Fueling the System

April 28, 2012
Renewable Energy From Waste

The first and most critical thing is to understand when researching equipment for a refuse-derived fuel (RDF) production system is to know what the end goal for the fuel really is, says Brian Wells, inside sales manager for Bulk Handling Systems (BHS), Eugene, Ore. He notes that RDF is a catch-all term that can mean many things depending on the conversion technology being used.

Mass burn, gasification, plastics-to-oil and other technologies can have very different end goals. “Understanding the back-end conversion technology and its requirements is of paramount importance,” Wells says.

Andreas Schwarz, president of Lindner America LLC, Raleigh, N.C., says having a defined or expected waste stream is a key consideration. “The more defined the waste stream, the less expensive the line, since there is less to sort out,” he notes. However, he says it is more of a capital investment issue.

The secret to success is to identify and separate the waste fractions that are high in caloric value and will produce good fuel from those that are more valuable in other applications in the waste stream or those with no value. That means using magnets to pull out the ferrous fractions and finding a way to get rid of rocks or dirt that is worthless.
“Why would you burn PET bottles that sell for $600 per ton?” Wells asks. He says capturing high quality RDF starts with three core concepts:

  • Accurate material sizing with compound, inline disc screen design
  • Separation of low density,-high BTU materials from high density-low BTU materials with air separation
  • Further separating the low density materials into two-dimensional (paper and plastic film) and 3-Dimensional (containers) fractions to maximize opportunity for recovering high value commodities and producing two very high quality fuel streams

“There are many exciting technologies that can be employed to subsidize the process including optical sorters, shredders, magnetic separators, even x-ray and induction based sorters,” Wells says.

Certain cleaner or single-material waste flows can get by with a single shredding station. “Dirtier and multiple waste flows require multiple shredding stations,” says Randy Baerg of Warren & Baerg Manufacturing Inc., Dinuba, Calif.
Grinders for wood and papers can be similar or versatile for 50/50 blends.

“Plastics and film plastics (or waste flows with this in them) require very specific type grinders and are not particularly good for woods or papers,” Baerg says. That said, it is possible to design for variable inputs depending on the end-use of those inputs.

“In any combustion-based conversion technology the key items to remove are metals and inert items that don’t burn, as well as items containing chlorides (PVC) which is corrosive and can create air quality problems,” Wells says.
Testing RDF must be done constantly. Chlorine content, moisture, and BTU value are the most important metrics. Some operations run sophisticated tests to look for other undesirables like heavy metals or other poisons.

In addition to magnets, many companies offer an array of eddy current separators for non-ferrous and stainless steel separation. Segregation by density is achieved by float/sink separation or various types of air classification including wind sifters, aspiration and elutriation separation. Optical separation technologies, including near infrared separation (NIRS) is required for other fractions such as plastics and glass.