In-Situ High-Temperature Magnetic Resonance Imaging of Coals

Energy & Fuels 2002, 16, 575-585

575

In-Situ High-Temperature Magnetic Resonance Imaging of Coals Using Prepared Magnetization SPRITE Techniques Koji Saito,*,† Ken-ichi Hasegawa,‡ Ikuo Komaki,§ and Kenji Katoh| Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu City, 293-8511, Japan, JEOL Ltd., Tachikawa, Tokyo 190-0012, Faculty of Environmental Engineering, University of Kita-Kyushu, Kita-Kyushu, Japan, and Steel Research Laboratories, Fattsu, Japan Received June 15, 2001. Revised Manuscript Received October 17, 2001

In this paper, we are at first demonstrating 3D-SPRITE which has been shown to be successful for studying short relaxation time systems and which is free from distortions due to susceptibility variations for coals. The results obtained were discussed in relation to the three-dimensional distribution of mobile components for two kinds of coal. At the same time, inversion recovery preparation experiments, for example, T1 mapping and T2* mapping based on SPRITE methods are presented in order to clarify the chemical heterogeneity of coals. Also, we have carried out the first systematic in-situ variable-temperature NMR microimaging study of coals between 25 and 600 °C with our newly developed high-temperature microimaging probe and systems in order to clarify the behavior of mobile components at high temperatures in heterogeneous coal specimens. Both the mechanism of softening and melting process for coals and the coal mapping for the softening and melting properties using in-situ imaging experiments which are based on the local information are proposed.

1. Introduction Thermally induced changes in coals are of interest from the standpoints of both fundamental and applied research for the steel-making process.1 Furthermore, for very inhomogeneous coals, there is a fascination in studying about the influence of thermal dynamical changes. To monitor the dynamical changes in coals with a change in temperature, an in-situ method must be used, because it is well-known that the properties of coals change dramatically in a high-temperature range (from 350 to 550 °C).2-12 However, the main problem with standard empirical tests, such as the Gieseler plastometer and Audibert-Arnudilatometer to study the properties of coals change, is that they have no relation* Author to whom correspondence should be addressed at Nippon Steel Corporation, Advanced Technology Research Laboratories, 20-1 Shintomi, Futtsu City, 293-8511, Japan. Tel: +81-439-80-2270. Fax: +81-439-80-2746. E-mail: [email protected]. † Advanced Technology Research Laboratories, Nippon Steel Corporation. ‡ JEOL Ltd. § University of Kita-Kyushu. | Steel Research Laboratories, Nippon Steel Corporation. (1) Dryden, I. G. C.; Pankhurst, K. S. Fuel 1955, 34, 363. (2) Hager, J. J. Soc. Chem. Ind. 1914, 33, 389. (3) Rillingworth, S. Fuel 1922, 1, 213. (4) Dryden, I. G. C.; Pankhurst, K. S. Fuel 1955, 34, 363. (5) Fitzgerald, D. Fuel 1956, 35, 178. (6) van Krevelen, D. W.; Hntjensm, F. J.; Dormans, H. N. M. Fuel 1956, 35, 462. (7) Chermain, H. A. G.; van Krevelen, D. W. Fuel 1957, 36, 85. (8) Fitzgerald, D.; van Krevelen, D. W. Fuel 1959, 38, 17. (9) Dryden, I. G.; Joy, W. K. Fuel 1961, 40, 473. (10) Ouchi, K. Fuel 1961, 40, 485. (11) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 17. (12) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 41.

ship with the actual structural changes which have occurred. High-temperature 1H NMR is a powerful technique for the in-situ investigation of molecular motions in coals during carbonization. The 1H NMR spectra obtained for coals basically comprise contributions from a mobile and an immobile (rigid) component.13-15 The technique was shown by Sanada et al.14 in the late 1970s to clarify fluid material from coal during early carbonization stages. From the early 1980s, Lynch et al. detected both mobile and immobile components using a simple spectrometer (PMRTA; proton magnetic resonance thermal analysis).16-23 They used mainly the empirical parameter M2T16, corresponding to the second moment integration limited at a width of 16 kHz, for monitoring changes in fluidity. PMRTA techniques have been producing many useful works17,19,20 which are the quantification of interactive effects between different components in coal blends. (13) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 138. (14) Miyazawa, K.; Yokono, T.; Sanada, Y. Carbon 1979, 17, 223. (15) Parks, T.; Cross, L. F.; Lynch, L. J. Carbon 1991, 7, 921. (16) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, R. N. Energy Fuels 1987, 1, 167. (17) Lynch, L. J.; Webster, D. S. Am. Chem. Soc. Symp. Ser. 1983, No. 230, 353. (18) Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W. A.; Maher, T. P. Fuel 1988, 67, 579. (19) Lynch, L. J.; Webster, D. S.; Barton, W. A. Adv. Magn. Reson. 1988, 12, 385. (20) Clemens, A. H.; Matheson, T. W.; Lynch, L. J.; Sakurovs, R. Fuel 1989, 68, 1162. (21) Sakurovs, R.; Lynch, L. J.; Barton, W. A. Adv. Chem. Ser. 1993, No. 229, 229. (22) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743. (23) Lynch, L. J.; Sakurovs, R.; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036.

