Energy Fuels 2009, 23, 4318–4324 Published on Web 08/03/2009
: DOI:10.1021/ef900390e
Enhanced Oxidative Reactivity for Anthracite Coal via a Reactive Ball Milling Pretreatment Step Angela D. Lueking,*,†,‡,§ Apurba Sakti† Dania Alvarez-Fonseca,‡ and Nichole Wonderling§ †
Department of Energy and Mineral Engineering, ‡EMS Energy Institute, §Materials Research Institute, and 120 Hosler, The Pennsylvania State University Received May 1, 2009. Revised Manuscript Received July 20, 2009
Reactive ball milling in a cyclohexene solvent significantly increases the oxidative reactivity of an anthracite coal, due to the combined effects of particle size reduction, metal introduction, introduction of volatile matter, and changes in carbon structure. Metals introduced during milling can be easily removed via a subsequent demineralization process, and the increased reactivity is retained. Solvent addition alters the morphological changes that occur during pyrolysis and leads to a char with significantly increased reactivity. When the solvent is omitted, similar effects are seen for the milled product, but a significant fraction of the char is resistant to oxidation.
However, direct addition of catalysts is expensive, and reaction of metals with inherent mineral matter may make catalyst regeneration difficult. Alternatives include addition of waste products7-9 or biomass,10 both of which often include alkali metals and may increase volatile matter. In the former case, the use of waste products may increase the environmental emissions, in particular sulfur-containing gases.9 In a previous publication, we reported the low-temperature H2 evolution from Buck Mountain anthracite coal after reactive ball milling in cyclohexene.11 The H2 evolution was attributed to dehydrogenation of cyclohexene within the ball mill. Subsequently we showed similar behavior for a Summit semianthracite, with H2 evolution observed 1 year after preparation; behavior not seen for graphite milled under these conditions.12 In addition to H2, several volatiles evolved with heating, including cyclohexene, benzene (a product of cyclohexene dehydrogenation within the mill), and other volatiles associated with coal. Here, we show how the ball milling process alters the oxidation behavior of the coal, consistent with established methods.13 However, ball milling not only decreases the particle size and increases the amount of volatile matter as previously reported,13 but may also hinder ordering of the carbon structure during pyrolysis, thereby increasing the reactivity of the resulting char.
Introduction Increasing concerns about energy efficiency, international relations concerning energy supply, environmental pollution, and CO2 emissions have led to several “clean coal” initiatives. Coal gasification is considered a clean coal technology as it produces syngas (CO and H2) for further combustion in a gas turbine, increases the overall conversion efficiency, is amenable to coproduction of transportation fuels, and is better suited for pollution control and CO2 capture. Practical considerations in coal gasification are residence time, reaction temperature, amount of generated fly ash, and full recovery of the heating value of the coal (i.e., elimination of unburned carbon). Thus, coal gasification is often carried out at high temperatures to minimize unburned carbon in a practical residence time. As a coal is gasified, volatile matter is quickly released and burned, the fixed carbon is pyrolized to form a char, and the rate-limiting step becomes oxidation of the residual char.1 The order of the char structure is increased with pyrolysis, and the increased order decreases its reactivity. The rate of gasification in high rank coals, such as anthracite, is governed by the intrinsic carbon reactivity of the coal, determined largely by carbon structure.2 The low reactivity of the anthracite structure is reflected by its high fixed carbon content, high degree of order, and low volatile matter, all of which slow its rate of ignition and sustained oxidation, thereby limiting the utilization of anthracite in industrial gasification facilities. Methods to utilize anthracite include high reaction temperature and/or pressure, and the use of metal catalysts,3 particularly alkali and alkaline earth salts.4-6
Experimental Section Buck Mountain PSOC-1468 (BMT) was obtained from the Pennsylvania State University coal sample bank. To remove the inherent mineral matter, BMT was demineralized as (7) Valenzuela-Calahorro, C.; Pan, Y. G.; Bernalte-Garcia, A.; Gomez-Serrano, V. Energ Fuel 1994, 8, 348–354. (8) Jaffri, G. E. R.; Zhang, J. Y. Chin. J. Chem. Eng. 2007, 15, 670–679. (9) Jaffri, G. E. R.; Zhang, J. Y. Chin. J. Chem. Eng. 2008, 16, 575–583. (10) Zhu, W. K.; Song, W. L.; Lin, W. G. Fuel Process. Technol. 2008, 89, 890–896. (11) Lueking, A. D.; Gutierrez, H. R.; Fonseca, D. A.; Narayanan, D. L.; VanEssendelft, D.; Jain, P.; Clifford, C. E. B. J. Am. Chem. Soc. 2006, 128, 7758–7760. (12) Sakti, A.; Wonderling, N. W.; Clifford, C. E. B.; Badding, J. V.; Lueking, A. D. J. Phys. Chem. C. 2008, 112, 17427–17435. (13) Dobbs, R. J.; Dolhert, L. E. Coal Particle Compositions and Associated Methods. Feb. 8, 2007, 2007.
