This article is an historic document. For a current treatment of this subject, see the article: AP 42 Air Pollutant Emission Factors.

The combustion of fossil fuels to produce energy results in the emissions of air pollutants. The exact levels and types of each pollutant depend on the type of fuel used, the efficiency and age of the equipment, emissions controls that may be mandated by regulatory agencies, and other factors.

An emissions factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of pollutant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., kilograms of particulate emitted per megagram of coal burned). Such factors facilitate estimation of emissions from various sources of air pollution. In most cases, these factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average).

Note: TNMOC = total non-methane organic compounds; PM10 = particulate matter 10 micrometers in diameter and smaller; and GWP = global warming potential.

Emissions factors have long been the fundamental tool in developing national, regional, state, and local emissions inventories for air quality management decisions and in developing emissions control strategies. More recently, emissions factors have been applied in determining site-specific applicability and emissions limitations in operating permits by federal, state, and local agencies, consultants, and industry. These users rely on emissions factors and other emissions quantification tools (e.g., emissions testing and monitoring, mass balance techniques) in developing permits that are practical in their enforcement.

The general equation for emissions estimation is:

E = A x EF x (1-ER/100)

where:

E = emissions;
A = activity rate;
EF = emission factor, and
ER =overall emission reduction efficiency, %

One source of information recently published by the National Renewable Energy Lab (NREL) estimates the total emissions resulting from energy use, tracing these back to the actual ultimate energy source to extract, convert, and transmit that energy to a user in the United States. Table 1 shows the emissions factors for various pollutants expressed as kilograms of material generated per kilowatt-hour of electricity delivered to the end user. The data represent a national average of all forms of electric power generation.

Further reading

Source Energy Emissions Factors Deru, M and P. Torcellini, 2006. Source Energy and Emissions Factors for Energy Use in Buildings, NREL/TP-550-38617