Energy & Fuels 1996,9, 20-24
20
Density Measurements of Argonne Premium Coal Samples He Huang,? Keyu Wang,? David M. Bodily,* and V. J. Hucka$ Department of Chemical and Fuels Engineering, and Department of Mining Engineering, University of Utah, Salt Lake City, Utah 84112 Received February 22, 1994. Revised Manuscript Received September 21, 1994@
The densities of the Argonne Premium Coal samples were measured using H2, He, and a H2He mixture (49.13vol % of H2 in He) in a gas pycnometer. Significant differences between the apparent hydrogen densities and helium densities were observed. The larger H2 densities observed in the tests are attributed to H2 gas adsorptionlabsorptionon coal. The H2 gas adsorption coefficient increases with increasing carbon content. The “true” or intrinsic densities of coals extrapolated from plots of the measured density versus Ha gas partial pressure are very close to the He-densities. The organic coal density (dmmf coal density) was obtained after a correction for the mineral matter in the coals. It decreases with increasing carbon content initially, reaches a minimum at about 82 wt % carbon, and then increases with increasing carbon content.
Pressure transducer
Introduction The principle of density measurement using a gas pycnometer is based on the determination of the volume occupied by a known weight of coal sample in a chamber of known volume. The volume not occupied by the sample is measured by expansion of a probe gas at constant temperature into a second chamber of known volume. For porous coal samples, density measurements depend on properties of both the probe molecule and the coal structure. The size of the probe molecule, adsorptiodabsorption of probe molecules, and the pore structures of the coal become important. The ideal probe molecule for intrinsic coal density measurements should have no adsorptionlabsorption in the coal and should be able to access all of the pore volume. The helium molecule is the smallest spherical molecule. The hydrogen molecule is the smallest linear, cylindrical molecule. The kinetic diameter of H2 is larger than that of He, but the cross-sectional diameter perpendicular to the cylindrical axis would be smaller. Thus, both molecules will have high accessibility and the hydrogen molecule might have greater pore accessibility than helium. Preliminary results show that H2 densities of coal samples measured with a pycnometer are significantly larger than He densities. The differences between H2 and He densities could be due to greater Ha pore access, in which case the Hz densities would be true densities, or H2 gas adsorptionlabsorption on coal samples, in which case the densities would be in error. If the He molecule had greater accessibility, the larger H2 densities could only be explained if there was also H2 adsorption. A H2-He gas mixture (49.13 vol % of HQin He) was used in this study to explore these possibilities. In this paper, we report the measurements of the density for the eight Argonne Premium Coal samples using Hz, He, and a H2-He mixture in a gas pycnom-
C e l l chamber
A I
I 11 1 coal sample
F i l l valve
Expansion valve
~
Expansion chamber
vent valve
Figure 1. Schematic diagram of gas pycnometer.
eter. The significant increase in apparent density in H2 gas compared to that in He gas is discussed.
Apparatus and Methodology The AccuPyc 1330 pycnometer used in these experiments determines density by measuring the pressure change of a probe gas in a calibrated volume. A schematic diagram illustrating the methodology used in the gas pycnometer is shown in Figure 1. The gas pycnometer includes a cell chamber, an expansion chamber, three valves (fill valve, expansion valve, and vent valve), and a pressure transducer. The analysis directly measures the pressure change due to the expansion of the gas from the cell chamber into the expansion chamber, from which sample volume and density can be calculated if the sample weight is known. The P-V-T (pressure-volume-temperature) in cell and expansion chambers before and after expansion are summarized in Table 1. Since the number of moles, n, is constant, (ncell
+ nexp)before expansion = (ncell + nesp)afler expansion (l)
+ Present address: Department of Chemical Engineering, University of Delaware, Newark, DE 19716. Department of Mining Engineering. @Abstractpublished in Advance ACS Abstracts, November 1, 1994.