*To whom correspondence should be addressed. E-mail: lueking@ psu.edu. Fax: 814-865-3248. (1) Russell, N. V.; Gibbins, J. R.; Williamson, J. Fuel 1999, 78, 803– 807. (2) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 1461– 1475. (3) Rewick, R. T.; Wentrcek, P. R.; Wise, H. Fuel 1974, 53, 274–279. (4) Walker, P. L.; Matsumoto, S.; Hanzawa, T.; Muira, T.; Ismail, I. M. K. Fuel 1983, 62, 140–149. (5) Freund, H. Fuel 1985, 64, 657–660. (6) Mckee, D. W.; Spiro, C. L.; Kosky, P. G.; Lamby, E. J. Fuel 1985, 64, 805–809. r 2009 American Chemical Society
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following demineralization process: a 4 M HCl (Sigma Aldrich) acid treatment, followed by a 10 M NaOH (J.T. Baker) base treatment (denoted by “-B”), followed by a second 4 M HCl treatment. Each of the three steps used a washing step as described in the preceding process. It should be noted that this particular demineralization process (samples subjected to this demineralization process will have a subscript “-A”) for the milled products differs from the Ward and Bishop demineralization process used for the as-received coals. The A demineralization process was used as an adaptation of a synthesis sequence described previously in which 10 M NaOH was originally substituted for the HF acid treatment for safety reasons. Subsequently, we have shown evidence that the first acid and base treatments of the A process lead to changes in carbon structure.15,17 All samples, at each stage of processing, were characterized with dynamic TGA and X-ray diffraction (XRD). TGA was performed by flowing UHP grade air with a flow rate of 100 cm3/min in a Perkin-Elmer TGA. Samples (10-14 mg) were heated from 30 to 1000 °C with a ramp rate of 5 °C/min. TGA is plotted as the negative derivative weight versus temperature (T); weight is smoothed using a central moving average of 30 points prior to taking the derivative. Ash content is calculated based on the initial mass versus the mass after cooling to room temperature to negate potential buoyancy effects; numbers in parentheses indicate the standard deviation of multiple runs. Carbon containing minerals, however, complicate determination of ash content: (1) conversion of iron carbides to iron oxides during oxidation (at ∼400 °C) will artificially deflate ash content (see Supporting Information). As all samples in this paper may contain carbon containing minerals, the term TGA ash content is used throughout and quantitative XRD is also considered as discussed in the next paragraph. X-ray diffraction (XRD) measurements of BMT anthracite were performed on a Philips X’pert MPD with Cu KR radiation operated at 40 kV and 40 mA with a beam mask of 10 mm and divergent slit of 2°. XRD measurements of all other samples were taken with a Scintag Model X2 θ/θ goniometer with Cu radiation KR1/KR2 with Si (Li) Peltier detector operated at 45 kV and 40 mA. Instrument error (in terms of 2θ and instrumental peak broadening) was accounted for using NIST 640c silicon as an external reference sample. Diffraction patterns for representative samples of the BMT series are shown later in Figure 3. Profile fitting for determination of fwhm and d-spacing values was performed with a pseudo-Voigt fitting algorithm using Jadeþ 8 software (Materials Data Inc., Livermore, CA). ICSD and ICDD PDF4 (2006) databases were used for phase identification. The Lc(002) and La(10) values have been calculated following the Scherrer Equation using K constants equal to 0.9 for Lc(002) and 1.84 for La(10).18,19 We conservatively estimate the errors as ( 0.05 A˚ for d-spacing and 10 A˚ for Lc(002) and La(10). For select samples, thermal gravimetric-mass spectroscopy (TG-MS) data were obtained on a thermal gravimetric analyzer 2050 coupled to a Thermostar GSD 301T, Pfeiffer Vacuum Inc., mass spectrometer (MS) via a temperaturecontrolled quartz capillary at 473 K. The mass range of the
Figure 1. Rate of mass loss in TGA of BMT and DBMT series, as indicated. Rate of mass loss is the normalized negative derivative of the weight profiles, or rate of mass loss, as defined in the text. Shorthand notation is as follows: “*” precursor; “m” milled; “HT” char of precursor; “mHT” milled, then annealed to form char; “mA” milled, then acid treated.
