Rhinelander is a small town in the North Woods of Wisconsin. In addition to its nearly 8,000 permanent residents, Rhinelander is home to the Hodag, a mythical creature that is said to resemble a cross between a dinosaur and a dog. It also happens to be home to the Rhinelander paper mill, which was established in 1903.
In 2010, the United States Environmental Protection Agency (EPA) issued a new National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO2) of 75 parts per billion (ppb), or 196.5 μg/m3, based on a 1-hour standard. When the NAAQS for sulfur dioxide went into effect, a sensor located at the city’s water tower, located about 600 meters northeast and across the Wisconsin River from the paper mill, indicated SO2 concentrations that were as much as twice that allowed under the new 1-hour standard.
Emissions from the Rhinelander paper mill—and, in particular, from its 63 meter (207 foot) cyclone boiler stack—were identified as the primary source of the elevated pollutant concentrations. Strangely, though, the EPA’s preferred regulatory modeling tool, AERMOD, continued to predict sulfur dioxide concentrations that were well within the threshold for compliance, even the threshold defined by the revised standards of 2010. Expera and AECOM turned to CPP’s air quality experts to uncover answers.
While studying a 1:240 scale model of the Rhinelander Mill facility in CPP’s atmospheric boundary layer wind tunnel, our engineers found that under the right wind conditions, one corner of the building that houses the boiler can generate powerful vortices, which actually worsen downwash near the water tower monitoring station. AERMOD, however, does not model this enhanced downwash effect. In fact, vortex physics are nowhere to be found in the software package.
Consequently, AERMOD consistently under-predicts pollution concentrations in winds that are dominated by vortices. In order to accurately account for the additional downwash created during these cornering wind events, CPP’s engineers conducted a series of tests with the 1:240 wind tunnel model to determine a suitable stack height that could successfully help the facility comply with EPA regulations: The higher the stack, the lower the downwash, and, consequently, the lower the pollution.
According to the Code of Federal Regulations (Title 40, Section 51.110 (ii)), the Good Engineering Practice (GEP) stack height is defined as the greater of the following:
- 65 meters
- 5H (for stacks in existence in January 12, 1979), or H + 1.5L (for all other stacks), where H is the height of the building itself or any significant nearby structure and L is the lesser of the projected height or width of the building
- The height determined by a wind tunnel modeling study.
The first criterion simply establishes a minimum stack height, and the second is a generic approach that attempts to capture a wide variety of structures in one simple formula. The third acknowledges the limitations of the first two and recognizes that a wind tunnel study can accurately predict GEP height in a broader variety of situations.
Using the formula above, the suggested GEP stack height would have been 75 meters, a full 12 meters higher than the stack’s actual size. But based on CPP’s wind tunnel study, a stack height of 90 meters was recommended, even higher than that calculated using the formula. Even the regulatory formula underestimated how high the stack should be. But the wind tunnel got it right.
The EPA and the Wisconsin Department of Natural Resources ultimately used CPP’s results along with the EPA’s software to study solutions to the sulfur dioxide problem. Thanks to CPP’s expert guidance, Expera and the regulatory agencies responsible for ensuring the Rhinelander mill’s compliance with air quality standards were able to proceed with modifications to the facility, knowing that these were based upon the best available science.