WATER POLLUTION CONTROL RESEARCH SERIES • 14010 FYY 09/71
Study of Sulfur Recovery
from
Coal Refuse
ENVIRONMENTAL PROTECTION AGENCY • WATER QUALITY OFFICE
-------
WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Series describes
the results and progress in the control and abatement
of pollution in our Nation's waters. They provide a
central source of information on the research, develop-
ment, and demonstration activities in the Water Quality
Office, Environmental Protection Agency, through inhouse
research and grants and contracts with Federal, State,
and local agencies., research institutions, and industrial
organizations.
Inquiries pertaining to Water Pollution Control Research
Reoorts should be directed to the Head, Project Reports
System, Office of Research and Development, Water Quality
Office, Environmental Protection Agency, Room 1108,
Washington, D. C. 20242.
-------
Study of Sulfur Recovery From Coal Refuse
by
Black,Sivails & Bryson, Inc.
Applied Technology
135 Delta Drive
Pittsburgh, Pennsylvania 15238
for the
Environmental Protection Agency
Project #14010 FYY
Contract # 14-12-929
September 1971
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 70 cents
Stock Number 6501-0137
-------
EPA Review Notice
This report has been reviewed by the Water
Quality Office, EPA, and approved for publication.
Approval does not signify that the contents
necessarily reflect the views and policies of
the Environmental Protection Agency, nor does
mention of trade names or commercial products
constitute endorsement or recommendation for
use.
-------
ABSTRACT
During coal preparation, a coal refuse of no commercial value is
produced and discarded in piles. As rainfall percolates through the
piles, acid waters are formed.
A feasibility study has been performed on a process producing sulfur from
such coal refuse. In this process, limestone and coal refuse are
ground, pelletized and preheated before entering a desulfurizing shaft
reactor where a hard, fired ash pellet and an H2S-S02 bearing offgas
are produced. After sulfur, tar, and other gases are removed, the
resulting H.2S-S02 gas proceeds to a conventional sulfur recovery plant.
Experimental results and economics of this study indicate that the
process is a profitable means of minimizing coal refuse pile water
pollution. For a sulfur and ash pellet selling price of $20 and
$1.50/ton respectively, it is estimated that a coal producer will have
a before tax return on investment up to 53 percent for a one MM
ton/year plant utilizing an eight percent sulfur refuse.
This report was submitted in fulfillment of Project Number 14010 FYY,
Contract 14-12-929, under the sponsorship of the Water Quality Office,
Environmental Protection Agency.
iii
-------
CONTENTS
Section
I Conclusions
II Recommendations
III Introduction
IV Raw Materials Used
V Experimental Technique and Apparatus
VI Sulfide Formation Experimentation
VII Desulfurization Experimentation
VIII Process Description
IX Economic Evaluation
X Acknowledgements
XI References
XII Appendices
Page
1
3
5
7
9
13
23
37
43
55
57
59
-------
FIGURES
PAGE
1 EXPERIMENTAL EQUIPMENT FOR HIGH TEMPERATURE
DESULFURIZATION U
2 EFFECT OF RESIDENCE TIME ON CONVERSION OF COAL REFUSE
SULFUR 15
3 EFFECT OF LIMESTONE ON CONVERSION OF COAL REFUSE SULFUR 16
4 EFFECT OF LIMESTONE SURFACE AREA ON CONVERSION OF
COAL REFUSE SULFUR 18
5 EFFECT OF TEMPERATURE ON CONVERSION OF COAL REFUSE SULFUR 19
6 EFFECT OF ORGANIC SULFUR CONTENT OF COAL REFUSE ON
LIMESTONE REQUIREMENTS 21
7 EFFECT OF TIME ON FRACTION OF UNREACTED SULFIDES 25
8 EFFECT OF TIME ON REACTION PRODUCTS MIX 26
9 EFFECT OF STEAM RATE ON PELLET SULFIDE CONTENT 28
10 EFFECT OF STEAM RATE ON DESULFURIZATION OFFGAS
COMPOSITION 29
11 EFFECT OF TEMPERATURE ON FRACTION OF UNREACTED SULFIDES 31
12 EFFECT OF TEMPERATURE ON REACTION PRODUCTS MIX 33
13 RESIDENCE TIME REQUIREMENTS TO ACHIEVE COMPLETE
DESULFURIZATION 34
14 PROCESS FLOW CHART 38
15 SHAFT REACTOR ZONES 40
16 CAPITAL INVESTMENT VS. PLANT CAPACITY FOR VARIOUS
COAL REFUSE SULFUR CONTENTS 45
17 ROI VS. SULFUR SELLING PRICE AT VARIOUS ASH PELLET
SELLING PRICES 43
18 ROI VS. COST OF COAL REFUSE AT VARIOUS CAPITAL
INTEREST RATES 49
vi
-------
FIGURES (CONT'D)
PAGE
19 ROI VS. PLANT CAPACITY AT VARIOUS COAL REFUSE
SULFUR CONTENTS 50
20 BREAK-EVEN PLANT CAPACITY VS. COAL REFUSE SULFUR
CONTENT FOR VARIOUS ECONOMIC SITUATIONS 52
vii
-------
TABLES
No. Page
1 Coal Refuse Screen Analysis 7
2 Screen Analysis of Limestone Samples 7
3 External Specific Surface Area of Limestone Samples 8
4 Raw Material Inputs and Products 43
5 Estimated Fixed Capital for a One MM Ton/Year
Coal Refuse Plant 44
6 Operating Revenue for a One MM Ton/Year Coal
Refuse Plant 46
vxn
-------
SECTION I
CONCLUSIONS
The experimental results and process economics generated in this
feasibility study show that the sulfur recovery process is a profitable
means of utilizing coal refuse to recover sulfur. Since the process is
economically attractive, it should entice communities and coal
producers to utilize rather than stockpile coal refuse; thereby, min-
imizing water pollution caused by coal refuse,, In particular, the
following conclusions may be drawn from the results of this study.
1. The experimental laboratory study has shown that commercially
feasible operating temperatures (1900-2100°F) and reasonable residence
times (30 minutes or less) are required to operate the process.
Specifically, the results of experimentation simulating the sulfide
formation zone in the shaft reactor show that a residence time of
30 minutes will suffice at 1900°F to yield a maximum conversion of
coal refuse sulfur to the sulfide form, and to completely retain the
organic sulfur. Experimentation to simulate the desulfurization zone
in the shaft reactor has shown that 1/4 to 3/4 inch diameter sulfide
bearing pellets can be desulfurized in less than thirty minutes at
2100°F and a 5 mole per minute per mole pellet sulfides steam rate.
2. Based on the experimental findings of this study, a suggested
process flow sheet has been generated and no unusual equipment or unit
operations are required to operate the process.
3. For the suggested process flow sheet, the return on investment
for the process under a number of financial situations has been
generated. Specifically, for a coal producer using the process selling
sulfur at $20/ton it is estimated that a return on investment of
53 percent would be realized for a one MM ton/year-eight percent sulfur
coal refuse plant costing $2.3 million. The size of a sulfur
recovery plant operating under the above conditions could be reduced
to as low as 100,000 tons/year and still yield a no cost operation.
Thus the process can be a profitable means of eliminating coal refuse
pollution for large quantities of high sulfur coal refuse or it can be
a no cost pollution abatement process for small or moderate quantities
of coal refuse.
-1-
-------
SECTION II
RECOMMENDATIONS
As originally planned, the development of this process should proceed
through three phases: a laboratory feasibility study, a pre-pilot
plant study, and a demonstration pilot plant. The laboratory feasibility
study has been completed and results indicate that the process is a
profitable means of reducing coal refuse stockpile pollution. Because
of the excellent experimental results and the favorable process economics
obtained in the feasibility study, it is highly recommended that the
pre-pilot plant study be immediately funded to insure continuity in the
project and a rapid solution to the water pollution problem caused by
coal refuse stockpiling.
The pre-pilot plant study is recommended to investigate the operation of
the shaft reactor so as to provide scale-up data for a pilot plant design
and determine those conditions in the shaft reactor which maximize the
formation of sulfur. The important process parameters which affect the
shaft reactor operation are the temperature profile (temperature in each
of the shaft reactor zones), and the steam and air flow rate relative to
the pellet retention time. The effects of these process parameters on
the shaft reactor offgas composition and the quality of the fired ash
pellet product are of primary concern. Maximizing the elemental sulfur
content of the shaft reactor offgas will minimize the cost of the sulfur
recovery plant thereby reducing operating costs. The quality of the
fired ash pellets will directly affect their market value.
-3-
-------
SECTION III
INTRODUCTION
This study involves the laboratory investigation of a process that
eliminates water pollution problems arising from the stockpiling of
coal refuse. During the preparation of coal, a coal refuse is produced
which has a high ash and pyritic content and a low heating value.
Presently, coal refuse has no commercial value and is discarded in
piles near the coal preparation plant. As rainfall percolates through
the piles, conditions become suitable for the formation of acid from
the pyrites contained in the coal refuse. Additional products resulting
from the interaction of water, air, and pyrites are ferrous and ferric
sulfate. These reaction products are dissolved by surface and ground
waters and impart ecological and esthetic damage to streams, rivers, and
lakes„ Consequently, great need exists for a process that will eliminate
or minimize such pollution. The process discussed in this report
represents a profitable means of recovering elemental sulfur from coal
refuse as it is produced or from existing coal refuse piles. Since the
process is economically attractive, it should entice communities and coal
producers to utilize rather than stockpile coal refuse; thereby min-
imizing the water pollution caused by coal refuse.
In this process, limestone and coal refuse are ground, pelletized, and
preheated prior to entering a high temperature desulfurizing shaft
reactor. In this reactor, the pellets proceed in series through a
preheat zone, a sulfide formation zone where coal refuse sulfur is
converted to sulfides, a desulfurization zone where the pellets are
contacted with steam, a gasification zone where coal refuse carbon is
burned to supply the shaft heat requirements, and finally a pellet
cooling, air preheating zone. The products of the reactor are hard,
fired, ash pellets and a hydrogen sulfide-sulfur-sulfur dioxide
(H2S-S-S02) bearing offgas. After sulfur, tar, and other gaseous
materials are removed from the offgas, the resulting H2S-S02 gaseous
mixture is sent to a conventional Glaus sulfur recovery plant.
