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Kinetics of change in sulphur forms in hydropyrolysis of coals
Kinetics of change in sulphur hydropyrolysis of coals Takuo
Sugawara,
Katsuyasu
Department of Chemical Engineering Akita 010, Japan (Received 37 January 1989)
Sugawara
forms
and the late Hiroyasu
for Resources,
Mining
College,
Akita
in
Ohashi University,
Eight kinds of non-caking steam coals were pyrolysed using fixed beds in hydrogen streams at atmospheric pressure and at 30 kg cme2 up to 773 K. Sequential changes were followed in various sulphur forms: pyritic and ferrous sulphide sulphur in the char, and hydrogen sulphide in the gas. The internal surface area was also determined sequentially by the CO, adsorption method. The observed changes in the contents of sulphur forms were successfully simulated kinetically, based on the desulphurization scheme proposed in the authors’ previous paper and with help of a volume reaction model. The effects of hydrogen pressure and type of coal on desulphurization behaviour were characterized by a set of frequency factors determined for all the steps included in the scheme proposed. The frequency factors for volatile organic sulphur decomnosition were correlated with chances in the internal surface area within the heat-up period and with the exinite contents in raw coals. (Keywords: kinetics; sulphur; hydropyrolysis)
Ermelo (South Africa), Plateau (Utah), PSOC-156 (Utah), 522 (Illinois), 592 (Illinois), 830 (Indiana), and 979 (Wyoming). Joban-azukidaira, Ermelo and PSOC156 coals include similar amounts of inorganic and organic sulphur, while the other coals contain predominantly organic sulphur.
Precise knowledge of the dynamic behaviour of sulphur forms in hydropyrolysis is important for developing and evaluating efficient chemical coal-cleaning processes. In a previous paper’, dynamic behaviour of sulphur forms was followed for three kinds of steam coals during hydropyrolysis, and the desulphurization process was qualitatively characterized as follows: rapid release of hydrogen sulphide and sulphur-containing tar at an initial stage including a heat-up period; the liberation of hydrogen sulphide accompanying the reduction of pyrite; the capture of hydrogen sulphide as a form of refractory organic sulphur in the char matrix during atmospheric pressure treatment; suppression of the capture of hydrogen sulphide at high hydrogen pressure, which promotes the decomposition of organic sulphur; and the reduction of ferrous sulphide to metallic iron in some coals. The purpose of this study is to kinetically evaluate the desulphurization process described above. To begin with, change in sulphur forms for eight non-caking steam coals was sequentially observed in gas-flow fixed beds under atmospheric and 30 kg cme2 hydrogen pressure. The desulphurization behaviour was then simulated kinetically based on the volume reaction model, and the effects of type of coal and hydrogen pressure were discussed in terms of the frequency factors determined for the desulphurization reactions. Change in the internal surface area with reaction time was also measured, by the CO, adsorption method, to investigate the influence of physical structures in the solid phase on the desulphurization behaviour.
Hydrogen-sulphide sulphur, liberated from the solid phase during pyrolysis, was determined using an ion electrode (JIS K0108). Some improvements’ to the ASTM (D2492) method and the Gladfelter and Dickerhoof method were applied to analyse sulphur forms (pyritic, ferrous sulphide, sulphate, sulphite, and organic sulphurs) in chars.
EXPERIMENTAL
KINETIC
Samples
Coal is a poorly characterized heterogeneous mixture from the viewpoint of both chemical and physical structures. When a coal is exposed to high temperature,
Table 1 shows
analyses
proximate, ultimate and sulphur-form of the coals used; Joban-azukidaira (Japan),
0016-2361/89/081005-07$3.00 0 1989 Butterworth & Co. (Publishers)
Ltd.
Apparatus
and procedure
Two sets of gas-flow fixed beds were used for the pyrolysis experiments. The reactors included a fused silica tube (i.d. 21 mm) for the atmospheric pressure treatment and a stainless steel autoclave (i.d. 36mm, volume 2OOcm”) for the high pressure treatment. The details of the apparatus and procedure have been described elsewhere’. From preliminary experiments2, it was concluded that an average particle size < 0.43 mm allows the influence of size dependency on the char yield and desulphurization behaviour to be neglected under the conditions studied. Internal surface areas of the coals and chars were measured with a YANACO GSA-10 instrument by applying the Dubinin-Polanyi equation to CO, adsorption isotherms at 273 K. Analysis
of sulphur forms
SIMULATION
FUEL, 1989, Vol 68, August
1005
Kinetics of change Table 1
Ultimate,
in sulphur forms in coal hydropyrolysis:
proximate
and sulphur-form
analyses
T. Sugawara
for coals used
Ultimate (wt%, dry basis) Sample
C
H
N
Joban-azukidaira
62.7
3.3
Ermelo
71.5
4.9
1.5
Plateau
75.2
5.5
2.9
PSOC- I56
65.4
4.1
PSOC-522
71.2
4.7
PSOC-592
69.2
4.9
PSOC-830
71.6
PSOC-979
74.3
et al
Proximate (wt%, dry basis)
Sulphur form (wt”? of total sulphur)
S
Diff.
V.M.
F.C.
Ash
Pyritic
1.8
32.2”
40.5
42.2
17.3
42
2
1.0
21.1
30.6
55.8
13.6
42
1
57
0.7
15.7
43.3
48.9
7.x
12.
I
81
1.6
1.4
26.9
41.8
51.8
6.4
48
3
49
2.0
0.9
21.2
41.3
57.1
1.6
14
3
x3
1.6
2.0
22.3
40.3
52.3
1.4
4
3
93
5.1
1.7
1.2
20.4
41.1
54.2
4.7
11
11
78
4.8
1.8
0.7
18.4
31.1
63.5
5.4
19
6
15
Sulphate
Organic 56
a N + Diff.
a part of the solid decomposes rapidly into gas and tar at an initial stage, showing drastic and complicated change in internal structure. Although various devolatilization models of coal have been presented3p5, they are all based on the volume reaction model6 to avoid chemical and physical complexities. Sulphur behaviour should show more intricacy, since it also depends on the petrographic distribution of organic as well as inorganic sulphur, coupled with change in the internal structure during pyrolysis. Here, a kinetic simulation is conducted for the sequential changes in the contents of sulphur forms in hydropyrolysis, based on the volume reaction model with a desulphurization scheme’ shown in Figure 1. It is assumed that organic sulphur in raw coal (symbolized by A) changes partly into refractory organic sulphur (B) in the char, partly to tar as organic sulphur (C), and partly to hydrogen sulphide (D); pyritic sulphur (E) forms ferrous sulphide sulphur (F) and hydrogen sulphide, and ferrous sulphide sulphur undergoes further reversible reactions to form metallic iron at high hydrogen pressure; and hydrogen sulphide produced is captured reversibly as a form of refractory organic sulphur in chars. Sulphate sulphur in raw coal is not involved in the desulphurization process because it forms sulphite sulphur only. Preliminary simulation showed that the reactions of organic sulphur in tar to the refractory form in the char and to hydrogen sulphide could be neglected in the present study. Reaction orders for hydrodesulphurization of refractory organic sulphur B and reduction of pyritic sulphur E are taken as 2 and 0.5, respectively, and the other reactions are assumed to be first order, according to the work of Yergey et a1.7. They used a mass spectrometer to follow the production rate of hydrogen sulphide during the pyrolysis of pyrite and ten USA coals in a hydrogen stream of heating rates of l-100 K min-I, and a terminal temperature of 1300 K. The proximate analyses of samples used in the present work are in the same range as these samples (F.C. 42.5-64.2%). The material balance for each sulphur form in the reactor gives Equations (l)-(6), where ki (i= 1-8) is the apparent rate constant possibly affected by the physical and chemical structure of the char and by the surrounding atmosphere, and TA is the space time of flowing gas in the reactor: dC,/dt
= - (k, + k, + k,)C,
dC,/dt=k,C,-k,CR2+khCD
1006
FUEL, 1989, Vol 68, August
A
(11 4
E
7
8
F
Figure 1 (%&a,;
Desulphurization FL (SorgLr;
scheme of coal in reduced
C> (S,,,),,,;
D> Su,,; E, SF&
dCJdt
= k,C,-
dC,ldt
= k,C, + ( 1/2)k,C,0.5
atmosphere:
A,
F, S,,s
(l/TA)C,
(3) + k,C,’
+k,C,-(k,+k,+l/TA)C, dC,/dt
= - k,C,0.5
dC,/dt
= ( 1/2)k,C,“.5
(4) (5)
- k,C, + k&I’,
(6)
The Arrhenius-type dependency of the rate constant on temperature was assumed as in Equation (7) including the heat-up period. ki = kio exp( - EJRT)
(7)
Activation energies were adopted from the values of Yergey et al.’ because a preliminary test for parametric sensitivity revealed that the values proposed simulated the present experimental results. Equations (8) and (9), respectively, were used to calculate the organic sulphur in tar (Cc,) and the hydrogen sulphide collected outside the reactor (C,,): dC,,/dt dC,,/dt= Equations
= (l/TA)C,
(8)
(l/TA)C,
(9)
(10))(12)
hold for the temperature
history:
dT/dt=m
(Oct
(10)
(1)
dT/dt=m
(O
(11)
(2)
T= Tl = 773
(tl dtdtf)
(12)
Kinetics of change in sulphur forms in coal hydropyrolysis:
T. Sugawara
et al.
