| OCR Text |
Show temperature is often observed. However, based on a heat flux distribution analysis of PDI, it is appeared that flame temperature is both lower and more unifonn due to delayed ignition and slow mixing of gas and air. Remarkable reduction of NOx has been obtained even under the severe conditions of a high temperature furnace using high temperature preheated air. 1.2 BASIC STRUCIURE OF THE FDI BURNER The burner consists of several air injectors around the burner tile and one gas injector in the center of the burner. Gas is directly injected into the furnace with the high velocity of about 100 m/s. The air is injected remotely from the gas injector and velocity of the air is also high at about 120 m/s. During preheating the furnace, (e.g. furnace temperature up to 900°C), the combustion air is delivered through an annular air duct around the gas injector which is equipped with a conical flame stabilizer. A typical stabilized flame can be generated to avoid ignition problems in a cold furnace environment When the furnace becomes hot FDI mode is initiated 'with combustion air directly injected into the furnace. 2. MATIlEMATICAL MODELING OF FDI FLAMES 2.1 FLOW PAlTERN Mathematical modeling of FDI flame was carried out in order to obtain further infonnation on the effect of process parameters such as velocities of air and gas. PHOENICS code was used and the calculation conditions are : k -£ turbulence Model Eddy-dissipation Model for turbulent diffusion flame 'Adiabatic Wall No buoyancy effect The furnace model had a dimension of 1 X 1 X 3.6m(H,W,L). One quarter of the furnace was evaluated with a calculation grid of 23 X 23 X 31. Results have been shown in Fig.2. Two flames are compared. For flame 1, where the air velocity is 40 mls and gas velocity is 100 mIs, a small external recirculation zone is observed between the air flow and the furnace wall. No internal recirculation on the axis of the furnace is obtained. On the other hand, for flame 2, where the air velocity is 100 mls and gas is 40 mIs, a larger external recirculation zone is fonned. Moreover, a large internal recirculation zone is obtained in center of the furnace which entrains substantial amount of combustion products. 3 |