10.1021/ef0101311 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/10/2002

576

Energy & Fuels, Vol. 16, No. 3, 2002

In most of the studies with PMRTA, the overall concentrations of fluid and inert material over the thermoplastic range have not often been reported,18 because of a combination of the convenience of measurements for handling large data sets and the likelihood of truncating broad Gaussian signals at high temperature, the sensitivity is much lower. Recently, Snape et al.24,25 have approached this problem using a Doty probe operating at a frequency of 100 MHz for 1H. The effects of particle size, mild oxidation, and different heating regimes on plasticity development are reported. Furthermore, these in-situ measurements have confirmed that plasticity development is a reversible phenomenon provided relatively fast quenching rates (ca. 75 °C/min). However, the results obtained from PMRTA and standard in-situ 1H NMR investigations are limited to the average data for heterogeneous coals. It has long been established that coal’s response to process conditions during coking and combustion is strongly related to coal’s heterogeneous nature.28,29 Traditional means of characterizing coal at the microscopic level by direct observation are optical and electron microscopic techniques. Both methods have some disadvantages. Reflected light optical microscopic analysis, ROM, of coal is typically performed using oil immersion techniques.30 Additionally, a given coal sample is suitably prepared for analysis and this process irreversibly alters the sample. On the other hand, scanning electron microscopy (SEM) can surpass ROM in image quality and resolution, but at the same time, is limited to the necessity of coating the sample with conductive materials and high vacuum conditions are generally required during analysis, necessitating desiccation of the coal sample prior to imaging. Also, the result is that, prior to and during analysis, the sample is damaged irreversibly. Both methods cannot perform an in-situ investigation during heating without destructive sample preparation. Another visualization method of coals is the X-ray microscopy techniques. However, the conventional X-ray microscopy method is powerless to monitor with high resolution while synchrotron techniques are very effective in achieving high resolution but at the same time, not conventional and it is very difficult to set up in-situ measurement.31-33 A relatively new technique, which has a great potential in the area of coal research, is nuclear magnetic resonance imaging microscopy (NMR microimaging).34 This method has its own special advantages, the sample (24) Mercedes Maroto-Valer, M.; Andresen, J. M.; Snape, C. E. Energy Fuels 1997, 11, 236. (25) Mercedes Maroto-Valer, M.; Andresen, J. M.; Snape, C. E. Fuels 1997, 76, 1301. (26) Stach, E.; Mackowsky, M.-Th.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, R. Stach’s Textbook of Coal Petrogy, 2nd ed.; Gebruder Borntaeger: Berlin, 1975. (27) Hayamizu, ; Hayashi, S.; Kamiya, K.; Kawamura, M. Adv. Chem. Ser. 1993, No. 229, 15 (28) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (29) Hou, L.; Cody, G. D.; Hatcher, P. G.; Gravina, S.; Mattingly, M. A. Fuel 1994, 73, 199. (30) Jincheng, X.; Maciel, G. E. Energy Fuels 1997, 11, 866. (31) Botto, R. E.; Cody, G. D.; Kirz, J.; Ade, H.; Behal, S.; Disko, M. Energy Fuels 1994, 8, 151. (32) Cody, G. D.; Botto, R. E.; Ade, H.; Behal, S.; Disko, M.; Wirick, S. Energy Fuels 1995, 9, 75 (33) Cody, G. D.; Botto, R. E.; Ade, H.; Behal, S.; Disko, M.; Wirick, S. Energy Fuels 1995, 9, 525. (34) Bluemich, B.; Kuhn, W., Eds. In Magnetic Resonance Microscopy; VCR: Weinheim, 1992.

