cpe 406 6060 lecture 1 handout(2)

7/26/2019 Cpe 406 6060 Lecture 1 Handout(2)

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CPE406/6060 Energy from Biomass and WasteLecture 1 handout Prof V Sharifi

1 - Characteristics of Waste CompositionThe following properties are important in the assessment of waste as a fuel:

Moisture content is obviously important since ignition will not occur if thematerial is wet. Moisture diminishes the gross CV value of a fuel.

Volatile mattercontains the combustible fraction of the waste and consists ofgases such as hydrogen, CO, CO2, CH4 etc

Ash content is important since a high ash percentage will lower the calorificvalue of the waste and must be removed/disposed of after combustion. Wasteash is highly heterogeneous and contains inert packaging material such asglass and metal cans.

Most organic wastes hold significant stored energy. There is considerable variation in

the calorific value of municipal waste. Clinical waste would show an even widervariation in calorific value, but has a higher value on average than municipal wastebecause of its plastics content. In general the higher the calorific value, the greaterthe net benefit in energy terms, and the more attractive the case for investment torecover the energy. But other factors may also be important. Although scrap tyres, forexample, have a high calorific value, there are problems in handling and shreddingthem. General industrial waste is more attractive than municipal waste as a source ofenergy, not only because of its higher calorific value (about 16 GJth/tonne) but also

because it has a lower moisture content (about 10%) and leaves less ash (6-10% ofthe original volume).

2 - Waste typesmaybe classified into 4 broad categories:

1. Municipal solid waste2. Industrial waste3. Sewage Sludges4. Special waste (e.g. clinical waste)

Municipal Solid Waste (MSW)

Typical composition: paper & card, plastic film, textile, glass, ferrous metals,miscellaneous combustibles and non-combustibles

Typical CV: 7 to 11 MJ/kg

Typical Moisture content: 30-35%

MSW may be incinerated directly or shredded

Production of RDF pellets (refuse derived fuel) where the CV is approx twicethat of raw MSW

RDF pellets are used in conventional boilers with thermal efficiencies up to80%

Currently MSW waste is segregated at source or at dedicated facilities. The main

categories of segregated wastes are paper, cardboard, plastics, textile, glass and


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metals. The residue of segregation includes dirty paper, mixed plastics and finedusts, which are disposed by incineration and/or landfilled.

Industrial waste

Composition and CV depend on the original process

CVs are often much higher than MSW (up to 30 MJ/kg)

Waste quantities and feed rates have to be limited to avoid excessivetemperatures in the incinerator

Sewage Sludge

Sewage sludge maybe incinerated after being de-watered!

Primary sludges from settling tanks have from 90- 97% water content! Thesludge must be dried before use and levels of moisture below 65% arerequired before incineration can be self-sustaining without the addition of an

auxiliary fuel system. Sludges maybe considered as having a useful CV only when the heat

released by combustion exceeds the latent heat of evaporation of the watercontent.

Incineration of sewage sludge is an energy consuming operation, hence thereis much interest in co-incineration with coal and/or other wastes.

Special Waste

is defined as waste containing hazardous materials such asflammables, explosives, toxic, radioactive, pathogenic or clinical waste.

Requires special incinerator design and handling procedures becauseof its toxic nature

Has higher CV compared to MSW, e.g hospital waste CV is approx.17 MJ/kg (higher plastic content), hence potential for a cost effectiveenergy recovery scheme with incineration

Refuse-derived fuel (RDF)There are two main forms of refuse-derived fuel (RDF):1) Coarse refuse-derived fuel (cRDF) is typically produced by separating frommunicipal waste its bulky and noncombustible fractions (glass, tins, rubble) andchopping the rest to a consistent size. The moisture content is not substantiallyreduced.2) To produce densified refuse-derived fuel (dRDF), separation of bulky and non-

combustible fractions is followed by substantial reduction of moisture content. Thematerial is then formed into pellets.Of the two types, dRDF has the virtue that it can be stored, whereas cRDF starts torot immediately. From the technical point of view it is possible to sell dRDF toindustrial users; in practice it has not been an attractive form of fuel, and has notovercome the cost penalty imposed by the dewatering part of the process.

The refuse-derived fuel option- The advantages of using cRDF are summarised asfollows:a) cRDF has a higher calorific value than raw municipal waste, and is more uniformin its combustion characteristicsb) cRDF has a lower heavy metal content than raw municipal waste, so reducing the

demands on the gas-cleaning equipment


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c) cRDF has a much lower proportion of non-combustible materials than rawmunicipal waste, so there is less ash to be handledd) efficiency is increased because the combustion conditions can be tailored moreprecisely to the feedstock specification.Sweden and the USA have considerable experience of using cRDF. The USA has inthe past tended to mix cRDF with coal (typically 1:4); but newer incinerators,integrated with the waste processing plant, run efficiently on 100% RDF. Sweden hascarried out trials of fluidised bed incinerators fuelled entirely with cRDF at Sundsvalland Lidkoping, and regards this technology as largely proven.

Solid Recovered Fuel (SRF)-Solid recovered fuel (SRF) contains a large proportion (up to 68%) ofbiomass/organic material found in the commercial/municipal solid wastes. Industryuses SRF for the production of power and heat. SRF contains smaller proportion ofash (compared to other wastes) and has a CV of approx 11 to 14 MJ/kg.

Co-mingled

Source-segregated Unsorted

Recyclate Recyclate

Municipal solid waste (MSW)

Residue SRF orshredded

Energy Fuel Gas Char, Oil, Gas

Combustion Co-combustion

(fossil, biomass)Gasification

Anaerobic

digestion

MethaneCompost

Composting

Biodegradable

Digestate

(Soil conditioner)

MRF MBT

Food/garden waste Unsorted

Segregation

Food/garden waste,

Biodegradable in MBT

Pyrolysis

Unsorted MSW,

Residue from MRFSRF or shredded waste

Energy

Biological Processes Thermal Processes

Thermal/BiologicalProcesses

Finaldisposal

Landfill

Unsorted MSW,Residue from treatment processes

Ash from thermal processes

Ferrous metals

Feedstock (construction industry)

Residue

B BLTT

LT

L

T

B to biological process

to thermal process

to landfill

MRF material recovery facility

MBT mechanical/biological treatment

SRF solid recovered fuel

Waste flows and available treatment technologies.