Equation (10) applies when the assigned terminal temperature Tf is lower than the highest terminal temperature Tl (=773 K), and Equations (11) and (12) when treatment time t, is longer than the time needed to reach the highest terminal temperature. Initial conditions equations:
are given
by the following
set of
T= T, C, = C,,,
C, = C,,
C,=Cc=C,=C,=O
at t=O
(13)
Equations (I)-( 13) were solved by means of the Runge-Kutta-Gill algorithm. Each of the frequency factors ki, was determined to simulate the experimental results. Summation of C, and C, was used to evaluate the observed organic sulphur content. A small computing time increment was necessary to follow the rapid change in the sulphur form distribution at an initial stage of heating. This was selected as 0.01-0.1 min, with checks on the precision of the calculated material balance for total sulphur. The effect of cooling rate was examined, and proved to be negligible under the conditions where the actual cooling rate was around 20Kmin-’ in both atmospheric and high pressurized reactions.
I I I 1
2-
RESULTS
AND DISCUSSION Heat-up
Atmospheric pressure treatment Figures2 and 3 show the changes in char yield and sulphur-form distribution with time for Ermelo and Plateau coals under atmospheric pressure of hydrogen
I
Sulphate sulphur + Sulphite sulphur
OA
0
I
!
I
I
I
I
I I
Organic sulphur
I
I I
0 0
IO
Sulphate sulphur + Sulphite sulphur 20
30 TimeImln)
I I IO
I 20
I
L
30 Tune (mm)
40
I 50
4 60
Figure 3 Change in char yield and sulphur form distribution with time for Plateau coal at atmospheric pressure of hydrogen (final temperature 773 K, heating rate 40 K min- I). Solid lines represent the simulated values of change in sulphur-form distribution
0
601
Orgamc sulphur
40
50
60
Figure 2 Change in char yield and sulphur-form distribution with time for Ermelo coal at atmospheric pressure of hydrogen (initial temperature 293 K, final temperature 773 K, heating rate 40 K min-I). Solid lines represent simulated values for change in sulphur-form distribution
and 773 K final temperature with a heating rate of 40 K min- ‘. Solid lines indicate the simulated changes in the contents of the sulphur forms computed from Equations (1 t( 13). The total sulphur in the char (indicated by circles in Figure2) rapidly decreased from for Ermelo coal within 14min 9.6 to 5.6mgS, gcoal-’ (including the heat-up period), and attained an ‘equilibrium’ value in 33min. The organic sulphur content, however, decreased from 5Smg S, gcoal-’ in raw coal to a minimum value of 2.5 mg S, gcoal- l at 14min, and then increased again to attain a constant value of 3.2mg S, gcoal-‘. This unique behaviour of organic sulphur, as noted elsewhere’, was also observed in the case of Joban-azukidaira coal reported previously’. Figure3 shows that the organic sulphur in Plateau coal decreased monotonically, without reaching a minimum value, after the rapid decrease within the heat-up period. The characteristic desulphurization behaviour was successfully simulated as shown in Figures 2 and 3 (solid lines). The frequency factors determined are listed (with the values of activation energies from the literature’) in Table2. It could be concluded from the simulation that the desulphurization steps from 1 to 4 were completed within the heat-up period and that steps 5 and 6 played an important role in the final temperature region. The frequency factors for both coals are of the same order of
FUEL,
1989,
Vol 68, August
1007
Kinetics
of change
Table 2 treatment
Frequency
in sulphur factors
forms
determined
for
in coal hydropyrolysis: atmospheric
pressure ~-
k%l Step i
Ermelo
Plateau
1 2 3 4 5 6 7 8
4.0 3.0 2.0 2.5 1.3 7.0
3.5 1.3 2.0 2.5 1.0 2.3
x x x x x x
106 106 106 10” 10” lo9
k,,, li,,, k,,, k,,, k,,, k,, k,, (g coal, mgS-‘min-‘)
(min-I);
x x x x x x
Ei (kcalmol-r) (Ref. 7)
106 106 106 10” 1013 10s
k,,
et al.
T. Sugawara
desulphurization. Pyritic sulphur, 1.4 mg S, g coal- ’ in raw coal, continued to convert to ferrous sulphide sulphur at 49min. On the other hand, ferrous sulphide sulphur first increased to a maximum of 0.6mgS, at 47min and then decreased to 0.3mg S, gcoall’
22.0 22.0 22.0 42.1 56.1 32.0 43.1 18.0 ((mgS,
g coal~‘)05min-‘):
magnitude for steps 1-3, the desulphurization reactions of organic sulphur, and identical for step 4, the reduction of pyrite. Reaction steps 7 and 8, the reversible reactions of iron sulphide and metallic iron, were negligible under atmospheric pressure of hydrogen and up to 773 K. Differences in the desulphurization characteristics between Ermelo and Plateau coal can be judged through the quite different frequency factors for steps 5 and 6, the decomposition and the formation of refractory organic sulphur, respectively. Plateau coal has a k,, value larger by two orders of magnitude and a k,, value smaller by one order of magnitude, compared with Ermelo coal. Cordes et al.’ used the activated complex theory to show that the frequency factors of solid-state thermal decomposition may vary from lo5 to lOi s-i depending on the type of reactions. The frequency factor k,, for the decomposition step of refractory organic sulphur is estimated to be between 10’ and 10i6s-‘, by considering the surface decomposition suggested by Cordes et al.. It can be seen from the larger value of k,O and the smaller of value of k,, in Table2 that the effective reduction organic sulphur is accomplished by the large decomposition rate and the small formation rate of thiophenic structures in Plateau coal. Consequently, acceleration of reaction step 5 and reduction in the rate of step 6 should reflect one aspect of the differences in the structures of Plateau coal and of Ermelo coal. Figure 4 shows the change in the internal surface area (determined by the CO, adsorption method) with reaction time for Plateau and Ermelo coals during atmospheric pressure treatment. The internal surface areas of both coals increased with time to a maximum (64m2g-’ for Ermelo and 88 m2 g-i for Plateau coal) and then decreased. Plateau coal has a larger surface area than Ermelo coal throughout the treatment time.
