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Pulverizing mills in boiler plants play a major role in preparing the coal for firing in a boiler. Coal, being a heterogeneous fuel, varies in its properties; the variation depends on many factors like type of origin of coal, formation, etc. Two major parameters which decide the mill capacity and performance are the hardness of the coal and the total moisture level in coal. The hardness of coal decides the amount of load required to grind and recirculate within the mill. The moisture level decides the amount of hot air required and has a direct effect on the milling capacity in many cases. To make sure the fuel is prepared to an acceptable level for combustion, the mill outlet parameters of coal is maintained within a specified range. The fineness of coal which gives the measure of pulverizing is maintained to a level of 75% though 200 mesh and less than 1 to 2% over 50 mesh. The mill outlet temperature of coal-air mixture is maintained between 65 to 90 degree centigrade in the case of low and medium volatile coal. In the case of high volatile coal, while the lower limit generally remains as 65 degree centigrade, the upper limit is restricted to a lower level depending on percentage of volatile content in coal. This is mainly done to avoid mill fire and mill explosion.
While all these are regarding the mill performance, the coal fineness and mill out let temperature also affect the boiler performance. The coal fineness has an effect on the combustion performance like the bottom ash percentage combustibles when the plus 50 mesh percentage goes up. It has been very well established that in the case of tangential firing system, the percentage of combustibles has a direct bearing on the plus 50 mesh percentage. However the changes in the case of 200 mesh passing though has only a very minor or no effect on the percentage of combustibles in fly ash. This is mainly because of the fact that the residence time available in a large capacity boiler and the type of burning system will be able to address this in a wide range. It is found that in tangential firing system the reduction in fineness even to a level of 65 percent through 200 mesh did not affect the combustibles. But the main change was in bottom ash combustibles due to increase in 50 mesh retention percentage. It is to be noted that all this results in combustibles loss in boiler and the magnitude is low, however it makes a difference when we talk about finer aspects of boiler performance levels.
The mill outlet temperature has a major impact on the boiler performance levels. The mill outlet temperature directly impacts the dry gas loss in boiler which is a major loss. This depends on the moisture level in coal, the higher the moisture level the quantity of hot air required in mill for the same mill load is higher. In fact in cases where moisture levels are very high the tempering air required will be zero. But in the case of moisture levels being on the lower side the quantity of tempering air keeps increasing. It is at this time the boiler operating engineers will have to decide to go to higher level of mill outlet temperature to reduce the tempering air quantity. The tempering air bypasses the air pre-heaters in boiler thus reducing the heat recovery from the flue gases leaving the boiler. This increases the boiler exit gas temperature which in turn increases the dry gas loss in boiler.