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Coal gasification in a stirred bed reactor
Fuel Vol. 75, No. 14, pp. 1617-1624, 1996
ELSEVIER
PII: S0016-2361(96)00133-0
Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/96$15.00+ 0.00
Coal gasification in a stirred bed reactor
Jiejie Huang and A. P. Watkinson* Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P.R. China * Department of Chemical Engineering, The University of British Columbia, Vancouver, B.C., Canada V6T 1/24 (Received 28 July 1995,"revised 12 June 1996)
Two Canadian non-caking coal chars of millimetre particle size were gasified in a stirred bed reactor at atmospheric pressure using steam-nitrogen mixtures. Experimental results showed that the reaction atmosphere changed with carbon conversion and the unit operated as a typical integral reactor. Some properties of the partially reacted chars were measured. The effects of gas superficial velocity, char particle size, steam partial pressure and temperature on conversion were examined. The role of transport resistance was assessed and empirical kinetic models were tested. Copyright © 1996 Elsevier Science Ltd. (Keywords: coal gasification; stirred bed reactor; kinetics)
Engineering research on the reactivities and kinetics of coal conversion aims to provide the data and basis for the mathematical modelling, development, design and operation of coal gasifiers. Because of the variability of coal quality and structure, no universally applicable kinetic model has been developed. Rather, many different models have been presented for specific ranges of application. Kinetic data can be gathered in a differential reactor such as a thermobalance; however, particle properties such as size and density, which change with conversion and have important effects on the design and operation of gasifiers, cannot be readily obtained using samples from t.g.a, studies. The fixed bed, which is an integral reactor, is closer to the conditions of an industrial gasifier, but because of axial temperature and solid composition variations presents certain difficulties in analysing kinetic data. To obtain data which are more useful for a large-scale gasifier, an atmospheric slowly stirred bed reactor has been developed 1'2. It can hold a significant quantity of coal (100 g) of millimetre particle size. Sufficient amounts of char can be recovered for characterization of particle properties as conversion proceeds. An unexplained difference in reaction rate has been reported 3 between the t.g.a, and the stirred bed, which uses a sample size I000 times that of the former. In this work, the stirred bed reactor was used to explore the kinetics of reaction of two coals of different rank whose performance in a spouted bed gasifier had been reported previously. The influence of transport on the kinetics was also assessed. One of the most common kinetic models for charCO2 and char-steam reactions uses the LangrnuirHinshelwood approach based on concepts of adsorption, reaction and desorption of gases on the solid surface.
For the carbon-steam reaction, the rate expression is given by R=
klPH20 1 + k2PH20 + k3PH2
(1)
Although it includes a term involving the product H2 pressure, direct application of this equation is limited because of the requirement to specify three rate constants, each of which may be a function of temperature and other variables. Johnson 4 proposed a rather complicated rate expression somewhat like the LangmuirHinshelwood form. It was developed to cover high pressures, and included terms for attack on carbon by hydrogen, which can generally be neglected at moderate pressures. The Gibson-Euker expression 5 represents a simpler two-constant kinetic scheme which considers the effect of thermodynamic reversibility: ~-~= k ( 1 - x) (PH20
PH2KPC° )
(2)
where k is the rate constant and K is the equilibrium constant of the C-steam reaction. For practical purposes, many empirical rate expressions which involve only one or two rate constants have been proposed. The rate constants are determined comparatively easily from experimental time-conversion data. However, extrapolation beyond the range of experiment should be avoided. The general rate expression for an irreversible volumetric reaction can be written in the following form: dx
-dt = kvfl(x, T)C~,0(1 - x) m
Fuel 1996 Volume 75 Number 14
(3)
1617
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson where /3(x, T) represents the relative available pore surface area of the particles. Structural models such as the random capillary and random pore models 6'7 are also used in coal gasification. The latter model expresses the reaction rate as dx _ ksCn(1 - x)[1 - ~ln(1 - x)] 1/2 dt 1 - e0
(4)
where ~b = 47rL0(1 - eo)/S2o. The unification of coal gasification data proposed by Raghunathan and Yang 8, which is based on the random pore model, fitted carbon conversion as a function of dimensionless time '7- = t/t!/2, where tl/2 is the time at which x = 0.5. The expression is given as
Table 1 Coal analysis
Proximate analysis (wt%) Volatiles Fixed carbon Moisture Ash Ultimate analysis (wt% as-received) Carbon Hydrogen Nitrogen Oxygen (by diff.) Sulfur Free swellingindex
Highvale (HV)
Coal Valley (CV)
40.6 30.3 18.8 10.3
32.8 51.8 7.9 7.5
52.7 3.33 0.82 13.9 0.16 1
66.9 4.07 0.90 12.7 0.17 1
1 - x = exp[-p(T +p~T2/4)] Table 2 Coal ash analysis (wt%)
p = 2[(1 + ~ln2) 1/2 - 1]/~
(5)
Values of ~ = 2.7 and p = 0.514 were obtained by fitting 110 sets of data. This integral approach is best for carbon conversions of <80%, and appeared to work for prior studies with the stirred bed reactor 3, although the dependence of rate on conversion of Equation (4) was not followed. In this paper, experiments with two coal chars are analysed for effects of process variables on reaction rate. Changes in particle properties with conversion are also reported, such that the results may be used for modelling fluid or spouted bed gasifiers. EXPERIMENTAL The stirred bed semi-batch reactor was described previously 1'2. It consists of a preheater furnace which generates steam from the feed water, and a gasifier furnace in which further heating takes place and in which a 6.3cm dia. reaction chamber is fitted. The latter is equipped with a sintered plate distributor, on which a ~6 cm deep bed of coal is placed. A six-pronged stirrer is used to keep the bed well mixed and to improve mass and heat transfer to the chars. Two Canadian coals, Highvale subbituminous coal and Coal Valley bituminous coal, were used. Their properties are shown in Tables 1 and 2. Both are noncaking coals and high in volatile matter. Coal samples of 0.85-3.0 mm were screened into different size fractions for the experiments. For each run, a 100 g coal sample was devolatilized in flowing N2 for 2h at the same temperature as the subsequent ~gasification step and at a low stirring speed of 17 r e v m i n - " to minimize comminution. Then a 400°C N2-steam mixture was fed to the reactor and the stirrer was adjusted to 26revmin -I (the upper limit of stable stirrer operation) to ensure complete mixing of the solids. Part of the gas produced was passed through a dryer containing magnesium perchlorate, for subsequent analysis. The gas, which was sampled every 10-15 min, was analysed by a gas chromatograph with a column of Carbosphere (80-100 mesh and 2 m long). For each run the calibration of the gas chromatograph was checked using a standard gas mixture. In separate experiments, the reaction was interrupted at different times to recover partially reacted chars for subsequent property measurements. Random densepacked bulk densities were determined in a 2.7 cm dia.
