Processing Municipal Solid Waste to Produce Refuse-Derived Fuel (RDF)

Waste to Energy Approach

Municipal governments throughout the world are facing choices about how to manage the unending stream of waste generated by their residents and businesses. In some places, landfills and dumpsites are filling up, and all landfills and dumpsites leak into the environment. As populations continue to grow, the issue of waste becomes more urgent and more complicated. Although enhancing recycling technologies reduces a significant fraction of waste, still a large portion of municipal solid waste ends up in landfills. Landfilling waste means the loss of resources and landfill sites. Also, risk toxins leach into soil and water and produce emissions of methane (CH₄) and carbon dioxide (CO₂) that contribute to climate change.

Using a combustible fraction of solid wastes that cannot be recycled as an energy source is one efficient way to reduce the amount of landfilled waste. The combustible fraction of waste is known as “Refuse Derived Fuel (RDF)”. RDF covers a wide range of waste materials which have been processed to fulfill guidelines, regulatory or industry specifications mainly to achieve a high calorific value. RDF production technology contributes to the “waste to energy” approach, reduces the carbon footprint and is essential for diverting waste from landfill. RDF captures the energy in non-recyclable and combustible waste and turns it into a replacement for fossil fuels like coal or oil. RDF is a renewable solid fuel that is used to generate energy. The RDF could be used in the cement industry; steel furnaces; power stations, substituting coal and oil; or be incinerated in energy-from-waste plants.

The benefits of harnessing this otherwise wasted energy are clear. Energy harvesting from RDF eliminates a huge amount of carbon dioxide equivalent gases (mostly carbon dioxide and methane) from being emitted every year from the burning of fossil fuels. Not only that but also every tonne of waste that is diverted from landfill eliminates 0.54 tonnes of carbon dioxide equivalent from being emitted in landfill gas. This is a significant saving of greenhouse gas emissions and we’re proud to be investing in and using this technology.

The further processing of RDF to enhance its physical and chemical properties strongly depends on the following application. Even taking into account transport costs, this often offers a cheaper alternative to landfill.  It also reduces emissions of the air pollutants that contribute to climate change, as the waste is burnt cleanly and efficiently to extract as much energy as possible.

Processing MSW to produce RDF:

To produce a standard solid fuel, waste goes through a range of processes. Municipal solid waste (MSW) and commercial and industrial waste (C&I) contains many different materials that need to be separated mechanically into a high- and a low-calorific fraction. This process involves shredding, screening and classifying of the material. Metals, inerts, and organics are removed; light fractions with high-calorific value (e.g. plastics, textile, and paper) remain. Table 1lists the typical components in input and output streams of an RDF production facility.

Table 1 – Typical components in input and output streams of an RDF production facility.

RDF production starts with the separating non-combustible wastes such as metal and glass from combustibles. The larger items must be broken into smaller pieces. Nest stage is the collection of un-segregated municipal waste, including organic waste (primarily food waste) and materials like paper, cloth, plastic, and wood that provide the calorific value required to burn. Ideally, during the separation stages, hazardous materials would be removed completely, but unfortunately, this is rarely possible. Another serious challenge in making RDF, particularly in less developed or tropical countries, is moisture. Since organic materials are not separated out at the source, MSW has a very high moisture content. Many RDF plants separate out some of the organic matter and sell it as compost.

The production of RDF includes a series of steps. The steps are taken and their sequence – as well as the specific machinery used – may differ depending on the waste characteristics, climatic conditions, technologies available, and final treatment(s) planned in a given location. The separation of waste mostly happens based on their physical properties such as size, weight (moisture content), electromagnetic properties etc.

In a more detailed process flow diagram, Figure 2 shows the sequences of a process in an RDF-production facility developed by ANDRITZ (Stattegger, Austria). For the sake of more clarification, the following clips, Figure 1 and Figure 2, show the industrial MSW treatment processes to shred, sort, separate, dry and produce final RDF either in bale or pellet form.

Figure 1 – A video showing various stages in MSW processing such as shredding, sorting and separation and baling.

Figure 2 – A video showing processes taken to produce RDF by ANDRITZ.

In continue, some of the processes that are used in the processing of MSW are explained. Obviously, the existence and sequence of the described stages depend on the waste characteristics and final product quality/application.

Manual separation

Bulky items such as large pieces of wood, rocks, long pieces of cloth, etc. are removed by hand before mechanical processing begins. Equipment involved in manual separation usually includes a sorting belt or table. Handpicking of refuse is perhaps the most prevalent MSW handling technique; it is also the only technique for removal of PVC plastics.

