7.7 Oil Refineries - External Combustion
Categories 298, 299, 300, and 301
7.7.1 Introduction
Categories 298, 299, 300, and 301 account for emissions from the external combustion operations at refineries, respectively.
External combustion refers to combustion of a fuel fed from the outside into a vessel primarily for the purpose of heat generation. Whereas, heat from fuel combustion inside a combustion engine is used for mechanization or movement of the pistons, the heat produced from oil refinery external combustion is used mostly to aid critical processes requiring high heat such as crude oil cracking/separation, hydrogen production, steam generation, and catalyst regeneration. The emissions reflected herein include all five criteria pollutants (PM, VOC, NOx, SOx, CO).
7.7.2 Methodology
Point Sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has issued a permit to operate, e.g. refinery cooling towers. These could also be a collection of similar equipment / sources located across multiple facilities, e.g. reciprocating engines.
During the permit to operate (PTO) issuance process, the BAAQMD collects information from the operating facility and/or determines from published literature, e.g. EPA’s AP-42, characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. These characteristics are then stored for future use in the BAAQMD’s internal database. Facilities that hold a permit to operate are required to renew this 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 last 12 months. This throughput, in combination with the emission factors and controls factors stored in the internal database, are used to estimate annual emissions at the source level. These source level emissions are then sorted and aggregated into categories.
Further speciation and quality assurance of emissions are performed as a part of the inventory process. The BAAQMD staff also perform a systematic crosswalk between CEPAM’s source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids), which ensures consistency in the annual emissions reporting process (CEIDARS) to California Air Resources Board. The last part of the inventory development process includes forecasting and back casting, and aggregation into sub-sectors and sectors for documentation purposes. For those years where no data is available, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using either interpolation or another mathematical approach (see Trends section). Finally, emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated for anomalies that are then investigated and addressed. Refinery external combustion categories are considered point source categories and follow the above methodology for emissions estimates.
Emission information from numerous sources is grouped into the above mentioned categories. EPA’s AP-42, Chapter 5 contains description of petroleum refining processes and emission factors. Criteria pollutants and greenhouse gas emissions, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), are produced during refining processes. Since refinery emissions are part of point sources, criteria pollutants as well as greenhouse gas data are obtained from the refinery plant as part of the Bay Area Air District permit renewal process.
The refineries in the Bay Area are permitted by the District under the Title V program, a Federally-mandated operating permit program for major sources of air pollution, and the refinery emissions are regulated by the District as well. Over the years, the District has enacted regulations to limit emissions from refinery sources. Regulation 9-10 306 controls emissions of NOx due to external combustion. Significant reduction in NOx emissions was observed between the period from year 2000 to 2004 for refinery external combustion operations as a result of this rule.
7.7.3 Changes in Methodology
There are no changes in the methodology to estimate emissions in the current base year inventory as compared to the previous base year inventory (year 2011).
7.7.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
7.7.5 Trends
Due to enactment of District regulations and modernization of refinery equipment in accordance with Title V permits, refinery emissions have been trending down over the years. Most notably, in 2010, SOx decreased significantly as a result of an abatement device installation at Valero refinery. Over 90% of SOx emissions were reduced at Valero refinery from prior years. Other reductions worthy of mention that helped to reduce air pollutant emissions include the flare minimization rule1 and the requirement of continuous emission monitoring devices to monitor and prevent unanticipated emission of a criteria pollutant, toxic air contaminant, and/or greenhouse gas into the atmosphere.
(a) Historical Emissions / History
Historical emissions are derived from source-specific throughputs provided by the permitted facility, compiled/reported emission factors, and regulation-based control factors. This information is archived in the BAAQMD’s internal database which is queried to retrieve the data for historical and current years. Interpolation techniques to account for missing data are used when necessary, this is the case for years 1991-1992.
Emissions prior to 1987 were derived from base year 1983 trend values. Past base year historical emissions include interchangeable emission reduction credits (IERC) which were part of category 10. IERC banking emissions are now inventoried under categories 298-300. Also, flare source emissions from categories 298 and 299 are now accounted for in category 15.
Emissions prior to 2015 were derived from base year 2011 trend values. Past base year historical emissions include interchangeable emission reduction credits (IERC) which were part of category 10. IERC banking emissions are now inventoried under categories 298-300. Also, flare source emissions from categories 298 and 299 are now accounted for in category 15.
(b) Future Projections / Growth
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 2020. The growth profiles for the current base year inventory have been verified and updated to represent the most likely surrogate for growing emissions for a given category up to year 2040. 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.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \]
\(PE\) = projected emissions of pollutant i in a future year
\(Gr\) = growth rate by economic profile of industry or population
\(Ci\) = control factor of pollutant i based on adopted rules and regulations
\(Ei\) = base year emissions of pollutant i
Projected growth for all refinery related categories was based on the even extrapolation of capacity reduction from 2010 to 2020, i.e., it’s assumed a 0.67% decrease annually from 2021 to 2040. The data used in the extrapolation is taken from US Energy Information Administration website307. The decrease is primarily driven by the shift to further increase fuel efficiency of vehicles through the U.S. Department of Transportation’s National Highway Traffic Safety Administration Corporate Average Fuel Economy (CAFÉ) standards as well as steady transition to electric vehicles and other alternative fuels (hydrogen fuel cell and others) in the California market.
7.7.6 Uncertainties
Throughputs for these categories are reported by facility via permit system requirement on a year by year basis and are assumed to reflect the most current data available at the time. Throughput data that are taken based on source test is considered the most accurate, followed by engineering calculations such as mass/material balance, and then published data via literature such as AP-422. The emission factor is estimated using historical data and could change or be improved as new data is published.
A step increase in TOG emissions for these categories may be seen for the periods of 2000-2001 and 2007-2008. This is due to a sustained Air District effort to gradually include and update methane emissions for various source types over these two periods of time. For years 1990-2008, high uncertainty in the TOG emissions estimates is expected; further refinement in backcasting of historical TOG emissions is planned in future inventory updates.
7.7.7 Contact
Author: Tan Dinh
Reviewer: Abhinav Guha, Tan Dinh
Last Update: November 06, 2023
7.7.8 References & Footnotes
BAAQMD. 2021. Reg 9-10. https://www.baaqmd.gov/rules-and-compliance/rules/reg-9-rule-10-nitrogen-oxides-and-carbon-monoxide-from-boilers-steam-generators-and-process-heaters↩︎
US Energy Information Administration. Data ; [accessed 2023 March 13]. https://www.eia.gov/dnav/pet/pet_pnp_cap1_dcu_SCA_a.htm↩︎