Page 279 - Materials Chemistry, Second Edition

CAT3525_C09.qxd 2/8/2005 10:11 AM Page 250
250 Waste Management Practices: Municipal, Hazardous, and Industrial
3
Air contains about 21% oxygen, therefore 4348 m of air is required to supply this volume of
3
oxygen. This value converts to 4.35 m of air per kg of dry combustible material.
There are many other formulas which have been devised to calculate the air required for waste
combustion. For example, Dvirka (1986) devised the following:

W 0.0431[ 2.667C 8H S – O ] kg of air/kg of waste (9.2)
a
where W is the mass of dry stoichiometric air (at STP) required to burn 1 kg of combustible waste
a
and C, H, S, and O are the mass % of carbon, hydrogen, sulfur, and oxygen, respectively, of the
moisture- and ash-free material.
Other critical factors influencing the completeness of combustion are temperature, time, and tur-
bulence, labeled the “three T’s of combustion.” Each combustible substance has a minimum ignition
temperature that must be attained in the presence of oxygen for combustion to be sustained. Above
the ignition temperature, heat is generated at a higher rate than it loses to the surroundings, which
makes it possible to maintain the elevated temperatures necessary for sustained combustion. The res-
idence time of the input wastes in the high-temperature region of the combustion zone should exceed
the time required for combustion to take place. Such a requirement will affect the size and shape of
the furnace. Turbulence (i.e., the thorough mixing of MSW as it passes through the combustion
chamber) will expose particle surfaces to oxygen and high temperatures and will speed the evapora-
tion of liquids for combustion in the vapor phase. Inadequate mixing of combustible gases and air in
the furnace will lead to the generation of PICs, even from a unit containing sufficient oxygen.

9.3 THE MASS-BURN INCINERATOR
Mass burning is unquestionably the most straightforward incineration technology, involving com-
bustion of MSW as received from the collection vehicle. The only processing involved is simple
blending of wastes and removal of large bulky items such as white goods (stoves and washing
machines), bulky combustible items (mattresses, furniture, etc.), and hazardous items. The crane
operator often accomplishes the removal in the waste storage pit. Therefore, a major benefit of
mass-burn systems, beyond its relative simplicity, is the avoidance of capital and operating costs
associated with extensive waste processing. Some incinerators may utilize shredding equipment for
reducing bulky items to workable sizes. As will become apparent, the convenience of mass burn is
matched by a number of significant health and environmental concerns.
The major components of the mass burn incinerator include:
● Tipping area or receiving floor
● Storage pit
● Equipment for charging, or loading the waste into the incinerator hopper. This is often a
crane or front-end loader
● The combustion chamber
● Energy recovery system
● Pollution control equipment
● Flue
A typical mass burn system is shown in Figure 9.1.
Mass-burning incineration can be divided into four broad areas (Hickman, 1984):

● Incineration without energy recovery
● Incineration using modular furnaces
● Incineration using refractory furnaces with heat recovery boilers
● Incineration using waterwall furnaces