Fuel Processing Technology, 23 (1989) 75-85 75 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Regenerat ion of Iron Oxide Containing Pel lets Used for Hot Gas Clean Up

ALLAN PALMER*, PHYLLIS HEENEY and EDWARD FURIMSKY

Energy Research Laboratories, CANMET, 555 Booth Street, Ottawa, Ontario KIA OGI (Canada)

ABSTRACT

Four iron-containing pelletized solids used for H2S removal from hot gas were oxidized in a Cahn electrobalance and in a fixed bed reactor. The main reactions included the sequence in which FeS was oxidized to iron sulphate which then decomposed rapidly yielding S02 and iron oxides. The oxidation occurred predominantly on the outer surface of the pellets.

INTRODUCTION

The oxidation sequence when iron sulphides are converted to oxides has been the focus of several investigations. The pyrite (FeS2) oxidation studies reported by Thorpe et al. [1,2] were aimed at improving pyrite separation efficiency from a bituminous coal by increasing its magnetic properties. The mechanism of pyrite oxidation during coal combustion was reported by Groves et al. [3]. Chemical changes which occur during pyrite oxidation at room tem- perature in air and air saturated aqueous solutions were described by Buckley and Woods [4 ]. It appears that pyrrhotite (FeS) is an important intermediate in the oxidation of pyrite to iron oxides.

The formation of iron sulphides accompanies the H2S removal from hot gases in the presence of iron-containing sorbents [5]. At the end of purifica- tion, i.e., when all active Fe is consumed, the sorbent is regenerated in air or diluted air and reused. An efficient and stable sorbent should maintain its sorption capacity after many utilization-regeneration cycles. The chemical changes which occur during the formation of iron sulphides and subsequent sulphide oxidation to iron oxides, i.e., during utilization-regeneration cycles were studied in detail by Schrodt [6] and Joshi and Leuenberger [7]. These authors used fly ash particles containing iron oxides for H2S removal from a model gas mixture.

*To whom correspondence should be addressed.

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This report describes an investigation on pelletized solids containing iron sulphide. The samples are spent sorbents, which were used for H2S removal from a gaseous mixture at high temperatures. The main focus was on the re- action of HzS with iron oxides. For this reason the model gas mixture contain- ing only N2 and H2S was used. The FeS oxidation experiments in a Cahn bal- ance reactor were paralleled by tests in a fixed bed reactor, which allowed a continuous analysis of oxidation products. Chemical changes incurred by the solids during oxidation were followed by X-ray diffraction analysis.

EXPERIMENTAL

Materials

Samples A and B, byproducts from the iron industry, were obtained from Dofasco Ltd. in Hamilton, Ontario. Solid C is almost pure iron oxide, Fe2Q, from the Northern Pigment Ltd. Company. Solid D "red mud", a byproduct from the aluminum industry, was supplied by Alcan International Ltd. Sieved to minus 325 mesh these solids were used to prepare cylindrical pellets 2 mm high and 2 mm in diameter. Two grams of these pellets were sulphided in a fixed bed reactor either at 650 ° C or 850 ° C in a stream of N2 + H2S (0.11 min - 1 )

containing 2 vol.% H2S. Details of the sulphidation procedure were published previously [8 ].

Oxidation procedure

Oxidation of sulphided pellets was performed either in a Cahn electrobal- ance or in a fixed bed reactor. For the former, oxidation was carried out on about 200 mg of pelletized sample in a flow of air (2 1 min - I ). The temperature was increased at 10°C min -1 until 350°C after which the temperature was increased at 5 °C min-1 until the desired temperature for each sample was reached. The flow of air in the fixed bed was maintained at 0.5 1 min - 1 and the temperature was increased at 10 °C min-1 until 250 °C and then at 5 °C min-1 until 500 ° C.

Analysis

The Cahn 1000 electrobalance was used to follow the weight variation of the samples. The oxidation products could not be analyzed because of the large gas dilution. However, the SO2, 02 and N2 concentrations in the gas exiting the fixed bed reactor were continuously monitored by an on-line mass spectrom- eter and an infrared analyzer. These systems could not measure SOe concen- trations higher than 2.5 vol.%.

