Selective oxidation pretreatments for the enhanced desulfurization of coal

Fuel Vol 74 No. 2, pp. 193%2OO,1995 Copyright c 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/95/$10.00 +O.OO

Selective enhanced Stephen

oxidation pretreatments desulfurization of coal

R. Palmer,

Edwin

J. Hippo

and Xavier

for the

A. Dorai

Department of Mechanical Engineering and Energy Processes, Southern University at Carbondale, Carbondale, IL 62901, USA (Received 78 May 7993; revised 7 March 7994)

lllinois

The desulfurization of selectively oxidized coals and unoxidized control coals was investigated using mild pyrolysis and various base treatments. Both an Illinois No. 6 and an Indiana No. 5 coal were selectively oxidized with peroxyacetic acid in the pretreatment step and then treated with various hydroxide and carbonate bases, using either water, methanol or ethanol as the solvent. Reaction variables investigated include reaction temperature, reaction time, pyrolysis pressure, and the level of oxidation in the pretreatment step. In general, it was found that selective oxidation, when combined with subsequent desulfurization reactions, always led to greater sulfur removal. In addition, the reactivity of the sulfur in the coal towards desulfurization was apparently enhanced by the selective oxidation pretreatment. Thus, the severity of desulfurization conditions can be reduced by employing this pretreatment. Sulfur removals of up to 95% were obtained in some cases. Usually the most effective treatments involved carbonate bases under supercritical alcohol conditions. The desulfurization of some selectively oxidized, sulfur containing model compounds was also observed under similar reaction conditions. (Keywords:desulfurization; selective oxidation; coal)

of sulfur from coal prior to its combustion could lead to considerable reductions in the emissions of acid rain precursors ‘. Although inorganic sulfur species can often be removed by physical cleaning processes, the organic sulfur cannot be removed unless chemical desulfurization strategies are employed. Over the years, many chemical desulfurization approaches have been applied to coal with differing levels of successz-4. For example, the use of aqueous ferric sulfate and ferric chloride solutions5*6 has received attention for the desulfurization of coal and led to the development of the Meyers process73s. Unfortunately, no or very little organic sulfur removal is obtained by this process. Many studies have used bases such as sodium carbonateg.“, sodium hydroxide”*‘2, potassium hydroxide13 and selected organic bases14 to effect the removal of sulfur from coal. Unless used in the molten state15-lg, bases usually remove inorganic sulfur only. In molten caustic leaching (MCL) processes, desulfurizations around 90% are routinely obtained and high levels of organic sulfur removal are reported’g.20. Other processes, such as perchloroethylene (PCE) extraction2’, have shown some potential. However, results to date have been far from conclusive and, since PCE emissions may be very harmful to the protective ozone layer, there are environmental concerns regarding its use. A two-stage reaction involving electron transfer agents and base catalysed desulfurization with a The removal

Presented at ‘Coal Utilization Orlando, USA

and the Environment’,

18-20 May 1993,

superbase was recently investigated by Chatterjee et ~1.~~. Although very good organic desulfurization results were reported (5(r90°/, removal), the chemical reagents used are too expensive for the development of a desulfurization process. Other recent work involves the use of nickel boride, which has the ability to remove organic sulfur from model compounds23. However, most chemical desulfurization approaches that have been applied to coal have involved oxidation reactions. These approaches include the metal salts solutions mentioned earlier, air oxidation procedures24, oxygen dissolved in various solvents25*26,the use of chlorination2’, and treatments with hydrogen peroxide28-30, sodium hypochlorite (bleach)31*32, potassium permanganate33 and a number of other chemical oxidants. Desulfurization is obtained during all of these reactions, but very few effect a significant removal of organic sulfur at low temperatures. In a previous investigation, peroxyacetic acid was examined for the chemical desulfurization of coa134,3s. This reagent was chosen because it generally reacts with organic sulfur species preferentially, and it was hoped that this selectivity would result in high levels of sulfur removal. Unfortunately, it was found that only lO-25% of the organic sulfur could be removed without extensive dissolution of the coal. However, mineral forms of sulfur were effectively dissolved. Although this result was disappointing, it was realized that the remaining organic sulfur in the oxidized coals should have been converted to the sulfoxide, sulfone or sulfonic acid form. Since the C-S bond strength of these

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of coal: S. R. Palmer et al.

