Sulfur removal from lignite by oxydesulfurization using fly ash

FuelVol. 76, No. 1, pp. 73-77, 1997 Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/97 $17.00+ 0.00

PIh S0016-2361 (96)00167-6

ELSEVIER

Sulfur removal from lignite by oxydesulfurization using fly ash Serdar Yaman and Sadriye KO~Likbayrak Istanbul Technical UniversRy, Chemical and Metallurgical Engineering Faculty, Chemical Engineering Department, 80626 Maslak, Istanbul, Turkey. E-mail:kmyaman@ cc.itu.edu.tr (Received 14 May 1996; revised 23 July 1996) The sulfur removal potential of the water-soluble fraction of fly ash in oxydesulfurization of coal was investigated using some high-sulfur Turkish lignites. The effects of the amount of fly ash used, temperature, partial pressure of oxygen and time were studied in the ranges 5-40 g, 403-498 K, 0.0-1.5 MPa and 1590 min respectively. The extents of pyritic and organic sulfur removal and recoveries of coal and calorific value were investigated for each of these variables. Reactivity and some combustion characteristics (ignition temperature, end temperature of combustion and combustion rate) of original and desulfurized lignite samples were compared using combustion curves obtained from thermogravimetric analysis (t.g.a.) results. FT-i.r. spectroscopy was used to determine the effects of the desulfurization process on the coal structure. Copyright © 1996 Elsevier Science Ltd.

(Keywords: lignite; desulfurization; fly ash)

Combustion of high sulfur coals is one of the fundamental reasons for problems with sulfur dioxide emissions. These emissions can be controlled by taking precautions which can be classified as coal desulfurization, sorbent injection into combustion systems and removal of sulfur oxides from flue gases. Desulfurization of coal before utilization is carried out by physical, chemical and biological methods. However, physical methods are incapable of removing finely disseminated pyritic sulfur and organic sulfur from coal. Biological methods are time-consuming and they can be applied only on the bench scale. Most of the effective coal desulfurization techniques are based on chemical methods. In these methods, expensive and unrecoverable chemicals are usually needed and the operating conditions are so severe that it is impossible to apply them economically. Furthermore, low-rank coals cannot withstand these severe conditions and as a result of decomposition, important losses in their calorific value are observed. However, certain chemicals used in these methods have the ability to remove either the pyritic or the organic sulfur from coal 12 ' . Oxydesulfurization is a method by which considerable portions of both pyritic and organic sulfur can be eliminated from coal using mainly water and oxygen. Since sulfur compounds are converted into soluble sulfates and sulfuric acid, alkaline additives, especially Na2CO3, are used to prevent corrosive effects. Moreover, the addition of different alkalis improves organic sulfur removal to different extents in oxydesulfurization 3'4. The behaviour of some alternative chemicals or natural minerals in oxydesulfurization has been investigated 4'5. Certain abundant and cheap materials

are potential reagents. Of these materials, fly ash is of interest. As fly ash is one of the derivatives of coal, addition of its constituents to coal will not cause any incompatible problems with coal structure, and no additional pollutants will form by the combustion of coals that are desulfurized using fly ash. Since fly ash of coals has abundant alkaline components (Fe203 + CaO + MgO + K 2 0 + Na20), it is possible to use the water-extractable components of fly ash as a desulfurization reagent. Hence the chemical potential of fly ash was investigated in the present work. EXPERIMENTAL Fly ash samples from a number of Turkish lignites were investigated and it was concluded that fly ash from (~ayirhan lignite is the richest in alkaline oxides. Therefore this fly ash was used instead of the fly ashes from the lignite samples used in this study. It was obtained from the cyclones of a power station burning lignite from the (~ayirhan region. The chemical composition of the fly ash is shown in Table 1. Proximate and sulfur analyses of the lignite samples are presented in Tables 2 and 3. Extraction of fly ash and oxydesulfurization of coal were carried out using a magnetically stirred 1 litre Parr autoclave. A given amount of fly ash of < 250#m particle size was extracted with 200 mL of distilled water at 473 K under the vapour pressure for 60 min and then the autoclave was cooled and its contents were filtered. The filtrate with the extractable components of the fly ash was poured on to a 5g lignite sample in the autoclave. After the autoclave was sealed, the desired partial pressure of oxygen was established and heating

