Pollutant Formation inPollutant Formation inCombustion SystemsCombustion Systems

Pollutant Formation in Combustion SystemsPollutant Formation in Combustion Systems NONOxx Formation Formation

Thermal NOThermal NOxx

Prompt NOPrompt NOxx

Fuel NOFuel NOxx

Carbon Monoxide (CO)Carbon Monoxide (CO) Volatile Organic Compounds (VOC)Volatile Organic Compounds (VOC) Polycyclic Aromatic Hydrocarbons (PAH), Soot and Polycyclic Aromatic Hydrocarbons (PAH), Soot and

Sub micron Particulates (solid-phase pollutants)Sub micron Particulates (solid-phase pollutants) Sulphur CompoundsSulphur Compounds

NOx formationNOx formation

What is NOWhat is NOXX?? NONOxx = Oxides of Nitrogen which are produced by = Oxides of Nitrogen which are produced by

combustion:combustion:Nitric Oxide (NO)Nitric Oxide (NO)

Nitrogen Dioxide (NONitrogen Dioxide (NO22))

Nitrous Oxide (NNitrous Oxide (N22O)O)

Nitric Oxide (NO)Nitric Oxide (NO) Highly reactiveHighly reactive due to lone electron at N atom due to lone electron at N atom Not particularly toxicNot particularly toxic Major precursor of photochemical smog Major precursor of photochemical smog ((NO NO NO NO22)) It is produced by It is produced by mostmost of combustion systems of combustion systems

Nitrogen Dioxide (NONitrogen Dioxide (NO22)) Brown, poisonous gasBrown, poisonous gas Emissions of NOEmissions of NO22 from most combustion < 10% of from most combustion < 10% of

NONOXX

Adverse health effects include: lung irritation, Adverse health effects include: lung irritation, bronchitis, pneumonia and a lowering respiratory bronchitis, pneumonia and a lowering respiratory resistanceresistance

Ambient limit = 120ppb [NEPC, 1998]Ambient limit = 120ppb [NEPC, 1998] Significant direct emissions of NOSignificant direct emissions of NO22 occur from occur from

processesprocesses involving involving premixed flamespremixed flames::

- Indoor gas appliances (20‑100% of NO- Indoor gas appliances (20‑100% of NOxx))

- Gas turbines - Gas turbines yellow/brown plumes yellow/brown plumes

Nitrous Oxide (NNitrous Oxide (N22O)O) Relatively inertRelatively inert Uses: Dental anaesthetic Uses: Dental anaesthetic Strong absorber of infrared radiation (~300 x COStrong absorber of infrared radiation (~300 x CO22)) Stability = long atmospheric residence times (~Stability = long atmospheric residence times (~150 150

yrsyrs)) Hence, potentially significant greenhouse gasHence, potentially significant greenhouse gas Long life‑time also allows its transportation into Long life‑time also allows its transportation into

stratosphere and participates in ozone depletionstratosphere and participates in ozone depletion Only significant from Only significant from low-temperature processeslow-temperature processes (eg. (eg.

Fluidised bed combustion) Fluidised bed combustion)

combustion activities

chemical reactions

Sources of NitrogenSources of Nitrogen Formation of NOFormation of NOXX requires a source of nitrogen requires a source of nitrogen Two sources of nitrogenTwo sources of nitrogen::

a. a. Molecular nitrogenMolecular nitrogen from airfrom air (1/2 N (1/2 N22 (from air) +1/2 (from air) +1/2

OO22 → NO) → NO) Thermal or Zeldovich Mechanism. Thermal or Zeldovich Mechanism. Prompt‑Fenimore Mechanism (HC + NPrompt‑Fenimore Mechanism (HC + N22). ). Other minor mechanismsOther minor mechanisms

b. b. Nitrogen chemically bound within fuelNitrogen chemically bound within fuel Fuel NOFuel NOxx,,

Most of NOMost of NOxx in the form ofin the form of NO NO

Thermal NOThermal NO One of the most important issues for combustion One of the most important issues for combustion

engineers is: 'What are my NOengineers is: 'What are my NOxx emissions?' emissions?' In most cases, unusually high NOIn most cases, unusually high NOxx emissions are due to emissions are due to