and the contents immediately filtered. The filter was washed with 100 ml hot distilled water and dried in a vacuum oven. Each sample was submitted for total sulfur, ash and moisture analysis and then examined for potential desulfurization at 35o”C, under both pyrolysis and supercritical methanol (SCM)/base conditions36,37. The oxidized sample that provided the greatest level of desulfurization under these conditions was used to identify the level of selective oxidation required in the pretreatment step. Once this was established, larger quantities of both coals were oxidized under the same conditions for use in all subsequent desulfurizations. The proximate, ultimate and sulfur forms data for these samples are reported in Table 1.

oxidized sulfur species is weaker than that of the unoxidized divalent sulfur forms, it was anticipated that these coals would be easier to desulfurize after an oxidative pretreatment. This paper investigates this possibility by examining the enhanced desulfurization of the selectively oxidized coals under a wide variety of desulfurization conditions. Thus, this paper is significantly different from the previous published work because it explores selective oxidation as a pretreatment for subsequent desulfurization and not for the desulfurization itself. To evaluate selective oxidation as a pretreatment for desulfurization, both an Illinois No. 6 and an Indiana No. 5 coal were selectively oxidized with peroxyacetic acid. Initially, a variety of oxidation conditions were studied to establish the level of oxidation necessary for subsequent desulfurization. Each selectively oxidized coal, together with its unoxidized parent control coal, was then reacted with various bases and solvents under a variety of conditions. Experimental variables investigated include reaction temperature, reaction time, reactor pressure, type of base and type of solvent. In every case, the level of desulfurization obtained for the pretreated coal was compared to that of the untreated coal. Desulfurization of oxidized sulfur containing model compounds was also performed under the conditions found successful for coal desulfurization.

Pyrolysis experiments

A 1 g sample of coal (pretreated or unoxidized) was placed in a 10ml stainless steel microreactor equipped with a three-way valve, purged with nitrogen and immersed in a fluidized sand bath set at the desired temperature. After reaction the reactor was cooled in cold water and the contents submitted for total sulfur, ash and moisture analysis. Lower pressure pyrolysis was obtained by attaching either a vacuum line (approximately 400 Pa) or a nitrogen purge (approximately 1 x lo5 Pa) to the reactor valve assembly. Base desulfurization reactions

EXPERIMENTAL Cocrl prepuration Two coal samples were obtained from the Illinois Basin Coal Sample Program (IBCSP). These coals have the sample bank codes IBC-101 and IBC-106. The IBC-101 coal is a sample of Herrin No. 6 (Illinois No. 6) and the IBC-106 coal is a sample of Indiana No. 5 (known as Illinois No. 5 for the same coal seam in Illinois). These coals were used because they have high organic sulfur contents. Aliquots (500 g) of each coal were stage ground to -400 mesh. Each was submitted for proximate, ultimate and sulfur forms analysis (see Table 1).

A 1 g sample of the selectively oxidized or unoxidized (control) coal was placed in a lOm1 stainless steel microreactor together with 2.5 mmol of base dispersed in 5 ml of water, methanol or ethanol. The microreactor was then purged with nitrogen, sealed and immersed in the fluidized sand bath for 1 h. Temperatures used included 250, 350 and 450°C. After reaction the microreactor was cooled in cold water and the contents washed with dilute HCl and then distilled water. The dried products were analysed for total sulfur, ash and moisture. Blank experiments were also performed in which the solvent, but no base, was added. Model compound reactions

Selective oxidation pretreatment

To investigate the level of selective oxidation required in the pretreatment step six oxidations were performed on each coal. These included 1, 6, 24 and 72 h at 21°C (room temperature), 6 h at 50°C and 5 min at 104°C. The typical procedure involved dispersing 4g of coal into 120ml of glacial acetic acid, warming to the desired reaction temperature and then adding 40ml of H,O, solution (30% w/v). After reaction, the vessel was cooled

The model compounds that were investigated include dibenzothiophene sulfone, phenyl sulfone and dodecyl sulfone. Each of these model compounds was treated for 1 h at 350°C with water, methanol, methanol/KOH, methanol/K&O,, water/Na,CO, and without additive (standard pyrolysis). Microreactors, 2 ml in size, were used. The base to solvent ratio was the same as that used in the coal experiments and was prepared in slurry form. The model

Table 1 Proximate and ultimate data (wt% dry, except for moisture) S

Sample

Moisture

Ash

C

H

N

SO,

Pyritic

Organic

Total

Volatile matter

Fixed carbon

IBC-101 Raw Oxidized

2.9 4.2

10.5 8.1

65.8 64.9

4.0 3.5

1.2 1.1

0.1 0.2

1.2 0.1

3.1 2.4

4.5 2.1

39.6 41.4

44.5

3.6

8.9

71.4

4.1

1.6

0.3

1.8

2.0

4.1

39.3

51.8

7.1

6.9

65.1

4.1

1.4

0.2

0.0

1.5

1.7

48.6

44.5

IBC-106 Raw Oxidized

194

Fuel 1995 Volume 74 Number 2

49.8

Desulfurization Table 2 sample

Effect ofextent ofoxidation on desulfurization for the IBC-101

Dry sulfur content (%) Oxidative pretreatment Time (h) 1 6 24 72 6 0.083

Temperature (“C)

