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Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride
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Fuel 87 (2008) 467–474 www.fuelfirst.com
Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride Shengji Wu1, Masaki Ozaki, Md. Azhar Uddin *, Eiji Sasaoka Department of Material and Energy Science, Graduate School of Environmental Science, Okayama University, Tsushima Naka, Okayama 700-8530, Japan Received 25 November 2006; received in revised form 18 April 2007; accepted 7 June 2007 Available online 24 July 2007
Abstract Laboratory studies were conducted to develop an elemental mercury (Hg0) removal process based on the reaction of H2S and Hg0 using iron-based sorbents for coal derived fuel gas. It is well known that hydrogen chloride (HCl) is present in fuel gases derived from some types of coal, but the effect of HCl on the Hg0 removal performance of iron-based sorbents in coal derived fuel gas is not yet well understood. In this study, the effects of HCl on the removal of Hg0 from coal derived fuel gases over iron-based sorbents such as iron oxide (Fe2O3), supported iron oxides on TiO2, iron oxide–Ca(OH)2, and iron sulfides were investigated. The Hg0 removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80 C using simulated fuel gas. In the case of iron oxide (Fe2O3), the presence of HCl suppressed the Hg0 removal rate. In the case of Fe2O3 (2 or 5 wt%)/TiO2, the presence of HCl did not suppress the Hg0 removal rate and the activity was stable. The Hg0 removal performance of reagent FeS2 was higher than that of the iron oxide, and not affected by the presence of HCl. The Hg0 removal rate of iron oxide–Ca(OH)2 was not effected by the presence of HCl, because HCl was captured by Ca(OH)2. The reagent FeS2 showed higher Hg0 removal activity than that of FeS2 ore. However, the Hg0 removal performance of ground and kneaded FeS2 ore was comparable to that of reagent FeS2 probably due to the increase in porosity of the FeS2 ore by grinding and kneading. 2007 Elsevier Ltd. All rights reserved. Keywords: Removal of mercury vapor; Coal derived fuel gas; Iron-based sorbent; HCl
1. Introduction Mercury is contained in coal (about 0.1–0.15 ppm) and it is emitted to the atmosphere in the process of combustion and gasification. Other anthropogenic source of mercury emission is municipal waste incinerators. There are three forms of mercury which may present in the coal combustion flue gas viz., elemental mercury (Hg0), oxidized mercury (Hg2+), and particle bound-mercury (HgP). Oxidized mercury can be removed in the wet flue gas desulfurization (WFGD) facilities of coal combustion processes, because it *
Corresponding author. Tel./fax: +81 86 251 8897. E-mail address: [email protected] (M.A. Uddin). 1 Present address: Department of Environmental Science and Technology, School of Mechanical Engineering, Hangzhou Dianzi University, Hangzhou, China. 0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.06.016
is soluble in most of the aqueous solutions. Particle-bound mercury can be captured along with fly ash in the typical air pollution control devices (APDC) such as electrostatic precipitators and or baghouses. However, elemental mercury vapor is difficult to capture with typical APCD because Hg0 is highly volatile and insoluble in water. Adsorption of elemental mercury with solid sorbents such as carbonaceous materials (activated carbons and fly ash), metals and metal oxides is an effective method to capture elemental mercury from coal combustion flue gas. It has been reported that sulfur impregnated carbon showed high Hg0 adsorption performance compared to the unimpregnated carbon at elevated temperature (150 C) due to the mercury reacting with sulfur on the carbon surface to form HgS [1]. Krishnan et al. also reported that enhanced adsorption of Hg0 occurred at high temperature through the reaction of Hg0 and S on sulfur impregnated carbon
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[2]. The main form of sulfur which improved the performance of sulfur impregnated ACF for the removal of Hg from coal combustion flue gas was reported to be the surface elemental sulfur [3]. Activated carbon injection is a well established method for removal of Hg0 and Hg2+ from coal combustion flue gas. Most of the research in carbon-based sorbents has focused on fly ash. An alternative to fly ash is Thief carbon. Thief carbon is partially combusted coal drawn from the furnace after short residence time [4]. Thief carbon in a packed bed showed that it oxidized 75% of the Hg0 in a bench-scale slipstream of flue gas at 140 C. Recently, integrated gasification combined cycle (IGCC) power generation from coal is receiving increased attention for its high efficiency. Fuel gas generated from coal gasification also contains elemental mercury. To date, most of the research activities, both practical and fundamental, have focused on the removal and conversion of elemental mercury from real or simulated coal combustion flue gas. While on the contrary, very little attention has been paid to the studies on capture of elemental mercury from coalderived fuel gas. Lopez-Anton et al. have demonstrated that the retention of Hg compounds on both sulfur impregnated and unimpregnated carbon decreased with the increase of temperature (120–270 C) for coal combustion flue gas and coal gasification fuel gas [5]. It has been reported that chemically reactive solid sorbents removed Hg0 from coal-derived synthesis gas at elevated temperature (300 C), however the exact name or composition of the sorbents have not been disclosed in the paper [6]. From the studies on the interaction of various synthesis gas constituents that effect mercury speciation, it has been suggested that the reducing environment is not favorable for Hg oxidation via gas-phase reactions alone and that more elemental mercury is expected to remain in the synthesis gas from coal gasification [7]. Granite et al. have reported that supported noble metals such as palladium, platinum, iridium, ruthenium and silver are effective in capturing elemental Hg from simulated fuel gas at elevated temperatures [8]. Flue gas species such as moisture, hydrogen sulfide, carbon monoxide impact the adsorption of mercury from fuel gas at temperature greater than 204 C. Another effective method being considered to control the emission of elemental mercury in the exhaust gas is oxidation of elemental mercury with oxidants such as HCl or Cl2 or ozone over a catalyst followed by the removal of oxidized mercury (Hg2+) with scrubber solution in the WFGD facilities. Recently, Presto and Granite have surveyed about the catalysts for oxidation of mercury in flue gas [9]. The effective catalysts for Hg oxidation are commonly used SCR catalysts, metal and metal oxides and carbon-based catalysts. The extent of Hg0 oxidation is affected by the presence of coexisting gases such as NH3, NO, SO2 and concentration of the oxidant (HCl). It was concluded in this review that one of the major source of uncertainty in understanding catalytic oxidation is the lack of reaction mecha-
