OCR Text |
Show an equal or greater secondary zone residence time is provided for carbon burnout. Although up to 90 percent reduction of NO^ has been observed in coal fired laboratory systems, the maximum practical reduction achieved on field operating boilers under actual or experimental conditions is 30 to 50 percent. Staged combustion is used in a number of configurations to achieve the NSPS for coal fired steam generators and is also employed to meet standards for oil and gas fired units. Burner design. Although diffusion flame burners of many designs have been used in fuel combustion for years, only recently has the modification of design approaches to achieve emission control received strong emphasis. The essential elements of a diffusion flame burner are a fuel introduction system and a burner throat to supply combustion air. Design variables used to achieve stable combustion and good fuel conversion efficiency include fuel distribution (controlled by injector design) and the rate of air mixing (controlled by throat velocity, use of swirl, and/or design of the flame holder). These same variables can control fuel and air mixing histories for emission control; however, the flame characteristics required may be significantly different than the conventional practice. The fuel and air mix initially in a primary reaction zone which contains a wide range of stoichiometries, from very rich to very lean. This characteristic of diffusion flames appears to result in the partial conversion of fuel nitrogen to N0X. The balance of the combustion air is mixed with the primary zone products farther downstream and combustion is completed. In addition, relatively cool combustion products recirculate within the combustion chamber and are entrained by the flame. This entrainment can provide a diluent effect which reduces peak temperature and, therefore, reduces thermal N0X. Several pulverized coal burners designed to reduce N0X have been tested by boiler manufacturers: reductions of 30 to 50 percent relative to uncontrolled levels have been achieved. A modified burner design is currently used by one manufacturer to achieve the NSPS for coal fired utility boilers. Entrained combustion gas recirculation burners have achieved N0X reductions in excess of 50 percent for clean fuels (natural gas and distillate oil). In addition, several studies of advanced burner designs for heavy oil and coal have shown the potential for 65 to 90 percent reduction relative to uncontrolled emissions. 3-9 9 |