Mechanical-Biological Stabilisation – the Process
The mechanical-biological treatment plant (MBT) Neumünster was designed and approved as a stabilisation plant. Its main purpose is the production of secondary fuel out of household and commercial waste. The fuel meets high quality standards and contains a minimum of rocks, sand and sheds.
According to its permit MBT Neumünster consists of two subsystems, the mechanical-biological one and the fuel refinement system. The first was approved by the Federal Immissions Control Ordinance and the latter by the Technical Instructions on Air Quality. MBT Neumünster was put into operation June 1st 2005.
The first step of the treatment is a mechanical treatment. It starts with the shredding of the waste in the delivery hall. Afterwards a screening takes place. It results in two fractions, a coarse one bigger than 60 mm and a finer on that is smaller than 60 mm in diameter. The coarse fraction usually has a high calorific content. It is transported into the fuel refinement section of the plant. Via conveyor belts the fine fraction gets into the biological drying section. About 30 % of the input end up in the fuel refinement hall after the first screening.
Commercial and bulk waste are shredded, too. Usually, that kind of waste is dry and doesn’t need to undergo the biological drying procedure. The shredded commercial and bulk waste is transported on conveyor belts into the fuel refinement delivery hall.
Biological Drying of the Fraction < 60 mm
Magnetic separators isolate ferrous metals. Afterwards wheel loaders transport the remaining material into rot boxes (BIODEGMA® System). In 48 closed and aerated boxes the biological stabilisation takes place. This process is in need of oxygen. It takes 20 to 25 days. During this time the input material becomes biologically active. The temperature inside the BIODEGMA® boxes rises. Thus water evaporates. The moisture content decreases to 20–25 %. Finally wheel loaders empty the rot boxes.
After the biological drying another screening takes place. It is followed by an air separation. It results in a light fraction and a mineral fraction. The latter is landfilled. The light fraction mainly consists of foil, cloth and paper tatters. It is transported into the fuel refinement section. The finest fraction resulting from the screening is a sandy material smaller than 10 mm in diameter. It contains organic matter and thus it is called fine organic fraction. It is transported to MBT Lüneburg where it undergoes an aerobic treatment until it meets the regulations for landfilling.
Fuel Refinement Section
Here refuse derived fuel (RDF) is produced out of the high calorific material from MBT Neumünster and from external deliveries (other waste treatment plants for non-hazardous waste). The RDF meets exactly the standards that the combined heat and power (CHP) plant of the municipal utilities (SWN Stadtwerke Neumünster) requires. Those are a grain size of less than 120 mm, a metal content of less than 1 % and a chlorine content of less than 1 %. To fulfil these criteria another screening, shredding and several steps of metal separation take place.
The end of the fuel refinement section is the compactor station, where the material is pressed into cylindrical transport containers. Natural gas driven trucks transport these giant „cans“ to our CHP plant (thermal RDF plant) in Neumünster. In order to store RDF for a longer term bales are produced. There are two storage areas in the vicinity of MBT Neumünster where the RDF bales can be stored for up to three years before usage. Usually, the storage areas are filled up in the summer and emptied in the winter. Within the past 10 years the RDF output rose continuously. It covers about 170,000 Mg per year. At the same time the amount of residues that need to be landfilled has decreased. Their share is only 6 % of the original input.
Quality Criteria of Refuse Derived Fuel (RDF)
The RDF that leaves MBT Neumünster is analysed regularly. Every hour the staff takes samples of the final product. The samples are shredded on site. A representative daily sample is mixed in a standardized procedure. X-ray-analyses provide information about the ingredients and the energetic quality of the RDF. Also, at the end of a week a representative weekly sample is taken from of the daily samples. It then undergoes a wet chemical analysis. All analyses are conducted by an accredited independent laboratory.