Improvement of Combustion Stability at Low Load for Low NOx Pulverized Coal Burner

Abstract

To improve the combustion stability at lower load for a specific low NOx burner, which enabled to reduce both NOx and unburned carbon extremely, an adjustment method to raise local concentration of coal particle was invented. Coal parti-cle was concentrated at outside of primary air nozzle by using centrifugal force and coal concentration was controlled by a ring. At the outlet of nozzle, the swirl of primary air was prevented by straightener, because this swirl disturbed the forming of the reduction flame for NOx decomposition. So, the influence of straightener length and number on re-ducing the swirl was examined by cold flow test with a primary air nozzle model. Secondary, the coal concentrating effects in changing the ring position was investigated with a nozzle of combustion test burner. Finally, the combustion stability at low load was tested with the coal combustion test furnace.The swirl at the outlet decreased with the increase of the coefficient of straightener, which was defined by the ratio of the gross area of straightener to the cross section area of primary air nozzle. The swirl number was decreased to zero when the straightener coefficient is greater than 1.2. The ring placed close to the outlet concentrated coal particle most efficiently and the local pulverized coal concentration rose 1.5 times as high as the mean coal concentration in primary air. With this arrangement, the combus-tion state at lower load became stable and the minimum load was improved to 20% as same as an oil burner. At lower load, the coal concentrating was very effective to re-duce the unburned carbon with little increase of the NOx emission. On the other hand, the coal concentration was excessive when the ring was close to the outlet at higher load. So, the ring was placed far from the outlet and the coal concentrating was modified so that the emission characteristics of NOx and unburned carbon in fly ash of the burner with coal concentration adjustment was the same as the characteristics of the burner without these instruments.