Saito et al.

being preparable without destruction, because NMR microimaging35 is accomplished by placing the sample in a controlled magnetic field gradient. An image is obtained by measuring the spatial variation of the magnetic resonance frequency created by the magnetic field gradient, and contrast (spin density, spin-spin relaxation, and spin-lattice relaxation time, etc.) and spatial resolution are selected by analysis conditions (number of voxels, strength of gradient, and variable parameters etc.). Some excellent work on microimaging studies for coals at room temperature is reported by Botto et al.36-39 However, the advanced technique of the multi-pulse sequence and the difficulty of experimental setup are needed in the case of coals because of the line broadening effects of the strong dipole-dipole interaction among protons with the rigid macromolecular network. In addition, the application of NMR microimaging to coal without a solvent swelling method has so far been limited by a typical low resolution (less than 200 µm) and has just given unclear images, resulting from the influence of various magnetic susceptibilities (some mineral components and some paramagnetic components, etc.) in coals. But conventional imaging methods have not been overcome by many difficulties (the influence of various magnetic susceptibilities, the effects of the strong dipole-dipole interaction, the limitation of k-space sampling to make prepared magnetization, etc.). Recent modified SPI, single-point ramped imaging with T1 enhancement (SPRITE), consist of a ramped phase encode gradient in the primary phase encode direction and conventional phase encode gradients in the other direction. The use of a ramped phase gradient permits imaging with greater speed and with lower dB/ dt, which minimizes gradient vibration compared with SPI. At the same time, a great time improvement is achieved with a sample where T1 relaxation times are on the order of the gradient rise time. Magnetization preparation is easily incorporated into the SPRITE sequence by sampling a single k-space point after each magnetization filter application. Balcom et al. have demonstrated that magnetization preparation permits accurate T2 and T1 mapping of samples with short relaxation times.40 In this paper, we are at first demonstrating 3DSPRITE which has been shown to be successful for studying short relaxation time systems and which is free from distortions due to susceptibility variations for coals. The results obtained were discussed in relation to the three-dimensional distribution of mobile components for two kinds of coal. At the same time, inversion recovery preparation experiments, for example T1 mapping and T2* mapping based on SPRITE methods, are presented in order to clarify the chemical heterogeneity (35) Bluemich, B.; Bluemler, P.; Saito, K. In Solid State NMR of Polymers; Elsevier: Amsterdam, 1998; p 123. (36) Dieckman, S. L.; Gopalsami, N.; Botto, R. E. Energy Fuels 1990, 4, 417. (37) Cody, G. D.; Botto, R. E. Energy Fuels 1993, 7, 561. (38) French, D. C.; Dieckman, S. L.; Botto, R. E. Energy Fuels 1993, 7, 90. (39) Botto, R. E.; Cody, G. D.; Dieckman, S. L.; Gopalsami, N.; Gopalsami, N.; Rizo, P. Solid State NMR 1996, 6, 389. (40) Beyea, S. D.; Balcom, B. J.; Prado, P. J.; Cross, A. R.; Kennedy, C. B.; Armstrong, R. L.; Bremner, T. W. J. Magn. Reson. 1998, 135, 194.

MRI of Coals Using SPRITE Techniques

Energy & Fuels, Vol. 16, No. 3, 2002 577

Table 1. Characteristics of Representative Examined Coalsa K-9 Saraji Goonnyella Workwarth Witbank Witbank slow heating (heating rate; 10 °C/min) Witbank rapid heating (heating rate; 104 °C/min) Bayswater K-prima a

C

H

N

O

ash (%)