described by Bishop and Ward.14 Demineralized BMT (DBMT) was treated with 6 M HCl, 6 M HNO3, then concentrated (47-52%) HF in series. For each acid treatment, the coal was mixed for 1 h at 300 K, then left undisturbed in the acid for 24 h at 300 K. The acid supernatant was then decanted and rinsed with centrifugation; rinsing was repeated until the rinsewater was pH neutral, at which point the coal was recovered by gravimetric filtration. After the final acid treatment, DBMT was heated under vacuum at 423 K to remove moisture. The ball milling procedure described in previous studies11,15,16 was repeated here. In brief, a Fritsch Planetary Mono Mill Pulverisette 6 LC - 106A with a 250 mL stainless steel (Fe-Cr-Ni) vessel and a set of twenty 10 mm diameter stainless steel (Fe-Cr) balls milled the materials for 80 h at 400 rpm in ultrahigh purity (99.99999%) grade argon atmosphere to prevent oxidation and minimize air exposure. Cyclohexene (99% purity, JT Baker) was added in the ratio of 6 g solid to 20 mL. For samples with added metal, 10% of the 6 g was either Fe powder (-200 mesh, STREM Chemicals) or 316 stainless steel (SS) powder (-325 mesh, STREM Chemicals), the remaining 90% was DBMT; these samples will be referred to as mDMBT-Fe and mDBMT-SS, respectively. Milled materials were annealed at high-temperature (denoted “-HT”) at 1673 K for 3 h in Argon to produce a char, using a ramp rate of 2.5 °C per minute. The choice of 1673 K was based on compatibility with previous publications. Separately, milled materials were subjected to the
(17) Lueking, A. D.; Li, Q.; Sakti, A.; Wonderling, N. W. Carbon 2009, Submitted. (18) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39, 1821–1833. (19) Bustin, R. M.; Rouzaud, J. N.; Ross, J. V. Carbon 1995, 33, 679–691.
(14) Bishop, M.; Ward, D. L. Fuel 1958, 37, 191–200. (15) Lueking, A. D.; Gutierrez, H. R.; Jain, P.; Van Essandelft, D. T.; Burgess-Clifford, C. E. Carbon 2007, 45, 2297–2306. (16) Narayanan, D. L.; Lueking, A. D. Carbon 2007, 45, 805–820.
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Figure 2. (a) TG signals as samples are heated to 1000 °C in Ar, with (b-c) associated (select) MS signals, including (b) cyclohexene and its dehydrogenation products and (c) other select volatiles. MS signals have been normalized by initial sample mass, then scaled as indicated due to differences in signal intensity.
MS is between 1 and 300 AMU and the pressure inside the vacuum system was in the range of 1.7 10-6 mbar. The TGMS is purged with argon (100 cc/min) at all times: reducing;but not eliminating;the trace oxygen and water in the balance. The argon was passed through a 3A zeolite trap to minimize moisture content. Before each run, peak alignment
and mass scale calibrations were performed for the MS. Samples were loaded into the TG-MS with minimal air exposure and then heated to 1273 at 10 K/min. The MS signals monitored were m/z 1, 2, 12, 14, 16, 17, 18, 20, 28, 32, 44, 78, and 82. MS signals have been normalized by initial sample mass, then scaled as indicated due to differences in 4320
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: DOI:10.1021/ef900390e
Lueking et al. Table 1. Quantitative TGA and XRD Data Tonset
Figure 3. XRD of BMT series. Carbon phases [002] and [10] are labeled with brackets. Mineral phases present in BMT and BMTHT were not identified. Metal phases are labeled as follows: “þ” iron and metal carbides, “O” iron and iron alloys. Full phase identification and quantification are provided in the Supporting Information.
signal intensity. For some samples, cyclohexene was added directly prior to the TG-MS analysis in the ratio of a 3.3 mL per gram of solid; this was done in order to compare evolution of cyclohexene (and its derivatives) when it was added in the milling process versus simple desorption of the sample. The TG-MS analysis is meant to supplement the primary TGA and XRD analysis and thus was performed only on select samples. The TG-MS provides some indication of volatile gaseous species that evolve with heating, but the masses selected are not all-inconclusive of potential evolving species. The mass loss with temperature in TG should not be confused with volatile matter, as defined by standard ASTM methods (i.e., 950 ( 20 °C for 7 min).
