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Lecture 3: Oxyfuel Combustion Science: Mass and energy balances, heat transfer, coal
combustion and emissions
Professor Terry Wall and Dr Jianglong Yu
University of Newcastle, Australia
APP OFWG capacity building course
University of Newcastle, Australia
Oxy- fuel flowsheet for first generation technology, showing additional units for a retrofit in red
Air Separation unit (ASU)
Air
Recycled Flue Gas (RFG)
Nitrogen
V Boiler or Ash removal /
cooler / Purification /
Oxygen
Fuel CO2 (SO2)
CO2 –rich Flue Gas
Recycled Flue Gas (RFG)
Conc. Stream of CO2
Vent
Gas Turbine cooler /
condenser / FGD compression
Steam
2 ( 2)
Steam Turbine
Power
Oxy- fuel flowsheet for first generation technology, showing additional units for a retrofit in red
Issues with Oxy-combustion CCSASU
g
ASUCO2 handlingPurification; compression; transportation; storage
Replacement of nitrogen with recycled CO2
Combustion environment of fuel•Fuel reactivityR t fit•Retrofit
•Impurity formation and emissionI it i t CO handlingImpurity impact on CO2 handling
Lecture context and content
Oxyfuel science used here to compare air and oxy-fuel furnace f f t fit f i ti i fi d b il hilperformance, for retrofit of an existing air-fired boiler while
maintaining heat transfer, considering
Conditions for matched heat transfer– 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
Mass and energy balances and heat transfer
AFT of air and oxy cases
2600
2200
erat
ure
(K)
air
1800
flam
e Te
mpe oxy-wet
oxy-dry
1400
Adia
batic
10000.18 0.22 0.26 0.30 0.34 0.38
O2 fraction at burner inlet
Oxy-fuel: differences of combustion in O2/CO2compared to air firingp g
•To attain a similar AFT the O2 proportion of the gases through the burner is ~ 30%g g
•The high proportions of CO2 and H2O in the furnace gases result in higher gas emissivitiesgases result in higher gas emissivities
•The volume of gases flowing through the furnace is reducedreduced
•The volume of flue gas (after recycling) is reduced by about 80%by about 80%.
Recycle gases have higher concentrations in theffurnace
Gas property differences 1: EmissivityTriatomic gas (H2O+CO2) emissivity ~ beam length
comparisonscomparisons
30 500 1050 MWe
0.8
1
(-)
Oxy-fuel fired furnace
30 500 1050 MWe
0
0.6
mis
sivi
ty (
Air fired furnace
0.2
0.4
Gas
Em
00 10 20 30 40 50 60 70 80 90
Beam Length (L) (m)
Gupta et al (2006)
Beam Length (L) (m)
CFD radiative transfer inputs
0.8 4 grey gas modelOxy-fuel fired furnace
0.6
0.7
(-)
4 grey gas model,
4-GGM
Gupta et al, (2006)
0 4
0.5
issi
vity
( Gupta et a , ( 006)
3 grey gas model,
0 2
0.3
0.4
Em 3-GGM
Smith et al, (1982)30 MWe 500 MWe0.2
0 10 20 30 40 50 60Beam length (L) (m)
]1[)( )(
0,
22 Lppk
ii
OHCOieTa
Property/ratio Gas property differences 2: Heat capacity etc
Impact for air to oxyfuel retrofitHigher O2 thru burner
Lower burner velocity, higher coal residence time in furnace
Slower flame propagation velocity
Properties from Shaddix 2006Gas property ratios for CO2 and N2 at 1200 K
Properties from Shaddix, 2006
Burner flow comparisons for a retrofit
Therefore secondary RFG reduced
27 O2 % v/v fixed for same
RFG reduced
2HT
Fi d l it
~3% v/v O2Fixed velocity 2
Burner
1 MWt test conditionsParameter 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 8Primary 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
Preheated air/RFG: primary 350 - 400K and secondary 450 - 550 K, Wall 1200 K
IFRF Flame types from swirl burners
Type-0 Lo SType 0 Lo S
Type-1 Hi S (S>0.6) , Lo v
T 2
Lo v2
Type-2Hi S, Hi v2
1 MWt – Temperature contours at full load
X 1.5m
Air-case
Type-0 flame
Oxy-case
Coal combustion in Sandia’s entrained flow reactor under the intermediate gas temperature conditions.intermediate gas temperature conditions.
(Murphy and Shaddix 2006)
Sensitivity analysis – full & partial load
Effect of momentum flux
Confirms the significance of momentum flux and Gas properties on flame ignition
1 MWt Temperature comparisons for matching furnace heat transferfurnace heat transfer
Flame
FEGTFEGT
Coal combustion in Sandia’s entrained flow reactor under the intermediate gas temperature conditions.intermediate gas temperature conditions.
