In many nations, the blending of high grade imported coal with low grade high ash coals has long been adopted. Many methods may be used. The blending can occur at the coal mine, preparation plant, trans-shipment point, or at the power station. The method selected depends upon the site conditions, the level of blending required, the quantity to be stored and blended, the accuracy required, and the end use of the blended coal. Normally in large power stations handling very large quantities of coal, the stacking method with a fully mechanized system is followed.
To decide to blend or not, it is very important to understand the composition of the coals that are to be blended. This means one will have to understand the origin of coal, the organic and inorganic chemistry of coal, and the behavior of the coals in questions. It has been established that coals produced by the drift theory of coal formation and coals formed by the swamp theory of coal formation have to be blended with caution. The main difference is that coal formed by drift theory exhibits pronounced regional variation in thickness and quality of seams. They also have enormously high ash content with varying inorganic chemistry. The organics of drift origin coal also present problems mainly because the vegetation that lead to the forming of the coal drifted from different places having different kind of vegetation. In contrast, the coals formed by the swamp theory have much more uniform organic properties and much lower ash content with consistent inorganic chemistry.
During combustion, it is necessary to understand the physical conditions and coal properties during heating of the particles, devolatalisation, ignition and combustion of the volatile matter, and ignition and combustion of the char. It is also equally important to know the phase changes in mineral matter and other inorganics present in coal. The combustion efficiency and carbon loss will have to be also addressed during blending of coals. It is also necessary to look into the aspects of slagging, fouling, and emission characteristics like NOx, Sox, and particulates.
Because of the complexity of the combustion process and the number of variables involved (which are still not fully understood), it is difficult to extrapolate small scale results to a full scale power plant. Thus, operational experience with a wide range of plant configurations with a variety of coal feedstock is essential for determining the practical significance of results from bench – and pilot – scale tests. More published research about how the behavior of the coals and coal blends utilized in tests differ from their actual performance in power station boilers is required.
Predicting the risk of spontaneous combustion of coal stocks is another aspect of current fuel quality research. In addition to the inherent dangers, uncontrolled burning can lead to the release of pollutants. The economic issues associated with the loss of a valuable energy resource are also a concern.
For more basic information, read about how coal power plants generate electricity by burning coal and find some other interesting facts about the process.