FINAL REPORT
CHEMICAL REMOVAL
OF NITROGEN AND ORGANIC SULFUR FROM COAL
CONTRACT NO. EHSD 71-7
17270-6007-R-OO 14 MAY 1971
PREPARED FOR
THE AIR POLLUTION CONTROL OFFICE
DURHAM, NORTH CAROLINA 27701
TRW
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FINAL REPORT
CHEMICAL REMOVAL
OF NITROGEN AND ORGANIC SULFUR FROM COAL
CONTRACT NO. EHSD 71-7
17270-6007 -R-OO
14 MAY 1971
PREPARED FOR
THE AIR POLLUTION CONTROL OFFICE
DURHAM, NORTH CAROLINA 27701
TRW
SYSTEMS GROUP
UNITED STATES
GOVERNMENT
PROPERTY
This book Is the property ot
the United States Government
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17270-6007-RO-OO
FINAL REPORT
CHEMICAL REMOVAL OF NITROGEN AND ORGANIC SULFUR FROM COAL
Contract No. EHSD 71-7
by
R. A. Meyers, J. S. Land, and C. A. Flegal
Approved by:
Efti~
Applied Chemistry Department
TRW
SYSTEMS a"au"
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10669-6007-RO-00
FO REWO RD
This document constitutes the final report for the work accomplished
between 27 August 1970 and 14 May 1971 by TRW Systems for the Air Pollution
Control Office of the Environmental Protection Agency, under Contract EHSD
71-7 for "Chemica1 Removal of Nitrogen and Organic Sulfur from Coa1".
This work was conducted under the technical direction of Messrs Lawrence.
Cramer and T. Kelly Janes of the Air Pollution Control Office, Durham, North
Carolina.
The project was performed within the Applied Chemistry and Chemical
Engineering Departments of the Chemistry and Chemical Engineering Laboratory,
Applied Technology Division. Dr. E. A. Burns, Manager of the Applied Chemistry
Department, provided line supervision and Dr. R. A. Meyers was Program Manager.
Major technical contributions were provided by Drs. J. W. Hamersma, J. L.
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CONTENTS
Page
l.
2.
INTRODUCTION AND SUMMARY...................................
1
ORGANIC SULFUR REMOVAL..................................... 7
2.1 Chemistry of Organic Sulfur Removal.................. 7
2.2 Screening of Extraction Solvents ... .................. 9
2.3 Selection of Extraction Solvents for Detailed
Characterization ..................................... 18
2.4 Preliminary Engineering and Economic Analysis
of a Baseline Processing Scheme for Organic
Sul fur Removal....................................... 18
2.5 Detail ed Characteri zati on ............................ 27
2.5.1 Studies for Increase in Sulfur Removal....... 28
2.5.2 Assessment of Critical Engineering
2.5.3
Parameters. ........... ..... ...... .. .. .. .. .. .. 31
Characterization of Extracted Coals ... .... ... 36
3.
REMOV AL OF NITROGEN........................................ 39
3.1 Need for Removal of Nitrogen......................... 39
3.2 Chemistry of Nitrogen Removal........................ 40
3.3 Experimental Results ............. .................... 41
3.4 Engineering Analysis of a Process Scheme ............. 44
4.
5.
CONCLUSIONS AND RECOMMENDATIONS............................ 49
NEW TE CHNOLOGY ............................................. 51
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11.
12.
13.
14.
15.
16.
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1 7270-6007-RO-OO
Tl\BLES
l.
2.
3.
4.
5.
6.
7.
8.
9.
Typical Analyses of Coals Used for Screening Studies .........
,Particle Size Analysis.......................................
Solvent Screening - Organic Acids............................
Solvent Screening - Organic Bases and Neutral Solvents .......
Solvent Screening - Inorganic Acids and Bases .......... ......
Sulfur Forms in Extracted Indiana No. V Coals ................
Example Case for Organic Sulfur Removal Process ....... .......
Single and Double Pass Extractions with p-Cresol .............
Effect of Retention Time on Removal of Sulfur With
p-Cresol [[[
Extraction of -14 Mesh Lower Freeport Coal...................
Effect of Particle Size on Extraction of Sulfur From
Pittsburgh Coal..............................................
Effect of Multiple Recycles of Nitrobenzene Solvent on
Sul fur Removal From Pi ttsburgh Coa 1 ..........................
Effect of Solvent Treatment on Heat Content of Coals .........
Ultimate and Proximate Analyses of Extracted Coals. ..........
Initial Nitrogen Removal Screening Tests .....................
Nitrogen Removal Screening Tests.............................
Example Case for Organic Nitrogen Removal Process ............
Page
10
11
13
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10.
17270-6007-RO-OO
ILLUSTRATIONS
1.
Photomicrographs (400X, Polarized Light) of Pulverized
Coa 1 Samples[[[
2.
3.
4.
Process Diagram for Organic Sulfur Removal From Coal...........
Leaching Section Costs VS Residence Time.......................
Separation Train Feed and Operating Costs as a Function
of Allowable Sulfur Compound Buildup in the Leaching
Sol v en t [[[
5.
Capital Cost for Separation Train as a Function of Feed
Rate to Distillation Units .... .................. .~.... .........
6.
Dryer Operating Cost as a Function of Solvent Retained
on Coal After Filtering ........................................ 23
7.
Dryer Capital Cost as a Function of Solvent Retained on
Coal After Filtering ...........................................
8.
Solvent Makeup Cost VS % of Solvent Recovered From Coal
i n the 0 rye r [[[
9.
Processing Scheme for Removal of Nitrogen Compounds by
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1.
INTRODUCTION AND SUMMARY
Approximately 30-mi11ior. tons of sulfur oxides are emitted into the
atmosphere yearly in the United States, and of this total, 60% of the oxides
of sulfur come from coal-fired power plants. It is projected that sulfur
oxide emissions will increase (in the absence of control processes) to
42-mi11ion tons in 1980 and 60-mi11ion tons in 1990, as more fossi1-fue1-
fired power plants are constructed to meet this country's power needs.
Combustion of the sulfur content of coal (which is on the average present
in equal portions as iron pyrite, FeS2' and organic sulfur compounds) gives
rise to the sulfur oxide emissions. Presently, about 4-mi11ion tons of
NOx are emitted annually from coal-fired power plants. It is generally
agreed that organic nitrogen compounds contained within the coal matrix are
a significant contributor to the oxides of nitrogen emission problem in
coal-burning power plants.
Recently, TRW has discovered and demonstrated the principle of the
basic feasibility of some exciting proprietary low-cost techniques for
extracting organic sulfur and nitrogen compounds from coal without signifi-
cantly altering the remainder of the coal matrix. Preliminary engineering
cost analyses of the processes indicate that organic sulfur and nitrogen could
be removed separately at a cost on the order of $1-1.50/ton of coal each (without
byproduct credit). Further, in combining the processes, total costs could
well be in the $1-1.50/ton range for organic sulfur and nitrogen removal.
Under the present APCO contract, entitled IIChemica1 Removal of Nitrogen and
Organic Sulfur from Coa111, it has been demonstrated that up to 80% of the
organic sulfur may be removed from coal, and it is indicated that significant
quantities of nitrogen may be removed by simple liquid extraction. Heretofore,
the organic sulfur and nitrogen compounds were thought to be unremovah1e.
The invention and laboratory investigation at TRW of processes for
removing pollutants from coal was prompted by an assessment of the current
technology for control of sulfur oxides, which indicated that a new approach
to improve the economics of commission control was needed.
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1 7270-6007-RO-00
Sulfur oxide emissions from coal-fired power plants may be controlled,
in theory, by one or more of the following methods:
.
Burn low sulfur coal
.
Remove sulfur oxides from flue gas, or
.
Desulfurize coal.
The first of these options, that of burning low sulfur coal, is capable
of solving only a small part of the problem, as low sulfur coal reserves
are not available to utilities in sufficient quantities. Although considera-
ble low sulfur (and low heat content) reserves are available in the western
United States, it is generally agreed that transportation costs (ranging up
to $20/ton) for utilizing these reserves in the east are excessive.
The second option, that of removing sulfur oxides from flue gas, is
being actively investigated by the Environmental Protection Agency and the
Tennessee Valley Authority. The results obtained thus far show that many
problems remain to be resolved before these processes will find wide appli-
cability for the control of sulfur oxide pollution. These include a) neces-
sary major modifications to existing power plants, b) disposal of waste
sorbents and c) high capital costs of the large size of reactors necessa~y
for treating the dilute sulfur oxide gas streams.
The third method, that of desulfurizing coal, is under active investi-
gation by the Environmental Protection Agency, as well as the Office of
Coal Research. Three approaches are under consideration, a) deep cleaning
of coal, b) solvent refining, and c) coal gasification. Deep cleaning (a
physical separation), because of its low selectivity, results in a signifi-
cant coal loss such that only 10-20 million tons annually out of a total
United States production of 350-million tons can be economically deep cleaned. .
Solvent refining, a high pressure hydrogenative desulfurization and dissolu-
tion, suffers from the high capital costs of high pressure reactors and
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l7270-6007-RO-00
catalyst fouling, as well as the necessity to consume hydrogen. In addition,
the product (at present) still contains a significant organic sulfur content.
Coal gasification (coal hydrogenative pyrolysis, under pressure, to form
gaseous and liquid products and coke) is limited by high capital costs, as
is the case for solvent refining. In addition, the coke or char, which often
constitutes more than 50% of the original coal, contains about the same
sulfur content as the starting coal. Hence, the char is not, at present,
a low sulfur fuel source.
The TRW program, supported by APCO, for Chemical Removal of Nitrogen
and Organic Sulfur from Coal was organized into three tasks, a) screening of
solvents for removal of organic sulfur, b) screening of solvents for removal
of nitrogen, and c) more detailed characterization of the extraction process
and extracted coal. In addition, TRW supplemented (under its own funding)
the effort for preliminary engineering analyses of processes based on the
data obtained during the program, in order to direct the detailed characteri-
zation studies toward the evaluation of economically sensitive parameters.
