Windboxes

     The overall performance of a power plant depends significantly on how fuel and combustion  air  enter  the furnace.  Large fossil fuel boilers can have up to 100 individual burners that inject fuel and air.  The proportion  of flow through each of these plays an important role in the efficiency of the combustion process.  By properly designing the combustion system, plant emissions of NOx and CO can be minimized while boiler efficiency  and equipment longevity are maximized.
     Often the engineering design process for a combustion system involves a flow model of the air and/or fuel delivery equipment.  The figures show two different types of models used for the  design of a power plant windbox (the duct that feeds combustion air to the burners).

Figure 1:  Physical Model	Figure 2:  CFD Model

    Figure 1 depicts a 1/18th scale physical model of an actual windbox.  A fan supplies air to the model,  and  laboratory experiments are performed to analyze the velocity, pressure, flow rate, and other fluid dynamic properties.  These models are usually built with clear materials to allow for smoke flow visualization.  Results  from a computational fluid dynamics (CFD) windbox model are shown in Figure 2.  In a CFD model, the geometry is represented virtually using a computer.  Sophisticated software calculates the air flow properties including velocity patterns, pressures, flow balance, temperatures, etc.

    For this windbox, flow modeling allowed deficiencies in the basic design to be pinpointed.  Design of flow control devices such as turning vanes and baffle plates were optimized using the model before any actual construction occurred at the plant.  The final design from the model was implemented to achieve optimal combustion of a low-NOx burner system. 

    After implementing ASC's windbox design changes to balance secondary air, Deseret G&T's Bonanza plant  Unit 1 realized an 8% decrease in NOx, a 40% reduction in unburned carbon, and a net heat rate improvement of 0.7%.