Landfill gas utilization
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Landfill gas utilization is a process of gathering, processing, and treating the
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
or another gas emitted from decomposing garbage to produce electricity, heat, fuels, and various chemical compounds. After fossil fuel and agriculture, landfill gas is the third largest human generated source of methane. Compared to , methane is 25 times more effective as a greenhouse gas. It is important not only to control its emission but, where conditions allow, use it to generate energy, thus offsetting the contribution of two major sources of greenhouse gases towards
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. The number of landfill gas projects, which convert the gas into power, went from 399 in 2005 to 519 in 2009 in the United States, according to the US Environmental Protection Agency. These projects are popular because they control energy costs and reduce
greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and lar ...
. These projects collect the methane gas and treat it, so it can be used for electricity or upgraded to pipeline-grade gas. These projects power homes, buildings, and vehicles.


Generation

Landfill gas Landfill gas is a mix of different gases created by the action of microorganisms within a landfill as they decompose organic waste, including for example, food waste and paper waste. Landfill gas is approximately forty to sixty percent methane, ...
(LFG) is generated through the degradation of
municipal solid waste Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. "Garbage" can also refer specifically to food waste, ...
(MSW) and other biodegradable waste, by
microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
. Aerobic conditions, presence of oxygen, leads to predominately emissions. In anaerobic conditions, as is typical of landfills, methane and are produced in a ratio of 60:40.
Methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
() is the important component of landfill gas as it has a calorific value of 33.95 MJ/Nm^3 which gives rise to energy generation benefits.Scottish Environment Protection Agency. Guidance on Landfill Gas Flaring. November 2002. Web. . The amount of methane that is produced varies significantly based on composition of the waste. Most of the methane produced in MSW landfills is derived from
food waste Food loss and waste is food that is not eaten. The causes of food waste or loss are numerous and occur throughout the food system, during production, processing, distribution, retail and food service sales, and consumption. Overall, about o ...
, composite paper, and corrugated cardboard which comprise 19.4 ± 5.5%, 21.9 ± 5.2%, and 20.9 ± 7.1% respectively on average of MSW landfills in the United States. The rate of landfill gas production varies with the age of the landfill. There are 4 common phases that a section of a MSW landfill undergoes after placement. Typically, in a large landfill, different areas of the site will be at different stages simultaneously. The landfill gas production rate will reach a maximum at around 5 years and start to decline. Landfill gas follows first-order kinetic decay after decline begins with a k-value ranging 0.02 yr-1 for arid conditions and 0.065 yr-1 for wet conditions.U.S. Environmental Protection Agency. "Landfill Gas Modeling." LFG Energy Project Development Handbook. 30 January 2009. Web. 26 November 2009. . The Landfill Methane Outreach Program (LMOP) provides first order decay model to aid in the determination of landfill gas production named LandGEM (Landfill Gas Emissions Model). Typically, gas extraction rates from a municipal solid waste (MSW) landfill range from 25 to 10000 m3/h where Landfill sites typically range from 100,000 m3 to 10 million m3 of waste in place. MSW landfill gas typically has roughly 45 to 60% methane and 40 to 60% carbon dioxide, depending on the amount of air introduced to the site, either through active gas extraction or from inadequate sealing (capping) of the landfill site. Depending on the composition of the waste in place, there are many other minor components that comprises roughly 1% which includes , , , ,
non-methane volatile organic compound Non-methane volatile organic compounds (NMVOCs) are a set of organic compounds that are typically photochemically reactive in the atmosphere—marked by the exclusion of methane. NMVOCs include a large variety of chemically different compounds, suc ...
s (NMVOCs),
polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
s (PAHs),
polychlorinated dibenzodioxins Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of long-lived polyhalogenated compound, polyhalogenated organic compounds that are primarily anthropogenic, and contribute toxic, Persistent organic pollutant, persistent org ...
(PCDDs),
polychlorinated dibenzofurans More thorough treatise of all groups with similar actions and binding to aryl hydrocarbon receptor is given in Dioxins and dioxin-like compounds. Polychlorinated dibenzofurans (PCDFs) are a family of organic compounds with one or several of the ...
(PCDFs), etc. All of the aforementioned agents are harmful to human health at high doses.