This study was undertaken to determine the feasibility of the process
and the technical aspects of the sulfide formation and desulfurization
reactions. In addition to engineering studies, experimentation was
completed on the formation of sulfides from the coal refuse sulfur and
the desulfurization of the sulfide bearing coal refuse pellets. In the
first phase of experimentation, the operating conditions necessary to
transform the sulfur contained in coal refuse to a sulfide form (either
calcium sulfide or ferrous sulfide) were determined. When the trans-
formation to sulfides has been accomplished, desulfurization of the
sulfide bearing pellets is possible by a high temperature method in which
the pellets are reacted with steam to form calcium and ferrous oxides and
hydrogen sulfide. The hydrogen sulfide can then be converted to elemental
sulfur using the standard Glaus sulfur recovery process.
-5-
-------
Sulfide formation occurs by two reactions. The first is the thermal
decomposition of pyrites to ferrous sulfide and sulfur. The reaction is
well known and occurs readily at about 1200-1300 F. The second reaction
(which produces calcium sulfide) is not fully understood but apparently
sulfides are formed by reactions of CaO with sulfur liberated from the
pyrite decomposition and/or the volatile organic sulfur. In the sulfide
formation study the effects of reaction temperature, limestone content,
limestone particle size, limestone type, and residence time on the
formation of sulfides were investigated.
The second phase of experimentation concerned the desulfurization of the
sulfide bearing coal refuse pellets. As stated previously, steam is
reacted with the sulfide bearing pellets to form ferrous and calcium
oxides and hydrogen sulfide,, However, it was found that the desulfur-
ization offgas also contained elemental sulfur and sulfur dioxide in
addition to hydrogen sulfide. It was speculated that sulfur dioxide was
being formed by the reaction of H2S with steam. The presence of elemental
sulfur probably results from the high temperature l^S decomposition to
molecular hydrogen and sulfur and/or from the Glaus reaction between
H2S and S02« The following variables were studied to determine their
effects on the desulfurization offgas composition and the rate of desul-
furization of the pellet: reaction time, steam rate, reaction temp-
erature, and pellet size. The objective was to determine those operating
conditions which maximized the production of sulfur while minimizing
the formation of sulfur dioxide. By maximizing the production of sulfur,
the size and consequently the cost of the sulfur recovery plant can be
minimized.
Based on the experimental results of the study, a process flow chart
has been devised and is discussed in a later section. Using this
flow chart, the economics of the process were investigated and are also
presented.
-6-
-------
SECTION IV
RAW MATERIALS USED
Coal refuse was obtained from the Truax-Traer Coal Company of Pickney-
ville, Illinois. The material was crushed in a jet impact pulverizer
to yield a finer sized, homogeneous sample for experimentation. Table 1
presents the screen analysis of the coal refuse.
TABLE 1
COAL REFUSE SCREEN ANALYSIS
U. S. Screen Percent Weight
+ 40 1.6
-40 + 60 3.3
-60 + 100 6.6
-100 + 200 13.4
-200 + 325 9.5
-325 65.6
The coal refuse was analyzed for its total sulfur content. Several
samples were analyzed in an attempt to obtain a representative value for
the sulfur content. The following percentages of sulfur in the coal
refuse were obtained for the four samples analyzed: 15.1, 15.2, 17.5,
and 18.2. An average value of 16 percent sulfur was used for the
experimental calculations. The coal refuse was also analyzed for the
pyritic and organic sulfur. The method used is described by the
American Society of Testing Materials in A.S.T.M. designation D2492-66T.
The amounts of pyritic and organic sulfur found were 12.1 and 3.9
percent respectively.
Several different types and particle size distribution limestone samples
were prepared for experimentation. The bulk of the experimental work
was conducted using limestone obtained from the U. S. Gypsum Company
in Bellefonte, Pennsylvania. This limestone contained 97.7 percent
CaC03o Other limestones used were from the Greer Limestone Company of
Germany Valley, West Virginia, and the Elkins Limestone Company in Elkins,
West Virginia. Their CaC03 contents were 98.8 percent and 58.75 percent
respectively as stated by the manufacturers. Screen analyses and
external surface area measurements were conducted on the limestone
samples and appear in Tables 2 and 3.
TABLE 2
SCREEN ANALYSIS OF LIMESTONE SAMPLES
U. S. Gypsum Limestone
Percent Weight
Sample -40 Mesh -60 Mesh -100 Mesh
Screen As Received Sample Sample Sample
+ 60 15.9 19.9 nil nil
-60 + 100 78.6 28.9 35.9 nil
-100 + 200 1.2 21.2 26.5 41.4
-200 4.3 30.0 37.6 58.6
-7-
-------
TABLE 2 CONTINUED
Elkins Limestone
Percent Weight
Screen Sample As Received -100 Mesh Samples
+ 20 33.6 nil
-20 + 100 41.6 nil
-100 +200 3.2 59.2
-200 + 325 0.9 16.7
-325 20.7 24.1
Greer Limestone (Germany Valley)
Percent Weight
Screen Sample As Received -100 Mesh Samples
+ 20 15.5 nil
-20 + 100 71.3 nil
-100 + 200 9.8 69.1
-200 + 325 2.6 25.4
-325 0.8 5.5
TABLE 3
EXTERNAL SPECIFIC SURFACE AREA OF LIMESTONE SAMPLES
f\
Surface Area, cm / gram
-40 Mesh -60 Mesh -100 Mesh
U. S. Gypsum Limestone 910 1603 2161
Elkins Limestone - - 1615
Germany Valley Limestone - - 1575
The external surface area was determined using an air permeability
technique in which the Carman-Kozeny equation was used to relate the
external surface area to the pressure drop of air passing through a
packed bed of limestone.
-8-
-------
SECTION V
EXPERIMENTAL TECHNIQUE AND APPARATUS
Coal refuse pellets were prepared in a laboratory disc pelletizer. Coal
refuse, limestone, and water (spray) were continuously fed onto the
rotating disc to produce pellets of a desired size. The most important
variables in the pelletizing operation were the amount of water added
and the disc rotation speed. The speed of the disc must be regulated
so that the pellets can roll across as large a portion of the disc face
as possible. A maximum amount of rolling is important to make compact,
homogeneous pellets. If an excess of water is used, the pellets will
cling together resulting in agglomeration and seeding (small pellets
clinging to the sides of larger ones). No attempt was made to correlate
pelletizing variables since the technology of commercial pelletizers
has been developed and no problems are anticipated in the manufacture
of coal refuse pellets.
Experimentation was divided into two distinct phases. The first phase
entailed reacting the coal refuse-limestone pellets in a high temperature
furnace for various times to convert the coal refuse sulfur into sulfide
forms (ferrous sulfide and calcium sulfide). The second phase concerned
desulfurization experiments aimed at converting the sulfides to hydrogen
sulfide and elemental sulfur. The experimental apparatus and methods
will now be described.
A high temperature electric tube furnace was used for the sulfide for-
mation experimentation. The furnace was heated by four symmetrically
arranged glow bars which surrounded and were parallel to the ceramic
reaction tube. The temperature was controlled by two voltage taps located
on the side of the furnace. One tap was for a coarse control of temp-
erature and the other was a fine voltage tap capable of changing the
furnace temperature by 30-50°F. The ceramic reaction tube was 1-3/4 inches
I.D. by 48 inches long.
In the sulfide formation investigation, pre-weighed (± 0.001 gram)
pellets were placed in ceramic reaction boats and pushed into the hot
zone of the furnace with a long, thin rod. In most instances, two or
three pellets were reacted simultaneously. A flow of nitrogen (about
20 standard cubic feet per hour) was maintained before, during, and
after the reaction of the pellets. It was discovered that the sulfur
contained in the pellets was being oxidized to sulfur dioxide if both
ends of the ceramic reaction tube were open to the atmosphere. After
the nitrogen purge was installed, no sulfur loss as sulfur dioxide was
3xperienced0 After a given period of time, the pellets were pulled from
the furnace and placed in an air-tight iron heat sink. The purpose of
the heat sink was to cool the pellets, stop the various reactions at the
time the pellets were withdrawn from the furnace, and prevent hot pellet
oxidation due to contact with air. The pellets were analyzed for their
ferrous and calcium sulfide contents* A method for determining the sulfide
contents is described in the Appendix.
-------
The second phase of experimentation concerned the desulfurization of
sulfide bearing pellets. The desulfurization experiments were conducted
at temperatures of 1600-2600°F. A schematic diagram of the desulfurization
equipment is shown in Figure 1. The electric tube furnace used for the
sulfide formation study was also used for the high temperature desul-
furization experiments. Steam was generated in a flask outside the
furnace and introduced at one end of a 1-1/2 inch I.D., ceramic lined,
Inconel reaction tube. A nitrogen flow was maintained in the furnace
countercurrent to the eventual steam flow until steam was introduced
into the furnace. A sulfide bearing pellet was placed on a ceramic boat
and inserted into the furnace; simultaneously, steam was introduced
into the reaction tube and the nitrogen flow was terminated. The excess
steam and gaseous products of reaction exiting the reaction tube passed
through a water cooled condenser where steam was condensed and the gases
cooled. The gases then passed through an absorbing solution of GuSO^
which trapped both SQ? and l^S. The gas exiting the absorption solution
was bubbled through a solution that became colorless (it was initially
blue) if either SC>2 or I^S was present. There were no instances in which
either gas escaped the CuSO^ solution. After a specified reaction time,
the steam flow was terminated and the CuSO^ solution analyzed to determine
the cumulative amounts of H2S and SC>2 generated in the experiments. An
explanation of these analyses is given in the Appendix. The coal refuse
pellet was removed from the furnace, cooled in a heat sink, and analyzed
for unreacted sulfides to determine the extent of reaction.
