Understanding Landfill Gas and RNG

Landfill gas (LFG) is produced through the natural decomposition of organic materials such as food scraps, paper products, yard waste and other biodegradable materials. A typical landfill gas stream contains approximately 50% to 55% methane and 45% to 50% carbon dioxide.

While methane is a valuable energy source, it is also a potent greenhouse gas with a significantly higher warming potential than carbon dioxide over a shorter time horizon. In fact, one-third of the food produced globally reaches the landfill instead of the consumer’s table, creating about 8% of total global greenhouse emissions.

As such, capturing and utilizing methane before it escapes into the atmosphere offers substantial environmental benefits. LFG is converted to renewable natural gas (RNG) by removing carbon dioxide and other contaminants, resulting in a methane-rich fuel that meets pipeline quality standards. Once upgraded, RNG can be injected into existing natural gas infrastructure and used for heating, electricity generation, industrial processes or transportation.

Unlike conventional natural gas extracted from fossil reserves, RNG originates from renewable waste streams, making it an attractive option for organizations seeking lower-carbon energy solutions. In fact, forecasts indicate that LFG-to-RNG production could reach 2.2 billion cubic feet per day (bcfd) by 2050 — up from 0.3 bcfd today.

Why Upgrading Matters

Although landfill gas contains methane, it cannot be directly injected into natural gas pipelines in its raw form. Utilities maintain strict gas quality specifications to ensure pipeline safety, equipment compatibility, and reliable performance.

To meet these standards, landfill gas must undergo a series of treatment and upgrading steps. The process typically begins with contaminant removal, including moisture, particulates, sulfur compounds and siloxanes. After pretreatment, advanced upgrading systems separate methane from carbon dioxide and other remaining gases.

The result is a high-purity methane product that closely resembles conventional natural gas. This transformation is what allows landfill-derived gas to become a marketable renewable fuel rather than simply a waste by-product. RNG can be used in place of fossil natural gas, as pipeline-quality gas, compressed natural gas or liquefied natural gas, with about 20% of operating LFG energy projects creating RNG.

Membrane Separation

A widely adopted upgrading technology is membrane separation. This approach uses specially engineered membranes that selectively allow certain gas molecules — particularly carbon dioxide — to pass through while retaining methane.

As landfill gas passes through multiple membrane stages, the methane concentration steadily increases until it reaches 98% purity, producing biomethane that meets strict pipeline standards. Modern membrane systems offer several advantages that have accelerated their commercial adoption.

First, they feature relatively compact footprints, making them suitable for landfills with space constraints. Second, modular system designs allow operators to expand capacity as gas production grows. Third, membrane technologies generally require fewer moving parts than some alternative systems, reducing maintenance requirements and operational complexity.

Continuous improvements in membrane materials have also increased methane recovery rates and system efficiency, making the technology increasingly attractive for large-scale RNG projects.

Pressure Swing Adsorption

Pressure swing adsorption (PSA) is another established technology helping drive RNG commercialization. PSA systems use adsorbent materials that selectively capture carbon dioxide and other unwanted gases under high pressure.

The process operates in cycles. During the adsorption phase, contaminants adhere to the adsorbent material while methane continues through the system. When the adsorbent reaches capacity, pressure is reduced, releasing the captured gases and regenerating the material for reuse.

PSA technology has gained popularity for its ability to achieve high methane purity while maintaining reliable performance across varying gas compositions. Landfill gas quality can fluctuate depending on factors such as waste composition, weather conditions and landfill age. While PSA systems can often accommodate these variations effectively, making them suitable for many RNG applications.

The Economics of Commercial Scale

Historically, RNG projects faced challenges related to equipment costs, methane recovery efficiency, and operational complexity. As such, many landfill operators found it difficult to justify the investment required for gas upgrading systems. However, several factors have changed that equation.

Technology costs have gradually declined as equipment manufacturers gain experience and production volumes increase. At the same time, system performance has improved, enabling greater methane recovery and more reliable operation. In fact, in 2023, biomass accounted for about 5% of the U.S.’s total energy consumption, showing its growing role in the renewable energy sector.

There is also a growing demand for low-carbon fuels, which has strengthened project economics. Utilities, transportation fleets and industrial users are increasingly seeking renewable fuel alternatives to support emissions reduction commitments. Regulatory programs and renewable fuel incentives in various markets further enhance revenue opportunities for RNG producers.

