addressing uncertainty in oil and natural gas industry greenhouse

5.2.4 Propagating Assymetric Uncertainty Distribution
Appendix F demonstrates the use of an asymmetric uncertainty distribution. Where the uncertainty values for
emissions data were greater than 100%, the lower bound estimate was truncated at -100% and tracked
separately from the upper bound estimate. The upper and lower bound uncertainties were then propagated
through to the total CO 2 equivalent emissions for the facility. For this example, the resulting difference
between the upper and lower uncertainty values was very small.
5.3 Example 2: Refinery
5.3.1 Background
Example 1 provides a very detailed, step-by-step demonstration of the uncertainty calculations for a crude oil
production facility. Because many of the source types are similar across all industry sectors, this example
focuses on a few distinct refinery process units. A comparison in the uncertainty calculations is provided for
methodologies presented in the API Compendium for fluid catalytic cracking unit (FCCU) and hydrogen plant
emissions. As with the previous example, much of the information is reprinted directly from the API
Compendium without change.
5.3.2 Uncertainty Comparison for FCCU Emission Estimation Methods
The following example applies uncertainty calculations to Exhibit 5.6 from the API Compendium for the
FCCU GHG emission estimation methods presented in the API Compendium. The same operating
parameters specified in the API Compendium are applied here, with the following assignment of uncertainties
added.
• The catalytic cracking unit has a coke burn rate of 119,750 tonnes per year ± 15% and a blower air
capacity of 2,150 m 3 /min ± 15% (assigned by expert judgment). The air blower is assumed to
operate continuously for the year (a ± 2% uncertainty is applied to this assumption).
• The carbon fraction of the coke is 0.93 ± 5.5% based on site-specific data (determined from
measured compositions).
• The flue gas concentrations are 11% for CO 2 and 9% for CO exiting the regenerator. Table D-3 of
this uncertainty document provides reproducibility values for the precision of Reformed Gas
Samples based on ASTM 1946-90. For molar compositions between 5 and 25 percent, a
reproducibility factor of 0.5 applies. An additional 5% uncertainty is assigned by expert judgment
to account for potential variability in the composition.
• It is assumed that no CH 4 is formed during the regeneration process.
• A CO boiler is used for control of the flue gas stream. Supplemental firing with natural gas is
employed at a rate of 100×10 6 ± 5% Btu/hr on a higher heating value basis.
The API Compendium presents three equations for estimating CO 2 emissions from FCCUs. The following
demonstrates the uncertainty quantification for each of the three methods.
Pilot Version, September 2009 5-10