Effect of Primary Air Velocity on Emission of Nitric Oxide from Pulverized Coal Combustion System

The effect of primary air velocity on the emission of NOx from pulverised coal combustion was investigated by using a model furnace which was designed to realize practical combustion conditions. The primary air velocity is an important aerodynamic parameter in the combustion of volatile matter. Measurements of velocity profile in the furnace were carried out by using a symmetrical type probe. This probe was newly developed for the measurement of velocity under the combustion conditions of pulverized coal. The following results were obtained. The emission of NOx was decreased to less than half of that at maximum velocity as the primary air velocity was decreased from 68 m/s to 11 m/s. The measurement of NOx distribution in the furnace revealed that this decrease of NOx at the furnace exit was associated with the decrease in emission of volatile NOx in the volatile combustion region. From the measurement of stable species concentration distribution, it was indicated that the emission of volatile NOx was suppressed by the appearance of a local reduced region which was formed in the volatile combustion region under the condition of low primary air velocity. The formation mechanism of this local reduced region, which significantly contributed to the reduction of NOx was revealed from the measured result of the axial change of the primary air jet flow quantity and the observed result of the ignition point.