## Technical Memorandum

TO: EPA Docket No. EPA-HQ-OAR-2010-0682

FROM: Brenda Shine, Environmental Engineer Refining and Chemicals Group, SPPD (E143-01)

DATE: March 15, 2012

SUBJECT: Derivation of Hourly Emission Rates fo r Petroleum Refinery Emission Sources Used in the Acute Risk Analysis

I. Purpose:

This memorandum documents the assumptions used to derive the hourly emission rates used to estimate acute risks from emissions of petroleum refinery emission sources. In past Risk and Technology (RTR) rulemakings, when we have lack ed hourly emissions data or other specific information about the source cate gory processes, we assumed that the 1-hour emission rate for any emission point could be 10 times higher than its average hourly emissions, calculated by dividing annual emissions by 8760 hours per year. The basis for this assumption was derivedfrom an analysis of short term release information collected from facilities in the HoustonGalveston area and a comparison against routine emission rates for an entire facility. The conclusions of this analysis were that the hourly emissions from any single release event to the average annual VOC release rate for an entire fa cility was seldom greater than a factor of 10. For the petroleum refinery source category, we have additional knowledge of the emission point release characteristics that enable us to refine th e default factor of 10 to something more realistic for this source category.

II. Acute Factors for the Petroleum Refinery Source Category:

Instead of using the default factor of 10 descri bed above, we estimated variability of hourly emissions based on the operating characteristics of refinery emission sources in the following

manner:

a. For many sources that are associated with continuous operations and are essentially steady state, such as Fluid Catalytic Cracki ng Units (FCCU), and sulfur recovery plants (SRU), we would not expect significant vari ability in hourly emissions. Typical source test variability is typically ±30 to 50%. We have applied an hourly multiplier of 2 to estimate maximum hourly rates as an upper ra nge of the expected variability. This assumption was used for the following t ypes of sources: FCCU, SRU, Chemical Processes, miscellaneous process vents, refinery processes (not specified) and
incinerators.

b. For delayed cokers, we considered the aver age venting cycle time in calculating our hourly emission rate. The process cycle for de layed cokers is 1 da y (24 hours), with the2 depressurization vent having an average vent ing cycle time of, on av erage, 2 to 3 hours per cycle. Since the depressurization vent opens for 730 to 1,095 hours per year, a factor of 8 to 12 times the annual average emission rate could be used to account for this emission process. The emissions are expect ed to be somewhat variable, with higher emissions at the start of the venting cycle. We therefore applied an hourly multiplier of 20 to estimate maximum hourly rates for delayed coker emissions.

c. There are three types of catalytic reformers: Continuous, cyclic, and semi-regenerative. Continuous reformers (SCC 30601602) are expected to have stable operations (much like the FCCU and SRU), so a factor of 2 is appropriate for estimating the maximum hourly emissions rate. Cyclic reformers (SCC 30601603) typically vent for 2,000 to 4,000 hours per year, suggesting a factor of 2 to 4 for these units. However, their emissions over the venting period are expected to be somewhat va riable, so a factor of 10 is applied as a conservative estimate. Semi-regenerativ e reformers (SCC 30601604) are expected to vent approximately 10-15 days per year, suggest ing a factor of 30 or 40. The venting is not expected to be uniform over this period, so a factor of 60 is applied for these units.
d. For evaporative loss sources such as wastew ater, we expect moderate variability in maximum hourly emissions from annual averag e emissions. Emissions from wastewater are dependent on organic loading to the unit and on the mass tran sfer (volatilization) rate from the wastewater surface (which is depe ndent on the wastewater temperature and air wind speed). The loading rates are expected to vary by a factor of 2 during normal operations and the mass transfer rates are expe cted to vary by a factor of 2 due to seasonal/meteorological variations. We ther efore applied an hourly multiplier of 4 to estimate maximum hourly rates fr om wastewater sources.

e. For fugitives from equipment leaks, we not e that the current methods of estimating emissions make use of correlation equations in conjunction with Met hod 21 readings that ultimately provide an hourly emission rate for the monitoring period in question. The
actual emission rates estimated based on the direct Method 21 r eadings are commonly divided by two to estimate the average emissi on rate between monitoring intervals (i.e., assuming the leak started mid-way between monitoring intervals as described in the emissions protocol). We expect the emi ssion estimates from the direct Method 21 readings to provide a direct measure of the maximum hourly emissions (as leak repair will be applied to reduce hourly emissions immediately after a monitoring cycle). Therefore, we applied an hourly multiplier of 2 to estimate maximum hourly rates from the annual average emission rates.
f. For storage vessels, higher hour ly rates from loading operatio ns can occur; however, the majority of tanks are floating roof tanks; em issions from these tanks are associated with wind-driven losses from fitting controls and clingage from exposure of external floating
roof tank shells. These emissions would not be expected to vary as significantly as fixed roof tanks, but the emissions from floating r oof tanks are expected to be dependent on temperature (seasonal variability) and, in the ca se of external floating roofs, wind speed.3 There may also be variability in the crude oi l processed or the inte rmediates and products stored in a given tank. As with wastewater sources, each of these factors is expected to cause a factor of 2 variability. We therefore applied an hourly multiplier of 4 to estimate maximum hourly rates from storage tanks.

g. For other transfer and loading operations, maximum hourly rates can be approximated from pumping rates; a factor of 2 for th ese sources would not adequately account for higher maximum hourly emissions in most cases. Considering the hours of actual product loading during the year (commonly 1,000 to 2,000 hours per year), we applied a
factor of 10 to these emission sources.