-10-
-------
Electric Tube Furnace
0
Steam
Generator
••*
Pellets
Temperature
Controller
Inconel Reaction Tube
with Ceramic Liner
a
Condenser
=—»• Water Outlet
Water Inlet
CuSO absorbing
solution
FIGURE 1-EXPERIMENTAL EQUIPMENT FOR HIGH TEMPERATURE DESULFURIZATION
-11-
-------
SECTION VI
SULFIDE FORMATION EXPERIMENTATION
Sulfide Formation
Several chemical reactions occur in the limestone-coal refuse pellet
during sulfide formation at high temperatures. The most important
reactions are:
FeS2 (pyrites) »- FeS + S (1)
CaC03 •- CaO + C02 (2)
CaO + S (elemental or combined) »~ CaS + Sulfur Oxides (3)
Coal Refuse Volatile Matter »- Tars + Organic Sulfur (4)
Compounds
The first reaction describes the thermal decomposition of iron pyrites
and is well known. At a temperature of 1290 F, the pyrite becomes
unstable under normal atmospheric pressure and decomposes readily . The
second reaction is the standard calcination reaction of CaCOo and is also
well known. The third reaction is not understood completely but was
suspected to occur when experimentation proved that a greater amount of
sulfides were produced in the pellet than could be accounted for by the
decomposition of pyrites to ferrous sulfide. During experimentation, it
was observed that a portion of the sulfur originally contained in the
coal refuse pellet was escaping in a volatile form. This volatile sulfur
was probably a mixture of organic sulfur and elemental sulfur, the latter
resulting from the thermal decomposition of pyrites. The volatile sulfur
leaving the pellet during the sulfide formation reactions was dependent
upon the reaction temperature and the quantity and particle size of the
limestone in the pellet. It was found that by increasing the limestone
and/or decreasing the particle size, the sulfides formed were increased.
As this occurred, the volatile sulfur leaving the pellet decreased. These
facts support the supposition that the lime in the pellet reacted with
the volatile sulfur to form calcium sulfide as shown in the third
reaction above.
The fourth reaction was the devolatization of the coal refuse to yield
tars and organic sulfur compounds which were not converted to the sulfide
form. According to equations (1) to (4) the coal refuse sulfur originally
present was either converted to FeS, CaS, or escaped as volatiles. The
results of chemical analyses performed to determine the sulfur forms
remaining in the sulfide bearing pellet indicated that essentially all
of the sulfur was in the form of sulfides with only traces of organic and
pyritic sulfur present. Therefore, essentially all of the pyritic sulfur
decomposed to ferrous sulfide by evolving sulfur and the volatile or
organic sulfur compounds reacted with lime to produce calcium sulfide.
Effect of Residence Time
The effect of residence time on the extent of sulfide formation in coal
refuse-limestone pellets was studied at 1900°F using minus 60 mesh U.S.
Gypsum Limestone (see Table 2 for particle size and Table 3 for surface
area). One half inch (O.D.) pellets consisting of 35 percent limestone
-13-
-------
and 65 percent coal refuse were reacted in the tube furnace for times of
5 to 40 minutes. The results are shown in Figure 2 where it is seen
that conversion of coal refuse sulfur to sulfides (defined as the moles
of sulfur as sulfides in the reacted pellet divided by the moles of sul-
fur initially present) does not significantly increase after 15 minutes
where a maximum conversion of about 0.6 was reached. Based on this
finding, all subsequent experimentation was conducted using residence
times of 30 minutes.
As previously indicated, pyrites, FeS_, will decompose to form ferrous
sulfide and sulfur. At the reaction temperature used, the sulfur left
the pellet in gaseous form. Consequently, due to the manner in which
sulfide formation or conversion is defined, a conversion with a value
of one cannot be attained since sulfur is leaving as a gas and does not
appear as a sulfide. Assuming that the sulfur leaving the pellet
results only from the decomposition of pyrites, then the sulfur to
sulfide conversion as defined above has a maximum theoretical value of
0.62 for the 16 percent sulfur coal refuse (12.1 percent pyritic, 3.9 per-
cent organic) used in these experiments. The value 0.62 assumes that
all the organic sulfur initially present in the pellet reacts with lime
to form calcium sulfide. The maximum theoretical conversion (Figure 2)
agrees quite well with the maximum conversion obtained by experimentation.
This finding supports the theory that calcium sulfide is formed by
the reaction of coal refuse organic sulfur with lime. More importantly,
it indicates that essentially all of the coal refuse sulfur is either
trapped as sulfides for subsequent sulfur recovery or appears as re-
coverable, gaseous, elemental sulfur.
Effect of Limestone Content
The effect of limestone content on the conversion of coal refuse sulfur
to sulfides was determined by reacting pellets of various limestone
contents (all minus 60 mesh particle size, U.S. Gypsum) at 1900°F for
30 minutes, and analyzing the reacted pellets for their sulfide content.
The percentage of limestone was varied between 25 and 65 percent; the
results are presented in Figure 3 where the sulfide conversion is
0.57 for 25 percent limestone and 0.70 for 65 percent limestone in the
pellet. The figure also shows the effect of limestone content on con-
version for pellets containing limestones of other particle sizes (to
be discussed in the next section). It is evident that increasing the
limestone content increases the conversion of coal refuse sulfur to a
sulfide form.
In Figure 3, sulfur to sulfide conversions higher than the theoretical
maximum of 0«62 (based on the assumption that all the sulfur formed
from the decomposition of pyrites leaves in elemental form) were realized.
This is attributed to the excess limestone (lime) which probably
reacted with some of the gaseous sulfur produced from the pyrites.
-14-
-------
0.70
g 0.60
o
S 0.50
ci
0.40
0)
o 0.30
0.20
0.10
.Maximum JThepre tica l_Convers.ion
— e ©
Temperature
Limestone
1900°F
-60 mesh, 357»
0
0 10 20 30 40 50
Time, Minutes
60
FIGURE 2-EFFECT OF RESIDENCE TIME ON CONVERSION
OF COAL REFUSE SULFUR
-15-
-------
0.70
0.68
o 0.66
4-1
O
(0
J-l
0.64
e
o
g 0.62
0)
fl
« 0.60
0.58
0.56
0.54
-100 mesh
60 mesh
-40 mesh
Temperature = 1900°F
Time = 30 minutes
10 20 30 40 50 60
Percent Limestone in Pellet
70
80
FIGURE 3-EFFECT OF LIMESTONE ON CONVERSION
OF COAL REFUSE SULFUR
-16-
-------
Effect of Limestone Particle Size
To determine the effect of limestone particle size on the formation of
sulfides in coal refuse pellets, pellets were reacted as above using
various U.S. Gypsum limestone contents and particle size distributions.
Three limestone particle sizes were used, -40 mesh, -60 mesh, and
-100 mesh, with external surface areas of 910, 1603, and 2161 cm^/gram
respectively. As was the case in the limestone content study, a reaction
temperature of 1900°F, pellet size of one half inch, and a residence
time of 30 minutes were used. The results of this study are shown in
Figure 3 where it is seen that the limestone particle size has a
significant effect upon sulfide formation. For example at a limestone
content of 35 percent, the conversion to sulfides is 0.58, 0.59, and
0.65 for -40, -60, and -100 mesh limestone respectively.
For the sulfide formation reactions, the conversion will be related
to the external surface area of the lime formed by the decomposition
of th® U.S. Gypsum limestone. The external surface area of a mass of
particles is related to the particle size distribution of these
particles. Therefore, while the designation of a -40, or -100 mesh
sample will provide a rough, relative comparison of external surface
area it is better to consider the external surface area of the lime-
stone to provide data which is more general and adaptable to any
particle size. Assuming that the limestone surface area (Table 3) is
directly related to the resulting lime surface area, the effect of
limestone surface area on sulfide formation at various limestone
contents is shown in Figure 40 It is evident from Figure 4 that
increasing the external surface area of the U. S. Gypsum limestone at
a given limestone content increases the conversion of sulfur to
sulfide forms.
Effect of Temperature
To determine the effect of reaction temperature on the formation of
sulfides, one half inch pellets were made containing 35 percent lime-
stone (minus 60 mesh) and were reacted for 30 minutes at temperatures
ranging from 1760°F to 2500°F. These values were selected because
they yielded a sulfur to sulfide conversion approaching the maximum
theoretical conversion at 1900°F. The experimental results are presented
in Figure 5 where it is seen that the sulfur to sulfide conversion
increases as the temperature increases to 1900°F<, At this point,
the temperature dependence becomes less important and the conversion to
sulfides remained essentially constant at about the maximum theoretical
conversion as the temperature increased to 2500°F. This infers that
temperature has no effect on trapping the gaseous sulfur produced from
the decomposition of pyrites as CaS but does affect the quantity of
organic sulfur converted to sulfides.
Effect of Different Types of Limestone
As part of the experimental study, it was decided to determine whether
limestone mined at different geographical locations and having different
chemical analyses had an effect on sulfide formation. Limestones
-17-
-------
a
o
o
to
a
o
•r-l
CO
J-J
0)
O
u
0.69
0.67
0.65
0.63
0.61
0.59
0.57
0.55
Temperature = 1900 F
Residence Time = 30 minutes
Limestone
Limestone
257<, Limestone
800 1000 1200 1400 1600 1800 2000 2200
External Surface Area, cm^/gram
FIGURE 4-EFFECT OF LIMESTONE SURFACE AREA ON CONVERSION
OF COAL REFUSE SULFUR
-18-
-------
d
o
0.62
o 0.61
CD
1-1
Pu
e" 0.60
CO
^ 0.59
c
o
0.58
0.57
0.56
Residence Time
Limestone
30 minutes
-60 mesh, 45 weight-
percent
1600 1700 1800 1900 2000 2100 2200 2300 2400 2500
o
Temperature, F
FIGURE 5-EFFECT OF TEMPERATURE ON CONVERSION
OF COAL REFUSE SULFUR
-19-
-------
obtained from the Elkins Limestone Company and from the Greer Limestone
Company were investigated in addition to the U.S. Gypsum Company limestone,
The Greer Limestone had a high purity (98.8 percent CaC03) whereas the
Elkins limestone had a CaCO_ content of only 58.75 percent. The Elkins
limestone is further characterized by having a high percentage of SiC^
(27.5 percent) compared to 1.2 percent for the Greer limestone. Screen
analyses of both limestones on an as received basis and on minus 100
mesh samples appear in Table 2.
Pellets were made from each type of limestone using a minus 100 mesh
sample and were formulated so that the ratio of coal refuse to CaCO^
(from the limestone) in the pellet was 65/35. Therefore, the relative
amounts of lime to coal refuse were the same in each pellet. The
conversion to sulfides was 0.57, 0.62, and 0.64 for Greer, Elkins, and
U.S. Gypsum limestones, respectively. Based on these results, it is
believed that any type of limestone can be used for sulfide formation
provided the proportion used with the coal refuse is adjusted for the
content.