The X-ray diffraction (XRD) analysis was conducted on a Phillips powder

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d i f f r ac tomete r us ing Cu K~ (~ = 1.54178 A) radia t ion. T h e su lphur c o n t e n t of the samples was d e t e r m i n e d by a Leco analyzer . Scann ing e lec t ron micro- graphs were ob ta ined wi th a J E O L scanning e lec t ron microscope, wi th an ac- ce lera t ing voltage of 25 kV and a magni f i ca t ion of X 660. T h e samples were spu t t e r coa ted wi th gold to p r even t discharges and to provide a s t anda rd pat- t e rn for energy dispersive X- r ay analysis .

RESULTS AND DISCUSSION

T h e chemical compos i t ion of the f resh solids is shown in Table 1. T h e resul ts are expressed in oxide con ten t s because the sample p r ep a ra t i o n for the analysis involved an ashing step. T h e X R D analysis of the fresh and sulphided solids is shown in Table 2, where the comp o u n d s are l is ted in order of decreas ing spec t ra abundance . Th i s order m a y no t ref lect the re la t ive c o n t e n t of the com- pone n t s because of the degree of c rys ta l l in i ty effect on X R D signals. Fo rmula FeS, in Table 2 and t h r o u g h o u t the whole text , refers to p y r rh o t i t e a l though it

TABLE1

Chemical analysis of solids as received (wt.%)

Solids A B C D

SiO~ 4.2 5.0 0.2 1.3 AI~O:~ 2.9 3.2 - 22.7 Fe~O:~ 81.2 64.6 97.3 42.6 CaO 3.4 11.3 0.1 5.4 MgO 1.6 1.0 0.1 - Na~O 0.1 0.1 0.1 0.8

TABLE 2

XRD analysis of fresh and sulphided solids

Solids A B C D

As received CaMg(CO:0~ CaCO:~ Fe2Q CaO CaCO:~ Fe:~04 CaCO:~ Fez O:~ CaMg ( C O:~ ) 2 FeO (OH) Fe:~04 Fe~O:~ Fe20:~

Sulphided FeS FeS Fe:~04 FeS at 650 ° C CaS Fe:~O4 FeS CaS

Fe:,O4 CaS FeeO:~

Sulphided FeS FeS Fe304 FeS at 850 ° C CaS CaS FeS CaS

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was established that the ratio of Fe to S in this compound varies widely [9]. Oxides such as SiO2 and A1203 were also detected in some solids. However, since these oxides do not react with H2S they are not listed in Table 2. The presence of Fe304 in solid C sulphided at 650 ° C confirms that this oxide is an intermediate in the overall conversion of Fe203 to FeS [5 ]. The rapid buildup of SO2 on admission of the N2 + H2S mixture to Fe203 containing sorbents was attributed to H2S oxidation by Fe203, which leads to Fe304 formation [10]. This suggests that Fe304 plays a key role in retaining the sulphur. For gasifi- cation products the Fe203 reduction is enhanced due to the presence of H2 and CO. In such a case the formation of wustite (Fe0.gnvO) and even of metallic Fe is possible indicating a rather complex mechanism of HzS removal. Pyrite was not detected in any sulphided solid. Small amounts of pyrite were found after sulphidation using a much higher H2S/Fe ratio than that used in the present work [ 10 ]. The higher ratio may be required to prevent a thermal decompo- sition of FeSz to FeS. Such a situation may occur during hot gas clean-up of gases containing large quantities of HzS, e.g., the gases produced during up- grading of high S-containing heavy oils.

Weight changes which occur during temperature programmed oxidation of the sulphided pellets (weighing about 200 mg) are shown in Fig. 1 for solids A and B and in Fig. 2 for solids C and D, respectively. For the former the contin- uous weight increase was interrupted by an abrupt weight loss which occurred at about 380 °C and 440 °C for pellets sulphided at 650 °C and 850 ° C, respec- tively. For solid C the weight increased continuously except for a small devia-

Sol id B ..-...~ ....... 4 0 . . . . . . . used of 6,50 °C

. . . . . . . 850 "C . "'"

20 ............ i . . • . . . . . . ~ , ~ .

o, o i E k

Z

"r" (.9

4 0 i Solid A .::

. . . . . . . . used o1" 6,50°C ..'" - , .Vf . . . . . . . e ~ o ' c : " . . - i "

2 0 .... ; ~ i ~ " i . . . . . .

