7.1 Domestic Natural Gas Combustion
Categories 283, 284, 285, and 2540
7.1.1 Introduction
Categories 283, 284, 285, and 2540 account for criteria pollutant emissions (NOx, CO, PM, PM10, PM2.5, ROG, SO2, and TOG) from combustion of natural gas (NG) in the residential sector. The combustion of natural gas in the residential sector is broken down into four usage categories: Space Heating, Category 283; Water Heating, Category 284; Cooking, Category 285; and Other, Category 2540. The Other category accounts for NG combustion emissions from appliances such as clothes dryers, pools/spas/hot tub water heaters, and other miscellaneous NG appliances. Design of residential boilers and furnaces for natural gas combustion heat transfer generally resemble fire tube type boilers with flue gas traveling through several channels or tubes with water or air circulated outside the channels or tubes.
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). The average gross heating value of natural gas is approximately 1,020 British thermal units per standard cubic foot (Btu/scf), usually varying from 950 to 1,050 Btu/scf.
7.1.2 Methodology
Categories 283, 284, 285, and 2540 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
More details on throughput, county distribution, emission factors and controls is provided in the following subsections.
(a) Activity Data / Throughput
Residential sector natural gas consumption data for the San Francisco Bay Area was obtained from the California Energy Commission (CEC)270. Natural gas consumption data is provided at the county-level by CEC and is presented in units of therms. The NG consumption data for the Bay Area was converted to millions of standard cubic feet (mil. ft3, or MMscf) using a conversion factor of 97.02 (ft3/therm) based on natural gas heat content values.
Natural gas consumption is 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”271 for the Pacific Gas and Electric company (PG&E) service area. The annual end-use category breakdown may vary from year to year depending on weather conditions. 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)272, it is assumed that 0.35% of the NG by volume is lost due to post-meter leaks. The post-meter natural gas emissions are fugitive emissions or leakage losses that take place during the transitory period from the utilities meter to the point of gas combustion. Thus, the Residential NG throughputs for combustion categories 283, 284, 285, and 2540 are adjusted by subtracting 0.35% of NG by volume from the total NG usage at the gas meter. The post-meter residential sector emissions due to NG leaks from pipeline infrastructure are accounted for in categories 2535-2537, and 2541.
The natural gas end-use percent breakdown 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
Emission factor information for natural gas combustion was obtained from the U.S. Environmental Protection Agency (EPA) document AP-42273 the Energy Information Administration (EIA)274 and the California Energy Commission (CEC).
(d) Control Factors
The Air District adopted Regulation 9, Rule 6275 on April 1, 1992, to control the amount of NOx emissions from natural gas fired water heaters. This rule has a control of 46% with a rule effectiveness of 94% reached by the year 2002 and provides 43% net NOx emissions control. For all other pollutants (other than NOx), there are no specific mandated controls.
(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 |
|---|---|---|
| Domestic Natural Gas Combustion | 1.000 | 0.791 |
The PM2.5/PM and the PM10/PM ratios applied to this category or this group of related categories are consistent with size fractions of speciation profiles developed by the California Air Resources Board (CARB) and published on their emissions inventory web-page276. For this category(s), CARB’s speciation profile number is 121; PM2.5 constitutes 100% of total PM and PM10 constitutes 100% of total PM
(f) Sample Calculations
The following example shows TOG emission calculation from combustion of natural gas for Space Heating, Category 283, Alameda County, year 2015:
TOG Emissions (tons/ year) = Throughput (millions ft3/ year) x TOG Emission Factor (lbs/ millions ft3) x Alameda County Fraction x Growth Factor x Control Factor x (ton/ lbs)
TOG Emissions (tons/ year) = 45,747 x 11 x 0.2057 x 1.0 x 1.0 x 1/2000 = 51.76 tons/year
7.1.3 Changes in Methodology
To be consistent with California Air Resources Board methodology, the ‘Other’ category was added to the residential sector emissions in this base year inventory.
7.1.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
7.1.5 Trends
Annual demand for natural gas in the residential sector follows household population growth and variations in 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, and, building and appliance efficiency standards, utility and public agency programs, and price and other effects. For these reasons, natural gas consumption is predicted to grow in future years but at a slightly lower rate than household population growth.
(a) Historical Emissions / History
For years 1990 to 1999, natural gas consumption data from the California Energy Commission were used to estimate the historical emission trends.
(b) Future Projections / Growth
The residential natural gas emissions estimates were developed for years 1990 through 2040. For years 2000 to 2040, the natural gas consumption growth profile was obtained from the California Air Resources Board (CARB) for the Bay Area residential sector emission estimations; the 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; the overall growth is estimated to be 2.2% from 2015 to 2040. The growth of natural gas consumption is associated with historically low number of heating degree days, building and appliance efficiency standards, and utility and public agency programs.
7.1.6 Uncertainties
This methodology uses emission factors published in EPA’s AP-42 which are listed for uncontrolled residential furnaces. It is assumed that these emission factors are applicable to other appliances. It is unknown how current changes in the design of residential furnaces to reduce emissions have affected the emission factors.
7.1.7 Contact
Author: Sukarn Claire
Reviewer: Abhinav Guha
Last Update: November 06, 2023
7.1.8 References & Footnotes
The California Energy Commission (CEC). http://www.ecdms.energy.ca.gov/gasbycounty.aspx↩︎
2009 California Residential Appliance Saturation Study. < https://planning.lacity.org/eir/CrossroadsHwd/deir/files/references/C18.pdf>↩︎
Lawrence Berkeley National Laboratory (LBNL). An Estimate of Natural Gas Methane Emissions from California Homes. https://pubmed.ncbi.nlm.nih.gov/30071722/↩︎
EPA. AP-42. Compilation of Air Pollutant Emissions Factors. Volume 1, Fifth Edition. < https://www.epa.gov/sites/default/files/2020-09/documents/1.4_natural_gas_combustion.pdf>↩︎
The Energy Information Administration (EIA). https://www.eia.gov/energyexplained/natural-gas/use-of-natural-gas.php↩︎
The Air District Regulation 9 - Rule 6. Nitrogen Oxides Emissions from Natural Gas-Fired Boilers and Water Heaters. < https://www.baaqmd.gov/rules-and-compliance/current-rules>↩︎
PMSIZE. CARB 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