OCR Text |
Show gas mixing and reburn zone in which a sufficient flow rate of natural gas is injected to form an overall fuel-rich mixture. The total gas flow rate is typically equivalent to 10-20% of the heat input in the primary combustion process. Reburn reactions in this overall fuel-rich, zone reduce N O to N2, but produce relatively high levels of CO. This CO is then reduced by injection of overfire air to produce overall lean conditions in which oxidation of the reburn gas is completed. Such conventional gas reburn technology has demonstrated NOx reductions above 5 0 % in many installations. Motivated by process economics, a related technology termed "fuel-lean gas reburn" (FLGR™) has recently been proposed to achieve comparatively moderate NOx reductions, but at much lower gas input than in conventional reburn, and without the need for an overfire air system to achieve CO burnout. FLGR is a patented technology which the Gas Research Institute (GRI) owns, and which Energy Systems Associates (ESA) has developed under contract to GRI. In FLGR technology, natural gas is injected into the furnace at sufficiently low flow rates to maintain overall fuel-lean conditions. The NOx reburning reactions then occur within the locally fuel-rich regions formed by the gas injection and mixing process. Mixing between the injected gas and furnace gas is key to effective NOx removal. CO burnout is achieved by the excess 02 available in the overall fuel-lean furnace gas, without the need for a separate overfire air system. This overall fuel-lean approach to gas reburning offers the potential to meet the NOx emissions targets applicable to many installations at lower capital costs and lower operating costs than are typically associated with conventional gas reburn. Based on early field experience, FLGR technology is expected to achieve 35-45% NOx reductions at 7 % gas heat input without significant impacts to the primary combustion process. The resulting economics make this technology a competitive emissions compliance option for many installations. Successful application of FLGR technology to any given installation hinges on achieving proper mixing of the injected gas with furnace gases to achieve high NOx removal and low CO emissions. Uniform mixing of the injected gas will in most cases not produce the highest NOx removal efficiencies. The NOx and CO performance of an FLGR system thus depends on the location, size, shape, and placement of the gas injectors, which determine details of the resulting gas mixing process. Proper choice of these parameters varies with each installation, and must therefore be determined by site-specific design and modeling. Design principles for major types of candidate installations thus are essential for FLGR technology development. With this objective in mind, FLGR has been applied in initial field demonstrations involving two different types of coal-fired utility boilers. The first of these (Elrama Unit 2) was a 112 M W e roof-fired pulverized coal unit operated by Duquesne Light Company. The second (Joliet Unit 6) was a 327 M We cyclone-fired unit operated by Commonwealth Edison. Results from these initial demonstrations were summarized in a paper at the previous AFRC International Symposium [41. Here we describe the application of FLGR technology to a 140 MWe tangentially-fired boiler (Riverbend Unit 7) operated by Duke Power Company. Tangentially-fired designs account for 4 0 % of all coal-burning utility boilers in the U.S. For this reason, adaptation of previous FLGR design guidelines to tangential boilers, and successful |