The Chemical Analysis of Argonne Premium Coal Samples
Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144
Rehydration of Desiccated Argonne Premium Coal Samples
By Marta R. Krasnow and Robert B. Finkelman
Eight Argonne Premium Coal samples stored in polyethylene bottles for several years exhibited substantial moisture losses. The samples retained an average of 23 percent of their original moisture content, with a range of 5 to 59 percent retention. Resaturated samples averaged 53 percent of the original moisture, with a range of 14 to 95 percent. If desiccated coal samples are to be analyzed, we recommend that moisture contents be determined prior to analysis. Drying coal samples prior to analysis would be an acceptable alternative if the original moisture content is known and the sample does not pick up moisture between drying and weighing for analysis.
The Argonne National Laboratory has sealed its eight Premium Coal samples in glass ampoules filled with an inert gas (argon) to ensure that they retain their original properties during storage (Vorres, 1990). Once the ampoules are opened in a laboratory, however, sample alteration, such as dehydration and oxidation, can occur.
We experienced a situation in which Argonne Premium Coal samples were stored in our laboratory for 2 to 4 years in polyethylene bottles, which allowed the samples to dehydrate (see columns 1 and 2 of table 1). A substantial amount of moisture had been lost from each sample during storage. This paper is intended to illustrate the degree of dehydration and to discuss the possibility of rehydrating the samples.
From 41 to 95 percent of the original moisture was lost during the unregulated storage of the 'aged' samples. Sample WY PC-2 lost almost 20 weight percent moisture, and ND PC-8 lost more than 30 weight percent. Sample POC PC-5 lost the least absolute amount of moisture, slightly more than 0.5 weight percent.
To resaturate the samples, weighed splits of -100 mesh coal were placed in uncovered petri dishes in a vacuum desiccator containing a barometer and a thermometer. Approximately 800 mL of deionized water was added to the base of the desiccator. The samples were kept under atmospheric pressure in the desiccator at 90 percent relative humidity and 22°C for 24 hours, after which they were removed from the desiccator and weighed. Several samples had increases far in excess of the original moisture loss. For example, sample UF PC-1, which had lost about 1 weight percent moisture, increased over 5 weight percent. This 'excess' moisture may be due to condensation on the coal particles. We therefore allowed the resaturated samples to equilibrate with the ambient atmosphere for 24 hours (22°C -23°C, 60-64 percent relative humidity) prior to determining the moisture (referred to as resaturated moisture) by the American Society for Testing and Materials (ASTM) (1996a) procedure (table 1, column 3).
After the 24-hour residency in the desiccator at the 90 percent relative humidity and subsequent equilibration, moisture increased substantially in most samples. Sample IL PC-3 regained 67 percent of the original moisture (table 1, column 1), and UT PC-6 regained 66 percent. Sample IL PC-3 regained 5.36 weight percent moisture, and ND PC-8 gained almost 10 weight percent. Only sample UF PC-1 showed no gain in moisture. Despite the substantial increases in moisture for most samples, none of the samples, with the possible exception of PITT PC-4, recovered all the moisture lost on drying.
Experiments on water desorption and adsorption have been conducted by varying vapor pressure and holding the temperature constant, usually less than 40°C (Gauger, 1945; Allardice and Evans, 1978). Results of the experiments indicate that, once dry, a coal sample will not adsorb enough water to regain its original moisture content. There is no generally accepted mechanism to explain this phenomenon (Allardice and Evans, 1978); however, several theories have been offered. These include (1) the shrinking of coal on drying, which causes a collapse of some capillaries, so that the dried material can no longer hold or take up as much water as it held originally, and (2) the replacement of moisture on the walls of some capillaries by adsorbed gases, making it difficult to re-wet the capillaries (Gauger, 1945).
Vorres and Kolman (1988) and Vorres and others (1988) conducted drying and rehydration studies of Argonne Premium Coal samples. They concluded that coal rank, particle size, and degree of oxidation affected moisture removal and replacement.
The Argonne Premium Coal samples behave in a typical fashion with respect to rehydration. Improper storage can lead to substantial moisture loss. Resaturation generally will restore some of the lost moisture. These observations on moisture loss are important if measurements of physical properties are to be made on samples that have been stored under noncontrolled conditions for any length of time. These observations are also important for calculations involving chemical analysis of the raw coal (for example, instrumental neutron activation analysis or X-ray fluorescence analysis). Assuming that the coal samples have retained their original moisture contents can lead to errors of as much as 30 percent for low-rank coals (lignites and subbituminous: ND PC-8 and WY PC-2, respectively). Even for higher rank coal (for example, IL PC-3), the errors can be as high as 6.5 percent.
We recommend the following procedures to minimize errors caused by desiccation (especially for low-rank coal):
The ASTM method for determining equilibrium moisture (ASTM, 1996b, D 1412-93) could be used to rehydrate the sample, but the method requires at least a 20-g sample, more time, and more equipment than the procedure described in this paper.
An alternative method would be to analyze a moisture-free sample by drying it (105°C for 24 hours) prior to analysis. Two assumptions are necessary: (1) that the published moisture value is applicable so that the chemical analysis can be recalculated to an as-received basis, and (2) that the sample does not pick up moisture between drying and weighing for analysis.
Allardice, D.J., and Evans, D.G., 1978, Moisture in coal, in Karr, C.C., Jr., ed., Analytical methods for coal and coal products (v. 1): New York, Academic Press, chap. 7, p. 247-262.
American Society for Testing and Materials (ASTM), 1996a, ASTM Designation D 3173Ð87, Standard test method for moisture in the analysis sample of coal and coke; 1996 Annual Book of ASTM Standards, v. 05.05, Gaseous fuels; Coal and coke, p. 289Ð290.
------1996b, ASTM Designation D 14120-93, Standard test method for equilibrium moisture of coal at 96 to 97 percent relative humidity and 30°C; 1996 Annual Book of ASTM Standards, v. 05.05, Gaseous fuels; Coal and coke, p. 188-191.
Gauger, A.W., 1945, Condition of water in coal, in Lowery, H.H., ed., Chemistry of coal utilization (v. 1): New York, J. Wiley & Sons, p. 600-626.
Vorres, K.S., 1990, The Argonne Premium Coal Sample Program: Energy and Fuels, v. 4, no. 5, p. 420-426.
------1993, Users handbook for the Argonne Premium Coal samples: Argonne National Laboratory [Report] ANL/PCSP-93/1, 200 p.
Vorres, K.S., Kolman, R., and Griswold, T., 1988, Kinetics of vacuum drying and rehydration of Illinois #6 coal samples. Implications for pore structure: Preprints, Fuel Chemistry Division, American Chemical Society, v. 33, no. 2, p. 333-342.
Vorres, K.S., and Kolman, R., 1988, Kinetics of the vacuum drying and rehydration in nitrogen of coals from the Argonne Premium Coal Sample Program: Preprints, Fuel Chemistry Division, American Chemical Society, v. 33, no. 3, p. 7-16.
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