By Douglas J. Smith, IEng Senior Editor
USING COMPUTER simulation to evaluate the performance of wet scrubbers has saved Babcock & Wilcox (B&W) up to $70,000 and four weeks of work. Because the design of wet scrubbers depends upon the fuel being burned and the amount of sulfur in the fuel, B&W must custom design each scrubber they sell. When designing a scrubber the engineer has to balance a number of competing factors including its geometry, gas flow velocity, sulfur dioxide removal rate, slurry droplet carry-over rate and the pressure drop.
In the past design engineers constructed a 1/8-scale model of the scrubber, which was then used to evaluate the performance of the design. The model, made from Plexiglas, made it possible to observe the movement of the gas and liquids within the absorber cylinder. A major drawback of using models is they cost $50,000 to $80,000 each. In addition to the cost is the length of time to construct the model and/or modify the model.
Wet Scrubber Absorber Module
To reduce engineering costs B&W decided to augment the physical model testing of wet scrubbers with computational fluid dynamics (CFD) analysis software to simulate scrubber performance. The CFD software, CFX purchased from AEA Technology, Pittsburgh, Pennsylvania, is capable of two-phase modeling. This was very important to B&W because their wet scrubbers must deal with gas and liquids.
The first step in the process was the construction of a large-scale physical model that B&W used to develop the following quantitative data:
- Pressure drop through the scrubber
- Tray characteristics
- Liquid fractions within the absorber
- Mist eliminator pressure drop
- Droplet loading to the mist eliminators
- Amount of carryover and gas
- Liquid flow patterns
The model was tested with a wide range of gas velocities, slurry fluxes and spray header arrangements. This phase of the project took B&W four years to collect and analyze all of the data.
Using computer simulation, B&W engineers have designed a wet scrubber for which the base design was established in the large physical model. The first step, before they could use the simulation program, was to update the existing digital scrubber model to show the scrubber inlet, absorber trays, the spray zones and the mist eliminators. In operation, the computer simulates the flue gas as it enters uniformly through the side inlet and exits through the top of the scrubber. It also simulates the water entering the spray headers and exiting, at uniform pressure, at the bottom of the scrubber.
Modifying the model to match the new design parameters took approximately one week. Another week was spent running the CFX simulations to gauge the performance of the system. Using the scrubber simulation allowed the gas and water flow patterns to be observed under six different operating conditions. With the simulator the engineers are able to modify the scrubber's configuration and optimize its design very quickly. One person at a cost of less than $15,000 did the initial modeling, simulation and analysis. Previously three people were required when using the Plexiglas model.
"Being able to evaluate many different configurations has improved the quality of our designs," says Paul Williams, senior advisor at B&W. According to Williams, the computer simulation gives B&W the ability to evaluate multiple design options quickly and at lower costs.