### Methane Leaker Emission Factors Ca lculated using Canadian Energy Partnership for Environmental Innovation Data

Emission surveys for the Canadian Energy Partnership for Environmental Innovation (CEPEI) were used in the de velopment of methane leaker emission factors (LEF) for the transmission and distribution sectors under Subpart W of the current proposed rule. This document discusses the methodology used to develop the methane LEFs from the CEPEI data and the statistica l analysis conducted.

The CEPEI data consists of emission su rveys conducted by Cl earstone Engineering Limited in 1998 and 2007 and leak measuremen t surveys submitted by individual private sector companies between those years. Th e surveys examined numerous natural gas transmission, storage and dist ribution facilities in Canada. The results of these surveys were used to write two se parate reports for CEPEI: Handbook for Estimating Methane Emissions from Canadian Natural Gas Systems 1 (referred to as CEPEI 1998) and Measurement of Natural Gas Emissions from the Canadian Natural Gas Transmission and Distribution Industry 2 (referred to as CEPEI 2007). CEPEI 1998 uses data from the survey conducted in 1998 to present available methods for assessing methane emissions from non-combustion sources of natural ga s transmission, storage, and distribution systems. It was designed as a detailed re ference guide for use by both management and practitioners. CEPEI 2007 uses data from both the 1998 and 2007 surveys, as well as data from surveys conducted for private sect or companies, to develop a protocol for detecting and measuring emission. Its objectiv e was to improve the estimation of fugitive methane emissions from the Canadian natura l gas transmission, st orage and distribution industries.

The data used in this analysis includ es the emission surveys from 1998 and 2007 and those conducted for individual companies. Sp ecific facilities we re segregated and methane leaker emission factors were calcu lated for component cat egories covered under Subpart W.

Methodology The collective data is a representative sample of block valve stati ons, booster stations, border meter stations, compresso r stations, control valve sta tions, transmission farm taps, receipt meter stations, sales meter stations, storage stations, and storage wells for the transmission/storage sector; and commercial me ter sets, gate stati ons, distribution farm taps, district regulating stations, industri al meter sets, residential meter sets, and miscellaneous pipeline equipment in the distri bution sector. As specified by Subpart W, the EPA isolated the data for compressor sta tions, storage stations, and storage wells to develop leaker factors for transmission and storag e, and isolated gate stations and district regulating stations for distribution.

1 Clearstone Engineering Ltd. Handbook for Estimating Methane Emissions from Canadian Natural Gas Systems, (for CEPEI). 1998
2 Clearstone Engineering Ltd. Measurement of Natural Gas Emissions from the Canadian Natural Gas Transmission and Distribution Industry, (for CEPEI). 2007.Components, in the raw data, we re associated with a site id entifier, tag identity, source, component count and leakage rate. The site iden tifiers designate the f acility types and the tag identities distingui sh the leaking components from the non-leaking components. The components were categorized as: blowdown vent, connector, controller, centrifugal compressor seal, reciprocating compressor seal , control valve, open- ended line, orifice meter, other flow meter, pre ssure relief valve, regulator and valve. The leakage rates provided in the raw data indica tes the total hydrocarbon leaker rates. In cases where there were more than one component associated w ith a leakage rate, it was assumed that the leakage rate was a total of those components.

Using the site identifiers, the components fr om the compressor stations, gate stations and district regulating station were separated out from the raw data. Unlike the Clearstone I and Clearstone II studies, it was not possibl e to separate the compressor station components into centrifugal, re ciprocating and non-compresso r related. Consequently, it was assumed that any component found at co mpressor stations was compressor related. In addition, the compressor unit could not be identified as centrif ugal or reciprocating given the labeling system used in the raw da ta. Hydrocarbon LEFs for distribution were derived from components found in gate stati ons and district regul ating stations as specified by Subpart W.

Components found at compressor stations were grouped together to formulate hydrocarbon LEFs for the transmission sector wh ere as components found at gate stations and district regulating stations were groupe d together to calculate hydrocarbon LEFs for the distribution sector. The data includes non-leaking and leaking components in their raw data requiring the isolat ion of leaking components usin g their tag identities. The leaking components were then grouped according to source category. Finally, hydrocarbon LEFs for the transmission and di stribution sectors were calculated by dividing the sum of the leakage rates by the sum of the component counts in each source category. To convert the hydrocarbon leak er emission factors to methane LEFs, a composition factor of 0.934, as stipulated by the GRI report 3 for transmission gas quality, was used

The data analysis process is summarized visually in Figure 1.

3 GRI (1992) GRI Report, Methane Emissions from the Natural Gas Industry, Volumes 1- 15. Energy Information Agency (U.S. DOE)Emission FactorCalculationsSource CategorizationFacility TypeLeakingComponents Raw Data

Compressor

Stations

Leaking Components Non-leaking Components

Component Counts Hydrocarbon Leaker Emission Factors Connector

Block Valve

Control Valve

Compressor Blowdown Valve

Pressure Relief Valve

Orifice Meter

Regulator

Open-ended Line

Methane Leaker Emission

Factors DISTRIBUTION TRANSMISSION

Component Counts Hydrocarbon Leaker Emission Factors Connector

Block Valve

Control Valve

Compressor Blowdown Valve

Pressure Relief Valve

Orifice Meter

Regulator

Open-ended Line

Methane Leaker Emission

Factors District Regulating

Stations Gate Station

Leaking Components Non-leaking Components Figure 1 – Summary of CEPEI data analysisStatistical Analysis The CEPEI source categories were reduced to the categories found in Clearstone I and II to facilitate a comparison of hydrocarbon LEFs between the studies. CEPEI LEFs for transmission were compared to LEFs for co mpressor related components that occur after the de-methanizer in Clearstone I and II. The CEPEI LEFs for distribution were not compared to Clearstone I and II because these studies had no data points from distribution facilities.