NO formed by the Thermal (Zeldovich) mechanismNO formed by the Thermal (Zeldovich) mechanism Thermal NO mechanism involves the attack of molecular Thermal NO mechanism involves the attack of molecular

nitrogen (nitrogen (NN22) and atomic nitrogen () and atomic nitrogen (NN) by oxygen () by oxygen (OO22) ) and oxygen‑containing radicals (and oxygen‑containing radicals (O, OHO, OH). This can occurs ). This can occurs in in oxygen richoxygen rich mixture. mixture.

First identified by Zeldovich (1946) and extended by First identified by Zeldovich (1946) and extended by Fenimore and Jones (1957)Fenimore and Jones (1957)

Described by the following reactions:Described by the following reactions:NN22 + + OO → → NONO + N + N (R.1)(R.1)NN + + OO22 NONO+O+O (R.2)(R.2)NN + + OHOH NONO + H + H (R.3)(R.3)

Westenberg (1971) invoked the steady‑state Westenberg (1971) invoked the steady‑state approximation and determined that the maximum NO approximation and determined that the maximum NO formation rate is given by:formation rate is given by:

Hence, [NO] depends on:Hence, [NO] depends on: Temperature (Temperature (the higher the temp, strongly the the higher the temp, strongly the

higher the NO formedhigher the NO formed) ) high temp environmenthigh temp environment OO22 concentration (the concentration (the higher the oxygenhigher the oxygen conc, the conc, the

higherhigher the NO formed) the NO formed) oxygen-rich oxygen-rich environmentenvironment

Residence timeResidence time

1/ 217 1/ 2 3

2 2

694601.45 10 exp . . / .

eq eq

d NOx T O N mol cm s

dt T K

Fenimore (1971) observed an additional formation of Fenimore (1971) observed an additional formation of NO which could not be explained by the thermalNO which could not be explained by the thermalmechanismmechanism NO formed close to the burner (hence ‑ "prompt' NO)NO formed close to the burner (hence ‑ "prompt' NO) Effect is Effect is not observednot observed under under very fuel‑leanvery fuel‑lean conditions conditions

or in systems with Hor in systems with H22 or CO as fuel or CO as fuel Mechanism involves Mechanism involves the attack of Nthe attack of N22 by hydrocarbon by hydrocarbon

fuel fragmentsfuel fragments, mainly , mainly CH radicalsCH radicals and and CC‑atoms.‑atoms. The Prompt‑Fenimore mechanism is initiated mainly The Prompt‑Fenimore mechanism is initiated mainly

by R.4 (the formation of HCN) with a lesser by R.4 (the formation of HCN) with a lesser contribution from R.5:contribution from R.5: CH CH + + NN22 → → HCNHCN + N + N (R.4)(R.4) CC + + NN22 → CN + N → CN + N (R.5)(R.5)

Prompt NOPrompt NO

HCNHCN is subsequently is subsequently oxidised to NOoxidised to NO (see diagram)(see diagram) Prompt mechanism dominates for hydrocarbon Prompt mechanism dominates for hydrocarbon

combustion in combustion in fuel‑richfuel‑rich, in both premixed and diffusion , in both premixed and diffusion flamesflames

Minor MechanismsMinor Mechanisms

1. N1. N22O‑Intermediate Mechanism:O‑Intermediate Mechanism: It occurs under It occurs under fuel‑lean, low‑temperaturefuel‑lean, low‑temperature conditions conditions Minor source of NOMinor source of NO in most practical combustors in most practical combustors Mechanism is given by:Mechanism is given by:

OO + + NN22 + M → + M → NN22OO + M + M (R.6)(R.6)

OO + + NN22OO → → NONO + NO + NO (R.7)(R.7)

H + NH + N22O → NO + NHO → NO + NH (R.8)(R.8)