Comments
The sulfide formation experimentation has shown that a conservative
residence time of 30 minutes will suffice under most operating conditions
to yield a maximum conversion of coal refuse sulfur to the sulfide form,
and to completely retain the organic sulfur. There is an upper limit
to the quantity of limestone (depending on particle size distribution)
which should be used so that the gaseous sulfur produced from the decom-
position of pyrites is not converted to the sulfide form. Economically
it is more attractive to recover this sulfur by condensation rather than
desulfurize an increased quantity of sulfides. Experimental results
indicate that while the particle size distribution (external surface area)
of the limestone is important, the type of limestone has only a minor
effect on the extent of sulfide formation.
The results of the sulfide formation study can be used to obtain an
estimate of the limestone requirements for coal refuse containing
various amounts of sulfur and having various combinations of pyritic
and organic sulfur. The effect of temperature on organic sulfur to
sulfide conversion is insignificant above 1900°F for residence times
of 30 minutes. The quantity of ferrous sulfide produced by pyritic
decomposition is not related to the limestone content; consequently, the
limestone requirements for complete conversion of organic sulfur to
the sulfide form depend on the limestone external surface area and
organic sulfur content of the coal refuse. Figure 6 is a plot of lime-
stone area vs. the percent limestone required for sulfide formation at
various organic sulfur contents. It was beyond the scope of this
feasibility study to investigate coal refuses of different sulfur
contents; consequently, the lines shown in Figure 6 for organic sulfur
-20-
-------
e
CO
M
60
E
y
o
o
o
0)
y
CO
M-l
1-1
^-l CO
CO •
a r-t
1-1
a)
vO
*
CM
CM
CO
•
CM
CM
•
CM
r-l
•
CM
o
•
CM
Pellet Size
Residence Time
Temperature
1/2"
30 minutes
1900°F
(or higher)
1%
47« Organic Sulfur
0 5 10 15 20 25 30 35 40 45 50 55 60
7o Limestone
FIGURE 6-EFFECT OF ORGANIC SULFUR CONTENT OF COAL REFUSE
ON LIMESTONE REQUIREMENTS
-21-
-------
contents of one to three percent were prepared assuming a linear
relationship between the limestone requirement and organic sulfur content
as determined in this study. Figure 6 should be considered only as an
estimate of the limestone requirement for sulfide formation when high
CaCC>3 content limestones are used and can be used for a coal refuse
of known organic sulfur content.
-22-
-------
SECTION VII
DESULFURIZATION EXPERIMENTATION
Desulfurization
The next phase of experimentation concerned the transformation of the
calcium and ferrous sulfides contained in the pellet to hydrogen sulfide.
Hydrogen sulfide can then be converted to elemental sulfur using the
Glaus reaction. Hydrogen sulfide can be produced by reacting the sulfide
bearing pellets with steam at temperatures ranging from 1600°F to 2100°F
according to the following reactions:
FeS + H20 »• FeO + H2S (5)
CaS + H20 »- CaO + H2S (6)
The sulfurous offgas from the desulfurization experiments was found to
contain sulfur and sulfur dioxide in addition to hydrogen sulfide. It
was suspected that sulfur dioxide was being formed by the reaction of
steam and hydrogen sulfide (formed via equations 5 and 6) according to
the following equation:
H2S + 2H20 *• S02 + 3H2 (7)
Elemental sulfur was probably formed by the decomposition of hydrogen
sulfide to molecular hydrogen and sulfur at the high temperatures in-
volved:
H2S »• H2 + S (8)
The occurrence of this reaction (8) was supported by the fact that
hydrogen was detected in the desulfurization offgas. The presence of
sulfur in the offgas may also occur by reaction of hydrogen sulfide
and sulfur dioxide according to:
2H2S + S02 * 2H20 + 3S (9)
Equation 9 is the Glaus reaction and is catalyzed by alumina which is
a component of coal refuse ash.
Preparation of Sulfide Bearing Pellets
The sulfide bearing pellets used for the desulfurization experiments
were prepared using 35 percent, -100 mesh U. S. Gypsum Company limestone
and 65 percent coal refuse. One half inch diameter coal refuse-limes tone
pellets were reacted for 30 minutes at 1900°F to yield a 0.62 conversion
of the coal refuse sulfur to sulfides. These sulfide bearing pellets
were used for all subsequent experimentation except the pellet size
study. The parameter studies and their effects on the reaction products
and residence time requirements for desulfurization will now be discussed.
Effect of Time on Pellet Sulfide Content
To initiate the desulfurization experimentation, a study was made of the
effect of time on the desulfurization of sulfide bearing pellets. Pellets
were reacted with steam using the equipment shown in Figure 1 for various
times and were analyzed after reaction to determine their unreacted sulfide
-23-
-------
content„ A reaction temperature of 2100°F and a steam rate of 50 times
the stoichiometric requirement was used. In this discussion, the steam
rate will be represented as moles of steam contacting the pellet per
minute per mole of sulfides originally present in the sulfide bearing
pellet. For this experiment, the steam rate was 60 moles per minute per
mole of sulfides. The experimental results are shown in Figure 7 where
the fraction of unreacted sulfides is plotted versus reaction time.
As seen in Figure 7, after 20 minutes, the coal refuse pellet was
completely reacted and contained no sulfur in the form of calcium sulfide
or ferrous sulfide. Also, after a period of only five minutes about
86 percent of the initial sulfides had reacted leaving only 14 percent
available for further reaction. The results of this experiment indicate
that reasonable commercial and experimental residence times could be
achieved using 1/2 inch pellets and a reaction temperature of 2100°F_
Effect of Time on Desulfurization Offgas Composition
Experiments to determine the effect of time on the composition of desul-
furization gaseous reaction products were conducted in the same manner
as above except that after various times the accumulated desulfurization
offgas was retained and analyzed for S02, H£S, and sulfur by difference.
A steam rate of 60 moles per minute per mole initial pellet sulfides
was employed as was a reaction temperature of 2100°F. The results are
given in Figure 8 where the mole fraction of I^S, S, and 862 produced
is shown as a function of time. From Figure 8, it is seen that between
five and fifteen minutes the proportions of the sulfur bearing gases are
not significantly affected by time. Between fifteen and twenty minutes
the fraction of hydrogen sulfide in the total evolved sulfur bearing gas
decreased; however, the fraction of sulfur increased and the fraction of
S02 remained essentially constant.
As seen in Figure 8, the fractions of H2S, S, and S02 in the accumulated
desulfurization offgas were 43, 42, and 15 percent respectively after 20
minutes when desulfurization of the pellet was essentially complete.
This result infers that the process is workable since the ratio of l^S to
SC>2 is approximately three. A minimum H^S/SOo ratio of two is required
for the Glaus sulfur recovery process; therefore, excess S02 is not
produced in the desulfurization offgas. While the above experimental
operating conditions suggest that the process is workable, it would be
more advantageous to select operating conditions which yield a maximum
of sulfur in the offgas rather than l^S. This situation would minimize
the size of the sulfur recovery plant and reduce operating costs.
Therefore, the following studies concerning the effect of steam rate on
the extent of desulfurization and the desulfurization offgas composition
were undertaken.
Effect of Steam Rate on Pellet Sulfide Content
Using the equipment of Figure 1, the effect of steam rate on the desul-
furization of sulfide bearing pellets was investigated for a residence
-24-
-------
1.00
0.90
0.80
co
01
13
[
4-1
3
Unreacted
0
c
0
Fracti
0
0
0
0
0
0
0
0
.70 LI
n
.60
.50
.40
o30
.20
.10
.05
0
C
I Steam Rate = 60
- \ Temperature = 21
1 Pellet Size = I/
\
&•
) 5 10 15
20
25
Time, Minutes
FIGURE 7-EFFECT OF TIME ON FRACTION OF UNREACTED SULFIDES
-25-
-------
O
O
CO
CO
60
S-i
-en
•a
a
CO
y
4!
c
•r-l
CO
CM
O
CO
CO
CM
£
O
•H
4-)
O
CO
o
•
o
o
CO
•
o
o
o
o
•
o
o
o
o
•
o
o
CO
o
CM
•
O
O
I-l
•
o
Steam Rate = 60-70 moles/min/mole sulfides
Temperature = 2100°F
Pellet Size = 1/2"
SO,
5 10 15 20
Time, Minutes
FIGURE 8 EFFECT OF TIME ON REACTION PRODUCTS MIX
-26-
-------
time of 30 minutes and a temperature of 2100 F. Several sulfide bearing
pellets were individually reacted at various steam rates and analyzed
for residual sulfide content. In each case, the amount of steam con-
tacted with the pellet was determined by measuring the amount of water
removed from the steam generation flask. The rate was defined as in the
previous studies. The results are shown in Figure 9 which is a plot of
the fraction of unreacted sulfides in the pellets versus steam rate in the
range of 1.6 to 7.2 moles per minute per mole of initial sulfides. Low
steam rates were selected for investigation because they are commercially
more attractive than high steam rates which would require larger con-
densers for removal of unreacted steam. They also represent a con-
siderably higher evaporative heat loss if water rather than steam is
fed to the desulfurization zone of the shaft reactor.
As seen in Figure 9, the fraction of unreacted sulfides remaining in
the pellet after 30 minutes increases as the steam rate is decreased.
This is expected since less steam per time is available for the desul-
furization reactions (5 and 6):
FeS + H20 *• FeO + H2S (5)
CaS + H20 »• CaO + H2S (6)
More importantly, the results indicate that at a steam rate of 5 moles
per minute per mole of initial sulfides, a residence time of about
30 minutes will be required for desulfurization in the shaft reactor.
While the extent of desulfurization as a function of steam rate is
important, the composition of the desulfurization offgas is of prime
importance and was the subject of the next investigation.
