Converting Landfill Gas to RNG
Landfill gas can be upgraded to renewable natural gas (RNG) by removing carbon dioxide and other constituents. RNG can be used as a substitute for natural gas in a variety of applications including vehicle fuel (e.g., CNG or LNG), electricity generation, thermal energy or as a feedstock for chemicals (e.g., hydrogen or methanol). RNG can be delivered to end users via pipeline injection, used locally at CNG or LNG fueling stations at or near the landfill or transported to either an injection point or fueling station via a tube trailer (“virtual landfill methane pipeline”). Some projects may use more than one of these delivery mechanisms.

While not a new concept, the prevalence of this project type increased steadily between 2005 and 2017 and then began a sharp upward trend in 2018 with more new LFG-to-RNG projects coming online than other uses. In addition to financial incentives, RNG pipeline injection projects capitalize on the RNG being versatile for numerous end uses and accessible to non-local energy demands. Capital costs of RNG processing equipment are approximately $6,200 to $8,300 per standard cubic foot per minute (scfm) of landfill gas (2020 dollars). Electricity demand to operate LFG gas systems is often a significant portion of the O&M costs, consuming 0.009 kilowatt-hours per cubic foot of landfill biogas processed. Total O&M costs including electricity, pipeline injection fees, labor and parts, and supplies range from $1.4 million for a 1,000-scfm LFG project to $7.4 million for a 6,000-scfm LFG project (2020 dollars). These costs are just for conversion of LFG to RNG so do not include fueling station costs. Project costs depend on the purity of the gas required by the receiving pipeline or end user and the size of the project. Some economies of scale can be achieved when larger quantities of RNG can be produced.

Landfill gas can be converted into RNG by increasing its methane content and, therefore, reducing its carbon dioxide, nitrogen and oxygen content. The exact spec for RNG will depend on how and where the product will be used. In the United States, four methods have been commercially employed (beyond pilot testing) to remove carbon dioxide from LFG gas.

Figure 3. Amine Scrubbing System Process Flow Diagram for Landfill Gas Application
Solvent Scrubbing (Figure 3) Solvent scrubbing involves use of a chemical solvent such as amine or a physical solvent like Selexol to strip CO2 and H2S from the landfill biogas. Carbon dioxide is adsorbed into the solvent and methane passes through as the RNG product. In a chemical solvent system the solution is heated to release CO2 into the tail gas while in a physical solvent system the solvent is depressurized to release the CO2. NMOCs are generally hundreds to thousands of times more soluble than methane, while CO2 is about 15 times more soluble than methane. Solubility is enhanced with pressure, facilitating the separation of NMOCs and carbon dioxide from methane.

Water Scrubbing (or water wash) consists of a high-pressure landfill gas flow into a vessel column where CO2 and some other impurities, including H2S, are removed by dilution in water that falls from the top of the vessel in the opposite direction of the gas flow. Methane is not removed because it has less dilution capability. The pressure is set at a point where only the carbon dioxide can be diluted, normally between 110 and 140 pounds per square inch (psi). The water that is used in the scrubbing process is then stripped in a separate vessel to be used again, making this system a closed loop that keeps water consumption low. The gases resulting from the stripping process (the same that were removed from the landfill biogas) are then released or flared. Generally, no chemicals are required for the water scrubbing process. It is important to note that this technology will not remove certain contaminants such as oxygen and nitrogen that may be present in the raw landfill methane. This limitation is an important variable when the end use of the cleaned landfill gas is considered.

Pressure Swing Adsorption (PSA). A typical PSA plant employs compression, moisture removal and H2S removal steps but relies on vapor-phase activated carbon to remove NMOC and a molecular sieve to remove carbon dioxide. A difference in molecular size allows landfill methane to pass through into the RNG product while the media capture carbon dioxide and, to a lesser extent, nitrogen. The media are depressurized after saturation to release the carbon dioxide and nitrogen into the tail gas. Once exhausted, the activated carbon can be regenerated through a depressurizing heating and purge cycle.

Membrane Systems. A typical membrane landfill gas facility uses compression, moisture removal and H2S removal steps but relies on activated carbon to remove NMOCs and membranes to remove carbon dioxide. Activated carbon removes NMOCs and protects the membranes. The membrane process takes advantage of the physical property that landfill gases, under the same conditions, will pass through polymeric membranes at differing rates. CO2 passes through the membrane approximately 20 times faster than methane. Pressure is the driving force for the separation process. Project-specific RNG quality specifications and project size will help determine if a single-pass or multiple-pass membrane system is needed.

Regardless of how far along you are in your project, please do not hesitate to reach out to us, and we will find a solution that is custom made for your needs.
Landfill Gas Collection System
Facility Review
Landfill gas facility conditions conditions and operational goals both influence the design of a gas collection system (GCS). Site conditions such as landfill geometry, moisture, compaction rates, waste types, waste depths, cover soils permeability and final cover all affect GCS design. The greater the moisture within the waste mass, the faster landfill gas (LFG gas) will be generated and the higher the peak landfill methane generation rate. A more rapid biogas generation rate also leads to a landfill waste mass that tends to settle faster, which may cause damage to collectors that will need to be inspected and potentially replaced. Liquids within the waste mass may decrease the pore space within the waste mass, decreasing the ability of landfill gas to move to the LFG gas extraction wells. Thus, landfills with higher moisture content may have a smaller effective zone (radius) of influence for individual collectors and may require more collectors for the same area of coverage. Also some facilities choose to add moisture to facilitate decomposition, which increases landfill biogas generation but may increase GCS operational costs due to additional wells, increased settlement and larger header sizing.

