (J = 40 Id.mol”.
A significant factor in dictating the accuracy of the particle temperature model
is the selection of the thermophysical properties of coal substance, i.e. heat capacity
and thermal conductivity.
Experimentally these properties are difficult to
determine due to the complicating effects of the heat of pyrolysis and enthalpy of
moisture vaporisation etc [13]. Tomeczek and Kowol [4] investigated various
correlations for the heat capacity and thermal conductivity of coal using data for the
temperature response of a Polish lignite. The heat capacity proposed by Agroskin
et al. [ 141 and the thermal conductivity proposed by Agroskin [ 151 were found to
yield the best results. The suitability of these correlations for predicting the
temperature response of Bowmans coal particles will be investigated.
Speculation remains as to the significance of the heat of pyrolysis on the
temperature response of coal particles during pyrolysis due mainly to the difficulty
in obtaining accurate estimates of its magnitude. Adesanya and Pham [3]
summarised previously published heats of pyrolysis which suggested that above
650°C exothermic heats of pyrolysis in the range 266 - 441 kJ.kg-’ and 616 - 653
kJ.kg-’ had been observed. Below 650°C the heat of pyrolysis was generally agreed
595
C.A. Heidenreich, H.M. Yan and D.K. Zhang
to be endothermic however no specific values were given. Lopez-Peinado et al.
[161 suggested that the majority of the heat effects involved in coal pyf.olysis are
endothermic. Interestingly, exothermic heat effects were observed exclusively for
low rank coals. Generally they reported three distinct regions; 1) at low
temperatures (300
-
450OC) the heat effect is endothermic and increases with
decrease in rank, 2) for moderate temperatures (450 - 750OC) the heat effects are
less defined and are exothermic for low rank coals, and 3) above 750°C the heat
effect is exclusively endothermic. Tomeczek and Palugniok [I31 found that
exclusively endothermic heat effects were observed up to 800°C after which strong
exothermic and endothermic heat effects were observed. Considering the
disagreement in the observed heats of pyrolysis, various trends will be tested to
investigate the effect on the predicted temperature response.
Davies and Brown [17] proposed that the intensive outflow of products from
coal particles during pyrolysis may inhibit the flow of heat to the particle surface.
Kalson [18] used an energy balance equation at the particle surface to develop a
correlation for the hctional reduction in the external heat transfer coefficient as a
result of the gaseous flux from the particle. Using this correlation, Kalson [18]
performed a theoretical investigation of the significance of this effect based on a
standard set of operating conditions (74-pm particle in entrained flow gasifier at
600°C). The results indicated that the external heat transfer coefficient could be
reduced by between 7% and 67% thus suggesting that the gaseous flux may have a
substantial influence on the particle temperature response. This approach will be
used to assess the influence of the gaseous flux on the temperature response of
large particles.
Moisture is likely to play a significant role during heating of wet particles as a
result of the large amount of energy required to carry out the evaporation and
desorption of inherent moisture. Wildegger-Gaismaier[ 121 looked at the influence
of moisture on the evolution of volatile matter and found that the presence of
moisture resulted in a delay in the initial release of volatiles. Wildegger-Gaismaier
[I21 modelled the devolatilizatiotdpyrolysis of wet particles by assuming that
596
Mathematical modelling of low-rankcoal particles during pyrolysis
moisture was removed at a wet/dry interface which occurred at 373K and receded
into the particle as moisture was progressively removed. No attempt was made to
compare the predicted temperature response with experimental data so no
assessment can be made of the proposed approach. More sophisticated models exist
for predicting the drying of coal particles which incorporate pressure effects.
However these models are inherently difficult to solve [191 and would significantly
increase the complexity of the current model. Due to the difficulties arising from
the presence of moisture, the bulk of the modelling work has been performed
assuming dry coal particles while experimental data were collected using both wet
and dry coal particles for comparative purposes.
The aim of this paper is to investigate the proposed model for predicting the
temperature response of large coal particles during pyrolysis under inert conditions,
and to compare the model predictions with experimental temperature data.
Furthermore, a theoretical analysis of the effect of the heat of pyrolysis and the
gaseous flux exiting the particle on the temperature response will be examined
using the model, while the effect of moisture on the temperature response will be
experimentally investigated.
Experimental
Spherical particles of Bowmans coal (-1Omm) were produced using a linear shear
to smooth the sides of lOmm cubes cut from a large lump of Bowmans coal. The
proximate analysis of Bowmans coal is given in Table 1. Holes were drilled to the
centre of the particles and the particles were dried by firstly allowing them to dry
under ambient conditions for approximately 24hrs and removing the remaining
moisture in a N, purged oven at 105°C until no further mass loss was observed.
