Modeling the slag behavior in three dimensional CFD simulation of a vertically-oriented oxy-coal combustor

摘要

Slagging and fouling are important phenomena associated with ash handling and discharge in coal combustion and gasification. Slagging has strong impacts on the heat transfer characteristics in the reactors, and may cause reliability and safety concerns. A slag model has been developed for modeling the ash/slag fate including char/ash deposition, char wall burning, molten slag flow, as well as solid slag layer formation on the reactor wall. The sub-models are implemented as user defined functions in a three-dimensional computational fluid dynamics (CFD) code, and applied for the simulation of a pilot scale coal slagging combustion facility. The results on the two-dimensional cylindrical reactor wall show uneven char/ash deposition distribution due to the non-axisymmetric flowfield and char trajectories in the reactor. The char and ash particle capture efficiency depends on the char carbon conversion, particle temperature and velocity, as well as the local stickiness of the slag. Molten ash properties such as the temperature of critical viscosity (T_(cv)) and slag viscosity are critical to the slag layer buildup and solid slag layer formation, and eventually affect the ash partition between slag and fly ash. The proportion of ash captured on the cylindrical wall decreases from 44.1% to 23.5% when T_(cv) increases from 1580K to 1780 K. If the reactor is configured with refractory-brick wall, the slag layer has limited impact on the heat transfer through the wall due to its small thickness; however it would have a major impact if the wall is water-membrane lined.