Removal of Organic Sulfur from Coal by Reaction with Supercritical

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7 Removal of Organic Sulfur from Coal by Reaction with Supercritical Alcohols C. B. Muchmore, J. W. Chen, A. C. Kent, and Κ. E. Tempelmeyer Department of Mechanical Engineering and Energy Processes, Southern Illinois University at Carbondale, Carbondale, IL 62901 The processing of high organic sulfur content coals with methanol and ethanol as s u p e r c r i t i c a l solvents has resulted in reduction in sulfur concentrations exceeding 50%. Since the t o t a l sulfur removed exceeds the mineral sulfur content of the coal processed, it is evident that organic sulfur is being removed during the reaction. The r e s u l t i n g s o l i d product maintains over 50% of the concentration of v o l a t i l e matter compared to that of the parent c o a l . A high BTU gas is also produced, and some conversion of coal to liquid products occurs. Pre-treatment of the coal with potassium hydroxide prior to the s u p e r c r i t i c a l desulfurization reaction has resulted in improved sulfur removal For some coals, the combination of a physical cleaning process for removal of p y r i t i c s u l f u r , followed by s u p e r c r i t i c a l desulfurization with an alcohol, can reduce sulfur emissions below 1.2 l b S0 / m i l l i o n BTU. 2

Growing concern over environmental e f f e c t s of acid r a i n has resulted in increased i n t e r e s t in development of pre-combustion removal of s u l f u r from c o a l . Physical coal cleaning processes are e f f e c t i v e for p y r i t i c sulfur removal but do l i t t l e t o reduce the organic sulfur content of coal* This paper reports the removal of organic s u l f u r from coal, employing ethyl or methyl alcohols as the solvent/ reactant* The process is based on the observation that, under s u p e r c r i t i c a l conditions, reactions occur that s e l e c t i v e l y remove organic s u l f u r from the coal matrix. L i t e r a t u r e Review The maximum conversion of coal to l i q u i d products has been the primary objective of most work on s u p e r c r i t i c a l extraction of coal reported in the l i t e r a t u r e . A 1975 a r t i c l e by Whitehead (1), one of the f i r s t references t o s u p e r c r i t i c a l coal extraction, presented data on s u p e r c r i t i c a l extraction of coal by coal tar or petroleum

