Coal Coal CombustionCombustion

Nadine SpitzNadine SpitzEnvironmental EngineeringEnvironmental Engineering

Ben-Gurion UniversityBen-Gurion University

ContentsContents What is coal? What is coal? Formation, sources, Formation, sources,

applications.applications. Coal combustion description.Coal combustion description. Coal power plants and air pollution: Coal power plants and air pollution:

Mechanisms and control Mechanisms and control technologies.technologies.

Coal and air pollution in Israel. Coal and air pollution in Israel.

Eshkol power stationHaifa

Coal – what is it?Coal – what is it?

CC 65-65-95%95%

HH 2-7%2-7%OO <25%<25%SS <10%<10%NN 1-2%1-2%CharChar 20-20-

70%70%AshAsh 5-15%5-15%HH22OO 2-20%2-20%VMVM 20-20-

45%45%

• Inhomogeneous organic fuel formed mainly from decomposed plant matter.

• Over 1200 coals have been classified.

Time, TemperatureCoal Rank

• Coalification forms different coal types:

(Peat)Lignite

Bituminous coalAnthracite(Graphite)P

roxi

mat

e A

naly

sis

Ele

men

tal C

ompo

sitio

n

CoalCoal

Coal SourcesCoal Sources Coal is the world’s most plentiful fossil fuel.Coal is the world’s most plentiful fossil fuel. Recoverable world coal reserves are estimated Recoverable world coal reserves are estimated

at about 1X10at about 1X101212 tons. tons.

World Coal Reserves (1989)

Coal ApplicationsCoal Applications Homes – heat and cookingHomes – heat and cooking Transportation – steam Transportation – steam

enginesengines Industry – metal worksIndustry – metal works Electricity – power plantsElectricity – power plants

Main Processes in Coal Main Processes in Coal CombustionCombustion

coal particlep-coal, d=30-70m

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO2, H2O, …

CO2, H2O, …

tchar=1-2sectvolatiles=50-100mstdevolatile=1-5ms

t

The physical processes The physical processes influencing pulverized coal influencing pulverized coal

combustioncombustion Turbulent/swirling flow of air and coal.Turbulent/swirling flow of air and coal. Turbulent/convective/molecular diffusion Turbulent/convective/molecular diffusion

of gaseous reactants and products.of gaseous reactants and products. Convective heat transfer through the gas Convective heat transfer through the gas

and between the gas and coal particles.and between the gas and coal particles. Radiative heat transfer between the gas Radiative heat transfer between the gas

and coal particles and between the coal/air and coal particles and between the coal/air mixture and the furnace walls. mixture and the furnace walls.

From From FumifugiumFumifugium by John Evelyn by John Evelyn (1661) (1661)

- on London’s air pollution - - on London’s air pollution - “…“…but so universally mixed with the otherwise wholesome and but so universally mixed with the otherwise wholesome and excellent excellent AerAer, that her , that her Inhabitants Inhabitants breathe nothing but an breathe nothing but an impure and thick Mist, accompanied by a fuliginous and filthy impure and thick Mist, accompanied by a fuliginous and filthy vapour, which renders them obnoxious to a thousand vapour, which renders them obnoxious to a thousand inconveniences, corrupting the inconveniences, corrupting the Lungs, Lungs, and disordering the entire and disordering the entire habit of their Bodies; so that habit of their Bodies; so that Catharrs, Phthisicks, Coughs and Catharrs, Phthisicks, Coughs and Consumptions, Consumptions, rage more in this one City, than the whole Earth rage more in this one City, than the whole Earth besides.besides. For when in all other places the For when in all other places the Aer Aer is most Serene and is most Serene and Pure, it is here Ecclipsed with such a Cloud of Sulphure, as the Pure, it is here Ecclipsed with such a Cloud of Sulphure, as the Sun itself, which gives day to all the World besides, is hardly able Sun itself, which gives day to all the World besides, is hardly able to penetrate and impart it here; and the weary to penetrate and impart it here; and the weary Traveller, Traveller, at at many Miles distance, sooner smells, than sees the City to which many Miles distance, sooner smells, than sees the City to which he repairs. This is that pernicious Smoake which sullyes all her he repairs. This is that pernicious Smoake which sullyes all her Glory, superinducing a sooty Crust or Fur upon all that it lights, Glory, superinducing a sooty Crust or Fur upon all that it lights, spoyling the moveables, tarnishing the Plate, Gildings and spoyling the moveables, tarnishing the Plate, Gildings and Furniture, and corroding the very Iron-bars and hardest Stones Furniture, and corroding the very Iron-bars and hardest Stones with those piercing and acrimonious Spirits which accompany its with those piercing and acrimonious Spirits which accompany its Sulphure; and executing more in one year, than exposed to the Sulphure; and executing more in one year, than exposed to the pure pure Aer Aer of the Country it could effect in some hundreds.”of the Country it could effect in some hundreds.”