1618
Fuel 1996 Volume 75 Number 14
Coal SiO2 A1203 Fe203 TiO2 P205 CaO MgO SO3 Na20 K20 HV 54.77 18.03 3.62 0.63 0.12 12.32 1.36 4.07 0.94 0.34 CV 55.52 19.00 4.24 0.65 0.16 10.06 1.39 2.22 0.88 0.57
cylinder. Particle size was determined by sieving. The particle elutriation (carryover) velocity, defined below, was determined using closely sized fractions in a 0.9 cm dia. x 42 cm long transparent plastic tube with nitrogen at room temperature. The carbon conversion was calculated based on inlet N2 flow and gas composition: x = nc°2 ÷ n c ° + ncH4
(6)
nc
where nco2, nco and riCH 4 a r e moles of outlet gas and nc is moles of initial carbon in char. RESULTS AND DISCUSSION
Effect of superficial gas velocity Though a stirrer is used to improve the mass and heat transfer, a suitable superficial gas velocity at which the external mass transfer resistance could be ignored was first determined. The tests were carried out for both coals at different gas flows, and the results are shown in Figure 1. It is observed that the effect of superficial velocity is significant when it is <0.233 m s - l . For the char particles of dp ~ 1.4mm and a temperature of 900°C, this corresponded to a particle Reynolds number of 2. Conversion increased with velocity for both chars up to 0.233ms -l. Increasing the velocit~ to 0.3ms -l gave identical results to those at 0.233 m s- over the full range of conversion for CV char, whereas for HV char at conversions >50% the rate decreased. This could have been due to higher fines generation with the more porous HV char, which was derived from the coal of much higher volatile matter. The initial rate of gasification of the HV char was about three times that of the CV char. The gas velocity was set at 0.233 m s -1 in experiments to test other variables.
Effect of initial coal particle size on conversion In the absence of external transport resistance, the effect of coal particle size on char gasification can be attributed mainly to the action of internal diffusion of gas, which decreases in effect as the size is reduced. There
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson
100 - (a) Highvale
100
=
(a) Highvale
/'./7" /-'/./:// /,,-
80
'2
6o
!
40
//'/
20
0
6 e. o
~..~. ;'--"0~.303 m/s
/ / /
-
~ I
I
0
20
I
I
40
60
u - 0.121 I
I
80
100
I~s I
120
40
d~ /~/¢
I
.•"=
/
0
./
/
• I ..
/
T = 900°C ~ u o = 0.3 m/s "*--" uo = 0.233 m/s ~ u o = 0.18 m/s
I
50
20
I
I
40
80
-,:// /
= ,0.85"-1.4 = mm I
60
I
80
t
100 t rain
t
150
t
200
Figure 1 Effect of superficial gas velocity on carbon conversion for (a) Highvale, (b) Coal Valley coals (dp = 1.4 mm, PH:O,in = 30 kPa)
is a limit below which the diffusional resistance effect is negligible. Yang 2 examined two typical coal sizes of 1.41.65 mm and 0.85-1.4 mm and found little difference in carbon conversion. A wider range of coal size of 0.853.0 mm was chosen in this work to cover typical spouted bed gasifier feed sizes. The gasification results are given in Figure 2. It is noted that for Highvale coal the carbon conversion decreases with increasing coal particle size, but for Coal Valley coal there is no significant difference up to a carbon conversion of 60%; above this value there are slightly higher conversions for the larger particles, but the differences are within experimental error. The effect of particle size is small for this coal, as was found previously.
Variation of char properties during gasification If gasification occurs according to the shrinkingparticle-segregated-ash model, then the average particle size will decrease with conversion according to dp/d~ = (I - x ) °333, whereas if reaction occurs mainly on the internal surfaces, particle size will not change markedly until very high conversions. Figure 3 shows that virtually no change in size of the 0.7ram chars occurred until x was ~0.7. For the 1.4mm char, 10% shrinkage occurred by x ,~ 0.5. Clearly the particle size change with reaction is much less than expected from the
/
'°F / 20t-
/ /
I'¢
120
,• •
//
:/•
_
:c
900°C
-d°= l'0-2"0mm "--*--"d° = 2.0-2.36 mm ~ m d o = 2.36-3.0 mm
h
/ 0
i
100
100[r- (b) Coal Valley
6o-
20
[
0
8(3
4o
~
O-
I
140
(b) Coal Valley
"~
T 900°C
I/
20
100
'i
//
60
I
50
I
100 t min
I
150
I
200
Figure 2 Effect of coal particle size on carbon conversion for (a) Highvale, (b) Coal Valley coals (PH20,in = 30 kPa)
shrinking-particle model. The production rate of fines by spalling and attrition in the stirred bed is also seen to be negligible until x > 0.7. For x < 0.8, random dense-packed bulk density could be fitted by a single equation for chars from both coals of 0.6-1 mm size and for HV coal of l - 2 m m size: p = 7 8 0 - 571x
(7)
Hence at x = 0.8 the bulk density is ~42-52% of the initial value. As shown in Figure 4, at x ~ 0.8, the bulk density began to increase again with conversion, reaching values of 400-550kgm -3 at x = 0.95. This effect may be due to increased ash content and reduced particle size. Because of the decreases in density with reaction, particles may be more readily elutriated from a gasifier. The gas velocity at which initial carryover of narrowly sized particles occurred, defined as the carryover velocity, was found to decrease markedly with increasing conversion, to about one-half its initial value, and was correlated with conversion as follows:
Uc/Uco = 1 - 0.973X + 0.447x 2
(8)
Values are compared with the particle terminal velocity in Figure 5, where calculated terminal velocities are based on an empirical relation 9 valid for particle Reynolds numbers from 2 to 500.
Fuel 1996 Volume 75 Number 14
1619
Coal gasification
i n a s t i r r e d b e d reactor." J. H u a n g
a n d A . P. W a t k i n s o n
--•-- Highvale char 0.85 mm - - * - Highvale char 0.6 mm --&--Coal Valley char 0.6 mm ~Calculated values, chat size 0.85 mm ---Calculated values, char size 0.6 mm
09
A
3.0 0.7 2.5-
•
i.,, Hi hvale --~vale char d~ = 0.71 mm N N --•-- .Coa_lValleych_.ar-dv0= 0.75 mrn ~ •
0.6
0.5
ar/a~0= (I-X) ~/3
2.0
\
/I
/
- • - - Highvale char 0,6~0.85 mm - O - C o a l Valley char 0.6-1.0 mm
•
S =
1.5
# 1.0
/
~ SS~'
all d,~j,~, A
s
S
2.". ,,s" l'
sS
•
0.5-
O
0.0
I 200 300
F I I I I 400 500 600 700 800 Char bulk density kg/m 3
I 900
Figure 5 Comparison between experimental carryover velocity and calculated terminal velocity
t 20
0
I 40
i 60
I 80
--a- HV --e-HV --A-- CV --v-- CV
I 100 '~
Carbon conversion % Figure 3 Variation of char diameter and fines generation during gasification (T = 900°C, stirrer speed = 26 rev r a i n - ' )
~•
-a
- - a - - Highvale char 1.0-2.0 m m - - * - Highvale char 0.6-0.85 m m - - • - - C o a l Valley char 0 . 6 - 1 . 0 m m
.\, !
~bs S
300
I
0 Figure 4
20
I
I
I / !