Fuel 1997 Volume 76 Number 1

73

Sulfur removal from lignite by oxydesulfurization using fly ash. S. Yaman and S. Kucokbayrak Table 1 Chemicalcomposition of the fly ash (wt%) SiO 2

CaO

MgO

Fe203

42.8

17.4

7.1

8.4

A1203 Na20 16.2

K20

Other

2.2

0.5

5.4

Table 2 Proximate analyses and calorific values of the lignite samples Dry basis

Lignite Gediz G6ynfik Tunqbilek

Moisture (wt%)

Volatile matter (wt%)

Ash

Fixed carbon

(wt%)

(wt%)

Gross calorific value (MJ kg l)

1.8 32.2 19.1

30.7 43.9 33.1

31.9 17.5 31.9

37.4 38.6 35.0

22.9 23.7 20.8

Table 3 Sulfur analyses of the lignite samples" % of total sulfur

Wt% of lignite (db) Lignite Gediz G6yniik Tun~bilek a

ST

Sp

So

Ss

Sp

So

Ss

7.38 3.09 4.44

2.85 1.22 1.97

4.16 1.83 1.94

0.37 0.04 0.53

38.6 39.5 44.4

56.4 59.2 43.7

5.0 1.3 11.9

ST: total S, Sp: pyritic S, So: organic S, Ss: sulfate S

4 Analysis(mg L i) of the solution obtained by the extraction of 5 g fly ash Table

Na +

K~

Ca *+

Mg~"

15.6

26.5

35.2

0.8

Fe ~

SO4

pH

91.4

10.5

was started while stirring the suspension at 500 rev min ~. The mixture was allowed to stand at a given temperature for a desired period. At the end o f this period, the autoclave was rapidly cooled using pressurized air, its contents were filtered and the treated coal was washed with hot distilled water until soluble sulfates were eliminated and the p H o f the washings became neutral. The coal was dried at 373 K for 24 h in a v a c u u m oven under nitrogen. Calorific value and total pyritic and sulfate sulfur contents were determined according to the relevant A S T M standards. Solid p r o d u c t yield (SPY) and recovery o f calorific value (RCV) were calculated using the following equations: SPY = (dry weight o f solid product/ dry weight o f coal) x 100wt%

(1)

R C V = (calorific value o f treated coal/ calorific value o f original coal) × S P Y % (2) The effects o f the extracted a m o u n t o f fly ash, oxydesulfurization temperature, partial pressure o f oxygen and oxydesulfurization time were investigated in the ranges 5 - 4 0 g , 403-498 K, 0.0-1.5 M P a and 15 90 min respectively. Thermogravimetric data were obtained using a Shimadzu T G 41 thermal analyser. A 2 0 m g sample o f lignite g r o u n d to pass a 2 5 0 # m sieve was spread

74


uniformly on the b o t t o m o f the alumina crucible and oxidized in a dry air flow o f 4 0 m L m i n i. The temperature was raised from atmospheric to 1223 K at 1 0 K m i n 1. Infrared spectra o f the samples were obtained using a M a t t s o n 1000 Series FT-i.r. spectrometer. 200 mg o f K B r pellets containing 1 wt% lignite were dried at 383 K for 36 h and spectra were obtained at a resolution o f 8 cm 1 with 16 scans.