*
0887-0624/95/2509-0020$09.00/0 0 1995 American Chemical Society
Density Measurements of Argonne Premium Coal Samples Table 1. P-V-T in Cell and Expansion Chambers before and after Expansiona before expansion after expansion expansion expansion cell chamber chamber cell chamber chamber
a p , gauge pressure (psig); V, volume (cm3);cell, cell chamger; exp, expansion chamberl; patm,atmospheric pressure (psig); Ta, ambient temperature.
if the probe gas is an ideal gas under operating pressure (120 psig) and ambient temperature (298 K). Vfis the free volume occupied by the gas in the cell chamber. Rearranging eq 2 gives
(3)
Energy & Fuels, Vol. 9, No. 1, 1995 21
where
E
is the inaccessible porosity, i.e.
Based on eq 9, &Ne > e m if E > 0. Effect of the Adsorption and/or Absorption of the Probe Gas. If the probe gas adsorbs on the measured sample (it is impossible t o differentiate between adsorption on the pore surface and absorption into the coal matrix using this method, so adsorption will be used in this paper), similar to eq 1, we have (ncell
+ nexp + nads)before
expansion
- (ncell + nexp + nads)a&er expansion
(
'1
If the probe gas is an ideal gas under operating pressure (< 20 psig) and ambient temperature and the adsorption isotherm follows
On the basis of (4)
Equation 11becomes
and substituting eq 3 into eq 4,we have
Finally, sample density is determined using eq 6: Rearranging eq 13 gives
W
e = WN, = Vce11
-
(6)
Pcell,z Pce11,l
Vf =
Vexp
- Pce11,a
PceU,l
The pressures, PceU,l and Pcell,z in eqs 2, 3, 5,and 6 are measured by the pressure transducer attached to the cell chamber (see Figure 1). The cell volume, Vcell, and expansion volume, Vexp, in eqs 2, 3, 4, 5, and 6 are determined by calibration using a steal ball of known volume. The sample density measured by this method is not the true density under two circumstances: (1)if the probe gas cannot access all of the pore volume for porous materials; and (2) if the probe gas adsorbs on andor absorbs in the measured sample. In summary, the intrinsic density of the sample is underestimated by the effect of the pore accessibility and overestimated by gas adsorption. Effect of the Accessibility of the Probe Gas. If the probe gas cannot access all of the pore volume, the true volume of the sample (Vs,tNe) should be (7) where Vs,mis the measured sample volume, and Vpmy the pore volume which is not accessible to the probe gas. Therefore, from eq 7, we have
Vexp- ItRTaW
PceU,z
-p c e ~ , ~
substituting into eq 4, we have
Finally, the correlation between measured density and true density is given in
,
+ kRTa em --I @true
+kRTa
(17)
em
Based on eq 17, e t n e < e m if k > 0. Combining eqs 9 and 17, the effects of both porosity and gas adsorption on the true density of sample can be represented by
Btne
(9)
(16)
i.e.,
-=--
i.e.,
(14)
E+kRTa
(18)
em
If gas A in a mixture is an adsorptive gas and gas B is a nonadsorptive gas, the adsorption isotherm eq 12
22 Energy & Fuels, Vol. 9, No. 1, 1995
Huang et al.
