Report No. SRIC 70-14
I
KINETIC STUDIES ON THE PYROLYSIS, DESULFUR-
IZATION, & GASIFICATION OF COALS WITH EMPHA-
SIS ON THE NON-ISOTHERMAL KINETIC METHOD
I
i
!
Marvin L. Vestal, Alan G. Day, III, J.S. Snyderman,
Gordon J. Fergusson, F.W. Lampe, R.H. Essenhigh,
and Wm. H. Johnston
With contributions from
Charles E. Waring, A.L. Wahrhaftig, J.H. Futrell,
and Pamela P. Farkas
Final Report December 1969
Phase II Contract No . PH 86-68-65
Scientific Research
ln»trumenta Corporation
Specializing in Air and Water Pollution Control Systems
6707 WhJteston* ltd. Baltimore, Md. 21207 Cable: SRICORP Tel. (301) 944-4020
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Report No. SRIC 70-14
KINETIC STUDIES ON THE PYROLYSIS, DESULFUR-
IZA TION, & GASIFICA TION OF COALS WITH EMPHA-
sIs ON THE NON -ISOTHERMAL KINETIC METHOD
Marvin L. Vestal, Alan G - Day, III, J. S. Snyderman,
Gordon J. Fergusson, F. W. Lampe, R.H. Essenhigh,
and Wm. H. Johnston
With contributions from
Charles E. Waring, A.L. Wahrhaftig, J.H. Futrell,
and Pamela P. Farkas
Final Report December 1969
Phase II Contract No . PH 86-68-65
with the
NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
Paul W. Spaite, Director, Bureau of Engin~ering and Physical Sciences
E. D. Margolin, Chief, New Process Development Unit
Leon Stankus, Contract Project Officer
Schmtlflc Rcuearch Instruments Corporation
Baltimore, Maryland
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TABLE OF CONTENTS
Page
INTRODUCTION
1
THE EXPERIMENTAL APPARATUS
3
THE INTERPRETA TION OF NON -ISOTHERMAL
KINETIC EXPERIMENTS
4
Example of CH3SH from Coal
11
HzS EVOLUTION IN REACTIONS CHARACTERISTIC
OF COAL DESULFURIZA TION
14
Iron Pyrite
15
Organic Sulfur
] 7
KINETICS OF HzS EVOLUTION FROM TEN COALS
25
Identification and ASTM Analyses of Ten Coals
28
DEPENDENCE OF DESULFURIZA TION KINETICS ON
HYDROGEN PRESSURE
48
FORMS OF SULFUR IN CHAR AS FUNCTIONS OF
CARBONIZING TEMPERATURE
52
THE KINETICS OF HzS REACTIONS WITH COAL CHAR
56
KINETICS OF HzS REACTION WITH CARBON
60
KINETICS OF HzS CAPTURE BY Fe AND BY CaO
71
KINETICS OF CALCINA TION OF DOLOMITES AND
LIMESTONES
76
KINETICS OF DECOMPOSITION OF IRON SULFATES
80
PYROL YSIS AND GASIFICA TION OF COAL MIXED WITH
CALCIUM OXIDE'
84
-i -
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TABLE OF CONTENTS
KINETICS OF REVERSIBLE DESULFURIZATION REACTIONS
SUMMARY
Chemical Kinetics
Applications of Kinetic Data
Diffusion and Mass Transport
Future Work
REFERENCES
-ii -
Pa~
87
92
93
96
99
100
101
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Figure
10
11
12
LIST OF FIGURES
1
Typical outgassing kinetics curve vs. temperature.
2
Graph for determining the activation energy for first
order reaction from the experimental parameters,
defined in Figure 1.
3
Evolution of CH3SH in a non-isothermal pyrolysis in
hydrogen.
4
HzS evolution in a non -isothermal experiment at one at-
mosphere of hydrogen on Illinois 5% sulfur coal, SRI No.4
5
HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on pyrite.
6
Kinetic analysis of the non-isothermal measurement on
pyrite.
7
Graph for obtaining the kinetic parameters from non-
isothermal experimental data.
8
HzS evolution in non-isothermal
phere of hydrogen on sulfurated
charcoal.
experiment at one atmos-
carbon prepared from
9
Non-isothermal HzS evolution in Hz from coals SRI,
Numbers 1 through 5.
Non-isothermal HzS evolutions in Hz from coals SRI
Numbers 6 through 10.
Kinetic analysis and resolution into individual processes
of the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Illinois 5% sulfur coal, SRI No.4.
Kinetic analysis and resolution into individual processes
of the HzS evolution in a non-isothermal experiment at
one atmosphere of hydrogen on Illinois 5% sulfur coal,
SRI No.1.
III
Page
8
10
13
18
19
20
21
23
30
31
32
38
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Figure
13
14
15
16
20
LIST OF FIGURES
Kinetic analysis and resolution into individual processes
of the HzS evolution in a non-isothermal experiment at
one atmosphere of hydrogen on Illinois 1% sulfur coal,
SRI No.2.
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Illinois 2.5% sulfur coal,
SRI No.3.
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Illinois 4.5% sulfur coal,
SRI No.5.
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Ohio 3% sulfur coal, SRI No.6.
17
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
~tmospher.e of hydrogen.on Maryland 3% sulfur coal, SRI
No.7.
18
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Ohio 3.5% sulfur coal,
SRI NO.8.
19
Kinetic analysis and resolution into individual processes of
the HzS evolution in a non-isothermal experiment at one
atmosphere of hydrogen on Pennsylvania 1% sulfur coal,
SRI No.9.
Kinetic analysis and resolution into individual processes
of the HzS evolution in a non -isothermal experiment at one
atmosphere of hydrogen on Kentucky 4% sulfur coal, SRI
No. 10.
21
1 st order dependence on Hz pressure: comparison
of non -isothermal rate constants (straight lines) with
isothermal data at 4750 C and pressures of 1 4 9
, ,
and 10 atms. of Hz.
IV
Page
39
40
41
42
43
44
45
46
51
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Figure
22
23
24
25
26
27
28
29
30
31
32
LIST OF FIGURES
Variations in the amount and forms of sulfur in coke
produced from the nominally 5% sulfur Illinois coal
carbonized in hydrogen as functions of carbonizing
temperature for a Hz flow rate of 1000 ml Imino
Variations in the amount and forms of sulfur in char and
gas produced from the normally 5% sulfur Illinois coal
carbonized in hydrogen as a function of carbonizing tem-
perature for a Hz flow rate of 100 ml Imino
Relative HzS concentrations in the effluent gases from six
non -isothermal measurements 011 the kinetics of the re-
actions of HzS with coke.
HzS absorption in non-isothermal experiments on char,
charcoal and iron.
Non-isothermal absorption of hydrogen sulfide on (a) 1
gram activated charcoal (b) 1 gram graphite and (c) a
blank in which the quartz wool plug normally used to hold
the sample in place was used but with no sample. These
experiments used 0.1% HzS in He at a flow of 220 ml/min.
Non-isothermal measurement of the desulfurization of high
sulfur char from 5% sulfur Illinois coal, SRI No.1 resolved
into contributions from FeS and organic sulfur III from
sulfurated charcoal.
Desorption in He of HzS adsorbed on (a) charcoal (b) char
from SRI coal No.1, (c) graphite.
Non-isothermal measurements of the desulfurization of sul- 70
furated charcoal.
Arrhenius type plot of the HzS absorption by iron in a non-
isothermal kinetic experiment.
HzS absorption in non-isothermal kinetic experiments on
calcium ox~de.
Arrhenius type plot of the HzS absorption by calcined No.
1930 dolomite in a non-isothermal kinetic experiment.
v
Page
54
55
58
61
63
68
69
72
74
75
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F i gu r e
33
34
35
36
37
38
. -
LIST OF FIGURES
Runs N -46 and N -47 are calcination runs on calcium
carbonate in helium with residence times of 0.47 sec-
onds and 0.024 seconds, respectively.
Carbon dioxide evolution from the non -isothermal cal-
cination in helium of precipitated calcium carbonate
(q, Limestone sample 1683B (2), Dolomite sample
1930 (3), and Dolomite sample 1380 (4).
Sulfur gas evolution from pyrolysis in 4 litres /minute of
( a) 1 gram and (b) 0.036 grams of fe rric sulfate.
Sulfur gas evolution from pyrolysis in 4 litres /minute
of (a) 1 gram and (b) 0.028 grams of ferrous sulfate.
Sulfur gas evolution from pyrolysis of ferrous sulfate in
(a) 4 litres/minute of Helium and (b) 4 litres/minute of
Hydrogen.
Rate constants for coal desul£urization reactions and for
important back reactions as functions of temperature.
vi
Page
77
78
81
82
83
95
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TABLE NO.
I
II
III
IV
V
VI
VII
VIII
IX
X
LIST OF TABLES
DESCRIPTION
Detailed Analysis of Non-Isothermal Experiment
on Pyr ite
Sum.m.ary of Analyses of Ten Coals - Coal Sample
Identification
Summary of Analyses of Ten Coals - Forms of
Sulfur (Percent of Coal)
Summary of Analyses of Ten Coals - Mineral
Analyses (Percent of Coal)
Summary of Analyses of Ten Coals
Analys is (Percent of Coal)
Proxirnate
Summary of Analyses of Ten Coals - Sulfur in Ash
and Coke
Resolution of the HzS Evolution Curves into Individual
Reactions
Summary of Data on Reactions of HzS :vith Coke
Sulfur Comparison Data for Isothermal Reactions of
Nominally 5% Sulfur Coal Mixed with Calcium Oxide,
in Helium and in Hydrogen
S~mary of Kinetic Data
vii
PAGE
22
33
34
35
36
37
47
59
86
94
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INTRODPCTION
This is the final report on work accomplished during Phase II
of contract PH 86-68-65 on sulfur control by means of coal gasifica-
tion. Our studie~l~ave emphasized the theoretical and experimental
application and extens ion of the new method of non- isothermal kinetic
( 2-9)
measurements developed originally by Juntgen and co -workers.
Previous work has shown that desulfurization reactions on coal dur-
ing pyrolys is and gas ification are inefficient under equilibrium con-
ditions; therefore, knowledge of the kinetics of the parallel and
oppos ing reactions are important to the efficient development of a
useful process.
During Phase I of this work the non- is othermal kinetic method
evolved as a most powerful and useful technique.
The theory of the
non-isothermal technique for study of the kinetics of complex heter-
ogenous reactions was extended to include reactive flush gases and
back reactions of the products.
An experimental laboratory was
constructed for performing these non-isothermal experiments and
twenty-three non-isothermal experiments were completed during
Phase I.
ducted.
In addition, nineteen isothermal experiments were con-
During Phase II minor modifications and improvements were
made to the exper imental apparatus and a total of one hundred and
forty two non-isothermal experiments and forty eight isothermal
experiments were accomplished.
These experiments included
measurements on the desulfurization kinetics for ten bituminous
-1-
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coals from a variety of sources throughout the East and Midwest.
A systematic procedure was developed for interpreting the HzS
evolution data obtained in the non- is othermal kinetic measurements
on the reactions of hydrogen with coal.
Measurements were con-
ducted on the dependance of the HzS evolution from coal on the
hydrogen pressure.
An extensive series of measurements were conducted 011 the
kinetics of HzS reactions with coal char and with the principal re-
active constituents of char including carbon, iron, and calcium
oxide.
The results indicate that calcium oxides obtained from
calcination of various dolomites and limestones fulfill some of the
requi rements of a suitable HzS absorbent for use in conjunction
with gas ification and desulfur ization.
The kinetics of calcination
for several dolomites and limestones were investigated.
Non-
isothermal kinetic measurements were also conducted on the de-
cornpos ition of iron sulfates.
During the course of this work attention has been paid to develop.
ing the techniques for applying the kinetic data by the non-isothermal
method to the development of processes for the practical desulfuriza-
tion of coal during gas ification or combustion. Exper irnents were
conducted on the pyrolysis of coal mixed with calcium oxide and on
the gas ification of coal with steam and oxygen both in the presence
and absence of calcium oxide.
-2-
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THE EXPERIMENTAL APPARATUS
The apparatus used in this research was described in Report No.
SRIC 68-13; however, some modifications and improvements to the
apparatus were made.
Improvements in the experimental aFparatus accomplished during
Phase II include the installation of a thermocouple directly into the re-
actant bed inside the reactor to allow more precise measurement of tem-
perature during the non-isothermal kinetic measurements.
A series of
experiments were conducted to estimate the error resulting from tempera-
ture measurement with the thermocouple located at the outside surface of
the reactor.
These measurements showed that the bed temperature was
in the range from 10 to 20°C below the temperature at the outside surface
of the reactor over the entire range from 100 ° C to 1100 ° C.
In the non-
isothermal experiments using the mass spectrometer, this error is par-
Hally compensated for by the time delay between gas evolution at the sample
and detection by the mass spectrometer.
The overall results of the tem-
perature measurements indicate that in the Phase I non-isothermal experi-
ments the maximum errors in the indicated temperatures wer~ in the range
from 0 to +200 C from the true bed temperatures.
An electrical flow meter has been installed into the gas handling system
replacing the rotometer flow meter to improve the accuracy and reliability
of the measurements of the flow of the sweep gas.
Also, some modifica-
tions to the mass spectrometer have been made to improve the stability,
absolute accuracy and flexibility of the mas s spectrometer for these
measurements.
These modifications include a new ion source, anim-
proved electron emission regulator, and a new power supply for the mass
spectrometer magnet which allows the scan speed to be varied over a wide
range.
