Preparation and Steam Gasification of Fe-Ion Exchanged Lignite

Aug 18, 2014 - The dissolved ferrous iron was loaded to a Victorian lignite by ion exchange. The iron-loaded lignites with different iron concentratio...
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Preparation and Steam Gasification of Fe-Ion Exchanged Lignite Prepared with Iron Metal, Water, and Pressurized CO2 Hyun-Seok Kim,† Shinji Kudo,‡ Koyo Norinaga,‡ and Jun-ichiro Hayashi*,†,‡ †

Research and Education Center of Carbon Resources, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

‡

ABSTRACT: This paper proposes an environment-friendly method to load ferrous ion (Fe2+) to lignite with water, iron metal and carbon dioxide (CO2) but without mineral acids or iron salts containing potentially harmful components. Iron metal was dissolved into 25 °C water that had been saturated with CO2 at its gaseous pressure of 4.0 MPa. The solubility of ferrous iron was 1 070 mg-Fe/L-water. This solubility was much higher than that predicted from the solubility product (Ksp) of FeCO3 with an assumption that Fe2+ represented the dissolved ferrous iron. Rather, another assumption of coexistence of Fe2+, FeHCO3+, and Fe(HCO3)2 reasonably explained the measured solubility of ferrous iron. The dissolved ferrous iron was loaded to a Victorian lignite by ion exchange. The iron-loaded lignites with different iron concentrations (0.3−6.7 wt %-dry-lignite) were subjected to a sequence of pyrolysis and steam gasification to produce synthesis gas in a thermogravimetric analyzer. The gasification was catalyzed by the loaded iron regardless of its concentration, while the quickest at 1.0 wt % where dispersion of iron-derived catalytic species in the char matrix was maintained until its complete gasification with steam.

1. INTRODUCTION Many studies have been dedicated to low temperature gasification of lignite expecting advantages of its use over that of higher rank coals due to higher intrinsic reactivity and effectiveness of inherent/extraneous catalysts.1−9 Lignite is often featured by abundance of acidic protons that can be exchanged by metallic cations4−7,9 as catalyst precursors. Ferrous and ferric ions are candidates for catalyst precursors. Conventional impregnation or ion-exchange1−3,8,10 can load ferrous/ferric ions to lignite, but the use of inorganic salts causes necessity of exhaustive removal of Cl-, N-, or S-containing ions, and otherwise, these provoke emission of gases such as HCl, NH3, and H2S, increasing risk of corrosion of gasifier utilities and burdening gas cleaning.1−3,10 Many efforts have hence been made for converting a raw material to an active catalyst that is free from potential risks, e.g., FeCl3 to Cl-free catalyst.2,10 The present study proposes a new method to prepare ferrous-iron loaded lignite with not water-soluble iron salts but with iron metal, which provides a way to reuse iron waste. It is well-known that CO2 causes corrosion of iron and its alloys in aqueous solution of carbonic acid (H2CO3).11 Iron metal is dissolved into carbonated water being converted to Fe2+. CO2 + H 2O → H 2CO3(aq)

Fe + H 2CO3(aq) → FeCO3(aq) + H 2

increases the acidity of water and thereby promotes the dissolution of metal iron. Among the previous studies, none of them investigated the solubility of ferrous iron in water carbonated with high pressure CO2. The present authors have been investigating the solubility of ferrous iron in water saturated with CO2 at its gaseous pressure of 4.0 MPa in the presence/absence of a type of lignite or ionexchange polymer resin. This study also examined thermochemical transformation of the loaded iron and its catalytic activity in a sequential pyrolysis and steam gasification.

