Energy Fuels 2011, 25, 1–16 : DOI:10.1021/ef1008192 Published on Web 01/05/2011
A Critical Review of Coal Demineralization and Its Implication on Understanding the Speciation of Organically Bound Metals and Submicrometer Mineral Grains in Coal Niken Wijaya and Lian Zhang* Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia Received June 29, 2010. Revised Manuscript Received December 6, 2010
The state-of-the-art of two coal demineralization technologies, acid/alkali leaching for ultraclean coal (UCC) and solvent extraction for hypercoal (HPC), has been critically reviewed in this paper. UCC or HPC here refers to a coal-derived solid fuel with overall ash content in the order of 0.1 wt %, which has the potential to burn directly in gas turbine combined cycle (GTCC) systems with a net power generation efficiency of no less than 48% on the higher heating value (HHV) basis. The UCC or HPC can also be potentially used in direct carbon fuel cell (DCFC) systems with a net power generation efficiency larger than 60% on HHV basis. Its gasification-derived syn-gas is more energy-intensive, and can be used as a hydrogen source for high-efficiency electricity generation with zero emissions and as a feedstock for the synthesis of value-added chemicals and liquid fuels. In this paper, two typical processes for the generation of UCC and HPC have been comprehensively reviewed to address both fundamentals of the elution of metals and the practical feasibilities. In particular, direct information related to the properties of the inorganic metals remaining in HPC has been intensively addressed. Its implications to the speciation of original metals in coal, especially those embedded as organically bound metals and/or submicrometer particles in coal matrix, were summarized. Finally, the research requirement for the generation of ultraclean coal from low-rank coal was proposed.
mately 48%,1,7thereby mitigating CO2 emission by 25-35% compared to conventional coal-fired power stations.8 Utilization of demineralized coal in direct carbon fuel cell (DCFC) systems is more efficient, potentially leading to the net power generation efficiency of larger than 60% on the higher heating value (HHV) basis. A simple substitution of ultraclean fuel for raw coal in existing power plants is also beneficial for eliminating ash-related problems and flue-gas cleaning requirement. Apart from causing slagging and fouling in the furnace, ash formed from coal combustion also increases the cost and environmental burden during transportation and handling.9 Extensive studies to date have proven the better performance of demineralized coal than its original raw coal during combustion,9-11 gasification,12-19 and coke making process.20 It has been confirmed that the char derived from demineralized bituminous coal can completely burn at the stoichiometric air/fuel ratio of ∼1 under typical air-fired conditions and GTCC condition. Catalytic gasification of hypercoal derived from solvent extraction favors the sustainable use of catalyst
Introduction Coal is the most widely distributed, abundant, and cheapest energy source in the world.1 Its utilization in industry will continue to play an important role in the foreseeable future. Conversion of coal into electricity is its major utilization, resulting in the emissions of greenhouse gases and a variety of air pollutants including SOx, NOx, and particulate matter.2-4 The ultrafine particulates and trace metals emitted from coal are potentially carcinogenic.5,6 With the deregulation of the electricity market in the event of a carbon tax or similar scheme, together with arising public awareness of the environmental burdens related to coal utilization, efforts must be intensified in the short term to develop advanced technologies for clean coal combustion with higher efficiency and near-zero emissions. Demineralization of coal to generate ultraclean fuel (i.e., demineralized coal) is a potential step-change technology to simultaneously increase coal combustion efficiency and eliminate its air pollutant emissions. Direct combustion of demineralized coal in gas turbine combined cycle (GTCC) systems yields a net power generation efficiency of approxi-
(7) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947–967. (8) Clark, K.; Langley, J. Developments in Ultra Clean Coal. UCC Energy Pty Limited. www.coal21.com.au/Media/Conference/clark_ paper.doc. (9) Sloss, L. L.; Smith, I. M.; Adams, D. M. B. Pulverised Coal AshRequirements for Utilisation; IEA Coal Research: London, 1996. (10) Okuni, H.; Saito, I.; Shinozaki, S.; Okuyama, N. In Proceedings of the 2nd Japan-Australia Coal Research Workshop, Tokyo, Japan, November 11-13; NEDO Yoko Kaikan: Tokyo, Japan, 2002; pp 9-18. (11) Sasahara, S.; Inada, M.; Yamashita, T.; Kozai, Y. In Proceedings of the 2nd Japan-Australia Coal Research Workshop, Tokyo, Japan, NEDO Yoko Kaikan: Tokyo, Japan, 2002; pp 55-62. (12) Koyano, K.; Takanohashi, T.; Saito, I. Energy Fuels 2009, 23, 3652–3657. (13) Sharma, A.; Kawashima, H.; Saito, I.; Takanohashi, T. Energy Fuels 2009, 23, 1888–1895.
