Energy & Fuels 1992,6,603-608
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High-Temperature H2S Removal from Coal-Derived Gas by Iron Ore Eiji Saaaoka,* Tomoo Ichio, and Shigeaki Kasaoka Faculty of Engineering, Okayama University, Thushima-naka, Okayama 700, Japan Received February 20,1992.Revised Manuscript Received May 26, 1992
Six kinds of hematite and two kinds of magnetite were examined as solid sorbents of H2S for high-temperature desulfurization. The hematite samples were more reactive than the magnetite samples, and reactivity of the sample of iron ore depended on the specific ore. The hematite ores were confirmed to be effective sorbents of H2S in H2O-poor and/or H2-rich gas. The reactivities of the samples of the hematite were decreased by the partial reduction to Fe304 with the simulated coal-derived gas (in the absence of H2S). Impurities and thermal history of an iron ore seemed to affect its reactivity. Pretreatment of iron ore with a gas mixture of 50% H2-N2 at higher than 600 "C for 1.5 h was found to be a useful activation method. Introduction
Experimental Section
High-temperatureprocesses for desulfurization of coalderived fuel gas are receiving attention from thermal efficiency and environmental points of view. Current commercial desulfurization processes are based on wet scrubbing, resulting in considerablethermal efficiency loss as well as costly waste water treatment. For new emerging technologies using coal-derived gas suchas combined-cycle power generation and molten carbonate fuel cells, highly efficient removal from several thousand ppm down to ca. 1 ppm for fuel-cell plants or ca. 100 ppm for power generation is claimed.' Many kinds of single- and mixed-oxide sorbents have been examined as candidate solids for high-temperature desulfurization.2 Also, thermodynamic studies on reaction between H& and oxideshave been r e p ~ r t e d . From ~ * ~ these studies, current research seems to be concentrated on iron oxide and zinc oxide. From the economic point of view, iron oxide sorbent is more attractive than zinc oxide; from the standpoint of desulfurization efficiency, zinc oxide is more attractive than iron oxide because of more favorable sulfidation thermodynamic^.^ This work has focused on the potential use of iron ore and the developmentof an activation method for iron ore. In this paper, simulated coal-derived-gas desulfurization using six kinds of hematite and two kinds of magnetite and effects of pretreatments of iron ore on ita reactivity are reported. The practical application of iron ore as a sorbent has been reported? but the activity differences among kinds of ore has not been clarified. Furthermore, the causes of activity differences are unknown, and a n y a t t e m p t of activation of iron ore has not been reportad.
Calcination of Iron Ore and Preparation of Synthetic Iron Ore. Seven kinds of hematite and two kinds of magnetite were used as sorbents. Bulk ores were calcined in an air stream (300cmS/min at STP) from room temperature to 650 OC (10 OC/min,total 3 h), crushed, and then sieved to average diameter of 1.0 mm.
(1)Lew,5.; Jothmurugeam, K.; Flytzani-Stephanopoulos,M.hd.Eng. Chem. Rea. 1989,!28,535-541. (2)Uchida, H.Nenryo Kyokaiehi, 1983,62,792-802. (3)Schrodt, J. T.;Hilton, G.B.;Ftogge, C.A. Fuel 1976,54,264-272. (4)We&norehnd, P.R.;Harrison, D.P . Enuiron. Sci. Technol. 1976, 10,660-661. (6)Inhii, T.; Haeimoto, K.; Kobayaehi, S.;Kobayaehi, J.; Sugitani, T.; Hono, M.Proceedings of the 21th Autumn Meeting of the Society of Chemical Engineers, Japan, l9W,SN316,p 698.