The screening studies for the removal of organic sulfur were performed
on four coals varying in total sulfur content from 1-5% and organic sulfur
content from 0.6 to 2%. It was found (as determined by analyses of the
coal sulfur before and after extraction) that organic sulfur could be ex-
tracted from the four coals. The solvents which were effective for removal
of organic sulfur fall into two classes, a) weak organic acids, e.g. phenol,
o-chlorophenol, cresol and nitrobenzene, and b) caustic solutions, e.g.
aqueous or alcoholic sodium or potassium hydroxide. A summary of the
screening results for these promising extraction solvents (in which the amount
of removal is not optimized) is shown below.
Solvent
% Organic Sulfur Removed
Weak Organic Acids
Phenols
Nitrobenzene
Caustic Solutions
17-48
10-32
0-28
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The caustic solutions were effective on only two of the four coals which
were investigated, while the weak organic acids were effective on all of
the coals.
The screening studies for nitrogen removal indicate that weak organic
acids and strong aqueous inorganic acids remove nitrogen from coal (accord-
ing to analyses of the coals before and after extraction by the Dumas ana-
lyti cal procedure)' as shown below.
Solvent
% Nitrogen Removal
p-cresol
20% HCl
Conc. Hl04
39
32
27
The use of weak organic acids, e.g. p-cresol, to remove nitrogen is particu-
larly attractive as nitrogen could then be removed essentially at no cost
along with organic sulfur. However, analysis of the coal samples by the
standard ASTM Kjeldahl analytical method for nitrogen indicated that no
nitrogen had been removed from the coal samples. An assessment of these
two analytical methods indicates that it is likely that nitrogen is being
. removed from the coal samples, and that these nitrogen compounds are probably
coal-tar-base type materials which are not readily sensed by the standard
Kjeldahl determination. Because of the importance of control of nitrogen
oxides from fossil fuel combustion, it appears that the problem of nitrogen
analysis of coal requires investigation.
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The weak organic acid class of solvents was selected for detailed
characterization of parameters for organic sulfur removal. In particular,
the two representatives, p-cresol and nitrobenzene were chosen for their
effectiveness, low cost, availability in large quantity, and resistance
to attrition" through oxidation or reaction. An engineering analysis indica-
ted that organic sulfur could be removed from coal at an incremental cost of
approximately $l/ton, and that important cost factors in the process were
the following, a) coal particle size, b) retention volume of solvent, and
c) ability to recycle solvent allowing a sulfur compound buildup in the
liquid. The detailed characterization studies were then directed toward
obtaining data for these parameters and also toward increasing the amount
of organic sulfur removed by the solvents, as well as characterizing the
extracted coal.
It was found that "the amount of organic sulfur removed could be in-
creased by the use of better solvent filtration from the coal and also by
double extractions. These results are summarized below for Pittsburgh coal.
% Organic Sulfur
Solvent No. of Extractions Removed
Nitrobenzene 1 X 77
p-Cresol 1 X 63
p-Cresol 2 X 80
It was found that coal particle
be utilized in the process, and
recycled at least three times.
size up to 1/2-inch (Pittsburgh seam) could
that the solvent (nitrobenzene) could be
These results were highly encouraging, as they indicate positive
results for all of the process parameters which were investigated. However,
a number of important factors remain to be investigated for accurate
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1 7270-6007-RO-OO
assessment of the potential of the method for cQal desulfurization. These
factors includes a) recovery and identification of extracted organic sulfur
compoundss b) regeneration of solvent by distillations c) overall solvent
lossess and d) maximum allowable sulfur compound buildup in the extraction
solvent.
The detailed results of the programs conclusions and recommendations
based on the resultss descriptions of new technology developed in the pro-
jects and TRW supplemental engineering analyses are presented in the five
sections to follow: Organic Sulfur Removals Removal of Nitrogens Conclu-
sions and Recommendationss New Technology and Appendix.
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2.
ORGANIC SULFUR REMOVAL
This section provides the results of the laboratory effort for removal
of organic sulfur from coal. The objectives were to, a) screen candidate
liquids for ability to remove organic sulfur from coal, and b) characterize
the process and extracted coals in more detail. In order to accomplish a
more effective laboratory investigation, TRW performed a supplemental pro-
cess engineering analysis prior to engaging in the process characterization
phase of the program.
Discussions of, a) the theory associated with chemical removal of sulfur,
b) the results of extraction solvent screening studies, c) an engineering
analysis of a proposed process based on the experimental results, and d) the
results of detailed extraction studies, are presented in the following
sections.
2. 1
CHEMISTRY OF ORGANIC SULFUR REMOVAL
For a number of years, chemists have studied the ability of solvents
to extract compounds from coal. Hundreds of solvents have been investigated
as to their dissolving power for coal (but not for removal of organic sulfur),
these include mostly organics such as pyridine, tetra1in, phenanthrene, anthra-
cene oil and so on. Generally, the "solvent extractions" were run at elevated
temperatures (400-800°F) , extended reaction times, and very often under hydro-
gen pressure (400-5000 psi) which generally resulted in a total dissolution
of the coal matrix. In the course of these processes, decomposition
occured due to the high temperatures and in some cases due to hydrogenation
of some of the bonds tying the coal polymer matrix together. In each case
it was found that a nonselective "dissolution" of coal had taken place in
which the main portion of coal which was extracted was actually present in
the form of very fine particles, so that the composition of the extract was
almost the same as the composition of the original coal.
The TRW approach toa process for removing organic sulfur from coal is
based on low temperature, atmospheric pressure, acid catalyzed, depo1ymeri-
zation and subsequent dissolution of organic sulfur compounds from
the coal matrix. The aim of this approach is to break down the coal
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polymer matrix selectively at the points occupied by organic sulfur compounds
without disturbing the bonding present in the major part of the coal material.
Although the structure of coal has been the subject of numerous investigations,
no truly concrete structural analysis is available at the present time. It
is thought from pyrolysis studies that organic nitrogen heterocycles and
amines, as well as sulfides, disulfides and sulfur heterocycles exist in
coal. It may be postulated that the nitrogen and sulfur compounds occur in
coal largely in the forms as stated above and that a part of the hydrocarbon
structure of bituminous coal exists in a partially saturated or unaromatized
form. Equation 1 shows a possible mechanism for the selective extraction
involving an acid catalyzed decomposition of the coal matrix at the positions
occupied by sulfur compounds, with aromatization shown as a possible driving
force. Consequently, it is postulated that solvents which would promote the
seler.tive dp~f"\'\,I"prization and dissolution of organic sulfur c0moounds would
be acidic In nature. It may be possible to utilize these acidic liquids in
small amounts in a carrier solvent such as kerosene or gas oil.
H+
&:
-:::.
~
~S'H
V
COAL
MAT RIX
COAL
MATRIX
+
+ H+(l)
~lternatively, the organic sulfur compounds may be considered as bonded
by strong hydrogen bonds to the coal matrix. Here again, acidic liquids or
those with high polarity would tend to extract the sulfur compounds away from
the coal matrix provided the liquids could effectively penetrate the organic
structure of the coal.
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2.2 SCREENING OF EXTRACTION SOLVENTS
Five classes of solvents have been assessed for their ability to remove
sulfur from coal. These are: 1) organic acids, 2) organic bases, 3) neutral
organic solvents, 4) aqueous inorganic acids, and 5) aqueous caustic solu-
tions. Since the initiation of TRW's studies in this area (including both
the present contract and previous company funded efforts), a significant
amount of data has been generated utilizing the above classes of solvents on
four coals: Illinois No.6, Indiana No. V, Pittsburgh and Bevier. The analy-
ses of those coals are shown in Table 1, together with an additional coal
(Lower Freeport) the extraction of which will be described in Section 2.6.
The coals vary in total sulfur content from 1% to nearly 5%, and in organic
sulfur content from less than 1% to near 2%. Three of the coals utilized
in the screening studies were pulverized in a hammer mill to pass 200 mesh,
giving average particle sizes of 1.5 - 2.0 microns, which was comparable in
size to the Illinois No.6 coal, received as pulverized for use in an
Illinois power plant (see Table 2, and Figure 1 ).
Table 3 presents the data obtained for extraction of the four coals
with organic acids (see Appendix A for experimental method). The extraction
durations were selected for sequential screening and should not be taken
as indicative of required contact times. Extraction temperatures normally
were chosen as the boiling point of the solvent. Extractions 1-12 show that
phenols, a class of weak Bronsted acids (k 10-9 to 10-10), are effective in
a
removing a significant portion of organic sulfur from the four coals, in
extraction batch times as low as one hour. Interestingly, o-ch10ropheno1
was ineffective on Illinois No.6 but was highly effective on the Pittsburgh.
This may indicate the dependence of the process on the chemical and/or
physical structure of coal. More effective solvents for Illinois No.6 are
p-creso1 or nitrobenzene (10 and 18.). These may probably be improved in su1-
. fur removal ability on Illinois -No.6 by adjustment of extraction temperature,
retention time, and as we shall show later, filtration techniques. Experi-
ments 13 and 14 show that dilution of o-ch1oropheno1 with methanol or concen-
trated HC1 destroys the sulfur removal properties of that solvent. Nitroben-
zene (15-18), a weak Lewis acid, is also effective for the removal of sulfur.
Conversely, the strong organic acids, acetic (ka10-5) and formic (ka10-4)
acid, do not appear to remove sulfur.
A number of organic bases and neutral organics were evaluated (Table 4)
and found to be totally ineffective for the removal of sulfur.
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TABLE 1.
TYPICAL ANAlYSES OF COAlS USED FOR SCREENING STUDIES
Coal Fixed Sulfur* Weight. %
Ash Volatile Carbon ~
Bed Locati on % % % Btu/1b Total Organic C H N C1 0
No.6 Randolf,I11. 12.84 40.05 47.11 12271 3.35 1. 81 68.62 4.73 1.23 0.10 9.1.:1
No. V Hawthorne, Ind. 9.71 40.13 50.16 13064 3.28 1. 57 72.52 4.92 1.20 0.04 8.33
Pittsburgh Bruceton, Pa. 4.76 38.65 56.59 14442 1.23 0.61 80.84 5.37 1.35 0.06 6.39
Bevier Macon, Mo. 8.74 42.01 49.25 12710 4.75 1.95 70.57 5.05 1.17 0.02 9.70
Lower Freeport Lecontes Mills 14.36 24.72 60.92 13095 4.13 0.43 73.05 4.16 1.23 0.15 2.92
*These are typical values which varied from sample to sample.