0097-6156/ 86/ 0319-0075Î06.00/ 0 © 1986 American Chemical Society

Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


76

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

naphtha f r a c t i o n s . Tugrul and Olcay (2) reported in 1978 extraction y i e l d s and a n a l y t i c a l r e s u l t s obtained by supercritical-gas extraction of 250 mesh l i g n i t e with toluene at 400 °C and 16 MPa. They found extraction nearly complete a f t e r 30 minutes; extract y i e l d s of about 24% were reported. Gas chromatography/masβ spectrometry analyses of several extract f r a c t i o n s indicated dozens of p a r a f f i n s , alkylated hydrocarbons, phenolic and oxygenated compounds; however, no sulfur compounds were reported. A k i n e t i c study of a h i g h - v o l a t i l e bituminous coal u t i l i z i n g s u p e r c r i t i c a l toluene was reported by Slomka and Rutkowski Ç 3 ) . A close f i t of t h e i r experimental data on time dependence of extraction y i e l d was found when a second order equation was used. The use of s u p e r c r i t i c a l toluene extraction of coal in p i l o t plant studies supported by the B r i t i s h Coal Board was reported by Maddocke ( 4 ) . The major objective of that study was also maximum conversion of coal to l i q u i d products; reduction of s u l f u r in the unconverted s o l i d was not reported. Some work has been reported u t i l i z i n g alcohols f o r s u p e r c r i t i c a l extraction of c o a l . Makabe et a l . ( 5 ) reported extraction of coal with ethanol-sodium hydroxide mixtures with the objective of maximizing extraction y i e l d ; no s u l f u r data were reported. Methyl alcohol reaction with a low v o l a t i l e bituminous West V i r g i n i a coal at higher temperatures (460-600 °C) was reported by Garner et a l . ( 6 ) . Promotion of coal g a s i f i c a t i o n was the objective of that study; s u l f u r content of the resultant char was not reported. An a r t i c l e by Amestica and Wolf (7) describes r e s u l t s of experiments c a r r i e d out in a batch reactor system s i m i l a r to that used in our investigations. These Investigators u t i l i z e d an I l l i n o i s No. 6 coal, employing either toluene or ethanol as s u p e r c r i t i c a l extractants. A batch autoclave was u t i l i z e d in t h e i r studies, and temperature and pressure ranges (350-450 °C, 5.89-19 MPa) were comparable to those we employ. As was the case with previous investigations reported in the l i t e r a t u r e , the major objective of t h e i r research was to study the d i s s o l u t i o n of coal materials by the s u p e r c r i t i c a l phase. They studied the e f f e c t s of temperature, pressure, solvent/coal r a t i o , and reaction time. Similar to the batch reactor system be employ, the f l u i d reaction products were vented through a condenser f o r c o l l e c t i o n of l i q u i d products, and the non-condensable components were vented through a sulfur scrubber system, employing zinc acetate s o l u t i o n . When ethanol was used as the s u p e r c r i t i c a l phase, sulfur r e j e c t i o n , expressed in terms of the s u l f u r recovered in the gas scrubber system, was reported to vary from 1.7% to 11.7% of the s u l f u r in the feed c o a l . However, no data were presented on s u l f u r content of e i t h e r the s o l i d or l i q u i d products. They noted that at 400 °C t h e i r l i q u i d y i e l d was higher than conversion, i n d i c a t i n g incorporation of carbon from the solvent. Incorporation in either the s o l i d or l i q u i d products could account f o r t h i s observation. In contrast to the previously reported work u t i l i z i n g s u p e r c r i t i c a l solvent extraction of coal, the major objective of our research e f f o r t is to develop a d e s u l f u r i z a t i o n process that w i l l r e s u l t in a s o l i d product suitable f o r combustion in e x i s t i n g coal f i r e d utility b o i l e r s .


7.

M UCH MORE ET AL.

Removal of Organic Sulfur from Coal

11


Experimental These experiments u t i l i z e d a 300cc s t i r r e d autoclave reactor, with associated purging, venting and product c o l l e c t i o n equipment, Figure 1. The coal, previously dried and ground to the desired p a r t i c l e size (generally -40 mesh), is charged to the reactor and alcohol is added. Heating the s t i r r e d mixture to above the critical temperature and pressure of the solvent (for ethyl alcohol 243°C and 6.38 MPa) r e s u l t s in a sequence of extraction and reaction processes that remove organic sulfur from the c o a l . After the desired reaction time (generally one hour), the f l u i d phase is vented from the reactor through a condenser system and l i q u i d and gaseous products are c o l l e c t e d . After cooling, the s o l i d product is collected from the reactor. For some experiments, pre-treatment of the coal with potassium hydroxide was employed in amounts of 5% of the weight of the coal charged to the reactor. Other pre-treatment methods Investigated include contact with 10% HC1 under varying conditions and soaking in N,N - dimethylacetamlde (DMA) to promote swelling of the coal p r i o r to s u p e r c r i t i c a l d e s u l f u r i z a t i o n in alcohol. The t o t a l sulfur concentration of the coal charged to the reactor and that of the s o l i d product were determined by use of a Fischer Total Sulfur Analyzer. Liquids were analyzed on a Perkin Elmer Sigma 3 gas chromatograph and gas analyses were performed on a Varian Model 3400 gas chromatograph. Results and

