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Energy & Fuels 2008, 22, 2474–2481
Sequential Leaching of Coal to Investigate the Elution of Inorganic Elements Into Coal Extract (HyperCoal) Lian Zhang,† Toshimasa Takanohashi,*,† Tetsuya Nakazato,‡ Ikuo Saito,† and Hiroaki Tao‡ Energy Technology Research Institute and Institute for EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, 305-8569, Japan ReceiVed February 20, 2008. ReVised Manuscript ReceiVed April 8, 2008
Two Argonne premium coal samples, Illinois No. 6 (IL) and Wyodak-Anderson (WY), were extracted by 1-methynaphthalene for 1 h at 360 °C and under 1 MPa nitrogen (cold) protection. Elution of inorganic elements into coal extracts as well as their chemistry has been mainly investigated. An indirect method for metallic speciation was employed by initially washing coal with a variety of acids. Subsequently, the washed coals as well as the respective raw coal were extracted. For a given metal, elution of its ion-exchangeable fraction was defined as the difference between its amounts eluted into the extracts of raw coal and acetic acid-washed coal. Elution of submicrometer discrete particles was defined as the difference between the extracts of acetic acidwashed coal and nitric acid-washed coal. Elution of its fraction insoluble in nitric acid was assigned as organometals which are chemically associated with coal carbonaceous matrix and/or those incorporated into fine clay minerals. About 822 and 1110 ppm inorganic elements were eluted into the extracts of IL and WY coals, respectively. Fe was the most prevalent. The transition metals including Cr, Ni, Mn, Co, Cu, and Zn were also abundant. These metals were mostly nitric-acid insoluble. Electron spin resonance spectroscopy characterization suggested the high-spin Fe3+ state for Fe that is virtually totally associated with coal functional groups. A portion of it may have a porphyrin-like structure that is soluble in sulfuric acid (7% in methanol, v/v). While the left Fe can be high-spin Fe3+ stored in the organometallic complex having a distorted axial symmetry. The other transition metals mainly partitioned between organometallic complex and cations incorporated into fine clay minerals. Regarding the remaining metals in coal extracts, they are mainly submicrometer discrete particles in IL extract. Elution of the ion-exchangeable carboxylates was however prominent during WY coal extraction.
Introduction The presence of inorganic elements in coal can cause corrosion in furnace as well as release of pollutants into air during coal combustion. Solvent extraction is one cost-effective strategy for producing clean fuel, which can efficiently remove the inorganic elements from a coal carbonaceous matrix. Since it was first reported by Penganathan et al. in 1988,1 coal extraction technology has been developed rapidly in the past decades. One representative technology is the HyperCoal (HPC) generation process developed in Japan, which features the following characteristics: extraction of coal by nonpolar or weakly polar solvent that can be readily separated and recycled and adoption of mild conditions such as 300∼400 °C and 1∼5 MPa nitrogen and use of no hydrogen.2 The resulting coal extract has very low ash content, typically of the order of 0.1 wt % or less.1–5 Up to now, studies on how to improve coal extraction * Corresponding author. Tel.: 81-29-861-8038. Fax: 81-29-861-8441. E-mail:
[email protected]. † Energy Technology Research Institute. ‡ Institute for Environmental Management Technology. (1) Penganathan, K.; Zondlo, J. W.; Mintz, E. A.; Kneisl, P.; Stiller, A. H. Fuel Process. Technol. 1988, 18, 273. (2) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947. (3) Miura, K.; Nakagawa, H.; Ashida, R.; Ihara, T. Fuel 2004, 83, 733. (4) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Fuel 2002, 81, 1463. (5) Yoshida, T.; Li, C.; Takanohashi, T.; Matsumura, A.; Sato, S.; Saito, I. Fuel Process. Technol. 2004, 86, 61.