BMT DBMT graphitea
420 395 575
mBMT mDBMT mDBMT-Fe mDBMT-SS mBMT(dry) mGa
300 280 200 220 340 200
BMT-HT DBMT-HT mBMT-HT mDBMT-HT mDMBT-Fe-HT mDBMT-SS-HT mBMT(dry)-HT
530 550 350 465 475 440 480
mBMT-A mDBMT-A mDBMT-Fe-A mDBMT-SS-A
235 305 200 320
dC (A˚)
Lc (A˚)
6.7 0.6 0.1
3.495 3.460 3.359
16 13 >1000
30.7 40.2 58.5 57.0 6.8b 8.4
3.482 3.443 3.526 3.509 3.433 3.3652
11 11 15 13 32 176
895 995 13.1b 775 890 505 700 49.6 555 630 43.6 550 625 76.4 540 675 67.3 840 910 Metal Removed
3.448
19
3.379 3.371 3.359 3.358 3.429
62 51 120 82 37
3.460 3.443
13 11
c
c
Tmax
Tb
ash (wt %)
Coal/ Carbon 555 820 525 650 780 845 Milled 480 520 430 470 375 500 480 525 525 620 460 695 Annealed
425 435 445 410
505 475 480 480
1.5 0.7 3.8 1.5
c
a Graphite and milled graphite had shoulders on the carbon [002] peak. XRD values for the strongest reflection are given. b As reported in ref 30. c XRD of the Fe-A and SS-A samples were not collected, as they were expected to be largely amorphous.
to build-up of oxygen complexes and activation of the material.20 Relative to BMT, Tonset and Tmax of graphite are both shifted to higher temperatures; the peak is narrower, reflective of a more homogeneous sample, yet the burnout temperature is comparable to BMT at 845 °C (Table 1). Before discussing TG-MS, we should note that the mass profiles in TGA differs, as the oxygen component of air (in TGA) is expected to chemisorb,2,21 which may lead to possible swelling and softening behaviors (as demonstrated in ref 22) and thus affect gas evolution. In TG-MS, BMT loses ∼3% of its mass as it is heated to 200 °C in Argon (Figure 2a); this low-temperature decrease is attributed primarily to desorption of water, nitrogen, and oxygen (Figure 2c). From 200-450 °C, the mass of BMT is relatively stable, although there is a slight evolution of N2 and O2 in this region (m/z=28, 32, Figure 2c); at ∼600 °C, an increase in the m/z=2 (H2, Figure 2b) and 16 (O or CH4) signals is noted; and at ∼700 °C there are increases in the signals at m/z=18, 28, and 44. As stated in the methods section, the TG-MS analysis differs from the ASTM definition of volatile matter, and the masses probed with the MS may not be comprehensive. Effect of Demineralization of the Coal. Mineral matter of BMT is significantly reduced via the Bishop-Ward demineralization process, to 0.6% (as measured by the TGA, Table 1) or 0.5% based on proximate analysis (reported in ref 16). Unlike BMT, the symmetric TGA (Figure 1b*) for DBMT suggests homogeneity. The disappearance of the high-temperature TGA tail with demineralization suggests that mineral matter inhibits oxidation. As metals often catalyze gasification, the effect is attributable to increased
Results and Discussion Coal Precursor and Graphite. BMT is an anthracite coal from the bottom of the Llewellyn formation of the Buck Mountain Seam, collected from Luzerne County, Pennsylvania. As previously reported,16 BMT has a low volatile matter content (3.65 wt %, dry) and high fixed carbon content (89.52 wt %, dry). The ash content of BMT is 6.7% (dry); elements associated with the mineral matter have been identified through neutron activation analysis as traces of aluminum, silicon, antimony, arsenic, barium, bromine, cerium, chromium, hafnium, iron, lanthanum, magnesium, potassium, scandium, and tungsten. The particle size of as-received BMT ranged from 0.27 to 0.58 μm.16 The rate of mass loss in the TGA of BMT is broad (Figure 1a*), indicative of a heterogeneous sample with multiple regions of oxidative reactivity. Reactivity in TGA is a function of particle size, carbon structure, and possible catalytic effects of inherent mineral matter. BMT begins to oxidize (Tonset) at 420 °C, has a maximum oxidation rate (Tmax) at 555 °C, and oxidation is complete (Tb) by 820 °C (Table 1). For comparison, the TGA profile of graphite is shown in Figure 1f*. Graphite has a uniform carbon structure, a regular gas-carbon interface, and no significant catalytic effect of metal in the oxidation profile. The TGA profile of graphite is asymmetric;shifted toward lower temperature;reflecting a slow onset of oxidation. This slow onset of oxidation shown by carbon samples is attributable
(21) Miura, K.; Makino, M.; Silveston, P. L. Fuel 1990, 69, 580–589. (22) Maloney, D. J.; Jenkins, R. G.; Walker, P. L. Fuel 1982, 61, 175– 181.