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TABLE 2.
*
PARTICLE SIZE ANALYSIS
Coal Type Average Particle Size (Microns)
Pittsburgh 2.0
Illinois No.6 4.1
Bevi er 1.5
Indi ana No. V 2.0
*Average particle sizes were determined utilizing
Fischer Sub-Sieve Analyzer
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FIGURE 1. PHOTOMICROGRAPHS (400X, POLARIZED LIGHT)
OF PULVERIZED COAL SAMPLES*
Illinois No. 6 Coal
Indiana No. V Coal
Pittsburgh Coal
Bevier Coal
*Least division = 3 microns
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TABLE 3.
SOLVENT SCREENING - ORGANIC ACIDS
I
......
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I
Extraction Extraction % % SUlfurc,d
a b Temperature Duration Sulfur After % Organi c Sul fur.
Solvent' °C hrs Coal Extraction Removed Removed
1. o-chlorophenol 175 1 Pittsburgh 0.83 23 46
2. o-chlorophenol 175 3 Pittsburgh 0.82 24 48
3. o-chloropheno1 175 3 Indi ana No. V 3.00 14 28
4. o-chlorophenol 175 3 Bevier 3.28 15 30
5. o-chlorophenol 175 1 III i no i s No.6 3.20 0 0
. 6. o-chlorophenol _175 0.5 III i noi s No.6 3.15 0 0
7. phenol 180 24 Bevier 3.17 18 29
8. phenol 180 3 Indiana No. V 3.12 10 20
9. p-cresol 200 3 Indiana No. V 2.76 21 47
10. p-cresol 200 1 Illinois No.6 2.86 .10 16
11. p-cresol 200 1 Bevi er 3.42 11 17
12. p-cresol 200 1 Pittsburgh 0.88 19 38
13. 1:1 methanol:o-chlorophenol 65 3 Indiana No. V 3.53 0 0
14. 1:1 conc. HC1:0-chlorophenol 100 3 Indiana No. V 3.45 0 0
15. nitrobenzene 210 3 Indiana No. V 3.07 10 20
16. nitrobenzene 210 1 Bevier 3.36 13 20
17. nitrobenzene 210 1 Pi ttsburgh 0.91 16 32
18. nitrobenzene 210 1 III inois No.6 2.97 6 10
19. acetic acid 20 24 Bevier 3.82 0 0
20. formic acid 100 3 Indiana No. V 3.50 0 0
......
.......
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a
a
.......
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a
a
aWeight ratio of solvent to coal was 5:1
bRuns 1, 2, 7 and 19 were washed with hot water to remove adsorbed solvent, all others were washed with cold methanol.
All were dried in oven to constant weight.
cSulfur content was determined by the standard ASTM Eshka procedure for all samples ~xcept nos.: 2, 4, 7 and 19, which
were analyzed by the Leco method.
dCalculations based on coal composition = organic sulfur content of 0.54% w/w and total sulfur content of 1.08% w/w
for Pittsburgh coal. organic sulfur content of 1.73% w/w and total sulfur content of 3.48% w/w for Indiana No. V
coal. organic sulfur content of 1.95% w/w and total sulfur content of 3.86% w/w for Bevier coal. organic sulfur
content of 1.81% w/w and 3.17% w/w for Illinois No.6 coal; all on dry basis, dssuming sulfur removed is entirely
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TABLE 4.
SOLVENT SCREENING - ORGANIC BASES AND NEUTRAL SOLVENTS
Extraction Extracti on % % Sul furd
Solventa,b Temperature Durati on Sulfur After
°C hrs Coal Extraction Removed
1. pyri di ne 115 1 Pittsburgh 1.04 0
2. pyri di ne 115 24 Pittsburgh 1.07 0
3. pyridine 115 24 Bevier 3.90 0
4. pyri di ne 115 3 Indiana No. V 3.44 0
5. aniline 180 3 Indiana No. V 3.51 0
6. methanol 65 3 Indiana No. V 3.45 0
7. l-methyl-2- 200 3 Indiana No. V 3.44 0
pyrroli done
8. dimethyl formami de 150 3 Indi ana No. V 3.45 0
9. triethylphosphite 240 1 Illinois No.6 3.13 0
10. xylene 140 1 Illinois No.6 3.20 0
aWeight ratio of solvent to coal was 5:1
bRuns 1-3 washed with hot water to remove adsorbed solvent, all others washed
with cold methanol. All were dried in oven to Gonstant weight.
d As i n Tab 1 e 3.
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A variety of inorganic acids and bases were assessed for sulfur removal
(Table 5). Caustic did not remove sulfur from the Bevier or Pittsburgh
coals, but did remove sulfur from Indiana No. V (1-5). It would thus appear
that the Indiana No. V and Illinois No.6 coals may contain free organic
thiols which are readily neutralized and dissolved by the caustic, while
the Bevier and Pittsburgh coals contain none of these compounds. Concentra-
ted hydrochloric acid removes sulfur from Indiana No. V, but not from Bevier
(6-8). In the case of hydrochloric acid, it is possible that some of the
sulfur removed is sulfate which was obtained by air oxidation of pyrites
during the course of the extraction; however, it is also likely that the
strong acid cleaves organic sulfides or disulfides from the coal matrix and
dissolves them. Hydrobromic acid removes sulfur from Indiana No. V, Illinois
No.6 and Bevier coals (9-11), where again, a part of the sulfur removed may
be inorganic in nature. Phosphoric acid appears to be ineffective for the
removal of sulfur (12-1-4), except on long extraction time as in the case of
the Illinois No.6 coal (15). Hexafluorophosphoric acid removes sulfur (16)
while sulfuric acid (17) dramatically increases the sulfur content by sul-
fonating the activated aromatic rings present in the coal matrix. Nitric
acid, in varying concentrations, was evaluated for its ability to remove
sulfur from the Bevier coal (18-22). As is well known, nitric acid removes
all of the inorganic sulfur and in addition, on prolonged contact time, a
small, portion of the organic sulfur is also removed (18 and 19). Of course,
at the same time the coal is oxidized and nitrated, hence the small amount
of organic sulfur removal is not worth the price in consumption of nitric
acid and modification of the coal matrix.
The type of sulfur which was removed by the two major candidate solvent
types, weak organic acids and caustic from Indiana No. V coal, was determined
by analysis of the total and inorganic sulfur content, before and after leach-
ing of the coal (Table 6). For both solvents only organic sulfur is removed,
as the pyrite content remained constant, while the sulfur content dropped
appreciably.
The utilization of a continuous flow through method for the extraction
of coal was briefly investigated using a TRW apparatus for continuously moni-
toring effluent sulfur content. However, it was found that coal particles
tended to restrict solvent flow at extraction temperatures and this approach
was discontinued (see Appendix for details).
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TABLE 5.
SOLVENT SCREENING - INORGANIC ACIDS AND BASES
I
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~
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Extraction Extracti on % % Sulfurc,d
SOlventa,b Temperature Duration Sulfur After % Organic Sulfur
or. ~rc:: Coal Extraction Removed Removed
1. 20% Sodium Hydroxide 110 1 Bevier 3.82 0 0
2. 20% Sodium Hydroxide 110 1 Pittsburgh 1. 10 0 0
3. 20% Sodium Hydroxide 115 3 Indiana No. V 2.95 15 30
4. 20% Methano1ic Potassium 70 3 Indi ana No. V 3.01 14 28
Hydroxi de
5. 20% Sodium Hydroxide 110 1 Illinois No.6 2.98 6 10
6. 36% Hydrochloric Acid 120 24 Bevier 3.87 0 0
7. 36% Hydrochloric Acid 120 24 Indiana No. V 3.13 10 (20)
8. 36% Hydrochloric Acid 120 24 Indi ana No. V 3.20 8 (16)
9. 48% Hydrobromic Acid 110 24 Bevier 3.44 11 (22)
10. 48% Hydrobromic Acid 110 3 Indi ana No. V 2.72 22 (8)
11. 48% Hydrobromic Acid 110 1 III i noi s No.6 3.03 4 (10)
12. 85% Phosphoric Acid 150 20 Bevier 3.90 0 0
] 3. 85% Phosphoric Acid 150 3 Indiana No. V 3.45 0 0
14. 85% Phosphoric Acid 150 1 Illinois No.6 3.21 0 0
15. 85% Phosphoric Acid 150 16 Illinois No.6 2.92 8 (13)
16. 75% Hexafluorophosphoric Acid 20 24 Bevier 3.47 10 (20)
17. 98% Sulfuric Acid 20 1 Bevier 5.21 +35 -
18. 70% Nitric Acid 20 24 Bevier 1. 43 63 (13)
19. 50% Nitric Acid 20 24 Bevier 1. 47 62 (12)
20. 50% Nitric Acid 20 0.2 Bevier 1. 73 55 (5)
21. 50% Nitric Acid 0 0.2 Bevier 1. 74 55 (5)
22. 10% Nitric Acid 100 0.5 Bevier 1. 93 50 -
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aWeight ratio of solvent to coal was 5;,1 ,
bAll runs were washed with water"neutralized, again washed with water and dried in oven to constant weight
cSu1fur content was determined by leco procedure ,fo~ all samples, except nos. 2-5, 7, 8, 10, 11, 13-15
d(As in Table 3). Parentheses indicate that all ~r part of the sulfur removed may have been inorganic rather
than organic. For nitric acid, the values in parentheses are the sulfur content removed in excess of the
-------
TABLE 6. SULFUR FORMS IN EXTRACTED INDIANA NO. V COALSa
Extracted Sample % Pyrite S % Pyrite S % nrganic Sb % Organi c Sb % Change in % Change in
Before After Before After Organi c S Pyrite S
I
-'"
"'-J 1. p-cresol-3 hr-2000C 1. 73 1. 78 1. 75 1.00 -42 +2
. I
2. 20% NaOH-3 hr-115°C 1. 73 1.82 1. 75 1. 19 -31 +6
aSu1fate content is negligible
bOrganic sulfur by difference
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1 7270-6007-RO-00
2.3 SELECTION OF EXTRACTION SOLVENTS FOR DETAILED CHARACTERIZATION
Nitrobenzene, phenols (o-chlorophenol, p-cresol and phenol), and caustic
solutions all were found to remove significant quantities of organic sulfur,
in short extraction times, as a result of the screening tests. Of these sol-
vents, the phenols and nitrobenzene are more attractive candidates for the
development of a process for the removal of organic sulfur from coal, as
these liquids removed sulfur from all four coals which were investigated,
while the caustic solutions removed sulfur from only two coals. p-Cresol
appears to be the most attractive representative from among the phenol deri-
vatives because of its effectiveness, moderate cost, availability in large
quantities and thermal stability.