Ss'
I
40 60 80 Carbon conversion %
I
100
Variation of char bulk density during gasification
Kinetic model test As is evident from Figures I and 2, the gasification rate remains essentially constant up to carbon conversions of ,,~50-60%. Over the same range, the specific rates increase with carbon conversion by varying amounts, depending on the coal and the temperature as shown in Figure 6. In addition, it is found from Figure 7 that the
1620
Fuel 1996 Volume 75 Number 14
./
•
•
0..'¢
~ ,. .. • ..
A e,
.vv.v-v'v*v~v_Yov.vv.Vy ,, I i I I 20 40 60 80 Carbon conversion %
I 1O0
Figure 6 Variation of specific gasification rate with carbon conversion (PH20,in = 30 kPa)
:%.X,," %-~
|
,,,
,Y
I 0
/
II
/
~- J
- ~m •
500
| 1.e
,.A.A ° A f
70(
600
•s ss
S
? st
•
• i'll
SI
-
800
9000C 850"C 900"C 850"C
steam content in the outlet gas increases with carbon conversion, due to the char weight loss with time, and the concentrations of product gas species also vary. Here a fixed inlet gasifying agent composition (30 vol.% H 2 0 70 vol. %N2, or 50 vol. % H20-50 vol. %N2) was used. The N2 contents are not shown in the plots. Over the first 5060% of carbon conversion, the steam partial pressure remains essentially constant. As the reaction nears completion, the steam concentration rises to the inlet value and the carbon-containing gas species approach zero concentration. To be valid up to high conversions, a kinetic model should not just take carbon conversion as the only variable but also allow for changes in gas composition. The effect of buildup of product gases on the rate was considered. In the work of Goyal and Rehmat 1°, the rate of gasification in a 50% steam-50% N2 mixture was double than that in a 50% steam-50% H2 mixture. In
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson 60
55 -
P ~ o = 30 kPa • H 2 • CO
50 -
• CO 2 •
45 -
PH20 = 30 kPa • H 2 + CO ~ CO~ * H20
4O "~ e~
-
~-
Table 3 Parametersdeterminedfor Equation (10)
H20
in
.~
30
0.07414 28.045
m
56.51 131.68
n
no
-0.489 0.351 0.380 0 0.542 0.245
./* ~/
(a) -6.6
Y I"*"*"v
25
>
ko (s-1 kPa-" mm-") E (kJ mo1-1)
HV CV
/*
.___.,..,1. . /
35
Coal
(a) Highvale
E 2o 15
V-~V~'O-'V,~O~_~V
no
o+_...~•~+ "~+,•
~ =
• H V coal
_
• C V coal
-7.0
-7.4
_
=~ - . ~ .
"'~
I~ -7.8
_=
I 20
0 60
I
~ 60
40
"r'o-~.,,t.._. 100 80 -8.2
55
(b) Coal Valley
50
-8.6 -
45
•
. . . , . . . , °*'*'*'w**~* -9.0
-~ 40
8.0
35 ,-- 30 ~ 25
8.4
~
8.8
9.2
9.6
104fr K -i
(b) -6.8
2o 15
• HV coal
_
i
=
~
c°al
-7.0
10 5 0
I 0
I
20
I
40
~ -*']'L**~ .q'~',K4~&e ' I
60
80
100
-7.2
,\
Carbon conversion% Pin H20 =
30 kPa PinH20=
• H 2 • CO • CO 2 • }12O
30 ld~a
• H2 + CO x CO 2 . H20
-7.4
Figure 7
Variation of gas composition with carbon conversion for (a) Highvale, (b) Coal Valley chars (T = 900°C, d~ : 1.4mm)
the present case, the concentration of H 2 is < 15 vol.% (Figure 7), so the rate effects are not expected to be significant. The absence of product gas inhibition was also assessed via the Gibson-Euker model. Calculations using the data of Figure 7 showed that the term Pn2" Pco/K could be neglected compared with PH2o. Internal diffusion resistances can affect the parameters extracted from kinetic experiments. As pore diffusion becomes increasingly important, the observed value of the activation energy will be reduced, and the reaction rate will show an inverse dependence on the particle diameter ~1. These effects can be captured by the introduction of an effectiveness factor, for example in Equation (3), which for m = 1 yields dx (1 - x) dt
- - kvT](dP'
T)/~(x, T ) ~ H 2 0
(9)
The data were fitted empirically to the following equation: d__fx dt = k° exp ( - RE-T)d~'~2°xn° (1 - x)
(10)
-7.6 0.0
I 0.2
Wl 0.4
I 0.6
I 0.8
I 1.0
J 1.2
ln(dp) Figure 8
Variation of rate functionwith (a) temperature,(b) particle
size
where it is evident that the observed temperature effect lumps both diffusion and chemical reaction together. During char gasification, PH20 and x vary at the same time, so n and no had to be determined by two-variable least-squares regression. From the data at different inlet steam partial pressures, the values of n and no were found (Table 3). A rate function was defined as follows:
dx/dt a = (1 - x)P~zoxno
(11)
E and m were determined through the fitting of ln(~) to 1/T and ln(dp), where c~is the average value for each run. Good linearity is shown in Figure 8, except for the plot of ln(~) versus ln(dp) for Coal Valley coal. The comparison of calculated dx/dt values from Equation (10) with the parameter values of Table 3 is shown in Figure 9. This
Fuel 1996 Volume 75 Number 14
1621
Coal gasification in a stirred bed reactor: J. Huang and A. P. Watkinson 6-
3.5-
T(°C) PinH2o(kPa) 3.0
2.5
• • •
900 900 850
50 30 30
•
800
30
J
• HV coal; T = 900°C • CV coal; T = 900oc
/• •/
"/./_
2.0
.
/
5-
.,/
"= 1,5 OS• •
•
,/v
• ,,v=
I.C
•
0.5
(a) Highvale
i 0.5
I 1.0
I I [ 1.5 2.0 2.5 Exp. dx/dt *104 s 1
I
I
3.0
3.5 3
I 0.5 1.0
I 1.5
I 2.0
I 2.5
I I~1 3.0 3.5
I 4.0 4.5
In (PH20)
1.6 1.4
• • • •
1.2
900 930 900 850
50 30 30 30
Figure 11
...//
T(°C) PinH2o(kPa)
Variation of half-life with steam pressure (dp = 1.44 ram)
~ / o • •
0.8
• HV coal; n' = 0.448 • CV coal; n' = 0.227
061
./
0.2 0
S I 0.2
Oe~ 6 ,.p
N
(b) Coal Valley I 0.4
I I I I 0.6 0.8 1.0 1.2 Exp. dx/dt "104 s "1
L 1.4
_=
J 1.6
11
100
4 +,%.
8.0
I
I
8.4
I
8.8
ffP
80
I
9.2
I
9.8
10.0
104/1 ` K-t
SIB
/
O
",~ 60 ¢) ;>
6.07 5.8
.I
o
# i.
40
/
~ O ~ o - - ~ o . . . .
-
5.6
&
x 2Or- "t ~
0
J
5
Figure 9 Comparison of experimental and calculated rates using Equation (10) for (a) Highvale, (b) Coal Valley chars
=O
/
.f.