To determine whether the emission factor s can be combined, a paired, two-tailed t -test was performed. The test consists of a null hypothesis and an alte rnate hypothesis which are validated using the t -statistic. The null hypothesis st ates that both the population means are equal and the alternate hypothesis refutes this by stati ng the population means are not equal. In this case , the population means are the leaker emission factors from Clearstone I or II (study 1) and CEPEI (st udy 2), (subscripts in formulas). Stated mathematically, the hypotheses are:

Null Hypothesis, H o : μ 1 = μ 2 , i.e., the two means (LEFs) are combinable Alternative Hypothesis, H 1 : μ 1 ≠ μ 2
The test statistic represents the difference in the two means from the two studies under consideration and is calculated using

2 2 2

1 2 1 2 1

n S

n S M M t o
+ −
= ,

where M 1 and M 2 are the means from studies 1 and 2, S 1 and S 2 are the standard deviations from studies 1 and 2,

n 1 and n 2 are the component counts from studies 1 and 2.

The population sizes (i.e. compone nt counts) and the standard deviations of study 1 and 2 were not equal. Therefore, a corrected degree of freedom, Dof , that accommodates such differences is calculated using



# ( ) ( )

1 1 2 2
2 2 2

1 2
1 2 1 2

2 2 2

1 2 1

− +

− ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛
+

=

n n S

n n S n S

n S
Dof

,

where S 1 and S 2 are the standard deviations from studies 1 and 2,

n 1 and n 2 are the component counts from studies 1 and 2.A standard t value found at a 95% confidence interval from a t -distribution curve is compared to t 0 . If the test statistic is below the standard t -value then the null hypothesis is accepted. Therefore, the population is considered to have similar characteristics and the emission factors from the compared st udies can be combined. However, if t 0 was above the standard t -value, then the null hypothesis is re jected and the emission factors cannot be combined.

The steps followed are summarized visually in Figure 2.

Figure 2 – Summary of comparative statistics methodology

The results of the t -test are shown in Table 1 and 2. Raw Data

Combinable or Not Combinable Statistical Analysis Statistical Data Clearstone I or II Raw Data CEPEI Raw Data

M 1 , Emission

Factors S 1 , Standard Deviation

Test statistic, t 0 Standard t -value at 95% confidence

If t 0 > t , then do FAIL Degree of Freedom, Dof

If t 0 < t , then PASS n 1 , Component Count M 2 , Emission

Factors S 2 , Standard Deviation n 2 , Component CountTable 1 – Comparing Clearstone I LEFs to CEPEI 1998/2007 LEFs.

Condensed Categories Clearstone I 2002 (scf/hr) CEPEI 1998/2007

(scf/hr) t -test result

Valve 18.26 11.14 Fail Connector 13.25 2.87 Fail Open-ended Line 29.61 23.06 Pass Pressure Relief Valve 30.46 39.82 Cannot Compare i
Meter 36.27 14.13 Pass

Table 2 – Comparing Clearstone II LEFs to CEPEI 1998/2007 LEFs.

Condensed Categories Clearstone II 2006 (scf/hr) CEPEI 1998/2007

(scf/hr) t -test result

Valve 17.78 11.14 Fail Connector 8.31 2.87 Fail Open-ended Line 7.00 23.06 Fail Pressure Relief Valve - 39.82 Cannot Compare i Meter 72.34 14.13 Cannot Compare i i. There are not enough data points to make a comparison

In most cases, the test indicated that the CEPEI data cannot be combined or compared with Clearstone I and II data.

Leaker Emission Factors The resulting hydrocarbon emission factors fo r transmission and distribution are shown in Table 3 and 4.

Table 3 – Table W-3 of Subpart W - Default methane emission factors for transmission.

Transmission Emission Factor (scf/hour/component) Leaker Emission Factors - All Components, Gas Service CEPEI (Picard 1998) & CEPEI (Picard 2007) Connector 2.7 Block Valve 10.4 Control Valve 3.4 Compressor Blowdown Valve 543.5 Pressure Relief Valve 37.2 Orifice Meter 14.3 Other Meter 0.1 Regulator 9.8 Open-ended Line 21.5Table 4 – Table W-7 of Subpart W – Default methane emission factors for distribution.

Distribution Emission Factor (scf/hour/component) Leaker Emission Factors - Above Grade M&R Stations Components, Gas Service CEPEI (Picard 1998) & CEPEI (Picard 2007) Connector 1.67 Block Valve 0.557 Control Valve 9.34 Pressure Relief Valve 0.270 Orifice Meter 0.212 Regulator 0.772 Open-ended Line 26.131