Article pubs.acs.org/EF
Rearrangement in Brown Coal Microstructure upon Drying As Measured by Ultrasmall-Angle Neutron Scattering Richard Sakurovs,*,† Liliana de Campo,‡ and Christine Rehm‡ †
Energy Technology, Commonwealth Scientific and Industrial Research Organisation (CSIRO), 11 Julius Avenue, North Ryde, New South Wales 2113, Australia ‡ Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Lucas Heights, New South Wales 2234, Australia ABSTRACT: Small-angle scattering can supply information about the microporosity of wet materials, unlike techniques such as gas sorption and mercury porosimetry. In this study, we applied the ultrasmall-angle neutron scattering (USANS) technique to compare the microstructure of brown coals before and after drying and after rewetting the dried coal, in the size range from 25 nm to 10 μm. We found that scattering of neutrons from brown coal is substantially greater when dried than when wet. Because we found that materials that shrink uniformly on all size scales must scatter neutrons to a lesser extent when dried than when wet, the microstructure of brown coal must be different when it is wet from when it is dry. This difference in the brown coal microstructure is on sizes of 25 nm), the effect is expected to be small, but densification effects may become important if pores of less than 1 nm diameter were to be investigated. Reich et al.3 found that, when their Victorian brown coals were dried, the scattering intensity also increases, when using X-rays, by a factor of about 4. This was attributed to the greater density contrast between brown coal and air compared to brown coal and water (water has a positive SLD in X-rays). Reich et al. found a change in the slope of the scattering curve
Figure 1. I(Q) versus Q for Yallourn brown coal before drying (circles) and after drying (triangles).
Figure 2. Ratio of the scattering intensity of coals after and before being dried. The horizontal line indicates the ratio expected if the pore size distribution was unaffected by drying. The indicative pore radius associated with the Q values is shown at the top of this graph.
should decrease when the sample is dried. The scattering intensity from pores is proportional to the square of the density contrast between the matrix and the pores. The SLD of these brown coals when dry is about 2.8 × 1010 cm−2. Because the SLD of water is negative, −0.5 × 1010 cm−2, the density contrast between coal and water is greater than that between the coal and air. This means that scattering from the dry brown coal should be reduced to about [2.8/(2.8 + 0.5)]2 (72%) of that of the wet coal. Figure 2 shows that the ratio of scattering intensity seen for the heat-treated Yallourn brown coal between dry and wet samples is about 70−90% over the entire Q range (open squares). This means that the pore size distribution of these treated brown coals when wet, after removing the initial moisture, is relatively unchanged by drying. The temperature treatment stabilizes the brown coal structure, so that it is unaffected by later drying. The scattering intensity of both the original Loy Yang and Yallourn brown coals, in contrast, increased substantially upon drying (Figure 2), by up to 40%, a factor of 2 greater than expected. A doubling of scattering intensity upon drying over the expected value demonstrates that the pore structures of the coals before and after drying are very different. 233
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sizes) than the Yallourn coal. This ratio is not affected by drying. This result suggests that techniques such as mercury porosimetry that measure porosities of the dried coal, even though they may not provide good indications of wet brown coal structures, can nevertheless provide information about the relative differences in the structure between wet coal samples. Recovery of Pore Structures upon Rewetting. The shrinkage of brown coal when it is dried is only partially reversible upon rewetting. The samples after being measured when dry were rewet with water, and then their scattering intensities were remeasured. Rewetting reduced the scattering intensity from the coals that had not been heat-treated but not to the same level as the scattering intensity from the original coals. This shows that neutron scattering could identify that reintroduction of water into the coal causes some recovery of the pore structures but also that the recovery is not fully reversed. The ratio of scattering intensity between the rewet samples and the original wet samples is shown in Figure 5. At
upon drying, which they attributed to pore collapse upon drying. In another study of gel drying, Chiang et al.12 found that drying increased scattering intensity from calcium silicate gels but used separate samples for wet and dry measurements. They attributed this increase to a greater number density of nanoparticles as a result of more efficient packing and shrinkage of the silicate lamella. SANS is used to characterize gels present in foodstuffs and is often used to study their interactions with water13,14 but has not been used to investigate the influence of drying on scattering intensity in these types of gels. Comparison between Coals. Calcium exchange modifies the structure of brown coal by introducing cross-links between carboxylic acids and compressing it15 and may thus be expected to modify the drying behavior. Figure 3 shows that the
Figure 3. Effect of calcium pretreatment on the ratio of scattering intensity of Loy Yang coal before and after being dried. Figure 5. Ratio of intensities from the coal after being rewet with water compared to that of the original wet sample.