-3-
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THE INTERPRETATION OF NON-ISOTHERMAL KINETIC EXPERIMENTS
We may write a general irreversible heterogenous reaction representing
on e of the desulfurization reactions of coal as follows:
A +B
solid gas
k
~C . + D
sol1d gas
( 1)
The rate of the reaction is given by the rate equation,
- d( A) =k( B) (A)
dt
( 2)
d(A)
Where --
dt
is the rate of 10s s of spe de A, (A) is the concentration
of the solid reactant, (B) is the concentration of the gaseous reactant
usually expressed as partial pressure, and k is the rate consta.nt as given
by the Arrhenius equation,
k-k -E/RT
- oe
( 3)
In this equation k is the rate constant of Equations (1) and ( 2), ko is
the temperature-independent Arrhenius constant, E is the activation
energy of the reaction, and Rand T are the universal gas constant and
the absolute temperature, respe ctively.
In the rate expression, Equation (2). (A) is actually the instan-
taneous concentration of specie A in the solid phase. Since continuous
measurement of this solid phase concentration is generally difficult, it
is helpful to expres s (A) in terms of the volwne of evolved gas D. In
this way the solid concentration of A at time t, which is written as (A)
-~-
-------
is proportional to the total amount present at zero time minus the
amount evolved up to the time, t.
This may be written in terms of the
volume of evolved gas, 1), as follows:
(A ) = b (V 0 - V)
( 4)
whe re V 0 is the total volume of gas evolved as Reaction 2 goes to com-
pletion,
V
is the volume of gas evolved up to time t, and
b
is the
proportionality factor.
With this beginning, we may derive the kinetic expression for the
non- isothermal desulfurization kinetics leading to an expres s ion for the
rate of gas evolution as a function of instantaneous temperature when
the sample is heated at a constant rate of M degrees per minute.
The
details of this derivation are given in the report SRIC 68-13 on Phase 1.
In terms of all of the parameters which have been defined above, this
basic equation for non-isothermal kinetics of reaction (1) , neglecting
any back reaction, is given by,
dV - Voko x (£ koRTz
dT - M e p - R T + ME
e E/R T)
( 5)
Before discussing changes in this expression which result
from
desulfurization by pyrolysis with the absence of a reactive gas, or dis-
cussing the extensions to include back reactions, let us review the
actual experiment and see how this equation may be used to obtain the
kinetic parameters of the rate constant from the experimental data.
A sample of solid, in this case coal, is heated at a constant rate of
- 5-
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heat ing, M, from a temperature from which no reaction occurs to an
elevated temperature at which reaction is completed. Dur ing this
period a constant gas flow is maintained of reactant gas, specie B; and
continuous analyses of the effluent gas stream are made to identify the
product gas, I3pecie D, and measure its rate of evolution as a function of
instantaneous ternperature.
In our experiments I we initially analyzed
by a series of techniques including gas-liquid chromatography. plasma
spectrometry, and mass spectrometry.
The most'l+seful technique for
thesE: measurements was the continuous recording mass spectrometer.
For most of the kinetic measurements the product gas measured waS Hj!$
(specie D in Eq\lation (1)) .
Although we measured and identified anum.
ber of other sulfur containing gases including methyl mercaptan and carbon
disulfide during pyrolysis or gasification of coal in an inert atmosphere,
in hydrogen, and in steam, most of the sulfur evolved (typically 98% or
more) is as HzS.
With the mass spectrometer we obtain essentially instantaneous
measurernents 9f the concentration of the product gas D in. the effluent stream.
This instantaneous concentration is uniquely related to the rate of gas evolu-
tion dV or dV by the expres s ion,
dt dT
dV 1
dT - M
dV - PQ
dt - MG
( 5a)
where P is the concentration of the product gas D as determined by the mass
spectrometer, Q is the flow rate, M is the heating rate, and G is the initial
weight of the solid sample, A. Since for a given experiment Q, M, and G
are constant, either the concentration P or the rate dV may be used in the
kinetic interpretations. dT
-6-
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A typical curve for Equation (5) showing the change in the instan-
taneous concentration of gas product, dV /.dT, as a function of tempera-
ture { or time) is shown in Figure 1. The definition of V 0 defined in
Equation (4) is shown as the shaded integral. This curve in Figure 1
is a graphical representation of Equation (5) for a given activation
energy, E, and rate constant, ko.
It is interesting to note that in
Equation (5) there are two terms in the exponential.
One of these
terms controls the rise of the curve in Figure 1 and the other term pro-
duce s the fall of this curve.
We next define To and (dV /dT) To which are shown in Figure 1. To
is the temperature at the maximum in this curve. (dV/dT)To is the value
of dV I dT at this maximum.
Mathematically, the next step is to evaluate the activation energy E
and the frequency factor ko from the experimental measurement of a
curve such as shown in Figurel '.
In order to accomplish this, Equation
(5) is differentiated with respect to temperature and set equal to zero,
thus defining the conditions for the maximum of the curve in Figure 1.
When this is done the resulting equation can be rearranged to give us
expres s ions for E and ko.
E = eRTo2
Vo
(dV/dT) e2RTo/.E
To
( 6)
EM
ko= R T
o
E/RTo
e
( 7)
Note that the expression for activation energy in Equation (6) is a trans-
cendental equation which cannot be solved explicitly for E.
We can,
however, solve this equation graphically and, of course, one could also
-7-
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i
'j
I
OJ
I
dv
dr'
(ARBITRARY):-
UNITS
4 (:; ho
3
2
o
400
~o
I
500
600
Temperature °C
Figure 1. Typical Qutgas sing kinetics curve vs. temperature.
( dV /dT ) To is peak height, V 0 is area.
.
NoD. Isothermal
Kinetics
Specific
Chemical
(eg HzS or CSz)
700
To is temperature at peak,
-------
~ aiii;
solve it by successive approximation.
To do this we define two
dimens ionles s parameters, alpha and beta
0(.
E
= RTo
( 8)
(3=
To ( dV)
va dT T
o
( 9)
Vfhen these dimensionless parameters, defined by Equations (8) and (9),
are substituted in our transcendental Equation (6), we obtain the follow-
ing relationship
oc
=
( 1-2/«")
(Je
( 10)
which by taking logarithms may be written as,
2
lnCl(+ ~ = 1 + In{3
( 11)
Experimentally we may immediately compute beta from the experimen.
tal measurement of the curve in Figure 1 from a non-isothermal run.
The experimental quantities of this curve are:
v 0 area
(12)
( dV / dT) To peak height
To peak location
M heating rate
-
-------
I + IntJ~ (~~) rJ
I
i-'
o
I
1+ In/3
2.8
14
3.4
3. 3
3.2
r n"( +2/0(
3. 1
3.0
2.9
16
18
20
22
24
0< :E/RTo
?igure 2. . Graph for determining the act ivation energy for first order
reactions froDl the experimental parameters, defined in
Figure 1.
28
-------
VIe ha ve now the relationship,
2
lna( + ~ = experimentally determined number
( 13)
This equation can be solved graphically by plotting this function of",.
This graph is shown in Figure 2.,
We read from the abscissa the
value of 0( corresponding to the experimentally determined 1 + Inti.
I
-------
The dotted lines on Figure 2 show 1 + In/J and the corresponding otfor
this non-isothermal measurement of the desulfurization of methyl mer-
captan from coal.
If the resulting kinetic parameters shown in expres-
sion (16) are inserted in our basic Equation (5) we obtain the dotted
curve shown in Figure 3.
This agreement is quite good and provides
evidence that the assumption of a single reaction like ,reaction (1) is
valid for this case.
We may now proceed to measurements on the
more abundant reactions to produce hydrogen sulfide during the pyrolysis
and gas ification of coals.
-12-
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100
So
~
t-..
......
~
~
~ 1: '0
~~
V)
VI
~
~
40
20 -
300
~oo ..rOO
7J::.MPCRATl/R.E: (Oc)
Figure
3.. Evolution of CH3SH in a non-isothermal pyrolys is in hydrogen.
_0- experimental data
---;(-- calculated from values of E and ko by analysis of
experimental results.
-13-
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HzS EVOLUTION IN REACTIONS CHARACTERISTIC
DESULFURIZA TION
OF COA L
-----.
In a hydrogen atmosphere the sulfur in the coal reacts with
hydrogen to produce hydrogen sulfide.
A typical HzS evolution
curve for a non- isothermal experiment on coal heated in hydrogen
is given in Figure 4.
These results were obtained using a hydrogen
flow rate of 1 litre per minute over a 250 mg sample of Illinois coal
No.4, as identified in our Report No. SRIC 69 -10.
The heating rate
was 4°C per minute.
Clearly the HzS evolution does not occur by a
single process such as that producing methyl mercaptan, as shown in
Figure 3.
Since sulfur exists in coal in many different forms, e. g.
pyrite, sulfide, sulfat e and several different types of organic sulfur,
this result is expected.
Each individual reaction of the form
COAL -S + Hz -? HzS
should yield an HzS evolution curve similar to that shown in Figure 1.
The parameters characterizing that curve To. Vo, and (dV/dT)To
should reflect the kinetics for the individual proces s.
The overall HzS
evolution curve will be composed of the sum of the set of overlapping
curves characterizing each of the individual reactions.
In the absence
of any knowledge on the individual proces ses an experimental result
such as given in Figure 4 can be resolved into individual processes
in infinitely many ways.
However, if the kinetics of the individual
processes are known a priori, a unique resolution of the experin1ental
results can be achieved.
-14 -
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Iron Pyrite
Since iron pyrite is known to be a major source of sulfur in coal,
we conducted non-isothermal experiments on samples of iron pyrite
obtained from the U.S. Bureau of Mines.
In these experiments the
back reaction of HzS with iron was suppres sed by us ing a very high
hyd rogen flow rate and a very small sample of iron pyrite.
The heating
rate enlployed was 4: °C per minute.
The experimental data on the non-
i sotherrnal evolution of HzS from pyrite are shown in Figure 5.
These
results clearly indicate two reactions producing hydrogen sulfide.
is' reasonable to suppose that these reactions are the following:
It
Hz + FeSz --? FeS + HzS
Hz + FeS
--? Fe + HzS
Independent measurements on the reactions of hydrogen with iron sulfide
have confi rrned that the higher temperature peak corresponds to the sulfide
reduct ion.
"he expe rilnental data shown in Figure I) was obtained using
a lIye! I'ogen pn'ssure or one atmosphere.
Isotherrnal TY1eaSUrenlents on
tht' kinetics or these reactions as a [unction of pres sure have established
the fi 1'st 0 rder dependence on hydrogen pres sure.
These experimental results on pyrite may be analyzed in a straight
forward manner to yield the kinetic parameters for the two reactions.
The procedure used is as follows:
First we sketch in two peaks of the
type shown in Figure 1 which give a reasonable fit to the experimental
points.
j
The values of the parameters characterizing the curves are
read off of these curves.
These parameters are the temperature cor-
responding to each of the peaks, T o( OK) , the area of the peak, V 0' and
the amplitude of the peak at To.
From these values a dimensionless
-15 -
-------
parameter 13 as given by Equation(9)is computed and using this value the
value of the dimensionless parameter 0( is read off the curve given in
Figure Z.
The values of the activation energies E and pre -exponential
factor ko are then computed using ECfuations (14) and ( 15). The
results are then double checked by recomputing the HzS evolution
peak corresponding to these parameter values using Equation (5) .
The calculated peaks are then replotted with the experimental data
and the accuracy of the fit is checked.
for the pyrite reaction
By these procedures we find
E = 47 kilocalor ie s pe r mole
ko= 2.8 x 101z (atm Hz) -1 min-l
and for the sulfide reduction
E = 55 kilocalories per mole
ko = 2. 1 x 10 13 (a tm Hz) - 1 min - 1
The calculated HzS evolution curves for these two reactions are compared
with the experimental data in Figure 6.
The calculated HzS evolution for
the sum of the two proces ses is shown in the dash line in the figure.
The
fit between calculation and exper imental could obviously be improved by
slightly adjusting the amplitudes of the two peaks.
However, the ampli-
tudes reflect the stoichiometries of the reaction while the locations and
shapes of the peaks reflect the kinetics.
These results suggest that the
pyrite sample was not pure FeSz but rather initially contained a small
amount of sulfide.
The details of the analysis of the non-isothermal results for pyrite
are illustrated in Table I and Figures 6 and 7.
The temperature at the
pyrite peak is read from Figure 6 as 510°C which, for the calculation,
is converted to oK giving To = 783°K.
The peak drawn in on Figure 6
-16 -
-------
to approximate the experimental results has an alnplitude of 51 units
where in this case the concentration unit is taken arbitrarily as milli-
volts of mass spectrometer response to the mass 34 HzS+ peak. The
area of the peak V 0 is 3360 oK mv. From this parameter value the
calculated value of the dimensionless parameter fJ = 11.9 and the
quantity req uired in graphical analysis, 1 + In tJ = 3.48. To define
O(we located this value of 1 + In~ on the ordinate of Figure 7. Then,
as illustrated in Figure 7, the corresponding value of the abscissa for
the plotted function gives the value ofC¥'which, in this case, is 30.6.
The
activation energy and pre-exponential factor are then calculated from th~
value ofO(according to Equations (14)and (15), giving the values shown
on Figure 6 and Table 1.
A simple detailed step by step analysis of the
sulfide reduction is also illustrated in Table I and Figures 6 and 7.
