Coal Coal CombustionCombustion
Nadine SpitzNadine Spitz
Environmental 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 and coal particles and between the coal/air mixture and the furnace walls. coal/air 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
FORMATION
Concentration 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 StructurStructuree
~ ~ 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 Mechanisms
1.1. 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 Reduction
Combustion 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) 20-3020-30
Low Nox Burners Low Nox Burners (LNB)(LNB)
35-5535-55
LNB + OFALNB + OFA 40-6040-60
ReburnReburn 50-6050-60
SNCRSNCR 30-6030-60
SCRSCR 75-8575-85
LNB with SCRLNB with SCR 50-8050-80
LNB 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
02000
400060008000
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
0
100
200
300
400
500
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).).