2. NNH-Intermediate Mechanism:2. NNH-Intermediate Mechanism: It is observed under It is observed under laboratorylaboratory conditions conditions HH22 and CH and CH44 (high H) fuel‑rich (high H) fuel‑rich, , laminarlaminar premixed premixed

flamesflames Yet to be definitely observed in practical combustorsYet to be definitely observed in practical combustors Mechanism is given by:Mechanism is given by:

HH + + NN22 → NNH (R.9) → NNH (R.9)

NNH + O → NO + NH (R.10)NNH + O → NO + NH (R.10)

1. The Nature of Fuel‑Nitrogen:1. The Nature of Fuel‑Nitrogen: Nitrogen in coal (and oil) originates in the plant Nitrogen in coal (and oil) originates in the plant

material from which the fuel is formedmaterial from which the fuel is formed

Plants contain nitrogenPlants contain nitrogen in the form of proteins, amino in the form of proteins, amino acids, alkaloids, chlorophyll and porphyrinsacids, alkaloids, chlorophyll and porphyrins

These were transformed, during the coalification These were transformed, during the coalification process, into polycyclic aromatic compounds with process, into polycyclic aromatic compounds with pyridinic, pyrrolic or other functional groupspyridinic, pyrrolic or other functional groups

Nitrogen content of coals typically vary between Nitrogen content of coals typically vary between 1‑2.5 wt% and is largely independent of rank1‑2.5 wt% and is largely independent of rank

Fuel‑NOFuel‑NO

2. Coal Combustion and the Release of Fuel‑Nitrogen:2. Coal Combustion and the Release of Fuel‑Nitrogen: First stage of coal combustion is rapid First stage of coal combustion is rapid devolatilisationdevolatilisation

of the coalof the coal DevolatilisationDevolatilisation: volatile components such as : volatile components such as light light

hydrocarbons and tarshydrocarbons and tars are released and then are released and then oxidised in oxidised in the gas~phase at very short timescalesthe gas~phase at very short timescales (< 10 ms) (< 10 ms)

The solid product of coal pyrolysis is the The solid product of coal pyrolysis is the charchar which is which is oxidised at much slower timescalesoxidised at much slower timescales (~ 1s) (~ 1s)

Fuel‑nitrogenFuel‑nitrogen is released during is released during bothboth pyrolysis and char pyrolysis and char combustion but in very different wayscombustion but in very different ways

Partitioning of fuel‑nitrogen between the volatiles and Partitioning of fuel‑nitrogen between the volatiles and char depends on pyrolysis conditions (normally approx. char depends on pyrolysis conditions (normally approx. equal)equal)

3. Nitrogen released with the volatiles:3. Nitrogen released with the volatiles: Nitrogen contained within the volatiles is released as, Nitrogen contained within the volatiles is released as,

or rapidly converted to compounds such HCN, NHor rapidly converted to compounds such HCN, NH33 or or

HNCOHNCO These simple nitrogenous species then react in the gas These simple nitrogenous species then react in the gas

phase to form either phase to form either NO, NNO, N22O or, underO or, under fuel‑rich fuel‑rich

conditions to Nconditions to N22 (see next slide)(see next slide)

Relative amounts of NO, NRelative amounts of NO, N22O and NO and N22 depends depends strongly strongly

on the local Oon the local O22 concentration during pyrolysis and concentration during pyrolysis and

temperaturetemperature

NONOXX formation is reduced if formation is reduced if volatiles are releasedvolatiles are released under fuel‑richunder fuel‑rich conditions conditions

Pulverised‑fuel combustors ‑ NOPulverised‑fuel combustors ‑ NOXX control techniques control techniques include: low‑NOinclude: low‑NOXX burners, flue gas recirculation burners, flue gas recirculation (fuel-lean) and air staging(fuel-lean) and air staging

NO emissions are reduced in these techniques by NO emissions are reduced in these techniques by changes in changes in stoichiometrystoichiometry and/or and/or temperaturetemperature near the near the burner thus burner thus reducing both volatile‑NO and reducing both volatile‑NO and thermal‑NO (thermal‑NO (important to take into account in important to take into account in combustioncombustion))