/ "

0 / \ . . . . . . . . .

-I0 I I I 1 I 1

tO0 2 0 0 300 400 500

T E M P E R A T U R E , °C

Fig. 1. Weight changes during temperature programmed oxidation of solids A and B in Cahn balance ( 200 mg of pellets; 2 1/min of air) .

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I00

~ 5 0

E

Lu" (.9 Z 0

"1- 0

I-- "i- (.9

50

Solid C

. . . . . . . . . used el Co50°C . . ' " . . . . . .

. . . . . . . 850°C . ' " " ~ . /

. . ' "

Solid D . . . . . . . . used et 6 5 0 ° C

. . . . . 8 5 0 % . . . . . . . . . . . . . . .

... '/- I~

0 / ' I I I I I I 0 I O0 200 300 400 500

TEMPERATURE, °C

Fig. 2. Weight changes during temperature programmed oxidation of solids C and D in Cahn balance (200 mg of pellets; 2 l/min of air).

tion at about 390 °C. The higher temperature of weight loss for pellets sul- phided at 850 ° C is attributed to a more extensive surface sintering compared with that for pellets sulphided at 650 °. In other words, the physical strength of the surface layer formed at 850 ° C is believed to be greater than that of the layer formed at 650 ° C. The higher strength may also be maintained during oxidation. Then, more severe conditions (higher temperature) might be re- quired to sufficiently change the molar volume of the layer.

XRD analysis was carried out on the solids for which the oxidation was interrupted about 20 °C below or above the abrupt weight loss. The results in Table 3 indicate the formation of Fe20~ and Ca and/or Fe sulphates during oxidation in the Cahn balance before the sudden weight loss. These reactions account for the weight increase, i.e., the 02 uptake to convert Fe304 and Fe and Ca sulphides to sulphates which represent molecular weight increase when expressed per unit of Fe and/or Ca. The disappearance of the sulphate after the sudden weight decrease suggests it decomposed rapidly. This agrees with the present results as well as those published by Kasaoka et al. [ 11 ]. The weight increase after the transition is attributed to conversion of Fe304 to Fe203. The pellets sulphided at 850 °C exhibited greater transitions than those sulphided at 650 ° C indicating a larger content of sulphate in the former. This is in line with the higher S content in pellets A, B and D (Table 4) sulphided at 850°C compared with those sulphided at 650 ° C. Pellets of sample C exhibited differ- ent patterns. Thus, a very small transition was observed for pellets sulphided at 650°C and no transition for those sulphided at 850 °C. This may suggest

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TABLE 3

XRD analysis of partially oxidized solids from the Cahn balance runs

Conditions A B C D

Sulphided at 650 ° C End of oxid 365 405 365 415 365 400 305 345

( ° C ) FeS Fe~O:~ FeS Fe20:l FeS Fe20:~ F % O ~ (Fe20:~)a FeSO4 Fe:~O~ FeSO~ Fe:~O4 Fe:~O4 Fe:~O4 (Fe:~O4) CaS CaS Fe:~O4 CaS Fe20:~ FeS Fe~O:~ FeS CaS (FeS) FeSO4 Fe:~O4 Fe20:~

Sulphided at 850°C Endofoxid 410 460 425 465 380 470 320 400

( ° C ) FeS Fe.,O:~ FeS Fe20.~ Fe20:~ Fe20:~ Fe~O:~ Fe20:l CaS Fe:~O4 Fe:tO4 CaS Fe:~O~ Fe:~O4 FeS (CaS)

(FeSO4) CaS CaS Fe:~Q FeS (CaS) (CaSO4) FeS (FeSO4) CaO FeSO4

CaO

"Brackets indicate some uncertainty in identifying the compound; rather low crystallinity of ox- idized solid D which was sulphided at 650 ° C prevented the component identification.