RESULTS AND DISCUSSION

Effbct of amount o['fly ash Oxydesulfurization o f Gediz lignite with distilled water containing dissolved oxygen under 1 M P a partial pressure o f oxygen at 473 K for 30 min resulted in the removal o f 41.8% o f the total sulfur content and recovery of 92.5 w t % of coal. To investigate the effects o f the solution containing water-soluble c o m p o n e n t s o f fly ash on sulfur removal and coal recovery, 5 - 4 0 g o f fly ash was extracted with 2 0 0 m L o f distilled water under the above-mentioned conditions and these solutions were used in oxydesulfurization. The use o f the water-soluble fraction of 5 g o f fly ash at 473 K under 1 M P a partial pressure o f oxygen for 30 rain resulted in removal o f 50.4% o f the total sulfur content from Gediz lignite and 91.1 w t % coal recovery. The use o f this alkaline solution derived from fly ash neutralized the acidic solution which formed during oxydesulfurization. An analysis o f the solution obtained by the extraction of 5 g o f fly ash in 200 m L o f distilled water is given in Table 4. As can be seen, the cations o f the principal alkaline components, except for Fe +++, were transferred into solution in different concentrations. Unfortunately some SO4 at a concentration o f 91.4mg L - I was also transferred into the solution. Increasing the a m o u n t of fly ash from 5 to 40 g slightly improved the sulfur removal capacity o f the process. These improvements were from 50.4 to 54.5% for the removal o f total sulfur content and from 21.5 to 29.9% for the removal o f organic sulfur content. However, a slight decrease from 91.5 to 89.7% took place in the removal o f pyritic sulfur content. This limited extent o f improvement can be explained by the saturation o f the water with the soluble c o m p o n e n t s o f the fly ash, since its volume was fixed. For this reason, effects o f the other variables were investigated using solutions derived from 5 g o f fly ash. The other coals investigated (Tunqbilek and G6yn~ik) showed considerable degradation as a result o f desulfurization, although almost all of the sulfur content was eliminated. Considerable fractions o f these very y o u n g lignite samples could not be recovered at 473 K under the oxidizing atmosphere. Both formation o f CO2 and dissolution o f organic matter are responsible for the loss of coal. To increase the recovery o f these lignite samples after the treatment, the temperature was reduced to 423 K. However, other variables such as the partial pressure o f oxygen, time and stirring rate were applied at the same values as used in the desulfurization experiments on Gediz lignite. Total sulfur removal was 45.3 and 50.5% for G6ynfik lignite when distilled water and the water-soluble fraction o f 5 g o f fly ash were used, respectively. N o significant change in the recovery o f coal

Sulfur removal from lignite by oxydesulfurization using fly ash."S. Yaman and S. KOcOkbayrak was observed in the two cases. The extents of sulfur removal and coal recovery from Tunpbilek lignite were approximately the same when distilled water or the solution of fly ash was used. These results showed that temperature should be ,-,473 K for satisfactory desulfurization. The decomposition problem is closely related to the age of the coal to be treated and therefore it would not be considered as serious unless very young lignite samples are exposed to the conditions of this wet oxidation process. To investigate the effect of temperature with the solution of 5 g of fly ash, Gediz lignite was subjected to oxydesulfurization under the same conditions except that the temperature was held at 423 K. By this process, 38.6% of total sulfur content, 75.2% of pyritic sulfur content and 11.9% of organic sulfur content were removed; the coal and calorific value recoveries were 91.9wt% and 97.4% respectively. The results of oxydesulfurization carried out using distilled water under the same conditions showed that removal of sulfur forms and coal and calorific value recoveries at 4 2 3 K were very close to those obtained using the solution of fly ash: total, pyritic and organic sulfur removal was 37.3, 76.4 and 8.3%; coal and calorific value recoveries were 93.1 wt% and 97.3% respectively. The increase in desulfurization of Gediz lignite due to an increase in temperature became more evident when alkaline solutions of fly ash were used instead of distilled water. The water-soluble fraction of fly ash can be considered as a superior solution to water, provided that the oxydesulfurization temperature is ~473 K.

100

v

"~

~

E

,a.