-----------------------------------------------------------------------------------------------------------------Waste Materials HandlingMajor functions of an incinerating plant include:1 - receiving,


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2 - handling,3 - storing,4 - blending,5 - feeding waste.Materials-handling systems are specific to waste type (solid, liquid, sludge, etc.), andto the mode of generation of that stream.Bulk Liquids UnloadingTypically, liquids and other pumpable materials are delivered to incineration facility inbulk by tank trucks or rail tank cars. Liquids from bulk systems are normallytransferred through a piping system by pumps, gravity flow, or compressed gassystems. Pumping systems, using positive displacement or centrifugal pumps aremost commonly used for moving liquid wastes. Positive displacement pumps aregenerally preferred over centrifugal pumps because their construction preventssiphoning when not in use. Gravity unloading may be preferred when liquids have arelatively high vapour pressure.Container UnloadingContainers, such as steel or fiber drums, barrels and special bulk units loaded on rail

box cars or semi-trailer trucks, are used to transport waste to an incineration facility.The containers can be unloaded from the trailers and rail cars manually with specialdrum-handling equipment. Forklift tracks are normally used to unload drums onpallets. Once a container has been unloaded it can be placed in storage, and thecontents can be transferred to other storage.Bulk Solids UnloadingThree mechanisms are used to unload bulk solids transported by truck or rail: gravity,pressure differential, and fluidized systems. Gravity systems are typically used withdischarge pits for unloading. They are also used with mechanical conveyors (screwconveyor, belt conveyor, bucket elevator, etc.) to transfer the solid waste to storageor directly to the incinerator. Pressure differential (pneumatic) systems are commonlyused to transfer dry powdered materials or granular solids up to approximately -

inch mean particle size. Pneumatic conveyors require that the materials must movethrough piping as well as auxiliary equipment (valves) without clogging, degradation,or segregation, and must be easily separated from the conveying air stream.Waste Liquid-Storage SystemsThree types of tanks are commonly used for liquid-waste storage. Temporaryholding tanks provide initial storage of liquid wastes, allowing the transport vehicle toleave the facility. Batching tanks allow preparation of wastes prior to feeding to theincinerator. Preparation might include blending for viscosity control or mixing toreduce the net chloride content of the waste feed. Main storage tanksare used tostore wastes that have been accepted by the facility. There may be a number ofstorage tanks in a single facility to hold wastes segregated by heating content,moisture content, viscosity, reactivity, etc.

Bulk-Solids StorageBulk solids received at an incineration facility can be stored in enclosed bins or silos;concrete pits or below-grade stockpiles; and stockpiles on grade. Materials withpotential toxicity problems or wastes with explosive, flammable, or corrosiveproperties, are generally stored in totally enclosed units, such as single-outlet bins,multiple-outlet silos, and portable bins. Concrete hoppers are normally not used forstorage of hazardous materials.Solids Processing and FeedingAvailable systems for charging solid wastes into an incinerator make use ofpneumatic, mechanical, or gravity techniques. Heterogeneous solid wastes aregenerally subject to some form of size reduction (shredding or pulverizing, forinstance) to meet feed-system requirements and to facilitate proper injection,distribution, combustion within the incinerator. Conveyors are often used for solid-waste transport. The three most common type of conveying systems are:


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1 - the belt conveyor,2 - the apron conveyor,3 - the flight conveyor.A belt conveyor, utilizes a neoprene, synthetic, or metal belt to move materials. It isusually limited in incline to no more than 15 from the horizontal. Variations of thisconveyor have intermediate flights or rib, which allow higher belt inclination.The apron conveyor, consists of a series of metal pans moving along a track. Thesepans move more slowly than the belt in a belt conveyor; however, they can withstandgreater impact from dropped waste than can belt system. A drag (or flight) conveyorsystem is used for ash disposal, and, occasionally for solid-waste transfer.Sludge HandlingGenerally, sludges are generated at the incinerator location and extensive storagefacilities are not required. Vacuum filters, belt-filter presses, and centrifuges arecommon sludge-dewatering devices, and each of these generate a continuoussludge feed to an incinerator.Another common method of sludge dewatering is theplate-and-frame press, which generates sludge in batches, one 15 to 30 minutebatch every 2 or 3 hours. The feed to an incinerator should be constant and

consistent. To even out this batch generation, sludge storage must be providedbetween a plate-and-frame press and an incinerator. This storage can be in ahopper beneath the press or it can be in a separate silo.Sludge handling can be a difficult problem. Sludge can adhere to conveyor andfeeder surfaces and the physical sludges characteristics are sensitive to changes inmoisture content. Normally, sludges are moved with belt conveyors, screwconveyors, plunger pumps or progressive-cavity pumps.Belt and screw conveyors are used when gravity feedings of an incinerator ispossible. An incinerator that is under positive internal pressure, such as a fluid-bedincinerator, requires that sludge be injected at positive pressure. The plunger pumpand progressive-cavity pump will inject sludge into pressurized systems. Thedistance limit for sludge pumping is normally 50 feet. The longer the pipeline, the

greater the load on the pump will be. By reducing the distances to the incinerator,the feed system can be simplified.

Waste pre-processing operations are:1. Size reduction

2. Compaction

3. Baling

4. Separartion

Densification of wastes may be achieved by size reduction (pulverisation), bulkywaste shredding, compression and high density baling, screening and separation into

relatively homogeneous components , baling of waste paper , crushing of glass andbottle containers for use as cullet, compaction and shredding or baling of metalcontainers, compaction and crushing of plastic containers, grinding of organismputrescibles for composting.Pulverisation is the mechanical shredding and size reduction of solid wastes toparticles of approx 5 to 10 cc top size. It helps in reducing space requirements atstorage area, in transportation and at the ultimate disposal site. Pulverising plant isavailable in continuous feed pulverisers or batch fed units. Commercially availablepulversiers fall into 3 main groups: hammer-mill pulverisers, rotary drum pulverisersand impact breakers and bulky waste-shears.Main benefits:

Increased material recovery for recycling

Remove non-combustible materials :

Landfill diversion


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Waste Incineration vs LandfillAdvantages:

Incineration is a fast method of disposal. The stabilisation of waste in a landfilltakes many years, whereas incineration in a grate takes a matter of hours.

Incinerators yield a more complete reduction in volume typically the residue ofincineration is about 25 40% by weight and only 8 12% by volume of theoriginal waste.

The residue of combustion is dense.

Option of energy recovery to reduce costs (production of steam, electricityand heat)

Current incineration capacity is low and incineration with energy recovery canstill provide significant environmental benefits.

