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Modeling of a Low Temperature Pyrolysis Process Using ASPEN PLUS.

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Dev. Chem. Eng. Mineral Process., 7(S/6),pp.577-591,1999.
Modeling of a Low Temperature Pyrolysis
Process Using ASPEN PLUS
H. M. Yan and D. K. Zhang*
CRC New Technologies for Power Generation fiom Low-Rank Coal,
Department of Chemical Engineering, The University of Adelaide,
Adelaide, South Australia 5005, Australia.
An ASPEN PLUS based simulation model has been developed for a low temperature
pyrolysis process, incorporating triple fruidised be& with solid circulation, for both
valuable fuel oil production and power generation from low-rank coal. This paper
reports preliminary simulation results using a Loy rang brown coal as the feed.
Simulations indicate that, for the coal modelled combustion of char and remaining
volatiles ajier pyrolysis not only provide enough energy requiredfor the endothermic
pyrolysis reactions and the wet coal e i n g , but also can supply extra energy for
power generation. About 2.7 MW power and 0.26 kg s-1 fuel oil can be produced
from 1 kg s-1 dry coal fied to the process. Simulation also shows that the initial
moisture content in the coal does not aflect the tar yield in the pyrolyser but has an
effect on the power generationfrom the process. Fraction of the non-condensablefuel
gas burnt in the combustor does not have signifcant effects on the power generated
from the process.
Introduction
Low rank coals have been used for power generation using conventional pulverisedcoal firing technology[l]. Two main problems exist when the low rank coals are used
in power generation industry[ 11. Firstly, the high moisture content (as high as 60%)
*Authorfor correspondence.
577
H.M. Yan and D.K.Zhang
of brown coal made it a poor competitor with black coal because of a much larger
size boiler required. As a consequence the capital cost, maintenance and operating
costs of the brown coal boiler are greater. Secondly, the usually high alkaline content
in brown coals leads to severe fouling and ash deposit on heat transfer surfaces and
this significantly influences boiler performance causing operational problems [2].
Low-temperature pyrolysis provides an alternative way for utilisation of low rank
coal.
The low-temperature pyrolysis technology thermochemically decomposes the low
rank coal at moderate temperatures (450
-
600°C) to yield liquid (fuel oil), non-
condensable gas (NCG)and residue char (and ash). The liquid fuel oil can be used as
a substitute or blending agent for transport fuels. The char and the NCG are byproducts which can be burnt on-site to provide the energy required for the coal drying
and endothermic pyrolysis process, and possibly for auxiliary electric power
generation. This paper presents our preliminary study using ASPEN PLUS to model
such a pyrolysis process.
Process Description
Triple fluidised beds are used for coal drying, low-temperature pyrolysis and high
temperature combustion, respectively, and solids are circulated among the three
fluidised beds.
The coal is fed into the drier and most of moisture is driven out at a temperature
between 200
-
250°C.
The dried coal with 10% moisture is transferred to the
pyrolyser, in which the tar and combustible gas are generated at low temperature of
450
- 600°C.
Products of tar oil and the non-condensable gas can be separated by
condensation. The remaining char is fed into the fluidised bed combustor in which
char and air react to generate energy for the coal drying and pyrolysis processes, and
the extra energy, if any, is used to generate high pressure and high temperature steam
for power generation. A part of energy generated in the combustor is carried by the
hot solids and hot flue gas, which is also used as a fluidisation gas, to the drier and
pyrolyser, respectively. More circulating solids go to the pyrolyser to provide most of
578
Modeling of low temperature pyrolysis process using ASPEN PLUS
energy required in it and minimise contamination of N2 to the NCG by the hot flue
gas. More hot flue gas goes to the drier to provide most of energy required.
Model Development
Assumptions
1. The yields of tar oils and gases from the pyrolysis reactor are based on the
experimental data in the literature [3]. Remaining volatiles in the char after the
pyrolysis are assumed to be completely released and burnt in the combustor.
2. Since components of tar oils from the pyrolysis process are very complex and
many of them can not be found in the ASPEN data bank, it is assumed that
chemical formula of tar oils is described by four components, ie. C12H26S,
C15H33N, C14H1202 and ClOH1204.
3. As an initial efforts to model the low temperature pyrolysis process, we assume
excellent hydrodynamics exists in the fluidised beds and reaction kinetics are not
considered.
