Energy Fuels 2009, 23, 4774–4781 Published on Web 07/02/2009
: DOI:10.1021/ef900014g
Effects of Metal Cations Present Naturally in Coal on the Fate of Coal-Bound Nitrogen in the Fixed-Bed Pyrolysis of 25 Coals with Different Ranks: Correlation between Inherent Fe Cations and N2 Formation from Low-Rank Coals† Yasuo Ohtsuka* and Zhiheng Wu‡ Research Center of Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan. ‡Current address: Fuel and Energy Centre, School of Chemical, Environmental and Mining Engineering, Nottingham University, Nottingham NG7 2RD, U.K. Received January 9, 2009. Revised Manuscript Received May 12, 2009
The fate of coal-N in the fixed-bed pyrolysis of 25 coals with 62-81 wt % (daf) C has been studied with a quartz reactor at 1000 °C under ambient pressure to examine the effects of metal cations present naturally in these coals on the partitioning of coal-N into N2, NH3, HCN, tar-N, and char-N. Nitrogen mass balances for all runs fall within the reasonable range of 100 ( 5%, and N2 is the predominant product for all of the coals. As the N2 yield increases, the sum of NH3, HCN, and tar-N is unchanged significantly, whereas the char-N yield decreases almost linearly, showing that most of N2 originates from char-N. When eight kinds of inherent metals, such as Na, Mg, Al, Si, K, Ca, Fe, and Ti, are determined by the conventional method and related with the N2 yield, there exists a strong, direct correlation between the Fe content and N2 formation for low-rank coals with less than 75 wt % (daf) C. Transmission electron microscopy coupled with an energy-dispersive analysis of X-rays (TEM-EDAX) measurements after pyrolysis at 1000 °C of a German brown coal, which provides the highest N2 yield of about 60%, reveal the existence of lamella structures because of graphitized carbon as well as nanoscale Fe particles with different sizes and shapes. The mechanism for conversion reactions of char-N to N2 is discussed in terms of the catalysis by nanoparticles of metallic Fe formed from inherent Fe cations.
efforts to understand the chemistry of formation reactions of volatile-N have been made thus far, according to several reviews.6-10 Nevertheless, further elucidation may be needed to realize near-zero emissions in coal combustion technologies in the future. The present authors’ research group has been working on the catalysis of the pyrolysis of coal-N by Fe and Ca cations for the main purpose of converting the precursors of NOx and N2O to N2, because the efficient conversion to N2 may lead to significant reduction of such pollutants. We have found that Fe cations, which are added to Australian low-rank coals by the precipitation method, increase N2 yield drastically in the fluidized-bed pyrolysis at temperatures of more than 700 °C.11-13 The detailed characterization of Fe catalysts and pyrolyzed chars has demonstrated that nanoscale particles of metallic Fe catalyze predominantly the formation of N2 from char-N through solid-solid interactions.14 It has also been shown that Ca cations, which are incorporated into acid-washed low-rank coals by the ion-exchange method, are effective for enhancing N2 yield in the pyrolysis at high temperatures of more than 1000 °C.15,16
Introduction It has been well-accepted that most of the total NOx emitted in pulverized coal-fired power plants originates from the nitrogen in coal, expressed as coal-N throughout this paper, and NOx and N2O emissions in fluidized-bed coal combustion at temperatures of less than 1000 °C arise exclusively from coal-N alone.1-5 Because the partitioning of coal-N into the nitrogen in volatile matters (tar, HCN, and NH3) and char, denoted as volatile-N and char-N, respectively, during coal pyrolysis is one of the crucial factors controlling NOx emissions in the subsequent combustion process, the fate of coal-N in the pyrolysis step has been studied extensively, and many † Progress in Coal-Based Energy and Fuel Production. *To whom correspondence should be addressed. Telephone: þ81-22217-5653. Fax: þ81-22-217-5655. E-mail:
[email protected]. jp. (1) Hjalmarsson, A. K. NOx Control Technologies for Coal Combustion; IEA Coal Research: London, U.K., 1990; IEACR/24. (2) Unsworth, J. F.; Barratt, D. J.; Roberts, P. T. Coal quality and combustion performance. Coal Science and Technology; Elsevier: Amsterdam, The Netherlands, 1991; Vol. 19, p 579. (3) Boardman, R.; Smoot, L. D. Fundamentals of coal combustion for clean and efficient use. Coal Science and Technology; Elsevier: Amsterdam, The Netherlands, 1993; Vol. 20, p 433. (4) W ojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1. (5) Takeshita, M.; Sloss, L. L.; Smith, L. M. N2O Emissions from Coal Use; IEA Coal Research: London, U.K., 1993; IEAPER/06. (6) Davidson, R. M. Nitrogen in Coal; IEA Coal Research: London, U.K., 1994; IEAPER/08. (7) Leppalahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1. (8) Johnsson, J. E. Fuel 1994, 73, 1398. (9) Li, C.-Z. In Advances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Amsterdam, The Netherlands, 2004; pp 286-359.
