The behavior of iron-sulfur species in fluidized-bed gasification on coal

The behavior of iron-sulfur species in fluidized-bed gasification on coal

215 Fuel Processing Technology, 30 (1992) 215-226 Elsevier Science Publishers B.V., Amsterdam The behavior of iron-sulfur species in fluidized-bed g...

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215

Fuel Processing Technology, 30 (1992) 215-226 Elsevier Science Publishers B.V., Amsterdam

The behavior of iron-sulfur species in fluidized-bed gasification on coal D.M.Mason Institute of Gas Technology, 3424 S. State Street, Chicago, Illinois 60616 (USA)

(Received June 26th, 1990; accepted in revised form November 29th, 1991)

Abstract The behavior of iron-sulfur species derived from pyrite, especially that present in the coal feed as particles of free pyrite, is of concern in the operation of fluidized-bed coal gasifiers. Work conducted at the Institute of Gas Technology (IGT) has shown that the decomposition of pyrite in a [email protected] pilot plant has yielded a liquid phase that spreads on the surface of ash particles. Formation of a liquid phase in decomposition of pyrite in an inert atmosphere has also been observed in connection with the investigation of boiler deposits. To aid in understanding the behavior of iron-sulfur species, we have prepared an Fe-S phase diagram at temperatures relevant to coal gasification and with equilibrium sulfur potentials exhibited, for the most part, as hydrogen sulfide to hydrogen mole ratios; in a fluidized-bed gasifier, the latter can easily be obtained from the composition of the reactor product gas, which closely approximates that in the back-mixed fluidized bed. To show, in addition, the effect of oxygen potential in the bed, we have also prepared phase stability diagrams at WO°F intervals from 1500 to 2000°F (816 to 1093°C). According to the Fe-S diagram, formation of an iron-sulfur liquid in the gasifier or in an inert atmosphere cannot be explained on the basis of equilibrium considerations. Instead, a kinetic explanation, based on increased concentration of iron in the exterior regions ofthe decomposing pyrite particle, as sulfur is removed from the surface, is much more plausible. Phase stability diagrams indicate that a liquid composed of iron, sulfur, and oxygen could be formed in steam-oxygen gasification ofhigh-sulfur coal at temperatures above 1850 °F (1010 C). However, no evidence ofits formation was found by optical petrography in the examination of solids from the V-GAS pilot plant in which steam-to-hydrogen mole ratios ranged up to about 2, but formation of a submicroscopic liquid layer on iron sulfide particles could have been responsible for deposition of these particles in the hot cyclone of the V-GAS pilot plant. Its formation in steam-air gasification, where the steam-to-hydrogen mole ratio typically is about 0.6, is less likely. 0

INTRODUCTION

In fluidized-bed gasification of high-sulfur coals, such as the KRW and UGAS processes (without a sulfur sorbent in the bed), relatively low-melting Correspondence to: Dr. D.M. Mason, Institute of Gas Technology, 3424 S. State Street, Chicago, Illinois 60616 (USA)

0378·3820/92/$03.50 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

ash is produced by reaction of iron from pyrite with the siliceous components produced from clays and quartz [1]. To prevent the uncontrolled growth of ash clinkers and defluidization of the bed, ash concentration in the bed must be limited. To control the ash content of the bed, the ash must be agglomerated to facilitate its removal from the bed without an unacceptable loss of char carbon and a decrease of gasification efficiency. The formation of agglomerates was found to be dependent on production of an iron-rich, relatively low-melting matrix, in which a substantial portion of other types of ash may be embedded. The agglomerative matrix was determined to consist predominately of ferrous aluminosilicate of variable composition together with smaller amounts of other oxides. From analysis of phase diagrams and ash fusion temperatures, it was concluded that the iron from pyrite (FeS2) reacts (as oxide) with siliceous minerals (quartz and clays) to produce the low-melting matrix that acts as the binder for agglomerate formation. Most of the iron in coal occurs as pyrite, which readily decomposes to ferrous sulfide (FeSl +x, pyrrhotite) in the gasifier. Oxidation of the ferrous sulfide by oxidizing gases is, therefore, a prerequisite for formation of the low-melting silicate mixture. However, ferrous sulfide itself may play an additional role in agglomeration as indicated by the photomicrograph of an ash particle obtained from the fluidized bed in a pilot-plant run in which run-of-mine (ROM) high-sulfur Kentucky No.9 seam coal was fed (Fig. 1). Ferrous sulfide is shown as the binding medium between ash particles. Particles with layers of ferrous sulfide on shale or ash agglomerate cores or similarly of ferrous sulfide partially converted to iron oxide, were ubiquitous in the bed materials from some pilot-plant runs on

Fig. 1. FeS (white) joining ash particles in bed material from a pilot-plant run on ROM coal [1].

