September 6, 1995

MEMORANDUM

SUBJECT: Calculating Potential to Emit (PTE) for Emergency

Generators 

FROM: John S. Seitz, Director

Office of Air Quality Planning and Standards (MD-10)

TO: Director, Air, Pesticides and Toxics 

 Management Division, Regions I and IV

Director, Air and Waste Management Division,

 Region II

Director, Air, Radiation and Toxics Division,

 Region III

Director, Air and Radiation Division,

 Region V

Director, Air, Pesticides and Toxics Division,

 Region VI

Director, Air and Toxics Division,

 Regions VII, VIII, IX, and X

The purpose of this guidance is to address the determination

of PTE for emergency electrical generators. 

Background

In a memorandum dated January 25, 1995, the Environmental

Protection Agency (EPA) addressed a number of issues related to

the determination of a source's PTE under section 112 and title V

of the Clean Air Act (Act). One of the issues discussed in the

memorandum was the term "maximum capacity of a stationary source

to emit under its physical and operational design," which is part

of the definition of "potential to emit." The memorandum

clarified that inherent physical limitations, and operational

design features which restrict the potential emissions of

individual emission units, can be taken into account. This

clarification was intended to address facilities for which the

theoretical use of equipment is much higher than could ever

actually occur in practice. For such facilities, if their
physical limitations or operational design features are not taken 

into account, the potential emissions could be overestimated and

consequently the source owner could be subject to the Act

requirements affecting major sources. Although such source

owners could in most cases readily accept enforceable limitations

restricting the operation to its designed level, EPA believes

this administrative requirement for such sources to be

unnecessary and burdensome. 

On the topic of "physical and operational design," the

January 25 memorandum provided a general discussion. In

addition, EPA committed to providing technical assistance on the

type of inherent physical and operational design features that

may be considered acceptable in determining the potential to emit

for certain individual small source categories. The EPA is

currently conducting category-specific analyses in support of

this effort, and hopes as a result of these analyses to generate

more general guidance on this issue as well.

The purpose of this memorandum is to address the issue of

PTE as it relates specifically to emergency generators. There is

a significant level of interest in this source category because

there are many thousands of locations for which an emergency

generator is the only emitting source. Moreover, based on a

review of this source category, there exists a readily

identifiable constraint on the operational design of emergency

generators. Hence, the EPA believes it would be useful to

provide today's guidance before the entire effort is complete. 

The policies set forth in this memorandum are intended

solely as guidance, do not represent final Agency action, and

cannot be relied upon to create any rights enforceable by any

party.

Guidance for Emergency Generators

For purposes of today's guidance, an "emergency generator"

means a generator whose sole function is to provide back-up power

when electric power from the local utility is interrupted. The

emission source for such generators is typically a gasoline or

diesel-fired engine, but can in some cases include a small gas

turbine. Emissions consist primarily of carbon monoxide and

nitrogen oxides. Other criteria pollutants, and hazardous air

pollutants, are also emitted, but at much lower levels. 

Emissions occur only during emergency situations (i.e., where

electric power from the local utility is interrupted), and for a

very short time to perform maintenance checks and operator

training. 
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The EPA believes that generators devoted to emergency uses

are clearly constrained in their operation, in the sense that, by

definition and design, they are used only during periods where

electric power from public utilities is unavailable. Two factors

indicate that this constraint is in fact "inherent." First,

while the combined period for such power outages during any one

year will vary somewhat, an upper bound can be estimated which

would never be expected to be exceeded absent extraordinary

circumstances. Second, the duration of these outages are

entirely beyond the control of the source, and when they do occur

(except in the case of a major catastrophe) rarely last more than

a day.

For emergency generators, EPA has determined that a

reasonable and realistic "worst-case" estimate of the number of

hours that power would be expected to be unavailable from the

local utility may be considered in identifying the "maximum

capacity" of such generators for the purpose of estimating their

PTE. Consequently, EPA does not recommend the use of 8760 hours

per year (i.e., full-year operation) for calculating the PTE for

emergency generators. Instead, EPA recommends that the potential

to emit be determined based upon an estimate of the maximum

amount of hours the generator could operate, taking into account

(1) the number of hours power would be expected to be unavailable

and (2) the number of hours for maintenance activities.

The EPA believes that 500 hours is an appropriate default

assumption for estimating the number of hours that an emergency

generator could be expected to operate under worst-case

conditions. Alternative estimates can be made on a case-by-case

basis where justified by the source owner or permitting authority

(for example, if historical data on local power outages indicate

that a larger or smaller number would be appropriate). Using the

500 hour default assumption, EPA has performed a number of

calculations for some typically-sized emergency generators. 

These calculations indicate that these generators, in and of

themselves, rarely emit at major source levels. (Of course,

there may be unusual circumstances where these calculations would

not be representative, for example where many generators are

present that could operate simultaneously).

Cautions

Today's guidance is only meant to address emergency

generators as described. Specifically, the guidance does not

address: (1) peaking units at electric utilities; (2) generators

at industrial facilities that typically operate at low rates, but

are not confined to emergency purposes; and (3) any standby
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generator that is used during time periods when power is

available from the utility. This guidance is also not intended

to discourage permitting authorities from establishing

operational limitations in construction permits when such

limitations are deemed appropriate or necessary. Additionally,

this memorandum is not intended to be used as the basis to

rescind any such restrictions already in place.

Distribution/Further Information

The Regional Offices should send this memorandum to States

within their jurisdiction. Questions concerning specific issues

and cases should be directed to the appropriate Regional Office. 

Regional Office staff may contact Tim Smith of the Integrated

Implementation Group at 919-541-4718. The document is also

available on the technology transfer network (TTN) bulletin

board, under "Clean Air Act" - "Title V" - "Policy Guidance

Memos". (Readers unfamiliar with this bulletin board may obtain

access by calling the TTN help line at 919-541-5384).

cc: Air Branch Chief, Region I-X

Regional Air Counsels, Region I-X 

Adan Schwartz (2344)

Tim Smith (MD-12)