scattering from the Ca-treated Loy Yang coal is increased proportionally less than for the untreated coal upon drying, but the shape of the curve is unchanged, indicating that this treatment had little effect on the drying behavior. Figure 4 shows the ratio between scattering intensities of the two coals compared both before and after being dried. Loy Yang has relatively more scattering at high Q (smaller pore
low Q, the scattering intensity in the coals that had not been heat-treated was greater after rewetting than before, and this ratio decreased with increasing Q. In contrast, the sample that had been heat-treated showed very little dependence of this ratio with Q. Although the difference between the scattering intensity of the samples containing the original moisture and when moisture was reintroduced is probably due to the coal shrinkage upon drying not being fully reversed upon rewetting, the influence of added ethanol (required to allow water to rewet the coal in these experiments) may also contribute to the measured irreversibility in this case. Varying the Contrast. For the experiments involving D2O/H2O mixtures, the scattering intensities of the coal in the D2O/H2O mixtures were divided by the scattering intensity of the rewet samples. Because of the large difference in SLD values between D2O and H2O, by varying the D2O/H2O ratio, we can study the influence of the SLD of the fluid on the scattering intensity. If the ratio is selected such that the SLD of the fluid is equal to that of the solid, then the contrast is zero. If all pores are accessible to the fluid and the sample is otherwise homogeneous, then there is no scattering from pores under these conditions. Thus, any residual scattering observed in the material in the presence of this fluid is from inaccessible or
Figure 4. Ratio of the scattering intensity between Loy Yang and Yallourn samples. 234
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closed pores and heterogeneities in the sample.16 This technique is known as “contrast matching”. For materials such as brown coal, the SLD of the coal is difficult to obtain accurately because it depends upon knowing the real density, and for highly microporous materials, “real” density may not be clearly defined. Moreover, in brown coals, deuterium can exchange with any carboxylic acids and phenols in the coal, releasing protons back into solution, which can affect both the SLD of the coal and that of the fluid. The approach taken here was to determine the scattering intensity using a number of D2O concentrations. Because the scattering intensity is proportional to the square of the density contrast, plotting the scattering data at any given Q value against the deuterium concentration should yield a parabola, with the minimum value being the minimum scatter, and the deuterium concentration at which this occurs provides an estimate of the SLD of the material containing the pores. Figure 6 shows an example for
Figure 7. Ratio of I(Q) of rewet coal with 50% D2O compared to rewet coal with H2O.
but even at the largest Q values reached in this experiment, nearly all of the pores are accessible. Figure 8 shows the ratio of the scattering intensity of Yallourn (original) coal in various concentrations of D2O. The
Figure 6. Plot of the scattering intensity ratio versus the D2O concentration for wet Yallourn brown coal at Q = 0.000 316 Å−1. The parabolic fitted curve is indicated.
scattering intensity of Yallourn coal plotted against the D2O concentration. In this example, the minimum is at weight fraction of 0.6, with a minimum value of 0.12 that indicates that at least 88% of pores of 80 nm radius are accessible. By doing this for every Q value, we can determine the variation of closed porosity versus Q. Moreover, we can determine if the minimum SLD value varies with Q. A variation could indicate that the nature of the pores varies with the pore size.17,18 Figure 7 shows the ratio of the scattering intensity from brown coals using a 50 wt % D2O solution compared to that in 100% H2O. The scattering intensity in 50% D2O is less than 0.2 of the value in H2O. For all samples, this ratio remains fairly constant, except for larger Q values (Q > 0.001 Å−1). A constant ratio means that D2O has replaced H2O in all pores to the same extent, irrespective of the size. The small values confirm that nearly all of the pores in brown coal on this size scale are accessible to water. The residual scatter could be due to closed pores, heterogeneities in the brown coal, or not having the exact zero contrast conditions. The pores in the heat-treated Yallourn sample appear to be equally accessible to water as the other samples: there is no evidence of structural change that impairs solvent penetration introduced by the heat treatment on this size scale. The 50% D2O/H2O ratio tends to increase with all four samples at high Q. This may reflect an increased fraction of closed pores occurring on this size scale,
Figure 8. Ratio of I(Q) of rewet Yallourn coal (original) with various mass fractions of D2O compared to rewet Yallourn coal (original) with H2O.