Table 2. Proximate and Ultimate Analyses of the Argonne Premium Coals coal sample proximate analysis (wt %) ultimate analysis (wt %, daf) steam state rank moisture ashu VMa C H 0 S Beulah-Zap ND lignite 32.24 9.7 44.94 72.9 4.83 20.3 0.70 Wyodak-Anderson subbituminous WY 28.09 8.8 44.73 75.0 5.35 18.0 0.47 Illinois No. 6 high vol. bit. IL 7.97 15.5 40.05 77.7 5.00 13.5 2.38 high vol. bit. Blind Canyon UT 4.63 4.7 45.84 80.7 5.76 11.6 0.37 high vol. bit. wv Lewiston-Stockton 2.42 19.8 30.17 82.6 5.25 9.8 0.65 Pittsburgh No. 8 high vol. bit. PA 1.65 9.2 37.82 83.2 5.32 8.8 0.89 med. vol. bit. Upper Freeport PA 1.13 13.2 27.45 85.5 4.70 7.5 0.74 low vol. bit. Pocahontas No. 3 VA 0.65 4.8 18.60 91.1 4.44 2.5 0.50 Dry basis. ~
~
~~~
N 1.15 1.12 1.37 1.57 1.56 1.64 1.55 1.33
Table 3. Measured Densities of the Argonne Premium Coals measured density, g/cm3 coal Beulah-ZaD Wyodak-kderson Illinois No. 6 Blind Canyon Lewiston-Stockton Pittsburgh No. 8 Upper Freeport Pocahontas No. 3 a
He 1.4527 1.4069 1.4587 1.3127 1.4637 1.3718 1.4198 1.3796
Hz 1.5251 1.5124 1.6011 1.4046 1.6011 1.4776 1.5738 1.5837
H2-Heu 1.4867 1.4558 1.5221 1.3538 1.5268 1.4207 1.4897 1.4695
k,b ,umoll(g)(bar) 1.472 2.234 3.140 2.238 3.076 2.414 3.360 4.120
dcm3 1.4514 1.4050 1.4529 1.3081 1.4613 1.3689 1.4166 1.3738
@truel
@He
- @truet dcm3
0.0013 0.0019 0.0058 0.0046 0.0024 0.0020 0.0032 0.0058
49.13 vol % of Hz in He. Dmmf basis.
should be modified by the partial pressure of adsorptive gas, i.e.,
where
(20) Substituting eq 19 into eq 11 and going through the steps by which eq 17 was derived, we obtain (21) In operation, the following parameters can be modified to optimize the analysis: number of purges, purge fill pressure, number of runs, run fill pressure, equilibration rate, and run precision. The runprecision option allows early and automatic termination of the analysis if the last five runs are all within a user-specified tolerance. Purge is used for sample and system clean up, i.e., removal of any residual air andor moisture from the sample surface and matrix and the chamber. It is accomplished by closing off the pycnometer block and filling both the sample and expansion volumes to the designated purge pressure. The chambers are then vented to the atmosphere, resulting in elimination of water vapor, air, or other contaminants. The zero function of the pressure transducer is used t o calculate a new zero pressure offset. It is measured when the system is at atmospheric pressure and thermal equilibrium and is subtracted from all subsequent pressure readings in order to obtain a true gauge pressure.
Experimental Section The eight Argonne Premium Coal Bank samples were carefully collected and preserved.l They represent a range of coal rank from lignite to low-volatile bituminous. The proximate and ultimate analyses1 of the eight coals are shown in (1)Vorres, K. S. Energy Fuels 1990,4, 420.
Table 2. The particle size of these coal samples is -100 mesh. The distribution of the particle size for these coal samples is given in the User's Handbook.2 The samples were dried at 80 "C in nitrogen gas under a vacuum of about 45 kPa (-8 psig) for 48 h before the measurements. The densities of each coal sample were measured separately in the order of He, Hz-He mixture gas, and HZusing the gas pycnometer (AccuF'yc 1330 Pycnometer, Micromeritics, Inc.) described above. The cell volume and expansion volume are 12.3848 and 8.0184 cm3, respectively. Both the purge pressure and the run pressure were set a t 19.500psig. The number of purges was 20 for the best results. A very low pressure equilibration rate of 0.0010 psig/min was chosen to give enough time for diffusion and adsorption before the fill valve was closed. The density datum of each coal reported in this paper is a n average value of five measurements and the standard deviation is less than 0.0005 g/cm3. About 5 g of coal sample was used for each analysis. The measurement temperature (T,)was about 25 "C. The helium and hydrogen gases had guaranteed minimum purity of 99.99%. The helium-hydrogen gas mixture was composed of 49.13 .% hydrogen by volume in helium.