Yield” (%)

After oxidation

After oxidation and pyrolysis

_

100.0 94.0 93.3 93.2 85.5 78.9 79.5

4.5 3.4 3.1 2.9 2.6 2.5 2.5

4.2 3.4 3.1 2.8 2.8 2.3 2.4

21 21 21 21 50 104

After oxidation and SCM/base 2.0 2.0 1.5 1.3 0.9 0.7 1.5

“Yields based on weight of oxidation residue only. If weight of soluble portion is included, then yields approach 100%

compound was added to 1 ml of the base slurry before being placed in the reactor. The amount of model compound used was such that the sulfur to base ratio involved was about the same as that used in the coal desulfurizations. After reaction the reactor was cooled, carefully vented, and the contents washed out with 20 ml methylene chloride. The methylene chloride solutions were then dried over anhydrous magnesium sulfate before gas chromatography-flame photometric detector (g.c.-FPD) analysis. Instrumental analysis

Proximate analyses were performed using a Leco MAC-400 proximate analyser. Elemental determinations were made using a Leco CHNS-920 elemental analyser. Total sulfur analyses were obtained from a Leco total sulfur analyser. Some ash and moisture determinations were made using a Perkin Elmer TGA-7, thermogravimetric analyser. Gas chromatography was performed on a Varian 3400 g.c. using a sulfur selective FPD. RESULTS

AND DISCUSSION

Selective oxidation pretreatments

Peroxyacetic acid was originally used for the selective oxidation of organic sulfur compounds in coal for their eventual characterization36*37. Organic peroxide reagents, such as peroxyacetic acid, are well known for their ability to oxidize sulfur selectively. The faster rate of oxidation of sulfur over carbon is due to the strong electrophilic nature of the oxidizing species (hydroxyl cation) and the fact that sulfur is a stronger nucleophile than carbon38. This is why dibenzothiophene can be reacted for extended periods of time with a many fold excess of peroxide, and yet the sulfone is the predominant product. No or very little reaction at carbon is observed39. Thus, as long as the oxidation conditions are controlled, the result is a product where organic sulfur species have been converted to their sulfoxides, sulfones and sulfonic acids, but most of the carbon structures remain unoxidized. This is why the term selective oxidation is used. To investigate the level of selective oxidation required in the pretreatment step both the IBC-101 -400 mesh and the TBC-106 -400 mesh coal samples were oxidized with peroxyacetic acid using six different

of coal: S. R. Palmer et al.

sets of conditions as described in the Experimental section. These conditions were chosen because previous experimentation had demonstrated that yields of solid selectively oxidized coal of around 80% and better could be obtained34.35. If the weight of the solubilized portion of the coal is taken into account, then yields approach 100%. The oxidized samples were then tested for enhanced desulfurization using standard pyrolysis and supercritical methanol (SCM)/NaOH conditions of 1 h at 350°C. The dry sulfur contents of each oxidized IBC- 101 and IBC- 106 coal sample, together with that of their subsequent pyrolysis and SCM/NaOH reaction products, are shown in Tables 2 and 3. The yields of the oxidized coals are also reported. From Tables 2 and 3 it is clear that the extent of selective oxidation has little effect on the amount of sulfur removed by pyrolysis at 350°C. Indeed, in some cases the sulfur content apparently increases slightly after pyrolysis. However, under the conditions of the SCM/base reaction, the extent of oxidation in the pretreatment step has a pronounced effect and large variations in the sulfur contents of the products can be seen. It appears that as the level of oxidation in the pretreatment step increases (as measured by the yield of solid products after oxidation) the greater is the desulfurization obtained in the subsequent SCM/base treatment. The exceptions to this are the coals that were oxidized for 5 min at 104°C. Although these samples have the lowest sulfur contents after the selective oxidation treatment, the sulfur that remains in these oxidized coals appears less susceptible to further desulfurization with SCM/base as compared to most of the other oxidized coals. The reasons for this are not clear, but it may be due to the very short reaction time and the subsequent lack of reagent penetration into the coal structure. On comparing the oxidized coals with the unoxidized control coals, it is clear that the oxidative pretreatment is necessary for the formation of very low sulfur products. Indeed, on a concentration basis, up to 84% of the sulfur in the IBC-101 coal and 86% of the sulfur in the IBC-106 coal was removed. Further examination of the data in Tab/es 2 and 3 revealed that the selective oxidation pretreatment with peroxyacetic acid at 50°C for 6 h gave the lowest