LFG collection systems

Landfill gas collection is typically accomplished through the installation of wells installed vertically and/or horizontally in the waste mass. Design heuristics for vertical wells call for about one well per acre of landfill surface, whereas horizontal wells are normally spaced about 50 to 200 feet apart on center.U.S. Environmental Protection Agency. "Project Technology Options." LFG Energy Project Development Handbook. 9 September 2009. Web. 26 November 2009. . Efficient gas collection can be accomplished at both open and closed landfills, but closed landfills have systems that are more efficient, owing to greater deployment of collection infrastructure since active filling is not occurring. On average, closed landfills have gas collection systems that capture about 84% of produced gas, compared to about 67% for open landfills. Landfill gas can also be extracted through horizontal trenches instead of vertical wells. Both systems are effective at collecting. Landfill gas is extracted and piped to a main collection header, where it is sent to be treated or flared. The main collection header can be connected to the leachate collection system to collect condensate forming in the pipes. A blower is needed to pull the gas from the collection wells to the collection header and further downstream. A landfill gas collection system with a flare designed for a 600 ft3/min extraction rate is estimated to cost $991,000 (approximately $24,000 per acre) with annual operation and maintenance costs of $166,000 per year at $2,250 per well, $4,500 per flare and $44,500 per year to operate the blower (2008). LMOP provides a software model to predict collection system costs.


Flaring

If gas extraction rates do not warrant direct use or electricity generation and, in order to avoid uncontrolled release to the atmosphere, the gas can be flared off. One hundred m3/h is a practical threshold for flaring in the US. In the U.K, gas engines are used with a capacity of less than 100m3/h. Flares are useful in all landfill gas systems as they can help control excess gas extraction spikes and maintenance down periods. In the U.K and EU enclosed flares, from which the flame is not visible are mandatory at modern landfill sites. Flares can be either open or enclosed, but the latter are typically more expensive as they provide high combustion temperatures and specific residence times as well as limit noise and light pollution. Some US states require the use of enclosed flares over open flares. Higher combustion temperatures and residence times destroy unwanted constituents such as un-burnt hydrocarbons. General accepted values are an exhaust gas temperature of 1000°C with a retention time of 0.3 seconds which is said to result in greater than 98% destruction efficiency. The combustion temperature is an important controlling factor as if greater than 1100ºC, there is a danger of the exponential formation of thermal NOx.


Landfill gas treatment

Landfill gas must be treated to remove impurities, condensate, and particulates. The treatment system depends on the end use. Minimal treatment is needed for the direct use of gas in boiler, furnaces, or kilns. Using the gas in electricity generation typically requires more in-depth treatment. Treatment systems are divided into primary and secondary treatment processing. Primary processing systems remove moisture and particulates. Gas cooling and compression are common in primary processing. Secondary treatment systems employ multiple cleanup processes, physical and chemical, depending on the specifications of the end use. Two constituents that may need to be removed are
siloxanes A siloxane is a functional group in organosilicon chemistry with the Si−O−Si linkage. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H(OSiH2)''n''OH and (OSiH2)n. Siloxanes also include branched compou ...
and
sulfur compounds Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
, which are damaging to equipment and significantly increase maintenance cost. Adsorption and absorption are the most common technologies used in secondary treatment processing.


Use of landfill gas


Direct use


Boiler, dryer, and process heater

Pipelines transmit gas to boilers, dryers, or kilns, where it is used much in the same way as natural gas. Landfill gas is cheaper than natural gas and holds about half the heating value at 16,785 – 20,495 kJ/m3 (450 – 550 Btu/ft3) as compared to 35,406 kJ/m3 (950 Btu/ft3) of natural gas.Bade Shrestha, S.O, G Narayanan, and G Narayanan. "Landfill Gas with Hydrogen Addition a Fuel for SI Engines." Fuel, 87.17/18 (2008): 3616-3626. Boilers, dryers, and kilns are used often because they maximize use of the gas, limited treatment is needed, and the gas can be mixed with other fuels. Boilers use the gas to transform water into steam for use in various applications. For boilers, about 8,000 to 10,000  pounds per hour of steam can be generated for every 1  million metric tons of waste-in-place at the landfill. Most direct use projects use boilers.
General Motors The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
saves $500,000 on energy costs per year at each of the four plants owned by General Motors that has implemented landfill gas boilers.U.S. Environmental Protection Agency. "Adapting Boilers to Utilize Landfill Gas: An Environmentally and Economically Beneficial Opportunity." September 2008. Web. 26 November 2009. Disadvantages of Boilers, dryers, and kilns are that they need to be
retrofitted Retrofitting is the addition of new technology or features to older systems. Retrofits can happen for a number of reasons, for example with big capital expenditures like naval vessels, military equipment or manufacturing plants, businesses or go ...
in order to accept the gas and the end user has to be nearby (within roughly 5  miles) as pipelines will need to be built.


Infrared heaters, greenhouses, artisan studios

In situations with low gas extraction rates, the gas can go to power infrared heaters in buildings local to the landfill, provide heat and power to local greenhouses, and power the energy intensive activities of a studio engaged in pottery, metalworking or glass-blowing. Heat is fairly inexpensive to employ with the use of a boiler. A microturbine would be needed to provide power in low gas extraction rate situations.


