Techniques for the combustion of landfill gas have undergone many changes over the last 15 years.
From the initial pipe-flares of the early 1980s where vertical tubes were simply forced into the surface of a site and the emitting landfill gas lit with a burning oily rag, the technology employed has advanced significantly.

In many ways the combustion of landfill gas may be seen as leading waste-gas flaring technology in other industries. This is largely because of the extremely
difficult nature of landfill gas.

As well as containing significant percentages of carbon dioxide and methane, landfill gas has been found to contain as many as 557 trace components.

Higher standards demanded for the landfilling of wastes, including the need to control emissions rather than just migration, have added impetus for the flaring of landfill gas, where undertaken, to be carried out in an acceptable manner.
It is a publicly stated aim of the Government to reduce greenhouse gas emissions in the United Kingdom. Based upon a 1994 estimate, landfill gas was recognised to account for approximately 20% of total UK methane emissions. The general trend in this contribution however is reducing with the increased

use of enclosed flares, greater landfill gas collection efficiency and an increased number of landfill gas utilisation schemes, particularly at modern

engineered landfills.

Large-scale passive venting of landfill gas is no longer considered an acceptable practice. To achieve pollution control and global atmosphere objectives relative to gas production, each landfill site must achieve the highest sustainable position

The technology involved in the collection, combustion and utilisation of landfill gas in the United Kingdom is at a comparatively advanced stage. In many ways the United Kingdom leads the world in the scale of application of such technologies.

More recently, research has commenced using low calorific burners and reticulation techniques which partition oxygen and carbon dioxide from the


Landfill Gas
  • Is mainly composed of methane and carbon dioxide
  • Has many small amounts of trace elements with potential health effects
  • Is potentially explosive
  • Is a potent greenhouse gas
  • Can be used as a fuel
  • Is legally considered a waste
  • Is an asphyxiant
The purpose of landfill gas flaring is to dispose of the flammable constituents safely and to control odour nuisance, health risks and adverse environmental impacts. Consideration needs to be given to the efficiency of destruction achieved during flaring and, hence, to the environmental impact and possible health risks associated with the combustion products resulting from flaring with systems of differing designs.

Landfill gas utilization

From Wikipedia, the free encyclopedia
Landfill gas utilization is a process of gathering, processing, and treating the gas to produce electricity, heat, fuels, and various chemical compounds. The number of landfill gas projects, which convert the methane gas that is emitted from decomposing garbage into power, went from 399 in 2005 to 519 in 2009 according to the Environmental Protection Agency. These projects are popular because they control energy costs and reduce greenhouse gas emissions. These projects collect the methane gas, which is released with twenty times the global warming potential of carbon dioxide, and treat it, so it can be used for electricity or upgraded to pipeline-grade gas. These projects power homes, buildings, and vehicles.[1]



1 Generation

2 LFG Collection Systems

2.1 Flaring

2.2 Landfill Gas Treatment

3 Use of Landfill Gas

3.1 Direct Use

3.1.1 Boiler, dryer, and process heater

3.1.2 Infrared heaters, Greenhouses, Artisan studios

3.1.3 Leachate Evaporation

3.1.4 Pipeline-quality gas, CNG, LNG

3.2 Electricity Generation

3.2.1 Internal Combustion Engine

3.2.2 Gas Turbine

3.2.3 Microturbine

3.2.4 Fuel Cell

4 Project Incentives

5 Environmental Impact

6 See also

7 References

[edit] Generation

Landfill gas (LFG) is generated through the degradation of municipal solid waste (MSW) by microorganisms. The quality (higher percent methane gases signify higher qualities) of the gas is highly dependent on the composition of the waste, presence of oxygen, temperature, physical geometry and time elapsed since waste disposal.[2] Aerobic conditions, presence of oxygen, leads to predominately CO2 emissions. In anaerobic conditions, as is typical of landfills, methane and CO2 are produced in equal amounts. Methane (CH4) is the important component of landfill gas as it has a calorific value of 33.95 MJ/Nm3 which gives rise to energy generation benefits.[3] 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, 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.[4] The rate of landfill gas production varies with the age of the landfill. Figure 1 shows the common phases that a section of a MSW landfill undergoes after placement towards equilibrium (typically, in a large landfill, different areas of the site will be at different stages simultaneously).
Figure 1: Percent composition of each major component of landfill gas with time.[5]