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D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

the high-sulfur Kentucky ROM coal. We believe that these phenomena result from formation of a molten ferrous sulfide phase at some stage in the life of particles derived from free pyrite. Ferrous sulfide spread on particles was not observed when well-washed coal (presumably lacking free pyrite) was fed. Ferrous sulfide is also of interest because it has sometimes been found to deposit in the hot cyclone of the IGT pilot plant [2]. Also noteworthy is a report by Raask [3] that particles from decomposition of FeS2 in a nitrogen atmosphere began to lose their sharp edges at 1880°F (1027°C) and melted at 1970°F (lonOC). For a better understanding of these phenomena, we have prepared a phase diagram of the Fe-S system and phase stability diagram of the Fe-8-0 system in forms facilitating their use by chemists and engineers dealing with fluidized-bed gasification of coal. PREPARATION OF PHASE DIAGRAMS

Fe-S system A phase diagram of a part of the Fe-S system relevant to coal gasification is presented in Fig. 2. Temperatures from 1650 to 2300°F (899-1260°C) and compositions from 31 to 41.5 weight percent (44.5 to 55.3 atomic % ) sulfur are covered. Variation of sulfur potential with temperature and composition is shown as the mole ratio of hydrogen sulfide to hydrogen at low sulfur potentials and as pressure or as fugacity of 8 2 at values of one atmosphere or more. Data for construction of the diagram were obtained from Burgmann et a1. [4] and Bale and Toguri [5], except that the liquidus from 31 to 33.5 wt% sulfur and the liquidus and solidus from 38.5 to 41.5 wt% were modified ac2300 H2 S/K 2 MOLE RATIO ,0.2 ,0.05

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D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

cording to the correlation of Chuang et al. [6 J. Pressure of 8 2 vapor was converted to H 2S/H 2 ratio by means of thermochemical data on formation of H 2 8 from JANAF [7]. The H 2 S/H2 ratios shown are probably accurate to 10% of the value or better. Fe-S-O phase stability diagrams

Phase stability diagrams at intervals of lOO°F from 1500 to 2000°F (816 to 1093°C) are presented in Figs. 3(a) to (f). They cover a range of oxygen potential from 0.1 to 10 H 20/H2 mole ratio and sulfur potential from 0.001 to 0.01 H 28/H2 mole ratio. In constructing these diagrams, data for relations among iron metal and its oxides were taken from Giddings and Gordon [8 J who correlated oxygen activities and composition limits of wlistite (FeOl+Y) from a large number of studies. Note that data of compilations such as those of JANAF [7 Jand Barin and Knacke [9], which are specific for a single wtistite composition (i.e., Fe O.947 0) or for the quasi-compound FeO, are less appropriate here than the composition-dependent ones of Giddings and Gordon [8]. For magnetite, we selected the data of Barin and Knacke [9] over those of JANAF [7 J. Activities of sulfur and iron in ferrous sulfide (FeS1+Y) were obtained as functions of temperature and composition from correlation equations reported by Chuang et al. [6], which agree well with the experimental data ofBurgmann et al. [4]. At each temperature and at intervals of H 2 S/H 2 ratio above that for equilibrium of FeS1+x with metallic iron, the H 2 0/H2 ratio for each H 2S/H2 ratio was obtained by finding the equivalent iron activity in wlistite from its oxygen activity - according to Giddings and Gordon [8] - via the GibbsDuhem relationship. The ratios of O2 activity to H 2 0/H2 mole ratio were obtained from JANAF [7]. Water-gas shift equilibrium constants were also obtained from JANAF tables to determine CO 2 /CO ratios corresponding to the H 2 0/H2 ratios. The Fe81+y-Fe304 lines were established according to the equilibrium for the reaction: Fea04 +4H 2 +3(1+x)H 2 S :;=: 3FeS1+X +4H2 0+3(1+x)H 2

(1)

as follows: log K=4Iog(H 2 0/H2 ) -3 (1 +x)log(H 2 S/H2 )

(2)

The equilibrium constant at each temperature was evaluated at the FeOl+YFeaOrFeS1+x invariant point, that is, at the intersection ofthe vertical line of the FeOl+y-Fea04 boundary with the slanting line of the FeS1+",-FeOl+Y boundary. These calculations sufficed for construction of the 1500 and 1600°F (816 and 871 DC) phase stability diagrams, in which only solid phase appear. In