2. EXPERIMENTAL SECTION 2.1. Materials. A Victorian lignite, Loy Yang, was employed as the starting lignite. It is featured by abundance of oxygen-containing functional groups.13 As-received lignite was milled, sieved to sizes of 710−2 000 μm, and then dried at a reduced pressure below 100 Pa and room temperature for 24 h. The dried sample is hereafter referred to as LY. Properties of the as-received lignite are listed in Table 1. LY was washed with an aqueous solution of 3.0 N HCl at 80 °C for 24 h, washed with deionized water until complete removal of chlorine ion, and then dried at conditions as above. This acid-washed LY (LYA) was prepared as a reference material that was free from ion-exchangeable metallic species such as Na, K, Ca, Mg, and also Fe. A metacrylic-acid based ion-exchange resin (IER; Mitsubishi Chemical, WK-11) was used as another reference material. The contents of carboxylic groups of LY/LYA and IER were 2.4 mequiv/g-dry14 and 10.2 mequiv/g-dry (guaranteed by the manufacturer), respectively. Foils of iron metal with a thickness of 100 μm and purity of 99.99 atomic % were purchased from The Nilaco Corporation. Liquified CO2 with a purity over 99.995 vol % was purchased from Taiyo Nippon Sanso Corporation. 2.2. Dissolution of Iron Metal and Loading of Fe2+ to Lignite by Ion Exchange. An autoclave with a capacity of 500 mL (Taiatsu

(I) (II)

The reaction II suggests us to change our way of thinking from corrosion of iron metal to its dissolution and loading to lignite by ion exchange. The solubility product of FeCO3, i.e., Ksp = [Fe2+][CO32−] limits the concentration of ferrous iron in the aqueous phase, while excessive dissolution results in precipitation of FeCO3.12 In situ loading of ferrous iron to lignite is a potential way to avoid such precipitation. It is also expected that increasing pressure of gaseous CO2 in contact © 2014 American Chemical Society

Received: May 8, 2014 Revised: August 13, 2014 Published: August 18, 2014 5623

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hexamethylenetetramine for inhibiting further corrosion.11 The specimens were subsequently washed again with acetone and deionized water and dried. The mass of the iron foil was then measured for determining the amount of iron that had been dissolved into the water and loaded to the solid. 2.4. Steam Gasification. LY and Fe-loaded ones were pulverized to sizes smaller than 75 μm and subjected to steam gasification under atmospheric pressure in a thermogravimetric analyzer (TGA; SII Nanotechnology, EXSTAR TG/DTA7200). The gasification employed char samples that had been prepared by the pyrolysis of LY or Fe-loaded LY in atmospheric N2 with a peak temperature and holding time of 600 °C and 3 min, respectively. Details of the pyrolysis are reported elsewhere.18 In the gasification, the char (initial mass; ∼2 mg) was heated at a rate of 10 °C up to 800 °C in a flow of N2 at 700 mL-stp min−1. Then the N2 flow was switched to that of H2O/N2 (20/80 vol/vol) mixed gases for commencing the gasification. 2.5. Characterization of Fe-Loaded Solid and Char. Original and Fe-loaded solids and their chars were analyzed by X-ray diffractometry (XRD), Fourier transform-infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). XRD was performed on a diffractometer (Rigaku, RINT-Ultima III) using a Cu−Kα radiation. The FT-IR employed a PerkinElmer model (Spectrum Two) in an attenuated total reflectance (ATR) mode. The TEM observation of Fe-loaded LY and chars was carried out with a JEOL JEM-2010F.

Table 1. Proximate and Ultimate Analyses of LY Proximate Analysis [wt %, as-received basis] moisture ash volatile matter fixed carbon Ultimate Analysis [wt %-dba] C H Ob N a

59.3 0.4 22.1 18.2 70.9 4.6 23.2 0.6

On dry basis. bDetermined by difference.

