When first extracted, natural gas is a blend of methane; water; natural gas liquids (NGLs) such as ethane, butane, and propane; and condensate, a light hydrocarbon.
Gas processing is the bridge between the extraction of gas and its delivery as a finished product—typically composed mostly of methane—for use in homes, businesses, and utilities.
Gas processing has two steps:
The steps and technologies used at processing plants depend on the composition of gas, which varies across formations and regions. For example:
- “Sour” gas contains the toxic gas hydrogen sulfide, and is extracted from some western and southern states
- “Wet” gas has a high NGL content and is found in some parts of the Marcellus and Utica Shale
- “Dry” gas has a high proportion of methane
As of 2014, there were about 550 natural gas processing plants nationwide. The number has risen in recent years alongside shale gas production, which has yielded more liquids-rich gas. Operators have sought additional processing capacity to satisfy the growing market for NGLs to feed the petrochemical and plastics industries.
Air pollution is the biggest impact of gas processing. Processors pollute the air when operating because they –
- Rely on combustion to operate
- Use flares to burn off gases
- Often simply vent gas
- Have a wide range of equipment that can and does leak
Air sampling near large gas processing plants in Pennsylvania indicates that such facilities can release a wide variety of chemicals. They are known to release several climate and health-harming pollutants, including:
- carbon monoxide (CO)
- nitrogen oxides (NOx)
- fine and coarse particulate matter (PM2.5 and PM10)
- sulfur dioxides (SOx)
- volatile organic compounds (VOCs)
- hazardous air pollutants (HAPs) (e.g., formaldehyde, benzene, toluene, and xylene)
- greenhouse gases (methane, carbon dioxide, and nitrous oxide)
Safety and health
Safety and health are key concerns for workers at, and residents near, gas processing plants. Gas and gas liquids can be toxic to breathe, and because they are highly volatile, plants can catch fire or explode. Processing and storing gas that contains hydrogen sulfide is also very risky, since it is a toxic gas with significant health impacts.
A recent study concluded that gas development facilities can be noisy enough to pose health risks such as stress, sleep deprivation, and elevated blood pressure. Earthworks measured noise near a large gas processing facility in Pennsylvania, finding that levels were often in the upper 50-70 decibel (dBA) range, which exceeds state and federal standards.
Under the federal Clean Air Act, state regulators review permits and plans for processing plants and determine whether, based on projected pollution levels, they require only a state air permit (for “minor” sources) or also a federal one (for “major” sources). State regulatory agencies provide notice when federal air permits for major sources (known as Title V) are open for public comment.
In 2016, the US Environmental Protection Agency (EPA) issued a new rule to reduce pollution of methane and volatile organic compounds from oil and gas operations, including at gas processing plants.
As a result of a legal petition from 19 environmental organizations, EPA proposed a new rule in early 2017 to require gas processing plant operators to report their air pollution to the federal Toxics Release Inventory (TRI). This action was based on the health risks of regulated chemicals and hazardous air pollutants like benzene, formaldehyde, and xylene, that processing plants emit. Currently, the only processing facilities that are subject to TRI reporting requirements are those that remove sulfur from sour gas.