Effect of Steam Rate on Desulfurization Offgas Composition
The most important concern of the desulfurization study was the effect
of various parameters on the composition of the desulfurization offgas
because the technical and economic feasibility of the process depends
on this composition. In this project, it was determined that the steam
rate was the most critical variable affecting the sulfur dioxide, hydrogen
sulfide, and elemental sulfur content of the offgas. To ascertain the
effect of steam rate on the offgas composition, one half inch sulfide
bearing pellets were reacted at 2100°F with steam at various rates. The
offgas was collected over a 30 minute period and was subsequently an-
alyzed. Although it was previously determined that a steam rate of about
5 moles per minute per mole initial sulfides at the above operating
condition was sufficient to desulfurize the coal refuse pellet in 30
minutes, it was felt that investigation of higher steam rates was
justified to observe the effect on offgas composition. Therefore,
rates ranging from 1.6 to 32 moles per minute per mole initial sulfides
were used. The experimental results appear in Figure 10 and are
presented as a plot of the composition of the offgas as a function of
steam rate. The important trends observed were that the elemental sulfur
-27-
-------
0.08
AJ
0)
T-I
•0" 0.07
d
•1-1
ra 0.06
-------
o
a,
e
O
O
T)
0)
4-1
TO
O
•r-l
-a
d 60
m o
o
60
ci d
O •!-)
•H M
4-J CQ
o a)
RJ P5
t-l
*
I
o
1.00 -
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0
Residence Time = 30 minutes
Temperature = 2100°F
H2S
SO,
05 10 15 20 25 30 35 40
Steam Rate, moles/min/mole pellet sulfides
FIGURE 10-EFFECT OF STEAM RATE ON DESULFURIZATION OFFGAS COMPOSITION
-29-
-------
content decreased and the sulfur dioxide content increased as the steam
rate increased. The fraction of hydrogen sulfide in the offgas increased
to a maximum of about 0.68 at a steam rate of 23 moles per minute per
mole initial sulfides and then decreased as the steam rate increased to
32.
The reactions that were postulated to occur during desulfurization are
reproduced here for convenience:
FeS + H20 * FeO + H2S (5)
CaS + H20 > CaO + H2S (6)
H2S 4- 2H20 * S02 + 3H2 (7)
H2S » H2 + 1/2S2 (8)
2H2S + S02 • *• 2H20 + 3S (9)
Elemental sulfur is formed via equations (8) and (9). At the high
desulfurization temperatures used, it is likely that hydrogen sulfide
partially dissociates into its elemental constituents. According to
Riesenfeld^, the extent of dissociation of hydrogen sulfide is 37 and
47 percent at 2192°F and 2372°F, respectively. Reaction (9) probably
occurs since this reaction is catalyzed by alumina which is present in
the pellet.
Sulfur dioxide is formed according to equation (7) which is the oxidation
of hydrogen sulfide by steam. The occurrence of reaction (7) during
desulfurization was supported by these facts: decreasing the steam
rate decreased the SC>2 content of the offgas, and hydrogen was detected
in the offgas.
The important conclusion from Figure 10 is that as low a steam rate as
practical should be employed in the process to produce a maximum of
sulfur consistent with a reasonable pellet residence time requirement.
The results of this and the prior study indicate that a steam rate of
5 moles per minute per mole of initial sulfides will completely desul-
furize the pellet in 30 minutes at 2100°F and yield a desulfurization
offgas containing about 45 percent sulfur.
Effect of Temperature on Pellet Sulfide Content
The effect of temperature on the extent of desulfurization of one half
inch sulfide bearing pellets was determined by reacting the pellets at
various temperatures for 30 minutes at the same steam rate. The steam
rate used was 5 moles per minute per mole initial sulfides. After
reaction, the pellet was removed from the furnace and analyzed for un-
reacted sulfides,, Figure 11, which is a plot of the fraction of unreacted
sulfides in the pellet versus temperature, indicates that the effect
of increasing temperature is to reduce the unreacted sulfide content in
the pellet. As seen, to achieve complete desulfurization under the
above experimental conditions, a temperature of about 1900°F is required.
If the temperature is lower, longer pellet residence times will be
required. At temperatures greater than 1900°F, it is noted that
-30-
-------
1.00
0.90
0.80
£ 0.70
,-i
" 0.60
-------
the pellets were completely desulfurized after 30 minutes. These results
indicate that to achieve pellet residence times of 30 minutes or less,
a minimum desulfurization temperature of 1900°F is required.
Effect of Temperature on Desulfurization Offgas Composition
The effect of temperature on the desulfurization offgas composition was
determined for temperatures ranging from 1900°F to 2100°F. For these
experiments, the steam rate was 5 moles per minute per mole initial
sulfides and the reaction time was 30 minutes. The desulfurization
offgas was collected and analyzed for sulfur dioxide, hydrogen sulfide,
and elemental sulfur. Experimental results are presented in Figure 12
which is a plot of the fraction of each constituent in the offgas
versus temperature. The hydrogen sulfide content of the offgas does
not exhibit temperature dependence to a noticeable extent; however, the
sulfur content does as it increased from about 0.40 at 1920°F to
0.49 at 2100°F. Conversely, the sulfur dioxide in the offgas decreased
from 0.16 at 1920°F to only 0.06 at 2100°F. These results suggest that
higher temperatures result in the production of more sulfur and less
sulfur dioxide and should, therefore, be employed in the commercial
process.
Effect of Pellet Size on Desulfurization
The final variable studied in the desulfurization phase of experimentation
was pellet size and its relation to the residence time required for
complete desulfurization. Pellets ranging in size from 1/4 to 3/4 inch
were reacted with steam at a rate of 5 moles per minute per mole initial
sulfides. The reaction temperature used was 2100°F. These values of
temperature and steam rate were chosen because prior experimentation
proved them to be the most feasible from the standpoint of application
to a commercial process. Several pellets of each size were reacted
for times of 5 to 30 minutes to determine the exact residence time
required for complete desulfurization. After a specified time, the
pellets were withdrawn from the furnace and analyzed for their residual
sulfide content. Figure 13 indicates the residence time required to
achieve complete desulfurization for each pellet size. As seen, residence
times ranged from 12 to 25 minutes for 1/4 to 3/4 inch pellets. The
pellet size used in the commercial process will depend on which size is
the most economical to manufacture and/or is most marketable; however,
each size yielded desulfurization residence times less than 30 minutes
at the operating conditions specified above.
Comments
Desulfurization experimentation has shown that 1/4 to 3/4 inch diameter
sulfide bearing pellets can be desulfurized in less than 30 minutes at
2100°F and at a 5 moles per minute per mole pellet sulfides steam rate.
This steam rate and operating temperature is commercially feasible; con-
sequently, a reasonable shaft reactor residence time is expected. It was
found that the effect of decreasing the steam rate is to increase the gas-
eous elemental sulfur and to decrease the sulfur dioxide formed. It would
be advantageous in a commercial unit to produce as much elemental sulfur
-32-
-------
OJ
d
o
a
1.00
0.90
0.80
3 0.70
CO CO
O CO
•H 60
5S°-60
M-l 60
O C
§£0.50
O
CO
0) 3
i-l CO
.50.30
0)
•r-l
J2 0.20
3
o
0.10
Time
Pellet Size
Steam Rate
30 minutes
1/2"
5 moles/min/
mole sulfides
1800 1900 2000 2100
Temperature, F
FIGURE 12-EFFECT OF TEMPERATURE ON REACTION PRODUCTS MIX
-33-
-------
CO
01
d
ro ci
0) O
M -r-1
•rl -U
3 nl
cr N
0) -i-l
PH J_|
IS
H co
0)
P
0)
4J
01
1-1
Ou
a
O
u
iri
Steam Rate = 5 tnoles/min/mole sulfides
Temperature = 2100°F
1/4 3/8 1/2 5/8 3/4
Pellet Diameter (inches)
FIGURE 13-RESIDENCE TIME REQUIREMENTS TO
ACHIEVE COMPLETE DESULFURIZATION
-34-
-------
as possible since the sulfur exiting the reactor can be easily and
economically recovered by condensation. The conversion of hydrogen
sulfide to sulfur requires a Glaus sulfur recovery plant. If the
quantity of hydrogen sulfide is reduced, the Glaus plant can be smaller
thereby enhancing the economics of the process. Therefore, as low a
steam rate as possible should be used for desulfurization consistent
with residence times of about 30 minutes.
Since pellets ranging in size from 1/4 to 3/4 inch were completely
desulfurized in less than 30 minutes at a temperature of 2100°F and a
steam rate of 5 moles per minute per mole pellet sulfides, the pellet
size(s) to use in the process will depend on which size ash pellet is
the most marketable.
-35-
-------
SECTION VIII
PROCESS DESCRIPTION
Process Flow Sheet
Bench scale laboratory results indicate that a commercial process can
be conceived for the recovery of sulfur from coal refuse. One possible
process flow sheet is presented in Figure 14. Limestone and coal
refuse from an existing pile or directly from a coal preparation plant
are crushed simultaneously in a crusher. The crusher reduces these
components to a fine product and provides intimate mixing. The mixture
proceeds to a disc pelletizer where pellets are formed and sent to a
preheater in which their temperature is raised to 900 -- 1100°F prior
to entering a shaft reactor. The energy requirement for preheating is
supplied by burning a carbon monoxide-bearing offgas from the H2S-S02
absorption-scrubbing system. The pellet preheater offgas enters a wet
scrubbing system where a tar-coal refuse dust mixture is separated from
the offgas. In the shaft reactor, sulfur is removed from the preheated
pellets by reaction with steam,, As the desulfurized pellet travels down
through the shaft reactor, air is introduced counter-currently into the
shaft to combust the carbon contained within the desulfurized pellet.
The heat of combustion not only produces a hard, fired pellet suitable
for use as road aggregate, but also produces a hot reducing gas. This
reducing gas, as it travels up the shaft reactor, raises the temperature
of the preheated pellets so that steam desulfurization occurs. The
fired ash pellets are removed and sent to storage for sale. The shaft
reactor offgas which contains tars, sulfur, hydrogen sulfide, sulfur
dioxide, nitrogen, water, carbon monoxide, and carbon dioxide proceeds
to the sulfur-tar recovery system where sulfur and tar are removed from
the gaseous stream. The resulting H2S-S02-C02~N2-H20 gas mixture is
sent to a H2S-S02 absorption scrubbing system where the gaseous com-
ponents are separated. The H2S-S02 gas enters a conventional Glaus
sulfur recovery plant. In the sulfur recovery plant, steam is generated
which can be used in the shaft reactor. The CO-C02-N2 gas from the
H2S-S02 absorption scrubbing system proceeds to the pellet preheater
where it is burned to supply the energy requirements for preheating.
