Experimental study: The effect of the variation of the mill air fuel ratio on fineness in the pulverizer of the 600 MW Indonesian steam power plant

Steam power plants in Indonesia with capacities above 600 MW all use coal as fuel. Electricity is generated from electric generators that are rotated by steam turbines. Regarding the boiler type, There are some of the steam power plants that use "front-back fried coal powder boilers". This steam power plant was designed using sub-bituminous species containing carbon around 55% - 86% originally from Bukit Asam. Because there is supply limitation from Bukit Asam it will be changed by using coal from East Kalimantan such as Berau coal. The coal specification from Bukit Asam is deferent with that from Berau. It will give an effect of changing in the number of feed-rate of coal which is different from the original design. At loads between 500-600 MW (Full Load load) the feed rate increases by 25.91 T/h using Berau coal. The increasing coal feed rate will influence strongly to the capacity and performance of the pulverizer. Decreasing performance of the pulverizer will cause that the quality of coal fineness will be less than 70% of 200 mesh. It will result delay of coal combustion and finally the metal temperature at the Secondary superheater increase. Experimental tests were carried out with the object pulverizer "C" in unit 6 of the This steam power plant to determine the relationship of mill characteristics to mill air-fuel ratio. The quality of pulverized coal depends on setting of the variation of mill air-fuel ratio (MAFR) such as (1.7: 1), (1.8: 1), (1.9: 1), and the variations of openings vane classifier such as 40%, 45%, 50%, 55%, 60%. From this experimental study, it is known that in certain coal characteristics, the setting of opening the vane classifier, and MAFR settings will affects the quality of the 200 mesh pulverized coal. Experimental data can be observed that feeding Berau coal having the characteristics of Sub-bituminous coal with total moisture of 21.42% Wt and HGI 50, by setting MAFR is (1,8:1), and classifier opened of 45% for the coal pulverizer unit 6 will produce 200 mesh fineness and also has an impact on decreasing metal temperature of the secondary superheater to the level of 564, 59 °C below the operational limit.