Page 219 - Materials Chemistry, Second Edition

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190 Waste Management Practices: Municipal, Hazardous, and Industrial
The shredding process involves three types of action: crushing, shearing, and grinding.
Crushing involves the reduction of particles by pounding; shearing involves forcing two parts of an
item in different directions; and grinding is friction applied to the surface of an object. All shred-
ding units employ two or more of these actions simultaneously.
Shredding can impart a number of benefits to waste. Shredded wastes are more amenable to
sanitary landfilling by virtue of decreased odor and therefore less rodents and insects; it also pro-
vides for greater ease of movement of landfill equipment. Shredding is extremely useful for the
manufacture of RDF as it increases the surface area of the fuel particles.
Shredding can process demolition debris and yard wastes. However, the most important appli-
cation of shredders is for materials recovery. Shredded MSW provides a number of advantages to
recycling and waste-to-energy systems:

• Waste volume is significantly reduced
• The waste becomes more homogenous
• Waste separation processes (e.g., Fe, paper, etc.) are facilitated
• Separation of the noncombustibles from the combustibles is made easier
• RDF is sized for convenient burning in power plant boilers

In an MRF, shredders can be installed in one or more positions along the processing scheme.
Primary shredders are used to reduce incoming raw MSW, whereas secondary shredders are used
to further reduce the size of the output product from the primary shredder.
Shredders are available in a variety of shapes and sizes, from portable paper shredders to huge
units that shred flattened automobiles at the rate of 1/min. The three most common types of shred-
ding units used for size reduction of MSW are the hammermill, the flail mill, and the rotary shear
(Figure 7.20). The tub grinder is also used; however, it is primarily devoted to the processing of yard
wastes or construction and demolition debris (Figure 7.21).

7.4.3.5 Hammermills

The most common shredder type is the hammermill. A hammermill is a large cylindrical or tapered
unit equipped with a central rotor with a series of attached rapidly rotating hammers (Figures 7.20a
and 7.22). Hammers are either fixed or swing on the rotating shaft to allow for rotation over bulky
or very dense waste components. The rotor and hammers are enclosed within a heavy-duty housing.
The housing interior may be lined with stationary breaker plates or mounted cutter bars. Shredding
relies on heavy force breakage of particles by rapidly swinging hammers in the enclosed vessel. Size
reduction occurs by the combined actions of tearing and impaction. Wastes are further reduced in size
by being struck against breaker plates or cutting bars fixed around the inner walls of the chamber.
Feedstock can be commingled or sorted MSW; in other applications, however, entire automo-
biles or steel I-beams are processed for shredding.
A shredder used for MSW processing usually has a width/diameter ratio greater than 1.0, a
hammer weight of 70 kg (150 lb), a hammer tip speed of 4260 m/min (14,000 ft/min), four rows of
hammers, and a starting time of 30 sec (Vesilind et al., 2002). Rotational speed ranges from about
700 to over 3000 r/min with power often set at approximately 500 to 700 kW. Hammermills vary
drastically in terms of horsepower, electrical needs, and type of acceptable input. For high-speed
shredders, rotational speeds are usually set between 1000 and 3500 r/min. A high-speed shredder
relies on brute force and is very noisy. High-horsepower motors, i.e., 50 Hp and higher, are neces-
sary. As a result, electrical costs can be substantial for high-speed shredders.

7.4.3.6 Horizontal-Shaft Hammermills

The hammer shaft can be oriented in either the horizontal or the vertical direction. In the horizon-
tal configuration, input is from the top and the materials flow through the machine by gravity and