Techno Corporation, TVS-N2-500) was used for dissolving the iron foil into carbonated water. Pieces of the iron foil (10 mm × 10 mm or 30 mm × 30 mm square) were charged into the autoclave together with 100−250 mL of deionized water (specific electrical resistance > 18.2 MΩ cm). After the autoclave was closed, the air in the headspace was replaced by CO2. Then, the autoclave was pressurized with CO2 up to 4.0 MPa, which was maintained afterward. The choice of 4.0 MPa is mainly due to that further increase of the pressure up to the critical one (7.45 MPa at 25 °C) does not induce significant increase in the CO2 concentration in the aqueous phase. The pH of carbonated water decreases with increasing pressure of CO2 but very slightly over 3−4 MPa.15 The mixture of water and the iron foil pieces was stirred at 600 rpm while the temperature was kept at 25 °C. After a prescribed period of stirring, the autoclave was depressurized and opened. The water was transferred quickly to a glass container. The iron foil pieces were isolated from the water, wiped with sheets of paper (Elliare, Prowipe Soft Microwiper S220), washed with acetone, washed with deionized water, dried, and then weighed. Atmospheric CO2 was continuously bubbled into the transferred water, which is herewith denoted by Fe-water, for suppressing the release of CO2 into the atmosphere. The release of CO2, in fact, seemed to be slow, and the precipitation of FeCO3 was not detected in the transferred water for at least 60 min, unless precipitation had occurred before opening the autoclave. It was also confirmed that the Fe-water was colorless, which indicated that the dissolved iron was ferrous. Thus, relying on the absence of precipitated FeCO3, the amount of the iron dissolved into carbonated water was accurately determined from the decrease in the mass of iron foils due to its dissolution. Either of a prescribed amount of LY or IER was suspended into the precipitate-free Fe-water for 60 min during which the bubbling of CO2 was continued. The suspension was then subjected to filtration for recovery of the Fe-loaded solid and Fe-water. The Fe-loaded solid was dried under vacuum at room temperature for 24 h. The Fe contents of Fe-loaded solids were determined by the mass of iron metal that had been dissolved into the carbonated water because the Fe concentration in the spent Fe-water was low enough to ignore. The ferrous iron dissolved in the Fe-water was thus loaded to the solid quantitatively. This was also confirmed by quantification of iron by combustion of the solid, dissolution of resulting Fe2O3 into an aqueous solution of HCl and subsequent quantification of Fe3+ by a general colorimetric method16,17 with 1,10-phenanthroline after reducing Fe3+ to Fe2+ with hydroxylammonium chloride. Hereafter, Fe-loaded solid with Fe content of α wt % on the basis of dry mass of the solid will be denoted by LY-Feα or IER-Fe-α. The ranges of α for LY and IER were 0.3−6.7 and 4.3− 9.8, respectively. 2.3. In Situ Loading of Iron to Solid. The iron metal (0.72 g), solid (10.0 g-LY, 10.0 g-LYA, or 2.34 g-dry-IER) and deionized water (100 mL) were charged together into the autoclave, and it was pressurized with CO2 in the same way as described previously. The amount of iron metal or period for iron metal dissolution was set to be low and short, respectively, enough to avoid precipitation of FeCO3. It was found that brownish solid material was deposited on iron foils when LY or LYA was used. The deposit was removed by cleaning the surface of the foil with acetone and deionized water, and then pickled in aqueous solution of HCl that contained 10 g L−1 of

3. RESULTS AND DISCUSSION 3.1. Solubility of Ferrous Iron in Carbonated Water. Table 2 shows results from experiments for determination of Table 2. Total Mass of Dissolved Iron and Precipitated Iron, mFe, in Water at Temperature and CO2 Pressure of 25 °C and 4.0 MPa, Respectively

a

run no.

mFe [mg-Fe/L]

precipitatea

1 2 3 4 5 6 7 8 9 10 11 12

513 517 738 771 825 881 1 070 1 150 1 226 1 290 1 513 1 600

nd nd nd nd nd nd nd d d d d d

nd, not detected; d, detected.

the solubility of ferrous iron in the 25 °C water that was carbonated with 4.0 MPa gaseous CO2. Different amounts of iron metal were charged in different experiments. As seen in the table, no precipitation of FeCO3 occurred when the total mass of Fe dissolved in the water, mFe, was 1 070 mg/L or smaller. The solubility of ferrous iron was thus determined as 1 070 mg/L. According to the overall chemical reaction II, dissolution of 1 mol of Fe is associated with formation of 1 mol of H2. For some runs of those listed in Table 2, the formation of H2 was confirmed quantitatively. 3.2. Discussion on Solubility of Ferrous Iron. The measured solubility of ferrous iron is considered based on two different models: Models A and B. Model A assumes that ferrous iron is dissolved in the carbonated water solely in the form of Fe2+. This model also considers the chemical species, elemental balance, charge balance, chemical equilibrium, and other parameters for the dissociation of carbonic acid, which are summarized in Table 3. 5624

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Table 3. Chemical Species and Equations Involved in Model Aa CO2, H2CO3, HCO3−, CO32−, H+, OH−, Fe2+

chemical species dissolved in water charge balance equation

2[Fe2 +] + [H+] =1 + 2[CO32 −] + [OH−]

[HCO3−] elemental balance equation

[H H2O] [OH2O]

chemical equilibrium (A) at 25 °C

=

[HCO3−] + [H+] + [OH−] + 2[Fe2 +]

