Methanization- Biogas in Our Waste

Biogas is naturally produced through the fermentation of many different types of animal- and plant- derived organic matter. New plants and even landfills ncreasingly tap into this secret energy from waste. Methanization can provide heating, electricity or fuel.

A Natural Phenomenon

Methanization is an anaerobic fermentation process. This involves bacteria breaking down rotting (putrescible) matter with a total absence of air, a process that occurs naturally in marshes (marsh gas).

Methanization produces biogas that contains methane (CH₄), among other things. This gas makes up over 90% of natural fossil gas content.  

The composition of methanization biogas is as follows:

   •    55-70% methane CH₄

   •    30-45% carbon dioxide CO₂

   •    Small quantities of ammonia NH₃ and hydrogen sulfide H₂S

   •    Solid residue from methanization or digestate- this can be dried and used as fertilizer.

Methanization can be used with any naturally fermenting organic matter.

True. The following matter commonly used in human activity can be used for methanization:

   •    Paper and cardboard

   •    Kitchen waste (peelings, stalks) and meal leftovers

   •    Agricultural waste

   •    Manure and slurry from domestic animals

   •    Sludge from water treatment plants

Industrialized Production

A methanization plant includes a large, covered tank into which the waste to be treated is deposited. These tanks are called reactors, fermenters or digesters.

In the newest reactors, methanization takes several days, producing 1-10 m³ of biogas per m³ of waste per day.

The Penkun methanization plant (Germany) produces enough power to supply 40,000 homes annually.

The size of biogas production plants can vary. A small agricultural digester on a farm has a capacity of about 100 m³. On the other end of the scale, the Penkun plant in Germany produces 20 MW. Its 40 500 kW digesters annually use 300,000 tons of corn (both stalks and ears), 50,000 tons of cereal, 50,000 tons of slurry, and 100,000 tons of water to produce 160 million kWh, enough to power 40,000 homes every year. The plant also produces heat through cogeneration (i.e. the simultaneous generation of heat and electricity), 30% of which is used to heat the digesters and 70% to make fertilizer.

A methanization reactor uses up some of the energy that it produces.

True. A temperature of 35°C is needed to keep the bacteria inside the reactor happy. However, anaerobic fermentation generates very little heat. Therefore, reactors often have to be heated up to this temperature. To do this, some of the biogas produced is used.

Landfills as Energy Sources

The other main method of producing biogas is through the recovery of landfill gases. Over long periods of time, landfills spontaneously emit methane.

In waste storage plants, refuse is compacted and placed in trenches (also known as cells) that are sometimes sealed to give better results. The cells are then covered with several meters of earth. A system of horizontal and vertical pipes run throughout these cells, the former to collect the biogas produced; and the latter to bring the biogas to the surface. This underground fermentation can take 25 years.

Underground fermentation can take 25 years.

Recovering methane, a powerful greenhouse gas, is essential to prevent it from being dispersed into the atmosphere. Where waste methane is not recovered, it is burnt off. The carbon dioxide produced in this way is much less harmful than methane. It has less capacity to absorb infrared radiation, thus making it a less harmful greenhouse gas.

Biogas' Many Applications

Before it can be used, all traces of hydrogen sulfide- a powerful metal corrosive- must be removed from the biogas. Hydrogen sulfide is highly toxic when burned as it releases sulfur dioxide (SO₂), which can damage both forestry (through acid rain) and our lungs.

Biogas can be burned to produce heat, electricity or both, through cogeneration (170 kWh of electricity + 340 kWh of thermal heat per ton of methanized waste). After all the impurities have been removed, including carbon dioxide, the gas can be used in the main gas network. It can also be used to fuel NGVs (natural gas vehicles) that use compressed methane.