Simplified model of SO2 oxidation and KCl sulphation for...
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Simplified model of SO2 oxidation and KCl sulphation for CFD applications
Peter Glarborg and Hao Wu
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DTU Chemiccal Engineering, Technical University of Denmark
Challenges in combustion of solid fuels
• Process efficiency • Pollutant emission control
– NOx
– SOx, HCl – Unburned, PAH – Soot, other aerosols – Heavy metals
• CO2 emission control – Fuel substitution – Carbon capture
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DTU Chemiccal Engineering, Technical University of Denmark
Biomass fuels in Europe
• Woody biomass fuels: – Bark – Industrial wood chips – Sawdust – Forest wood chips – Waste wood – Pellets, briquettes
• Herbaceous biomass fuels: – Straw, cereals – Grasses (miscanthus, giant reed)
• Alternative biomass fuels: – Kernels, shells, olive stones, shea nuts
Frandsen, 2012
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DTU Chemiccal Engineering, Technical University of Denmark
KCl related issues in biomass combustion
KCl Ash deposition
Bed agglomeration
Corrosion
SCR deactivation
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DTU Chemiccal Engineering, Technical University of Denmark
Potassium speciation in ash: concerns
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Deposition Corrosion SCR deactivation
Fly ash quality
KCl XXX XXX XXX XXX K2SO4 XX X XXX K-silicates XX K-alumina-silicates
X
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DTU Chemiccal Engineering, Technical University of Denmark
Combustion of straw on a grate
Department of Chemical Engineering
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0 100 200 300 400 500 600 700 800
MeasurementEmission limit
SO2 /
mg/
Nm
3
Time / min
Data from the Ensted Power Plant (Knudsen and Sander, 2004)
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DTU Chemiccal Engineering, Technical University of Denmark
Grate-firing of biomass: fate of K, S, Cl
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DTU Chemiccal Engineering, Technical University of Denmark
Sulfation of KCl
Proposed gas-phase sulfation mechanism: SO2+½O2 → SO3 (global, rate limiting) KCl + SO3 (+M)→ KSO3Cl (+M) (fast) KSO3Cl + H2O → KHSO4 + HCl (fast) KHSO4 + KCl → K2SO4 + HCl (fast) 2KCl+SO2+½O2+H2O→K2SO4+2HCl (net)
KSO3Cl
KHSO4
Reference reaction: NaOH+HCl → NaCl+H2O k298 ∼ 1014 cm3 mol-1 s-1
Silver et al. (1984) Glarborg and Marshall (2005)
K2SO4
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DTU Chemiccal Engineering, Technical University of Denmark
A mechanism for sulfation of KCl
Hindiyarti et al., 2007
Detailed reaction mechanism: 50 species, 200+ reactions
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DTU Chemiccal Engineering, Technical University of Denmark
Flame experiments at Lund University
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• Multi-jet (91) burner • Atm pressure flame • Hydrocarbon/oxygen/nitrogen • Add KCl w/wo SO2
• Measure temperature, KCl, HCl, SO2
Li et al. 2013
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DTU Chemiccal Engineering, Technical University of Denmark
Temperature profiles
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Rayleigh
Thermocouple
Model
Tem
pera
ture
/ K
Height above burner / cm
Flame 1
Flame 3
Flame 5
Li et al. 2013
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DTU Chemiccal Engineering, Technical University of Denmark
Effect of SO2 addition on post-flame KCl
12 Li et al. 2013
1030 K 1100 K
1300 K 1410 K
Without SO2
With SO2
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DTU Chemiccal Engineering, Technical University of Denmark
In-furnace KCl sulfation Gas-phase mechanism: SO2+Ox → SO3 KCl + SO3 (+M)→ KSO3Cl (+M) KSO3Cl + H2O → KHSO4 + HCl KHSO4 + KCl → K2SO4 + HCl K2SO4 → aerosol
Post-flame sulphation of KCl
Li et al. (2012)
Gas-solid reaction: 2KCl+SO2+½O2+H2O→K2SO4+2HCl (net)
Sengeløv et al. (2013)
Without SO2
With SO2
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DTU Chemiccal Engineering, Technical University of Denmark
From basic science to technology
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Semi-industrial scale experiments
Model (CFD)
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DTU Chemiccal Engineering, Technical University of Denmark
Objectives
• Develop a simplified model for gas-phase sulphation of KCl –Describe oxidation of SO2 to SO3
–Describe sulphation of KCl by SO3
–Describe homogeneous nucleation of K2SO4
• Questions: –Is it possible to reduce the detailed model (50 species,
200+ reactions – without fuel oxidation) to an operational size model?
–Where does the important chemistry occur? –Is superequilibrium of radicals important?
