ATS has been awarded of the contract for the design and construction of the flue gas treatment for Rennes WtE (France)
In Rennes, the 144,000 tons of waste, that the Villejean WtE plant receives each year, warms the equivalent of 20,000 homes in the neighborhood...
The Waste to Energy plant in Villejean (WtE) has been completed in 1968. Nowadays, despite good performances, some devices are at the "end of their lifes". The evolution of technologies, coupled with a desire to optimize the energy and environmental performance of the plant, have led Rennes Métropole to reflect on a plant modernization project. As part of this project, ATS will be in charge of the design and construction of the new flue gas treatment for the two lines.
Aereal view of the existing plant
The waste arrives by truck from all over the metropolis and from neighboring towns: every day, the solid urban waste and the non-recyclable bulky waste are poured into an immense pit of 5,500 m³ of the Villejean WtE plant.
From the control room, with a view of the pit, an operator works an overhead crane to mix waste at its best: "this allows a more stable mixture to be obtained, in order to have a better energy", explains Nathalie Gaillard, head of waste treatment and supply chain department, in Rennes Métropole.
The waste is then instered into the furnaces, "brought to around 1,000 degrees". Two line of furnace-boiler of about thirty meters high, each burning five tons of waste per hour. Unfortunately both date 1968, the year of the construction of the plant, and "they are saturated. The third line, dated 1995, treats 8 tons of waste per hour, continues Nathalie Gaillard. More modern, it allows you to recover more energy. »
And then there's the "valorisation" phase: the temperature of flue gases heats the water into the boiler which also allows to feed with hot water the district heating network that supplies the houses of the neighborhoods of Villejean and Beauregard. With the modernization of the plant planned for 2022-23, the two older lines will be replaced with a more performing line. «We will always treat 18 tons of waste per hour, all with a better performance and energy efficiency.» Corresponding to a few thousand additional houses heated in this way...
"We also measure pollutants continuously, in order to react immediately to any overruns," explains Nathalie Gaillard. Screens connected to computers show data in the control room, from which operators follow trends. At the slightest increase of emissions, "the injection of reagents is increased (today lime slurry, in the future bicarbonate) in the flue gas treatment, to neutralize acid pollutants".
Probes are installed on the stacks: "one for each furnace, in order to clearly understand what to stop in case of issues" but also if "we have the autorization of having up to 60 h/year of overruns, the contract signed with Rennes Métropole will obliged not to have more than 20."
All the collected data are transferred immediately to the Direction Régionale de l’Environnement, de l’Aménagement et du Logement (Dreal), 24 h/24. "We are below the autorized emission limits, also with the revamping on 2005, where 20 millions of euros have been invested. But nowadays we have the oldest plant of the entire country and we need a modernization", confirms Nathalie Gaillard. "We reached the limit of what is repairable, said Jean-Yves Doaré, plant manager. Some equipments are no longer on the market and we have difficulties in finding people that can repair them…"
Scheme of the new FGT plant
During the plant modernization, the flue gas treatment will be modified by the use of a dry technology that will "anticipate future regulation". Today the emission limits of nitrogen oxides are at the level of 200 mg/Nm3, in the future Villejean WtE the given guarantees are, followin ATS intervention, as low as 80 mg/Nm3.
But the NOx emission level won't be the only strong point of the flue gas treatment plant: all the pollutants will remain below the values given in the new BREF document for waste incineration, that will allow Villejean WtE to have performances at the same level of the best European plants.