Organic: Sulfur
The pyrite sulfur clearly accounts for most of the inorganic sulfur
found in coal, but there is also generally substantial amounts of organic
sulfur and it is well known that this sulfur may exist in many different
kinds of bonding arrangerrE nts within the coal.
In an attempt to investi-
gate behavior of the or ganic sulfur on a somewhat simpler, but,related
system, we prepared artificially some organic sulfur-containing material.
A sample of es sentially mineral free charcoal was reacted with hydrogen
sulfide in a stream of heliUITl to produce a sulfurated carbon which con-
tained approximately 2.5% sulfur.
Non-isothermal measurements on
the desulfurization of this material in both hydrogen and heliUITl were
carried out.
The results of one such experiment are given in Figure 8.
In this experiment the sample size and flow rate of hydrogen used were
the same as that employed on the major series of non-isothermal experi-
-17-
-------
I
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rature
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CD
Il' ..
..... . .
8 . . ...
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rn i .. .
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::r'
CD 300 400 500
'1
CD
Temperature (C n)
.
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-
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600
.
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-------
fT1
,....
(JQ
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ro
0'
.
~
,....
::J
ro
;:;: ;t:
n IV
PJ r./)
::J ()
PJ 0
I-' ::J
~ n
,.... (1)
CIJ ::J
~
o '"1
....., PJ
~ ~
;:r ....
(1) g
::J ..........
o »
::J "i
~. cr
ID ....
o ~
~ "i
;:r PJ
('1) "i
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8 c
PJ ::J
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I
PYRITE+SULFIDE ~;--- \ \
I \ 8,
I .\ ~I
. \ 1;
\ -/
.Jj
Sol-
40
30
PYRITE
E:A7
12
K..= 2.BX 10
20r-
I
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\
.
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10 r-
i
1..
. ..~
.. . .,~
~-r-' '--
40G
300
I"
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k
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Temperature (C 0)
,
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\. &
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.
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\
\
X
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\ E=55
" Ko=-2.1 X 10'3
I
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(
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700
-------
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N
>--
..... (I) f:rj
g- x ...
'\jCIQ
PJ (I) s::
::s "1 "1
PJ ... ro
-<"3-.)
enro.
... ::s
.....
en PJ C1
o ...... "1
""'a.PJ
.....1lI'\j
::r""'::r
(I) PJ
. .....
(I) 0
x ~ "1
'\j ::r 0
(I) (I) 0-
"1 .....
... PJ
~ a. ...
;J PJ ::s
(I) en ...
::s ::r ::s
..... ...... CIQ
o ..... .....
::s ::s ::r
ro
'\j en ro
'< ..... '"
"1 ...... .....
..... ...... ::s
..... s:: (I)
(1) en .....
. C""'t- t-I"
"1 ()
PJ'\j
~ PJ
"1
..... PJ
g-3
(I)
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()
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--
SULFIDE
---
-------
3.5
---
PYRITE
--
3,3
3.2
3.1
20
22
I
24
I
26
2~
0(
30
32
34
36
-------
TABLE I DETAILED ANALYSIS OF NON-ISOTHERMAL EXPERIMENT
ON PYRITE
REACTION FeSz+Hz ---7 FeS+HzS FeS+Hz ---7 Fe+HzS
To ( OK) 783 853
(dV/dT)T (mv) 51 51
o
V o( oK. my) 3360 3360
8= To dV
Vo dT T 11.9 12.8
o
l+lnJj 3.48 3.55
0( 30.6 32.5
E=R T cP<'( kcal/mo1e) 47 55
M"f ct
k v e ( . - 1) 2 . 8 x 10 12 2. 1 x 10 13
o =T mln
0
-22-
-------
I
N
W
I
o 'Tj
...... ,....
;:r()Q
'< s::
0.. ~
~ (1)
ell) C):)
OQ ...
(1)
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o N
:::I rn
en (1)
s:: <:
...... 0
.............
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~ <"t-
~ o'
(1) :::I
0.. .....
() :::I
\II :::I
~ 0
cr :::I
o I
:::I ,....
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(b ::r
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:r. 8
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~ :::I
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rn
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lOt--
t
I
o RGANIC."".m
300
400
500
600
700
Temperature (C 0)
-------
ments on coals.
It is clear from the results of studies to date on the
sulfurated carbon that a single simple reaction does not account for
the behavior of this material.
Pending the completion of the kinetic
investigation on the complex sequence of reactions involved in the de-
sul£urization of these relatively stable organic sulfur species the
empirical result corresPQnding to the smooth curve shown in Figure 8
has been used in the analysis on the results on coal.
this form of organic s\ll£ur as Organic III.
We have designated
As will be discussed in the next section of this report, the
kinetic evidence strongly suggests that the low -temperature peak in
Figure 4 arises from reactions of two types of organic sulfur.
have designated these types as Organic I and II.
We
-24 -
-------
KINETICS OF HzS EVOLUTION FROM TEN COALS
In the previous section it was shown how the non-isothermal
evolution of HzS from the coal designated as SRI -4 (see Figure 4)
could not be attributed to a single reaction, as was the case for
CH3SH (see Figure 3).
In point of fact, a minimum of five reactions
is necessary to account for the non-isothermal HzS evolution curve
in Figure 4.
The HzS evolution in some characteristic reactions
probably involved in coal desulfurization is discus sed in the previous
section.
To proceed further in our analysis it is useful to consider HzS
evolution studies, similar to that depicted in Figure 4, of ten diverse
samples of coal.
The results of non-isothermal kinetic experiments
for these ten coals are summarized in Figures 9 and 10.
Certain
points in common and certain differences should be noted in these
results.
All of the HzS evolution curves show a peak in the range
between 380°C and 430°C.
All of the coals high in pyrite show
see ondary peaks very close to those found experimentally for the
sample of pyrite as illustrated in Figure 6.
However, in general for
these coals these peaks appear to occur at slightly lower temperatures
typically from 10 - 20°C.
If we as sume that results on the pyrite are
valid for coal we would expect that the presence of.the carbon should
have little effect on the activation energies for these reactions.
But,
because of both the production of hydrogen from within the coal and
the possibility of the absorption of hydrogen on the carbon surface, we
might expect that the pre -exponential factor, which is expres sed in
terms of the concentration of hydrogen in the bulk gas stream, might
be increased in the case of the coal by these effects, increasing the
-25-
-------
surface concentration of hydrogen for a given bulk gas concentration
of hydrogen. A downward shift in the temperature corresponding to
the peak in the HzS evolution from pyrite of 200 C corresponds to an
increase in the pre -exponential factor of about 40%. Data obtained by
us and by Powell in his earlier work on the forms of sulfur in char, as
a function of carbonizing temperature, support the hypothesis that the
secondary peaks in these non-isothermal results do correspond to the
reaction of the pyritic and sulfide sulfur s.
Similarly from this work,
we suggest that the low temperature peaks in the non-isothermal
results correspond to the reaction of relatively more reactive organic
sulfur compounds in coal.
A single reaction cannot account for the
variation in the location and shape of the low temperature peak for all
of these ten coals.
However, we have found that two proces ses, one
with To corresponding to 3800 C and a second with To corresponding to
4300 C satisfactorily account for the low temperature peak in all ten
coals.
The kinetic parameters for these proces ses which we have
designed as Organic I and Organic II are as follows:
ORGANIC I
ORGANIC II
E=34.5 kcal/mole
ko=3.1 x 1010 atm Hz -1 min -1
E =41.5 kcal/mole
ko=2.8 x lOll atm Hz
-1 . -1
,mln
It is, of course, possible that more than two processes contribute to this
low temperature peak, however, only the two are required to account for
the experimental results.
Weare now in a position to discus s the resolution of the HzS evolution
curve for SRI No.4 given in Figure 4, into individual processes. The five pro
cesses we have identified in the preceeding discussion are the conversions to
HzS of three forms of organic sulfur, of pyrite, and of sulfide. This
resolution into the individual processes was peTformed graphically by
drawing in the peaks corresponding to the individual processes and
-26 -
-------
adjusting the amplitude of the peak, without changing the peak location,
until a best fit to the experimental data is obtained.
The fit is determined
by comparing the sum of all of the HzS evolution peaks with the experi-
mental data.
The result of this analysis is shown in Figure 11.
In the
figure the dashed line represents the sum of the individual processes
shown in the figure and the agreement with the experimental points is
quite satisfactory with one exception.
In the region about 5300 there
appears to be a significant amount of sulfur evolution which is not ac-
counted for by these five processes.
This discrepancy occurs in most
of the coals studied but is particularly prominent in coal No.7, the
Maryland coal.
This discrepancy may indicate that an additional desul-
furization proces s occurs which we have not taken into account; however,
our recent experiments, in an attempt to further understand the Organic III
set of reactions, have indicated that the results obtained on the artificial
sample of sulfurated carbon may not be directly applicable to coal.
It now
appears that a proper representation of the Organic III sulfur removal may
remove this discrepancy.
One additional point that should be mentioned concerning this analysis
of the desulfurization of the coal designated as SRI -4 is that the total amount
of sulfur evolved from the pyrite and sulfide processes appears rather lower
than would be expected from the amount of iron pyrite in the coal from the
ASTM analysis. However, this coal contains an unusually high calcium
content.
Our separate experiments on the reaction of HzS with calcium
oxide and calcium carbonate have shown that the reaction of HzS with these
materials in the temperature range above 5000 is extremely fast so that
nearly one half of the sulfur, which might otherwise be evolved in the pyrite
and sulfur peaks, is converted to calcium sulfide and retained in the char.
In view of the succes s achieved in resolving the HzS evolution curve
for SRI-4,we have applied the same technique to the resolution of the non-
-27 -
-------
isothermal HzS evolution curves for nine other representative coal
samples.
The overall evolution curves are shown in Figure 9 and
10 while in Figures 12 -20 each curve is shown in more detail and
is resolved into the contribution from the five characteristic reactions
previously discussed.
In each case a sample of coal was heated at 4° per minute in
hydrogen at one atmosphere.
Flow conditions were maintained with
appropriate control of residence time.
The exit gases were analyzed
continuously for hydrogen sulfide by mas s spectrometry.
Preliminary
calibrations showed that the continuous mas s spectrometric measure-
ments were in agreement with analyses by gas liquid chromatography.
Details of the experimental methods are given in the Appendix of
report SRI 68 -13.
The ten coal samples investigated were from Illinois, Ohio, Mary-
land, Pennsylvania, and Kentucky.
With the exception of two coals these
are all high in sulfur.
The Illinois coals were provided by Mr. Jack
Simon of the Illinois State Geological Survey.
were provided by the U. S. Bureau of Mines.
The coals from other states
Identification and ASTM Analyses of Ten Coals
Tables II-VI summarize the identification and analyses of the ten.
coal samples.
In Table II the source, nominal percent sulfur, and the
bed are shown.
In addition, permission was obtained to list the mine from
which each sample was obtained.
In Table III the forms of sulfur are shown as percent of coal. These
analyses and the ones summarized in Table IV through VI were done accord-
ing to the ASTM procedures. Table IV shows the mineral analysis, Table V
-28-
-------
shows the proximate analysis, and the sulfur in ash and in coke are given
in Table VI.
As mentioned previously the HzS evolution curves from the non-
isothermal desulfurization of ten diverse coals are shown in Figures 11-20.
For each coal the overall evolution curve is resolved into contributions
from the five specific desul£urization reactions of Organic I, II, III, pyrite
and sulfide and in all cases these five processes account for from 87-990/0
of the total HzS evolution. The successful decomposition of anyone HzS
evolution curve into five characteristic proces ses is by itself, not too sig-
nificant. However, the successful decomposition of the HzS evolution
curves for all ten coals into the same characteristic five processes is
highly significant and strengthens our conclusion that to a reasonably good
approximation the temperature -programmed desulfurization may be viewed
as a sequence of characteristic and independent chemical reactions applicable
to coal in general.
The principal di'8crepancy in this approximation appears to occur
in the temperature region around 5200 C and above 6000.
This may be due
to an oversimplified treatment of the poorly-understood series of reactions
cCl'q)rising the desulfurization of Organic III, to additional reactions not yet
identified, or both.
Fin.Hy, in Table VII we summarize the relative contributions of
the five characteriltic processes to the total desulfurization process of
each coal.
-29-
-------
z
0
I-
« 40
a:::
I- ......
Z II)
LLJ -
.-
(.) c
Z ::>
0 >-
(.) ~
I a
...
VJ C/) -
0 .- 30
C\I ..c
I I '"
I-
«
....J
LLI 20
a:::
1'-----, /4
, \
, \
I " \
! I \
", \
, \
, \
I \
, \
I I .. \
I': ;' \~, \ ./""-" c::~
I I \ \ ~\/ \ /
I / j \ \ ''\.,..,,- "
5/¥ ! \ " \
I I / \ " '5
/ , '.....- ...- -- - - \. " l
I,' ,I ~--\ '\',,~
I I / ",- -- -""
I " I -7r" ',,-', ",
I I / 4...', ""'-
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I /, ... ........ "
~ " "
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60
50.
10
o
300
500
600
400
TEMPE RATU R E. (Oc.)
RESIDENCE 1.\\11':
25 millisel.und:-;
700
FIGURE 9-
NON-ISOTHERMAL HzS EVOLUTIONS IN Hz FROl\l.[Cc>~LS SRI
---........... - --,." --....-~ c::;...