2.4 PRELIMINARY ENGINEERING AND ECONOMIC ANALYSIS OF A BASELINE
PROCESSING SCHEME FOR ORGANIC SULFUR REMOVAL
In parallel with the current APCO Contract, TRW has funded a preliminary
engineering and economic analysis of the new chemical extraction technique
for selective removal of organic sulfur compounds from coal. This initial
work was aimed at:
.
Defining a baseline processing scheme which utilizes
standard unit operations and conventional hardware.
Determining those aspects of the baseline scheme which
will have the greatest impact on the processing costs.
.
.
Defining key laboratory experiments in the cost sensi-
tive areas which will help to determine the processing
economics better and point the direction to minimize
processing costs.
Figure 2 presents a baseline processing scheme for extraction of the
organic sulfur compounds from coal with p-cresol, although the treatment
applies also to nitrobenzene (with minor modification, i.e. slight boiling'
point difference, lower cost). For purposes of calculation, 100 tons per
hour of organic sulfur compounds (containing 1.2 tons/hour of sulfur)
-------
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1.0
I
P-CRESOL RECYCLE
P-CRESOL
LIGHT
ORGANIC
SULFUR
P-CRESOL
ORGANIC
SULFUR
H EA VY
ORGANIC
SULFUR
P-CRESOL
MAKEUP
P-CRESOL
r--------l
COAL COAL DRYER PYRITE I . .......
LEACHER I REMOVAL I COAL -....J
PREPARATION N
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17270-6007-RO-00
extracted from the coal. The baseline process can be divided into four
primary sections or operations as follows:
, LeaChing
, Filtering
, Coal drying and solvent recovery
, Distillation
In the leaching section makeup plus recycled cresol solvent is con-
tracted with pulverized coal in 20,000 gallon stainless-steel stirred
reactors (three to five units if retention time 0.5 hr and 5 tons solvent/ton
coal) which are insulated and heated to 200°C. The leaching solution chemically
separates the organic sulfur compounds from the coal matrix and dissolves them
in the creso.1 solvent. Rotary vacuum disk filters are used to separate the
bulk of the cresol solution from the coal. The cresol solution is recycled
to the 1eacher, while the wet coal filter cake is transferred to a drying and
solvent recovery unit.
Continuous, indirect heated, rotary, steam tube dryers are used to
evaporate off the residual cresol which remains with the coal after filter-
ing. Laboratory experimentation has shown that depending on particle size
the coal entering the dryer may contain from 0.1 to 1.0 tons of absorbed
cresol per ton of coal. This residual cresol solvent is evaporated from
the coal and carried by a stream of nitrogen gas to a condenser where it
is condensed and recycled to the leaching unit.
In order to remove the dissolved organic sulfur compounds from the
leaching solvent, part of the cresol recycle stream is fed to the distil-
lation section. In the distillation section,4 tons per hour of organic
sulfur compounds are separated from the cresol solvent. The fraction of
the cresol recycle stream which must be fed to the distillation unit will
depend upon the allowable buildup of dissolved sulfur compounds in the
leaching solution. It is of course desirable to allow the sulfur compounds
to buildup in the cresol recycle stream to the highest possible value
(commensurate with not interfering with the extraction process) so that
-------
17270-6007-RO-00
the feed to the distillation section is minimized. For purposes of
calculation, it was assumed that 50% of the organic sulfur compounds boil
above p-creso1 and 50% boil below p-cresol. Two distillation columns .are
used. The first column separates the light organic sulfur compounds from
the p-creso1 and heavy sulfur compounds and the second column separates the.
heavy organic sulfur compounds from the cresol. In sizing the distillation
columns it was "assumed that the heavy organic sulfur compounds were equiva-
lent to thionaphthene (bp 220°C) and that the light organic compounds were
equivalent to thiophene (bp 84°C).
Capital and operating costs for each of
baseline processing scheme were investigated
are summarized in Figures 3-8.
the four major sections of the
parametrically and the results
Figure 3 shows the effect of coal residence time in the leaching sec-
tion on the operating, 'capita1, and labor costs (based on ~/ton of coal).
Although the total cost of operating the leaching section is a function of
residence time, the cost differential between fifteen minute residence time
in the 1eacher and two hour residence time is only about 10~ per ton of
coal.
Figure 4 shows the rate at which recycled cresol solution must be fed
to the distillation unit as a function of the allowable buildup of sulfur
compounds in the cresol recycle stream. Operating costs for the distillation
unit are also plotted in Figure 4 as a function of feed rate to the distil-
lation column. Capital costs of the distillation unit as a function of feed
rate are presented in Figure 5. It is apparent from Figures 4 and '5 that
the-operating and capital costs in the distillation section are strongly
dependent on the maximum allowable buildup of sulfur compounds in the
. cresol recycle stream. In order to keep the operating costs within rea-
sonable bounds a sulfur compound buildup of 10% or more in the cresol re-
cycle solution is required. Although experimental data for the actual
extracted compounds is not yet available, the solubility in cresol of many
of the compounds which are expected to be extracted from the coal is high
and hence it appears likely that a sulfur compound buildup of 10% or greater
is possible.
-------
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30'110
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.IG'NCI 1M IN LlACHlNG SlCTION . HIS
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l7270-6007~RO-OO
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Leaching Section Costs vs Residence Time
100 200 300
F££D TO SfPAtATION TIAIN-TONS LfACHING SOLUTION/loo TONS COAL
3
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o
400
Separation Train' Feed and Operating Costs as a
Function of Allowable' Sulfur Compound Buildup
in the Leaching Solvent
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FEED TO SEPARATION TRAIN-TONS LEACHING SOLUTION/loo TONS COAL
400
Figure 5.
Capital Cost for Separation Train as a
Function of Feed ~ate to Distillation Units
0.25
o
0.2 0.3 0.4 0.5 0.6 0.7 0.8
. LEACH SOLUTION RETAINED ON COAL ENTERING OIIYER- WT SOLVENT/WT COAL
0.9
1.0
0.1
Figure 6.
Dryer Operating Cost as a Function of
Solvent Retained on Coal After Filtering
-------
1 7270-6007-RO-00
Operating and capital costs for the drying section are shown as a
function of the cresol solution retained on the coal entering the dryer in
Figures 6 and 7 respectively. It is apparent from Figures ~ and 7 that the
drying costs will differ significantly depending on the amount of cresol
retained on the coal after filtering. Laboratory tests have shown that
cresol retention after filtering depends very strongly on particle size.
-200 mesh coal was found to retain from 0.75 to 1.00 tons of solvent per
ton of coal after filtering, while -14 mesh coal retained only 0.1 to 0.2
tons of solvent per ton of coal. Therefore, from the point of view of
minimizing the costs in the drying section there is a great advantage to
utilizing larger particle size coal. The question which remains to be
answered, is how particle size effects the rate of leaching of the sulfur
compounds from the coal. The best tradeoff must be established between
maximizing sulfur removal and minimizing processing costs.
In Figure 8, the cresol solvent makeup cost is plotted as a function
of the percentage of cresol recovery in the drying section. It is apparent
from Figure 8, that a high degree of cresol recovery is required in order
for the process to be economical. This is based on an expense of lO~/lb for
cresol solvent. It is possible that cresol acts as a catalyst in the sep-
aration of the sulfur compounds from the coal matrix and hence only small
amounts may be required. Thus it could be possible to use a leaching sol-
vent which consists of a small amount of expensive cresol dissolved in a
large amount of very cheap carrier solvent such as kerosene or gas oil.
The function of the carrier solvent would be to dissolve the organic sulfur
compounds after they were severed from the coal matrix by the cresol. If
this type of a low cost combination leaching solvent proves to be feasible,
the requirement for a high degree of solvent recovery in the drying section
would be considerably reduced.
Table 7 summarizes the estimated capital, operating, and labor costs
(based on tlton of coal) for an example case together with the primary
assumptions used in the cost estimate. No credit was taken for the sulfur
compounds which were extracted from the coal nor was any credit taken for
the energy released in the condensation of evaporated solvent. Three
operating positions are assumed in calculating the labor costs. A value
-------
0.60
17270-6007-RO-OO
0.50 2.0
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0.20 0.30 0.40 0.50 0.60 0.70 0.80
LEACH SOLUTION RETAINED ON COAL ENTERING DRYER - WT SOLVENT /Wi COAL
0.90
Fi gure 7.
Dryer Capital Cost as a Function of
Solvent Retained on Coal After Filtering
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98.5
99
% OF SOLVENT RECOVERED FROM COAL IN THE DRYER
99.5
Fi gure 8.
Solvent Makeup Cost vs % of Solvent
Recovered from Coal in the Dryer
-25-
-------
1 7270-6007-RO-OO
Tab le 7.
Example Case for Organic Sulfur Removal Process
DESIGN BASIS:
100 TONS COAL/HR
1.2% WT ORGANIC SULFUR REMOVED FROM COAL (4 TON/HR,OF
ORGANIC SULFUR COMPOUNDS)
UTON COAL
CAPITAL COST OPERATING COST
LABOR
LEACHING SECTION
3.6
5.1
1.8
.
5:1 Wt Basis P-creso1 to Coal
.
0.5 Hr at 330°F
.
30% Wt Solubility of Organic
Sulfur Compounds in P-cresol
FILTRATION SECTION
2.1
0.2
7.0
.