*To whom correspondence should be addressed. Tel: þ61-3-99052592. Fax: þ61-3-9905-5686. E-mail:
[email protected]. (1) Brooks, P.; Clark, K.; Waugh, B. UCC as A Gas Turbine Fuel. CSIRO, Division of Energy Technology. http://www.australiancoal. csiro.au/pdfs/ucc.pdf. (2) Couch, G. R. Non-OECD Coal-Fired Power Generation-Trends in the 1990s; IEA Coal Research: London, 1999. (3) Nalbandian, H.; Carpenter, A. M. Prospects for Upgrading CoalFired Power Plants; IEA Coal Research: London, 2000. (4) Sloss, L. L. Halogen Emissions from Coal Combustion; IEA Coal Research: London, 1992. (5) Shoji, T.; Huggins, F. E.; Huffman, G. P. Energy Fuels 2002, 16, 325–329. (6) Sloss, L. L. Trace Emissions from Coal Combustion: Measurement and Control; IEA Coal Research: London, 2002; pp 110-125. r 2011 American Chemical Society
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without deactivation upon ash particle deposition, benefiting coal conversion to clean syn-gas with a higher yield of hydrogen at relatively low temperatures.12-19 For noncoking low-rank coals, a slight addition of the corresponding hypercoals to coking coals can substantially enhance the thermoplasticity and tensile strength of the carbonization-driven coke.20 Obviously, once the cost for ultraclean coal generation is substantially reduced and becomes comparable to the total costs for coal mining/transportation plus ash handling and flue-gas cleaning, its utilization instead of raw coal is a promising way to deliver a number of advanced clean energy technologies with higher efficiency and near-zero emissions. In this paper, two major strategies working in opposite manners for coal demineralization have been critically reviewed: acid/alkali leaching to dissolve mineral matter and solvent extraction to extract organic moieties. To avoid ambiguity, the product generated from the former process is termed “ultraclean coal (UCC)”, consistent with the literature.9,21-24 On the other hand, the term “hypercoal (HPC)” is used hereafter for the clean extract delivered from the latter process.7 Both UCC and HPC denote ultraclean demineralized coal, referring to a solid fuel containing less than 0.1 wt % ash, which otherwise causes blade corrosion and erosion problems when burns directly in a gas turbine.1,7 In particular, the concentrations of Na and K in a gas turbine must be below 0.5 mg/kg; Ca and V must be less than 2 mg/kg and 0.5 mg/kg, respectively.10,25 A maximum sulfur content of 0.5 wt % in the demineralized coal is also required.10 These criteria preclude the physical methods such as hydraulic or heavy media separation techniques, oil-agglomeration processes, and froth flotation, which are only capable of reducing the ash content by 8% or a maximum of half of the original amount.26,27 Reviewing the processes of converting coal into liquids such as direct coal liquefaction and solvent-refinedcoal (SRC) process is also beyond the scope of this paper, considering that the severity of the operation conditions and the huge hydrogen consumption have significantly hindered the commercialization of these two processes over a very longterm. Moreover, a prior demineralization is indeed useful for coal liquefaction as the removal of residual solid matter from
