When organic waste is composted it produces heat : the temperature of the waste can rise up to 80°C ! It is this property that the team led by Amaury de Guardia, Research Associate at the Rennes Irstea center, used to develop two, now patented, innovations to recover heat and exploit it. "Heat is produced by the activity of microorganisms involved in the composting process," explains the researcher. "Specifically, this increase in temperature can be used to sanitize matter by neutralizing pathogenic microorganisms as well as drying it."
With his colleagues, he conducted several experiments on the conditions in which heat is transferred to the external environment. The verdict ? When forced aeration is implemented and air is sucked or blown through the composting matter, installing water-filled tubes through the center of the composting matter to recover heat is fairly ineffective: only 5 to 10 % of calories are gathered. In contrast, 80% of the heat produced can be recovered from the gas let off by the composting matter. "It is therefore possible to recover this heat by condensing steam contained within this gas," explains de Guardia. In addition, this recovery method does not change the composting process.
These recovery and exploitation innovations are based on composting within a box or tunnel. Tubes sunk into a box's concrete slab suck air down from the top of the composting matter, ensuring a constant flow. "Using this common base, we have developed two resource recovery procedures: using the heat to heat a space and to dry another batch of composted matter," explains the researcher.
For the first procedure, researchers started with the idea that livestock farms, which also produce organic waste, and greenhouses could benefit from the heat emitted from a local composting platform. Gas flows from the tubes are funneled into an exchange network where condensation and heat recovery take place. The exchanger is then used to heat the area (greenhouse, livestock farm) in which it is installed. The scientists have done their calculations. A platform capable of composting 10,000 tons of biowaste per year would cover the heating needs of a 2.3 to 3.6 ha greenhouse. In another example of a breeding and fattening farm for 170 sows, composting slurry centrifugation waste would meet 22 to 35 % of livestock heating needs.
The second process allows batches of various materials such as compost, waste or agricultural matter to benefit from the heat produced during the composting process. "In particular, it can be useful to dry digestates prior to composting them," says de Guardia. Other uses include drying overly damp compost before spreading, as well as producing solid recovered fuels from residual household waste that has no other agricultural value. The way this procedure works is ingenious : at the end of the tubes, the gas outflow is mixed with a second gas flow circulating in the opposite direction. The second flow is heated then injected into a tubing network installed in a slab supporting the material to be heated and dried.
Relocate the production of renewable energy
By recovering energy that has until now been lost, these innovations also help relocate the production of renewable energy, providing a virtuous circle of composted heat recycling. This also provides energy savings. The researcher is enthusiastic: "So many people benefit from these procedures: composting platform operators and greenhouse operators with significant heating needs who are located near composting platforms, as well as farmers and livestock farmers who compost their waste and then use it to heat their facilities. We believe that local communities, some of whom have set up regulated composting, could also use the heat for nearby facilities, public buildings, etc." These two innovations are now subject to patents.