In Situ Diagnostics on the Dynamic Processes of Ash Deposit Formation, Shedding, and Heat Transfer in a Self-Sustained Down- Fired Furnace

摘要

Ash deposition is the most acute problem that hinders safe and efficient operations of coal-fired boilers. In this paper, we investigate the ash deposition process by incorporating in situ observations with standard procedures of ash deposit sampling. A specially calibrated digital camera is used. A sodium-rich Zhundong (ZD) lignite, a Si/Al-dominated bituminous coal, and their blend were burned in a 25 kW down-fired furnace. The deposits were collected at a position with flue gas temperatures of 850 degrees C. The newly developed RGB pyrometry quantitatively measures the deposit surface temperature in a range of 800-1150 K. During the dynamic deposition process, the deposit surface temperature approaches the local flue gas temperature within 1 h of elapsed time. The heterogeneity of the deposit surface is caused by either the deposition of burning coarse particles or the shedding event of the deposit layer with a low cohesive strength. The deposit structure is characterized by its packing ratio, which positively correlates with the deposit mass load. Moreover, the effective thermal conductivity of the deposit can be derived from a "two-resistance" model and ranges from 0.14 to 0.32 W m(-1) K-1. After ZD lignite is blended with the bituminous coal, the shedding period of the ash deposit is greatly reduced. The effective conductivity of the deposit is also enhanced in a nonlinear manner.