Experimental study on bituminous coal blending in a down‐fired boiler with anthracite combustion system under low load

Abstract

AbstractThe industrial experiments were carried out to study the bituminous coal blending in a 660‐MWe supercritical down‐fired boiler with anthracite combustion system under low load. The influence of the bituminous coal ratio on the boiler thermal efficiency, flue gas temperature, flue gas species, and heat transfer of various heating surfaces was analyzed in detail. The ignition distances of three experimental coals were measured, and the influence mechanism of bituminous coal blending on the combustion, heat transfer, and NOx emission in the furnace was proposed. The results show that increasing the bituminous coal blending ratio is beneficial to reduce NOx emissions. However, it leads to the rise of the exhaust gas temperature and the decrease of the boiler efficiency, which can be ascribed to the large amount of the cold primary air sent into the pulverizing system to control the air pulverized coal flow temperature at the mill outlet. The ignition distances of three experimental coals are different. The ignition distance from short to long follows the following order, that is, bituminous coal, lean coal, and anthracite. The earlier ignition of the bituminous coal leads to a sharp rise in the temperature of the airflow at the burner nozzle outlet, a rapid expansion of the airflow volume, and a sharp attenuation of the airflow rigidity, resulting in a short flame downward penetration depth and the upward movement of the flame center in the furnace. As the bituminous coal is blended, the change of the flame center and the flame fullness in the furnace affects the flue gas temperature distribution. Similarly, the change of the radiant heat transfer intensity and the convective heat transfer intensity affects the heat absorption distribution of the heating surface in the furnace.