Discussion

Solvent e f f e c t s . For one set of experiments, a comparison of ethyl and methyl alcohols as s u p e r c r i t i c a l extractants was made over a temperature range of 275-450 °C, u t i l i z i n g three d i f f e r e n t coals of varying r a t i o of organic to p y r i t i c sulfur content. The coals were provided by the I l l i n o i s State Geological Survey (ISGS); they have been kept under a nitrogen atmosphere since the i n i t i a l size reduction following mining of the c o a l . Analyses of these coals, provided by the ISGS, are given in Table I. Sulfur forms were determined by wet chemical analysis according to the ASTM D2492 standard method for organic s u l f u r . The organic s u l f u r / p y r i t i c s u l f u r r a t i o varied from 0.72 to 2.82 for these coals. For a l l rune, the reaction time was 1 hr, and a solvent/coal r a t i o of 1/1 was used. The results of these batch runs are summarized in Figure 2, where the s u l f u r reduction obtained (evaluated on a concentration basis, comparing the t o t a l sulfur in the product char to that of the o r i g i n a l coal) is shown as a function of the organic e u l f u r / p y r i t i c s u l f u r r a t i o of the o r i g i n a l coal, with temperature as a parameter. E t h y l alcohol resulted in greater d e s u l f u r i z a t i o n (48%) than methyl alcohol at higher temperatures (400 °C) with the higher organic sulfur content coal, and methyl alcohol gave comparable desulfurization to ethyl alcohol at the lower temperature investigated (300 °C) with the lower organic sulfur content c o a l . The r e s u l t s with the high organic s u l f u r content coal confirm that organic sulfur is being removed by the s u p e r c r i t i c a l



ι

b

Figure 1.

CONDENSER

COLD TRAP

#Γ}-*«

VENT

o GAS SAMPLE BULB

Flow diagram of batch reactor system.

PRIMARY LIQUID COLLECTION

VARIAC

300 cc PACKLESS AUTOCLAVE BATCH REACTOR

α

COAL/SOLVENT MIXTURE TRANSFER LINE

GAS VOLUME APPARATUS

GC ANALYSIS

SECONDARY LIQUID COLLECTION


7.

MUCHMORE ET AL.

extraction/reaction process, since the t o t a l sulfur removal was excess of the p y r i t i c and sulfate s u l f u r content of the coal.


Table I.

79

Removal of Organic Sulfur from Coal in

Analyses of Coals (Mine Washed Coals Provided by the I l l i n o i s State Geological Survey)

ASTM ANALYSIS (as received basis) MOISTURE ASH VOLATILE MATTER FIXED CARBON SULFATIC SULFUR PYRITIC SULFUR ORGANIC SULFUR TOTAL SULFUR FREE SWELLING INDEX BTU(Parr Rapid Method)

Coal #1 13.7 8.5 37.7 40.1 0.06 0.94 2.65 3.65 5.5 10,903

Coal #2 13.4 5.6 37.0 44.0 0.06 1.69 0.96 2.71 4.5 11,562

Coal #3 4.98 7.90 37.18 49.9 0.11 0.98 1.53 2.62 4.5 12,630

c o l l e c t e d June 1, 1983 "Colchester (No. 2) coal from a north western I l l i n o i s s t r i p mine, collected June 23, 1983 Mixture of 80% Springfield (No. 5) coal with 20% Herrin (No. 6) coal from southern I l l i n o i s slope and s t r i p mines, respectively, blended in the washing plant, collected J u l y 15, 1983

c

Sulfur forms analyses on the product s o l i d s are not reported here, since r e s u l t s of the ASTM standard procedure can be misleading in terms of i n d i c a t i n g which type of sulfur ( p y r i t i c , sulfate, organic) has been removed. T y p i c a l l y , both sulfate and p y r i t i c sulfur are indicated to be present in low concentrations (generally less than 0.2%) when the ASTM procedure is applied to the s o l i d product from s u p e r c r i t i c a l d e s u l f u r i z a t i o n of coal with alcohols. The organic sulfur is defined as the difference between the t o t a l sulfur in the sample and the sum of the sulfur of the two mineral forms. I t has been recognized that during pyrolysis of coal, pyrite decomposition can r e s u l t in trapping of sulfur released from the pyrite within the coal matrix (8). Although the temperatures employed in t h i s work are lower than those at which pyrite decomposition might be expected, microscopic examination of the s o l i d product indicated conversion of pyrite to lower sulfur content mineral forms. Thus, it is l i k e l y that application of the ASTM procedure f o r determination of sulfur forms in the s o l i d product measures this "new" organic s u l f u r , in addition to the remaining - o r i g i n a l - organic s u l f u r . Potassium hydroxide pre-treatment. Another series of four runs was performed to compare methyl and ethyl alcohols as s u p e r c r i t i c a l f l u i d reactants, with and without Κ0Η (5% of the coal charge). Previous studies had Indicated enhanced d e s u l f u r i z a t i o n by pretreatment of the coal with a potassium hydroxide-alcohol solution. Reaction time was 2 hours at a reaction temperature of 340 °C; maximum reaction pressures were 17.3 to 30.8 MPa. The high organic



