3.1 Refinery Fuel Combustion

Category ID	Description	EIC
15	Flares and Blowdown Systems	Various
298	Oil Refineries External Combustion - Refinery Make Gas	Various
299	Oil Refineries External Combustion - Natural Gas & Other Gases	Various
300	Oil Refineries External Combustion - Liquid Fuel	Various
301	Oil Refineries External Combustion - Solid Fuel	4007002400000

Introduction

Greenhouse gas (GHG) emissions from the Refinery subsector primarily result from two types of combustion: (1) fuel combustion to generate heat for process operations or working fluids, and (2) fuel combustion to directly power equipment. This chapter focuses on the former—external combustion for heat production—and includes the following source categories:

Category 298 – Combustion of Refinery Make Gas
Category 299 – Combustion of Natural Gas and Other Gaseous Fuels
Category 300 – Combustion of Liquid Fuel
Category 301 – Combustion of Solid Fuel

GHG emissions from these categories are influenced by both the type of fuel used and the specific combustion technology. Regulatory requirements often dictate fuel quality and control measures, which are reflected in the category structure above.

These categories primarily cover external combustion processes in refinery heaters and boilers, where fuels such as natural gas, refinery gas, liquid waste, and petroleum coke are burned to generate steam or heat a working fluid.

This chapter also includes:

Category 15 – Flares and Blowdown Systems (Process Gas and Natural Gas)

Flaring is a safety mechanism used to combust excess gases during routine operations or unplanned events (e.g., equipment malfunctions). While intended to prevent overpressure and reduce hazardous emissions, flaring can release significant quantities of GHGs, carbon monoxide, sulfur dioxide, and unburned hydrocarbons.

Emissions from on-site cogeneration systems used to generate both electricity and heat are excluded from the categories covered in this chapter. These emissions are covered in the Cogeneration subsector/chapter. Source categories associated with fuel combustion at refineries to power engines and turbines are covered in the Fuel Combustion -Turbines and Engines 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)

Emissions data is finally aggregated under sub-sectors and sectors for tracking trends and documentation purposes.

Local Controls

A few local regulations directly target GHG emissions from refinery combustion. Among them, Regulation 9, Rule 10 (BAAQMD, 2021) indirectly affects GHGs by limiting nitrogen oxides (NO_x) and carbon monoxide (CO) emissions from boilers, steam generators, and process heaters installed before 1994. While this rule does not set limits on CO₂ or other GHGs, the associated improvements in combustion efficiency may have resulted in marginal, incidental GHG reductions. However, because the rule only applies to older equipment and its requirements were fully implemented by 2002, no further GHG reductions are expected from this regulation (BAAQMD, 2010).

Regulation 12, Rule 12, adopted in 2005, addresses emissions from refinery flares. Although the rule primarily focuses on reducing flaring frequency and magnitude through the development of Flare Minimization Plans (FMPs), these plans can also reduce associated GHG emissions—particularly methane and CO₂—by minimizing the volume of waste gas combusted. However, the degree of GHG reduction is highly variable across refineries and difficult to quantify, especially since FMPs do not apply to emergency flaring events. For these reasons, GHG reductions from this regulation are considered limited and uncertain, and Category 15 (Flares and Blowdown Systems) is projected to follow the general trend of other refinery combustion categories.

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

For fuel use at refineries, forecast for energy use as allocated by fuel type were applied from CARB’s E3 Pathways model as used in their 2022 Scoping plan (CARB, 2022b). E3 forecasting assumes a ramp down in petroleum refining in-line with in-state petroleum demand. This demand is forecasted to decrease as zero-emissions technologies are promoted and adopted. The forecast also accounts for historical emissions, which show an overall downward trend at the state level for the industrial sector.

Emissions

The detailed breakdown of the year 2022 Refinery subsector fuel combustion GHG emissions in units of metric tons of CO₂ equivalents (MTCO₂eq) is provided in the table below.

ID	Description	CH2Cl2	CH4	CO2	N2O	Total
299	Oil Refineries External Combustion - Natural Gas & Other Gases	27.0	4992.3	3280703.2	8488.1	3294210.6
298	Oil Refineries External Combustion - Refinery Make Gas	5.5	4480.0	2415582.0	7810.3	2427877.8
15	Flares and Blowdown Systems	0.0	2612.7	54950.9	84.2	57647.8

Summary of Base Year 2022 Emissions

The relative contribution of refinery external combustion and flaring GHG emissions to the region-wide and sector-level GHG emissions totals are highlighted in the table below. Refinery Fuel Combustion is the top source of emissions in the Industrial sector.

Contribution of Refinery Fuel Combustion Emissions by Sector

Subsector	Sector	Subsector GHG Emissions (MMTCO2eq)	Sector GHG Emissions (MMTCO2eq)	% of Sector
Refinery Fuel Combustion	Industrial	5.78	17.90	32.28%

Contribution of Refinery Fuel Combustion Emissions to Regional Total

Subsector	Subsector GHG Emissions (MMTCO2eq)	Regional Total GHG Emissions (MMTCO2eq)	% of Regional Total
Refinery Fuel Combustion	5.78	65.68	8.80%

Trends

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

Summary of Trends

GHG emissions from refinery combustion increased steadily from 1990 through 2017. However, beginning in 2018, emissions declined sharply—primarily due to reduced activity at the Martinez Refinery following its ownership transfer from Shell to PBF Holding Company LLC. Since the change in ownership, the facility has not returned to pre-2018 production levels. Additionally, several refineries in the San Francisco Bay Area have either initiated or completed transitions to renewable diesel and gasoline production, further reducing GHG emissions beginning in 2019. Category 301, representing solid fuel combustion, has not had any significant GHG sources for over a decade.

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 petroleum refineries, 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: 08/15/2025

References

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

BAAQMD. 2010. Bay Area Air Quality Management District. Staff Report, Proposed Amendment to BAAQMD Regulation 9, Rule 10: Nitrogen Oxides and Carbon Monoxide from Boilers, Steam Generators, and Process Heaters in Petroleum Refineries. J. Julian Elliot.

BAAQMD. 2021. Bay Area Air Quality Management District. Regulation 9, Rule 10: Nitrogen Oxides and Carbon Monoxide from Boilers, Steam Generators, and Process Heaters in Petroleum Refineries. https://www.baaqmd.gov/rules-and-compliance/rules/reg-9-rule-10-nitrogen-oxides-and-carbon-monoxide-from-boilers-steam-generators-and-process-heaters

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.

USEPA. 2024. US Environmental 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.