79.0 78.9 79.4 73.8 73.4 73.4

4.1 4.0 4.8 5.2 4.4 4.1

0.8 1.6 1.8 1.6 1.8 1.8

4.1 3.7 4.2 4.2 10.5 10.1

11.5 11.1 9.1 9.9 9.1 9.1

73.4

4.3

1.8

10.5

9.1

63.4 63.0

4.6 5.1

1.6 0.7

15.2 23.8

14.4 6.7

C, H, N, and O on a dry and weight percent basis

of coals.41-44 Also, we have carried out the first systematic in-situ variable-temperature NMR microimaging study of coals between 25 and 600 °C with our newly developed high-temperature microimaging probe and systems45-47 in order to clarify the behavior of a mobile component at a high temperature in heterogeneous coal specimens, although high temperature in-situ imaging of coals was thought to be impossible. 2. Experimental Section 2.1. Coal Sample. The samples used in this study were Witbank coal (ash; 7.6%, volatile matter; 32.0%, total carbon as dry base; 75.8%, hydrogen; 4.8%, nitrogen; 1.9%, sulfur; 0.7%, MF (Maximum Fluidity); 1.25 (log ddpm) and Goonyella coal (ash; 9.8%, volatile matter; 24.0%, total carbon as dry base; 78.8%, hydrogen; 4.6%, nitrogen; 1.7%, sulfur; 0.5%, MF (Maximum Fluidity); 3.3 (log ddpm)). Both coals are widely used in the steel-making industries, but Witbank has a slight coking property, while on the other hand, Goonyella has a high coking property. Typical sample size was about 2 mm × 2 mm × 2 mm. All coal samples were predried and then soaked in pyridine-d5 vapor for 24 h. A total twenty-four kinds of coal were demonstrated using in-situ high-temperature imaging in order to evaluate their softening and melting properties and representative coals of chemical characteristics are shown in Table 1. 2.2. NMR Imaging Experiments. 1H NMR images were recorded using a JEOL R-400 NMR system (9.45T) with micro imaging accessory. We have carried out the first systematic in-situ variable-temperature NMR microimaging study of coals between 25 and 600 °C with our newly developed hightemperature microimaging probe and systems in order to clarify the behavior of a mobile component at a high temperature in heterogeneous coal specimens.48 The main specifications of our developed probe are (1) maximum sample temperature is 600 °C, (2) maximum field gradient strength is 250 G/cm, (3) maximum current is 50 A, and (4) sample size is 5 mm. These specifications can be completely achieved by adopting a rectangular enameled wire for the coil producing gradient (41) Murphy, P. D.; Gerstein, B. C.; Weinberg, V. L.; Yen, T. F. Anal. Chem. 1982, 54, 522. (42) Newman, P. C.; Pratt, L.; Richards, R. E. Nature (London) 1955, 125, 645. (43) Vanderhart, D. L.; Retcofsky, H. L. Fuel 1976, 55, 202. (44) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 15, 1776. (45) Hasegawa, K.; Yamakoshi, R.; Tsuno, H.; Saito, K. Proceedings of 13th-ISMAR, Berlin 1998, 1, 396. (46) Saito, K.; Komaki, I.; Hasegawa, K.; Tsuno, H. Proceedings of 13th-ISMAR, Berlin 1998, 2, 581. (47) Saito, K.; Kanehashi, K.; Komaki, I. Annual Report on NMR Spectroscopy; Academic Press: New York, 2001; Vol. 44. (48) Saito, K.; Komaki, I.; Hasegawa, K.; Tsuno, H. Fuel 2000, 79, 405.

Figure 1. The heating system in our developed probe and each point of temperature during heating test when the sample temperature was 502 °C. field, cooling system using both water and alcohol mixture, coil molding technique, and a very efficient heating system in probe (Figure 1). The rectangular wire, made by Mitsubishi Cable Ind. Ltd., has an extremely high space-factor (over 95% is available), so it is possible to get a high total current density, and it has also good heat dissipation. The Founer-Bessel expansion method49 was used for optimal coil design. Following this design, the coil was wound as a stand-alone type coil using Asahi Electric Institute’s technology. Moreover these coils are cooled by a closed circulation cooling system. These coils and cooling pipes are molded by resin for enabling heat dissipation and for preventing coil vibration. Figure 2 shows our variable temperature probe. It has a very long spiral heating wire, and the path of the heated N2 gas is very close to this heating wire, so that a highly efficient heat exchange is achieved. We have tried the SPRITE method at 500 °C using this probe. Both of these two methods are very tolerant of gradient-switching time. For example, we applied large pulsed field gradients up to 250 G/cm to this probe, which made the gradient-switching time as long as 150 µs. But the fixed phase-encoding time tp can be shortened as short as 64 µs (method: SPI, FOV: 5 mm, points). So, the combination of high temperature and the SPRITE method is very effective for coal analysis. We estimate the errors in the sample temperatures measured in this work to be (2 °C. Images reconstruction were accomplished by 2D or 3D FT using DEC 3000. The size of this bulk sample was about 2 mm × 2 mm × 2 mm for 3D-SPRITE. Only one specimen was placed into a 5-mm diameter standard NMR tube. 3D-SPRITE sequence was applied to observe the distribution of mobile components in (49) Turner, R.; Bowley, R. M. J. Phys. E. 1986, 19, 876.

578


Saito et al.

