Page 26 - Materials Chemistry, Second Edition

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Current Practice and Future Sustainability
7
solid/liquid waste or, briefly, incineration is a thermal treatment process.
There are various types of incinerators and the type used depends on the
type of waste to be burnt. Conceptually, incinerators can be classified into
the following common types of systems (Dasgupta and El-Haggar, 2003):
• Liquid feed incinerator
• Rotary kiln incinerator
• Grate-type incinerator
• Fluidized-bed incinerator

A liquid feed incinerator can handle liquid waste while a rotary kiln incin-
erator can handle both liquid wastes and solid wastes. A grate-type inciner-
ator is used for large irregular-shaped solid waste and allows air to pass
through the grate from below into the wastes. The fluidized-bed type incin-
erator is used for liquid, sludge and/or uniformly sized solid waste.

Liquid feed incinerator
A large number of hazardous liquid waste incinerators used today are of this
type. The waste is burned directly in a burner or injected into a flame zone
or combustion zone of the incinerator chamber through atomizing nozzles.
The heating value of the waste is the primary determining factor for the
location of the injection point of the liquid waste. Liquid injection-type
incinerators are usually refractory-lined chambers, generally cylindrical in
cross-section, and equipped with a primary burner (waste and/or auxiliary
fuel fired). Often secondary combustors or injection nozzles are required
where low heating value waste liquids are to be incinerated. Liquid inciner-
ators operate generally at temperatures from 900°C to 1,500°C. Residence
time in the incinerator may vary from milliseconds to as much as 3 seconds.
The atomizing nozzle in the burner is a critical part of the system because it
converts the liquid waste into fine droplets. Two fluid atomizers, using com-
pressed air or steam according to fuel/waste type as an atomizing fluid, are
capable of atomizing liquids. The atomizer design is therefore an important
parameter of this system. The reasons for injecting the liquid waste as a fine
spray are: (1) to break up the liquid into fine droplets, (2) to develop the
desired pattern for the liquid droplets in the combustion zone with suffi-
cient penetration and kinetic energy, and (3) to control the rate of flow into
the combustion zone. In a good atomizer, the droplet size will be small pro-
viding greater surface area and resulting in rapid vaporization. The type of
atomizer depends on the type of liquid waste and the combustion conditions
required. Proper mixing of air with the atomized droplet is very important
for complete oxidation. The method of injection of the liquid waste is one of
the critical factors in the design and performance of these incinerators. If the
liquids contain fine solids, the design must allow the particles to be carried
to the gas stream without agglomeration affecting proper combustion.