Last year, the EPA announced its intention to add or modify a number of AP-42 emission factors, primarily for petroleum refineries, pursuant to a consent decree entered into with Air Alliance Houston, Community In-Power and Development Association, Inc., Louisiana Bucket Brigade and Texas Environmental Justice Advocacy Services (“Plaintiffs”).  The EPA informally solicited comments on its proposal with a final comment period extended to December 22, 2014. After considering the comments, on April 20, 2015, the EPA made a decision to issue or revise eight emission factors.

Probably the most noteworthy part of the decision is that the EPA decided not to modify the nitrogen oxide (“NOx”) emission factor for industrial flares.  In 2014, the EPA initially proposed to increase this factor forty-fold based on information gleaned from various flare stack test intended to determine the combustion efficiency of flares under various operating conditions (i.e., varying steam and air to fuel ratios).  However, in its final decision, EPA determined not to revise the factor for NOx at this time. According to the EPA:

Based on our review of NOx emissions data for flares and additional information received after proposal, we have determined that the data was not adequate to support revising the NOx emissions factor for flares. Based on comments received, EPA determined that the NOx data used for the proposal contained certain flaws that rendered the data quality suspect.[1]

The EPA also decided not to revise emission factors for tanks and wastewater systems.  Emissions from storage tanks containing organic liquids are estimated based on a series of correlation equations developed by the American Petroleum Institute (“API”) and incorporated into Section 7.1 of AP-42.  As part of this review, the EPA determined that actual measured data from several studies agreed well with emission estimates based on these correlations.[2]  Unlike storage tanks, “there have been no studies to specifically investigate wastewater treatment systems.”[3]  As such, the EPA concluded that the existing AP-42 factors for wastewater treatment provided reasonably accurate estimate of VOC emissions and it was not necessary to make revisions.[4]

“AP-42, Compilations of Air Pollutant Emission Factors has been published since 1972 as the primary compilation of EPA’s emission factor information.”[5]  Whereas other more accurate and specific emission levels may (at times) be obtained through other means, AP-42 factors provide an industry average benchmark or default value. Often, as in the case of storage tank emission calculations, no other reasonable method is available to industry or government to determine emission levels on a day-to-day basis.  Industry and regulators have long relied on the emission factors contained in AP-42 to estimate annual emission inventories and to develop the basis for and show compliance with air emission permitted levels.  These changes are designated as final and are effective immediately.

Otherwise the EPA has indicated that it has determined to add several new and modified factors as shown below:


Note 1:  Many sulfur plants have incinerators fired on refinery or natural gas that combust the sulfur recovery unit tail gas.  Many refiners have used natural gas combustion factors as presented in §1.4 of AP-42 or a manufacturers burner rating to estimate NOx emissions from a SRU.

Note 2:  Refiners would have previously used AP-42 factor for natural gas combustion or manufacturers burner ratings to estimate emissions.  As hydrogen production heaters fire much hotter than many typical furnaces, it is apparent that the EPA believed that hydrogen production furnaces required a specific factor.


[1] New and Revised Emission Factors for Flares and New Emission Factors for Certain Refinery Process Units and Determination for No Changes to VOC Emission Factors for Tanks and Wastewater Treatment Systems, found at (last visited April 21, 2015).

[2] EPA Review of Available Documents and Rationale in Support of Final Emissions Factors and Negative Determinations for Flares, Tanks, and Wastewater Treatment Systems, Section 5, found at (last visited April 22, 2015).

[3]Id. at Chapter 6, p. 38.

[4] Id. at 41.

[5] Emissions Factors & AP 42, Compilation of Air Pollutant Emission Factors, found at (last visited Apr. 22, 2015)