Additionally, landfill operators are recognize that RNG production can transform an environmental liability into a long-term revenue-generating asset. Rather than simply flaring excess gas, operators can monetize captured methane while supporting sustainability objectives.

The Environmental Benefits Beyond Energy Production

The environmental value of landfill RNG extends beyond replacing fossil fuels, as methane capture itself delivers substantial climate benefits.

Landfills are the third-largest human-generated source of methane emissions in the United States, releasing an estimated 100.9 million metric tons of carbon dioxide equivalent to the atmosphere in 2022 alone. Capturing it before it enters the atmosphere can significantly reduce the overall climate impact of waste management operations.

Additionally, producing RNG also supports circular economy principles by extracting value from materials that would otherwise be discarded. Organic waste effectively becomes a feedstock for renewable energy production, extending the usefulness of resources that have reached the end of their original life cycle.

Lastly, because RNG can utilize existing natural gas infrastructure, it provides a practical pathway for decarbonization without requiring extensive new distribution networks. This compatibility allows renewable fuel adoption to scale more rapidly than some alternative energy solutions.

Looking Ahead

The RNG industry is entering a period of accelerated growth. Advances in membrane separation and pressure swing adsorption technologies have significantly improved project feasibility, enabling landfill operators to produce pipeline-quality gas at larger scales than ever before.

As climate policies become more ambitious and demand for renewable fuels continues to rise, landfill gas is increasingly being recognize as a valuable energy resource rather than a waste by-product. As such, RNG represents a compelling combination of waste management, renewable energy and emissions reduction, that turns landfill gas into a reliable source of renewable energy for years to come.

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The Table below shows a comparison of performance for these techniques at 8 bar (grid) injection.

Table:  Comparison of performance for various upgrading techniques (result at 8 bar) (Robert Lems, 2010) , (Lems R., 2012)

^*2 If no oxygen of nitrogen is present in the raw biogas

^*3 Activated carbon (AC) consumption is depending on the presence of certain pollutants (trace components) within the raw biogas.

From the above Table, it can be concluded that the differences between technologies with respect to performance seem to be relatively small. However, some “soft factors” can have a significant impact on technology selection. For example, water scrubber technology is a broadly applied technology. The requirement for clean process water, to make up for discharge and condensation, could be a challenging constraint for remote locations.

Moreover, PWS systems are prone to biological contamination (resulting in clogged packing media and foaming), especially when operated at elevated temperatures. Without additional preventative measures this will result in an increase of operational issues and downtime.

Amine scrubbers are a good choice when surplus heat is available for the regeneration of the washing liquid. The transport and discharge of this washing liquid could however be a burden, as well as the added complexity of operation. With respect to cryogenic Liquefaction (CL) one may conclude that, this technology has a questionable track-record, is highly complex, hard to operate, and should therefore not be selected for small-medium scale applications.

Both PSA and MS provide a “dry” system, both technologies operate without the requirement for a solvent/washing liquid, which significantly simplifies operation and maintenance. Distinctive factor between these technologies is that the membrane based system operates in a continuous mode, while the PSA technology is based on columns filled with absorption materials which operate in a rotating/non-continuous mode.

Moreover, the membrane based system has a more favourable methane slip, energy consumption and turndown ratio. The biggest advantage over PSA however, is that membrane systems do not require any transport of absorbents, its ease of operation and superior up-time.

Main disadvantage of membrane systems are that they are sensitive to pollution by organic compounds, which can decrease efficiency. However, by applying a proper pre-treatment (generally based on activated carbon and condensation) in which these compounds are eliminated, this disadvantage can be relatively easy nullified.

Based on membrane technology, DMT Environmental Technology, developed the Carborex ®MS. A cost-effective plug and play, containerized (and therefore), easy to build in remote locations) biogas upgrading system. The Carborex ®MS membrane system has relatively little mechanical moving components (compared to other upgrading technologies) and therefore, ensures stability of biomethane production, and consequently, the viability of the biogas plant operation.

Moreover, its design for ease of operation and robustness makes this technological platform perfectly suitable for operation at locations with limited experience and expertise on handling of biogas plants.

Conclusions

Capture of biogas through application of closed ponds or AD’s is not only a necessity for mitigation of greenhouse gas emissions, it is also a method of optimizing liquid waste treatment and methane recovery. Billions of cubic meters of biomethane can be produced on a yearly basis, facilitating a significant reduction of fossil fuel dependency.

