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Show Table I: Cofiring Burner Inlet Conditions Gas Burner Coal Flow Gas Burner Coal Firing Gas Burner Gas Burner Azimuthal Excess Air Excess Air Rate Firing Rate Axial Velocity Velocity (mmBtulhr) (mmBtulhr) (m/s) (m/s) High Load Maximum Cofiring 42% 00/0 237.5 37 67 -20 Low Load Minimum Cofiring 700/0 900/0 123.5 3 15 -4 The pendant superheater was represented as porous flow cells with a specified resistance to flow which develops approximately Pa (0.5" w.c.) of pressure drop for the flue gases in the furnace. The porous cells used for the pendant superheater only develop a pressure drop and do not remove heat from the flue gases. The pendant superheater cells are represented in Figure 5 with dashed lines. Flue gases from the furnace can exit without passing through the superheater. Boundary and Inlet Conditions: The boundary conditions for the furnace model involve noslip conditions at the walls of the furnace. The water walls of the boiler were set to a constant temperature of 800K to represent the temperature of the saturated steam. The inlets to the boiler include the coal grate flow which was represented as reacted flue gas with the appropriate concentrations of C02, H20, 02 and temperature for the typical quantity of excess air observed in the furnace. The velocity of the upward flowing coal flue gas was detennined to maintain a mass balance for the boiler. While one of the goals of the gas cofiring burners was to reduce the amount of oxygen in the furnace and increase boiler efficiency, the excess air level of the coal combustion for the gas cofiring cases was maintained at the levels which were measured with coal firing only during the baseline boiler tests.2 The gas cofiring burner inlet conditions were detennined by evaluating the total boiler load, gas cofiring load percentage, and desired excess air level for the gas cofiring burners (see Table I). MODELING RESULTS The modeling of the furnace was initiated with a baseline coal only model in which the general furnace flow patterns in the furnace were investigated. The results of this baseline model allowed the definition of a proper pressure drop across the pendant superheater in the furnace. Once the general flow patterns and superheater conditions were determined, the CFD investigation continued with two boiler load and cofiring percentage cases for each of the three cofiring burner separation distances considered (4, 6, and 8 ft.). 5 |