80


0 I 0

, 1 1 2 Ratio organic S/ p y r i t i c S

Γ"

3

Figure 2 . Comparison of ethyl and methyl alcohols as s u p e r c r i t i cal d e s u l f u r i z a t i o n f l u i d s , f o r coals of varied organic s u l f u r / p y r i t i c sulfur ratio, (ethanol , methanol — · — )



7.

MUCHMORE ET AL.


81

sulfur content coal ( t o t a l s u l f u r content 4.23%, organic S / p y r i t i c S r a t i o - 2.82; I.e., 73 % of t o t a l sulfur being organic sulfur ) was used for these runs; results are summarized In Table I I . Inspection of Table I I Indicates that t o t a l sulfur removals exceed the amount of p y r i t i c sulfur present In the coal processed, Indicating that organic sulfur Is being removed under the reaction conditions employed. Addition of potassium hydroxide decreased s o l i d product and l i q u i d recoveries and Increased gas production; t h i s was an anticipated r e s u l t due to the reported Influence of caustic on decomposition rates of alcohol at the reaction temperatures u t i l i z e d . The greatest desulfurization ( 54.0% reduction In s u l f u r concentration ) resulted In the methanol-KOH system. I t Is of interest to note that from 56 to 69% of the v o l a t i l e matter is retained in the s o l i d product compared to that of the o r i g i n a l coal, on a concentration basis. Heating values of the s o l i d products are one to seveη per cent greater than the o r i g i n a l coal, in spite of a s l i g h t increase in ash content. The higher ash r e s u l t i n g from the KOH treatment r e f l e c t s the greater conversion of coal, as well as the KOH i t s e l f . Gas and l i q u i d product analyses. Analyses of the gas products r e s u l t i n g from the batch experiments reported in Table I I are also given in that Table. The greater production of hydrogen when KOH was used is evident for both ethanol and methanol. As anticipated, no ethylene and much less ethane resulted from the methanol runs, compared to the ethanol rune; methane concentration was higher in the methanol runs. Gas chromâtograms of the l i q u i d products f o r two of the batch experiments reported in Table I I , were obtained where ethyl and methyl alcohols were used without KOH addition. A c a p i l l a r y column was used f o r the analyses, and the sample was s p l i t to two d i f f e r e n t detectors after passing through the column, giving dual traces on each chromatogram. A flame photometric detector (FPD), s p e c i f i c f o r sulfur containing compounds, and a flame Ionization detector (FID), sensitive to e s s e n t i a l l y a l l organic compounds, were used. Of the more than half dozen major sulfur-containing compounds observed on the FPD chromatograme, several have been i d e n t i f i e d . E t h y l s u l f i d e , ethyl d i s u l f i d e , thioacetal, and thiophene were indicated by GC/HS analysis. The p a r t i c i p a t i o n of the ethanol solvent (or a two carbon degradation product) in the d e s u l f u r i z a t i o n reactions is suggested by the structure of the sulfur-containing products thus far i d e n t i f i e d . The gas chromatogram of the l i q u i d product from the methanol run Indicates only two major sulfur compounds and lesser amounts of coal-derived organic material, compared to the ethanol run. E f f e c t s of alternate pre-treatment procedures. A v a r i e t y of pretreatment procedures, previously described, were performed to study the e f f e c t on s u p e r c r i t i c a l d e s u l f u r i z a t i o n . Table I I I presents the elemental analyses of the s o l i d products r e s u l t i n g from these runs. A l l pre-treatment procedures resulted in greater d e s u l f u r i z a t i o n than tests when no pre-treatment was used. In a l l cases the H/C r a t i o was reduced compared to that of the coal processed, suggesting that incorporation of the hydrogen-rich solvent In the coal



