Waste is usually delivered to the plant by road, but deliveries via rail or water are also possible. Once the incoming waste has been weighed, the delivery vehicles bring the waste to the designated tipping points – for household waste, commercial waste similar to household waste, bulky waste, street cleaning waste and other waste – where it is fed into the residual waste bunker. Each residual waste bunker has six tipping points where waste can be added. Waste is delivered to the plant from Monday to Saturday.
The residual waste bunkers are large-volume structures made of reinforced concrete with a net capacity of 6,000 Mg (12,000 m3) each. They are dimensioned to ensure the reliable operation of the plant, even at the weekend and on bank holidays. Even in the case of scheduled or unscheduled plant downtime at short notice, the size of the bunkers guarantees enough space for incoming residual waste.
Waste that cannot be directly transferred to the incineration zone due to its size, for example bulky waste, is pre-treated in a crushing unit (with a bulky waste cutter).
Four crane systems (maximum load: 4 x 12.5 t) transport the waste within the residual waste bunker to perform the following steps:
Special feeders (rams) distribute the waste to be incinerated evenly across the water-cooled incineration grate. The incineration grate is comprised of four zones and three grate runs. Its structural design causes the distributed waste to be turned, mixed, incinerated and moved to the end of the grate.
In accordance with the statutory requirements of the 17th German Federal Emission Control Act (BlmSchV), the combustion chamber is designed to ensure that a minimum temperature of 850°C is guaranteed for the two-second dwell time of the flue gases under all operating conditions and that good burnout is achieved. The chamber contains two additional burners, which not only help to start up and shut down the incinerator but are also automatically activated when the temperature falls below the minimum limit to ensure that the combustion chamber temperature remains at 850°C. These burners run on fuel oil (extra light).
A multitude of comprehensive measures have been implemented to reduce emissions from the incineration process that can be influenced, for example modifying the design of the grate and combustion chamber and regulating the combustion air.
The organic material in the residual waste burns almost completely. All that is left is a slag comprised of inert (incombustible) residue, which is transported through a discharge point (1 per line), where it first goes through a wet de-slagger before being fed into the slag bunker via a steel plate conveyor. The slag bunker can hold a total of approx. 1,500 tons of slag. The automatic crane system continuously moves the slag into storage, before it is transported via lorry to an external treatment plant to be processed and reused by a contractor.
The plant uses a denitrification system based on the selective non-catalytic reduction (SNCR) method to limit its nitrogen oxide emissions. The SNCR method uses ammonia (NH3) to reduce the nitrogen oxides in the flue gas in a non-catalytic selective process.
The nitrogen oxides in the flue gas (approx. 95% NO and 5% NO2) are reduced to nitrogen (N2) and steam (H2O). The ammonia is delivered to the plant in an aqueous solution and stored in a tank farm.
The hot flue gases flow from the combustion chamber into the convection sections of the steam generator. Each steam generator produces around 77.5 tons of steam with a pressure of 40 bar and heats it to a temperature of 400°C. This steam is then used in the plant’s turbo-generator to produce electricity and district heat and to cover the plant’s own power requirements.
The flue gases leave the steam boiler at a temperature of 200-230°C and enter the flue gas cleaning system.
Calcium hydroxide silo (7), HOK (lignite coke) silo (8), spray absorber (9), white lime silo (10), quenching container (11), diversion reactor (12), fabric filter (13)
The main components of the flue gas cleaning system are a spray absorber to separate harmful acidic gases and to cool the flue gases and a fabric filter, which is used as a bag filter to separate flue dust, salts, heavy metals (especially mercury) and dioxins/furans.
Lime milk and lignite coke (HOK) are injected into the spray absorber to separate the harmful acidic gases. Automatic controls ensure the supply of lime milk from the lime milk treatment system (white lime silo, quenching container and diversion reactor) and lignite coke from the HOK silo in line with requirements. To guarantee compliance with the emission limits in the case of high pollutant concentrations, slaked lime is additionally injected between the spray absorber and the fabric filter where needed.
The interaction of the elements in the flue gas cleaning system ensures that pollutants are removed from the flue gas and the MHKW Rothensee therefore reliably complies with or even stays below the emission limits of the 17th German Federal Emission Control Act (BlmSchV). Automatic devices ensure that the process of loading waste onto the grate is suspended if a fault in the flue gas cleaning system may cause the plant to exceed the continuously monitored emission limit. Flue gas cleaning is an effluent-free process.
In both systems, the cleaned flue gases are discharged via the chimney connected to the incineration lines.
Generator (15), turbine (16), district heat exchanger (17)
The MHKW Rothensee operates based on the principle of combined heat and power generation, in which electricity and district heat are used simultaneously. Pipes lead the steam produced from the heat of the flue gas in the steam generators to the steam turbine. The steam turbine, an extraction condensing turbine with controlled extraction and tapping for heat extraction, drives the generator, which in turn converts the mechanical energy into electrical energy.