4. Char combustion and the Fate of Char‑Nitrogen:4. Char combustion and the Fate of Char‑Nitrogen: The fate of char‑nitrogen is still The fate of char‑nitrogen is still leading edge researchleading edge research Primary product of char‑N oxidation is NOPrimary product of char‑N oxidation is NO However, both However, both HCN and HNCOHCN and HNCO are observed at are observed at low low

temperaturestemperatures (Ashman et al., 2000) (Ashman et al., 2000) NO may be NO may be reduced to Nreduced to N22 by direct reaction with the by direct reaction with the

char surfacechar surface or by a char‑catalysed reaction with CO or by a char‑catalysed reaction with CO Effective techniques aimed specifically at controlling Effective techniques aimed specifically at controlling

char‑NO are not availablechar‑NO are not available5. 5. In principle, NOx is reduced if it is reacted with C or In principle, NOx is reduced if it is reacted with C or

CxHy CxHy concept of reburn concept of reburn

Summary of NOx FormationSummary of NOx Formation

ReferencesReferences Ashman, P1, Haynes, B.S., Nicholls, P.M. and Ashman, P1, Haynes, B.S., Nicholls, P.M. and

Nelson, P.F. (2000). Interactions of gaseous NO with Nelson, P.F. (2000). Interactions of gaseous NO with char during the low temperature oxidation of coal char during the low temperature oxidation of coal chars. chars. 28th Symp. (Int.) on Combustion, 28th Symp. (Int.) on Combustion,

Bowman, C.T. (1992). Conrol of combustion- Bowman, C.T. (1992). Conrol of combustion- generated nitrogen oxide emissions: technology generated nitrogen oxide emissions: technology driven by regulation, driven by regulation, 24th Symp. (Int.) on 24th Symp. (Int.) on Combustion, Combustion, The Combustion Institute, pp 859‑878.The Combustion Institute, pp 859‑878.

Fenimore, C.P. (1971). Formation of nitric oxide in Fenimore, C.P. (1971). Formation of nitric oxide in premixed hdrocarbon fames. premixed hdrocarbon fames. 13th Symp. (Int.) on 13th Symp. (Int.) on Combustion, Combustion, The Combustion Institute, pp 373‑379.The Combustion Institute, pp 373‑379.

Fenimore. C.P. and Jones, G.W. (1957). The water Fenimore. C.P. and Jones, G.W. (1957). The water catalysed oxidation of carbon monoxide by oxygen at catalysed oxidation of carbon monoxide by oxygen at high temperatures. high temperatures. J. Phys. Chem, J. Phys. Chem, 61, pp 651‑654.61, pp 651‑654.

Pershing, D.W. and Wendt, LO.L. (1976). Pulverized Pershing, D.W. and Wendt, LO.L. (1976). Pulverized coal combustion: the influence of flame temperature coal combustion: the influence of flame temperature and coal composition on thermal and fuel NOx, and coal composition on thermal and fuel NOx, 16th 16th Symp. (Int.) on Combustion, Symp. (Int.) on Combustion, The Combustion The Combustion Institute, pp 389‑399.Institute, pp 389‑399.

Westenberg, A.A. (1971). Kinetics of NO and CO in Westenberg, A.A. (1971). Kinetics of NO and CO in lean premixed hydrocarbon air flames, lean premixed hydrocarbon air flames, Combust. Sci. Combust. Sci. Technol., Technol., 4, pp 59‑64.4, pp 59‑64.

Zeldovich, Ya.B. (1946). The Oxidation of nitrogen Zeldovich, Ya.B. (1946). The Oxidation of nitrogen in combustion explosions. in combustion explosions. ActaActa Physiochimica, Physiochimica, USSR, USSR, 21, pp 577‑628.21, pp 577‑628.