The markets for recycled materials are uncertain, and may become quicklysaturated as material is diverted from landfills.

This will lead to a significant expansion in incineration with energy recoveryover the next 10 15 years as a contribution to a more sustainable wastemanagement system

Disadvantages

High construction costs

High operation and maintenance costs

MSW managementoption

Impacts

Incineration Emissions of toxic substances to air;

Emissions from transport;

Production of hazardous waste (fly ash); Contaminated waste water;

Combustion produces carbon dioxide, a greenhouse gas; Odours and possible vermin at waste storage prior to

incineration.Landfill

Leachate of toxic substances, nutrients, etc to surfaceand ground waters; Methane and carbon dioxide as greenhouse gases;

Combustion products from flaring/utilisation, eg nitrogenoxides;

Emissions from transport of waste;

Health risk from vermin; Explosions;

Dust;

Odour.-----------------------------------------------------------------------------------------------------------------

Incineration Technologies


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The primary function of an incinerator is to burn waste to an inert residue, the thermalefficiency and quality of heat transfer for heat recovery are secondary to their primaryfunction. Incineration plants may be classified on a variety of criteria, for example,their capacity, the nature of the waste to be combusted, the type of grate system etc.However, a broad classification may be:-

1. Single stage combustion

A single chamber unit in which complete combustion or oxidation occurs. Typicalexamples of single stage combustion are the large municipal incinerator systems,fluidised bed combustors and cyclonic combustors. Single stage type of combustionsystems require gas clean up systems of some form.

2. Two stage combustion

Two stage combustion is also known as starved air combustion. The two stagesinvolved are gasification of the waste where semi-pyrolysis occurs and secondarycombustion where complete oxidation of the formed gases occurs. Two stagecombustors include the pyrolytic combustor with static hearth, the rotary kiln androcking kiln.

1. Single stage combustion

a) Travelling grate (mass burn) Municipal Solid Waste Incinerators (MSW)

A typical municipal waste incineration plant may be divided into four main areas:-

1. Bunker and feeding system

2. Furnace and combustion chamber

3. Heat recovery

4. Gas cleaning

Example of travelling grate incineration plant


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1. Collection vehicle, 2. Waste bunker, 3. Crane, 4. Hopper,5. Ram feeder, 6. Grate7. Combustion chamber/boiler, 8. Acidic gas removal, 9. Particulate removal, 10.Bottom ash, 11. Control room, 14. stack

1. Bunker and Feeding System

The bunker and feeding system includes the provision for storing the waste to allowcontinuous operation of the plant. A crane transfers the waste to the feeding system.The feeding system is a steel hopper where the waste is allowed to flow into theincinerator under its own weight.

2. Furnace and Combustion Chamber

The waste is fed into the furnace and combustion chamber either by anindependently controlled ram or by the action of the first part of the stoker.Incineration of waste takes place broadly in three stages; drying, ignition and

combustion. The gases and vapours formed are also burned out to control airpollution. In practice the various stages merge, since the components of the wastestream differ in moisture and ignition temperature. Complete combustion of thewaste and gases generated during drying and thermal decomposition requiressufficient residence time, temperature and turbulent mixing. To ensure completecombustion the temperature in the combustion chamber should be at least 850 C. Itshould not exceed 1000 C as above this temperature ash fusion is likely to occurleading to a build up of slag on refractory material. Typical mean residence times forgases are 2-4s and 30-60 minutes for solids. Control of the air supply to the furnaceand combustion chamber is essential for combustion. Primary air is fed evenlythrough the fuel bed via the underside of the grate which assists in combustion andcooling of the grate. Secondary air is introduced through nozzles above the fuel bed

and has the purpose of creating turbulence in the combustion chamber gases andthus ensuring complete combustion and avoiding local reducing conditions which areresponsible for boiler tube corrosion. Tertiary air in some plants is added to cool theflue gases before gas cleaning treatment.

A number of different types of furnace grate exist for municipal waste incineration, forexample the roller (Dusseldorf) system, reciprocating systems, reverse reciprocating(Martin) systems, rocker (Nichols) system and continuous (L. Stoker) systems. Thegrates are automatic and serve to move the waste from the charging and to thedischarge end, whilst providing agitation or tumbling of the fuel bed. The grate has avariable speed drive to adjust the residence of time of the waste in the combustionzone to allow for changes in composition. The grates are generally arranged in

sections which assist the distribution and control of primary air.

The combustion residue, ash, metals and charred material is dischargedcontinuously at the end of the last grate section into a water trough and quenched.

3. Heat Recovery

i) Heat Recovery

The potential for heat recovery from the incineration process is due to the fact thatthe combustion gases must be cooled before they can be discharged through the fluegas cleaning system. The temperature of the gases leaving the combustion chamber,

between 800-1100 C is too high for direct discharge since gas temperature of 250-300 C are required for gas cleaning such as electrostatic precipitators.


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The hot water or steam produced in the boiler may be used to provide power fordriving auxiliary plants, for example soot blowers, space or district heating or in largeplants for electricity production. The design of the boiler has to ensure a reasonablygood heat transfer without the occurrence of excessive fouling, allow for the cleaningof the boiler surfaces using soot blowers etc., allow for optimum circulation of theboiler feed water, be mechanically stable at the operation conditions and beproduced at the lowest cost. A large number of different boiler design exist but maybe divided into two main types, those with vertical flues and those with horizontalflues.

In large scale waste incineration the boiler efficiency is adversely affected by a seriesof factors :-

a) the large amount of excess air required to complete the combustion and cool thecombustion chamber, increases the losses of sensible heat in the flue gases.

b) the ash content of municipal waste is high which increases the losses of sensible

heat and unburnt carbon in the residue.

c) the boiler is large and massive and increases the heat losses of the plant.

d) the boiler operates with short term and long term fluctuations in load.

ii) Boiler Tube fouling

The flue gases contain flyash, charred paper, volatised salts etc and these materialsgradually form a layer of deposits on the boiler tubes. The rate at which tube foulingbuilds up depends on the dust loading of the flue gases, stickiness of the flyashwhich depends on temperature, flue gas velocity and tube bank geometry. The

adherence of flyash to boiler tubes is mainly determined by the presence of moltensalts such as calcium, magnesium and sodium, sulphates, oxides, bisulphates,chlorides, pyrosulphates etc in the flyash. Tube fouling is increased in the presenceof SO3 and HCl. Tube fouling gradually increases the pressure drop over the tubebanks and decreases the rate of heat transfer and hence the steam generation andflue gas cooling. Scale deposits can be partially removed by means of soot blowers(using superheated steam), shot cleaning (dropping cast iron shot on the tubes toknock off the deposit) or by rapping the tubes (rapping the tube banks to knock offthe deposits). Soot blowers are the most common and are usually operated once pershift.

