Energy & Fuels 1992,6, 651-655
65 1
Moisture Determination and Structure Investigation of Native and Dried Argonne Premium Coals. A 'H Solid-state NMR Relaxation Study X. Yang,*9+JA. R. Garcia,? J. W. Larsen,tJ and B. G. Silbernagel'lt Exxon Research and Engineering Company, Annandale, New Jersey 08801, and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received April 3, 1992. Revised Manuscript Received July 13, 1992
Proton free induction decays (fid) have been recorded for as-received and dried North Dakota lignite, Illinois No. 6 coal, and Pittsburgh No. 8 coal. Two curve-fitting methods were used to deconvolute the free induction decays. The two methods are multiparameter nonlinear numerical fit and sequential linear-least-squares fit. The summation of two Gaussian and one Lorentzian components gives the best fit to the relaxation data. The first Gaussian component is due to the contribution of coal lattice protons and is comparable with model polymer systems. The structural nature of the second Gaussian componentwas studied. The Lorentzian component is almost entirely due to water molecules. The water content of the as-received coals determined from the intensity of the Lorentzian component agrees very well with those determined using other methods. Four different drying methods (He flushing,heating under low vacuum, high vacuum at room temperature, and high vacuum at 85 "C)were evaluated for their effectiveness for water removal. High vacuum at room temperature was found effective. No significant structural change in either of the bituminous coals was found by any of the drying methods used. Drying the lignite caused an increase in the amount of the least mobile part of the structure. Introduction The determination of moisture in coals, the evaluation of effective drying methods, and the possible impact of drying on structures are all important in coal science and utilization. Several experimental methods have been NMR relaxation developed to analyzemoisturein measurements have also been used to quantify water in NMR has an advantage in that it also provides structural information. Most relevant to this work is the determination of the water content of Argonne premium coals by Gerstein and co-workers using a solid-echopulse sequence and a nonlinear numerical fitting pr~cedure.'~ In NMR relaxation studies, it is very common to use a function consisting of several exponential terms (Gauss-
* Author to whom correspondence should be addressed.
+ Euon. t Lehigh
University. (1) Vorres, K. S. 'User's Handbook for the Argonne Premium Coal Sample Program", U S . Department of Energy, October 1, 1989. See also: Vorres, K. S.Energy Fuels 1990, 4, 420, and references therein. (2) Groud, G.;Visman, J. In Coal Handbook; Meyers, R. A., Ed.; Marcel Dekker, Inc.: New York, 1981; Chapter 2. (3) Finwth, D. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987, 32,260. (4) Bainbridge, J. R.; Scanlan, P. G.;Belyea, I. Fuel 1949,28, 88. (5) Allardice, D. J.; Evans, D. G. Anal. Methods Coal Coal Prod. 1978, 1. 247. (6) De., K. S.Fuel 1988, 67, 1020. (7) Klein, A. Aufbereit.-Tech. 1987,28, 10. (8)Geratain, B. C.; Chow, C.;Pembleton,R. G.;Wilson, R. C. J . Phys. Chem. 1977,81, 565. (9) Lynch, L. J.; Webster, D. S.Fuel 1979,58, 429. (10) Riley, J. T. Am. Lab. 1983,15, 17. (11) Cutmore, N. G.; Sowerby, B. D.;Lynch, L. J.;Webster, D. S.Fuel 1988,66,34. (12) Graebert, R.; Michel, D. Fuel 1990,69,826. (13) Wroblewski,A. E.;Reinartz,K.;Verkade,J. G.EnergyFuels 1991,
b 7.-". ",
(14) Rosa, L. D.; Prueki, M.; Gerstein, B. C. Quantitation of Protons in the Argonne Premium Coals by Solid State 1H NMR. In Techniques ofMagneticResonancein Carbonaceous Solids;Botto, C. E., Sanada,Y., Me.;Advances in Chemistry; American Chemical Society: Washington, DC, in press; Vol. 229.