Further ReadingFurther Reading Turns, S.R. (1996). "An Introduction to combustion Turns, S.R. (1996). "An Introduction to combustion

concepts and applications", McGraw‑Hill, Chapter 15 concepts and applications", McGraw‑Hill, Chapter 15 and pp143‑146 (1st Edition)and pp143‑146 (1st Edition)

Borman, G.L. and Ragland, KM. (1998). Borman, G.L. and Ragland, KM. (1998). "Combustion Engineering", WCB/McGraw‑Hill, pp "Combustion Engineering", WCB/McGraw‑Hill, pp 125‑138125‑138

Wendt, LO.L. (1995). Wendt, LO.L. (1995). Combust. Sci. Tech., 108, pp Combust. Sci. Tech., 108, pp 323‑344323‑344

van der Lans, R.P., Glarborg, P. and Darn‑Johansen, van der Lans, R.P., Glarborg, P. and Darn‑Johansen, K. (1997). Prog. K. (1997). Prog. Energy Combust. Sci., 23, Energy Combust. Sci., 23, pp pp 349‑377349‑377

All hydrocarbon fuels are converted to COAll hydrocarbon fuels are converted to CO22 via COvia CO

CHCH44 → CH → CH33 → CH → CH22O → CHO → CO → COO → CHO → CO → CO22

Conversion of CO to COConversion of CO to CO22 occurs largely occurs largely downstream downstream of the flame‑zoneof the flame‑zone and and is an equilibrium processis an equilibrium process which occurs relatively slowlywhich occurs relatively slowly

Emission of CO as a stable product is minimised if Emission of CO as a stable product is minimised if CO → COCO → CO22 is allowed to proceed to completion is allowed to proceed to completion

There are 3 processes leading to incomplete There are 3 processes leading to incomplete combustion:combustion: Insufficient Oxygen (Insufficient Oxygen (low Olow O22)) Aerodynamic Quenching (Aerodynamic Quenching (low templow temp)) Impingement Quenching (Impingement Quenching (low templow temp))

CO (carbon monoxide) formationCO (carbon monoxide) formation

1. 1. Insufficient OxygenInsufficient Oxygen In this case, there is simply not enough OIn this case, there is simply not enough O22 present in present in

the downstream gases to fully oxidise the CO to COthe downstream gases to fully oxidise the CO to CO22

Usually as a result of poor design or operation and, in Usually as a result of poor design or operation and, in flames, flames, often related to inadequate mixing of often related to inadequate mixing of secondary airsecondary air

2. Aerodynamic Quenching2. Aerodynamic Quenching Caused by a Caused by a rapid decrease in the temperaturerapid decrease in the temperature of the of the

postflame gasespostflame gases, either by expansion of the gases or , either by expansion of the gases or by mixing with ambient airby mixing with ambient air

Rapid decrease in temperature results in kinetic Rapid decrease in temperature results in kinetic "freezing" of the CO at the equilibrium condition"freezing" of the CO at the equilibrium condition

Aerodynamic quenching is a serious problern for Aerodynamic quenching is a serious problern for spark-ignition engine when too much air used (not spark-ignition engine when too much air used (not really for open‑flame systems)really for open‑flame systems)

3. Impingement Quenching3. Impingement Quenching Quenching occurs as a result of the Quenching occurs as a result of the hot postflame hot postflame

gases impinging on a relatively cold surfacegases impinging on a relatively cold surface Again, a problem in internal combustion enginesAgain, a problem in internal combustion engines

CO ToxicityCO Toxicity

Balance between CO and NOBalance between CO and NO We have seen that We have seen that COCO is largely a product of is largely a product of

insufficient combustion (insufficient combustion (low Olow O22, , rapid coolingrapid cooling, etc), etc) In comparison, the formation of In comparison, the formation of NONO which is which is

enhanced by conditions which could be termed as enhanced by conditions which could be termed as over‑aggressive combustion (over‑aggressive combustion (high Ohigh O22, , high temphigh temp, etc), etc)

There is a fine balance between the formation of NO There is a fine balance between the formation of NO and CO such that control measures which are and CO such that control measures which are effective against one species often result in an effective against one species often result in an increase in the other speciesincrease in the other species

Fortunately, the two mechanisms are decoupledFortunately, the two mechanisms are decoupled somewhat since somewhat since NO formation occurs primarily in the NO formation occurs primarily in the near‑burner zonenear‑burner zone and and CO consumption occurs in the CO consumption occurs in the postflame gases. postflame gases.

