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PRINCIPLES AND DESIGN
OF THERMAL TREATMENTEVT 627
HAZARDOUS WASTE TECHNOLOGY AND MANAGEMENT
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TOPIC OF DISCUSSION
Introduction
Status and Regulation
Thermal treatment technology Instrumentation
Air pollution control
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INTRODUCTION
Hazardous waste is nonexclusive in
its content ~ contain combustible
organic + noncombustible inorganic.
Hazardous waste come in all physical
forms: liquid, solid and somewhere in
between.
Good combustion is good oxidation
of organic components.
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Example
A waste mixture (by weight ) of 30% toluene,66 % acetone and 5% water is to be burn in a
liquid injection type incinerator at rate of 1000
lb/h with 20% air.
What is the total heat release in the incinerator?
Compound Formula Heating value, Btu/lb
Toluene C6H5CH3 18,252
Acetone CH3COCH3 13,120
Water H2O 0
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Solution
Toluene heat release = 0.30 x 18,252 = 5,476 Btu/lb
Acetone heat release = 0.65 x 13,120 = 8,659.2 Btu/lb
Water heat release = 0 Btu/lb
Heat release per pound of mixture = 14,135.2 Btu/lb
Heat release in the incinerator = 1000 lb/h x
14,135.2Btu/lb
= 14,135,200 Btu/lb
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STATUS & REGULATION
In United States the law governing
incineration system is the Resource
Conservation and Recovery Act (RCRA)
as amended by the Hazardous and SolidWaste Amendments of 1984 (HSWA).
This statute provide far more technical
details than other environmentallegislation.
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THERMAL TREATMENT
TECHNOLOGY
Combustion
Excess air
Fuels
Metals
Gases and Vapors
Flares
Catalytic VOC Incinerators
Thermal VOC Incinerators
Liquid Injection Incinerators
Solid Waste Incineration
Grate-Type Incinerators
Hearth-Type Incinerators
Fluidized-Bed Incinerators
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COMBUSTION
Combustion of hazardous waste does
not differ greatly from the
combustion of conventional fuel
except that the wastes may containmany different organic compounds.
Has a measurable heating value.
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Excess Air
When organic wastes + stoichiometric amount of
air (oxygen) complete combustion
Perfect combustion not possible in commercial
burners or incinerators.
Incinerators must always utilize excess air to
achieve combustion
May accomplish in two ways:
Operating under starved air
With an excess air
Also used in incinerators for temperature control
because the excess air absorbs heat generated
during the combustion reaction.
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Fuels
The fuels used in an incineration
system to provide auxiliary heat.
May be any commercially available
fuel such as natural gases (methane),
propane (LPG), light or heavy fuel
oil, which may not be hazardous
waste.
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Metals
Inorganic components of wastes fed
in an incinerator cannot be destroyed.
Only oxidized.
Most of the inorganic materials are
chemically classified as metals and
enter the combustion process as a
component of waste. Will exits combustion process as
oxides of the metal that enters.
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GASES AND VAPORS
Not covered by federal RCRA
regulations.
Waste gases consist of hydrocarbon
or a mixture of hydrocarbon in air.
Waste gaseshigh concentration
Higher Explosive Limit (HEL)
Lower Explosive Limit (LEL)
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Flares
are used for waste gases that are
above the HEL and may be mix
with air, ignited and burned
cleanly.
Can be elevated
Primarily used to disposed
combustible gases during theprocess.
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Catalytic VOC Incinerators
To remove VOC with low concentration.
The VOC-air stream is directly heated with product
of combustion from a fuel burner to a temperature
at which the VOC will begin to burn on the surface
of the catalyst.
The temperature is determined by:
Type of VOC
The catalyst employed
VOC concentration in air.
Catalystenhances the combustion reaction
- Causes oxidation of VOCs on the
catalyst surface with very little heat loss.
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Thermal VOC Incinerators
Is a catalytic incinerator without the
catalyst.
The incoming mixture must be heated
to a temperature where VOC will be
oxidized.
Consist of fuel burner firing into a
chamber where the VOC-air mixtureis adequately mixed with the burner
combustion products.
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LIQUID INJECTION INCINERATORS
Is the greatest proportion of hw incinerators in
operation today.
The waste is burn in a burner (combustor) or
injected by atomizing nozzles into a flame zoneor combustion zone of the incinerator chamber
(furnace).
Are usually refractory-lined combustion
chambers, generally cylindrical in crosssection, and equipped with a primary burner.
Operate at temperatures ranging from 1000C
(1832F) to 1700
C (3092
F).
