The quality of coals can vary over a wide range due to coalification history, mining and upgrading of a coal after mining. A range of parameters determines coal quality, since power plant operations cover a wide range of processes. Coal quality criteria therefore range from basics like the lower heating value via moisture and sulphur content to grindability and slagging propensity.

In the Netherlands only internationally traded coals are utilized for power generation. This means that for more than thirty years different coals from all over the world have been utilized in the Netherlands. The wide ranges of coal quality could only be fired due to blending facilities available for all the power plants in the Netherlands. In close co-operation with the power plants KEMA has since 1988 performed a large number of studies and experiments to optimize coal blending in relation to power plant operation. Most of these studies have been performed in full scale practice using all seven coal-fired boiler installations in the Netherlands. 

It appears that coal blending is a powerful tool to optimize between fuel costs and power plant operation. Striking examples are the successful results obtained at the Maasvlakte power plant at the port of Rotterdam. This power plant had to combat several problems at their plant operation, which was symbolized by the so-called Diabolic Triangle. In order to simultaneously fulfil the constraints for thermal efficiency, NOx-emission and unburned carbon, blending proved to be a very effective means to meet these constraints at acceptable fuel costs. From systematic analysis of power plant results with more than thirty different coals and even more coal blends, a number of relations has been formulated. Using these relations the power plant was able to effectively compose coal blends within the existing constraints.

Extensive experience has been gained by KEMA in full-scale measurements with element distributions in a power plant. This applies to the main elements in the coal and the ash as well as the trace elements. It has resulted in a large database with the results of these measurements obtained from firing single coals as well as coal blends. On the basis of these data a model has been developed in order to predict element distributions over ashes (fly-ash, bottom ash), gaseous emissions, FGD by-products and water effluent emissions. 

For some coal parameters simple optimization by coal blending is possible based on the calculation of the arithmetic mean of the parameters from the individual coal in the blend. For other coal quality parameters one or two of the coals dominate the behavior of a blend. This is the case for example for grindability, slagging, fouling, free swelling index and sometimes for NOx. Based on full-scale experience KEMA has found that installation-specific blending rules related to such operational effects can however be developed to enable optimized use of coal blends.

KEMA has extensive experience in dealing with the effects of coal quality on the quality of fly-ash and bottom-ash. The quality is related to industrial applications such as cement and concrete manufacturing, road building and also to environmental properties (leaching behavior). This work has also been conducted in relation to the coal blending studies and the studies on element distributions.

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