nism and kinetics. A kinetic study of the catalytic oxidation of mercury in flue gas using HCl treated and untreated iridium (1 wt%) supported on alumina, HCl treated and untreated thief carbon and commercial activated carbon (Darco) (HCl treated and untreated) as catalysts was reported [10]. The kinetic parameters such as reaction order with respect to Hg0 and HCl, activation energy, reaction rate, rate constant etc. were determined and it was reported that reaction rate normalized for catalysts mass was in the order of Ir > Ir/HCl > Darco > Thief/HCl. We have reported for elemental mercury removal from simulated coal derived gases that Hg0 reacts with H2S to form HgS over solid sorbents [11–13]. The gas composition of a coal derived gas before purification is favorable for this method as this process requires H2S which is usually present in the unrefined gas derived from coal gasification. We have reported that elemental mercury can be removed from the coal derived fuel gas containing H2S with iron oxides (bulk and unsupported) at temperatures ranging from 60 to 100 C according to the following reactions: iron oxide reacts with H2S to form FeSx, and some surface elemental sulfur (–S) species which then react with elemental mercury (Hg0) to form HgS [14,15]. We have also reported that surface elemental sulfur species may react with CO to form COS under some reaction conditions, depending upon the nature of iron oxide sorbent [16]. Iron sulfides (FeS and FeS2) also exhibited high activities for Hg0 removal [17]. HCl may be present in the coal derived fuel gas if the coal contains chlorine species [7]. Generally, HCl capture device has to be installed in the gas cleanup process, but some HCl may still escape the device. As the Hg capture device may be located before or after HCl capture unit, it is necessary to study the effect of HCl on the Hg removal activity of the sorbents. In this study, the effects of HCl in the coal derived fuel gas on the Hg0 removal performance of iron-based sorbents, such as iron oxide (Fe2O3), iron oxide supported on TiO2, iron oxide–Ca(OH)2, and iron sulfides, were investigated. 2. Experimental 2.1. Sorbents Previously, we have reported that the bulk Fe2O3 and supported Fe2O3 on TiO2 showed different Hg0 removal performance. The Hg0 removal performance of 1–5 wt% Fe2O3 supported on TiO2 was higher than that of bulk Fe2O3. Although, we suggested that iron oxide reacts with H2S to form FeSx, and some surface elemental sulfur (–Sad) species which then react with elemental mercury (Hg0) to form HgS, but it is possible that the surface sulfur species formed on reagent FeS2 may react with Hg0 differently than with the surface elemental sulfur species produced by reaction of Fe2O3 with H2S. We also thought that different kind of sulfur species may also have different reactivity for Hg0 removal in the presence of HCl. Taking above
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points into consideration, we paid attention to FeS2 (reagent and ore) and the Fe2O3 with TiO2 or Ca(OH)2 as well as Fe2O3 as iron-based sorbents to study the effect of HCl on Hg removal from coal-derived fuel gas. 2.1.1. Iron oxide (Fe2O3) Iron oxide sample was prepared by precipitation at room temperature using an aqueous solution of reagent grade Fe(NO3)3 Æ 9H2O as a iron precursor and an aqueous solution of NH3 as a precipitating agent. The precipitant was washed with de-ionized water and dried for 25 h at 110 C. The sample was calcined at 300 C for 3 h before use. The calcined sample was crushed and sieved to 1 mm size. This sample is designated as Fe2O3. 2.1.2. Iron oxides supported on the TiO2 A commercial TiO2 sample was purchased from Sakai Chemicals Co. Fe2O3/TiO2 was prepared by a conventional impregnation method using aqueous solution of Fe(NO3)3. The amount of Fe2O3 loading was varied from 0.6 to 5 wt% of Fe2O3. A part of the impregnated sample was treated with aqueous NH3 solution and then washed with deionized water, followed by drying at 110 C for 25 h. Other impregnated samples were dried at 110 C for 25 h. The sample treated with aqueous NH3 solution is denoted as Fe2O3/TiO2(NH3). The dried samples were calcined at 300 C for 3 h before use. 2.1.3. Iron disulfides A commercial FeS2 (iron pyrite, 95%) was purchased from STREM Chemicals Co. (USA). Bulk density FeS2 is 1.42 g/cm3. The BET surface area of FeS2 is <1 m2/g. The granular FeS2 particles were sieved into average diameter of 1 mm. Two types of FeS2(A, B) ore were used in this study. The FeS2 ore samples were dried at 110 C for 25 h. The FeS2 ore particles were sieved into average diameter of 1 mm. Bulk density of FeS2(A) ore and FeS2(B) ore were 2.42 g/cm3 and 1.89 g/cm3, respectively. The BET surface areas of the two FeS2 ore could not be measured because of very low surface area. The kneaded FeS2(A, B) ore samples were prepared from FeS2 ores by grinding and kneading with water, followed by drying at 110 C for 25 h. The dried samples were crushed and sieved to 1 mm size. Table 1 Bulk density and surface area of the samples used in this study Sample
Method
BET surface area (m2/g)
Bulk density (g/cm3)
Fe2O3
Precipitation
59
1.20
1 wt% Fe2O3/ TiO2(NH3) 1 wt% Fe2O3/TiO2
Impregnation
70
0.84
Impregnation
65
0.84
Fe2O3–Ca(OH)2
Mixed up
36
0.78
<1
1.42
FeS2 (reagent)
469
2.1.4. Iron oxide–Ca(OH)2 sample The iron oxide–Ca(OH)2 sample was prepared by mixing Fe2O3 with Ca(OH)2 (particle size: 150 lm under) mechanically. The mixed sample was pressed into cylindrical pellets at 3 MPa pressure. The pellets were crushed and sieved to average diameter of 1 mm. Bulk densities and surface areas of the samples used in this study are shown in Table 1. 2.2. Apparatus and procedure 2.2.1. Removal of Hg0 The reactivity of the samples for Hg0 removal was investigated using a flow-type packed bed reactor under atmospheric pressure at 80 C. The apparatus consisted of an Hg0 vaporizer, a feed system, a quartz glass reactor, a furnace with temperature controller and a cold vapor Hg0 analyzer (Fig. 1). About 0.125 cm3 of the sorbent sample (particle diameter: 1 mm) was packed into a quartz tube reactor. The reaction of commenced when a mixture of Hg0 (4.8 ppb), HCl (0–10 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) was fed into the reactor at 500 cm3 STP/min (SV: 24.0 · 104 h1). Measurements of the inlet and outlet concentrations of mercury were carried out using a cold vapor mercury analyzer. 2.2.2. Effect of the presence of HCl on iron oxide sulfidation The reactivity of iron oxide with H2S was measured at 80 C for 4 h using a flow-type thermo-gravimetric apparatus (Shimadzu, TGA50S) equipped with a quartz tubular reactor (1.5 cm inside diameter). About 0.05 g of the sample was placed in a platinum wire net sample holder (1.3 cm diameter). In the sulfidation experiments, a mixture of HCl (0, 1 ppm), H2S (1500 ppm), H2 (10%), CO (30%), H2O (8%), and N2 (balance gas) was fed into the reactor at 500 cm3 STP/min. The surface area of the samples was measured by conventional N2 adsorption method (Micromeritics Gemini 2375). The powder X-ray diffraction (XRD) patterns of the samples were recorded using a Shimazu XRD-6100 diffractometer with Cu Ka irradiation (30 kV, 30 mA). 3. Results and discussion 3.1. Effect of the presence of HCl on the mercury removal activity of Fe2O3 The effect of the presence of HCl on the Hg0 removal activities of Fe2O3 sample was investigated in a fixed bed flow-type reactor using model fuel gas with a composition of Hg0 (4.8 ppb), HCl (1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) at 80 C. Fig. 2 shows the effect of HCl on the Hg0 removal activity of the Fe2O3. The Hg0 removal activity of Fe2O3 decreased in the presence of 1 ppm HCl. However, a steady level of Hg0 removal was maintained after 3 h. So, it is evident that
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Fig. 1. Experimental setup for removal of Hg0.
80 60
0 ppm
40 20
1ppm
0 0
60
120
180
240
300
Time on stream /min Fig. 2. Effect of the presence of HCl on the Hg0 removal by Fe2O3. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
the presence of HCl suppressed the Hg0 removal activity of Fe2O3. After the reaction of the Hg0 removal in the presence of HCl, the reaction system was purged with N2 in order to replace the HCl, and then the model fuel gas in absence of HCl was flowed through the reactor in order to investigate reaction of Hg0 removal. It was observed the HCl deactivated Fe2O3 was not recovered to the original level in the absence of HCl. We have presented a mechanism for Hg0 removal in which an active surface sulfur Sad produced from surface oxygen plays important role in the Hg0 removal [14]. Therefore, in order to understand the reasons for the deterioration of Hg0 removal activity of Fe2O3 in the presence of HCl, we decided to investigate the following two issues: (1) Does HCl suppress the production of the active surface sulfur Sad produced from the iron oxide surface oxygen (–O) and H2S? (2) Is there any particular kind of surface sulfur for the adsorption of HCl on iron oxide (Fe2O3)?
