Landfill gas

Landfill gas is generated in landfills mainly by the bacteriological and chemical degradation of organic substances contained in the waste. It consists of the main constituents methane (CH4) and carbon dioxide ( CO2).

• 4.4.1 Criticality of H2S and C2H3Cl ( chloroethene, VC ) for humans

Formation of landfill gas production

Landfill gas is caused 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 beginning in temporally sequential phases and running simultaneously at the end of the processes.

• Aerobic phase: In these reactions, the embedded oxygen residual products is depleted and water, nitrogen (N2 ), carbon dioxide (CO2) and higher molecular weight formed. In loose garbage pile or a mixture of rubble and domestic waste can take place in the peripheral zones of constant topped up oxygen so that aerobic processes for a long time to run stable.
• Anaerobic non-methane phase: In these so-called " acid fermentation " phase bacteria become active, get along with little or no oxygen and produce primarily nitrogen, hydrogen, carbon dioxide and lower fatty acids. The carbon dioxide content can increase up to 80 % by volume. The pH value drops during this phase to 5.5.
• Anaerobic methane stable phase: Under anaerobic conditions, the organic components to methane (CH4) and carbon dioxide are (CO2) removed. The pH rises to 8.5. The result of biochemical degradation processes is a water- saturated gas, consisting of 50-70 % by volume of methane and 30 substantially - is 50% by volume carbon dioxide. This gas mixture is referred to as 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 starts into the landfill body, because the gas flow over the surface is too low to constantly maintain an effluent gas stream, for example, at atmospheric pressure changes. The process increases with further decreasing gas production at a constant air inlet by pressure fluctuations and diffusion.

As a result of these reactions occur in the course of 15-20 years from a ton of household waste approximately 100 - 200 cubic meters of landfill gas with a methane content of 55 % by volume. After one year storage of the waste is achieved, the stable methane phase is generally in the interior of the waste body: In this case, changes over time, the composition of the gas. The landfill gas continues in the stable methane phase consisting essentially of 60 vol % CH4 and 40 vol % CO2 together. 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 gas composition as a function of biochemical waste age is an indicator of the degree of degradation of the gas-forming waste components and thus on the timing and amount of future landfill gas formation.

Chemical composition and amount of gas produced

The chemical composition of the gases depends greatly on the nature of the deposited material, the nature of the bed, and the age of the landfill. All data listed below relate to a typical municipal solid waste landfill.

At the beginning - in the first eight weeks - the gas contains even higher levels of nitrogen and oxygen from the air registered. The content of oxygen falls rapidly from about 20 % to virtually 0 % within the first 2-3 weeks after the closing and sealing of the landfill. The initial nitrogen content of 80% is reduced by about eight weeks at 40 % and continues to decrease within the first two years of almost 0%. At the beginning hardly carbon dioxide and methane are not included. While the content of carbon dioxide rises sharply immediately after sealing of the landfill within a few weeks, methane is formed only with the beginning of the anaerobic phase after a few months and then rapidly increases to 60%. The average composition of the gas after about two years of relatively stable for a period of 20 to 25 years or more and contains mainly coarse:

• 50 vol - % methane ( CH4)
• 40 vol - % carbon dioxide ( CO2)
• 4-0 vol - % nitrogen ( N2)
• 5-7 vol - % water ( H2O)
• 20 ppm hydrogen sulfide ( H2S)
• 30 ppm thiols ( mercaptans ) (R · SH)

In addition to these main components is from the second month with the beginning of the aerobic phase for about two years up to a maximum hydrogen. 20 % contained. Furthermore, traces of many organic, partly toxic compounds are detectable.

The amount of gas produced depends on the

• Material
• Warehousing amount
• Storage time
• Water balance in the landfill
• Climate ( temperature, atmospheric pressure, wind direction and speed)
• Deponieart / fill ( eg dumps, pits, hillside landfill )

The half-life of degradable carbonaceous material is about six years after emplacement, when sewage sludge is about three years. The amount of gas produced is - if not particularly favorable conditions (optimal water and temperature budget ) are present - after six years of decline sharply. Then only the safety for humans, animals and equipment in the foreground. After about 20 years, the hazard potential is considerably lower. Then only is the climate, small animal and plant protection in the foreground.