TABLE4

Sulphur content and surface area of sulphided pellets

Solid S Surface area O,, consumed (wt. % ) (m ~ g-1 ) (mol × 102)

A 8.1 Sulphided at 650 ° C 17.7 10.3 3.0

850 ° C 30.0 7.3 3.4

B 13.1 Sulphided at 650 ° C 16.8 8.2 3.6

850 ° C 28.8 13.4 6.8

C 11.0 Sulphided at 650 ° C 15.2 10.0 4.4

850 ° C 9.7 7.0 0.7

D 78.9 Sulphided at 650 ° C 12.9 41.2 1.7

850°C 15.9 15.7 3.0

t h a t for C pe l l e t s t he s u l p h a t e d e c o m p o s i t i o n was m u c h less e v i d e n t c o m p a r e d

w i t h t he o t h e r pe l le t s . T h e p r e s e n t r e s u l t s m a y be i n t e r p r e t e d in t e r m s of t he F e - S - O e q u i l i b r i u m

s y s t e m r e l a t i n g p a r t i a l p r e s s u r e s of $2 a n d 02 to t h e o c c u r r e n c e of Fe com-

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pounds such as oxides, sulphides and sulphates [ 12 ]. According to this system a critical partial pressure of 02 is required to cause the oxidation of FeS to Fe304. Once such a pressure is attained the oxidation occurs rapidly and is accompanied by the formation of SO2. High partial pressures of $2 and 02 ensure a high concentration of SO2. According to the phase diagram shown in Fig. 3, high partial pressures of SO2 and 02 favor sulphate formation. The 02 partial pressure used in the present work {e.g. log Po2 -~ - 0 . 7 ) indicates the co-existence of iron oxides and sulphates. It is believed that the most favorable situation for sulphate formation is on the outer surface of the pellets. Thus, the diffusion problems may cause different partial pressures in the pellet's interior. This suggests that different equilibrium factors apply for the exterior and the interior of the pellet. This may explain the co-existence of several iron- containing species before and after the transition, although according to the Fe-S-O diagram the Fe sulphides should be completely converted. Then, both kinetic and equilibrium factors must be considered when interpreting chemical changes occurring in the pellet.

The sulphate decomposition is accompanied by a drastic change of molar volume. If such a reaction occurs on the pellet exterior, it will inevitably break surface layers thus improving 02 diffusion into and SO2 diffusion out of the

E eu

t~

-5

T = 1000 K

FeS 2

FeSO 4

Fe304 I Fe203

Fe2(S04) 3

-IO 'i

-15 -25

ql Fe'9470

i -20

l II I -15 -IO -5 log P02 , atm

Fig. 3. Stable equilibria in the Fe-S-O system (log Pso~ vs. log Po~ ).

82

O _

Fig. 4. Scanning electron micrographs of sulphided and regenerated solids A and B. {a): solid A sulphided at 850 ° C. {b): solid A sulphided at 850 ° C, then regenerated in air up to 410 ° C. (c): solid A sulphided at 850 ° C, then regenerated in air up to 460 ° C. (d): solid B sulphided at 850 ° C. { e): solid B sulphided at 850 ° C, then regenerated in air up to 425 ° C. { f): solid B sulphided at 850 ° C, then regenerated in air up to 465 ° C.

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24

~: 20

Z Q ~6

t.--

Z 12 W

Z o

d~ s

g ~ 4

0 o IO0 2 0 0 5 0 0 4 0 0 5 0 0

TEMPERATURE, °C

Fig. 5. Formation of S02 and consumption of 02 during temperature programmed oxidation of solids A and D in fixed bed reactor. ~ Solid A used at 650 ° C, A 850 ° C, [] Solid D used at 650 ° C, + 850°C.

TABLE 5

Tentative reactions during sulphide oxidation and sulphate decomposition a

- AG7ooK (kcal mo1-1)

FeS + 202 CaS + 202 FeSO4 FeSO4 + FeS + 02 FeSO4 + 2FeS + 302 FeSO4 + CaS + 02 CaSO4 + CaS + 02

FeSO4 138.3 CaSO4 168.4 FeO + S03 - 30.2 2FeO + 2S02 63.3 Fe304 + 3SO2 213.6 FeO + CaO + 2SO2 58.0 2CAO+2S02 23.1

"Wustite is identified as FeO.

