Lecture 3:
The Science of Oxy-fuel
System differences in flames, heat transfer, coal combustion and emissions
APP OFWG Capacity Building Course, Sunday/Monday 11/12 September, 2011 Capricorn Resort, Yeppoon AUSTRLIA
Rohan Stanger University of Newcastle, Australia
Related Literature
Woodhead Publishing
February 2011
Volume 5, Supplement 1, Pages S1- S238 (July 2011)
Oxyfuel Combustion Technology -
Working Toward Demonstration
and Commercialisation
TEXTBOOK JOURNAL Special Edition WEBSITE
Lecture Outline
• Flames & heat transfer
• Coal Behaviour
• Emissions
• Impurity Impacts during CO2 compression
Lecture context and content
Oxyfuel science used here to compare air and oxy-fuel furnace performance, for retrofit of an existing air-fired boiler while maintaining heat transfer, considering
– Conditions for matched heat transfer– Changed burner flows, with flame and heat transfer impacts– Coal reactivity and burnout impacts
Developments and gaps in knowledge
will be suggested
Basic Oxy-fuel Circuit
ESP Sequestration Site
Transport (pipeline, truck, etc)
Air Separation Unit
Coal Handling
O2
N2
Recycled Flue Gas ~70%
CO2compression
Heat Transfer
Flame & Coal Behaviour
Emissions & Impurities Impacts
~30%
Oxy-fuel: differences of combustion in O2 /CO2 compared to air firing
• To attain a similar AFT the O2 proportion of the gases through the burner is ~ 30%
• The high proportions of CO2 and H2 O in the furnace gases result in higher gas emissivity's
• The volume of gases flowing through the furnace is reduced (longer residence time)
• The volume of flue gas (after recycling) is reduced by about 80%.
Recycle gases have higher concentrations in the furnace
Flames and heat transfer
Fixed velocity
27 O2
% v/v fixed for same HT
Therefore secondary RFG reduced
~3% v/v O2
Burner
Burner flow comparisons for a retrofit
Gas property ratios for CO2
and N2
at 1200 KProperties from Shaddix, 2006
Impact for air to oxyfuel retrofitHigher O2 thru burner
Lower burner velocity, higher coal residence time in furnace
Slower flame propagation velocity
Property/ratio Gas property differences
AFT of air and oxy cases
1000
1400
1800
2200
2600
0.18 0.22 0.26 0.30 0.34 0.38
O2 fraction at burner inlet
Adia
batic
flam
e Te
mpe
ratu
re (K
)
airoxy-wetoxy-dry
Gas property differences : Emissivity Triatomic gas (H2 O+CO2 ) emissivity ~ beam length
comparisons
Gupta et al (2006)
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60 70 80 90
Beam Length (L) (m)
Gas
Em
issi
vity
(-)
Oxy-fuel fired furnace
Air fired furnace
30 500 1050 MWe
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10 20 30 40 50 60Beam length (L) (m)
Emis
sivi
ty (-
)
4 grey gas model,
4-GGM
Gupta et al, (2006)
3 grey gas model,
3-GGM
Smith et al, (1982)
Oxy-fuel fired furnace
CFD radiative transfer inputs
]1[)( )(
0,
22 Lppk
ii
OHCOieTa
30 MWe
500 MWe
Preheated air/RFG: primary 350 - 400K and secondary 450 - 550 K, Wall 1200 K
Parameter Full load Partial load
Air case Oxy case Air case Oxy case
Coal flow rate kg/hr 120 120 72 72
Primary velocity m/s 20 23 17 21
Secondary velocity m/s 35 21 18 12
Secondary swirl number - 0.2 0.2 0.2 0.2
Primary momentum flux kg/s.m2/s2 35.7 54.1 20.9 36.8
Secondary momentum flux kg/s.m2/s2 270.2 74.1 38.2 16.4
Momentum flux ratio (Pri/Sec) - 0.13 0.73 0.55 2.25
1 MWt
test conditions
Type-0
Type-1
Type-2
Low S
Hi S (S>0.6) , Low v2
Hi S, Hi v2
IFRF Flame types from swirl burners
Air-case
X 1.5m
Type-0 flame
Oxy-case
1 MWt
–
Temperature contours at full load
30 MWe
–
heat transfer results
150
170
190
210
230
250
45 51 56 56Furnace wall area (m2)
Tota
l sur
face
hea
t flu
x (k
w/m
2)
airoxy-fuel
Front wall Rear wall Side wall1 Side wall2
1 MWt
Temperature comparisons for matching furnace heat transfer
Flame
FEGT
Sensitivity analysis –
full & partial load
Confirms the significance of momentum flux
and Gas properties
on flame ignition
Effect of momentum flux
21% O2 /CO2
Particle imaging of ignition and devolatilisation of pulverized coal during oxy-fuel combustion.
