This article highlights about process of incineration that offers French municipalities a number of advantages. First, it reduces the volume of solid waste to a fraction of its bulk. A metric ton of household waste leaves about 250 kg of bottom ashes and 30 to 50 kg of fly ash. This is an important consideration in France, which has much less landfill space than New World giants such as the United States or Canada. Combustion also sanitizes by destroying any microbes present, and the heat produced by the waste furnace can generate steam or hot water to heat residences, supply factories, or generate electricity in a turbine. Two popular French waste incinerator designs are rolling hearth and inclined grate furnaces. The former type contains a sloping series of rollers turned continuously by motor. As fuel slides down the rollers from top to bottom, solid residues drop out. Electricite de France is researching several ways of reducing the costs of vitrification. In laboratories near Fontainebleau, fly ash is fed into an electric arc furnace equipped with a graphite electrode.
THE FRENCH CITY of Nantes is the birthplace ofJules Verne, the 19th century visionary whose novels predicted futuristic wonders such as the deep-sea submarine and the airliner. Verne would likely be pleased with how his hometown, like many French communities, treats its household waste: incinerating it to generate electricity for the local power grid and steam for a neighboring metal refinery.
Incineration offers French municipalities a number of advantages. First, it reduces the volume of solid waste to a fraction of its bulk. A metric ton of household waste leaves about 250 kg of bottom ashes and 30 to 50 kg of fly ash. This is an important consideration in France, which has much less landfill space than New World giants such as the United States or Canada.
Combustion also sanitizes by destroying any microbes present, and the heat produced by the waste furnace can generate steam or hot water to heat residences, supply factories, or generate electricity in a turbine.
Household waste is about 85 percent combustible, containing paper, plastic, and organic matter that will burn when combined with air. The chief noncombustibles are metals and glass.
However, incinerating household waste creates fly ash, and solid residues, or clinker. The challenge facing French utilities and environmental technology companies is to prevent the incineration itself from damaging the environment.
It costs an estimated $2.83 million per metric ton of capacity to build a medium-size waste incinerator capable of handling 5 to 20 metric tons an hour. About half the investment goes into technology to treat flue gases and to recover energy. The sale of energy means extra money for the incinerator operators, whose primary revenue comes from trash-handling fees charged to municipalities. Operating the plant can cost between $75 and $94 for each metric ton of household waste burned.
French engineering firms are designing cleaner, more economical technologies to optimize the combustion of waste to obtain the maximum amount of energy while producing the minimum of ash and solid residues. On the other end of the waste incineration process, other French engineering companies are desulfurizing flue gases and developing ways of rendering incineration ash harmless by stabilizing it.
With the cultural sensitivity for which the French are noted, companies are making their incineration plants as attractive as possible. For example, the Arc en Ciel incineration plant in Nantes is used to exhibit contemporary sculpture, while an incinerator operated by Tiru on the edge of Paris is lit at night for dramatic effect.
KEEP THOSE WASTES A-RoLLIN'
Two popular French waste incinerator designs are rolling hearth and inclined grate furnaces. The former type contains a sloping series of rollers turned continuously by motor. As fuel slides down the rollers from top to bottom, solid residues drop out. SGE Environnement of Rueil Malmaison built a waste incineration plant in the Alpine city of Grenoble in April 1994 that burns 240,000 metric tons per year at about 2,012°F, in roller hearth furnaces.
The grate furnaces receive waste on a horizontal grate that moves back and forth to promote combustion. Air is injected at the level of the grate as well as in the combustion chamber to enhance burning. In addition, the injected air serves as a coolant, preventing an excessive rise in temperature that would otherwise damage the grate.
Electricite de France in Paris, the world's largest electricity distributor, is the major shareholder in Tiru, a Paris neighbor. Tiru operates most of the incineration plants in the Paris region. The Compagnie Parisienne de Chauffage Urbain uses some of the heat from these incinerators to supply several thousand homes in the City of Light with hot water.
In Saint-Ouen, on the outskirts of Paris, Tiru built an incineration plant with three grate furnaces, each furnace possessing a capacity of 28 metric tons per hour. The furnaces, built by Alstom in Belfort, consume more than 630,000 metric tons of waste a year. These furnaces accept highly combustible household waste such as plastic.