Volatile Organic Compounds (VOC’s)Volatile Organic Compounds (VOC’s) Volatile organic compoundsVolatile organic compounds (VOC's) are also (VOC's) are also

products of products of incomplete combustionincomplete combustion VOC consist of various unburned fuel fragments, VOC consist of various unburned fuel fragments,

aliphatic and aromatic hydrocarbons and partially aliphatic and aromatic hydrocarbons and partially oxidised fuel fragmentsoxidised fuel fragments

The formation and destruction of VOC's occurs much The formation and destruction of VOC's occurs much closercloser to the flame front (to the flame front (higher temp zonehigher temp zone) than does ) than does CO and hence CO and hence greater extents of incomplete greater extents of incomplete combustion cause the emission of VOC'scombustion cause the emission of VOC's

VOC's play a key role in photochemical smog cycles VOC's play a key role in photochemical smog cycles (reactions in atmosphere triggered by photon of the (reactions in atmosphere triggered by photon of the sunray)sunray)

PAH's, Soot & PM10PAH's, Soot & PM10 ( (solid phase pollutantssolid phase pollutants))

What are PAH's, Soot & PM10?What are PAH's, Soot & PM10? PAH'sPAH's (polycyclic aromatic hydrocarbons) are (polycyclic aromatic hydrocarbons) are

clusters of clusters of aromatic rings up to about 4 ringsaromatic rings up to about 4 rings in size. in size. Soot Soot is formed when, under appropriate conditions, is formed when, under appropriate conditions,

PAH's grow in sizePAH's grow in size until they reach approximately until they reach approximately 20‑50 nm20‑50 nm

PM10 PM10 is a general term given to all air‑borne is a general term given to all air‑borne particulates which have a size particulates which have a size < 10 microns< 10 microns

PAH's (polycyclic aromatic hydrocarbons):PAH's (polycyclic aromatic hydrocarbons): PAH's are formed in, and may be emitted from, PAH's are formed in, and may be emitted from,

combustion processes under combustion processes under very fuel‑richvery fuel‑rich conditions conditions The main cause of concern with PAH's is that many The main cause of concern with PAH's is that many

species are known mutagens, co‑carcinogens, or species are known mutagens, co‑carcinogens, or carcinogenscarcinogens

While While PAH's are oxidised quite rapidlyPAH's are oxidised quite rapidly in the urban in the urban atmosphere, they may be stabilised considerably by atmosphere, they may be stabilised considerably by adsorption to the surface of particulate (e.g. soot)adsorption to the surface of particulate (e.g. soot)

PAH's then become quite long-lived and are capable PAH's then become quite long-lived and are capable of penetrating deep into the human lungsof penetrating deep into the human lungs

Soot:Soot: The health effects of soot are serious and are related The health effects of soot are serious and are related

to the transport of fine particles within the lungsto the transport of fine particles within the lungs PM10 is the environmental standard used to quantify PM10 is the environmental standard used to quantify

airborne particulates from all sources airborne particulates from all sources Soot particles also contribute to visible pollution and Soot particles also contribute to visible pollution and

haze within the urban environmenthaze within the urban environment Soot formationSoot formation is encouraged within some is encouraged within some

combustion applications on the basis of the combustion applications on the basis of the significantly significantly enhanced radiative heat transferenhanced radiative heat transfer obtained from sooting flamesobtained from sooting flames

PAH & Soot Formation:PAH & Soot Formation: PAH's and soot are formed mainly in PAH's and soot are formed mainly in diffusion flamesdiffusion flames

and in diesel engines (also from I.C. engines with and in diesel engines (also from I.C. engines with lubricant oil leakage)lubricant oil leakage)

A A precursor speciesprecursor species (believed to be (believed to be acetyleneacetylene) is ) is formed under formed under fuel‑rich fuel‑rich conditionsconditions