It is not possible to directly investigate the effect of the presence of HCl on the production of the surface active surface sulfur Sad, because the separation of the active surface sulfur and the common surface sulfur of the sulfate produce from the iron oxide is difficult. Therefore, we examined the effects of the presence of HCl on the weight gain of the iron oxide sample in the simulated fuel gas (excluding Hg0) with the Fe2O3 by thermo-gravimetric analysis (TGA) in a thermo-balance using HCl (0, 1 ppm), H2S (1500 ppm), H2 (10%), CO (30%), H2O (8%), and N2 (balance gas) as a model fuel gas at 80 C. Fig. 3 shows the weight gain of the sample as a function of time on stream. As the weight gain of the sample was not observed in the absence of H2S and presence of HCl (results not shown), it is worth mentioning that the weight gain of the Fe2O3 sample may be expressed by the following two reactions [14].
Weight gain of sample / mg·mg-1
Percent of Hg0 removal / %
100
0.01
0 ppm
0.008 0.006
1 ppm 0.004
0.002 0
0
40
80
120
160
200
Time on stream / min Fig. 3. Effect of the presence of HCl on the weight gain of Fe2O3. Reaction temperature: 80 C; Inlet gas: HCl (0, 1 ppm), H2S (1500 ppm), CO (30%), H2 (10%), H2O (8%), and N2 (balance gas).
S. Wu et al. / Fuel 87 (2008) 467–474
H2 S þ O ¼ Sad þ H2 O H2 S þ Fe2 O3 ¼ 1=xFeSx þ Fe21=x O2 þ H2 O
ð1Þ ð2Þ
If the reaction of Eq. (1) predominantly occurs at first stage followed by the reaction of Eq. (2), then we can evaluate the effect of the presence of HCl. Furthermore, if the presence of HCl suppressed both of the reactions, the effect of the presence of HCl can also be evaluated. As shown in Fig. 3, the weight gain of the Fe2O3 was observed in both cases, i.e., with and without HCl: the presence of HCl slightly suppressed the weight gain. From these results, it may be concluded that HCl suppressed weight gain of the Fe2O3. However, it is difficult to conclude that the HCl suppressed the production of the surface active sulfur Sad. As we could not clarify the suppression of the production of the surface active sulfur by HCl using a thermogravimetric method, different type of iron-based sorbents were employed in the Hg0 removal tests. If some kind of the surface active sulfur has tolerance for the presence of HCl then the second assumption may be valid, i.e., there are particular kind of surface sulfur for the adsorption of HCl on iron oxide (Fe2O3).
observed. From the comparison of Fig. 4a and b, the effects of the remaining NH3 in the sample cannot be confirmed. The surface areas of the packed Fe2O3, 1 wt%Fe2O3/ TiO2(NH3) and 1 wt%Fe2O3/TiO2 were 9, 7 and 7 [m2/ 0.125 cm3], respectively. It is evident from these results (as shown Figs. 2 and 4) that the effect of the presence of HCl on the Hg0 removal did not depend on the surface area of the sorbents. We suggest that the activity may depend on the surface active sulfur produced on the sorbents. The effect of the loading amount of the Fe2O3 on the Hg0 removal activity of Fe2O3/TiO2 in the presence of HCl was examined for 0.6, 1, 2 and 5 wt% Fe2O3. As shown in Fig. 5, the Hg0 removal of the sorbent with greater than 2 wt% iron oxide loading was high and stable even in the presence of HCl. From these results it may be confirmed that when the amount of the supported iron oxides increased more than 2 wt%, the supported iron oxide has resistance to HCl. However, this conclusion is not consistent with the results of Fe2O3 sample (100% Fe2O3). If the resistance to HCl appears when the loading amount of iron oxide decrease, the results may be reasonable and consistent with that of 100% Fe2O3 sample. Therefore, we have to consider the
3.2. Effect of the presence of HCl on the mercury removal with Fe2O3/TiO2
80 60
5 wt% 2 wt% 1 wt% 0.6 wt%
40 20 0
0
60
120
180
240
300
Time on stream /min Fig. 5. Effect of Fe2O3 loading amount on the Hg0 removal activity of Fe2O3/TiO2(NH3) in the presence of HCl. Reaction temperature: 80 C; inlet gas: Hg0 (4.8 ppb), HCl (1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
100
Percent of Hg0 removal / %
100
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100
The effect of the presence of HCl was studied using 1 wt%Fe2O3/TiO2(NH3) sorbent at 80 C and SV: 24 · 104 h1. As shown in Fig. 4a, the presence of HCl suppressed the Hg0 removal rate slightly; however a steady level of Hg0 removal (60%) was maintained after 3 h. Since the 1 wt% Fe2O3/TiO2(NH3) was prepared using an aqueous solution of NH3, it is possible that if NH3 remained in the sample after the preparation, the NH3 can react with HCl and reduces the effect of the presence of HCl as a result. In order to clarify the effect of NH3 treatment, the Hg0 removal performance of an untreated sample (1 wt% Fe2O3/TiO2) was examined in the presence of HCl. As shown in Fig. 4b, the effect of the presence of HCl was also
0 ppm 80 60
1 ppm
40 20
(a) 1wt% Fe2O3/TiO2(NH3) 0
471
0
60
120
180
Time on stream /min
240
300
0 ppm
80 60
1 ppm
40 20
(b) 1wt% Fe2O3/TiO2 0 0
60
120
180
240
300
Time on stream /min
Fig. 4. Effect of the presence of HCl on the Hg0 removal by supported iron oxides (a: 1 wt% Fe2O3/TiO2(NH3); b: 1 wt% Fe2O3/TiO2). Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
472
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other reasons for the inconsistency. If there are several kinds of surface sulfur produced on the sorbents and some kind of the surface active sulfur has tolerance for HCl presence, then the results shown in Fig. 5 can be explained. 3.3. Effect of the presence of HCl on mercury removal by FeS2 Previously, we have reported that iron sulfides (FeS and FeS2) also exhibited activity for Hg0 removal [16]. In order to investigate the kind of surface sulfur that is resistant to HCl for the Hg0 removal, we used reagent grade iron disulfide which has a known composition and two FeS2 ores. The reagent FeS2 was treated in a thermo-balance using N2 at 150 C (until the sample weight became constant) prior to the Hg0 removal experiments for prevention of the sulfur desorption during the Hg0 removal experiment. In the case of reagent FeS2, it was observed that weight of FeS2 decreased with time during heating and continued to decrease for 2–4 h at 150 C. Eventually, the weight of samples became stable. However, the weights of two FeS2 ores were not decreased at 150 C. From the results obtained from themo-balance experiment, it was thought that over the reagent FeS2, there are some active sulfur species which can be easily desorbed by heating. Although the weight loss was not observed from the FeS2 ores, the sample gave off sulfurous smell when the ore was crushed. From these observations, it was thought that some active sulfur existed in the powdered FeS2 ore samples. Therefore, we assumed that this active sulfur may be similar to the surface active sulfur which we consider a key species for Hg0 removal. Therefore, we made an effort to use the ground and kneaded FeS2 ore samples in the Hg removal experiments. The heat treated iron sulfide samples were then used in the Hg0 removal experiments in a fix bed flow-type reactor. Fig. 6 shows the effect of HCl concentration on the Hg0 removal activity of the reagent FeS2. It is evident that the Hg0 removal performance of FeS2 was higher than that
of the iron oxide sorbent and the performance was unaffected by the presence of HCl. As the surface area of the reagent FeS2 was considerably lower than that of the Fe2O3, this result also suggest that a large amount active sulfur is presence over unit area of the reagent FeS2. Furthermore, as the Hg0 removal rate slightly decreased, the active sulfur seemed to consumed by the reaction: it was suggested that the active sulfur was not regenerated by H2S. Fig. 7 shows the comparison of Hg0 removal activity of reagent grade FeS2 and two FeS2 ores in the presence and absence of HCl. The reagent FeS2 showed higher activity than the FeS2 ore for Hg0 removal in the presence of HCl. In all cases of iron sulfides, the presence of HCl did not suppress the Hg0 removal rate. The reagent grade FeS2 and FeS2 (A, B) ore were identified as FeS2 by SEM analysis. Reagent grade FeS2 appears to consist of aggregates of fine particles. However, the FeS2 ore appears to be an aggregate of coarse particles. From these observations, the difference of the activity among the reagent FeS2 and FeS2 ores can be explained by the difference of surface area. To confirm this hypothesis, FeS2 ores are ground and then kneaded with water. The kneaded-FeS2 ore samples were dried, crushed and sieved to 1 mm size. We suggest that the pores were generated during the kneading of FeS2 ore with water. Fig. 8 shows the scanning electron microscope (SEM) images before and after the kneading of FeS2(A) ore. It is apparent from the SEM images that large particles of FeS2 have become small particles after kneading. Furthermore, the bulk densities of kneadedFeS2(A) ore and kneaded-FeS2(B) ore decreased to 1.56 and 1.66 g/cm3, respectively. However, the difference in specific surface area of kneaded-FeS2(A) ore and kneaded-FeS2(B) ore was not compared because the specific surface areas of these samples were too small to be measured by N2 adsorption method. Fig. 9 shows the effect of the kneading of FeS2 ore on the Hg0 removal activity after 3 h time-on-stream in the presence of HCl. Both kneaded-FeS2 (A, B) ores showed higher activity than the unkneaded ore for Hg0 removal in the presence of HCl. rom this examination, the hypothesis was confirmed.
