2.3 Refinery Process Vessel Depressurization

Category 20

2.3.1 Introduction

Category 20 accounts for organic emissions (ROG and TOG) from refinery process vessel depressurization.

Emissions for this category are generated from depressurization of process vessels at petroleum refineries. Generally a standard procedure requires repeated purging of the process vessel with inert gasses to reduce the concentration of organic vapors to below 300 PPM, before venting a vessel to the atmosphere.

2.3.2 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. Category 20 in whose emissions are captured in this chapter is considered a point source category and follows the above methodology for emissions estimates.

Refinery vessel depressurization generates and releases organic compound vapor as the main criteria pollutant into the atmosphere. The ROG/TOG ratios applied to this category 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 CARB⁶ and the US Environmental Protection Agency⁷. For this category, ROG constitutes 100% of TOG. Further assessment and improvement of ROG/ TOG speciation profiles have been planned in future inventory updates.

The District adopted Regulation 8, Rule 10 ⁸ on July 20, 1983. The purpose of this Rule is to limit emissions of organic compounds from depressurizing and opening of process vessels at refineries and chemical plants in Bay Area. Overall control afforded by this rule is estimated to be 95% reduction of organic compound releases to the atmosphere from the operation of process vessel depressurization at District’s permitted refineries. Since the adoption of the regulation, emissions of category 20 have been relatively insignificant, less than 5 pounds per day for the entire region.

2.3.3 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).

2.3.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

2.3.5 Trends

Refineries in the Bay Area are expected to produce refinery outputs based primarily on gasoline and diesel demands from consumers in Northern California. Since refineries have not expanded production capacity, refinery throughput can vary from year to year but the overall maximum capacity output is expected to remain relatively constant. District’s rules and regulations, particularly Regulation 8, Rule 10³ as previously mentioned, has helped significantly reduce organic emissions at refinery vessel depressurization sources over the years.

(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.

(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 past or 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 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 website⁹. 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.

2.3.6 Uncertainties

Throughputs for this category 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 EPA AP-42. The emission factor is estimated using historical data and could change or be improved as new data is published.

2.3.7 Contact

Author: Tan Dinh

Reviewer: Abhinav Guha, Yuan Du

Last Update: November 06, 2023

2.3.8 References & Footnotes