Githu,
Many things can be causing unburnt carbon in boilers. If it is a coal fired boiler, there should be soot blowers installed in the system. If it is fired on natural gas or diesel, the fuel should be clean enough and you should check the FD and ID fan performance to insure the proper amount of combustion air is being supplied to the boiler. As always, better information means a better answer. Good Luck.