Figure 2. The developed high-temperature imaging probe of gradient parts. coals using an encoding time of 80 µs (T2* is about 100 µs for coal samples of mobile component). On the other hand, as T2* is about 30 µs for coal samples of immobile components, the contribution of immobile component to signal intensity is negligible. The recycle delay time was 10 ms and gradient strength was about 104 G/cm for three axes. The data matrix size was 128 × 128 × 128 (the digital resolution: 50 µm), and the number of signals accumulated was 8 for 3D-measurement. The total data acquisition required about 16 h. The RF pulse length was 10 µs and the flip angle θ was equal to 90°. On the other hand, the data matrix size was 128 × 128 (the digital resolution: 50 µm), and the number of signals accumulated was 16 for in-situ measurement. The data acquisition required about 8 min. The experimental sample scheme is shown in Figure 3. The rate of heating was 3 °C/min, which is the same as that of the industrial coking process. This means that the sample temperature was increasing about 24 °C(8 min × 3 °C/min) during NMR microimaging measurements. The sample of temperatures was calibrated on the basis of the direct measurements for Al2O3 sample with thermocouples. Single-point imaging (SPI) methods50 have proven their worth for studies of short relaxation time systems, e.g., for rigid types of polymer and concrete.51-53 The SPI pulse sequence does not employ band-selective pulses, instead it relies on broadband RF pulses of limited duration. The pulse bandwidth (1/pulse length) must be greater than the maximum spectral width (Gmax × sample length) to ensure uniform excitation. SPI is a pure phase encoding NMR microimaging technique. Therefore, a position is encoded in reciprocal space, S(k), where k ) 1/(2 π) γ Gt, by amplitude cycling of the applied (50) Choi, S.; Tang, X. W.; Cory, D. G. J. Imaging Syst. Technol. 1997, 8, 263. (51) Gravina, S.; Cory, D. G. J. Magn. Reson. 1994, B104, 53. (52) Mastikhim, I. V.; Balcom, B. J.; Prado, P. J.; Kennedy, C. B. J. Magn. Reson. 1999, 136, 159. (53) Prado, P. J.; Balcom, B. J.; Jama, M. J. Magn. Res. 1999, 137, 59-64.

Figure 3. The pulse sequence of T1 and T2*-SPRITE imaging. phase gradients G. The signal is acquired at t ) tp after a short excitation RF pulse in the presence of magnetic field gradients dB/dt with a quadrature method. Unlike frequency encoded images, SPI images are free from distortions due to B0 inhomegeneity, susceptibility variations, and chemical shift variations.54 (54) Saito, K.; Shinohara, M.; Hasegawa, K.; Tsuno, H. Bull. Magn. Reson. 1995, 18, 154.


Energy & Fuels, Vol. 16, No. 3, 2002 579 k-space point. From eq 1 we see that for a large flip angle, R, and a TR on the order of the T1, we can introduce T1 contrast into a SPRITE image. The measurement is, however, more sensitive if we introduce a longitudinal recovery, following magnetization inversion, before sampling the SPRITE point, as shown in Figure 4b. By varying the delay time between the 180° inversion pulse and sampling pulse, R, we obtain images with local intensity weighted by T1, as shown in eq 3:

S ∝ F exp(-tp/T2*)*(1 - 2 exp(-τ/T1))

(3)

assuming the TR is long enough so as to allow complete T1 recovery between sequential RF pulses. Since tp is a constant, the T2* decay is simply a constant weighting term, and the only variation between images is due to T1 recovery.

3. Results and Discussion

Figure 4. Experimental sample scheme. The resolution, even for short T2* species such as coals, is limited only by the maximum gradients which can be applied to the sample. The signal intensity, S, from any point in the image is related to local proton density, F, by eq 1:

S ) F exp(-tp/T2*) × R(x)

(1)

where R(x) ) (1 - exp(-TR/T1))/(1 - cos θ exp(-TR/T1)) The expression for R(x) in eq 1 suggests that there will be a minimum acceptable repetition time TR, for a given RF flip angle θ, to achieve an acceptable signal. This time restriction can be avoided in certain instances by reducing the pulse flip angle at the expense of the observable signals. We will consider several basic features of SPRITE methods (Figure 4). In the SPRITE sequence, unlike the basic SPI sequences, we do not switch gradient on/off for each acquisition. Instead we ramp this gradient in discrete steps and apply an RF pulse, collecting a single data point, at each gradient step in Figure 4a. The step length is less than 5 ms so that the overall gradient duration is brief and the mechanical force on the gradients is minimal (because of low overall dB/dt). An extremely loud SPI sequence becomes nearly inaudible when reconfigured as an equivalent SPRITE experiment. The phaseencode gradients in the SPRITE sequence inherently spoil transverse magnetization. When identical measurement conditions are set, typically SPRITE methods are quite time-saving, while at the same time the damage to the probe which is expected using SPI sequence is much less. SPRITE T2* Mapping. By examining eq 1 we can see that by using a long TR, and /or a small RF flip angle, R, we obtain an image of spin density, F, weighted purely by local T2*,

S ∝ F exp(-tp/T2*)

(2)

When T2* mapping with variable tp we must adjust the magnitude of gradient step size in order to maintain a constant k-space step,

∆k ) (2π) ∆G* tp (FOV ) 1/∆k) SPRITE T1 Mapping. Our proposed T1 and T2 mapping methods employ SPRITE to obtain the image, with some form of spin preparation being imposed before acquiring the SPRITE