Table3 sample

Effect ofextent ofoxidation on desulfurization for the IBC-106

Dry sulfur content (%) Oxidative pretreatment Time (h)

Temperature (“C)

Yield” (%)

After oxidation

After oxidation and pyrolysis

21 21 21 21 50 104

loo.0 95.7 95.2 93.9 87.9 82.8 90.9

4.1 2.6 2.1 1.9 1.8 1.8 1.4

4.2 2.5 2.0 1.9 1.9 1.9 1.7

_ 1 6 24 72 6 0.083

After oxidation and SCM/base 1.7 1.4 0.9 0.8 0.6 0.6 1.2

“Yields based on weight of oxidation residue only. If weight of soluble portion is included, then yields approach 100%

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of coal: S. R. Palmer et al.

%DryS sr Raw 4r

* . . . . . . . ------.---*--.----.-----

OL0

. . . . . . . . . . . .._._............................

Oxidized

c 15

60

30 Pyrolysis

90

120

Tme

(Min)

Figure 1 The effect of increasing pyrolysis time on the sulfur content of the raw and selectively oxidized IBC-101 coal at 350°C

sulfur products after subsequent desulfurization with SCM/base. Large scale oxidation reactions were then performed on both coals using these conditions. The products were analysed and stored under nitrogen until required. The analysis results are given in Table 1. It is clear that 40% of the sulfur in the IBC-101 coal and 59% of the sulfur in the IBC-106 coal can be removed by the oxidative pretreatment. This is primarily due to pyrite dissolution, but removal of up to 20% of the organic sulfur has also been observed under similar oxidative treatments34v35. It should be noted that the sulfur contents of the oxidation products from the larger scale oxidation reactions are similar, but not identical, to those shown in Tables 2 and 3. This is most likely due to slight variations in the degree of oxidation and the experimental error associated with the sulfur determination. Variable pyrolysis conditions

Having established a level of selective oxidation necessary for effective desulfurization during subsequent treatments, it was necessary to evaluate the behaviour of the oxidized coals under mild pyrolysis conditions in more detail. The pyrolysis experiments conducted at 350°C for 1 h during the screening of the various oxidative pretreatments served as the basis of subsequent pyrolysis studies in which the pyrolysis temperature was varied from 250 to 450°C the pyrolysis time varied from 15 to 120min, and the pyrolysis pressure varied from 400 to -2.5 x 10’ Pa. Figure 1 shows how the sulfur content of both the raw and oxidized IBC-101 coal varies with increasing pyrolysis time at 350°C. It is clear that increasing the duration of pyrolysis has very little effect on the level of desulfurization that can be obtained for either the pretreated or untreated coal. Indeed in both cases the most significant drop in sulfur content occurs in the first 15 min of pyrolysis, after which the sulfur content remains almost constant. Previous workers have observed sulfur contents initially decrease and then increase with increasing pyrolysis time4’s4r. This phenomenon was attributed to the reincorporation of sulfur gases into the coal by retrogressive reactions. Although some of the data show increases in sulfur content with increasing pyrolysis time, the levels are quite small and it is difficult to be conclusive regarding sulfur reincorporation.

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The effects of varying the pyrolysis pressure and temperature are shown in Figure 2. In general, it can be seen that increasing temperature and reducing pressure lower the sulfur content of the product. By comparing the pyrolysis data of the selectively oxidized coal with that for the control unoxidized coal, it is found that sulfur removal starts at an earlier temperature when the coal has been selectively oxidized. For example, virtually no sulfur removal (on a concentration basis) is observed for the control coal at 350°C. However, over 13% of the sulfur is removed from the selectively oxidized coal under the same conditions. This suggests the sulfur moieties in the oxidized coal have been activated towards thermal decomposition by the selective oxidation pretreatment. Greater levels of sulfur removal are obtained for both the oxidized and unoxidized coals at a temperature of 450°C. However, the selectively oxidized coal continues to desulfurize more effectively than the unoxidized control samples. Indeed, under enclosed pyrolysis conditions of 450°C (~2.5 x 10’ Pa) for 1 h, only 17% of the sulfur is removed from the unoxidized sample, while 46% of the sulfur can be removed from the selectively oxidized coal. If the amount of sulfur removed in the pretreatment step is combined with that obtained during pyrolysis, then over 65% of the sulfur in the IBC-101 coal has been removed. This can be increased to over 70% by performing vacuum pyrolysis on the selectively oxidized coal. Similar vacuum pyrolysis of the unoxidized coal removed only 22% of the sulfur. Although the yields for the oxidized coal pyrolysis products were approximately 2-10% lower than those obtained for the unoxidized coals, these results clearly demonstrate that large amounts of sulfur can be removed from the selectively oxidized coal using simple thermal and low pressure procedures. Desulfirization using bases

