Landfill gas

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Compressor station for landfill gas with flare (right)
Gas well with filter sheets

Landfill gas is mainly produced in landfills by the bacteriological and chemical breakdown of organic substances in the waste . It consists mainly of methane (CH 4 ) and carbon dioxide (CO 2 ).

Emergence

Landfill gas is created by biochemical degradation processes of organic compounds and materials in the waste body. The processes are divided into aerobic and anaerobic degradation processes, which can be divided into successive phases at the beginning and which run simultaneously towards the end of the processes.

  • Aerobic phase: In these reactions, the stored atmospheric oxygen is used up and water, nitrogen (N 2 ), carbon dioxide (CO 2 ) and residual products of higher molecular weight are formed. In the case of loose garbage or a mixture of construction rubble and household waste , a constant supply of oxygen can take place in the edge zones, so that the aerobic processes run stably for a long time.
  • Anaerobic non-methane phase : In this phase, known as “acid fermentation”, bacteria become active that manage with little or no oxygen and mainly produce nitrogen, hydrogen, carbon dioxide and lower fatty acids . The carbon dioxide content can rise to 80% by volume. The pH value decreases in this phase up to 5.5.
  • Anaerobic unstable methane phase: The conditions (pH value, temperature) in the landfill stabilize. There are methanogenic microorganisms active.
  • Anaerobic stable methane phase: Under anaerobic conditions, the organic components are broken down into methane (CH 4 ) and carbon dioxide (CO 2 ). The pH rises to 8.5. The result of the biochemical degradation processes is a water-saturated gas that essentially consists of 50–70% by volume methane and 30–50% by volume carbon dioxide. This gas mixture is called the actual landfill gas .
  • Decaying methane phase: The methane formation takes place only at a low level, and even when not technically degassed landfills entry of air into the landfill body starts because the gas flow is low over the surface to be in order z. B. to constantly maintain an outflowing gas flow when the air pressure changes. The process intensifies as the gas production continues to decrease, resulting in constant air infiltration due to pressure fluctuations and diffusion.

As a result of these reactions, approx. 100-200 m³ of landfill gas with a methane content of around 55% by volume will be generated from one ton of household waste over the course of 15-20 years. The composition of the gas changes over time: the stable methane phase is usually reached inside the waste body just one year after the waste has been stored. In the stable methane phase, the landfill gas essentially consists of 60% by volume CH 4 and 40% by volume CO 2 . The ratio of the two components is then 1.5: 1. With increasing degradation of the organic waste components, this ratio shifts to values ​​significantly greater than 2: 1. The change in the gas composition as a function of the biochemical waste age enables a statement to be made about the degree of degradation of the gas-forming waste components and thus about the duration and amount of future landfill gas formation.

Chemical composition

The chemical composition of the gases depends to a large extent on the type of material deposited, the type of bedding and the age of the landfill. All of the data listed below relate to a typical household waste dump.

At the beginning - in the first eight weeks - the gas contains even higher levels of nitrogen and oxygen from the air introduced. The oxygen content drops very quickly from around 20% to practically 0% within the first 2 to 3 weeks after the landfill has been closed and sealed. The initial nitrogen content of 80% drops to 40% after about eight weeks and continues to drop to almost 0% within the first two years. At the beginning there is hardly any carbon dioxide or methane. While the carbon dioxide content increases sharply within a few weeks after the landfill has been sealed, methane is only formed after the anaerobic phase begins after a few months and then rises quickly to up to 60%. The mean composition of the gas is relatively stable after about two years for a period of 20 to 25 years and more and essentially contains roughly:

  • 50 vol% methane (CH 4 )
  • 40% by volume carbon dioxide (CO 2 )
  • 0-4 vol .-% nitrogen (N 2 )
  • 5–7% by volume water (H 2 O)
  • 20 ppm hydrogen sulfide (H 2 S)
  • 30 ppm thiols (mercaptans) (R SH)

In addition to these main components, from the second month with the start of the aerobic phase, hydrogen up to max. 20% included. Furthermore, traces of many organic, partly toxic compounds can be detected.

Amount of gas produced

The amount of gas produced depends on the

  • Material type
  • Storage quantity
  • Storage time
  • Water balance in the landfill
  • Climate (outside temperature, air pressure, wind direction and speed)
  • Landfill type / embankment (e.g. dump, pit, slope landfill)

The half-life of degradable, carbonaceous material is around six years after storage, and around three years for sewage sludge. The amount of gas produced declines rapidly after six years, unless the conditions are particularly favorable (optimal water and temperature balance). Then only the safety for people, animals and facilities is in the foreground. After approx. 20 years, the potential risk is considerably lower. After that, only the protection of the climate, small animals and plants is in the foreground.