0
I I
I
I
0
I
I
I
40
20
6
TimeCmin)
Figure 4 Change in internal surface area (CO, adsorption) for 0, Ermelo and 0, Plateau coals, treated in atmospheric of hydrogen
0
with time pressure
1
I I
0
’ 01
0 J
High pressure treatment
As the first example of high pressure treatment, shows the sequential changes in char yield and sulphur form distribution for PSOC-830 under 30 kg cm - * hydrogen pressure and 773 K final temperature with a heating rate of lOKmin_‘. The sample coal, PSOC-830 was desulphurized most effectively. Total sulphur content decreased rapidly from 12.4mg S, to < 5.0mgS, gcoal-’ within the heat-up gcoal-’ period, and then reached 1.9mgS, gcoal-’ at 1 lOmin, corresponding to desulphurization of 84.7%. Organic sulphur decreased from 9.7 mg S, g coal- ’ in raw coal to 0.2mgS, gcoal-’ at 110 min, equivalent to 98% Figure5
1008
FUEL, 1989, Vol 68, August
T 01
0
I
Sulphatel sul@wr + Sulphrfe sulphrr I
I
20
40
I
I
I
I
60
80
100
120
Timetmin)
Figure 5 Change in char yield and sulphur form distribution with time for PSOC-830 at 30 kgcm-’ of hydrogen (tIna temperature 773 K, heating rate 10 K mini ‘), Solid I’mes represent the simulated values for change in sulphur-form distribution
Kinetics
0
of change
in sulphur
/
b I
I
0
I
_l-LL--
--
r
t
/\ 0 0
I
Sulphate I 20
sulphur+t Sulphite sulphur I I I I 80 40 60
/
1
forms
in coal hydropyrolysis:
T. Sugawara
et al.
30 kgcme2. The k,, value relating to the decomposition of refractory organic sulphur increased by three orders of magnitude and one order of magnitude for Ermelo and Plateau coals, respectively, while the value of k,, for the capture of hydrogen sulphide decreased by one order of magnitude for both coals. The frequency factors klo, k,, and k,, for the decomposition of organic sulphur in raw coal were not as much affected by hydrogen pressure. The effect of the coal type on the desulphurization process can also be seen in Table3. The progressive reduction of organic sulphur was observed even after the heat-up period for PSOC-830 and Plateau coals, which had larger k,, and smaller k,, values than other sample coals. The reduction from ferrous sulphide to metallic iron was appreciable in PSOC-156, 522, 830, Plateau and Joban-azukidaira coals, for which frequency factors are listed as k,,s in Table3. The reaction step from metallic iron to ferrous sulphide was negligible in the simulation for all coal samples used. While Zielke et a1.13 found experimentally that 0.5% hydrogen sulphide was enough to reverse the reaction between metallic iron and hydrogen sulphide, the momentary maximum concentration of hydrogen sulphide was less than 0.01% under the experimental conditions studied. Figure 7 demonstrates the change in internal surface area (CO, adsorption) with reaction time for seven coals treated under high pressure. Each coal showed different behaviour in the development of internal structure. A local minimum or maximum was observed, but otherwise there was a monotonic increase with time during the heat-up period, reflecting the characteristic changes
Tlme(mm)
Figure 6 Change in char yield and sulphur form distribution with time for PSOC-979 at 30 kg cm-’ of hydrogen (final temperature 773 K, heating rate 10 K min ‘)
at 1lOmit-1. This implies that the reducing g coal-’ reaction proceeded from ferrous sulphide to metallic iron. Similar behaviour was observed in the case of Joban-azukidaira’, Plateau’, PSOC-1561 ‘, and PSOC522l’. Figure 6 shows the sequential changes in the sulphur form distribution for PSOC-979 under a hydrogen pressure of 30 kg cme2 with a heating-up rate and a final temperature of 773 K. The of lOKmin_’ figure shows that the decomposition reaction of the organic sulphur was not remarkable and the reduction from ferrous suphide to metallic iron was not appreciable. The extents of desulphurization were respectively 66.7% and 59.7% for organic sulphur and total sulphur at 140min. The dynamic features of the sulphur-form change were similar for PSOC-59211 and Ermelol. Simulation was also successful in the case of atmospheric pressure treatment, as shown in Figures5 and 6 (solid lines). The consistency of the simulated results with the observed ones was comparable for the other six coals8~“~‘2. Table3 shows the frequency factors determined for eight coals treated under 30 kgcme2 of hydrogen pressure. The effect of hydrogen pressure on the desulphurization of coal can be clearly understood by comparing the values in Tables2 and 3. The frequency for the reduction of pyrite increased factor i& remarkably from z 10” to = 1014 for both coals, as hydrogen pressure increased from atmospheric to
A
X
A
k0
0
Heat-up
I
--wl
I I I
I
20
40
I
I 60
I 80
I 100
I
Time (min)
Figure 7 Change in internal surface area (CO, adsorption) with time for 7 coals treated at 30kgcmm2 of hydrogen: A, PSOC-156; V, PSOC-522; +, PSOC-592; v, PSOC-830; A, PSOC-979; 0, Ermelo; 0, Plateau
FUEL,
1989, Vol 68, August
1009
Kinetics Table 3
of change Frequency
in sulphur forms in coal hydropyrolysis:
factors
determined
for high pressure
T. Sugawara
et al.
treatment k 10
Step i
Ermelo
Plateau
Joban azukidaira
_.
PSOC-156
PSOC-522
PSOC-592
PSOC-830
PSOC-979
1
4.0 x 106
2.0 x lo6
8.0 x 106
7.0 x lo5
8.3 x 10’
9.0 x 105
8.0 x 10s
1.0 x 106
2
5.0 x lo5
3.0 x 106
4.0 x 106
9.0 x 105
5.5 x lo5
4.0 x 10s
5.5 x lo5
7.0 x 105 2.0 x 106
3
2.0 x 106
1.0 x 106
9.0 x lo6
1.0 x lo5
4.7 x lo5
1.5 x 106
1.7 x 106
4
6.0 x lOI4
6.0 x lOI4
5.0 x 10’5
7.0 x 10’3
5.0 x 1014
1.0x 1015
1.0 x 1014
1.0 x 10’4
5
1.5 x 10’4
1.5 x 10’4
2.0 x 10’3
2.0 x 1or3
2.0 x 10’3
3.0 x 1o’3
1.0 x 1or4
3.0 x 10’3
6
1.0 x 10s
1.0 x 10’
5.0 x 10s
1.0 x 10’
1.0 x lo9
3.0 x lo9
5.0 x 10’
5.0 x 10s
6.0 x 10”’
8.0 x 10’”
6.0 x 10”
7.0 x 10’0
7
5.0 x 10’0
8 k,,,
k,,,
k,,,
k,,,
k,,,
k,,
(min-I);
k,,
((mgS,
g coal~1)0~5min-‘);
k,,
