Development of the middle exhaust air load (in g per hour) from a rotting box (with 385 t input) during the course of the six-week rotting phase. The load occurring per tonne of rotting material is shown highlighted in blue.
© RWTH Aachen University, Department of Processing and Recycling

Carbon concentrations in the exhaust air during the course of the rotting process: initially exhaust air treatment using RTO, in the intermediate phase, depending on contamination, RTO and/or washers and biofilters; these are sufficient in the end phase.
© RWTH Aachen University, Department of Processing and Recycling
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Exhaust air purification through regenerative thermal oxidation (RTO)

Over half of the 45 MBT plants in Germany also use biofilter systems in addition to RTO technology. These systems make it possible to treat exhaust air flows with lower levels of contamination, e.g. from the mechanical treatment, comparatively easily and in a less energy-intensive way. Thanks to cycle management of exhaust air flows, the exhaust air volume to be treated can be reduced. As a result, the specific energy content of the exhaust air increases at the same time. The RTO therefore needs less external energy for auxiliary combustion. The researchers are using the MBT in Großefehn in the Aurich district as an example to study how such a plant can be operated more efficiently. For this purpose, optimisation potential is being investigated in the mechanical and biological treatment stage. Additionally, in an alternative exhaust air management system, the exhaust air from the biological treatment is divided according to contamination grades. With the exhaust air concept at the MBT in Großefehn used to date, a part of the exhaust air flow from the hall suction device with low contamination from the mechanical treatment is used for ventilation in the biological treatment. The excess exhaust air flow from the mechanical treatment is conveyed to biofilter systems. The entire exhaust air flow from the biological treatment stage is treated using RTO.

For the more efficient operation of the plant, the researchers examined among other things what measures they could take to reduce the exhaust air volume flow to the RTO and therefore the energy consumption for the exhaust air treatment. They aim to achieve this by dividing up the exhaust air flows from the BT according to degree of contamination and treating them separately. For thi  selective recording of the exhaust air flows, they are installing a new exhaust air collection pipe, which in each case captures the exhaust air from all reactors of a block of 15 boxes in total with a medium and low total carbon load.

In a project completed in mid-2015, it was possible to demonstrate that an enlarged sieve opening width in the mechanical treatment leads to improved ventilation behaviour in the biological treatment. As a result, less energy is required for ventilation. The material influx into the rotting box was changed, and rougher material could be introduced below and finer material above. Depending on the method of operation and rotting stage, the quantity and contamination of the exhaust air change with time.

Within the six-week rot, three phases with strongly differing concentrations can be seen. The methods for purifying the exhaust air can be selected accordingly. The highly contaminated exhaust air in the initial phase (the first 7 days) is treated using RTO, the medium-contaminated exhaust air (up to day 28) is treated either with RTO or with a combination of filling body, nozzle floor washers and biofilters, while the exhaust air with low contamination between days 28 and 42 is also treated with a combination of washer systems and biofilters.

Reference plant in Großefehn

The MBT in Großefehn has a treatment capacity of around 60,000 tonnes per year. The plant is divided into mechanical and biological treatment, as well as an exhaust air treatment plant. First, the waste mixture is separated into different fractions: The light fraction, which has a high calorific value, can substitute fossil fuels in power stations. Further material flows such as metals are used as raw materials. The fraction of the domestic waste which is rich in organic matter is subjected to a six-week aerobic treatment in 30 rotting boxes during the biological treatment stage of the MBT in Großefehn. The final product of the biological treatment can then be deposited in landfill sites. The exhaust air from the plant is purified using biofilters, washers and a regenerative thermal oxidation plant (RTO).

Dividing up exhaust air flows for more efficient treatment

If only the exhaust air flows with a high level of contamination are conveyed to the RTO, the exhaust air quantity in the RTO to be treated is reduced; at the same time, the carbon content in the remaining volume flow increases: as a result, less supporting gas is needed to operate the RTO.

The aim is to achieve the highest and most constant carbon concentration possible in the exhaust air. In order to throttle the exhaust air flow to the RTO, the researchers divide the exhaust air flows from the biological treatment stage according to their degree of contamination and convey them to separate treatment methods. For this purpose, they are constructing a new exhaust air collection pipe, which can capture the exhaust air from an entire box block. The process control system can, thanks to a newly-developed regulation algorithm, convey the exhaust air flows to the respective collection pipe using automated vents. This exhaust air management is used with 15 rotting boxes out of a total of 30. The exhaust air with medium to low contamination is purified using the alternative exhaust air treatment.

They are also transferring the MBT process to a model. On the basis of the results, guidelines on energy-efficient exhaust air treatment in MBT plants are being produced which can be used in other plants.

Projektinfo 15/2016:
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Coordination, modification of the process chain in MT and BT
RWTH Aachen, IAR

Operator of the MBT system, test implementation

Development of exhaust air purification plant
PlasmaAir AG

Exhaust air monitoring:
Universität Stuttgart, ISWA

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