OCR Text |
Show 5. DISCUSSION The qualitative value of the gas addition to the CWS firing, as gas/air atomizing fluid for improving flame stabilization, was no surprise. The range of firing conditions is still limited, which restricts generalization of the results. Notably, we need more complete data on the windows of stability as: swirl is reduced; as the gas proportion is reduced (as a function of swirl and wall temperature); as the local thermal loading in the flame stabilization region is increased; as the gas/air proportioning is changed; and as all factors are modified with turn-down. Gas addition, however, is clearly beneficial, and this conclusion is not likely to change as a wider range of conditions is examined. The quantitative difference between the slurry/gas firing and the oil, however, were surprising; notably, the difference in thermal efficienc between slurry firing and oil firing of about 50%. The source of the difference is far from clear. The system is a complex balance of heat exchange fluxes in the furnace. The primary source of heat is the flame, of course, but the heat received by the load (the water tubes) is provided by the following sources: direct radiation from the flame; convection from the flame (generally less than 5% of the total received); direct radiation from the walls (with a fraction absorbed during transmission through the flame); radiation from the flame that was wall-directed but was absorbed by intervening gas and re-radiated to the load; and wall-to-wall radiation that was absorbed by the gas and re-radiated to the load (14). In a small furnace as much as 2/3 of the heat is derived from the roof and walls, with only 1/3 supplied directly from the flame. This proportioning is sensitive to the optical width and length of the furnace. |