0887-0624/92/2506-0603$03.00/0
Synthetic iron ores (FezO3, FezOs-Alz03, FezOs-SiOz) were prepared by a coprecipitation method using metal nitrate, sodium silicate, and NH3. Products of coprecipitation were dried at 110 OC for 25 h, mainly calcined in the air stream from room temperature to 650 "C (10OC/min, total 3 h), crushed, and sieved to average diameter of 1.0 mm. Macro properties of the iron ores and the synthetic iron ores are listed in Tables I and 11. Apparatus and Procedure. The sulfidation experiments were carried out using a flow-type packed-bed tubular reactor systems under atmospheric pressure at 400 (450,500)"C. The reactor consisted of a quartz tube, 1.5 cm i.d., in which 0.5-4.0 mL of sorbent were packed. In these experiments, a mixture of H2S (500-1OOO ppm), H2 (0-50%), HzO (0-19.3%), and NZwas fed into the reactor at 200 cms/min at STP. Some of the samples were used after pretreatment to be described later. The H S concentrations of inlet and outlet gases were measured by an iodine method. Reducibilities of the iron ores were characterized using a flowtype thermobalance. Temperatureprogrammed reduction (TPR) profiles of the samples were measured under a gas flow of 50% Hz-Nz, 200 cms/min at STP, and a heating rate of 10 OC/min.
Results and Discussion Reactivity of Iron Ore with HzS. The reactivities of six samples of hematite, the synthetic Fe203, and two samples of magnetite with H2S at 400 O C are shown in Figure 1. Reactivity of the sample depended on the specific ore, and the hematite samples were more reactive than the magnetite samples. The order of initial reactivity (6)Kaaaoka, 5.;Saaaoka, E., Inari, M.;Sada, N.Nenryo Kyokaiahi 1987,66,21&223. Q 1992 American Chemical Society
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604 Energy &Fuels, Vol. 6, No.5, 1992
Table I. Iron Ore Studied main component, wt % totalFe FeO Si02 AlnOa sample" 1.37 1.77 65.1 0.34 hematite CP, Capnema 1.67 1.29 66.9 0.12 hematite SN, Sandur 0.90 0.30 1.13 hematite 67.8 BL, Biladila 0.36 0.94 1.52 66.4 hematite CR, Carajaa 0.15 4.20 2.45 62.8 hematite HM, Ham-F 3.63 4.28 59.9 3.97 hematite GA, Goa 65.4 27.8 4.39 0.96 magnetite RM, Romeral 1.85 16.3 8.30 magnetite 59.5 AR, Arugarobo Calcined sample; room temperature 650 "C, 10 OC/min, totd 3 h in air. ~~~~
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Table 11. Synthetic Iron Ore Studied bulk density, sample" raw salt g/cm3 composition,w t % (precipitantNH3) ~~~
CaO 0.02 0.03 0.02 0.04 0.04 0.01 1.19 1.14 1.01
bulk density, g/cm3 1.7 2.0
Ma0 0.03 0.02 0.02 0.05 0.07 0.07 0.19 1.72
2.1
2.4 1.7 1.5 2.0 1.8
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surface area, mZ/g 8.7 0.5 0.2 3.6 19 39 0.1 7.9
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Calcined sample; room temperature 3 h in air.
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650 OC, 10 OC/min, total
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Figure 2. Reactivity of iron ore with HzS and surface area change
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of sample. Precalcination in the air stream, room temperature 650 O C , 10 OC/min, total 3 h. Reaction conditions: 500 ppm &S-50% Hz-11.8% H&Nz (200 cmg/min at STP, 2.4 X 10.' h-l), 400 OC. (a b): surface area (m2/g) of sample; (a, upper panel) before use; (b, lower panel) after use.
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Figure 1. Reactivity of iron ore with H2S. Precalcination in the air stream, room temperature 650 "C, 10 W m i n , total 3 h. Reaction conditions: 500 ppm HzS-25% H2-19.3% H20-Nz (200 cmg/min at STP,2.4 X 104 h-l), 400 OC.