Operating Technology and Suggested Operating Parameters
The crushing and pelletizing operations are standard and require no
further elaboration as to their operation. Crushing and pelletizing are
required to minimize dust during subsequent operations, to provide
maximum contact between the limestone and coal refuse, and to yield
an ash pellet suitable for use as a road aggregate. The pellet
preheater may be a direct fired, refractory lined, continuous flow
shaft furnace or rotary kiln. The purpose of preheating the pellets
prior to entering the shaft reactor is to prevent the sulfur contained
in the shaft reactor gas from condensing on the surface of the incoming
pellets. If sulfur were to condense on the pellet surface and be
carried into the shaft reactor, it would be converted to K^S and/or
S0° While this would not affect the overall recovery of sulfur, it would
-37-
-------
Coal Refuse
Limestone
Water
Crushing
and
Pelletizing
Coal Refuse
Pellets
Pellet
Preheater
Offgas
Preheated
Pellets
Steam
(From S-
Recovery
Plant)
Shaft Reactor
Air
Fired Ash
^Pellets
Offgas
Wet
Scrubbing
System
Tar-Dust
CQ-C02-N
Offgas
Sulfur-Tar
Recovery
System
Tar
Offgas
H2S-S02
Absorption
Scrubbing
System
Sulfur
Recovery
Plant
Sulfur
\y Sulfur
Sulfur Recovery from Coal Refuse
FIGURE 14-PROCESS FLOW CHART
Steam to
Shaft Reactor
-38-
-------
increase the size and cost of the sulfur recovery plant since a higher
throughput of the H^S-SC^ feed gas would result. The wet scrubbing
system consists of a standard scrubber using a closed-loop water scrub-
bing system. The tars and dust are separated from the water using
conventional techniques„
A schematic diagram of the reaction zones of the continuous flow shaft
reactor is shown in Figure 15. As seen, the preheated coal refuse
pellets entering the shaft reactor proceed in series through five zones
(pellet preheating, sulfide formation, desulfurization, gasification,
and air preheater) and exit as fired ash pellets.
The reactions occurring in the pellet preheating and sulfide formation
zones of the shaft reactor are identical to the ones studied in the
sulfide formation phase of the laboratory study; the only significant
difference being that the laboratory study used an inert gas whereas in
the shaft reactor a highly reducing gas would pass through the pellets.
Laboratory results indicate that the amount and particle size of lime-
stone used in the coal refuse pellets be such that the conversion of
coal refuse sulfur to sulfides be 0.60-0.65. This can be achieved using
35 percent limestone (-100 mesh) and 65 percent of a 16 percent sulfur
coal refuse in the pellet. Of course for lower sulfur content coal
refuse, the fraction of limestone would be reduced.
The resulting products of the pellet preheating and sulfide formation
zones are FeS-CaS bearing coal refuse pellets which proceed to the
desulfurization zone and an offgas containing tars, sulfur, hydrogen
sulfide, sulfur dioxide, nitrogen, water, carbon monoxide, and carbon
dioxide. The pellets at this point will still contain carbon but be
essentially free of volatiles, these volatiles having been removed in
the previous two zones.
The same reactions occur in the desulfurization zone that occurred in
the pellets during the laboratory investigation of desulfurization.
Again, the only difference from the laboratory study is that a highly
reducing gas from the gasification zone passes through the pellets.
The presence of a highly reducing gas will be beneficial to the process
because it will tend to minimize or prevent the formation of sulfur
dioxide.
Based on the results of the laboratory study, a low steam rate should
be used for desulfurization consistent with complete desulfurization
in about 30 minutes. In the laboratory study, a rate of 5 moles per
minute per mole initial sulfides was found to accomplish complete
desulfurization in less than 30 minutes for pellets up to 3/4 inch
diameter. As pointed out in the laboratory study, a low steam rate
maximizes production of sulfur and minimizes the formation of SC>2.
Temperatures in the range of 1900°F-2100°F should be used in the desul-
furization zone since the rate of the desulfurization reaction between
steam and sulfides increases with temperature.
-39-
-------
Preheated Coal Refuse Pellets
Shaft Reactor Offgas
Steam
Air
Pellet
Preheat
Zone
Sulfide
Formation
Zone
Desulfurization
Zone
Gasification
Zone
Air Preheat
Zone
Preheated, Partially
Devolatilized, Coal Refuse
Pellets
Devolatilized, CaS-FeS
Bearing Pellets
Desulfurized, Carbon Bearing
Ash Pellets
Hot, Fired Ash Pellets
Cooled, Fired Ash Pellets
FIGURE 15-SHAFT REACTOR ZONES
-40-
-------
The pellets exiting the desulfurization zone will be essentially free
of sulfur in any form, completely devolatilized, and will contain carbon
and ash constituents. These desulfurized carbon bearing ash pellets
then enter the gasification zone. In the gasification zone, the carbon
contained in the pellets is reacted with air from the air preheat zone
to provide a hot carbon monoxide rich offgas, which supplies the energy
requirements of the desulfurization and sulfide formation reactions.
The carbon monoxide also provides a reducing atmosphere in which the
above operations are carried out, thereby minimizing the formation of
sulfur dioxide. In addition, the carbon monoxide is recovered later in
the process and is burned to provide the pellet preheating energy
requirements. Any sulfur remaining in the desulfurized pellets will be
burned to sulfur dioxide to produce sulfur-free fired ash pellets. The
air preheat zone serves two purposes. The fired ash pellets from the
gasification zone will be used to preheat the incoming air to minimize
the energy requirements of the reactor, and will serve to cool the
fired ash pellets for ease in removal from the shaft reactor. The
experimental results of the laboratory study have shown that both the
sulfide formation and desulfurization zone require a maximum of one-
half hour pellet residence time. Since the gasification reaction is
rapid, the residence time requirement for the desulfurization of the
pellets in the shaft reactor will be in the order of two hours — one
half hour pellet preheating, one half hour sulfide formation, one
half hour desulfurization, and one half hour for gasification and air
preheating.
Several possibilities exist for the design of the sulfur-tar recovery
system,, These include (1) a staged condenser operation in which high-
boiling-point organics, sulfur, and tars are separately recovered in
sequence, (2) a tank in which the offgas is directly contacted with
water producing a floating tar layer and a suspension of sulfur which
can be recovered by filtration, or (3) by sulfur scrubbing of the
offgas to remove the tar with subsequent condensation of the scrubbed
sulfur vapor in a condenser.
The H2S-SC>2 absorption-scrubbing system will require standard absorption
scrubbing equipment. The sulfur-tar recovery system offgas will contain
H2S, SOo, CO, C02» and N2. One possible scheme is to remove the
H2S-S02 and G02 using water or ammonia as the absorption reagent. The
CO-N2 offgas would then be used for pellet preheating. The C02 content
of the H2S-S02-C02 mixture could then be removed using propylene car-
bonate as the absorption reagent to yield an H2S-S02 gaseous feed for
the sulfur recovery plant.
The above suggested operating parameters and process flow chart should
be considered only as one of many possible alternatives.
-41-
-------
SECTION IX
ECONOMIC EVALUATION
Estimated Fixed Capital Investment
Based upon a one million ton per year coal refuse plant (process flow
chart—Figure 14) the daily material input and output of the streams
of economic significance for an eight percent coal refuse is shown in
Table 4. The composition of the coal refuse (dry basis) was assumed
to be six percent pyritic and two percent organic sulfur, 40 percent
ash, and 45 percent volatile matter and carbon. The results of Table 4
were based on a process requiring the addition of 20 percent by weight
limestone to the eight percent coal refuse. According to Figure 6,
the limestone requirement will be 20 percent for a coal refuse containing
two percent organic sulfur if the limestone external surface area is
about 1800-1900 cm^/gram. In the discussion which follows, the coal
refuse sulfur content is assumed present as pyrite and organic sulfur in
a 3:1 ratio. The iron remaining in the ash pellets was assumed to be
in the form of FeO. From Table 4, the quantity of sulfur produced will
be 211 tons/day assuming a 95 percent overall recovery and the production
rate of ash pellets will be 1824 tons/day.
TABLE 4
RAW MATERIAL INPUTS AND PRODUCTS
(1MM Ton/Year Coal Refuse Plant)
Inputs Tons/Day
Coal Refuse (Dry) 2778
Limestone 695
Outputs
Sulfur (95% Recovery) 211
Fired Ash Pellets (Ash-FeO-CaO) 1824
For a one million ton per year coal refuse plant using a three percent
moisture coal refuse, the crusher will handle 2850 tons/day of coal
refuse plus 695 tons/day of limestone. The pelletizer is sized to
produce approximately 3600 tons/day of pellets depending upon the
moisture content of the coal refuse pellet. The pellet preheater is
sized to accommodate about 3600 tons/day of pellets. If all the water
and 20 percent of the total volatile matter and carbon contained in the
coal refuse is driven off in the pellet preheater then the shaft reactor
must accommodate about 3000 tons/day of pellets.
Experimental laboratory results indicate that approximately 40 percent
of the original sulfur contained in the coal refuse can be converted to
sulfur prior to the sulfur recovery plant. Thus, the sulfur recovery
plant is sized to yield 125 tons/day of sulfur.
-43-
-------
Based upon the tonnage requirements as specified above, and assuming a
two hour residence time requirement for the shaft reactor, the estimated
fixed capital required for a plant using one million tons per year of
coal refuse is shown in Table 5. As indicated, the fixed capital in-
vestment will be $2.3 million for a one MM ton/year coal refuse plant.
Black, Sivalls & Bryson constructs sulfur recovery plants; therefore,
the estimated capital expenditure for the sulfur recovery plant was
based on internal sources. By scaling the equipment costs of Table 5,
the capital investment for plants of various capacities and coal refuse
sulfur contents were determined and are shown in Figure 16.