yield had been conducted extensively;2,4–8 less systematic knowledge, however, is available for the elution of overall and individual elements into coal extract, which is believed to be one significant factor for selecting the coal and solvent. For a given element, its mode of occurrence in coal is primarily important in determining its elution. Presumably, the organically bound cations can be extracted preferentially. For instance, in a study of extracting coal by 1-methynaphthalene (1-MN) at 360 °C and by N-methylpyrrolidinone (NMP) at 350 °C, siloxane-Si was found in coal extracts.9 A further study on extracting raw coal and acetic acid-washed coal suggested the elution of ion-exchangeable cations, especially the alkaline earth elements in the case of a sub-bituminous coal.10 Although some information had been obtained, knowledge on this issue is still scarce. No quantitative interpretation has been made to investigate if only the organically bound metals transform into coal extract. In a high-rank bituminous coal, certain lithophile elements such as Ti, V, Cr, and Zr likely occur in small clusters of cation polyhedra, forming poorly crystallized (6) Li, C.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 97. (7) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I. Energy Fuels 2002, 16, 1006. (8) Masaki, K.; Kashimura, N.; Takanohashi, T.; Sato, S.; Katsumura, A.; Saito, I. Energy Fuels 2005, 19, 2021. (9) Sakanishi, K.; Saito, I.; Ishom, F.; Watanabe, I.; Mochida, I.; Okuyama, N. Fuel 2002, 81, 1471. (10) Wang, J.; Li, C.; Sakanishi, K.; Nakazato, T.; Tao, H.; Takanohashi, T. Fuel 2005, 84, 1487.
10.1021/ef800127g CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
Elution of Inorganic Elements Into Coal Extract
oxides or hydroxides around 2-10 nm in maceral voids.11,12 For a laboratory-scale process adopting a filter to isolate coal extract,2–10 it is reasonable that some submicrometer particles potentially pass through the filter as well. Moreover, the organometallic chemistry in coal matrix is much complicated. The ion-exchangeable cations are those soluble in a weak acid like acetic acid. Removing them prior to coal extraction influenced their elution. Some elements were slightly reduced while some were not affected.10,13 No attempt has been made to elucidate the chemical forms of the acetic acid-insoluble metals in coal extracts. Some metals, especially the transition metals can exist as organometallic complex such as metalloporphyrin or its derivatives in coal.14–20 Metalloporphyrins are believed to inherit from coal precursors including heme and chlorophylls of the biological deposits that have transition metals as the nutrients for their growth. In such a configuration, a metal is bound with nitrogencontaining functional groups to form a formula like C32H36N4Fe21 or presents as a complicated structure formed during coalification, which can be intimately associated with the oxygen and/or nitrogen-containing organic macromolecules. Study in reference pointed out the possibility of extracting these species by boiling NMP.14 The final evidence is however still lacking, owing to the difficulty for directly measuring the organometallic complex. For an individual metal in coal extract, its content is on the order of hundreds of parts per million (milligrams per kilogram) or much less, which is too low to be measured by the conventional instruments like X-ray diffraction, electron microscopes, and even the Mo¨ssbauer spectrometer.22 On the above considerations, an indirect method was employed in this study to elucidate the modes of occurrence of metals that can transform into coal extract. This method involves a sequential acid leaching of coal prior to its extraction. The acids tested have the different strength of acidity. The washed coals as well as the raw coal were first extracted under the identical conditions. Subsequently, concentrations of individual metals in the extracts of raw coal and washed coals were compared. For a given metal eluted out of a raw coal, the differences of its quantity to that eluted out of the respective washed coals can reflect the chemical associations of this metal as well as its modes of occurrence in coal extracts. This is because acid leaching is a fairly useful method for the speciation of trace metals in coal.23 No such attempt was made to treat the coal extract directly since the large errors were observed during coal extract transferring. Compared to raw coal, coal extract is quite light and electrostatic. Its contact with any surface can lead to the significant mass loss, which in turn biases the (11) Huggins, F.; Srikantapura, S.; Parekh, B.; Blanchard, L.; Robertson, J. Energy Fuels 1997, 11, 691. (12) Huggins, F.; Huffman, G. Int. J. Coal Geology 2004, 58, 193. (13) Sakanishi, K.; Akashi, E.; Nakazato, T.; Tao, H.; Kawashima, H.; Saito, I. Fuel 2004, 83, 739. (14) Richaud, R.; Lazaro, M.; Lachas, H.; Miller, B. B.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 317. (15) Li, Z.; Ward, C.; Gurba, L. Int. J. Coal Geology 2007, 70, 137. (16) Bonnett, R. Int. J. Coal Geology 1996, 32, 137. (17) Bonnett, R.; Burke, P. J.; Reszka, A. Fuel 1987, 66, 515. (18) Bonnett, R.; Czechowski, F. Fuel 1987, 66, 1079. (19) Bonnett, R.; Czechowski, F.; Latos-Grazynski, L. Energy Fuels 1990, 4, 710. (20) Callot, H.; Ocampo, R.; Albrecht, P. Energy Fuels 1990, 4, 635. (21) Bonnett, R.; Burke, P. J. Geochim. Cosmochim. Acta 1985, 49, 1487. (22) Richaud, R.; Lachas, H.; Lazaro, M.; Clarke, L.; Jarvis, K.; Herod, A. A.; Gibb, T. C.; Kandiyoti, R. Fuel 2000, 79, 57. (23) Senior, C. L.; Zeng, T.; Che, J.; Ames, M. R.; Sarofim, A. F.; Olmez, I.; Huffman, G. P.; Kolker, A.; Mroczkowski, S.; Palmer, C.; Finkelman, R. Fuel Process. Technol. 2000, 63, 215.
