Synthetic minor permit limits

Why propose a synthetic minor permit limit?

There may be some times when you will want to keep your allowable emissions — emissions as set by permit limits — below certain levels. For example, you may be able to accept limits to keep your facility non-major under the federal Title V/Part 70 permit program to avoid the need for a Part 70 permit or under the federal New Source Review (NSR)/Prevention of Significant Deterioration (PSD) regulations to avoid the facility being classified as a major PSD source. If you already have a major PSD facility, you might want to restrict the size of a modification to a level below what constitutes a major modification under the NSR/PSD regulations.

Taking limits like these is referred to as making your facility or modification a "synthetic minor;" the limit is a “synthetic minor limit” and the permit is a “synthetic minor permit.” You may benefit from applying for a synthetic minor permit by avoiding a more complex permitting process or by receiving simpler monitoring or modeling requirements. This discussion will introduce you to what you should propose in your permit application for synthetic minor limits.

Types of limitations

Acceptable synthetic minor limits must include specific emission limits as well as operational or production limits to restrict emissions of specific or multiple pollutants. Emission limits by themselves are not adequate. For example, we can't just say in your permit "do not exceed 100 tons per year" — that is too vague to be enforceable. Operation and production limits must apply to something that can be readily measured. Examples of acceptable restrictions that limit emissions include limiting the hours of operation of your equipment, or limiting material or fuel usage. Limiting emissions may also require a combination of restrictions such as limits on materials, hours, and the requirement to operate control equipment meeting some specified control efficiency. If you choose to limit the hours of operation, note that your potential emissions have to be calculated assuming that the maximum design capacity operation occurs for all the hours allowed by the permit. This might make a limit on the hours of operation more restrictive for you than process limits.

Whichever type of limit is selected, there needs to be a time period associated with it. The default time period preferred by the EPA is daily (See EPA's 6/13/89 Guidance on Limiting Potential to Emit in New Source). However, it is often too cumbersome to do daily limits, particularly for operations that are seasonal or unpredictable in variation. In such cases, a longer time period may be acceptable, if it can be justified.

For many sources, the limit may be proposed in any of several ways. It could be a straight monthly limit. It could be an annual limit where, every month, you calculate the total over the previous 12 months (called a 12-month rolling sum) or where, every month, you calculate the average over the previous 12 months (called a 12-month rolling average).

Either of the rolling limits will give you a bit more operating flexibility than a monthly limit by compensating for high months with other months that are lower. This type of rolling limit is used when a facility has unpredictable variability in their operations and where there are no short term concerns that must be addressed through the proposed limit.

Some emission units are subject to federal New Source Performance Standards, which often require the use of a 30-day rolling period. For some facilities (often those that are complex), a 365-day rolling average may be necessary. If you propose a rolling average or rolling sum for a new unit, you must provide an additional way to show that you comply with your limits during the first 12 months of operation. This could be a formula, as shown in the examples, below. It could be a temporary monthly limit. If you are requesting new limits on an existing source, you have the option of using existing operating data in the rolling period. That would eliminate the need for a special limit for the first 12 months.

In some instances, the synthetic minor limit may be used to restrict the emissions of a pollutant for which there is an ambient air quality standard. Depending on the size of your facility or proposed modification, you may need to show modeled compliance with the standard. If the form of the ambient standard is a 1-hr, 3-hr, or 24-hr average, you may need to propose your limits using a shorter averaging period in order to restrict your short term emissions for the purpose of modeling compliance. Twelve-month rolling limits do not restrict your short term potential to emit, only the annual average emissions.

Margin below applicable threshold

Synthetic minor limits are set in order to ensure that a given operation does not trigger a regulatory requirement, therefore it is important that the proposed limit leave an adequate margin in order to account for the accuracy of the proposed compliance method and any emissions that may be left out of the on-going compliance tracking (e.g., insignificant activities). Facilities that propose a limit to avoid a threshold and hope to rely on general or non site-specific emissions factors typically receive limits with a margin of 10%. If you are willing to collect on-going site-specific data (e.g., continuous emissions monitoring, analyze contents of raw materials, etc.), a narrower margin might be appropriate.


Once you have your synthetic minor permit, you will have to keep records to show that you are complying with the permit limits. As part of your application requesting the limit, you need to propose your compliance method. Typically, we would expect you to propose a method involving some combination of performance testing, monitoring, on-site daily record keeping, and calculations at least monthly. Again, EPA’s default policy is usually daily, unless a longer period is justified as described above. Every year, you will have to certify that you are in compliance with these limits and send the certification to the MPCA.

The level of monitoring that you propose should take into account the likelihood of violating the limit, whether or not add-on controls are necessary to meet the emissions limit, the variability of emissions over time and different modes of operation, the type of monitoring, process, or control equipment data already available, the technical and economic feasibility of possible continuous and periodic monitoring methods, and the kind of monitoring found on similar units.


