OCR Text |
Show 2 This heating method requires varying the periodic time of the alternate combustion to provide a suitable heat flux profile for each particular application. A further improved heating method is suggested here, combining heat recirculation with hot combustion products recirculation in a furnace, which decreases the maximum flame temperature and creates a more uniform profile of combustion gas temperature in the furnace. In the present paper, the advantages of this proposed heating method is compared to the conventional excess enthalpy method without hot combustion products recirculation by using a simple heat balance calculation. 2. IMPROVED HEATING METHOD 2.1 Heat and Combustion Product Recirculation To avoid localized overheating of the load, there has been a restriction of heat input and amount of heat recirculation for excess enthalpy combustion using fuel with high heating value. It is, however, possible to overcome this thermal restriction by controlling the following two parameters in combustion process which influence adiabatic flame temperature. They are (1) initial temperature of the air and fuel and (2) vitiation of the inlet air and fuel when combustion occurs under the condition of constant air ratio, constant fuel heating value and atmospheric pressure in a furnace. Thus if there is a way to control the flow rate of combustion products to vitiate the inlet air and fuel, it is possible to increase the amount of heat recirculation, particularly for high heating value fuels, over the restriction level without rising flame temperature. This heating method combining heat recirculation with high temperature combustion product recirculation in furnace, schematically shown in Fig.1, can significantly improve the uniformity of the temperature profile formed in furnace when compared to periodical alternate excess enthalpy combustion. The macroscopic mass balance and the heat balance of the system can be expressed by equation (1) : { min + mrec = mexh + m,ec f2cold + Q f + Oev + a Qrec = Qm + Qloss + O-exh + Qev + Qrec where m = mass flow rate Q = heat flow rate (1 ) \ |