Physical properties of the waste mass such as waste density (compaction), type and depth vary by location and affect the moisture level and methane generation potential of the landfill. Many facilities accept special waste streams such as sludges, ash, construction and demolition (C&D) and liquids, which affect the gas collection system design, landfill gas generation rates and the suitability of the LFG gas for beneficial use. For example, gypsum wall board and onions are known to elevate hydrogen sulfide (H2S) within landfill methane, which may need to be removed.

The materials used for daily, intermediate and final cover also change depending on local availability of soils, climate and approvals for alternate cover methods. Daily cover prevents blowing litter and odors and is usually not considered part of the gas collection system design. Sites that use a low-permeability soil such as clay for daily and intermediate cover can greatly reduce the influence of the LFG gas collectors and the effectiveness of the GCS. If this low-permeability soil cover is not completely stripped between placement of waste lifts, the landfill waste mass can be isolated from other landfill components, which adversely affects the ability to collect LFG and drain leachate. It also increases the potential of landfill biogas emissions and perched leachate (pooling of leachate on top of an impermeable layer) within the waste mass.

At the landfill surface, intermediate and final cover are designed to give a seal between the landfill and the atmosphere. A more impermeable seal on the surface of the landfill allows more vacuum to be applied to landfill methane collectors while minimizing the potential for atmospheric air and water to seep into the waste mass and ultimately into the LFG gas collectors. The more impermeable the intermediate and final cover, the greater the potential well spacing and the better the landfill gas wells are likely to operate.

Gas collection system (GCS) design can change dramatically due to local climatic conditions. The two most critical elements are temperature and precipitation. Accounting for temperature involves considering how GCS components will respond both during typical and extreme weather events. For example, facilities in areas that experience prolonged temperatures below 0 C (32 F) require winterization of equipment and vessels, and all header pipes and laterals should be buried to prevent freezing. Alternately, sites in very warm, sunny areas can have exposed gas collection system components experience significant thermal movement as they expand during the day and then contract overnight.
Figure 4. Example of a Gas Collection System (GCS)
Landfill Gas Collectors
Once the review of the landfill is complete, design of the gas collection system (GCS) can begin. One of the key components of the GCS is the landfill methane collectors. LFG gas collectors are typically composed of slotted or perforated plastic pipe, surrounded by stone or other aggregate backfill material, that are installed in borings (for vertical placement) or trenches (for horizontal placement) in the waste mass, below the top of the landfill. Design considerations for both vertical and horizontal wells, as well as other early collector techniques, are discussed below.

Vertical Extraction Wells
Vertical wells are the most common well type due to their potential to be installed across most landfill areas and effectively operated to meet a variety of the gas collection system operational goals. Vertical wells have the advantage of being capable of operation as soon as they are installed and being more effective at controlling surface emissions than horizontal wells. Vertical wells can also be adjusted or “tuned” to accommodate a wide range of operational requirements, including compliance and various utilization goals and to assist in liquids removal. One downside is the need for operators to continue compacting waste around vertical wells installed in operational areas of the landfill and the need to extend or re-drill the wells as the landfill continues to fill up.

The components of a vertical landfill well include the borehole, well casing, backfill materials and well seal. Vertical well boreholes typically range from 24 inches to 36 inches in diameter. Larger diameter boreholes increase the surface area of the well perimeter, which in turn can increase landfill biogas collection. Larger boreholes also allow additional space for gravel backfill, which can prevent adjacent waste fines from clogging the well casing perforations. Borings less than 24 inches in diameter are generally discouraged as they provide less filter between the waste mass and the well casing and may necessitate the use of smaller well casings. Smaller casings have a reduced structural integrity and limit the ability to remove liquids from the extraction well.

It is critical to generate an accurate survey of the proposed boring location and compare it to known areas of waste deposition (including wet waste, asbestos, other “special” wastes, C&D debris) and previously constructed GCS components. Impacting any of these items results in varied levels of construction and/or operational concern. It is critical to conduct an accurate survey of the proposed boring location and compare it to known areas of waste deposition (including wet waste, asbestos, other “special” wastes, C&D debris) and previously constructed gas collection system components. Impacting any of these items results in varied outcomes of construction and/or operational activities.

Borehole depths typically range from 40 to 140 feet below the surface of the landfill, but depths can increase in quarries and canyon fills. The maximum depth is usually limited by the drilling equipment. There are several challenges associated with very deep boreholes, including: 1) Vacuum dispersion. 2) Well integrity (due to higher potential of settlement or crushing). 3) High waste compaction, which decreases the waste permeability and prevents LFG gas extraction. 4) High degree of decomposition, which can potentially lead to saturated wastes, borehole collapse and limited landfill methane extraction.
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