Figure 5 presents a 3D-SPRITE image of Goonyella (a) and Witbank (b) coal. The lighter areas in this sample corresponding to higher concentrations of mobile components, which means pyridine-mobilized materials, as the conditions of this SPRITE experiment were set to T2* of the mobile component. The distribution of protons in mobile components was mapped exclusively in this way. A user-adjusted threshold was applied to define the minimum intensity used to calculate the contiguous surface in 3D rendering.54 This threshold was chosen to obtain the most accurate representation of the sample topology while suppressing low signal-tonoise features near the sample surface. The lighter regions correlate with increased nuclear spin density; the lighter the region the greater the density. It is clearly evident that the three-dimensional distribution of mobile components is very heterogeneous and these components have some three-dimensional connection with each other. The thickness of one plane about nine images is about 50 µm because of the setup measurement condition. It is obvious that SPRITE gave significantly clear images of coal as compared with those of the same sample (Figure 5c) that was obtained by the standard spin-echo imaging sequence.55 And the amount of mobile components, which was calculated from all of the images using a program routine supplied by JEOL Ltd, was 22% in the case of Goonyella. This result was almost consistent with that of 1H wide-line method line which was calculated from sub-spectra line simulation data. On the other hand, the result obtained by the SPRITE method (22%) is compared to that obtained by the spin-echo method (36%). As the estimated contribution to the total amount of proton signals from residual protons in pyridine-d5 was less than 0.1%, there is a substantial enhancement of image distortion because of the effect of various susceptibilities in the amount of mobile components due to swelling when the spin-echo method is applied. The results from SPRITE are definitely correct about the fractional intensity for mobile components, which means pyridine-mobilized materials. In addition, surprisingly, the region of mobile components is basically independent in a plane and its distribution is not a random and has heterogeneous orientation vertically on the scale of about 100 µm. It is interesting that the mobile domain of Goonyella has a very complex connectivity and a vertical orientation (55) Saito, K.; Hatakeyama, M.; Matsuura, M.; Komaki, I. Tetsuto-Hagane 1999, 85, 111.

580


Saito et al.

Figure 5. The NMR 3D and 2D images of (a) Goonyella coal, (b) Witbank coal obtained by SPRITE method, and (c) Goonyella coal obtained by spin-echo method.

on the three-dimensional image, because Hou et al. reported the mobile regions have a heterogeneous distribution.29 The amount of mobile components which means pyridine-mobilized materials in Witbank(17%), is less than that in Goonyella, and at the same time, the size of mobile components in Witbank is smaller. According to the properties of both coals, it is reasonable, that is to say, that the data of Goonyella obtained from Gieselar-Plastmeter56,57 and Ro (vitrinite refraction

rate %) which are common methods for estimating the properties of coals are better than those of Witbank for coking. The question is what the mobile component is. It is well-known that coals have two chemical structure parts (56) van Krevelen, D. W. Coal, 3rd ed.; Elsevier: Amsterdam, 1993; Chapters 23, 24. (57) Stach, E.; Mackowsky, M.-Th.; Teichmuller, M.; Taylor, G. H.; Chandra, D.; Teichmuller, R. Stach’s Textbook of Coal Petrogy, 2nd ed.; Gebruder Borntaeger: Berlin, 1975.



Figure 6. T2* mapping SPRITE and T1 mapping SPRITE images of Goonyella with various T1 values (from top left, T2* mapping SPRITE image, T1 mapping SPRITE images τ ) 20 ms, 40 ms, 60 ms, 80 ms, 100 ms).

Figure 7. T2* mapping SPRITE and T1 mapping SPRITE images of Witbank with various T1 values (from top left, T2* mapping SPRITE image, T1 mapping SPRITE images τ ) 10 ms, 30 ms, 50 ms, 70 ms, 90 ms, 110 ms).

(aliphatic and aromatic parts) and the T1 value of the aliphatic part is different from that of the aromatic part, except for high rank coal.30,58 In reality, the evidence derived from CRAMPS-based T1 measurements30 in Table 2 shows the difference of T1 value between the aliphatic part and the aromatic part is small because of spin diffusion effects in the case of Goonyella. Therefore, we applied T1 mapping SPRITE because there is a difference between the T1 value of the aliphatic part and that of the aromatic part. Figure 6 shows T2* mapping SPRITE and T1 mapping SPRITE images of Goonyella with various T1 values. It is clear that the observed pattern of mobile components which means pyridine-mobilized in these images are quite similar. As it is common that there are few aliphatic parts, in the case of Goonyella, the observed mobile components are not the aliphatic part because the amount of mobile components are too much. Therefore, it is likely that the mobile components, which means (58) Harmer, J. R.; Callcott, T.; Maeder, M.; Smith, B. E. Fuel 2001, 80, 417.