Shaddix, C. R. and A. Molina (2009)
CO2 atmosphere delays ignition and
devolatilisation
• higher Cp of gas
• Reduced radical pool
• Lower diffusivity of O2, CxHy
(Murphy and Shaddix 2006)
Coal combustion in Sandia’s entrained flow reactor under the intermediate gas temperature conditions.
O2% affects flame
length and type
Controlled through
recycle rate
Important when matching heat transfer in retrofit applications
Recycle Rate
Normal Air Fired
OXY recycle ratio = 0.76
(low O2%)
OXY recycle ratio=0.58
(high O2%)
O2 Flow is set by stoichiometry
Recycle Rate changes O2%, AFT, radiative/convective HT
Tan, Corragio, Santos, IFRF 2005 Review
Flame & Heat Transfer Summary
• Properties of oxy-fuel recycle gas
• Lowers adiabatic flame temperature (higher Cp)• Delays flame ignition (higher Cp, lower O2 diffusion)• Affects radiative heat transfer (higher emissivity)
• Matching heat transfer in a retrofit is a balance of
• adiabatic flame temperature, • recycle rate of flue gas (for O2%)• gas emissivity (model validation needed)
• Managing oxy-fuel flame has more parameters to consider (eg flame type, length) but offers more control (eg O2 Injection)
Coal Behaviour
25
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
500 600 700 800 900 1000 1100 1200 1300
Temperature, T p (K)
Rea
ctiv
ity, R
m,p
(s-1
)
Coal 100% N2Coal 100% CO2
Pyrolysis of Coal D in N2 & CO2TGA Experiments
Char gasification begins
CO2
N2
Devolatilisation reactivities are similar in both N2 and CO2 atmospheres.
Char-CO2 gasification reaction is clearly evident in a CO2 atmosphere.
CO2
reactivity with char
devolatilisation reactivities
are similar
Rathnam et al, OCC1, 2009
26Comparison of Apparent Volatile Yields in N2 & CO2
DTF Experiments at 1400 oC
Higher apparent volatile yield at higher temperatures and heating rates in DTF at 1400 oC
compared to the proximate analysis volatile matter.
Higher apparent volatile yield in CO2
atmosphere -
attributed to the char-CO2
gasification reaction.
76.3
6
46.7 60
.0
50.9
66.6
36.1
8 48.5
7
87.4
5
48.7
74.3
59.4 70
.2
35.8
0 49.1
9
57.1
9
37.0
1
37.9
3
35.5
1 51.4
6
32.6
1
36.5
1
0
10
20
30
40
50
60
70
80
90
100
67.51 75.00 78.40 79.10 81.60 81.86 83.91
App
aren
t vol
atile
yie
ld (w
t. %
daf
bas
is)
N2CO2Proximate Analysis
C in Coal (wt. %, daf basis)
Rathnam et al, OCC1, 2009
27Char Swelling in N2 & CO2
0102030405060708090
100
1 10 100 1000Particle diameter (μm)
Cum
ulat
ive
volu
me
perc
enta
ge CoalN2 CharCO2 Char
Coal A
0102030405060708090
100
1 10 100 1000Particle diameter (μm)
Cum
ulat
ive
volu
me
perc
enta
ge CoalN2 CharCO2 Char
Coal B
0102030405060708090
100
1 10 100 1000Particle diameter (μm)
Cum
ulat
ive
volu
me
perc
enta
ge CoalN2 CharCO2 Char
Coal C
Chars were formed in the DTF at 1400 oC
Coal A Coal B
Coal C
Swollen chars
Similar N2
and CO2
chars
Larger CO2
chars
Rathnam et al, OCC1, 2009
28
100
150
200
250
300
350
400
450
500
60 65 70 75 80 85 90
C content in coal (wt. % daf basis)
Mic
ropo
re s
urfa
ce a
rea
(m2 /g
) N2 CharCO2 Char
Char Micropore
Surface Areas of Chars
Chars were formed in the DTF at 1400 oC
Rathnam et al, OCC1, 2009
29Coal A Reactivity in O2/N2 & O2/CO2 TGA Experiments
REACTIVITY
Endothermic gasification
Constant Heating Rate – 25C/min
- Lower reactivities & heat flows at low temperatures in O2 /CO2 combustion
- Endothermic gasification reduces heat flows and increases reactivities at high temperatures
0
0.01
0.02
0.03
0.04
0.05
0.06
0 200 400 600 800 1000
Temperature (oC)
Rea
ctiv
ity (m
in-1
)
0
0.04
0.08
0.12
0.16
0.2Coal A
21% O2
CO2
3% O2
10% O2
N2
-100
-50
0
50
100
150
200
0 200 400 600 800 1000
Temperature (oC)
Hea
t Flo
w (m
W)
Coal A
3% O2
10% O2
21% O2
N2
CO2
HEAT FLOW
Rathnam et al, OCC1, 2009
30
405060708090
100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Oxygen concentration (atm)
Coa
l bur
nout
(wt.
