OCR Text |
Show High Load, Maximum Cofiring: The results of the high boiler load and maximum cofiring percentage (30%) indicated improved mixing within the furnace and the general swirling pattern caused by the gas cofiring burners. The predicted furnace temperatures for a vertical slice through the centerline of the boiler, see Figure 7, revealed significant differences between the various burner separation distances considered. The 4 ft. burner separation case indicated that the flames from each of the two gas cofiring burners bent towards each other creating a localized region of high temperature in the center of the furnace. As a result, a large volume of the furnace, near the front and back walls, was not affected by the flames of the gas cofiring burners. These were the regions in the furnace where conditions were least likely to promote complete combustion of the coal. It was evident from this result that fly ash caught in the regions surrounding this condensed flame region would not be affected by the gas cofiring burners. The 6 ft. burner separation case indicated improved furnace coverage by the gas cofiring flames while cooler regions persisted near the front and back walls of the boiler. When the burner separation was increased to 8 ft., the entire furnace above the cofiring burners was affected by the gas cofiring burner flames. The penetration of the cofiring burners into the furnace is illustrated in Figures 8 and 9 where temperature contours and velocity vectors are given for a vertical slice through the centerline of the cofiring burner on the right side of the furnace. The gas cofiring burners penetrated nearly the full distance across the furnace for each of the burner separation distances tested. The penetration of the cofiring flame across the furnace was reduced as the burner separation distance was increased. The strength of the furnace swirl pattern created by the burners increased as the burner offset distance increased as shown in the horizontal plane of the furnace, see Figure 10. The 8 ft. burner offset distance resulted in a significant increase in the flow near the boiler front and rear walls which caused cooler flue gases to be convected into the tails of the cofiring flame and reduced the penetration of the flame to the opposite wall of the furnace, see Figure 11. The sides of the cofiring flames did not impact on the walls of the boiler as this general rotation of the furnace flue gases pushed the flames into the center of the furnace. Low Load, Minimum Cofiring: The effects of tum down on both the gas cofiring burners and total boiler load were investigated for each of the cofiring burner separation distances considered. A dramatic reduction in the penetration of the gas cofiring burner flame into the furnace is evidenced by the velocity plots for a vertical slice through the cofiring burners at an 8 ft. burner separation, see Figure 12. The momentum of the cofiring burner flow was reduced as the cofiring burner firing rate was decreased. To improve the cofiring burner momentum at the low cofiring rates, the excess air through the cofiring burner was increased to 90%. The problem of reduced penetration of the cofiring burner flame into the furnace was also exacerbated by the increased excess air requirement for the coal firing on the stoker required at these low boiler loads to prevent excessive smoke production. The reduced influence of the gas cofiring burners on the swirl pattern generated in the furnace is also evidenced by the velocity plot for a horizontal slice of the furnace at the cofiring burner centerline elevation, see Figure 13. The low load, minimum cofiring cases did not indicate the significant furnace swirl pattern that was present at the high load, maximum cofiring case. It is important to note that this reduced cofiring burner flame penetration and furnace mixing at minimum gas cofiring levels would become even 6 |