How is RNG Created?
How is RNG Created?
Raw biogas has a methane content between 45 and 65 percent, depending on the source of the feedstock, and must go through a series of steps to be converted into RNG. Treatment includes removing moisture, carbon dioxide (CO2) and trace level contaminants (including siloxanes, volatile organic compounds, or VOCs, and hydrogen sulfide), as well as reducing the nitrogen and oxygen content. Once upgraded, the gas has a methane content of 90 percent or greater. Typically, RNG injected into a natural gas pipeline has a methane content between 96 and 98 percent.
What is Landfill Gas?
Basic Information about Landfill Gas
Landfill gas (LFG) is a natural byproduct of the decomposition of organic material in landfills. LFG is composed of roughly 50 percent methane (the primary component of natural gas), 50 percent carbon dioxide (CO2) and a small amount of non-methane organic compounds. Methane is a potent greenhouse gas 28 to 36 times more effective than CO2 at trapping heat in the atmosphere over a 100-year period, per the latest Intergovernmental Panel on Climate Change (IPCC) assessment report (AR5)
Methane Emissions from Landfills
Municipal solid waste (MSW) landfills are the third-largest source of human-related methane emissions in the United States, accounting for approximately 15.1 percent of these emissions in 2018. The methane emissions from MSW landfills in 2018 were approximately equivalent to the greenhouse gas (GHG) emissions from more than 20.6 million passenger vehicles driven for one year or the CO2 emissions from more than 11.0 million homes’ energy use for one year. At the same time, methane emissions from MSW landfills represent a lost opportunity to capture and use a significant energy resource.
Example Case of LFG Composition
|Constituent||Typical % In Feed To System||Typical Pipeline Specification|
|Carbon Dioxide||39.35%||< 2%|
|Hydrogen Sulfide||0.0040%||< 0.0004%|
|VOCs + Siloxanes||0.1460%||Varies|
*Note that Digester Gas Is Typically Predominantly Methane & Carbon Dioxide with less Nitrogen & Oxygen, and more Hydrogen Sulfide.
Upgrading LFG to RNG
LFG can be upgraded to renewable natural gas (RNG), a high-Btu gas, through treatment processes by increasing its methane content and, conversely, reducing its CO2, nitrogen and oxygen contents. RNG can be used in place of fossil natural gas, as pipeline-quality gas, compressed natural gas (CNG) or liquefied natural gas (LNG). About 13 percent of currently operating LFG energy projects are creating RNG.
High-btu RNG can be used locally at the site where the gas is produced or can be injected into natural gas transmission or distribution pipelines for delivery to another location. Typical end-uses for RNG range from similar to those of traditional natural gas to vehicle fuel, particularly as a substitute for diesel.
Given that all landfills generate methane, it makes sense to use the gas for the beneficial purpose of energy generation rather than emitting it to the atmosphere. It is estimated that an LFG energy project will capture roughly 60 to 90 percent of the methane emitted from the landfill, depending on system design and effectiveness.