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220

n.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

higher temperature diagrams a liquid phase is also present; according to Turkdogan and Kor [10], a ternary Fe-S-O eutectic appears at about 1697°F (915°C). This temperature is an average from three investigations that yielded values ranging from 1666 to 1688°F (908 to 920°C). These values are substantially below the eutectic temperature of 1724 °F (940 °C) obtained by Giani as reported by Asanti and Kohlmeyer [11]. In the 1700 and 1800°F (927 and 982°C) diagrams, corners of the liquid field were obtained from the figures ofthe Turkdogan and Kor [10] paper that show variation with temperature of sulfur and oxygen potentials for univariant equilibria of three phases. Their diagram for sulfur potentials is shown in Fig. 4. The equilibria required are: Fe, Fe01+y, and liquid; Fe, FeSl+x, and liquid; FeOl+y, FeSl+x, and liquid. The oxygen potentials of their diagram agree well with the oxygen potentials reported by Giddings and Gordon [8]; the latter were used in preparation of our diagrams. The values obtained at 1800, 1900, and 2000°F (982,1038 and 1093°C) for Fe, Fe01+Y, and liquid, were compared with values from Hyashi and Iguchi [12]; sulfur potentials agreed within 0.0004 in H 2S /H 2 ratio, but their oxygen potentials were lower by from 0.03 to 0.05 in H 20/H 2 ratios. -26 (i)

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221

D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226 20%5 20%0 60%Fe

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and log

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with composition of iron sulfide-oxide liquid at 1120·C

Additional data to establish the boundary between pyrrhotite (FeSl+x) and liquid in the 1900 and 2000 °F (1038 and 1093 °C) diagrams were obtained from Naldrett [13]. He correlated potential and composition data from Rosenqvist [14], Nagamori and Rameda [15] and Bog and Rosenqvist [16] to obtain sulfur and oxygen potential curves on phase diagram isotherms at 1050, 1120 and 1200°C, as illustrated in Fig. 5 for 1120°C. To obtain the boundaries at 1900 and 2000°F (1038 and 1093°C), the sulfur and oxygen potentials at the pyrrhotite liquidus on the 1050 and 1120°C figures from Naldrett [13] were first plotted on a phase stability diagram, and sulfur potentials (as H 2 S/H2 ratios) at selected values of H 20/H2 ratio were plotted against reciprocal absolute temperature; values of H 2S/H2 ratio at 1900 and 2000°F (1038 and 1093°C) were then obtained from the latter plot. In addition, values of the H 2 S/H2 ratio at the H 2 0/H2 ratio of 0.1 were taken at or only slightly above those of Burgmann et a1. [4] for FeS1+x' The possibility of vapor transport and deposit of ferrous sulfide was not considered in this work; later reference to thermochemical data [17] indicates that at reactor bed temperatures with an equilibrium FeS gas pressure of 10- 11 atm, such transport is insignificant. APPLICATION TO FLUIDIZED-BED GASIFICATION

The agglomerate shown in Fig. 1 was produced during a test at the IGT pilot

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D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

