OCR Text |
Show Fig. 15 Skematic drawing of exhaust gas re-circulation combustion Burner r y tt Boiler * Smoke Stock &z ^ tx X Air Mixing device forced blower Hexaust gas re-circulation combustion. Exhaust gas recirculation combustion can roughly be divided into . forced re-circulation (F. E.G.R.) and self recirculation (S.E.G.R.), and in this text, the former is described. By this method, NOx reduction is attained while mixing a part of combus~ tion exhaust gas into the air for combustion as shown in Fig-15. The mechanism of NOx reduction by exhaust gas re-circulation can be explained by flame temperature lowering effect and by oxygen concentration diluting effect in flame area. The major part of exhausted combustion gas is C02and-N2 with a little content of 02. The furnace temperature is reduced if an excessive air is supplied to the furnace. From the point of low oxygen combustion, however, generation of NO is increased up to a certain quantity because of the high content of active oxygen, and above the limit, thermal efficiency is lowered abruptly. Accordingly, a part of non-active combustion gas which contains little oxygen is mixed with combustion air to be supplied to the furnace to lower the flame temperature for NOx controlling. Although this method is effective for the reduction of thermal NO, it has almost no effect on the reduction of fuel NO. The equipment can roughly be divided into two types; premixed type by which the combustion gas is mixed in the wind box or in the air duct to the wind box and after-mixed type by which the combustion gas is sent into the furnace through an independent nozzle exclusively for 8 - 19 - |