0.50 Tons of Leach Solution
Retained Per Ton of Coal After
Filtering
DISTILLATION SECTION
6.0
10.0
8.7
.
Two Distillation Units Required
to Separate Light and Heavy
Organic Sulfur Compounds from
P-cresol
.
0.14 Tons of Leaching Solution
Fed to Separation Train per Ton
of Coal
COAL DRYING AND P-CRESOL RECOVERY SECTION
DRYING
SOLVENT LOSS
34.0
10.2
29.0
3.5
.
99.7% of Solvent Recovered from
Coal in Dryer
SUBTOTAL
45.7
54.5
21.0
TOTAL COST FOR ORGANIC SULFUR REMOVAL:
$1.21/TON COAL
-------
1 7270-6007-RO-00
of 0.5 tons of cresol solvent retained on the coal per ton of coal enter-
ing the dryer was assumed, although, laboratory data shows that 0.25 ton
retention is the actual value for 1/2-inch coal. Capital costs were depre-
ciated on a 10% per year basis with an additional 10% per year added for
taxes, maintenance, etc.
From Table 7, it is seen that without byproduct credit, a conservative
estimated cost of removing organic sulfur from a ton of coal is $1.21.
2.5 DETAILED CHARACTERIZATION
The screening studies indicated that it.was technically feasible to
remove varying amounts of organic sulfur compounds from all four coals which
were investigated, utilizing p-cresol or nitrobenzene. It was the objective
of the detailed characterization studies to, a) determine the maximum amount
of organic sulfur which can be removed by the above two solvents, b) obtain
preliminary data relating to important engineering cost parameters identified
in Section 2.4, and c) characterize the extracted coal.
Studies for increasing the amount of organic sulfur removal were based
on improving extraction techniques to maximize the amount of solvent filtered
away from the coal, utilizing double extractions and adjustment of retention
time.
The preliminary engineering and economic analysis for the removal of
organic sulfur utilizing an organic solvent was performed in order to define
clearly economically important process parameters for laboratory investigation.
The process which was designed consists of (1) coal leaching, (2) coal-solvent
filtration, (3) solvent distillation to remove sulfur compounds and (4) solvent
recycle (see Figure 2 ).
The following were shown to be important cost facto~s: coal particle
size, retention volume of solvent remaining in coal after filtration, and the
ability to recycle the extraction solvent several times and allow a high
sulfur compound buildup. It would be desirable to be able to utilize the
largest possible coal particle size in a process for the removal of organic
sulfur because, (1) fine mesh coal cannot be shipped by rail so that a coal
desulfurization plant is limited to a location adjacent to a power plant
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17270-6007-RO-00
(note: -200 mesh is the size normally used for a pulverized coal-fired power
plant), (2) fine mesh coal is difficult to filter on an industrial scale,
and (3) the solvent retention on fine particles tends to be greater than
on larger particles. The retention volume of solvent on coal must be mini-
mized, as any solvent retained not only limits the amount of organic sulfur
which may be removed by liquid-solid partition, but must be vaporized in the
drying step of the process. Finally, it is critical to the economics of a
process for the leaching of organic sulfur from coal, that the allowable
buildup of organic sulfur compounds in the leach liquid be high (i.e.,
multiple recycle of solvent), so that only a minimum amount of extraction
solvent need be sent to the distillation columns for separation on each
pass.
The experimental results of the assessment of each of the above factors,
as well as additional studies on heat content and coal analysis are presented
in the following sections: Studies for Increase in Sulfur Removal, Assessment
of Critical Engineering Economic Parameters and Extracted Coal Characterization.
2.5.1 Studies for Increase in Sulfur Removal
In contrast to the screening studies, the coal samples were not washed
with water or methanol to remove residual solvent, but instead the residual
solvent was vaporized from the coal (see Appendix for details). This procedure
more closely approximates the envisioned process steps (Figure 2) and seems to
lead to somewhat higher sulfur removal.
The effect of extracting coal twice with p-cresol is shown in Table 8.
It can be seen that two extractions remove considerably more organic sulfur
than was removed by one extraction alone. The removal of 80% of the organic
sulfur from the Pittsburgh coal was particularly encouraging as this is far
higher than has been obtained previously.
The effect of varying retention time from 0.5 to 3 hr. is shown in
Table 9. The screening technique of water washing residual solvent was utili-
zed for these runs, giving lower amounts of organic sulfur removal. No obvious
change was observed in the amount of sulfur removed as a function. of retention
time. The apparent increase in sulfur removal for 0.5 hr extraction of
Pittsburgh coal over 1.0 hr extraction is within experimental error for the
-------
TABLE 8. SINGLE AND DOUBLE PASS EXTRACTIONS WITH P-CRESOLa
Coa1b No. of Extractions % Sulfur After Extractions % Organic Su1furc Removed % Total Su1 furc Removed
Bevi er 1 X 2. 69, 2. 71 29 15
Bevi er 2 X 2.32, 2.31 54 27
Pittsburgh 1 X 0.82, 0.86 63 32
Pi ttsburgh 2 X 0.73, 0.74 80 40
I
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I
aExtraction with 5 vo1s/wt solvent to coal for 1 hr at 200°C, filtered, then dried
to constant wt @ 150°C/20 min
bpartic1e size =-200 mesh
cBevier: total sulfur = 3.10%, organic sulfur assumed = 1.6%
Pittsburgh: total sulfur = 1.23%, organic sulfur = 0.61%
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TABLE 9.
EFFECT OF RETENTION TIME ON REMOVAL OF
SULFUR WITH p-CRESOLa
I
eN
a
I
Coalb,c Retention Time % Sulfur After % Organi c Sul fur % Sul fur Total
Hrs Extracti on Removed Removed.
Indiana No. V 3 2.76 42 21
Indi ana No. V 0.5 2.80,2.78 39 20
Pittsburgh 1 0.99 39 19
Pittsburgh 0.5 0.87, 0.89 57 28
aExtraction with 5 vols/wt solvent/coal for 1 hr @ 200°C, filtered,
washed several times with hot water on filter, then dried to con-
stant wt @ 100°C/20 mm.
bparticle size = -200 mesh.
cIndiana No. V: total sulfur = 3.48%, organic sulfur = 1.95%.
Pittsburgh: total sulfur = 1.23%, organic sulfur = 0.61%.
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1 7270-6007-RO-00
Eshka sulfur analysis method*. Thus, it appears that p-cresol penetrates the
coal organic matrix very rapidly (on a scale of minutes) and rapidly dissolves
organic sulfur compounds. Similar results were obtained during the screening
studies for the extraction of Pittsburgh coal with o-chlorophenol (at 1 hr,
46% removal, and at 3 hrs, 48% removal), in which there was no significant
difference in organic sulfur removal (Table 3 ).
A series of experiments were performed on the Lower Freeport coal in
order to assess the ability of p-cresol and nitrobenzene to remove organic
sulfur in retention times as short as 15 minutes (Table 10). Normally, the
Lower Freeport coal analyzed 4-4.1% total sulfur with approximately 0.4%
organic sulfur (Table 1). However, during the course of these experiments.
it was found that the analyses performed by Commercial Testing Company began
to give highly variable sulfur content for each unextracted sample sent for
analysis and, in addition, where replicates were run, the variance was greatly
above that normally obtained with the ASTM method. Thus, although the sulfur
analyses of the extracted Freeport coals were lower than those for most of
the unextracted samples, which were run by Commercial Test Company, no firm
interpretation can be made.
This variance may be due to a sampling problem, or difficulty in the
sulfur analysis method for this particular coal. This type of problem is of
great importance in assessing the results obtained for the treatment of coal
by any process for the removal of sulfur, and should be investigated further.
2.5.2 Assessment of Critical Engineering Parameters
The effect of varying the particle size of Pittsburgh coal is shown in
Table 11. The organic sulfur removed decreased somewhat for nitrobenzene
extracted coal, in going from -200 mesh pu1verized coal to 1/2-inch pieces,
while the solvent remaining sorbed on the coal decreased dramatically in
*Duplicate determinations on the same sample with same operator should not
differ by more than 0.05% for sulfur contents of 2% or less. For different
samples, the difference is 0.1%. Hence, for a 1.2% Pittsburgh coal, there
is no significant difference between the 1.0 and 0.5 hr samples.
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17270...6007-RO-00
TABLE ,10.
EXTRACTION OF -14 MESH
LOWER FREEPORT COALa
b Retenti on Time % Sulfur After
Solvent Hrs Extraction
p-Cresol 0.25 3.00
p-Creso 1 0.25 2.83
p-Cresol 0.50 2.96
Nitrobenzene 0.25 2.97
Ni trobenzene 0.50 3.01, 3.03
Ni trobenzene 1 2.75,2.77
aStarting Lower Freeport coal analysis had changed
from a normal 4.13% sulfur to give the following
chronological results on separate samples from the
same container: 4.00, 3.37, 3.40 (2.97 on repeat)
3.13, 3.00, 3.04 (the average of all of these being
3.27) .
bExtraction with 5 vols/wt solvent/coal @ 200° and
210°C, respectively for p-cresol and nitrobenzene
filtered, then dried to constant wt @ 150°C/20 torr.
-------
TABLE 11. EFFECT OF PARTICLE SIZE ON EXTRACTION OF SULFUR
FROM PITTSBURGH COAL
I
W
W
I
a Parti cle Solvent % by wt on % Sulfur % Organic Sulfur % Total Sulfur
Solvent Size Coal Before Drying After Extraction Removed Removed
Nit roben zene -200 Meshb 200 0.78 72 36
Nitrobenzene -14 meshc 85 1. 29 31 13
Ni trobenzene 1/2 inchc 25 1. 17 48 20
p-Cresol -200 meshb 90 0.84 63 32
p- Creso 1 -14 meshc 85 1.44 0 0
p-Cresol 1/2 inchc 60 1. 43 0 0
aExtraction with 5 vols/wt solvent/coal for 1 hr @ 200° and 210°C, respectively for
p-cresol and nitrobenzene, filtered, then dried to constant wt @ 150°C/20 torr
bpittsburgh coal sample: total sulfur = 1.23%. organic sulfur = 0.61%
CPittsburgh coal sample: total sulfur = 1.48%, organic sulfur = 0.63%
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17270-6007-RO-00
going to larger size coal*. In the case of p-cresol, it appears that organic
sulfur may be removed only when the coal is pulverized to the -200 mesh range,
at least for Pittsburgh coal. It should be noted that the analyzed sulfur
content of the unextracted Pittsburgh coal increased from 1.23 to 1.48%
during the latter part of these experiments (one sample analyzed 1.60%).
