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Show formation, and subsequent molecular mixing of the resulting CH radicals with N O in the furnace gases under locally sufficiently oxygen-depleted conditions to avoid the NO reforming reactions. 1.1 Conventional Gas Reburning The gas reburning principle outlined above can be implemented in a variety of ways. The traditional method is via overall-fuel rich gas reburning, where the NOx-containing product gases enter a gas mixing and reburning zone in which sufficient natural gas is injected to form an overall fuel-rich mixture. The total amount of gas injected is typically equivalent to 10-20% of the heat input in the primary combustion zone. Mixing between the injected gas and the product gases from the primary zone are key to effective NOx removal, and thus in some cases a carrier gas may be injected with the natural gas to increase the momentum of the injected stream and achieve increased cross-stream penetration of the gas jets. The resulting overall-rich zone reduces N O to N2. but produces relatively high levels of CO. The CO from the reburn zone is thus reduced by injection of overfire air to produce lean conditions in a burnout zone, where oxidation of the reburn gas is completed. Conventional gas reburning has been demonstrated to achieve NOx reductions of 50-70% in many installations. When used in conjunction with SNCR agents, increased removal efficiencies are possible while maintaining acceptable levels of ammonia slip and N2 0 formation. 1.2 Fuel-Lean Gas Reburning A rather different gas reburn technology called "fuel-lean gas reburning" has recently been proposed for achieving comparatively moderate NOx reductions, but at much lower gas input levels than are typical of conventional reburning. and without the need for an overfire air delivery system to achieve CO burnout. In this case, natural gas is injected into the reburn zone at sufficiently low levels to maintain overall fuel-lean conditions in the furnace. The NOx reburning reactions then proceed within locally fuel-rich regions formed by the gas injection and mixing process. CO burnout is achieved by the excess 0 2 available in the overall fuel-lean furnace gas, without the need for a separate overfire air system. Consequently this overall fuel-lean approach to gas reburning offers the potential to meet the NOx emissions targets applicable to a large number of utility boilers, but at much lower initial capital costs and at lower operating costs than are required for conventional gas reburning. It is anticipated that if overall fuel-lean gas reburning can achieve 35 - 4 5 % NOx reductions at 5- 7% gas input, then economic considerations could make this new technology a competitive option for seasonal NOx reduction in a large number of utility boilers. 1.3 Advanced Modeling of Gas Reburning As in all reburning technologies, the success of fuel-lean gas reburning hinges on achieving effective mixing of the injected gas with NOx-containing product gases from the primary combustion zone. Since NOx and temperature distributions can vary significantly over the furnace cross section and along the furnace length, uniform mixing of the injected gas will in most cases not produce the highest NOx removal efficiencies. Accurate numerical simulations of the gas injection and mixing processes can assist in identifying practical reburn system designs tailored to the characteristics of any given furnace. Such simulations offer rapid assessment of |