Coal Combustion Air PollutantsCoal Combustion Air Pollutants COCO22 COCO NOxNOx SOxSOx Particulate matterParticulate matter Trace metalsTrace metals Organic compoundsOrganic compounds

Carbon Dioxide, COCarbon Dioxide, CO22

C + O2 CO2

Almost 99% of C in coal is converted to CO2.In order to lower CO2 emission levels, coal power plants will have to leave steam-based systems (37% efficiency) and go towards coal gasification technology (60% efficiency).Meanwhile, CO2 sequestration is being tested.

Carbon monoxide, COCarbon monoxide, CO

C + ½O2 CO

CO is minimized by control of the combustion process (air/fuel ratio, residence time, temperature or turbulence).

Particulate MatterParticulate Matter

PM composition and emission levels are a complex function of:

1. Coal properties,2. Boiler firing configuration,3. Boiler operation,4. Pollution control equipment.

Bottom Ash Fly Ash

In PC power plants, since combustion is almost complete, the emitted PM is primarily composed of inorganic ash residues.

PM controls PM controls (AP-42, EPA)(AP-42, EPA)

Mainly post combustion methods:Mainly post combustion methods:

Electrostatic Electrostatic precipitator (ESP)precipitator (ESP)

99% (for 0.1>d(99% (for 0.1>d(m)>10)m)>10)<99% (for 0.1<d (<99% (for 0.1<d (m)<10)m)<10)

Fabric filter (or Fabric filter (or baghouse)baghouse)

As high as 99.9%As high as 99.9%

Wet scrubberWet scrubber 95-99%95-99%

CycloneCyclone 90-95% (d90-95% (d((m)>m)>10)10)

Trace metalsTrace metals

Class 1Elements that are approximately equally concentrated in the fly ash and bottom ash (Mn, Be, Co, Cr)

Class 2Elements that are enriched in fly ash relative to bottom ash (Ar, Cd, Pb, An)

Class 3Elements which are emitted in the gas phase (mainly Hg).

Control of total particulate matter emissions

Collection of fine particles.

Sorbents ???

CONTROL

FORMATIONConcentration of metal in coal, physical and chemical properties of the metal, combustion conditions.

Organic CompoundsOrganic CompoundsInclude volatile, semivolatile and condensable organic

compounds either present in the coal or formed as a product of incomplete combustion.

Characterized by hydrocarbon class: alkanes, alkenes, aldehydes, alcohols and substituted benzenes.

The main groups of environmental concern are:1) tetrachloro- through octachloro- dioxins and

furnans.2) Polycyclic organic matter (POM).

Emissions dependent on combustion behavior in the boiler (air/fuel ratio, residence time, temperature or turbulence).

Sulfur Oxides, SOxSulfur Oxides, SOx

Sulfur in coal Sulfur in coal (<10%)(<10%)Organic sulfur (40%)Chemically bonded to the hydrocarbon matrix in the forms of thiophene, thiopyrone, sulfides and thiol. Inorganic sulfur (60%)Imbedded in the coal, as loose pyrite - FeS2 or marcasite, and calcium/iron/barium sulfates.

Sources of sulfur in coal: Seawater sulfates, Limestone

Coal-S )CS, S2, S, SH(

char

COS, CS2H2S

SO SO2SO3

O2, M-SO4

SO2 molecule

radicals

SOx FormationSOx Formation

SOx reductionSOx reduction Pre combustion removal:Pre combustion removal:

– Physical cleaning (30-50% removal inorganic sulfur)Physical cleaning (30-50% removal inorganic sulfur)– Chemical and biological cleaning (90% removal Chemical and biological cleaning (90% removal

organic sulfur)organic sulfur) Combustion configuration:Combustion configuration:

– No benign sulfur species!No benign sulfur species!– gasification combined-cycle systems (IGCC systems)gasification combined-cycle systems (IGCC systems)

Post-combustion removal:Post-combustion removal:– Wet Flue Gas Desulfurization (FGD) (80-98%)Wet Flue Gas Desulfurization (FGD) (80-98%)

In situIn situ sulfur capture:sulfur capture:– Dry Sorbent Injection (DSI) (50%)Dry Sorbent Injection (DSI) (50%)

Nitrogen Oxides, NOxNitrogen Oxides, NOx

Nitrogen in Coal Nitrogen in Coal (1-2%)(1-2%)NameName StructurStructur

ee~ ~

Relative Relative amountamount

StabilityStability

PyridinePyridine11 15-40%15-40% More More stablestable

PyrrolePyrrole11 60%60% Less Less stablestable

Aromatic Aromatic aminesamines

6-10%6-10% StableStable

N

NH

NH2··

··

1Including structures made up of 2-5 fused aromatic rings.

Main NO MechanismsMain NO Mechanisms1.1. Thermal NOThermal NO

2.2. Prompt NOPrompt NO

3.3. Fuel NO: volatiles-NO and char-NOFuel NO: volatiles-NO and char-NO

Thermal NOThermal NO(Zeldovich mechanism)(Zeldovich mechanism)

NN22 + O + O NO + N NO + N N + ON + O22 NO + O NO + O

Strong temperature-dependence: >1300-1500°C

Not a major source of NO in coal utility boilers.