Landfills are ranked sixth in the world for the most climate-damaging methane producers. In the case of increased gas production, active degassing (see below) should therefore be considered, provided there is no risk of fire or explosion. The assessment of a life cycle assessment on the basis of climate-endangered emitters (freely flowing methane and carbon dioxide) during the operation and maintenance of the plant can serve as an approach.

The amount of landfill gas that is generated in Germany can be estimated at around 2.5 billion  Nm³ per year or around 285,000 Nm³ per hour. Of this, about 1.5 billion Nm³ / a are methane and about 1.0 billion Nm³ / a are carbon dioxide (170,000 Nm³ / h CH 4 and 115,000 Nm³ / h CO 2 ). The thermal energy content of this amount of methane is around 15,000  GWh per year (corresponding to around 1.3 million tons of oil) or 1,700 MW of continuous output. Both gases, CH 4 and CO 2 added together, have a greenhouse effect of around 16 billion Nm³ / a CO 2 equivalent.

Hazards from landfill gas

Risk of fire or explosion

Explosion triangle of the gas mixture methane / air (oxygen content) / inert gas (nitrogen or CO 2 )
Landfill gas warning signs

Methane, the main component of landfill gas, can form an ignitable gas mixture with atmospheric oxygen. For this purpose, methane must be present in the mixture in concentrations between 4.4 and 16.5 % by volume and an oxygen content of at least 12% by volume.

The explosive properties of landfill gas can be described using the safety-related characteristics of methane. On the other hand, the non-flammable components in the landfill gas (especially CO 2 ) have an inertizing effect and reduce the risk of explosion.

Smoking, naked flames and fire are not permitted in unprotected and monitored rooms on the landfill. In buildings, rooms, shafts or other facilities on the landfill in which gas development is possible, strict care must be taken to refrain from smoking, naked flames and fire or other actions that could trigger an explosion or fire. Compliance with accident prevention regulations (GUV-R 127, formerly GUV 17.4) and corresponding operating instructions must be observed.

On December 1, 1990, there was a fire in the landfill gas power plant at the district waste disposal site in Beselich, Hesse ( Limburg-Weilburg district ). This DM 9.5 million plant from Main-Kraftwerke AG (MKW) was commissioned on July 3, 1989 as a pilot project with a net output of 1200 kW and a planned annual net feed-in of 6.6 million kWh and was one of the first of this kind in Germany. The fire was extinguished after a short time by the local volunteer fire brigade Beselich-Obertiefenbach . However, the amount of damage was 1.9 million DM.

Choking hazard

Carbon dioxide in concentrations> 9% by volume leads to asphyxiation within a few minutes. There is a risk of suffocation especially in deep pits or shafts into which landfill gas flows. There is a further risk of suffocation from the lack of oxygen in the air mixed with landfill gas.

Toxic hazard potential

Landfill gas contains a large number of trace substances which can be harmful to health even in low concentrations. These are mainly LHKW , BTEX and sulfur compounds . The strong dilution of the landfill gas (> 10,000-fold) when it escapes above the landfill surface leads to a reduction in the concentration of pollutants in the surrounding air.

Representing many of the above-mentioned pollutants, here is information on the danger of H 2 S and C 2 H 3 Cl ( vinyl chloride , chloroethene):

Climate effects

Methane is the second largest contributor to the anthropogenic greenhouse effect after carbon dioxide . The climate impact of one kilogram of methane over a period of 100 years is 21 times as strong as that of one kilogram of carbon dioxide. Landfills, along with power generation, livestock and rice cultivation, are among the largest human-made sources of methane in the world. The collection and generation of electricity from landfill gas therefore has considerable potential to counteract climate change by means of waste management.

However, it should also be borne in mind that hydrocarbons such as methane are slowly oxidized in an oxygen-containing atmosphere, especially by hydroxyl radicals . A methane molecule once it has entered the atmosphere has an average residence time of twelve years. This effect has already been taken into account when calculating the relative global warming potential of 21 for methane.

Ground elevation

In the case of landfills that are not actively ventilated (see below), the cover can noticeably lift with higher gas production. If no countermeasures are taken, this will eventually lead to the cover tearing open and the abrupt discharge of large amounts of gas, which can be fatal for living beings in the vicinity. Even before the final breakthrough, larger amounts of gas can escape through cracks and crevices and endanger the immediate vicinity.