ian or Lorentzian) to fit the observed free induction decay because these exponential functions can be related to different relaxation mechanism^.^^ The functions and their relative intensities provide information about the structure and behaviors of the samples. For example, a Gaussian function is associated with the strong dipolardipolar interactions in a solid lattice. A Lorentzian has its origin in the interactions of highly mobile groups. The use of several exponential functions to fit a complex curve gives rise to concerns about the reliability of the fitting process because it is an inverse Laplace transformation, which is mathematically unstable.Is It is important to examine the reliability of the curve-fitting process. We have done that by using two independent methods. On the other hand, the insight into structure which can be obtained by deconvolutingthe observed fid has been amply justified by many w o r k e r ~ . ~ ~ J There ' J ~ is a balance between the risk of an unreal fit and utilizing the full information contained in the fid. Drying is often the first step in coal utilization, both in the laboratory and in industry. There is a need for a systematicevaluation of the effectivenessof various drying methods and an understanding of the impact of the drying processes on the remaining coal structure. NMR relaxation studies provide both water content and structural informationbut have not previously been used to compare different drying methods. We have conducted a series of experiments on asreceived, dried, and solvent-swollen Argonne premium coals using pulsed solid-state NMR techniques. Here we (15) For example, see: Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, U.K., 1961. (16) For example, see; Carslaw,H. S.;Jaeger, J. C. Operational Methods in Applied Mathematics; Clarendon Press: Oxford, U.K., 1941. (17) Barton, W. A,; Lynch, L. J.; Webster, D. 5.Fuel 1981,63,1262. (18) Jurkiewicz, A,; Bronnimann, C. E.; Maciel, G. E. Fuel 1990,69, 804.
0887-0624/92/2506-0651$03.00/00 1992 American Chemical Society
652 Energy & Fuels, Vol. 6, No. 5, 1992
present part of these results: 1H NMR T2 relaxation measurements of as-received and dried Illinois No. 6 coal, Pittsburgh No. 8 coal, and North Dakota lignite. Our purpose was to investigate (1) the reliability of water quantificationusing NMR relaxation, (2) the effectiveness of different dryingprocesses, and (3)the impacta of various drying procedures on coal structure.
Experimental Section
12
1. Sample Preparation. The as-received samples were prepared by transferring (N2 atmosphere) well-shaken 100-mesh
Argonne premium coals from the original ampules to 5-mm NMR tubes. The NMR tubes were frozen with liquid N2 and then sealed under a vacuum of 1X 10-2 Torr. Helium-dried samples were prepared by evacuating the as-received coals and then pressurizingto about 1 atm with He. This procedure was repeated 5 times and then the evacuated tube was sealed. Samples were also dried by heating at 100 O C for 15 h under a vacuum of about 10 Torr. A vacuum of about 1 X lo4 Torr for 48 h at room temperature was used to prepare the high vacuum dried samples. The high vacuum/heated samples were obtained by heating the samples at about 85 O C for 24 h at about 1 X 104 Torr. All of the sealed sample tubes were about same length as that of the NMR probe coil (2 cm), and about two-thirds of each tube was filled with coal. 2. NMR Measurements. The relaxation measurements were carried out on a Bruker MSL-360 spectrometer operated at 360 MHz. A two-pulse (the output of the RF pulse is about 1kW) W,-r90y solid-echo sequence was used to obtain the entire fid. The signal was recorded by a Bruker BC-131 transient digitizer with a sampling speed of 0.2 pa. The data was stored in an Aspect 3000 computer. The parameters used in the measurement, e.g., pulse width, pulse spacing, repetition time, and number of scans were systematically optimized; typical values used were 1.0 ps, 8.0 pa, 6 a, and 512, respectively. Special efforts were made to shim the NMR magnet to reduce the inhomogeneityas much as possible. With liquid cyclohexane, the measured line width is less than 100 Hz. Since the narrowest line width in the experiment is about 1300 Hz (for the as-received North Dakota lignite sample), the effect of the magnet inhomogeneity is negligible. A Kel-F probe base was made to reduce the background proton signal to about 0.3 % relative to the intensity of dried coals. The original Bruker probe ceramic base contains water which gives a background proton signal of about 4% that obtained with dried coals. 3. Data Treatment. Two curve-fitting methods are commonly used for NMR relaxation studies: linear-least-squares and nonlinear numerical fitting. Both methods were used in this work. The equation used to fit the complex fid has the form
