An emission 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).
The general equation for emission estimation is
(3.3)
where
- E = emissions,
- A = activity rate,
- EF = emission factor, and
- ER = overall emission reduction efficiency, %.
ER is further defined as the product of the control device destruction or removal efficiency and the capture efficiency of the control system. When estimating emissions for a long time period (e.g. one year) both the device and the capture efficiency terms should account for upset periods as well as routine operations.
AP‐42 by Chapters: Emission Factors for Quantifications
AP‐42, Compilation of Air Pollutant Emission Factors, has been published since 1972 as the primary compilation of EPA’s emission factor information. It contains emission factors and process information for more than 200 air pollution source categories. A source category is a specific industry sector or group of similar emitting sources. The emission factors have been developed and compiled from source test data, material balance studies, and engineering estimates. The fourth edition of AP‐42 was published in 1995 (USEPA 1985a). Since then EPA has published supplements and updates to the 15 chapters available in Volume I, Stationary Point and Area Sources. Use the AP‐42 Chapter webpage links to access the document by chapter (Table C.2).
EXAMPLE 3.3 USE OF EMISSION FACTORS
A electrical power–generating station with four electrical generating units burned 1.0 million T of coal, 22 000 000 gal of No. 6 fuel oil, and 600 million ft3 natural gas to generate electricity during 1990. Use emission factors given in the table below to estimate the releases of arsenic and mercury from the stack at the power plant.
| Pollutant | Coal emission factor (lb/T) | Oil emission factor (lb/103 gal) | Natural gas emission factor (lb/106 ft3) |
| Arsenic | 4.10 × 10−4 | 1.32 × 10−4 | 2.30 × 10−4 |
| Cadmium | 5.10 × 10−5 | 3.98 × 10−4 | 2.52 × 10−4 |
| Chromium | 2.60 × 10−4 | 8.45 × 10−4 | 1.10 × 10−4 |
| Lead | 4.20 × 10−4 | 1.51 × 10−3 | 2.71 × 10−4 |
| Mercury | 8.30 × 10−5 | 1.13 × 10−4 | 7.15 × 10−4 |
| Nickel | 2.80 × 10−4 | 8.45 × 10−2 | 3.61 × 10−3 |
Source: From USEPA, Office of Air Quality Planning and Standards (1998).
SOLUTION
- Arsenic:(1 000 000 T)(4.10 × 10−4 lb/T) + (22 000 × 103 gal)(1.32 × 10−4 lb/103 gal) + (600 × 106 ft3)(2.30 × 10−4 lb/106 ft3) = 413 lb (approximately).
- Mercury:(1 000 000 T)(8.30 × 10−5 lb/T) + (22 000 × 103 gal)(1.13 × 10−4 lb/103 gal) + (600 × 106 ft3)(7.15 × 10−4 lb/106 ft3) = 86 lb (approximately).
Emission factor ratings in AP‐42 (discussed below) provide indications of the robustness, or appropriateness of emission factors for estimating average emissions for a source activity. Usually, data are insufficient to indicate the influence of various process parameters such as temperature and reactant concentrations. For a few cases, however, such as in estimating emissions from petroleum storage tanks, this document contains empirical formulae (or emission models) that relate emissions to variables such as tank diameter, liquid temperature, and wind velocity. Emission factor formulae that account for the influence of such variables tend to yield more realistic estimates than would factors that do not consider those parameters.
The extent of completeness and detail of the emissions information in AP‐42 is determined by the information available from published references. Emissions from some processes are better documented than others. For example, several emission factors may be listed for the production of one substance: one factor for each of a number of steps in the production process such as neutralization, drying, distillation, and other operations. However, because of less extensive information, only one emission factor may be given for production facility releases for another substance, though emissions are probably produced during several intermediate steps. There may be more than one emission factor for the production of a certain substance because differing production processes may exist or because different control devices may be used. Therefore, it is necessary to look at more than just the emission factor for a particular application and to observe details in the text and in table footnotes.
The fact that an emission factor for a pollutant or process is not available from USEPA does not imply that the Agency believes the source does not emit that pollutant or that the source should not be inventoried, but it is only that USEPA does not have enough data to provide any advice.
Emission factors may be appropriate to use in a number of situations such as making source‐specific emission estimates for area wide inventories. These inventories have many purposes, including ambient dispersion modeling and analysis, control strategy development, and in screening sources for compliance investigations. Emission factor use may also be appropriate in some permitting applications such as in applicability determinations and in establishing operating permit fees.
Emission factors in AP‐42 are neither USEPA‐recommended emission limits (e.g. best available control technology or BACT or maximum achievable control technology or MACT or lowest achievable emission rate or LAER) nor standards (e.g. National Emission Standard for Hazardous Air Pollutants or NESHAP or New Source Performance Standards or NSPS). Use of these factors as source‐specific permit limits and/or as emission regulation compliance determinations is not recommended by USEPA. Emission factors essentially represent an average of a range of emission rates, approximately half of the subject sources will have emission rates greater than the emission factor and the other half will have emission rates less than the factor. As such, a permit limit using an AP‐42 emission factor would result in half of the sources being in noncompliance.
Also, for some sources, emission factors may be presented for facilities having air pollution control equipment in place. Factors noted as being influenced by control technology do not necessarily reflect the best available or state‐of‐the‐art controls, but rather reflect the level of (typical) control for which data were available at the time the information was published. Sources often are tested more frequently when they are new and when they are believed to be operating properly and either situation may bias the results.
As stated, source‐specific tests or continuous emission monitors can determine the actual pollutant contribution from an existing source better than emission factors. Even then, the results will be applicable only to the conditions existing at the time of the testing or monitoring. To provide the best estimate of longer‐term (e.g. yearly or typical day) emissions, these conditions should be representative of the source’s routine operations.
A material balance approach also may provide reliable average emission estimates for specific sources. For some sources, a material balance may provide a better estimate of emissions than emission tests would. In general, material balances are appropriate for use in situations where a high percentage of material is lost to the atmosphere (e.g. sulfur in fuel, or solvent loss in an uncontrolled coating process). In contrast, material balances may be inappropriate where material is consumed or chemically combined in the process or where losses to the atmosphere are a small portion of the total process throughput. As the term implies, one needs to account for all the materials going into and coming out of the process for such an emission estimation to be credible.
If representative source‐specific data cannot be obtained, emissions information from equipment vendors, particularly emission performance guarantees or actual test data from similar equipment, is a better source of information for permitting decisions than an AP‐42 emission factor. When such information is not available, use of emission factors may be necessary as a last resort. Whenever factors are used, one should be aware of their limitations in accurately representing a particular facility, and the risks of using emission factors in such situations should be evaluated against the costs of further testing or analyses.
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