Energy & Fuels, Vol. 22, No. 4, 2008 2475 Table 1. Proximate and Ultimate Properties of Two Coals Illinois No. 6 (IL) volatile matter fixed carbon ash C H N S O (by difference) SiO2 Al2O3 CaO Fe2O3 MgO MnO Na2O K2 O TiO2 P2O5 SO3 BaO
Wyodak-Anderson (WY)
proximate analysis, wt %, dried 40.1 44.4 15.5
44.7 46.5 8.8
ultimate analysis, wt %, daf 77.7 5.0 1.4 4.8 11.1
68.4 4.9 1.0 0.7 25.0
ash composition, wt % 43.7 18.3 7.9 18.0 1.2 0.0 2.9 1.0 0.2 6.8
28.7 15.4 15.0 10.2 3.6 0.0 1.5 0.8 1.2 1.2 21.9 0.5
results for metallic speciation. In addition, electron spin resonance spectroscopy (ESR) was employed to determine the coordination structure of transition metals, which is a promising technology for the studies of organically bound transition metals in a variety of natural organic matter including humic acid, soil, and water sediment.24–27 Experimental Details Coal Properties. The coals used are two samples of the Argonne premium coal sample (APCS) series.28 As shown in Table 1, the bituminous Illinois No. 6 (IL) coal contains a lot of ash and S relative to the sub-bituminous Wyodak-Anderson (WY) coal. Regarding the ash compositions, Si, Al, Ca, and Fe are the major elements in both ashes. The discrete minerals are abundant in IL coal ash, including calcite, pyrite, and illite.23 On the other hand, WY coal contains lots of ion-exchangeable cations and other organically bound species, due to its low-rank characteristic. Sequential Leaching of Coal. Prior to solvent extraction, the parent coals were treated by sequential acid leaching as illustrated in Figure 1. The raw coals were initially leached by acetic acid (1 M, ultrapure grade), hydrochloric acid (3 M, reagent grade), and nitric acid (2 M, ultrapure grade) in sequence. Accordingly, the ion-exchangeable cations like carboxylates were removed from washed coal 1.9,20 The discrete particles including oxides, hydroxides, and carbonates could be mostly removed from washed coal 2, and sulfides like pyrite were further removed from washed coal 3.23,29 Elements remaining in washed coal 3 include aluminosilicates and organometals encased completely in the coal matrix. Each leaching was carried out by stirring a mixture of coal with acid for 24 h under ambient condition. Subsequently, approximately half the residual solid was removed, vacuum-dried, and saved for further experiments. Another leaching procedure was carried out by mainly taking into account the organically bound metals. In this procedure, two (24) Senesi, N. Anal. Chim. Acta 1990, 232, 51. (25) Biological and biochemical applications of electron spin resonance; Ingrem, D. J. E., Ed.; Adam Hilger Ltd: London, 1969; Vol. 99, p 1. (26) Senesi, N.; Saiz-Jimenez, C.; Miano, T. M. Sci. Tot. EnViron. 1992, 117/118, 111. (27) Porphyrins and metalloporphyrins; Smith, K. E., Ed.; Elsevier: New York, 1976; p 555. (28) Vorres, K. S. Energy Fuels 1990, 4, 420. (29) Kolker, A.; Huggins, F.; Palmer, C.; Shah, N.; Crowley, S. S.; Huffman, G. P.; Finkelman, R. B. Fuel Process. Technol. 2000, 63, 167.
2476 Energy & Fuels, Vol. 22, No. 4, 2008
Zhang et al.
