COAL ANALYSIS – Effects and Corrections due to Moisture
Moisture content in the coal affects the reported Heating Value of coal. Corrections have to be applied to get the real values. Read on to learn about these corrections
Fuel cost is the most important single component that determines the cost of electric power. Coal is the primary fuel used for power generation. All utility companies are concerned about how much has to be paid to the coal suppliers. Fuel cost is directly linked to the Heating Value or the energy content of the coal. Determining the correct heating value becomes an important commercial issue and directly affects the profitability of a power plant.
In its journey from the mines to the power plant, Combustibles and the Ash in the Coal remain unchanged. What changes is the moisture content. A Coal sampled at the mines will have a moisture content higher than what enters the boiler.
The moisture in Coal consists of two parts.
- One is the moisture held within the molecular structure of the coal called “inherent moisture.” Removal takes place only at temperatures greater than 100 deg C.
- The second part is the moisture that is on the surface of the coal. This is normally due to the conditions and locations of the mines. This moisture can evaporate in exposed atmospheric conditions. How much evaporates depends on
- The time of exposure,
- Atmospheric contact because of spreading,
- Ambient temperature and humidity.
- The Moisture in the Coal is both these put together and is the “Total Moisture.”
In real life, Coal analysis takes place in the laboratory. The coal is spread out and air-dried to remove most of the surface moisture. The Coal Analysis and Calorific Value is determined and reported as on Air Dried Basis (ADB).
This ADB analysis is corrected to the actual conditions in the field to the extent of the difference in the moisture content.
Since there is a lag between the sampling and the lab analysis, some moisture can evaporate.
To avoid this error, coal is sampled in two parts.
- The first part kept in a sealed container and is used to determine only the Total Moisture (TMactual).
- The second part is used to find the ADB analysis. Moisture is also determined in the ADB analysis. (Madb)
The correction factor is based on simple proportion accounting for the moisture difference.
HHVactual = HHVadb x [100 – TMactual] / [100- Madb].
This gives you the exact GCV at the point of sampling be it at the point of receiving (AS Received Basis) or when it enters the boiler (AS Fired Basis).
The same is correction is applied for Proximate analysis values and Ultimate Analysis value also.
VMactual = VMadb x [100 – TMactual] / [100- Madb]
Ashactual = Ashadb x [100 – TMactual] / [100- Madb]
Often when comparing different coals it is a practice to take zero percentage Moisture as the reference (Moisture Free basis).
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