(20) Jenkins, R. G.; Nandi, S. P.; Walker, P. L. Fuel 1973, 52, 288–293.
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gas-carbon interface. BMT mineral matter must inhibit access of oxygen to active sites, and its removal must “open up” pores for oxidation and thereby homogenize the burning of DBMT. A similar effect was seen previously for a bituminous coal20,23 and is generally attributed to mineral matter located in “feeder pores”.20 Relative to BMT, Tonset of DBMT is decreased by 25 °C (Table 1); this small shift in Tonset suggests particle size reduction with filtration or a slight catalytic effect by residual acid (despite repeated washing, some may be retained in the structure). The former is supported by particle size data, reported elsewhere.16 The loss of small particles may be responsible for changes in XRD parameters with demineralization. Char Formation (Coals). Samples were annealed at 1673 K to form a high-temperature char. Although the char formation temperature may affect subsequent reactivity,1,21 use of a constant temperature was used to eliminate this effect. In all cases, char formation led to decreased oxidative reactivity, as expected, due to the combined effect of ordering of the carbon structure and removal of volatile matter. Specifically, the TGA profile of the BMT-HT char (HT, Figure 1a) is shifted by 110, 340, and 175 °C for Tonset, Tmax, and Tb, respectively, relative to BMT. The TGA parameters of the BMT-HT exceed that of graphite: the burnout temperature of graphite is 845 °C compared to a Tb of 995 °C for BMT-HT. Despite the high oxidation temperature of the BMT-HT char, XRD data suggests the material does not have long-range threedimensional order (i.e., dc =3.448 A˚ and Lc =19 A˚, Table 1). TGA of the DBMT-HT char (HT, Figure 1b) also reflects an increased carbon ordering relative to DBMT. Removal of mineral matter prior to char formation is expected to bring about changes in porosity and surface area,24 and the oxidation profile of DBMT-HT more closely resembles graphite than BMT-HT, highlighting the effect of mineral matter on char formation. This low oxidative reactivity of BMT-HT highlights the limitations of using anthracite in gasification facilities. Effect of Milling. Milling BMT in cyclohexene reduces the onset of oxidation by 120 °C and reduces burnout temperature Tb by ∼300 °C (Figure 1a, Table 1). The narrow TGA of mBMT indicates that milling homogenizes the sample. Milled DBMT also has reduced oxidation parameters (Table 1). mDBMT has a lower oxidation temperature than mBMT, consistent with the trends seen for the precursors. The decreased oxidation temperatures with milling can be attributed to the combined effects of introduction/formation of volatile matter, particle size reduction, and introduction of metal, a lesser effect is expected based on changes in carbon structure; each of these effects will be considered separately below. XRD indicates only very subtle changes in the carbon structure (dc and Lc, Table 1), for both BMT and DBMT with milling. Although ball milling graphite is expected to amorphitize the carbon structure and reduce particle size, either with12 or without25-28 solvent, we have demonstrated
the mobile/reactive fractions of anthracite lead to quite different behavior for reactive milling in cyclohexene.12 In short, milling a semianthracite led to significant ultramicroporosity, increased cross-link density, and increased sp2 clustering, behavior not seen for graphite or the semianthracite char when milled.12 Expectedly then, the shifts in TGA for BMT and DBMT with milling are less pronounced than graphite milled under the same conditions (Figure 1f, Table 1). Milling leads to a significant increase in volatile matter, as expected from the addition of cyclohexene to the mill and/or an increase due to effects of milling such as possible crushing of organic macromolecules inherent to the coal (discussed in a later section where cyclohexene is omitted from the milling process), and/or dissolution of organic macromolecules in the added cyclohexene. TG-MS in Ar demonstrates that mBMT loses 31-37% of its weight with heating to 1000 °C (Figure 2a), compared to an 8% loss for BMT. (The range of mass loss for mBMT is due to dependence on time between preparation and analysis, as well as sample heterogeneity;see Supporting Information). Previously we speculated possible reactions between cyclohexene and coal could include: free radical, Diels-Alder, and hydrogen abstraction reactions.11,15 Although the reaction between anthracite and cyclohexene is not necessarily simple or straightforward, the following observations provide evidence that cyclohexene reacts with the coal structure in the mill. First, mBMT binds cyclohexene stronger than BMT: cyclohexene (m/z=82) evolves up to ∼480 °C in mBMT, whereas it ceases by ∼100 °C when