coal-derived liquids and liquid carrier has always been problematic.28 Apart from technical issues related to UCC and HPC generation processes, fundamentals governing the elution of inorganic impurities have also been intensively discussed to address which inorganic elements and which forms are preferentially removed in a process. Understanding these issues is imperative for advancing coal demineralization process. It is also essential for managing the potential ash problems during downside UCC or HPC utilization. Moreover, elucidating the modes of occurrence of the metals remaining in UCC or HPC is expected to shed light onto their original species associated within carbonaceous matrix of a raw coal. In this regard, this paper is arranged in the following sequence: [1] Modes of occurrences of the original inorganic elements in raw coal will be discussed first. The mode of occurrence is a description of how an element occurs in coal, the understanding of which is imperative to answer the question of which metals are difficult to remove while preferentially remain in UCC or HPC. [2] State-of-the-art of UCC and HPC processes will be reviewed second. In particular, the partitioning of inorganic impurities in each process will be intensively and quantitatively discussed to further address the properties of the inorganic elements that are difficult to remove. Implication of the inorganic impurities remaining in UCC and HPC to the speciation of original inorganic elements in coal will be addressed. The interference of major metals (e.g., mineral grains) is the cause of failing to appropriately elucidate the properties of the metals with minor or trace contents in a raw coal, which is expected to be eliminated in the ultraclean demineralized coals. [3] Comparison between the two strategies for coal demineralization will be discussed next from the perspectives of the removal efficiency of inorganic elements and others such as economical assessment. [4] The problems and potential new technologies for lowrank coal demineralization will be finally concluded to address the fact that the low-rank coals in some areas such as Victoria of Australia are the predominant source for primary energy consumption. To date low-rank coal has been less considered in the existing ultraclean coal generation processes.
(14) Sharma, A.; Saito, I.; Takanohashi, T. Energy Fuels 2008, 22 (6), 3561–3565. (15) Sharma, A.; Saito, I.; Takanohashi, T. Energy Fuels 2009, 23, 4887–4892. (16) Sharma, A.; Takanohashi, T. Energy Fuels 2010, 24, 1745–1752. (17) Sharma, A.; Takanohashi, T.; Morishita, K.; Takarada, T.; Saito, I. Fuel 2008, 87, 491–497. (18) Takanohashi, T.; Shishido, T.; Kawashima, H.; Saito, I. Fuel 2008, 87, 592–598. (19) Wang, J.; Sakanishi, K.; Saito, I. Energy Fuels 2005, 19, 2114– 2120. (20) Takanohashi, T.; Shishido, T.; Saito, I. Energy Fuels 2008, 22, 1779–1783. (21) Cottrell, A.; Scaife, P.; Wibberley, L. CCSR report: Systems assessment of hypercoal for electricity supply in Japan 2007. http://www. ccsd.biz/publications/files/TA/TA%2065%20Hypercoal_web.pdf. (22) Steel, K.; Besida, J.; O’Donnell, A.; Wood, D. Fuel Process. Technol. 2001, 70 (3), 171–192. (23) Steel, K.; Patrick, J. Fuel 2003, 82 (15-17), 1917–1920. (24) Wu, Z.; Steel, K. Fuel 2007, 86 (14), 2194–2200. (25) Sakanishi, K.; Akashi, E.; Nakazato, T.; Tao, H.; Kawashima, H.; Saito, I.; Takarada, T. Fuel 2004, 83 (6), 739–743. (26) Alam, H.; Moghaddam, A.; Omidkhah, M. Fuel Process. Technol. 2009, 90 (1), 1–7. (27) Wang, Z.; Ohtsuka, Y.; Tomita, A. Fuel Process. Technol. 1986, 13, 279–289. (28) Bowling, K. M.; Rottendorf, H. In National Conference on Chemical Engineering, Adelaide, 1976.