The landfill gas production rate will reach a maximum at around 5 years and start to decline.[6] 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.[5] 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).[5] 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.[3] MSW landfill gas typically has roughly 45 to 60 % methane and 40 to 60 % carbon dioxide.[7] There are many other minor components that comprises roughly 1% which includes H2S, NOx, SO2, CO, NMVOCs, PAHs, PCDDs, PCDFs, etc. All of the aforementioned agents are harmful to human health at high doses.[3]

[edit] LFG Collection Systems

Landfill gas is gathered from landfills through extraction wells placed depending on the size of the landfill. Roughly one well per acre is typical.[8] A typical gas extraction well is shown in figure 2.
Figure 2: Gas extraction well.[8]

A layout of a landfill gas collection system is shown in figure 3. 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, as shown in figure 3, to collect condensate forming in the pipes.
Figure 3: Gas Landfill gas blower.

A blower is needed to pull the gas from the collection wells to the collection header and further downstream. A 40-acre (160,000 m2) 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).
Figure 4: Landfill gas collection system.[9]

LMOP provides a software model to predict collection system costs.[8]

[edit] Flaring

If gas extraction rates do not warrant direct use or electricity generation, the gas can be flared off. ‎
Figure 5: Open (left) and enclosed (right) flare.[8]

One hundred m³/h is a practical threshold for flaring.[3] Flares are useful in all landfill gas systems as they can help control excess gas extraction spikes and maintenance down periods. Flares can be either open or enclosed. Enclosed flares are typically more expensive, but 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.

[edit] 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 and sulfur compounds, which are damaging to equipment and significantly increase maintenance cost. Adsorption and absorption are the most common technologies used in secondary treatment processing.[8]

[edit] Use of Landfill Gas

The use of landfill gas is divided into electricity generation and direct use. Direct use is the use of the gas for various reasons, usually within 5 miles (8.0 km) of the landfill.

[edit] Direct Use

[edit] 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 that 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.[10] Boilers, dryers, and kilns are used often because they maximize utilization 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.[8]
Figure 6: Boiler retrofitted to accept landfill gas.[8]

Most direct use projects use boilers. General Motors saves $500,000 on energy costs per year at each of the four plants owned by General Motors that has implemented landfill gas boilers.[11] Disadvantages of Boilers, dryers, and kilns are that they need to be retrofitted 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.

[edit] 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.[8]

[edit] Leachate Evaporation

The gas coming from the landfill can be used to evaporate leachate in situations where leachate is fairly expensive to treat. Figure 7 shows the overall system to evaporate the leachate. ‎
Figure 7: Leachate evaporation system.[8]

The system 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.[8] Estimates are in 2007 dollars.

[edit] 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) or LNG (liquid natural gas). 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.[8] 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 a the Frank R. Bowerman Landfill in Orange County, California. The same process is used for the conversion to CNG, but on a smaller scale. The CNG project at Puente Hills Landfill in Los Angeles has realized $1.40 per gallon of gasoline equivalent with the flow rate of 250 scfm.[8] Cost per gallon equivalent reduces as the flow rate of gas increases. LNG can be produced through the liquidfication 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 dollar facility is estimated to achieve $0.65/gallon for a plant producing 15,000 gallons/day of LNG (3,000 scfm).[8] Estimates are in 2007 dollars.

[edit] 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.

[edit] Internal Combustion Engine

More than 70 percent of all landfill electricity projects use internal combustion (IC) engines because of relatively low cost, high efficiency, and good size match with most landfills.ch3 IC engines (shown in figure 9) usually meet an efficiency of 25 to 35 percent with landfill gas. ‎
Figure 9: IC engines.[8]

IC engines have relatively high maintenance costs and air emissions when compared to gas turbines. Each IC engine requires 300 to 1100 cubic feet per minute (cfm) to operate. However, IC engines can be added or removed to follow gas trends. Each engine can achieve 800 kW to 3 MW depending on the gas flow. An IC engine (less than 1 MW) can typically cost $2,300 per kW with annual operation and maintenance costs of $210 per kW. An IC engine (greater than 800 kW) can typically cost $1,700 per kW with annual operation and maintenance costs of $180 per kW.[8] Estimates are in 2010 dollars.

[edit] Gas Turbine

Gas turbines (shown in figure 10) usually meet an efficiency of 20 to 28 percent at full load with landfill gas. ‎
Figure 10: Gas turbines.[8]

Efficiencies drop when the turbine is operating at partial load. Gas turbines have relatively low maintenance costs and nitrogen oxide emissions when compared to IC 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 IC 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.[8] Estimates are in 2010 dollars.

