Indian coals, by virtue of their formation, are in some characteristics different from the European and American coals. Indian coal formation due to the drift origin gives more chances for variations in property and a high amount of inorganic inclusions resulting in high ash content. Petrographic analysis of Indian coals indicates the presence of very high percentages of low reactive constituents like semifusinites, fusinites, etc. apart from subjugation to oxidation/weathering. The above variations have their impact on combustion performance in large capacity boiler furnaces.
Pulverised coal combustion involves two main sequel, but possibly overlapping, stages: (1) rapid heating and devolatilisation as a result of pyrolysis followed by (2) combustion of char residue emanating from the devolatilisation stage. The first stage is fast, taking times of the order of 0.1 sec. only. The second stage is slow, requiring time of the order of 1 sec. to 2.5 sec. and therefore, thus has a major effect on the size of combustion chamber.
Optimization of combustion in Indian high ash coal fired boilers is of special interest due to the organic and inorganic mix up and the large amount of variation in the organics. It is found that the high percentage of ash, the low reactives in the organics of coal, the encapsulation of organics in inorganics, the presence of oxidized coals in many cases, and the blending of many types of coals are some of the reasons for varying behavior of coals during combustion.
Detailed measurements of furnace flame temperature profile in the field were carried out while varying the air distribution pattern. The results of the study shows that the proper understanding of the fuel’s combustion characteristics could help in tuning the combustion regime whereby the flat furnace gas temperature profile could be changed to match closely the normal profile experienced with reactive coals. The temperature in the burner zone could be increased by about 50 Degree C and the furnace outlet temperature could be reduced by about 90 Degree C.