iii) Corrosion

Corrosion is another primary consideration in the design and operation of incineratorboilers. The formation of HCl by the combustion of PVC may cause seriouscorrosion of tubes due to low temperature acid corrosion. Critical control oftemperature is required to prevent high temperature and low temperature corrosionof the boiler. High temperature corrosion involves superheater tubes, and ofevaporator in the combustion chamber and in the first flues. High temperaturecorrosion involves a series of interactions between tube metal, tube scale deposits,slag deposits and flue gases. The rate of corrosion is influenced by temperature, thepresence of low melting phases such as alkali bisulphates and pyrosulphates andHCl and SO3, the nature of the tube metal and the periodic occurrence of reducingconditions. The low melting phases are eutectic mixtures formed between metal

salts and the metal surface,


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Low temperature corrosion is due to condensation of acid gases as temperature fallsbelow the dew point. Therefore gas temperatures of more than 200 C and metaltemperatures of more than 140 C are required. The corrosion due to acid gases ismost pronounced for HCl which occurs in high concentration in the flue and is due todew point corrosion and is most likely to occur in the air pre-heater, electrostaticprecipitator, ducts and stack. The dew point of HCl is low, falling in the range of27 C to 60 C, depending on the HCl concentration and the water content of the fluegas.

Boiler tube Fouling The flue gases contain fly ash , charred paper, volatilisedsalts and these materials gradually form a layer of deposits on the boiler tubes. Therate at which tube fouling builds up depends on the:

Dust loading of flue gases

Stickiness of the flyash which depends on temperature

Flue gas velocity

Tube bank geometry

The adherence of fly ash to boiler tubes is mainly determined by the presence ofmolten salts such as calcium, magnesium and sodium sulphates, oxides chloridesand pyrosulphates in the fly ash. Tube fouling is increased in the presence of SOxand HCl.

Tube fouling gradually increases the pressure drop over the tube banks anddecreases the rate of heat transfer and hence the steam generation and flue gascooling. Scale deposits can be partially removed by means of:

Soot blowers (using superheated steam)

Shot cleaning (dropping cast iron shot on the tubes to knock off the deposit)

Or by rapping the tubes (rapping the tube banks to knock off the deposits)

Soot blowers are the most common and are usually operated once per shift

4. Gas Cleaning

Finally the emissions from the incinerator are passed to the chimney which serves to

disperse the combustion products to be surrounding environment within acceptablelevels. The chimney also provides a draught to drive the combustion gases throughthe furnace.

a) Other types of single stage combustors

There are several types of furnaces that are used primarily for the incineration ofsludge waste. Sludges are those materials with semi-liquid consistency thatcomprise small particles within a liquid mixture, as opposed to slurries, which arelarge particles in a liquid medium (usually water).

1. Fluidised Bed Incinerators


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This consists of a bed of sand particles contained in a refractory lined chamberthrough which the primary combustion air is blown from below, the sand particles arehence fluidised by adjusting the air flow. The fluidised bed reactor promotes thedispersion of incoming solid fluid particles, heats them rapidly to ignition temperatureand promotes sufficient residence time in the reactor for their complete combustion.Secondary functions include the uniform heating of excess air, good heat transfer forheat exchange surfaces within the bed, and the ability to reduce gaseous emissionsby control of temperature, or the addition of pollutant absorbing material directly tothe bed.

The fluidised bed reactor greatly increases the burning rate of waste since the rate ofpyrolysis of the solid waste material is increased by direct contact with the hot inertbed material; also gases in the bed are continuously mixed by the bed material, thusenhancing the flow of gases to and from the burning solid surface and enhancing thecompleteness and rate of gas phase combustion reactions; finally the charredsurface of the burning solid material is continuously abraded by the bed material,enhancing the rate of new char formation and the rate of char oxidation.

The fluid bed may be used to incinerate solid, liquid or gaseous waste streams andthose with difficult combustion properties such as acid tars and sewage sludge.

Fluidised bed Incinerator

Three main types:Bubbling fluidised bed(BFB) : Low air velocity ( 1- 3 m/sec), Intense mixing, Nearisothermal conditions in bedCirculating fluidised bed(CFB) : High air velocities ( 5 6 m/sec), Recirculation ofsolid particles via hot cyclone,Revolving turbulent Fluidised bed: Internal geometry provides greater solid mixing

than BFB

Fluidised bed Systems (main features):

Rapid mixing of solids creates isothermal conditions throughout the reactor

Thermal flywheel effect limits temperature variations

Minimisation of hot spots when combusting high CV materials

Heat & mass transfer between gas and solids is very high

In-situ removal of acid gases by addition of limestone

Reduced corrosion risk allows for higher steam temp giving increased thermal

efficiency Typical operating temp creates low level of NOx

Can handle low CV and high CV wastes

Can handle high moisture and high ash wastes


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Rotary Kiln

The rotary-kiln incinerator can handle solid, liquid gaseous and sludge wastes.Throughout the world, more rotary-kiln incinerators are used for the destruction ofnon-liquid hazardous wastes than any other incinerator. A source of heat (gasburner) is required to bring the kiln up to operating temperature and to maintain itstemperature during incineration of waste.

Kiln Application

The rotary-kiln can incinerate a wide variety of wastes; however, its application haslimitations. Advantages and disadvantages in the use of a rotary kiln as anincinerator can be summarised as follows:

Advantages:

Ability to incinerate a variety of waste streams

Minimal waste pre-processing

Direct disposal of wastes in metal drums

Ability to incinerate varied types of wastes (solids, liquids, sludges, etc.) at thesame time

Availability of many types of feed mechanisms (ram, feeder, screw, direct injection,etc.)

Readily controlled residence time of waste in kiln

High turbulence and effective contact with air within kiln

Disadvantages

Relatively high particulate carryover to the gas stream

Separate afterburner normally required for destruction of volatiles

Conditions along kiln length are difficult to control

Relatively high amount of excess air, normally 100 percent of stoichiometric,

required

Effective kiln seal difficult to obtain

Significant amount of heat is lost in the ash discharge

Operation in a slagging mode to process inorganic wastes or metal drumsincreases kiln maintenance requirements

There are a number of variations in kiln design, including the following: Parallel orcounterflow, Slagging or non-slagging, Refractory or bare-wall

The most commonly used kiln design, referred to as the conventional kiln, is aparallel-flow, non-slagging, refractory-lined system.