[HCO3−][H+] = K a1 [CO2 *] pK a1 = pK a1° + log γH+ + log γHCO * − log γCO

2*

3

chemical equilibrium (B) at 25 °C

=2

[HCO3−] + [CO32 −] + [OH−]

19

(pK a1° = 6.351)

[CO32 −][H+] = K a2 [HCO3−] pK a2 = pK a2° + log γH+ + log γCO 2 − − log γHCO − 3

chemical equilibrium (C) at 25 °C

19

3

KW = [OH−][H+] pKW = pKW ° + log γH+ + log γOH−

(pKW ° = 13.997)

−2

1.92 × 10 mol/L (given from experimental result) 1.08 mol/L (literature value20) Assumption: [CO2*] ≈ [CO2]

2+

[Fe ] [CO2] a

(pK a2° = 10.329)

γi: the activity coefficients of species i. [CO2*] = [H2CO3] + [CO2].

All of hydrogen (H) as H+, HCO3−, and H2 are derived from that of H2O. Then, the total concentration of H derived from H2O is given by

Model A numerically gives the optimized set of HCO3−, CO32−, H+, OH−, and I concentrations that satisfy the five equations in Table 3. The result is shown in Table 4. The

[H H2O] = [HCO3−] + [H+] + [OH−] + 2[H 2] = [HCO3−] + [H+] + [OH−] + 2[Fe*]

Table 4. Concentrations of H+, OH−, HCO3−, and CO32− Calculated by Model A

(1)

species

[Fe*] is the total concentration of ferrous iron, the molar amount of which is twice that of H in H2, and assumed to be equal to [Fe2+]. It is also valid that oxygen (O) from H2O is given from the following relationship: The total concentration of species that contain O originating from H2O ([OH2O]) =3([HCO3−] + [CO32 −]) + [OH−] − 2([HCO3−] + [CO32 −])

(2)

(3)

The three equilibrium constants, Ka1, Ka2, and Kw, are all based on the concentration of relevant species. [CO2*] is given by [CO*2 ] = [H 2CO3] + [CO2 ]

(4)

1.08 1.92 × 10−2

[H+] [OH−] [HCO3−] [CO32−] [Fe2+][CO32−] Ksp of FeCO3

1.92 8.02 3.83 2.19 4.20 5.03

× × × × × ×

10−5 10−10 10−2 10−7 10−9 10−11

mol/L mol/L mol/L mol/L mol/L mol/L mol2/L2 mol2/L2

remark literature value20 given from the experimental result calculated calculated calculated calculated literature value24

log K sp = − 59.3438 − 0.041 377Tk −

Strictly saying, the equilibrium should be based on not the concentrations but activities of the species involved in the reaction. As shown in Table 3, the concentration-based equilibrium constants (Ka1, Ka2, and Kw) are expressed by the corresponding activity-based constants (Ka1°,Ka2°,Kw°) and the activity coefficients of the chemical species. Further, the activity coefficients of species i (γi) can be approximated by the Davies equation.21 ⎛ ⎞ I log γi = 0.5zi 2⎜ − 0.15I ⎟ ⎝1 + I ⎠

[CO2] [Fe2+]

unit

calculation based on Model A was also performed assuming the absence of ferrous iron in the carbonated water. The model predicted pH = 3.16, which agreed well with the report by Mohamed et al.23 Model A gives the product of [Fe2+] and [CO32−] as 4.20 × 10−9, while Ksp of FeCO3 is given as 5.03 × 10−11 by the following equation.25

The third term of the right-hand corresponds to O derived from CO2. From the eqs 1 and 2, the following one is derived. [H H2O] = 2[OH2O]

concn

2.1963 Tk

+ 24.5724 log Tk + 2.518I 0.5 − 0.657I

(6)

where Tk is the temperature in the unit of Kelvin. As seen in Table 4, [Fe2+][CO32−] is greater than Ksp by a factor of about 80. Such disagreement is arisen from one of the assumption of Model A in which Fe2+ represents the dissolved ferrous iron. Consideration of another or other ferrous iron species is therefore needed. Model B assumes three different ferrous iron species: Fe2+, FeHCO3+, and Fe(HCO3)2. Previous researchers considered importance of bicarbonate ion and the presence of FeHCO3+ and Fe(HCO3)2.26−29 Yin et al.28 found Fe(HCO3) 2 in solids that had occurred during corrosion of a type of carbon steel. Castero et al.29 focused on FeHCO3+ based on their finding of

