10.17 Natural Gas Distribution - Leakage

Categories 2535-2539, 2541

10.17.1 Introduction

These categories account for natural gas fugitive emissions or leakage losses that take place during the transitory period from the utilities meter to the point of gas combustion. Categories 2535-2537 and 2541 cover leakage from the residential sector for space heating, water heating, cooking, and other sources, respectively. Categories 2538-2539 cover leakage from the industrial and commercial sector, respectively.

Natural gas consists of a high percentage of methane and varying amounts of ethane, propane, butane, and inert gases (typically nitrogen, carbon dioxide, and helium). Therefore the majority of emissions from these categories falls under the pollutant category: total organic gases (TOG).

Category	Description
2535	NG leakage - Space Heating
2536	NG leakage - Water Heating
2537	NG leakage - Cooking
2538	NG leakage - Industrial
2539	NG leakage - Commercial
2541	NG leakage - Other

10.17.2 Methodology

Estimation of emissions for these categories uses a top-down methodology. Natural gas consumption data for the San Francisco Bay Area counties was obtained from the California Energy Commission (CEC)⁴⁹². This data is further broken down by the CEC into residential, industrial, and commercial sectors. Using this county and sector level consumption data and applying leakage percentages as derived from literature, an approximate estimate of natural gas leaked to the atmosphere post-meter was calculated. A pollutant speciation profile for pipeline NG in the Bay Area was then applied to estimate TOG emissions.

(a) Activity Data / Throughput

As previously stated, NG consumption for the Bay Area by sector was obtained from the CEC. This consumption data is presented in units of therms and converted to millions of standard cubic feet (mil. ft3, or MMscf) using a conversion factor of 97.02 (ft3/therm) based on a range of natural gas heat content values.

Based on the natural gas leak study, “An Estimate of Natural Gas Methane Emissions from California Homes” by Marc L. Fischer of Lawrence Berkeley National Laboratory (LBNL)⁴⁹³, it is assumed that 0.35% of the NG by volume is lost due to post-meter leaks for the residential sector and 0.45% for the industrial and commercial sectors.

Natural gas consumption in the residential sector is further broken down into four end-use categories; Space Heating, Water Heating, Cooking, and Other, based on information from the CEC report “2009 California Residential Appliance Saturation Study” ⁴⁹⁴ for the Pacific Gas and Electric company (PG&E) service area. The natural gas end-use percent breakdown for the residential sector is as follows:

Space Heating - 50.34%

Water Heating - 41.15%

Cooking - 4.47%

Other - 4.04%

(b) County Distribution / Fractions

Natural gas consumption information provided by CEC includes residential, commercial, and industrial usage for the nine Bay Area counties. Solano and Sonoma Counties are partially under Bay Area Air District’s jurisdiction; therefore, data were used for the cities that are located within the Air District. For Solano County, data used were for Benicia, Fairfield, Suisun, and Vallejo; for Sonoma County, data used were for Cotati, Petaluma, Rohnert Park, Santa Rosa, Sebastopol, and Sonoma.

(c) Emission Factors

Natural gas is composed of a majority methane (CH4) with trace amounts of other organic compounds and inert gases. For estimating emissions, a speciation of NG as provided by PG&E staff⁴⁹⁵ was used. This speciation profile specified a 96.5% methane content.

(d) Control Factors

There are currently no District regulations that apply to these categories, therefore no controls are applied.

(e) Speciation

Organic speciation is based on the methane emissions relative to the TOG emissions, such that ROG emissions are the result of subtracting methane from TOG emissions.

Organic speciation fractions are shown below:

Cat. description	TOG	ROG/TOG Fraction
Natural Gas Leakage	1.000	0.965

10.17.3 Changes in Methodology

Prior to this base year, these categories did not exist. For this base year inventory, emissions from these categories were estimated and included in the inventory to provide a more holistic account of natural gas usage within the Bay Area.

10.17.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

10.17.5 Trends

(a) Historical Emissions / History

For years 1990 to 2000, natural gas consumption data from the California Energy Commission were used to estimate the historical emission trends.

(b) Future Projections / Growth

For years 2000 to 2040, the natural gas consumption growth profile was obtained from the California Air Resources Board (CARB); CARB’s growth profile was derived from the California Energy Commission’s 2019 Integrated Energy Policy Report, which reports historical and forecasted natural gas consumption by sector, planning area, and year. Based on this growth profile, the year-to-year change of Bay Area natural gas consumption is not significant.

10.17.6 Uncertainties

There are several areas of uncertainty for NG leakage estimates. According to a study done in 2020 and published by the American Chemical Society⁴⁹⁶, the percent of post-meter leakage for the residential water heaters ranges from 0.35% to 0.44%. The composition of natural gas may also need to be updated with more recent information concerning the composition of pipeline natural gas in the Bay Area. According to more recent PG&E pipeline data, the percentage of natural gas ranges from 90% to 97%.

10.17.7 Contact

Author: Sukarn Claire

Reviewer: Ariana Husain

Last Updated: November 06, 2023

10.17.8 References & Footnotes