ASPEN Simulation Flowsheet
An ASPEN simulation flowsheet is illustrated in Figure 1. The wet coal (FSLGE)
with 50% of initial moisture is fed into a fluidised bed drier (DRY-REAC) in which
operating temperature is maintained at 230 "C. The drier is modelled as an adiabatic
reactor. The energy required in the drier is supplied by the hot flue gas (GASDRY)
and circulating solids (SAND2) from combustor to the drier. Another hot gas stream
(FGAS4) is also supplied to the drier to meet energy required at different initial
moisture level of coal. From the drier, 10% of moisture is remained in the coal. The
exiting stream (DRYSLG) from the drier is a mixture of water vapour, flue gas and the
dried coal (10% moisture), which is separated in a separator (Dry-Flash). After
separation, the gas (DRYGAS) goes to a two-phase heat exchanger (HEXI), in which
the latent heat of water vapour in the gas is used to preheat feed air to the system
(FDAIR), and the dried char (SLG1) goes to the pyrolyser (PYRO-R).
The energy requirement in the pyrolyser is mainly supplied by circulating solids
(SAND1)and also by the flue gas (FGAS3) from the combustor (COMBS), the latter
is also used as a fluidising gas in the reactor. The pyrolyser is also modelled as an
579
H.M. Yan and D.K. Zhang
adiabatic reactor and its maximum operating temperature is controlled below 600 "C
by varying the solid circulation rate. The yields of tar oil, some components in the
pyrolysis gas and char in the pyrolyser are temperature-dependent. The pyrolysis
product (PY 1) from the pyrolyser is a mixture of char, and gas that consists of light
and heavy hydrocarbons, and is separated into two streams in a separator (SPLIT1).
The stream (PROGAS) of pyrolysis gas goes through two heat exchangers (HEX4 and
HEX3) and a flasher (FLASHl), and cooled down to 80 "C to obtain the final oil
product (OILS) and the non-condensable gas (NCG). The stream (SLD) of char and
solids goes to a fluidised bed combustor.
Three reactor models (COMB1, RYLD and COMBS) are used to model the
fluidised bed combustor. In the COMB 1, the operating temperature is controlled at
900 "C by controlling the extent of char combustion. The hot flue gas (GASI) from
the COMB 1 is divided (SPLIT3) into two streams to drier and pyrolyser, respectively.
The circulating solids (CIP1) are directly goes to the COMBS. The char (SLD1) goes
to the yield reactor (RYLD), in which the remaining volatiles in char are completely
released and then fed into the combustor (COMBS). The heat required in the RYLD
reactor is directly supplied by a heat stream (YLDHS) from the COMBS which also
operates at 900 "C. The COMBS is modelled as an equilibrium reactor, in which all
the combustible components is assumed to be burn out. Energy generated in the
COMBS is transported to the heat mixer by a heat stream (CMHS).The combustion
products (PROD21 from the COMBS are separated into two streams. The ash (FASH)
is discharged from the system. The flue gas which consists of a large amount of NZ,
goes to a separator (SPLIT8) in which flue gas is divided separated into two streams,
one (FGAS4) goes to the drier and the other (TOPY) to the pyrolyser. If the NCG is
considered to be burnt in the combustor, it is fed into the COMBS by stream (NCG2)
directly.
The extra energy generated in the process is accumulated in a heat mixer (HMX)
and then transported by a heat stream HST to a heat exchanger (HEATS), in which
superheated steam above 550 "C at 10 atm is generated. Then the steam is transported
to a steam turbine (TURBINE) to generate electricity PWRl. The turbine is modelled
using a compressor model in the ASPEN and its efficiency is 72%.
580
Modeling of low temperature pyrolysis process using ASPEN PLUS
J
f
Figure I . ASPEN simulationjlowsheet for low-temperaturepyrolysis.
581
H.M. Yan and D.K. Zhang
Solid Circulation
Since only continuous and steady state operation can be treated in ASPEN; and
ASPEN does not provide a convenient way for closing the circulating solids loop
(which is different from a recycle stream), the solution is achieved by matching the
solid flow rates in (FDSAND)and out (SANDOUT).The model uses the two heater
models (HEAT1 and HEAT2) to simulate hot inert solid circulation. The amount of
energy required in one heater (HEATI) is balanced by the energy supply (SDHS1)
from the other (HEAT2) as shown in Figure 1.