(5)

I and zi are the ionic strength of the system and ionic valence of species i, respectively. The activity coefficient of CO2* can be approximated as unity according to the literature.22 5625

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Table 5. Chemical Species and Equations Involved in Model B CO2, H2CO3, HCO3−, CO32−, H+, OH−, Fe2+, FeHCO3+, Fe(HCO3)2

chemical species in water charge balance equation

2[Fe2 +] + [H+] + [FeHCO3+]

=1

[HCO3−] + 2[CO32 −] + [OH−] elemental balance equation

[H H2O] [OH2O]

=

[HCO3−] + [H+] + [OH−] + [FeHCO3+] + 2[Fe(HCO3)2 ] + 2[Fe*] [HCO3−] + [FeHCO3+] + 2[Fe(HCO3)2 ] + [CO32 −] + [OH−] =2 The equations are the same as those in Model A. [Fe*] = [Fe2+] + [FeHCO3+] + [Fe(HCO3)2] = 1.92 × 10−2 mol/L (given from experimental result) The condition is the same as that in Model A.

chemical equilibrium (A), (B), and (C) [Fe*] [CO2] relative abundances of FeHCO3+ and Fe(HCO3)2

[FeHCO3+] =β [FeHCO3+] + [Fe(HCO3)2 ]

0≤β≤1

Table 6. Results of Calculation by Model B β

log[OH−]

log[HCO3−]

log[CO32−]

log[Fe2+]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

−9.807 −9.775 −9.738 −9.696 −9.651 −9.604 −9.557 −9.510 −9.465 −9.423 −9.383

−2.127 −2.095 −2.058 −2.017 −1.972 −1.925 −1.877 −1.831 −1.786 −1.743 −1.703

−8.173 −8.108 −8.033 −7.949 −7.857 −7.759 −7.660 −7.563 −7.470 −7.381 −7.296

−2.433 −2.497 −2.570 −2.649 −2.734 −2.823 −2.910 −2.996 −3.076 −3.151 −3.222

the assistance of Fe dissolution by the presence of HCO3−. Although it was recognized that Fe(HCO3)+ was an unstable intermediate in weakly alkaline aqueous solutions, its stability was expected in acidic solutions with pH < 5.29 Model B assumes the following reversible reactions according to the above-mentioned previous studies.26−29 Fe2 + + CO32 − ↔ FeCO3

(III)

Fe2 + + HCO3− ↔ FeHCO3+

(IV)

FeHCO3+ + HCO3− ↔ Fe(HCO3)2 Fe(HCO3)2 ↔ FeCO3 + CO2 + H 2O

log[FeHCO3+] −2.796 −2.482 −2.295 −2.159 −2.054 −1.968 −1.896 −1.834 −1.780 −1.731

log[Fe(HCO3)2]

pH

−1.810 −1.842 −1.880 −1.927 −1.983 −2.054 −2.144 −2.264 −2.436 −2.734

4.096 4.128 4.165 4.204 4.247 4.291 4.335 4.378 4.418 4.456 4.492

on the result from their examination of the stability of FeHCO3+. −log([Fe 2 +][HCO3−]) = pH − 0.57

(8)

2+

However, they measured not [Fe ] but total concentration of ferrous iron, i.e., [Fe*] dissolved in the carbonated water. Equation 8 should therefore be corrected (eq 7) as above. Figure 1 shows the relationship between pH and −log[Fe*][HCO3−] given by Model B (see Table 6) and compares it to that by eq 7. It is seen that the relationships by Model B and eq 7 agree with each other at pH = 4.25, where β ≈ 0.40. In the present study, the pH of Fe-water was measured only after

(V) (VI)

Table 5 summarizes the chemical species, equations and conditions that are involved in Model B. A particular feature of Model B is introduction of a factor, β, which is defined in the table. The calculation of the concentrations of HCO3−, CO32−, H+, OH−, Fe2+, FeHCO3+, and Fe(HCO3)2 is thus performed based on Model B varying the value of β. The results of calculation are summarized in Table 6. Model B cannot predict the concentrations of Fe2+, FeHCO3+, and Fe(HCO3)2 without β, but the introduction of the following equation proposed by Singer et al.30 enables one to determine those concentrations. −log([Fe*][HCO3−]) = pH − 0.57