As mentioned earlier, preliminary screening with SCM/NaOH treatments was performed to establish the level of selective oxidation necessary in the pretreatment step. Many other combinations of additives were subsequently investigated for the desulfurization of both oxidized and unoxidized IBC-101 and IBC-106 coals. In addition, the effect of varying the reaction temperature was investigated for the IBC-101 coals. The various treatments used are outlined in Tables 4 and 5. (It should be noted that the data in Tables 4 and 5 were obtained

%DryS

‘----,:

. .. . ..--------

=:x:::::z

. . . . . . . .._._......

.____. ____ --.--- .___

_._

*----.....__

-:~:~:II_x.__..__.~

2

.-.---

.

__

.

.

..__

---.....

Dxldized IBC 101 . . .._.____ ___

--- . . . . 1::::::::

oL__250

:::_;

-____)

450

350 Temperature

(”C,

Figure 2 The effect of pyrolysis temperature and pressure on the sulfur content of the raw and selectively oxidized IBC-101 coal (1 h duration): 0, enclosed; x , atmospheric N,; 0, vacuum

Desulfurization Table 4 Table of additives and conditions used to desulfurize the IBC-101 coal (selectively oxidized and unoxidized control coals) % dry sulfur Unoxidized coal (“C)

Oxidized coal (“C) .

Treatment/additives

250

350

450

250

350

450

1 2 3 4 5 6 7 8 9 10

4.5 4.2 3.7 3.6 3.6 3.7 3.6 3.7 3.8 3.6 3.8

4.5 4.2 3.4 3.0 2.7 2.9 3.3 3.5 2.8 2.5 1.9 2.3 2.5 1.6 2.9 2.1 1.9 2.8 3.5 2.9

4.5 3.5 3.2 2.7 2.2 2.5 2.8 2.4 1.7 2.1 1.1 1.6 2.3 2.1 2.8 2.4 1.6 2.2 1.8 1.7

2.1 2.6 2.4 2.3 2.4 2.1 2.5 2.4 2.3 2.3 2.1 2.4 ~ 2.1 2.7 2.8 2.1 2.7 2.5

2.7 2.3 2.0 2.1 2.0 1.6 1.8 2.1 2.1 1.3 1.0 0.9 1.8 0.8 1.6 1.4 1.0 1.7 1.2 1.1

2.1 1.5 1.6 1.4 1.3 1.2 1.7 1.8 0.9 _ 0.2 0.8 0.8 0.3 1.3 1.3 0.7 1.0 0.5 0.7

11 12 13 14 15 16 17 18 19 20

None Pyrolysis Water Water/NaOH Water/KOH Water/Na,CO, Water/CaCO, Water/lime Methanol Methanol/NaOH Methanol/KOH Methanol/Na,CO, Methanol/NaHCO, Methanol/K,CO, Ethanol Ethanol/NaOH Ethanol/KOH Ethanol/Na,CO, Ethanol/NaHCO, Ethanol/K&O,

_ 3.9 _ _ 3.9 _ _ _

using the products from the large scale oxidations, and slight discrepancies between the data in Tables 2 and 3, and Tables 4 and 5, are to be expected.) From the data in Tubles 4 and 5 it is clear that the sulfur contents of the samples that received both the selective oxidation pretreatment and the base desulfurization treatment are consistently much lower than those that received only the base desulfurization treatment. For example, by examining the 350°C data in Tubles 4 and 5, it can be seen that levels of desulfurization for the unoxidized control IBC-101 coal were rarely over 50%. For the selectively oxidized IBC-101 coal, combined desulfurizations over 70% and even 80% were routinely obtained. Desulfurization of the selectively oxidized IBC-106 coal was even more successful, with sulfur removals in the 7&90% range obtained in all but a few cases. Without the oxidative pretreatments, sulfur removal levels were only occasionally better than 50%. This demonstrates that the sulfur removal obtained during selective oxidation pretreatment and that obtained in subsequent treatments is substantially additive. In addition, since most of the pyritic sulfur is removed in the selective oxidation step it is clear that the subsequent desulfurization must involve organic sulfur. Indeed, the sulfur contents of most of the products are too low for there not to have been organic sulfur removal. Another way to examine the desulfurization data is to compare the percentage desulfurizations for the oxidized and unoxidized samples obtained in the base-solvent reactions only. In this way the amount of sulfur removed in the pretreatment step is ignored, and hence the susceptibility of the sulfur remaining in the oxidized coals towards desulfurization can be compared with that of the sulfur species in the raw coals. If this is done, it becomes clear that for most of the base-solvent reactions studied the removal of sulfur appears easier after selective oxidation. Although the increases in percentage desulfurization obtained after selective oxidation may be