-------
100
RELATIV.E
/-1 Sl S
COlleENT/?ATION
(ARBlrJ?ARY lJlJlr5J 6 °
r
W
.....
I
IJ
,.-
I
1
/
I
I
,
,
"
,
,
I
8,\/ I I
'/ f\
,: I \
f / / \
.'/ I
if /
. 1 I
II ~
'/ I
I~ f
// ~
,'1 II
// 'I
~ '
/; f"
/'" /- ,..
,,~ 7
,.::"~_.__.__...__._-_. --------L-.-
300 ./fO 0
80
40
;w
-.- ..IE'::: - or=-- -== -...0.. -r- "-' ~ -=0: - <-C=:-:JI
- ..-----a- "'-"''''''JL.'' l. l~"ll...
25 rnilliseconds
\\\
~\
\ \
\ \
\ \
\ . \ /'\ 10
\ \j~ ;1 \/
, \ \
\ \ vi --~.
~ / ~
~ ,,/ '\.\.
~ ~-~ ~
"'- - - ~....
" ",. ~
"\' /;// , \..
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~-..:~ \.- ...-
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..... "",
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"""'"- " ----
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FIGURE lOG
-----=--..--7
-. -.. ---- -. . .--._~. J -...... 9
7QO
-L
5"OQ
1
600
T.£MPERATl/RE (Oc)
NON-ISOTHERMAL HzS EVOLUTIONS IN Hz FROM COALS SRI
NUivU3E:RS 6 THROUGH 10.
-------
..... (II 'TJ
,...... <' -.
~O(IC.
::s ........ c
ot:.,
....,. C""f' ro
C/J o' .....
.J1 :3 -
--'.::1 ...... .
':> :3
ar
~ III ,-:
..... -.
..... ::1 ::J
~ 0 (1)
'i ::J ~
n :... n
o C/J III ::r::
III 0 ;j ~I
1-' n- III "'en .;Iv'
~ ::r-
en(O-<
~ ~ ~.
H ;J en
Z~III
o (1) ::J
. X ~
'~'"C;j 'i
. (1) (II
'1 C/J
3' ~
(1)~ ----
;j ;:!".
..... 0
I 1II;j
VJ ..... .....
N 0 :;j
I ;j n-
(1) 0
III 5'
g~
o ,....
C/J ~
'tj ~
::rill
(1) .....
'1'tj
(1) 'i
o 0
.....n
::T(1)
-< m
""m
'i (1)
o C/J
(JQ 0
(1) .....
;j n-
o ::r
;j (1)
::r:
en
I
I
I
bU~
()
o
;j
n
(1)
;j
.....
'i
III
n-
,....
o
;j
:t>
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cr'
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III
'i
-<
G
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C/J
30
20r
I
~
.
.
~
Ii
./.
-..
it
~
\
,
I ~
\\- ......J.. ~t\ SULFIDE
....-. I ~
\ \ \ \ / \" -..
\ . . / \.
\ \ I
\ \~/
.
8~
( :.. \
" .~.
I .,
.
300
/
I
/
\
.
,/
400
o RGAf jf C -]
o RGANIC- It
\
\
\
\
( c:")
tu. r e
T..~pe ra.
\
1
i
~
-------
TABLE II. SUMMAR Y OF ANALYSES OF TEN C OA LS
COAL SAMPLE IDENTIFICATION
SRI % SULFUR
NO. SOURCE NOMINAL MINE BED
- -
1 ILLINOIS 5 CROWN SEAM 6
ISGS: 1-1
l ILLINOIS 1 NO. 21 SEAM 6
ISGS : 5-1
3 ILLINOIS 2.5 NOR THERN ILLINOIS SEAM 2
ISGS: 25-1
4 ILLINOIS 5 WILL SCARLET DA VIS
ISGS: 28-1
5 ILLINOIS 4.5 LITTLE DOG SEAM No.6
I ISGS: 32-1-1. 60F
\io)
\,»
I
6 OHIO 3 STANLEY PITTSBURGH
USBM: 205 NO.8
7 MARYLAND 3 ROYAL FRANKLIN
USBM: 106
8 OHIO 3.5 CRAVAT PITTSBURGH
USBM: 107 NO.8
9 PENNSYLVANIA 1 GREENWICH LOWER KITTAN-
USBM: 109 NO.8 NING ( B)
10 KENTUCKY 4 SHAMROCK NO. 14
USBM: 110
-------
TABLE III. SUMMAR Y OF ANALYSES OF TEN COALS
FORMS OF SULFUR (Percent of Coal)
SRI PYRITIC ORGANIC
NO. SAMPLE SULFATE SULFUR SULFUR TOTAL
---
1 ILLINOIS 5% 0.38 0.96 2.48 L HZ
0.79 2.71 0.91 ~ -- 4~ 4 1
0.36 1. 89 2.47 4.72
2 ILLINOIS 1% 0.02 0.25 0.62 0.89
0.00 0.29 0.62 0.91
3 ILLINOIS 2.5% 0.14 1. 30 1.18 2.62
0.35 1. 28 0.84 2.47
4 ILLINOIS 5% 0.13 3.10 2.06 5.29
0.13 3.22 1. 27 4.62
I
VJ
~ 5 ILLINOIS 4. 5% 0.13 1.15 2.86 4.14
I
0.10 1. 21 2.87 4.18
6 OHIO 3% 0.00 1. 70 1.08 2.78
0.00 1. 26 1. 51 2.77
7 MARYLAND 3% 0.00 2.13 0.83 2.96
0.01 1. 56 1.18 2.73
8 OHIO 3.5% 0.14 1.68 1. 25 3.07
0.00 1. 33 1.91 3.23
9 PENNSYLVANIA 1 % 0.02 0.17 0.72 0.91
0.00 0.23 0.86 1. 09
10 KENTUCKY 40/0 0.00 3.50 1.15 4.65
0.00 1. 81 1. 35 3. 16
-------
TABLE IV. SUMMARY OF ANALYSES OF TEN C OA LS
MINERAL ANALYSES (Percent of Coal)
~.
SRI TOTAL PYRITIC
NO. SAMPLE CALCIUM MAGNESIUM IRON IRON
--
1 ILLINOIS 5% 0.18 0.006 1. 06 0.84
2 ILLINOIS 1% 0.007 0.02 0.34 0.22
3 ILLINOIS 2.5% 0.06 0.09 1. 76 1.04
4 ILLINOIS 5% 1. 39 0.07 3.54 2.70
I 5 ILLINOIS 4. 5% 0.10 0.06 1.12 0.74
VJ
U1
I
6 OHIO 3% 0.14 0.03 1.29 1. 08
7 MARYLAND 3% 0.14 0.05 1.43 1. 35
8 OHIO 3.5% 0.18 0.03 1.61 1.14
9 PENNSYLVANIA 1% 0.05 0.02 0.78 0.23
10 KENTUCKY 4% 0.29 0.05 2.41 1. 55
-------
TABLE V. S D 1\'1 MAR Y OF ANALYSES OF TEN C OA LS
PROXIMATE ANALYSIS (Percent of Coal)
SRI FIXE D
NO. SAMPLE MOISTURE VOLATILES CARBON AS!I
I ILLINOIS 50/0 8.0 36.2 44.7 11. I
2 ILLINOIS 10/0 5.4 33.8 54.6 6.Z
3 ILLINOIS 2.5% 7.8 33.7 42.5 16.0
4 ILLINOIS 5% 1.9 37.6 43.2 17.3
, ILLINOIS 4.5%
~ 5 5. I 40.8 44.1 10.0
(t.
'
6 OHIO 3% 1.6 38.1 49.b 10.7
7 MARYLAND 3% 0.6 20.5 62.-8 16.1
8 OHIO 3.5% 1.7 37.2 50.4 10.7 '.
9 PENNS YL VANIA 1% 0.8 27.9 64.2 7. 1
10 KENTUCKY 4% 1.8 31. 3 46.5 20.4 'Ii!
-------
SULFUR IN ASH AND COKE
SRI SULFUR IN ASH SULFUR IN
NO. SAMPLE % of ash FIXED CARBON
1 ILLINOIS 5111/0 1.18 3.84
2 ILLINOIS 1 % 0.51 0.60
3 ILLINOIS 2.5% 0.26 2.00
4 ILLINOIS 50/0 4.80 4.96
I
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I 5 ILL! NOIS 4.5% 0.12 1.99
6 OHIO 3% 0.03 2.47
7 MARYLAND 3% 0.07 2.29
8 OHIO 3.5% 0.31 2.73
9 PENNS YL VANIA 1% 0.31 0.80
10 KENTUCKY 4% 0.00 2.70
-------
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-------
TABLE VII .
RESOLUTION OF THE HzS EVOLUT ION CURVES INTO INDIVIDUAL REACTIONS
PERCENT OF TOTAL HzS EVOLVED IN EACH DESULFURIZATION REACTION
COAL
SRI ORGANIC ORGANIC ORGANIC
NO. SAMPLE I II PYRITE SULFIDE III UNKNOWN
1 ILLINOIS 5% 25 8 20 25 10 9
2 ILLINOIS. 1 % 17 30 15 6 20 11
3 ILLINOIS 2. 5% 9 26 24 19 19 2
I
oj::.
-J
I 4 ILLINOIS 5% 33 27 15 6 16 3
5 ILLINOIS 4.5% 29 18 12 9 24 7
6 OHIO 3% 9 26 15 13 35 2
7 MARYLAND 3% 6 34 23 14 10 13
8 OHIO 3. 5% 11 36 13 7 29 4
9 PENNSYLV ANIA 10/0 25 15 6 50 1
10 KENTUCKY 4% 8 40 11 17 15 8
-------
DEPENDENCE OF DESULFURIZA TION KINETICS ON HYDROGEN
PRESSURE
Several isothermal experiments were conducted to determine
the dependence of the desulfurization kinetics on hydrogen pressure.
The initial series of experiments each used approximately three grams
of the nominally 5% sulfur coal and were conducted at a temperature of
5000 C. Substantial agglomeration of the sample 'occurred in these funs
and it was felt that this might affect the validity of the data. Therefore,
a second series of experiments was conducted using approximately one
gram samples of char prepared by pyrolysis in helium at 500°C for
twenty minutes. The char was ground, and resieved to 2(>-40 mesh
size before the hydrogen experiments were cdnducted.
This second
series of experiments was conducted at a temperature of 475°C and at
hydrogen pressures of one atmosphere, 4.9 atmosphere, and 10 atmos-
pheres.
In these experiments the sample was heated to the reaction
temperature in helium.
The gas stream was then quickly switched
to hydrogen at the desired pressure and the HzS concentration was
lnonitored us ing the mass spectrometer.
In these experiments the
flow rate was adjusted so that a residence time of appro:x:imately 0.25
seconds was maintained for all three exper iments.
The logar ithm of
.
the height of the HzS peak in the mass spectrum is plotted as a function
of time after hydrogen introduction, in FigureZI. Under the conditions
used in these experiments the HzS evolved corresponds to the reduction
of pyrite and the reduction of sulfide and stable organic sulfur. The
kinetics for these processes was determined at a hydrogen pressure of
one atmosphere using the non-isothermal method.
-48-
-------
I .-",
In the non-isothermal measurements the rate parameters for the
reduction of pyrite to sulfide were found to be
ko = 2.8 x 101Z min-l
( 17)
E = 47 kcal/mole
Correspondingly the parameters found for the reduction of the sulfide were
ko = 2. 1 x lO 13 m in - 1
( 18)
E = 55 k cal/mole
These values were obtained for a hydrogen pressure of one atmosphere.
If these processes are first order in hydrogen, then under isothermal
conditions the rate constant for a given reaction is given by
k = ko PH e;E/RT
z
( 19)
where ko and E are given above, PHz is the hydrogen pres sure in atmos-
pheres, T is the absolute temperature in oK, and R is the gas constant
in k cal' oK -mole.
,In the isothermal experiment the concentration of HzS
in the effluent stream is given by
C=C °e-kt
(20) ,
where t is the time after introduction of hydrogen, and Co is the initial
concentration.
Taking the logarithm and differentiating, gives
:t 0n C/C~=-k
d
ill (In C-ln CO) = -k
dIn C
---
dt
= -k
( 21)
-49-
-------
and changing to log base 10 gives,
d log C -
dt -
-k
2.303
(-22)
The data of log C versus t are given for three different pressures
in Figure 2J .
If the reactions of Hz with pyrite and sulfide are first order
in hydrogen, then the kinetic parameters from the non- isothermal measure-
ments give the straight lines shown for comparison in Figure 21.
The over-
all agreement between the calculated lines from the non- isothermal data and
the experimental data in the present runs is excellent.
It is reasonable,
therefore, to consider the reactions first order with respect to hydrogen
pressure over a range from 1 to 10 atmospheres.
- 50-
I
-------
I Atm.
I OO()
~
~
-
... .8
:t:
~
~ ..
:t: .
"C
"C
I ~
\J1 100
..-
I ~
~
. .
8
10 At 71'\.
.
..
.
8
.
.
.
. .
.
8
.
..
. .
. . ..
. .
It>
o
.....
..
. .
.
. .
-.
. ...
.
..........
...
. .
. .
. ...
.
10
. -
''-0
Figure 21. 1st order dependence on Hz pressure: comparison of non-isothermal rate-eon-
stants (straight lines) with isothermal data at 475°C and pressures of 1~ 4.9 and 10 atms.
of Hz.