Table 2. T1 Proton of Relaxation Parameters Derived for Two Typical Coals Goonyella slow component aliphatic aromatic rapid component aliphatic aromatic Witbank slow component aliphatic aromatic rapid component aliphatic aromatic

percent (%)

T1 (s)

12 11

1.02 0.92

88 89

0.032 0.047

percent (%)

T1 (s)

39 14

0.71 0.97

61 86

0.031 0.054

pyridine-mobilized, could be hydroaromatic parts which have the higher mobility.58 On the other hand, T2* mapping SPRITE and T1 mapping SPRITE images of Witbank with various T1 values in Figure 7 show an observed pattern of mobile components which means pyridine-mobilized are quite different. Therefore, in the case of Witbank, the mobile component could be not only

582


Figure 8. The variations in the existing rate and their half width at half-height of mobile component between 25 °C and 550 °C for Goonyella coal and Witbank coal.

hydroaromatic parts which have a higher mobility and also aliphatic rich parts, as the amount of aliphatic parts for Witnbank is more than that for Goonyella. And more evidence derived from CRAMPS-based T1 measurements30 in Table 2 shows the difference between the T1 value of aliphatic parts and that of aromatic parts in Witbank as low rank coal is not small, though in general the aliphatic parts in high rank coal are in close (nm scale) contact with the dominant aromatic domains. At room temperature there is a significant difference at the origin of the mobile components which means pyridinemobilized among two typical coals and SPRITE methods

Saito et al.

are pretty effective in clarifying the property of coals that have very high inhomogeneity. According to the properties of both coals, it is reasonable, that is to say, that the data of Goonyella obtained from Gieselar-Plastmeter and Ro (vitrinite refraction rate %) which are common methods for estimating the properties of coals are better than those of Witbank for coking. It is well-known that coals start to soften and melt at about 400 °C and then to cake and carbonize at more than 450 °C. Figure 8 shows the variations in the existing rate and the half widths at a half-height of a mobile component between 25 °C and 550 °C in both coals are obtained by 1H wide line method with only 1 accumulation because our developed probe has very high sensitivity. As the temperature became higher, the existing rate of the mobile component increased gradually and attained a maximum mobility at about 450 °C for Goonyella, corresponding to a minimum in the halfwidth values. And then, the existing rate of the mobile component decreased suddenly at above 475 °C. Though a very short time measurement was acquired, good agreement has been obtained for the temperature of maximum mobility between Gieseler plastometry and rheology experiments58 and comparable behavior has been reported earlier by PMRTA16-23 and the method of Marto-Valer et al.24,25 Figure 9a shows an NMR image of Goonyella coal (only 1 piece) which was cut to a thickness of 100 µm at 25 °C and it was found that the distribution of mobile components which means pyridine-mobilized is not

Figure 9. Figure 27(a) of NMR image of Goonyella coal at 25 °C, (b) at 350 °C, (c) at 375 °C,(d) at 400 °C, (e) at 425 °C, (f) at 450 °C, (g) at 475 °C, (h) at 500 °C.



Figure 10. The image of Witbank coal at 425 °C.

random but very heterogeneous. As the temperature became higher, the mobile components increased but their distribution remained heterogeneous (Figure 9b,c). The images of Figure 9(d-f) show the distribution of mobile component became more homogeneous. This means that coal starts to soften and melt, and then the existence rate of mobile components increased dramatically. It is likely that these areas which are easy to soften and melt are fixed initially, because there are rich areas of the mobile components which means pyridinemobilized, that were cut to a thickness of 100 µm on the right side of Figure 9a at 25 °C mainly. It is very interesting that the softening and melting areas are moving from the right side to the left side in these images slightly and gradually. This means that, in the case of Goonyella, mobile components, which means pyridine-mobilized at room temperature, play the role of the starting point when heating occurs. And it is well established that mobile components emanate from the pyridine-extractable present in the coal. According to the half-width of the mobile component at 400 °C (where 400 °C means average data from 400 to 425 °C during the measurement, as the temperature was increasing at the rate of 3 °C/min), as compared to other materials measured at the same frequency, the mobility of the mobile components is the same as that of gels, not liquids.59 Therefore, it is likely that the expansion of the mobile component area occurs since some immobile component parts change some mobile component parts by a heating process, and at the same time, some mobile component parts which have good fluidity initially are melting at first and expanding upon a slow transfer of them to other areas. As the optimized variable encoding (59) Nomura, S.; Katoh, K.; Komaki, I.; Fujioka, Y.; Saito, K.; Yamaoka, I. J. Jpn. Inst. Energy 1998, 77, 55.

Figure 11. Our proposed mechanism of the softening and melting process for coals.

time at each temperature was used in order to avoid the contribution of immobile components in these experiments, it is noted that all results show the behavior of mobile components only. When the temperature was over 450 °C, the resolidification was starting and then the amount of the mobile component was

584


Saito et al.

