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Show (top), with the outlet of the model corresponding to the beginning of the convection section. The main body of the furnace measures 5.12 m by 5.12 m , and has one burner and two overfire air ports on each side. The burners are located at an elevation above the grate with a separation of 1.92 m. The overfire air ports are positioned 2.33 m above the burner centerline, with one port 8.4 c m to the left of the burner and the other port 71.2 c m to the right of the burner. The overfire air have an open area equivalent to an 18 c m hole. This model has three inlets and one outlet. Flow enters the computational domain from the bottom inlet (pre-burned gases from the wood grate), from the sander-dust burners, and from the over-fire air ports. The actual boundary conditions used for each inlet will be discussed in the next section. The superheater tubes at the top of the furnace are modeled as a simple pressure drop, and the outlet is located at the beginning of the heat exchanger tubing. The remainder of the model boundaries are all heat transfer walls. The main cross-section of the furnace is modeled with 25 by 25 cells, or about one cell every 20 cm. Overall, the model contains 51,000 cells, which is sufficient to provide a high degree of accuracy. Inlet and Boundary Conditions: Flow into the bottom of the computational domain is simulated as pre-burned flue gas coming from the wood grate. The total flow rate is 67,500 lb/hr of gas at 1600°F. The composition of this gas is shown in Table 2. Flow through the burners is approximated by a mixture of methane and air, which delivers a total firing rate of 86 MBtu/hr with 8 0 % of the stoichiometric air (i.e., fuel rich). The flow velocity angles have been chosen to simulate burners that are firing straight into the furnace with no tilting. In addition, a swirl velocity component was added to provide a 20° swirl angle. The temperature of the mixture is 268°F. Flow through the over-fire air ports delivers 4 0 % of the stoichiometric air at a velocity of 113 ft/s and a temperature of 300°F. The superheater tubes at the top of the furnace are modeled as a simple pressure drop, which is approximated as a perforated plate with 5 0 % open area. The walls of the furnace were specified to be 1/8-in thick steel plate with a constant temperature of 360°F on the outside and a radiative emissivity of 0.8 on the inside. Figure 9: Sander Dust Burner/Boiler Computational Grid CFD Model Results: Many versions of this model were run to determine the effectiveness of different geometric configurations and different firing angles. Initial results indicated insufficient penetration into the corners of the furnace, which leads to poor mixing and incomplete Table 2: Wood Bark Flue Gas Composition CO, H20 o2 N2 13.18% by vol. 21.26% by vol. 2.78% by vol. 62.78% by vol. 8 |
