What is Total Moisture in Coal and How to Determine It?

Total moisture in coal specified in ARB basis (As Received Basis).This basis is the fairest in the coal trade. For example, the value of the calorie coal stated in ARB for purposes of trade. While other parameters are usually analyzed in ADB (Air Dried Basis) where the sample has dried first by air at a certain temperature.

Total moisture is one of the important parameters in coal trading

Total moisture in coal is the total amount of water in the form of inherent and adherent when it is taken at the time of sampling or when coal was received. The amount of water may be expressed in terms of a percentage of the weight of the coal.

The Drying Stage of The Sample in Determining The Total Moisture In Coal

The sample preparation process for total moisture is very important. Because the value will be related to the calculation of the caloric value in the ARB  or for the calculation of other parameters on the same basis.

a. First stage

In the process of determination of total moisture of coal, this is usually dried in two stages. In the first stage of drying (ADL 1) or also called free moisture (FM), samples of the specific size and weight are dried in the preparation room until it reaches a constant weight. The temperature used in the first stage is ambient room temperature not exceeding 40 degrees Celcius to prevent damage to components of the coal. Especially for the coal with low caloric value content will be vulnerable to oxidation.

This constant weight is achieved when the percentage difference in moisture vapor is below 0.2% / hour (or 0.1 %/h in ASTM). This difference applies to the last two weighings. The constant weight is very important to achieve. Because the coal condition is estimated to no longer absorb or release water vapor so that the test results will be more accurate.

Other alternative can be done using a drying oven with a good flow of air circulation and use trays with certain size as sample containers.

Air dry oven is used for second stage air-dry loss process

Actually, it depends on the condition of the coal. If coal is known to have a high content of moisture then two-stage drying must be done. It aims to help the drying process in the second stage later. However, if the total moisture content is low, then single stage drying can be done.

Coal in tray

Furthermore, the particle is crushed to a smaller size and then allocated for residual moisture analysis.

b. Second stage

In this second stage, the residual moisture is determined by drying the sample with a certain weight and size by using a minimum free space (MFS) oven that is fed with dry air flow with a certain flow rate. An example of a good quality MFS Oven is the Carbolite brand.

Minimum free space oven used to determine residual moisture in coal

Nitrogen can be used as a dry gas (or regular air that has been passed first using silica gel or other materials that can absorb water from the air in ASTM standard). The preheated oven temperature is 105 - 110 degrees Celsius and drying can be done for one hour. Then the value of the residual moisture can be calculated as the percentage of moisture that evaporates during heating.

Total Moisture Calculation

Furthermore, the total moisture value of coal can be calculated by the following formula:

TM = ADL + RM*(1-ADL/100)

Where:

TM = total moisture, % ar

ADL = air dry loss, % ar

RM = residual moisture, % ad

Total moisture is a very important parameter in coal. Because the value will be directly related to the value of the main parameters of calories in the ARB base. This base is commonly used in coal trading agreed by shipper and buyer.

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