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addressing uncertainty in oil and natural gas industry greenhouse

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Uncerta<strong>in</strong>ty Estimate for CH 4 Emissions from Storage Tanks:<br />

Methane emissions from flash<strong>in</strong>g losses are estimated us<strong>in</strong>g the simple emission factor provided <strong>in</strong> the API<br />

Compendium. (Note that this simple emission factor approach is used <strong>in</strong> absence of the more detailed<br />

<strong>in</strong>formation necessary for the other calculation approaches provided <strong>in</strong> the API Compendium).<br />

-4<br />

6,100 bbl 343 day 8.86 × 10 tonne CH ⎛<br />

4<br />

0.80mole CH<br />

4(facility)<br />

⎞<br />

E<br />

CH<br />

= × ×<br />

4<br />

×⎜ ⎟<br />

day yr bbl ⎝0.788 mole CH<br />

4<br />

(default EF) ⎠<br />

E = 1,880 tonnes CH /yr<br />

CH4<br />

4<br />

As noted <strong>in</strong> Table 5-1, there is no <strong>uncerta<strong>in</strong>ty</strong> associated with the count of equipment. The <strong>uncerta<strong>in</strong>ty</strong><br />

associated with the volume of <strong>gas</strong> is 5% <strong>and</strong> the <strong>uncerta<strong>in</strong>ty</strong> associated with the days of operation is 2%.<br />

The <strong>uncerta<strong>in</strong>ty</strong> <strong>in</strong> the emissions factor is 90% based on expert judgment <strong>and</strong> the <strong>uncerta<strong>in</strong>ty</strong> <strong>in</strong> the site<br />

specific CH 4 mole % is 4 % based on Exhibit 5-1. The <strong>uncerta<strong>in</strong>ty</strong> <strong>in</strong> the CH 4 content is 5.53% from the<br />

GRI/EPA Methane Study (Shires <strong>and</strong> Harrison), which is provided <strong>in</strong> Table E-4 of the API Compendium.<br />

The <strong>uncerta<strong>in</strong>ty</strong> of the emissions is calculated by apply<strong>in</strong>g Equation 4-6 <strong>and</strong> us<strong>in</strong>g the relative <strong>uncerta<strong>in</strong>ty</strong><br />

values.<br />

Urel ( )<br />

=<br />

U( rel) +U( rel) +U( rel) +U( rel) +U( rel)<br />

emissions 2<br />

+U( rel)<br />

Default mole%<br />

2 2 2 2 2<br />

Number of Units Gas processed Days EF Facility mole%<br />

( )<br />

Urel ( ) = 0 + 5 + 2 + 90 + 4 + 5.53 =± 90.4% tonnes CH /yr<br />

2 2 2 2 2 2<br />

emissions 4<br />

Uncerta<strong>in</strong>ty Estimate for CO 2 Emissions from Storage Tanks:<br />

E =<br />

CO2<br />

4<br />

-4<br />

8.86 × 10 tonne CH4 tonne mole CH4 0.80 mole CH<br />

4<br />

(facility)<br />

6,100 bbl 343 day<br />

× × × ×<br />

day yr bbl 16.04 tonne CH 0.788 mole CH (default EF)<br />

4 4<br />

tonne mole <strong>gas</strong> 0.12 tonne mole CO2 44.01 tonne CO2<br />

× × × = 775 tonnes CO<br />

2<br />

/yr<br />

0.80 tonne mole CH tonne mole <strong>gas</strong> tonne mole CO<br />

2<br />

The same approach applies to estimat<strong>in</strong>g the <strong>uncerta<strong>in</strong>ty</strong> for the CO 2 emissions. The <strong>uncerta<strong>in</strong>ty</strong> associated<br />

with the mole% of CO 2 is 4%, so the results are the same as for CH 4 .<br />

Urel ( )<br />

=<br />

U( rel) +U( rel) +U( rel) +U( rel) +U( rel)<br />

emissions 2<br />

+U( rel)<br />

Default mole%<br />

2 2 2 2 2<br />

Number of Units Gas processed Days EF Facility mole%<br />

( )<br />

Urel ( ) = 0 + 5 + 2 + 90 + 4 + 5.53 =± 90.4% tonnes CO /yr<br />

2 2 2 2 2 2<br />

emissions 2<br />

Pilot Version, September 2009 F-22

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