Scale-up of Advanced Low NOx and High Turndown Pulverized Coal Burner
Masayoshi KIMOTO, Hirofumi TSUJI, Hisao MAKINO, Takashi KIGA
The specific low NOx burner, which is enabled to reduce both NOx and unburned carbon extremely and to perform the stable combustion at 20% load as like an oil burner, has been developed with a small size burner whose coal feed rate is 0.12 t/h in the previous study. To apply this burner to utility boilers, the influence of burner capacity on the combustion characteristics was investigated by comparison between the small burner (0.12 t/h) and a large burner (1.5 t/h) in this paper. The concept of this burner is follows. Coal particle is concentrated at outside of primary air nozzle by centrifugal force, and the coal concentration is controlled by a ring. At the exit of nozzle, the swirl of primary air is inhibited by straightener to reduce NOx efficiently.
The swirl at the burner exit decreased with the increase of the straightener coefficient, which is a ratio of the gross area of the straightener to the cross section area of the primary pipe. When the straightener coefficient became greater than 1.2, the swirl was inhibited completely as same as 0.12 t/h burner. When the pulverized coal concentration control ring was placed close to the exit of nozzle, the local concentration of pulverized coal rose 1.7 times as high as the mean concentration in primary air. With this arrangement, the combustion flame kept stable at lower load and the minimum load of the burner was improved to 20% as like an oil burner. The unburned carbon in the fly ash was reduced very efficiently with a little increase of the NOx emission at lower load by controlling the coal concentration higher.