July 2013: How to Estimate Potential to Emit (PTE) for Air Permits

This is the ninth article in a series about air quality in Indiana. I am planning to publish an article every two weeks to cover air pollution topics. Subjects covered in previous articles include national standards, ambient air monitoring, and air toxics. This article will discuss how to estimate potential to emit for air permits.

A permit is a tool for making sure air quality meets national standards. IDEM issues several different levels of permits, each providing a certain set of requirements for operations to monitor and control criteria pollutants or hazardous air pollutants. The level of air permit required, or the need to get one at all, is based on the potential emissions that a source can generate. The calculation, known as potential to emit (PTE), reflects the maximum amount of emissions a source would generate if there were no restrictions in place, operating continuously, at full physical capacity.

For example, a boiler may be physically capable of burning 50 tons per hour of coal operating 8760 hours per year. If the source takes a permit limit to only operate its boiler to operating 2000 hours per year and keeps records of its operation, the PTE may be based on 2000 hours, and would then be considered its limited PTE. If the source installs a control device that will limit emissions, and gets the air permit revised to require use of this control device whenever operating, the PTE can be further limited. In some cases it may seem that a source needs a permit so that it does not need a permit. In reality, a source often needs a permit at one level so that it does not require a permit at a higher level.

Other restrictions can be put in place to limit the PTE. For the example above, if the source were modified so that it could not burn more than 40 tons per hour of coal, then the PTE could be reduced. The key to remember is that without these restrictions in place in an enforceable permit, the PTE is based on emissions at full capacity and 8760 hours per year. When a company builds a source without getting a permit, the PTE is based on these assumptions because it does not have a permit in place to limit the use of the unpermitted source. Once it gets the proper permit, the PTE can be minimized. However, during the period operating without a valid permit, the source may have been subject to permitting requirements for a much higher level of permitting.

One other concept that often gets confused in the estimation of PTE is the concept of “integral to the process”. In short, IDEM has to determine if the controls are required to be operated in order to operate the facilities at the source. For example, if a material produced by a process is valued at $100,000 per ton, then losing lots of product up the stack while a control device is not operating properly may mean that the source would not realistically operate while the control device is malfunctioning. On the other hand if the product is not valuable or is not a product at all, but a waste stream, then there is no incentive for the source to shut down when the control device is malfunctioning. In concept this idea is easy to understand. However, exactly where is the breakpoint? What cost of product is sufficient to make this determination? Does it depend upon the source or the size of the source? There are several elements to this determination and each case is different, so a separate assessment is made each time a source claims that its controls are integral to the process.

When the source is applying for a new permit, the PTE calculation is a critical element of the permit application. The applicant must be aware of the physical limits of a new source and how many hours it will operate. If they are willing to take limits on either production capacity, throughput, or hours of operation, they will be required to keep records demonstrating that they are meeting these limits. Failure to maintain records will be a violation of the permit and can be subject to enforcement action.

More on the level of permit a new source needs and other information can be found on the Air Permits Frequently Asked Questions page.

Keith Baugues