scattering intensity decreases from 8 to 20 and then to 50% D2O. In pure D2O, scattering increases again. This shows that the zero contrast condition lies at a solution with less than 100% D2O. The scattering intensity ratio between the coal in D2O and H2O increases markedly with increasing Q. This can be caused if the SLD of the part of the coal containing the pores decreases with increasing Q. This has the effect of increasing the contrast with D2O and, hence, increasing the scattering. After a parabola was fit to the data, the resulting minimum fraction of scattering intensity and the fraction of D2O at which it occurs were plotted against Q in Figure 9. The minimum scattering intensity ratio calculated from parabolic fitting (Figure 9) is comparable to that from the sample exposed to 50% D2O, showing that 50% D2O provides close to the zero contrast condition. The percentage of D2O at which the minimum scattering occurs decreases substantially with increasing Q, from 60 to 50%. This shows that the nature of the pores changes with a decreasing pore size. This can occur if 235
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contrast. For this reason, we have not directly calculated the SLD value of the zero contrast solution shown in Figure 9. What we hope to have demonstrated here is the usefulness of neutron small-angle scattering techniques to monitor the structure of brown coals upon drying and how new information can be obtained by this approach.
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CONCLUSION
USANS can measure the pore size distribution in wet materials, unlike most other techniques used to estimate the pore size distribution. In this study, we measured the neutron scattering from brown coals and brown coals that had been heat-treated before and after being dried. This exploratory study has identified a number of previously unobserved effects of drying on the brown coal microstructure. The primary finding is that the scattering intensity from brown coal increases substantially upon drying. This increase cannot be caused by shrinkage of the coal being uniform on all scales upon drying and means that major changes in the microstructure of brown coals occur upon drying. It does not happen if the coal is treated wet at 300 °C prior to experimentation. It would be useful in coal dewatering studies to identify the temperature that the coal needs to be heated to “fix” its structure this way and if this temperature is dependent upon the constituents of the brown coal. If this increase is due to gel collapse, it would also be important to identify the point on the coalification step at which this no longer occurs. In this study, we also found that water rapidly penetrates pores of brown coal, even in the brown coal that had been heattreated, down to sizes of 50 nm at least. Nearly all pores of 50 nm radius are accessible to water, even a sample that had been heat-treated wet at 300 °C. Additionally, there is evidence that 1 μm pores in the wet coal are different in nature from those of 50 nm radius. Extending the study to higher Q values using SANS would also test whether the trends reported here extend to smaller pores. This approach may also be applicable to other gel structures to understand how they are restructured by drying.
Figure 9. Yallourn coal (original): Matchpoint for different Q values, expressed as weight fraction D2O/(D2O + H2O) (black). Calculated intensity ratio of the sample at the matchpoint compared to that in H2O: this is a measure of the fraction of pores that are closed at each Q value (red). Root-mean-square (rms) residuals of the parabolic fit to demonstrate fit quality (green).
the hydrogen-rich parts of the coal contain more fine pores and less large pores than the remaining part of the coal. Parts of brown coal can contain considerable concentrations of aliphatic material,19,20 and this part, by virtue of its greater hydrogen content, would have a lower SLD than the bulk coal. In any event, in wet brown coal, 1 μm pores appear on average to be different in nature to those of 50 nm radius. This unexpected finding would need to be confirmed by further studies. Influence of Ethanol Prewetting. Of the four samples examined, only the dried calcium-treated coal was able to be rewet without prior ethanol addition, presumably as a result of its more polar nature. Figure 10 shows the ratio of scattering
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APPENDIX Materials with fractal porosity that either do not shrink or shrink uniformly upon drying must have a lower neutron scattering intensity when dry than when wet. Derivation: At a given pore size, the wet brown coal can be divided into pores filled with water and the remaining brown coal, which also contains the remaining water (which is present in pores much smaller than the given size). The intensity of scattering is related to the square of the difference in the densities of the scattering length between the water in the pores in the coal and the brown coal. The SLD of water is negative. The brown coal itself contains water, and therefore, its SLD is a weighted average of the SLD values for the bulk brown coal (SLDbc) and its internal water (SLDw). The pore volume of the pores that are non-resolvable on this size scale is xw mL/g. The density of brown coal (without pores) is ρc. Thus, the total volume of brown coal plus water is (1/ρc + xw). The average SLD of the wet coal (SLDwc) is the sum of the volume contributions of the coal and embedded water.