Modes of Occurrence of Inorganic Impurities in Coal Overall Inorganic Elements in Coal. Distribution of an element in a raw coal differs distinctively from one element to another. For the two major elements, S and N, a large fraction of them are covalently bound with organic macromolecules forming heterocyclic aromatics.29,30 The halogens are composed of water-soluble halides and ions covalently bound with organic moieties in a form which has not been exactly clarified.30-32 Regarding the metals, their properties are much more complex, exhibiting a broad variation in terms of physical and chemical properties. As illustrated in Figure 1, the inorganic metals in a raw coal can be broadly (29) Calkins, W. H. Fuel 1994, 73 (4), 475–484. (30) Gong, B.; Buckley, A. N.; Lamb, R. N.; Nelson, P. F. Surf. Interface Anal. 1999, 28, 126–130. (31) Cressey, B. A.; Cressey, G. Int. J. Coal Geol. 1988, 10, 177–191. (32) Domazetis, G.; Raoarun, M.; James, B. D.; Liesegang, J.; Pigram, P. J.; Brack, N.; Glaisher, R. Energy Fuels 2006, 20, 1556–1564.
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Figure 1. Classification scheme for elemental modes of occurrence in coal (reproduced with permission from ref 35. Copyright 2000 Elsevier Ltd.).
divided into two major categories: ions either chemically associated with organics via covalent or ionic bond or dissolving in the inherent moisture in coal matrix, and mineral grains with or without substitution elements.33-36 Mineral Grains. Mineral grains are the most abundant inorganic components in high-rank coals, as shown in Figure 2a for the cross-section of a bituminous coal, pulverized bituminous Illinois No. 6 coal (IL) containing 15.5 wt % ash.37 Observation was made using scanning electron microscopy (SEM) under the backscattered electron (BSE) mode. The bright spots denote mineral grains, which are abundant in high-rank coal and principally consist of silicates, oxides, carbonates, sulfides, sulfates, and phosphates with different structures and sizes. Methods for the direct characterization of these species have been well established.38-40 Association of individual mineral grains with a coal matrix can be quantitatively examined
through the use of computer-controlled SEM (CCSEM), which defines a mineral particle associated within the coal matrix as included grain, whereas a mineral that is not associated with organic material is termed excluded grain.33,39,41 Coal rank plays an important role in the properties of minerals. As visualized in Figure 2b for an Australia brown coal (i.e., lignite), Yallourn (YL) coal, the number density of the visible bright spots (i.e., mineral grains) within it is extremely low when compared with IL coal in panel a. The minerals within lignite are also dominated by small grains, some of which are on the submicrometer scale and are difficult to observe by SEM. Compared to that of coarse particles, the properties of submicrometer mineral grains have been less understood. Organically Bound Metals. Direct characterization of organically bound metals is still a challenge, as it is beyond the capabilities of most of the conventional analytical instruments. Understanding of the chemical forms of organically bound metals and their variation with coal rank is thus still qualitative. Conventionally, the indirect methods with an initial segregation of coal or its mineral matter followed by characterization of each fraction are employed, including sequential leaching and size/density-segregation.33,34,39,42 The first method aims to separate the original mineral matter into different groups according to their solubility in
(33) Benson, S. A.; Jones, M. L.; Harb, J. N. In: Fundamentals of Coal Combustion and Efficient Use, Smoot, L.D. (Ed.) Elsevier Amsterdam 1993, 299-373. (34) Huggins, F. E.; Srikantapura, S.; Parekh, B. K.; Blanchard, L.; Robertson, J. D. Energy Fuels 1997, 11, 691–701. (35) Senior, C. L.; Zeng, T.; Che, J.; Ames, M. R.; Sarofim, A. F.; Olmez, I.; Huggins, F. E.; Shah, N.; Huffman, G. P.; Kolker, A.; Mroczkowski, S.; Palmer, C.; Finkelman, R. Fuel Process. Technol. 2000, 63, 215–241. (36) Wang, J.; Yamada, O.; Nakazato, T.; Zhang, Z.-G.; Suzuki, Y.; Sakanishi, K. Fuel 2008, 87 (10-11), 2211–2222. (37) Vorres, K. S. Energy Fuels 1990, 4, 420–426. (38) Gupta, R. P. Energy Fuels 2007, 21, 451–460. (39) Huggins, F. E. Int. J. Coal Geol. 2002, 50, 169–214. (40) Vassilev, S.; Tascon, J. M. D. Energy Fuels 2003, 17, 271–281.