Figure 1. Scheme of sequential acid leaching of two coals. Table 2. Extraction Yields of Raw Coals and Washed Coals, wt %, daf raw coal washed coal washed coal washed coal washed coal
1 2 3 4
washed coal 5
washing time 48 h washing time 100 h
IL
WY
54.3 52.9 49.2 42.1 41.0 39.3 46.7
28.6 32.7 33.1 34.8 35.0 35.2
raw coals were initially washed by acetic acid to remove the ionexchangeable cations. Subsequently, sulfuric acid (7%, v/v) in methanol and 0.5 M ethylenediaminetetraacetic acid (EDTA) were employed to further wash coals. The former agent is capable of removing metalloporphyrins,16 while the latter one useful for mobilizing the complex chelates in the carbonaceous materials like soil.30 The mixture of coal with sulfuric acid was mainly shaken for 48 h.15 A longer stirring of 100 h was also undertaken on the consideration that some metalloporphyrins might be dissolved incompletely in 48 h. This method was adopted for a sub-bituminous coal elsewhere.18 The resulting coal sample is termed washed coal 4 in Figure 1, a portion of which was finally washed by EDTA for 24 h, generating washed coal 5. Here again, half of the coal treated at each step was removed, vacuum-dried, and saved. EDTA, sulfuric acid, and methanol used are of reagent grade. All solutions were prepared using milli-Q water. Extraction of Coal Samples by 1-MN. The raw coals and washed coals were pulverized to less than 125 µm and extracted by 1-MN in a batch-scale device. As introduced elsewhere,2,31 ca. 2.5 g coal sample was mixed with 25-30 g solvent and stirred under 1 MPa nitrogen (cold) protection for 1 h at 360 °C. The resulting slurry was then quickly charged into a separation unit and filtered at 360 °C and under nitrogen protection too. The filter used is a stainless mesh-type filter having a pore diameter around 0.5 µm. The filtrate include 1-MN and the extractable fraction of coal, which was further added with 400 mL of n-hexane to promote the precipitation of coal extract. The resulting precipitate is coallike powder and termed HyperCoal (HPC) hereafter. The insoluble part of coal was recovered by washing with toluene and acetone in sequence. Its amount was used for calculating coal extraction yield on a dry ash-free basis.4–8,10 All solutions used here are of reagent grade. The coal recovery is around 90% in each run. About 54.3 wt % of IL coal was extracted, compared to a low yield of 28.6 wt % for WY coal (see Table 2). Both are quantitatively consistent with what reported elsewhere.2,10 Acid leachings of IL coal reduced its extraction yields, which is however converse to the cases of WY coal. Their difference can be attributed (30) Tandy, S.; Bossart, K.; Mueller, R.; Ritschel, J.; Hauser, L.; Schulin, R.; Nowack, B. EnViron. Sci. Technol. 2004, 38, 937. (31) Zhang, L.; Kawashima, H.; Takanohashi, T.; Nakazato, T.; Saito, I.; Tao, H. Energy Fuels 2008, 22 (2), 1183. (32) Li, C.; Takanohashi, T.; Yoshida, T.; Saito, I.; Aoki, H.; Mashimo, K. Fuel 2004, 83, 727.
Table 3. Definitions of the Species Containing a Certain Metal Eluted into the HPC of a Raw Coal definition ion-exchangeable cation
submicrometer particles in forms of oxides, hydroxides, carbonates, and sulfides sulfuric acid-soluble organometallic complex that may have a porphyrin-like structure sulfuric acid-insoluble organometallic complex and cations that are incorporated into clay
difference between the absolute amount of a metal eluted into HPC of raw coal and HPC of washed coal 1 difference between the absolute amount of a metal eluted into HPC of washed coal 1 and HPC of washed coal 3 difference between the absolute amount of a metal eluted into HPC of washed coal 3 and HPC of washed coal 4 absolute amount of a metal eluted in HPC of washed coal 5
to the difference of coal structures. The reason for a reduction on IL coal extraction yield is difficult to specify. In contrast, for the low-rank WY coal, an acid washing can cleave its highly crosslinking macrostructure. More of its organic moieties can be extracted accordingly. A detailed discussion on it is beyond the scope of this study, which can be found in refs 6 and 32. Quantification of Inorganic Elements and Definition of Eluted Species in HPCs. Eight major elements (Fe, Na, Mg, Al, K, Ca, P, and Ti) and thirteen trace elements (Ba, Be, V, Cr, Mn, Co, Ni, Cu, Zn, As, Pb, Li, and Sr) in HPCs were analyzed by inductively coupled plasma optical emission spectroscopy (ICPOES). Each sample was digested and measured in duplicate. The methods for sample preparation and measurement are identical to that reported previously. 33,34 A sample to final solution dilution factor of 150 was used. A reliability of determination of