-------
FORMS OF SULFUR IN CHAR AS FUNCTIONS OF CARBONIZING
TEMPERA TURE
A series of isothermal experiments were conducted in which
coal was carbonized in a hydrogen atmosphere at several different
temperatures in the range from 3250 C to 7000 C. These experi-
ments were conducted each using approximately three gram samples
riffled from the nominally 50/0 sulfur Illinois coal.
A flow rate of
1000 mJ/min of Hz at one atmosphere corresponding to a residence
time of 0.08 sees was used in each experiment.
The samples were
heated at a rate of 330 per minute to the desired reaction temperature
and held at this temperature for a period of ten minutes.
At this
time the hydrogen flow was replaced by helium and a helium flow was
maintained during the time required to cool the reactor to room tem-
perature.
The resulting char samples were then removed from the
reactor.
weighed. and analyzed for forms of sulfur and total sulfur
by conventional techniques.
summarized in Figure 22.
The results of these experiments are
A second series of ten isothermal experiments were conducted
in which coal was carbonized in hydrogen at one atmosphere and at
250 C increments in the range from 3000 C to 5250 C . The results
of these experiments are summarized in Figure 23. The experi-
mental conditions and procedures were similar to those used in the
first series of experiments on forms of sulfur as functions of carbon-
izing temperature, except that in this present series a much lower
hydrogen flow rate ( 100 ml/min) was used to allow efficient cryo-
genic trapping of the condens ible gases evolved.
The trapped gases
were measured using the gas chromatogra ph. The results for hydro-
gen sulfide, which accounts for more than 95% of the gaseous sulfur
compounds detected, are included in Figure 23.
-52-
-------
The purpose of these experiments was to confirm our conclusion from
the non-isothermal studies concerning the origins of the evolved HzS and the
order in which the various HzS peaks evolve. While there is considerable
scatter of the data in Figures 22 and 23, the results are quite consistent with
our conclusion that an organic type (or types) of sulfur in first removed,
followed by pyritic sulfur and then sulfide. A comparison of Figures 22 and
23 with the non-isothermal data appears to indicate that sulfate sulfur is
removed at about the same temperature as the first organic sulfur.
-53-
-------
.Q:
~
"t(..
~
t.')
"4
<
~
...
.. a
. "
I ~
U1 '-'
oj::.
~
A.:
- ---------- ----',
"
"
."
,.
\
,
. ". .
" .
--- --
.
JOo
80
J/O
20
o
TOTAL S IN COKE
ORGANIC (:BYDIFFER£NCE)
PYRITI C
.
.
--
.
.----:1
$00 '00 700
T;:MP.ERATV~£ (oC)
Figure 22. Variations in the amount and forms of sulfur in coke produced fron"} the
no:rninally 5% sulfur Illinois coal carbonized in hydrogen as functions of carbonizing
temperature for a Hz f10"W rate of 1000 rn.l/rn.in.
SUl.FA rE'
100
~oo
.300 ~O
C ARlJO/l1 ZINC
~oo
-------
PER CENT OF
COAL SULFUR
I
U1
U1
I
100
80
40
TOTAL S IN CH/'\R
.
PYR1TIC
ORGANIC (By Difference)
SULFATE
o
100
500
200
400
TEMPERA TU RE (0 C)
Figure 23. Variations in .the amount and forn!s of sulfur in char and gas produced
from the norn!ally 5% sulfur Illinois coal carbonized in hydrogen as a
function of carbonizing temperature for a Hz flow rate of 100 ml/min.
-
600
-------
THE KIT\ETICS OF HzS REACTIONS WITH COAL CHAR
Previous work has demonstrated that reactions of HzS with coal
char inhibit desulfurization; however, little data was available on the
kinetics of HzS reactions with coal char and its components. In this
work the non- isothermal method has been applied to a study of the kin-
etics of these reactions.
The experimental and theoretical procedures
developed for this purpose are described in our report on Phase 1.
A series of non-isothermal measurements on the kinetics of the
reactions of hydrogen sulfide with coal char were conducted.
Thos e
experiments were carried out using a heating rate of 5° /minute and HzS
concentrations in helium in the range from 500 ppm to 2300 ppm.
Data
on sulfur content of the original coal and the coke before and after reaction
with HzS are given in Table VIII.
The relative HzS concentrations in the
gaseous effluents from the reactant beds are given for several of tp.ese
experiments in Figure 24.
These results indicate that the kinetics of
the reactions of HzS with char are quite dependent on properties of the
char sample.
Since the char samples used in these measurements were
all prepared from the coal in a similar manner, it appears that these
differences are due to variations in some component of the original coal.
In duplicate analyses of several samples riffled from the same batch of
coal, one of the components which is most variable in amount is the pyrite.
-
The effect of pyrite variability was investigated using the 10/0 sulfur
Illinois coal SRI No.2 which is initially very low in pyrite and in total iron
(see Tables II-VI). The results of runs using char prepared from the 10/0
sulfur coal (N45) and char prepared from the 10/0 sulfur coal with 5% by
-56 -
-------
weight added pyrite (N43) are give6. in Figure 24 and Table VIII.
The
addition of the pyrite substantially increases the amount of sulfur absorbed
by the coke, both as inorganic S and organic S. It is also interesting to
note that in none of the experiments conducted to date is the per cent sul-
fur in the char after reaction with HzS higher than the percentage in the
original coal, despite the fact that a considerable excess of HzS was avail-
able for reaction in the temperature range in which the reactions would be
expected to be rapid. A si gnificant part of the excess sulfur is desorbed
as esz in the range from 900Ge to lOOOGe in all of these experiments.
In an effort to inve stigate separately the several reactions apparently
involved in the absorption of HzS by coal char a series of non-isothermal
experiments were conducted on the reactions of HzS with the principal re-
active components of coal char including carbon, iron, and calcium oxide.
The results are described in the next two sections of the report.
-57 -
-"~ ..:....,...~<
-------
H2S+
INTENSITY
I N MASS
SPECTRUM
(Arbitrary
Units)
I
\J1
00
I
-....,...:--:~;:-: ""'1,.-:...""c-"=':'.....no..:~ -~~r_~i--~--".------~I~~.
.
~
-~-'--"'-"'''"W1._~:-~-
5
4
N45
N43
N39
3
N40
N22
N34
2
--.-.."Z:io~r-!.~~
o
100
"
~
-
.
=
.
~, .
.
200
400
1100
300
500
600
700
800
900
1000
TEMPERATURE (OC)
Figure 2.4. Relative HzS concentrations in the effluent gases from six non -isothermal
rr.casurernents on the kinetics of the reactions of HzS ,"vith coke.
-------
TABLE VIII . SUMMARY OF DATA ON REACTIONS OF H,S WITH COKE
PERCENT SULFUR H,S RESIDENCE
RUN COAL ( 1) COKE( 2) COKE + HzS( 3) CONCENTRA TION( 4) TIME( 5)
N22 4.2 1.9 4.2 o. 1 O. 14
N34 4.2 O. 1 . 3. 1 0.05' 0.04
N:39 3.8 1.6 3.8 0.2 0.3
N40 3.8 1.0 2.4 0.2 0.25
I N41 3. 8 1.0 2.5 O. 1 0.13
U1
-.0 N43 inorganic 2.75 1.31 0.23 0.22
, ~ O. 1
organic 0.62. 0.90
N45 inorgahic 0.27 L 0.1 0.36 o. 1 0.22
organic 0.62 0.58
( 1) In coal sample from same riffling as sample used in coke prepared
for the HzS run.
( 2) In coke sample before reaction with H,S.
(3)In coke sample after reaction with H,S.
(4) Concentration of H,S in helium sweep gas.
( 5) Average time in seconds required for a volume element of gas to
sweep through the reactant bed.
-------
KINETICS OF HzS REACTIONS WITH CARBON
In Figure 25 the results of several non-isothermal measurements
of HzS absorption by chars from three different coals are compared to
similar results for iron and charcoal. These results show that the
absorption of HzS on coal char at temperatures below 500°C is princi-
pally due to reaction with iron to form iron sulfide while at higher temp-
erature the reactions with carbon also become important.
Non-isothermal kinetic measurements were carried out on the re-
actions of HzS with carbon in the form of activated charcoal.
One gram
of the powdered charcoal was placed in the reacto'r and heated to 800 ° C
in a flow of helium and held at that temperature until gas evolution from
the charcoal had ceased as determined by monitoring the effluent with the
mass spectrometer.
The reactor was then cooled to room temperature
with the helium flow maintained.
The non-isothermal kinetic measure-
ments were accomplished by flowing a mixture of 1000 ppm of HzS in
helium through the reactor at a rate of 220 ml/min.
The heating rate
was 4° Imin.
The relative HzS concentration in the effluent for one such
exoeriment is given as a function of temperature in Figure 25.
Detectable
absorption of HzS begins at about 450°C and becomes substantial in the
region between 600 and 700°C. Above 750°C the HzS concentration in
the effluent remains constant with temperature at about 100/0 of the inlet
concentration. In the range between 900 and 950 ° C, CSz evolution occurs.
Beginning at about 600°C deposition of sulfur on the walls of the reactor
outside the hot zone of the furnace is observed. This deposition of free
sulfur indicates that not all of the HzS loss involv~s reaction with the car-
bon, but rather indicates sorne los s due to the reaction
-
-60 -
-------
I
400 500 600
Temperature ( o.C) -
Figure 25. HzS absorption in non-isothermal experiInents on char, charcoal and iron. l, 000 ppm
of HzS in helium was passed over approximately 1 gram of the solid at a total flow rate of 220 ml/min;
the heating rate was 4 °c lmin, (1) Char from coal SRI No.1, a nominally 5% sulfur Iliinois coal,
(2) Char from coal SRI No.2, a nominally 1% Illinois coal, (3) Char from coal SRI No.4, a nominally
50/0 sulfur Illinois coal with a high calcium content, (4) Iron filing:;, (5) Charcoal. -
I
0'
~
I
::r:
N
C/)
o
o
z
o
M
:;; Z
t-3
~ ::0
~ )-
t-3
H
~
H
Z
M
t-rj
t-rj
L'
cj
M
Z
t-3
-, ---- -----"--r-------:~.'-
1000
.~-
800
-I
J
600
.~~.
400
200
100
I ------_L-._____.L~- ---..:.--- __L_- --- -- ---
.700 800 - 900 1000
200
300
-------
2 Hz5
~ 2Hz + 5z
( 23)
which may occur on the carbon surface. The solid residue from the Hz5
absorption on carbon contained 2.3% sulfur, while the original charcoal
sample contained no detectable sulfur.
The results of some additional non -isothermal kinetic measurements
on the reaction of HzS with carbon are given in Figure 26.
The experiment
on the activated charcoal was carried out as described above and a similar
experiment was performed using graphite.
In all of these experiments the
sample has been held in place in the reaction chamber by a plug of quartz
wool placed above and below the sample.
The third run in this series was
done as a blank using only the quartz wool plug, other wise the procedures
were the same as those used in the experiments on activated charcoal and
graphite.
The blank experiment shows that the Hz5 in the effluent from
the reactor begins to drop below the imput HzS concentration at about 7400C,
even when no reactant is present.
This loss in Hz5 coupled by the observa-
tion of sulfur formations shows that under the conditions employed a homo-
genous decomposition of HzS occurs.
Since (23) is the most favored decom-
position on thermodynamic grounds, the blank experiment describe suggests
strongly that above 7000 C (23) occurs homogenously.
-
Analysis of the data from this experiment indicates that this reaction
occurs with an activation energy of 38 kcal/mole. Since similar effects
are observed in the HzS reaction on char and carbon presumably the drop in
effluent HzS concentration in the region of 700 ° C is due primarily to the
homogenous reaction (23) rather than to a heterogenous decomposition on
the carbon surface at this temperature. However, since the steep drop in
HzS concentration occurs at a lower temperature in the case of the car bon
-62-
-------
--,-----
1----'--
----- r - ___d ------r---- -'-- - --I
I ()<) i I
::r:
N
C/) i
,
0 1 !
o i
;:J ~Ol j
(') ( a)
(D r-- Activated Carbon -j
:::; / I
..... ,
'1
~
.....
,.....
0
;j , /- ( b) Graphite
i
> 6t !
I '1
'" r;j /
v.I ,.....
I .....
'1 ,i(c) Blank
~
'1
'< 40[ /
r.:
;:J J
,.....
..... I
en
:
I
20 ~
!
300
400
500
I
600
I
700
I
80:"
I
900
I
1000
Temperature ( 0 C)
Figure 26. Non-isothermal absorption of hyr1:!:'ogen sulfide on (a) 1 gram activated charcoal
(b) 1 gram graphite and (c) a blank in which the quartz wool plug normally used to hold the
salnple in place was used but with no sample. These experil1H:,nts used 0.10/(1 HzS in He at a
itow of 220 111l /111 in.
-------
as compared to the blank it appears likely that the dissociation rl'.H~bon may
also be catalyzed by the carbon surface.
An understanding of the details of the interactions of HzS with coal
char is of great importance both to the interpretation of the non -isothermal
kinetic data and to the applications of these data to the design of practical
processes. The sequence of reactions involved in the desulfurization of
coal with hydrogen may be written as follow s:
Hz -> Hz
(gas) (adsorbed)
(24 )
Hz ( d d) + Coal-Sulfur -> HzS
a sorbe (adsorbed)
( 25)
HS
z (adsorbed)
:> HzS (gas)
(26 )
where reaction (25) represents several reactions corresponding to reac-
tion of the several different kinds of sulfur in the coal, and reaction (26)
may represent more than one desorption process, if (as is often the case
in heterogenous reactions) more than one type of HzS adsorption site is
present. It certainly is reasonable to suppose that adsorption sites in the
neighborhood of an iron pyrite particle are different than those in the neigh-
borhood of an organic sulfur compound.