%, d
af b
asis
)
O2/N2
O2/CO2
Coal A
Coal A Burnout & Reactivity
60
70
80
90
100
800 900 1000 1100 1200 1300 1400 1500
Furnace temperature (oC)
Coa
l bur
nout
(wt.
%, d
af b
asis
)
O2/N2
O2/CO2
Coal A
DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS
0
0.2
0.4
0.6
0.8
1
0 0.05 0.1 0.15 0.2 0.25Oxygen Concentration (atm)
Rea
ctiv
ity (m
in-1
)
Air - O2/N2Oxy - O2/CO2
COAL A 1400N2 CHAR1000 OC, 50% CONVERSION
0
0.2
0.4
0.6
0.8
1
600 700 800 900 1000 1100 1200
Temperature (oC)
Rea
ctiv
ity (m
in-1
)
Air - O2/N2Oxy - O2/CO2
COAL A 1400N2 CHAR10% O2, 50% CONVERSION
Rathnam et al, OCC1, 2009
Char reactivity comparison for air and oxyfuel conditions at the same O2
level
1/Tp (K-1)
Low temperatures Reaction kinetics controlled (Regime I)
Moderately high temperatures Reaction kinetics & internal diffusion limited (Regime II)
Very high temperatures Bulk diffusion controlled (Regime III)
Oxy-fuel (O2/CO2) combustion
Air (O2/N2) combustion
Rea
ctio
n ra
te
32
Variable Technique Difference in O2
/CO2
or CO2atmosphere
Apparent Volatile Yield DTF Higher in CO2Pyrolysis Rate TGA Similar peak devolatilisation
Enhanced at higher temperature
(due to gasification)
Char Swelling DTF/PSD VariableMicropore Surface Area
DTF/BET Different results in CO2 to N2
Coal Burnout DTF Higher in O2 /CO2 conditions in some coals
Char Reactivity TGA Higher in O2 /CO2 conditions
SUMMARY OF COAL RESULTS
33
There are significant differences in pulverised coal reactivity
in air and oxy‐fuel
conditions. The extent of the differences depend on the coal type (rank).
The devolatilisation reactivity of coal in N2
and CO2
is similar
as shown by TGA
results. The apparent volatile yield appears to be higher in CO2
due to mass loss
during the char‐CO2
gasification reaction.
Depending on the coal type, there are also significant differences in the
characteristics of char formed in N2
and CO2
atmospheres. Some coals exhibit
greater swelling in a CO2
atmosphere.
Higher char reactivity in O2
/CO2
conditions, especially at high temperatures and
low O2
levels, is attributed to the char‐CO2
gasification reaction.
Reactivity parameters for char combustion need to be estimated separately for oxy‐
fuel conditions including the char‐CO2
gasification reaction and other relevant
differences need to be accounted for when modelling pulverised coal combustion
in O2
/CO2
conditions.
Coal Behaviour Conclusions
Emissions
PM, SOx, NOx, Hg Concentration in the flue gas and in furnace
Particulates
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30
Dust Air, g/Nm3
Dus
t O
xy, g
/Nm
3
Coal ACoal BCoal C
0
200
400
600
800
1000
0 200 400 600 800 1000
Gas Flow Air, Nm3/h
Gas
Flo
w O
xy, N
m3/
h
Coal ACoal BCoal C
Lower gas flow due out of Furnace (before recycle) due to higher O2% at burner
Dust concentration higher due to lower gas flow
Higher concentration of dust & longer residence time for flue gas/particulates
NOx Concentration & Emission – 1MWt
Coal ACoal BCoal C
Coal ACoal BCoal C
Wall, T., et al., An overview on oxyfuel coal combustion--State of the art research and technology development. Chemical Engineering Research and
Design, 2009. 87(8): p. 1003-1016.
