Considerations for Coal Blending in Power Stations

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.

The presence of trace elements in coal combustion has also received increased attention throughout the world during the last few years, with elements such as mercury of particular concern. One way to reduce trace element emissions is cleaning the coal prior to combustion. The use of cleaner coals – those with lower ash and sulfur content – can have the added advantage of substantially reducing operating costs. Again, however, some effects may be detrimental (ash deposition may be exacerbated, and the effects on corrosion and precipitator performance are uncertain), which makes testing vital.

It has been found from field data that even if the blended coal closely resembles the design coal for the boiler, the blend need not perform the same way. This is mainly due to the transformation of inorganic particles during combustion and the way in which the organics are dispersed in coal. A limitation to blending coals is the compatibility of the coals themselves, and problems are more likely when blending petrographically different coals or coals with different ash chemistry. Non-additive properties make blend evaluation for power generation inherently complex. More work is required on understanding how the inorganic components of coals in the blend interact and how it affects ash behavior including its emissivity, reflectivity, and thermal conductivity.

Blending decisions should be based on the knowledge of the specific behavior of a given pair of coals, rather than an assumption of linear variation of properties with blend traction. The ever more stringent constraints placed on coal-fired power stations worldwide and the continuing development of new technologies means that the issue of fuel quality improvement will remain a primary factor.

About the Author

Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.

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