it is added directly to BMT without milling (Figure 2c). Dehydrogenation products of cyclohexene, including benzene (m/z = 78) and H2 (m/z = 2), evolve from mBMT; the evolution is over a wider temperature range than when cyclohexene is added directly to BMT (i.e., BMT þ CH, Figure 2a-b). Furthermore, mBMT evolves gases immediately upon loading, with a secondary peak at 75 °C (Figure 2a); whereas samples with added cyclohexene evolve mass immediately and with a secondary peak at 52-55 °C (for BMT þ CH and mBMT_dry þ CH, Figure 2a). The amount of mass evolved from 25 to 200 °C is 2.9% for BMT, 21.6% for mBMT, 10.3% for mBMT-dry þ CH, and 8.5% for BMT þ CH (Figure 2a), suggesting that although all samples have low-temperature evolution of volatiles, cyclohexene adds to the amount of volatiles, milling liberates certain volatiles, and milling in cyclohexene extracts even more low-temperature volatiles. Room temperature H2 evolution is somewhat unexpected and has been discussed in detail elsewhere.11 It decreases with the length of time between the milling and the TG-MS analysis. The low-temperature H2 evolution is unique to anthracites and is not observed for graphite milled under similar conditions.12 Additional volatiles for select samples are shown in Figure 2c: volatiles associated with hydrocarbons and water are increased for mBMT relative to BMT (i.e., m/z = 2, 18, 44), whereas volatiles associated with air (i.e., m/z= 28, 32) are greater for BMT than mBMT at low temperatures, suggesting these materials may have evolved during the milling process. There is a pronounced difference in char formation after milling: mBMT-HT oxidizes at slightly higher temperature than mBMT (Figure 1, Table 1), but the burnout temperature is increased more significantly (by 180 °C, Table 1). The emergence of a shoulder in the TGA profile of mBMT-HT is a sign of carbon restructuring and the formation of an
(23) Zhang, S. Y.; Lu, J. F.; Zhang, J. S.; Yue, G. X. Energ Fuel 2008, 22, 3213–3221. (24) Tomita, A.; Mahajan, O. P.; Walker, P. L. Fuel 1977, 56, 137–144. (25) Walker, P. L.; Seeley In Proceedings of the third conference on carbon Naugatuck, CT, 1959; Mrozowski, S., Studebaker, M. L., Walker, P. L., Eds.; Pergamon: Naugatuck, CT, 1959; p 481. (26) Shen, T. D.; Ge, W. Q.; Wang, K. Y.; Quan, M. X.; Wang, J. T.; Wei, W. D.; Koch, C. C. Nanostruct Mater 1996, 7, 393–399. (27) Tang, J.; Zhao, W.; Li, L.; Falster, A. U.; Simmons, W. B.; Zhou, W. L.; Ikuhara, Y.; Zhang, J. H. J. Mater. Res. 1996, 11, 733–738. (28) Fukunaga, T.; Nagano, K.; Mizutani, U.; Wakayama, H.; Fukushima, Y. J. Non-Cryst. Solids 1998, 232, 416–420.
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oxidative resistant fraction with anneal. The shift in the TGA profile with anneal for the milled material (mBMT vs mBMT-HT) is much less pronounced than that for the coal precursor and its corresponding char (i.e., BMT vs BMT-HT). Quite notably, the char of the milled material (mBMT-HT) has a lower oxidation temperature than the original BMT coal. For the BMT series, Tmax trends as follows: mBMT < mBMT-HT < BMT , BMT-HT (Table 1). A similar effect was seen for milled graphite (without solvent) after 80 h of milling, followed by treatment at 2800 °C: annealing the milled product did not lead to reordering to form the graphite structure.29 Light scattering demonstrates milling leads to a ∼100-fold reduction in particle size.30 The reduced particle size may be partly responsible for the increased reactivity of the milled chars.31 However, cyclohexene addition also plays a role in char formation, as will be discussed separately. mBMT-HT was significantly more ordered than BMT-HT (dc and Lc, Table 1). TEM (reported elsewhere15,32) indicates mBMTHT has a very different morphology, with a significant fraction of nanographene ribbons and multiwalled nanopolyhedral particles. The aspect ratio of these structures would lead to increased reactivity. These carbon structures may have been due in part to carbon deposition, particularly in the presence of introduced metal and with the introduced volatile matter that made up 31-37% of the sample. Similarly, the mDBMT-HT char has a higher oxidation temperature than mDBMT. For the DBMT series, Tmax trends as follows: mDBMT < DBMT < mDBMT-HT , DBMT-HT (Figure 1b, Table 1). Unlike the BMT series, mDBMT-HT has a lower Tb than DBMT. Thus, the milled char is also relatively reactive, but the effect is less pronounced than for the BMT series. The TGA of mDBMT-HT is narrow (compared, e.g., to mBMT-HT) and is indicative of a homogeneous carbon structure. Significant metal is introduced with milling (30.7 wt % ash for mBMT, Table 1), as shown both here and in previous studies. The metal is both zerovalent and in the form of carbides (Figure 3), with an estimated metal content (of the ordered fraction in XRD) of 44%. Milling DBMT also introduces substantial (i.e., 40.2%) TGA ash content and is made up of both metals and metal carbides (Table 1, Figure 3). The ash content of mDBMT is ∼10% greater than mBMT, despite the lack of inherent mineral matter in mDBMT. DBMT may be more prone to break down the milling materials, due to either residual acid from the demineralization process or due to the opening up of pores with demineralization (as was seen in TGA of BMT relative to DBMT). Metal introduced from ball milling can be easily removed with a subsequent demineralization process: mBMT-A has 1.5 wt % ash; mDMBT-A has 0.7 wt % ash. mBMT-A oxidizes at a low temperature, with Tb of 505 °C; Tb for mDMBT-A is 475 °C. mBMT-A has a more pronounced