"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," said Rene Presles, technical manager at Tiru. This system directs heated air from the incinerator chimney back into the furnacetwo-thirds injected under the grates and one-third sent over the flame-to promote combustion.
Tiru has exported the waste burning expertise it accumulated in France, and is operating a waste sorting and incineration plant in Mataro, Spain. It is also taking over the operation of the incineration plant that serves Quebec City in Canada.
A French competitor of Tiru, Onyx, is based in Nanterre and is adapting the fluidized bed technologies used to burn coal more efficiently to improve waste incineration. Onyx injects air at the hearth of its test furnace, causing a bed of sand to become suspended. Waste falls into the sand bed, becomes suspended in turn, and burns more freely because of the mixture of fuel and air.
The fluidized bed is easier to maintain than a roller hearth because it does not contain moving parts. It is suited to burn waste with high calorific value and has a thermal efficiency on the order of 80 percent, compared with 75 percent provided by grate furnaces . The extra heating value is recovered in the boiler. An additional benefit of the fluidized bed furnace is that it can burn the same amount as a grate furnace but is only half the size.
However, a limitation of the fluidized bed furnace is that it requires its waste fuel to be distributed at an even size. As a result, the fluidized bed is particularly effective at incinerating the residual waste from other recycling operations, such as plastic and paper sorting and composting, said Eric Le Sueur, an innovation manager at Onyx. Residual waste that cannot be recycled is ground up and then incinerated.
Onyx is testing three such industrial sorting and incineration facilities in the Paris region. Monthyon, east of Paris, incinerates 130,000 tons, composts 30,000 tons, and sorts 30,000 tons of household waste per year for recycling. North of Paris, the Cergy-Pontoise plant incinerates 15,000 tons, composts another 20,000, and sorts 70,000. Orleans, south of the capital, incinerates 105,000 tons, composts 40,000 and sorts 15,000. Both Monthyon and Orleans use the energy of their furna ces to generate electricity, while Cergy-Pontoise uses it to provide heating as well as electricity.
Some smaller Fre nch cities that do not generate enough household waste to justify purchasing an incinerator may use thermolysis as an alternative. This process is promoted by Thide Environnement, an engineering firm headquartered in the Parisian suburb of Corbeil-Essones. Organic waste, including cardboard, paper, food , and wood, is fed into a reactor that is steam heated to 930°F. Because there is no oxygen in the reactor, the waste does not combust, but decomposes. Operators remove the inert glass and metals for recycling, leaving an odorless, chlorine-free powder with a high calorific value. This powder can be comseilles to develop its thermolysis process and find ways to burn its residues safely.
The widespread incineration of French household waste has spurred the development of pollution control technologies to treat the gaseous and solid emissions that incineration produces. The engineering firm Lab in Lyon specializes in designing wet systems that spray water or a lime and water solution into flue gases to absorb sulfur dioxide. The water evaporates, leaving a sulfur residue that can be sold conunercially. Waste incineration plants in Amsterdam and Tokyo use Lab equipment.
A Tip FROM CAESAR
Solid incinerator wastes are another concern. France generates about 300,000 metric tons of fly ash annually from incinerating household waste. The fly ash contains heavy metals, including lead and mercury, and chlorinated compounds that could leach into groundwater if the ash is interred in a landfill.
One promising method of controlling fly ash involves depositing powdered activated carbon on the walls of the filtering ducts where flue gases circulate. The heavy metals and dust in the fly ash are adsorbed into the carbon, which can be stored in controlled landfills. For example, the Remival company uses activated carbon in its incineration plant in Reims for that purpose.
Another technique for tatTung fly ash is to stabilize it, to prevent toxic compounds from entering the water table, by adding mineral, hydraulic, or organic binders that will render the ash inert. This combines mechanical and th ermal engineering, as well as chemistry.
The researchers at Sarp Industries in Limay studied the ancient Roman recipe for making mortar to help develop their fly ash stabilization process.
"The famous Pont du Gard bridge has withstood erosion and wear for 2,000 years," said Jean-Louis Biros, a technical director at Sarp Industries. "It owes this performance to binders based on silicates and calcium aluminates."
The binders Sarp uses in its Ashrock proc ess are commonly found on the Earth's surface. They are defined as hydraulic because after being combined with fly ash, they create compounds containing water molecules. When these binders are cold mixed with fly ash, the resulting mixture becomes viscous, then forms very stable hydrated aluminates and silicates. Because these compounds possess a large developed surface area-about a hundred square meters per gram-they can adsorb potentially toxic molecules of hazardous materials and be stored safely in a landfill.