(Murphy and Shaddix 2006)
Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion.pulverized coal during oxy fuel combustion.
Shaddix, C. R. and A. Molina (2009)
Devolatilization of coals under fluidized bed conditions in oxygen-enriched airyg
(Borah, Ghosh et al. 2008)
30 MWe Burner plane – Temperature contoursAir case Oxy case
30 MWe – heat transfer results
250
m2)
Front wall Rear wall Side wall1 Side wall2
210
230
flux
(kw
/m
airoxy-fuel
190
face
hea
t f
150
170
Tota
l sur
f
45 51 56 56Furnace wall area (m2)
425 MWe – burner zone temperatures
2200
2000
re (K
)
Air
1600
1800
s te
mpe
ratu
r
Oxy-0.28-3GGMOxy-0.28-4GGM
1400Peak
ga
12000 2 4 6 8 10 12
Burner horizontal level
Summary plot relating gas emissivity changes to burner oxygen ……yg
2.0ai
r) wet recycle dry recycle
1.8
y (o
xy to
a
1.2 MWt
1.6
s em
issi
vity
425 MWe
30 MWe
1.2 MWt
1.4
atio
of g
as
30 MWe
425 MWe1
1.20.20 0.25 0.30 0.35 0.40
O f ti t b i l t ( )
R
0 2
0.4
0.6
0.8
Gas
Em
issi
vity
(-)
Oxygen fraction at burner inlet (-)0
0.2
0 10 20 30 40 50 60 70 80 90
Beam Length (L) (m)
G
Heat transfer: Other relevant study (CFB (Zhejiang 2003))(Zhejiang 2003))
Through early modelling although CO2 has a differentth d i ti d th t tthermodynamic properties, under the same oxygen content,oxy-combustion heat transfer does not differ much from aircombustion because oxy-combustion has a lower flametemperature;temperature;
CFD based modelling indicated that when oxygen content inthe oxy-combustion increases from 21% to 29%, combustionthe oxy combustion increases from 21% to 29%, combustionefficiency and boiler efficiency increase. But further increasein oxygen content does not significantly improve the efficiencyand 30% above oxygen causes other problems and safetyyg p yconcerns. The oxygen content in oxy-combustion thereforevary from 21% to 29%.
….. And O2 fraction at the burner can be determined by flue gas recycle ratio (here, for wet recycle)y g y ( , y )
Coal combustibilityCoal combustibility
1 MWt Combustibility comparison
6 Coal A
Coal B
Parity, equal C-in-ash air/oxy
4
5
ode ?%
Coal C
3
n-ash, Oxy m
1
2
Carbon-in
0
1
0 2 4 6 8 10
Carbon-in-ash, Air m ode (%)
Unburnt carbon in ash: oxy vs air combustion
El t i P D l t & IHI Ki 1995Electric Power Development & IHI, Kimura 1995
Illustrative differences in air and oxyfuel which influence burnoutinfluence burnout
For matched furnace heat transfer:For matched furnace heat transfer:
O f l h l f id ti 20% G dOxyfuel has longer furnace residence time, ~20%
Oxyfuel has lower temperatures, ~ 50 C
Good
BadIn oxyfuel, coal experiences an environment with higher O2
Good
Pyrolysis and oxidation reactivities of Coal A & Coal B in TGA
0.12Coal A N2C CO2
0.08
0.1
min
-1)
Coal A CO2Coal A AirCoal A OxyCoal B N2C l B CO2
Coal BPyrolysis
0 04
0.06
ctiv
ity (m
Coal B CO2Coal B AirCoal B Oxy
Coal BOxidation
Coal AOxidation
Coal A
0.02
0.04
Rea
c Coal APyrolysis
00 200 400 600 800 1000 1200
Temperature (oC)Temperature ( C)
Heat flow during pyrolysis & oxidation of Coal A & Coal B in TGA
200Coal A N2Coal A CO2
100
150
mW
)
Coal A CO2Coal A AirCoal A OxyCoal B N2Coal B CO2
Coal BOxidation
Coal AOxidation
0
50
at fl
ow (m Coal B Air
Coal B OxyOxidationExothermic
-50
0
Hea
Coal BPyrolysis
Coal A Pyrolysis
Pyrolysis & gasificationEndothermic
-1000 200 400 600 800 1000 1200
Temperature (oC)
Coal A PyrolysisEndothermic
Temperature ( C)
Volatile yields in DTF at 1400 C
Coal B Coal C Coal DCoal B Coal C Coal DV* (N2) 36.7 30.9 53.5Q factor (N )
1.52 1.43 1.76(N2)V* (CO2) 43.3 32.2 66.2Q factor (CO )
1.79 1.49 2.18(CO2)
V* - Volatile yield at 1400 oCQ factor – Ratio of V* and volatile yield obtained by proximate analysis