Figure 10. Scattering intensity from the calcium-treated Loy Yang brown coal with a 50% D2O/H2O mixture, as a fraction of the intensity with 100% H2O added.
intensity from the coal with 50% D2O/H2O solvent against the corresponding intensity in water, with and without ethanol prewetting. The sample with ethanol in the D2O mix had a lower scattering intensity than without it. We attribute this difference to the dilution of the D2O/H2O mixture by ethanol, reducing the SLD of the bulk liquid and, thus, changing the
SLDwc = (SLDbc /ρc + SLDw x w )/(1/ρc + x w) 236
(1)
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Notes
The density contrast between the wet coal and water (ΔSLDw) is the difference between SLDwc and SLDw, hence
The authors declare no competing financial interest.
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ΔSLDw = (SLDbc /ρc + SLDw x w)/(1/ρc + x w) − SLDw
ACKNOWLEDGMENTS The authors thank Peter Scaife and Louis Wibberley (Energy Technology, CSIRO) for the coal samples used in this study. The authors also thank Yuri Melnichenko (Oak Ridge National Laboratories) and the Jülich Centre for Neutron Science at FRM II for organizing beam time on the KWS-3 instrument in March 2013, where the increase in neutron scattering intensity from brown coal upon drying was first identified.
(2)
which simplifies to ΔSLDw = (SLDbc − SLDw )/(1 + x wρc )
(3)
The calculation for the SLD of the dry coal is approached in a similar way. At a given pore size, the effective SLD of dry coal in contrast calculations is the SLD of the bulk coal reduced by the volume fraction of pores from all pores of smaller sizes. The volume of these pores in dry brown coal is xd mL/g [Note that xd and xw are dependent upon Q. At large Q (small pores), xd and xw tend to zero as the volume of pores of smaller sizes decreases. Thus, the effective SLD decreases with an increasing pore size, because more pores are included in the average SLD. This effect has been observed in other highly microporous carbons.18 Here, we are considering the low Q region, where xd and xw are large.]. Thus, the SLD measured for the dry coal is SLDdc = SLDbc /(1 + xdρc )
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(4)
Brown coal pores shrink upon drying. The total pore volume is reduced from xw to xd mL/g. If we assume the shrinkage is by the same percentage on all size scales, then the ratio of the diameter of each pore in the dry coal to what it was in the wet coal would be (xd/xw)1/3. At a given pore size, as a pore shrinks, its place is taken by previously larger pores. If the number of pores follows a fractal distribution, then a plot of log[I(Q)] versus log Q would yield a straight line with slope s (with s usually being between −3 and −4). The number of pores varies as r−(7 + s),21 and thus, the exponent is negative (with these values of s, there are always fewer larger pores). Thus, upon drying, the number of pores at a given size is reduced, by a ratio of (xw/xd)−(7 + s)/3. Because the scattering intensity is proportional to the product of the number of pores and the square of the density contrast, the ratio of intensity between dry and wet brown coal (Rdw) at a given pore size is the square of the ratio of the wet and dry SLD contrast (eqs 3 and 4) multiplied by the ratio in the number of pores at a given size or ⎧ ⎫2 ⎧ x ⎫−(7 + s)/3 SLDbc (1 + x wρc ) ⎬ ⎨ w⎬ R dw = ⎨ (SLD − SLD )(1 + x ρ ) ⎩ bc w d c ⎭ ⎩ xd ⎭ ⎪
⎪
⎪
⎪
(5)
Provided that 0 < xd ≤ xw < 2 and s is between −3 and −4, Rdw is always less than 1. In other words, if the original specimen contains water, the mass of water adsorbed is twice or less than that of the dry material, and the shrinkage of a material upon drying is proportionally the same on all size scales, the neutron scattering intensity must decrease upon drying. If, at any Q value, there is an increase in scattering intensity upon drying brown coal, its pores cannot shrink by a percentage that is independent of the pore size.
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REFERENCES
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[email protected]. ORCID
Richard Sakurovs: 0000-0003-0967-6560 237
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