Methane from Landfills
Methane is a hydrocarbon gas (CH4). It is a greenhouse gas and it is explosive. Methane is generated by decomposition (in landfills, swamps, in the stomachs of cows, etc.) and is considered a biomass energy fuel. In this discussion we will focus on methane from landfills.
Natural gas is approximately 80-99% methane, with the remainder being mostly other hydrocarbons (ethane, propane, butane, etc.) as well as some nitrogen, oxygen, water, CO2, sulfur and various contaminants.
Landfill gas is about 40-60% methane, with the remainder being mostly carbon dioxide (CO2). Landfill gas also contains varying amounts of nitrogen, oxygen, water vapor, sulfur and hundreds of other contaminants — most of which are known as “non-methane organic compounds” or NMOCs.
Methane present in the atmosphere as a result of landfills, manure, or coal mining activities, is a health and environmental hazard. When compared molecule to molecule, methane is a more potent greenhouse gas than carbon dioxide. It contributes to unhealthy smog over cities and presents safety hazards when it accumulates in underground pockets. From a health, environmental, and economic perspective it makes good sense to use this gas for a purposeful objective. Methane recovered from landfills, coal mines, and farm manure can generate electricity and, in some cases, supply gas that can be burned to heat buildings.
Although methane is a much cleaner form of energy than coal or oil, landfill and coal mine methane are, arguably, not renewable energy sources because the methane from these sources is a product of unsustainable activities. However since these sources of methane currently exist, it is worthwhile to collect the methane until those sources of methane are no longer present in our society. Farm methane is a renewable energy source because it is a natural byproduct of animal manure. This makes it a good source of renewable energy.
Landfills are responsible for about 20% of the methane produced in the United States. There are several EPA regulations for municipal solid waste landfills that have a bearing on the eligibility of methane collection and combustion. These include:
- New Source Performance Standards (NSPS) for Municipal Solid Waste Landfills, codified in 40 CFR 60 subpart WWW – Targets landfills that commenced construction or made modifications after May 1991.
- Emission Guidelines (EG) for Municipal Solid Waste Landfills, codified in 40 CFR 60 subpart Cc. – Targets existing landfills that commenced construction before May 30, 1991, but accepted waste after November 8, 1987.
- The National Emission Standards for Hazardous Air Pollutants (NESHAP), codified in 40 CFR 63 subpart AAAA – Regulates new and existing landfills.
Per regulations, landfills of certain size thresholds must either flare (burn) or in some other way eliminate methane. It is in their best interest to partner with industry and sell the methane at a rate that creates a win-win partnership.
A landfill methane system can be broken into 3 parts:
- An extraction, cleaning and pumping system at the landfill. Two-foot to three-foot diameter wells are drilled in the landfill and perforations in these pipes allow the methane gas to enter the pipes. The pipes are filled with gravel, sealed at the top, and connected through a header system to equipment that cleans and dries the gas to remove moisture. The gas is then compressed and sent into the transport pipeline.
- The pipeline. Because of the corrosive gases present in the methane mixture, polyethylene pipe is used to transport the gas from the landfill to the point of use.
- Monitoring, mixing and regulating equipment at the point of use. Typically, for commercial applications, landfill methane is used to supplement natural gas. Since landfill methane has a lower energy content than commercial natural gas, most users burn a mixture of the two gases to achieve an energy content meeting the specifications of the end-use equipment.
Facts about methane from landfills
- Landfill methane typically has a heat content of 400 to 500 million Btu per standard cubic foot (scf). This compares with a heat content of 1030 million Btu per scf for commercial natural gas. The lower Btu content requires piping that is about twice the size of that used for natural gas to provide an equivalent Btu value.
- Per EPA standard conversion², about 300 scf per minute of landfill gas is available for utilization for every million tons of landfill waste in place. That provides about 9.1 million Btu per hour (mmBtu/hr).
- Landfill gas is typically dried to a dew point of 40F and compressed before being sent into the pipeline.
