French use household waste for heating and resource recovery

By André Larané, Technical Freelance Writer, Paris

In Paris, France, the Compagnie Parisienne de Chauffage Urbain supplies several thousand homes with hot water from incinerators located on the outskirts of the French capital. The system is claimed to be a source of low cost and pollution-free energy. In Nantes, on the Loire River estuary, the Arc en Ciel incineration plant produces electricity which it resells to the public distributor. It also provides steam to a neighboring metal refinery.

The investment required to build a medium-sized incinerator with a nominal capacity of five to twenty metric tonnes per hour is around three million euros ($2.83 million US) per metric tonne/hour of capacity, with half devoted to the treatment of flue gases and the recovery of energy. Taking into account current regulations for the protection of the environment and the treatment of flue gases, incineration costs around 80 to 100 euros ($75-$94 US) per metric tonne of household waste.

The French love of art is reflected in the Saint-Ouen incineration plant, north of Paris.

In France, where incineration is widespread, large companies are using their expertise to develop clean and economical technologies. Their objective is to optimize the combustion of waste in order to obtain a maximum of energy and a minimum of ash and solid residues.

Recently built incineration plants attest to the French taste for eye-pleasing architecture, even in a waste treatment facility. In Saint-Ouen, on the edge of Paris, the incineration plant operated by the Tiru company is lit up at night like a cathedral. Nantes, the home town of Jules Verne, with its Arc en Ciel (rainbow) plant, even houses exhibitions of contemporary sculpture. The plant was built by Onyx, part of the CGEA group. Onyx operates the largest incineration plant in the world in Florida, near Miami. This facility processes a million metric tonnes of waste per year.

Combustion Technologies

There are several competing technologies used in combustion furnaces. The best known uses a slightly inclined grate, consisting of parallel bars or rollers. The rollers turn and cause the combusting material to slide down and remove the clinker. Air is injected at the level of the grate as well as in the combustion chamber so that the waste is fully burnt. This prevents an excessive rise in temperature which might damage the furnace. In a plant, constructed by SGE Environnement in Grenoble, in the Alps, the waste is burnt at around 2,012°F, in roller-hearth furnaces.

In Saint-Ouen, the Tiru plant has three grate furnaces, each with a capacity of 28 metric tonnes per hour. It processes more than 630,000 metric tonnes of waste per year, in furnaces built by Alstom. These can accept very combustible household waste with a high calorific value. “So that the excess heat does not damage the coating of the furnace and grates, we designed and patented a very effective system of airflow cooling,” notes René Presles, technical manager at Tiru.

The main shareholder of Tiru is EDE (Electricité de France), the world’s largest electricity distributor. Tiru operates most of the incineration plants in the Paris region, and in 1994, took over operation of the incineration plant for the city of Quebec.

Onyx is currently develop ing technology for fluidized bed furnaces derived from techniques used in coal-fired power stations. Air is injected at the hearth of the furnace, which puts a bed of sand into suspension into which falls the material to be burnt. A fluidized bed has the advantage of simple maintenance since it does not contain any moving parts. It is suitable for waste of high calorific value and produces a thermal efficiency of the order of 80%, higher than a grate furnace, whose efficiency is around 75%. This represents a large amount of heat that is recovered in the boiler and that does not escape through the walls of the furnace or out of the chimney. A fluidized bed furnace is half the size of a grate furnace of the same capacity.

However, it needs fuel with an even size distribution. “The fluidized bed is especially suitable for residual waste from other recycling operations such as sorting and composting,” emphasizes Eric Le Sueur, innovation manager at Onyx. “The fraction of waste that avoids recycling is ground up before being incinerated.”

Thermolysis (thermal decomposition) is a new technique that has potential for energy recovery from household waste in small cities. It breaks down the organic content of waste into a combustible product that is easy to store and transport. The decomposition takes place at less than 932°F in the absence of oxygen. Inert matter contained in household waste (glass or metals) is recovered in an unchanged state after the thermolysis and can be recycled.

The end product of the thermolysis resembles charcoal but has the drawback of trapping heavy metals and other toxic compounds. It, therefore, cannot be burnt without special precautions. Nexus Technologies has set up an industrial pilot plant nearby, close to Marseilles, to develop its thermolysis process and overcome the remaining technical obstacles.

Incineration residue

In an incineration plant, combustion of a metric tonne of household waste at around I,832°F produces 45 pounds of fly ash and 550 pounds of clinker. Fly ash must be recovered because it contains heavy metals such as lead and mercury, as well as chlorinated compounds that can combine with oxygen in the air to produce dioxins. The scrubbing of the flue gases is the most expensive part of the incineration.

Dealing with the solid residues

The incineration of household waste produces a solid residue or clinker which contains around 14% of ferrous metals and 1% of non-ferrous metals such as copper, aluminum, and lead. All these metals are worth recovering because they have a market value, and extracting them makes it possible to use the clinker as ballast in construction projects.

Andrin has developed equipment for extracting metals from the clinker. The first machine was recently delivered to the incineration plant for the French city of Toulon, on the Mediterranean coast.

This equipment can continuously treat up to 15 metric tonnes of clinker an hour and could pay for itself in three years. It consists of a screen that removes pieces larger than one foot in diameter at the out let of the furnace, a magnetic screen that removes pieces larger than three inches and recovers the bulk of the ferrous scrap, a magnetic separator that extracts the smaller pieces of ferrous scrap, and lastly, an eddy current separator, patented by Andrin, to extract the small particles of non-ferrous metals in the clinker.