(Figure 1)and initial surface area (Table I) of the samples were as follows: reactivity:
Fe203> CP = HM = GA> CR > AR>> SN E BL N RM
surface area: GA > HM > Fe,03 > CP > AR > CR >> SN > BL > RM Roughly speaking, the initial surface areas of the samples seemed to correlate with the reactivity, but the orders were not the same. From these results, it might be suggested that reactivity of the sample is affected by the chemical property of the sample as well as its physical property. The dependency of the samplereactivityon stream time also was affected by the ore kind. That is, reactivities of
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the iron ore samples except Sandur and Bailadila monotonically decreased with increasing elapsed time; in contrast, reactivities of Sandur and Bailadila increased with increasing elapsed time and then reached a maximum value. For confirmation of these time dependencies, the reactivities of the hematite sampleswere examined under another reaction condition. As shown in Figure 2, almost the same time dependencies as those of Figure 1 were obtained. From the surface area variation of the fresh and used samples (shown in Figure 2), it is suggested that the surface area changes of Sandur and Bailadila due to the reaction might be partial causes of their special time dependences. Effects of Reaction Condition on Reactivity of Iron Ore. From a thermodynamicstudy, it had been reported that the reaction of iron oxide with of H2S was affected by the equilibrium limitation of the following equation: Fe,O, + 3H,S + H, = 3FeS + 4H,O (1) As the first step, the effects of reaction conditions on the reaction rate were studied using Capnema (hematite). The reaction rate of Capnema with H2S was accelerated by H2 and decelerated by H2O as shown in Figure 3. For the other hematites, almost the same results as that in Figure 3 were obtained. From these results, it was concluded that iron ore was suitable for the removal of HzS from Hz-rich and HzO-lean gas. The effect of reaction temperature on the reaction rate was studied using Capnema. Little temperature dependency of the Capnema-H2S kinetics was found within the range of 400-500 "C, as shown in Figure 4. Although the cause of this temperature dependency is uncertain, from
Energy & Fuels, Vol. 6, No. 5, 1992 605
HB Removal from Coal-Derived Gas .."I
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Figure 3. Effecta of HzO and/or H2 concentration on reactivity of Capnema with H2S. Precalcination in the air stream, room temperature-650 O C , 10 OC/min,total3 h. Reactionconditions: 500 ppm H~S-H~-H~O-NZ (200 cmS/min at STP, 2.4 X lo" h-9, 400 "C.
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Figure 5. Reactivity of reductively pretreated iron ore. Precalcination in the air stream, room temperature 650 OC, 10 OC/min, total 3 h. Pretreatment conditions: 25% H2-19.3% H20-N2 (200cm3/minat STP, 2.4 X lo" h-l), 400 OC, 3 h. Reaction conditions: 500 ppm HzS-25% H2-19.3% H20-Nz (200 cm3/ min at STP, 2.4 X 104 h-l), 400 OC. I
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Figure 4. Effecte of temperature on reaction of Capnema with H2S. Precalcination in the air stream, room temperature 650 O C , 10 "C/min, total 3 h. Reaction conditions: 500 ppm HzS50% H2-11.8% H20-Nz (200 cm3/min at STP, 2.4 X lo4 h-9. this result it was supposed that the reaction rate at high temperature may be controlled by the equilibrium limitation. From the above studies, it was concluded that the concentration of H2O in the coal-derived gas should be depressed if the iron ore is to be used as a sorbent at high temperature. Effects of Pretreatment of Hematite to Magnetite on Reactivity. In a packed-bed reactor, hematite is reduced to magnetite with HdCO in coal derived gas (coexisting HzO), and then reacts with H2S; magnetite ores were earlier shown to have lower reactivity (Figure 1). Reactivities of hematite samples pretreated with the simulated desulfurized gas (25 7% Hp19.3 7% H&Nd at 400 OC for 3 h were examined. As shown in Figure 5, the activity of the sample depended on the specific ore. From the comparison of the fractional conversions at 3 h from the start of the reaction in Figures 1and 5, it was found that the activities of the pretreated (reduced) hematite samples were lower than those of nontreated hematite samples. Effects of Impurity of Iron Ore on Reactivity. From the above examination, the establishment of the criteria for the iron ore selection seemed to be very important for practical use of iron ore. As the fundamental study for the establishment of the criteria, the effects of main impurities (SiO2,A1203) of the iron ore on the reactivity were studied using synthetic iron ores (Fe203-Al203, Fe203-Si02). In this study, the samples were used after reductive pretreatment.
