PAEL Applicability Notice & Guidance

Pre-Approved Emission Limit Applicability Notice

On October 1, 2004 the Alaska Department of Environmental Conservation made some significant changes to 18 AAC 50, the Air Quality Control Regulations, including the addition of the minor permit program. Some of these changes have an effect on the Pre-Approved Emission Limits (PAELs) in 18 AAC 50.230 that facilities use to avoid a Title V Permit and minor permit.

This website is a PAEL Applicability Guide and by following the guidance and instructions can help determine whether or not a PAEL is needed for a facility. EPA guidance relating to standby or emergency generators can also help in the PAEL requirement verification process.

Pre-Approved Emission Limit Applicability Guidance

PAELs for small stationary diesel engine installations established in 18 AAC 50.230 are designed to hold potential emissions from this type of stationary source to less than 40 tons per year (tpy) of nitrogen oxides, thereby avoiding the requirement for an air quality control minor permit under 18 AAC 50.502(c)(1)(B).

There are situations where if the stationary source has a combination of diesel engines and other fuel burning equipment not included in the emission and fuel consumption limit under the PAEL. However, for such combination stationary sources, the owner or operator must also consider and account for potential emissions of other stationary equipment when calculating the stationary source's potential emissions under 18 AAC 50.230(c)(1)(F) for the 40 tpy threshold. Note that the PAEL for diesel engines authorized under 18 AAC 50.230(f) is not designed for stationary sources with other emissions units outside of those listed under 18 AAC 50.230(f)(1)(B).

Total Generating Capacity more than 294 hp or 219 KW

The emission factor on which the potential to emit (PTE) calculation for the PAEL issued under 18 AAC 50.230(c) is based, comes from the EPA publication AP-42, Table 3.3-1. The emission factor is 0.031 lb of NOx / hp-hr. Using this emission factor, a total diesel engine installation must have no greater than 294 installed horsepower (219 kW) capacity to stay below the 40 tpy NOx threshold requiring a minor permit.

(294 hp x .031 lbs NO_x / hp- hr x 8760 hr / yr) / (2000 lbs/ton) = 39.9 tons NO_xper year

If the total rated capacity of all stationary diesel engines at the installation does not exceed 294hp or 219kW, and there are no other emitting activities, then the stationary source does not have the potential to emit 40 tons or more of NO_x and does not require a minor permit. If there are other emitting activities at the stationary source and NO_x potential to emit is not certain, or the stationary source exclusively uses newer emissions units with EPA Nonroad or Marine Compression Ignition Engine Tier Certifications, then contact Air Quality for assistance at 907-465-5100.

Emergency Generators

If all stationary diesel electric generating equipment installed at the stationary source is used strictly as emergency or standby power in the event that there is an interruption of the normal power supply, a PAEL may not be necessary.

The Department uses EPA's September 6, 1995 Guidance to calculate PTE from emergency generators. The Guidance provides that diesel electric emergency generator PTE need not be estimated at 8,760 hours per year. Instead the permitting agency may determine PTE by using a reasonable and realistic estimate of the number of hours that the normal power supply can be expected to be unavailable, plus the number of hours for maintenance and training activities for the emergency equipment. EPA lists a value of 500 hours per year as an appropriate default assumption if historical information is unavailable.

If a stationary source with a PAEL for diesel emergency generators has a plant-wide PTE of less than 40 tpy, calculated using this guidance, the source owner or operator no longer needs to retain this PAEL. The Department will rescind the existing PAEL upon the owner or operator's specific request to rescind the PAEL.

(Total backup emergency engine hp x .031 lb NOx/hp-hr x operating hr/yr) / (2000 lb/ton) = NOx PTE from engines

As stated above, if other emitting activities also occur at the installation, the owner or operator may need to account for the PTE from these activities to be combined with the PTE from the stationary diesel engines to calculate a total plant-wide PTE. If plant wide NOx PTE is below 40 tpy, a PAEL is not needed.

Mixed Use – Power Generation and Emergency Backup

In certain instances where the stationary source has a mixed-use of diesel electric generators for electric generation and backup emergency capacity, a PAEL may not be necessary. To determine applicability, the owner or operator must first designate those diesel electric generators located at the stationary source used solely for emergency power generation. Then the owner or operator may determine the potential to emit using the below formula: In order to convert kW to hp, multiply the kW rating by 1.34 hp/kW.

((Total electric generating (non-backup) engine hp x .031 lb NOx/hp-hr x 8,760 hr/yr) / (2000 lb/ton)) + ((total backup emergency engine hp x .031 lb NOx/hp-hr x Operating hr/ yr) / (2000 lb/ton)) = NOx PTE from engines

The owner or operator can determine how many hours a backup unit may operate in any one year based on historical use. If no history is available, the owner or operator may opt to use 500 hours per year as the default multiplier in this formula based upon EPA policy. However, the Department prefers actual data. As stated above, if other emitting activities also occur at the installation, the owner or operator may need to account for the PTE from these activities to be combined with the PTE from the stationary diesel engines to calculate a total plant-wide PTE. If plant wide NOx PTE is below 40 tpy, a PAEL is not needed.

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