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Show 2. Reburning Zone The reburning fuel (normally 15 to 20 percent) is injected downstream of the main combustion zone to create a slightly fuel rich zone (about 90 percent theoretical air) where the NOx from the main combustion zone is reduced. 3. Burnout Zone In the third and final zone, additional combustion air is added to oxidize any remaining fuel fragments and produce overall fuel-lean conditions. Extensive bench and pilot scale tests have been conducted to compare the performance of alternate reburning fuels ~nd to evaluate NOx control effectiveness and process design considerations. The results have shown that reburning with natural gas is more effective than reburning with other fuels, particularly at low baseline NO~ levels. Also, CO and smoke emissions are much lower when residence time is llmited. The remainder of this paper focuses on natural gas as the reburning fuel and the resulting technology is termed Gas Reburning (GR). In a utility boiler, GR is applied in the furnace between the burners and convective pass. Since the burners are not altered, GR is compatible with all firing types including wall, tangential, cyclone and stoker configurations. Figure 2 illustrates the typical retrofit of GR to a wall fired boiler. In the conventional system, all the coal is fired through the existing burners. for GR, the coal firing is reduced by 18 percent, but no changes to the firing system are required. The burners are simply operated at slightly lower load and the lowest excess air compatible with the burner and furnace designs. The lower load and excess air increase residence time in the lower furnace (improving burnout) and reducing NOx emissions. Natural gas is injected above the burners to form the reburning zone. It is important to note that the gas injectors are not burners. They simply inject natura 1 gas into the furnace wi thout air. The des i gn of these injectors is critical to NOx control. The gas must be injected to mix rapidly and uniformly with the furnace gases. Since the amount of gas is small, in some cases it is necessary to use a carrier gas to provide sufficient momentum for mixing the natural gas across the furnace cross section. A small amount of recirculated flue gas (about 3 percent) is ideal since it has little excess oxygen thus minimizing the natural gas required to achieve the desired slightly fuel rich conditions. To complete combustion, overfire air is added above the reburning zone. The overfire air ports are similar to those used for conventional air staging except that the conditions are more stringent. The amount of overfire air is greater and the space and residence time available for mixing are more limited. The specific design of the overfire air ports has little affect on NOx control but is key to achieving complete combustion of the natural gas. The GR system must be designed to achieve the desired stoichiometries and optimize NO control without affecting the thermal performance of the boiler. To achieve this goal, EER uses a combination of analytical and physical modeling. Combustion, NO~ kinetics, and heat transfer are evaluated analytically and mixing is evaluated Vla isothermal flow modeling. These models are applied in parallel and iterated to define optimum parameters based on the unique characteristics of each boiler application. 3 |