82


Table I I . Comparison of Product D i s t r i b u t i o n s f o r Methanol and Ethanol as S u p e r c r i t i c a l Extractants, and E f f e c t of Potassium Hydroxide

Ethanol, No KOH

Solvent System** Methanol, Ethanol, No KOH 5% KOH

Maximum Reaction 20.8 17.3 pressure (KPa) S o l i d y i e l d (% of coal 81.9 83.0 charged) L i q u i d recovery (% of 74.8 83.5 l i q u i d charge) Gas produced ( l i t e r s 13.8 9.2 at STP) % desulfurization 37.6 36.3 (cone, basis) Overall material 90.5 91.5 balance(%) S o l i d analyses (Moisture free) ISGS Std. Coal #1 (moisture free) % Total 2.64 2.69 Sulfur 4.23 % Volatile 23.2 23.8 matter 41.6 17.4 12.2 % Ash 10.4 12,990 13,263 Btu/lb 12,375 Gas Analyses (volume %) 17 9 Ho 23 35 CHa 1 1 2*4 27 30 9 fi L 0 22 20 COo 10 5 Other "Reaction Conditions: 60 g coal, 60 g ethanol , 340°C. 3.07% organic s u l f u r b

C

C

H

Methanol, 5% KOH

18.4

30.8

89.5

86.8

90.3

59.8

5.8

24.6

30.5

54.0

94.3

89.2

2.94

1.95

28.8 11.3 12,826

25.0 17.9 12,501

38 4 31 68 0 0 3 7 25 17 3 4 2 hr. reaction at

b


MUCHMOREETAL.

7.


83


structure did not occur to a s i g n i f i c a n t extent. Note that although the t o t a l sulfur concentration was reduced in a l l cases, the nitrogen content exhibits a very s l i g h t increase, due to conversion of coal material t o l i q u i d and gaseous products less r i c h in nitrogen than the parent coal* The greater increase in nitrogen r e s u l t i n g from the DMA pre-treatment was l i k e l y the r e s u l t of contribution from the solvent* Oxygen content was reduced f o r a l l the processing conditions employed.

Table I I I . Elemental Analyses of Solid Products from S u p e r c r i t i c a l Solvent Treatment of Coal under Various Reaction Conditions "S H/C RATIO H SOLVENT & CONDITION Ν 881 4.23 1.17 4.85 Coal Treated, ISGS std coal #1 66.05 625 2.75 1.39 3.95 75.82 EtOH 667 2.99 1.31 4.04 72.63 MeOH 539 2.60 1.24 3.32 73.90 EtOH + 5% KOH 634 1.96 3.72 1.25 70.37 MeOH + 5% KOH 747 2.41 4.42 2.56 70.97 DMA soaked 24 hr; EtOH 502 1.88 3.07 1.30 73.32 Acid leached^; MeOH + 5% KOH 661 1.53 1.23 3.96 71.91 Acid leached ; MeOH + 5% KOH 474 2.16 1.32 77.17 3.05 HC1 refluxed 1 hr; MeOH 552 1.47 1.12 69.16, 3.18 MeOH + 5% KOH; Two stage N,N-Dimethylacetamide A l l run at 350 C, 1 hr Soaked f o r 48 hrs in 10 % HC1 Soaked f o r 4 hrs, in 10 % HC1 d

