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Gas Cofiring Model

The primary benefits of gas cofiring with coal are (1) reduced carbon loss, (2) reduced NOx, SOx, and CO emissions, (3) recovery of load derate, and (4) improved heat transfer in the furnace. It is believed that most of the emissions of unburned carbon and CO in a spreader stoker boiler are the result of poor mixing within the furnace, called channeling. With the introduction of gas cofiring burners, the mixing of the flow above the coal grate is improved so that the more uniform temperatures and oxygen concentrations promote burnout.

In this study, Computational Fluid Dynamics (CFD) was used to determine the proper location of gas cofiring burners in a coal-fired spreader stoker boiler. The goal was to estimate the offset distance of these burners so that mixing above the coal grate is maximized without causing flame impingement on the boiler walls. The burner placements were evaluated at a maximum cofiring rate of 30% of the total boiler load and at a minimum cofiring rate of 5% of the boiler load. An optimum burner separation of 8 feet was determined and the burner operational characteristics defined as a result of this study.

The actual cofiring burner combustion performances was excellent with the burners demonstrating high stability, wide turn down, and no flame impingement on the boiler walls. The gas cofiring flames, as observed with an in-situ video camera, generated the furnace circulation pattern predicted by the CFD analysis when operating at high cofiring rates. However, the gas burners were significantly less dominant on the furnace flow at lower cofiring rates. Cofiring burner effectiveness at more economical gas cofiring rates (under 10%) was improved with the redesign of the gas burner spud for greater axial gas penetration.

Computational Grid
Computational grid.

Flame Shape
Flame shape at different firing rates.

 Top View
Top view comparison.

Side View
Side view comparison.

Front View
Front view comparison.
Flame shape at 4-foot separation.
Flame shape at 4-ft separation.		 Flame shape at 8-foot separation.
Flame shape at 8-ft separation.

CFD Features Used in This Model:

  • A 3D structured grid (hex mesh).
  • The standard k-e turbulence model.
  • Variable properties (viscosity, thermal conductivity, and specific heat are functions of temperature.
  • The buoyancy model.
  • The DTRM radiation model.
  • The finite reaction rate combustion model.

 

 

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