Emmanuel Thevenon of Actualite en France says rubbish is no longer what it used to be. In future, he says, waste could provide an important source of energy. Subsidiaries of the Vivendi group, Onyx, Dalkia and REP, recently provided proof of this by building what are the two largest power stations in the world running on biogas.

Each of these plants produces 85 million kWh a year, enough to meet the electricity needs of more than 30,000 people. All the electricity produced is bought up by the French electricity company, Electricite de France (EDF), in the framework of a 12-year contract adapted to this specific type of installation.

This gas results from the anaerobic (without air) decomposition of the organic matter in buried waste. It consists mainly of methane (about 50 per cent), carbon dioxide (35 per cent) and nitrogen (15 per cent); it also contains traces of hydrogen sulphide. Biogas, which is 21 times more active than carbon dioxide, also actively contributes to the Greenhouse Effect. So to avoid such problems, in the largest installations, it has to be eliminated. It is usually burnt in flares, but can also be used to produce electricity. A cubic metre of biogas is equivalent to half a cubic metre of natural gas, i.e. 5 kWh.

So far, this alternative was limited by the corrosive components of biogas and by its fluctuating composition. Indeed, the latter varies according to climatic conditions. Depending on humidity, temperature, atmospheric pressure and the method used to gather it, its methane content, which gives biogas its calorific properties, wavers between 30 and 60 per cent. As a result, in addition to considerable corrosion and dirtying, the engines have to be very finely tuned. So the biogas is usually mixed with some natural gas before being used in power stations.

The two power stations of the Vivendi group are a spectacular innovation as they run exclusively on biogas. Each of them backs onto a rubbish dump in which 800,000 tonnes of waste are buried a year in an area of 200 hectares, representing the waste produced by two million people. The biogas is gathered by means of a network of horizontal and vertical conduits installed at the same time as the rubbish is buried. A control and regulation unit, operating 24 hours a day, supplies a gas whose pressure and quality are constantly checked.

When the gas reaches the production unit, it is burnt using burners especially developed by Vivendi subsidiary Dalkia, which form part of boilers with stainless steel fireboxes. High pressure, high temperature steam (400 degrees C) is thus produced and sent to a turbine driving an alternator. The latter converts mechanical energy into electrical energy, which is then delivered to an EDF station three kilometres away from the plant.

The two power stations are expected to go on operating for about 20 years. Income from the sale of electricity (around 5 cents a kWh) helps finance the investment made in the power station ($17 million), and its running and maintenance costs.

According to the French Environment and Energy Agency, the two French production units for valorising biogas will make it possible to save three million tonnes of carbon dioxide from being dispersed into the atmosphere in the next 12 years. There is also a saving of electricity as the biomass power stations supply the Paris area (where there is a shortage) and save the need to bring that extra amount into the region using high-voltage cables, which lose some of the electricity they carry.