OCR Text |
Show painted) area is that expanded by the wide load range burners for coal of FR=2.3. The further expansion was achieved for coal of FR=1.1, drawn by the hatched areas. The use of the conventional burners can reduce the boiler load down to 17% by operating two mills. However, the boiler load when using the new burners becomes less than 15% even if operating three mills for coal of FR=2.3 or five mills for coal of FR=1.1. That is, the new burner can minimize the cut-down number of mills in the lower range of load, which is desirable in the continuous operations of power plants. CONCLUSIONS Based on fundamental studies and practical operations, we developed the new burner to obtain stable flames at extremely low load firing condition in coal combustion boilers. Analyzing coal particles behavior around the concentrator, we succeeded to enlarge the high coal concentration area near the burner outlet by condensation. 1) It is recognized from both the theoretical and the single-burner examinations that the speed of coal particles should be less than 12 m/s in the coal concentration(C/A) more than 0.20 to have a stable ignition. 2)Reducing an interference between the lean and dense phase flows is important to enlarge dense area around the burner outlet and a longer nose of the concentrator is effective to avoid the mixing the two flows. 3 ) A minimum burner load was reduced by using a conventional burner mounting the newly developed coal concentrator. NOMENCLATURE A: turbulent correction factor, Ac: frequency factor, Cd: solid concentration in gas suspension, CO2 : oxygen concentration, C: specific heat of gas at constant pressure, C : specific heat 6f solid, D 0: particle diameter, E: activati6~ energy, h: heat transferPcoefficient, k: thermal conductivity of gas, L: average distance between adjacent particles in gas suspension, lL: scale of turbulence, M : mass of a particle, Q: rate of heat loss, Q : o r rate of radiant heat, Tf : flame temperature, T : gas temperature, T s: gas temperature at burner exit, To: initIal temperature of g~s, Ts: particle temperature, T8i: ignition temperature of particle, U: intensity of turbulence, V = dx/dt: gas velocity, x: distance from burner exit, xH: burner axial length, Z: rate of heat generation, p: gas density, A: thermal diffusivi ty of turbulence REFERENCE 1 S. Nomura and T. G. Callcott, Powder Technol., 45, (1986) P .145 2 Y.Mizutani, Kikaigakkai-Ronbunshu, 38, (1972) p.2935 6 |