Fortran and Design-Specifibn Blocks
There are five Fortran sub-routines and two design-specification blocks combined
in the simulation currently which are not shown in Figure 1. The Fortran sub-routines
are used to calculate changes of proximate and ultimate analyses of different feeds due
to processes of drying, pyrolysis and combustion. One of them is also used to
calculate the yields of tar oil and pyrolysis gas based on the experimental data 131 and
to perform element balances of C, H, 0, N and S for all components. One designspecification block is used to control the operating temperature in the drier by varying
the flue gas (FGAS4) flow rate to the drier. The other design-specification block is
used to specify that oxygen content in the flue gas (GAS1) from the COMB 1 is zero
by varying the feed air flow rate into it. This ensures that no oxygen goes into the
pyrolyser with the flue gas so that oil production can be maximised.
Simulations
Model Inputs
Model inputs in the simulation runs are obtained from literature[3]. They investigated
the pyrolysis behaviours of three low rank coals in a fluidised bed reactor. The data of
a Loy Yang brown coal is chosen in the current study. The proximate and ultimate
analyses of both coal and char after pyrolysis are given Table 1.
582
Modeling of low temperature pyrolysis process uring ASPEN PLUS
Table 1: Analysis of Loy Yang coal and char after pyrolysis [3]
Proximate analysis
(air-dried, w%)
10.8
Moisture
Ash
0.6
VM
45.8 1
FC
42.79
Ultimate analysis
(air-dried, w%)
C
62.9 1
H
4.25
0
20.64
N
0.53
S
0.27
Ultimate analysis
(daf, w%, for char)
C
77-78
H
4-4.5
OJS
17-18
The following three polynomial equations are obtained by correlating the data
given by Edwards et a1.[3] to calculate the temperature-dependent (450 5 T 5 600 "C)
yields of tar oils and components of CH4and C2H4in the pyrolyser.
Y UIr = -0.66+ 2.7 x
YCH,
YC2H4
T -2 x
T'
(1)
= L5 1- 9.95 x 10-3 T + 1.54 x 10-5 T 2
= L8 1- 9.57x
T + 1.26 x
T2
(3)
where yi is yields of pyrolysis products (kdkg coal, daf basis) and T is "C.
Edwards' experiments confirmed that the maximum yield of tar is found at 600 "C,
above which the tar yield declines for all three coals tested due to the secondary
decomposition of the tar vapours. Other components in non-condensable gas are
C2H6,C3H6and C3H8,the yields of which are 0.2%, 0.3% and 0.1% respectively and
are not varying with temperature. The pyrolysis products of Hz, CO and C02 from the
Lot Yang brown coal are in trace quantities [3].
Simulation Runs
At first, the ASPEN base simulation run is performed according to conditions
given in Table 2.
Then, sensitivity study runs are performed according to the
conditions in Table 3.
583
H.M. Yan and D.K.Zhang
Table 2: Conditions for the ASPEN base simulation run:
Coal Feed Rates
Solid circulation rates
Air feed rates
Drier temperature
Pyrolysis temperature
Combustor temperature
NCG burning
Percentage of solid circulation
Percentage of flue gas (FGAS3)
Percentage of flue gas (FGAS4)
kg/s
kg/s
kg/s
"C
"C
"C
%
%
%
%
%
%
%
2.0
1.0
variable
230
600
900
Yes
10
90
80
20
43
O
57
wet (50% moisture)
silica sand
oxygen dependent
in COMBS
to drier
to pyrolyser
to drier
to pyrolyser
to drier
to pyrolyser
discharge
Table 3: Conditions for the sensitivity study runs:
Varying
Pyrolysis temperature
Solid circulation rates
Initial moisture content of coal
Percentage of NCG burning
Units
Range
Notes
"C
450 - 600
0.4 - 1.7
40-50
0- 100
in combustor
kg/S
%
%
Results and Discussion
The compositions of pyrolysis products predicted from the ASPEN based simulation
run are given in Table 4.
