The total moisture content of coal is the percentage of water in all forms (except water of crystallization of the mineral matter) that resides within the coal matrix.
The various forms of moisture in coal are described according to the manner in which they are measured by some prescribed standard method. These forms are
(1) inherent moisture, (2) surface or free moisture, (3) total moisture, (4) air-dry loss moisture, (5) residual moisture, (6) as-received moisture, (7) decomposition moisture, and (8) water of hydration of mineral matter.
Inherent moisture (bed moisture, equilibrium moisture, capacity moisture) is assumed to be the water held within the pore system and capillaries of coal and is not to be identified with residual moisture. Surface moisture (free moisture) is, as the term implies, water held on the surface of the coal. Total moisture is the moisture determined as the loss in weight in an air atmosphere under rigidly controlled conditions of temperature, time, and airflow (ASTM D-3302) and is the sum of inherent moisture and free moisture and is also the sum of the air-dry loss and residual moisture. Air-dry loss moisture is the loss in weight resulting from the partial drying of coal, and residual moisture is that remaining in the sample after determining the air-dry loss moisture. As-received moisture also is equal to the total moisture, or is the sum of the inherent and free moisture present in the coal at the time of the analysis. Decomposition moisture is produced from the thermal decomposition of organic constituents of coal. Water of hydration of mineral matter is the water that is incorporated into the crystal lattices of the clay and inorganic minerals in coal.
The moisture content of coal ranges from 6% by weight in anthracite coals to 45% by weight in lignite.
There are various standardized tests to determine the moisture content of coal and they are classified depending upon the type of moisture content tested (1) ASTM D-1412 (ISO 1018) for determination of the equilibrium moisture of coal at 96 to 97% relative humidity and 30◦C, (2) ASTM D-2961 for determination of the total moisture of coal reduced to number 8 (2.38-mm) top sieve size (limited-purpose method), (3) ASTM D-3173 for determination of the moisture in the analysis sample of coal and coke, and (4) method D-3302 for determination of the total moisture in coal. In addition to these test methods, the method of preparing coal samples for analysis (ASTM D-2013) gives directions for air-drying coal samples. It has been suggested that the best technique is to determine loss during drying in air (ASTM D-3302; ISO 11722) followed by co distillation of moisture with xylene. Alternatively, moisture can be determined in an oven (at a fixed temperature) that is swept with dry nitrogen or another inert gas (ISO 589).
Moisture determination (ASTM D-3173; ASTM D-3302) depends on the extent of sample preparation and the condition of the coal sample. The entire procedure for determining the total moisture in coal, after collecting the gross sample, begins with preparing the sample for analysis (ASTM D-2013). If the gross sample is sufficiently dry, it may be sieved immediately and air-dried. If the sample is too wet to reduce in size, it should be weighed before size reduction and air-dried using an oven that is set 10 to 15◦C (18 to 27◦F) above room temperature with a maximum oven temperature of 40◦C (106◦F); under ambient temperature conditions, ambient temperature should be used. In this manner, the moisture in the sample is reduced to an equilibrium condition with the air in the laboratory, and changes in moisture content are minimized during the crushing and grinding operations and even during analysis. After reduction of the gross sample to number 4 or number 8 top size, it is divided and a laboratory sample taken. The laboratory sample is then air-dried and reduced to number 8 top size if necessary. If the total moisture is to be determined (ASTM D-3302), residual moisture is determined by heating at 104 to 110◦C (219 to 230◦F) for approximately 1 hour. Air drying removes most of the surface moisture of coal, while a temperature of approximately 107◦C (225◦F) is needed to remove inherent moisture. At temperatures of approximately 200 to 300◦C (392 to 572◦F), moisture from the decomposition of organic materials is driven off, but water of hydration requires a considerable amount of energy for expulsion. For example, the water of hydration in clay minerals may require a temperature in excess of 500◦C (932◦F). However, the issues of decomposition moisture and water of hydration of mineral matter are not usually dealt with in conventional analysis because the temperatures specified in the test methods for moisture determination are well below those needed to remove such moisture. Usually, the first moisture value to be obtained on a coal sample is the air- dry loss moisture. This moisture loss occurs during an attempt to bring the coal sample into equilibrium with the atmosphere in the sample preparation room. The practice of using temperatures above room temperature may accelerate oxidation but shortens the time needed for air drying; hence, temperatures above 40 to 50◦C (104 to 122◦F) are not recommended for air drying.
In the test method for determination of the equilibrium moisture in coal (ASTM D-1412; ISO 1018), a sample is brought into equilibrium in a partially evacuated desiccator with an atmosphere of 96 to 97% relative humidity at 30◦C (86 ◦ F). However, as in all methods of determining moisture, precautions must be taken to obtain reliable results from this test method. Over-dried and/or oxidized coal results in low moisture values. To prevent overdying, the sample should be kept wet before this test is run, and using a dry nitrogen atmosphere can prevent oxidation of the coal during the test. During the test itself, it is important to observe the temperature and time limits for equilibration (as specified in the test method). Furthermore, sudden lowering of the temperature or a sudden surge of air into the desiccator after equilibration may cause condensation of moisture on the coal. In addition, loss of part of the coal sample when a sudden surge of air is allowed into the evacuated desiccator voids the results of the test.
Sample handling should be kept at a minimum during moisture determination, thereby eliminating the potential for loss or gain of moisture during prolonged handling. Heat generated by the crushing and grinding operations used during reduction of the gross sample may be sufficient to cause moisture loss. Alternatively, or in addition, the relative humidity of the sample during preparation and the relative humidity in the testing laboratory may also change during the time that is required for a complete analysis. Air-drying steps in the analysis and efficient sample handling help minimize the effects of relative humidity changes. Exposure of the coal sample to the atmosphere for extended periods of time increases the chances of oxidation, which results in a weight gain by the coal sample that offsets part of the loss of moisture and gives moisture results that are incorrect. In the determination of moisture by a weight-loss method, it is necessary to attain a constant weight, which requires alternate heating and cooling of samples. Prolonged heating or an excessive number of alternate heating and cooling cycles should be avoided, to minimize the chances of oxidation.
The natural bed moisture of coal is determined (ASTM D-1412; ISO 1018) by wetting the coal, removing the excess water by filtration, and allowing moisture equilibration to occur by standing the coal over a saturated solution of potassium sulphate in a closed vessel, thereby maintaining the relative humidity at 96 to 97%. The vessel must be evacuated to about 30 mm Hg and the entire sample maintained at 30 ± 0.2◦C (86 ± 0.4◦F) for 48 hours for coals of higher rank than lignite; lignite will require 72 hours to reach equilibrium. The method can also be used to estimate the surface for extraneous moisture of wet coal; such moisture is the difference between the total moisture of the coal and the natural bed moisture.