High primary airflow contributes to poor coal fineness by increasing classifier air velocities and reducing retention time of coal in grinding zones. Increases in primary airflow correlate to an increase in velocities at the classifier outlet of the pulverizer. Higher velocity air at the classifier outlet has sufficient energy to entrap larger more massive coal particles. Lower velocities allow larger particles to be returned to the pulverizer grinding zones. High primary airflow also increases the velocity differential between air and combustion air. This delays combustion, allowing a large percentage of heat to be released above the burner belt zone. Lower burner belt zone heat release results in less water wall heat absorption and subsequent elevation of the furnace exit gas temperature.
Our experience has indicated high primary air flow will increase NOx emissions. Our observations have been that increased NOx emissions caused by high primary air flow is the result of the following:
To ensure this, the flame must be attached to the burner nozzle, high primary air flow tends to push ignition points out to the furnace. Flame attachment not only controls the near burner air:fuel ratio, but enhances the devolatilization of coal hence the release of fuel N2 into this critical reducing zone of the flame.
Primary air flow must be accurately measured and controlled. We prefer utilization of a flow nozzle (shown in illustration below) when possible. If flow nozzles are not installed, primary air flow measurement accuracy must be within ±5% of actual flow.