Thermal waste treatment and energy utilisation plant
TREA stands for Thermal Residue Treatment and Energy Utilisation Plant. In our two TREA plants, we utilise high-calorific, i.e. particularly energy-rich, processed waste as fuel. TREA I is designed purely as a heating plant. In TREA II, two CHP units are used in addition to a steam turbine. Not only to generate additional heat and electricity. Their exhaust gas is at least as important.
Important waste gas? To produce electricity from processed waste, a turbine is needed to drive a generator. Hot steam causes the turbine's blade wheel to rotate. The emphasis here is on hot. This is because the temperatures reached by the waste fire under the boiler of a TREA are not sufficient to produce hot steam. This is where the CHP exhaust gas comes into play. We use it to bring the steam up to the required temperature. It is precisely this new and very special combination of two essentially familiar technologies that makes this special CHP plant probably unique in the world.
Since 2010, TREA I (Thermal Residue Treatment and Energy Utilisation Plant) has been making a significant contribution to the generation of heat in Giessen. Processed waste is used as fuel here, replacing valuable primary energies such as natural gas, heating oil or coal. In this way, TREA I makes an important contribution to reducing the greenhouse gas carbon dioxide (CO2).
Fuel
The fuel for TREA I and TREA II is supplied by fuel treatment plants from the Central Hesse region, which process energy-rich waste from commercial, trade and industrial operations. Among other things, odour-emitting substances are reduced in the process.
This processed fuel consists of up to 50 % biogenic materials such as wood, cardboard, paper and cellulose and has a high calorific value of 11.0 to 14.5 MJ/kg. This means that a lot of energy can be recovered during combustion and made available in the form of heat.
The technology
TREA I is divided into the following sections:
Lorries deliver the fuel from the processing plant and unload it into the deep bunker (see illustration). The delivery and fuel quality are monitored during tipping.
A fully automatic crane system transports the fuel from the deep bunker to the storage area, which is separated by a concrete wall. If required, the crane conveys the fuel into the feed hopper when the level sensors send a corresponding signal to the crane system.
The storage area is used for intermediate buffering and mixing of the delivered fuel. The primary air required for combustion is fed to the furnace from the bunker. This creates a constant negative pressure in the bunker. This prevents dust emissions and odour nuisance in the surrounding area.
The fuel stored in the bunker is fed into the feed hopper onto an apron conveyor belt using the crane system's grab. The conveyor belt transports the fuel into a shaft. At the bottom of the shaft, 14 hydraulically driven rams push the fuel onto the first grate stage. The stamps, the apron conveyor and the grate are electronically controlled so that the transport speed of the fuel can always be adapted to the combustion heat output.
The fuel is dried, degassed, burnt and finally burnt out on a grate divided into four zones. The air volumes required for combustion are channelled into the combustion chamber as primary and secondary air.
The combustion process itself must take place at temperatures above 850 °C. For this reason, the walls directly above the grate and the afterburner zone above it are lined with refractory materials. This area forms the radiation section of the boiler.
The slag that accumulates at the end of the grate is discharged into the wet deslagger and discharged using a plate conveyor belt. The slag is then loaded directly into the transport containers using a conveyor belt. One container is removed per operating day.
In the convection section of the boiler, the flue gas heat is transferred to the hot water circuit via 5 economisers (heat exchangers). The hot water heats up to 120 °C by the time it leaves the boiler, while the flue gases cool down to around 160 °C.
Flue gas cleaning consists of the following stages:
- Urea injection (reduces the nitrogen oxides (NOX) in the flue gases by around 90 %)
- a reaction section for dry absorption (binding of pollutants in the flue gas to sodium hydrogen carbonate and activated carbon)
- a fabric filter (fly ash and the bound pollutants are separated here, removal via silo trucks)
- a heat exchanger (extracts heat from the flue gases and transfers it to the return flow of the heating water circuit)
- a condenser (utilises part of the condensation heat present in the flue gas; the flue gas heat extracted here can be used for primary air preheating)
- a droplet separator (separates water droplets present in the flue gas)
After passing the emission measurement, the flue gases are released into the atmosphere via the 50 m high steel-jacketed chimney with an inner flue made of glass fibre plastic.
Thanks to the multi-stage flue gas cleaning system, the TREA I not only naturally remains within the limits of the applicable BImSCHV (Federal Immission Control Ordinance), but also undercuts some of the specifications by more than 95 %. You can download the values from previous years as a PDF file below.