Leachate evaporation

The gas coming from the landfill can be used to evaporate
leachate A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed. Leachate is a widely used term in the environmental sciences wher ...
in situations where leachate is fairly expensive to treat. The system to evaporate the leachate costs $300,000 to $500,000 to put in place with operations and maintenance costs of $70,000 to $95,000 per year. A 30,000 gallons per day evaporator costs $.05 - $.06 per gallon. The cost per gallon increases as the evaporator size decreases. A 10,000 gallons per day evaporator costs $.18 - $.20 per gallon. Estimates are in 2007 dollars.


Pipeline-quality gas, CNG, LNG

Landfill gas can be converted to high-Btu gas by reducing its carbon dioxide, nitrogen, and oxygen content. The high-Btu gas can be piped into existing natural gas pipelines or in the form of CNG (
compressed natural gas Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in cy ...
) or LNG (
liquid natural gas Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the vol ...
). CNG and LNG can be used on site to power hauling trucks or equipment or sold commercially. Three commonly used methods to extract the carbon dioxide from the gas are membrane separation, molecular sieve, and amine scrubbing. Oxygen and nitrogen are controlled by the proper design and operation of the landfill since the primary cause for oxygen or nitrogen in the gas is intrusion from outside into the landfill because of a difference in pressure. The high-Btu processing equipment can be expected to cost $2,600 to $4,300 per standard cubic foot per minute (scfm) of landfill gas. Annual costs range from $875,000 to $3.5 million to operate, maintain and provide electricity to. Costs depend on quality of the end product gas as well as the size of the project. The first landfill gas to LNG facility in the United States was the Frank R. Bowerman Landfill in
Orange County, California Orange County is located in the Los Angeles metropolitan area in Southern California. As of the 2020 census, the population was 3,186,989, making it the third-most-populous county in California, the sixth-most-populous in the United States, a ...
. The same process is used for the conversion to CNG, but on a smaller scale. The CNG project at
Puente Hills Landfill Puente Hills Landfill was the largest landfill in the United States, rising high and covering . Originally opened in 1957 in a back canyon in the Puente Hills, the landfill was made to meet the demands of urbanization and waste-disposal east ...
in Los Angeles has realized $1.40 per gallon of gasoline equivalent with the flow rate of 250 scfm. Cost per gallon equivalent reduces as the flow rate of gas increases. LNG can be produced through the liquification of CNG. However, the oxygen content needs to be reduced to be under 0.5% to avoid explosion concerns, the carbon dioxide content must be as close to zero as possible to avoid freezing problems encountered in the production, and nitrogen must be reduced enough to achieve at least 96% methane. A $20 million facility is estimated to achieve $0.65/gallon for a plant producing 15,000 gallons/day of LNG (3,000 scfm). Estimates are in 2007 dollars.


Electricity generation

If the landfill gas extraction rate is large enough, a gas turbine or internal combustion engine could be used to produce electricity to sell commercially or use on site.


Reciprocating piston engine

More than 70 percent of all landfill electricity projects use reciprocating piston (RP) engines, a form of internal combustion engine, because of relatively low cost, high efficiency, and good size match with most landfills. RP engines usually achieve an efficiency of 25 to 35 percent with landfill gas. However, RP engines can be added or removed to follow gas trends. Each engine can achieve 150kW to 3 MW, depending on the gas flow. An RP engine (less than 1 MW) can typically cost $2,300 per kW with annual operation and maintenance costs of $210 per kW. An RP engine (greater than 800 kW) can typically cost $1,700 per kW with annual operation and maintenance costs of $180 per kW. Estimates are in 2010 dollars.


Gas turbine

Gas turbines A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
, another form of internal combustion engine, usually meet an efficiency of 20 to 28 percent at full load with landfill gas. Efficiencies drop when the turbine is operating at partial load. Gas turbines have relatively low maintenance costs and nitrogen oxide emissions when compared to RP engines. Gas turbines require high gas compression, which uses more electricity to compress, therefore reducing the efficiency. Gas turbines are also more resistant to corrosive damage than RP engines. Gas turbines need a minimum of 1,300 cfm and typically exceed 2,100 cfm and can generate 1 to 10 MW. A gas turbine (greater than 3 MW) can typically cost $1,400 per kW with annual operation and maintenance costs of $130 per kW. Estimates are in 2010 dollars.


Microturbine

Microturbines can produce electricity with lower amounts of landfill gas than gas turbines or RP engines. Microturbines can operate between 20 and 200 cfm and emit less nitrogen oxides than RP engines. Also, they can function with less methane content (as little as 35 percent). Microturbines require extensive gas treatment and come in sizes of 30, 70, and 250 kW. A microturbine (less than 1 MW) can typically cost $5,500 per kW with annual operation and maintenance costs of $380 per kW. Estimates are in 2010 dollars.


