Combustion. The immediate effect of high primary air (PA) flows is on combustion of pulverized coal. Burner coal nozzle exit velocities are vey dependant on the level of PA flow. High PA flows cause excessive nozzle exit velocities and hamper the ability of secondary air fed around the PA/coal stream to mix with the coal. Because the relative velocity between the PA/coal stream and secondary air remains stratified near the burner throat, PA travels into the furnace volume without initiating combustion of the entire coal stream. As coal particles are blown farther from the nozzle exits, the concentration of oxygen from the secondary air decreases, lowering the probability of complete combustion for most coal particles. The negative consequences are high loss-on-ignition values in flyash and reducing environments and wall slagging in the furnace, due to the lack of adequate mixing and residence time near the burner, where oxygen concentrations are higher.

NOx generation. Low-NOx burners firing pulverized coal are designed to enhance staged combustion of the coal from a fuel-rich environment at the burner nozzle exits to a more complete sequence as the coal streams enter the furnace. Staging of the combustion sequence allows volatile matter in the coal to diffuse from the particles as the coal streams exit the coal nozzles and are exposed to flame temperatures. The volatile matter, in the form of free radicals, then strips the oxygen from the NOx formed as the devolatized coal particles are further oxidized.

The presence of high PA flows diminishes this desired staging effect by oxidizing most of the volatile matter reagents when the coal exits the burners’ nozzles. The greater concentration of oxygen from the high PA flow (for the same coal flow rate) reacts with more of these radicals, decreasing their concentration and availability to chemically reduce the NOx formed at the burners. If the PA flows are high enough to sufficiently diminish the reducing reagent concentrations, the result will usually be unacceptably high NOx emissions from the boiler—even if it is equipped with low-NOx burners.

Furnace water wall energy absorption. Water wall energy absorption is affected by high PA flows due to insufficient combustion of the coal at the burners and its subsequent energy release at higher elevations in the furnace. As with combustion, the resulting excessive burner exit velocities discharge the coal stream into the furnace at velocities greater than the secondary air velocity; the results are poor mixing into the coal stream and reduced combustion at the burners. As unburned coal particles are carried upward with the air/gas stream, most are oxidized upon collision with the remaining free oxygen not consumed at the burners. Therefore, more of the combustion’s total heat release occurs at higher elevations within the furnace. This is very significant, because almost all the energy transferred to the furnace water walls is radiated from the fired coal suspended with the furnace.

Boiler gas temperatures and unit performance. Because one of the initial effects of high PA flows is reduced energy absorption in the furnace, the resulting high gas temperatures are then carried from the furnace into the rest of the unit. Higher-than-design gas temperatures and increase attemperation spray flows trying to control them. Higher-than-design economizer and air heater outlet gas temperatures are indicative of the overwhelming heat load on the boiler’s convection and backpass sections. This is why high PA flows increase induced-draft fan power consumption; the higher gas temperatures increase the volumetric flow at a given unit load while lowering static pressure capabilities because of lower gas densities (that is, the effect of lower density ratio on fan static pressure curves). The overall effect of high PA flows here is a possible limitation on the units load rating.