100
10 ppm
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100 80 60
0 ppm
0 ppm
1 ppm
40 20 0
0
60
120
180
240
300
Time on stream /min Fig. 6. Effect of the presence of HCl on the Hg0 removal by reagent FeS2. Reaction temperature: 80 C; inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
0 ppm
1ppm
80
Reagent FeS2
60 40
FeS2 ore (A)
20
FeS2 ore (B) 0
0
120 60 180 Time on stream /min
240
Fig. 7. Comparison of Hg0 removal activity of reagent FeS2 and two FeS2 ore. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
S. Wu et al. / Fuel 87 (2008) 467–474
473
Fig. 8. Scanning electron microscope (SEM) images of FeS2(A) ore before kneading and after kneading.
100
Kneaded 80
Unkneaded
60
40
20
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100
1ppm 80 60 40
FeS2(A) or) ore
Fig. 9. Effect of kneading of FeS2 ore on the Hg0 removal activity in the presence of HCl. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
From the above study, we can conclude that some active sulfur over the FeS2 samples is active for Hg0 removal and resistant to HCl for Hg0 removal. Therefore, if the same active sulfur is produce over the iron oxide sorbent by H2S without disturbance of HCl, the sorbent should have tolerance for HCl. However, the type of the active sulfur over the FeS2 samples and the active surface sulfur of the iron oxide samples is not clear. Whether HCl disturb the formation of the active sulfur over some iron oxide sorbent or not also was unknown. Furthermore, it is possible that some kind of active sulfur can be produced without disturbance of HCl, but HCl adsorbs on the sulfur and prevent the Hg0 removal. From above consideration, we concluded that much more study is needed to clarify the various kinds of the active sulfur species over the sorbents.
Fe2O3 0
60
120
180
240
300
Time on stream /min
FeS2(B) ore
Samples / -
0 ppm
20 0
0
Fe2O3-Ca(OH)2
Fig. 10. Comparison of the Hg0 removal activity of Fe2O3–Ca(OH)2 and Fe2O3 in the presence of HCl. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
Fe2O3 with Ca(OH)2 mechanically. Fig. 10 shows a comparison of the Hg0 removal activity of Fe2O3–Ca(OH)2 and Fe2O3 in the presence of HCl. In the case of iron oxides, the presence of HCl suppressed the Hg0 removal rate. However, the Hg0 removal rate of Fe2O3–Ca(OH)2 was almost perfectly not affected by the presence of HCl. This result suggested that HCl preferably reacted with Ca(OH)2 even in the presence of Fe2O3. As HCl simultaneously attacks to Ca(OH)2 and Fe2O3, this also suggest that if HCl attacked Fe2O3, the HCl was not so strongly connected to prevent the HCl migration to Ca(OH)2 as a second step reaction. However, it was already confirmed that the effect of the presence of HCl on the removal of Hg0 by Fe2O3 was irreversible. Much more studies is needed to understand the reason for the strong effect of the addition of Ca(OH)2 for HCl tolerance. 4. Conclusion
3.4. Comparison of mercury removal activity of Fe2O3(300)Ca(OH)2 and Fe2O3(300) in the presence of HCl Calcium hydroxide is used as a sorbent for HCl capture from municipal waste incinerator exhaust gas. We thought that the sorbent containing Fe2O3 and Ca(OH)2 would be an interesting material for Hg0 removal from fuel gas containing HCl. Fe2O3–Ca(OH)2 was prepared by mixing
In this study, the effect of HCl in the coal derived fuel gas on the Hg0 removal performance of iron-based sorbents such as Fe2O3, iron oxide supported on TiO2, iron oxide–Ca(OH)2, and iron sulfides was investigated. The following results were obtained: (1) in the case of Fe2O3, the presence of HCl suppressed the Hg0 removal rate. (2) In the case of Fe2O3 (2, 5 wt%)/TiO2, the presence of
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HCl did not suppress the Hg0 removal rate and the activity was stable. (3) The Hg0 removal performance of reagent grade FeS2 was higher than that of Fe2O3, and the performance was not affected by the presence of HCl. (4) The natural FeS2 ore also showed Hg0 removal activity. (5) The Hg0 removal rate of iron oxide–Ca(OH)2 was not affected by the presence of HCl. From this study, we can conclude that some active sulfur over the FeS2 sample is active for Hg0 removal and resistant to HCl for Hg0 removal. Therefore, if the same active sulfur is produce over the iron oxide sorbent by H2S without the interference of HCl, the sorbent has tolerance for HCl. However, the type of the active sulfur over the FeS2 samples and the active surface sulfur of the iron oxide samples is unknown. We need much more study to clarify the various kinds of the active sulfur species formed over the sorbents for the development of high active sorbent for Hg0. Acknowledgements This work was partly supported by the Steel Industrial Foundation for Advancement of Environmental Protection Technology, 21th COE program of Okayama University, and a Grant-in-Aid for Scientific Research on Priority Areas (B) from Ministry of Education, Science, Sports and Culture, Japan (No. 18310056). The JSPS is gratefully acknowledged for the financial support and postgraduate scholarship for Shengji Wu.
References [1] Sinha RK, Walker PL. Carbon 1972;10:754–6. [2] Krishnan SV, Gullett BK, Jozewicz W. Environ Sci Technol 1994;28:1506–12. [3] His HC, Rood MJ, Rostam-Abadi M, Chen S, Chang R. Environ Sci Technol 2001;35:2785–91. [4] O’Dowd WJ, Pennline HW, Freeman MC, Granite EJ, Hargis RA, Lacher CJ, et al. Fuel Process Technol 2006;87:1071–84. [5] Lopez-Anton MA, Tascon JMD, Martinez-Tarazona MR. Fuel Process Technol 2002;77–78:353–8. [6] Portzer JW, Albritton JR, Allen CC, Gupta RP. Fuel Process Technol 2004;85:621–30. [7] Lu DY, Granatstein DL, Rose DJ. Ind Eng Chem Res 2004;43:5400–4. [8] Granite EJ, Myers CR, King WP, Stanko DC, Pennline HW. Ind Eng Chem Res 2006;45:4844–8. [9] Presto AA, Granite EJ. Environ Sci Technol 2006;40:5601–9. [10] Presto AA, Granite EJ, Karash A, Hargis RA, O’Dowd WJ, Pennline HW. Energy Fuel 2006;20:1941–5. [11] Nakajima W, Tougaki N, Wu S, Nagamine S, Sasaoka E, In: Trace element workshop, Yokohama; July 18–19, 2002. [12] Togaki N, Uddin MA, Nakasima W, Wu S, Nagamine S, Sasaoka E, In: 20th Pittsburgh coal conference, Pittsburgh; September 15–19, 2003. [13] Wu S, Morimoto T, Togaki N, Nagamine S, Uddin MA, Sasaoka E, In: 227th ACS national meeting, Anaheim; March 28 to April 1, 2004. [14] Wu S, Uddin MA, Sasaoka E. Fuel 2006;85:213–8. [15] Wu S, Kawakami J, Oya N, Ozaki M, Uddin MA, Sasaoka E, In: 22nd Pittsburgh coal conference, Pittsburgh; September 12–15, 2005. [16] Wu S, Oya N, Kawakami J, Ozaki M, Uddin MA, Sasaoka E. Fuel; in press, doi:10.1016/j.fuel.2007.03.004. [17] Wu S, Ozaki M, Uddin MA, Sasaoka E, In: 23rd Pittsburgh coal conference, Pittsburgh, September 25–28, 2006.