NOx emission much lower due to “reburning effect” in furnace and lack of N2
NOx concentration higher due to lower gas flow & recycled flue gas
SO2 Concentration & Emission – 1MWt
Coal A
Coal BCoal C
Coal A
Coal BCoal C
Wall, T., et al., An overview on oxyfuel coal combustion--State of the art research and technology development. Chemical Engineering Research and
Design, 2009. 87(8): p. 1003-1016.
SO2 concentration higher due to lower gas flow & recycled flue gas
SO2 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition
SO3 Concentration & Emission- 1MWt
Wall, T., et al., An overview on oxyfuel coal combustion--State of the art research and technology development. Chemical Engineering Research and
Design, 2009. 87(8): p. 1003-1016.
Coal ACoal BCoal C
Coal A
Coal BCoal C
SO3 concentration higher due to lower gas flow & recycled flue gas
SO3 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition
SO3 Pilot Comparison
0
10
20
30
40
50
60
70
0 500 1000 1500 2000
SO2, ppm
SO3,
ppm
IHI/Callide Coal A-Air
IHI/Callide Coal A-Oxy
IHI/Callide Coal B-Air
IHI/Callide Coal B-Oxy
IHI/Callide Coal C-Air
IHI/Callide Coal C-Oxy
ANL-Air [17]
ANL-Oxy [17]
CANMET-Air [13,30]
CANMET-Oxy [13,30]
IVD Stuttgart-Air [31]
IVD Stuttgart-Oxy [31]
Utah-Air
Utah-Oxy
E.on-Air
E.on-Oxy
Alstom-Air
Alstom-Oxy
0
1
2
3
4
5
0 1 2 3 4 5
SO3/SO2 Conversion -Air Firing, %
SO3/
SO2
Con
vers
ion-
Oxy
fuel
Firi
ng, %
Stuttgart
ANL
CANMET
IHI/Callide
Utah
E.on
Alstom
0
20
40
60
80
100
120
140
160
180
200
0 5000 10000 15000 20000
SO2, ppm
SO3,
ppm
Low range SO3 High range SO3
• Uncertainty in SO3 due to difficulty of measurement
• Controlled condensation method with inertial separation of fly ash prevents gas-solid interactions
• Results are approaching consensus, but understanding?
SO3 Behaviour
Eddings, Ahn, Okerlund, Fry,
IEA SO2/SO3/Hg Workshop 2011
1.5MW PC Firing
Marier and Dibbs, Thermochimica Acta, 1974.
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500
Temperature,oC
Equi
libriu
m
SO2H2SO4
SO3
TEMPERATURE• Cooling rate
FLY ASH CATALYSIS & CAPTURE
• Fly Ash species + dust load
• Residence time
EQUILIBRIUM• Gas Species
Hg in oxyfuel
• Becoming more important due to downstream corrosion of aluminium heat exchangers in inerts (O2, N2, Ar) separation in cold box
• Difficult to measure due to low concentration and manual methods (similar to SO3)
• Removal depends on species of Hg
– Hg0 (elemental Hg) not water soluble, but absorbed on unburned carbon in fly ash (HgP), potentially captured as Hg(NO3 )2 (aq) in compression
– Hg2+ (ionic Hg, mainly HgCl2 ) is water soluble
• Hg tends to coat walls and piping, taking a long time to reach equilibrium due to low concentrations
• Measurements in large pilot systems may never reach this point
Hg0 (g)
HgCl2(g)Halogenation
Cl/HCl/Cl2NO/NO2
SO2/SO3 Hg0 Sorption
Postcombustion
CatalyticOxidation Hg2+ X(g) Species
Hg(p) Species
Hg(NO3)2HgO
HgCl2HgOHgSO4HgSHgSe
Combustion
Vaporization
Ash Formation andParticle Growth
AmalgamAuHgAgHg
FuelHgSpecies
HgBr2(g)HgF2(g)
Hg0 (g)
HgCl2(g)Halogenation
Cl/HCl/Cl2NO/NO2
SO2/SO3 Hg0 Sorption
Postcombustion
CatalyticOxidation Hg2+ X(g) Species
Hg(p) Species
Hg(NO3)2HgO
HgCl2HgOHgSO4HgSHgSe
Combustion
Vaporization
Ash Formation andParticle Growth
AmalgamAuHgAgHg
FuelHgSpecies
HgBr2(g)HgF2(g)
Pavlish, J. Understanding the fate of Hg during oxyfuel combustion. IEA Workshop on
SO2/SO3/Hg/Corrosion in Oxyfuel. 2011.
