Air Quality Compliance: Understanding Dispersion Modeling
By Mike Craig, Technical Director | Exhaust Dispersion & Design
What tools does your company use to assess air quality and compliance?
Dispersion modeling is a vital method of assessing compliance and mitigating existing and future air quality issues. They provide a link between exhaust emissions and changes in air quality at sensitive receptor locations.
The use of practical modeling tools to accurately evaluate the transport and dispersion of stack exhaust is critical to assessing your company’s compliance – with regards to the design of new facilities, additions to existing facilities, or processes for offsite compliance – as well as onsite worker comfort and safety.
Selecting A Dispersion Model
When it comes to selecting the most effective dispersion model, there are several vital factors to consider, including equipment size, the buoyancy of the exhaust, as well as the location of the receptors of interest. For receptors near the exhaust, wind flow interacting with building geometry can produce recirculation zones, which can have a significant influence on dispersion performance. Finally, the contaminants of concern such as heat, moisture, and traditional pollutants can also influence which dispersion model is most effective.
So, what modeling tools are used?
Modeling Assessment Techniques
There are three major tools currently employed for exhaust dispersion modeling, though they are not the only ones available.
- EPA approved dispersion models, such as AERMOD
- Wind tunnel testing
- Computational fluid dynamics (CFD)
Regulatory EPA dispersion models are considered the standard methods of assessing compliance within the industry sector. However, these models are not able to consider the influence of wind flow around buildings on dispersion in the nearfield, which may be important buildings with complex geometries. In cases where this is important, the use of alternate models may be appropriate.
Achieving accuracy in modeling results starts with understanding each of the major tools and when they are best used.
EPA Approved Dispersion Models
AERMOD, one of the EPA approved dispersion models, is quite useful for mid-range transport looking at impacts off-site. This modeling tool considers atmospheric stability as well as terrain and some building downwash. AERMOD is a steady state model that is not able to consider the effect of mechanical turbulence and wind flow patterns on dispersion. Thus, this model is generally not appropriate for nearfield dispersion where stacks are expected to be heavily influenced by building recirculation zones.
In some jurisdictions, building wake equations as published by ASHRAE are the approved dispersion model for situations involving same structure contamination. However, these equations can be challenging to apply for buildings with complex geometry.
Wind Tunnel Testing
Wind tunnel testing is considered the most accurate and reliable of these three major tools for simulating the aerodynamic influence of building geometry on exhaust dispersion. It offers the ability to optimize designs and takes into account complex building design and surrounding topography. As such, some jurisdictions allow wind tunnel testing for compliance purposes.
Wind tunnel testing is completed by constructing a scale model of the site and surroundings. In the boundary layer wind tunnel, wind interacts with the physical site and building geometry and tracer gas is released from the exhaust stack. Sensors on the model at key receptor locations measure the tracer gas to predict dilution levels achieved between the exhaust and receptor for a range of wind speeds and directions.
Wind tunnel results can be combined with local meteorological data to predict the frequency of potential air issues at nearby air intakes and adjacent buildings. Testing can be used to explore various mitigation options for improving dispersion performance, such as increased stack height and velocity, or optimizing exhaust and intake locations.
Computational Fluid Dynamics (CFD)
Computational fluid dynamics is another innovative design, troubleshooting, and efficiency improvement tool in the industry sector. CFD provides the virtual environment to simulate complex flow and thermal problems without the scaling limitations or infrastructure requirements of wind tunnel testing. Heat transfer, buoyancy, mixing, chemical reactions, vaporization, and particulate dispersion are some of the physics that can be modeled using CFD.
The standard approach to model turbulence in CFD is Reynolds Average Navier Stokes (RANS) turbulence models under steady-state assumptions. Such standard CFD turbulence models, however, struggle to accurately predict wind flows around buildings. This is especially the case with regards to separation and reattachment zones, and consequently contaminant dispersion around buildings. CFD can also be quite computer intensive to iteratively solve for each wind direction.
In addition to the added computational cost associated with implementing advanced turbulence modeling techniques, special set of skills and experiences are required to overcome some of the difficulties associated with such models. Large eddy simulations (LES), for instance, can present with difficulties in setting up the atmospheric boundary layer.
The way in which wind interacts with a building, particularly when the building design is fairly complex, has a significant effect on nearfield dispersion, and as such, assessing air quality, including the potential for poor air quality on adjacent buildings, is essential. In some cases, EPA approved dispersion models such as AERMOD are not sufficient in predicting dispersion impacts when the exhaust is influenced by wind flow around buildings.
In these cases, specialty dispersion models, including wind tunnel testing and CFD are recommended. These models offer increased accuracy in predicting nearfield dispersion, which is particularly beneficial when accounting for building effects. While CFD is quite useful in handling heat transfer, phase change, or even chemical reactions – which cannot be modeled in a wind tunnel – wind tunnel testing remains the preferred dispersion model in most cases.