It would be very desirable to design a process based on the extraction of
half-inch range coal, as the plant could be set up either at a mine face or
at a power plant, filtration is extremely facile and it would be necessary
to vaporize a relatively small amount of solvent from the coal in the drying
step. The engineering analysis (Section 2.4, Figures 5, 6, and 7) shows
that the results obtained for nitrobenzene leach of 1/2-inch coal, 0.25 ton
solvent/ton coal, would reduce process operating costs (e.g., $0.06/ton of
coal operating costs, $0.23/ton of coal capital costs for the dryer section).
By contrast, the example case (Table 7) assumed 0.50 tons solvent/ton coal
leading to a higher dryer section cost than may be the case. It should be
noted that extraction of the 1/2-inch coal resulted in fragmentation (as a
result of the swelling action of the nitrobenzene) so that the dried coal
was approximately 1/8-inch particle size.
It is important for the economic utility of the process (Figure 4) that
the leach solvent be capable of multiple recycling so that the organic
sulfur buildup in the solution may reach at least 10% without necessity of
sending the liquid to the separation train. In order to get a first cut in
the evaluation of this factor, a sample of nitrobenzene was utilized three
times for extraction of fresh Pittsburgh coal (Table 12). It was found that
the organic sulfur removal decreased after the first cycle but remained con-
stant into the third cycle indicating that organic sulfur buildup may be
allowed to continue to some reasonable concentration. A calculation based
on the measured amount of organic sulfur removal, assuming that the sulfur
compounds are modeled after benzothiophene, indicates a roughly 1% buildup
of organic sulfur compounds in the nitrobenzene after the third cycle. This
*The apparent increase in sulfur removal in going from -14 mesh to 1/2-inch
is within experimental error for the Eshka procedure.
-------
TABLE 12.
EFFECT OF MULTIPLE RECYCLES OF NITROBENZ6NE SOLVENT
ON SULFUR REMOVAL FROM PITTSBURGH COALa,
I
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(J1
I
Solvent Cycle % Sulfur After Extraction % Organic Sulfur Removalc % Total Sulfur Removedc
1 0.76 77 38
2 0.97 43 21
3 0.95 46 23
aExtraction with 5 vols/wt solvent to coal
then dried to constant wt @ 150°C/20 min
bparticle size = -200 mesh
~otal sulfur = 1.23%, organic sulfur = 0.61%
for 1 hr @ 210°C, filtered,
........
N
........
a
I
CT\
a
a
........
I
:;:0
a
I
a
-------
1 7270-6007-RO-00
is, of course, far short of that which will
course of process development studies, e.g.
recycles will have to be obtained.
2.5.3 Characterization of Extracted Coals
need to be measured during the
about 10 times this number of
The heat content of, the coals should not appreciably change as a result
of extraction of organic sulfur compounds. Typically, where 0.5% of the
total sulfur is removed, and this sulfur is borne on molecules of the type
of benzothiophene (about 2.5% of the organic matrix is removed), the ash to
organic ratio would remain about constant and hence, the heat content should
not change. A number of heat contents were determined before and after
extraction and these are shown in Table 13. The Bevier and Lower Freeport
coals showed no measurable heat content change, while the Illinois No.6
decreased 51ightly and the Pittsburgh had a mean decrease of 5%. These
heat losses are probably due to a small amount of coal oxidation during
drying.
Detailed analyses of some extracted coals were obtained for cresol,
nitrobenzene and o-chlorophenol extraction (Table 14 ). It is difficult
to interpret the slight changes which can be noted in these analyses com-
pared with those in Table 1 for unextracted samples, as it has been shown
that the sulfur content of the Pittsburgh and Lower Freeport coals tends
to vary significantly according to the sample, hence it can be anticipated
that the ultimate and proximate analyses of the starting coals also vary
according to the particular sample. As the analyses reported in Table 1
(which were obtained near the beginning of the program) cannot be definitely
assigned as those of the unextracted coals for the particular extractions
reported in Table 14 (obtained near the end of the program), no meaningful
treatment of data can be made at this time.
-------
I
eN
........
I
TABLE 13. EFFECT OF SOLVENT TREATMENT ON HEAT CONTENT OF COALS
Heat Content Heat Content
Retention Time Be fore After % Heat Content
Coal Solventa Hrs Btu Btu Change
Illinois No.6 Ni trobenzene 1 12271 11961 -2
Bevier p-Cresol 1 12710 12670 0
Bevier Nitrobenzene 1 1271 0 12660 0
Pittsburgh p-Creso 1 1 14354 13662 -5
Pittsburgh o-Ch1oropheno1 1 14165 12416 -12
Pittsburgh Nitrobenzene 1 14165 13654 -4
Pittsburgh Ni trobenzene 1 14442 13711 -5
Pittsburgh Nit robenzene 1 14442 13745 -5
Pittsburgh Nitrobenzene 1 14442 12000 -16
Lower Freeport p-Cresol 0.25 13460 13560 0
Lower Freeport p-Cresol 0.50 13460 13488 0
Lower Freeport Nitrobenzene 1 13460 13512 0
aExtraction with 5 vols/wt solvent/coal @ 200° and 210°C, respectively for p-creso1 and nitrobenzene,
filtered, then dried to constant wt @ 150°C/20 torr.
.......
........
N
........
a
I
C7'I
a
a
........
I
~
I
a
-------
TABLE 14. ULTIMATE AND PROXIMATE ANALYSES OF EXTRACTED COALS
I
W
eX>
I
Fixed Weight, %
Ash Vol atile Carbon
Coal Treatment % % % Btu/1b Sulfur,% C H N C1 0
Lowe r
Freeport Creso1-1 hr 11.42 27.37 61.21 13560 3.00 77.04 4.43 1. 12 0.12 2.87
Pittsburgh Nitrobenzene-1 hr 4.96 32.50 62.54 12000 0.76 73.53 2.28 3.19 0.02 15.26
Pittsburgh Ch1oropheno1-1 hr 4.49 36.31 59.20 12416 0.83
......
.......
N
.......
a
I
0'\
a
a
.......
I
;;tI
a
I
a
-------
17270-6007-RO-00
3.
REMOVAL OF NITROGEN
This section provides the results of the laboratory effort for removal
of nitrogen from coal. The objectives of the effort were to determine the
ability of selected solvents to remove nitrogen from coals. In addition, .
TRW performed a process engineering analysis for the use of one of the candi-
date solvents to leach nitrogen compounds in a cyclic batch-mode process.
Discussions of, a) the need for removal of nitrogen from coal, b) the
theory associated with chemical removal of nitrogen, c) experimental results,
and d) and engineering analysis (TRW funded) of a proposed process based on
the experimental results are presented in the following sections.
3. 1 NEED FOR REMOVAL OF NITROGEN
Approximately l4-million tons of nitrogen oxides are emitted into the
atmosphere yearly and of this total, 25-30% come from coal fired power plants.
The potential requirement for removal of nitrogen from coal has only
recently been recognized. Heretofore, it has been assumed that nitrogen
fixation during combustion is the only significant source of nitrogen oxides
emitted from utility stacks. However, it is likely that a portion of the
organic heterocyclic components of coal are converted during the combustion
process to their N-oxides which then decompose directly to nitric oxide (the
major component of NOx emission), as shown in Equations 2 and 3 below.
o (0] a
a.-
N N!P
I
0
co [0] OCJ
.
~ N"" ~ N
I
o
C02
t [0]
HYDRO- (2)
. CARBON + NO
~
.. HY DRO- + NO (3)
6. CARBON
~ [0]
C02
-------
17270-6007-RO-00
3.2 CHEMISTRY OF NITROGEN REMOVAL
Nitrogen occurs in coal in an organic form. It is thought that nitrogen
compounds in coal may be categorized in three compound classes, a) ni trogen
heterocycles, also known as coal tar bases, b) aromatic amines and c) aroma-
tic ami des. Of these three classes, the first is thought to be the major
structural form in which nitrogen is found in coals and other fossil fuels.
The experimental basis for the assignment of the heterocyclic structure to
nitrogen in the coal is via destructive distillation or pyrolysis investiga-
tions, not direct determination. These compounds are thought to be bonded
to the coal polymer matrix by covalent bonds, although a more likely inter-
pretation of the bonding structure may actually be simple hydrogen bonding.
Up to the present time, no method has been available for removing these com-
pounds from coal.
The TRW approach to removing nitrogen from coal is similar to that
advanced in Section 2.1 for removal of organic sulfur, that is, low tempera-
ture, atmospheric pressure, acid catalyzed, depolymerization and subsequent
dissolution of organic nitrogen compounds from the coal matrix. As before,
the aim of this approach is to break down the coal matrix polymer selectively
at the points occupied by organic nitrogen compounds without disturbing the
bonding present in the major part of the coal material. Equation 4 shows a
possible mechanism for the selective extraction involving acid catalyzed
decomposition of the coal matrix at the positions occupied by the nitrogen
compounds, with aromatization shown as a possible driving force. Here again,
it appears that solvents which would be effective would be acidic in nature.
COAL
MATRIX
......
6
N
COAL
MATRIX
~
+
+ H+ (4),
H
-------
17270-6007-RO-00
3.3 EXPERIMENTAL RESULTS
The initial screening studies (Table 15) were performed on a rather
unusual Bevier coal containing 0.5% nitrogen, as determined by the Kjeldahl
method. The nitrogen removal results (Kjeldahl analysis) showed that hydro-
chloric acid and phosphoric acid were effective at removing organic nitrogen
from the coal, while sulfonic acid type liquids were ineffective. Multiple
leach experiments were performed with the phosphoric acid and found to give
increasin9 removal of organic nitrogen compounds.