plan in which ROM Kentucky No.9 seam coal from the Providence No.1 mine was gasified. Steam and air were fed to the distributor grid and central venturi opening during the 12-hour period preceding the bed sample that contained the above agglomerate. The temperature of the bed was 1870 to 1900°F (1021 to 1038° e) during this period, and the bed pressure was 1.4 to 1.6 atm. Similar occurrence of ferrous sulfide spread on ash particles was apparent in samples from periods when the bed temperature was 1800 to 1850°F. The temperature near the grid, which was fed with stream and air, would be higher. Thus, the temperature of melting of these pyrite-derived particles does not differ greatly from the melting temperature of 1975°F (1077°C) reported by Raask [3] during decomposition of pyrite particles in nitrogen. The iron-sulfur compound initially in the reactor bed is pyrite (FeS2) at 53.45 wt% sulfur, off scale to the right in Fig. 2. Observations on the behavior of pyrite at elevated temperatures show that it does not melt congruently (without change in composition) at temperatures up to 1490°F (810 e), where the pressure of sulfur vapor in equilibrium with pyrite (Fe8 2) and liquid sulfur containing a small amount of dissolved iron is 4935 atm [18] . At higher temperatures' pyrite decomposes to FeS1+x and gaseous sulfur at low pressures, or to FeS1 + x and a liquid at S2 fugacities above those shown on the right hand border of FeS 1+ X in Fig. 2. Below 1976°F (1080°C) the liquid is sulfur with a small amount of dissolved iron; above this temperature the liquid does contain a substantial amount of iron. Total pressures of sulfur vapor are somewhat less than the 8 2 fugacities; for example, the three phase equilibrium pressure among FeS1+x and the two liquids at 1976°F (1080°C) is 110.6 atm [6], compared to the S2 fugacity of 126 atm. However, such equilibrium pressures are much too high for either stable or even momentary existence in a fluidized-bed gasifier. Instead, FeS 1+ x from decomposition ofthe pyrite should continue to lose sulfur (which reacts with hydrogen to form HzS) until its composition is such that its nearly vertical H 28/H2 mole ratio line in the FeS1+x field is equivalent to the H 2SjH2 mole ratio of the gas atmosphere in the gasifier. The latter ratio depends on the amount of sulfur in the feed coal; in pilot plant steam/oxygen runs with about 1.7 wt% sulfur in the coal, the ratio was about 0.01, and was about 0.032 with the Kentucky ROM coal which contained about 4.6 wt% sulfur. The FeS1+x so formed is sufficiently stable in the back-mixed bed atmosphere that it can be oxidized for reaction with siliceous ash only in the oxygen inlet region. The conclusion from the above considerations is that there is little or no possibility of formation of an Fe-S liquid that is thermodynamically stable in the gasifier atmosphere. Instead, an explanation based on kinetics needs to be explored. In decomposition of pyrite, diffusion through the solid is of paramount importance. In solid systems of this kind, it is well known that cations diffuse much more rapidly than anions. For example, the self diffusivity of oxygen in silica glasses at 2192°F (1200°C) was determined to be 10- 12 cm2 0

D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

223

s-t, whereas that of calcium was 10- 6 cm2 S-l - a million times faster [19]. Similarly, in ferrous sulfide the self diffusivity of sulfur from 662 to 1292°F (350 to 700 °C) is reported to be many orders of magnitude smaller than that of iron [20]. Thus, when molecules of S2 (or H 2S) leave the surface of pyrite, iron cations are left at the surface. These must diffuse inwardly through the immobile sulfur anions to enable additional sulfur to be exposed at the surface. If such diffusion is the limiting rate of the decomposition reaction, the concentration of iron will build up and sulfur concentration decline in the outer regions of the particles. Figure 2 shows that, at temperatures above 1814 of (990 °C) and at sulfur concentrations below those ofFeS1+ x, mixtures ofliquid FeS and FeSl+x occur. Such a mixture can be expected to flow readily at sulfur concentrations such that only about 25 to 30% of solid pyrrhotite is present that is, below about 35 wt% Sat 1900°F (1038°C). We believe that the above is a likely explanation for the occurrence offerrous sulfide spread on ash particles observed in samples of bed material from gasification of the Kentucky ROM coal. We think this also applies to the melting during pyrite decomposition in nitrogen that was observed by Raask at 1975 °F (1077 °C ) [3]. Substantiation of this explanation could probably be obtained by scanning electron microscopical analysis of layers of rapidly quenched particles of pyrite undergoing thermal decomposition in the absence of oxygen, as when pyrite particles are dropped through a heated tube against upflowing inert gas. This effect is not likely to be observed in particles obtained from the gasifier bed, such as that in Fig. 1, or in those deposited in the hot cyclone, because of their long residence at elevated temperatures, resulting in the attainment of equilibrium with the gas atmosphere. We also suspect that if a similar ROM coal containing free pyrite is fed to a gasifier along with limestone for sulfur capture, molten Fe-S liquid will spread on the limestone-derived particles and react to yield FeS-CaO mixtures. According to Kopylov et a1. [21], these have eutectics that melt as low as 1508 °F (820°C), as shown in Fig. 6. Problems, including ash deposition, may be encountered as a result of occurrence of such low melting material. Conditions in a gasifier under which a liquid eutectic between FeSl+x and FeOl+y may form are also of interest (these are shown in Figs. 3 (a) to (f) at lOO°F intervals from 1500° to 2000°F (816 to 1093°C)). Scales of H 2S/H2 mole ratio against H 2 0/H2 and CO 2 / CO mole ratios indicate sulfur and oxygen potentials in the gasifier atmosphere. In some typical V-GAS pilot plant runs with washed Kentucky No.9 coal at a bed temperature of about 1870°F (1021 °C), the product gas atmosphere was borderline for formation of the Fe-S-O liquid at 1800°F (982°C) and was favorable for its formation at higher temperatures, according to Figs. 3(e) and (f). If the liquid does form, samples after cooling should contain intermixed ferrous sulfide and ferrous oxide in characteristic forms such as those observed by Hilty and Crafts [22]. However, intermixed sulfide and oxide particles were

n.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

224

I

t,Or;

I

l.