Fuel 87 (2008) 467–474 www.fuelfirst.com
Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride Shengji Wu1, Masaki Ozaki, Md. Azhar Uddin *, Eiji Sasaoka Department of Material and Energy Science, Graduate School of Environmental Science, Okayama University, Tsushima Naka, Okayama 700-8530, Japan Received 25 November 2006; received in revised form 18 April 2007; accepted 7 June 2007 Available online 24 July 2007
Abstract Laboratory studies were conducted to develop an elemental mercury (Hg0) removal process based on the reaction of H2S and Hg0 using iron-based sorbents for coal derived fuel gas. It is well known that hydrogen chloride (HCl) is present in fuel gases derived from some types of coal, but the effect of HCl on the Hg0 removal performance of iron-based sorbents in coal derived fuel gas is not yet well understood. In this study, the effects of HCl on the removal of Hg0 from coal derived fuel gases over iron-based sorbents such as iron oxide (Fe2O3), supported iron oxides on TiO2, iron oxide–Ca(OH)2, and iron sulfides were investigated. The Hg0 removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80 C using simulated fuel gas. In the case of iron oxide (Fe2O3), the presence of HCl suppressed the Hg0 removal rate. In the case of Fe2O3 (2 or 5 wt%)/TiO2, the presence of HCl did not suppress the Hg0 removal rate and the activity was stable. The Hg0 removal performance of reagent FeS2 was higher than that of the iron oxide, and not affected by the presence of HCl. The Hg0 removal rate of iron oxide–Ca(OH)2 was not effected by the presence of HCl, because HCl was captured by Ca(OH)2. The reagent FeS2 showed higher Hg0 removal activity than that of FeS2 ore. However, the Hg0 removal performance of ground and kneaded FeS2 ore was comparable to that of reagent FeS2 probably due to the increase in porosity of the FeS2 ore by grinding and kneading. 2007 Elsevier Ltd. All rights reserved. Keywords: Removal of mercury vapor; Coal derived fuel gas; Iron-based sorbent; HCl
1. Introduction Mercury is contained in coal (about 0.1–0.15 ppm) and it is emitted to the atmosphere in the process of combustion and gasification. Other anthropogenic source of mercury emission is municipal waste incinerators. There are three forms of mercury which may present in the coal combustion flue gas viz., elemental mercury (Hg0), oxidized mercury (Hg2+), and particle bound-mercury (HgP). Oxidized mercury can be removed in the wet flue gas desulfurization (WFGD) facilities of coal combustion processes, because it *
Corresponding author. Tel./fax: +81 86 251 8897. E-mail address: [email protected] (M.A. Uddin). 1 Present address: Department of Environmental Science and Technology, School of Mechanical Engineering, Hangzhou Dianzi University, Hangzhou, China. 0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.06.016
is soluble in most of the aqueous solutions. Particle-bound mercury can be captured along with fly ash in the typical air pollution control devices (APDC) such as electrostatic precipitators and or baghouses. However, elemental mercury vapor is difficult to capture with typical APCD because Hg0 is highly volatile and insoluble in water. Adsorption of elemental mercury with solid sorbents such as carbonaceous materials (activated carbons and fly ash), metals and metal oxides is an effective method to capture elemental mercury from coal combustion flue gas. It has been reported that sulfur impregnated carbon showed high Hg0 adsorption performance compared to the unimpregnated carbon at elevated temperature (150 C) due to the mercury reacting with sulfur on the carbon surface to form HgS [1]. Krishnan et al. also reported that enhanced adsorption of Hg0 occurred at high temperature through the reaction of Hg0 and S on sulfur impregnated carbon
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[2]. The main form of sulfur which improved the performance of sulfur impregnated ACF for the removal of Hg from coal combustion flue gas was reported to be the surface elemental sulfur [3]. Activated carbon injection is a well established method for removal of Hg0 and Hg2+ from coal combustion flue gas. Most of the research in carbon-based sorbents has focused on fly ash. An alternative to fly ash is Thief carbon. Thief carbon is partially combusted coal drawn from the furnace after short residence time [4]. Thief carbon in a packed bed showed that it oxidized 75% of the Hg0 in a bench-scale slipstream of flue gas at 140 C. Recently, integrated gasification combined cycle (IGCC) power generation from coal is receiving increased attention for its high efficiency. Fuel gas generated from coal gasification also contains elemental mercury. To date, most of the research activities, both practical and fundamental, have focused on the removal and conversion of elemental mercury from real or simulated coal combustion flue gas. While on the contrary, very little attention has been paid to the studies on capture of elemental mercury from coalderived fuel gas. Lopez-Anton et al. have demonstrated that the retention of Hg compounds on both sulfur impregnated and unimpregnated carbon decreased with the increase of temperature (120–270 C) for coal combustion flue gas and coal gasification fuel gas [5]. It has been reported that chemically reactive solid sorbents removed Hg0 from coal-derived synthesis gas at elevated temperature (300 C), however the exact name or composition of the sorbents have not been disclosed in the paper [6]. From the studies on the interaction of various synthesis gas constituents that effect mercury speciation, it has been suggested that the reducing environment is not favorable for Hg oxidation via gas-phase reactions alone and that more elemental mercury is expected to remain in the synthesis gas from coal gasification [7]. Granite et al. have reported that supported noble metals such as palladium, platinum, iridium, ruthenium and silver are effective in capturing elemental Hg from simulated fuel gas at elevated temperatures [8]. Flue gas species such as moisture, hydrogen sulfide, carbon monoxide impact the adsorption of mercury from fuel gas at temperature greater than 204 C. Another effective method being considered to control the emission of elemental mercury in the exhaust gas is oxidation of elemental mercury with oxidants such as HCl or Cl2 or ozone over a catalyst followed by the removal of oxidized mercury (Hg2+) with scrubber solution in the WFGD facilities. Recently, Presto and Granite have surveyed about the catalysts for oxidation of mercury in flue gas [9]. The effective catalysts for Hg oxidation are commonly used SCR catalysts, metal and metal oxides and carbon-based catalysts. The extent of Hg0 oxidation is affected by the presence of coexisting gases such as NH3, NO, SO2 and concentration of the oxidant (HCl). It was concluded in this review that one of the major source of uncertainty in understanding catalytic oxidation is the lack of reaction mecha-
