The NTH Advantage
NTH has over 30 years of experience providing air quality permitting solutions through modeling and qualitative analysis demonstrations. NTH has been involved with the stakeholder group currently working with MDEQ to refine its guidance, and we understand the agency’s expectations from this type of modeling and qualitative analysis.
Dispersion modeling is a technique that utilizes models to predict the location and concentration of air contaminants as a result of atmospheric dispersion. Sources of air contaminants include industrial, manufacturing and utility facilities, and can be emitted from stacks and fugitive sources (storage piles, equipment leaks, etc.). Dispersion modeling is utilized to determine if sources can comply with state and federal air quality standards for various pollutants.
U.S. EPA has promulgated standards for regulated air pollutants including nitrogen oxides, carbon monoxide, fine and coarse particulate matter (PM10/PM2.5), sulfur dioxide, and lead. Further, many states have promulgated health-based ambient standards for toxic air contaminants such as formaldehyde, acrolein and benzene. Both federal and state ambient health standards are developed for various averaging time periods to consider health impacts from both acute and chronic exposure (i.e., hourly, daily, annual). To properly assess impacts for each averaging time period, source emissions must be determined and account for the maximum emission rate for any given hour, day or year. Ambient standards are extremely stringent and monitored background levels in some locations are close to the standards. As such, new industrial sources or changes to existing sources can have difficulty during the permit application review process. Ultimately, dispersion modeling defines the location, equipment configuration and operational flexibility for a source.
Typically, dispersion modeling is not required for sources with de minimis emissions as impacts are often negligible. As standards are ratcheted down, dispersion modeling is an increasingly important part of any permit application review. The Michigan Department of Environmental Quality (MDEQ) has recently released new guidance addressing when air quality modeling - and in some cases, qualitative assessments of dispersion will be required. These stringent requirements mean that modeling, and/or qualitative assessments, will be necessary for approval of most air permit applications in which the project involves installation of moderate to large industrial equipment (i.e., equipment for electrical and steam generation, gas and oil production, manufacturing, materials processing, etc.). More importantly, it is also likely that either dispersion modeling or a qualitative assessment will be necessary for those projects that involve installation of air quality control systems, replacement of equipment, and/or provide a net reduction in emissions; thereby improving local air quality. Other examples include replacing a coal boiler with a smaller natural gas boiler, or installation and operation of temporary equipment without tall stacks.
NTH Case Study - Qualitative Assessment
NTH developed one of the first qualitative analyses under this guidance. As part of the Boiler MACT compliance plan, our industrial client decided to remove coal-firing capability from the facility boilers, convert to natural gas, and install a backup boiler to provide process steam while the facility boilers were down. Although there was a reduction in pollutant emissions as a result of removing coal capabilities, MDEQ’s guidance required further analysis regarding impacts from the new backup boiler. NTH developed a qualitative assessment demonstration to show that ambient impacts as a result of the project would not increase. The analysis included an evaluation of the existing air quality, the difference in facility actual and allowable emissions as a result of the project, and location of the backup boiler related to the facility fence line, nearby residents, and waterways. MDEQ approved the qualitative analysis and proceeded to approve the air permit without further restrictions on the backup boiler or associated stack parameters.
NTH Case Study - A Unique Solution
NTH recently assisted one of our waste-to-energy clients with a compliance demonstration related to health-based screening levels for formaldehyde emissions using dispersion modeling. In order to more accurately account for how formaldehyde reacts and breaks down in the atmosphere, NTH recommended use of a Gaussian puff modeling system called CALPUFF. CALPUFF was ideal for this project as it includes pollutant removal due to wet scavenging, dry gas deposition, and reactive decay. It also allows the use of more sophisticated meteorology that incorporates multiple weather stations, topography, water temperatures and precipitation. CALPUFF is not a conventional model, and therefore, approval from MDEQ was required prior to using it.
As part of this project and modeling demonstration, NTH performed an extensive scientific assessment in order to demonstrate that the source would not cause or contribute to injurious effects to human health or safety. All available information on the health effects of formaldehyde exposure based on the level of emissions and duration of exposure were reviewed. This included a description on how formaldehyde is formed, how it reacts in the atmosphere, how it is present in the atmosphere, its half-life, and methods of exposure (i.e., inhalation, ingestion, dermal). An evaluation of toxicological affects from exposure, both acute and chronic, was performed after an extensive review of available documents and studies.
Finally, NTH reviewed the distance of the facility to nearby residents and current and future land use to more accurately determine exposure from actual emissions from the facility. This information, coupled with the CALPUFF modeling analysis, was used to demonstrate that the existing facility will not result in adverse impacts. MDEQ approved the analysis and the facility could continue operating without restrictions or changes to the equipment configuration and stack heights.