From this study, the following results were obtained (1)addition of a small amount of A1203 to Fez03 increased
ita reactivity for H2S, but the number of active sites was small, as shown in Figure 6; (2) a small amount of Si02 addition to Fez03 did not induce considerable change of activity; and (3) a large amount of A1203 added to Fez03 induced deactivation. These results were not explained by the surface areas of the samples (Table 11). From this consideration,it was supposed that the interaction between the additive and Fez03 stabilized the Fez03 and, hence, the reactivity of the Fez03 was decreased. A1203 and Si02 in iron ore may be expected to play the same role as that of the synthetic ore. The thermal stability of iron ore was examined using Capnema and compared with that of the synthetic iron ore. As shown in Figure 7,the thermal stability of Capnema was stronger than that of the synthetic Fe2O3, and the thermal stability of the synthetic Fez03 at 900 O C was improved by the addition of A1203. From this result, it was suggested that the reactivity of the iron ore had been affected by ita own thermal history and that the impurities in the iron ore contribute to ita thermal stability. Activation of Iron Ore. While the establishment of a criteria for the selection of iron ore was difficult, the development of an activation method for iron ore could be claimed. In general, hematite as a sorbent of H2S was converted to FeS via FesOr. The activation of iron ore is the creation of an active Fe30r from the ore via Fe.
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606 Energy & Fuels, Vol. 6, No.5, 1992
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Figure 7. Thermal resistance of Capnema, Fe200aand Fe203Precalcination in the air stream, room temperature setting temperature, 10 OC/min, total 3 h. Pretreatment conditions: 25% H ~ 1 9 . 3 % H20-N2 (200 cm3/minat STP, 2.4 x 104 h-l), 400 OC, 3 h. Reaction conditions: 500 ppm &S-25% H2-19.3% H20-N2 (200 cm3/minat STP, 2.4 X l(r h-l), 400 O C . 7- 1.0
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Figure 9. Effects of temperature of reduction and oxidation on activation of Capnema. Precalcination in the air stream, room temperature 650 O C , 10 OC/min, total 3 h. Pretreatment: (a b); (a) reduction temperature, (b) oxidation temperature. Reduction: 50%H2-N2, 1.5h. Oxidation: 25%H2-19.3% HzONe, 1.5 h. Reaction conditions: 500 ppm &S-25% H2-19.3% H20-N2 (200 cm3/min at STP, 2.4 X l(r h-l), 400 O C .
1 2 Time on stream (h)
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CAPNEMA
d .
0
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or R"Terrp650C,Wmin
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l+s reacted ( c d a t STPQSmI'',3h-') Figure 10. Effectsof pretreatment of Capnema on ita reactivity. Reaction conditions: 500 ppm HzS-25% H2-19.3% H20-N2 (200 cm3/min at STP, 2.4 X l o 4 h-l), 400 O C . Amount of
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Figure 8. Activation of Capnema by pretreatment. Precalcination: in the air stream, room temperature 650 O C , 10 OC/min, total 3 h. Pretreatment conditions: (1) 25% Hz19.3%H20-N2,4OO0C, 3 h, (2) 50%H2-N2,400 O C , 1.5 h,-25% H2-19.3% H20-N2,400 O C , 1.5 h, (3) 25% H2-N2,500 OC, 1.5 h, 25% H2-19.3% H20-N2, 500 O C , 1.5 h, (4) 50% Hz-Nz, 500 "C, 1.5 h, 25% Hr19.3% H20-N2, 500 O C , 1.5 h, (5) 25% H2-N2,600 'C, 1.5 h, 25% H2-19.3% H20-N2,600 OC, 1.5 h, (6) 50% Ht-Nz, 600 OC, 1.5 h, 25% &-19.3% H20-N2, 600 "C,1.5 h, (7) 50% H2-N2,600 O C , 1.5 h, 25% H2-19.3% H20N2,600OC,1.5h,+25% H2-19.3% H&N2,400OC,3h,(8) 50% Hz-Nz, 700 "C, 1.5 h, 25% H2-19.3% H20-N2,400 "C, 1.5 h. Reaction conditione: 500ppm HzS-25% H2-19.3 % HzO-NZ(200 cm3/min at STP, 2.4 X l(r h-9, 400 O C .