It is seen that the pyrolysis product (PY 1) from the pyrolyser consists of 79% noncombustible components (N2, C02 and H20) and only 21 % combustible components
(hydrocarbons). The high concentrations of N2 (25.98%) and C02 (14.11%) in the
pyrolysis product are a result of the fact that the flue gas (75% Na. see GAS 1 in Table
4) from the combustor enters the fluidised bed pyrolyser as a fluidising agent. The
higher water concentration (38.85%) in the pyrolysis product comes from the moisture
(10% remains after drying) in the coal and a low flow rate of the flue gas to the
pyrolyser (see Table 2). The weight percentage for light hydrocarbons (C,-C3)and H2
in the pyrolysis product is 6.458, for heavy hydrocarbons ( C l ~ # 2 137.43%
~)
and for
water 18.9%.
584
Modeling of low temperature pyrolysis process using ASPEN PLUS
The NCG product consists of about 83% inert gases and only 17% combustible
gases. The weight percentage of final tar oils consists of about 11% water and 89%
heavy hydrocarbons.
Table 4: Compositions of the pyrolysis products.
PYl
PYl
GAS1 NCG OILS FGAS
(w%) (v%) (v%) (w%) (v%)
H20
38.85 18.90
- 35.25 10.87 10.12
co2
14.11 16.77 18.15 16.78 0.04 16.04
N2
25.98 19.65 75.21 30.88 0.01 71.92
co
5.86 4.43 4.54 6.97
- 0.00
02
0.03 0.03 0.10 0.04
1.91
CH4
3.74 1.62
- 4.53
H2
2.92 0.16
- 3.47
C2H4
1.19 0.90
1.44
C3H8
0.29 0.34
- 0.34
- 0.01
so2
C2H6
0.21 0.17
0.25
Cl2H26S
0.34 1.85
- 4.45
C15H33N
0.30 1.85
- 4.44
- 17.92
C14H1202
1.29 7.41
C10H1204
4.89 25.92
- 0.04 62.28
[notes]: PY1- gas products from the pyrolyser; GAS1- flue gas enter the pyrolyser;
NCG - non-condensable fuel gas; OILS - final tar oil products and FGAS - flue gas
discharged from the process.
(v%)
Effects of Temperature
The yields of tar oils, C& and C2H4 in the pyrolyser increase with increasing the
pyrolyser temperature as shown in Figures 2. The yields of C2&, C3H6 and C3H8are
temperature-independent [3]. The yields of
H2,CO
and C02 are assumed and treated
as temperature independent variables in the model currently. Therefore, they are not
included in Figure 2.
Figure 2 illustrates that the maximum yields of pyrolysis products (including 10%
of moisture in coal released) at 600 "C are about 0.27 kg/kg dried ash free basis (daf)
for tar and 0.017 kg/(kg daf coal) for CH4 and CzH4. The mass yields of the tar (not
including water) and the non-condensable gas (light hydrocarbons C I - C ~CO,
, CO2
and H2) at 600 "C are 23.7% and 9.4% ( k g k g daf coal), respectively (not shown in
585
H.M. Yan and D.K.Zhang
Figure 2). The total volatile yield is 33% that is equivalent to about 64%of volatiles
(VM=5 1.5% daf basis, see Table 1) in the raw coal.
1.2
=
m
8
8
1.0
c
2
0.8
0.6
g 0.4
v
2 0.2
0.0
L
2
I
20
760
780
800
820
840
860
880
Temperature of Pyrolyser (K)
Figure 2. Effect of temperature of pyrolyser on the yields of pyrolysis products
Table 5 illustrates changes of elements C, H and 0 in char with varying temperature of
pyrolyser. The calculated results match closely with experimental data (C: 77-78%, H:
4-4.5% and 0: 17-18%) as given in Table 1 .
Table 5: Effect of temperature of pyrolyser on elemental compositions of C, H and
0 in the char.
723
773
823
873
586
75.61
76.11
76.52
76.80
4.66
4.46
4.23
4.00
17.90
17.54
17.30
17.21
Modeling of low temperature pyrolysis process using ASPEN PLUS
36.6 1
w
36.4
36.1
e
Q
27.0
Tar
0
0
r
2
25.0 --
n
880
1
i
s
f 850 -->
0
w
i
i
760
0.5
I
0.7
0.9
1.1
1.3
1.5
1.7
Solid circulation rate (kg/s)
Figure 3: Eflects of solid circulation rate on yields of the NCG and tar and
temperature of pyrolyser.
587
H.M. Yan and D.K. Zhang
The high oxygen and hydrogen contents in the char show that the extent of pyrolysis
for the coal used at a range of 450-600 "C is not significant when compared with the
volatile content in the raw coal; only about 64% volatiles in the raw coal is released in
the pyrolyser.
