6.Principles and Design of Thermal Treatment

8/13/2019 6.Principles and Design of Thermal Treatment

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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:


Hearth-Type Incinerators



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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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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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6.Principles and Design of Thermal Treatment