Figure 12. Our proposed coal mapping for the softening and melting properties.

decreasing (Figure 9g) and at last, there were a few mobile parts at 500 °C (Figure 9h) because of the resolidification means that some mobile component parts convert to some immobile component parts and the carbonization means that the amount of protons in coal is extremely decreasing. On the other hand, Figure 10 shows the image of Witbank coal at 425 °C which indicates the maximum mobility. It is surprising that it shows low mobility areas and some unsoftened areas remain, corresponding to inert areas, although temperature of the maximum mobility for Witbank is set. It is likely that low rank coals for coking like Witbank have some inert areas which are difficult to soften and melt at their maximum mobility temperature in their specimen and to expand to other areas. The reason for this is that the amount of mobile components, which means pyridine-mobilized, in the case of Witbank, is less at room temperature, and judging from the results of T1SPRITE experiment, there is a significant difference at the origin of the mobile component between Witbank and Goonyella. As we already mentioned, the mobile components, which means pyridine-mobilized at room temperature, play an important role for softening and melting, and this mobile component as the starting point is expanding and increasing the melting area. And also, it is clear that T1 SPRITE is a very powerful technique for deciding the difference between the properties of two coals, when softening and melting phenomena have occurred. Additionally, it is clear that the in-situ variable-temperature NMR imaging method based on SPRITE is very useful for investigating and clarifying

the thermal change of coal. Figure 11 shows the mechanism of the softening and melting process for coals we proposed using in-situ imaging method. At the same time, we propose the coal mapping for the softening and melting properties shown in Figure 12. The imaging data can provide the maximum T2 value with the information of local positions at the maximum fluidity temperature for individual coal. Judging from the mechanisms of the softening and melting process, as the distribution of mobile components which means pyridine-mobilized, is not random, the average of T2 value for coal is not essential and T2 value at a local position which shows the maximum fluidity area is dominant. Therefore, both the amounts of the mobile components at the temperature of the maximum fluidity and T2 value at local position which shows the maximum fluidity area decide the softening and melting properties of coal. According to this coal map, we can easily understand coal properties quantitatively. Also, we can distinguish the slight difference between the raw coal (Witbank) and rapid heated coal which are treated by SCOPE 21 (Super Coke Oven for the Productivity and Environment enhancement toward the 21st century) process,60-62 although it was very difficult to detect this rapid heated effect by the ordinary Gieseler plastometer and Audibert-Arnudilatometer. This coal map is now (60) Saito, K.; Shinohara, M.; Suzuki, A. Proceeding of EENC, Paris 1996, 1, 318. (61) Nishioka, K. Tetsu-to-Hagane 1996, 82, 353. (62) Sasaki, M.; Komaki, I.; Matsuura, M.; Saito, K.; Fukuda, K. ICSTI-98 1998, 33, 44. (63) Saito, K.; Hatakeyama, M.; Matsuura, M.; Komaki, I. Tetsuto-Hagane 2000, 86, 79.


usefel and helpful in our company and will be used widely to understand the properties of softening and melting for steel industries. 4. Conclusion In this paper, we are at first demonstrating 3DSPRITE which has been shown to be successful for studying short relaxation time systems and which is free from distortions due to susceptibility variations for coals. The results obtained were discussed in relation to the three-dimensional distribution of mobile components for two kinds of coal (Goonyella and Witbank). The SPRITE imaging method can give useful and correct information about the fractional intensity for mobile components, which means pyridine-mobilized materials, compared with the spin-echo method. In addition, the region of mobile components is basically independent in a plane and its distribution is not random but it has a heterogeneous orientation vertically on the scale of about 100 µm. The amount of mobile components which means pyridine-mobilized, in the


case of Witbank, is less at room temperature, and judging from the results of T1-SPRITE experiment, there is a significant difference at the origin of the mobile component between Witbank and Goonyella. This paper shows that at first the mobile components, which means pyridine-mobilized at room temperature, is a very important factor for softening and melting using in-situ NMR imaging method, as these mobile component as the starting point are expanding and increasing the melting area. In-situ NMR imaging method is the best tool to estimate the properties of coals and our proposed map for the softening and melting properties will be used widely in steel industries. Acknowledgment. We are grateful to Prof. Dr. B. Bluemich (RWTH-Aachen), Prof. Dr. B. J. Balcom (University of New Brunswick), and Dr. R. E. Botto (Argonne National Laboratory) for the helpful discussions we had with them and their suggestions. EF0101311

In-Situ High-Temperature Magnetic Resonance Imaging of Coals

Recommend Documents