Landfills worldwide are in sixth place in the ranking of klimabeeinträchtigendsten methane producers. With increased gas production should therefore have a Aktiventgasung ( see below) be given, provided that no hazard exists due to fire or explosion hazard. The estimation of an LCA on the basis of climate- vulnerable issuers ( free effluent methane and carbon dioxide ) for operation and maintenance of this system can serve as an approach.

The amount of landfill gas that is produced in Germany can be estimated at about 2.5 billion Nm3 per year, or about 285,000 Nm3 per hour. Of these, about 1.5 billion Nm3 / a methane and about 1 billion Nm3 / a carbon dioxide ( 170,000 Nm3 / h CH4 and 115,000 Nm3 / h CO2). The thermal energy content of the methane amount is about 15,000 GWh per year (equivalent to about 1.3 million tons of oil ) or 1,700 MW. Both gases, CH4 and CO2 added together, causing a greenhouse effect of about 16 billion m3 / a of CO2 equivalent.

Pollutant loads and potential hazards

Hazards of landfill gas

Fire or explosion

Methane as the main constituent of landfill gas can combine with oxygen in the air, an explosive gas mixture. To do this in the mixture of methane in concentrations from 4.4 to 16.5 vol - % and an oxygen content of at least 12 vol - % be available. The explosion characteristics of landfill gas can be described by means of the safety-related characteristic data of methane conservative (see note).

Safety characteristics of methane

Note: The non-combustible components in landfill gas (mainly CO2) have an inerting influence and cause a narrowing of the explosion range.

Smoking, open flames shall not be permitted in non-protected and monitored facilities at the landfill. In buildings, rooms, shafts or other facilities at the landfill where gas development is possible, care should be taken strictly that smoking, open flames or other actions that could cause an explosion or fire, be omitted. Compliance with the accident prevention regulations (GUV -R 127, formerly GUV 17.4) and corresponding operating instructions must be strictly observed.

Suffocation

Carbon dioxide in concentrations of> 9 % by volume of leads within a few minutes to suffocation. The risk of choking, especially in deep pits or shafts, flows into the landfill gas. More choking hazard arises there from lack of oxygen in the mixed with landfill gas air.

Dangers of CO2 for humans

Toxic hazard potential

In landfill gas a variety of trace substances is included, which can already be harmful in low concentrations. This is mainly to CHC, BTEX and sulfur compounds. The strong dilution of the landfill gas ( > 10,000 - fold) in the outlet on the landfill surface leads to a reduction in pollutant concentrations in the air of the environment.

Hazards of H2S and C2H3Cl ( chloroethene, VC ) for humans

Climate effects

Methane contributes carbon dioxide after the second worst in the anthropogenic greenhouse effect. The climate impact of one kilogram of methane is considered a period of 100 years, 21 times as strong as the one kilogram of carbon dioxide. Landfills are among the world, together with energy production, animal husbandry and the cultivation of rice, the largest man-made sources of methane. In the collection and generation of electricity from landfill gas, therefore considerable potential is seen to counter climate change through waste management.

However, it is well to remember that hydrocarbons such as methane are slowly, especially by hydroxyl radicals, oxidized in an oxygen- containing atmosphere. Once a product comes into the atmosphere methane molecule has an average residence there of twelve years. This effect is already taken into account in the calculation of the relative global warming potential of 21 for methane.

Land uplift

In landfills, which are not actively vented (see below), the cover can lift significantly at higher gas production. If no action is taken, this eventually leads to tearing open the cover and the abrupt outflow of large amounts of gas, which can fatally affect organisms in the vicinity. Even larger amounts of gas may before final breakthrough leak through cracks and crevices and endanger the immediate environment.

The process itself is relatively slow, usually forms over the years gradually a small hill. This was occasionally observed at smaller landfills until the 1980s, where you had underestimated the gas production.

