| OCR Text |
Show natural gas, leading to a decrease in flame temperature. In this paper,the principles of FDI combustion and results from the experimental research along with mathematical modeling of flames are described. Effects of process parameters such as gas and air velocities and burner geometry on NOx emission are shown in detail. 1. FDI COMBUSTION PRINCIPLES AND BURNER STRUCTURE 1.1 FDI COMBUSTION PRINCIPLES In burning fuel without fuel-bound, such as natural gas, NOx is mainly generated via the thermal path as described in the Zeldovich mechanism. Effective methods for low NOx com bustion are reduction of peak flame temperature and oxygen availability in the flame. FDI combustion has been developed to fully utilize these methods. Principles of FDI combustion technique are shown in Fig. 1. Gas is injected directly into a furnace with very high velocity. Air is injected remotely from the gas, also with high velocity. Relatively slow mixing between gas and air is created which reduces peak flame temperature. Furthermore, both gas and air entrain substantial amount of recirculated combustion product before ignition. A self-induced flue gas recirculation (FGR) effect is created and oxygen availability in the furnace is reduced, resulting in low NOx level. Air Switching 0 Valve Gas c:::> .". . - - ------------- ( Air Flow (120m/s) , ~-~~~-;;:= (100m/ s) -+-""-:---:-f-+-O...-H-";"'--+ r-----..,.,.,..~~." Flame:, HOlder("'---'-'-'-'-"'- AH : Combustion Air for FDI Mode AL : Combustion Air for Preheat Mode ~""""""'~ ' .. .- .. -.--_.-... ------. Fig. 1 FDI com~llstion Another aspect of FDI is enhancement of radiation from a flame. Since gas is directly injected into a furnace, part of the gas can be cracked by the high temperature environment. Emissivity of the flame is increased and flame temperature is thus decreased due to higher flame emissivity. For conv~ntional burners with flame being stabilized inside a burner tile, high peak flame 2 |
