OCR Text |
Show Diverting Portions of the Fuel and Combustion Air Streams from the Main Burner(s) lor Injection into the Post Flame Gases Mechanistic Model for NOx Destruction NOx Formation Inhibited Due to Fuel Rich Conditions in Reburn Zone; NOx Destruction is Promoted Due to Secondary Flame Radical Attack on N O Produced in Primary Zone to Form Molecular Nitrogen Burnout Air Reburn Fuel Primary Fuel-Air Burnout Zone Reburn Zone Primary vk Zone /) Hypothesized NOx Destruction Mechanism: CH OH H °H,H NO-^-^HCN °H,H» NH Figure 2: Basic Rebum Process Description diate nitrogen-containing species (e.g., N H 3 , H C N ) , in a high-temperature reducing zone, react with previously formed N O x to form the desired product, molecular nitrogen. A n y unburned fuel leaving the reburn zone is subsequently burned to completion in the burnout zone, where additional combustion air is added. Reburning can be used on all types of fossil-fuel-fired boiler configurations using coal, oil, or gas as primary fuels. The technology is particularly adaptable to slagging furnaces employing either cyclone burners5 or swirl-stabilized burners similar to those used in the Ladyzhin units. Since these burners m a y not be able to tolerate significant changes to their operation, such as lower excess air or staged air injection, without the possibility for creating other problems (such as problems with slag tapping; i.e., problems with the removal of coal-ash slag through the furnace bottom while still in its molten state), they are limited to an in-furnace N O x reduction technology that does not depend on significant changes to either the existing m o d e of operation or the burner hardware. Reburning does not require that any significant operational changes be made to the primary combustor or burners. The key requirement is that the fuel feed rate to the primary combustor be reduced by an amount equivalent (on a Btu basis) to the fuel feed rate to the reburn zone, usually not more than 20 percent of the total fuel input. With reburning, the excess air or air/fuel stoichiometry within the main burner or primary zone can be essentially identical to the baseline, pre-reburn operating conditions, thereby avoiding the potential for operational problems in the lower furnace. 4 11-12 |