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DTU Chemiccal Engineering, Technical University of Denmark
Representing chemistry in CFD
•Full reaction mechanism
•Skeletal mechanism
•Analytically reduced mechanism
•Global mechanism
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Accuracy Complexity
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DTU Chemiccal Engineering, Technical University of Denmark
Effect of cooling rate on SO3 formation - predictions with full mechanism
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
SO3 equilibrium
Cooling rate 100 K/sCooling rate 200 K/sCooling rate 400 K/sCooling rate 800 K/s
SO3 m
ole
fract
ion
Temperature / KInlet composition: 500 ppm SO2, 4% O2, 10% H2O, 5% CO2; balance N2
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DTU Chemiccal Engineering, Technical University of Denmark
Skeletal mechanism for SO3 formation
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Minimum reaction subset that provides a satisfactory description of the relevant chemistry
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DTU Chemiccal Engineering, Technical University of Denmark
Radical concentrations during cooling
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EquilibriumCooling rate 100 K/sCooling rate 200 K/sCooling rate 400 K/sCooling rate 800 K/s
O m
ole
fract
ion
Temperature / K
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EquilibriumCooling rate 100 K/sCooling rate 200 K/sCooling rate 400 K/sCooling rate 800 K/s
OH
mol
e fra
ctio
n
Temperature / K
Inlet composition: 500 ppm SO2, 4% O2, 10% H2O, 5% CO2; balance N2
Radical partial equilibrium largely maintained during cooling
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DTU Chemiccal Engineering, Technical University of Denmark
Prediction of SO3 formation
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
EquilibriumFull mechanismSkeletal mechanismReduced mechanism
SO3 m
ole
fract
ion
Temperature / K
Skeletal mechanism: • Detailed H2/O2 subset • Skeletal SOx subset
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DTU Chemiccal Engineering, Technical University of Denmark
Prediction of SO3 formation
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
EquilibriumFull mechanismSkeletal mechanismReduced mechanism
SO3 m
ole
fract
ion
Temperature / K
Reduced mechanism: • Assume HOSO2 in steady state • Assume O in partial equil. O2+M = O+O+M • Assume OH in partial equil. H2O+½O2 = OH+OH • Rates of formation: ωSO2 = – ω1 – ω3 – ω4
ωSO3 = ω1 + ω3 + ω4
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DTU Chemiccal Engineering, Technical University of Denmark
Effect of cooling rate on sulphation rate - predictions with full mechanism
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
SO3 equilibrium
Cooling rate 100 K/sCooling rate 200 K/sCooling rate 400 K/sCooling rate 800 K/s
Mol
e fra
ctio
n
Temperature / K
SO3
K2SO
4
K2SO
4(s)
Inlet composition: 500 ppm SO2, 50 ppm KCl 4% O2, 10% H2O, 5% CO2; balance N2
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DTU Chemiccal Engineering, Technical University of Denmark
Skeletal mechanism
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DTU Chemiccal Engineering, Technical University of Denmark
Predictions of sulphation - performance of skeletal mechanism
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
Full mechanismSkeletal mechanism
Mol
e fra
ctio
n
Temperature / K
SO3
K2SO
4
K2SO
4(s)
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DTU Chemiccal Engineering, Technical University of Denmark
Analytically reduced model
Simplifying assumptions: • Assume H2SO4 in steady state • Assume O in partial equilibrium
(O2+M = O+O+M) • Assume OH in partial equilibrium
(H2O+½O2 = OH+OH) • Replace the fast gas-phase alkali
sulphation steps by one global reaction (G):
SO3+2KCl+H2O → K2SO4+2HCl
Model: • Components: SO2, SO3, KCl, K2SO4, K2SO4(c) • Rates of formation: ωSO2 = – ω1 – ω3 – ω4
ωSO3 = ω1 + ω3 + ω4 - ωG ωKCl = – ωG ωK2SO4 = ωG – ωNUCLEATION ωK2SO4(c) = ωNUCLEATION
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DTU Chemiccal Engineering, Technical University of Denmark
Predictions of sulphation - performance of reduced mechanism
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0
5 10-6
1 10-5
1,5 10-5
2 10-5
2,5 10-5
8001000120014001600180020002200
Full mechanismReduced mechanism
Mol
e fra
ctio
n
Temperature / K
SO3
K2SO
4
K2SO
4(s)
KHSO4+K
2SO
4
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DTU Chemiccal Engineering, Technical University of Denmark
Conclusions
• A simplified model for gas-phase sulphation of KCl in the post-flame region has been developed
–Oxidation of SO2 to SO3
• O/H radicals in partial equilibrium • HOSO2 in steady-state
–Sulphation of KCl by SO3
• One-step global reaction for sulphation • One-step global reaction for homogeneous
nucleation of K2SO4 • Provides a good estimate of the Cl/S ratio in the
condensed alkali salts • Needs refinement to predict concentration profiles
(time / temperature)
27