In performing a combustion tune-up at a coal plant many considerations have to be made. The first is the purpose of the tuning. In many cases tuning is performed to lower NOx formation and to minimize carbon losses. Tuning can also be performed to minimize slags and ash deposits.
In many cases tuning for lower NOx can increase slag formation, so please temper the NOx levels to support good slag control. Reducing (high CO) conditions tend to lower NOx, but they can lower ash fusion temperatures, particularly with higher iron (Fe2O3) and higher sulfur coals. The lengthening of the flame using over fire air, OFA, and low NOx burners can raise the furnace exit gas temperatures, FEGT, making for more slags in the inlet of the convection pass.
Many sub-bituminous coals have high volatiles and good combustibility. They like to spontaneous combust, cause fire/explosion issues in the pulverizer, and they burn well in the furnace. Using carbon in the ash as an indicator of good combustion can be misleading if you are experiencing wall slags.
Air
The amount of air flowing to a boiler should be measured and verified. O2 monitors can be influenced by air in leakage, and not all the forced draft, FD fan air makes it to the boiler due to air preheater, AH, leakage.
The required air to fuel ratio, A/F can be calculated from ultimate coal analyses and heat input to the boiler. The heat input to the boilers can be calculated at least two ways, 1) coal flow times coal CV, and unit heat rate times load. All these calculations should be made and compared to actual measurements made at the plant and look for conformation and consistency of measurements and calculated values.
Fuel
The fuel entering the burners has two important parameters, size and velocity. It is also important to get the fuel balanced between burners so the proper A/F ratio can be present at the burner. It is therefore necessary to measure burner line velocities and coal flows. This is typically done using clean air, dirty air, and isokinetic fuel sampling in each burner line.
The particle sizes should be at least 70% passing a 200 mesh (75um) screen with less than 0.1% remaining on the 50 mesh (300um) screen. Many combustion engineers use a 1% remaining on the 50 mesh screen as a standard, but this author sees many wall slag issues minimized by using the 0.1% standard. To achieve the 50 mesh standard, the 70% passing the 200 mesh screen may increase to near 75% passing. This high level of fineness is not just for combustion and slag control, it helps balance the burner lines and tends to decrease fuel line velocities by minimizing primary, PA, air flow.