Make sure you can live with your proposed synthetic minor limits. If, in the future, you want to modify your permit to increase your limits, a major permit amendment would be required. You may have to revise your project analysis and address the requirements of the program for which you were previously made synthetically minor. For example, if at any time in the future you want to raise a limit that was taken to avoid a PSD major modification, PSD may be triggered including requirements such as the installation of control equipment or computer modeling of the emissions.


Example 1 — An existing spray painting operation with no control for Volatile Organic Compound (VOC) emissions. Without limits, the facility has potential emissions which make the facility major for both Part 70 and NSR/PSD. Since their past actual emissions were less than 100 tons per year, they accepted permit limits on the amount of material used per month. This limit keeps the facility's potential emissions below 100 tons per year for VOC. Therefore, the facility as permitted is considered a synthetic minor source under NSR/PSD. [The permit limit does not affect the potential emissions of hazardous air pollutants (HAPs). With the limit in this example, the company could still be a major source for HAPs, and therefore a still major source under Part 70].



Volatile Organic Compounds (VOCs)

VOC-containing material use shall not exceed 12,900 pounds per month, calculated each calendar month by the 5th of the month.

12,900 lbs. per month/2000 lbs. = 6.45 tons per month
6.45 tons per month x 12 months = 77.4 tons per year)

Proposed Compliance Method

Daily records of materials used using flow meters and usage logs and monthly calculations as detailed above

Example 2 — Installation of a new diesel generator set at an existing minor facility; the unrestricted potential emissions of the generator set would cause the installation to be a major modification under NSR/PSD. The proposed limit keeps potential emissions from the modification to less than 100 tons per year, and PSD review is avoided.



Nitrogen Oxides (NOx)

Diesel generator use: Not to exceed 300 hours of operation in any 12 month period

Compliance with the limit shall be based upon a monthly, 12 month rolling sum. For the first 12 months of operation, compliance shall be based upon the following formula:
Hn < 27.27 + 22.73n
Hn = the total number of hours of operation through month n
n = number of months of unit operation

Proposed Compliance Method

Install hour meter on the generator, record hours daily, calculate hours operated monthly as detailed above

Where did that formula come from? It was set up to keep the total number of hours of operation for the first year less than or equal to 300 hours. The company decided that having 50 hours in the first month was more than enough for the set. That meant that there were 250 hours available for the next 11 months. Dividing, 250/11 = 22.73 hours. The first term of the formula is then (50 - 22.73) = 27.27. There's nothing requiring the choice of 50 hours for the first month. If 30 hours were enough, the formula could use 30. If more were needed in the first month, the formula would use that higher number.

Example 3 — Existing boilers which utilize several different fuels; the total capacity is about 70,000 lb of steam per hour (total rated heat input is about 100 MMBtu/hr). While the actual emissions from the facility have never exceeded major source thresholds, the potential emissions (if all boilers operated at full capacity all the time) cause the facility to be a major source under NSR/PSD. By limiting facility-wide fuel use to realistic amounts, the source can limit their potential emissions to below major source thresholds, and avoid being classified as a major source. All the boilers are lumped together for the purposes of limiting fuel use at the facility.



All criteria pollutants



The types and total amounts of each fuel consumed in any 12 month period shall not exceed:

  • Natural gas/propane: 102 million cubic feet
  • Distillate oil (not to exceed 0.5% sulfur by weight): 720,000 gallons

Proposed Compliance Method

Track fuel usage daily using meters and usage logs, calculate total usage monthly as detailed above; fuel supplier certification with each shipment re: sulfur content

Example 4 – A new grain unloading operation will be installed at an existing major facility. The potential to emit of the modification, without limits, would be a major modification for NSR. The facility proposed to limit the new operation by installing a total enclosure around the unloading area as well as emissions controls and limiting the actual throughput. Based on emissions factors and the proposed control efficiencies, PM10 is the limiting pollutant (i.e., if the PM10 emissions are less than the applicable NSR threshold, PM and PM2.5 are also below their thresholds).



Particulate Matter, Particulate Matter less than 2.5 and 10 microns


Grain throughput limited to 6,500,000 lbs/yr on a 12-month rolling sum basis

Install and operate a total enclosure to capture emissions during grain unloading and install and operate a fabric filter connected to the total enclosure with a control efficiency of 99% for PM and 93% for PM10 and PM2.5

(Calculation: 6,500,000 lbs/year x 0.059 lb of PM10/lb of grain x (1-.93)*ton/2000 lb = 13.5 tons of PM10 (threshold of 15 tpy)

Proposed Compliance Method

  • Track grain throughput daily and calculate the 12-month rolling sum throughput each month by the 10th of the month
  • Propose to do one-time stack test to measure PM10 and PM2.5 emissions to confirm the emissions factor used to derive the throughput limit
  • Keep doors and other functional openings into the grain unloading area totally closed during grain unloading operations; maintain signage in area around doors instructing staff to keep doors closed
  • Monitor the fabric filter daily by pressure drop and checking for VEs; proposed fabric filter pressure drop range of 2-5 inches of water column