Emission characteristics of NOx and unburned carbon in fly ash during combustion of blends of bituminous/sub-bituminous coals☆

Because of large recoverable reserves, sub-bituminous coal is anticipated to become a substitute for the bituminous coal used presently as fuel in Japan. Most sub-bituminous coal contains more than 20% moisture. In the case of the utilization of sub-bituminous coal in Japan, it is considered that, in the near future, sub-bituminous coal will be applied to conventional boilers designed for bituminous coal combustion. We have already clarified that moisture in coal causes low combustion efficiency and a high conversion ratio of fuel nitrogen to NOx compared with a coal having the same fuel ratio with little moisture, in pulverized coal combustion. In particular, oxygen consumption during combustion of sub-bituminous coal with high moisture is more delayed than that during combustion of bituminous coal. Therefore, sub-bituminous coal is utilized in blended combustion with bituminous coal. In this paper, the emission characteristics of NOx and unburned carbon in fly ash in blended combustion of sub-bituminous coal with bituminous coal is investigated using a pulverized coal combustion test furnace with a single burner (coal combustion rate: 0.1 t/h). NOx concentration in blended combustion approaches the NOx concentration in sub-bituminous coal combustion with the increase of the amounts of sub-bituminous coal. As the remaining moisture content in sub-bituminous coal increases, NOx concentration becomes higher in blended combustion. On the other hand, the unburned carbon concentration in fly ash in blended combustion is higher than that in non-blended combustion of each coal. Furthermore, unburned carbon concentration in fly ash becomes much higher as the moisture in sub-bituminous coal increases. This tendency is caused by the influence of the moisture in the sub-bituminous coal on the decrease of the combustion efficiency of bituminous coal, since moisture in sub-bituminous coal delays oxygen consumption at the burner exit and the shape of the combustion flame is diffused. When the swirl vane angle of the burner is controlled in order to suppress diffusion of the combustion flame, oxygen consumption progresses and emissions of both NOx and unburned carbon in fly ash are reduced.