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When gas flow through the kiln is in the same direction as the waste flow, the kiln issaid to have parallel or co-current flow. With counter-current flow, the gas flowopposite the flow of waste.

Slagging mode At temperature in the range of 1000 to 1200 C ash will start todeform and as the temperature increases, the ash will melt. A kiln can be designedto generate and maintain molten ash during operation. Operation in a slagging modeprovides a number of advantages over non-slagging operation. The construction of aslagging kiln is more complex than that of a non-slagging kiln, requiring a lip at thekiln exit to contain the molten material. A non-slagging kiln will normally operate attemperatures below 1000C. The destruction of organic compounds is achieved by acombination of high temperature and residence time. Generally the higher thetemperature, the shorter residence time required for destruction. Conversely, thehigher the residence time, the lower the required temperature will be. The use ofhigher temperatures in the slagging kiln reduces the residence time requirements forthe off-gas. The after-burner associated with a slagging kiln can often be muchsmaller than that required for a non-slagging kiln.

A danger in a slagging kiln operation is that the slag will solidify. When this happensthe kiln will be off balance. One reason for the slag freezing besides a drop in thetemperature, is a change in the feed quantity. To assure the maintenance ofadequate eutectic parameters, additives may have to be employed. These additivesmay include CaO, Al2O, SiO2 depending upon the nature of the waste. Additives willhelp maintain the eutectic point to assure that the slag will remain molten.

The waste residence timein a kiln can vary. It is a function of kiln geometry and kilnspeed as shown below:

T = 2.28 x (L/D) / (SxN)

Where t = mean residence time (minutes)

L/D = internal length to diameter ratio (m/m)

N = rotational speed (revolution per minute)

S = kiln rake (cm per meter of length).

For a given L/D ratio and rake, the solids residence time within the unit is inverselyproportional to the kiln speed. By doubling the speed, one halves the residence time.

Multiple Hearth Furnace/IncineratorThe multiple hearth furnace is the most prevalent furnace used for sludgeincineration. There are over 400 of these furnaces in operation in UK today, themajority of which are used for sludge incineration at waste water treatment plants.They are also used for carbon regeneration and lime recalcining.The multiple hearth furnace is a vertical steel shell , lined internally with refractory.Sludge cake is fed by gravity at the top of the furnace. Sludge can also be fed to thefurnace from side, through a screw type feeder. The furnace interior is composed of aseries of circular refractory hearths, one above the other. The hearths are self-supporting, each acting as an arch supported from the refractory lining the furnaceshell. The hearths are designated with number 1 as the top hearth, number 2 next to

the top, etc. There are from 5 to 9 hearths in a typical furnace, and they are normallybuilt in diameters of from 3 to 8 m.


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A vertical shaft is positioned in the centre of the furnace. Rabble arms are attached tothe centre shaft above each hearth. The centre shaft rotates and drives the rabblearms. A series of rabble teeth on each rabble arm wipes sludge across each hearth.The shaft is driven at a speed of to 2 revolutions per minute by a variable speeddrive at the bottom of furnace. Sludge combustion air is introduced at the bottom ofthe furnace. It rises through the furnace, passing over each hearth, picking up theproducts of combustion and elutriated ash particles with it. Additional combustion airis often supplied above additional hearths within the furnace. Generally centre shaftcooling air id recycled as sludge combustion air in the furnace. The temperature ofthis air stream ranges from 130 C to 230 C. It suse in the furnace represents wasteheat energy reclamation. Off gas exists from the top of furnace. Approx 10 to 20 % ofnon-combustible (ash) component of the sludge will be airborne and will exit with theoff gas. The upper hearths of the furnace comprise the drying zone, where the sludgecake gives up moisture while cooling the hot flue gas. Approx 5 kg of moisture arereleased per square meter of hearth area per hour. When the moisture content ofsludge within the furnace is reduced to 30%, the sludge begins to burn. The burningrate is approx 6 kg of combustibles per m2of hearth area per hour, applied above the

burning hearths.In a typical six hearth furnace, the upper two hearths will normally be the dryinghearths, where sludge loses most of its moisture. The two middle hearths are theburning hearths, where both the air and other gases passing through the furnace andthe sludge are heated to combustion temperature. The sludge residual (ash) will burnout to a sterile ash on the two lowest hearths. The ash cools , heating the air passingover it. All sludge off gas must pass through the afterburner.

Schematic diagram of a multiple hearth incinerator

2. Two Stage Combustion

Starved air waste incineratorStarved air incinerator is a two stage combustion type incinerator which is widelyused for toxic solid wastes from industry. The two stages consist of a pyrolytic stageand a combustion stage. The advantages of the starved air incinerator are a morecontrolled combustion processes leading to lower releases of volatile organiccompounds and carbon monoxide. In addition, the low combustion air flow results in

low entrainment of particulate in the flue gases which also reduces other particulatebourne pollutants such as heavy metals.


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1st stage: the waste is combusted under sub-stoichiometric conditions, i.e. wherethere is insufficient air to provide complete combustion and therefore there is a highproportion of the products of incomplete combustion which pass through to thesecond stage e.g soot, higher hydrocarbons, carbon monoxide, hydrogen, methane.The temperature of the gases leaving the pyrolytic section is of the order of 700 800 C2nd stage: These gases will then pass to the secondary section where secondaryexcess air approx 200% stoichiometric is added, to give a temperature of 1000-1200C which completes the combustion process.The gases entering the secondary chamber have a sufficient calorific value to be selfsustaining in combustion. Secondary air is introduced to provide the excess airconditions with a high degree of turbulence to create sufficient mixing to sustaincombustion without the use of support fuel at typical operating temperatures ofbetween 1000 and 1200 C-------------------------------------------------------------------------------------------------------------Alternative Waste Treatment Technologies

