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]