Internal and external water in coal
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What are the internal and external waters in coal? I found a post on a forum that described it as follows: The moisture in coal can be divided into three types, based on its state of existence within the coal, namely external moisture, internal moisture, and bound water. Free moisture (or surface moisture) refers to the water that adheres to the surface of coal particles and in capillaries with a diameter greater than 10-5 cm during coal mining, transportation, storage, and washing processes (denoted by the symbol Mf). The external moisture is combined with the coal mechanically; its vapor pressure is equal to that of pure water, making it easier to evaporate. When coal is placed in air at room temperature, the external moisture continuously evaporates until equilibrium is reached with the relative humidity of the air. The water lost at this point is external water. Coal that contains external moisture is called received coal, while coal that has only lost its external moisture is called air-dried coal. Coal quality testing is usually carried out using air-dried coal samples. (Intrinsic moisture (Wnz) refers to the water that is adsorbed or condensed within the capillaries of coal particles (with a diameter of <10-5 cm). What does “water in capillaries with a diameter greater than 10-5 cm” mean? Are capillaries with a diameter greater than 10-15 centimeters that big? Is it wrong? Please provide the correct explanation.
Coal is a porous material that contains both internal and external water. Based on capillary pores with a diameter of less than 10-5 centimeters, when the diameter is below this value, the adsorption effect of capillary pores results in a vapor pressure for this water that is lower than that of pure water; as a result, it is difficult for this water to evaporate, and a temperature higher than the normal boiling point of water is required to remove it