owing to the rapid formation, deposition, and release of volatiles within coal particles up to 773 K. After the heat-up period, the internal surface area attained a maximum value for PSOC- 156,592, and 979, and showed an increasing tendency for PSOC-522 and 830, and a decreasing trend for Ermelo and Plateau coals. The release of volatile matter still continued, together with an increase of the internal surface area for PSOC-522 and 830. The internal surface area reached a maximum value as the release of volatile matter ended for the other coals. The highly developed internal structure and high partial pressure of hydrogen may enable the reducing reaction to proceed from ferrous sulphide to metallic iron, because these factors increase the frequency of collision between ferrous sulphide and hydrogen molecules and decrease the partial pressure of hydrogen sulphide produced. Since part of the organic sulphur in raw coal changes into refractory organic sulphur in chars (frequency factor k,,) and part moves into tar and gas as volatile matter (k,, and k,, respectively), the ratio between the release rate of volatile sulphur and the overall reaction rate of organic sulphur is defined as (k,, + k,,)/(k10 + k,, + k,,), when the activation energies are the same for the steps 1 to 3. A drastic change in internal structure may be related to a release in volatile matter from inside and an easy diffusion of hydrogen from the surroundings into a coal particle. Consequently, the time-averaged internal surface area within the heat-up period, defined by Equation (14), t1 L=
l/t1
S(t)dt
(14)
s0
was considered to be a correlation parameter for characterizing the release behaviour of volatile organic sulphur because the reaction steps from 1 to 3 were practically complete within the heat-up period. Figure8 shows the correlation between (k,, + k,o)/(k,o + k,, + k,,) and S,, for seven types of coal in atmospheric and high pressure treatments. While the behaviour of the internal structure was very complicated, as shown in Figure 7, a fair correlation can be seen from Figure& This suggests that the reaction rate of volatile organic sulphur was promoted, i.e. the transformation rate decreased from thiols and sulphide to thiophenic sulphursr’, with an increase in the internal surface area. The ratio of frequency factors defined above did not correlate well with the internal surface area measured by nitrogen
1010
FUEL,
1989,
Vol 68, August
(g coal, mgS-‘min-‘)
/ -& -t” +
b’
-
0
X
+0
‘t
l/
0.5-
,4
v n
0
$ -
A 0 0
*i+
0’ 0
-tr: -
-
0
0
0
0’
0’ 0
/’
I
0
0
20
I 60
I
40 S,,
I 00
IC
( m2g-t)
Relation between the frequency factor ratio (k,,+ k,,)/ and the averaged internal surface area S,, in the heat-up period: A, PSOC-156; V, PSOC-522; +, PSOC-592; v, PSOC-830; A,, PSOC- 979; 0, Ermelo; 0, Plateau; 0, Ermelo in atmospheric pressure treatment; n , Plateau in atmospheric pressure treatment Figure 8 (k,,+
kzo+ k,,),
adsorption at 773 K. It could be deduced that the dynamic behaviour of volatile organic sulphurs was characterized by the kinetics in the micropore structure formation represented by the CO, adsorption. Figure 9 shows that the value of (k,, + k30)/(k10 + k,, + k,,) seems to increase as the exinite contentI in raw coals increases. This might be related to the fact that exinite contains organic sulphur mostly15 in macerals, and decomposes easily in the initial stage of heating. According to the petrographic observation of the chars by a reflective microscope, exinite decomposed first and disappeared by 673 K before pores began to generate in vitrinite16. CONCLUSIONS Sequential changes in the sulphur-form distribution and the internal surface area were followed during hydropyrolysis of eight non-caking steam coals. Kinetic
Kinetics of change
in sulphur forms in coal hydropyrolysis:
T. Sugawara
et al.
REFERENCES 1
Sugawara, T., Sugawara, K. and the late Ohashi, H. Fuel 1988, 67, 1263 Sato, S. Bachelor OJ’engineering thesis, Akita University, Japan, 1988 Anthony, D. B., Howard, J. B., Hottel, H. C. and Meissner, H. P. Fuel 1976, 55, 121 Russel, W. B., Saville, D. A. and Greene, M. I. AIChE J. 1979, 25( 1), 65 Suuberg, E. M. in ‘Chemistry of Coal Conversion’, Plenum Press. New York. USA. 1985. D. 67 Lacey, D. T., Bowen, J. H. and Basden, K. S. Ind. Eng. Chem. 1965, 4, 275 Yergey, A. L., Lampe, F. W., Vestal, M. L. et ul. Ind. Eng. Chem. Proc. Des. Dev. 1974, 13(3), 233 Ohashi, H., Sugawara, T. and Sugawara, K. ‘Proceedings of the Third Congress of Asian Pacific Confederation of Chemical Engineering’, Bangkok, Thailand, October 1984, pp. 125-130 Cordes, H. F. J. Phys. Chem. 1968, 72(6), 2 185 Attar, A. Fuel 1978, 57, 201 Sugawara, T. and Sugawara, K. ‘Studies on Utilization of Coal through Conversion’, Reports of Special Project Research on Energy under Grant in Aid of Scientific Research of the Ministry of Education, Science and Culture, Japan, SPEY 16, 1987, pp. 2977302 Ohashi, H., Sugawara, T. and Matsunaga, T. ‘Proceedings of the Third Pacific Chemical Engineering Congress’, Seoul, Korea, May, 1983, pp. 282-287 Zielke, C. W., Curran, G. P. and Goring, G. E. Ind. Eny. Chem. 1954, 46(l), 53 Coal Data Base of The Pennsylvania State University Tseng, B., Buckentin, M., Hsieh, K., Wert, C. and Drykacz, R. Fuel 1986, 65, 385 Sugawara, T., Sugawara, K. and Enda, Y. ‘Proceedings of 24th Conference on Coal Science’, Tokyo, Japan, October 1987, pp. 2644267
2 3 4 5 6 7 8
9 10 I1 5
IO Eximte content tdmmf %)
15
20
Figure 9 Relation between the frequency factor ratio (k2c+k3a)/ (k,,+k,,+k,,) and the exinite content in raw coals: & PSOC-156; v, PSOC-522; x , PSOC-592; v, PSOC-830; A, PSOC-979
12
13 14 15
simulation based on the authors’ desulphurization scheme and the volume reaction model successfully represented the dynamic behaviour of sulphur forms and clarified the effects of hydrogen pressure and coal type on each step of the desulphurization process. The reduction rate of pyrite and the decomposition rate of refractory organic sulphur were respectively promoted by three orders of magnitude and one to three orders of magnitude, and the capture of hydrogen sulphide was suppressed by one order of magnitude by increasing the hydrogen pressure from atmospheric pressure to 30kgcm-‘. The effective decomposition of organic sulphur observed in some coals was related to a large rate of release of volatile organic sulphur during the heat-up period, the large decomposition rate of refractory organic sulphur and a small uptake rate of hydrogen sulphide. Although changes in the internal surface area were very complicated, the extent of volatile organicsulphur release increased linearly with increase in the internal surface area within the heat-up period. A fair correlation was also observed between the extent of volatile organic sulphur release and the exinite content in raw coals. ACKNOWLEDGEMENTS The authors with to thank S. Saruta, Z. Zhang, K. Shimoko, Y. Enda, Y. Nishiyama, K. Ishizuka and S. Sato for their assistance, and the Pennsylvania State University and Electric Power Development Co. Ltd. for the supply of sample coals. This work was supported by Special Project Research on Energy under Grant in Aid of Scientific Research of the Ministry of Education, Science and Culture, Japan, in 1984 (No. 59040006), 1985 (No. 6004005), 1986 (No. 61040004).