0.4
P" =-~- 5.4
Calculated values
• HV coal; n' = 0.448 • CV coal; n' = 0.227
~5.2 I 1
I 2
I 3
I 4
/
I 5
5.0
Dimensionless time "r Figure 10
Comparison of experimental carbon conversion data with predictions from Equation (5) for Highvale and Coal Valley chars
4.8
'•
4.6
shows that Equation (10) is suitable for fitting the data with the carbon conversion up to 90%. The data unification test using Equation (5) is shown in Figure 10. It is evident that Equation (5) is in good agreement with the experimental data up to carbon
1622
Fuel 1996 Volume 75 Number 14
0.0
I
0.2
i
0.4
I
0.6
I
0.8
I
1.0
i
1.2
ln(dp) Figure 12
Variation of kinetic parameters with temperature and particle size (T = 900°C; PH20,in = 30 kPa)
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson
conversions of 95%. It is important to know the half-life so that the design calculations can be made. Since the initial rate is constant at least up to 50% conversion, the half-life is proportional to the inverse of the rate, and hence depends on parameters such as temperature, pressure and particle size:
[" E'~dm'p( tl/2=kloexp~-~) p H20
(12)
Figures 11 and 12 show that the half-life correlates well with the steam partial pressure, and that normalized by the steam pressure, good correlations of the resulting kinetic parameters with temperature and particle size are found. The regressed parameters are given in Table 4. Figure 13 shows that the combination of Equations (5) and (12) provides a good basis for prediction of the rate of reaction, and how it changes with the gasification parameters.
Differences in kinetic parameters for the two chars are evident in Tables 3 and 4. The apparent activation energy for the more reactive Highvale char is low, and a significant particle size effect is found. This suggests that an internal diffusional resistance is involved for the particle size tested. The intrinsic gasification rate for Highvale coal is known to be high because of catalytic effects of minerals. Sakata and Watkinson 12 have reported that leaching alkaline metals from Highvale char with hydrochloric acid results in a roughly fourfold reduction in gasification reactivity. Hence transport effects are expected to influence the present results. The more slowly reacting Coal Valley char exhibits a higher apparent activation energy than does Highvale char, and no particle size effect, which suggests that diffusional effects are less important for this inherently less reactive material. CONCLUSIONS
Table
4
Parameters determined for Equation (12)
Coal
k~ (s -1 k P a - ' mm -m')
E' (kJ mol -l)
m'
n~
HV CV
31.266 115.53
52.48 134.09
0.375 0
-0.448 -0.227
3.5
T(°C) Pinli20 (kPa)
3.0
• •
900 900 850 750
•
2.5
• "
¢#
d
•/ •/
50 30 30 30
/ • •/~e~
2.0
•Y"
1.5
/,
•/.
;,':
e~
r.,,) 1.0
y~O 0.5
,,T
ACKNOWLEDGEMENTS
•
A&e
0.C
I
0.0
0.5
(a) Highvale I
]
I
I
-
].2"
• • • •
T(°C) PinH20(kPa) 900 50 930 30 900 30 850 30
I
3.5
0.0f I 0.0 0.2
1 •/ L •f/ _ "I I •
2 3 4 5
•~ •
<.
6 7 8 (b) Coal Valley
$ 0.4
This work was carried out at The University British Columbia, Canada. Financial assistance Jiejie Huang was supplied by the Chinese Academy Sciences. Ongoing support to A.P.W. by NSERC acknowledged.
$ I I I 0.6 0.8 1.0 1.2 Exp. dx/dt * 104 s l
of to of is
REFERENCES
=,•2;" •
-~ 0.6e5
0.2
3.0
/
o8-
0.4
I
1.0 1.5 2.0 2.5 Exp. dx/dt "104 s l
1.61.4
Study of the gasification of two Canadian coal chars with steam in a stirred bed reactor has shown an effect of gas superficial velocity on carbon conversion at velocities <0.233 m s -1 for coal particle size 0.85-3.0 mm at 900°C. Highvale char is about three times as reactive as Coal Valley char. For the more reactive Highvale char, coal particle size is important and the apparent activation energy is low. For the less reactive Coal Valley char, particle size effects can be ignored, and the apparent activation energy is high. It has been shown that both a semi-empirical kinetic expression and the data unification master curve of Raghunathan and Yang 8 can be used to fit the experimental data. Quantification of the changes with conversion of particle size, bulk density and elutriation carryover velocity provides a basis for modelling of spouted and fluid bed gasifiers using the established kinetics.
$ 1.4
9 $ 1.6
Figure 13 Comparison of experimental and calculated rates using Equations (5) and (12) for (a) Highvale, (b) Coal Valley chars
10 11
Nguyen, Q. T. and Watkinson, A. P. Can. J. Chem. Eng. 1990, 68, 814 Yang, Y. M.A.Sc. Thesis, The University of British Columbia, 1991 Yang, Y. and Watkinson, A. P. Fuel 1994, 73, 1786 Johnson, J. L. In 'Coal Gasification', Advances in Chemistry Series No. 131, American Chemical Society, Washington, DC, 1974 Sundaresan, S. and Amundson, N. R. Chem. Eng. Sci. 1979, 34, 345 Gavalas, G. R. AIChEJ. 1980, 26, 577 Bhatia, S. K. and Perlmutter, D. D. AIChE J. 1980, 26, 379 Raghunathan, K. and Yang, R. Y. K. Ind. Eng. Chem. Res. 1989, 28, 518 Davidson, J. F. and Harrison, D. 'Fluidization', Academic Press, London, 1971, p. 630 Goyal, A. and Rehmat, A. In 'Clean Energy from Waste and Coal' (Ed. M. R. Khan), Symposium Series 515, American Chemical Society, 1991 Fogler, H. S. 'Elements of Chemical Reaction Engineering', 2nd Edn, Prentice-Hall, Englewood Cliffs, NJ, 1992
Fuel 1996 Volume 75 Number 14
1623
Coal gasification in a stirred bed reactor: J. Huang and A. P. Watkinson Sakata, Y. and Watkinson, A. P. In 'Developing Advanced Process for Efficient Use of Coal' (Ed. K. Hashimoto), Ministry of Education, Science and Culture, Japan, 1989, pp. 36-41
12
NOMENCLATURE C
CA Cs,o
dp E' E, k K
ko, k~
kv Lo f
m,m n, no, n t rl c
nco, nco~, riCH4
1624
local reactant gas concentration concentration of reactant gas initial concentration of reactant solid coal particle diameter (mm) apparent activation energy (kJ mol- l) rate constant equilibrium constant pre-exponential factor intrinsic rate constant rate constant initial length of overlapped system per unit volume reaction order reaction order moles of initial carbon in char moles of outlet gas
Fuel 1996 Volume 75 Number 14
P Prt2o, PH2, Pco R
So t
T tu2
uc ut W
X
o~
3(x, T) ~0
(dp, T)
# P T
¢
parameter in Equation (5) partial pressure of gas (kPa 1 specific gasification rate (s-') initial surface area of particle reaction time (s) temperature (K) half-life (s) particle carryover velocity (ms -1) particle terminal velocity (rn s-1) fines generation rate (g g-l min-l) carbon conversion based on initial carbon moles in char defined term in Equation (11) relative available pore surface area of particle initial porosity of particle effectiveness factor viscosity of gas (Pa s) density of char (kgm -3) dimensionless time = t/h/.2 pore structure parameter in random pore model
ELSEVIER
PII: S0016-2361(96)00133-0
Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/96$15.00+ 0.00
Coal gasification in a stirred bed reactor
Jiejie Huang and A. P. Watkinson* Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P.R. China * Department of Chemical Engineering, The University of British Columbia, Vancouver, B.C., Canada V6T 1/24 (Received 28 July 1995,"revised 12 June 1996)