Temperature measurements were carried out in a horizontal tube furnace
comprising a 26mm i.d. ceramic tube set inside a Carbolite CFM 14/2 furnace. The
furnace was heated to the desired temperature and nitrogen blown through the
597
C.A. Heidenreich, H.M. Yan and D.K. Zhung
ceramic tube. The particle was placed on a thermocouple lead connected to a
supporting rod and the rod was subsequently inserted into the furnace such that the
particle was maintained inside the heated zone. The temperature response was
recorded using the Picolog data logging package and sufficient time was allowed
for devolatilization to be complete prior to the particle being removed fiom the
furnace. initial and final particle diameters and weights were also recorded.
Experiments were carried out at furnace temperatures of 150°C, 35OoC, 6OO0C,
700°C,and 800°C. To investigate the effect of moisture, similar experiments were
conducted using particles with moisture contents of 0.55 g/g wet coal, 0.35 g/g wet
coal, and 0.15 g/g wet coal. The particles of moisture content 0.55 g/g wet coal
represent the raw coal particles while the remaining moisture contents were
achieved by allowing the particles to dry in air to the required mass assuming all
the mass loss is that of moisture.
Table 1. Proximate anabsis of Bowmans coal.
Proximate Analysis
Fixed Carbon
38.7 w?t!
d.b.
Volatiles
49.3 w?t!
d.b.
Ash
12.0 wt?! d.b.
Moisture
52.0 w?t!
d.b. dry basis
w.b. wet basis
598
w.b.
Mathematical modelling of low-rank coal particles during pyrolysis
Model Development
The particle energy balance equation is based around the unsteady state heat
conduction equation in spherical coordinates to which an energy accumulation term,
4 0 , has been added, as seen in Equation (1).
The heat transfer to the particle surface can be described by :
The convective heat transfer coefficient,
Lnv,
can
be calculated using the
correlation of Ranz and Marschall as reported by Kunii and Levenspiel [20].
Linjewile [21] determined the effective emissivity, E+ad, of petroleum coke particles
as a function of temperature and, according to the data presented, the emissivity
could be expressed by Equation (3).
E,&
= 0.377+ 0.00039Tp
(3)
As mentioned, the release of volatile matter was predicted using the DAEM
which is described elsewhere [3] and results in the fractional amount of volatile
matter remaining in the particle, [(V'-V)N*],. This enables the mass loss and the
mass loss rate to be determined based on the ultimate volatile matter content and
particle density according to Equation (4).
Mass loss rate = dAMdt?" , AM&v = vpp,v
dr
599
C.A. Heidenreich, H.M.Yan and D.K.Zhang
The release of volatile matter results in a decrease in the particle density and the
density at any time was determined from the mass loss. The particle size was taken
as the average of the initial and final particle sizes measured during the experiments.
The heat of pyrolysis can be incorporated into the energy balance equation as the
energy accumulation term, q o , according to Equation (5).
The heat of pyrolysis, [AH(T)]pyrolysis,
has been varied in order to evaluate the
effect of both the nature (ie endo or exothermic) and the magnitude of its value on
the temperature response in accordance with the trends observed in previous studies
[3,16].The reduction in the external heat transfer coefficient by the gaseous flux
leaving the particle was predicted using the correlations of Kalson [I81 which
predict the Ackermann correction factor, &', defined as:
Gsis a dimensionless parameter which relates the heat transferred by the bulk
gas to the conductive heat flux across the boundary layer, and is given by :
COS
=
+
CO
1 2/ NU,
where Co =- NRCp,g
h
(7)
The mass flux leaving the particle, NR,can be calculated from the mass loss rate,
Eqn (4), and the surface area of the particle.
600
Mathematical modelling of low-rank coal particles during pyrolysis
Results and Discussion
In order to investigate the applicability of the thermophysical properties in terms of
predicting the particle response, model predictions for the measured temperature
responses of the bone dry coal particles were made. In these calculations the
influence of the heat of pyrolysis and the gaseous flux were ignored, and no
moisture was present. The parameters used in these model predictions are
summarised in Table 2. The model predictions are compared with the experimental
data at furnace temperatures of 150°C and 350°C in Figure 1, and at furnace
temperatures of 6OO0C, 700°C, and 800C in Figure 2. At the higher temperatures,
the radiation shape factor between the ceramic tube and the particle was estimated
at F, = 0.4 based on similar experiments conducted using a copper sphere. From
both Figures it can be seen that the thermophysical properties assumed in the
current model enable accurate predictions of the particle temperature response to be
made. Good agreement between the model prediction and the experimental data, as
shown, suggests that the influence of the heat of pyrolysis and the gaseous flux can
be confidently assessed using the model predictions.
Table 2. Model and experimental parameters used in the original model
predictiom.