Air separation

In this step, fans are used to create a column of air moving upwards. Low-density materials are blown upwards, and dense materials fall. The air carrying light materials, like paper and plastic bags, enters a separator where these items fall out of the air stream. The quality of air separation depends on the strength of the air currents and how materials are introduced into the column. Moisture content is also critical as water may weigh down some materials or cause them to stick together.

Size reduction

Two types of devices are commonly used for this process: hammer mills and shear shredders. Hammer mills consist of rotating sets of swinging steel hammers through which the waste is passed, and shear shredders are used for materials that are difficult to break apart such as tires, mattresses, plastics, etc. The hammers need frequent resurfacing or replacement. Both are energy and maintenance-intensive. Hammer mills shatter items such as fluorescent light bulbs, compact fluorescent lamps, and batteries.

Trommel screening

A trommel screen, also known as a rotary screen, is a mechanical screening machine used to separate materials, mostly solid-waste processing industries. It consists of a perforated cylindrical drum that is normally elevated at an angle at the feed end. For an inclined drum, objects are being lifted and then dropped with the help of lifter bars to move it further down the drum; otherwise, the objects roll down slower. Furthermore, the lifter bars shake the objects to segregate them. Lifter bars will not be considered in the presence of heavy objects as they may break the screen.

Physical size separation is achieved as the feed material spirals down the rotating drum, where the undersized material smaller than the screen apertures passes through the screen, while the oversized material exits at the other end of the drum.

In municipal solid waste industry, trommel screens classify sizes of solid waste. By removing inorganic materials such as moisture and ash from the air-classified light fraction segregated from shredded solid waste, trammel screening improves the fuel-derived solid waste.

Figure 3 – Schematic of a trommel screening apparatus to classify materials based on their dimensions.

Another available design of trommel screen is concentric screens with the coarsest screen located at the innermost section. It can also be designed in parallel in which objects exit one stream and enter the following. A trommel in series is a single drum whereby each section has different apertures size arranged from the finest to the coarsest.

Advantages and limitations of competitive processes

One of the competitors in the screening process is vibrating screens. Trommel screens are vibration-free which cause less noise than vibrating screens. Trommel screens are cheaper to produce than vibrating screens, too. Trommel screens are more mechanically robust than vibrating screens allowing it to last longer under mechanical stress.However, the trammel screen has a lower capacity of processing material than vibrating screens. This is because only one part of the screen area of the trommel screen is utilized during the screening process whilst the entire screen is used for a vibrating screen.

Drying

Drying process reduces the moisture content of waste and prevents the leachate production. Dried materials are inactive biologically and are easier to store. The result is a homogeneous refuse-derived fuel. The partially decayed waste should be dried, either under the sun, by hot air, or by a combination of both. This important step in the process differs in each facility depending on the investment or land availability. Solar drying is not possible during rainy seasons, and most facilities run at a fraction of their capacity during the rains, sending most of the waste to landfills. Mechanical drying, on the other hand, requires significant amounts of energy that could easily render RDF plants unprofitable without huge government subsidies.

Metal separation

Ferrous metal separation (Magnetic separation)

Electro-magnets are used in this step so they can be switched on or off to allow removal of collected metals. However, not all metals can be removed by magnets. Non-ferrous metals do not have iron and do not respond to the magnetic field. Stainless steel, copper, and aluminum, for example, are only weakly magnetic or are not magnetic at all. A further limitation of this technique is that small magnetic item will not be picked up if they are buried in non-magnetic materials, and larger magnetic items can drag unwanted items like paper, plastic, and food waste along with them.

Non-ferrous metal separation (Eddy current separator)

The clip showing in Figure 4 displays a separation of non-ferrous metals from inert materials in an eddy current separator. Eddy current separators, or non-ferrous separators, use the current induced in little swirls (“eddies”) on a large conductor and separate non-magnetic metals. An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. If a large conductive metal plate is moved through a magnetic field which intersects perpendicularly to the sheet, the magnetic field will induce small “rings” of current which will actually create internal magnetic fields opposing the change.

Eddy current separators handle high capacities because the conveyor belt separates and carries away non-ferrous metals continuously and fully automatically. An important factor for good separation is an even flow of material, supplied by a vibrating feeder or conveyor belt, for example, to provide a uniform monolayer of materials across the belt. It is especially important with smaller fraction sizes.

Figure 4 – A video showing the eddy current separator operating mechanism.

Producing the final product

Once all of the separating and size reduction steps are complete, the final RDF product can be formed into bricks or pellets or can be left as fluff. Each form is derived from material separated at a particular stage in the process. Large pieces that escape the trommel screening stage and lighter materials like plastic bags that get blown off during air separation are baled together as RDF bricks. The shredded material from the hammer/flail mill and medium-size rejects from the trommel screens are used for the RDF fluff. Finally, the residual waste is mixed with binders like agricultural husk and passed through a pelletizing machine that converts the waste into pellets.