Prompt NOPrompt NO

N2 + CHx HCN + N + …

N + OH NO + H

Prevalent only in fuel-rich systems.

Not a major source of NO in coal utility boilers.

Fuel NO Fuel NO (-N in volatiles)(-N in volatiles)

Fuel-N HCN/NH3 volatiles

(formation)

(destruction)

HCN/NH3 + O2

N2

NO

NO + HCN/NH3

The major source of NO in coal utility boilers (>80%).

Char NO Char NO (-N in the char)(-N in the char)

Char-N + ½O2 NO

Char-C + NO ½N2 + Char(O)

(formation)

(destruction)

[char-NO = ~25%] < [volatiles-NO = ~75%]

NO ReductionNO ReductionCombustion controls:Combustion controls:1.1. Modification of combustion configuration:Modification of combustion configuration:

ReburningReburning Staged Combustion (air/fuel)Staged Combustion (air/fuel)

Post combustion controls:Post combustion controls:1.1. Injection of reduction agents in flue gas.Injection of reduction agents in flue gas.2.2. Post-combustion denitrification processes.Post-combustion denitrification processes.

ReburningReburning

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO2, H2O, NO…

Excess air

CO2, H2O, NO…

CO2, H2O, N2…

CHi·

CHi· + NO HCN

HCN + NO N2 + …

Staged CombustionStaged Combustion

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO, CO2, H2O, N2…

Fuel Rich

CO, CO2, H2O, N2…

CO2, H2O, N2…

O2

NOx control optionsNOx control options(from AP-42, EPA)(from AP-42, EPA)

Control TechniqueControl Technique NO Reduction NO Reduction Potential(%)Potential(%)

Overfire air (OFA)Overfire air (OFA) 20-3020-30Low Nox Burners Low Nox Burners (LNB)(LNB)

35-5535-55

LNB + OFALNB + OFA 40-6040-60ReburnReburn 50-6050-60SNCRSNCR 30-6030-60SCRSCR 75-8575-85LNB with SCRLNB with SCR 50-8050-80LNB with OFA and LNB with OFA and SCRSCR

85-9585-95

(Selective Non Catalytic Reduction)

(Selective Catalytic Reduction)

Fuel Oil and Coal Consumption for Fuel Oil and Coal Consumption for ElectricityElectricity

in Israel (1980-2001) (1000 Tons)in Israel (1980-2001) (1000 Tons) Source: Israeli CBS, 2001Source: Israeli CBS, 2001

02000400060008000

1000012000

1980 1990 1999 2000 2001

Fuel Oil Coal

Fuel Combustion Emissions in IsraelFuel Combustion Emissions in Israelby Fuel, 2002 (1000 Tons)by Fuel, 2002 (1000 Tons)

Source: Israeli Central Bureau Statistics (CBS), Source: Israeli Central Bureau Statistics (CBS), 20022002

0100200

300400500

LPG Gasoline Diesel Oil Coal Heavy FuelOil

CO SOx NOx SPM

Fuel Combustion Emissions in IsraelFuel Combustion Emissions in Israelby Sector, 2002 (1000 Tons)by Sector, 2002 (1000 Tons)

Source: Israeli CBS, 2002Source: Israeli CBS, 2002

0

100

200

300

400

500

Motor Vehicles Industry ElectricityProduction

CO SOx NOx SPM 0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

Motor Vehicles Industry ElectricityProduction

CO2

1) Coal combustion in Israel has 1) Coal combustion in Israel has tripledtripled since 1990. Almost all of coal use is since 1990. Almost all of coal use is for electricity production.for electricity production.

2) Coal combustion emissions in Israel:2) Coal combustion emissions in Israel: 71% of total SO71% of total SO22 emissions. emissions. 62% of total CO62% of total CO22 emissions. emissions. 39% of total NOx emissions.39% of total NOx emissions. 38% of total SPM emissions.38% of total SPM emissions. 1% of total CO emissions.1% of total CO emissions.

ReferencesReferences Compilation of Air Pollutant Emission Compilation of Air Pollutant Emission

Factors, AP-42, Fifth Edition, Volume I: Factors, AP-42, Fifth Edition, Volume I: Stationary Point and Area SourcesStationary Point and Area Sources ( (http://www.epa.gov/ttn/chief/ap42/ch01/http://www.epa.gov/ttn/chief/ap42/ch01/).).

““Fundamentals of coal combustion: for Fundamentals of coal combustion: for clean and efficient use”, edited by L. clean and efficient use”, edited by L. Douglas Smoot, Elsevier Science Douglas Smoot, Elsevier Science Publishers, 1993.Publishers, 1993.

Israel Central Bureau of Statistics, Shanton Israel Central Bureau of Statistics, Shanton 54, 2003 (54, 2003 (http://www.cbs.gov.ilhttp://www.cbs.gov.il).).