16
NOMENCLATURE Cj O’=A-E) Cj, O’=A-E) C CGI Cno Ej
(i=l-8)
ki (i=l-8)
ki, (i=l-8) m R S 2”
S(t) T
T, Tl
G
TA t1
4
concentration of each sulphur form (mgSgcoall’, d.b.) initial concentration of each sulphur form (mgSgcoal_‘, d.b.) concentration of evolved tar or hydrogen sulphide (mg S g coal- i, d.b.) activation energy for each step of reaction (kcal mol- ‘) reaction rate constant (mini) (for i= l-3, 6-8) ((mg S, gcoal-‘)0.5 min-‘) (for i =4) (gcoal, mgS-‘mini) (for i=5) frequency factor (units are the same as for ki) constant heating rate (K min-‘) gas constant (cal mol- ’ K- ‘) time averaged internal surface area (m’g-‘) internal surface area as a function of time (m” g- ‘) temperature (K) initial temperature (K) highest final temperature (K) assigned final temperature (K) space time of gas in reactors (min) time (min) time needed to reach Tl (min) treatment time (min)
FUEL, 1989, Vol 68, August
1011
Sugawara,
Katsuyasu
Department of Chemical Engineering Akita 010, Japan (Received 37 January 1989)
Sugawara
forms
and the late Hiroyasu
for Resources,
Mining
College,
Akita
in
Ohashi University,
Eight kinds of non-caking steam coals were pyrolysed using fixed beds in hydrogen streams at atmospheric pressure and at 30 kg cme2 up to 773 K. Sequential changes were followed in various sulphur forms: pyritic and ferrous sulphide sulphur in the char, and hydrogen sulphide in the gas. The internal surface area was also determined sequentially by the CO, adsorption method. The observed changes in the contents of sulphur forms were successfully simulated kinetically, based on the desulphurization scheme proposed in the authors’ previous paper and with help of a volume reaction model. The effects of hydrogen pressure and type of coal on desulphurization behaviour were characterized by a set of frequency factors determined for all the steps included in the scheme proposed. The frequency factors for volatile organic sulphur decomnosition were correlated with chances in the internal surface area within the heat-up period and with the exinite contents in raw coals. (Keywords: kinetics; sulphur; hydropyrolysis)
Ermelo (South Africa), Plateau (Utah), PSOC-156 (Utah), 522 (Illinois), 592 (Illinois), 830 (Indiana), and 979 (Wyoming). Joban-azukidaira, Ermelo and PSOC156 coals include similar amounts of inorganic and organic sulphur, while the other coals contain predominantly organic sulphur.
Precise knowledge of the dynamic behaviour of sulphur forms in hydropyrolysis is important for developing and evaluating efficient chemical coal-cleaning processes. In a previous paper’, dynamic behaviour of sulphur forms was followed for three kinds of steam coals during hydropyrolysis, and the desulphurization process was qualitatively characterized as follows: rapid release of hydrogen sulphide and sulphur-containing tar at an initial stage including a heat-up period; the liberation of hydrogen sulphide accompanying the reduction of pyrite; the capture of hydrogen sulphide as a form of refractory organic sulphur in the char matrix during atmospheric pressure treatment; suppression of the capture of hydrogen sulphide at high hydrogen pressure, which promotes the decomposition of organic sulphur; and the reduction of ferrous sulphide to metallic iron in some coals. The purpose of this study is to kinetically evaluate the desulphurization process described above. To begin with, change in sulphur forms for eight non-caking steam coals was sequentially observed in gas-flow fixed beds under atmospheric and 30 kg cme2 hydrogen pressure. The desulphurization behaviour was then simulated kinetically based on the volume reaction model, and the effects of type of coal and hydrogen pressure were discussed in terms of the frequency factors determined for the desulphurization reactions. Change in the internal surface area with reaction time was also measured, by the CO, adsorption method, to investigate the influence of physical structures in the solid phase on the desulphurization behaviour.
Hydrogen-sulphide sulphur, liberated from the solid phase during pyrolysis, was determined using an ion electrode (JIS K0108). Some improvements’ to the ASTM (D2492) method and the Gladfelter and Dickerhoof method were applied to analyse sulphur forms (pyritic, ferrous sulphide, sulphate, sulphite, and organic sulphurs) in chars.
EXPERIMENTAL
KINETIC
Samples
Coal is a poorly characterized heterogeneous mixture from the viewpoint of both chemical and physical structures. When a coal is exposed to high temperature,
Table 1 shows
analyses
proximate, ultimate and sulphur-form of the coals used; Joban-azukidaira (Japan),
0016-2361/89/081005-07$3.00 0 1989 Butterworth & Co. (Publishers)
Ltd.
Apparatus
and procedure
Two sets of gas-flow fixed beds were used for the pyrolysis experiments. The reactors included a fused silica tube (i.d. 21 mm) for the atmospheric pressure treatment and a stainless steel autoclave (i.d. 36mm, volume 2OOcm”) for the high pressure treatment. The details of the apparatus and procedure have been described elsewhere’. From preliminary experiments2, it was concluded that an average particle size < 0.43 mm allows the influence of size dependency on the char yield and desulphurization behaviour to be neglected under the conditions studied. Internal surface areas of the coals and chars were measured with a YANACO GSA-10 instrument by applying the Dubinin-Polanyi equation to CO, adsorption isotherms at 273 K. Analysis
of sulphur forms
SIMULATION
FUEL, 1989, Vol 68, August
1005
Kinetics of change Table 1
Ultimate,
in sulphur forms in coal hydropyrolysis:
proximate
and sulphur-form
analyses
T. Sugawara
for coals used
Ultimate (wt%, dry basis) Sample
C
H
N
Joban-azukidaira
62.7
3.3
Ermelo
71.5
4.9
1.5
Plateau
75.2
5.5
2.9
PSOC- I56
65.4
4.1
PSOC-522
71.2
4.7
PSOC-592
69.2
4.9
PSOC-830
71.6
PSOC-979
74.3
et al
Proximate (wt%, dry basis)
Sulphur form (wt”? of total sulphur)
S
Diff.
V.M.
F.C.
Ash
Pyritic
1.8
32.2”
40.5
42.2
17.3
42
2
1.0
21.1
30.6
55.8
13.6
42
1
57
0.7
15.7
43.3
48.9
7.x
12.
I
81
1.6
1.4
26.9
41.8
51.8
6.4
48
3
49
2.0
0.9
21.2
41.3
57.1
1.6
14
3
x3
1.6
2.0
22.3
40.3
52.3
1.4
4
3
93
5.1
1.7
1.2
20.4
41.1
54.2
4.7
11
11
78
4.8
1.8
0.7
18.4
31.1
63.5
5.4
19
6
15
Sulphate
Organic 56
a N + Diff.
a part of the solid decomposes rapidly into gas and tar at an initial stage, showing drastic and complicated change in internal structure. Although various devolatilization models of coal have been presented3p5, they are all based on the volume reaction model6 to avoid chemical and physical complexities. Sulphur behaviour should show more intricacy, since it also depends on the petrographic distribution of organic as well as inorganic sulphur, coupled with change in the internal structure during pyrolysis. Here, a kinetic simulation is conducted for the sequential changes in the contents of sulphur forms in hydropyrolysis, based on the volume reaction model with a desulphurization scheme’ shown in Figure 1. It is assumed that organic sulphur in raw coal (symbolized by A) changes partly into refractory organic sulphur (B) in the char, partly to tar as organic sulphur (C), and partly to hydrogen sulphide (D); pyritic sulphur (E) forms ferrous sulphide sulphur (F) and hydrogen sulphide, and ferrous sulphide sulphur undergoes further reversible reactions to form metallic iron at high hydrogen pressure; and hydrogen sulphide produced is captured reversibly as a form of refractory organic sulphur in chars. Sulphate sulphur in raw coal is not involved in the desulphurization process because it forms sulphite sulphur only. Preliminary simulation showed that the reactions of organic sulphur in tar to the refractory form in the char and to hydrogen sulphide could be neglected in the present study. Reaction orders for hydrodesulphurization of refractory organic sulphur B and reduction of pyritic sulphur E are taken as 2 and 0.5, respectively, and the other reactions are assumed to be first order, according to the work of Yergey et a1.7. They used a mass spectrometer to follow the production rate of hydrogen sulphide during the pyrolysis of pyrite and ten USA coals in a hydrogen stream of heating rates of l-100 K min-I, and a terminal temperature of 1300 K. The proximate analyses of samples used in the present work are in the same range as these samples (F.C. 42.5-64.2%). The material balance for each sulphur form in the reactor gives Equations (l)-(6), where ki (i= 1-8) is the apparent rate constant possibly affected by the physical and chemical structure of the char and by the surrounding atmosphere, and TA is the space time of flowing gas in the reactor: dC,/dt
= - (k, + k, + k,)C,
dC,/dt=k,C,-k,CR2+khCD
1006
FUEL, 1989, Vol 68, August
A
(11 4
E
7
8
F
Figure 1 (%&a,;
Desulphurization FL (SorgLr;
scheme of coal in reduced
C> (S,,,),,,;
D> Su,,; E, SF&
dCJdt
= k,C,-
dC,ldt
= k,C, + ( 1/2)k,C,0.5
atmosphere:
A,
F, S,,s
(l/TA)C,
(3) + k,C,’
+k,C,-(k,+k,+l/TA)C, dC,/dt
= - k,C,0.5
dC,/dt
= ( 1/2)k,C,“.5
(4) (5)
- k,C, + k&I’,
(6)
The Arrhenius-type dependency of the rate constant on temperature was assumed as in Equation (7) including the heat-up period. ki = kio exp( - EJRT)
(7)
Activation energies were adopted from the values of Yergey et al.’ because a preliminary test for parametric sensitivity revealed that the values proposed simulated the present experimental results. Equations (8) and (9), respectively, were used to calculate the organic sulphur in tar (Cc,) and the hydrogen sulphide collected outside the reactor (C,,): dC,,/dt dC,,/dt= Equations
= (l/TA)C,
(8)
(l/TA)C,
(9)
(10))(12)
hold for the temperature
history:
dT/dt=m
(Oct
(10)
(1)
dT/dt=m
(O
(11)
(2)
T= Tl = 773
(tl dtdtf)
(12)
Kinetics of change in sulphur forms in coal hydropyrolysis:
T. Sugawara
et al.