Subsequently, the screening studies were continued on the APCO program
for Chemical Removal of Nitrogen and Organic Sulfur from Coal. In these
studies, some more widely used coals (see Appendix for extraction experi-
mental methods) were investigated (Table 16) , with some of the same acids
utilized in the previous study but also including p-cresol, the liquid of
choice for removal of organic sulfur compounds from coal. It was found
that p-cresol ,.20% aqueous hydrochloric acid and concentrated phosphoric
acid remove significant amounts of nitrogen from two coals according to
the Dumas analytical technique, while the Kjeldahl method indicates that
nitrogen is not removed from the coal samples. It should be noted that the
starting coals showed a higher nitrogen content by the Dumas method than by
the Kje1dahl. Consequently, it is apparent that the analytical techniques
utilized for analysis of coal nitrogen require further evaluation.
The Kjeldahl method is the most widely used for determining nitrogen
in coal. The method consists of boiling pulverized coal with concentrated
sulfuric acid which contains potassium sulfate and a catalyst such as mercury
or selenium, until the mixture reaches a straw-colored stage. The method
depends on complete reduction of the organic nitrogen compounds to ammonium
salts by treatment with the above solution. Following the digestion, the
solution is made alkaline and the resulting free ammonia is distilled into
a trap and determined titrimetrical1y. However, it has long been known that
nitrogen contained in the form of pyridine rings is highly resistant to this
type of treatment, and may not be totally reduced in the first step. Because
nitrogen compounds in coal are believed to consist partially of pyridine
derivatives (e.g. coal tar bases) and partially of amino type compounds, it
-------
Table 15.
Initial Nitrogen Removal Screening Testsa
Extraction
Temperature Extraction Duration Nitrogen Removed
Li qui d °C Mins %
Methane Sulfonic Acid 100 1200 0
Benzene Sulfonic Acid 100 1200 0
98% Sulfuric Acid 0 60 Ob
85% Phosphoric Acid 20 1200 56
85% Phosphoric Acid (lX) 150 1200 60
85% Phosphoric Acid (2X) 150 1200 84
85% Phosphoric Acid (3X) 150 1200 91
36% Hydrochloric Acid 100 150 48
I
~
N
I
aUntreated dry Bevier coal nitrogen content: 0.52%, Kje1dahl method (TRW)
bExtracted sample showed an increase in sulfur content arising from su1fonation
......
.......
N
.......
C>
I
0'1
C>
C>
.......
I
:::0
C>
I
C>
-------
TABLE 16.
NITROGEN REMOVAL SCREENING TESTS
I
.J::o
W
I
Change in tlitrogen Composition
Extraction Extraction Ni trogen Content
Te~erature Duration Coala,b Dumas Ana 1Y5 i 5 Durnas Ana lys i s r,jeldahl Analysis Kjeldahl Analysis
Liquid °C Mins :lethod ~,: Nethod r flethod ," Method ~,
P-cresol 200 60 Illinois No.6 1. 16 -39 0 1. 24
201 HC 1 100 60 Illinois No.6 1.28 -32 +9 1.31
Conc. H3P04 (IX) 150 900 Illinois No.6 1. 55 -18 +13 1. 44
Conc. H3P04 (2X) 150 900 Illinois rlo. 6 1.39 -27 +9 1. 38
P-creso! 200 60 Pittsburgh 2.28 -20 - -
Conc. Hl04 150 60 Pi ttsburgh 2.43 -25 - -
aUntreated dry Illinois coal nitrogen content: 1.90%. Dumas I;iethod (Elek nicroanalytical Laboratories. TprrancC'. CalifClrnia);
1.231, I(jeldahl method (Coomercial Test Laboratory, Chicago, l11ino;s); 1.27", Y.jeldahl l'1ethod (TR':).
bUntreated dry Pittsburgh coal nitrogen content: 3.03%, Dumas method (Elek Microanalytical Laboratories, Torrance, California);
1.35%, l(je1dahllllethod (Cannercia1 Test Laboratory, Chicago. Illinois). .
--'
'-I
N
'-I
a
J
0\
a
a
'-I
t
;;c
a
I
a
-------
l7270-6007-RO-00
appears that special digestion treatment may be necessary to ensure deter-
mination of the former compounds by the Kje1dah1 method.
The Dumas nitrogen determination method analyzes the total nitrogen con-
tent of coal by a completely different approach. In this method, pulverized
coal mixed with copper oxide is heated in a stream of carbon dioxide and
oxygen at 950-1000°C in contact with platinum. All nitrogen present in the
sample is converted to nitrogen gas which is measured vo1umetrica11y after
sorption of oxygen gas on hot metallic copper and carbon dioxide in an
aqueous sodium hydroxide scrubbing media.
Thus, it is apparent that further work is required in the area of
evaluating the effectiveness of the various nitrogen analysis methods so
that precise information can be obtained as to the effectiveness of the
various leaching liquids for removal of organic nitrogen from coal.
3.4 ENGINEERING ANALYSIS OF A PROCESS SCHEME
From the initial feasibility experiments it would appear that both p-
cresol and concentrated inorganic acids such as phosphoric acid or hydro-
chloric acid are candidates for selectively removing organic nitrogen
compounds from coal. Of the two approaches, p-creso1 extraction is by far
the more attractive from a cost standpoint since the organic nitrogen com-
pounds would be removed (essentially for free) along with the organic sulfur
compounds. The only question wit~ t~e p~cresol approach is whether or not
sufficient nitrogen is removed from the coal. Additional nitrogen may be
removed from some coals, if required, by the phosphoric acid leaching approach.
In order to obtain a preliminary estimate of the costs associated with a phos-
phoric acid leaching process, a preliminary analysis of one possible proces-
sing scheme was made and is described below.
Figure 9 presents a processing scheme for removal of nitrogen
compounds by phosphoric acid leaching. The plant is sized to process 100
tons/hr of coal and it is assumed that 3.5 tons/hr of coal tar bases (nitro-
gen compounds) are extracted from the coal. The process' can be divided into
6 major sections, as follows:
.
Leaching
-------
f"103.5TONSiHRCOAL--l
~~T~'y~R~~:.:~~~>j
I
I
I
I
COAL
MAKE-UP
PHOSPHORIC
ACID
I
~
<.TI
I
I
I
~~~~~~5~~~~
F i gu re
LEACHER
I
I
rjOOTONsiHR~OAL-- --,
: 500 TONS/HR H3P04 (85%) i
L~.!.o~~~ ~O~L2. A~~SE~J
I
fSoo TC;NS/HR ~P04(a5%)-1
L.&'...::!~~~~C~A~ ~'!....BA...:E~
r:------------,
122.1 TONS/HR Hl04 (85%) I
L~2.~~~~o~:!~~~sr
PHOSPHORIC ACID RECYCLE
flooTON"SiHRcoAl---l
: 4.3 TONS/HR H3PO 4 (85%) 1
L?:.:.~~~R-i~~~~~
I
I
I
I
ROT MY VACUUM
DISK FILTER
17-----------'
122.1 TONS/HR H3P04 (85%) 1
-l::.92~.Y~~~:.:~~~~
I
I
I
I
I
I
1
I r.;:-----------"
L_.~473.6 TONS/HR H3P04 (85%) I
l.8!:~~'y~R.;:~!:.!~~~~
COAl TAR BASES EXTRACTOR
PHOSPHORIC ACID RECYCLE
r.----------,
-.14.3 TONS/HR Hl04 (85%) I
~..:.~~~~~L2;\~~~
~5TONS/HR ';;o----j
-, 4.3 TONS/HR Hl04 (85%) I
La:.:. ~~~R~O~L2.A~~~
~~~N~~~OJ--
MAKE-UP
WASH WATER
-E52~~~~~
~ TONS/HR ;;;oJ
--,---
H20
FOUR STAGE COAL WASHING
i2s0TON5iHRCc~----1
L02~~S~!..'r£~ ~'!....BA~~
I
[looToNS7HRCOALl
L.:..T~N~~R~2~J
E."oOTONS~RCOA~
---T---
I
cw --1250 TONS/HRCCL4- - --1
1 3.B TONS/HR COAL TAR BASES I
L______---_J
COAL TAR BASES TO STORAGE
9 .
Processing Scheme for Removal of Nitrogen Compounds by Phosphoric Acid Leaching
CCl4
I
I
~~~~~~~A0~~~sJ
lOW
NITROGEN
COAL
.......
.......,
N
.......,
a
I
0"1
a
a
.......,
I
;0
a
I
a
-------
1 7270-6007-RO-00
. Filtering
. Coal washing
. Acid concentration
. Extraction
. Distillation
In the leaching section 100 tons/hr of pulverized coal are slurried
with 500 tons/hr of concentrated phosphoric acid and stirred at 300°F.
Residence time of the coal in the leaching section is 30 minutes. Insulated,
atmospheric pressure, stainless steel tanks are used for the leaching
operation.
Rotary vacuum disc filters separate the bulk of the concentrated
phosphoric acid from the coal. It is assumed that 0.05 tons of acid solution
are retained on the coal filter cake per ton of coal after filtering. The
concentrated p~osphoric acid is recycled to the leaching unit while the
coal filter cake is transferred to the washing section.
Four stages of counter current washing with intermediate filtration
are used to remove the residual phosphoric acid from the coal. After
washing the coal contains less than 100 ppm of phosphorous impurity.
After washing,
residual wash water
the washing unit.
the extracted coal is fed to a drying unit where the
is evaporated from the coal, condensed and recycled to
The dilute phosphoric acid solution leaving the washing unit is con-
centrated in an evaporator or acid concentrator and the concentrated
phosphoric acid recycled to the leaching unit. The water evaporated from
the acid solution is condensed and recycled to the washing unit.
Part of the phosphoric acid recycle stream is fed to a liquid-liquid
extractor where the coal tar bases are extracted from the phosphoric acid
with carbon tetrachloride. It is assumed that the coal tar bases can be
allowed to buildup in the phosphoric acid leaching solution to approximately
20% without significantly affecting the leaching capability of the acid.
-------
17270-6007-RO-OO
In the distillation section the coal-tar bases are separated from the
carbon tetrachloride solution. The CC14 is recycled to the liquid-liquid
extractor and the coal tar bases are collected and sold as a by-product of
the process.
