Selective Non-Catalytic Reduction (SNCR) Systems
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Cold Flow Modeling


Cold Flow Modeling

Applied Utility Systems, Inc.'s (AUS) scope of services for NOx emissions control includes cold flow modeling, mathematical heat and mass transfer calculations, furnace temperature and emissions measurements, and special equipment fabrication. AUS routinely uses mathematical and cold flow models to design commercial systems for both combustion based and post-combustion based NOx controls.

AUS has established an in-house cold flow modeling facility. This facility is equipped with state-of-the-art diagnostic techniques to document boiler flow field as a function of load. AUS has also established modeling subcontractors to rapidly fabricate models to meet short deadlines.

The AUS cold flow modeling facility is equipped with two (2) 40 HP blowers that will allow the modeling of up to an 800 MW utility boiler. This boiler modeling can be achieved while duplicating Reynold's numbers in the convective section of the boiler. This condition is an important requirement, especially in modeling, flue gas flow distribution and NH3 mixing for SCR systems, and in achieving uniform air distribution to all burner positions in low NOx burner (LNB) retrofit applications.

Flow field documentation is non-intrusive and yields large amounts of data at minimum cost. Using light modulating techniques and helium-filled soap bubbles, the boiler flow field can be sliced vertically or horizontally to characterize the flow in each slice. This provides detailed information on the flow characteristics within the boiler, augmenting the accuracy of the heat transfer modeling.

AUS has augmented its cold flow modeling capabilities with analytical modeling for both boiler heat transfer and droplet trajectories. The heat transfer model used is commercially proven. As a matter of fact, in certain applications, heat transfer modeling identified errors in field temperature measurement by showing inconsistencies in the measured temperature values.

Cold Flow Modeling Methodology

Cold flow modeling involves the design and fabrication of a Plexiglas model at a suitable scale to geometrically duplicate the full-scale boiler. A scale of 12:1 is generally used, but for large boiler applications (in excess of 500 MW), a larger scaling factor is required. A typical model of a 480 MW boiler is shown in the photograph below. Applicable ductwork sections, including all turning vanes, airfoils, duct dampers, and all identifiable structures are geometrically duplicated. The boiler model may include combustion air ducts, flue gas ducts, and/or furnace and convective regions of the boiler, depending upon the application. Similarity between the model and field installation is based on the following criteria:

  • Furnace gas flow momentum and injected stream momentum are matched in the model.
  • The model is built with geometrical similarity at a reduced scale.

  • The Froude number of the model injection ports (e.g., OFA ports or NH3 injection nozzles) is matched to the "would be" full-scale injection port and/or nozzles Froude number.
  • Thring-Newby modeling criteria are applied in sizing burner throats to account for the temperature difference between the model and full-scale application.
  • A Reynolds number in excess of 20,000 is used in the model to simulate turbulent flow conditions.

480 MW Boiler Model

Flow field documentation is performed by a non-intrusive method. Helium-filled soap bubbles are injected into the flow field. Having essentially no density, the soap bubbles follow the direction of the flow field. Projecting a modulated light onto the soap bubbles produces broken lines as presented below. The line length provides an indication of the flow velocity, and the line direction provides an indication of the flow direction. The photograph below represents the documentation of the flow field at the boiler nose near the furnace exit. The photograph visually shows the presence of a recirculation zone that is established due to the geometry of the boiler.

Quantitative combustion air or flue gas distribution data are obtained in the model with the use of a miniature pitot tube. After documenting the base flow condition, baffles, turning vanes, and/or static mixing devices are introduced to the model to balance combustion air distribution to all burner positions.

Use of Modulated Light with Neutrally Buoyant Soap Bubbles for Flow Visualization

Quantitative evaluation of the modeling success is provided by the results of the modeling itself. Flow distribution before and after installing static mixing devices is compared to document the improvement achieved. Since the before and after measurements are both obtained in the model, they are directly comparable.

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