Moreover, upgrading of raw biogas-to-biomethane (grid, CNG or LNG quality) provides additional utilization routes that have the extra advantage to be independent of existing infrastructure. To sum up, membrane based technology is the best way forward due to its ease of operation, robustness and the high quality of the end-products.

References

• Lems R., D. E. (2012). Next generation biogas upgrading using high selective gas separation membranes. 17th European Biosolids Organic Resources Conference. Leeds: Aqua Enviro Technology .
• Robert Lems, E. D. (2010). Making pressurized water scrubbing the ultimate biogas upgrading technology with the DMT TS-PWS® system. Energy from Biomass and Waste UK . London: EBW-UK .

Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)

Note: This is the final article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first two articles can be viewed at these links

http://www.bioenergyconsult.com/biomethane-utilization/

http://www.bioenergyconsult.com/pome-biogas/

The post Biogas-to-Biomethane Conversion Technologies first appeared on BioEnergy Consult.

At present four different biogas upgradation methods are generally used for removal of carbon dioxide from biogas, either to reach vehicle fuel standard or to reach natural gas quality for injection to the natural gas grid. These methods are:

• Water absorption
• Polyethylene glycol absorption
• Carbon molecular sieves
• Membrane separation

Water Scrubbing

Water scrubbing is used to remove carbon dioxide but also hydrogen sulphide from biogas since these gases is more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurized and fed to the bottom of a packed column where water is fed on the top and so the absorption process is operated counter-currently.

Polyethylene Glycol Scrubbing

Polyethylene glycol scrubbing is a physical absorption process. Selexol is one of the trade names used for a solvent. In this solvent, like in water, both carbon dioxide and hydrogen sulphide are more soluble than methane.

The big difference between water and Selexol is that carbon dioxide and hydrogen sulphide are more soluble in Selexol which results in a lower solvent demand and reduced pumping. In addition, water and halogenated hydrocarbons (contaminants in biogas from landfills) are removed when scrubbing biogas with Selexol.

Carbon Molecular Sieves

Molecular sieves are excellent products to separate specifically a number of different gaseous compounds in biogas. Thereby the molecules are usually loosely adsorbed in the cavities of the carbon sieve but not irreversibly bound. The selectivity of adsorption is achieved by different mesh sizes and/or application of different gas pressures.

When the pressure is released the compounds extracted from the biogas are desorbed. The process is therefore often called “pressure swing adsorption” (PSA). To enrich methane from biogas the molecular sieve is applied which is produced from coke rich in pores in the micrometer range. The pores are then further reduced by cracking of the hydrocarbons. In order to reduce the energy consumption for gas compression, a series of vessels are linked together.

The gas pressure released from one vessel is subsequently used by the others. Usually four vessels in a row are used filled with molecular sieve which removes at the same time CO2 and water vapour.

Membrane Purification

There are two basic systems of biogas purification with membranes: a high pressure gas separation with gas phases on both sides of the membrane, and a low-pressure gas liquid absorption separation where a liquid absorbs the molecules diffusing through the membrane.

• High pressure gas separation

Pressurized gas (36 bar) is first cleaned over for example an activated carbon bed to remove (halogenated) hydrocarbons and hydrogen sulphide from the raw gas as well as oil vapour from the compressors. The carbon bed is followed by a particle filter and a heater. The raw gas is upgraded in 3 stages to a clean gas with 96 % methane or more.

The waste gas from the first two stages is recycled and the methane can be recovered. The waste gas from stage 3 (and in part of stage 2) is flared or used in a steam boiler as it still contains 10 to 20 % methane.

• Gas-liquid absorption membranes

Gas-liquid absorption using membranes is a separation technique which was developed for biogas upgrading in the recent past. The essential element is a micro-porous hydrophobic membrane separating the gaseous from the liquid phase. The molecules from the gas stream, flowing in one direction, which are able to diffuse through the membrane will be absorbed on the other side by the liquid flowing in counter current.

The absorption membranes work at approx. atmospheric pressure (1 bar) which allows low-cost construction. The removal of gaseous components is very efficient. At a temperature of 25 to 35°C the H₂S concentration in the raw gas of 2 % is reduced to less than 250 ppm.

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