Landfill Gas to Energy
Design Factors
Selecting the best technology for a landfill gas (LFG) project involves consideration of several key design factors, beginning with estimating the LFG gas recovery potential for the landfill. In general, the volume of waste controls the potential amount of landfill methane that can be extracted from the landfill. Local conditions, LFG gas collection efficiency and the flow rate for the extracted landfill biomethane also significantly influence the types of technologies and end use options that are most feasible for a project. Landfill Gas Collection is described in detail in a separate tab.


LFG Gas Treatment Systems
Before LFG can be used in an energy conversion process, it must be treated to remove condensate, particulates and other impurities. Treatment requirements depend on the end use. Landfills that are selling gas for beneficial use and are subject to gas collection and control requirements under federal MSW landfill rules (40 CFR part 60, federal or state plan implementing 40 CFR part 60, or 40 CFR part 63 ) are required to develop a site-specific treatment monitoring plan and keep records of the parameters noted in the plan.

• Gas treatment for LFG gas electricity projects typically include a series of filters to remove contaminants that can damage the engine or turbine and reduce efficiency.
• Minimal treatment is required for direct use of landfill biogas in boilers, furnaces or kilns (direct use).
• Advanced treatment is needed to produce renewable natural gas (RNG)
for injection into natural gas pipelines or production of alternative fuels. The most growth is currently occurring in RNG applications.

Treatment systems can be divided into primary and secondary treatment processing. Most primary processing systems include de-watering and filtration (required for electricity generation). Gas cooling and compression have been used for many years and are standard elements of active landfill gas collection systems. Secondary treatment systems are designed to provide greater landfill methane cleaning than is possible using primary systems alone. Secondary landfill gas treatment systems may employ both physical and chemical treatments. The type of secondary treatment depends on the constituents that need to be removed for the end use. Two of the trace contaminants that may have to be removed from LFG gas are siloxanes and sulfur compounds, which can significantly increase the cost of landfill gas to energy project.

Table 1. Sizes of available internal combustion engines
Landfill Gas to Electricity
Converting Landfill Gas (LFG) to electricity continues to be the most common application, accounting for about 70 percent of all U.S. LFG energy projects operating during 2021. Renewable electricity can be produced by sending landfill gas to devices such as an internal combustion engine, a gas turbine or a microturbine.

Internal Combustion Engines
The internal combustion engine is the most commonly used conversion technology in LFG gas to energy projects because of its low cost, high efficiency and engine sizes that complement the landfill biogas output of many landfill gas project locations. Internal combustion engines have generally been used at sites where the methane from landfill quantity is capable of producing 800 kilowatts (kW) to 3 megawatts (MW), or where sustainable LFG gas flow rates to the engines are approximately 300 to 1,100 cubic feet per minute (cfm) at 50 percent landfill methane. Multiple engines can be combined together for biomethane projects larger than 3 MW. Table 1 provides examples of available sizes of internal combustion engines.

Internal combustion engines are efficient at converting LFG gas into electricity, achieving electrical efficiencies in the range of 30 to 40 percent. Even greater efficiencies are achieved in combine heat and power (CHP) applications, also known as cogeneration, where waste heat is recovered from the engine cooling system to make hot water or from the engine exhaust to make low-pressure steam.
Figure 1. Gas Turbine
Gas Turbines
Gas turbines (Figure 1), are typically used in larger LFG energy projects, where landfill gas flows exceed a minimum of 1,300 cfm and are sufficient to generate a minimum of 3 MW. Gas turbine systems are used in larger LFG gas electricity generation projects because they have significant economies of scale. The cost per kW of generating capacity drops as the size of the gas turbine increases, and the electric generation efficiency generally improves as well.

Simple-cycle gas turbines applicable to LFG energy projects typically achieve efficiencies of 20 to 28 percent at full load; however, these efficiencies drop substantially when the unit is running at partial load. Combined-cycle configurations, which recover the waste heat in the gas turbine exhaust to generate additional electricity, can boost system efficiency to approximately 40 percent. As with simple-cycle gas turbines, combined-cycle configurations are also less efficient at partial load. Advantages of gas turbines are that they are more resistant to corrosion damage than internal combustion engines and have lower nitrogen oxides emission rates. Additionally, gas turbines are relatively compact and have low operation and maintenance (O&M) costs compared with internal combustion engines. However, landfill methane treatment to remove siloxanes may be required to meet manufacturer specifications.

A primary disadvantage of gas turbines is that they require high gas compression of 165 pound-force per square inch gauge (psig) or greater. As a result, more of the plant’s power is required to run the compression system (causing a high parasitic load loss)
Microturbines have been sold commercially for landfill methane and other biogas applications since early 2001 (Figure 2). Generally, microturbine project costs are higher than internal combustion engine project costs based on a dollar-per-kW installed capacity. However, reasons for using microturbines instead of internal combustion engines include:
• Require less LFG gas volume than internal combustion engines
• Can use landfill gas with a lower percent methane (35 percent methane)
• Produce lower emissions of nitrogen oxides
• Can add and remove microturbines as gas quantity changes
• Interconnection is relatively easy because of the lower generation capacity

Microturbines typically come in sizes of 30, 70 and 250 kW. Projects should use the larger capacity microturbines where power requirements and LFG availability can support them. The following benefits can be gained by using a larger microturbine:
• Reduced capital cost (on a dollar-per-kW of installed capacity basis) for the microturbine itself
• Reduced maintenance cost
• Reduced balance of plant installation costs — a reduction in the number of microturbines to reach a given capacity will reduce piping, wiring and foundation costs • Improved efficiency — the heat rate of the 250-kW microturbine is expected to be about 3.3 percent better than the 70-kW unit and about 12.2 percent better than the 30-kW unit

Figure 2. Microturbines
Table 2. Landfill Gas to Electricity Generation Summary
Is Landfill Gas Renewable Energy?
Yes, since it is generated as a result of decomposition of organic substances, and capturing it prevents the release of a potent greenhouse gas. 
How is Landfill Gas Turned into Electricity?
Landfill Gas is turned into electricity using several approaches. Some of the most common of them is sending pretreated landfill gas to a combustion engine, gas turbine or injecting it into gas pipeline, and afterwards using it a feedstock to a power plant.
How can Landfill Gas be used for Energy?
In addition to using Landfill gas for generating electricity, it be used for fueling commercial vehicles as compressed natural gas (CNG) or liquified natural gas (LNG). Additionally, it can have direct used for any process that requires heating, such as furnace or a boiler, and in this case, pretreatment of landfill methane can be minimal.
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