(7)

Singer et al.30 investigated the solubility of ferrous iron in carbonated water and proposed the following equation based

Figure 1. Relationship between pH and −log[Fe*][HCO3−]. 5626

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Table 7. Results of In Situ Iron Loading to LY, LYA, and IERa

opening the autoclave, in other words, after the gaseous pressure of CO2 was reduced to the atmospheric one. The pH for Run 7 (see Table 2) was around 4.5, which was higher than 4.25 but acceptable. Thus, Model B explains the measured concentration of ferrous iron in the carbonated water by considering the coexistence of Fe2+, FeHCO3+, and Fe(HCO3)2 and more abundances of FeHCO3+ and Fe(HCO3)2 than Fe2+. There have been a number of studies on the chemistry of ferrous iron in carbonated water, but the solubility of ferrous iron under highly pressurized CO2 was not available. The solubility measured in the present study has demonstrated that it is much greater than expected with the assumption of [Fe*] ≈ [Fe2+]. The measured solubility of ferrous iron, 1 070 mg/L-water, determined by the experimental result has a significant meaning. For example, with this value, the amount of Fe loaded to lignite, assuming a process that mixes the Fe-water and lignite (dry) at a mass ratio of 10:1, can be calculated to 1.07 wt %. 3.3. Rate of Dissolution of Iron Metal. Regardless of the presence or absence of the organic solid, the dissolution of iron metal into the carbonated water was a slow process. The rate of dissolution, i.e., mass of iron dissolved per unit time, increased with total mass of iron foils (in other words, total available surface area of the foils) and also stirring speed. Figure 2 shows

solid

mass of solid [g-dry]

mass of dissolved iron [mg]

iron loading [wt %-dry]

none IER LYA LY

2.34 10.0 10.0

68.2 70.1 61.6 57.8

2.99 0.616 0.578

Conditions: temperature, 25 °C; CO2 pressure, 4.0 MPa; time, 6 h; mass of water, 100 g; and mass of iron foil, 0.72 g. a

lower pH.27,31−36 Progress of the ion exchange was confirmed by analyzing IERs before and after the iron loading by FT-IR. The absorption around 1710 cm−1, which was attributed to −COOH groups, was suppressed by the iron loading. On the other hand, the presence of LY and that of LYA slowed down the rate of dissolution of iron. It was found that brown-colored organic material had been deposited on the surfaces of the spent iron foils. The deposits were detected by a FT-IR. Their spectra were similar to those of LY and LYA. Thus, small portions of the inherent organic matter of LY and LYA could inhibit the dissolution of iron. Although the mechanism of the inhibition was unknown, it was suggested that ex situ iron loading was more preferable to in situ one because a higher concentration of ferrous iron in the carbonated water is preferred to the iron loading to lignite. It was also implied from the difference between LY and LYA that the inherent metallic species such as Na and Ca inhibited the dissolution of iron, but the mechanism was unknown and its clarification was left to future studies. 3.5. Mechanism of Iron Loading to Lignite. It is known that ion exchange is the main mechanism for the loading of metal ions to lignite in acidic aqueous media.37,38 The ion exchange process occurs mainly at carboxylic groups (−COOH) if pH < 5 as the present solutions of ferrous iron are so, while also at aromatic hydroxylic groups at pH > 5.37 L−COOH ↔ L−COO− + H+(aq)

Fe2 +(aq) + 2L−COO− ↔ Fe(L−COO)2

Figure 2. Ferrous iron concentration as a function of time. Conditions: Initial mass of iron foil, 70.4 mg; stirring speed, 500 rpm; temperature, 25 °C; mass of water, 250 g.