nism and kinetics. A kinetic study of the catalytic oxidation of mercury in flue gas using HCl treated and untreated iridium (1 wt%) supported on alumina, HCl treated and untreated thief carbon and commercial activated carbon (Darco) (HCl treated and untreated) as catalysts was reported [10]. The kinetic parameters such as reaction order with respect to Hg0 and HCl, activation energy, reaction rate, rate constant etc. were determined and it was reported that reaction rate normalized for catalysts mass was in the order of Ir > Ir/HCl > Darco > Thief/HCl. We have reported for elemental mercury removal from simulated coal derived gases that Hg0 reacts with H2S to form HgS over solid sorbents [11–13]. The gas composition of a coal derived gas before purification is favorable for this method as this process requires H2S which is usually present in the unrefined gas derived from coal gasification. We have reported that elemental mercury can be removed from the coal derived fuel gas containing H2S with iron oxides (bulk and unsupported) at temperatures ranging from 60 to 100 C according to the following reactions: iron oxide reacts with H2S to form FeSx, and some surface elemental sulfur (–S) species which then react with elemental mercury (Hg0) to form HgS [14,15]. We have also reported that surface elemental sulfur species may react with CO to form COS under some reaction conditions, depending upon the nature of iron oxide sorbent [16]. Iron sulfides (FeS and FeS2) also exhibited high activities for Hg0 removal [17]. HCl may be present in the coal derived fuel gas if the coal contains chlorine species [7]. Generally, HCl capture device has to be installed in the gas cleanup process, but some HCl may still escape the device. As the Hg capture device may be located before or after HCl capture unit, it is necessary to study the effect of HCl on the Hg removal activity of the sorbents. In this study, the effects of HCl in the coal derived fuel gas on the Hg0 removal performance of iron-based sorbents, such as iron oxide (Fe2O3), iron oxide supported on TiO2, iron oxide–Ca(OH)2, and iron sulfides, were investigated. 2. Experimental 2.1. Sorbents Previously, we have reported that the bulk Fe2O3 and supported Fe2O3 on TiO2 showed different Hg0 removal performance. The Hg0 removal performance of 1–5 wt% Fe2O3 supported on TiO2 was higher than that of bulk Fe2O3. Although, we suggested that iron oxide reacts with H2S to form FeSx, and some surface elemental sulfur (–Sad) species which then react with elemental mercury (Hg0) to form HgS, but it is possible that the surface sulfur species formed on reagent FeS2 may react with Hg0 differently than with the surface elemental sulfur species produced by reaction of Fe2O3 with H2S. We also thought that different kind of sulfur species may also have different reactivity for Hg0 removal in the presence of HCl. Taking above
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points into consideration, we paid attention to FeS2 (reagent and ore) and the Fe2O3 with TiO2 or Ca(OH)2 as well as Fe2O3 as iron-based sorbents to study the effect of HCl on Hg removal from coal-derived fuel gas. 2.1.1. Iron oxide (Fe2O3) Iron oxide sample was prepared by precipitation at room temperature using an aqueous solution of reagent grade Fe(NO3)3 Æ 9H2O as a iron precursor and an aqueous solution of NH3 as a precipitating agent. The precipitant was washed with de-ionized water and dried for 25 h at 110 C. The sample was calcined at 300 C for 3 h before use. The calcined sample was crushed and sieved to 1 mm size. This sample is designated as Fe2O3. 2.1.2. Iron oxides supported on the TiO2 A commercial TiO2 sample was purchased from Sakai Chemicals Co. Fe2O3/TiO2 was prepared by a conventional impregnation method using aqueous solution of Fe(NO3)3. The amount of Fe2O3 loading was varied from 0.6 to 5 wt% of Fe2O3. A part of the impregnated sample was treated with aqueous NH3 solution and then washed with deionized water, followed by drying at 110 C for 25 h. Other impregnated samples were dried at 110 C for 25 h. The sample treated with aqueous NH3 solution is denoted as Fe2O3/TiO2(NH3). The dried samples were calcined at 300 C for 3 h before use. 2.1.3. Iron disulfides A commercial FeS2 (iron pyrite, 95%) was purchased from STREM Chemicals Co. (USA). Bulk density FeS2 is 1.42 g/cm3. The BET surface area of FeS2 is <1 m2/g. The granular FeS2 particles were sieved into average diameter of 1 mm. Two types of FeS2(A, B) ore were used in this study. The FeS2 ore samples were dried at 110 C for 25 h. The FeS2 ore particles were sieved into average diameter of 1 mm. Bulk density of FeS2(A) ore and FeS2(B) ore were 2.42 g/cm3 and 1.89 g/cm3, respectively. The BET surface areas of the two FeS2 ore could not be measured because of very low surface area. The kneaded FeS2(A, B) ore samples were prepared from FeS2 ores by grinding and kneading with water, followed by drying at 110 C for 25 h. The dried samples were crushed and sieved to 1 mm size. Table 1 Bulk density and surface area of the samples used in this study Sample
Method
BET surface area (m2/g)
Bulk density (g/cm3)
Fe2O3
Precipitation
59
1.20
1 wt% Fe2O3/ TiO2(NH3) 1 wt% Fe2O3/TiO2
Impregnation
70
0.84
Impregnation
65
0.84
Fe2O3–Ca(OH)2
Mixed up
36
0.78
<1
1.42
FeS2 (reagent)
469
2.1.4. Iron oxide–Ca(OH)2 sample The iron oxide–Ca(OH)2 sample was prepared by mixing Fe2O3 with Ca(OH)2 (particle size: 150 lm under) mechanically. The mixed sample was pressed into cylindrical pellets at 3 MPa pressure. The pellets were crushed and sieved to average diameter of 1 mm. Bulk densities and surface areas of the samples used in this study are shown in Table 1. 2.2. Apparatus and procedure 2.2.1. Removal of Hg0 The reactivity of the samples for Hg0 removal was investigated using a flow-type packed bed reactor under atmospheric pressure at 80 C. The apparatus consisted of an Hg0 vaporizer, a feed system, a quartz glass reactor, a furnace with temperature controller and a cold vapor Hg0 analyzer (Fig. 1). About 0.125 cm3 of the sorbent sample (particle diameter: 1 mm) was packed into a quartz tube reactor. The reaction of commenced when a mixture of Hg0 (4.8 ppb), HCl (0–10 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) was fed into the reactor at 500 cm3 STP/min (SV: 24.0 · 104 h1). Measurements of the inlet and outlet concentrations of mercury were carried out using a cold vapor mercury analyzer. 2.2.2. Effect of the presence of HCl on iron oxide sulfidation The reactivity of iron oxide with H2S was measured at 80 C for 4 h using a flow-type thermo-gravimetric apparatus (Shimadzu, TGA50S) equipped with a quartz tubular reactor (1.5 cm inside diameter). About 0.05 g of the sample was placed in a platinum wire net sample holder (1.3 cm diameter). In the sulfidation experiments, a mixture of HCl (0, 1 ppm), H2S (1500 ppm), H2 (10%), CO (30%), H2O (8%), and N2 (balance gas) was fed into the reactor at 500 cm3 STP/min. The surface area of the samples was measured by conventional N2 adsorption method (Micromeritics Gemini 2375). The powder X-ray diffraction (XRD) patterns of the samples were recorded using a Shimazu XRD-6100 diffractometer with Cu Ka irradiation (30 kV, 30 mA). 3. Results and discussion 3.1. Effect of the presence of HCl on the mercury removal activity of Fe2O3 The effect of the presence of HCl on the Hg0 removal activities of Fe2O3 sample was investigated in a fixed bed flow-type reactor using model fuel gas with a composition of Hg0 (4.8 ppb), HCl (1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas) at 80 C. Fig. 2 shows the effect of HCl on the Hg0 removal activity of the Fe2O3. The Hg0 removal activity of Fe2O3 decreased in the presence of 1 ppm HCl. However, a steady level of Hg0 removal was maintained after 3 h. So, it is evident that
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Fig. 1. Experimental setup for removal of Hg0.