11 10
B 8 7
6 5 4
0
0.5
1
1.5
2
2.5
3
0.1 0.3
3.5
0.5 0.7 0.9 1.1 1.3 1.5
[tbs)Pnm
It(r)P/looo
Figure 1. The natural logarithmof the fid intensity of as-received Illinois No. 6 as a function of relaxation time t and t2. is the experimental data point and the solid line is the linear-leastsquares fitting. (A) the original fid, (B) after subtracting the Lorentzian component from (A), (C) same as (B) except plotted as function of t2, and (D) after subtracting the intermediate Gaussian component from (C). Table I. Comparison of Linear and Nonlinear Curve Fittings. Gaussian 1 Gaussian 2 Lorentzian fitting methods (pa) T@lc Zgl(0) TQZ ZeCO) T21 Z(0) x~~ linear 3 9.9 72 18.6 8 151 20.5 nonlinear nonlined
8 3 8 3 8
9.9 2.5 4.3 10.5 10.2
74 6.4 8.9 61 63
18.8 5 10.7 73 10.7 70 11.6c 18.8 11.7c 17.8
155 143 149 150 155
20.4 20.7 21.3 19.4 20.5
27 17 13 6
As-received Illinois No. 6. Standard deviation of data points from fitted line, defined by eq 2 in the text. c In microseconds. d One Gaussian and two Lorentzian components are used. e This is a Lorentzian component. fid),the long-decayingcomponent is clearly a Lorentzian function. After fitting the Lorentzian component and subtracting it, we plotted the natural logarithm of the rest of the signal in Figure 1B as a function o f t (Lorentzian) and in Figure 1C as a function of t2 (Gaussian). The Lorentzian fitting in Figure 1B for the second component would lead to an erroneous extrapolation to t = 0, while the Gaussian fitting in Figure 1C not only gives a reasonable value for Ze(0) but also leads to an almost-perfect Gaussian fit for the last component as shown in Figure 1D.
Results where Z(t) is the observed intensity at time t, T2i is the transverse relaxation time of the ith component, and Zi(0) is the intensity extrapolated to t = 0 for the ith component. The subscripts g and 1 indicate Gaussian and Lorentzian, respectively. The nonlinear fit was carried out using an x2numerical fitting procedure.l8 The goodness of the fit is measured by the x2 value which is defined as
x2
CIbi- y(xi;a,...aM)l/ui) N
2
where ui is the standard deviation of each data point (xiyi), N is the total number of data points, and M is the total number of sequential linear-least. the adjustable parameters a ~ The squares fit is demonstrated in Figure 1. In Figure 1A (the full (19)Prees, W.; Flannery, B.;Teukoleky,S.; Vetterling, W. Numerical Recipes; Cambridge University Press: New York, 1987.
1. The Reliability Study of the Two Fitting Methods. Coal is a complexsystem. The protonsdetected by IH NMR can be structurally different (lattice, rotating methyl, absorbed water etc.). These “different” protons can relax to equilibrium by different mechanisms and at different rates, each making ita own contribution to the free induction decay signal. A proper deconvolution of the fid into different components is crucial. Table I lists the results for as-received Illinois No. 6 coal using both fitting methods, and two different pulse spacing values ( 7 ) . The linear-squares fitting gives three components: two Gaussians and one Lorentzian. For the nonlinear numerical fitting, different combinations of Gaussian and Lorentzian functions were tried. It was found that either two Lorentzians and one Gaussian or two Gaussians and one Lorentzian give good fits. The fit
Energy & Fuels, Vol. 6, No. 5, 1992 683
Moisture Determination in Argonne Premium Coab
