addressing uncertainty in oil and natural gas industry greenhouse

EXHIBIT 4-1: Uncertainty Example for a Simple Emission Estimation
Input Data: A gas production facility operated 45 high-bleed pneumatic devices during the previous year.
The average production gas composition is 80% CH 4 and 5% CO 2 .
Emission Factor: The API Compendium provides a default CH 4 EF for high-bleed pneumatic devices of
4.941 tonne CH 4 /device-yr ± 33.1% (Table 5-15 of the 2009 API Compendium. Uncertainty is expressed at
the 95% confidence interval.)
Emission Estimate: Emissions for this source are estimated based on the following calculations.
4.941 tonne CH 80 mole % CH
CH : (45 pneumatic devices) × × = 226 tonnes CH /yr
CO :
4 4
4 4
device - yr 78.8 mole % CH4
2
226 tonne CH4 tonne mole CH4
tonne mole gas
× ×
yr 16 tonne CH4 0.80 tonne mole CH4
0.05 tonne mole CO 44 tonne CO
38.8 tonnes CO /yr
2 2
× × =
tonne mole gas tonne mole CO2
2
Uncertainty Assessment:
Measurement Uncertainty:
o For this example, the activity data are based on a single point measurement. All of the devices were
accounted for, so there is no bias and the uncertainty of the activity value (i.e., the number of
pneumatic devices) is 0.
o The composition measurements are based on multiple measurements representing a sampling of the
composition. Equations 4-1, 4-2, and 4-3 are applied to calculate the standard deviation of the
composition samples collected throughout the year. The standard deviation accounts for the
measurement error and natural variability of the sampled values.
o For this example, the precision uncertainty of the composition data is assumed to be ± 1%. (A more
detailed demonstration of quantifying uncertainty for measured composition data is provided in a
separate example.) On an absolute basis, this equates to 0.8 for the CH 4 composition, and 0.05 for
the CO 2 composition.
o Bias associated with the sampling and analysis is assumed to be small due to equipment
maintenance and calibration practices. A value of 3 % is assigned for this assessment. On an
absolute basis, this equates to 2.40 for the CH 4 composition, and 0.150 for the CO 2 composition.
U = U + U
U
U
2 2
Composition Data Bias Precision
CH4
CO2
= + =
2 2
0.80 2.40 2.53
= + =
2 2
0.05 0.150 0.158
Emission Factor Uncertainty:
As noted above, an uncertainty of ± 33.1% was specified for the default emission factor. Any bias in the
default emission factor is accounted for in the associated uncertainty value.
Pilot Version, September 2009 4-9