Fa r the capture of HzS by coal char the sequence of reactions may be
written as follows:
HzS ( -> HzS
gas) (adsorbed)
( 27)
.
HzS + X -> XS + H
(adsorbed) z (adsorbed)
( 28)
HzS
(adsorbed)
> Hz ( )
gas
( 29)
-64 -
--,,-= ,- -~--
-
-------
where X is a reactive component of the coal, for example, iron,
calcium oxide, or carbon. Non-isothermal kinetic measurements
have been conducted on the reactions of HzS with iron and calcium
oxide; the results of these investigations are presented in the next
S'-e.ction.
For this section we are concerned with the reaction of H S
z
with carbon.
The reaction model proposed here is a reasonable working hypoth-
esis within which to examine kinetically the desulfurization inhibition
that is known to arise because of the reaction of HzS with various con-
stituents of coal char.
For (27) '- (28) to be important, it is only
necessary for the desulfurization (24) - (26) to have proceeded to a
measurable extent because (27) - (29) are merely thE7 back reactions
. .( 24) - (26), the desulfurization proce s ses.
In connection with the measurement on the kinetics of desulfuriza-
tion of the ten c~als studied in this work, it was found that the behavior
of the most stable form of organic sulfur in coal (des ignated as
Organic III in the previous discussion) could be approximated by a
sulfuratoo carbon prepared by reacting HzS with activated charcoal.
Non-isothermal kinetic data on the reaction of Hz with the sulfurated
carbon and with a high sulfur coal char are compared in Figure 27.
The coal char was prepared from the 5% sulfur Illinois coal, SRI No. I,
by pyrolysis in helium at 800°C for one hour.
The char contained
3.4% sulfur by standard ASTM analys is.
A non-isothermal measure-
ment on this char us ing helium sweep gas showed that no significant
desorption of HzS occurred in the absence of an external source of
hydrogen. The sulfurated carbon was prepared as described above.
The difference between the non-isothermal HzS evolution curves for these
-65-
-------
b as crl'bed to the FeS Pres ent in the coal char.
two experiments may e
The resolution of the result into two processes corresponding to re-
action of Hz with FeS and with organic-S III is illustrated in Figure 27.
In addition to the strongly-bound organic- Sill, HzS may be
adsorbed on the carbon surface apparently without 10s ing its chemical
identity. The organic-S III is stable in an inert atmosphere to at
least 1000 ° C while the adsorbed HzS may be desorbed at high temperatures
in an inert atmosphere.
The results of three non-isothermal experiments
on the desorption of HzS from char are summarized in Figure 28. These
results are on the desorption of HzS from sulfurated charcoal, sulfurated
coal char from 5% sulfur Illinois coal SRI No.1, and sulfurated graphite.
The sulfur contents of the chars were 2.3% for the charcoal; 3.4% for the
coal char; and 0.2% for the graphite.
The sulfur contents were not increased
by further reaction with 1000 ppm of HzS in helium at temperatures up to
800 ° C, indicating that the chemically stable forms of sulfur were fully
saturated.
Each of these chars was then treated at 500°C by passing
1000 ppm of HzS in helium over 5 g of the sample at a flow rate of 200 ml/min
for one hour.
The temperature was then lowered to 300 ° C while main-
taining the flow of the HzS-helium mixture.
The flow was then switched
to pure helium and the linear temperature programmer started to give a
heating rate of 4°C per minute. The results given in Figure 28 indicate
that the high temperature desorption of HzS from coal char may be quite
slow and may be somewhat variable from coal to coal depending on the
structure of the carbon matrix.
.
To sunlmarize the results of this set of experiments:
( 1) Figure 27 compared to Figure 8 shows clearly that the difference in
desulfurization behavior between a sulfurated carbon and a high sulfur
coal char is the presence of FeS in the latter. Further, it is shown in
Figure 27 that in the desulfurization of a high sulfur coal char, the
-66 -
-
-------
desulfurization process can be represented quite by desulfurization from
FeS sites and from organic -S III type sites.
(2) Figur'e 28 shows that HzS .may also be physically adsorbed on a carbon
surface such as charcoal (curve a) without losing its identity. This figure
also indicates that the situation is not always as simple as might be thought
from Figure 27 because as curve (b)shows the desulfurization from coal char
can require considerably higher temperatures than those for Organic I and
II. Also as a comparison of curves (a) and (b) indicates, the desulfurization
from coal to coal might be very dependent upon the structure of the carbon
matrix.
The non-isothermal evolution of HzS from sulfurated carbon by reaction
with HzS is given for different Hz flow rates in Figure 29.
The desorption
of HzS from charcoal in helium is included in Figure 29 for comparison.
The kinetics corresponding to this HzS evolution represents the maximum
rate at which HzS may be evolved from coal in this temperature range,
even if the chemical reaction were infinitely fast and if there were no back
chemical reaction.
This relatively slow desorption of HzS may account
for some of the small discrepancies observed in the resolution of the de-
sulfurization kinetics for the ten coals into individual proces ses.
-67 -
-------
6 C _u_--
I
0"
00
I
r: (/) 'I1
- C -~
~ -'J':J
,.-. -,.....
,.. .....
. ~ ..,
;;:;r '";)
~ ~
:::r r",
... ~ I
:: '"1 ~ 50
u- ..-, ~ ..,.,
" 0 t'"
..-, C ;J Ib
j .., I ,-
o ;:J ...... ~
~ 'n CfI ::.
. v' 0 .
1-rje;"!;'r3
(t 'J; (t) ~r<
(J) C "1 .,-<
~ ~~ ~ 'C'n 4 0
.. ~ ii>
Q. 'i ...... ,J
c:::sg
"i ~ (t) ()
(JQ ;J PJ CD
PJ 0 CfI ::s
::s ...... C n-
...... CfI "i
(') (') ~ ~ 30
en 0 3 o'
g.~CD ::J
~h g.--
"i ClJo»
H;:d......'i
~H....g
...... ~ :::r n-
'1 0 CD '1 20
o . 0.. \IJ
3 ...... CD ~
en
CfI'1CC::
C CD ...... ....
...... CfI ...., .....
...., 0 C ~.
C ...... "i CfI
"i < ......
PJ CD ::<; -
n- 0.. PJ
CD n-
o.. ...... ,....
::J 0
() ("'to ::s
:::r 0 0
PJ ...,
"i
~
..
10
o
PJ
......
:::r
,....
I)Q
:::r
I -.- ---------------1-
300 400
200
---,- - --~~I-~~-~
r ..,. -- . ,- .- r ..-_.
.. ... .. T'" ..-.,.--.- -.- --.
------
/ \
/ ,..'"
I \.
I. ,.
/ : ,~
I . ,8.
I. , .
,. \ \
, , .
\.
I \ \,
, \ -.
FeS " .
50C
Temperature
I
600
, --.-- ._--"..- --- .1- '.-"'--
700 BOe
( 0 C)
..----- J --..-
~OO
1000
-------
60
~ 50 ( b)
ro
......
P>
M-
....
~
ro
::r:
N
en 40
0
0
;j
n
ro
;j
M-
"i
P>
M- 30
....
I 0
0' ;j
'-D
I ~
"i
0"
....
M-
"i
P> 20
"i
'<
C
;J
....
M-
r:n
10
300
400
500
800
600
700
Figure 28.
Temperature (CD)
Desorption in He of HzS adsorbed on (a)
coal No.1, (c) graphite.
charcoal (b) char from SHI
900
1000
1100
-------
::r:
N
(JJ
n
o
~
n
(!)
~
,....
'"'I
Il>
....
,.,
'"'
~
:x>
I '"'I
~ 0-
a
, (""t-
'1
Il>
'"'I
-...:::
c:
~
~.
(""t-
oo
€o C,
5j I~
LlOJ-
I
301-
I
I
20~
j
!
I
I
lOt-
I
40C
----------
--~--- ~---
- -- - -- - ----~-'-1--- ----- -- -
\ ,/'
>,
~
, ; r .
-- --
, -
~/ ~~'~~-d~~-----~
5)(\
-----------~.~--- -~
4
~
"
,
I
700
8e -:-
c.. j ,-
u-...\...
Figure 29. Non- isothermal measurements of the desulfurization of sulfurated charcoal. The
nun"1.oers identifying the curves give the Hz flow in litres Imin/g of san"1ple. Desorption in He
-------
KINETICS OF HzS CAPTURE BY Fe AND BY CaO
The reactions of HzS with Fe and CaO are important, not only
because of the role of these reactions in inhibiting desulfurization
but also because of the potential use of these materials as sulfur
acceptors.
We have performed several non.,
isothermal kinetic e~perirpents usi:ng iron prepared by hydrodesulfur-
ization of pyrite in hydrogen at 8000 C.
One such experiment was per-
formed by passing a stream of helium over the iron with 1,000 ppm of
HzS in the helium.
These data are plotted in an Arrhenius type plot
in Figure 30.
The region of reaction limitation is shown, at approx-
ima.tely oI\e half this rate the region of di£fus ion limitation takes over
at higher temperatures.
This
reaction
of iron with HzS to form FeS and Hz is
extremely fast.
The kinetic parameters are
E = 18 kcal/mole
ko =6.5 x 106 (atm - HzS) -1 min-l
whe re k = k
o
in terms of the solid reactant;
-E/RT
and the rate equation for the reaction is written
d[ S l K [S] PH S
dt z
,
( 30)
where [S ] is the concentration of sulfur in the solid product.
-71-
-------
+1.0
I
-.J
tV
I
~. (-L. CYco)
0.00
-1. 00
"0,.
~~;
~~0e
~"._',
. .
I .
Diffus ion Limited
Reaction Limited
I
1. 60
1. 80
2.00
1. 20
1.40
IO/T (OK)
Figure 30. A rrhenius type plot of the HzS absorption by iron in a non- isothermal kinetic experiment.
The iron was prepared by the hydrodesul£urization of pyrite with a 4 litre flow of Hz at 8000 C.
The absorption. experiment was perforn"led by pas sing aI, 000 pp.rn of HzS in heliurn over 1 gram of
-------
The reactions of HzS with calciUIll oxide are also of cons ide rable
significance, both becaus e of the importance of GaO as a potential sul-
fur absorbent, as well as the contribution to the back reaction for coals
containing calcium in the ash.
The results of three non-isothermal
kinetic measurements on the reactions of HzS with GaO from different
precursers are shown in Figure 31.
Similar measurements using
MgO formed by calcining precipitated MgG03 indicate that the MgO
component of the calcined dolomites is es sentially unreactive to HzS.
32.
The Arrhenius plot of the data for the dolomite is shown in Figure
The kinetic parameters for the reaction limited regions are
E = 38 kcal/mole
ko = 1.7 X 1013 (atm. HzS) -1 rnin-l
'whe re the form of the rate equation is the same as given above for the
iron reactions.
-73-
-------
..--.
":"
~
P-.
P-.
f-< 1000
~.(I
~
;:J
~
(.L.
~ 750
W
Z .
........
Z
0
........
f-<
< 500
~
b
'1
t..
W
U
Z
0 250
U
(f)
N
::c
0 200 400 600
TEMPERA TURE ('oC)
Fil5ure 31. HzS absorption in non-isothermal kinetic experiments on calcium
oxic.e. 1 J 000 ppm of HzS in helium was pas sed over 1.5 grams of solid at
a total flow rate of 150 ml/min, the heating rate was 4°C/min. (1) Calcium
oxide from calcination of -200 mesh precipitated calcium carbonate, (2) Ca~cium
oxide from calcination of -140 mesh No. 1930 dolomite. (3) Calcium oxide
from calcination of -140 mesh No. 1683 dolomite.
-74-
-------
-, 1.00
--
Diffusion Limited
----.r,'\-r..-, 0
\;J ~- . -'7.....,-.~ r-:
v-\:)-~-0-0 r;
Reaction Limited
0.00
t,(-k CYCo)
I
-..J
\J1
I
-1.00
-2.00
1.50
1. 60
-- -~-~--~~<_,__l=__-_~-~~<,<~~~,--~~~-~~~-~~=~-
1.,0 1.80 2.00
=~-~~-----L~~~~-
,ofT (')()
Figure 3$. Arrhcnius type plot of the HzS :::.bsorplion by calcined No. 1930 dolo111ite in a non-
isothel"lllcLI kinetic cxpcrin1ent. The absorption experin1cnt was perforn1cd by p3.ssing a 1,000
ppm of IllS in helium over 1 grain of the solid at 150 11)1/111in; heating rate W(\s -l"Chnin.
-------
KINETICS OF CALCINA TION OF DOLOMITES AND LIMES TONES
A series of kinetic experiments were conducted on the calcination
of precipitated calcium carbonate. The results of three such experi-
ments are summarized in Figure 33.
Run N -46 is the non-isothermal
(heating rate 5°C/min) calcination of 3.4 g of CAC03 with He flowing
over the sample at flow rate of 500 ml/min.
These parameters cor-
respond to a mean gas residence time in the sample of 0.47 sec.
Run N-4t is a similar experiment with the sample reduced to 0.35 g and
the helium flow increased to 1000 ml/min.
This approximately 20 fold
decrease in residence time (to 0.024 see) causes a shift in the peak COz
evolution from 775°C to 650°C.
