SAB sends letter to EPA regarding review of biogenic framework

By Erin Voegele | April 01, 2019

A recent letter authored by the chair of the U.S. EPA’s Science Advisory Board and addressed to EPA Administrator Andrew Wheeler outlines the SAB’s findings with regard to its review of the EPA’s 2014 Framework for Assessing Biogenic CO2 Emissions from Stationary Sources.

The SAB’s review of the framework has been ongoing for several years. In November 2014, the EPA released a revised framework for assessing biogenic carbon dioxide (CO2) emissions from stationary sources, updating an original draft published in 2011. Since that time, the second draft has been undergoing additional review by the biogenic emissions panel.

In late March, a letter authored by SAB Chair Michael Honeycutt and addressed to Wheeler was posted to the agency’s biogenic carbon framework webpage. That letter, dated March 5, discusses the panel’s review of the 2014 biogenic carbon framework.

The letter explains that SAB was asked by the EPA to review the 2014 framework, which “considers the scientific and technical issues associated with accounting for emissions of carbon dioxide (CO2) from biogenic feedstocks used at stationary sources.”

According to the letter, the purpose of the 2014 framework was to develop a method for calculating the adjustment, or biogenic assessment factor (BAF), for CO2 emissions associated with the combustion of biogenic feedstocks at stationary facilities by accounting for the biological carbon cycle effects associated with the growth, harvest, and processing of those feedstocks. The letter also explains the BAF is an accounting term developed by EPA to adjust stack emissions to reflect a feedstock’s net carbon emissions after accounting for subsequent sequestration of carbon in regrown biomass or soil, and after considering emission that might have occurred with an alternative fate had the biomass not been used for fuel.

The letter notes that the 2014 framework may be used to develop BAFs for multiple regulations and associated climate objectives. As a result, the BAF must be able to accommodate a wide range of potential time and special scales and all relevant greenhouse gas (GHG) emissions. “Lack of specificity in the BAF objectives to be addressed under the framework has made it difficult for the SAB to address many of the charge questions fully,” Honeycutt wrote.

The letter lists a variety of suggestions the SAB made in reference to the 2011 framework that the EPA did incorporate into the 2014 framework. However, the letter criticizes the 2014 framework for failing to provide regulatory context, specific BAF calculations for that context, or the implementation details that SAB previously requested.

“In fact, the lack of information in both frameworks on how the EPA may use potential BAFs made it difficult to fully evaluate these frameworks,” Honeycutt wrote. “The BAF is a construct designed to evaluate the importance of the stack emissions of CO2 at a given time relative to their climate impacts at some point in the future when some of the emitted CO2 may have been sequestered by regrowth of biogenic feedstocks. As such, the computation of the BAF for a feedstock in a region depends upon the climate impact of concern and the future point in time that is of interest, which is a choice that depends upon the specific regulation or policy that will rely on that BAF. If the objective of interest for the BAF computation is defined by short term processes, then the relevant time-period for the BAF computation needs to include relevant details on short term climate phenomena, which might be less important if the objective of interest is much longer term. In addition to identifying the relevant analytic time frame, knowing the objectives of interest would provide other information necessary to the assessment of the science underpinning the BAFs, such as the scale of demand for biogenic feedstocks, the anticipated time frame for that demand and eligible feedstocks to meet it, relevant spatial scope, and importance of including each type of GHG in the analysis.”

While Honeycutt noted that the SAB agreement with many recommendations developed by the Biogenic Carbon Emission Panel in previous drafts of the report, he said it disagreed with the extended time frame recommended for BAF computation. He explains that “SAB favors selecting the time horizon for calculating the BAF to comport with the objective under consideration, which is generally dependent on the regulation mandating use of that particular BAF,” and stresses “this SAB review would have been enhanced in the agency offered a specific regulatory application that, among other things, provided explicit proposed BAF objects, which would in turn have defined the applicable boundaries regarding upstream and downstream emissions in the feedstock life cycles.”

“The 2014 framework lacks specificity and is written in a way that is too generic with too many possibilities that would require assessment of different underlying science,” Honeycutt wrote. “Rather than offering a lengthy menu of calculating options, the EPA framework needs to define its scenarios and justify those choices. This would enable the SAB to evaluate the science underpinning those decisions and justifications.”

Despite those limitations, Honeycutt said the SAB does offer overarching suggestions for moving forward with the framework, along with specific responses to EPA’s charge questions when possible and general guidance regarding the calculation of BAFs. He said the EPA’s equations were based on emissions (fluxes) with some adjustment terms to account for carbon mass escaping the system between the point of assessment and the point of emissions. The letter references a report sent to Wheeler in which the SAB recommends an alternative formulation based on changes in terrestrial carbon stocks, such as the live stocks in biomass, dead stocks, soil stocks, etc., that explicitly incorporates the principle of conservation of mass.

The letter also notes the EPA did not ask the SAB for feedback on its modeling approach. “We think this was an oversight, given that modeling is critical to the development of the BAF and different modeling approaches can yield different results,” Honeycutt wrote, noting the SAB believes EPA should identify and evaluate its criteria for choosing a model or models that examine the sensitivity of BAF estimates to key modeling features.

A full copy of the letter can be downloaded on the EPA website.