Wet Air Oxidation TechnologyWet air oxidation is a process for oxidising organic contaminants in water. Wetair oxidation refers to the aqueous phase oxidation of dissolved or suspendedorganic substances at elevated temperatures and pressures. Water whichmakes up the bulk of the aqueous phase serves to modify oxidation reactionsso that they proceed at relatively low temperatures i.e. 350 650 C and at thesame time serves to moderate the oxidation rates removing excess heat byevaporation. Water also provides an excellent heat transfer medium whichenables the wet oxidation process to be thermally self sustaining withrelatively low organic feed concentrations.An oxygen containing gas usually air is bubbled through the liquid phase in a

reactor used to contain the process, thus the commonly used term wet airoxidation. The process pressure is maintained at a level high enough toprevent excessive evaporation of the liquid phase, generally between 300 and3000 psi.A waste water stream containing oxidizable contaminants is pumped to thesystem by means of a positive displacement type pump. The waste waterpasses through a heat exchanger which preheats the waste by indirect heatexchange with the hot oxidised effluent. The temperature of the incomingwaste feed is increased to a level necessary to support the oxidation reactionin the reactor vessel. Air and the incoming liquid are injected into the reactorwhere the oxidation begins to take place. As oxidation progresses up through

the reactor, the heat of combustion is liberated, increasing the temperature ofthe reaction mixture. This heat of oxidation is recovered by a heat exchangethat utilizes the incoming feed. Thus it is thermally a self-sustaining operation.After energy removal, the oxidised effluent consisted mainly of water, carbondioxide and nitrogen is reduced in pressure through a specially designedautomatic valve. Of all variables affecting wet air oxidation, temperature hasthe greatest effect on reaction rates.Molten salt TechnologyThe molten salt process is designed for solid and liquid waste streams. It isespecially applicable to highly toxic wastes and to highly halogenated wastestreams. Waste streams with high percentages of ash and non-combustibles


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are not very good for the system since such waste makes it necessary toreplace the molten bed more often.Molten salt process is a method of burning organic waste material while at thesame time scrubbing in-situ any toxic by products of that burning and thuspreventing their emission in the effluent gas stream. This process of

stimulating combustion and scrubbing is accomplished by injecting the wastematerial to be burned with air or oxygen enriched air under the surface of apool of molten sodium carbonate. The melt is maintained at temperatures ofthe order of 900 C causing the hydrocarbons of the organic waste material tobe immediately oxidised to carbon dioxide and water. The combustion byproducts containing such elements such as phosphorous, sulphur, arsenicand halogens react with sodium carbonate. These by products are terained inthe melt as inorganic salts rather than being released to the atmosphere asvolatile gases. In time inorganic products resulting from the reaction of organichalogens, phosphorous sulphur etc build up and must be removed from themolten bed to retain its ability to absorb acidic gases. Ash introduced by

waste must be removed to preserve the fluidity of the melt. An ashconcentration in the melt of about 20 % by weight provides an ample marginof safety to maintain melt fluidity.Plasma Arc TechnologyPlasma Arc is a process using the extremely high temperatures of plasma todestroy hazardous waste. A plasma is a substance consisting of charged andneutral particles with an overall charge near zero. A plasma arc is generatedby electricity and can reach temperatures up to 27000 C! When applied towaste disposal, the plasma arc can be considered as an energy conversionand energy transfer device. The electrical energy is transformed into a plasma.As the activated components of the plasma decay, their energy is transferredto the waste materials exposed to the plasma. The wastes are ultimatelydecayed and destroyed as they interact with the decaying plasma.

Supercritical Water ProcessIn the supercritical water process an aqueous waste stream is subjected totemperatures and pressures above the critical point of water i.e. that point atwhich the densities of the liquid and vapour phase are identical. For watercritical point is 379 C and 2018 atm. In this supercritical region, water exhibitsunusual properties that enhance its capacity as a waste destruction medium.Because oxygen is completely miscible with supercritical water, the oxidation

rate for organics is greatly enhanced. Also inorganics are practically insolublein supercritical water. This factor allows the inorganics to be oxidisedextremely rapidly and the resultant stream is virtually free of inorganics.

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Waste to Energy Legislation & Public Perception

Perception-All waste management options involve risks of pollution and releases of thepollutants to air, land and water. Landfill for example can pose a significant potential

for causing pollution in the form of leachates into groundwater and methane gaswhich is a greenhouse gas produced from the breakdown of waste in landfill sites.


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Other forms of waste re-use and recycling options also emit by-products such aswaste sludge, which must be subsequently disposed of by incineration. At present,the major public concern over emission from the waste incinerator plants are thelevels of dioxins in the solid residues and releases to air, particularly from municipalsolid waste incinerators.The incineration of waste is a field that has been changing very rapidly in recenttimes. The future of incineration seen from the present perspective will involve alarge increase in the number of incinerators primarily due to legislation banning thedisposal and re-use of wastes, such as spent cooking oil, via other routes. Releasesfrom waste incineration are expected to change in response to emissions legislation,recycling initiatives and ash re-utilisation practices as well as other social factorssuch as waste minimisation initiatives. Whilst technology can reduce emissions toalmost any level, this usually involves increased costs. Careful balance should bestruck between the level of pollution control required and environment and healtheffectsSome key points of discussion:- Public perception why is it important to understand?

- Public perception of risk different from experts?- Highlights differences between opinions of experts scientists/engineers and ofgeneral public TRUST!- Importance to policy/decision makers including local level- The need to win public acceptance can be a major constraint on how, and evenwhether particular policies/decisions can be made, examples objections of localresidents and environmental groups to new waste to energy plants, opposition leadsto increased consultation, planning enquiries etc, all of which are costly and timeconsuming creates a negative attitude !-How do we address these perceptions? - Science & Engineering avoid confusion!

Legislation/Policies:

The operation of waste to energy plants is controlled by UK Environment Agencyusing IPRs and applying the principles of BATNEEC (best available technology notentailing the excessive costs). These regulations set out the maximum levels of eachpollutant permitted for any incinerator plant. The pollutants present in the variouseffluents from the plant are specified separately. Applicants for an IPC authorisationmust be able to demonstrate that the process chosen represent BEPO (bestpracticable environmental option).Waste Strategy 2000

Need for change

Waste Hierarchy

Proximity Principle Regigional self-sufficiency

Market based strategies

Policy on waste incineration

Targets for Local Waste Strategies

The Waste Hierarchy

- Related to full environmental cost of the waste, leads to a theoretical ranking asfollows

- Waste reductionI) Recovery: Recycling/Composting


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Energy recovery (incineration)

ii) Disposal : Incineration (without energy recovery)

Volume reduction

Landfill

Proximity Principle

Dispose or manage as close as practicable to origin

- Encourage producers to take more responsibility

- Sustainable reduces transportation and risk of environmental harm

Regional Self Sufficiency

Aim to treat in the Government region in which produced. WPAs should plan for atleast 10 years of production

But treat flexibly e.g proximity principle sometimes says treat in adjacent region.

