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.
The heating value of coal depends on the Combustible Matter, mainly Carbon and Hydrogen, in the coal and how it reacts during the combustion process. Apart from the Combustible Matter the other main ingredients are Ash and Moisture.
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."
A coal sampled at the mines having a moisture of 20%, after storage for a few weeks, when entering the boiler can have moisture of only 8%. If the Coal sampled at the mines has an HHV of 18800 kJ/kg the same coal at the boiler will have a higher HHV. This is because the percentage of combustibles in one kg is more. Based on a simple proportion calculation, multiplying by a factor [100-8]/ [100-20] the HHV will be 21620 kJ/kg.
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 ADB analysis is corrected to the field conditions by using the Moisture content in the first sample. The HHV, the proximate analysis and the Ultimate analysis are all corrected by using the same factor.
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). All analysis values and HHV is corrected to this zero moisture.