OCR Text |
Show • Reduced combustion air temperature (for large watertube boilers utilizing combustion air preheat) • High excess air firing • Flue gas recirculation (FGR) • Water injection. The second group of techniques, however, would result in some loss in thermal efficiency and could increase combustible emissions. Following the study, IGT identified the cyclonic combustion concept (which it was developing for its cyclonic incinerator) as the ideal candidate for the application of the above techniques and initiated its development for boiler and fluid heaters. The concept (illustrated in Figure 1) involves inj ection of natural gas and combustion air mixture tangentially into a cylindrical combustion chamber at a relatively high velocity. The resulting cyclonic motion, with self-stabilization of the flames, creates internal recirculation of partially combusted hot gases. This intensifies and further stabilizes combustion by providing active radicals as well as improves temperature and combustion uniformity. Consequently, peak flame temperatures and localized pockets of high 02 availability are minimized, thereby reducing NOx formation. The swirl and internal combustion products recirculation are further enhanced by placing an orifice of appropriate size at a specific distance from the burner. Excellent combustion characteristics are achieved over a wide stoichiometric range - from highly fuel-rich to highly fuel-lean. Cyclonic combustion was considered ideal because 1) two of the best NOx control techniques high heat transfer and internal combustion products recirculation - are inherent and can be optimized for the desired performance, 2) earlier tests at IGT had shown that it provides very stable combustion over a wide range of combustion temperatures with premixed fuel and combustion air, and 3) its combustion characteristics are well-suited to the incorporation of the other NOx control techniques. The initial development work was carried out on a 16-inch-diameter burner for a 40-hp boiler. The burner was mounted on a water-cooled firetube boiler simulator and tested in the single-stage configuration with high excess air firing and with combustion air staging. In both configurations, the burner could be operated with emissions below 20 ppm NOx* and below 50 ppm CO and showed a potential for even lower emission levels. Since excess air primarily reduces NOx by decreasing the peak flame temperatures, it was also concluded that similar NOx reductions could be achieved with equivalent (in terms of adiabatic flame temperature) amounts of an alternate diluent, such as FGR. NOx may actually be even lower with FGR because of 1) the reduced 02 availability and 2) the higher * All emission data are corrected to 0% 02. |