The process itself takes place relatively slowly, usually a small hill gradually forms over the years. Until the 1980s, this was occasionally observed in smaller landfills, where gas production had been underestimated.

Hazards depending on methane emissions

  • Emissions below 100 ppm: There are currently no binding values ​​for permissible emissions. Long-term studies at several landfills where the emission values ​​were observed showed that practically no damage to vegetation occurs on areas with mean values ​​below 100 ppm methane. Odors (H 2 S) are imperceptible. Area emissions of around 100 ppm should not pose a source of danger to insects or other small organisms or to plants.
  • Emissions of 100 to 500 ppm can have a damaging effect on certain vegetation and soil organisms. Deep-rooted plants can die due to the lack of soil oxygen.
  • Emissions from 500 to 2000 ppm can be a source of danger. The soil air oxygen is displaced by the outflowing gases. Vegetation with a high demand for atmospheric oxygen can fail. Gases leave the landfill surface diluted. The smell accompanying the gas can be perceived.
  • Emissions of more than 2000 ppm: The landfill gas is a source of danger. The oxygen is displaced from the ground by the gases escaping. The vegetation is falling out, some flammable gases leave the landfill surface less diluted. Under certain weather conditions (e.g. inversion position) gas can accumulate near the ground. This increases the risk of fire if it is ignited from the outside.
  • Emissions of more than 5000 ppm: Explosive mixtures can form in pits, crevices, crevices, basements or other lower-lying rooms and represent a hazard when entering or working in these areas.

Recovery and treatment of landfill gas

The following forms of recovery and treatment of landfill gas are used:

  • Treatment of the landfill gas with membrane technology. The raw gas is separated into a product gas with> 90% methane and <4% carbon dioxide and a permeate with <15% methane and> 80% carbon dioxide. The product gas is used as an energy carrier. The permeate is usually flared. Most of the hydrogen sulphide is also eliminated.
  • Incineration in flares : Lean and good gas disposal 25–45% by volume methane> 25 m³ / h; Minimum requirement for safety and environmental reasons
  • Combustion in engines, heating, ovens: Good gas disposal Utilization> 45 vol .-% methane> 100 m³ / h. A good gas utilization can be calculated with heat utilization in the case of disposal via engines from 120 kW electrical output.
  • Utilization with micro gas turbines> 30 vol .-% methane; expensive to invest, but low-maintenance and flexible
  • catalytic and non-catalytic burns: averting danger, hazardous lean gas <25% by volume methane, <25 m³ / h; only makes sense if the gas quality is still in the Ex area and the amount of gas is low
  • Methane oxidation via bio-filters, bio-windows etc .: averting danger, hazardous low gas <25 vol .-% methane, <25 m³ / h; Inexpensive investment and operating costs, effect disputed.
  • Aerobic conversion of biomass : Can be used in old landfills with declining methane production. Costly procedures for the construction and operation of the plants. May be useful in individual cases. These procedures are still fully in the testing phase (as of 2013). Can only be used to a very limited extent (e.g. due to the dump height). A conversion of almost 100% is not guaranteed, a later methane production is therefore possible again. Very time-consuming and expensive procedures, which could, however, pay off if the use is changed. Previous publications are not yet convincing.

combustion

Landfill gas flare with compressor and control

When burning landfill gas, 1/3 of the methane is converted into CO 2 and 2/3 into H 2 O under optimal conditions . The greenhouse effect is reduced , assuming 60% methane and 40% carbon dioxide, from 6.4 Nm³ CO 2 equivalent per Nm³ landfill gas to 0.6 Nm³ / Nm³ and the ozone destruction effect is even reduced to zero. Based on investigations into trace substances in landfill gas, it can be assumed, based on current knowledge, that in addition to inorganic trace gases such as hydrogen sulfide (H 2 S), which is typical in a concentration of 20 to 500 mg / Nm³, ammonia (NH 3 ), hydrogen (H 2 ) and nitrogen oxides (NO x ), around 500 different organic hydrocarbons , including halogenated hydrocarbons , are present in the landfill gas.

