OCR Text |
Show I I j^ lb c • w 10 fO • ^ QH distance along flow (a) Normal Combustion 'g • > JT >x distance along flow (b) Excess Enthalpy Combustion i /.enthalpy; x. distance along flow; f, fraction of stoichiometric, Q, heat of comhustion/mass of stoichiometric mixture, I Qr, heat recirculated per unit time, Qw, heal abstracted pei unit time suffix 0, ambient, b, burnt, g. Hue gas at exit Fig. 2.1 Schematic Diagram of Enthalpy Profiles under Normal Combustion and Excess Enthalpy Combustion Conditions In this method the combustion in the main combustion chamber is allowed to occur under fuel lean conditions so that the average temperature remains unchanged through the use of excess enthalpy'combustion. The method has distinct advantages in almost all cases since it can utilize the sensible heat oi flue gases. The use of oxygen, in place of air, is less attractive since it would increase the operational costs and some sensible heat would still be lost in the flue gases. Therefore, in order to reduce the waste heat associated with the flue gases, the recovery of heat using the excess enthalpy combustion method has distinct advantages over the oxygen assisted combustion. The excess enthalpy combustion can, however, be applied to both the air or oxygen assisted combustion systems. Excess enthalpy combustion method can be applied to any fossil fuel combustion under fuel lean conditions or to the combustion of low BTU value fuels. In the former case the maximum flame temperature can be up to the same level or below than that obtained under the conventional operational conditions of the combustor. The reason why the flame temperature should not be raised above the conventional operational range is that there are certain restrictions when a higher temperature flame is used, For example, when higher heat flux is provided continuously to the boiler tubes, it may cause their burn-out. Good flame stability can be achieved using low B T U value fuels through the use of high temperature combustion air. 1000 Authors believe that the burn-out of boiler tubes would not occur under the ._, conditions of slightly higher llame temperature using HRS. This is because of the controlled high heal flux distribution using the approach of alternate firing within the given space of combustion chamber. The uniformization of heat transfer alleviates the problem of over heating of the boiler tubes under conditions of high heat flux. A true enhancement in heat transfer can only occur if the time averaged value of heat flux could be maintained uniform within the materials limit of the system. The excess enthalpy combustion method has provided an innovative means of uniformly raising the temperature within 100 r •§ IO > WD c 3 0.1 Normal Combustion Excess Enthalpy Combustion 200 400 600 800 1000 1200 Preheated Air Temperature [ *C ] Fig. 2.2 High Temperature Excess Enthalpy Combustion 3 II-8 |