What is the best suited ash fusion temperature of coal for boilers in power generation? Why reducing AFT is considered?

First of all , it is always to be considered that during combustion in PF boiers and in CFBC boilers the furnace temperatures during combustion are different. In PF boilers, the average furnace temperature at the combustion is about 1400 to 1600 degree(C), where as in CFBC boilers the furnace bed temoerature is 880 to 940 degree(C) . Now ash fusion temerature is in any grade of coal is always greater than 1000 degree(C).So in PF boilers , the ash fusion temperature always achieved, and it is known that clinkers also formed, and these clinkers are collected in bottom ash hopper and temperature of these clinkers are reduced by submersing these clinkers with water in the seal trough and cold water always supplied from bottom ash overflow tank.Now in case of CFBC boilers , the ash fusion temoerature can not achieved as the bed inventory in terms of bed height or bed perssure, is maintained so that the bed temperture could not increase beyond ash fusion temrature. If in case of CFBC boilers the bed temperature increases towards ash fusion temperature the boiler should have logic to cut the motive fuel(coal) and the temperature will decrese or the boiler should be tripped in auto or manual for avoiding clinkerisation.

For a boiler of coal, if the temperature exceeds 2000'C, there is a possibility of bottom ash operations. But also lower ash fusion temperatures may lead to clinker formation in boilers. In case of PF boilers, its better to operate with coals of high ash fusion temperatures. In case of FBC boilers, ash fusion temperatures more than 1000'C can be done.

Ash fusion temperatures(IDT) of indian coals are generally in the region of 1200 to 1500 deg C, whereas in case of Indonesian coals it ranges from 1150 to 1300 deg C. It is advisable to operate with coals of high ash fusion temperatures in case of PF boilers. In case of FBC boilers, ash fusion temperatures above 1100 deg C should not cause any problems.

lower ash fusion temperatures can lead to clinker formation in boilers.

The temperature could be anywhere between 1650' C to 2000' C...which is best suited for ash fusion for coal for boilers.