(41) Yu, D.; Xu, M.; Zhang, L.; Yao, H.; Wang, Q.; Ninomiya, Y. Energy Fuels 2007, 21 (2), 468–476. (42) Pusz, S.; Krzton, A.; Komraus, J. L.; Martinez-Tarazona, M. R.; Marinez-Alonso, A.; Tascon, J. M. D. Int. J. Coal Geol. 1997, 33, 369–386.
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Figure 2. Association of mineral grains with coal matrix, as observed by SEM at backscattered electron (BSE) mode: (a) bituminous coal (Illinois No. 6 coal, USA); (b) brown coal (Yallourn coal, Australia).
The elution behavior of Fe from the coal series listed in Table 1 is substantially different from that of alkali and alkaline earth metals, as indicated in Table 2.44 No relationship was found between its ammonium acetate solubility and coal rank, implying that ion-exchangeable cation is an insignificant fraction for Fe, even in the low-rank coals. Instead, as a large percentage of Fe remains insoluble in HF and HNO3, it is indicative that a complex association of Fe with organic moieties is apparently present within the carbonaceous matrix of coal. Similar observation was found for the sequential leaching of another suite of five differently ranked coal samples as shown in Table 3. Goonyella (GO) is a low-volatile bituminous coal of Australia. The other three coals are the same as those in Tables 1 and 2. Coal samples were washed by acetic acid (1 M), HCl (3 M), HNO3 (2 M), and HF (48%) in sequence.46 Regardless of coal rank, Fe is the most abundant element surviving from the sequential leaching. Electron spin resonance spectroscopy (ESR) analysis of coal residues revealed that the acid-insoluble Fe is mostly present as distorted octahedral Fe3þ complex associated with phenolic hydroxyl at maceral surfaces,46 as visualized in Figure 3. Small clusters of Fe3þ polyhedra could even be formed through precipitation into closed or highly restricted pores in coal, forming amorphous or cryptocrystalline oxide/oxyhydroxide microaggregates typically in the 2-10 nm size ossbauer analysis successfully detected the range.47 57Fe M€ presence of ferric oxide/hydroxide with a poorly ordered structure and a particle size of approximately 50 A˚ in YL coal.48 Similar structures were also confirmed for the other transition metals (Cr, Mn, V) and Ti and As through characterizing coal fractions with X-ray adsorption nearedge structure spectroscopy (XANES)34,43,47,49 and Ca and Al through microprobe observation of coal vitrinite using electron microscopy.50 Moreover, since the Fe-bearing species are common micronutrients in the biological materials, it
Table 1. Ammonium Acetate-Soluble Cations in APCS Coal Samples, in mg/kg (Reprinted from Ref 44. Copyright 1990 American Chemical Society) coal Wilcox Beulah-Zap Wyodak Blind Canyon Illinois No. 6 Stockton Pittsburgh Upper Freeport
rank
lignite lignite sub-bituminous high-volatile bituminous high-volatile bituminous high-volatile bituminous high-volatile bituminous medium-volatile bituminous Lower Bakerstown medium-volatile bituminous Pocahontas No. 3 low-volatile bituminous
Ca
Na
Mg
total
6400 340 1480 8220 6200 6600 2800 15600 4200 1000 1280 6480 3100 130 55 3285 4600 700 54 5354 360 37 70 467 2000 260 30 2290 2600 120 24 2744 780
38
34
852
3200
480
40
3720