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LIQUID INJECTION INCINERATORS
(Cont..)
The atomizing nozzle in the burner is a critical
part of the system because it divides the liquid
waste into fine droplets.
The reason for injecting the liquid as a finespray are:
To disperse the liquid as a fine mist mixed with air
for efficient combustion.
To develop the desired pattern for the liquiddroplets in the combustion zone with sufficient
penetration and kinetic energy.
To control the rate of flow of the liquid discharged
to the combustion system.
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Atomizer
Three basic types of atomizers for
liquid wastes:
Mechanical or pressure atomizer
Two-fluid internal mix atomizer
Two-fluid external mix atomizer
Atomizers must be designed to cause
shearing action of the liquid to breakit into many smaller diameter
particles.
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SOLID WASTE INCINERATION
Solid waste combustion occurs in
suspension, on a grate, or on a solid
hearth.
Three types:
Grate-Type Incinerators
Hearth-Type Incinerators
Fluidized-Bed Incinerators
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Grate-Type Incinerators
Burn the waste on metal grates, provide air
circulating below, above and through the
waste.
Not generally suitable for hw because thehigher temperature required for waste
destruction may destroy the grates.
Its depends on the waste to be of such
character that it will be supported on thegrate and will not fall through to the ash pit
until it is burned.
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Hearth-Type incinerators
Types;
Rotary Kiln
Control-air or two-chamber fixed-
hearth incinerator
Multiple-Hearth incinerator
Monohearth incinerator (seldomly used)
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Rotary kiln consist of a refractory-lined
cylindrical chamber that sits on trunnions
and rotates slowly on its longitudinal axis. Waste burns as it moves toward the ash
discharge end.
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The fixed-hearth incinerator consist ofprimary chamber having either single-level
hearth or a stepped hearth.
In smaller units- waste is intermittentlycharged to the primary chamber, but ash
not removed.
In larger units- a mechanical ram pushesthe charge through the incinerator, and the
ash is continually removed.
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The multiple-hearth incinerator
Originally utilized in an incineration
mode for sewage sludgeComplicated
Highly mechanical system used to burn
sludges
Limited use in hw incineration because
the temperatures required to provide
reasonable destruction efficiencies are
not compatible with long equipmentlife.
The system consist of two to six
horizontal hearths in a vertical array.
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Fluidized-Bed Incinerators
Utilizes a fluid hearth consisting of sand
or alumina on which combustion occur.
Waste is injected into the fluidized bed,
either as a liquid, sludge, or uniformlysized solid.
Ash will remain in the bed while some
exits the incinerator into the air pollution
control equipment.
Exiting hot flue gases can be used in a
boiler or to preheat combustion air.
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INSTRUMENTATION
Must be reliable
Must be able to measure all of the system
variables and to shut down the system if
there is any indication of multifunction thatmight cause discharge of the hazardous
chemicals to the environment.
The basic operating parameters measured
are temperature, flow, pressure, differentialpressure, pH and level.
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AIR POLLUTION CONTROL
Required two functional elements:
A system or equipment item that will remove
particulates from the flue gas stream
A system or item of equipment for the removal
of acid gases.
May be achieved with wet systems or dry
systems or with a combination of both.
10 metals: arsenic, beryllium, cadmium,antimony, barium, lead, mercury, silver
and thallium.
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Particulate Removal
Measures in grains/dscf or mg/dscm.
Varies widely, depending on two factors:
Gas velocity in the incinerator
Actual particle size.
Can be effected by gravity separation,
interference, centrifugal separation, filtration
through a media filter, electrostatic separation, etc.
The smaller the particles, the more easily it is
carried by the flue gas at a low gas velocity.
Shall not exceed 180 mg/dscm.
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Particulate Removal (Cont..)
The dry particulate removal methods include:
Impactionbaffles and screens
Centrifugal separationcyclone separators
Filtrationfabric filters
Electrostaticprecipitators
Wet methods that employ water as a medium
include:
Impactionpacked and tray columns
Centrifugal separationwet cyclones Particle wettingventuris and similar units
Particle conditioning and wettingcollision scrubber
Electrostaticwet ionizer/precipitator
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Acid Gas Removal
Normal acid gases encountered are HCl. SO2 and
HF. Occasionally HBr will be present.
Can be accomplished in either a wet or dry
system.
Efficienciesvary between the wet and dry
system.
Dry removal system:
Dry lime injectiona variety of systems
Dry scrubberspray dryer
Wet removal system:
Absorption/reactionpacked and tray columns
Wetting contactorsventuris/Calvert/Hydro-Sonic
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