Results and Discussion The measured helium, hydrogen, and hydrogenhelium densities of each of the eight Argonne Premium Coal samples are shown in Table 3. The Hz densities are significantly greater than the He-densities for all eight Argonne Premium Coal samples. As discussed in a previous section (Apparatus and Methodology), only two reasons have been identified to explain this observation, i.e., H2 pore accessibility and H2 gas adsorption, when He gas is taken as a reference gas. If the H2 densities of the coal samples are mainly affected by pore accessibility, the results obtained using the H2-He gas mixture as a probe should be nearly identical to those using pure Ha gas. This is because the H2 in the H2He mixture could access the pore structure of coal just (2)Vorres, K. S. Users' Handbook for the Argonne Premium Coal Samples Program ANLiPCSP-89/1,Argonne National Laboratory: Argonne, IL, 1989.
Energy &Fuels, Vol. 9,No. 1, 1995 23
Density Measurements of Argonne Premium Coal Samples I
a78,
a
7
6
4.5,
1
I
k
0.74
ai
0.2
a3
0.4
a5
0.6
0.1
aa
0.9
i a r b o n Content, m% (dnf)
Fraction of H y d r w n Oat in Helium
Figure 2. Reciprocal measured density (I,/@,) of Hz (YH~).
versus fraction
like pure Hz gas and the partial pressure of hydrogen in the smaller pores should be nearly equal to the total pressure outside these pores. In fact, Hz-He densities differ from the Hz densities and they are intermediate between the HZ and He densities. This suggests that HZadsorption is the predominant factor for the increase in density measured by Hz gas. Taking He as a reference gas, the correlation of the measured density with the intrinsic or true density and gas adsorption effect is given in eq 21. In this case, we have
--1 - 1 @true
em
-k k R T ~ H 2
Table 4. Mineral Matter and Organic Coal Densities of the Argonne Premium Coals mineral water, wt % clay eavg,mm, eoa, coal quartz pyrite calcite (dim g/cm3 g/cm3 1.7 6.1 2.883 1.387 0.6 0.3 Beulah-Zap 0.1 0.4 6.2 2.843 1.342 Wyodak-Anderson 2.0 7.3 3.231 1.301 5.5 1.9 Illinois No. 6 3.4 1.3 2.7 2.924 1.273 0.8 0.5 Blind Canyon 0.3 18.4 2.881 1.289 0.3 Lewiston-Stockton 2.6 0.5 6.3 3.134 1.284 1.7 2.4 Pittsburgh No. 8 1.0 9.4 3.150 1.292 1.5 3.4 Upper Freeport 3.4 2.845 1.339 0.1 1.7 0.3 Pocahontas No. 3
(22)
the true where e m is the measured density (g/cm3), density (g/cm3),T, the measurement temperature, k the gas adsorption constant @moY(g)(bar)),and Y H ~the hydrogen gas fraction. The true density could be estimated by eliminating the effect of gas adsorption, i.e., taking the limit Y H ~ 0 in eq 22:
-
(23) Plots of 1/em against Y H ~for the eight coals are illustrated in Figure 2. Because all derivations are based on helium as a reference gas, the true densities, shown in Table 3, extrapolated from the Hz and HzHe density measurements are very close to the He densities. The differences are shown in the last column of Table 3. The very small positive values of the differences for all samples may be an indicator of a very small effect of He adsorption in coal during He-density measurements. These results indicate that HZ gas adsorption on coal is the reason for the increase in density using Hz gas. This is in good agreement with the results obtained by other researchers on the adsorption strength of gases on Illinois No. 6 c o d 3 Adsorption strength was found to increase with polarizability. The greater polarizability of hydrogen compared to helium should lead to stronger adsorption. The parameter K is also given in Table 3. A plot of the H2 gas adsorption constant (k)versus carbon content of coal is shown in Figure 3. The Hz gas adsorption increases with increasing carbon content. The strong correlation of Hz gas adsorption with coal rank may be an indication that some H2 gas absorption (3) Glass,A. S.;Larsen, J. W. Energy Fuels 1993,7, 994.