Popular Science website
Hg in oxyfuel - results
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
CFB Air
CFB Oxy
L1500 Air
L1500 Oxy
Carbon
in Ash
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
CFB Air
CFB Oxy
L1500 Air
L1500 Oxy
HgP
Hg2+
Hg0
HgTG
Fry, A., et al. Mercury Speciation & Emission from Pilot Scale PC Furnaces under Air & Oxyfired Conditions. IEA Workshop on
SO2/SO3/Hg/Corrosion in Oxyfuel. 2011
• Little differences observed in Hg speciation between Air and Oxy firing
• Mainly attributed to differences in firing systems
• Further trials necessary before conclusions made
Impact of Emissions/Impurities on
CO2 processing
PM, SOx, NOx, Inerts, Hg
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process(A generic diagram)
CRR=92.15%
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process-Particulates
Natural SO2 capture by
CaO & MgO fly ash species
Natural Hg capture by
unburned carbon in fly ash
Fly Ash removed prior to recycle to avoid excessive fouling
Particulates can damage compressors
Fabric Filter/ESP
Absorbed SO3 can improve ESP performance
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process-SOx
SO3 absorbs on active Hg sites in activated carbon
reducing efficacy
SO3 forms H2SO4 which
condenses <160 C in colder
sections (eg recycle, mill)
If not removed SO2 would report with CO2 product,
corroding transport materials and acidifying storage media
Furnace corrosion enhanced
by high SO2/H2S
FGD placement before/after recycle depends on furnace corrosion (fuel-S<1%)
FGD & FGC
Remaining SOx removed as H2SO4 in corrosive
condensate
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process-NOx
SCR catalyst may enhance SO3 formation
If not removed NO2 would report with CO2 product,
corroding transport materials and acidifying storage media
Direct O2 injection can lower NOx
NOx recycled to furnace is “re-burned” and reduced to N2 in furnace
SCR
NO2 forms at higher pressure and oxidises
SO2 to H2SO4
Remaining NOx removed as HNO3 in corrosive
condensateRemaining NO would be vented as a stack
gas
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process-Inerts
N2 & Ar IN
• O2 purity
• Air ingress
N2 & Ar OUT
Dilutes CO2 + affects CO2 VLE & capture rate
Reduces storage media capacity
O2 in CO2 product can corrode pipelines &
support biological growth, lowering injection rates
Li, H., J. Yan, et al. (2009)
Impurity impacts on the oxyfuel process-Hg
Hg can corrode brazed aluminium HEX used in the
“cold box”
during CO2 distillation
Activated carbon “guard beds”
are affected by SO3 absorption at low pressure
and may be explosive under high pressure with O2
Experience with Hg comes from natural gas industry where high pressure activated carbon beds are commonly used
without O2
Hg is expected to be removed as Hg(NO3)2 with
NOx
condensates
Impacts Summary
• Particulates
– Can capture extra SOx and Hg in fly ash – Can increase the load on ESP x– Can be abrasive to compressor if not removed x
• SOx
– Can be corrosive in coal mill, boiler and convective pass– Can be corrosive if condensed as H2SO4 (in recycle lines & compression)– Higher acid dew point in oxy-fuel (
SO2,
SO3,
H2O)– Higher SO3 on fly ash can enhance ESP
• H2O
– Involved in corrosion (H2SO4, HNO3)– Can form solid “ice-like” hydrates with compressed CO2, causing blockages
Impacts Summary
• NOx
– Less of a problem as lower emission than with air firing (re-burning effect)– Environmental limit of NO in stack (as ppm and/or mg/MJ)– Could form HNO3 in compression (corrosive to plant, pipeline)
• Hg
– Hazardous to health, emissions becoming limited by regulation– Can be extremely corrosive to cryogenic aluminium heat exchangers (for
separation of inerts O2/N2)
• Inerts
– Lowers CO2 purity & reduces pipeline/storage capacity (affects economics)– O2 corrosive to pipeline, and some storage media– O2 supports biological growth, – Affects compression energy
Additional research required
• Heat transfer and combustion
• Hot spot evaluation, and burner impacts (cfd based)• Validate emissivity modelling at large scale
• Gas quality and furnace
• Sulphur gases and corrosion • Fly ash-SOx interactions (coal specific?)• NOx/SOx interaction at higher pressure• Mercury levels and form, and impact/control in CO2 handling
Gas quality for transport, compression and storage
• Plant impacts, regulation and safety issues• Pipeline corrosion and effect in storage media
Thank you for your attention!