a

c

Process p o t e n t i a l . The potential f o r processing a t y p i c a l high sulfur coal to produce a s o l i d product with less than 1% t o t a l sulfur by a sequence of physical b e n e f i c i a t i o n f o r pyrite reduction, followed by s u p e r c r i t i c a l extraction f o r removal of organic s u l f u r , is indicated by example In Table IV. A 3% t o t a l sulfur coal, containing equal amounts of p y r i t i c and organic s u l f u r , could be processed by e x i s t i n g technologies f o r removal of p y r i t i c sulfur to give a t o t a l sulfur concentration of, say, 1.8%. An a d d i t i o n a l 50% reduction of the remaining t o t a l sulfur would then give a f i n a l t o t a l sulfur concentration of 0.9%. The technical f e a s i b i l i t y of such an approach is demonstrated in Table IV, where data obtained on a sample of I l l i n o i s No. 5 seam cleaned coal from an I l l i n o i s Basin operating mine was processed by s u p e r c r i t i c a l extraction. The cleaned coal had a t o t a l sulfur content of 1.5 %; processing this p h y s i c a l l y cleaned coal f o r 1 hour at 350C in methanol with 5% KOH reduced the t o t a l sulfur concentration t o 0.75 %· The s o l i d product, which retained 56 % of the o r i g i n a l v o l a t i l e matter concentration, exhibited a value of 1.1 l b S0 / m i l l i o n BTU, thus meeting the e x i s t i n g new performance standard of 1.2 l b S0 / m i l l i o n BTU. 2

2


84


Table IV.

Example of Sequential Processing of Coal for Removal of P y r i t i c and Organic Sulfur. (Current experimental data are given in the l i n e d regions of the table.) PYRITIC S


COAL RUN OF MINE

1 PHYSICAL CLEANING L

1 SUPERCRITICAL [DESULFURIZATION

SULFATE S

TOTAL S

1.5%

1.5%

—

3.0%

—

—

—

>2.0%

0.3%

1.5%

—

1.8%

0.76%

0.06%

1.48%

0.66% 0.2%

1

ORGANIC S

0.02%

0.7%

—

0.9%

0.61%

0.12%

0.75%

I

J

1

Conclusions I t is apparent from the data thus f a r obtained that both ethyl and methyl alcohols are e f f e c t i v e f o r d e s u l f u r i z a t i o n of high organic s u l f u r content coals when used as extractants/reactants under s u p e r c r i t i c a l conditions. The data presented here does not indicate i f any s i g n i f i c a n t amount of p y r i t i c s u l f u r is being removed during the s u p e r c r i t i c a l d e s u l f u r i z a t i o n reactions; however t h i s p o s s i b i l i t y must be recognized in any proposed model of the system. Although s i g n i f i c a n t quantities of hydrogen s u l f i d e have not been detected in the gaseous products, the formation of hydrogen s u l f i d e as an intermediate compound when sulfur is released from organic moitiés in the coal matrix, as well as from p y r i t e , is a l i k e l y p o s s i b i l i t y . The detailed reaction pathways of the d e s u l f u r i z a t i o n reactions occurring in the presence of s u p e r c r i t i c a l alcohols has yet to be determined. The technical f e a s i b i l i t y of producing a compliance s o l i d f u e l by sequential processes of physical treatment f o r p y r i t i c sulfur removal, followed by s u p e r c r i t i c a l d e s u l f u r i z a t i o n with an alcohol for removal of organic s u l f u r , has been demonstrated. Acknowledgments This work was performed with grant support from the I l l i n o i s Coal Research Board and the U. S. Department of Energy.

Literature Cited 1. 2.

Whitehead, J.L.; Williams, D.F. Journal of the Institute of Fuel, December 1975. Tugrul, T.; Olcay, A. Fuel, July 1978, 57.


7. MUCHMORE ET AL. 3. 4.


5. 6. 7. 8.


85

Slomka, B.; Rutkowski, A. Fuel Processing Technology, 1982, 5. Maddocks, R.R.; Gibson, J.; Williams,D.F. Chem. Engr. Progress, June 1979. Makabe, M.; Hirano, Y.; Ouchi, K. Fuel, Hay 1978, 57. Garner, D.N.; Huffman, W.J.; Parker, H.W., Coal Processing Using Methanol as a Reactant, 79th National Meeting, AIChE, 8th Petrochemical and Refining Exposition, 1975, Houston, TX. Amestica, L. A; Wolf, Ε. E. Fuel, 1984, 63, 227. Cleyle, P.J.; Caley, W.F.; Stewart, I.; Whiteway, S.G. Fuel, 1984, 63, 1579.

RECEIVED April 1, 1986


Removal of Organic Sulfur from Coal by Reaction with Supercritical

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