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U.S. EPA Defends Its Major Source Boiler MACT Floor Calculation Methodology

Posted: July 21st, 2014

Author: All4 Staff 

Those of you following the saga of the Major Source Boiler MACT remand (see our March 17, 2014 and May 22, 2014 blogs for a refresher) will be interested to learn that latest milestone in the process occurred on July 14, 2014. Back in May 2014, the U.S. Environmental Protection Agency (U.S. EPA) was granted 60 days to provide further explanation to the U.S. Court of Appeals for the District of Columbia Circuit on the use of the upper prediction limit (UPL) statistical method to set the emission standards for certain subcategories in the Major Source Boiler MACT rule. On July 14, 2014, U.S. EPA submitted its formal response in the form of a 16 page document that provides a qualitative and quantitative justification of the validity of the UPL methodology.

As stated in the U.S. EPA’s response, the UPL is a value, calculated from a dataset, that identifies the average emissions level that a source or group of sources is meeting and would be expected to meet a specified percent of the time that the source is operating (99% in this case of Major Source Boiler MACT). U.S. EPA further states that “the 99% UPL is the level of emissions that we are 99% confident is achieved by the average source represented in a dataset over a long-term period based on its previous, measured performance history as reflected in short-term stack test data.” Furthermore, “the UPL predicts the level of emissions that the sources upon which the floor is based are expected to meet over time, considering both the average emissions level achieved as well as emissions variability and the uncertainty that exists in the determination of emissions variability given the available, short-term data.” U.S. EPA contends that the UPL methodology is preferred and appropriate because it considers variations in material inputs (such as fuels), control device performance, and operating unit performance (changes in process conditions that impact emissions). U.S. EPA states that average emissions performance [Section 112(d)(3) of the Clean Air Act] is not meant to be the average of three (3) stack test run, but rather the average emissions over time (long-term rather than a snap shot stack test).

U.S. EPA’s position is presented in a reasonable and logical matter. I will leave an evaluation of the statistical appropriateness of the UPL from a mathematical perspective to those that are more familiar with the nuances of statistical methods than me. At a minimum, it certainly will be interesting to read the Court’s response to this filing, particularly as we near the start of briefings in mid-August. 


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