Char burnout in DTF taking V*(N2) to estimate char yieldchar yield
100
80
90
100
basi
s (%
)
Coal B
60
70
80
nout
daf
b
40
50
60
Cha
r bur
n AirOxy
400 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Oxygen concentration (atm)
Summary of reactivity data for 63-90 micron size cuts of four coals from DTF experiments at 1400 Cfour coals from DTF experiments at 1400 C
Volatile yield Coal Burnout
5060708090
100
ld a
t 140
0 o C in
f b
asis
), %
889092949698
100
out a
t 140
0 o C
in
af b
asis
), % 8% O2
010203040
65 70 75 80 85
Vola
tile
yiel
DTF
(da
N2CO2 80
82848688
65 70 75 80 85F l C b (d f b i ) t %
Coa
l bur
noD
TF (d
a
Air, 10% O2Oxy, 10% O2
Fuel Carbon (daf basis), wt.%Fuel Carbon (daf basis), wt.%
99
100o C
in
% 21% O2
96
97
98
99bu
rnou
t at 1
400
TF (d
af b
asis
), %
Ai 21% O2
% O
95
96
65 70 75 80 85
Fuel Carbon (daf basis), wt.%
Coa
l b DT Air, 21% O2
Oxy, 30% O2
DTF-char reactivity in TGA
0.005Air 2% O2Oxy 2% O250% O2
Coal B
0 003
0.004
ity (s
-1)
y %Air 5% O2Oxy 5% O2Air 10% O2Oxy 10% O2Ai 21% O2
2
21% O2
0.002
0.003
Rea
ctiv Air 21% O2
Oxy 21% O2Air 50% O2Oxy 50%O2
10% O2
5% O
0.001Cha
r
2% O2
5% O2
0300 500 700 900 1100 1300
Temperature (oC)Temperature ( C)
Char reactivity comparison for air and oxyfuel conditions at the same O levelconditions at the same O2 level
Oxy-fuel (O2/CO2) combustion
Ai (O /N )
L t t
Air (O2/N2) combustion
e Low temperaturesReaction kinetics controlled (Regime I)
actio
n ra
te
Moderately high temperatures Reaction kinetics
Very high temperatures Bulk diffusion
Rea
Reaction kinetics& internal diffusion limited (Regime II)
Bulk diffusion controlled (Regime III)
1/Tp (K-1)
Combustion efficiency (cfb)cy nc
y
on e
ffici
en
ler
effic
ien
O2
Com
bust
io
O2
Boi
C
Emissions and impurity impacts
PM, SOx, NOx, HgPM, SOx, NOx, HgConcentration in the flue gas and in furnace
and Impacts on CO2 handling
Impurity impacts on the purification process in oxy-fuel combustion based CO2 capture and storage system
CRR=92.15%
Li, H., J. Yan, et al. (2009)
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air g y
combustion conditions Sheng, 2008
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air g y
combustion conditions Sheng, 2008
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air g y
combustion conditions Sheng, 2008
Formation of Submicron Particulates (PM1) from the Oxygen-Enriched Combustion of biomass
Zhang, 2007
Impurity impacts on the purification process in oxy-fuel combustion
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
2-stage-flash
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
2-stage-flash
Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion
distillation
Li, H., J. Yan, et al. (2009).
SO2 formation (CFB )
O2
SO2 emission Normalized SO2 emission
O2
(CFD, Zhejiang 2003)
2 2
NOx emission (PF )
El t i P D l t & IHI Ki 1995Electric Power Development & IHI, Kimura 1995
Fuel-N conversion into NOx (PF )
IHI T Ki 1997IHI, T.Kiga, 1997
NOx emission (CFB )
O2
SO2 emission Normalized SO2 emission
O2
(CFD, Zhejiang 2003)
Additional research required
Heat transfer and combustion
Hot spot evaluation and burner impacts (cfd based)Hot spot evaluation, and burner impacts (cfd based)
Quantify potential O2 reduction
Gas quality and furnace
Sulfur gases and corrosion (using pilot-scale data and g ( g pcalculation)
Mercury levels and form, and impact on CO2 handling
Gas quality for transport, compression and storage
Plant impacts, regulation and safety issuesp , g y
Thank you for your attention!Thank you for your attention!
Phase Diagram of CO2
Flame propagation speed (mm/s) vs oxygen content in different atmospherecontent in different atmosphere
(T. Kiga, 1997)