TABLE 5
ESTIMATED FIXED CAPITAL FOR A ONE MM TON/YEAR COAL REFUSE PLANT
Land & Building
Crusher-Pelletizer
Pellet Preheater
Shaft Reactor
Bucket Elevators - Belt Conveyors - Pumps
Wet Scrubbing System
Sulfur-Tar Recovery System
H2S-S02 Scrubbing System
1. Total Purchased Equipment 560
Installation, Piping, Electrical
Instrumentation, Utilities
(85% of 1)
2. Physical Plant Costs
Engineering & Construction
(30% of 2) 311
(thousands of dollars)
3. Direct Plant Cost 1,347
Contractors Fee & Contingency
(20% of 3)
4. Installed Sulfur Recovery Plant
TOTAL FIXED CAPITAL
Operating Revenue
The daily operating revenue for a one million ton per year coal refuse
plant is shown in Table 6 for an eight percent sulfur coal refuse. The
-44-
-------
CO
S-l
o
Q
CQ
G
O
4J
c
CD
-------
operating revenue "is defined as the revenue resulting from the profits
realized from the selling of products less the costs associated with
capital interest charges, labor and supervision, maintenance and raw
materials. Table 6 was calculated using the material balance shown
in Table 4 and the fixed capital requirements of Table 5. This
table was prepared assuming that sulfur and ash pellets can be sold
for $20/ton and at $1.5/ton respectively and that limestone can be pur-
chased at $2.5/ton.
TABLE 6
OPERATING REVENUE FOR A ONE MM TON/YEAR COAL REFUSE PLANT
(All Numbers, Dollars/Day - 360 Day/Year Basis)
Daily Production Cost
Capital Interest Charge @ 14%
Labor and Supervision (2 men--$50/day - 3 shifts)
Maintenance @ 10% of Fixed Capital
Coal Refuse (2778 tons @ $0.5/ton)*
Limestone (695 tons @ $2.5/ton)
4,970
Daily Production Credit
Sulfur (211 tons @ $20/ton) 4,220
Ash Pellets (1824 tons @ $1.5/ton) 2,736
Operating Revenue
Return on Investment
(yearly profit before taxes/capital investment) 317»
Payback Period
(capital investment/yearly profit before taxes) 3.2 years
*Note: This is a charge for coal refuse which coal producers would
not have.
At present, the cost of coal refuse is unknown; therefore, a conservative
haulage charge of $0.5/ton was assumed for a non-coal producer using the
process. This haulage charge assumes the coal producer will bear the
cost of loading the coal refuse on trucks which he would do anyway if he
were to stockpile his refuse. As seen in Table 6, the operating revenue
is $1986/day which yields a before-tax return on investment of
31 percent. The economic attractiveness of the process should encourage
the utilization of coal refuse rather than stockpiling which generates
air and water pollution problems.
-46-
-------
The operating revenue and the before-tax return on investment (ROI) are
a function of plant capacity, coal refuse sulfur content, selling price
of sulfur and ash pellets, coal refuse cost, and capital interest rate.
In Figures 17-19 are shown the relative effects of these variables on
ROI. The effect of the selling price of sulfur and ash pellets is shown
in Figure 17 for a one MM ton/year-eight percent sulfur coal refuse plant.
As seen, these economic variables have a significant effect upon ROI.
For example, the ROI is 15, 31, and 47 percent for sulfur selling prices
of 15, 20, and $25/ton respectively for ash pellets sold at $1.5/ton.
At present, the selling price of sulfur is in the $20-25/ton range; there-
fore, ROI's in the range of 31-47 percent are anticipated. The selling
price of ash pellets is an important economic consideration. In the
economic analysis of the process, a conservative estimate of $1.50/ton
for ash pellets as a road aggregate was used. However, these ash pellets
will contain lime, ash, and iron and can be more profitably used for
other industrial applications. Based on the lime content of the ash
pellet alone (approximately 20 percent lime) they can be valued at approx-
imately $3.5/ton. The selling price of ash pellets will depend on the
location of the plant and whether suitable users of ash pellets are in
close proximity to the plant. Using a conservative maximum selling
price of $3/ton, Figure 17 indicates that the ROI increases from 31 to
73 percent as the ash pellet selling price increases from 1.5 to $3/ton.
The effect of the purchase cost of coal refuse on ROI is shown in
Figure 18 for a one MM ton/year-eight percent sulfur coal refuse plant
at various capital interest charges. It is anticipated that coal refuse
will be available to the process at 0 to $0.5/ton depending on who is
operating the process (for coal producers the cost would be zero) and
the distance the coal refuse must be hauled. As seen in Figure 18, the
ROI at a capital interest rate of 14 percent is 53 and 31 percent as the
coal refuse cost increases from 0 to $0.5/ton. In the economic evaluation
presented in Table 6, a conservative coal refuse cost of $0.5/ton was
used.
As expected, the ROI decreases as the capital interest rate increases.
This is shown in Figure 18 where the ROI decreases from 39 to 24 percent
as the capital interest rate increases from 6 to 20 percent at a coal
refuse cost of $0.5/ton. The capital interest rate will depend on who
is financing the plant. Municipalities will probably be able to finance
at the six percent rate using bonds whereas non-government financing will
be at the 14 percent rate.
Figure 19 shows the effect of plant capacity and coal refuse sulfur
content on ROI. As seen, as the sulfur content increases from 4 to 12
percent, the ROI increases from 8 to 47 percent for a one MM ton/year
plant. Also the ROI increases from 4 to 47 percent as the plant capacity
is increased from 0.2 to one MM ton/year for a 12 percent sulfur coal
refuse. These results indicate that as large a plant as practical should
be built using coal refuse with a maximum of sulfur.
-47-
-------
4J
fl
HI
J-J
co
-------
o
o
o
03
4J
d
0)
4-1
CD
0)
ci
o
O
C-J
Basis:
1 MM Ton/Year Plant
$1.5/Ton Ash Pellets
$20.0/Ton Sulfur
8% Sulfur Coal Refuse
6% Capital Interest Rate
.25
Cost of Coal Refuse, $/Ton
.50
FIGURE 18- RETURN ON INVESTMENT VS. COST 0*1 COAL REFUSE
AT VARIOUS CAPITAL INTEREST RATES
-49-
-------
o
o
o
oo
4J O
d ^>
|
to
-------
Comments
If a coal producer should utilize the process on his site, the cost of
coal refuse would be zero. In fact, the cost of coal refuse could even
be considered a credit since the coal producer would not have to
stockpile his current production of coal refuse. Assuming the cost of
coal refuse to be zero (for a coal producer), the process becomes
extremely attractive. Using the data of Table 6, the operating revenue
becomes $3376/day ($1986 + $1390 savings in coal refuse cost per day)
with a before-tax return on investment of about 53 percent. The coal
producer has, therefore, been given great incentive to utilize his
coal refuse production. A municipal agency wishing to build a coal
refuse plant could obtain capital at a conservative rate of 6 rather
than 14 percent. Based on capital available at six percent, and using
the data of Table 6, the operating revenue becomes $2490 per day
($1986 + $504 savings in capital interest charges per day). This
represents a return on investment of about 39 percent. The economics of
the process are extremely favorable and should induce communities or
coal producers, saddled with coal refuse piles, to utilize this process
and eliminate a serious pollution problem.
Thus far, the discussion has concentrated on the profits that can be
realized on this pollution abatement process; however, many communities
or small coal producers have coal refuse piles or sources which are of
moderate quantity and sulfur content. To eliminate or minimize acid
water formation from coal refuse piles under these conditions, the con-
cept of a break-even process operation will be considered. Under a
given set of economic and process operating factors, there will exist a
plant capacity and coal refuse sulfur content in which the ROI is
zero: a break-even operation. In Figure 20 the break-even plant
capacity is shown as a function of the coal refuse sulfur content under
various economic conditions. For this figure, the selling prices of
sulfur and coal refuse have been fixed at $20 and $1.5/ton respectively.
Four cases have been considered in which the costs of coal refuse and
capital interest rate have been related to the following situations:
Cost of Capital Interest
Case Coal Refuse $/ton Rate, Percent
I $0.5 14
II 0.5 6
III 0. 14
IV 0. 6
Case I is for private financing at normal interest rates. From
Figure 20 it is seen that the minimum or break-even plant capacity
ranges from 640,000 to 160,000 tons/year of coal refuse of 4 to 12
percent sulfur respectively. Case II is for public financing of a plant
at a low interest rate (municipal bonds). As seen, break-even plant
capacities range from 400,000 to 80,000 tons/year (coal refuse sulfur
-51-
-------
M 800,000
TO
0)
W
a
o
H
600,000
O
CD
Ch
CO
4-J
d
CO
PL, 400,000
4)
CO
0)
200,000
Basis:
$20/ton sulfur
$1.5/ton ash pellets
III
4 8 12
Coal Refuse Sulfur Content, Percent
15
FIGURE 20-BREAK-EVEN PLANT CAPACITY VS. COAL REFUSE SULFUR CONTENT
FOR VARIOUS ECONOMIC SITUATIONS
-52-
-------
content of 4 to 12 percent). Case III is for private financing at a
normal interest rate and no charge for the coal refuse. In this case,
the break-even plant capacity varies from 260,000 to 60,000 tons/year.
Case IV is for a municipality financing the plant at a low interest rate
with no charge for the coal refuse. This is the most attractive
situation and yields a break-even plant size of 160,000 to 40,000
tons/year for sulfur contents of 4 to 12 percent. The above results
(particularly case IV) indicate that the process can be applied as a
no-cost pollution abatement process for communities or small coal
producers troubled with moderate to small quantities of low sulfur coal
refuse.
-53-
-------
SECTION X
ACKNOWLEDGEMENTS
The support of the project of the Environmental Protection Agency and
the help provided by Mr. Robert B. Scott, Project Officer, is acknowl-
edged with sincere thanks.
-55-
-------
SECTION XI
REFERENCES
1. Kim, J. H., et. al. "Kinetic Studies of the Thermal Decomposition
of Pyrite". Preprint of text presented at 100th annual meeting
of AIME, Feb., 1971.
2. Riesenfeld, J. F. Prakt, Chem.. 100, 115 (1920).
3. Flaschka, EDTA Titrations, Pergamon Press, 1964.
4. Fritz, J. S. and Schenk, G. H., Quantitative Analytical Chemistry,
Allyn and Bacon, 1969, pp. 211f.
5. Fischer & Peters, Quantitative Chemical Analysis, Saunders Co.,
1968, p. 524.
-57-
-------
SECTION XII
APPENDICES
Page No.
