Variability in writing and enforcing air permits
Contrary to what you might think, there is little uniformity in air emissions permits from plant to plant. While many look to the Federal Government’s standards for PM emissions (i.e. MACT and other regulations), even these are not always applied uniformly across all applications. The job of actually enforcing most regulations often falls to the State authorities, who then may even distribute the responsibility further to other smaller air control boards that serve specific regions (such as in California). State regulations must be at least as strict as the Federal regulations when there is overlap - but the states have broad discretion on their own enforcement requirements. Even then the exact requirements for each plant can be further modified depending on factors such as the exact location of the plant, the surrounding area’s pollution levels, number of employees, and process conditions among others.
That means there is a wide variety of language and terminology used on permit documentation.
For this reason, dust collector OEMs and filter manufacturers may not always provide emissions testing data that directly corresponds to the language used on your plant’s air permit. Environmental/compliance engineers often have to interpolate available test data from OEMs to determine if the use of this particular product will enable the plant to achieve compliance.
That leaves us asking....Why can’t there just be one standard method of calculating emissions data?
This is the first of two articles which will tackle that question and explain how to reconcile the two.
Different Methods for Measuring PM Emissions For Different Purposes
Different applications call for using different methods of measuring particulate emissions. The party requesting the data may have different motives and requirements and therefore different methods have been developed for these purposes. For example, a filter manufacturer will want to know a baseline of performance for its products under ideal conditions while end users may want to calculate actual PM emissions from their equipment/process/facility for compliance obligations. These two goals require the use of different measuring methodologies.
Filter Manufacturers Rating in Grains per Dry Cubic Foot
Choosing the correct filter media has a direct impact on whether a facility achieves compliance with its air permit or not. In order to guide plant decision makers in the process, filter media manufacturers often have their fabric medias tested in order to establish baseline PM emissions levels. This involves measuring total emissions through the fabric under laboratory test conditions. For consistency, these findings are usually stated in grains per dry standard cubic foot or g/dscf. This method of measuring works well as it provides a baseline that can then scale up to fit each unique application. However, it is not a good indicator of performance across all applications as the results are based on only one fixed dust loading level.
Another way of stating this is in terms of collection efficiency. Some manufacturers also choose to include a calculation showing what percentage of the test particles were captured by the fabric in the test. Unlike grains per dry cubic foot, it has more flexibility to be useful for comparing fabric performance under different dust loading levels.
So far, so good. That's pretty straightforward.
How to Use Filter Media Test Data When Preparing an Air Permit
With two common methods for measuring PM emissions from dust collector technology, grains per dry cubic foot and collection efficiency percentage, the next question naturally is how those compare to PM emissions measurements commonly used on air permits. Specifically it's important to understand how to convert emissions specifications used by equipment OEMs and those used in air permits. Next week we'll tackle those as well some points to keep in mind when considering whether or not to use certain emissions control technology.