OCR Text |
Show smaller at high boiler loads where the upward momentum of the furnace flue gases would be increased. Non-Uniform Coal Inlet, Maximum Cofiring: The baseline, coal only, emissions data obtained from within the furnace indicated significant non-unifonnity in flue gas composition existed.2 The effects of this coal combustion non-unifonnity on the cofiring burners were investigated at high boiler load with 30% gas cofiring. The coal flue gas inlet for the model was adjusted to represent coal combustion at 10% excess air at the rear of the furnace and 900/0 excess air at the front of the furnace, see Figure 14. The flue gas concentrations varied linearly across the depth of the furnace and the inlet velocity was constant. The enhanced mixing of the furnace gases with the cofiring burners are illustrated with temperature variations through the depth of the furnace for the 4 ft. and 8 ft. burner separations up the level of the pendant superheater, see Figures 15 and 16, respectively. Colder flue gases present near the front of the furnace were swept along the left wall towards the rear of the furnace and mixed well with the cofiring burner flames. The superior furnace mixing of the 8 ft. burner separation case was evident with no discernible remnant of the cofiring flame present at the furnace elevation directly below the pendant superheater. The larger burner separation distance effectively mixed the furnace oxygen concentrations as well resulting in enhanced oxidation rates for fly ash and carbon monoxide. Thus, the 8 ft. burner separation distance was chosen as the optimum burner location for improved furnace mixing and greatest cofiring benefit. Initial Burner Start Up Performance: The gas cofiring burners were installed into the boiler wall during a shut down in the Summer of 1995. Simultaneously, the control system for the boiler as a whole were upgraded and integrated into the gas cofiring burner control system. The gas cofiring burners were successfully used to wann up the boiler and bake out the superheater without coal on the stoker grate. During the initial gas cofiring burner test, without coal firing in the boiler, the cofiring burners exhibited (1) greater than 12: 1 cofiring burner load tum down, (2) stable operation at substoichiometric (SR :::: 0.7) and high excess air (SR :::: 4.0) cofiring burner stoichiometries, (3) maximum and minimum cofiring burner loads without flame impingement on furnace walls or the pendant superheater. Coal firing was then initiated and required the use of liquid kerosene to aid in coal ignition due to insufficient grate temperatures to provide ignition of the coal with only the gas cofiring burners. It is possible that the gas cofiring burners can ignite the coal on the bed if the undergrate air is reduced to allow increased temperatures above the grate. Coal firing was brought on line with significantly less smoke emissions from the boiler in both concentration and duration. Furnace oxygen levels were reduced with gas cofiring and boiler derate was recovered due to the reduced load on the induced draft fan. The gas cofiring flames, observed with an in-situ video camera, were found to generate the circulation pattern in the furnace at high cofiring rates predicted by the CFD analysis. 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 «10%) was improved with redesign of the gas burner spud for greater axial gas penetration. 7 |