Figure 3. Correlation of the Hz gas adsorption constant (k) with carbon content of coals.
Dmmf basis.
occurs during the measurements. A strong coal rank dependence was also observed in methane, carbon dioxide, and other gas adsorption using a gas chromatographic method.* The measurements are corrected to a dmmf (dry mineral-matter-free) basis to obtain the organic coal densities. The important constituents of mineral matter in coal include quartz (2.65 g/cm3),pyrite (5.00 g/cm3), calcite (2.71 g/cm3),and clay (2.90 g/cm3h5 The contents of these mineral constituents for the eight coals2 are given in Table 4. A formula to calculate the average density of the mineral matter is derived as follows:
or
-=czf i 1
gave
where
and wi is the mass of species i (g), vi the volume of species i (cm3),ei the density of species i, Wmmthe total mass of the mineral matter (g), and fi the mass fraction of species i. The average densities of mineral matter (eaVg,-)in the coal samples are listed in Table 4. ~~
~
(4! Huang, H. Methane Formation and Retention in Coal. Ph.D. Thesis, University of Utah, 1992. (5) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington, DC, 1985; pp 14-16.
24 Energy & Fuels, Vol. 9, No. 1, 1995
Huang et al.
._-
1 d')
1.41
2
1.361
1.26
/
4
0
\ i
\
75
80
a5
90
95
Carbon Content, IM% (dag
Figure 4. Correlation of the organic coal density (ee) with carbon content (data points with x are from ref 7).
From eq 24, the formula for the evaluation of the dmmf coal densities (ec)becomes:
1 ---+@He
fmm @avg,mm
1-f"
(27)
@c
where fmm is the mass fraction of mineral matter in coal or the fraction of low-temperature ash. The calculated results of the organic coal densities are shown in Table 4. They show a notable variation with rank or carbon content (open circles, Figure 4). The organic coal density decreases with increasing carbon content initially. After reaching a minimum a t about 82 w t %, it increases with increasing carbon content. Walker and c o - ~ o r k e r s ~have - ~ extensively studied the densities of coals and report results for a number (6) Mahajan, 0.P.;Walker, P. L., Jr. InAnalytical Methods for Coal and Coal Products, Karr, C., Jr., Ed..; Academic: New York, 1978; Vol. 1, Chapter 4. (7) Gan, H.;Nandi, S. P.; Walker, P. L., Jr. Fuel 1972,51, 272. (8)Walker, P.L.,Jr.;Verma, S. K.; Rivera-Utrilla, J.; Davis, D. Fuel 1988,67,1615.
of fluids including helium. In a review article, Mahajan and Walker6 cite reports that indicate some pores are not accessible to helium and that helium may be adsorbed on carbon surfaces. Both effects are thought to be small a t 25 "C. Gan, Nandi, and Walker' measured the helium densities of coals which covered a range of rank. The results of their measurements are also included in Figure 4. Walker et aL8 reported the helium densities of a number of maceral concentrates. The results for vitrinites are similar to the results shown in Figure 4 but show a much larger range of density values. Resinites show a lower density and fusinites and semifusinites show a higher density than vitrinites.
Conclusions The measured H2 densities of coal samples are significantlylarger than the corresponding He densities. This is attributed t o H2 gas adsorption during the density measurements. The Hz gas adsorptiodabsorption coefficient increases with increase of carbon content or rank of coal. The organic coal density or dmmf coal density (ec) is correlated with the rank of coal. It decreases with increase of carbon content initially. After reaching a minimum at about 82 w t % C, it increases with increasing carbon content. A small volume of pores inaccessible to helium could account for the small positive difference between extrapolated densities measured in hydrogen and densities measured in helium. Acknowledgment. We acknowledge the financial support of the US Department of Energy under grant NO.DE-FG22-88PC88939. EF9400272 (9) Walker, P.L.,Jr.; Mahajan, 0. P. Energy Fuels 1993,7,559.