Other general risks

Forms of active landfill gas

The following forms of active landfill gas are applied:

• Treatment of landfill gas with membrane technology. Here, the raw gas 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 energy sources. The permeate is usually flared. It also largely the entire hydrogen sulfide is removed with.
• Combustion in flares: Low and Gutgasentsorgung 25 - 45 vol methane > 25 m³ / h; Minimum requirements for safety and environmental reasons,
• Combustion in engines, heating, stoves: Gutgasentsorgung recovery > 45 % by volume of methane > 100 m³ / h; A Gutgasverwertung is calculable at disposal on engines from 120 kW of electrical power with heat utilization.
• Recovery with micro gas turbines > 30 vol % methane; expensive investment, but low maintenance and flexible
• Catalytic and non- catalytic burns: security, ex- risk vulnerabilities gas < 25Vol % methane, <25 m³ / h. Has sense only if the quality of gas is still in the hazardous area and the amount of gas is low
• Methane oxidation over biofilters, etc. Biofenster: security, ex- risk vulnerabilities gas < 25Vol % methane, <25 m³ / h. Cost-effective investment and operating costs. Effect controversial.
• Aerobic conversion of biomass: Suitable for old landfills with decaying methane production. Costly procedures for creation and operation of the facilities. Useful in individual cases may. These methods are still completely in the testing phase (as of 2013). Only very limited use ( eg due to the bed height ). An implementation of almost 100% can not be guaranteed, a subsequent methane production is therefore possible again. Very complex and expensive process, which could be expected in use change, however. Previous publications have not been convincing.

Combustion

The combustion of landfill gas, methane is converted to H2O under optimal conditions, to 1 /3 CO2 and 2/3. This case, therefore there is a reduction of the greenhouse effect, on the assumption of 60 % methane and 40 % carbon dioxide, 6.4 Nm3 of CO2 equivalent per Nm3 of landfill gas to 0.6 Nm3/Nm3 and the ozone - destroying effect is even reduced to zero. Based on studies of the trace compounds of landfill gas is based on current knowledge can be assumed that in addition to inorganic trace gases such as hydrogen sulfide ( H2S), which is typical in a concentration of 20 to 500 mg/Nm3, ammonia (NH3 ), hydrogen ( H2) and nitrogen oxides (NOx), about 500 different organic hydrocarbons, including halogenated hydrocarbons, are present in the landfill gas.

The majority of previously identified trace substances can be described as toxic, carcinogenic or harmful to health in the broadest sense. The total of the organic hydrocarbon compounds typically moves 500-1500 mg/Nm3 and the sum of halocarbons between 10 and 250 mg/Nm3. Exceptions with extremely high concentrations can occur locally and temporarily. Assuming an average concentration of 800 mg/Nm3 and 50 mg/Nm3 organic hydrocarbons halogenated hydrocarbons arise with the above Amounts of landfill gas following streams, which are released from landfills BRD per year:

• About 3,300 tons of organic hydrocarbons and which
• About 200 tons of halocarbons.

It also applies to those substances that thermal destruction during combustion means a significant reduction of environmental pollution, which, however new, undesirable compounds (see below) may arise.

Conservation

In summary, it should be noted that, for environmental reasons, an uncontrolled degassing of landfills into the atmosphere should not be done, and that at least one thermal treatment of the gas to minimize the negative impacts is necessary. It should be considered whether the amount of electrical energy, energy expenditure for maintenance, energy consumption during material production in a balance (in terms of CO2 emissions ) results in a saving. In addition, the cost has to be considered with. However, it should be noted that even when the combustion new, undesirable compounds may be produced. Here are only carbon monoxide (CO ), nitrogen oxides (NOx), referred to as salt and hydrofluoric acid (HCl and HF). However, these substances can be minimized by suitable incineration process, Proceedings of the flue gas cleaning or gas precleaning or even prevented.

As an alternative to combustion so-called passive methane oxidation windows are used meaningfully.

Landfills are in 6th place for the methane producers in the world. Firstly, there are cattle (about 400 l / d by rumination ) and rice fields ( digester gas ).

Through the use of energy by combustion of landfill gas to other sources of emissions can be reduced and it can also be almost neutralized in the sum of all gaseous emissions from a municipal solid waste landfill.

Deemed to be unsafe, which is banned in Germany since 1 June 2005, the landfilling untreated waste ( MSW TA ), which in fact the majority of the domestic waste is nowadays applied to the waste incineration. Although this produces much less methane emissions, but correspondingly larger amounts of CO2 emissions in the atmosphere, in contrast to the landfill methane emissions, not further decompose (methane oxidized with oxygen in about 12 years back to carbon dioxide can be and water) or collected and burned.