r 2009 American Chemical Society
(10) Tsubouchi, N.; Ohtsuka, Y. Fuel Process. Technol. 2008, 89, 379. (11) Ohtsuka, Y.; Mori, H.; Asami, K. Energy Fuels 1993, 7, 1095. (12) Ohtsuka, Y.; Mori, H.; Watanabe, T.; Asami, K. Fuel 1994, 73, 1093. (13) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 1022. (14) Ohtsuka, Y.; Watanabe, T.; Asami, K.; Mori, H. Energy Fuels 1998, 12, 1356. (15) Tsubouchi, N.; Ohtsuka, Y. Fuel 2002, 81, 2335. (16) Tsubouchi, N.; Abe, M.; Xu, C.; Ohtsuka, Y. Energy Fuels 2003, 17, 940.
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Table 1. Ultimate and Proximate Analyses of 25 Coals ultimate analysis (wt %) (daf) coal Uvdughdag Rhein Braun-1 Loy Yang Morewell Rhein Braun-2 Aban Shivee Ovoo Borodino Bienfait Wyoming Breau Adaro Zalainuoer Pasir Taiheiyo Obed Mountain Illinois No.6 Coal Valley Lower Lausitz Puertollano Upper Freeport Leopold Blair Athol Liddell Hunter Valley
countrya MON GER AUS AUS GER RUS MON RUS CAN USA IND IND CHI IND JPN CAN USA CAN POL SPA USA GER AUS AUS AUS
proximate analysis (wt %) (db)
C
H
N
S
O
ash
VM
FC
61.8 64.4 65.9 65.9 67.3 68.3 68.6 69.1 69.2 69.9 70.4 70.6 72.0 72.1 73.9 74.0 76.5 76.8 78.3 78.3 78.8 79.7 80.7 81.1 81.2
4.4 4.8 4.1 4.9 5.1 4.7 4.5 4.8 4.4 4.9 5.1 5.0 5.0 5.2 6.1 5.3 5.3 4.8 4.9 5.0 5.4 5.2 4.5 5.4 5.5
0.94 0.74 0.68 0.66 1.0 1.1 0.92 1.1 1.3 1.1 1.7 1.3 1.7 1.4 1.4 1.8 1.6 1.1 1.6 2.2 1.8 1.6 2.0 2.1 2.2
4.0 0.3 0.3 0.2 0.6 0.6 0.8 0.6 0.6 0.5 0.6 0.1 0.4 0.2 0.3 0.6 3.5 0.3 1.4 1.2 1.9 1.3 0.3 0.6 0.8
28.9 29.8 29.0 28.3 26.1 25.3 25.3 24.4 24.5 23.6 22.2 22.9 20.9 21.1 18.3 18.3 13.1 17.0 13.8 13.3 12.1 12.2 12.5 10.8 10.3
13.2 4.7 0.6 2.0 4.1 5.3 8.6 7.0 16.5 5.4 3.1 1.5 4.1 4.1 14.0 12.6 9.0 10.5 10.8 9.4 8.8 3.4 7.3 8.3 8.2
42.0 54.4 54.2 54.4 56.9 44.6 41.4 43.1 37.4 45.0 45.6 45.7 42.5 45.2 44.2 40.7 36.5 35.4 34.7 32.3 36.2 35.7 31.2 33.9 33.6
44.8 40.9 45.2 43.6 39.0 50.1 50.0 49.9 46.1 49.6 51.3 52.8 53.4 50.7 41.8 46.7 54.5 54.1 54.5 58.3 55.0 60.9 61.5 57.8 58.0
a MON, Mongolia; GER, Germany; AUS, Australia; RUS, Russia; CAN, Canada; USA, United States of America; IND, Indonesia; CHI, China; JPN, Japan; POL, Poland; SPA, Spain.