+
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Fe304 sampleswere derived from CapnemaviaFe under different conditions. As shown in Figure 8, the reactivities of the samples were improved by high temperature and high HZconcentration in reduction (Fez03 Fe). The temperature of the partial oxidation (Fe Fe304) also affected reactivity of the sample, as shown in Figure 9. The partial oxidation of the sample with 25% Hz-19.3 % HzO-Nz, a t 400 OC, after the high-temperature (>600 "C) reduction was the best. To clarify the effects of the partial oxidation on the reactivity, the reactivities of the high-temperature reduced
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Capnema sample were compared with the reactivities of the other pretreated samples. As shown in Figure 10, the reactivity of the reduced sample was the same as that of the partial oxidized sample. This resulta suggested that the surface of the reduced sample was immediately oxidized with HzO and then reacted with HPS. Furthermore, it was confirmed that the iron-ore preheating step in the activation procedure could be abbreviated. The surface areas of Capnema samplesin Figure 10 were as follows: nontreated sample, 8.7 m2/g;partially reduced sample (25% Hz-19.3% HzO-NZ,400 OC, 3 h), 9.9 mz/g; reduced and patially oxidized sample (50% HZ-Nz,700 O C , 1.5 h 25% H2-19.3% HzO-Nz, 400 OC, 1.5 h), 9.5 m2/g;and reduced sample (50%HZ-Nz,700 OC, 1.6 h), 9.5 m2/g. From a comparison of the surface areas and reactivities of these samples, it was concluded that surface area of the sample was not most important factor for reactivity of the sample. From the above results, the activation procedure was selected as follows: an iron ore sample was reduced with 50% HZ-Nz,a t 700 OC, 1.5 h, and then partially oxidized with 25% H219.3% H20-Nz, 400 "C, 1.5 h. The availability of this method was examined using the other six iron ores calcined. As shown in Figure 11,this activation method was useful for all of the iron ores; the activities of the ores were drastically increased. Regeneration. The stability of the activated Capnema for regeneration was examined. A noncalcined Capnema sample was activated with HZunder 50% Hz-NZ mixed gas flow at 650 OC for 1.5 h. The activated sample were
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Energy & Fuels, Vol. 6, No.5, 1992 607
H f i Removal from Coal-Derived Gas
Hematite
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Magnetite
Figure 11. Activation of iron ore. Precalcination in the air 650 O C , 10 OC/min, total 3 h. stream, room temperature Pretreatment: (1) no treatment; (2) 25% H2-19.3% HzO-N~, 25% H2-19.3% 400 "C, 3 h; (3) 50% Hz-N~,700 OC, 1.5 h HzO-N2, 400 "C, 1.5 h. Reaction conditions: 500 ppm H2S-25% H219.3% H20-N2 (200 cms/min at STP, 2.4 X l(r h-l), 400 OC.
-g 4 O t Li 30[
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o ,
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300
400 500 600 700 Temperature (TI
800
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Figure 13. Temperature-programmed-reductionspectraof iron ores. Precalcination in the air stream, room temperature 650 OC, 10 OC/min, total 3 h. Reduction conditions: 50% HrN2, room temperature 800 "C, 10 OC/min.
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1 u lo O
1 2 3 Times of regeneration
4
(-1 Figure 12. Stability of activated Capnema for regeneration. Sample: Noncalcined sample. Activation: 50% Hz-Nz, 650 O C , 1.5 h. Reaction: lo00 ppm HzS-25% H2-19.3% H&N2 (200 cm3/min at STP, 2.4 X l(r h-l), 400 OC, 3 h. Regeneration: 5% 0~-11.8%HzO-NZ,400 650 OC (ca. 12 min), held (total 2.5 h). Cycle: activation reaction regeneration.