pellets. Figure 4 indicates the importance of surface-dependent phenomena which occur upon oxidative regeneration. The sulphided pellets A and B (a and d) show a smooth clean surface. When these solids are heated to about 20 ° C (b and e ) lower than the recorded transition temperature, the surface of the pellets is still fairly smooth, but some cracks do appear and white partic- ulate matter forms in some spots. For those solids heated to about 20 ° C (c and f) above the reaction temperatures, the surface of the pellets is dramatically different. Cracks in the particles, rough edges and amorphous masses are ob- served and the white particulate matter is present in greater quantities. The

84

XRD results in Table 3 suggest that these changes are accompanied by the formation of additional Fe304 and Fe203 compounds. Then, a narrow temper- ature range exists in which drastic physical and chemical changes occur during regeneration of sulphided iron-containing sorbents. It appears that the tem- perature range depends on the sorbent composition and sulphiding conditions.

To complement the results obtained in the Cahn balance the sulphided pel- lets were oxidized in a fixed bed reactor. In this case the concentration of S02 and 02 in the gas exiting the reactor was monitored. Examples of a rapid in- crease in S02 concentration and decrease in 02 concentration for solids A and D are shown in Fig. 5. A temperature increase different from the programmed temperature was observed in this region and confirms the presence of exo- thermic reactions. The temperature ranges at which the rapid changes oc- curred agree with those observed in the Cahn balance. The amount of 02 con- sumed in those regions (Table 4) depends on a combination of several factors, i.e., the contents of Fe304 and sulphur, surface area and porosity. Thus the absence of any trend for the results shown in Table 4 is not surprising.

It is generally known that oxidation of iron and calcium sulphides to sul- phates is thermodynamically very favourable. The free energies of formation shown in Table 5 indicate that the decomposition of the sulphates is also very favourable. These reactions may account for the abrupt weight loss observed in the Cahn balance as well as the rapid S02 yield observed in the fixed bed. The sulphate decomposition yielding SO3 is unfavourable.

CONCLUSIONS

The experimental results for oxidative regeneration of sulphided sorbents give an overall picture of the role of certain chemical compounds. The oxida- tion of iron sulphide to iron oxides is accompanied by the formation of an intermediate iron sulphate. Its appearance and subsequent decomposition have been highlighted using the Cahn electrobalance to follow weight changes and XRD to identify sulphided products, intermediates and end products. The re- actions occurring on the surface of the pellets have been confirmed by scanning electron microscopy. The occurrence of a rapid exothermic transition requires careful control of the regeneration process to avoid overheating the solid. The temperature at which the rapid transition occurs depends on the origin of the solids and on the severity of the utilization cycle. This temperature may also vary between successive utilization-regeneration cycles. The global approach to understanding the regeneration reactions reported here will be used to eval- uate further multiple-cycle H2S adsorbents.

(c) Minister of Supply and Services, Canada, 1989.

85

REFERENCES

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1987. Fuel, 66(2): 142. 3 Groves, S.J., Williamson, J. and Sanyal, A., 1987. Fuel, 66(4): 461. 4 Buckley, A.N. and Woods, R., 1987. Appl. Surf. Sc., 27: 437. 5 Furimsky, E. and Yumura, M., 1986. ErdS1 u. Kohle, 39(4): 163. 6 Schrodt, J.T., 1981. Hot gas desulphurization; Use of Gasifier Ash in Fixed and Fluidized

Bed. U.S. DOE Report ET-10463-T1, Vols. 1 and 2, Washington, DC. 7 Joshi, D.K. and Leuenberger, E.L., 1977. Hot Gas Desulphurization in Fixed Bed of Iron

Oxide-Fly Ash. U.S. DOE Report FE-2033-19, Vols. 1 and 2, Washington, DC. 8 Yumura, M. and Furimsky, E., 1985. Ind. Eng. Chem. Proc. Des. Dev., 24:1165. 9 Toulmin, P. and Barton, P.B., 1964. Geochim. Cosmochim. Acta, 28: 641.

10 Furimsky, E., Palmer, A., Brett, M.E., Provencher, R. and Yumura, M., 1987. Adsorption Sci. Technol., 4: 230.

11 Kasaoka, S., Sakata, Y., Ito, M. and Ikeda, M., 1977. Int. Chem. Eng., 17: 308. 12 Chakraborti, N., 1983. Can. J. Chem. Eng., 61: 763.