of coal: S. R. Palmer et al.

due to the removal of pyrite in the pretreatment step (and hence a redistribution of the sulfur forms requiring removal), they are also entirely consistent with the weakening of the C-S bonds in the coal by the conversion of divalent organic sulfur in the raw coal to sulfones, sulfoxides and sulfonic acids in the oxidized coal, Temperufure effects. The effect of varying the reaction temperature of the base desulfurization reactions can be easily observed from Tubles 4 and 5. In general, treatments at 250°C had only a marginal effect on the sulfur contents of both the unoxidized and the oxidized samples. However, as the temperature was increased from 250 to 35O”C, and then to 45O”C, more desulfurization was always observed. At 45O”C, sulfur contents as low as 0.23 and 0.29% were obtained for the oxidized IBC-101 coal with methanol/KOH and methanol/K&O,, respectively. These levels of sulfur removal represent total sulfur removals of over 94%, and organic sulfur removals of over 92%. It should be noted that these levels of sulfur removal are beyond those demanded by the Clean Air Act and were obtained for a high sulfur coal with a very high organic sulfur content (75% of the total sulfur). Efict of dzfirent bases. As can be seen from the data in Tables 4 and 5, many of the carbonate base systems were generally more effective desulfurizing agents than the equivalent hydroxide base systems under equivalent conditions. For instance, the water/Na,CO, combination was found to be far superior for the desulfurization of the oxidized coals than any of the aqueous hydroxide bases. Indeed, nearly 65% of the sulfur in the pretreated IBC-101 coal and nearly 85% of the sulfur in the pretreated IBC-106 coal can be removed using aqueous sodium carbonate at 350°C. It was anticipated that the hydroxide bases would be more effective for desulfurization, because they are stronger bases and are known to provide good levels of desulfurization in the molten caustic leaching procedures 2o. The fact that carbonate bases, such as sodium carbonate and potassium carbonate, provide

Table 5 Table of additives and conditions used to desulfurize the selectively oxidized IBC-106 coal (all at 350°C) % dry sulfur Treatment/additives

Unoxidized coal

Oxidized coal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

4.1 3.7 2.5 2.4 1.9 2.4 2.8 2.5 2.0 1.1 1.9 1.7 1.6 2.3 2.5 1.8 2.2 1.2

1.7 _ 1.2 1.1 1.2 0.9 1.2 1.2 1.0 0.8 0.5 0.6 0.6 1.1 0.8 1.1 0.9 0.5

None Pyrolysis Water Water/NaOH Water/KOH Water/Na,CO, Water/CaCO, Water/lime Methanol Methanol/NaOH Methanol/KOH Methanol/Na,CO, Methanol/K,CO, Ethanol Ethanol/NaOH Ethanol/KOH Ethanol/Na,CO, Ethanol/K&O,

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Desulfurization

20

of coal: S. R. Palmer et al.

30

40

Desulfurized

50

60

aqueous

70

system

80

90

(%)

Figure 3 Percentage desulfurization obtained for the raw (EI) and selectively oxidized (+) IBC-101 coal obtained upon changing the solvent from water to methanol (350°C reactions)

outstanding levels of desulfurization is especially encouraging since these are among the cheapest chemical reagents presently available. At this time the role of the bases in these reactions, and the reason for the superiority of the carbonate systems over the hydroxide systems in many cases, is unclear. The reaction of hydroxide bases with organic sulfur moieties has been investigated with the primary aim of elucidating the chemistry occurring in the molten caustic leaching process20*42. Benzothiophene and dibenzothiophene were reacted under the molten caustic conditions and found to undergo a ring opening reaction. These displacement reactions resulted in desulfurization of the model compounds, but a number of different products were formed, suggesting the reaction is not simple 20*42. The reaction with the sulfoxide, sulfone or sulfonic acid derivatives, as would occur with the oxidized coals, may alter this reaction pathway. To complicate the issue further, the presence of a solvent with the base opens the possibility that other reactions may be responsible for the observed desulfurization. For example, in the case of the oxidized coals the treatment with methanol, or aqueous NaOH or aqueous KOH did not significantly desulfurize the coal any more than water alone. However, when methanol and the base are mixed together significantly higher levels of desulfurization are observed. This suggests that some synergistic interaction between the methanol and the base occurs. This interaction may involve the reaction between methanol and base to form a more active desulfurizing agent or it may involve a more complicated pathway. In previous studies it was determined that the alcohol and base could react together to form gases such as H,, CO and C0240*41,43. Thus, it is possible that these gases could contribute to the observed desulfurization.