secondary shoulder than mDBMT-A, but in each case the biphasic profile suggests the acid treatment changes the carbon structure. We have discussed this observation previously based on TGA, XRD, and TEM,15 as well as multiwavelength Raman spectroscopy.17 The role of mineral matter in carbon transformations during the milling process cannot be ruled out, but the similarity of mBMT-A and mDBMT-A in TGA suggests virtually equivalent gasification behavior and further suggests demineralization prior to milling adds an extra processing step while increasing potential attrition of the milling materials. In one series of experiments, additional stainless steel (SS) and Fe were added to DBMT in an attempt to “emphasize” the effect of metals on transformations that occur during milling. Perhaps not surprisingly, free Fe powder is much more reactive in the mill than stainless steel;either from break down or direct addition of SS powder. For both SS and Fe addition, the added metal powder lowered Tonset relative to mDBMT; in the case of Fe addition, the decrease in Tonset was significant and accompanied by a decrease in Tmax. The decrease in Tonset is a sign of the catalytic effect of added metal. For SS addition, Tmax of the milled product was shifted to higher temperatures relative to DBMT, suggesting perhaps the stainless steel powder alters the physics of milling. For both SS and Fe addition, the majority of added metal powder can easily be removed by the A demineralization process. No significant benefits in metal addition were seen. The TGA of BMT milled without cyclohexene (mBMTdry, Figure 1e) more closely resembles mBMT than the BMT precursor. The oxidation temperature of mBMT-dry is ∼40 °C higher than mBMT (Table 1, Figure 1e). The volatile matter is increased after milling in the absence of a solvent, particularly at high temperature: the overall mass loss of mBMT-dry in Ar is almost 40%, compared to 8% for BMT (Figure 2a), with the majority of mass loss for mBMTdry occurring at ∼700 °C and above. In MS, increases are seen in the evolution of several volatiles (i.e., m/z = 16, 28, 32 and 44, Figure 2c) at high temperatures. The combined TG-MS suggests milling leads to crushing of organic macromolecules in the coal to form volatile matter, and this volatile matter is retained in the coal structure unless cyclohexene is added to liberate the associated volatile matter. The similarity of mBMT to mBMT-dry in TGA suggests that devolatilization and desorption of added cyclohexene plays little role in subsequent oxidation. TG-MS further suggests the majority of cyclohexene has evolved before the onset of oxidation at 340 °C (Figure 2a-b), although TGA and TG-MS are not directly comparable due to swelling induced by oxygen chemisorption, as discussed above. Comparing the evolution of other volatiles (Figure 2c) shows little similarity between mBMTdry and mBMT: obviously, cyclohexene, benzene, and hydrogen do not evolve from mBMT-dry. Comparing the chars, the mBMTdry-HT char oxidizes at significantly higher temperatures than the mBMT-HT char (Figure 1e). mBMT-dry-HT is more ordered than BMT-HT but is less ordered than mBMT-HT (Lc and dc, Table 1), suggesting that milling facilitates carbon restructuring at high temperature and that cyclohexene further participates in this restructuring. The TGA profiles follow the reverse trends of the XRD ordering, suggesting the oxidation is inhibited by gascarbon interface rather than carbon reactivity. The BMTdry series may resemble traditional grinding. The dry-milled material has a decrease in oxidation temperature due to
(29) Salver-Disma, F.; Tarascon, J. M.; Clinard, C.; Rouzaud, J. N. Carbon 1999, 37, 1941–1959. (30) Narayanan, D. Exploratory Study of Exfoliated Graphite Nanofibers and Milled Antrhacite-Metal Composites for Hydrogen Storage; MS Thesis, The Pennsylvania State University: University Park, PA, USA, 2006. (31) Sahu, R.; Levendis, Y. A.; Flagan, R. C.; Gavalas, G. R. Fuel 1988, 67, 275–283. (32) Burgess-Clifford, C. E.; Narayanan, D. L.; Van Essendelft, D. T.; Jain, P.; Sakti, A.; Lueking, A. D. Fuel Process. Technol. 2009, Submitted.