was regarded as good if the addition of more terms did not yield a significant improvement. The data in Table I can be summarized as follows: (A) Both linear and nonlinear methods gave stable and essentially identical TZand Z(0) values for the longer Lorentzian component. (B)Different pulse spacing values ( 7 ) give the same results, indicating that the solid-echo pulae sequence refocused most of the spins. (C) The nonlinear deconvolution of the two Gaussian componentswith comparable Ta values is not stable and depends strongly on the smoothness of the decay curve and on the fitting functions used. The fit, blindly applied, may yield physically impossible results. For example, the T2g value (2.5 and 4.3 ps for 7 = 3 and 8 ps) for the short Gaussian component in the two Gaussian and one Lorentzian case is too small to be physically reasonable; the short 2'21 values (11.6 pa) obtained using the two Lorentzian and one Gaussian fit are also too small to be physically possible. (D) The linear fitting method leads directly to one Lorentzian and two Gaussian components and gives physically reasonable values for Tz.The results are similar to those reported by othera.lZJ4 From the above analysis, both fitting methods give reliable results on the water content of coals, which we w i l l show gives rise to the Lorentzian component. The linear fitting method is the more reliable one for all the samples and has been used throughout for data analysis. The statistical error of the fit is small in general, less than 2%. The exception is the Lorentzian component of dried coals where the low signal-to-noise level leads to fluctuations of up to 20%. The overall accuracies of the results were estimated for one as-received and one dried coalby independently repeating the measurement for three differentsamplesunder the same experimentalconditions. The statistical errors of the fit and the overall relative errors of the measurement for as-receivedand dried Illinois No. 6 coal are listed in Table 11. These data demonstrate the accuracy level of the results reported in this work. 2. Drying Effect. In Figure 2, the natural logarithms of the fid signals of the three coals are plotted as a function of time. Each plot contains data for an as-received coal and for the coal dried in four different ways. Using the linear fit procedures outlined in the data treatment section, the fitted values of TZand I(0) for each of these samples are given in Tables 111-IV. It is well-known that the North Dakota lignite contains a large amount of water.' This is clearly indicated by the high intensity of the long Lorentzian component in Figure 2A. After the sample is dried, this Lorentzian component is dramatically decreased in the following order: He flushing, heating, high-vacuum pumping, and high vacuum/heating. There is no significant difference in the relaxation behaviors of coals dried using the last two methods. Similar trends can be observed in Figure 2, B and C, for Illinois No. 6 coal and Pittsburgh No. 8 coal, respectively. Some patterns can be seen in these data. First, I(0)l and 2'1 are directly related to the moisture in coals. After the He-flush treatment, a significant amount of water is left in the coals. This drying process is the mildest and the least time-consuming of the four and is incomplete. While heating further reduces the moisture, use of high vacuum removes almost all of the Lorentzian components.
12L
4
I North Dakota
eC1
i
\
'!I/
-1 1
0
I,
I
' I I
20 40 60 80 100120 140160180200 t (I4
Figure 2. The natural logarithm of the fid intensity versus relaxation time for Illinois No. 6 coal, and Pittsburgh No. 8 coal, and North Dakota lignite as a function of relaxation time under five different drying conditions. Table 11. Error Analysis for Illinois No. 6 Coal. Gaussian 1 Gaussian 2 Lorentzian 2'21 Zi(0) samples errors Tklb ZdO) 2'2~2 Z,(O) 0.2 0.2 0.1 0.2 0.8 as received fitting 0.08 30 10 5 5 10 exptd 3 0.6 17 10 0.2 2 dried' fitting 0.5 30 30 50 5 15 3 expt 4 Relative errors in % . In microseconds. Statistical errors associated with the linear fitting. Obtained by repeating the measurement for three times using different samples under same experimental conditions. e Dried by high vacuum at room temperature.
Since the residual Lorentzian component after the highvacuum pumping is so small, the coals are well-dried. In contrast to the Lorentzian component, both Ta and I,(O) are essentially unchanged by all the drying treatments. Some differences are observed for North Dakota lignite, particularly for the ratio of the relative intensity of the short Gaussian and longer Gaussian component, which will be discussed in the following text.
Discussion 1. Coal Moisture Determination. The presence of water has a profound effect on the observed relaxation
Yang et al.