I I

1'100

Ga.O T l.

I

-0-----0-GuO+CnS+l.

o

o

0

GaO

TellS

+ CzF+l.

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en 0 + Gil S T CzF + ISO

-t L

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+G 2 F+Fe

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80

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CaO,mass%

Fig. 6. Phase equilibrium diagram for the crystallization of the melts ofthe CaO-FeB system [20].

not observed in samples from any U-GAS pilot plant run that we examined petrographically, but no concerted effort to identify such particles was made. They were not evident in ferrous sulfide deposits found in the hot cyclone of the pilot plant after some runs; the deposits were porous and consisted of grains up to about 50 11m in size with sharp edges at pore boundaries [12]. However, the reaction rate for its formation may be so slow that only an optically invisible layer is formed that nevertheless may make the particles sticky enough to cause the deposition. CONCLUSIONS

The behavior of iron-sulfur species derived from pyrite, especially that present in the coal feed as particles of free pyrite, is of concern in the operation of fluidized-bed coal gasifiers. The decomposition of free pyrite in the V-GAS pilot plant has been found to yield a liquid phase that spreads on the surface of ash particles. Formation of a liquid phase in decomposition of pyrite in an inert atmosphere has also been observed in connection with investigation of boiler deposits. To aid in understanding the behavior of iron-sulfur species, we have prepared an Fe-S phase diagram at temperatures relevant to coal gasification an with equilibrium sulfur potentials exhibited, for the most part, as hydrogen sulfide to hydrogen mole ratios; in a fluidized-bed gasifier, the latter can easily

D.M. Mason/Fuel Processing Technol. 30 (1992) 215-226

225

be obtained from the composition of the reactor product gas, which closely approximates that in the back-mixed fluidized bed. To show in addition the effect of oxygen potential in the bed, we have also prepared phase stability diagrams at intervals of 100°F from 1500 to 2000°F (816 to 1093°C). According to the Fe-S diagram, formation of an iron-sulfur liquid in the gasifier or in an inert atmosphere cannot be explained on the basis of equilibrium considerations. Instead, a kinetic explanation, based on increased concentration of iron in the exterior regions of the decomposing pyrite particle, as sulfur is removed from the surface, is much more plausible. Phase stability diagrams indicate that an Fe-S-O liquid could be formed in steam-oxygen gasification of high-sulfur coal at temperatures above about 1850°F (1010°C). However, no evidence of its formation was found by optical petrography in examination of solids from the U -GAS pilot plant in which steam to hydrogen mole ratios ranged up to about 2, but formation of a submicroscopic liquid layer on iron sulfide particles could have been responsible for deposition of these particles in the hot cyclone of the V-GAS pilot plant. Formation of the liquid in steam-air gasification, where the steam to hydrogen mole ratio typically is about 0.6, is less likely. ACKNOWLEDGEMENTS

This work was supported in part by the Morgantown Energy Technology Center, United States Department of Energy, under Contract No. DE-AC2184MC212313. Mr. E.D. Bolzan ofIGT's staff did the computer calculations on the thermodynamics of the Fe-S and Fe-O systems. We are grateful for the assistance of Professor K.C. Hsieh of National Sun Yat-Sen University, Taiwan, for interpretation of an equation in the paper by Chuang et al. [6].

REFERENCES 1 Mason, D.M. and Patel, J.G., 1980. Chemistry of ash agglomeration in the U-GAS'" process. Fuel Processing Technol., 3: 181-206. 2 Mason, D.M., Rehmat, A.G. and Tsao, K.C., 1983. Chemistry of ash deposits in the U-GAS process. In: R. W. Bryers (Ed.), Fouling of Heat Exchange Surfaces. Engineering Foundation, New York, NY, pp. 565-582. 3 Raask, E., 1985. Mineral Impurities in Coal Combustion. Behavior, Problems, and Remedial Measures. Hemisphere, Washington, DC, p. B6. 4 Burgmann, W., Urbain, G. and Frohberg, M.G., 1968. Iron-sulfur system in the iron monosulfide (pyrrhotite) region, Mem. Sci. Rev. Met., 65: 567-578. 5 Bale, C.W. and Toguri, J.M., 1976. Thermodynamics of the Cu-S, Fe-S, and Cu-Fe-S systems. Can. Metall. Q., 15: 305-317.

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