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sflided with H2S under lo00 ppm-25% H2-19.3 % H20N2 mixed gas flow at 400 OC for 3 h. The sflided sample was regenerated to Fe2O3 by heating up from 400 to 650 OC and holding (total2.5 h) under 5% 02-11.8% H20-Nz mixed gas flow. The regenerated samplewas alsoactivated with H2 under 50% H2-N2 mixed gas flow at 650 OC, 1.5 h, before next sflidation. The change of the amount of H2S reacted with the sample in five cycles of activation, sflidation, and regeneration is shown in Figure 12. Approximately 14% of the fractionalconversion of sample, as determined by the amount of reacted H2S, was maintained. Reduction Character of Ore and Synthetic Iron Ore. The reducibility of iron ore with H2 was measured by TPR, and the correlation of the reduhibility and the reactivities were examined. The TPR profiles of the ore depended on the specific ore, and the synthetic Fez03 was most easily reduced to iron, as shown in Figure 13. The samples of hematite seemed to be more easily reduced to iron than the samples of magnetite. This trend was not inconsistent with the trend of the reactivities of the samples, but among the reactivities shown in Figure 1 and the reducibility of the samples, no clear correlation was found. Figure 14showethe effect of the activation pretreatment on the reducibility of the sample. The temperature at which the reduction of the sample was started was ca. 270
200
300400500600700800 Temperature ('C)
Figure 14. Temperature-programmed-reductionspectra of
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samples of activated iron ores. Precalcinationin the air stream, room temperature 650 "C, 10 "C/min,total 3 h. Activation conditions: 50% H2-N2,700 "C, 1.5 h 25% Hz-19.3% HzONz, 400 "C, 1.5 h. Reduction conditions: 50% Hz-Nz, room temperature 800 "C, 10 OC/min.
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"C for the all samples. This result indicated that the activated samples contained almost the same reactive oxygen for H2S. Although correlations of sample reducibility and reactivity of sample with H2S were not found as described above, it was found that the reducibility of the activated ore was more uniform than that of the original ore sample. The weight loss of the activated sample was too small if the reduced sampleswas completely converted to FesO4 by the followingpartial oxidation as the second step in the activation procedure. For example, from the weight loss of the activated Capnema sample, the conversion of Fe to Fe304 was calculated as 26 % . Furthermore, in the XRD spectrumof the Capnema sample, strong diffraction peaks of Fe and moderate peaks of Fe304 were measured,but no peak of FeO was found. From these resulta, it was confirmed that the active Capnema sample consisted of Fe and FesOr. In general, oxidation of Fe proceeds from surface to bulk inside of porous Fe particle. If this assumptioncould be applied to this system, the surface layer of the activated Capnema sample consists of FeaO4. As the fractional sulfidation of the sample was smaller than the fraction of
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Fe304, it is considered that p--t of the FeaO4 layer was only sulfided. From these considerations it is suggested that the fresh FeaO4 via Fe may account for the reactivity. For confirmation of this mechanism of activation, more detailed study on surface character and composition is needed.
Conclusions The utility of iron ore as a sorbent of H2S was clarified, that is, (1)the iron ore was suitable for a H2O-lean and/or H2-rich coal-derived gas; (2) the reactivity of the iron ore
Sasaoka et al.
depended on the specific ore; (3) hematite ores were more reactive than the magnetite ores. For the utilization of both types ore, activation methods were examined. Treatment of iron ore with a gas mixture of 50% H2-Nz at higher than 600 "C for 1.5 h was found to be an useful activation method. For the development of the most suitable activation method for iron ore, the mechanism of activationis claimed to be reduction to Fe followedby partial oxidation to FeaOd. Registry No. HzS, 7783-06-4; hematite,1317-60-8; magnetite,
1309-38-2.