Table 17 summarizes the estimated capital, operating, and labor cost,
for the phosphoric acid extraction process. No credit is taken for the
coal-tar bases. Capital costs are depreciated asa 10% per year basis
with an additional ten percent of the capital costs taken for taxes, main-
tenance, etc. Based on the assumptions described above and in Table 12
the cost of processing the coal is $1.18 per ton.
It is interesting to note that if a $40 per ton credit is taken for
the coal tar bases the phosphoric acid leaching process more than pays for
for itself. For comparison purposes, the selling prices* of coal tar and
some of the chemicals present in coal tar bases are presented below:
Crude coal tar - $40/ton (tank cars, works)
Refined coal tar - $48/ton (tank cars, works)
Pyridine - $680/ton (denatured, drums)
Quinoline - $lOOO/ton (tank cars)
Aniline - $280/ton (tank cars)
*
Oil paint and drug reporter, 1969
-------
1 7270-6007-RO-OO
Table 17. Example Case for Organic Nitrogen Removal Process
DESIGN BASIS:
100 TONS/HOUR OF. LOW NITROGEN COAL
3.5 TONS/HOUR COAL TAR BASES REMOVED FROM COAL
LEACHING SECTION
CAPITAL COST
3.6
.
5:1 Wt Basis Phosphoric Acid
(85%) to Coal
0.5 Hr at 300°F and 1 Atm
.
.
15% Wt Solubility of Coal Tar Bases
in Phosphoric Acid Leaching Solution
COAL FILTRATION AND WASHING SECTION
13.2
.
Four Stage Washing with Intermediate
Fi ltering
0.05 Ton Solution/Ton Coal Retained
After Each Filtering
.
.
20 Tons/Hr Wash Water Entering Counter
Current Washing Sections
COAL TAR BASES EXTRACTION AND DISTILLATION
26.2
.
0.26 Tons of Coal Leaching Solution and
2.5 Tons of CC14 Fed to Coal Tar Bases
Extractor per Ton Coal
2.54 Tons of Extract Solution Fed to
Distillation Column Per Ton Coal
.
COAL DRYING AND H3P04 CONCENTRATION SECTION
3.2
.
20 Tons/Hr Wash Water Evaporated from
Phosphoric Acid Recycle
PHOSPHORIC ACID MAKEUP
.
Four Stage Washing
0.01 TONS/Hr of Phosphoric Acid (85%)
Remains with Washed Coal
.
SUBTOTAL
46.2
TOTAL COST FOR ORGANIC NITROGEN REMOVAL: $1.18
-48-
~/TON COAL
OPERATING COST
5.4
1.0
31.8
10.8
2.0
51.0
LABOR
1.8
7.0
8.7
3.5
-------
17270~6007~RO~OO
4.
CONCLUSIONS AND RECOMMENDATIONS
Laboratory results of the Chemical
Sulfur from Coal Program show that the
can be drawn:
Removal of Nitrogen and Organic
following preliminary conclusions
1.
The concept for chemical removal of organic sulfur and
nitrogen is feasible.
2.
Weak organic acids remove 45-80% of the organic sulfur from
the coals investigated in this program depending on process
conditions and the specific coal leached.
Nitrobenzene appears to be the most efficient solvent for
the removal of organic sulfur.
3.
4.
Aqueous caustic removes small amounts of organic sulfur
from some coals, but none from others.
5.
Particle size (-200 mesh to 1j2-inch) has no discernab1e
effect on the removal of organic sulfur by nitrobenzene.
The leaching solvent, nitrobenzene, may be recycled at least
three times to remove organic sulfur from coal.
6.
7.
Weak organic acids and strong inorganic acids appear to remove
nitrogen from the coals tested in this program according to the
Dumas analytical method, but no nitrogen is removed according
to the Kjeldah1 method.
The engineering analysis results show that:
1.
Weak organic acids for removal of organic sulfur from coal
may be readily adapted to engineering process schemes.
The cost of the process for removal of organic sulfur from
coal is highly dependent on, a) the degree of solvent loss,
and b) the ability to recycle solvent and allow high sulfur
compound buildup.
2.
3.
Utilizing the laboratory data and assuming reasonable values
for undetermined parameters, the process cost for removal for
organic sulfur is approximately $1.00-$1.50jton of coal.
Based on the above laboratory results and conclusions, it is recom-
mended that:
1.
2.
Information should be developed to support a pilot plant effort.
The process for utilization of weak organic acids to remove
organic sulfur from coal should be investigated on a bench
scale to assess important engineering parameters such as,
-------
17270-6007-RO-00
a) recovery and identification of extracted organic sulfur
compounds, b) regeneration of solvent by distillation,
c) overall solvent loss, d) maximum allowable sulfur com-
pound buildup in the extraction solvent and e) coal sampling
factors.
2.
Nitrogen removal screening studies should be continued in
order to determine, a) the correct analytical method for
determining the nitrogen content of extracted and unextracted
coal, and b) the amount of nitrogen removed from coal by the
various leaching liquids.
If nitrogen removal appears promising, bench scale evaluation
of the process should be performed.
3.
-------
17270-6007-RO-OO
5.
NEW TECHNOLOGY
An Invention Disclosure based on information developed under APCO
funding was submitted to the TRW Patent Office for filing entitled, liThe
Removal of Organic Sulfur from Coal with Caustic Solutions". A summary of
the disclosure is presented below.
The Removal of Organic Sulfur from Coal Uti1iz~ng Caustic Solutions -
Aqueous or alcoholic sodium or potassium hydroxide removes organic sulfur
from certain coals. Coal is pulverized and treated with aqueous caustic
for 1-3 hrs at reflux. The caustic is filtered from the coal, the coal is
washed to neutrality and dried in a vacuum oven to constant weight. The
extracted coal has a decreased sulfur content. The aqueous solutions con-
taining organic sulfur compounds may be regenerated by air oxidation and
separation of the organic phase.
-------
1 7270-6007-RO-00
APPENDIX A
EXTRACTION PROCEDURE
Organic Solvents. - The organic solvent (typically 350 ml) was added
to a three-neck round bottom flask fitted with a reflux condenser and an
overhead stirrer. For the screening studies, magnetic stirring was used.
The flask was heated to bring the well stirred solvent to near reflux, then
the coal {typically 70 g) was added. The mixture was maintained at mild
reflux for the specified retention time (typically 1 hr), then immediately
the dark brown liquid phase was separated from the coal while still hot on
a sintered glass or Whatman No.4 filter. In some cases, the coal was then
washed with water or methanol to remove the remaining solvent. The coal
was dried in a vacuum oven at 100-150°C/20 torr to a constant weight (typically
for 16-24 hrs).
Aqueous Acids or Bases. - The aqueous solution (typically 100 ml) and
coal (typically 20 g) were added to a single-neck round bottom flask fitted
with a reflux condenser and a magnetic stirrer. The flask was heated to
mild reflux for the specified retention time, then the aqueous phase was
separated from the coal on a sintered glass filter. The coal was washed
several times with water, neutralized and rewashed, then dried in a vacuum
oven to constant weight at 100°C/20 torr.
FLOW THROUGH EXTRACTION OF COAL
It was anticipated that if pulverized coal beds were amenable to con-
tinuous extraction with a solvent, and an extraction device could be con-
structed in which a moving front of liquid would pass through a stationary
pulverized coal bed and then into a device for continuously monitoring the
sulfur content of the effluent, such a device would be particularly useful
for measuring the rate of sulfur compound extraction. This could be done
as a function of residence time and solvent volume. The data thus obtained,
would be particularly useful if the eventual design of the process for the
removal of organic sulfur from coal was to operate in this process mode.
A continuous extractor apparatus with a sulfur detection system, capa-
ble of continuously monitoring effluent sulfur in a solvent carrier, was
constructed (Figure 10). The apparatus was constructed under TRW funds, as
-------
1 7270-6007-RO-00
both the extractor element and the sulfur detection system (the cpmbustor
and Faristor) would be generally useful for a wide range of general process
studies. The equipment consists of: (l) a pump capable of pumping up to
290 ml/hr of liquid at pressures up to 1000 psi (an adjustable spring type
relief valve is used to protect against over pressurizing the system and
Bourdon type gauges are used before and after the packed column to monitor
pressure, (2) a heat exchanger consisting of a six-foot length of 1/4-inch
stainless-steel tubing, (3) a packed (pulverized coal) column contained in
a thermostatic air circulating oven capable of maintaining temperatures up
to 200°C, (4) an automated fraction collector, (5) an oxygen combustor
system based on a quartz glass combustion tube Fedvia 1/16-inch capillary
tube, and (6) a Faristor sulfur detector with recorder output.
An initial assessment of the usefulness of the apparatus was performed
utilizing weak organic acid solvents with pulverized Pittsburgh coal (-200
mesh) at temperatures of 100°C. It was found that the solvent flow was
quickly reduced to zero even at maximum pressure due to the pulverized coal
forming an impenetrable mass. This problem was overcome by utilizing pul-
verized coal with no fines of less than -20 mesh and the recorder output
indicated some sulfur content in the effluent. However, when the column
temperature was adjusted to the typical extraction temperatures of l50-200°C .
which had been found to be favorable for removal of organic sulfur compounds
from coal in a batch mode, the column again became inpenetrable probably due
to agglomeration, swelling, or plastic deformation of the coal material, so
that solvent could not pass.
Due to the operating problems encountered with this mode of coal ex-
traction and to the choice of a batch mode for the large scale process for
removal of organic sulfur from coal, experiments utilizing the continuous
extractor with sulfur detector where discontinued.
-------
I
01
~
I
SOLVENT
RESERVOIR
PUMP
RELIEF
VALVE
STOP FLOW VALVE
i
PACKED COAL COLUMN
i
COMBUSTOR
TUBE
OVEN
COLUMN BYPASS LINE
CONSTANT PRESSURE
CHECK VALVE
FRACTION
COLLECTOR
Fi gure 10.
Continuous Extractor with Sulfur Detector
FARISTOR
RECORDER
OUTPUT
VENT
....
""-!
N
""-!
a
I
0'\
a
a
""-!
I
~
I
a
------- |