664 Energy & Fuels, Vol. 6, No.5, 1992 Table 111. Summary of Relaxation Measurements for Illinois No. 6 Coal Gaussian 1
a
Gaussian 2
T2gl" Is1 (0) 74 9.9 88 9.7 95 9.8 96 9.9 96 9.7
samples aa received He flushed heated high vacuum highvacuumlheat
T2g2 19 20 21 21 20
Lorentzian
4 (0)
Is2 (0) Tu 5 155 5 34 4 26 4 27 4 28
20 7 1 0.3 0.3
In microseconds.
Table IV. Summary of Measurements for North Dakota Lignite Gaussian 1
Tzpf 9.5 9.5 9.7 9.6 9.8
samples aa received He flushed heated high vacuum highvacuumlheat a
Zgl
Gaussian 2
(0) Tzgz
35 82 89 92 92
17 16 20 20 19
Lorentzian
zgz (0) TZl 9 258 9 34 8 26 I 25 7 28
ZI (0) 56
9 3 1 1
In microseconds. Table V. Summary of Relaxation Measurements for Pittsburgh No. 8 Coal Gaussian 1
T2gl' 10.1 10.1 10.1 10.1 10.1
samples
aa received He flushed heated high vacuum highvacuumlheat a
Zgl(0) 92 93 95 95 95
Gaussian 2
Lorentzian
T2g2
162 (0)
Tu
4 (0)
20 19 21 21 21
4 5 4 4 4
23 26 25 25 25
4 2 1 1 1
In microseconds. Table VI. Moisture in Coals ( W W )Determined by Various Methods
coals Pittsburgh No. 8 Illinois No. 6
North Dakota a
Hda 31P (dry%) NMRb 2.85 4.83 9.35 4.23 31.05 4.36
moisture W % 180 ASTMa diltc 2.5 1.65 9.6 7.94 32.24 34.4
1H NMRd 2.2 8.5 33
Reference 1. b Reference 13. Reference 3. This work.
dynamics of coal samples. A slow Lorentzian decay is usually due to protons with a high degree of mobility. The fact that the Lorentzian component for the as-received coals is almost completely gone after drying suggests that this Lorentzian component is entirely due to physically absorbed water protons. Gerstein et al. have heated coals at 100 OC under vacuum, observed the disappearance of the longer Lorentzian component, collected the distillate, and showed that this distillate is water.I4 Further support for this conclusion comes from the fact that the weight percentage of water in coals ( W % ) derived from Il(0) from the present experiments compares very well with those obtained using other established methods for coal moisture determination. This comparisonis made in Table VI, where the literature values of W % are listed and compared with ours derived from Il(0)using the equation I,(O) X 18/2
W % =II(0) X 18/2 + [lOO-Il(O)]/H~
(3)
where H e d is the weight percentage of hydrogen in dried coals obtained from elemental analysis.' The 1H NMR relaxation measurement not only servesfor understanding coal structures but also provides a quantitative method
for the estimation of moisture in coals. Water so strongly bonded that it is part of a solid will not be detected by this NMR method. 2. Coal Structures and Their NMR Relaxation Behavior. It is well-known that strongly coupled rigidlattice structures usually give an fid signal of Gaussian shape.ls The coal structure, which results in two Gaussian components, seems to be a tight one. As shown in Table 111-V, neither wetting nor drying has a significant effect on the Gaussian relaxation times. The ratio of intensities of the short to long Gaussians (I,~(O)/Z~(O)) increases with drying for the lignite. Structurally, a "free" part of the solid coal is increasing in rigidity. This is an entirely reaeonable observation. The presence of large amount of water in the solid enhances the mobility of part of that solid. This does not occur with the bituminous coals. This observation is consistent with a "hydrogel" structure for lignite in which water is an important part of the coal structure. The shorter time Gaussian component is dominant in all the dried coals and accounts for more than 90% of the fid signal. Its T2 values are nearly the same for all of the three coals. It has been proposed that the coal structure consists mainly of a three-dimensional cross-linked macromolecular networkemThe present experimental observations are consistent with this picture. Further support for this model comes from the fact that a similar NMR experiment on a cross-linked polystyrene (1.5 % divinylbenzene) in this laboratory gives just one Gaussian component with a TZof 9.7 pa, which is very close to the Tzglvalues for the first Gaussian component of the coals. The clear correlationbetween Tal and Tw (a ratio about 1:2) suggests that a relationship may exist between the structures giving rise to the two Gaussian