Equation (10) applies when the assigned terminal temperature Tf is lower than the highest terminal temperature Tl (=773 K), and Equations (11) and (12) when treatment time t, is longer than the time needed to reach the highest terminal temperature. Initial conditions equations:
are given
by the following
set of
T= T, C, = C,,,
C, = C,,
C,=Cc=C,=C,=O
at t=O
(13)
Equations (I)-( 13) were solved by means of the Runge-Kutta-Gill algorithm. Each of the frequency factors ki, was determined to simulate the experimental results. Summation of C, and C, was used to evaluate the observed organic sulphur content. A small computing time increment was necessary to follow the rapid change in the sulphur form distribution at an initial stage of heating. This was selected as 0.01-0.1 min, with checks on the precision of the calculated material balance for total sulphur. The effect of cooling rate was examined, and proved to be negligible under the conditions where the actual cooling rate was around 20Kmin-’ in both atmospheric and high pressurized reactions.
I I I 1
2-
RESULTS
AND DISCUSSION Heat-up
Atmospheric pressure treatment Figures2 and 3 show the changes in char yield and sulphur-form distribution with time for Ermelo and Plateau coals under atmospheric pressure of hydrogen
I
Sulphate sulphur + Sulphite sulphur
OA
0
I
!
I
I
I
I
I I
Organic sulphur
I
I I
0 0
IO
Sulphate sulphur + Sulphite sulphur 20
30 TimeImln)
I I IO
I 20
I
L
30 Tune (mm)
40
I 50
4 60
Figure 3 Change in char yield and sulphur form distribution with time for Plateau coal at atmospheric pressure of hydrogen (final temperature 773 K, heating rate 40 K min- I). Solid lines represent the simulated values of change in sulphur-form distribution
0
601
Orgamc sulphur
40
50
60
Figure 2 Change in char yield and sulphur-form distribution with time for Ermelo coal at atmospheric pressure of hydrogen (initial temperature 293 K, final temperature 773 K, heating rate 40 K min-I). Solid lines represent simulated values for change in sulphur-form distribution
and 773 K final temperature with a heating rate of 40 K min- ‘. Solid lines indicate the simulated changes in the contents of the sulphur forms computed from Equations (1 t( 13). The total sulphur in the char (indicated by circles in Figure2) rapidly decreased from for Ermelo coal within 14min 9.6 to 5.6mgS, gcoal-’ (including the heat-up period), and attained an ‘equilibrium’ value in 33min. The organic sulphur content, however, decreased from 5Smg S, gcoal-’ in raw coal to a minimum value of 2.5 mg S, gcoal- l at 14min, and then increased again to attain a constant value of 3.2mg S, gcoal-‘. This unique behaviour of organic sulphur, as noted elsewhere’, was also observed in the case of Joban-azukidaira coal reported previously’. Figure3 shows that the organic sulphur in Plateau coal decreased monotonically, without reaching a minimum value, after the rapid decrease within the heat-up period. The characteristic desulphurization behaviour was successfully simulated as shown in Figures 2 and 3 (solid lines). The frequency factors determined are listed (with the values of activation energies from the literature’) in Table2. It could be concluded from the simulation that the desulphurization steps from 1 to 4 were completed within the heat-up period and that steps 5 and 6 played an important role in the final temperature region. The frequency factors for both coals are of the same order of
FUEL,
1989,
Vol 68, August
1007
Kinetics
of change
Table 2 treatment
Frequency
in sulphur factors
forms
determined
for
in coal hydropyrolysis: atmospheric
pressure ~-
k%l Step i
Ermelo
Plateau
1 2 3 4 5 6 7 8
4.0 3.0 2.0 2.5 1.3 7.0
3.5 1.3 2.0 2.5 1.0 2.3
x x x x x x
106 106 106 10” 10” lo9
k,,, li,,, k,,, k,,, k,,, k,, k,, (g coal, mgS-‘min-‘)
(min-I);
x x x x x x
Ei (kcalmol-r) (Ref. 7)
106 106 106 10” 1013 10s
k,,
et al.
T. Sugawara
desulphurization. Pyritic sulphur, 1.4 mg S, g coal- ’ in raw coal, continued to convert to ferrous sulphide sulphur at 49min. On the other hand, ferrous sulphide sulphur first increased to a maximum of 0.6mgS, at 47min and then decreased to 0.3mg S, gcoall’
22.0 22.0 22.0 42.1 56.1 32.0 43.1 18.0 ((mgS,
g coal~‘)05min-‘):
magnitude for steps 1-3, the desulphurization reactions of organic sulphur, and identical for step 4, the reduction of pyrite. Reaction steps 7 and 8, the reversible reactions of iron sulphide and metallic iron, were negligible under atmospheric pressure of hydrogen and up to 773 K. Differences in the desulphurization characteristics between Ermelo and Plateau coal can be judged through the quite different frequency factors for steps 5 and 6, the decomposition and the formation of refractory organic sulphur, respectively. Plateau coal has a k,, value larger by two orders of magnitude and a k,, value smaller by one order of magnitude, compared with Ermelo coal. Cordes et al.’ used the activated complex theory to show that the frequency factors of solid-state thermal decomposition may vary from lo5 to lOi s-i depending on the type of reactions. The frequency factor k,, for the decomposition step of refractory organic sulphur is estimated to be between 10’ and 10i6s-‘, by considering the surface decomposition suggested by Cordes et al.. It can be seen from the larger value of k,O and the smaller of value of k,, in Table2 that the effective reduction organic sulphur is accomplished by the large decomposition rate and the small formation rate of thiophenic structures in Plateau coal. Consequently, acceleration of reaction step 5 and reduction in the rate of step 6 should reflect one aspect of the differences in the structures of Plateau coal and of Ermelo coal. Figure 4 shows the change in the internal surface area (determined by the CO, adsorption method) with reaction time for Plateau and Ermelo coals during atmospheric pressure treatment. The internal surface areas of both coals increased with time to a maximum (64m2g-’ for Ermelo and 88 m2 g-i for Plateau coal) and then decreased. Plateau coal has a larger surface area than Ermelo coal throughout the treatment time.