Two Canadian non-caking coal chars of millimetre particle size were gasified in a stirred bed reactor at atmospheric pressure using steam-nitrogen mixtures. Experimental results showed that the reaction atmosphere changed with carbon conversion and the unit operated as a typical integral reactor. Some properties of the partially reacted chars were measured. The effects of gas superficial velocity, char particle size, steam partial pressure and temperature on conversion were examined. The role of transport resistance was assessed and empirical kinetic models were tested. Copyright © 1996 Elsevier Science Ltd. (Keywords: coal gasification; stirred bed reactor; kinetics)
Engineering research on the reactivities and kinetics of coal conversion aims to provide the data and basis for the mathematical modelling, development, design and operation of coal gasifiers. Because of the variability of coal quality and structure, no universally applicable kinetic model has been developed. Rather, many different models have been presented for specific ranges of application. Kinetic data can be gathered in a differential reactor such as a thermobalance; however, particle properties such as size and density, which change with conversion and have important effects on the design and operation of gasifiers, cannot be readily obtained using samples from t.g.a, studies. The fixed bed, which is an integral reactor, is closer to the conditions of an industrial gasifier, but because of axial temperature and solid composition variations presents certain difficulties in analysing kinetic data. To obtain data which are more useful for a large-scale gasifier, an atmospheric slowly stirred bed reactor has been developed 1'2. It can hold a significant quantity of coal (100 g) of millimetre particle size. Sufficient amounts of char can be recovered for characterization of particle properties as conversion proceeds. An unexplained difference in reaction rate has been reported 3 between the t.g.a, and the stirred bed, which uses a sample size I000 times that of the former. In this work, the stirred bed reactor was used to explore the kinetics of reaction of two coals of different rank whose performance in a spouted bed gasifier had been reported previously. The influence of transport on the kinetics was also assessed. One of the most common kinetic models for charCO2 and char-steam reactions uses the LangrnuirHinshelwood approach based on concepts of adsorption, reaction and desorption of gases on the solid surface.
For the carbon-steam reaction, the rate expression is given by R=
klPH20 1 + k2PH20 + k3PH2
(1)
Although it includes a term involving the product H2 pressure, direct application of this equation is limited because of the requirement to specify three rate constants, each of which may be a function of temperature and other variables. Johnson 4 proposed a rather complicated rate expression somewhat like the LangmuirHinshelwood form. It was developed to cover high pressures, and included terms for attack on carbon by hydrogen, which can generally be neglected at moderate pressures. The Gibson-Euker expression 5 represents a simpler two-constant kinetic scheme which considers the effect of thermodynamic reversibility: ~-~= k ( 1 - x) (PH20
PH2KPC° )
(2)
where k is the rate constant and K is the equilibrium constant of the C-steam reaction. For practical purposes, many empirical rate expressions which involve only one or two rate constants have been proposed. The rate constants are determined comparatively easily from experimental time-conversion data. However, extrapolation beyond the range of experiment should be avoided. The general rate expression for an irreversible volumetric reaction can be written in the following form: dx
-dt = kvfl(x, T)C~,0(1 - x) m
Fuel 1996 Volume 75 Number 14
(3)
1617
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson where /3(x, T) represents the relative available pore surface area of the particles. Structural models such as the random capillary and random pore models 6'7 are also used in coal gasification. The latter model expresses the reaction rate as dx _ ksCn(1 - x)[1 - ~ln(1 - x)] 1/2 dt 1 - e0
(4)
where ~b = 47rL0(1 - eo)/S2o. The unification of coal gasification data proposed by Raghunathan and Yang 8, which is based on the random pore model, fitted carbon conversion as a function of dimensionless time '7- = t/t!/2, where tl/2 is the time at which x = 0.5. The expression is given as
Table 1 Coal analysis
Proximate analysis (wt%) Volatiles Fixed carbon Moisture Ash Ultimate analysis (wt% as-received) Carbon Hydrogen Nitrogen Oxygen (by diff.) Sulfur Free swellingindex
Highvale (HV)
Coal Valley (CV)
40.6 30.3 18.8 10.3
32.8 51.8 7.9 7.5
52.7 3.33 0.82 13.9 0.16 1
66.9 4.07 0.90 12.7 0.17 1
1 - x = exp[-p(T +p~T2/4)] Table 2 Coal ash analysis (wt%)
p = 2[(1 + ~ln2) 1/2 - 1]/~
(5)
Values of ~ = 2.7 and p = 0.514 were obtained by fitting 110 sets of data. This integral approach is best for carbon conversions of <80%, and appeared to work for prior studies with the stirred bed reactor 3, although the dependence of rate on conversion of Equation (4) was not followed. In this paper, experiments with two coal chars are analysed for effects of process variables on reaction rate. Changes in particle properties with conversion are also reported, such that the results may be used for modelling fluid or spouted bed gasifiers. EXPERIMENTAL The stirred bed semi-batch reactor was described previously 1'2. It consists of a preheater furnace which generates steam from the feed water, and a gasifier furnace in which further heating takes place and in which a 6.3cm dia. reaction chamber is fitted. The latter is equipped with a sintered plate distributor, on which a ~6 cm deep bed of coal is placed. A six-pronged stirrer is used to keep the bed well mixed and to improve mass and heat transfer to the chars. Two Canadian coals, Highvale subbituminous coal and Coal Valley bituminous coal, were used. Their properties are shown in Tables 1 and 2. Both are noncaking coals and high in volatile matter. Coal samples of 0.85-3.0 mm were screened into different size fractions for the experiments. For each run, a 100 g coal sample was devolatilized in flowing N2 for 2h at the same temperature as the subsequent ~gasification step and at a low stirring speed of 17 r e v m i n - " to minimize comminution. Then a 400°C N2-steam mixture was fed to the reactor and the stirrer was adjusted to 26revmin -I (the upper limit of stable stirrer operation) to ensure complete mixing of the solids. Part of the gas produced was passed through a dryer containing magnesium perchlorate, for subsequent analysis. The gas, which was sampled every 10-15 min, was analysed by a gas chromatograph with a column of Carbosphere (80-100 mesh and 2 m long). For each run the calibration of the gas chromatograph was checked using a standard gas mixture. In separate experiments, the reaction was interrupted at different times to recover partially reacted chars for subsequent property measurements. Random densepacked bulk densities were determined in a 2.7 cm dia.