decays. Speculations have been made about the species responsible for Candidate the second Gaussian component (Tw).14121t22 species include the methyl groups attached to the macromolecular framework, molecules detached from the lattice, and stronglyabsorbed residual water. The methyl group hypothesis does not agree with the present observations. First, the relative intensity of the second Gaussian component measured in this experiment is too low to account for all the methyl groups reported by I3CNMR,23 which would amount to about 4 0 4 0 % of the protons for Illinois No. 6 and Pittsburgh No. 8 coals. Second, we studied and compared the relaxation behaviors of a-methylpolystyrene and polystyrene. No significant difference in their fid is found for the two model polymers; no component could be identified for the a-methyl group. Additionalinsight on the nature of these speciesis provided in Table VII, where the relaxation parameters of Illinois No. 6 coal are listed as a function of temperature. The relative intensity of the second Gaussian component incream with increasing temperature, but the change is small and smooth,suggestingan increased motion of some of the terminal segments of the molecular structure. Clearly, more work is needed to provide a structural identification for the second Gaussian component. (20) Larren,.! W. In New Trends in Coal Science; Ybiim, Y., Ed.; Kluwer Acadermc Publisher: New York,,1988, pp 73-84. (21) Kamieiki, B.; Prueki, M.;Gentem, B. C.; Given, P. H. Energy Fuels 1987,1,45. (22)Mraw, 5. C.; Silbemagel, B. G. Am. Znst. Phys. Roc.: Chem. Phys. Coal Utilization 1981, 70, 332. (23)Solum, M.5.;Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.
Moisture Determination in Argonne Premium Coals
Energy & Fuels, Vol. 6, No. 5, 1992 6SS
Table VII. Effect of Temperature on the Relaxation Behaviors of Dried Illinoir No. 6 Coal
T(W 296 326 366 a
Gaueeian 1 Td' Zg1(0) 9.9 10.0 9.9
96 94 92
Gaueaian 2
Tm 21 21 21
ZgdO) 4 6 7
Lorentzian
Tu
Zl(O)
27 27 26
0.3 0.6 0.6
In microseconds.
3. Moisture Removal and Its Impact on Coal Structure. It is very common to dry coal by heating it to about 100 "C under vacuum. Very natural questions to ask are: How much of the water is removed and what possible impact does the drying process have on coal structures? We conclude that heating at house vacuum (about 1 Torr) removes most of the moisture in coals, leaving about 5% of the total water in the coals. Among the four drying methods we examined, the high-vacuum pumping is found to be the moet effective. No significant change was found for the relaxation times of the two Gaussian components after any one of the drying treatments, indicating that none of the drying processes had an effect on the rigidity of the coal structures. There is a shift from the more mobile solid to less mobile solid with drying the lignite. The rigid structure of the bituminous coals are unaffected by the drying procedures.
Conclusionr The linear-least-squaresfitting usedin the present work is preferred over the nonlinear numerical procedure. The fid signal was deconvolutedinto two Gaussian componenta and one Lorentzian component. In agreement with Gerstein et al.," the Lorentzian component arises from the protons of physically absorbed water and gives a quantitative measure of the moisture in coals. More than 90% of the coal structure behaves as a rigid lattice. The relaxation timee measured for coals are very similarto that of cross-linkedpolystyrene. Less than 10% of the dried coals' structure showshigh mobility, and more work is needed for the structural identification of this component. Drying by high-vacuum pumping is almost complete. No significantstructuralchangein either of the bituminous coals was caused by any of the four drying processes studied. Drying the lignite causes an increase in the amount of the least mobile part of the structure.
Acknowledgment. We acknowledgeuseful discussions with Dr. P. Sheng of E u o n on curve fittings. We warmly thank P.Mulieri of Lehigh University and M. Bernard0 and Q.Zhang of E u o n for technical assistance. We are also grateful to Prof. B. Gerstein of Iowa State University for providing us with a copy of ref 14before ita publication. Registry No. Water, 7732-18-5.
