Methanisation (or methanogenesis as it is more rigorously called) is an anaerobic fermentation process,
that is
a process of decomposition of matter by bacteria in the absence of air. This process produces biogas consisting largely of methane, CH4, the same gas that accounts for more than 90% of natural gas from fossil sources. This methanisation phenomenon goes on naturally in marshes (marsh gas). Methanisation can be applied to all organic matter that ferments naturally:
- Paper and cardboard,
- Kitchen waste (peelings, tops and tails of vegetables, and so forth) and the leftovers from meals,
- Agricultural waste,
- Liquid slurry and solid manure from domestic animals,
- Sludge from effluent treatment plants.
A methanisation unit consists of a large basin with a lid, in which the waste to be treated is placed. These basins are called reactors, fermentors, or digestors. A temperature of 35°C is necessary to ensure that the bacteria are quite happy. Anaerobic fermentation produces very little heat; it is therefore often necessary to heat the reactors by using some of the biogas produced. Methanisation, in the most recent reactors, takes several days and produces biogas levels of between 1 and 10 m 3/day per cubic meter of reactor volume. The size of biogas production units can vary a great deal, from a small agricultural digestor on a farm (about 100 cu m) up to a mega-plant of 20 MW, such as the Penkun plant in Germany (near the Polish village of Szczecin), the construction of which has just finished and boasts 40 digestors, each of 500 KW capacity, with an annual consumption of 300 000 t of maize (the whole vegetal including the corncob), 50 000 t of cereals, 50 000 t of slurry and 100 000 t of water, for an output of 160 million kWh, which represents the annual consumption of 40 000 households! At the same time, the plant produces heat (cogeneration), 30 % of which is used to heat the digestors and 70 % to manufacture fertilisers.
The composition of biogas from the methanisation process is as follows:
- 55 to 70% methane, CH4,
- 30 to 45% carbon dioxide, CO2,
- Trace quantities of ammonia, NH3, and hydrogen sulphide, H2S,
The solid residue from the process can be dried and used as fertiliser.
The other main method of biogas production is the recovery of gas from rubbish tips, which spontaneously discharge methane over a long period of time. In these centres of rubbish storage, the waste is compacted and then deposited in deep trenches known as landfills (which can be isolated from the surrounding environment for the best results). The trenches are subsequently covered with several metres of earth. The landfills are crisscrossed by a system of horizontal drains, collecting the biogas produced and feeding it into vertical pipes, which bring the gas to the surface. The underground fermentation can continue for 25 years.
The recovery of methane from rubbish tips is very important to avoid its dispersion into the atmosphere, because it is an extremely potent greenhouse gas. In situations where it is not yet recovered for use, the methane discharged is burnt off in a flare stack, the resulting CO2 being very much less harmful for the atmosphere.
Before use, the biogas must have all traces of hydrogen sulphide removed. As well as being toxic, this compound is a powerful corrosive agent for metal and burns giving off sulphur dioxide, SO2, the sworn enemy of forests (acid rains) and our lungs. Biogas can be burnt to produce heat, electricity, or both in a co-generation system (170 kWh of electricity + 340 kWh of thermal heat per ton of methanised waste). Once all its impurities (including carbon dioxide, CO2) have been removed, the gas can be injected into the town gas distribution network.
It can also be used as fuel for natural gas vehicles which work on compressed methane. |