(VII) (VIII)

L: lignite macromolecule

The fates of Fe(HCO3)2 and FeHCO3+ were unknown, but it was believed that these species were dissociated and converted to Fe(L−COO)2 with HCO3− or otherwise H2CO3. Figure 3 illustrates FT-IR absorption spectra from four different samples: LY, LY-Fe-1.0, LY-Fe-4.1, and LY-Fe-6.7. It is seen that increasing Fe loading attenuates the absorption around 1710 cm−1 that is arisen from −COOH groups in LY.10,39−41 It is also noted that relatively broad absorption with a peak around 1190 cm−1, which is attributed to both carboxylic and aromatic hydroxylic groups, is suppressed by the Fe loading. The ion exchange at the hydroxyls was, however, implausible, taking the pH of the present ferrous iron solutions. Figure 4 shows that the Fe loading for LY-Fe-6.7, i.e., 6.7 wt %, was the upper limit of the Fe loading under the present conditions. The maximum loading at 2.4 mequiv-Fe/g-LY, 6.7 wt %, is in good agreement with the content of carboxylic groups in LY.14 However, this agreement does not necessarily mean quantitative ion-exchange of carboxylic groups to (−COO)2Fe. As seen in Figure 3, the elimination of the absorption due to −COOH groups is incomplete even at the maximum loading of Fe. A possible explanation for the residual

a typical example of change in the ferrous iron concentration with time in a linear manner. 3.4. In Situ Loading of Iron to Lignite. The dissolution of iron into the carbonated water was also investigated in the presence of LY, LYA, or IER. Preliminary experiments showed that the coexistence of the solid resulted in a negligible concentration of ferrous iron in the carbonated water, and this was due to more rapid loading of the ferrous iron to the solid than dissolution of the iron metal. The result also suggested the loading by ion exchange. Table 7 shows the effect of the presence of solid on the dissolution of iron. The concentrations of ferrous iron were well below its solubility as reported in previous results. The presence of IER slightly promoted the dissolution of iron. The in situ transfer of ferrous iron species from the water phase to the IER phase, in other words, removal of the ferrous species from the water phase induced reduction of pH, due to ion exchange between ferrous iron and proton of carboxylic groups of IER. It is agreed that the dissolution of iron in water containing carbonate/bicarbonate ions favors 5627

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Figure 3. FT-IR spectra of LY and Fe-loaded LY.

Figure 5. XRD patterns for LY-Fe-2.5, LY-Fe-6.7, and their chars. The chars were prepared by the sequence of pyrolyses at 600 °C and that at 800 °C. The details are reported in the Experimental Section. α, α-Fe; γ, γ-Fe.

It is clear that Fe species in the char catalyze its gasification. The rate of gasification up to X ≈ 0.7 seems to increase with increasing Fe loading but saturate around 2.5 wt %. In the later stage at X > 0.7, the Fe loading over 1.0 wt % decelerates the gasification remarkably, while that at 1.0 wt % is not the case but maintains the rate within a certain range until the full conversion of the char. The Fe loading of 1.0 wt % thus minimizes the time required for the complete gasification of char. The rate of gasification for the Fe loading of 2.5−6.7 wt % is very small at X > 0.9, and though not shown in the figure, as low as that for the char from LYA. This indicates deactivation of the Fe. The above-mentioned characteristics are very similar to those recently reported by the present authors47 who investigated steam gasification of chars that were prepared from Ca-ion exchanged Loy Yang lignite with Ca-loading of 0.5−7.0 wt %. The initial rate of gasification seemed to be saturated around 4.0 wt % loading. It was also found that the time of gasification needed for the complete char gasification was minimized at 1.0 wt % Ca loading. Detailed kinetic analysis revealed that increasing Ca loading accelerated transformation of nanosized Ca particles to much less active coarser ones and that the concentration of highly dispersed Ca species had already been saturated in the char matrix even at the beginning of gasification and at 1.0 wt % Ca loading. The mechanism of Ca catalyst deactivation can hence apply to that of the present Fe catalyst. The deactivation of Fe catalyst was confirmed by XRD analysis of chars from LY-Fe-2.5 at different conversions. The results are shown in Figure 8. At the beginning of the steam gasification at 800 °C, the Fe species in the char from LY-Fe-2.5 was present as α/γ irons or, otherwise, as its mixture with highly dispersed iron species48,49 that were not detected by XRD. The α/γ-Fe irons were the major species in the earlier stage of the gasification but transformed to magnetite (Fe3O4) in the later stage. It is known that magnetite has no or much lower catalytic activity than the metallic iron,1,2 and this is consistent with the slow down of the gasification of the char

Figure 4. Effect of ferrous iron concentration in carbonated water on Fe loading to LY. Conditions: mass of LY, 10 g; mass of water, 1 000 g.