80 60
0 ppm
40 20
1ppm
0 0
60
120
180
240
300
Time on stream /min Fig. 2. Effect of the presence of HCl on the Hg0 removal by Fe2O3. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
the presence of HCl suppressed the Hg0 removal activity of Fe2O3. After the reaction of the Hg0 removal in the presence of HCl, the reaction system was purged with N2 in order to replace the HCl, and then the model fuel gas in absence of HCl was flowed through the reactor in order to investigate reaction of Hg0 removal. It was observed the HCl deactivated Fe2O3 was not recovered to the original level in the absence of HCl. We have presented a mechanism for Hg0 removal in which an active surface sulfur Sad produced from surface oxygen plays important role in the Hg0 removal [14]. Therefore, in order to understand the reasons for the deterioration of Hg0 removal activity of Fe2O3 in the presence of HCl, we decided to investigate the following two issues: (1) Does HCl suppress the production of the active surface sulfur Sad produced from the iron oxide surface oxygen (–O) and H2S? (2) Is there any particular kind of surface sulfur for the adsorption of HCl on iron oxide (Fe2O3)?
It is not possible to directly investigate the effect of the presence of HCl on the production of the surface active surface sulfur Sad, because the separation of the active surface sulfur and the common surface sulfur of the sulfate produce from the iron oxide is difficult. Therefore, we examined the effects of the presence of HCl on the weight gain of the iron oxide sample in the simulated fuel gas (excluding Hg0) with the Fe2O3 by thermo-gravimetric analysis (TGA) in a thermo-balance using HCl (0, 1 ppm), H2S (1500 ppm), H2 (10%), CO (30%), H2O (8%), and N2 (balance gas) as a model fuel gas at 80 C. Fig. 3 shows the weight gain of the sample as a function of time on stream. As the weight gain of the sample was not observed in the absence of H2S and presence of HCl (results not shown), it is worth mentioning that the weight gain of the Fe2O3 sample may be expressed by the following two reactions [14].
Weight gain of sample / mg·mg-1
Percent of Hg0 removal / %
100
0.01
0 ppm
0.008 0.006
1 ppm 0.004
0.002 0
0
40
80
120
160
200
Time on stream / min Fig. 3. Effect of the presence of HCl on the weight gain of Fe2O3. Reaction temperature: 80 C; Inlet gas: HCl (0, 1 ppm), H2S (1500 ppm), CO (30%), H2 (10%), H2O (8%), and N2 (balance gas).
S. Wu et al. / Fuel 87 (2008) 467–474
H2 S þ O ¼ Sad þ H2 O H2 S þ Fe2 O3 ¼ 1=xFeSx þ Fe21=x O2 þ H2 O
ð1Þ ð2Þ
If the reaction of Eq. (1) predominantly occurs at first stage followed by the reaction of Eq. (2), then we can evaluate the effect of the presence of HCl. Furthermore, if the presence of HCl suppressed both of the reactions, the effect of the presence of HCl can also be evaluated. As shown in Fig. 3, the weight gain of the Fe2O3 was observed in both cases, i.e., with and without HCl: the presence of HCl slightly suppressed the weight gain. From these results, it may be concluded that HCl suppressed weight gain of the Fe2O3. However, it is difficult to conclude that the HCl suppressed the production of the surface active sulfur Sad. As we could not clarify the suppression of the production of the surface active sulfur by HCl using a thermogravimetric method, different type of iron-based sorbents were employed in the Hg0 removal tests. If some kind of the surface active sulfur has tolerance for the presence of HCl then the second assumption may be valid, i.e., there are particular kind of surface sulfur for the adsorption of HCl on iron oxide (Fe2O3).
observed. From the comparison of Fig. 4a and b, the effects of the remaining NH3 in the sample cannot be confirmed. The surface areas of the packed Fe2O3, 1 wt%Fe2O3/ TiO2(NH3) and 1 wt%Fe2O3/TiO2 were 9, 7 and 7 [m2/ 0.125 cm3], respectively. It is evident from these results (as shown Figs. 2 and 4) that the effect of the presence of HCl on the Hg0 removal did not depend on the surface area of the sorbents. We suggest that the activity may depend on the surface active sulfur produced on the sorbents. The effect of the loading amount of the Fe2O3 on the Hg0 removal activity of Fe2O3/TiO2 in the presence of HCl was examined for 0.6, 1, 2 and 5 wt% Fe2O3. As shown in Fig. 5, the Hg0 removal of the sorbent with greater than 2 wt% iron oxide loading was high and stable even in the presence of HCl. From these results it may be confirmed that when the amount of the supported iron oxides increased more than 2 wt%, the supported iron oxide has resistance to HCl. However, this conclusion is not consistent with the results of Fe2O3 sample (100% Fe2O3). If the resistance to HCl appears when the loading amount of iron oxide decrease, the results may be reasonable and consistent with that of 100% Fe2O3 sample. Therefore, we have to consider the
3.2. Effect of the presence of HCl on the mercury removal with Fe2O3/TiO2
80 60
5 wt% 2 wt% 1 wt% 0.6 wt%
40 20 0
0
60
120
180
240
300
Time on stream /min Fig. 5. Effect of Fe2O3 loading amount on the Hg0 removal activity of Fe2O3/TiO2(NH3) in the presence of HCl. Reaction temperature: 80 C; inlet gas: Hg0 (4.8 ppb), HCl (1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
100
Percent of Hg0 removal / %
100
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100
The effect of the presence of HCl was studied using 1 wt%Fe2O3/TiO2(NH3) sorbent at 80 C and SV: 24 · 104 h1. As shown in Fig. 4a, the presence of HCl suppressed the Hg0 removal rate slightly; however a steady level of Hg0 removal (60%) was maintained after 3 h. Since the 1 wt% Fe2O3/TiO2(NH3) was prepared using an aqueous solution of NH3, it is possible that if NH3 remained in the sample after the preparation, the NH3 can react with HCl and reduces the effect of the presence of HCl as a result. In order to clarify the effect of NH3 treatment, the Hg0 removal performance of an untreated sample (1 wt% Fe2O3/TiO2) was examined in the presence of HCl. As shown in Fig. 4b, the effect of the presence of HCl was also
0 ppm 80 60
1 ppm
40 20
(a) 1wt% Fe2O3/TiO2(NH3) 0
471
0
60
120
180
Time on stream /min
240
300
0 ppm
80 60
1 ppm
40 20
(b) 1wt% Fe2O3/TiO2 0 0
60
120
180
240
300
Time on stream /min
Fig. 4. Effect of the presence of HCl on the Hg0 removal by supported iron oxides (a: 1 wt% Fe2O3/TiO2(NH3); b: 1 wt% Fe2O3/TiO2). Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
472
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other reasons for the inconsistency. If there are several kinds of surface sulfur produced on the sorbents and some kind of the surface active sulfur has tolerance for HCl presence, then the results shown in Fig. 5 can be explained. 3.3. Effect of the presence of HCl on mercury removal by FeS2 Previously, we have reported that iron sulfides (FeS and FeS2) also exhibited activity for Hg0 removal [16]. In order to investigate the kind of surface sulfur that is resistant to HCl for the Hg0 removal, we used reagent grade iron disulfide which has a known composition and two FeS2 ores. The reagent FeS2 was treated in a thermo-balance using N2 at 150 C (until the sample weight became constant) prior to the Hg0 removal experiments for prevention of the sulfur desorption during the Hg0 removal experiment. In the case of reagent FeS2, it was observed that weight of FeS2 decreased with time during heating and continued to decrease for 2–4 h at 150 C. Eventually, the weight of samples became stable. However, the weights of two FeS2 ores were not decreased at 150 C. From the results obtained from themo-balance experiment, it was thought that over the reagent FeS2, there are some active sulfur species which can be easily desorbed by heating. Although the weight loss was not observed from the FeS2 ores, the sample gave off sulfurous smell when the ore was crushed. From these observations, it was thought that some active sulfur existed in the powdered FeS2 ore samples. Therefore, we assumed that this active sulfur may be similar to the surface active sulfur which we consider a key species for Hg0 removal. Therefore, we made an effort to use the ground and kneaded FeS2 ore samples in the Hg removal experiments. The heat treated iron sulfide samples were then used in the Hg0 removal experiments in a fix bed flow-type reactor. Fig. 6 shows the effect of HCl concentration on the Hg0 removal activity of the reagent FeS2. It is evident that the Hg0 removal performance of FeS2 was higher than that
