INTEGRATED DRYING AND PARTIAL COAL GASIFICATION FOR LOW NOX PULVERIZED COAL FIRED BOILER

摘要

Coal bound moisture is a key issue in pulverized coal fired power generation. Coal being hygroscopic, accumulates considerable surface moisture with seasonal variations. A few varieties of coals are having unusually high inherent as well as surface moisture that affects the pulverizer performance and results lower thermal efficiency of the plant. A proper coal drying is essential for effective pulverization and pneumatic conveyance of coal to furnace. But, the drying capacity is limited by available hot airflow and temperature of hot primary air. Even, use of high-grade coal for blending would not provide the entire useful heat value due to moisture, when used for matching power plant design coal parameters. Besides, the inefficient mining, transportation, stacking and associated coal fleet management deteriorates the "as fired" coal quality affecting cost while purchased on "total moisture and gross heat value" basis. Partial devolatilisation of coal in a controlled heating process, prior combustion in fuel-rich environment ensures better in-furnace flame stability and less residual carbon in product of combustion. It improves the opportunity of a lower flame zone temperature, delivering better control over thermal NOx formation from fuel bound nitrogen. The pulverized coal fired power plants use coal feeders in either gravimetric or volumetric mode of feeding that needs correction for moisture in coal which changes the coal throughput requirement. In this paper an integrated coal drying and partial coal gasification system model is discussed to improve the useful heat value for pulverized coal combustion of high moisture typical power coals so that related improvement in coal throughput can be carried out by application of suitable coal drying mechanism like Partial Flue Gas Recirculation through Pulverizer '(PFGR® ) for mitigating the coal throughput demand with optimizing available pulverizing capacity along NOx control opportunity without derating steam generation capacity of the boiler.