Furthermore, we have reported that N2 is the dominant species among all of N-containing products evolved in the fixed-bed pyrolysis at 1000 °C of several raw coals without any catalysts added17 and that acid washing of low-rank coals decreases N2 yield considerably and also changes the proportion of tar-N, HCN, and NH3.18 These observations indicate that naturally present inorganic components can change the partitioning of coal-N to N2, volatile-N, and char-N upon pyrolysis. Although it has been well-documented that alkali, alkaline earth, and transition-metal cations, which are included inherently in low-rank coals, affect the distribution of pyrolysis products and promote the gasification of pyrolyzed chars, there have been no detailed studies on the effects of these cations on the pyrolytic behavior of coal-N. In this paper, therefore, we aim at examining this point in more detail, particularly making clear the correlation between inherent metal cations and N2 formation, in the fixed-bed pyrolysis at 1000 °C using a number of coals with different carbon, nitrogen, and metal contents.
(>99.9999%) was then passed over the sample until the concentration of N2 in the exit gas of the reactor was less than 30 ppm. Such prudent precautions against leakage were essentially needed for precisely determining N2 formed from coal-N. Finally, the reactor was heated electrically with an infrared image furnace in a stream of high-purity He at a rate of 400 °C/min, held at 1000 °C for 2 min, and quenched to room temperature.17 Nitrogen Analysis. Pyrolysis products were separated to recover as gas, tar, and char, according to the method mentioned in our previous work.17 Gaseous compounds were collected with an Al-foil laminated sampling bag; N2 in it was determined with a micro gas chromatograph; and HCN and NH3 were analyzed with Fourier transform infrared spectroscopy (FTIR) equipped with a long path gas cell. The nitrogen in the tar and char recovered as condensed materials, denoted as tar-N and char-N, were determined with separate combustiontype analyzers. The analytical procedure has been reported in more detail elsewhere.17,18 The yield of N2, NH3, HCN, tar-N, or char-N is expressed in percent of total nitrogen in feed coal. Nitrogen mass balances for all runs fell within the reasonable range of 100 ( 5%, which indicates that all of the analytical methods used for nitrogen determination are reliable. Metal Determination. Every coal was first burned up at 815 °C to obtain high-temperature ash. About 50 mg of the ash was then dissolved completely with a mixed aqueous solution of aqua regia and HF in a Teflon Parr bomb held at 115 °C. Eight kinds of metal cations, such as Na, Mg, Al, Si, K, Ca, Fe, and Ti, in the solution recovered were finally analyzed by the inductively coupled plasma (ICP) method.17 When it is assumed that these seven elements except Ca are present in the ash as the oxide forms and that the Ca in the ash derived from a low- or highsulfur coal is in the form of CaO or CaSO4, respectively, ash contents calculated for 25 coals according to this assumption fell within 100 ( 5% of those (Table 1) measured actually by the proximate analysis, showing that the above-mentioned metal determination method is acceptable. It should be noted that the method measures the total amount of each metal element, regardless of whether it was present naturally as the cations or not. For example, Ca and Fe elements of low-rank coals usually
Experimental Section Coal Sample. A total of 25 coals from 11 countries were used in the present study. The as-received sample was air-dried, ground, and sieved to coal particles with a size fraction of 150-250 μm. The ultimate and proximate analyses are provided in Table 1, which reveals that carbon and nitrogen contents of all coals are in the range of 62-81 and 0.66-2.2 wt % (daf), respectively. Pyrolysis. Pyrolysis was carried out with a horizontal, fixedbed quartz reactor at ambient pressure. The apparatus and procedure have been reported in details elsewhere17,18 and are thus simply explained below. About 0.5 g of the sample after dryness at 110 °C in inert gas was first charged into a graphite cell in the center of the reactor. After evacuation, high-purity He (17) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477. (18) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902.
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Figure 3. Relationship between the N2 yield and the char-N or volatile-N yield, which is equal to the sum of tar-N, HCN, and NH3, during coal pyrolysis at 1000 °C.