Particle size
dp.o = lOmm
Specific heat
C,, = 1150 + 2.03(T - 300) - 1.55 x lO3(T - 300)*
Thermal conductivity
k, = 0.19 + 2.5
Density
po= 780 kg.m"
N, flow rate
G = 3.5 L.min-' (@15OC, 1 a m )
Initial temperature
To= 25°C
x
104(T - 300)
A&olyrir
0 W.kg-' for T > 300°C
A,"
0 fort > 0
601
C.A.Heidenreich, H.M. Yan and D.K.Zhang
400
350
-
0
Y
g 300
J
+.
$
+
$.
250
0.
200
2
Z 150
1
-Model
predidion
T
8
$
.-
100
2
50
lz
0
0
100
200
300
400
500
600
Time [s]
Figure 1. Measured and predicted temperature response of lOmm Bowmans coal
particles in a horizontal tube furnace at 150°C and 350°C.
Figure 2. Measured and predicted temperature response of lOmm Bowmans coal
particles in a horizontal tube furnace at 6OO0C, 7OOOC and 800°C
602
Mathematical modelling of low-rank coal particles during pyrolysis
The effect of the heat of pyrolysis is minimal at 150°C and 350°C as no
significant pyrolysis has occurred, hence no comparisons can be made using the
data. Adesanya and Pham [lo] used a constant endothermic heat of pyrolysis of
300 kJ.kg-’ in their modelling work. Using this value, the effect on the predicted
temperature response is compared with the original model predictions in Figure 3
as denoted by the “Run 1” curves. It can be seen that this value for the heat of
reaction has a significant influence on the predicted temperature response,
especially in the region above -350°C in which the bulk of the pyrolysis occurs. To
investigate the effect of the trend suggested by Lopez-Peinado et al. [ 161, the heat
of pyrolysis was assumed to vary from endothermic (300450°C)to exothermic
(450-750°C)and returning to endothermic thereafter. Within each range, a
sinusoidal type curve was used to describe the variation of AHpymlysis
with
temperature, and the maximum value of AHpyrolyrir
in each range was varied. Above
750°C the heat of pyrolysis was assumed to be constant. The maximum values of
the heat of pyrolysis in the exothermic region correspond to the average values for
the two ranges quoted by Adesanya and Pham [3] in their review of previously
published values. The
s,,
values
, used in the runs performed are summarised
in Table 2, and the model predictions are again compared with the original model
predictions in Figure 3.
The results of these runs further indicate that the heat of pyrolysis can have an
effect on the predicted temperature response. The time required for the particle
centre to reach the furnace temperature was not greatly affected by the heat of
pyrolysis and, in general, any effect during heating was not substantial. The
predictions from Run 1 show the greatest variation when compared to the original
predictions. However a better understanding of the magnitude and nature of the
heat of pyrolysis is required to fully compliment the current model. Interestingly,
the measured temperature responses in Figure 1 do exhibit a faster initial heating
rate (up to 400°C)than predicted which is also the case in the model predictions
resulting from Runs 2 and 3. At this stage it is premature to suggest that the heat of
pyrolysis proposed in either Runs 2 or 3 are correct subject to further analysis.
603
C.A. Heidenreich, H.M. Yan and D.K.Zhang
900
i
~
800 +
700
i
600 500
t
a
400
L
-
c.
S
8
300
B
1
200
-
100
1
0
30
0
60
Time [s]
90
120
Figure 3. Comparison between original model predictions and those when the heat
ofpyrolysis is considered according to Runs 1-3. Gaseousjlm effects ignored
Table 3. Mpyrolysis values used in the various model runs.
Run No.
Run 1
Run 2
Run 3
Temp range
([~lpym~ysdmax
("C)
300 +
kJ.kg-'
300
300 - 450
450 - 750
750 +
300 - 450
450 - 750
750 +
300
-355
300
300
-635
300
Figure 4 compares the original model predictions with those calculated when
the effect of the gaseous flux leaving the particles is considered. These results
clearly indicate that the effect is minimal for large coal particles. This is as expected
considering that the heating rate is considerably lower than for micron-sized coal
604
Mathematical modelling of low-rankcoal particles during pyrolysis
particles and as a result the gaseous flux at the surface is minimal. The calculated
minimum values for the Ackermann correction factor in each run were found to
decrease with increasing temperature; (A:)min = 0.92 @ 600"C,0.89 @ 7OO0C, and
0.85 @ 800°C. This is again expected as the higher temperatures induce higher
maximum pyrolysis rates thus leading to a greater surface gas flux. Although
reductions in the convective heat transfer coefficient of up to 15% were calculated
at SOO"C,the contribution of radiation to the total external heat transfer rate reduces
the significance of this value somewhat. It can be safely concluded that the gaseous
flux does not need to be considered in future modelling efforts.