Fuel cell

Research has been performed indicating that molten carbonate
fuel cells A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
could be fueled by landfill gas. Molten carbonate fuel cells require less purity than typical fuel cells, but still require extensive treatment. The separation of acid gases (HCl, HF, and SO2), VOC oxidation (H2S removal) and siloxane removal are required for molten carbonate fuel cells. Fuel cells are typically run on hydrogen and hydrogen can be produced from landfill gas. Hydrogen used in fuel cells have zero emissions, high efficiency, and low maintenance costs.


Project incentives

Various landfill gas project incentives exist for United States projects at the federal and state level. The Department of the Treasury,
Department of Energy A Ministry of Energy or Department of Energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-rel ...
, Department of Agriculture, and
Department of Commerce The United States Department of Commerce is an executive department of the U.S. federal government concerned with creating the conditions for economic growth and opportunity. Among its tasks are gathering economic and demographic data for bu ...
all provide federal incentives for landfill gas projects. Typically, incentives are in the form of tax credits, bonds, or grants. For example, the Renewable Electricity Production Tax Credit (PTC) gives a corporate tax credit of 1.1 cents per kWh for landfill projects above 150 kW."EPA - LMOP - Funding Guide: Federal Resources." U.S. Environmental Protection Agency. Web. 8 November 2009. . Various states and private foundations give incentives to landfill gas projects. A Renewable Portfolio Standard (RPS) is a legislative requirement for utilities to sell or generate a percentage of their electricity from renewable sources including landfill gas. Some states require all utilities to comply, while others require only public utilities to comply."EPA - LMOP - Funding Guide: State Renewable Portfolio Standards (RPS)." U.S. Environmental Protection Agency. Web. 8 November 2009. < http://www.epa.gov/lmop/res/guide/state_rps.htm>.


Environmental impact

In 2005, 166 million tons of MSW were discarded to landfills in the United States. Roughly 120 kg of methane is generated from every ton of MSW. Methane has a
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time f ...
of 25 times more effective of a greenhouse gas than carbon dioxide on a 100-year time horizon. It is estimated that more than 10% of all global anthropogenic
methane emissions Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...
are from landfills. Landfill gas projects help aid in the reduction of methane emissions. However, landfill gas collection systems do not collect all the gas generated. Around 4 to 10 percent of landfill gas escapes the collection system of a typical landfill with a gas collection system.”Environmental Protection Agency LMOP: Benefits of Energy.” U.S. Environmental Protection Agency. Web. 27 November 2009. . The use of landfill gas is considered a green fuel source because it offsets the use of environmentally damaging fuels such as oil or
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, destroys the heat-trapping gas methane, and the gas is generated by deposits of waste that are already in place. 450 of the 2,300 landfills in the United States have operational landfill gas utilization projects as of 2007. LMOP has estimated that approximately 520 landfills that currently exist could use landfill gas (enough to power 700,000 homes). Landfill gas projects also decrease local pollution, and create jobs, revenues and cost savings. Of the roughly 450 landfill gas projects operational in 2007, 11 billion kWh of electricity was generated and 78 billion cubic feet of gas was supplied to end users. These totals amount to roughly of pine or fir forests or annual emissions from 14,000,000 passenger vehicles.U.S. Environmental Protection Agency. "Fueling the Economy and a Sustainable Energy Future While Improving the Environment." Landfill Gas Energy. December 2008. Web. 26 November 2009.


See also

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Anaerobic digestion Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the ferm ...
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Atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year globa ...
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Biogas Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a ...
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Biodegradation Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
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Cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elect ...
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Landfill gas migration Landfill gas migration is a complex process in which gases produced by waste in a landfill move from the site of original deposition to other places via diffusion, usually from areas of high concentration to low. The process is also affected by t ...
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Landfill gas monitoring Landfill gas monitoring is the process by which gases that are collected or released from landfills are electronically monitored. Landfill gas may be measured as it escapes the landfill ("Surface Monitoring") or may be measured as it is collecte ...
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Waste minimisation Waste minimisation is a set of processes and practices intended to reduce the amount of waste produced. By reducing or eliminating the generation of harmful and persistent wastes, waste minimisation supports efforts to promote a more sustainab ...
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Underground coal gasification Underground coal gasification (UCG) is an industrial process which converts coal into product gas. UCG is an ''in-situ'' gasification process, carried out in non-mined coal seams using injection of oxidants and steam. The product gas is brought t ...


References

{{DEFAULTSORT:Landfill Gas Utilization Waste management concepts Landfill Renewable energy Cogeneration Greenhouse gas emissions Methane