The Refuse Derived Fuel Production involves converting non-recyclable waste materials into a fuel source that can be used for energy generation. This conversion is also known as waste to energy. The final product can be used to generate heat and electricity in cement plants and power plants, where burners and furnaces consumes lots of fossil energy.
Turning non-recyclable waste into alternative fuel has benefits such as reducing the need for landfill and the reliance on fossil fuels. In terms of caloric value, 20 tons of RDF is equivalent to 14 tons of charcoal, or 12 tons of gasoline or 10 tons of fuel oil. Furthermore, RDF is easy to transport and store. This is why Wiscon has played a major role in providing a total solution of Refuse Derived Fuel Production. We will discuss more about RDF itself in the article “The Benefits of RDF“.
Input waste materials determine the quality of final RDF. Theoretically, the ideal material is dry, combustible and contains inert and contamination as least as possible.
Inert is defined as materials that contain a low caloric value like gravals, sands, glasses, and ceramics. It is heavier than combustible, which can be separated by a simple mechanical treatment like air swifter and trommel. Contamination refers to chlorine and halogen and any toxic substance after burning. As a thumb of rule, hazardous waste should never enter RDF plants. Here are some features of infeed materials.
Wiscon provides a briefing and past experience of making high quality RDF. There are six main steps in RDF production, which are applicable to make SRF or SRF. Only mechanical process is the focus, but thermal drying is not discussed. The process is various but the goal is to extract combustible materials at an appropriate size. As RDF varies from input materials and processes, to consult a one-to-one solution for your Refuse Derived Fuel Production plant, please contact us.
Size reduction of the input material with the target to break up bigger pieces so that the downstream screening and segregation systems are able to work efficiently. The target grain size after the pre-shredder is “90% <250mm”. Wiscon suggest a double shaft shredder for preshredding, therefore the grain size is determined by the blade thickness and number of hooks. As this is the first shredder of the system, this shredder should reserve a big allowance on the power and chamber size.
If the infeed materials are mostly plastic and paper, it is easier to go by a single-stage shredding, where one mega shredder realizes the output size of 90%<50mm.
Within the pre-shred material there is a certain quantity of fine particles (“Fines”) which have a low calorific value and/or are not suitable for a thermal recovery process. The use of a rotary trommel allows the plant to separate this Fines-fraction and deviate it from the main flow directly after the pre-shredder.
Based on the estimated grain size distribution inside the input material the screen opening and screen geometry are chosen, the RDF plant will be using a 60mm screen opening. The material falling through these screen holes are the Fines with a share of approx. 55% of the input material. The material floating across the screen is too big to fall through the screen holes and is collected for the further downstream treatment and equals approx. 45% of the incoming material. The screen cut can be modified by using different screen sizes. A change of the screen size always impacts also on the material flow rates of the Fines and the material quantity available for further treatment.
Within the pre-shred and screened material there is a certain quantity of heavy and/or inert material (“Heavies”) which has a low calorific value and/or is not suitable for a thermal recovery process. The use of a Wind sifter allows the RDF plant to separate these Heavies deviate them from the main flow. Based on the estimated material flow within the plant the Wind sifter machine type is chosen. The machine operates using a special, wide air nozzle system, which blows the light material fraction into the expansion chamber where it can settle onto an integrated extraction belt conveyor. The heavy material fraction is not picked up by the air flow, but instead follows gravity and falls onto a dedicated extraction belt towards a skip. The separation efficiency can be modified by air volume, the position of the air nozzle and the material drop off position at the air nozzle area.
Ferrous metal is not allowed in high quantities in the thermal recovery process, and therefore it should be separated from the main material flow. In the present plant the first magnet takes out bigger pieces from the pre-shredded waste stream, the second magnet takes out pieces from the remaining smaller sized waste stream just before the re-shredder. The third one locates after a RDF fine shredder to pick up the tiny bit of metals. The locations, sizes of the magnet and the strength of the magnetic field are not fixed.
The separation grade can be influenced only by the set distance between magnet and material flow. The ferrous metal separation is also necessary in a mechanical treatment plant in order to protect the fast-running RDF Re-shredder: Should a bigger metal part ends up inside the re-shredder, the blades and screens are be heavily damaged. Therefore multiple emergency switches avail along the line would be necessary. Another reason for ferrous metal separation is its worth as a resource, it can be resold to the market and generate income.
Non-ferrous metal is not allowed in high quantities in the thermal recovery process, and therefore it should be separated from the main material flow. In the present plant the Non-ferrous metal separation point is located after the wind sifter and before the re-shredder. The size of the separator and the strength of its Eddy-Current magnetic field are fix. The separation grade can be influenced only by a variable setup of an integrated splitter mechanism. The splitter unit can be set up as follows:
Move closer to the Eddy-Current drum in order to capture more Non-ferrous material. Impact: By doing so, the amount of main material inside the Non-ferrous fraction rises.
Move away from the Eddy-Current drum in order increase the main material flow. Impact: By doing so, the amount of Non-ferrous material inside the main material flow rises.
However, if non-ferrous metals are common in the waste stream, then you may skip this step.
Size reduction of the input material with the target to achieve the final material size of the mechanical treatment process which will then be used for thermal recovery. The reshredding machine is equipped with screens which determines the output particle size. The target grain size after the Re-shredder is “95% <40mm”; the grain size however can be modified by using different screen sizes during the fine shredding process. A change of the screen size always impacts also on the throughput capacity of the machine and must be balanced with the upstream plant configuration.
RDF is catergorize into two types: high quality RDF and low quality RDF, but wait, what is the difference? A high-quality RDF is
It is easy to conclude that high quality RDF is more expansive around $700 per ton (it varies in different countries). In addtion to the above process, manual sorting, thermal drying and compacting would increase the value of the products. The Refuse Derived Fuel Production from Wiscon is low-maintenance and had excellent levels of throughput capacity and energy efficiency.