WHAT ARE VOCS AND WHY THEY’RE IMPORTANT
Do you ever wonder what some of those abbreviated technical terms used by your environmental department means? Do you work in the oil and gas industry or operate crude oil storage tanks? If yes, then you need to know about the ABCs of VOCs.
VOC stands for Volatile Organic Compound.
By USEPA regulation, VOC emissions are defined as organic compounds which participate in atmospheric photochemical reactions. The definition of VOCs is found in 40 CFR 51.100(s). The definition includes a list of chemicals not defined as VOCs. Two organic compounds that are not classified as VOC are methane and ethane.
VOCs and Ozone Formation
Photochemically reactive VOCs are regulated because these VOCs plus oxides of nitrogen (NOx – a byproduct of fossil fuel combustion) in the presence of sunlight chemically react to form ozone (O3) in the troposphere (lowest part of earth’s atmosphere). VOCs are air pollutants that are a precursor to the formation of ozone. The regulation of VOCs is designed to prevent the emission into the atmosphere one of the precursors of ozone formation.
Ozone is a major component of smog and is a powerful oxidant. Prolong exposure to ground level ozone concentrations above 75 parts per billion (ppb) can damage mucus and respiratory tissues in humans and damage plants. This makes ozone a potent human respiratory hazard near ground level.
Note the earth’s so-called Ozone Layer (a portion of the stratosphere with a higher concentration of ozone) is beneficial to life on earth. This stratospheric ozone helps prevent damaging ultraviolet (UV) light from reaching the Earth’s surface.
VOCs and Oil and Gas
For the oil gas industry complying with the alphabet soup of air quality regulations, it important to know that VOCs do not include methane (CH4), ethane(C2H6), H2S, CO2 or N2. When calculating the mass of VOCs in a natural gas stream vented, you calculate the VOC mass based on the propane plus hydrocarbon (C3+) fraction in the natural gas. Of course, with the requirements to report greenhouse gases, the methane in the natural gas stream is still important.
For air permits and compliance with NSPS OOOO/OOOOa this is significant because venting from such sources as storage tanks only have to account for the VOC fraction in the gas.
Another thing to understand is that the VOC content of storage tank vent gas is typically much higher than gas from a high pressure (HP) separator. A storage tank vent may have VOC content of 35% to 50%+ by volume while a HP separator gas would typically range from 80% to 98%+ VOC by volume. So, a volume of storage tank vent gas will have a higher mass (pounds) of VOC emissions than a same volume of HP separator gas.
Components of natural gas can include:
- Methane C1
- Ethane, C2
- Propane, C3
- Butanes, C4
- Pentanes, C5
- Hexanes, C6
- Heptanes, C7
- Octanes, C8
- Nonanes, C9
- Decanes plus, C10+
- CO2, N2, H2S
Sample VOC Calculation
To calculate the mass of natural gas and VOC content, the generally accepted regulatory formulas are based on the ideal gas law. For this we use the relationship that one pound mole (lb-mole) of a gas at standard conditions (60°F, 14.7 psia) occupies approximately 379.3 standard cubic feet (SCF). The lb-mole being the molecular weight of the gas stream or natural gas component.
Example Natural Gas Stream VOC – High Pressure Separator
|Volume of natural gas:||500,000||standard cubic feet (SCF) at 60°F & 14.7 psia|
|Molecular weight of gas stream :||19||lbs/lb-mole|
|Methane content:||95%||by volume|
|Ethane content:||5%||by volume|
|Methane content:||76%||by weight|
|Ethane content:||8%||by weight|
|VOC content:||16%||by weight|
|Gas stream mass:||25,046||lbs|
|Gas stream VOC mass:||4,007||lbs|
Mass of natural gas = (SCF gas)(lb/lb-mole)(1 lb-mole/379.3 SCF)
Mass of VOCs in gas stream = (lbs gas stream)(VOC weight percent)
If 500,000 SCF per day of this gas stream were vented to the atmosphere, it would result in emissions of over 731 tons VOC per year.
Sources of VOC
Oil and gas industry VOC emission sources include:
- Storage Tanks – Crude Oil, Condensate and Produced water
- Emergency/Process Vents
- Natural Gas Driven Pneumatic Pumps (e.g., Wilden/Aro/Texsteam)
- Glycol Dehydrators Still Column & Flash Tank
- Reciprocating and Centrifugal Compressor Seals
- Natural Gas Pneumatic Devices (e.g., Pressure/Level Controllers)
- Loading/Unloading Facilities – Tank Trucks, Barges)
- Amine Gas Sweetening Units
- Stuck Dump Valves
- Well Venting from Well Liquids Unloading
- Fugitive Emissions from Equipment Leaks
- Facility Blowdown Vent Stacks
- Unburned Hydrocarbons from Engine Exhaust and Flares
- Well Casing Head Gas Venting
Many expect federal and state regulation of VOC emissions to become stricter in the future and cover more processes in the oil and gas industry. This could include the following:
- Gather data for existing oil and gas production sites (prior to August 24, 2011), emissions. This would include facilities not affected by the NSPS OOOO and OOOOa
- Emission controls for existing sites not affected by NSPS OOOO and OOOOa.
- Regulate methane emissions from oil and gas facilities.
- Reporting of greenhouse gases (GHG) from more facilities than currently required by the GHG reporting rules in 40 CFR Part 98.
Capturing these vent gas streams, where feasible, and sending the gas to sales can increase production and profits for a facility.
For more information on how Cimarron can assist your company to identify, quantify and rectify (IQR) these lost VOCs and comply with the Alphabet Soup of air quality regulations, contact us at:
+1 (844) 746-1676