1618
Fuel 1996 Volume 75 Number 14
Coal SiO2 A1203 Fe203 TiO2 P205 CaO MgO SO3 Na20 K20 HV 54.77 18.03 3.62 0.63 0.12 12.32 1.36 4.07 0.94 0.34 CV 55.52 19.00 4.24 0.65 0.16 10.06 1.39 2.22 0.88 0.57
cylinder. Particle size was determined by sieving. The particle elutriation (carryover) velocity, defined below, was determined using closely sized fractions in a 0.9 cm dia. x 42 cm long transparent plastic tube with nitrogen at room temperature. The carbon conversion was calculated based on inlet N2 flow and gas composition: x = nc°2 ÷ n c ° + ncH4
(6)
nc
where nco2, nco and riCH 4 a r e moles of outlet gas and nc is moles of initial carbon in char. RESULTS AND DISCUSSION
Effect of superficial gas velocity Though a stirrer is used to improve the mass and heat transfer, a suitable superficial gas velocity at which the external mass transfer resistance could be ignored was first determined. The tests were carried out for both coals at different gas flows, and the results are shown in Figure 1. It is observed that the effect of superficial velocity is significant when it is <0.233 m s - l . For the char particles of dp ~ 1.4mm and a temperature of 900°C, this corresponded to a particle Reynolds number of 2. Conversion increased with velocity for both chars up to 0.233ms -l. Increasing the velocit~ to 0.3ms -l gave identical results to those at 0.233 m s- over the full range of conversion for CV char, whereas for HV char at conversions >50% the rate decreased. This could have been due to higher fines generation with the more porous HV char, which was derived from the coal of much higher volatile matter. The initial rate of gasification of the HV char was about three times that of the CV char. The gas velocity was set at 0.233 m s -1 in experiments to test other variables.
Effect of initial coal particle size on conversion In the absence of external transport resistance, the effect of coal particle size on char gasification can be attributed mainly to the action of internal diffusion of gas, which decreases in effect as the size is reduced. There
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson
100 - (a) Highvale
100
=
(a) Highvale
/'./7" /-'/./:// /,,-
80
'2
6o
!
40
//'/
20
0
6 e. o
~..~. ;'--"0~.303 m/s
/ / /
-
~ I
I
0
20
I
I
40
60
u - 0.121 I
I
80
100
I~s I
120
40
d~ /~/¢
I
.•"=
/
0
./
/
• I ..
/
T = 900°C ~ u o = 0.3 m/s "*--" uo = 0.233 m/s ~ u o = 0.18 m/s
I
50
20
I
I
40
80
-,:// /
= ,0.85"-1.4 = mm I
60
I
80
t
100 t rain
t
150
t
200
Figure 1 Effect of superficial gas velocity on carbon conversion for (a) Highvale, (b) Coal Valley coals (dp = 1.4 mm, PH:O,in = 30 kPa)
is a limit below which the diffusional resistance effect is negligible. Yang 2 examined two typical coal sizes of 1.41.65 mm and 0.85-1.4 mm and found little difference in carbon conversion. A wider range of coal size of 0.853.0 mm was chosen in this work to cover typical spouted bed gasifier feed sizes. The gasification results are given in Figure 2. It is noted that for Highvale coal the carbon conversion decreases with increasing coal particle size, but for Coal Valley coal there is no significant difference up to a carbon conversion of 60%; above this value there are slightly higher conversions for the larger particles, but the differences are within experimental error. The effect of particle size is small for this coal, as was found previously.
Variation of char properties during gasification If gasification occurs according to the shrinkingparticle-segregated-ash model, then the average particle size will decrease with conversion according to dp/d~ = (I - x ) °333, whereas if reaction occurs mainly on the internal surfaces, particle size will not change markedly until very high conversions. Figure 3 shows that virtually no change in size of the 0.7ram chars occurred until x was ~0.7. For the 1.4mm char, 10% shrinkage occurred by x ,~ 0.5. Clearly the particle size change with reaction is much less than expected from the
/
'°F / 20t-
/ /
I'¢
120
,• •
//
:/•
_
:c
900°C
-d°= l'0-2"0mm "--*--"d° = 2.0-2.36 mm ~ m d o = 2.36-3.0 mm
h
/ 0
i
100
100[r- (b) Coal Valley
6o-
20
[
0
8(3
4o
~
O-
I
140
(b) Coal Valley
"~
T 900°C
I/
20
100
'i
//
60
I
50
I
100 t min
I
150
I
200
Figure 2 Effect of coal particle size on carbon conversion for (a) Highvale, (b) Coal Valley coals (PH20,in = 30 kPa)
shrinking-particle model. The production rate of fines by spalling and attrition in the stirred bed is also seen to be negligible until x > 0.7. For x < 0.8, random dense-packed bulk density could be fitted by a single equation for chars from both coals of 0.6-1 mm size and for HV coal of l - 2 m m size: p = 7 8 0 - 571x
(7)
Hence at x = 0.8 the bulk density is ~42-52% of the initial value. As shown in Figure 4, at x ~ 0.8, the bulk density began to increase again with conversion, reaching values of 400-550kgm -3 at x = 0.95. This effect may be due to increased ash content and reduced particle size. Because of the decreases in density with reaction, particles may be more readily elutriated from a gasifier. The gas velocity at which initial carryover of narrowly sized particles occurred, defined as the carryover velocity, was found to decrease markedly with increasing conversion, to about one-half its initial value, and was correlated with conversion as follows:
Uc/Uco = 1 - 0.973X + 0.447x 2
(8)
Values are compared with the particle terminal velocity in Figure 5, where calculated terminal velocities are based on an empirical relation 9 valid for particle Reynolds numbers from 2 to 500.
Fuel 1996 Volume 75 Number 14
1619
Coal gasification
i n a s t i r r e d b e d reactor." J. H u a n g
a n d A . P. W a t k i n s o n
--•-- Highvale char 0.85 mm - - * - Highvale char 0.6 mm --&--Coal Valley char 0.6 mm ~Calculated values, chat size 0.85 mm ---Calculated values, char size 0.6 mm
09
A
3.0 0.7 2.5-
•
i.,, Hi hvale --~vale char d~ = 0.71 mm N N --•-- .Coa_lValleych_.ar-dv0= 0.75 mrn ~ •
0.6
0.5
ar/a~0= (I-X) ~/3
2.0
\
/I
/
- • - - Highvale char 0,6~0.85 mm - O - C o a l Valley char 0.6-1.0 mm
•
S =
1.5
# 1.0
/
~ SS~'
all d,~j,~, A
s
S
2.". ,,s" l'
sS
•
0.5-
O
0.0
I 200 300
F I I I I 400 500 600 700 800 Char bulk density kg/m 3
I 900
Figure 5 Comparison between experimental carryover velocity and calculated terminal velocity
t 20
0
I 40
i 60
I 80
--a- HV --e-HV --A-- CV --v-- CV
I 100 '~
Carbon conversion % Figure 3 Variation of char diameter and fines generation during gasification (T = 900°C, stirrer speed = 26 rev r a i n - ' )
~•
-a
- - a - - Highvale char 1.0-2.0 m m - - * - Highvale char 0.6-0.85 m m - - • - - C o a l Valley char 0 . 6 - 1 . 0 m m
.\, !
~bs S
300
I
0 Figure 4
20
I
I
I / !
Ss'
I
40 60 80 Carbon conversion %
I
100
Variation of char bulk density during gasification
Kinetic model test As is evident from Figures I and 2, the gasification rate remains essentially constant up to carbon conversions of ,,~50-60%. Over the same range, the specific rates increase with carbon conversion by varying amounts, depending on the coal and the temperature as shown in Figure 6. In addition, it is found from Figure 7 that the
1620
Fuel 1996 Volume 75 Number 14
./
•
•
0..'¢
~ ,. .. • ..