As discussed in connection with the HzS
reactions, this shift may be ascribed to the back reaction of COz with CaO.
This is confirmed in non-isothermal run N-5l, in which a 50:50 mixture
by volume of COz and He at flow rate of 100 ml and a total pressure of 1
atmosphere was passed over 5.7 g of CaO prepared from precipitated
CaC03 in an earlier calcination run.
Detectable absorption of CO2 begins
below 400°C. At 725°C the highly exothermic absorption reaction becomes
sufficiently rapid to cause local thermal runaway within the bed. At
this point the CaO bed is quickly converted to CaC03 and no further absorp-
tion occurs.
A series of experiments were conducted on the calcination of lime-
stones and dolomites in helium. In these experiments 250 milligrams
of the solid was used with a helitun flow rate of 1 litre per minute and a
heating rate of 4° C per minute. The results of four such experiInents
are given in Figure 34 where the carbon dioxide evolutions are given as
-76-
-------
6
N-51
5
4
RELATIVE C 01
CONCENTRATION
I
-..J
-..J
,
o
300
3
(A rbltrary Units.>
2
400
500
600
100
800
900
10'00
T E M PERATURE. (0 C.)
Figure 33. Runs N-46 and N-41 are calcination runs on calcium carbonate in helium with residence times of 0.47
seconds and 0.02-1: seconds, respectively. The twenty-fold decrease in residence time shifts the -peak carbon
dioxide evolution Jrom 755°C to 650°C. Experiments on-recarbonization on calcilUTI oxide, run N-51, confirm
that this shift is due to the back reaction of carbon dioxide with calciun1 oxide.
-------
6 n
~
h I V
~
~ 6" I \
IV
~
~ \
~'f
~~ 4
~'.:::I
~)..
~t<
R
~~
~~ J
~~
",'-
~
~
~ ~
~
~
P<
/
.5'00
600
Tf:MP£:RATURE (oC)
700
Figure 34.. Carbon dioxide evolution from the non-
isothermal calcination in helium of precipitated
calcium carbonate (1), Limestone sample 1683B
( 2 ). Dolomite sample 1930 (3), and Dolomite
sample 1380 (4) 0 The heating rate was 40 Cimino
and the helium flow rate one 1 Imino
78-
-------
functions of temperature.
Results given in Figure 34 include precipi-
tated calcium carbonate, a limestone and two dolomites.
These results
indicate that under the conditions employed the calcination kinetics for
limestones and dolomites are ~ubstantially the same as the kinetics for
pure calciuro carbonate.
Kinetic parameters determined from these
results fall within the range
E = 58 + 5 kcal/mole
log ko= 12.5 + 1
This similarity of the calcination kinetics probably holds only for low
local concentration"s of carbon dioxide.
At higher COz concentrations
it is apparent from the literature that the back reaction inhibits the de-
composition of the calcium carbonate with little effect on the decompo-
sition of tbe magnesium carbonate.
-79 -
-------
KINETICS OF DECOMPOSITION OF IRON SULFA T ES
It is well known (see Figure 22 -23) that a significant part of the sulfur
In coal is in the form of sulfate.
Therefore in any kinetic study of the overall
coal desulfurization proces s we must consider the removal of sulfur from
this type of compound.
With this in mind an orientation series of non-iso-
thermal experiments were conducted on the decomposition and removal of
sulfur from ferrous and ferric sulfate in hydrogen and in helium.
The results for the reactions of ferric sulfate in a hydrogen atmosphere
are given in Figure 35.
Substantial shiftsin the shapes and locations of
the SOz and HzS evolution peaks occur as the ratio of Hz flow rate to sample
size is varied.
Similar resu.lts for ferrous sulfate are given in Figure 36.
The results for the two iron sulfates are qualitc-.tively similar; the princi-
pal difference being the low temperature evolution of SOz from the ferric sulfate.
As can be seen in Figures 35 -37, we should expect an HzS evolution
peak from sulfate sulfur having an onset of about 410 0, a maximum of
about 5000 C and then a slow decline, vanishing at 650 -700 0 .
Whethe r such
a peak will be visible in a coal desulfurization process depends of course
on the relative amounts of sulfate, organic I, organic II, and pyrite.
These
figures also show that even in a Hz-atmosphere sulfur will be removed from
the sulfates as SOz concurrently with the HzS.
SOz is evolved.
In a helium atmosphere only
- 80-
-------
I
00
......
I
s::
o
....
+'
II!
Jo<
+'
s::
Q)
u
s::
0-
u2
....
II) s::
II! :::s
o >-
Jo< ...
=' cG
'+of ...
- +'
-=' ....
oo.D
...
Q) II!
>-
....
+'
II!
-
Q)
~
6C
f
\
I
\
--1
50
40
~S02
30
\5C2
\
.\
\
,
20
. ~
\ I ,
'J. \
/ \
,
\
10
100
200
400
300
1"'"'\
I !
I .
\
I I
J I
I I
I I
I I
I 1
II
f I
~ :
,
( a)
----(b)
r-- ,.."
/
-- ....t - -
--
......,
-
-......
50C
boe
700
800
90(;
Figure 35.
Sulfur gas evoluti on from pyrolys is in 4 litree'minute of Hz for (a) 1 gram and
(b) 0.036 grams of ferric sulfate.
-------
6°1 I
I
I
50~ 1
I
", - - - ( a)
I I
~ I I (b)
(1) , " -..j I
;- i .:~ .-:: -
-- II' 401- , ,
II' '"'" I
,..... I
6- < .
,..... (1) i I I J
'"'"en I
'1 ~
II' r- I I
'1 .....
'< ~ i
I ~.; 30t- I I
00 ,
N '"'" II' I I I
I tIJ tIJ ! 1
() , I '" -
0 ! , /''' ---
;j I / r+--' h --
() I \ I \
(1) I I
~ 20t- , I \
g: I I / \
I / \
;j j /
I I I \
10 , I / \
50- /
c.. \ / \
\ ./ \
/
\/ ,
, /,
/. \ "
,-
100 200 300 400 500 600 700 800 900
Figure 36. Sulfur gas evolut8()n from pyrolys is in 4 litres Iminute of I;Iz for (a) 1 gram. and (b)
0.028 grams of.ferrous sulfate.
-------
60,----T -,-----"'-1
~
(t)
- !
111
.....
.... 50 t-
<,
(t) I
,
UJ '
~
-
1-+0
~ I 1"'\
'"! I \ ( a)
- - -
a t
I I \
111 40~ (b)
en I
() \
0 S'='-:> .., \
;:I .:...
n I
(t) \
;:I I
.....
'"! \
111 30 I
.....
I .... \
00 0 I I
\.N ;:I
I I \
111
'"! I
0"
.... I
.....
'"! I
111 20
'"! I
'<
~ I
;:I
....
..... I
en
10 I
,~ I
-,
'.......' --/2 ,
/
/
/
100
200
300
Figure 36.
400 500
Temperature (E: 0)
Sulfur gas evolution from pyrolys is of ferrous sulfate in (a)
and (b) 4 litres /minute of Hydrogen.
600
700
80e
900
4 litres /minute of He lium
-------
PYROL YSIS AND GASIFICA TION OF COAL MIXED WITH CALCIUM OXIDE
Isothermal experiments were performed on mixtures of the nom-
inally 5% sulfur coal mixed with calciwn oxide obtained by calcining
marble chips.
In two of these experiments approximately 7 grams of
coal was thoroughly mixed with approximately 7 grams of calcium oxide.
An additional gram of calciwn oxide was added to the bed on the exit
side. In one of these experiments a very low flow (20 millilitres per
minute) of helium was used as the sweep gas and in another a somewhat
higher flow rate of hydrogen ( 100 millilitres per minute) was used.
These experiments were conducted isothermally at a temperature of
7500 C and the reaction time was fifteen minutes.
The bed was heated to
reaction temperature at a rate of 330 per minute.
In both experiments
the gaseous effluent was monitored using the mass spectrometer and the
condensable effluents were trapped at liquid nitrogen temperature and
analyzed in the gas chromatograph.
No HzS was detected by either method
in either experiment giving an upper limit to the total amount of HzS evolved
of less than 0.1 milligram.
Sulfur comparison data for these experiments
is given in Table IX.
Sulfur in the gas cons isted entirely of trace amounts
of sulfur dioxide and methyl mercaptan.
The char formed in these experiments was separated manually from
the calciwn oxide.
It proved to be quite difficult to achieve a reliable
complete separation so that the coke samples analyzed probably contained
small amounts of calcium oxide and calciwn sulfide.
The lower values
for sulfur in the coke were obtained by combustion of the coke in oxygen
with measurement of the sulfur dioxide evolved using the gas chromato-
-84-
-------
graph.
method.
The higher values were obtained by the conventional Eschka
In previous investigations on coal we have found that these
two methods normally agree within 100/0.
However, in separate experi-
ments on oxidation of calciUIXl sulfide, less than one third of the sulfur
present was removed as sulfur dioxide.
The balance remained as sulfide
and sulfate in the solid.
Therefore, the difference between these two
methods of analysis may be due to the calcium oxide and calcium sulfide
present in the sample.
Experiments were conducted orr-the gas incaHon of coal with steam both
with and without added calcium oxide.
These experiments were conducted
at a temperature of 1000°C and one atmosphere of steam.
The reaction
time was approximately 100 minutes and the heating rate to reaction tem-
perature was 33°C per minute.
Helium flowed over the sample dur-
ing heat up. In the experiment conducted with coal alone the char yield
was 230/0 of the initial ,coal weight and this char contained 7% of the sulfur
initially present in coal. The tar yield was approximately 8% of the initial
coal weight and contained 50/0 of the entire coal sulfur. More than 99% of
the gaseous sulfur evolved was as HzS.
In experiments involving coal
mixed with calcium oxide a very small amount of gaseous sulfur was
evolved as HzS during gasification.
Analys is of the solid product for car-
bon at the conclusion of the experiment indicated that at least 99% of the
carbon in the coal had been gas ified.
-85 -
-------
TABLE IX SULFUR COMPARISON DATA FOR ISOTHERMAL REACTIONS
OF NOMINALL Y 50/0 SULFUR COAL MIXED WITH CALCIUM
OXIDE, IN HELIUM AND IN HYDROGEN
(all data are shown in mg of S per gram of coal)
SULFUR SPECIES SWEEP GAS
He !!.L
-
S in coal 42 42
S in gas 0.2 0.3
S in tar 0.7
S in char
by combustion 3 3
by Eschka 10 6
-86--
-------
KINETICS OF REVERSIBLE DESULFURIZATION REACTIONS
The reactions involved in the desulfurization of certain constituents of
coal can be treated as consisting of two oppos ing elementary reactions, viz:
( XS) 1" d + Hz ~ X 1" d + HzS
so 1 so 1
( 31)
where X is a component of the solid coal particle to which sulfur may be
bound.
Hence, X may represent iron, carbon, or more complex chemical
groupings in which the sulfur in coal may exist.
action is the reduction of the iron sulfide in coal.
An example of such a re-
Since the two reactions are elementary, the law of mas s action applies
and we may write for the rate of desulfurization
dNXS
dt
=
kf PHz NXS - kb IHzs NX
dNX
dt
( 32)
where NXS is the number of sulfur atoms in the coal bonded to X at time t,
NX is the number of XS linkages destroyed at time t, viz.
N2eS = NXS + NX
( 33)
and PHz + PHzS are the partial pres sures of Hz + HzS respectively in the bed
volume.
The material balance applied to HzS, which, in addition to being formed
and destroyed in the forward and reverse chemical reactions, is swept out
by the flush gas, is
-87-
-------
o Q
NX = NXS - NXS = NHzS + Vb
t
S :HZS (t) dt
( .H)
flow rate of the flush gas.
Differentiating we obtain
where NHzS is the number of HzS molecules in the volume of the bed... Qis the
dNX _-dNXS = dNHzS + Q NHzS
or- - dt dt Vb
So that for the time dependence of NH S we have
z
dNHzS
dt
= kf PHz NXS - kb PHzS NX
Q
Vb NHzS
The units of kf and kb are atm - ~ min -1.
We now have the two differential equations
dNXS
dt = kf PHz NXS - kb PHzS NX
dNH S Q
-'dt z = kf PHz NXS - kb PHzS NX - Vb NHzS
where the relationship between NHzS and PHzS is
L Vb
NHzS = ~ PHzS
L = Avogadro's number.
Since we have the stoichiometric relationship
o
NX = NXS
NXS
-~~-
( 35)
( 36)
( 32)
( 36)
-------
(where N~S is the initial concentration of XS in the solid and the assump-
tion is made that NSt =0 ). equations ~2) and (3~ are two simultaneous
differential equations in the unknowns NXS and NHzS (or PHzS) .
Note that if, as an approximation, we take dNHzS = 0 (a steady-state
dt
in gas -phase on HzS) , then from {32} and (36) we see that
.9.... NH S = - dNXS
Vb z dt
( 37)
or
Vb dNXS
NHzS = --0 dt
( 38)
or
RT
PHzS = - LQ
dNXS
dt
( 39)
Substituting (3q) into (32) yields
dN 0 RT (dNXS \
- d~S = kf PHz NXS + kb (NXS - NXS) LQ \: dt :J
.( 40.)
or { RT 0
. 1 + La ( NXS
o
let B= kbRT NXS
LQ
NXS) kbJ dNXS
dt
- kf PHz NXS
( 41)
C = kf PHz
( 42)
y = NXS
-:-:v-
NXS
-89 -
-------
With these definitions we get
51 + B (l-y ) d = jdt
Cy Y
( 43)
or
f,; + B fl;Y dy = - Ct + Constant
( 44)
J~ + B 57
- By ::= - Ct + Constant
( 1 + B) In~y- By = - Ct + Constant
( 45)
At t= Q J
1 80
y= .
