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Show the B & W LEBS boiler is advanced staged combustion. The concept is based on the use of advanced aerodynamically air-staged burners (AASBs) mounted in the lower furnace main burner zone operating at an air/fuel stoichiometric ratio of 0.7-0.9. Overfire air (OFA) is added at a higher furnace elevation to complete the combustion of the fuel. AASBs are the most common type of low-NOx burners for coal-fired utility plant applications. Current commercial burners In this class Include B & W s DRB-XCL® (Figure 4). In these burners, coal and transport (primary) air are introduced through a central feeder pipe. Swirl is imparted to the coaxial secondary air stream to control the aerodynamics of the flame and the rate of air/fuel mixing. For the advanced staged combustion system, the gases leaving the main burner zone in the lower furnace are quenched (cooled) by radiant heat transfer to the furnace walls prior to the addition of OFA. OFA is added in such a manner as to efficiently complete the combustion of the fuel while avoiding the formation of additional NOx. The application of this N O x control concept to the LEBS boiler involves the careful integration of several power plant subsystems. Furnace Geometry. One of the more interesting aspects of designing the LEBS boiler is that the boiler configuration can be designed to enhance the N O x and S 0 2 control concepts. Of the numerous furnace geometries and burner locations considered for implementing advanced staged combustion, the most promising Included box multi-sided, venturi, and roof-fired furnaces. Additional factors considered in the selection of a furnace configuration included: sensitivity of flue gas flow patterns to load or burners in service, separation between the fuel-rich and fuel-lean zones, optical access for control system sensors, and accommodation of heat release patterns and flame shapes. Through this analysis the B & W LEBS project team has designed a furnace to Implement advanced staged combustion. The size of the furnace has been chosen to provide: a compact fuel-rich flame zone where total fixed nitrogen species are minimized, ample lower furnace waterwall surface to cool the first-stage gases at the lower furnace exit, a ven-turi- shaped double arch to decouple first- and second- stage heat transfer and to establish the flow patterns In the upper furnace, ample waterwall surface and residence time in the upper furnace to make steam and assure complete carbon burnout, and a flexible, staged air injection system that will provide slow, complete mixing to minimize both N O x and unburned combustibles losses. These factors can be integrated in a single boiler design by the approach taken in the LEBS program. Pulverizer Enhancements. It is anticipated that increased coal fineness and improved control and monitoring of coal flow on an individual burner basis will be significant factors in minimizing loss of combustibles and N O x emissions. B & W s current commercial M P S pulverizer has been selected as a base from which to begin evaluations of potential performance improvement. Many available and/or potential design options have been identified which could further improve performance. These options include various modified and advanced types of classifiers and upper housing configurations to Improve coal fineness. The benefits of increased coal fineness and flow control will be evaluated in a later task of the program. Trade-off studies will be conducted to determine the cost-effectiveness of various design improvement options. New emissions reduction measures to be implemented in the L E B S plant, along with the general dynamic complexity of a supercritical, coal-fired, once-through boiler, together amount to a formidable task of multivariate control and optimization. The Sliding Air Damper Drive Coal1 Inlet Air Measuring Device Adjustable Spin Vanes ~1 Outer -^ Secondary **V"^ II* £+.-I Inner 1 Secondary I Air Coal Nozzle ^ 3 - ' Flame ' Stabilizing Ring ] Separation --^ Rate Figure 4. B&W DRB-XCU® burner 6 Paper No. 11-13 |
