Page 311 - Materials Chemistry, Second Edition

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282 Waste Management Practices: Municipal, Hazardous, and Industrial
PEF can be produced in either shredded (fluff) or densified form. Preselected postuse plastic
waste as well as wood, sawdust, or scrap paper is freed of glass and metal contaminants and ground
to uniform size by a primary grinder. In some cases, a secondary grinder further processes these
materials which are then densified. The most common methods for densifying PEF are cubing and
pelletizing. Densification makes the final fuel product easier to transport and handle and assures
consistent heating value. A typical commercial product is a densified pellet about 5 to 8 cm (2 to 3
3
in.) long and about in. in diameter.

4
Compared with conventional RDF, PEF is a more refined, low ash, low moisture, high heat value
fuel. The high heating value of PEF is due to its plastics content. Plastics contribute to heating values
in the range of 15,650 to 35,750 kJ/kg (7,000 to 16,000 Btu/lb). Conventional RDF has a heating
value of about 14,000 to 15,650 kJ/kg (6,500 to 7,000 Btu/lb), and most coals have in the range of
20,000 to 27,000 kJ/kg (9,000 to 12,000 Btu/lb). As is the case with RDF, using PEF as a supple-
ment for coal can also reduce some types of environmental emissions, particularly sulfur dioxide.

9.8.8 TIRE-DERIVED FUEL (TDF)
In recent years, tire burning for fuel recovery has increased (U.S. EPA, 1991). Facilities such as
cement kilns and pulp and paper mills use scrap tires (shredded or whole, depending on the indus-
try) as a combustion fuel, burning approximately 42% of all scrap tires generated annually and
utilizing the energy. Tires are shredded into small particles, the steel is removed magnetically, and
the particles are often shredded a second time to produce crumb rubber. The fine rubber particles
are mixed with coal, typically less than 10 to 20% (by wt), and fed directly into the combustion
chamber. The so-called “tire-derived fuel” or TDF possesses the equivalent amount of energy per
unit weight as oil and comparitively more energy than coal (average 32,500 kJ/kg or 14,000 Btu/lb).
New technologies and pollution control equipment allow facilities to burn tires at high tempera-
tures, reducing air emissions.
When tires are burned as fuel during controlled combustion, atmospheric emissions are similar
to those emitted when coal or petroleum are burned. As with other fossil fuels, emissions include
“criteria” pollutants such as particulates, carbon monoxide (CO), sulfur oxides (SO ), oxides of
x
nitrogen (NO ), and “noncriteria” hazardous air pollutants such as PAHs, PCDDs, PCDFs, and
x
trace metals. When operated properly, the burning of tires for fuel is a reasonably safe and eco-
nomical practice that has been approved by the U.S. EPA. Air emissions usually are improved with
the incorporation of TDF with coal due to its low sulfur and nitrogen content.

The California Integrated Waste Management Board (CIWMB) (1996) stated:

In general, test results have shown that tire derived fuels have no additional adverse effect on emissions
when compared to coal. In fact, test results indicate a net reduction of SO (sulfur oxides), NO (nitrous
x x
oxides) and particulate matter.
The EPA’s Office of Research and Development published a study in 1997 entitled “Air Emissions
From Scrap Tire Combustion”, which stated:

TDF has been used successfully in properly designed combustors with good combustion control and
appropriate add-on controls, particularly particulate controls, such as electrostatic precipitators (ESPs)
or fabric filters. Air emissions characteristic of TDF combustion are typical of most solid fuels, such as
coal and wood. The resultant air emissions can usually satisfy environmental compliance limits even
with TDF representing up to 10 to 20% of the fuel requirements.

Each facility that uses TDF must, after being permitted, pass an initial compliance test to ensure
that it complies with emissions limits and operating conditions stated in their permit. Following the