4.2 Power Plants

Category ID	Description	EIC
293	Power Plants - Boilers - Gas Fired Boilers	1000501100000, 6000501100000
294	Power Plants - Boilers - Oil Fired Boilers	13013201300000
297	Power Plants - Turbines - Oil Fired Turbines	1004512000000
1595	Power Plants - Turbines - Gas Fired Turbines	Various

Introduction

This chapter describes the methodology used to estimate greenhouse gas (GHG) emissions from power plants that combust natural gas or fuel oil to generate electricity. Emissions result from the operation of boilers and turbines and are categorized into the following source categories:

Category 293 – Combustion in Gas-Fired Boilers
Category 294 – Combustion in Oil-Fired Boilers
Category 297 – Combustion in Oil-Fired Turbines
Category 1595 – Combustion in Gas-Fired Turbines

Historically, oil-fired power plants were introduced in the late 19th century and reached peak deployment between the 1950s and 1970s. Over time, many facilities transitioned to natural gas to reduce emissions and improve combustion efficiency. In recent decades, the power sector has continued to evolve with increased adoption of low-carbon and renewable energy sources such as wind, solar, geothermal, and nuclear power.

In the San Francisco Bay Area (SFBA), Categories 294 and 297 account for emissions from oil-fired steam plants, while Categories 293 and 1595 encompass emissions from natural gas-fired boilers and turbines. These combustion processes are a significant source of carbon dioxide (CO₂), with smaller contributions from methane (CH₄) and nitrous oxide (N₂O).

These categories do not include emissions from cogeneration activities. Emissions from cogeneration at power plants and industrial facilities are also included in the Electricity Generation sector and covered under the Cogeneration methodology chapter.

In recent years, power plants have improved efficiency by incorporating heat recovery steam generators (HRSGs). These systems capture waste heat from gas turbines to produce steam, which is then used to power steam turbines. To enhance steam production, HRSGs may also be equipped with natural gas-fired duct burners that supplement the recovered heat. The emissions from duct burners are currently accounted for under the Industrial sector within the Non-Refinery Fuel Combustion subsector/chapter.

Methodology

Point sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, and for which the Bay Area Air Quality Management District (BAAQMD or the Air District) has issued a permit to operate (PTO), e.g. refinery cooling towers. These point sources could also be a collection of similar equipment and/or sources located across multiple facilities, e.g. reciprocating engines.

During the PTO issuance process, the Air District collects site-specific information from the operating facility and/or determines from published literature, e.g. United States Environmental Protection Agency’s (USEPA) AP-42 (USEPA, 2024), characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. This data is then compared against the Air District’s Regulations to ensure compliance. Facilities that hold a PTO are required to renew their permit periodically (this period varies based on facility and source type). Upon renewal, the facilities are requested to provide any updates to source characteristics as well as the source throughput for the past twelve months. This throughput, in combination with the emission factors and controls factors stored in the Air District’s internal database, are used to programmatically estimate annual emissions at the source level. The methodology used to calculate emissions for the reported base year(s) of a permitted point source is as follows:

Base Year(s) Emissions _{source,pollutant}=

Activity Data_source × Emission Factor_pollutant × Control Factor_pollutant × GWP_pollutant

Base Year(s) Emissions _county= ∑ ^N_i=1 Emissions_i

Where:

Base Year: is a year for which activity / throughput data is available from permit records.
Activity Data_source is the throughput or activity data for applicable base year(s) at the source/equipment level. This data is usually available from the internal permit records that are provided annually to the Air District at permit renewal by the facility operator.
Emission Factor_pollutant is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data. For example, tons CO₂ per gallons of gasoline burned or pounds of N₂O per million standard cubic feet of natural gas combusted. GHG emissions are calculated by using specific emission factors for every source/operation for which information has been supplied by the facility (and verified/validated through source tests). If no specific emission factors are available, generalized factors developed by Air District staff are used to determine emissions. These default factors typically come from published literature such as USEPA’s AP-42 (USEPA, 2024) or California Air Resource Board’s (CARB) Mandatory Reporting Requirement (CARB, 2019) for Greenhouse Gases.
Control Factor_pollutant is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of District rules and regulations.
GWP_pollutant is the Global Warming Potential. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO₂), 34 for methane (CH₄), and 298 for nitrous oxide (N₂O), when calculated on a 100-year basis with climate-carbon feedback included.
N is the number of permitted and similar sources in a county.

If available, a facility can provide emission factors specific to the source that are verified and validated through source tests to estimate GHG emissions. If no specific emission factors are available, general factors developed by Air District staff are used to estimate emissions. These source level emissions are then sorted and aggregated by year, county, and category.

Further speciation and quality assurance of emissions, including those of GHGs, are performed as a part of the inventory refinement process. A systematic crosswalk has been developed between CARB’s California Emissions Projection Analysis Model (CEPAM) source category classification using the primary sector emission inventory codes (EICs) and the Air District’s source category classification (category identification number - cat_ids), which ensures consistency when reporting annual emissions under the California Emissions Inventory Data Analysis and Reporting Systems (CEIDARS) to CARB (CARB, 2022a). This emissions data represents the reported base years emissions for a point source category.

Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:

Current Year Emissions_county = Base Year(s) Emission_county x Growth Factor

Where:

Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends.

For those years where no data is available, emissions data are backcast to the year 1990, as well as forecasted to year 2050 using either interpolation or another mathematical approach (see Trends section), or by applying a growth profile based on socioeconomic indicators. GHG emissions data from the years 1990 to 2050, including the projections outlined below, are analyzed for each source category and pollutant, with the trends evaluated for any observed anomalies and modified, if needed:

Historical Backcast (1990 – 2006): Association of Bay Area Governments (ABAG) Employment growth profiles (ABAG, 2024) and scaled District permitted data
Base Years (2007 – 2022): District permitted data
Future Projection (2023 – 2050) : CARB 2022 Scoping Plan projection profiles (CARB, 2022b)

The emissions estimation methodology for this sub-sector has not changed since the base year 2015 GHG emissions inventory, but updates to the 2022 base year inventory have improved the representativeness of the GHG emissions data.

Local Controls

Air District Regulation 9, Rule 11 – Nitrogen Oxides and Carbon Monoxide from Electric Power Generating Steam Boilers (BAAQMD, 2010) governs emissions under categories 293 and 294. This regulation establishes NO_x emissions limits based on boiler heat input and fuel type. The limits and compliance dates are summarized below:

Boiler Heat Rating (MMBtu/hr)	Fuel Type	NO_x limit (ppmv @ 3% O₂)	Compliance Date
>= 1.75 billion	Gaseous	10	December 2001
	Non-Gaseous	25
	Both (Gaseous & Non-Gaseous)	Weighted average of above
1.5 billion – 1.75 billion	Gaseous	25	December 2004
	Non-Gaseous	110
	Both (Gaseous & Non-Gaseous)	Weighted average of above
< 1.5 billion	Gaseous	30	December 2004
	Non-Gaseous	110
	Both (Gaseous & Non-Gaseous)	Weighted average of above

Since compliance deadlines have passed several years prior to the base year of this inventory, and the required controls have been in place for two decades, the reported/estimated emissions include the complete impact of the regulation. No additional controls are required to be accounted for the source categories under this rule.

Similarly, Regulation 9, Rule 9 – Nitrogen Oxides from Stationary Gas Turbines (BAAQMD, 2006) applies to emissions under categories 297 and 1595. This rule sets NO_x emissions limits based on turbine heat input and fuel type, with specific restrictions for turbines using refinery fuel gas. All compliance deadlines were set for the year 2010, which is well prior to the base year of this inventory. As a result, any emission reductions required by this rule are already reflected in actual emissions data, and no further adjustments are necessary.

Historical Emissions

Historical emissions for point sources are derived from source-specific data provided by the facility on throughputs, compiled or reported emission factors, and regulation-based control factors. This information is archived in the Air District’s internal database and is queried to retrieve the data for historical and current years. Interpolation techniques to account for missing data are used when necessary.

In the case of GHGs, up until the year 2006, the Air District was not engaged in systematic information collection during permit renewal process. This changed when AB32 bill was passed into a statewide law in 2006, and a statewide Cap and Trade system was introduced to reduce GHG emissions from specific facilities. Hence, GHG emissions data for years 1990-2006 are derived from the historical emissions data reported in the base year 2011 GHG inventory (released in year 2012). The historic emissions dataset is scaled to sync with the data in the permit database (which started systematic GHG data accounting from year 2006 onwards), to generate a complete GHG emissions time series for each point source category from 1990 to 2050.

Future Projections

Forecasting of point source emissions is done based on calculations as shown in the equation below using recently updated growth profiles and a base year of 2022. The growth profiles for the current base year inventory have been verified and updated to represent the most likely surrogate for forecasting emissions for a given category up to the year 2050. Forecasting for point source emissions includes impact of in-place regulations but does not include estimation of controls that will theoretically be implemented as part of future policy emission targets or proposed regulation and legislation.

PE = Gr × C_i × E_i

PE = projected emissions of pollutant i in a future year

Gr = growth rate by economic profile of industry or population

C_i = control factor of pollutant i based on adopted rules and regulations
E_i = base year emissions of pollutant i

California Senate Bill 100 (SB 100), enacted in 2018, establishes a target of 100% clean electricity by 2045 and updates the state’s Renewables Portfolio Standard (RPS) to require that 60% of retail electricity sales come from eligible renewable sources by 2030 (CEC, 2021). In alignment with this mandate, the California Air Resources Board (CARB) Scoping Plan (CARB, 2022b) includes two future emissions scenarios for power plant fuel combustion: a reference scenario and a scoping scenario. The reference scenario incorporates the 60% RPS target but assumes no further reductions beyond 2030 due to cost and feasibility constraints, as discussed in Appendix J of the Scoping Plan. To reflect business-as-usual conditions and the most likely emissions trajectory, the reference scenario is used for forecasting emissions from power generation in this inventory.

