6.3 Natural Gas Distribution
Categories 61 and 868
6.3.1 Introduction
Categories 61 and 868, account for fugitive organic emissions (TOG and ROG), including methane (CH4), from natural gas distribution systems in the San Francisco Bay Area. Emissions associated with the transmission of natural gas are mostly fugitive organic emissions that occur due to the leaks in pipeline. Components such as valves, flanges, pumps, compressors, and pressure relief valves (PRVs) are the potential leak sources. Emissions from Pacific Gas and Electric’s (PG&E’s) pipeline system that distributes natural gas for its own use are covered in Category 61. Emissions from PG&E’s pipeline system that distributes natural gas to residential, commercial, and industrial customers are covered in Category 868.
6.3.2 Methodology
Categories 61 and 868 are considered an area source category since they cover facilities / emission sources that are not directly permitted by the District, and hence not systematically cataloged. Emissions for area source categories are determined using the formula:
Current Year Emissions = Base Year Emission X Growth Profile, and,
Base Year Emission = Throughput X Control Factor X Emission Factor
where,
- throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
- emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
- control factor (if applicable) is determined by District and state rules and regulations in effect;
- and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter
Fugitive emissions occur from leaks in the natural gas pipeline distribution systems. Emissions for these categories were estimated by multiplying total amount of natural gas distributed (in thousand cubic feet or MCF) by a leak rate (% by volume) and emission factor (lb of pollutant/MCF).
More details on throughput, county distribution, emission factors and controls is provided in the following subsections.
(a) Activity Data / Throughput
Natural gas consumption data for the San Francisco Bay Area for year 2015 was obtained from the California Energy Commission (CEC)167.
(b) County Distribution / Fractions
County distributions for Categories 61 and 868 are based on natural gas consumption by each county in the San Francisco Bay Area.
(c) Emission Factors
Prior to 1990, a leak rate of 0.7% by volume of gas distributed was used. This rate was based on the South Coast Air Quality Management District report “Estimated Fugitive Emissions from Natural Gas Transmission for Calendar Year 1979”, July 1980. Since 1990, a leak rate of 0.2% by volume has been used for both distribution systems: PG&E’s own usage (Category 61) and PG&E’s customers (Category 868). The emission factor in pounds per thousand cubic feet of gas (lb/MCF) is based on the 1990 PG&E report titled “Unaccounted-For Gas Project”.
(d) Control Factors
No separate emission controls have been implemented.
(e) Speciation
This category accounts for fugitive leaks of natural gas ( which is large majority methane) therefore most of the emissions are assumed to be organic gas (TOG).
The ROG/TOG ratios applied to this category or this group of related categories are based on an Air District internal speciation profile. Multiple data sources have been used for developing speciation profiles, such as Air District-approved source tests, TOG speciation ratios used by other regional air quality agencies, and relevant literature including latest speciation profiles developed by CARB168 and the US Environmental Protection Agency169. For this category or group of categories, ROG constitutes 3% of TOG. Further assessment and improvement of ROG/ TOG speciation profiles has been planned in future inventory updates.
(f) Sample Calculations
The following example shows Category 868 TOG emissions calculation for Alameda County for year 2015:
TOG Emissions (tons/ year) = Throughput (millions ft3/ year) x Leak Rate (% of NG by Volume) x Emission Factor (lbs/ millions ft3) x County Fraction x Growth Factor x Control Factor x (ton/ lbs)
TOG Emissions (tons/ year) = 434,404 x 88 x 0.1289 x 1.0 x 1.0 x 1/2000 = 2,463.8 tons/ year
6.3.3 Changes in Methodology
No changes to methodology were made in this version of the base year emissions inventory.
6.3.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
6.3.5 Trends
Annual demand for natural gas varies in response to business conditions and annual temperatures but has been generally declining or flat over recent years. This is due in part to historically low number of heating degree days, increases in building and appliance efficiency standards, and increases in price.
(a) Historical Emissions / History
In general, emissions increase as activity increases and emissions decrease as activity decreases. In recent years, emissions for Category 868 have been increasing, while emissions for Category 61 have been on the decrease due to PG&E’s divestment in power plants.
For Base Year 1983, all distribution losses of natural gas were handled as a single category which had been designated as Category 43. Category 43 has been subsequently split into two categories (61 and 868) since Base Year 1987. As mentioned earlier, emissions from Pacific Gas and Electric’s (PG&E’s) pipeline system that distributes natural gas for its own use are covered in Category 61. Emissions from PG&E’s pipeline system that distributes natural gas to residential, commercial, and industrial customers are covered in Category 868.
For Category 61 up to base year 2008, it was assumed that annual emissions would tend to follow two components: services industry employment and population by county in the District. As such, a hybrid growth profile was developed based on 50% of the appropriate county services industry employment value and 50% of the appropriate county population value in combination with CEC’s natural gas data. For Category 868 up to base year 2008, it was assumed that annual emissions would tend to follow: manufacturing industry employment and population by county in the District in combination with CEC’s natural gas consumption forecasts. As such, a hybrid growth profile was developed based on 50% of the appropriate county manufacturing industry employment value and 50% of the appropriate county population value in combination with CEC’s natural gas data. For previous forecasts, the employment and population data were obtained from the Association of Bay Area Government’s (ABAG’s) 2009 “Projections” reports170.
(b) Future Projections / Growth
Natural gas consumption is predicted to grow in future years but at a lower rate than population growth.
6.3.6 Uncertainties
Use of one generalized emission factor for the whole gas distribution system can lead to inaccuracies in emissions estimates. Therefore, an accurate estimate of leak rate for each equipment component group for gas distribution infrastructure would be helpful in improving the emissions estimates.
6.3.7 Contact
Author: Sukarn Claire
Reviewer: Ariana Husain
Last Update: November 06, 2023
6.3.8 References & Footnotes
The California Energy Commission (CEC). https://www.energy.ca.gov/↩︎
ORGPROF. CARB. 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
SPECIATE. USEPA. 2022. https://www.epa.gov/air-emissions-modeling/speciate↩︎
The Association of Bay Area Governments (ABAG) Projections. 2009. https://abag.ca.gov/↩︎