Policy for Waste Incineration

- Incineration with energy recovery should not be undertaken withoutconsideration first being given to the possibility of composting and materialrecycling.

- Current incineration capacity is low and incineration with energy recovery canstill provide significant environmental benefits.

- The markets for recycled materials are uncertain, and may become quicklysaturated as material is diverted from landfills

- This will lead to a significant expansion in incineration with energy recoveryover the next 10 15 years as a contribution to a more sustainable wastemanagement system

Waste Strategy 2007 EfW

Recovering energy from waste which cannot sensibly be recycled is an essentialcomponent of a well-balanced energy policy. The UK Government plans to use PFI,Enhanced Capital Allowances and/or ROCs to encourage a variety of energyrecovery technologies so that unavoidable residual waste provides the greatestbenefits to energy policy. EfW is expected to account for 25% of municipal waste by2020!

There are a number of regulations that regulate the waste management industry,such as the EU directives, landfill tax and some national targets as listed in Error!Reference source not found.. The EU landfill directive (1999/31/EC) requiresdiversion of bio-degradable municipal waste (BMW) from landfills. The targets set forthe UK include reduction in the amount of BMW sent to landfill to 75% of the totalweight of BMW in 1995 by 2010, and further 50% reduction by 2013. The Landfill Taxwas introduced in 1996 in the UK in order to reduce landfill of waste. It was 24/ton in2007/8 but will continue to increase by 8/year to reach a level of 48/ton. Thepenalty for exceeding the BMW landfill targets is a stronger driver to reduce landfill -150/ton. The new Waste Strategy introduces targets on the waste prevention aspresented in Error! Reference source not found..


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Segregation residues of MSW have to be treated thermally to recover energy. Theenergy can be fully recovered directly by incineration or by other thermal processes.The potential electrical yield from the segregation residues (including commercial andindustrial wastes) could account for as much as 17% of total UK electricityconsumption in 2020 while meeting the recycling targets.

Regulations Target values

BMW landfill targets in the Landfill Directive75% of the amount produced in 1995 by 2010,50% by 2013 and 30% by 2010

Penalty for exceeding the BMW landfill targetsin LATS

150/ton

Landfill tax24/ton, to be increased by 8/ton a year toreach a level of 48/ton

Reduction of household waste in Waste Strategy2007

29% reduction from the level in 2000 in 2010,35% in 2015 and 45% in 2020

Recycling/composting targets of household waste in

Waste Strategy 2007 40% by 2010, 45% by 2015 and 50% by 2020Recovery (Recycling/composting/energy recovery) ofMW in Waste Strategy 2007

53% by 2010, 67% by 2015 and 75% by 2020.

BMW: Biodegradable municipal waste, LATS: Landfill Allowance Trading Scheme

Key national targets and tax for municipal waste management in England.

The major legislative influences on emissions from incineration plants in the UK are,89/369/EEC Municipal Waste Incineration, 94/67/EC Hazardous Waste Incinerationand the 2000/76/EC Incineration of Waste. Waste incineration is categorised as aPrescribed Process and as such requires an Authorisation from the Environment

Agency. Prescribed Processes are covered by Guidance Notes issued by theEnvironment Agency which set out benchmark emission limits. Processes Subject toIntegrated Pollution Control IPC Guidance Notes S2 5.01: Waste Incineration waspublished in 1996 under the system of Integrated Pollution Control. With the adventof the concept of Integrated Pollution Prevention and Control (IPPC), theEnvironment Agency has issued an internal pre-consultation draft sector guidancenote (IPPC Sector Guidance Note S5.01: IPPC & IPC Interim Sector Guidance forthe incineration of waste and fuel manufactured from or including waste) which setsout the benchmark limits for the incineration sector, taking into account the variousEuropean Directives including the Waste Incineration Directive 2000/76/EU. Thepresent air pollution control technologies are normally capable of reducing thereleases of pollutants to within the limits imposed by the 2000/76/EC WasteIncineration Directive.

UK legislation covering incineration involves the Environmental Protection Act (EPA)and covers Schedule A process which are deemed larger and technically complexand includes incineration of waste at throughputs of over 1 tonne per hour.Therefore schedule Acovers municipal solid waste incineration and chemical wasteincineration. Schedule A processes are under central control of Her MajestysInspectorate of Pollution (HMIP) and will be subject to integrated pollution control.There will be a system of prior approval established, and it will be a general conditionof authorisations that the process must use BATNEEC (Best Available TechniquesNot Entailing Excessive Costs). Incineration processes with combustion rates of less

than 1 tonne per hour except chemical waste incineration will come under ScheduleB of the EPA and will come under Local Authority control and be subject to air


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pollution control rather than integrated pollution control. Other incineration processessuch as sewage sludge, clinical waste and small packages incineration are alsosubject to legislation and will come under HMIP control.

EU Directives covering municipal waste incineration covers new plants fromDecember 1 1990. These limits apply to existing plants from 1 December 1996 andthere are interim limits for plants under 6 tonnes per hour from 1 December 1995.The Directives apply to plants incinerating domestic waste as well as commercialtrade waste of similar nature or composition and consequently apply to someindustrial incinerators. The Directives do not apply to incinerators specifically forsewage sludge, hazardous waste, hospital waste or other special wastes even ifthese plants incinerate municipal solid waste as well.

The Non-Fossil Fuel Obligation (NFFO)

The Electricity Act of 1989 makes provision for the UK Government to place a Non-Fossil Fuel Obligation (NFFO) on any or all of the Regional Electricity Companies

(RECs). The obligation is a requirement on the RECs to take a certain proportion ofelectricity generated from non-fossil fuel sources such as waste, but also includingother sources such as wind, wave and hydro-power. The obligation in the context ofwaste applies to the recovery of energy from waste via , for example, incineration,gasification, pyrolysis or landfill gas combustion via electricity generation orcombined heat and power schemes. The fossil fuel levy is placed on the fossil fuelpower generators as a percentage of their electricity sales revenue to compensatethe RECs for the higher electricity prices paid under the NFFO. An NFFO specifies aperiod for which the RECs must make available a defined capacity of non-fossilgenerating plant.

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District Heating/CHP (connected to Waste to Energy Plants)

District Heating/Cooling (DHC) provided by a CHP system offers an increase inoverall plant efficiency from approximately 55% for the best conventional electricitygenerating plant to approximately 85% for a CHP plant, whilst reducing CO2emissionby 30%. Furthermore, CHP can also be provided from MSW incineration, which canrepresent almost 20% of the energy needs of a city, again contributing a majorreduction to net CO2emissions.

