Flame instability/flame detatchment with low NOx burners

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:

• Flame detachment (ignition points far from burner nozzle tip). The most critical part of a low NOx burner flame is the region of the flame close to the nozzle, as the near burner conditions control the flame’s overall NOx production. In the “near burner” region of the flame the fuel N (largest producer of NOx) in the volatile fraction of the coal must be released into an atmosphere lacking O2.

  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.

• Injection of excess oxygen into the high temperature flame core. High primary air flows force more O2 into the flame core making more oxygen available for the conversion of nitrogen to nitric oxide.
• Increases slagging propensity. High primary air flow delays or prevents mixing of combustion air and fuel carbon in the furnace cavity resulting in lower water wall heat absorption and higher furnace temperatures.
• Decreased coal fineness and poor fuel balance associated with decreasing fineness levels.

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.