[edit] Microturbine

Microturbines (shown in figure 11) can produce electricity with lower amounts of landfill gas than gas turbines or IC engines.
Figure 11: Microturbine.[8]

Microturbines can operate between 20 and 200 cfm and emit less nitrogen oxides than IC 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.[8] Estimates are in 2010 dollars.

[edit] Fuel Cell

Research has been performed indicating that molten carbonate fuel cells 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.[12] Fuel cells are typically run on hydrogen and hydrogen can be produced from landfill gas. Hydrogen utilized in fuel cells have zero emissions, high efficiency, and low maintenance costs.[10]

[edit] 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, Department of Agriculture, and Department of Commerce 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.[13] Various states and private foundations give incentives to landfill gas projects. Figure 12 shows the states with various state or private incentives. ‎
Figure 12: States with state or private incentives.[14]

Figure 13 shows the states with RPS (Renewable Portfolio Standard). ‎
Figure 13: States with RPS.[15]

A 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.[15]

[edit] Environmental Impact

In 2005, 166 million tons of MSW were discarded to landfills in the United States.[16] Roughly 120 kg of methane is generated from every ton of MSW.7 Methane has a global warming potential of 23 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 are from landfills.[17] Landfill gas projects help aid in the prevention on methane emissions. However, landfill gas collection systems do not collect all the gas generated. 4 to 10 percent of landfill gas escapes the collection system of a typical landfill with a gas collection system.[18] 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, destroys the heat-trapping gas methane, and the gas is generated “naturally” by already in place deposits of waste. 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 utilize landfill gas (enough to power 700,000 homes). Landfill gas projects also decrease local pollution, and create jobs, revenues and cost savings.[18] 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 17,500,000 acres (71,000 km2) of pine or fir forests or annual emissions from 14,000,000 passenger vehicles.[19] Figure 14 shows the current landfill gas projects in the United States as well as the landfills that could utilize a landfill gas project.
Figure 14: Current landfill gas projects in the United States and landfills that could utilize a landfill gas project.[20]