Effect of Solid Circulation Rate
Effect of solid circulation rate (SCR) on the yields of tar and the NCG,
temperature of pyrolyser is shown in Figures 3. It is seen that the temperature of
pyrolyser increases with increasing the solid circulation rate because of more energy is
carried by solids into the pyrolyser. The yields of tar increase with increasing in the
SCR due to an increase in the temperature of pyrolyser.
The concentrations of the NCG changing with varying the SCR is shown in Figure
4, which shows that fraction of combustible gas in the NCG increases with an increase
in the SCR because of temperature in increase.
-
Y
gz
33
3 28 --
E
0
5
U
r
23
+H20
+N2
-n+
+Combustible
c02
gas
--
E 181~
0
-
db
."
m
a
w
Figure 4: Effects of solid circulation rate on concentrations of NCG.
Power generated by excess energy in the process is from 2.56 - 2.88 MJ/(kg dried
coal) and decrease with an increase in the SCR as shown in Figure 5 because more
energy is consumed in the pyrolyser for both tar and NCG production.
588
Modeling of low temperature pyrolysis process using ASPEN PLUS
!
2.5
0.5
0.9
0.7
1.1
1.3
1.5
7
1.1
Solid circulation rate (kgk)
Figure 5: Effects of solid circulation rate on power generated in the process.
Effect of Initial Moisture Content of Coal
The initial moisture content in the coal has no effect on the pyrolysis product yields
because most of it is driven out in the drier. As mentioned previously, the operating
temperature of the drier is fixed at 230 "C and controlled by the hot flue gas into it in
L.""
,
- 2.75
0
-8
2.70
CJ)
x
2 2.65
\
2.60 4
40
42
44
46
48
50
Initial moisture in coal (w%)
Figure 6: Effect of initial moisture content of coal on power generated in the process
589
H.M. Yan and D.K.Zhmg
the simulation. The higher the moisture content in the coal, the more the hot flue gas
is required.
The moisture in the dried coal fed into the pyrolyser is constant (lO%).However,
the initial moisture content in the coal does have an effect on the power generated in
the process because of more energy consumed in the drier. Figure 6 shows that the
power generated decreases with increasing initial moisture content in the coal.
Effect of NCG Burning
Figure 7 gives an effect of fraction of the NCG burnt in the combustor on the
power generated in the process by excess energy, which decreases with decreasing
fraction of the NCG burnt in the combustor. It is noted that the NCG does not
significantly influence the energy generation in the system because the NCG contains
a large amount of N2 and C02 and only 17% of combustible gas as mentioned
previously.
-8 2.5 --
2.1
Power
I
,
1
0
0.2
0.4
0.6
0.8
1
Fraction of NCG burnt in combustor
Figure 7: Effect of the fraction of NCG burnt in combustor on power generated.
Conclusions
The temperature of pyrolyser influences the yields of tar oils and pyrolysis gas and
the composition of char. Solid circulation is an effective way to carry energy among
the drier, pyrolyser and combustor. The higher the solid circulation rate, the higher
590
Modeling of low temperaturepyrolysis process using ASPEN PLUS
the temperature of pyrolyser and the greater the tar oil and NCG product (below 600
"C) but the lower the power generated in the process.
The initial moisture content in the coal does not affect the tar yield in the pyrolyser
but has an effect on the power generation from the process. Fraction of the NCG
burnt in the combustor does not have significant effects on the power generation in the
process. For the coal modelled, Combustion of char and remaining volatiles after
pyrolysis not only provides enough energy required for the endothermic pyrolysis
reaction and the wet coal drying but also supply extra energy to generate high pressure
and temperature steam, which is used to drive a steam turbine to generate about 2.6
MJ power per kilogram dried coal.
The current model can be used to evaluate other pyrolysis processes with different
feedstocks quickly.
Acknowledgment
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 in Australia.
References
1. Dune, R. A. The Science of Victorian Brown Coal: Structure, Properties and
Conseauences for Utilization, Butterworth-Heinemann, Great Britain, 1991.
2. Tyler, R. J, Fuel, 58,680- 686, 1979.
3. Edwards, J. H., Smith, I. W. and Tyler, R. J., Fuel, 59, 681- 686, 1980.
591
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