The efficiency of electricity generation from an incineration by a steam turbine is

about 25%, which is significantly low compared to about 35% for coal-fired powerplant. However, combined heat and power (CHP) significantly increases the energyefficiency, by using the low-grade steam/water from a steam turbine for domesticheating and cooling (DHC). The overall energy efficiency of CHP using conventionalor waste-derived fuels can be more than 65%.

Combined heat and power (CHP) systems both generate electricity and make use ofthe low grade heat. They are small scale units (compared to power stations). Theplant consists of a prime mover (usually an engine) which is used to drive agenerator from which electricity is produced. The heat recovery system extracts heatfrom the exhaust gases, the engine jacket cooling system and from the enginelubricating oil. Of the fuel input, around 25-35% is used to generate electrical power

and between 50-70% is used to produce heat. Thus the overall thermal efficiency ofthe whole plant can be as high as 85% .


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The heat may be delivered as :- Hot air e.g waste or sludge drying- Hot water e.g district heating, green houses etc- Steam e.g tyre manufacturing, paper & pulp manufacturing etc

The potential benefits:

An additional revenue stream, Greater favour in the planning process, because CHP is seen to be good

and the government has targets for CHP installations

Climate Change Levy exemption

The main disadvantages are:

Finding a long-term buyer for heat is exceedingly difficult because there isreluctance to buy heat over the fence in the UK especially when the sourceis an EFW plant,

The operating demands of the heat purchaser may be un-acceptable to the

EFW plant operator e.g heat may be required when EFW operator wishes toshut down.

Plant controllability may be a significant issue because there will be differentdemands on waste disposal, power generation and heat demand operations

The relative value of hot water and steam are low compared with power.

There are 4 alternative means of achieving a CHP system:- Heat from Exhaust steam using a back pressure turbine- Heat from Exhaust steam using a high pressure condenser- Heat from Bled steam using a condensing turbine- Heat from extracted steam using an extraction condensing turbine

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Example Heat & Mass BalancesIncineration generally operates at temperatures between 850oC and 1100oC with alarge excess of oxygen to convert waste materials into ashes and exhaust gas whileproducing thermal/electrical energy from the chemical energy of waste. The overallreaction for incineration can be written as:

w(H2O)CxHyOzAsh + (1+)a (O2+3.76 N2)

x CO2+ (w+2/y) H2O + aO2+ 3.76(1+)a N2+Ash

where w(H2O)CxHyOzAsh: waste, : excess air ratio and a = x+y/4-z/2.

The chemical composition of waste varies significantly between seasons andlocations.

Ultimate and proximate analyses of MSW

Material % by weight

Water 31.2

Carbon 24.0

Hydrogen 3.2

Oxygen 15.9Nitrogen 0.7


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Sulphur 0.1

Chlorine 0.7

Ash 24.2

Waste

1000 kg8.6 GJ (100%)

Solid ResidueBottom ashFilter ash

Incineration(70% Excess Air,

25% Energy Efficiency)

Exhaust GasO2N2

CO2H2O

Air

5377 kg

Electricity2.15 GJ (25%)

516 kg4124 kg

880 kg600 kg

227 kg25 kg

(a) electricity generation only

Waste1000 kg8.6 GJ (100%)

Solid ResidueBottom ashFilter ash

Incineration + CHP(70% Excess Air,

75% Energy Efficiency)

Exhaust GasO2N2CO2H2O

Air5377 kg

Electricity1.29 GJ (15%)Heat5.16 GJ (60%)

516 kg4124 kg880 kg600 kg

227 kg25 kg

(b) combined heat and power

Indicative mass and energy balance of incineration

Other Types of Waste/Biomass Thermal Technologies

Molten Salt TechnologyMolten salt destruction is a method of burning organic waste material while at thesame time scrubbing in-situ any toxic by-products of that burning and thus preventingtheir emission in the effluent gas stream. This process of stimulating combustion andscrubbing is accomplished by injecting the waste material to be burned with air oroxygen enriched air, under the surface of a pool of molten sodium carbonate. Themelt is maintained at temperatures of the order of 900 C , causing the hydrocarbonsof the organic waste material to be immediately oxidized to carbon dioxide and water.The combustion by-products are entrained in the melt as inorganic salts rather thanbeing released to the atmosphere as volatile gases. The molten salt process is

designed for solid and liquid waste streams. It is especially applicable to highly toxicwastes and to highly halogenated waste streams.


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Plasma Arc Technology

This process uses the extremely high temperatures of plasma to destroy hazardouswaste. The plasma is a substance consisting of charged and neutral particles with anoverall charge near zero. A plasma arc is generated by electricity and can reachtemperatures of up to 27000 C! When applied to waste disposal, the plasma arc canbe considered as an energy conversion and energy transfer device. The electricalenergy is transformed into a plasma. As the activated components of the plasmadecay, their energy is transferred to the waste materials exposed to the plasma. Thewastes are ultimately decayed and destroyed as they interact with the decayingplasma.

Molten Glass Incineration (Vitrification)

The integral part of this process is an electric furnace that has a pool of molten glasscovering the bottom. This type of furnace is used extensively in the glass

manufacturing industry to produce glass. When used as a waste incinerator theextremely high temperatures in the combustion chamber destroy organic wastestreams. Waste materials are charged directly into the combustion chamber abovethe pool of molten glass. Electrodes immersed in the pool maintain the temperatureof the pool of the molten glass above 1260 C. Combustible wastes are oxidizedabove the pool, and inorganics and ash fall onto the pool where they are melted intothe glass. Combustion off-gases pass through ceramic filters which are themselvescharged into the molten glass when they are no longer effective.

Supercritical Water Process

In the supercritical water process, an aqueous waste stream is subjected to

temperatures and pressures above the critical point of water, i.e that point at whichthe densities of the liquid and vapour phase are identical. For water, the critical pointis 379 C and 218 atm pressure. In this supercritical region, water exhibits unusualproperties that enhance its capacity as a waste destruction medium. Because oxygenis completely miscible with supercritical water, the oxidation rate for organics isgreatly enhanced. Also inorganics are practically insoluble in supercritical water. Thisfactor allows the inorganics to be oxidised extremely rapidly and the resultant streamis virtually free of inorganics.

cpe 406 6060 lecture 1 handout(2)

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