different acids. For instance, neutral water is first used to remove water-soluble ions. Ammonium acetate is second employed to mobilize ion-exchangeable cations associated with carboxylic acids. Third, coal residue is washed by strong acids including HCl, HNO3, and HF to remove carbonates, sulfides/sulfates, and alumino-silicates, respectively. The species surviving from this scheme are referred to as organometal complexes39 and/or fine mineral grains that are completely encased into and protected by host organic or silicate matrix from acid attack.43 Ion-exchangeable cations in the form of carboxylate, which can be readily washed away by weak reagent such as ammonium acetate, is one major fraction for alkali and alkaline earth metals, Na, K, Ca, and Mg, and even Al. Decreasing coal rank favors the presence of the ionexchangeable cations of these metals in a coal, as the oxygen-containing acidic functional groups are abundant in low-rank coals. This is supported by the results for a suite of Argonne Premium Coal Samples (APCS) washed by ammonium acetate in Table 1.44 As can be seen, for either individual or total metals from Ca, Na, and Mg, their ammonium acetate-soluble fractions are broadly the highest in two lignites, which decrease as the coal rank is increasing from lignite to low-volatile bituminous coal. For the inorganic metals of low-rank Victorian brown coal such as YL coal in Australia, a large fraction of them were proven ionexchangeable cations bound with the carboxylic acids.45
(45) Li, C.-Z., Advances In The Science of Victorian Brown Coal; Elsevier: Oxford, 2004. (46) Zhang, L.; Takanohashi, T.; Kutsuna, S.; Saito, I.; Wang, Q.; Ninomiya, Y. Fuel 2008, 87, 2628–2640. (47) Huggins, F. E.; Shah, N.; Huffman, G. P.; Kolber, A.; Crowley, S. S.; Palmer, C.; Finkelman, R. Fuel Process. Technol. 2000, 63, 79–92. (48) Cook, P. S.; Cashion, J. D. Geochim. Cosmochim. Acta 1987, 51, 1467–1475. (49) Huggins, F. E.; Huffman, G. P.; Kolker, A.; Mroczkowski, S.; Palmer, C.; Finkelman, R. Energy Fuels 2002, 16, 1167–1172. (50) Li, Z.; Ward, C. R.; Gurba, L. W. Int. J. Coal Geol. 2010, 81, 242–250.
(43) Kolker, A.; Huggins, F. E.; Palmer, C.; Shah, N.; Crowley, S. S.; Huffman, G. P.; Finkelman, R. Fuel Process. Technol. 2000, 63, 167–178. (44) Finkelman, R.; Palmer, C.; Krasnow, M. R.; Aruscavage, P. J.; Sellers, G. A.; Dulong, F. T. Energy Fuels 1990, 4, 755–766.
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Table 2. Percent Leachable Iron in ACPS Samplesa (Reprinted from Ref 44. Copyright 1990 American Chemical Society) coal
rank
H2O
NH4OAc
HCl
HF
HNO3
total
Wilcox Beulah-Zap Wyodak Blind Canyon Illinois No. 6 Stockton Pittsburgh Upper Freeport Lower Bakerstown Pocahontas No. 3
lignite lignite sub-bituminous high-volatile bituminous high-volatile bituminous high-volatile bituminous high-volatile bituminous medium-volatile bituminous medium-volatile bituminous low-volatile bituminous
0 25 -
0 15 5 0 10 0 13 7 15 0
41 26 60 18 8 18 0 20 26 65
19 0 17 8 0 45 24 10 8 17
8 7 þ 64 0 3 þ þ 4 4
68 48 82 90 18 66 37 37 53 86
a
- = no data; þ = increase; 0 = change not statistically significant.
Table 3. Percent Leachable of Individual Elements in Differently Ranked Coal Samples, in mg/kg (Reprinted With Permission from Ref 46. Copyright 2008 Elsevier Ltd.) Goonyella (GO)
Al Fe Ca Mg Na K Ti Cu Zn Sr Ba Ni Co Cr V Mn Pb As Li Se Be Fe, wt % transition metals, wt %
Illinois No. 6 (IL)
Wyodak (WY)
Beulah- Zap (BZ)
Yallourn (YL)
raw
washed
raw
washed
raw
washed
raw
washed
raw
washed
5400 1900 700 120 220 450 1600 10 20 30 30 5 5 4 40 10 10 1 10