Sarp Industries designed its first Ashrock plant to treat eight metric tons per hour of fly ash generated by household waste incineration in Normandy in 1995. The plant is located in Argence, near Caen. Fly ash is sent to a 3- meter-square chemical reactor to be mixed with water and hydraulic binders. The stabilized fly ash is currently stored in a landfill, but Sarp hopes to recycle it as construction aggregate. The company installed its second Ashrock plant, capable of treating up to 30 tons of fly ash per hour, near Paris in 1998.
Seche Eco-Industries operates its own fly ash stabilization center in Change in western France using hydraulic binders. Company scientists use X-ray fluorescence to analyze fly ash samples and select the appropriate binders to stabilize the waste. Thus, Seche Eco-Industries can reduce the soluble portion of the waste to less than 10 percent and neutralize heavy metals and toxic phenols.
Some French conuuunities stabilize incineration fly ash by using organic binders, a technique originally developed to treat low-level radioactive wastes. Unlike hydraulic or mineral binders, organic binders do not react chemically with waste. Instead, they encapsulate dehydrated waste and isolate in stable capsules.
SCE Environnement of Rueil-Malmaison found a way to combine plastic recycling and fly ash stabilization with its Plastibloc organic binder process. The company operates a plastics extrusion unit in the French Alps, where it manufactures pipes from mixed plastics extracted from the household waste sorting center for the city of Grenoble. SGE mixes heated plastic wastes and mineral filler to produce a homogeneous molten plastic that is extruded from a screw die.
SCE engineers realized that they could replace the mineral filler in their extrusion process with fly ash. They make a material whose total weight is 40 to 70 percent fly ash. The Plastibloc material is water resis tant and is sent to a landfill or accredited storage site.
BETTER STABILlZATlON OF FLY ASH
Vitrification, originally developed to treat nuclear wastes, offers better stabilization of fly ash than mineral· or organic binders do. This technique involves subjecting wastes to temperatures above 2,400°F to convert them into inert, glass-like material stable enough to be used as an aggregate in road building. However, vitrification is three times more expensive than using binders, and can cost $300 to $600 per metric ton of waste.
Electricite de France is researching several ways of reducing the costs of vitrification. In laboratories near Fontainebleau , fly ash is fed into an electric arc furnace equipped with a graphite electrode. The furnace is heat..: ed to 2,642°F. After exiting the furnace , the vitrified waste is molded into ingots . The furnace combustion gases are filtered so that the metallic salts they contain can be recycled into the furnace. The entire process reduces wastes to one-sixth of their former volume.
Engineers at Europlasma in Bordeaux began operating the first industrial-scale vitrification furnace to process fly ash derived from household waste incineration at the end oflast year. The furnace is equipped with a 700-kW plasnu torch made by Aerospatiale. The electric arc ionizes the gas at the torch's tip, heating it as high as 2,642°F.
Operators adjust the amount of time the fly ash spends in the plasma to ensure that all the waste is fused and to produce a homogeneous material. A second unit is being planned near Bordeaux.
If fly ash is a troublesome by-product of incineration, the clinker represents an economic opportunity, containing about 14 percent ferrous metals and one percent nonferrous, such as copper, aluminum, and lead. These metals are worth recovering because they have a market value and, after they are extracted, the clinker can be used as a construction aggregate, explained Marc Deplancq, a manager at Andrin, based in Villers-IaMontagne in Lorraine. Andrin has developed equipment for extracting metals from the clinker.
The first metal extracting system was delivered to the incineration plant that serves the Mediterranean city of Toulon. The Andrin equipment can treat up to 15 metric tons of clinker per hour and is mounted at the furnace outlet to receive the clinker directly. A screen removes debris larger than one foot in diameter. A second, magnetic screen removes pieces of ferrous metal larger than three inches. Next, a magnetic separator extracts smaller pieces of ferrous scrap. At the last stage of extraction, an eddy current separator removes the small particles of nonferrous metals. The Centre de Recherches de Voreppe, a research center of the Pechiney Group that specializes in aluminuIT1, confirmed the efficacy in the Andrin extracting system" in 1997 and 1998, encouraging the system's chances for commercialization.