OCR Text |
Show ABSTRACT Retrofitting Duct Burners for CO Control Richard T. Waibel and Steve Somers John Zink Company Tulsa, OK Duct burners are often installed in gas turbine cogeneration or combined cycle systems to add supplementary heat to the turbine exhaust gas (TEG) stream upstream of the heat recovery steam generator (HRSG). The turbine exhaust gas usually contains enough oxygen to sustain combustion and the duct burner is designed to use the TEG as combustion air. Although duct burners produce relatively low NOx levels due to the low oxygen content of the TEG, the levels of CO and VOC's can be greatly influenced by the composition, temperature, velocity and turbulence of the TEG stream. In some applications, steam is added to the TEG to reduce NOx emissions produced by the turbine or to augment the power produced by the turbine. This steam further depresses the oxygen content of the TEG and can lead to increases in combustible emissions from the duct burners. An improved, low emission duct burner design has been developed to significantly minimize the effect of steam addition, turbulence and TEG velocity on combustible emissions. Data are shown for a gas turbine application that has been retrofitted with the improved design. INTRODUCTION Many gas turbine/heat recovery steam generator systems used in cogeneration or combined cycle applications utilize duct burners to add supplementary heat to the turbine exhaust gas (TEG) prior to the TEG entering the heat recovery steam generator (HRSG). In general the oxygen content in the turbine exhaust gas is sufficient for combustion and duct burners are designed to use TEG as combustion air. While the composition of the TEG depends on the turbine and the specifics of the application, a typical composition will fall within the range of 11 to 15% oxygen on a volume percent, wet basis. Typical TEG temperatures fall within the range of 850°F to 1100°F. The duct burner is generally located in the expansion duct between the turbine outlet and the heat recovery steam generator inlet. The expansion section is needed to provide the proper TEG velocity through the steam generator. Figure 1 shows a schematic diagram of a typical turbine/heat recovery steam generator system with duct burners. The duct burner is designed to distribute the heat as uniformly as possible in the lEG stream using a series of linear runners extending across the duct at several elevations. The TEG velocity at the plane of the duct burners is normally in the range of 30 to 60 feet per second, although lower and higher velocities are occasionally encountered. |