Constant = - B
( 46)
Then (1 + B) In y- By = - Ct - B
(47)
-Ct = B-By + (1 + B) In" y
( 48)
= B(I-y) + (1 + B) ln~y= B [1-y)+ (1+ 113)ln~
( 49)
From the definitions we have
- k P ::= kb R T NSts r~
f Hzt LQ l\
) ( LQ ~ J
NXS 1 In N!S
N~S + + kbRT NStS NXS
( 50)
Letting Y::= PH t we get
z
-90 -
-------
¥={~)
~SRT
LQ
f(; - NXS.\£ + LQ -\ In NxsJ
~ N5(SJ \ kbR TN~S) NStS
-91-
( 51)
-------
SUMMARY
Three principal factors control the design and performance of a
gas - solid reactor; the reaction kinetics for single particies,
the
particle size distribution for the reactant solid, and the flow patterns
for both solids and gases in the reactor.
In many pra.ctical cases the
kinetics are complex and not well known, and, as a result, detailed
analys is of the reactor des ign is not pos sible.
In such cases designs
are based largely on the experience gained by many years of operation,
innovation, and small changes made on existing reactors.
The blast
furnace for producing non is perhaps a most important industrial
example of such a sys tem.
The non-isothermal kinetic method provides an extremely powerful
tool for obtaining the kinetic data needed for analys is of practical de-
sulfur iz.ation systems.
Coupled with modern high speed computer
techniques for performing the necessary integrations, the kinetic data
allow detailed analyses of idealized systems approximating the real
processes.
An approach to the analysis of practical systems may be summarized
as follows:
First the chemical kinetic and reaction mechanisms are
de te rm ine d us ing the non - is othe rmal me thod .
Secondly, a model of the
gas-solid reaction system is developed for individual particles, and
the validity of this model is determined by comparisons of calculations
with the results of closely controlled laboratory experiments.
Finally,
the results for individual particles are averaged over the appropriate
particle size, temperature, and compos ition distribution best approx-
imating the real reactor system.
-92 -
-------
The major problems involved in such a procedure are in determining
the necessary detailed kinetics of the reactions,
competing and oppos ing reactions are involved.
since,
in general, many
The complete analysis of
the rate equations requires the use of the computer.
The general approach
to such analyses for some simple cases which can be handled manually are
sumnlariz.ed in the present report.
Chemical Kinetics
The desul£urization of coal during pyrolysis in a hydrogen atmosphere
may be considered as a superposition of the parallel and opposing chemical
reactions shown in Table X.
In addition to identifying the occurrence of
these reactions dur ing pyrolysis, we have measured the kinetic parameters
given in Table X
The rate constant for one of these processes at an ab-
solute temperature T is expressed by the Arrhenius equation as
-E IR T
k=koe
( 52)
where E is the activation energy and ko is the frequency factor.
The dependence of the rate constants on temperature is shown in
Figure 38.
The rate constant for all of the desulfurizationreactions (reactions
1 through 5) fall within the indicated band. These results were obtained using
heating rates of approximately 4°C per minute and as a result the rate con-
stants are measured in the temperature range over which the rate constant
varies from approximately 0.01 to 1 atm-l min-l. In this range the results
should accurately reflect the kinetics of the chemical reaction.
-93 -
-------
TABLE X. SUMMARY OF KINETIC DATA
Reaction E( kcal/mole) ko
( 1) (Org-S) I + Hz ----'7 HzS 34.5 3. 1 x 10 10 ( 1)
(2) (Org-S) II + Hz -? HzS 41. 5 2.8xl011 ( 1)
(3) FeSz + Hz ----'7 HzS + FeS 47 2 . 8 x 10 lZ ( 1)
(4) FeS + Hz --7 HzS + Fe 55 2. 1 x 10 13 ( 1)
(5) (C -S) III + Hz -? HzS ads 52 2.3 X 108 ( 1)
( I) ( 3)
(6) HzS ads ----'7 HzSgas 10 50
( II) 43 2.4xl08 ( 3)
(7) HzS ads ~ HzSgas
(8) Fe + HzS ---? Hz + FeS 18 6. 5 x 106 ( 2)
(9) (C) + HzS --,>,HzS d 32 2.3 x 108 ( 2)
a s
(10) CaO + HzS ~ HzO + CaS 38 4. 7 x 10 13 ( 2)
(11) CaC03 ---? COz + CaO 58 3 . 8 x 10 lZ ( 3)
( 1) Units of (atm. Hz) -1 min -1
(2) Units of (atm. HzS) -1 min-l
(3) Units of min -1
-94 -
-------
7
10
106
105
104
j. 103
~ ........
c ...
{Ij I
~ c:
00 ....
c 2
0
u ...
I
V S 102
~ ....
{Ij C1!
~
Figure 38.
10
1
700
Temperature (" C)
Rate constants for coal desulfurization reactions
reactions as functions of temperature.
and for important back
-95 -
-------
-Applications of Kinetic Data
The kinetics of G.iffusion are relatively well understood, but in
combination with chemical kinetics the mathematics become somewhat
complex.
To avoid this added complexity, we first consider the applica-
tion of the kinetic data in the absence of diffusional effects with the under-
standing that for larger particle sizes and larger chemical kinetic rate
constants the diffusional effects must be taken into consideration.
Reactions 1, 2, and 3 in Table X are simple irreversible chemical
reactions.
The rate of desulfurization due to one of them, for example,
the pyrite, can be expressed by,
dN
FeS~
dt = k3PHzNFeSz
( 53)
where NFeSz is the instantaneous number of molecules of pyrite in the
reactor, PHz is the hydrogen partial pressure, and k3 is the rate con-
stant (units of atm -1 min -1). Integration of equation (53) gives,
N -N°
FeSz - FeS~ exp ( - k3PH t)
z
( 54)
where NFeS is the initial number of pyrite molecules in the reactor.
z
If we define the extent of desulfurization by the ratio
Y - N lNo
FeSz - FeSz/-FeSz
( 55)
then the time required for a given desulfurization of the pyrite is given
by
-k3PHzt = In YFeSz
( 56)
-96 -
-------
The teITlperature dependence of the rate constant k3 is given by the
Arrhenius forIn, Equation ( 52) with ko and E as given in Table X. Thus
Equation (56) , or the equivalent Equation (54) give the relationship between
reaction time, temperature, hydrogen pressure, and extent of desulfuriza-
tion for the pyritic sulfur.
Analogous equations can be written for the other
simple, irreversible reactions, namely reactions (1) and ( 2) .
For the reversible reactions the rate equations consist of a set of coupled
simultaneous differential equations which are difficult to solve for the general
case.
An analysis of the reversible reaction rate equations is given inan earlier
section.
This analys is shows that for a reaction such as
k
--?
FeS + Hz ~ Fe + HzS
kl
( 57)
the process variables in the steady state approximation are linked by the equation
-.kPH t -
z -
k lR TN 0 ~ '~ L Q j J
FeS 1- Y FeS) + 1 + ° ln Y FES
LQ . klR TN FeS
( 58)
where Y is the extent of desulfurization, N'FeS is the initial nUITIber of molecules
of iron sulfide, L is Avogadro's nUITIber, T the absolute temperature, R the gas
constant, t the reaction time, PHz the hydrogen partial pressure. Q the sweep
gas £low rate, and k and k1 the rate constant for the forward and reverse re-
ac tions .
Equation (58) can be rewritten in a somewhat simpler form by noting that
the total amount of HzS evolved by the complete reduction of the iron sulfide
is given by
(pv) HzS = R T N°FeS
L
( 59)
-97 -
-------
which when substituted into equation ( 58) gives
k 1 ( PV) 0 H Sr. (1 Q )
-kPHzt = Q z L0 - YFes) + ~ + kl( PV) fIzS .
1n YFeS J
( 60)
Note that when the flow rate of the sweep gas Q ( cm3/min) is large compared
to the overall rate of HzS production by reduction of FeS (cmj of HzS /min) ,
equation( (1)) reduces to the form of Equation (:61 for no back reaction. Thus
for Q»k1( PV) HzS the effect of the back reaction is small.
An alternate method to inhibit the back reactions consists of adding a
suitable sulfur adsorbent to the reactor, for example
kA
CaO + HzS ~ CaS + HzO
( 61)
The rate constant for this reaction has been determined by non-isother-
mal measurements.
Since the CaS is stable in Hz, the reverse reaction need
not be considered so long as the HzO concentration is small.
For the case
of absorbent added in an amount greatly in excess of the stoichiometry require-
ment the desulfurization equation becomes
-kPHzt=k:~A [0 - YFe0 ~ + kt{A) 1n Y FeS J
( 62)
where fA is the ratio of the total capacity of the absorbent for HzS to the total
amount of HzS evolved by complete reduction of the sulfide, kA is the ratio
constant (atm -1 min -1) for HzS reaction with the adsorbent, and the other
quantities are the same as in Equation (60' .
-98-
-------
Diffusion And Mass Trahsport
In the desulfurization of coal we are dealing with heterogenous re-
actions between gases and solids. Heterogenous reactions occur by a
series of diffusional and chemical steps which may be described as
follows:
1.
Diffusion of reactants from a fluid stream through a relatively stag-
nant film to the solid surface and to available capillary areas of pore
structure.
2.
Physical and/ or chemical adsorbtion of reactants on the solid.
3. Surface chemical reaction.
4.
Desorbtion of surface reaction products.
1.
Diffusion of products to the bulk gas stream.
Thus the heterogenous reaction mechanism comprises a diffusional
part and a chemical kinetic part.
A study of gas - solid reaction kinetics
must necessarily consider both diffusional and chemical kinetic effects.
If the reaction resistance attributed to diffusion comprises nearly the
total resistance, the reaction is considered "diffusion' controlled".
Similarly,
the reaction is "kinetically controlled" at negligible diffusion resis
tance. In the present work most of the kinetic data was obtained by working
in the kinetically controlled regime. Therefor e, the data obtained should
reflect the true chemical kinetics of the processes studied.
In the appli-
cation of these data to process design, the possibility of transition into the
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diffusion controlled regime must be considered.
These trans itions n\ay be
expected to occur with an increase in particle size or in chemical kinetic rate
constants.
Trans ition from one type of rate control to another is usually indi-
cated when a significant change in apparent activation energy with temperature
occurs.
Transitions of this type have been observed in our measurements on
the reactions of HzS with various solids.
Future Work
Our studies on desulfurization of coal during gasification have emphasized
the theoretical and experimental application and extension of the new method of
non-isothermal kinetic measurements developed originally by Juntgen and co-
workers.
A substantial amount of basic kinetic data relative to the desulfuriza.
tion of coal during pyrolys is and gasification has been obtained.
These data
have enabled us to construct a rational mechanistic picture of the process in
terms of a set of competing and opposing chemical reactions. the individual
rates of which are known under a variety of process conditions.
It is anticipated that the major emphasis in future work will be placed on
the applications of the kinetic data to the development and/or improvement of
processes for coal desulfurization.
Additional kinetic measurements will be
needed to apply the basic kinetic data to particular proposed proces~es.
In
particular, non-isothermal kinetic measurements will be made on specific feed
coals for specific processes; on a series of model organic sulfur compound
and limiting cases, such as lignite and anthracite; on the reactions of coal
sulfur in an oxidizing atmosphere; and on the absorption of sulfur dioxide by
calcined dolomites and limestones.
Some non-isothermal measurements
will study the transition from reaction controlled kinetics to diffusion con-
trolled kinetics, and for those reactions in which diffusion control is of
practical significance. to measure the diffusion controlled kinetics.
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REFERENCES
1 .
( a)
M. L. Vestal et al , I1Kinetic Studies on the Pyrolysis, De-
sulfurization, and Gasification of Coals with Emphasis on the
Non -Isothermal Kinetic Method ,II Scientific Research Instruments
Corporation Rept. SRIC 68-13 ( 1969) .
(b)
M. L. Vestal and W. H. Johnston, I'Desulfurization Kinetics of
Ten Bituminous Coals,ll Scientific Research Instruments Corpor-
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2..
Juntgen, Erdol and Kohle G 180 (1964).
3.
Van Heek, Juntgen, and Peters, Brennstoff -Chem. 48, No.6, 35
( 1967). Translation by Scientific Research Instruments, Inc.,
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.t.
Peters and Juntgen,
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16. Squires, A. M. - "Fuel Gasification, " Advances in Chemistry Series
69, Robert F. Gould, editor (American Chemical Society, Washington"
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17. Squires, A.M., Trans. Inst. Chern. Engrs. (London) 39, 3 (1961).
18. Squires, A.M., ASME paper 67-WA /PWR-3 (1967).
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(1928), 1, 776, 876 (1930).
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(1965); W.M. Goldberger, ASME paper 67-WA/FU-3 (1967).
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paper 66-WA/FU-2 (1966); J.W. Bishop, S. Ehrlich, A.K.Jain,
E.B. Robison, and P.M. Chen, ASME paper 68-WA/FU-4 (1968).
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G.C. Lammers, U.S. Bur. Mines Rept. Invest. 4954 (1953).
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