Moisture content is an important parameter in coal analysis. It is needed for determining the calorific (heating) value and handling properties of a coal. The moisture value given for a proximate analysis is the moisture measured as mass lost from a sample under specified conditions after heating in a moisture oven to 104 to 110°C (ASTM method D3173-11; American Society for Testing and Materials, 2013, p. 494–497). This measured result is termed residual moisture (see below). This is the moisture value used for calculating other analytical results to a dry basis. It can be used in conjunction with air-dried moisture to calculate on an as-received basis.

Coal samples contain several different types of moisture, which are measured and recorded in different ways.

Although the proximate analysis moisture value is the most common moisture value reported, there are several other moisture values, which are used for different reports and calculations. Following is a list of common moisture terms and what they mean:

  • As-received moisture: Moisture (water) in a sample upon delivery to a lab. The same as as-delivered moisture. Also used as a recording standard (denoted as “ar” or “AR”). As-received (AR) moisture equals air-dried loss moisture plus residual moisture (measured from the proximate analyzer).
  • Air-dried moisture: Moisture (water) in a sample after air-drying. Air-drying removes surface moisture prior to analysis. Because this term was also sometimes used for residual moisture, it was withdrawn by the American Society for Testing and Materials (ASTM method D121-09; American Society for Testing and Materials, 2013, p. 355–368). The air-dried recording standard (denoted as “ad” or “AD”) remains valid for inherent moisture.
  • Equilibrium moisture: Moisture in a coal sample determined using ASTM method D1412-07 (American Society for Testing and Materials, 2013, p. 425–429). It provides an estimate of inherent moisture in wet coals or coals with visible surface moisture. It is approximately equal to inherent moisture in medium- and high-rank coals, but may be lower than inherent moisture in low-rank coals (ASTM method D121-09; American Society for Testing and Materials, 2013, p. 355–368).
  • Inherent moisture: Moisture (water) that occurs in the microscopic structure of the coal, generally filling pores, but not visible in large fractures (ASTM methods D388 and D1412-07; American Society for Testing and Materials, 2013, p. 390–396, 425–429, respectively), so it excludes surface moisture. Inherent moisture is approximately equivalent to equilibrium moisture, except in low-rank coals. It is also approximately equal to or slightly greater than pore moisture (ASTM method D121-09; American Society for Testing and Materials, 2013, p. 355–368). Thomas (1992) stated that this term has many different definitions, or has been used differently in different reports.
  • Pore moisture: Moisture (water) specifically occurring in pores within the coal, it is equal to or less than the inherent moisture (ASTM method D121; American Society for Testing and Materials, 2013, p. 355–368), so should also be less than or equal to the equilibrium moisture, except in low-rank coals.
  • Residual moisture: Moisture (water) remaining in coal after air-drying a sample and minor heating in a moisture oven to 10 to 15°C (depending on coal rank) above room temperature (ASTM method D121-09; American Society for Testing and Materials, 2013, p. 355–368). It is recorded on an as-determined basis (denoted as “ad” or “AD”). This is also the moisture value measured as part of a proximate analysis.
  • Surface moisture: Moisture (water) adhering to the outside of a coal sample or particle. It is sometimes referred to as adventitious moisture (Thomas, 1992) (ASTM method D388; American Society for Testing and Materials, 2013, p. 390–396). It is that part of the total moisture in excess of the inherent moisture (ASTM method D121-09; American Society for Testing and Materials, 2013, p. 355–368). It is recorded on an as-received basis (denoted as “ar” or “AR”).
  • Total moisture: All moisture (water) in and on a coal sample that can be removed through heating in an oven at 107°C according to ASTM methods D3302-12 and D2961 (American Society for Testing and Materials, 2013, p. 518–525, 485–486, respectively). It is essentially equal to the inherent moisture plus the surface moisture. It is recorded on an as-received basis (denoted as “ar” or “AR”).

Moisture absorbs heat, so high moisture content in coal reduces the relative efficiency of heating when a coal is combusted. This is why low-rank, high-moisture coals have lower calorific values (Btu/lb) than high-rank, low-moisture coals. Moisture adds weight to coal, and also contributes to spontaneous combustion in low-rank coals, and as such, affects handling and transport. Moisture can also cause coal to freeze in rail cars in the winter, which can be a major issue in power plants in colder climates. Moisture is used to define low-rank coals in some rank classification systems, and is an important criteria for determining coal grade in low-rank lignite coals.

  • If you have specific questions about this analysis, please contact Jason Backus.

 

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Last Modified on 2017-03-30
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