0
I I
I
I
0
I
I
I
40
20
6
TimeCmin)
Figure 4 Change in internal surface area (CO, adsorption) for 0, Ermelo and 0, Plateau coals, treated in atmospheric of hydrogen
0
with time pressure
1
I I
0
’ 01
0 J
High pressure treatment
As the first example of high pressure treatment, shows the sequential changes in char yield and sulphur form distribution for PSOC-830 under 30 kg cm - * hydrogen pressure and 773 K final temperature with a heating rate of lOKmin_‘. The sample coal, PSOC-830 was desulphurized most effectively. Total sulphur content decreased rapidly from 12.4mg S, to < 5.0mgS, gcoal-’ within the heat-up gcoal-’ period, and then reached 1.9mgS, gcoal-’ at 1 lOmin, corresponding to desulphurization of 84.7%. Organic sulphur decreased from 9.7 mg S, g coal- ’ in raw coal to 0.2mgS, gcoal-’ at 110 min, equivalent to 98% Figure5
1008
FUEL, 1989, Vol 68, August
T 01
0
I
Sulphatel sul@wr + Sulphrfe sulphrr I
I
20
40
I
I
I
I
60
80
100
120
Timetmin)
Figure 5 Change in char yield and sulphur form distribution with time for PSOC-830 at 30 kgcm-’ of hydrogen (tIna temperature 773 K, heating rate 10 K mini ‘), Solid I’mes represent the simulated values for change in sulphur-form distribution
Kinetics
0
of change
in sulphur
/
b I
I
0
I
_l-LL--
--
r
t
/\ 0 0
I
Sulphate I 20
sulphur+t Sulphite sulphur I I I I 80 40 60
/
1
forms
in coal hydropyrolysis:
T. Sugawara
et al.
30 kgcme2. The k,, value relating to the decomposition of refractory organic sulphur increased by three orders of magnitude and one order of magnitude for Ermelo and Plateau coals, respectively, while the value of k,, for the capture of hydrogen sulphide decreased by one order of magnitude for both coals. The frequency factors klo, k,, and k,, for the decomposition of organic sulphur in raw coal were not as much affected by hydrogen pressure. The effect of the coal type on the desulphurization process can also be seen in Table3. The progressive reduction of organic sulphur was observed even after the heat-up period for PSOC-830 and Plateau coals, which had larger k,, and smaller k,, values than other sample coals. The reduction from ferrous sulphide to metallic iron was appreciable in PSOC-156, 522, 830, Plateau and Joban-azukidaira coals, for which frequency factors are listed as k,,s in Table3. The reaction step from metallic iron to ferrous sulphide was negligible in the simulation for all coal samples used. While Zielke et a1.13 found experimentally that 0.5% hydrogen sulphide was enough to reverse the reaction between metallic iron and hydrogen sulphide, the momentary maximum concentration of hydrogen sulphide was less than 0.01% under the experimental conditions studied. Figure 7 demonstrates the change in internal surface area (CO, adsorption) with reaction time for seven coals treated under high pressure. Each coal showed different behaviour in the development of internal structure. A local minimum or maximum was observed, but otherwise there was a monotonic increase with time during the heat-up period, reflecting the characteristic changes
Tlme(mm)
Figure 6 Change in char yield and sulphur form distribution with time for PSOC-979 at 30 kg cm-’ of hydrogen (final temperature 773 K, heating rate 10 K min ‘)
at 1lOmit-1. This implies that the reducing g coal-’ reaction proceeded from ferrous sulphide to metallic iron. Similar behaviour was observed in the case of Joban-azukidaira’, Plateau’, PSOC-1561 ‘, and PSOC522l’. Figure 6 shows the sequential changes in the sulphur form distribution for PSOC-979 under a hydrogen pressure of 30 kg cme2 with a heating-up rate and a final temperature of 773 K. The of lOKmin_’ figure shows that the decomposition reaction of the organic sulphur was not remarkable and the reduction from ferrous suphide to metallic iron was not appreciable. The extents of desulphurization were respectively 66.7% and 59.7% for organic sulphur and total sulphur at 140min. The dynamic features of the sulphur-form change were similar for PSOC-59211 and Ermelol. Simulation was also successful in the case of atmospheric pressure treatment, as shown in Figures5 and 6 (solid lines). The consistency of the simulated results with the observed ones was comparable for the other six coals8~“~‘2. Table3 shows the frequency factors determined for eight coals treated under 30 kgcme2 of hydrogen pressure. The effect of hydrogen pressure on the desulphurization of coal can be clearly understood by comparing the values in Tables2 and 3. The frequency for the reduction of pyrite increased factor i& remarkably from z 10” to = 1014 for both coals, as hydrogen pressure increased from atmospheric to
A
X
A
k0
0
Heat-up
I
--wl
I I I
I
20
40
I
I 60
I 80
I 100
I
Time (min)
Figure 7 Change in internal surface area (CO, adsorption) with time for 7 coals treated at 30kgcmm2 of hydrogen: A, PSOC-156; V, PSOC-522; +, PSOC-592; v, PSOC-830; A, PSOC-979; 0, Ermelo; 0, Plateau
FUEL,
1989, Vol 68, August
1009
Kinetics Table 3
of change Frequency
in sulphur forms in coal hydropyrolysis:
factors
determined
for high pressure
T. Sugawara
et al.
treatment k 10
Step i
Ermelo
Plateau
Joban azukidaira
_.
PSOC-156
PSOC-522
PSOC-592
PSOC-830
PSOC-979
1
4.0 x 106
2.0 x lo6
8.0 x 106
7.0 x lo5
8.3 x 10’
9.0 x 105
8.0 x 10s
1.0 x 106
2
5.0 x lo5
3.0 x 106
4.0 x 106
9.0 x 105
5.5 x lo5
4.0 x 10s
5.5 x lo5
7.0 x 105 2.0 x 106
3
2.0 x 106
1.0 x 106
9.0 x lo6
1.0 x lo5
4.7 x lo5
1.5 x 106
1.7 x 106
4
6.0 x lOI4
6.0 x lOI4
5.0 x 10’5
7.0 x 10’3
5.0 x 1014
1.0x 1015
1.0 x 1014
1.0 x 10’4
5
1.5 x 10’4
1.5 x 10’4
2.0 x 10’3
2.0 x 1or3
2.0 x 10’3
3.0 x 1o’3
1.0 x 1or4
3.0 x 10’3
6
1.0 x 10s
1.0 x 10’
5.0 x 10s
1.0 x 10’
1.0 x lo9
3.0 x lo9
5.0 x 10’
5.0 x 10s
6.0 x 10”’
8.0 x 10’”
6.0 x 10”
7.0 x 10’0
7
5.0 x 10’0
8 k,,,
k,,,
k,,,
k,,,
k,,,
k,,
(min-I);
k,,
((mgS,
g coal~1)0~5min-‘);
k,,
owing to the rapid formation, deposition, and release of volatiles within coal particles up to 773 K. After the heat-up period, the internal surface area attained a maximum value for PSOC- 156,592, and 979, and showed an increasing tendency for PSOC-522 and 830, and a decreasing trend for Ermelo and Plateau coals. The release of volatile matter still continued, together with an increase of the internal surface area for PSOC-522 and 830. The internal surface area reached a maximum value as the release of volatile matter ended for the other coals. The highly developed internal structure and high partial pressure of hydrogen may enable the reducing reaction to proceed from ferrous sulphide to metallic iron, because these factors increase the frequency of collision between ferrous sulphide and hydrogen molecules and decrease the partial pressure of hydrogen sulphide produced. Since part of the organic sulphur in raw coal changes into refractory organic sulphur in chars (frequency factor k,,) and part moves into tar and gas as volatile matter (k,, and k,, respectively), the ratio between the release rate of volatile sulphur and the overall reaction rate of organic sulphur is defined as (k,, + k,,)/(k10 + k,, + k,,), when the activation energies are the same for the steps 1 to 3. A drastic change in internal structure may be related to a release in volatile matter from inside and an easy diffusion of hydrogen from the surroundings into a coal particle. Consequently, the time-averaged internal surface area within the heat-up period, defined by Equation (14), t1 L=