Emissions

The detailed breakdown of 2022 Power Plants GHG emissions in units of metric tons of CO₂ equivalents (MTCO₂eq) is provided in the table below.

ID	Description	CFC-11	CH4	CO2	CO2_bio	N2O	Total
1595	Power Plants - Turbines - Gas Fired Turbines	1.5	6325.7	4095536.8	3959.5	705.7	4106529.2
293	Power Plants - Boilers - Gas Fired Boilers	0.0	19.8	57918.7	0.0	17.5	57956.0
297	Power Plants - Turbines - Oil Fired Turbines	0.0	5.1	18517.2	0.0	44.1	18566.4

Summary of Base Year 2022 Emissions

The relative contribution of Power Plants subsector GHG emissions to region-wide and sector-level GHG emissions totals are highlighted in the table below.

Contribution of Power Plants Emissions by Sector

Subsector	Sector	Subsector GHG Emissions (MMTCO2eq)	Sector GHG Emissions (MMTCO2eq)	% of Sector
Power Plants	Electricity Generation	4.18	8.47	49.37%

Contribution of Power Plants Emissions to Regional Total

Subsector	Subsector GHG Emissions (MMTCO2eq)	Regional Total GHG Emissions (MMTCO2eq)	% of Regional Total
Power Plants	4.18	65.68	6.36%

Trends

The time series chart below shows the emission trends for all categories.

Summary of Trends

Beginning in the 1980s, oil-fired steam plants in the SFBA were gradually phased out in favor of cleaner-burning natural gas-fired steam plants. A significant shift occurred in 2001 with the commissioning of two large natural gas-fired power plants—Metcalf Energy Center and Los Medanos Energy Center, which continue to be the primary contributors to emissions in Category 1595 (Gas-Fired Turbines). In 2010, the NRG Potrero Power Plant, a major oil-fired facility, was decommissioned, while the Gateway Generating Station, a new natural gas-fired plant, began operations the same year. These facility transitions are clearly reflected in the emissions trends for Categories 293, 294, 297, and 1595.

Uncertainties

As noted above, point source emissions are calculated at an individual source level. The accuracy of these calculations is limited by the accuracy of the specific emission factors applied and estimated throughput. As these emissions are aggregated to create category level summaries, it is difficult to define a quantitative error associated with the total.

For power plants, there is an additional state level requirement to report greenhouse gases under the CARB Mandatory Reporting Requirement (MRR) (CARB, 2019). These reported emissions are independently validated by a third-party verifier and are generally considered to be the best estimate of greenhouse gas emissions. The Air District’s calculated refinery emissions are compared against these reported and verified emissions to identify any significant outliers. If an outlier is identified, a detailed analysis is done to confirm whether the Air District estimates should be left “as-is” or corrected to align with reported emissions. This additional verification provides more certainty in the emissions presented for the base year.

Contact

Author: Ariana Husain

Reviewer: Abhinav Guha

Last Update: 08/19/2025

References

ABAG. 2024. Association of Bay Area Governments. Historical Growth Profiles from Archived Internal Database. Accessed October 3, 2022.

BAAQMD. 2000. Bay Area Air Quality Management District - Regulation 9, Rule 11: Nitrogen Oxides and Carbon Monoxide from Utility Electric Power Generating Boilers. https://www.baaqmd.gov/en/rules-and-compliance/rules/reg-9-rule-11-nitrogen-oxides-and-carbon-monoxide-from-utility-electric-power-generating-boilers

BAAQMD. 2006. Bay Area Air Quality Management District - Regulation 9, Rule 9: Nitrogen Oxides and Carbon Monoxide from Stationary Gas Turbines. https://www.baaqmd.gov/en/rules-and-compliance/rules/reg-9-rule-9-nitrogen-oxides-and-carbon-monoxide-from-stationary-gas-turbines

CARB. 2019. California Air Resources Board - Regulation for the Mandatory Reporting of Greenhouse Gas Emissions. https://ww2.arb.ca.gov/sites/default/files/classic/cc/reporting/ghg-rep/regulation/mrr-2018-unofficial-2019-4-3.pdf

CARB. 2022a. California Air Resources Board - Emission Inventory Documentation. https://ww2.arb.ca.gov/emission-inventory-documentation. Accessed October 3, 2022. Accessed October 3, 2022.

CARB. 2022b. California Air Resources Board - CARB 2022 Scoping Plan. https://ww2.arb.ca.gov/our-work/programs/ab-32-climate-change-scoping-plan/2022-scoping-plan-documents. Accessed October 3, 2022.

CEC. 2021. California Energy Commission – 2021 SB100 Joint Agency Report. Publication # CEC-200-2021-001. https://www.energy.ca.gov/publications/2021/2021-sb-100-joint-agency-report-achieving-100-percent-clean-electricity

USEPA. 2024. United States Environment Protection Agency - AP-42: Compilation of Air Emissions Factors from Stationary Sources. https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors-stationary-sources. Accessed November, 2024.