point falling on the equivalency line indicates that the choice of solvent is insignificant. Similarly, a data point not on the equivalency line indicates that one solvent is better than the other (for that particular base and temperature). As can be seen from Figure 3, most reactions using methanol were better than the equivalent aqueous reaction for both the oxidized and unoxidized coal. Using this form of analysis it was determined that in general the alcohol solvents were better than water, and that methanol was marginally better than ethanol. The only exception to this was the comparison of aqueous and ethanol desulfurizations for the IBC-106 coal. In this particular case, as can be seen from Figure 4, the water and ethanol were reasonably equivalent. Effect of increasing reaction time. The effect of increasing the reaction time of the base desulfurization reaction was investigated for four different systems. The results are shown in Figure 5. With the exception of the methanol/K&O, system at 250°C no improvements in

20

30

40

Desulfurized

solvents.

In order to clarify the effect of changing the solvent system on the base desulfurization reactions, the percentage desulfurization obtained with equivalent bases and temperatures, but with different solvents, were plotted against each other. Representative plots are shown in Figures 3 and 4. In this way a data

198

Fuel 1995 Volume 74 Number 2

60 aqueous

70

80

system

90

100

(%)

Figure 4 Percentage desulfurization obtained for the raw (m) and selectively oxidized (+) IBC-106 coal obtained upon changing the solvent from water to ethanol (350°C reactions)

I

Effect of d@rent

50

0

2

I

4

I

6

I

8

Reaction time (h) Figure 5 The effect of increasing reaction time on the desulfurization of the selectively oxidized IBC-101 coal for a selection of base treatments: ? ,?methanol, Na,CO,, 350°C; +, water, Na,CO,, 250°C; ?? , methanol, K,CO,, 250°C; and A, methanol, K&O,, 350°C

Desulfurization Table 6

Elemental composition of desulfurized products (dry wt%)

Treatment

Coal no.

C

H

N

S

Ash

H,O, Na,C03, 350-C CH,OH, KOH, 350-C CH,OH, K&O,, 350°C C,H,OH, K,CO,. 450-C CH,OH, KOH, 450-C CH,OH, KOH, 350 C

101 101 101 101 101 106

11.9 14.9 74.7 67.4 72.4 73.1

5.1 5.0 5.1 4.4 5.1 4.3

1.2 1.2 1.2 1.1 1.2 1.6

1.6 1.1 0.8 0.7 0.2 0.5

8.1 12.7 13.2 16.4 12.3 12.9

desulfurization were obtained by increasing the reaction time. This is significant, because it suggests that extended reaction times may not be necessary for enhanced levels of desulfurization. Indeed, similar levels of desulfurization may be obtained with reaction times substantially less than the 1 h minimum used in these experiments. Evaluation ofproduct quality. To evaluate the quality of the desulfurized products elemental analysis was performed on selected desulfurized products derived from the selectively oxidized coals. The results are shown in Table 6. On comparison with the elemental composition of the raw coals (Table 1) it can be seen that the typical final desulfurized products had appreciably higher carbon and hydrogen contents relative to the original coal. Nitrogen contents remained unchanged and ash contents were slightly higher. The increase in the carbon and hydrogen contents suggests that alkylation reactions between the coal and the alcohol solvent occur. The increase in ash content may be due to the concentration of the ash in the original sample, but it is suspected that at least some of the residual ash is due to the incomplete removal of the base and its salts during the washing of the products. Since the desulfurization reactions were performed on small sample sizes (a condition set by the size of the microreactors), insufficient sample was available for direct specific enthalpy measurement. However, using Dulong’s formulae44 and the available elemental analysis, a value of 32331 kJ kg-’ was calculated for a typical final product. This value is slightly higher than that of the original coal (31703 kJ kg-i). Thus it is clear a high quality product can be produced which has very low sulfur and a high specific enthalpy value. Due to the problems of handling the small sample sizes it was difficult to measure the yields of products accurately, and hence specific enthalpy recoveries could not be measured with certainty. Using a conservative estimate of 75% recovery of solid material, specific enthalpy recovery would be at least 75%. If the specific enthalpy values of any liquid and gaseous products are considered, the actual enthalpy recovery could be much higher. Studies involving sulfur model compounds