40

"-1

(,/)

20110 ~

,

I

'

I

420

400

'

440

I

''

460

I

'

480

500

Temperature (K) Figure 1 Relation between temperature and sulfur removal: O. total S; O, pyritic S; + organic S

0.6

0.50

Effect of temperature Effects of temperature on sulfur removal and recoveries of coal and heating value were investigated using the water-soluble fraction of 5 g of fly ash under 1 MPa partial pressure of oxygen for 30 min. Relations between temperature and total, pyritic and organic sulfur removal from Gediz lignite are shown in Figure 1. Removal of total sulfur content increased continuously with increasing temperature. For example, it was 26.5% at 4 0 3 K and 62.9% at 498K. The effect of rising temperature on pyritic sulfur removal was fairly distinct up to 448 K, but at higher temperatures no further increase took place. The pyritic sulfur removal was 63.6, 88.3 and 91.7% at 403, 448 and 4 9 8 K respectively. Organic sulfur removal increased exponentially with temperature; for instance, it was 0.12% at 4 0 3 K and 42.6% at 498 K. Coal recovery varied between 93.7 and 91.1 wt% in the temperature range 403-473 K. As the coal began to decompose heavily above 473 K, considerable reductions in coal recovery occurred; recovery fell to 79.9wt% at 498 K. The reduction in calorific value recovery was more rapid than that in coal recovery: 90.3, 79.2 and 56.2% of calorific value was recovered at 448, 473 and 498 K respectively. Some changes also took place in the combustion properties of the treated lignite samples depending on the desulfurization temperature. The burning profiles of treated lignite samples at 403 and 498 K are shown in Figure 2. The ignition temperature of the lignite sample treated at 403 K was determined as 528 K, whereas it decreased to 493 K when the treatment temperature was raised to 498 K. Another difference was detected in the maximum combustion rate and its temperature between

60

O

Q

0.4-

0

E O}

E

v

ooO0 O0~

0

°~o

go

gig O

•

0.3-

0

E 0

0.2-

0

0

0.1

0 0

o

0

0.0

%

~cs , 400

I

'

I

600 800 Temperature (K)

~ O0

I-"

1000

Figure 2 Burning profiles of the desulfurized lignite samples. Desulfurization temperature: 0, 403 K; Q, 498 K

these two treated lignite samples. The maximum combustion rate was 0 . 4 1 m g m i n - and it appeared at 753 K for the lignite sample treated at 403 K. As a result of desulfurization carried out at 498 K, the maximum combustion rate increased to 0.52mgmin -1 and the temperature of this maximum decreased to 653 K. The combustion of the lignite sample treated at 403 K was complete at 1028 K. Since the combustion rate of the lignite sample treated at 498 K was greater, combustion was complete at 933 K. The original Gediz lignite sample had an ignition temperature of 608 K and its combustion


75

Sulfur removal from lignite by oxydesulfurization using fly ash. S. Yaman and S. Ko,cOkbayrak

(;

~

.

j

80v

40

'

0.0

I

'

I

'

I

'

0.4 0.8 1.2 Partial pressure of oxygen (MPa)

1.6

Figure 3 Relation between partial pressure of oxygen and sulfur removal. Symbols as in Figure 1

0.5

i

_ID

Effect qf time

0.4

Effects of time on desulfurization and heating value recovery were investigated in the interval 15-90min (Figure 5). These experiments were carried out with Gediz lignite using solutions of 5 g fly ash at 473 K and under 1MPa partial pressure of oxygen. The results showed that the removal of pyritic sulfur content proceeded so rapidly that 81.5% was removed at 15min. Small increases took place in pyritic sulfur removal at longer times. Improvements in total and organic sulfur removal with increasing time were more

