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Air emissions modeling advances for oil and gas production facilities

Hydrocarbon processing systems and storage tanks are a significant source of volatile organic compound (VOC) emissions in the United States.

Barry L Burr and Adam M Georgeson, Bryan Research & Engineering, Inc.
Kha Mach, Chesapeake Energy
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Article Summary
Emissions rates from process units and tanks have historically been calculated individually for permit applications and at a single operating point once design engineering is complete. Now with EPA’s new NSPS OOOO rule, operating companies must perform far more VOC emissions calculations than ever before to comply with increased reporting requirements. With more focus on the quantities emitted and possible control alternatives, there is more interest in using the emission calculation methods during the oil and gas production site design stage.

More sophisticated modelling systems using chemical process simulators permit more accurate emissions estimation over a wider range of conditions and configurations. Recent advances in simulation interface technology permit automation of these modelling tasks for more efficient enterprise-wide reporting.

Oil, gas and process industries affect the atmosphere due to the quantity of emissions and types of chemicals emitted. Their environmental performance is becoming increasingly regulated by government agencies. Air emissions in the United States are regulated by the Environmental Protection Agency (EPA), often in conjunction with state or local agencies.

Coinciding with the rise in regulation and scrutiny of air emissions, there has been a rise in computer and simulation software performance. Such technology allows producers to create detailed emissions predictions using the best available scientific methods. It also allows permitting and reporting tasks to be executed with minimal economic burden and provides a framework for further engineering and optimisation tasks.

This paper will describe recent advances toward automated methods for calculating both tank flash emissions and working, breathing, and loading loss emissions from oil and gas site storage tanks.

Types of Air Emissions

Due to the wide variety of industrial activities in the United States, there are many different classifications of air emissions. Let’s take a look at the most common classifications affecting the upstream oil and gas sector:
• A hazardous air pollutant (HAP) is a chemical specifically designated by the EPA as having adverse human health or ecological impacts. At the time of the 1990 Clean Air Act, 187 chemicals were designated as hazardous, although additions and removals have been made.1
• Volatile organic compounds (VOC) are defined as any carbon compound which participates in atmospheric photochemical reactions, unless the compound is excluded or specifically exempted. Excluded are carbon monoxide, carbon dioxide, carbonic acid, metallic carbides and ammonium carbonate.  Methane and ethane are exempted along with a list of compounds in 40 CFR 51.100(s).2
• Benzene, toluene, ethyl-benzene and xylene (BTEX) are aromatic hydrocarbons classified both as VOCs and HAPs. These compounds make up a considerable fraction of the hydrocarbon produced from oil & gas wells, and thus are of concern to operating companies and environmental regulators alike.
• Greenhouse gases (GHG) are believed to contribute to climate change.  Carbon dioxide is the most prevalent greenhouse gas, released from both process and combustion sources. Other common greenhouse gases include methane and nitrous oxide, which are reported to have 21 and 310 times the global warming potential of carbon dioxide, respectively.3
• Nitrous oxides (NOx) refer to a group of highly reactive gases including NO, NO2 and N2O, which are formed in high temperature combustion. Of this group, NO2 is considered of the highest interest and tracked as an indicator for the larger group of nitrous oxides.4
• Sulphur dioxide (SO2) is a toxic gas which also contributes to acid rain through a reaction with atmospheric water. Sulphur dioxide is a product when sulphur-rich fuels or process streams are burned.5 Particulate matter (PM) is a measure of the tiny solids suspended in the atmosphere with a size between 0.1 and 10 µm. The majority of particulates are naturally occurring, including mineral dust and sea salt. Industrial activity such as fuel combustion can contribute to particulate pollution.6
• Criteria pollutants are ozone, lead, carbon monoxide, nitrogen oxides, sulphur dioxide and particulate matter. They may be directly emitted or the product of atmospheric reactions. Criteria pollutants are the subject of national ambient air quality standards.7

Air emissions from a facility are categorised as point source or fugitive. Point source emissions come from a single, identifiable location such as a tank vent or exhaust stack. Point sources may be equipped with continuous emissions monitoring systems to measure pollutant type and rate at regular intervals. For the vast majority of point sources, pollutant type and rate are estimated with engineering calculations or process simulation. Fugitive emissions escape through valve packing, pump seals, flanges and the like; they are usually estimated on a plant-wide basis using emissions factors. For most of oil and gas production sites, continuous emissions monitoring is excessive. Operating companies typically develop their own mechanisms for meeting emission tracking requirements by EPA and state agencies. The development of such tracking mechanisms is discussed in this paper.

Oil and Condensate Tanks
Oil and condensate tanks at production sites are point sources and several types of emissions are estimated from this type of source. The primary emissions are atmospheric tank flash emissions which result when production liquids originally at their bubble point in a high pressure separator are transferred to a storage tank held at near atmospheric pressure. The change to new equilibrium conditions at a lower pressure usually vaporises hydrocarbons which vent to the atmosphere or some emissions control system. Secondary types of VOC emissions which must be estimated are Working, Breathing, and Loading Losses produced from unsteady state operation of the atmospheric storage tank. The standard method for calculating these is defined in the EPA AP-42 specification8.

Usually these emissions are predicted for the purpose of obtaining construction and/or operating permits for a well-site from a state environmental regulatory agency. During this task, the operating company through their own employees or environmental consultants prepare permit documents containing estimates of well-site emission. Storage tank VOC emissions are also predicted annually at each well-site for the purpose of compiling operating emissions inventories for all sites and segregated by geographical regions. For permitting, compliance demonstration, and inventory purposes, the short term emissions (lb/hr or lb/day of VOC), and the long term emissions (12 month rolling total of VOC emissions) are usually required by EPA, states and local agencies.

Also in recent years, the emission thresholds differentiating permit requirement levels have been dropping. This motivates operators to reduce emissions and also estimates of those emissions. Operators are using more accurate emissions calculation methods and tools which let them design well-site facilities with emissions reduction in mind.
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