A. Determination of Sulfide Content of
Coal Refuse Pellets 61
Figure 1: Sulfide Determination
Experimental Equipment 62
Bo Determination of Desulfurization Offgas Composition 65
Figure 1: Experimental Equipment used in
S02 Determination „ 0 66
-59-
-------
APPENDIX A
DETERMINATION OF SULFIDE CONTENT OF COAL REFUSE PELLETS
The determination of sulfur in the coal refuse pellets which is present
as either FeS or CaS is based upon the fact that these species react
in an acidic media to form hydrogen sulfide. The reactions are as
follows:
FeS + 2HC1 *- FeCl2 + H2S
CaS + 2HC1 »- CaCl2 + H2S
By determining the H2S evolved, the total amount of sulfides present in
the sample can be determined. Unreacted pyritic sulfur, FeS2, which may
be present in the coal refuse is insoluble in HC1 and does not interfere
in the method. The amount of hydrogen sulfide evolved is determined by
passing the gas through an aqueous CuS04 solution. Hydrogen sulfide
reacts with CuSO^ to form H2SO^ and an insoluble precipitate of CuS
according to the following equation:
H2S + CuSO^ *. H2S04 + CuS
The solution is then filtered to remove the insoluble CuS and titrated
with ethylenediaminetetraacetic acid (EDTA) to determine the amount of
copper ion that remains in solution. The difference between the amount
of copper ion that remains in solution and the amount originally present
determines the amount of CuS precipitated and, therefore, the amount of
H2S evolved from the sample. The EDTA titration is a complexiometric
titration and is widely used to determine various ions in solution. A
complete explanation of the theory and practice of EDTA titrations is
given by Flaschka and Fritz .
The experimental setup used to determine the amount of sulfides contained
in the coal refuse pellets is shown in Figure 1. The sample pellet is
ground to approximately minus 100 mesh and placed in an evolution
flask. A dropping funnel containing concentrated hydrochloric acid is
inserted into a neck of the flask. Nitrogen is used to slowly force the
HC1 solution into the evolution flask. After the acid has been added
to the flask, the heating mantle is turned on and the solution gently
refluxed for 30 minutes. Nitrogen continually flows through the system
during this analysis to purge the lines of any residual hydrogen sulfide.
The evolved hydrogen sulfide is passed through a fritted cylinder
absorption column containing a measured volume of CuSO/ solution. A
small piece of moistened lead acetate paper was placed at the exit of
the absorption column. Any H2S leaving the column would have turned the
lead acetate paper from white to a brown-black color. In no instance
was this color change experienced. At the end of 30 minutes, the
absorption column is removed from the system and the CuSO^ solution
containing a black precipitate of CuS filtered. The clear filtrate was
then added to a 500 ml volumetric flask and was diluted to volume with
distilled water. A 50 ml aliquot was removed with a pipette and placed
in a 400 ml beaker. It was found that the end point in the EDTA titra-
-61-
-------
Nitrogen
Dropping
Flask ~
w/acid
H20 Outlet
H2S Offgas
Water Cooled Reflux
Condenser
H0 Inlet
-Thermometer
Evolution Flask
Heating Mantle
Lead Acetate Paper
Absorption Column
Containing
FIGURE 1-SULFIDE DETERMINATION EXPERIMENTAL EQUIPMENT
-62-
-------
tion was much sharper if an aliquot was titrated instead of the entire
solution. Twenty milliliters of a pH 5.5 buffer solution was added to
the aliquot of CuSC>4 solution in the beaker. The buffer
solution was made by adding 200 g. of ammonia acetate, 80 g. of hexa-
methylene-tetramine, and 175 ml of glacial acetic acid to 800 ml of
distilled water. The buffer solution is used to maintain a constant
pH during the EDTA titration. Five drops of PAN indicator (l-(2pyridylazo)
2-Naphthol) and 10-15 drops of xylenol orange indicator were added.
The solution is then titrated using a standard solution of EDTA. The
end point is reached when the color of the solution turns from a deep
blue to green. The EDTA reacts with the copper ion on an equimolar basis.
Therefore, the moles of EDTA added are equal to the moles of Cu++ remain-
ing in solution. As mentioned previously, the moles of H2S evolved from
the sample are equal to the moles of copper ion consumed in the ab-
sorption column. The moles of copper consumed are equal to the number
of moles of copper ion initially present less the number of moles present
after the evolution of l^S as determined by the EDTA titration. Once
the moles of H^S are determined, the sulfide content of the sample can
be established.
-63-
-------
APPENDIX B
DETERMINATION OF DESULFURIZATION OFFGAS COMPOSITION
The second phase of experimentation necessitated the development of an
analytical method to determine the amounts of l^S and S02 evolved
during desulfurization. The accumulation of these gases in solution was
done during experimentation using the apparatus shown in Figure 1
(Section V). A measured amount of CuSO^ solution was placed in an
absorption column and the gases exiting the desulfurization experiment
were passed through the column. The resulting CuS04 solution was then
analyzed for S02 and H2S using the following procedure.
The experimental procedure consisted of analyzing the CuS04 solution
initially for S02 since solutions of S02 in water (or in CuS04 solution)
are unstable and release gaseous S02 if exposed to the air. Nitrogen
is bubbled through the CuS04 solution to carry the S02 into another
solution (described below) where it can be directly determined by
titration. The analytical apparatus is shown in Figure 1. The
solution is gently heated for 45 minutes while the nitrogen purge
carries the volatile S02 into the indicating solution. The indicating
solution is made by adding the following to 100 ml of distilled water:
two ml concentrated HC1, one gram of soluble starch (A.C.S. purity), and
five grams of potassium iodide (A.C.S. purity). A drop of the potassium
iodate titrant will change the solution color from clear to blue due
to the formation of a starch-iodine complex. As S02 dissolves in the
solution, it is oxidized to 804 by the iodine, thereby, destroying the
starch-iodine complex and blue color. Addition of more titrant restores
the blue color. When a persistant blue color is obtained, the end
point in the titration has been reached and S02 is not issuing from
the CuS04 solution,, The stoichiometric reactions involved in the
titration are as follows:
KK>3 + 5KI + 6HC1 • *• 312 + 6KC1 + 3H20
S02 + 12 + 2H20 • »• H2SQ4 + 2HI
The first reaction produces iodine from the potassium iodate titrant.
The second reaction depicts the oxidation of S02 to I^SO^ by iodine and
results in'the destruction of the blue color (from the starch-iodine
complex) until more potassium iodate is added. It can be seen from the
above stoichiometry that three moles of KIO-j titrant added to the solution
are equivalent to each mole of S02 that dissolves in the solution.
Therefore, the amount of S02 evolved from the CuSO^ solution is directly
related to the number of moles of KIO^ consumed in the titration.
Approximately 50 percent of the S02 that was evolved from the coal
refuse pellet was determined by the above method. The remaining S02 is
dissolved in the CuS04 solution and will not escape if the solution is
filtered or exposed to the air. The next step in the analysis is to
filter the insoluble CuS (formed by the reaction of H2S with CuS04) from
the CuS04 solution. The clear filtrate is then added to a 500 ml
-65-
-------
Thermometer
Nitrogen_,
Purge
\
*-Flask Containing
Solution
Heating Mantle
Burette Containing
KIOo
Indicating Solution
FIGURE 1 -EXPERIMENTAL EQUIPMENT USED IN SC>2 DETERMINATION
-66-
-------
volumetric flask and diluted to volume. A 50 ml aliquot is extracted
with a pipette and titrated with a standard solution of KMnO^ to
determine the amount of dissolved SC>2. The reaction involved in this
oxidation titration is as follows:
2KMn04 + 5S02 + 6HC1 + 2H24 will be completely reduced to
MnCl2 if the reaction occurs in a solution with a pH of 1.0 or lower.
Therefore, four to five ml of concentrated HC1 were added to the 50 ml
aliquot to lower the pH to this level. From the stoichiometry of the
above reaction, it is seen that five moles of S02 react with two moles
of KMnC>4 titrant. From this ratio and the number of moles of KMn04
required to complete the titration (an end point is achieved when the
solution remains pink for 30 seconds as KMnC>4 sets as its own indicator)
the amount of dissolved S02 can be determined.
The total amount of S02 that results from the desulfurization reaction
is then the sum of the S02 determined by the above procedures. The H2S
is determined in the same manner described in the determination of
sulfide sulfur (i.e. an aliquot of the filtered CuS04 solution is
titrated with EDTA)„
The amount of elemental sulfur contained in the desulfurization offgas
was obtained from a material balance. Sulfur that could not be accounted
for as S02, H2S, or residual unreacted sulfides in the pellet was assumed
to be in an elemental form.
-67-
-------
1
Access/on Number
W
5
2
Subject Field & Group
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Urbanization
Blar-k. Rn-iraiia *, -R-rvnon. Inc.. ADDlied Tecnnolosrv TK vis-inn
135 Delta Drive, Pittsburgh, PA
Title
STUDY OF SULFUR RECOVERY FROM COAL REFUSE
1 r\ Authors)
IU LaRosa, Paul J.
Michaels, H. James
16
21
Project Designation
14010 FYY 09/71
Note
22
Citation
Environmental Protection Agency
Washington, D. C.
23
Descriptors (Starred First)
Acid Mine Water*, Coal Mine Waste*, Sulfur Recovery*,,
Pollution Abatement*, Ash Pellets*
25
Identifiers (Starred First)
Coal Refuse*, Sulfur Recovery*
27
Abstract
During coal preparation, a coal refuse of no commercial value is produced
and discarded in piles. As rainfall percolates through the piles, acid waters
are formeda
A feasibility study has been performed on a process producing sulfur from
such coal refuse. In this process, limestone and coal refuse are ground,
pelletized, and preheated before entering a desulfurizing shaft reactor where
a hard, fired ash pellet and an H2S-S02 bearing offgas are produced. After
sulfur, tar, and other gases are removed, the resulting H2S-S02 gas proceeds to
a conventional sulfur recovery plant.
Experimental results and economics of this study indicate that the process
is a profitable means of minimizing coal refuse pile water pollution. For a
sulfur and ash pellet selling price of $20 and $1.50/ton respectively, it is
estimated that a coal producer will have a before tax return on investment up
to 53 percent for a one MM ton/year plant utilizing an eight percent sulfur
refuse.
This report was submitted in fulfillment of Project Number 14010 FYY,
Contract 14-12-929 under the sponsorship of the Water Quality Office,
Environmental Protection Agency. (LaRosa--Blacks Sivalls & Bryson)
Abstractor
P.
J.
LaRosa
Institution
Black,
Sival
Is t
* Brysons
Applied
Technology
Division,
Pgh.,PA
SEND. WITH COPY OF DOCUMENT TO: WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 20240
* GPO! 1970-389-930
------- |