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: DOI:10.1021/ef900390e
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particle size reduction, metal introduction, and increased volatile matter due to crushing, yet a significant portion of the dry milled material still forms an oxidative resistant char at high temperature. The addition of an organic lubricant is thought to decrease the intensity of collisions and prevent aggregation of graphite particles.33 Cyclohexene may undergo various reactions with the coal structure, including possible Diels-Alder cyclo-addition reactions and/or radical reactions. These reactions strongly associate cyclohexene, and its derivatives, with the coal structure, enabling them to participate in reordering reactions during anneal and pyrolysis. Cyclohexene that is not strongly associated with the coal structure would volatilize during anneal and pyrolysis, and less effect on the carbon restructuring would be expected.
added solvent, this was attributable to a decrease in particle size, introduction of metal from the milling materials, and an increase in the volatile matter of the coal due to crushing. With added solvent, a larger effect was seen due to the added effect of introduced volatile matter and reactions that occurred between the coal and cyclohexene during milling. Most significantly, the char of the anthracite that had been reactively ball milled in cyclohexene maintained a low oxidation temperature and was thus susceptible to oxidation even after a high temperature anneal. This observation has the most potential significance to anthracite pretreatment steps used in industrial applications. The similarity of the ball milling process to current methods for coal pulverization and/or preparation for gasification suggest solvent addition may substantially lower oxidation temperatures. However, for practical applications, the effect of ball milling time, and use of various solvents and solvent-to-coal ratios need to be explored. Previously, beneficial effects of ball milling on oxidation have been attributed, largely, to particle size reduction and increased surface area.13 Here, we show that changes in carbon structure and metal addition may also affect oxidation behavior. Although metals introduced during milling may be easily removed with a demineralization process, the cost and environmental benefits of this additional step should be considered. Most significantly, milling in cyclohexene (and perhaps other organic solvents) inhibits char formation and substantially decreases burnout temperature of the char.
Summary and Conclusions The effect of ball milling, annealing, and demineralization (both before and after ball milling) on the oxidative reactivity of an anthracite coal have been discussed. As in previous studies, the annealed char is intended to be representative of the “oxidative resistant” fraction of a coal that is formed during more realistic oxidation conditions; and expectedly, annealing led to a large decrease in oxidative reactivity due to increased carbon order and decreased volatile matter. The effect of demineralization is generally coal-specific, dependent upon the relative location of the mineral matter, carbon fraction, and the catalytic activity of the former. Here, a slight increase in reactivity with demineralization suggested the mineral matter decreased the gas-carbon interface, and the inherent mineral matter played little to no catalytic effect; a similar effect was seen upon demineralization after milling (for samples without added metal). Demineralization after milling also led to a decrease in the temperature at which the oxidation rate was a maximum, suggesting the treatment further increased the reactivity of the ball milled coal, consistent with a previous report;15 this increased reactivity is a secondary effect of ball milling. The most significant effect was thus the increased reactivity due to ball milling. Without
Acknowledgment. The authors thank Drs. Caroline Clifford, Humberto Rodriguez-Guiterrez, and Jonathan Mathews for useful discussions in the preparation of this paper. The work was funded by the Consortium for Premium Carbon Products from Coal (DEFC2603NT41874, Internal Agreement No. 2875TPSU-DOE-1874), with partial support from PSU’s Energy Institute and Material Research Institute. Supporting Information Available: (1) Alternate data presentation of TPO figures, including mass profiles. (2) Oxidation of iron carbides and effect on TGA ash calculation. (3) Quantitative XRD phase identification. (4) Additional TG-MS figures. This information is available free of charge via the Internet at http://pubs.acs.org/.
(33) Janot, R.; Guerard, D. Carbon 2002, 40, 2887–2896.
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