To examine what types of oxidized organic sulfur species may be involved in the desulfurization reactions, phenyl sulfone, dodecyl sulfone and dibenzothiophene sulfone were treated as described in the Experimental section. The reaction products produced under the various desulfurization conditions were analysed using sulfur selective gas chromatography. Phenyl sulfone was essentially quantitatively recovered after reaction with aqueous sodium carbonate at 350°C

of coal: S. R. Palmer et al.

with only a very small amount of an unknown sulfur containing compound also being detected. However, only a trace of dodecyl sulfone, and no dibenzothiophene sulfone, could be detected after reaction under the same conditions; indicating that these sulfur moieties are susceptible to desulfurization under these conditions. Methanol with no base at 350°C appeared very effective for the desulfurization of both phenyl sulfone and dodecyl sulfone; with no sulfur compounds being detectable by gas chromatography. However, dibenzothiophene was recovered unchanged. The absence of any g.c. amenable sulfur containing compounds after reaction of methanol with the non-thiophenic sulfones is intriguing, for it suggests that the sulfur was converted to either a volatile form, such as H,S, that escaped upon opening the microreactor, or to a condensed form which is too involatile for g.c. analysis. The methanol and potassium hydroxide combination appears to be very effective in attacking sulfone structures, with no or very little of the starting materials being present in any of the reaction products. A small amount of dibenzothiophene was detected in the products from dibenzothiophene sulfone. This suggests that a reducing environment was established during the methanol-potassium hydroxide reaction. The methanol and potassium carbonate combination was also very effective in reactions with the sulfones. Of the three model compounds studied only a trace amount of phenyl sulfone could be detected after reaction. Both standard pyrolysis and treatment with water at 350°C did not appear to affect any of the model compounds studied, each being recovered unchanged after treatment. CONCLUSIONS It has been shown that selective oxidation of coal is a very effective pretreatment for the enhanced desulfurization of coal. Indeed, in every desulfurization reaction performed, the selectively oxidized coal always gave a much lower sulfur content product than that derived from the unoxidized control coal. The selective oxidation pretreatment itself removes virtually all pyritic sulfur, and some of the organic sulfur, and hence has value as a desulfurization strategy in its own right. In addition, the pretreatment also appears to activate the remaining organic sulfur towards desulfurization with many of the reagents investigated. Although this activation may be due to changes in the distribution of sulfur forms requiring removal (since pyrite is removed in the pretreatment step), it is entirely consistent with the weakening of C-S bonds in the coal via the conversion of divalent sulfur species to sulfones, sulfoxides or sulfonic acids during the oxidative pretreatment. The levels of desulfurization obtained at 350°C approach 85% for both the IBC-101 and IBC-106 coals. At 450’C the levels of desulfurization are around 95%, which is beyond that demanded by the Clean Air Act. The highest levels of desulfurization were obtained when the pretreated coal was reacted with bases dissolved in alcohol solvents. However, aqueous bases also provided good levels of desulfurization. In general, carbonate bases were found to be just as effective, if not more so, than hydroxide bases, and methanol was usually superior to ethanol. It is suspected that synergistic interactions

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Desulfurization

of coal: S. R. Palmer et al.

between the bases and the alcohol solvents are responsible for much of the desulfurization observed. It was determined that the extent of oxidation in the pretreatment step is important in governing the amount of sulfur removed in any subsequent treatments. In addition, increasing the temperature of the base-solvent desulfurization reactions led to increased levels of sulfur removal, but increasing the residence time beyond 1 h generally provided little additional sulfur removal. Lowering the pressure during pyrolysis experiments gave improved sulfur removals, which suggests that regressive reactions in which sulfur is reincorporated into the coal, may be involved at higher pressures. Lastly, the desulfurization of sulfur containing model compounds was indicated under a number of reaction conditions that also desulfurize coal.

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ACKNOWLEDGEMENTS Support for this study was received from the Illinois Department of Energy and Natural Resources (IDENR) through the Illinois Coal Board and the Illinois Clean Coal Institute (ICCI), and from the US Department of Energy. Coal samples were provided by the Illinois State Geological Survey (ISGS) through the Illinois Basin Coal Sample Program (IBCSP). We appreciate the guidance and encouragement of Dr D. Banerjee, ICC1 project manager. REFERENCES

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