While biogas is commonly used for generating site power and heat, biogas in its raw form is not a substitute for commercial natural gas due to its relatively low methane (energy) content and the presence of various contaminants. Typical biogas contains 40-60% methane with the balance composed principally of carbon dioxide, but also inert compounds such as nitrogen and several deleterious constituents. Biogas purification is the process of upgrading biogas to RNG by removing non-methane contaminants, which increases the methane content and gas quality to commercial pipeline specifications.
Biogas Contaminants
Raw biogas contains several contaminants, the type and composition largely determined by the feedstock. Below we summarize the most common biogas contaminants and the impact they have:
- Carbon dioxide, nitrogen, and water. The presence of these substances reduces the proportion of methane and heating value of biogas.
- Water in the biogas stream presents a process hazard from freezing and corrosion of infrastructure and must be removed from the raw biogas stream.
- Oxygen is corrosive in pipelines and other metal infrastructure and can be difficult to remove.
- Hydrogen sulfide (H2S) is also highly corrosive and presents a serious health hazard to humans.
- Volatile organic compounds (VOCs). H2S, and siloxanes can negatively impact downstream treating processes by damaging membranes used in CO2 removal and poison adsorbent medias and catalysts. Siloxanes can also impair downstream equipment due to sand deposition.
READ MORE: Biogas Upgrading Technologies for Purification to RNG
Factors Impacting the Biogas Purification Process
The biogas purification process varies from one operation to another, depending on the mix of contaminants and their proportion. Some of the variables affecting the design of the purification process include:
- Source of organic material (i.e., landfill, wastewater, swine, dairy, poultry, etc.)
- Degree of degradation of source organic material (e.g., manure collected from a field vs. in a dedicated collection facility)
- Digester design and agitator efficiency
- Feedstock volume
- Cost of power and materials
- Location and permitting requirements
- Planned use of the RNG (transmission pipeline, vehicle fuel, etc.)
- Project economics
- Pipeline or end use specifications (including tolerances of impurities)
READ MORE: Biogas Purification – Upgrading Biogas to RNG
Infographic: Biogas Purification Technology
Despite the variations in the purification process, most biogas to RNG facilities include many of the treatment steps outlined in the diagram below, which identifies the stage of purification and the typical process technologies used in each phase.
About EcoVapor
EcoVapor, a DNOW company, offers a suite of technology-based solutions for improving the sustainability of gas-powered energy systems by converting waste streams into revenue, which reduces emissions. Our flagship ZerO2™ removes oxygen from biogas (“deoxo”), landfill gas and tank vapors, allowing gas to meet pipeline specification and be sold instead of otherwise being flared. We serve the Oil and Gas and Renewable Natural Gas (RNG) markets.