Acetylene and other hydrocarbon fragments combine Acetylene and other hydrocarbon fragments combine through pyrolytic (through pyrolytic (without oxygenwithout oxygen) reactions ) reactions to form to form progressively larger ring structuresprogressively larger ring structures (PAH's) (PAH's)

Without competition with oxidation reactions, which Without competition with oxidation reactions, which break ring structures, the clusters continue to increase break ring structures, the clusters continue to increase in sizein size

Small particles of a critical size are formed by the Small particles of a critical size are formed by the growth of these aromatic clusters by both chemical growth of these aromatic clusters by both chemical and physical means, i.e. coagulationand physical means, i.e. coagulation

At this point, the molecules are identifiable as At this point, the molecules are identifiable as primary soot particlesprimary soot particles

The soot particles continue to grow by both chemical The soot particles continue to grow by both chemical ((surface growthsurface growth) and physical () and physical (particle particle agglomerationagglomeration) processes whilst they remain within ) processes whilst they remain within the bath of precursor species, clusters and adolescent the bath of precursor species, clusters and adolescent soot particlessoot particles

Sulphur CompoundsSulphur Compounds All of the sulphur contained within various fuels is All of the sulphur contained within various fuels is

emitted from combustion processes as either SOemitted from combustion processes as either SO22 or or SOSO33

The ultimate fate of oxides of sulphur in the The ultimate fate of oxides of sulphur in the environment is transformation to Henvironment is transformation to H22SOSO44 (acid rain) (acid rain)

Only Only two possible abatementtwo possible abatement procedures: procedures: remove S remove S before combustionbefore combustion (from the fuel) (from the fuel) remove SOremove SOXX after combustionafter combustion (from the flue gases) (from the flue gases)

Most common technique is Most common technique is the reaction of SOthe reaction of SO22 in the in the flue gas flue gas with limestonewith limestone (CaCO (CaCO33) or ) or lime lime (CaO)(CaO)

A slurry of limestone or lime is sprayed in an aqueous A slurry of limestone or lime is sprayed in an aqueous absorption tower through which the flue gas is passedabsorption tower through which the flue gas is passed

ReferencesReferences Muzio, L.J. and Quartucy, G.C. (1997). Muzio, L.J. and Quartucy, G.C. (1997).

Implementing NOx control: Research to Application, Implementing NOx control: Research to Application, Prog. Energy Cornbust. Sci., 23, pp 233‑266.Prog. Energy Cornbust. Sci., 23, pp 233‑266.

Wendt, LO.L. (1995). Mechanisms governing the Wendt, LO.L. (1995). Mechanisms governing the formation and formation and destruction of NOdestruction of NOxx and other and other nitrogenous species in low NOnitrogenous species in low NOxx Coal Combustion Coal Combustion Systems, Systems, Combust. Sci. Tech., 108, pp 323‑344Combust. Sci. Tech., 108, pp 323‑344

Further ReadingFurther Reading Turns, S.R. (1996). "An Introduction to combustion Turns, S.R. (1996). "An Introduction to combustion

concepts and applications", McGraw‑Hill, chapter 15, concepts and applications", McGraw‑Hill, chapter 15, pp 291‑293 and ppl43‑146 (1pp 291‑293 and ppl43‑146 (1stst Edition) Edition)

Borman, G.L. and Ragland, KM. (1998). Borman, G.L. and Ragland, KM. (1998). "Combustion Engineering", WBIMcGraw‑Hill, pp "Combustion Engineering", WBIMcGraw‑Hill, pp 125‑138 and pp 413‑421125‑138 and pp 413‑421

Wendt, J.0.L. (1995). Wendt, J.0.L. (1995). Combust. Sci. Tech., 108, pp Combust. Sci. Tech., 108, pp 323‑344323‑344

van der Lans, R.P., Glarborg, P. and Darn‑Johansen, van der Lans, R.P., Glarborg, P. and Darn‑Johansen, K. (1997). K. (1997). Prog. Energy Combust. Sci., 23, Prog. Energy Combust. Sci., 23, pp pp 349‑377349‑377