Figure 1. Yield of N2, NH3, or HCN against the carbon content of feed coal in the fixed-bed pyrolysis at 1000 °C.
many cases. It can therefore be summarized that there is a yield sequence of HCN < NH3 < N2 with many coals. Figure 2 shows yields of tar-N and char-N at 1000 °C as a function of % C. The former yield was in the range of 4-17%, and no distinct relationship between % C and tarN was observed. The comparison of Figures 1 and 2 reveals that tar-N yield was smaller than the sum of NH3 and HCN yields in many coals and thus suggests that secondary decomposition reactions of tar-N take place to a large extent, because tar-N is a primary pyrolysis product under the rapid heating rate conditions that can minimize the decomposition.10,19-21 As seen in Figure 2, the char-N yield tended to increase with an increasing % C, in contrast with the trend observed with N2 yield against % C in Figure 1. Although one might expect that N2 is formed by the secondary decomposition of volatile-N, such as tar-N, HCN, and NH3, the contrasting dependency of N2 and char-N on the coal rank shown in Figures 1 and 2 strongly suggests that N2 originates from char-N (and/or the precursors). To confirm that char-N is the source of N2, the yield of char-N or volatile-N (that is, the sum of tar-N, HCN, and NH3) was plotted as a function of the N2 yield using all of the data provided in Figures 1 and 2. The results are shown in Figure 3. As N2 increased, char-N decreased remarkably and almost linearly, whereas volatile-N was almost unchanged or decreased slightly. Such a strong, reverse correlation between N2 and char-N supports the conclusion that N2 arises not from tar-N, HCN, and NH3 but predominantly from char-N. Correlation between N2 Formation and Inherent Metal Cations. The determination of eight kinds of metals in all of the coals used revealed that Si and Al were the major elements for relatively high-rank coals, whereas Mg, Ca, and Fe were more significant with brown coals and lignites. Because Fe cations externally added to Australian low-rank coals promoted N2 formation considerably in the fluidizedbed pyrolysis at 750-1000 °C,11-13 Fe-containing inorganic components naturally present in coal may be related to N2 formation under the present conditions. Thus, all of N2 data observed in Figure 1 were plotted against the total content of
Figure 2. Yield of tar-N or char-N against the carbon content of feed coal in the fixed-bed pyrolysis at 1000 °C.
exist not only as ion-exchanged cations but also as discrete minerals. Char Characterization. The X-ray diffraction (XRD) measurements of several chars recovered after pyrolysis at 1000 °C were made with a conventional X-ray diffractmeter using Nifiltered Cu KR radiation. The observations with transmission electron microscopy coupled with an energy-dispersive X-ray analyzer (TEM-EDX) were also carried out with a highresolution TEM, with the acceleration voltage being 200 kV.
Results Nitrogen Distribution. Yields of N2, NH3, and HCN at 1000 °C against carbon content (% C) in feed coal on a dry, ash-free basis are shown in Figure 1, where several data reported elsewhere17 are plotted again for reference. Among these compounds, N2 was the predominant species with all of coal samples examined, and the yield was larger at a lower % C, as indicated in our earlier work using 16 coals,17 although the scattering of some data was observed. More than one fourth of the total nitrogen was converted to N2 for almost all of the low-rank coals with % C of less than 75 wt % (daf), and the highest yield of about 60% was observed with a German brown coal. On the other hand, NH3 yield was always less than 15% and larger than HCN in
(19) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749. (20) Solomon, P. R.; Hamblen, D. G.; Garangelo, R. M.; Krause, J. L. 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1139. (21) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264.
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Figure 7. Yield of tar-N as a function of the inherent Ca content for low-rank coals with carbon contents of less than 75 wt % (daf).
Figure 4. Yield of N2 as a function of the inherent Fe content of feed coal in the fixed-bed pyrolysis of 25 coals at 1000 °C.
Figure 8. Yield of HCN or NH3 as a function of the inherent Ca content for low-rank coals with carbon contents of less than 75 wt % (daf).
Figure 5. Correlation between inherent Fe content and N2 yield for low-rank coals with carbon contents of less than 75 wt % (daf).
