How a sewage sludge combined heat and power plant works
Walheim power plant was built between 1962 and 1967. It has two coal-fired blocks. In the winter of 1981/1982, a gas turbine fired using light fuel oil was commissioned at the power plant in Walheim. Since it can be started up within a matter of minutes and feed electricity into the grid, it is used during times of peak demand.
The two coal-fired power plant units were registered for decommissioning back in 2014, but have been categorised by the Federal Network Agency as system-relevant until March 2023. However, the power plant will not be able to remain in operation for that long, as it will no longer be possible to staff full shift teams due to the retirement of the personnel trained on this plant. The final decommissioning of the coal-fired units is planned for March 2025 at the latest. The gas turbine will continue to be operated.
The new sewage sludge combined heat and power plant will be built on part of the present site of the coal dump at the current power plant. The remaining coal dump area is also sufficient for the coal blocks’ reserve power plant status. The planned plant essentially consists of a closed storage bunker, a fluidized bed furnace for incinerating the sewage sludge with a downstream waste heat boiler and a flue gas cleaning system.
The flue gas produced during incineration flows into a waste heat boiler. This generates hot steam that drives a downstream steam turbine and in turn generates electricity. The cooled flue gas then reaches the multi-stage flue gas cleaning system. This is also where the ash is separated, from which the phosphorus is later recovered.
Any potential extraction for district heating takes place in the so-called vapor processing area. The vapors produced during the drying of the sewage sludge are cleaned and condensed here. Up to four megawatts can be extracted for district heating, enough to supply around 300 households with sustainable energy.
The plant does not smell
One important issue concerns questions around potential odor pollution. This can be 100 percent ruled out with the state-of-the-art plant: The combustion air is sucked out via a reception and storage bunker during operation of the plant, creating a vacuum, which, together with an airlock system, prevents odors from escaping outside.
Even if the plant is idle or out of service for an overhaul, the exhaust air is guided through a filter system. In such cases, odor pollution will also be reliably prevented.
Following storage, the sewage sludge is dried. Its water content is reduced to such an extent that it can be burned in an autothermal process in a so-called fluidized bed furnace at temperatures of up to 950 degrees Celsius. Organic matter is also completely incinerated at these temperatures.
Incineration as the method of choice
Sewage sludge is already being incinerated and added to the fuel in coal power plants. Special incineration plants, such as those planned for Walheim, are notably superior to this method. For one thing, they are designed for low-emission processing (in other words, they significantly cut carbon emissions). Furthermore, the ash produced during incineration contains valuable phosphorus that can be recycled – as even required by legislators from 2029/2030. In modern plants, the ash containing the phosphorus can be better treated after the incineration process and then sent to specialized companies for further processing. As things currently stand, this is not possible when the sewage sludge is incinerated with other fuels in a coal power plant.
Technical data for Walheim sewage sludge recycling plant
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Function
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specific figure
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Function
Plant capacity
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specific figure
around 180,000 t/a of dewatered sewage sludge (equivalent to 50,000 t/a of dry matter) and around 5,000 t/a of dried sewage sludge
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Function
Dry matter content of delivered sewage sludge
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specific figure
20-30% dry matter
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Throughput
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specific figure
approx. 20 metric tons of dewatered sewage sludge per hour
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Operating time
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specific figure
over 8,000 hours per year
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Function
Electrical outputv
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specific figure
approx. 1.5 megawatts electrical
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Function
Local heating
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specific figure
up to 4 megawatts thermal
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