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MODEL SETUP A commercial CFD software was used for the numerical analysis of the coal fired, spreader stoker boiler.3 The code employs a finite number of discrete control volumes over the computational grid to obtain a 3-D, steady-state solution of the governing equations of continuity, momentum, and energy. After integrating the governing equations across the control volumes, velocity and pressure coupling were resolved via the Semi-Implicit-Pressure-LinkedEquation (SIMPLE) algorithm. Turbulence closure was provided by the k-£ turbulence model with standard coefficients. The turbulent boundary conditions for momentum and heat transfer in the near wall region followed the logarithmic law of the wall. The transport equation for energy was solved using a conjugate heat transfer model Gas phase combustion was modeled as a one step global reaction, with user specified fuel and oxidant components reacting to form combustion products. Transport and chemical reaction rates were governed by both mixing of the eddies and simple Arrhenius rate equations and the Magnussen mixing model. The gas cofiring burner flame was assumed to be mixing limited and infinitely fast fuel reaction kinetics were used. Gas phase radiation and molecular dissociation at elevated temperatures were not included in the model. Gas Cofiring Burner Design: The burner chosen for the gas cofiring project was the Coen Circulating Fluidized Bed (CFB) burner which has been used extensively to warm up and provide temperature control of circulating fluidized bed boilers. The CFB burner (shown schematically as Figure 4) uses a nearly straight throat, fixed spinner, and center fired gas spud and provides the benefits of (1) extremely wide operating ranges in both tum down and excess air, (2) minimal boiler side wall penetrations reducing installation tube bending costs, and (3) high burner pressure drop performance for maximum furnace penetration and mixing with the cofiring burner flame. The swirl direction of the CFB was intentionally chosen to force fluid upward between the burners to allow the overfire air injected over the coal grate to be well distributed over the furnace. Grid Development: Initial grid development for the boiler represented the entire furnace, the pendant superheater, and the first pass of the convection section of the boiler. The results of this initial grid illustrated that a nearly constant pressure boundary existed at the exit of the furnace, after the superheater. Advantage of this was taken and the outlet of the furnace, after the superheater, was represented as a constant pressure boundary. A second grid was then developed for the furnace alone to improve the grid resolution with dimensions for length, width, and height of 5.03m x 5.26m x 11.00m (X x y x Z) (16.5 ft. x 17.25 ft. x 36 ft.), see Figure 5. This final grid arrangement was body fitted in order to represent the burners as circular with throat diameters matching the CFB burner throats of 0.3 m (13 in.). The grid used was 21 x 42 x 56 (i x j x k) resulting in 49,392 computational cells with the bottom of the grid representing the coal grate flow. The CFB burner inlets were defined with a circular group of nine cells with the center cell remaining a wall cell to represent the flow blockage caused by the spinner. Three different gas cofiring burner arrangements were used with burner offset distances of 4, 6, and 8 ft., see Figure 6. 4 |
