Aachen University of Technology

In coal furnaces, burned coal (ash) particulates hitting the walls form deposits that develop into a solid crust. If the temperature in the furnace is sufficiently high, the outermost layer of ash will melt, forming a viscous film on top of the crust. The film will creep downward over time, due to the action of gravity.

While the capture of ash on the furnace walls is desired, to protect turbine blades and other internals downstream of the furnace, the development of a crust on the walls is not, since it impacts heat transfer, and therefore the overall operation of the furnace. If the crust forms a film and the film can be removed at the base of the furnace, however, consistent operating conditions inside the furnace can be maintained.

Modeling the pressurized pulverized coal combustion (PPCC) facility at the Aachen University of Technology, researchers included the deposition and slagging processes through user-defined functions in FLUENT. The built-in coal combustion model is used, but the particles striking the wall are allowed to reflect or stick, depending upon their moisture content and the angle of incidence. The mass build-up of ash at the wall is tracked over time, and a separate calculation is done to compute the formation and transport of the slagging film.

The results of this calculation are used to modify the heat transfer properties at the wall, and the updated thermal boundary conditions are then used in subsequent coal combustion calculations. The coupled calculation, run for several hours, predicts the evolution of ash deposits and slagging film in the furnace. A model of the PPCC furnace, operating with adiabatic walls, was found to reach steady state after about 10 hours. At this time, the mass of ash striking the walls was equal to that being removed in the form of a film at the base of the furnace. The internal temperature remained sufficiently high that minimal crust development occurred.

Another design, using constant temperature walls, was found to be inferior, generating increasing crust thickness and not reaching steady state after 30 hours of simulation. The customized FLUENT models proved to be helpful tools in determining the best operating conditions for the PPCC at Aachen. Modeling the pressurized pulverized coal combustion (PPCC) facility at the Aachen University of Technology, researchers included the deposition and slagging processes through user-defined functions in FLUENT.

The built-in coal combustion model is used, but the particles striking the wall are allowed to reflect or stick, depending upon their moisture content and the angle of incidence. The mass build-up of ash at the wall is tracked over time, and a separate calculation is done to compute the formation and transport of the slagging film. The results of this calculation are used to modify the heat transfer properties at the wall, and the updated thermal boundary conditions are then used in subsequent coal combustion calculations.

The coupled calculation, run for several hours, predicts the evolution of ash deposits and slagging film in the furnace. A model of the PPCC furnace, operating with adiabatic walls, was found to reach steady state after about 10 hours. At this time, the mass of ash striking the walls was equal to that being removed in the form of a film at the base of the furnace. The internal temperature remained sufficiently high that minimal crust development occurred. The customized FLUENT models proved to be a helpful tool in determining the best operating conditions for the PPCC at Aachen.

The development of a steady-state film thickness on the coal combustor operating with adiabatic walls was reached after about 10 hours.

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