Writing a synthetic minor limit that holds up

What the limit has to do

A synthetic minor source could emit above a major source threshold at full design capacity, and takes limits that keep its potential to emit below it. The limit works only if it changes potential to emit, and it changes potential to emit only if it can be enforced.

The definition starts from design capacity. Anything that brings the number down, whether an hours cap, a throughput cap, or a control device, counts only when it is written into an enforceable requirement. Read the definition in the program you are applying under; the PSD definition is at 40 CFR 52.21(b)(4) and the Title V definition at 40 CFR 70.2.

The two tests in EPA's guidance

EPA's 1989 guidance on limiting potential to emit in new source permitting gives two criteria. The first is about the legal vehicle: the permit or plan the limit sits in. The second, enforceable as a practical matter, is about the wording: whether someone outside the facility can tell from the permit and the records whether the source complied.

The second test is where drafting matters. A limit can sit in a valid permit and still fail it, for example an annual emission cap with no stated way to calculate emissions.

Three things each condition must say

EPA's 1995 memo on options for limiting potential to emit lists what a source-specific permit has to specify. Use it as a drafting checklist for each limit:

• What is limited, stated precisely, and which units or parts of the source it covers
• The time period the limit applies over
• How compliance is determined: the monitoring, the calculation, the records, and the reports

Choose the averaging period on purpose

A long averaging period gives operations room, but it also lets a source run at full capacity for part of the period. EPA guidance prefers short-term limits and allows rolling limits where the permitting authority accepts that compliance can be readily determined. Look at what your agency has accepted for similar sources, and raise it early if you need a longer period.

A rolling limit is checked every period, not once a year. The Rolling 12-Month Limit Tracker sets up the monthly check and shows next month's headroom.

Limit something you can measure

Few minor sources measure emissions directly. The usual approach limits an operating parameter that drives emissions, such as fuel burned, throughput, or hours, and ties it to emissions with a stated factor or equation. Each limit should stand on its own: if the permit caps both throughput and hours, each is a separate condition that can be checked.

Keep the emission factor behind the limit in the permit record, with its source document and revision, so the basis stays clear if the factor is later revised.

When a control device does the work

If the limit relies on a control device, its efficiency alone is not enough. The permit needs the operating conditions the efficiency depends on, such as a minimum operating temperature, a pilot flame, or a closed vent system that stays closed, with monitoring of each.

Claim only the efficiency you can support: a performance test, a value the applicable rule allows, or a manufacturer guarantee the agency accepts.

Records that keep the source minor

The records are the proof. Plan them with the limit: what is metered or logged and how often, who calculates the total and when, where the factor comes from, and how long records are kept. A month without records is a month the source can't show it complied.

Date each calculation, and keep the original when a month is corrected, so the file shows both.

Some federal rules set their own list

A few rules say what a limit must contain before it can lower potential emissions for that rule's applicability. NSPS Subpart OOOOb does this for tank batteries. If a limit is meant to keep equipment out of a rule like this, check it against that rule's list as well as the general guidance.

Before you submit

Check the draft conditions against this list:

• Potential to emit without the limit is calculated first, so the limit's effect is visible
• Each limit names the units it covers, the limited quantity, and the averaging period
• The calculation from the monitored parameter to emissions is written out, with the factor's source
• Monitoring, records, and reports are specific enough for an inspector to check
• Control device conditions match the basis of the claimed efficiency
• Any rule-specific list, such as OOOOb's for tank batteries, is met
• Operations knows the limit and gets a monthly budget against it