Figure 6. Yield of N2 against the total content of inherent Fe and Ca for low-rank coals with carbon contents of less than 75 wt % (daf).
catalytically active for N2 formation; the Fe as ion-exchanged cations can catalyze the reaction, whereas discrete Fe-containing minerals, such as pyrite (FeS2) and siderite (FeCO3), are catalytically inactive. It has been reported that Fe cations in Australian and German brown coals are dominantly exchanged with the protons in COOH and phenolic OH groups22,23 and that the ion-exchanged Fe is readily transformed upon pyrolysis into nanoscale particles of metallic Fe.24 Further, we have indicated that nano-order Fe metal catalysts exhibit high activity in the conversion of char-N to N2.13,14 Thus, N2 yields and Fe contents for low-rank coals alone with % C of e75 wt % (daf) were selected from all of the data given in Figure 1, and both of them were plotted again in Figure 5. As the content increased from 0.03 to 0.7 wt % Fe, the yield also increased from 13 to about 60%. In other words, there existed a strong, direct correlation between the two. A German or Australian brown coal with 0.56 or 0.03 wt % Fe provided the highest or lowest N2 yield of 61 or 13%, respectively. Although some data are scattered in Figure 5,
inherent Fe metals in 25 coals. The results are illustrated in Figure 4. It was difficult to deduce any distinct correlation between the two because of the considerable data scattering. This suggests that all of the Fe may not necessarily be
(22) Schafer, H. N. S. Fuel 1977, 56, 45. (23) Huttinger, K. J.; Michende, A. T. Fuel 1987, 66, 1165. (24) Xu, C.; Tsubouchi, N.; Hashimoto, H.; Ohtsuka, Y. Fuel 2005, 84, 1957.
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Figure 9. (a) TEM picture and (b) EDX profile after pyrolysis at 1000 °C of a German brown coal showing the highest N2 yield. x in b denotes signals from the TEM grid.
the main reason may be that all of the Fe cations present in low-rank coals examined are not in the ion-exchanged forms. We have shown that, when an Indonesian lignite with 0.2 wt % Ca exchanged was first acid-washed to remove the Ca and 0.5 wt % Ca was incorporated into the washed lignite by the ion-exchange method, these two coal samples provide almost the same N2 yields in the pyrolysis at 1200 °C.15 The similar catalytic effects have been reported in slow-heating rate pyrolysis of sub-bituminous coals after acid washing and subsequent Ca addition.25,26 These observations suggest that Ca cations present naturally in low-rank coals may also promote N2 formation. Thus, N2 yield given in Figure 1 was plotted against the total content of inherent Fe and Ca metals for 15 coals with % C of e75 wt % (daf). The result is illustrated in Figure 6. The good correlation observed in Figure 5 was not improved, but the extent of data scattering was rather larger. These observations show that the catalysis by inherent Fe cations accounts mainly for N2 formation from low-rank coals under the present conditions of a heating rate of 400 °C/min and 2 min soaking at 1000 °C.
Effects of Metal Cations on Yields of Tar-N, HCN, and NH3. Acid washing of low-rank coals increased tar-N and HCN in the fixed-bed pyrolysis at 1000 °C but decreased NH3, and Ca ions exchanged with the washed coals provided the nearly reverse effect.10,18 Inherent Ca content might thus be one of the key factors controlling the partitioning of coal-N to tar-N, HCN, and NH3. Figure 7 shows the effect of the Ca on the formation of tar-N from low-rank coals with % C of e75 wt % (daf). As the Ca content increased up to 2 wt % (dry), tar-N tended to decrease. When the HCN or NH3 yield was plotted in place of tar-N, as seen in Figure 8, one might indicate that, as the Ca content was larger, HCN decreased gradually, whereas NH3 increased significantly. Although several data were scattered remarkably in Figures 7 and 8, this can be expected, because Ca species in low-rank coals exist not only in the ion-exchanged forms but also as discrete Cacontaining minerals, such as calcite (CaCO3) and dolomite [CaMg(CO3)2], which are much less effective for changing the fate of volatile-N than the ion-exchanged Ca.10,26 It is thus possible that ion-exchanged Ca cations naturally present in low-rank coals may control the proportion of tar-N, HCN, and NH3.
(25) Tsubouchi, N.; Ohshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158. (26) Tsubouchi, N.; Ohtsuka, Y. Fuel 2002, 81, 1423.
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Figure 10. (a) TEM picture and (b) EDX profile of a smaller particle observed in Figure 9a. x in b is the same as in Figure 9b.