A e,
.vv.v-v'v*v~v_Yov.vv.Vy ,, I i I I 20 40 60 80 Carbon conversion %
I 1O0
Figure 6 Variation of specific gasification rate with carbon conversion (PH20,in = 30 kPa)
:%.X,," %-~
|
,,,
,Y
I 0
/
II
/
~- J
- ~m •
500
| 1.e
,.A.A ° A f
70(
600
•s ss
S
? st
•
• i'll
SI
-
800
9000C 850"C 900"C 850"C
steam content in the outlet gas increases with carbon conversion, due to the char weight loss with time, and the concentrations of product gas species also vary. Here a fixed inlet gasifying agent composition (30 vol.% H 2 0 70 vol. %N2, or 50 vol. % H20-50 vol. %N2) was used. The N2 contents are not shown in the plots. Over the first 5060% of carbon conversion, the steam partial pressure remains essentially constant. As the reaction nears completion, the steam concentration rises to the inlet value and the carbon-containing gas species approach zero concentration. To be valid up to high conversions, a kinetic model should not just take carbon conversion as the only variable but also allow for changes in gas composition. The effect of buildup of product gases on the rate was considered. In the work of Goyal and Rehmat 1°, the rate of gasification in a 50% steam-50% N2 mixture was double than that in a 50% steam-50% H2 mixture. In
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson 60
55 -
P ~ o = 30 kPa • H 2 • CO
50 -
• CO 2 •
45 -
PH20 = 30 kPa • H 2 + CO ~ CO~ * H20
4O "~ e~
-
~-
Table 3 Parametersdeterminedfor Equation (10)
H20
in
.~
30
0.07414 28.045
m
56.51 131.68
n
no
-0.489 0.351 0.380 0 0.542 0.245
./* ~/
(a) -6.6
Y I"*"*"v
25
>
ko (s-1 kPa-" mm-") E (kJ mo1-1)
HV CV
/*
.___.,..,1. . /
35
Coal
(a) Highvale
E 2o 15
V-~V~'O-'V,~O~_~V
no
o+_...~•~+ "~+,•
~ =
• H V coal
_
• C V coal
-7.0
-7.4
_
=~ - . ~ .
"'~
I~ -7.8
_=
I 20
0 60
I
~ 60
40
"r'o-~.,,t.._. 100 80 -8.2
55
(b) Coal Valley
50
-8.6 -
45
•
. . . , . . . , °*'*'*'w**~* -9.0
-~ 40
8.0
35 ,-- 30 ~ 25
8.4
~
8.8
9.2
9.6
104fr K -i
(b) -6.8
2o 15
• HV coal
_
i
=
~
c°al
-7.0
10 5 0
I 0
I
20
I
40
~ -*']'L**~ .q'~',K4~&e ' I
60
80
100
-7.2
,\
Carbon conversion% Pin H20 =
30 kPa PinH20=
• H 2 • CO • CO 2 • }12O
30 ld~a
• H2 + CO x CO 2 . H20
-7.4
Figure 7
Variation of gas composition with carbon conversion for (a) Highvale, (b) Coal Valley chars (T = 900°C, d~ : 1.4mm)
the present case, the concentration of H 2 is < 15 vol.% (Figure 7), so the rate effects are not expected to be significant. The absence of product gas inhibition was also assessed via the Gibson-Euker model. Calculations using the data of Figure 7 showed that the term Pn2" Pco/K could be neglected compared with PH2o. Internal diffusion resistances can affect the parameters extracted from kinetic experiments. As pore diffusion becomes increasingly important, the observed value of the activation energy will be reduced, and the reaction rate will show an inverse dependence on the particle diameter ~1. These effects can be captured by the introduction of an effectiveness factor, for example in Equation (3), which for m = 1 yields dx (1 - x) dt
- - kvT](dP'
T)/~(x, T ) ~ H 2 0
(9)
The data were fitted empirically to the following equation: d__fx dt = k° exp ( - RE-T)d~'~2°xn° (1 - x)
(10)
-7.6 0.0
I 0.2
Wl 0.4
I 0.6
I 0.8
I 1.0
J 1.2
ln(dp) Figure 8
Variation of rate functionwith (a) temperature,(b) particle
size
where it is evident that the observed temperature effect lumps both diffusion and chemical reaction together. During char gasification, PH20 and x vary at the same time, so n and no had to be determined by two-variable least-squares regression. From the data at different inlet steam partial pressures, the values of n and no were found (Table 3). A rate function was defined as follows:
dx/dt a = (1 - x)P~zoxno
(11)
E and m were determined through the fitting of ln(~) to 1/T and ln(dp), where c~is the average value for each run. Good linearity is shown in Figure 8, except for the plot of ln(~) versus ln(dp) for Coal Valley coal. The comparison of calculated dx/dt values from Equation (10) with the parameter values of Table 3 is shown in Figure 9. This
Fuel 1996 Volume 75 Number 14
1621
Coal gasification in a stirred bed reactor: J. Huang and A. P. Watkinson 6-
3.5-
T(°C) PinH2o(kPa) 3.0
2.5
• • •
900 900 850
50 30 30
•
800
30
J
• HV coal; T = 900°C • CV coal; T = 900oc
/• •/
"/./_
2.0
.
/
5-
.,/
"= 1,5 OS• •
•
,/v
• ,,v=
I.C
•
0.5
(a) Highvale
i 0.5
I 1.0
I I [ 1.5 2.0 2.5 Exp. dx/dt *104 s 1
I
I
3.0
3.5 3
I 0.5 1.0
I 1.5
I 2.0
I 2.5
I I~1 3.0 3.5
I 4.0 4.5
In (PH20)
1.6 1.4
• • • •
1.2
900 930 900 850
50 30 30 30
Figure 11
...//
T(°C) PinH2o(kPa)
Variation of half-life with steam pressure (dp = 1.44 ram)
~ / o • •
0.8
• HV coal; n' = 0.448 • CV coal; n' = 0.227
061
./
0.2 0
S I 0.2
Oe~ 6 ,.p
N
(b) Coal Valley I 0.4
I I I I 0.6 0.8 1.0 1.2 Exp. dx/dt "104 s "1
L 1.4
_=
J 1.6
11
100
4 +,%.
8.0
I
I
8.4
I
8.8
ffP
80
I
9.2
I
9.8
10.0
104/1 ` K-t
SIB
/
O
",~ 60 ¢) ;>
6.07 5.8
.I
o
# i.
40
/
~ O ~ o - - ~ o . . . .
-
5.6
&
x 2Or- "t ~
0
J
5
Figure 9 Comparison of experimental and calculated rates using Equation (10) for (a) Highvale, (b) Coal Valley chars
=O
/
.f.
