Ducts in a row
As an example of the benefits that high-tech design techniques brought to the project, consider how the configuration and sizing of induced-draft fans was optimized. Given the size of the major pollution-control systems to be added, Washington Group designers realized that they would require a substantial increase in overall fan capacity. But would it make more sense to get that capacity by replacing or upgrading existing ID fans, or by adding an ID booster fan or two? What about constant-speed versus variable-speed fans? Centrifugal or axial? After many design iterations, and with an eye toward equipment life-cycle costs, the final decision was made to install two new single-speed axial booster fans with blade pitch control in series with the existing ID fans.
For this project, mercury control was not included in the scope of work. However, Washington Group's final design does anticipate the possible use of sorbent injection and fabric filter baghouses. It does so by making the ductwork between the old and new fans long enough to accommodate the addition of baghouses and conveyors, as well as silos for storing fresh and spent activated carbon sorbent, in the event that new rules require such control.
Once Washington Group's engineers had determined the fans' type and configuration, they next had to confirm the ductwork design. They outsourced that task to Airflow Sciences Corp. (www.airflowsciences.com), which used computational fluid dynamics (CFD) simulations and a 1/12th-scale physical model of P4's AQCS to confirm three design characteristics: overall system pressure loss, uniformity of flow, and the reduction of opportunities for flyash to accumulate in ducts.
The velocity distributions generated by the CFD model (Figure 6, left) confirmed that the velocity profile of the ductwork was sufficiently uniform in critical areas. The physical model (Figure 6, right) endorsed the confirmations of pressure loss and flow via smoke testing, and that of ash accumulation via salt entrainment testing.
6. Second opinions. Booster fans were added before the ID fans on both units to overcome the higher backpressure produced by the addition of the SCR and FGD systems. Stability of the system was a concern, so CFD testing was required. Shown on the left is a 3-D velocity distribution taken from the CFD model; the booster fans' discharge enters from the left, and the air exiting at right goes to the absorber inlet. A 1/12th-scale model (right) also was built and used to confirm that opportunities for ash accumulation in ductwork had been minimized. Courtesy: Washington Group International
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