A method to target and correct sources of unburned carbon in coal-fired utility boilers

► A general method was developed to identify sources of unburned carbon in utility boilers. ► Oxygen availability was found to play the key role in burnout rather than residence time. ► A simple strategy was designed to improve burnout by targeting air to specific regions. ► 80% of the unburned carbon is from the 6 out of the 16 coal burners. ► Coal particles larger than 140 μm contribute 70% of the total unburned carbon.