6.1 Storage Tanks
Categories 55, 56, 57, 58, 84, 85, 940, 1902, and 1903
6.1.1 Introduction
Categories 55-58, 84-85, 940, and 1902-1903 account for fugitive organic emissions from the usage of storage tanks.
The largest tank emissions are from the refinery industry, and these are captured as point source emissions in categories 55-58. Tanks are used at petroleum refineries to store raw materials, intermediate products, finished products, and waste. Most commonly occurring materials stored in tanks at refineries include liquefied petroleum gas, gasoline, kerosene, diesels, and fuel oils although they are also used to store unusable waste products such as waste water, spent acids, and oily sludge.
In addition to fugitive emissions from refinery tanks, this Chapter also accounts for non-refinery related tank point source emissions which are captured in categories 84-85 . Other categories in this Chapter include area source categories from activities such as cleaning of storage tanks (category 940) as well as 1902 and 1903, which account for non-refinery area source emissions from storage tanks.
| Category # | Description | Classification |
|---|---|---|
| 55 | Cone Roof Tanks (refinery) | Point |
| 56 | External Floating Roof Tanks (refinery) | Point |
| 57 | Internal Floating Roof Tanks (refinery) | Point |
| 58 | Other Tanks (refinery) | Point |
| 84 | Cone Roof Tanks (non-refinery) | Point |
| 85 | Other Tanks (non-refinery) | Point |
| 940 | Tank Cleaning (refinery) | Area |
| 1902 | Cone Roof Tanks (non-refinery) | Area |
| 1903 | Other Tanks (non-refinery) | Area |
Emissions from storage tanks at bulk plants and gasoline stations are inventoried in sections 6.4 and 6.9, respectively.
6.1.2 Methodologies
The categories for storage tanks herein contain emissions from both point and area sources. The primary criteria pollutant emitted from storage tanks include hydrocarbon vapors or volatile organic compounds (VOCs). The predominant types of storage tanks that are accounted for within permitted sources include fix roof tanks, external/internal floating roof tanks, and conservation tanks as well as pressure tanks. The parameters or data collected to calculate storage tank emissions include: tank diameter, paint factor, tank height, vapor space, temperature change, tank volume, throughput, stored material, and factors specific to stored material.
Point Sources
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.
Categories 55-58, 84-85, and 940 are considered point source categories and follow the above methods for emissions estimates. To limit and reduce emissions from these categories, District’s Regulation 8, Rule 5, Storage of Organic Liquids Rule 157 regulates organic compound emissions from storage tanks in the Bay Area. The regulation was adopted on January 1978.
For this group of point source categories, ROG constitutes 100% of TOG. For the current base year inventory, the magnitude of the ROG emissions amounts to approximately 1,200 tons/year. The bulk of the emissions are derived from Cat 55 and 56, which as an aggregate, comprised of approximately 86% of these group of point sources ROG emissions. Historical emission trend for these group of point sources has been declining over the years mostly as a result of the above District’s rule. It is expected that the future emission trend for refinery related storage tanks to follow data extrapolated from California Annual Operable Atmospheric Crude Oil Distillation 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 website158.
Area Sources
Categories 940, 1902, and 1903 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. Emissions from storage tanks used in non-permitted industries are usually estimated at the unit of a geographical area, e.g., county. These area source emissions are derived based on an estimated ratio assumed constant over time between area and point source throughputs for tank type.
(a) Activity Data / Throughput
Point source emissions are obtained from the District’s internal permit database. The permit database contains information submitted by the individual refinery by tank sources on an annual basis. Area Source emissions are derived based on an assumed constant ratio between area and point source throughputs via tank type.
(b) County Distribution / Fractions
The county distributions are based on the point source distribution of sources among individual counties.
(c) Emission Factors
Emission factors for point sources are collected and used as referenced in the Methodology section of this chapter.
(d) Control Factors
District Regulation 8, Rule 51 set limitation standards on storage tanks effective from January 1978. This Rule has helped to lower VOC emissions into the atmosphere from storage tanks. Control efficiencies are estimated to range from 17% to 96%, depending on the type of tank, percentage of sources with true vapor pressures above 0.5 psi, and average true vapor pressure of the corresponding category.
(e) Speciation
The ROG/TOG ratios applied to 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 CARB159 and the US Environmental Protection Agency160. For this group of categories, ROG constitutes 100% of TOG. Further assessment and improvement of ROG/ TOG speciation profiles has been planned in future inventory updates.
6.1.3 Changes in Methodology
There are no changes in the methodology to estimate emissions in the current base year inventory compared to the previous base year inventory (year 2011).
6.1.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
6.1.5 Trends
Albeit future rules and controls on both point and area sources which are unknown at this point in time, the growth in emissions of the storage tanks is expected to correlate with the growth of its activity level. The bulk of emissions in this chapter is derived from storage tanks at refinery and/or refinery related industry. The trend therein tends to follow refinery historical and future growth.
(a) Historical Emissions / History
Historical emissions for point source 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. Storage tank usage correlates with throughputs and productions at respective industries. Historical refinery storage tank emissions have varied with refinery throughputs.
(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.
Storage tanks related to refinery processes follow refinery growth. Projected growth for all refinery related categories was based on the even extrapolation of California Annual Operable Atmospheric Crude Oil Distillation 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 EIA website2. 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.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \]
\(PE\) = 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
6.1.6 Uncertainties
Throughputs for the point source categories are reported by via the BAAQMD 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.
For both point and area source emission factors, the main uncertainty in the updated methodology arises from the use of emission factors for certain categories that remain constant over several decades although technological controls may have been applied at facilities. Additionally, the emission factors themselves are seldom verified and validated against measurements, in part due to the difficulty in measuring fugitive emissions from area sources.
6.1.7 Contact
Author: Tan Dinh
Reviewer: Abhinav Guha, Yuan Du
Last Update: November 06, 2023
6.1.8 References & Footnotes
Bay Area Air District Management District (BAAQMD). 2021. Reg 8-5. https://www.baaqmd.gov/rules-and-compliance/rules/reg-8-rule-5-storage-of-organic-liquids?rule_version=2021%20Nov%20Amendment↩︎
US Energy Information Administration. Data; [accessed 2023 March 07]. https://www.eia.gov/dnav/pet/pet_pnp_cap1_dcu_SCA_a.htm↩︎
ORGPROF. CARB 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
SPECIATE. USEPA 2022. https://www.epa.gov/air-emissions-modeling/speciate↩︎