Early in my career, I struggled to conceptualize the difference between actual and potential emissions and when to use which. It was an arduous process to piece together the regulatory definitions, agency website definitions, and learning on-the-job. So this series will focus on explaining in simple terms the difference between potential and actual emissions and when to use them. Links to successive articles in this series will be added as they are published.

Note: There's a whole other set of emission calculations called projected actual emissions under New Source Review, however, this series will focus just on actual and potential.

Potential Emissions

Potential emissions are the maximum amount of emissions equipment or a process can emit. There are three key pieces of information you need to determine to calculate potential emissions:

  1. The maximum physical and operational capacity of the equipment or process
  2. The emission factor which is representative of operating at the maximum capacity
  3. If there are any existing federally enforceable limits for the equipment or process.

This article will focus on #1, specifically on maximum physical capacity.

Determining the maximum physical capacity

Imagine you're preparing a cake for your friend's upcoming birthday party. To avoid the cake being too dense, you decide to use an electric hand mixer to mix the ingredients together. 

Most modern electric hand mixers have speeds varying from slow (Level 1) to fast (Level 5). As you notch up the level higher, the two beaters spin more quickly. The beaters' revolutions per time increases. 

Depending on how much ingredients you have in the bowl to mix or how much of a hurry you're in, you may choose to mix the ingredients on varying levels. However, the fastest that you can mix the cake ingredients together is when you're on Level 5. The beaters are rotating at the fastest possible speed at Level 5.

While you may not choose to mix the ingredients on Level 5, the maximum physical capacity is at Level 5.  As part of determining potential emissions, the maximum physical capacity of the equipment or process must be determined. 

Conceptually with the hand mixer example in mind, here are some ways you can determine the maximum physical capacity for equipment you're trying to calculate potential emissions for:

  • Look at the nameplate capacity listed on the equipment. The nameplate is often a small metal plate affixed to the outside of the equipment that lists the maximum capacity.
  • Find the maximum capacity on the manufacturer's specification sheet or design document. If you know the equipment name and model, you may be able to find it through a search online. Otherwise, you can request a copy from your client if they have it in their records. Common capacities are represented as heat input (MMBtu/hr), pumping capacity (gallons/minute), horsepower (hp), or volume (gallons).
  • Review previous air permit applications or submissions where the maximum capacity for the equipment has been referenced in the past. If your company is new to the project or client, you may want to do extra due diligence by requesting the manufacturer's specification sheet to confirm past representations in air permit applications.
  • Perform an agency-approved engineering test to run the equipment at maximum operating conditions to determine the maximum physical capacity. 

Keep in mind

Equipment can be modified to increase its physical capacity after its original purchase, so you want to make sure that the manufacturer or design document you utilize represents the current or planned design of the equipment.

For example, physical changes could include a burner on a boiler could be replaced with one of higher capacity or which can burn different fuels, changing the catalyst in a chemical reactor could change the physical capacity, or disconnecting fuel oil supply pipe from a boiler so it can only operate on natural gas.

In the next few articles we'll discuss maximum operating capacity and how that can interact with maximum physical capacity to determine the overall capacity for potential emission calculations.

Regulatory References

According to 40 CFR Part 52.21(4), Potential to emit means "the maximum capacity of a stationary source to emit a pollutant under its physical and operational design. Any physical or operational limitation on the capacity of the source to emit a pollutant, including air pollution control equipment and restrictions on hours of operation or on the type or amount of material combusted, stored, or processed, shall be treated as part of its design if the limitation or the effect it would have on emissions is federally enforceable. Secondary emissions do not count in determining the potential to emit of a stationary source."

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