−COOH groups is loading of Fe in forms of not only (−COO)2Fe but also others such as (−COO)Fe(−OH) and (−COO)Fe(−HCO3). The Fe loading by the ion-exchange mechanism was further examined by XRD and TEM observation of the Fe-loaded LY. Figure 5 displays XRD patterns for LY-Fe-2.5 and LY-Fe-6.7 as examples together with the chars from their pyrolyses. The XRD detected no peak due to either iron hydroxide, carbonate, or other types of salts regardless of the Fe loading. According to the previous reports,10,42,43 it was concluded that the ferrous iron was loaded to LY following the ion-exchange mechanism. It was also clear that the pyrolysis at 800 °C formed phases of metallic iron. Figure 6 exhibits TEM images of LY-Fe-2.5 and a char from its pyrolysis at 600 °C. No Fe-containing particles were observed in/on the matrix of the Fe-loaded LY over the entire range up to 6.7 wt %. The pyrolysis of the Fe-loaded LY formed nanosized iron particles in the char matrix, and this was consistent with the XRD results. 3.6. Steam Gasification of Fe Loaded LY. The catalysis of the loaded Fe in the steam gasification has been demonstrated in Figure 7. The conversion profiles show characteristics crucial to the performance of the Fe-catalyzed gasification. The gasification of the reference char, i.e., that from LY, seems to take place obeying a zeroth order kinetics at X > 40%. This type of kinetics arises from the catalysis of inherent Na, of which dispersion in the char matrix is maintained while the rate of gasification is virtually determined by the amount of catalytic Na per char particle.44−46 5628

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Figure 6. TEM images of (a) LY-Fe-2.5 and (b) char from its pyrolysis in atmospheric flow of N2 with peak temperature at 600 °C and holding period of 30 min.

the char from LY-Fe-1.0. Wustite catalyzed the gasification of the char from LY-Fe-1.0 until its complete conversion, participating in redox cycles between itself and α/γ-Fe iron. The activity of wustite (i.e., reactivity with carbon) is much higher than that of magnetite.1,50 The presence of optimum Fe loading, i.e., ∼1.0 wt %-dry-LY for the steam gasification, in other words, no need of more Fe loading, is important in application of carbonic acid solutions of ferrous iron to its loading to lignite. As demonstrated by the present study, the solubility of the ferrous iron is as low as ∼1 g/L-water. This means that 1 wt % Fe loading requires 10 L of Fe-water per kg of dry lignite for the loading without repletion.

4. CONCLUSIONS The followings conclusions have been drawn within the range of the present experimental conditions. (1) The iron metal was dissolved at solubility of 1.07 g per liter of carbonated water at 25 °C and gaseous pressure of CO2 of 4.0 MPa. (2) The solubility of ferrous iron is explained by considering not only Fe2+ but its coexistence with Fe(HCO3)2 and FeHCO3+. (3) The ferrous iron in the Fe-water is loaded to LY by ion exchange without precipitation of either nanosized or greater particles. This is supported by the presence of an upper limit of Fe loading (6.7 wt %) and consistent with the results of XRD and TEM analyses. (4) The time required for complete steam gasification of LY char is minimized with Fe loading of 1 wt %-dry-LY. More loading results in loss of catalysis of Fe species and much longer time for the complete char gasification.

Figure 7. Time dependent changes in conversion of LY char and Fe-loaded LY chars during steam gasification.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81 92 583 7796. Fax: +81 92 583 7793. E-mail: [email protected].

Figure 8. XRD patterns for chars from the steam gasification of LY-Fe-2.5 char at X = 0.4 and 0.9 and char from the steam gasification of LY-Fe-1.0 char at X = 0.8. α, α-Fe; γ, γ-Fe; M, magnetite; W, wustite.

Notes

The authors declare no competing financial interest.

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from LY-Fe-2.5 in the later stage, as shown in Figure 7. The transformation of the metallic iron to magnetite was caused by irreversible oxidation by steam, which could result from isolation of Fe-based particles from the carbon matrix as the most important reducing agent. Not magnetite but wustite (FeO) was the major form of Fe in the late stage gasification of

ACKNOWLEDGMENTS A major part of this study was carried out in a project that was supported by Funding Program for Next Generation WorldLeading Researchers (NEXT Program) established by the Japan Society for the Promotion of Science (JSPS). 5629

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