00

0.3 E

v

(9

E

1.5MPa. In these experiments the alkalinity of the solution carrying dissolved oxygen to the coal particles was provided using 5 g of fly ash. The temperature and time were 473 K and 30 min respectively. The results of these experiments are illustrated in Figure 3. Sulfur removal was extremely low when no oxygen was supplied to the reaction medium: 19.0% of total sulfur content, 25.7% of pyritic sulfur content and 9.5% of organic sulfur content could be removed from the lignite sample. As a result of the use of oxygen pressure, desulfurization increased rapidly to 56.9, 89.7 and 34.1% under 1.5 MPa partial pressure of oxygen for total, pyritic and organic sulfur removal respectively. On the other hand, reductions occurred in heating value recovery, related to oxidation of the coal. In the former case, recovery of heating value was 98.4% , but it decreased to 74.1% in the latter case. The effect of partial pressure of oxygen on the combustion properties of treated lignite samples was also investigated. Figure 4 illustrates the burning profiles of the lignite samples desulfurized without oxygen supply and under 1.5MPa partial pressure of oxygen. The ignition temperature was 613 K in the absence of oxygen, but under 1.5 MPa partial pressure of oxygen, burning started at a lower temperature, 498 K. The presence of oxygen in the desulfurization medium did not lead to considerable changes in maximum combustion rate and end temperature of combustion.

0.2

0.1 100

,~5

0.0

..... 400

7 ' I ' 600 800 Temperature (K)

80

;~1~ 1000

Figure 4 Burning profiles of the desulfurized lignite samples. Oxygen partial pressure: ©, 1.5 MPa; O, zero

> 0

E t-

was complete at 1073 K; its maximum combustion rate was 0 . 3 9 m g m i n ] and this occurred at 768K. The reactivity of the treated lignite samples increased considerably as the temperature of treatment was raised.

60-

r'~

40 ......-r-- j

(f)

20

Effect of partial pressure of oxygen The partial pressure of oxygen is one of the important factors determining the extent of desulfurization. Therefore, removal of total, pyritic and organic sulfur contents from Gediz lignite was investigated under different partial pressures of oxygen ranging from zero to

76


'

0 Figure 5 Figure l

I

20

'

I

I

40 60 Time (min)

'

I

80

'

100

Relation between time and sulfur removal. Symbols as in

Sulfur removal from lignite by oxydesulfurization using fly ash: S. Yaman and S. KO,cOkbayrak were extremely high for original lignite and lower for the desulfurized sample. Although the intensities of the CH 2 and CH 3 bands decreased, an increase in the amounts of phenols caused the intensity of the peak at 1596 cm-t to increase.

< oD

-

~o

0.8-

0.6

8

CONCLUSIONS

=

0.4

p

473 K caused considerable decreases in heating value recovery. Some mineral species such as kaolinite, quartz and gypsum were concentrated in the desulfurized lignite, since they were inert under the conditions applied. Furthermore, the concentrations of haematite and sulfatic structures also increased in the desulfurized lignite, due to the oxidation of pyritic iron and sulfur compounds 3. In addition, oxidative treatment caused some changes in the organic coal matrix, such as increases in carbon-oxygen and oxygen-hydrogen bonds and decreases in c a r b o n carbon and carbon-hydrogen bonds. The reactivity of the lignite increased as a result of oxydesulfurization: the ignition temperature and the end temperature of combustion were lower and the maximum combustion rate was higher for the treated sample. In the technical realization of this kind of process in a power station, the fly ash supplied from the cyclones would be hot and it could therefore provide some of the energy requirement for its extraction at 473 K. Moreover, preheating of the fly ash extraction system can be carried out by the heat removed from the oxydesulfurization process, since the desulfurization reactions are exothermic.

REFERENCES 1. 2. 3. 4. 5.

Meyers, R. A., Hydrocarb. Process., 1975, 54(6), 93. Burrow, D. F. and Glaviencevski, B. M., Am. Chem. Soc. Div. Fuel Chem. Preprints, 1980, 25(2), 153. Yaman, S., Ph.D. Thesis, Istanbul Technical University, 1996. Chuang, K. C. Markuszewski, R. and Wheelock, T. D., Fuel Process. Technol., 1983, 7, 43. Yaman, S. and Kii~kbayrak, S., in Proceedings of the 8th International Conference on Coal Science, Vol. 2. 1995, pp. 1733-1736.


77