Measurements and Modeling of Coal Ash Deposition in an Entrained-Flow Reactor Measurements and Modeling of Coal Ash Deposition in an Entrained-Flow Reactor

by Blanchard, Ryan P

Coal plays a significant role in meeting the world’s need for energy and will continue to do so for many years to come. Economic, environmental, and public opinion are requiring coal derived energy to be cleaner and operate in a more narrow window of operating conditions. Fouling and slagging of heat transfer surfaces continues to be a challenge for maintaining boiler availability and expanding the use of available fuels and operating conditions. The work incorporates existing information in the literature on ash deposition into a User-Defined Function (UDF) for a commercial comprehensive combustion and CFD code. Results from the new submodel and CFD code is are then compared to deposition measurements in on a simulated boiler tube where particle mass deposited and ash size distribution are measured.

Several model components governing various aspects of ash deposition have been incorporated into the UDF which has been implemented in a quasi-unsteady Computation Fluid Dynamics (CFD) simulation. The UDF consists of models governing ash particle impaction and sticking, thermal and physical properties of ash deposits, unsteady growth of the ash deposits, and the effects of the insulating ash layers on the combustion processes. The ash layer is allowed to transition from an accumulation of individual particles, to a sintered layer, and finally to a molten or frozen slag layer. The model attempts to predict the deposit thickness, thermal conductivity, and emittance.

Measurements showed fly ash particle sizes that were much smaller than predicted under a non-fragmentation assumption. Use of a fragmentation model matched mean particle diameters well but did not match the upper tail of the particle sizes where inertial impaction takes place. Assuming 100% capture efficiency for all particles provided reasonably good agreement with measured deposition rates. The observed trend of lower deposition rates under reducing conditions was captured when the gas viscosity was calculated using the probe temperature.