TEM-EDX Analysis of Brown Coal Char. Because a German brown coal showed the highest N2 yield among 25 coals examined, the TEM-EDX measurements of the pyrolyzed char were made. The TEM picture of the char and the EDX profile of the whole field of the picture are provided in panels a and b of Figure 9, respectively. Figure 9a showed the formation of several particles with the nanosizes of 10-20 nm, and Figure 9b revealed the presence of many elements, such as C, N, O, Mg, Al, Si, S, Ca, and Fe, in this field. Among these elements, C provided the strongest intensity, followed by O, Mg, Ca, and Fe. The dominance of Mg, Ca, and Fe over other Na, K, and Al corresponded to the ICP analysis of the leaching solution of high-temperature ash prepared from this coal. Part of Figure 9a was magnified, and the EDX measurements were carried out. The two TEM pictures are shown in Figures 10a and 11a, and the corresponding EDX profiles are provided in Figures 10b and 11b, where the center of each particle observed in Figures 10a and 11a is analyzed by the EDX method. A nanoscale particle with the size of approximately 10 nm existed in Figure 10a, whereas another particle observed in Figure 11a was much larger, of about 25 nm in size, and the shape was distorted. Figures 10a and 11a also indicated the existence of lamella structures of graphitized carbon, which was also detectable by the XRD measurement
of the brown coal char. On the other hand, no diffraction lines of this carbon could be detected with the char from the Australian brown coal that had the lowest N2 yield. As seen in Figures 10b and 11b, the EDX profiles of the two particles were very similar, and these results proved the predominance of C and Fe elements. Because C originates from the char substrate, these particles are probably composed of metallic Fe. The larger particle with the distorted shape observed in Figure 11a may be formed through the agglomeration of two or more nanoparticles of metallic Fe in the process of pyrolysis. Discussion When 25 coals with different ranks were heated at a rate of 400 °C/min in inert He and pyrolyzed for 2 min at 1000 °C, as seen in Figures 1 and 2, N2 showed the highest yield with all of the coals and was more remarkable for a lower rank coal with a smaller carbon content, although some exceptions were observed. In this section, therefore, the mechanism of N2 formation from low-rank coals is mainly discussed. It has been accepted in the pyrolytic behavior of coal-N that tar-N, which is composed of pyridinic and pyrrolic forms, is first released and then decomposed into HCN and NH3, and the extent of this secondary decomposition is larger as the 4779
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Figure 11. (a) TEM picture and (b) EDX profile of a larger particle observed in Figure 9a. x in b is the same as in Figure 9b.
severity of coal pyrolysis is higher.10,19-21,27,28 Because the tarN yield was lower than the sum of HCN and NH3 with many coals (Figures 1 and 2), it is likely that the decomposition of tar-N occurs under the present low-heating rate conditions. One might thus expect that volatile-N, such as tar-N, HCN, and NH3, is converted further to N2, but this possibility was very minor, because only the slight dependency of N2 on volatile-N was observed in Figure 3. On the other hand, Figure 3 clearly showed an almost linear decrease of char-N with an increase in N2. It is therefore evident that char-N (and/ or precursors) is the major source of N2. In earlier work, we have shown that nano-order metallic Fe catalysts, which are formed on low-rank coal chars13,14 and added to polyacrylonitrile,29,30 can promote the remarkable formation of N2 from N-containing heterocyclic structures through solid-phase reactions. As evidenced by Figure 5, the N2 yield observed for low-rank coals with e75 wt % (daf) C increased with an increasing total content of inherent
Fe cations. Thus, this direct correlation between the two suggests that nanoparticles of metallic Fe formed from Fe cations, which are present naturally in these coals, play a catalytic role in the conversion of char-N to N2. Figures 10 and 11 proved the presence of nanoscale Fe particles on the surface of the char derived from the German brown coal that provided the highest N2 yield among the 25 coals examined. Figures 10 and 11 also revealed the formation of graphitized carbon with lamella structures on the char surface, which means strong chemical interactions between metallic Fe and char substrate, because such Fe-enhanced graphitized reactions proceed through the dissolution of amorphous carbon into metallic Fe to form solid solution phases and/or Fe carbide species (FexC), and the subsequent precipitation of graphitized carbon from these precursors.31 According to binary Fe-N and Fe-C phase diagrams,32 nitrogen atoms can dissolve in metallic Fe more readily than carbon atoms. It is thus likely that the nitrogen in pyridinic and pyrrolic functional forms as char-N dissolves in nanoparticles of metallic Fe in the process of carbon graphitization to form interstitial Fe nitrides (FexN and/or FexCyN), as proposed in
(27) Kelly, M. D.; Nelson, P. F. Proceedings of the 5th Australian Coal Science Conference, Melbourne, Australia, 1992; p 161. (28) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710. (29) Watanabe, T.; Ohtsuka, Y.; Nishiyama, Y. Carbon 1994, 32, 329. (30) Ohtsuka, Y. J. Jpn. Pet. Inst. 1998, 41, 182.