l/t1
S(t)dt
(14)
s0
was considered to be a correlation parameter for characterizing the release behaviour of volatile organic sulphur because the reaction steps from 1 to 3 were practically complete within the heat-up period. Figure8 shows the correlation between (k,, + k,o)/(k,o + k,, + k,,) and S,, for seven types of coal in atmospheric and high pressure treatments. While the behaviour of the internal structure was very complicated, as shown in Figure 7, a fair correlation can be seen from Figure& This suggests that the reaction rate of volatile organic sulphur was promoted, i.e. the transformation rate decreased from thiols and sulphide to thiophenic sulphursr’, with an increase in the internal surface area. The ratio of frequency factors defined above did not correlate well with the internal surface area measured by nitrogen
1010
FUEL,
1989,
Vol 68, August
(g coal, mgS-‘min-‘)
/ -& -t” +
b’
-
0
X
+0
‘t
l/
0.5-
,4
v n
0
$ -
A 0 0
*i+
0’ 0
-tr: -
-
0
0
0
0’
0’ 0
/’
I
0
0
20
I 60
I
40 S,,
I 00
IC
( m2g-t)
Relation between the frequency factor ratio (k,,+ k,,)/ and the averaged internal surface area S,, in the heat-up period: A, PSOC-156; V, PSOC-522; +, PSOC-592; v, PSOC-830; A,, PSOC- 979; 0, Ermelo; 0, Plateau; 0, Ermelo in atmospheric pressure treatment; n , Plateau in atmospheric pressure treatment Figure 8 (k,,+
kzo+ k,,),
adsorption at 773 K. It could be deduced that the dynamic behaviour of volatile organic sulphurs was characterized by the kinetics in the micropore structure formation represented by the CO, adsorption. Figure 9 shows that the value of (k,, + k30)/(k10 + k,, + k,,) seems to increase as the exinite contentI in raw coals increases. This might be related to the fact that exinite contains organic sulphur mostly15 in macerals, and decomposes easily in the initial stage of heating. According to the petrographic observation of the chars by a reflective microscope, exinite decomposed first and disappeared by 673 K before pores began to generate in vitrinite16. CONCLUSIONS Sequential changes in the sulphur-form distribution and the internal surface area were followed during hydropyrolysis of eight non-caking steam coals. Kinetic
Kinetics of change
in sulphur forms in coal hydropyrolysis:
T. Sugawara
et al.
REFERENCES 1
Sugawara, T., Sugawara, K. and the late Ohashi, H. Fuel 1988, 67, 1263 Sato, S. Bachelor OJ’engineering thesis, Akita University, Japan, 1988 Anthony, D. B., Howard, J. B., Hottel, H. C. and Meissner, H. P. Fuel 1976, 55, 121 Russel, W. B., Saville, D. A. and Greene, M. I. AIChE J. 1979, 25( 1), 65 Suuberg, E. M. in ‘Chemistry of Coal Conversion’, Plenum Press. New York. USA. 1985. D. 67 Lacey, D. T., Bowen, J. H. and Basden, K. S. Ind. Eng. Chem. 1965, 4, 275 Yergey, A. L., Lampe, F. W., Vestal, M. L. et ul. Ind. Eng. Chem. Proc. Des. Dev. 1974, 13(3), 233 Ohashi, H., Sugawara, T. and Sugawara, K. ‘Proceedings of the Third Congress of Asian Pacific Confederation of Chemical Engineering’, Bangkok, Thailand, October 1984, pp. 125-130 Cordes, H. F. J. Phys. Chem. 1968, 72(6), 2 185 Attar, A. Fuel 1978, 57, 201 Sugawara, T. and Sugawara, K. ‘Studies on Utilization of Coal through Conversion’, Reports of Special Project Research on Energy under Grant in Aid of Scientific Research of the Ministry of Education, Science and Culture, Japan, SPEY 16, 1987, pp. 2977302 Ohashi, H., Sugawara, T. and Matsunaga, T. ‘Proceedings of the Third Pacific Chemical Engineering Congress’, Seoul, Korea, May, 1983, pp. 282-287 Zielke, C. W., Curran, G. P. and Goring, G. E. Ind. Eny. Chem. 1954, 46(l), 53 Coal Data Base of The Pennsylvania State University Tseng, B., Buckentin, M., Hsieh, K., Wert, C. and Drykacz, R. Fuel 1986, 65, 385 Sugawara, T., Sugawara, K. and Enda, Y. ‘Proceedings of 24th Conference on Coal Science’, Tokyo, Japan, October 1987, pp. 2644267
2 3 4 5 6 7 8
9 10 I1 5
IO Eximte content tdmmf %)
15
20
Figure 9 Relation between the frequency factor ratio (k2c+k3a)/ (k,,+k,,+k,,) and the exinite content in raw coals: & PSOC-156; v, PSOC-522; x , PSOC-592; v, PSOC-830; A, PSOC-979
12
13 14 15
simulation based on the authors’ desulphurization scheme and the volume reaction model successfully represented the dynamic behaviour of sulphur forms and clarified the effects of hydrogen pressure and coal type on each step of the desulphurization process. The reduction rate of pyrite and the decomposition rate of refractory organic sulphur were respectively promoted by three orders of magnitude and one to three orders of magnitude, and the capture of hydrogen sulphide was suppressed by one order of magnitude by increasing the hydrogen pressure from atmospheric pressure to 30kgcm-‘. The effective decomposition of organic sulphur observed in some coals was related to a large rate of release of volatile organic sulphur during the heat-up period, the large decomposition rate of refractory organic sulphur and a small uptake rate of hydrogen sulphide. Although changes in the internal surface area were very complicated, the extent of volatile organicsulphur release increased linearly with increase in the internal surface area within the heat-up period. A fair correlation was also observed between the extent of volatile organic sulphur release and the exinite content in raw coals. ACKNOWLEDGEMENTS The authors with to thank S. Saruta, Z. Zhang, K. Shimoko, Y. Enda, Y. Nishiyama, K. Ishizuka and S. Sato for their assistance, and the Pennsylvania State University and Electric Power Development Co. Ltd. for the supply of sample coals. This work was supported by Special Project Research on Energy under Grant in Aid of Scientific Research of the Ministry of Education, Science and Culture, Japan, in 1984 (No. 59040006), 1985 (No. 6004005), 1986 (No. 61040004).
16
NOMENCLATURE Cj O’=A-E) Cj, O’=A-E) C CGI Cno Ej
(i=l-8)
ki (i=l-8)
ki, (i=l-8) m R S 2”
S(t) T
T, Tl
G
TA t1
4
concentration of each sulphur form (mgSgcoall’, d.b.) initial concentration of each sulphur form (mgSgcoal_‘, d.b.) concentration of evolved tar or hydrogen sulphide (mg S g coal- i, d.b.) activation energy for each step of reaction (kcal mol- ‘) reaction rate constant (mini) (for i= l-3, 6-8) ((mg S, gcoal-‘)0.5 min-‘) (for i =4) (gcoal, mgS-‘mini) (for i=5) frequency factor (units are the same as for ki) constant heating rate (K min-‘) gas constant (cal mol- ’ K- ‘) time averaged internal surface area (m’g-‘) internal surface area as a function of time (m” g- ‘) temperature (K) initial temperature (K) highest final temperature (K) assigned final temperature (K) space time of gas in reactors (min) time (min) time needed to reach Tl (min) treatment time (min)
FUEL, 1989, Vol 68, August
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