The influence of the presence of moisture is shown in Figures 5 and 6 for
furnace temperatures of 150°C and 350"C7and 600°C and 800"C, respectively. The
arrows in each figure indicate an increase in moisture content according to the
following; Owt?!, lSwt?!,
35wt?! and 52wt% on a wet basis. The data clearly
indicate that moisture has a marked effect on the temperature response at each of
the temperatures investigated. It can be seen that the effect on the temperature
response is not limited to the region around 100°C which indicates that the
assumption of a wet/dry interface at 100°C proposed by Wildegger-Gaismaier [ 121
may not be correct. Furthermore, several researchers [ 17,22-231 have reported the
presence of a "temperature plateau" which appears during the heating of large coal
particles. It has been proposed that the gaseous flux at the particle surface may
have caused the temperature plateau [24] however the results presented here
suggest that this is unlikely. While it is possible that the heat of pyrolysis may
contribute to this temperature plateau, the uncertainty which exists as to its
magnitude and nature does not enable any definite conclusions to be drawn at this
stage. It is very likely that the presence of moisture is the major contributor to the
measured temperature plateau. In general the effect that moisture has on the
temperature response is far more significant than the heat of pyrolysis and further
effort will be made to incorporate the effect of moisture into the present model.
605
C.A. Heidenreich, H.M. Yan and D.K. Zhang
-
900
800 --
-Modd predidion-flueffed ignored
.-....Model prediction - flweffedinduded
0
40
20
60
100
80
120
Tme [s]
Figure 4. Comparison between original model predictions and those when the
gaseousflux efect is considered. Heat of pyrolysis effects ignored.
400
I
-0 wt% moisture
- - - 35 wt% moisture
.- -.-.15 wt% moisture
- - - - 52 wt% moisture
--------..--
I
0 1
0
i
500
1000
1500
2000
Time [s]
Figure 5. Measured temperature response of IOmm Bowmans coal particles of
varying moisture content in horizontal tube furnace at 150 97 and 350 97.
606
Mathematical modelling of low-rank coal particles during pyrolysis
900
800
?!
700
Eal
600
E
500
3
c
p.
*
t__.__I
I
400
-0 wt% moisture
300 I
- .- .- -15 wt% moisture
- - - 35 wt% moisture
c
200
- - - - 52 wt% moisture
/-
100
0
o
40
80
120
160
200
240
280
320
360
400
Time [s]
Figure 6. Measured temperature response of IOmm Bowmans coal particles of
vatying moisture content in horizontal tubefurnace at 600
and 800 T.
Conclusions
The current model enables the temperature response of dry coal particles to be
accurately predicted using the proposed thermophysical properties. A sensitivity
study indicates that the gaseous flux at the particle surface has no significant effect
on the predicted temperature response and hence can be ignored in future modelling
work. The heat of pyrolysis can have a significant effect on the temperature
response, however, the uncertainty surrounding the magnitude and nature of the
heat of pyrolysis remains a problem. The effect of the presence of inherent moisture
on the temperature response is quite significant, and is superior to that of the heat of
pyrolysis. The reduction in the heating rate which results fiom the presence of
moisture would further reduce the significance of the heat of pyrolysis hence
emphasising the importance of the moisture.
Further model development is
required to incorporate the effect of moisture into the model after which a detailed
assessment of the kinetics of pyrolysis can be performed.
607
C.A. Heidenreich, H.M.Yan and D.K. Zhang
Acknowledgments
The authors gratefully acknowledge the financial and other support received for
this research from the Cooperative Research Centre (CRC) for New Technologies
for Power Generation from Low-Rank coal, which is established and supported by
the Australian Government’s Cooperative Research Centres program. Craig
Heidenreich and Hong Ming Yan would also like to thank the CRC for a
postgraduate scholarship and a research fellowship awards, respectively.
Nomenclature
Ahl
CP
F
h
AH
k,
AM
NR
q(T)
r
T
t
V
V’
VP
Subscripts
0
conv
g
pyrolysis
RP
rad
P
S
co
Ackermann correction factor
Specific heat
Radiation shape factor
Heat transfer coefficient
Heat of reaction
Thermal conductivity of coal
Mass loss
Surface gas flux
Energy accumulation term
Radial position
Temperature
Time
Volatile matter evolved
Total volatile matter evolved
Particle volume
.
Initial condition
Convection
Gas
Due to pyrolysis
Particle outer radius
Radiation
Particle
Spherical conditions1Solid
Bulk gas
Greek letters
&
P
d
608
emissivity
density
Stephan-Boltnans constant
Mathematical modelling of low-rank coal particles during pyrolysis
Dimensionless groups
Heat transferred by bulk flow/conductive heat flux
co
Nu
Nusselt number
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