GOOD Everything about landfill gas

Landfilled waste contains a significant amount of organic matter, in the form of food waste, green waste, paper, cardboard, leather, wood, etc. It represents about 50% of overall tonnage and still 25% when waste is sorted upstream.

Inside the cells, the organic matter is degraded by various microorganisms which thrive in humid and oxygen-deprived environments. The most active micro-organisms are so-called methanogenic bacteria, which produce methane (CH4) as a metabolic by-product in hypoxic conditions.

This spontaneous phenomenon produces a biogas consisting primarily of methane and carbon dioxide (CO2). It also contains traces of hydrogen sulfide (H2S), ammonia (NH4), hydrogen (H2), organic sulfur compounds (called mercaptans) and various acid gases or aerosols.

The proportion of each of these components varies over time as the organic matter passes through different degradation stages.

Methane is a highly flammable gas. It is also a greenhouse gas with a global warming potential 20 times larger than that of carbon dioxide. Landfill site operators have to collect this gas to prevent fire and explosions, but also to avoid direct emissions into the atmosphere. Collection is also a way of reducing odor nuisance (caused primarily by hydrogen sulfide and ammonia) and helping to regreen the site.

A standard LFG collection system consists of a blower hooked up to a network of wells, installed vertically and/or horizontally in the waste mass, which transport the gas to a flare stack or an energy recovery unit. Given that such networks are never completely airtight, they also draw in air (oxygen and nitrogen). The volume of air present in the LFG depends on various factors, such as blower settings, cell wall permeability and the airtightness of conduits.

The collection network also draws in volatile compounds produced by waste such as paint, solvents, detergents, etc. These compounds include hydrocarbons, siloxanes, aerosols, dust, particles and volatile metal compounds. The various trace compounds are grouped together under the term “volatile organic compounds” (VOCs).

The collection network also draws in volatile compounds produced by waste such as paint, solvents, detergents, etc. These compounds include hydrocarbons, siloxanes, aerosols, dust, particles and volatile metal compounds. The various trace compounds are grouped together under the term “volatile organic compounds” (VOCs).

The gas arriving at the flare stack or the energy recovery unit is a mixture of biogas, air and VOCs. It consists of methane (CH4), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2), as well as various trace compounds. The proportion of each gas varies from one site to another – it depends on the type of waste landfilled, the degradation phase and other external factors.

The blower settings influence the volume of air in the LFG directly. The stronger the pull, the more air infiltrates through imperfections in the collection system. The weather conditions play an important role too: a high level of humidity combined with warm temperatures stimulates bacterial activity and biogas production. Variations in atmospheric pressure also have a direct impact on the level of oxygen present in the LFG.

LFG production takes place over several decades. It regularly increases when the site is in operation, and peaks a few months after the last load of waste has been landfilled. It then gradually declines until all the organic matter has been degraded.

Gas production at a landfill site can be partly controlled by regulating the LFG collection system and adjusting blower settings in particular. There are two possible approaches:

Operating the network in “depollution” mode

The blower draws in the LFG at a high rate and maintains the vacuum in the landfill at a high level. It helps to reduce odor nuisance but can increase the concentration of air up to 30%.

Operating the network in “recovery” mode

The blower keeps the vacuum at a constant, low level. If the valves are precisely controlled and the LFG collection network is sufficiently airtight, the air content can be kept to below 15%.

A landfill site operator often has to combine the different settings according to the production level of the different cells and the various, sometimes conflicting, constraints: methane recovery, odor nuisance reduction, polluting emission reduction, etc.

Nunc sed neque fermentum, semper erat sit amet, volutpat mi. In hendrerit ullamcorper arcu at luctus.

Praesent gravida dignissim mi, vitae sodales leo.

• Sed libero dui, ornare et neque mollis, porta consequat dui.
• Pellentesque tincidunt turpis et porta dictum.
• Quisque bibendum auctor sem eget porttitor.

Pellentesque accumsan dignissim neque, eu pulvinar tortor rhoncus quis. Sed finibus pellentesque convallis. Aliquam accumsan feugiat est, id elementum enim mattis sed. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Suspendisse sit amet eleifend nulla. Fusce

Nunc sed neque fermentum, semper erat sit amet, volutpat mi. In hendrerit ullamcorper arcu at luctus.

Praesent gravida dignissim mi, vitae sodales leo.

• Sed libero dui, ornare et neque mollis, porta consequat dui.
• Pellentesque tincidunt turpis et porta dictum.
• Quisque bibendum auctor sem eget porttitor.

Pellentesque accumsan dignissim neque, eu pulvinar tortor rhoncus quis. Sed finibus pellentesque convallis. Aliquam accumsan feugiat est, id elementum enim mattis sed. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Suspendisse sit amet eleifend nulla. Fusce