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As the environmental impact and non-renewable nature of fossil fuels are receiving increasingly more attention, many countries are focusing on obtaining cleaner, less expensive and more reliable energy sources. One promising resource is the production of natural gas. While this has historically been extracted during oil drilling, newer technologies like fracking are also being used to produce natural gas from non-traditional sources like shale.
Natural gas burns ‘cleaner’ than other fossil fuels. The result is a cut in carbon dioxide emissions to approximately half of that created from coal burning, or a quarter of that from oil combustion. It also produces no mercury vapors, sulfur dioxide, or particulate matter, and a reduced amount of nitrogen oxide.
However, is natural gas clean when the whole fuel cycle is considered?
Air Pollution by Fugitive Gas Release
There are several ways in which natural gas contributes to air pollution. For example, fugitive gas that escapes via leakage or venting causes significant pollution during the production phase.
Venting is intentional, through pneumatic valves or vents on storage tanks, refinery equipment and wells following hydraulic fracturing, or from oil wells. However, leaks are unintentional, the result of corrosion, wear and tear, over-pressurized systems, or poor installation/maintenance.
The equipment used for processing may also release air pollutants. For example, separators or other units that remove byproducts and wastes from the gas before it is distributed as dry or pipeline quality natural gas for commercial use.
Leakage may occur from the gigantic transmission network, including the many pieces of equipment used in this process, and this consists almost entirely of methane.
A second type of air pollution due to natural gas thus occurs because of the use of gas combustion during the gas extraction operations. Thus, both combustion-related emissions and the leakage or venting of waste gases cause volatile organic compounds (VOCs), hazardous air pollutants (HAPs) and sulfur dioxides to enter the atmosphere.
Other associated pollutants come from emissions produced from burning fuel to power many operations of an oil well. This ranges from the initial building of the well pad, the roads and pipelines, to the drilling of the wells.
It can also include the flaring of natural gas at oil wells where its transport for commercial use is unviable, and all the following activities; processing activities and equipment, including compressors, controllers and dehydrators. Dust and particulate matter are also released in this phase.
What are the Pollutants Released from Natural Gas?
The most prominent pollutant is undoubtedly methane followed by volatile organic compounds (VOCs), which are produced in the highest amounts by the natural gas industry. Others include nitrogen and sulfur oxides, and HAPs.
Methane
Methane is a prime cause of smog (ground-level ozone) formation, which causes a variety of breathing problems. It is also a powerful greenhouse gas (GHG), although less long-acting than CO2.
Approximately 25% of total domestic methane emissions and 2.5% of total GHG emissions in the US come from natural gas systems, making this the second-most plentiful source of methane emissions from human activity.
VOCs
This is also a precursor of smog, through its interaction with nitrogen oxides and sunlight. Oil and natural gas systems produce 20% of all manmade VOCs in the US.
Nitrogen Oxides
This ground-level smog precursor is also emitted from natural gas sites because of the combustion of fossil fuels during well drilling and operation. It is independently linked to several respiratory illnesses.
Sulfur Dioxide
Oil and natural gas production and processing, especially when the gas is “sour” or sulfur-rich, involves sulfur dioxide emission, causing respiratory illness.
Hazardous Air Pollutants
HAPs or air toxics are pollutants with a known or suspected role in carcinogenesis or other health issues, including impaired fertility or teratogenesis. In relation to natural gas, these include mostly VOCs followed by n-hexane, BTEX group of chemicals (benzene, toluene, xylenes and ethylbenzene), and sometimes hydrogen sulfide at high concentrations.
These come from the natural gas itself, or due to leakage, as a byproduct of various operations, from incomplete burning of fossil fuel, or from chemical processes.
So is it Better to Use Natural Gas?
When it comes to burning natural gas in vehicles, the leakage throughout the production cycle to the final product would have to be as low as 1% to 1.6% to make it cleaner than the combustion of other fuels. With natural gas power plants, a leakage of <3.2% ensures they are cleaner than new coal plants.
Conclusion
It is possible that natural gas does not quite live up to its label as a greener fuel. Some areas where unconventional gas extraction is performed have shown higher levels of HAPs and two pollutants out of the six regulated by the Environmental Protection Agency (EPA) for their toxicity to humans and to the environment.
When emissions due to leakage or venting are considered along with the total emissions due to combustion of natural gas and other fossil fuels throughout the entire production cycle, it is clear that low levels of these air polluting emissions is crucial to ensuring that natural gas is a cleaner and greener form of fuel.
More accurate emissions data is important in managing this sector more logically, including the economic retrieval and sale of leaked natural gas.
Sources and Further Reading
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