[edit] See also
[edit] References
<ol><li><b><b><a href="" target="_self">^</a></b> Koch, Wendy (2010-02-25). <a href="" target="_self">"Landfill Projects on the rise"</a>. USA Today. Retrieved 2010-04-25.</b></li><li><b><b><a href="" target="_self">^</a></b> <a href="" target="_self">DoE Report CWM039A+B/92</a> Young, A. (1992)</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> <b><i><sup><a href="" target="_self">c</a></sup></i></b> <b><i><sup><a href="" target="_self">d</a></sup></i></b> Scottish Environment Protection Agency. Guidance on Landfill Gas Flaring. Nov. 2002. Web. <<a href="" target="_self"></a>>.</li><li><b><b><a href="" target="_self">^</a></b> Staley, Bryan, Morton Barlaz, and Morton Barlaz. "Composition of Municipal Solid Waste in the United States and Implications for Carbon Sequestration and Methane Yield." Journal of Environmental Engineering, 135.10 (2009): 901-909.</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> <b><i><sup><a href="" target="_self">c</a></sup></i></b> U.S. Environmental Protection Agency. "Landfill Gas Modeling." LFG Energy Project Development Handbook. 30 Jan. 2009. Web. 26 Nov. 2009. <<a href="" target="_self"></a>>.</li><li><b><b><a href="" target="_self">^</a></b> Whittington, H. "Electricity Generation: Options for Reduction in Carbon Emissions." , 360.1797 (2002): 1653-1668. .</b></li><li><b><b><a href="" target="_self">^</a></b> U.S. Environmental Protection Agency. "Landfill Gas Energy Basics." LFG Energy Project Development Handbook. 16 Feb. 2009. Web. 26 Nov. 2009. <<a href="" target="_self"></a>>.</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> <b><i><sup><a href="" target="_self">c</a></sup></i></b> <b><i><sup><a href="" target="_self">d</a></sup></i></b> <b><i><sup><a href="" target="_self">e</a></sup></i></b> <b><i><sup><a href="" target="_self">f</a></sup></i></b> <b><i><sup><a href="" target="_self">g</a></sup></i></b> <b><i><sup><a href="" target="_self">h</a></sup></i></b> <b><i><sup><a href="" target="_self">i</a></sup></i></b> <b><i><sup><a href="" target="_self">j</a></sup></i></b> <b><i><sup><a href="" target="_self">k</a></sup></i></b> <b><i><sup><a href="" target="_self">l</a></sup></i></b> <b><i><sup><a href="" target="_self">m</a></sup></i></b> <b><i><sup><a href="" target="_self">n</a></sup></i></b> <b><i><sup><a href="" target="_self">o</a></sup></i></b> <b><i><sup><a href="" target="_self">p</a></sup></i></b> <b><i><sup><a href="" target="_self">q</a></sup></i></b> <b><i><sup><a href="" target="_self">r</a></sup></i></b> <b><i><sup><a href="" target="_self">s</a></sup></i></b> U.S. Environmental Protection Agency. "Project Technology Options." LFG Energy Project Development Handbook. 9 Sept. 2009. Web. 26 Nov. 2009. <<a href="" target="_self"></a>>.</li><li><b><b><a href="" target="_self">^</a></b> U.S. Environmental Protection Agency. "An Overview of Landfill Gas Energy in the United States." Landfill Methane Outreach Program, June 2009. Web. 26 Nov. 2009.</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> 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.</li><li><b><b><a href="" target="_self">^</a></b> U.S. Environmental Protection Agency. "Adapting Boilers to Utilize Landfill Gas: An Environmentally and Economically Beneficial Opportunity." Sept. 2008. Web. 26 Nov. 2009.</b></li><li><b><b><a href="" target="_self">^</a></b> Urban, W, H Lohmann, J.I. Salazar Gomez, H Lohmann, and J.I. Salazar Gomez. "Catalytically Upgraded Landfill Gas as a Cost-effective Alternative for Fuel Cells." Journal of Power Sources, 193.1 (2009): 359-366.</b></li><li><b><b><a href="" target="_self">^</a></b> "EPA - LMOP - Funding Guide: Federal Resources." U.S. Environmental Protection Agency. Web. 08 Nov. 2009. <<a href="" target="_self"></a>>.</b></li><li><b><b><a href="" target="_self">^</a></b> "EPA - LMOP - Funding Guide." U.S. Environmental Protection Agency. Web. 08 Nov. 2009. <<a href="" target="_self"></a>>.</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> "EPA - LMOP - Funding Guide: State Renewable Portfolio Standards (RPS)." U.S. Environmental Protection Agency. Web. 08 Nov. 2009. < <a href="" target="_self"></a>>.</li><li><b><b><a href="" target="_self">^</a></b> KAPLAN, P. OZGE, JOSEPH DECAROLIS, SUSAN THORNELOE, JOSEPH DECAROLIS, and SUSAN THORNELOE. "Is It Better to Burn or Bury Waste for Clean Electricity Generation?." Environmental Science & Technology, 43.6 (2009): 1711-1717.</b></li><li><b><b><a href="" target="_self">^</a></b> Lohila, Annalea, Tuomas Laurila, Juha-Pekka Tuovinen, Mika Aurela, Juha Hatakka, Tea Thum, Mari Pihlatie, Janne Rinne, Timo Vesala, Tuomas Laurila, Juha-Pekka Tuovinen, Mika Aurela, Juha Hatakka, Tea Thum, Mari Pihlatie, Janne Rinne, and Timo Vesala. "Micrometeorological Measurements of Methane and Carbon Dioxide Fluxes at a Municipal Landfill." Environmental Science & Technology, 41.8 (2007): 2717-2722.</b></li><li>^ <b><i><sup><a href="" target="_self">a</a></sup></i></b> <b><i><sup><a href="" target="_self">b</a></sup></i></b> ”Environmental Protection Agency LMOP: Benefits of Energy.” U.S. Environmental Protection Agency. Web. 27 Nov. 2009. <<a href="" target="_self"></a>>.</li><li><b><b><a href="" target="_self">^</a></b> U.S. Environmental Protection Agency. "Fueling the Economy and a Sustainable Energy Future While Improving the Environment." Landfill Gas Energy. Dec. 2008. Web. 26 Nov. 2009.</b></li><li><b><b><a href="" target="_self">^</a></b> "Biomass Combined Heat and Power Catalog of Technologies." U.S. Environmental Protection Agency. Sept. 2007. Web. 26 Nov. 2009.</b></li></ol>