0.4
P" =-~- 5.4
Calculated values
• HV coal; n' = 0.448 • CV coal; n' = 0.227
~5.2 I 1
I 2
I 3
I 4
/
I 5
5.0
Dimensionless time "r Figure 10
Comparison of experimental carbon conversion data with predictions from Equation (5) for Highvale and Coal Valley chars
4.8
'•
4.6
shows that Equation (10) is suitable for fitting the data with the carbon conversion up to 90%. The data unification test using Equation (5) is shown in Figure 10. It is evident that Equation (5) is in good agreement with the experimental data up to carbon
1622
Fuel 1996 Volume 75 Number 14
0.0
I
0.2
i
0.4
I
0.6
I
0.8
I
1.0
i
1.2
ln(dp) Figure 12
Variation of kinetic parameters with temperature and particle size (T = 900°C; PH20,in = 30 kPa)
Coal gasification in a stirred bed reactor." J. Huang and A. P. Watkinson
conversions of 95%. It is important to know the half-life so that the design calculations can be made. Since the initial rate is constant at least up to 50% conversion, the half-life is proportional to the inverse of the rate, and hence depends on parameters such as temperature, pressure and particle size:
[" E'~dm'p( tl/2=kloexp~-~) p H20
(12)
Figures 11 and 12 show that the half-life correlates well with the steam partial pressure, and that normalized by the steam pressure, good correlations of the resulting kinetic parameters with temperature and particle size are found. The regressed parameters are given in Table 4. Figure 13 shows that the combination of Equations (5) and (12) provides a good basis for prediction of the rate of reaction, and how it changes with the gasification parameters.
Differences in kinetic parameters for the two chars are evident in Tables 3 and 4. The apparent activation energy for the more reactive Highvale char is low, and a significant particle size effect is found. This suggests that an internal diffusional resistance is involved for the particle size tested. The intrinsic gasification rate for Highvale coal is known to be high because of catalytic effects of minerals. Sakata and Watkinson 12 have reported that leaching alkaline metals from Highvale char with hydrochloric acid results in a roughly fourfold reduction in gasification reactivity. Hence transport effects are expected to influence the present results. The more slowly reacting Coal Valley char exhibits a higher apparent activation energy than does Highvale char, and no particle size effect, which suggests that diffusional effects are less important for this inherently less reactive material. CONCLUSIONS
Table
4
Parameters determined for Equation (12)
Coal
k~ (s -1 k P a - ' mm -m')
E' (kJ mol -l)
m'
n~
HV CV
31.266 115.53
52.48 134.09
0.375 0
-0.448 -0.227
3.5
T(°C) Pinli20 (kPa)
3.0
• •
900 900 850 750
•
2.5
• "
¢#
d
•/ •/
50 30 30 30
/ • •/~e~
2.0
•Y"
1.5
/,
•/.
;,':
e~
r.,,) 1.0
y~O 0.5
,,T
ACKNOWLEDGEMENTS
•
A&e
0.C
I
0.0
0.5
(a) Highvale I
]
I
I
-
].2"
• • • •
T(°C) PinH20(kPa) 900 50 930 30 900 30 850 30
I
3.5
0.0f I 0.0 0.2
1 •/ L •f/ _ "I I •
2 3 4 5
•~ •
<.
6 7 8 (b) Coal Valley
$ 0.4
This work was carried out at The University British Columbia, Canada. Financial assistance Jiejie Huang was supplied by the Chinese Academy Sciences. Ongoing support to A.P.W. by NSERC acknowledged.
$ I I I 0.6 0.8 1.0 1.2 Exp. dx/dt * 104 s l
of to of is
REFERENCES
=,•2;" •
-~ 0.6e5
0.2
3.0
/
o8-
0.4
I
1.0 1.5 2.0 2.5 Exp. dx/dt "104 s l
1.61.4
Study of the gasification of two Canadian coal chars with steam in a stirred bed reactor has shown an effect of gas superficial velocity on carbon conversion at velocities <0.233 m s -1 for coal particle size 0.85-3.0 mm at 900°C. Highvale char is about three times as reactive as Coal Valley char. For the more reactive Highvale char, coal particle size is important and the apparent activation energy is low. For the less reactive Coal Valley char, particle size effects can be ignored, and the apparent activation energy is high. It has been shown that both a semi-empirical kinetic expression and the data unification master curve of Raghunathan and Yang 8 can be used to fit the experimental data. Quantification of the changes with conversion of particle size, bulk density and elutriation carryover velocity provides a basis for modelling of spouted and fluid bed gasifiers using the established kinetics.
$ 1.4
9 $ 1.6
Figure 13 Comparison of experimental and calculated rates using Equations (5) and (12) for (a) Highvale, (b) Coal Valley chars
10 11
Nguyen, Q. T. and Watkinson, A. P. Can. J. Chem. Eng. 1990, 68, 814 Yang, Y. M.A.Sc. Thesis, The University of British Columbia, 1991 Yang, Y. and Watkinson, A. P. Fuel 1994, 73, 1786 Johnson, J. L. In 'Coal Gasification', Advances in Chemistry Series No. 131, American Chemical Society, Washington, DC, 1974 Sundaresan, S. and Amundson, N. R. Chem. Eng. Sci. 1979, 34, 345 Gavalas, G. R. AIChEJ. 1980, 26, 577 Bhatia, S. K. and Perlmutter, D. D. AIChE J. 1980, 26, 379 Raghunathan, K. and Yang, R. Y. K. Ind. Eng. Chem. Res. 1989, 28, 518 Davidson, J. F. and Harrison, D. 'Fluidization', Academic Press, London, 1971, p. 630 Goyal, A. and Rehmat, A. In 'Clean Energy from Waste and Coal' (Ed. M. R. Khan), Symposium Series 515, American Chemical Society, 1991 Fogler, H. S. 'Elements of Chemical Reaction Engineering', 2nd Edn, Prentice-Hall, Englewood Cliffs, NJ, 1992
Fuel 1996 Volume 75 Number 14
1623
Coal gasification in a stirred bed reactor: J. Huang and A. P. Watkinson Sakata, Y. and Watkinson, A. P. In 'Developing Advanced Process for Efficient Use of Coal' (Ed. K. Hashimoto), Ministry of Education, Science and Culture, Japan, 1989, pp. 36-41
12
NOMENCLATURE C
CA Cs,o
dp E' E, k K
ko, k~
kv Lo f
m,m n, no, n t rl c
nco, nco~, riCH4
1624
local reactant gas concentration concentration of reactant gas initial concentration of reactant solid coal particle diameter (mm) apparent activation energy (kJ mol- l) rate constant equilibrium constant pre-exponential factor intrinsic rate constant rate constant initial length of overlapped system per unit volume reaction order reaction order moles of initial carbon in char moles of outlet gas
Fuel 1996 Volume 75 Number 14
P Prt2o, PH2, Pco R
So t
T tu2
uc ut W
X
o~
3(x, T) ~0
(dp, T)
# P T
¢
parameter in Equation (5) partial pressure of gas (kPa 1 specific gasification rate (s-') initial surface area of particle reaction time (s) temperature (K) half-life (s) particle carryover velocity (ms -1) particle terminal velocity (rn s-1) fines generation rate (g g-l min-l) carbon conversion based on initial carbon moles in char defined term in Equation (11) relative available pore surface area of particle initial porosity of particle effectiveness factor viscosity of gas (Pa s) density of char (kgm -3) dimensionless time = t/h/.2 pore structure parameter in random pore model