The majority of the trace substances identified so far can be described as toxic, carcinogenic or, in the broadest sense, harmful to health. The sum of the organic hydrocarbon compounds is typically between 500 and 1,500 mg / Nm³ and the sum of the halogenated hydrocarbons between 10 and 250 mg / Nm³. Exceptions with extremely higher concentrations can occur locally and temporarily. With an assumed mean concentration of 800 mg / Nm³ organic hydrocarbons and 50 mg / Nm³ halogenated hydrocarbons, the above-mentioned results. Amounts of landfill gas the following material flows that are released from landfills in Germany per year:

  • about 3,300 tons of organic hydrocarbons and thereof
  • about 200 tons of halogenated hydrocarbons.

For these substances, too, thermal destruction during incineration means a significant reduction in environmental pollution, which, however, can create new undesirable compounds (see below).

environmental Protection

In summary, it can be stated that, for environmental reasons, uncontrolled degassing of landfills into the atmosphere should not take place, and that at least one thermal treatment of the gas is necessary to minimize the negative effects. Here, it must be considered whether the expenditure on electrical energy, energy expenditure for maintenance, energy expenditure in material production in a balance (measured in terms of CO 2 emissions) results in a saving. In addition, the cost must be considered. It should be noted, however, that new undesired compounds can also arise during combustion. Only carbon monoxide (CO), nitrogen oxides (NO x ), hydrochloric and hydrofluoric acid (HCl and HF) are mentioned here. However, these substances can be minimized or even prevented by suitable combustion processes , processes of flue gas cleaning or gas pre-cleaning.

As an alternative to combustion, passive so-called methane oxidation windows can be used sensibly.

Long-term studies on landfill gas incinerators have shown that the formation of dioxins in the flue gas could not be detected.

Landfills are the 6th most methane producer worldwide. In the first place are cattle (approx. 400 l / d due to rumination ) and rice fields ( digester gas ).

By using the energy by burning landfill gas, other sources of emissions can be reduced and, in addition, all the gaseous emissions from a household waste landfill can be almost neutralized.

Web links

Commons : Landfill gas  - collection of images, videos and audio files

Individual evidence

  1. ^ Roland A. Simonet: Energy generation from landfills . In: gas-water-sewage . Vol. 65, No. 4 , 1985, pp. 185 .
  2. ^ Roland A. Simonet: Energy generation from landfills . In: gas-water-sewage . Vol. 65, No. 4 , 1985, pp. 185-188 .
  3. ↑ GUV rule 127: landfills. (PDF; 0.5 MB) (No longer available online.) Federal Association of Accident Insurance Funds, February 2001, archived from the original on February 2, 2016 ; accessed on August 22, 2015 .
  4. ^ Franz-Josef Sehr : Fire in the landfill gas power plant Beselich . In: Florian Hessen 9/1991 . Munkelt Verlag, Wiesbaden 1991, p. 26-28 . ISSN 0936-5370 .  
  5. ↑ Piers Forster, Venkatachalam Ramaswamy, Paulo Artaxo, Terje Berntsen, Richard Betts, David W. Fahey, James Haywood, Judith Lean, David C. Lowe, Gunnar Myhre, John Nganga, Ronald Prinn, Graciela Raga, Michael Schulz, Robert Van Dorland : Changes in Atmospheric Constituents and in Radiative Forcing . In: Susan Solomon, Dahe Qin, Martin Manning, Zhenlin Chen, Melinda Marquis, Kristen B. Averyt, Melinda Tignor, Henry L. Miller (eds.): Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change . Cambridge University Press, Cambridge / New York 2007, Chapter 2, Table 2.14 ( ipcc.ch (PDF; 8.0 MB)).
  6. ↑ Alison Smith, Keith Brown, Steve Ogilvie, Kathryn Rushton, Judith Baites: Waste Management Options and Climate Change . Final report to the European Commission, DG Environment. July 2001 ( PDF; 1.0 MB ).
  7. ↑ Piers Forster, Venkatachalam Ramaswamy, Paulo Artaxo, Terje Berntsen, Richard Betts, David W. Fahey, James Haywood, Judith Lean, David C. Lowe, Gunnar Myhre, John Nganga, Ronald Prinn, Graciela Raga, Michael Schulz, Robert Van Dorland : Changes in Atmospheric Constituents and in Radiative Forcing . In: Susan Solomon, Dahe Qin, Martin Manning, Zhenlin Chen, Melinda Marquis, Kristen B. Averyt, Melinda Tignor, Henry L. Miller (eds.): Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change . Cambridge University Press, Cambridge / New York 2007, pp. 212 ( ipcc.ch (PDF; 8.0 MB)).
  8. ^ Roland A. Simonet: Energy generation from landfills . In: gas-water-sewage . Vol. 65, No. 4 , 1985, pp. 187 .