of the iron oxide sorbent and the performance was unaffected by the presence of HCl. As the surface area of the reagent FeS2 was considerably lower than that of the Fe2O3, this result also suggest that a large amount active sulfur is presence over unit area of the reagent FeS2. Furthermore, as the Hg0 removal rate slightly decreased, the active sulfur seemed to consumed by the reaction: it was suggested that the active sulfur was not regenerated by H2S. Fig. 7 shows the comparison of Hg0 removal activity of reagent grade FeS2 and two FeS2 ores in the presence and absence of HCl. The reagent FeS2 showed higher activity than the FeS2 ore for Hg0 removal in the presence of HCl. In all cases of iron sulfides, the presence of HCl did not suppress the Hg0 removal rate. The reagent grade FeS2 and FeS2 (A, B) ore were identified as FeS2 by SEM analysis. Reagent grade FeS2 appears to consist of aggregates of fine particles. However, the FeS2 ore appears to be an aggregate of coarse particles. From these observations, the difference of the activity among the reagent FeS2 and FeS2 ores can be explained by the difference of surface area. To confirm this hypothesis, FeS2 ores are ground and then kneaded with water. The kneaded-FeS2 ore samples were dried, crushed and sieved to 1 mm size. We suggest that the pores were generated during the kneading of FeS2 ore with water. Fig. 8 shows the scanning electron microscope (SEM) images before and after the kneading of FeS2(A) ore. It is apparent from the SEM images that large particles of FeS2 have become small particles after kneading. Furthermore, the bulk densities of kneadedFeS2(A) ore and kneaded-FeS2(B) ore decreased to 1.56 and 1.66 g/cm3, respectively. However, the difference in specific surface area of kneaded-FeS2(A) ore and kneaded-FeS2(B) ore was not compared because the specific surface areas of these samples were too small to be measured by N2 adsorption method. Fig. 9 shows the effect of the kneading of FeS2 ore on the Hg0 removal activity after 3 h time-on-stream in the presence of HCl. Both kneaded-FeS2 (A, B) ores showed higher activity than the unkneaded ore for Hg0 removal in the presence of HCl. rom this examination, the hypothesis was confirmed.
100
10 ppm
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100 80 60
0 ppm
0 ppm
1 ppm
40 20 0
0
60
120
180
240
300
Time on stream /min Fig. 6. Effect of the presence of HCl on the Hg0 removal by reagent FeS2. Reaction temperature: 80 C; inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
0 ppm
1ppm
80
Reagent FeS2
60 40
FeS2 ore (A)
20
FeS2 ore (B) 0
0
120 60 180 Time on stream /min
240
Fig. 7. Comparison of Hg0 removal activity of reagent FeS2 and two FeS2 ore. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
S. Wu et al. / Fuel 87 (2008) 467–474
473
Fig. 8. Scanning electron microscope (SEM) images of FeS2(A) ore before kneading and after kneading.
100
Kneaded 80
Unkneaded
60
40
20
Percent of Hg0 removal / %
Percent of Hg0 removal / %
100
1ppm 80 60 40
FeS2(A) or) ore
Fig. 9. Effect of kneading of FeS2 ore on the Hg0 removal activity in the presence of HCl. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
From the above study, we can conclude that some active sulfur over the FeS2 samples is active for Hg0 removal and resistant to HCl for Hg0 removal. Therefore, if the same active sulfur is produce over the iron oxide sorbent by H2S without disturbance of HCl, the sorbent should have tolerance for HCl. However, the type of the active sulfur over the FeS2 samples and the active surface sulfur of the iron oxide samples is not clear. Whether HCl disturb the formation of the active sulfur over some iron oxide sorbent or not also was unknown. Furthermore, it is possible that some kind of active sulfur can be produced without disturbance of HCl, but HCl adsorbs on the sulfur and prevent the Hg0 removal. From above consideration, we concluded that much more study is needed to clarify the various kinds of the active sulfur species over the sorbents.
Fe2O3 0
60
120
180
240
300
Time on stream /min
FeS2(B) ore
Samples / -
0 ppm
20 0
0
Fe2O3-Ca(OH)2
Fig. 10. Comparison of the Hg0 removal activity of Fe2O3–Ca(OH)2 and Fe2O3 in the presence of HCl. Reaction temperature: 80 C; Inlet gas: Hg0 (4.8 ppb), HCl (0, 1 ppm), H2S (400 ppm), CO (30%), H2 (20%), H2O (8%), and N2 (balance gas); SV: 24.0 · 104 h1.
Fe2O3 with Ca(OH)2 mechanically. Fig. 10 shows a comparison of the Hg0 removal activity of Fe2O3–Ca(OH)2 and Fe2O3 in the presence of HCl. In the case of iron oxides, the presence of HCl suppressed the Hg0 removal rate. However, the Hg0 removal rate of Fe2O3–Ca(OH)2 was almost perfectly not affected by the presence of HCl. This result suggested that HCl preferably reacted with Ca(OH)2 even in the presence of Fe2O3. As HCl simultaneously attacks to Ca(OH)2 and Fe2O3, this also suggest that if HCl attacked Fe2O3, the HCl was not so strongly connected to prevent the HCl migration to Ca(OH)2 as a second step reaction. However, it was already confirmed that the effect of the presence of HCl on the removal of Hg0 by Fe2O3 was irreversible. Much more studies is needed to understand the reason for the strong effect of the addition of Ca(OH)2 for HCl tolerance. 4. Conclusion
3.4. Comparison of mercury removal activity of Fe2O3(300)Ca(OH)2 and Fe2O3(300) in the presence of HCl Calcium hydroxide is used as a sorbent for HCl capture from municipal waste incinerator exhaust gas. We thought that the sorbent containing Fe2O3 and Ca(OH)2 would be an interesting material for Hg0 removal from fuel gas containing HCl. Fe2O3–Ca(OH)2 was prepared by mixing
In this study, the effect of HCl in the coal derived fuel gas on the Hg0 removal performance of iron-based sorbents such as Fe2O3, iron oxide supported on TiO2, iron oxide–Ca(OH)2, and iron sulfides was investigated. The following results were obtained: (1) in the case of Fe2O3, the presence of HCl suppressed the Hg0 removal rate. (2) In the case of Fe2O3 (2, 5 wt%)/TiO2, the presence of
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HCl did not suppress the Hg0 removal rate and the activity was stable. (3) The Hg0 removal performance of reagent grade FeS2 was higher than that of Fe2O3, and the performance was not affected by the presence of HCl. (4) The natural FeS2 ore also showed Hg0 removal activity. (5) The Hg0 removal rate of iron oxide–Ca(OH)2 was not affected by the presence of HCl. From this study, we can conclude that some active sulfur over the FeS2 sample is active for Hg0 removal and resistant to HCl for Hg0 removal. Therefore, if the same active sulfur is produce over the iron oxide sorbent by H2S without the interference of HCl, the sorbent has tolerance for HCl. However, the type of the active sulfur over the FeS2 samples and the active surface sulfur of the iron oxide samples is unknown. We need much more study to clarify the various kinds of the active sulfur species formed over the sorbents for the development of high active sorbent for Hg0. Acknowledgements This work was partly supported by the Steel Industrial Foundation for Advancement of Environmental Protection Technology, 21th COE program of Okayama University, and a Grant-in-Aid for Scientific Research on Priority Areas (B) from Ministry of Education, Science, Sports and Culture, Japan (No. 18310056). The JSPS is gratefully acknowledged for the financial support and postgraduate scholarship for Shengji Wu.
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