Pros and Cons of methane from landfills
- Burning landfill methane prevents the release of two powerful greenhouse gases (methane and carbon dioxide) into the atmosphere.
- It is a reliable energy source because landfill gas is generated 24 hours a day, every day of the year.
- Burning waste methane reduces the demand on fossil fuel and nuclear energy. It also reduces our country’s economic dependence on fossil fuels.
- There may be financial opportunities to sell greenhouse gas offsets to other businesses.
- Environmental Benefits include:
- Mitigating climate change
- Odor control and less localized air pollution
- Reduction of local water pollution from gases seeping into the earth beneath landfills
- Use of landfill methane avoids pipeline charges associated with natural gas distribution and fuel charges can be fixed in a long-term contract.
- Cost of fuel may be lower per Btu than natural gas, depending on current natural gas prices. Landfill methane cost is often around $5.00 per mmBtu. But depending on your location, current natural gas prices may be lower, e.g., under $4.00 per mmBtu, making the burning of landfill methane not financially feasible unless special circumstances make it available at a much lower cost.
- Selling landfill methane is in the best interest of landfill operators and helps them meet environmental requirements. Current EPA regulations under the Clean Air Act require many larger landfills to collect and burn landfill gas through flaring. Collecting and selling it for productive use can create a win-win situation for the buyer and seller.
- There is a high capital cost to build and install a landfill methane system. One million dollars for the landfill site costs and another one to two million for the pipeline and point of use design and construction are common.
- For businesses, existing natural gas purchase contracts might need amending to account for a reduction in natural gas usage.
- There can be EPA or other code and regulatory impacts associated with running piping from the landfill to the point of use.
- Profitable use of landfill methane could discourage solutions to overwhelming landfill waste problems. Landfill methane is, arguably, not a renewable energy source (although it is “green”) and is actually a product of unsustainable activity.
- The non-methane portion of landfill gas consists of many compounds that are toxic. These are released into the atmosphere when the gas is burned and could have adverse environmental impacts.
- Methane production at each drilling site in a landfill trails off over time requiring new holes to be drilled in different locations to regain sufficient production of methane. This causes production quantities to vary. Moving the points of extraction is a constant requirement throughout the life of the project. Sometimes air is injected into the landfill to increased the rate of decomposition and increase production from a hole, but the added production comes at a cost.
- Contaminants in the landfill gas have a negative impact on operation and maintenance costs. Equipment burning landfill gas requires more frequent cleaning to remove silica buildup. In addition, hydrogen sulfide in sufficient quantity can have adverse corrosive effects on equipment.
- Equipment modifications may be needed to burn landfill methane:
- Landfill methane typically has an energy content of 400 – 500 Btu per scf. Equipment designed for the higher energy content of commercial natural gas may not operate correctly with landfill methane, or may require modifications to operate as designed.
- Having the ability to run 2 gases with different energy contents might not be possible with some equipment or might require additional and costly modifications.
- Piping and controls must be made of materials immune to the corrosive deterioration from the landfill gas (particularly the acids and hydrogen sulfide – H2S). Polyethylene and stainless steel are commonly used.
- Since the energy content of landfill methane is lower, increased volumetric flow is required to achieve Btu’s equivalent to burning natural gas. This requires larger piping to handle the additional flow. Associated pressure and flow regulation is also needed.
- To mix and burn two different gases, controls are needed to control the Btu content of the mixture delivered to equipment.
In summary, methane from landfills has a high cost to collect, clean, pressurize and pipe to the point of use. It is a dirty fuel that leaves deposits on equipment when burned. Its use is most beneficial and cost effective for larger commercial and industrial applications where the point of use is very near the landfill the methane is collected from. Burning landfill methane is generally not advantageous or cost effective as a replacement for natural gas in residential applications.
Have you had some experience with landfill methane? Share your thoughts below in comments.
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² Source: EPA Landfill Outreach Program