(31) Oya, A.; March, H. J. Mater. Sci. 1982, 17, 309. (32) Binary Alloy Phase Diagrams, 2nd ed.; Massalski, T. B., Ed.; ASM International: Materials Park, OH, 1990; Vol. 2.
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Fe-enhanced formation of N2 observed in this work might appear in fluidized-bed coal combustion, because the particle size of 150-250 μm is appropriate and Fe cations added on low-rank coals can increase the N2 yield remarkably during the fluidized-bed pyrolysis.11-13
the Fe-catalyzed conversion of char-N to N2 observed in the fluidized-bed pyrolysis of an Australian brown coal with Fe cations added.13,14 These nitride species may subsequently decompose to evolve N2 at 1000 °C, because the transformation of nano-order Fe3N and Fe4N particles into N2 and metallic Fe takes place to a large extent at low temperatures of 500-600 °C.33 The nitrogen dissolution into metallic Fe may be the rate-determining step of the present Fe-catalyzed N2 formation. Because pyridinic and pyrrolic nitrogen forms are composed of single N atoms, there must be some migration process that can combine two Fe-N atoms to form N2 and metallic Fe, if the nitride intermediates are involved. As shown in Figures 10 and 11, the lamella structures existed even in the fields where any Fe particles were not observed and the relatively large Fe particle with the distorted shape appeared, respectively. These results strongly suggest that small Fe particles migrate in the char matrix and agglomerate. It is well-known that nanoscale metal particles are very mobile and quite reactive compared to the corresponding micrometer-sized ones. The atomic ratio of Fe/N in the German coal before pyrolysis was estimated to approximately 0.2, indicating that the number of Fe atoms in it was much lower than that of N atoms. Thus, the large mobility and high reactivity of Fe nanoparticles formed from naturally present Fe cations may account for high conversions of char-N to N2 observed with the Fe-rich low-rank coals. The results shown here may be irrelevant to actual pulverized coal-fired power plants because of the lower heating rate, longer residence time, and larger size of coal particles under the present conditions. In the pyrolysis of Chinese and Indonesian lignites at 1300 °C with a drop tube reactor, the N2 yield was smaller, whereas the char-N yield was larger, compared to those with the present fixed-bed reactor.34 The
Conclusions The atmospheric pyrolysis of 25 coals with 62-81 wt % (daf) C has been studied with a fixed-bed quartz reactor under the conditions of 400 °C/min, 1000 °C, and 2 min soaking, and the effects of inherent metal cations on the fate of coal-N, in particular N2 formation, have been investigated. The following conclusions are summarized: (1) Among N2, NH3, HCN, and tar-N evolved, N2 is the main product with all of the coals examined, tends to be larger at a lower coal rank, and increases almost linearly as char-N decreases, which means that char-N is the major source of N2. (2) A direct correlation between N2 yield (13-61%) and the total content of naturally present Fe ions (0.03-0.7 wt %) exists with low-rank coals with less than 75 wt % (daf) C, indicating that ion-exchanged Fe cations in these coals play a crucial role in N2 formation. (3) The TEM-EDAX analysis reveals the presence of not only nanoscale Fe particles but also graphitized carbon with lamella structures on the surface of the char derived from a German brown coal giving the highest N2 yield. It is likely that the conversion of char-N to N2 takes place through strong interactions between nanoparticles of metallic Fe and N-containing heterocyclic forms in the char. Acknowledgment. The present study was supported in part by the Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Program of the Center for Coal Utilization, Japan (CCUJ), commissioned by the New Energy and Industrial Development Organization (NEDO) with the subsidy of the Ministry of Economy, Trade, and Industry (METI) of Japan.
(33) Tsubouchi, N.; Hashimoto, H.; Ohtsuka, Y. Catal. Lett. 2005, 105, 203. (34) Tsubouchi, N.; Ohtsuka, Y. Energy Fuels 2003, 17, 940.
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