Pelletization, Municipal Solid Waste (MSW), and Refuse Derived Fuel (RDF)

"Renewable energy advisor" Salman Zafar gives an overview of the process:Pelletization of municipal solid waste involves the processes of segregating, crushing, mixing high and low heat value organic waste material and solidifying it to produce fuel pellets or briquettes, also referred to as Refuse Derived Fuel (RDF). The process is essentially a method that condenses the waste or changes its physical form and enriches its organic content through removal of inorganic materials and moisture. The calorific value of RDF pellets can be around 4000 kcal/ kg depending upon the percentage of organic matter in the waste, additives and binder materials used in the process. ...

The process starts with the delivery of MSW to the plant by the garbage pick-up trucks to the waste processing facility. The raw waste is dropped on to a tip floor, with the overhead grapple operator moving any obvious hazardous or large materials to the side for either later use or disposal. The remaining materials are then moved to the incoming material hoppers, where they are transferred to transverse conveyors, which feed onto the incline conveyors, and subsequently to the manual sorting station. The materials are then passed through a trommel, where smaller items are screened and separated. Rest of the waste is passed through separators, screens and plastic removal systems to positively select those materials that are to either be used in the pellet or sold as recyclable product. The remaining materials that require disposal is stored until sufficient amounts are retained to send via truck to the selected landfill site. ...

During the pelletization process, eddy current separation and magnets are used in several locations to select both ferrous and non-ferrous materials for delivery to the recycling markets. The materials suitable for manufacture of fuel pellets are shredded, fiberized and stored in a silo. This stored material is then mixed with high BTU materials such as carpet waste, polyfilm etc and then pelletized and stored for sale and transportation to their final destination for use as an alternate fuel. Pellets can be stored on-site in bags or in bulk in interior bins or storage rooms, or in exterior storage silos. ...

Various qualities of fuel pellets can be produced, depending on the needs of the user or market. A high quality of RDF would possess a higher value for the heating value, and lower values for moisture and ash contents. The quality of RDF is sufficient to warrant its consideration as a preferred type of fuel when solid waste is being considered for co-firing with coal or for firing alone in a boiler designed originally for firing coal. ...

Fly ash which is produced as by-product in the process is commonly used as an additive for brick manufacturing. A biological air filtration system is installed to ensure air exhaust within the MSW receiving and sorting section of the facility is cleaned prior to exhaust to atmosphere. Dust, odour and debris emissions are minimized in the process facility by maintaining a negative pressure in the tipping floor and pit area and continuously introducing fresh air. ...

But read on for the comments!

Mr Zafar,

As an engineer turned financier who has had long and continuing involvement in the finance of waste management projects and technologies, I have to profoundly disagree with your article.

There is only one reason to justify the production of RDF, and that is political. Some politicians think that using a mechanical-thermal process such as you describe (or a mechanical-biological process or a mechanical-chemical process) to separate and concentrate certain fractions of the waste prior to burning it somehow makes it less environmentally damaging and therefore more acceptable to the public at large than conventional “mass burn” energy from waste (EfW).

The reality is that the production of the RDF typically adds $35 to $90 per ton to the cost of trash disposal compared to the use of a mass burn EfW plant where no pre-treatment of the trash is required.

If the RDF is burnt for electricity generation in a dedicated RDF plant, it increases the capital cost of the plant (as a water-cooled grate and use of corrosion resistant cladding becomes essential) and the maintenance costs compared to a mass burn EfW plant.

If it is co-fired with coal in a conventional power station, unless the power station has been specifically designed to allow this, expensive modifications to the plant will be required first. ...

The fly ash cannot be used to make bricks (or do anything else) – it has high heavy metal concentrations. Unless it is vitrified (difficult and very expensive to do), anything made with it will fail any sensible toxicity leach test. ...

The only way that production of RDF can result in lower quantities of waste being sent to landfill is when there is insufficient mass-burn EfW capacity available in the area and there are cement kilns, in which case they can replace some (but not all) of the fossil fuels with RDF. The exact amount will depend on how the kiln is fired and what grade of cement it is making. As for the “recyclates” that are produced in the RDF production plant, saleable materials produced, mainly metal and hard plastic, are typically are no more than 3% of the input tonnage.

In practice, the energy content of the RDF is typically no nore than 160% of the energy content of the trash it is made from, unless the trash has very high plastic content and low organic content. To obtain the 4000kcal you quote, you have to have a very low production rate (7 tons per hundred), select for hard plastic and plastic film and exoend a lot of energy drying it. The additional costs of doing this far outweight any extra revenue you might get for the pellets (in most of the world, RDF producers have to pay people to take their product, as it is rightly seen as a means of avoiding landfill). ...

Zafar has a set of responses, which I recommend you read, but he does not respond specifically to the fly ash issue (and I would just hate to imagine fly ash being used as fines for daily cover, since the heavy metals would end up in our groundwater). I note also the focus on using pellets in coal boilers. It's not clear to me whether there are coal boilers available for pelletized fuel in the state of Maine. Don't our boilers burn mostly forestry products? And if I recall correctly, Red Shield recently had a series of unfortunate events burning fuel in their boiler based on "political" rather than engineering considerations.