Vaporization and Transformation of Mineral
Components in High CO2 Atmosphere of
Oxyfuel Combustion
Yongchun Zhao, Wenju Li, Junying Zhang, Chuguang Zheng
State Key Laboratory of Coal Combustion, Huazhong University of Science and
Technology, Wuhan, 430074, China
Ash deposit
Thickness 10cm！
Deposits formation on the heating tubes in coal-fired boilers reduces the energy
efficiency, shortens the lifetime and causes unplanned shutdowns of the boiler. More
than 80% coal-fired boilers are influenced by high temperature corrosion and
slagging; The problem is so seriously in China due to the varying of coal quality.
Mineral transformation & ash deposition
Bryers RW, Prog. Energy Combust. Sci. 1996, 22:29-120
 The vaporization, transformation and melt has significant influence
on ash deposition.
3
Outline
Air combustion
oxyfuel combustion
Mineral transformation --- Thermodynamic
calculation
Mineral component vaporization --- Tubefurnace experiments
Mineral quantitative melting thermal
analysis (MQTA)--- TG-DTG-DSC melting
experiments
4
Mineral components transformation
100
80
Mole percent w/%
60
50
CaCO3 (oxy)
CaSiO3(oxy)
Ca2Al2SiO7(oxy)
CaSO4(oxy)
CaMg(CO3)2(oxy)
Ca3Si2O7(oxy)
Ca(NO3)2(oxy)
80
Mole percent w/%
70
CaCO3 (air)
CaSiO3(air)
Ca2Al2SiO7(air)
CaSO4(air)
CaMg(CO3)2(air)
Ca3Si2O7(air)
Ca(NO3)2(air)
CaMgSiO44(air)
40
30
20
MgSiO3(air)
MgO(air)
Mg2SiO4(air)
MgCO3(air)
MgSiO3(oxy)
MgO(oxy)
Mg2SiO4(oxy)
MgCO3(oxy)
60
40
20
10
0
1000
800
600
400
Temperature ℃
200
0
1000
800
600
400
200
Temperature ℃
 Different atmospheres will has significant influence on mineral
partition and transformation. Especially for some carbonate
components.
5
Effect of CO2 concentration
100
80
Mole percent w/%
70
CaCO3 15%CO2
CaCO3 50%CO2
CaCO3 80%CO2
CaCO3 90%CO2
CaMg(CO3)2 15%CO2
CaMg(CO3)2 50%CO2
CaMg(CO3)2 80%CO2
CaMg(CO3)2 90%CO2
Mole percent w/%
90
10
60
50
40
30
KHCO3 15%CO2
KHCO3 50%CO2
KHCO3 80%CO2
KHCO3 90%CO2
5
20
10
0
1000
0
800
600
400
Temperature ℃
200
1000
800
600
400
200
Temperature ℃
 The CO2 concentration also has influence on carbonate mineral
components.
6
Effect of SO2 concentration
100
CaCO3
0ppmSO2
CaSiO3
0ppmSO2
Ca2Al2SiO7 0ppmSO2
CaSO4
0ppmSO2
CaMg(CO3)2 0ppmSO2
Ca3Si2O7
0ppmSO2
CaMgSiO4 0ppmSO2
60
100
80
Mole percent w/%
Mole percent w/%
80
CaSO4 2000ppmSO2
Ca2Al2SiO72000ppmSO2
CaSiO3 2000ppmSO2
40
20
Fe2O3 0ppmSO2
Fe3O4 0ppmSO2
FeO 0ppmSO2
60
Fe2(SO4)3 2000ppmSO2
Fe2O3
2000ppmSO2
2000ppmSO2
Fe3O4
40
20
0
1000
800
600
400
200
Temperature ℃
0
1000
100
800
600
400
200
Temperature ℃
Mole percent w/%
80
 The SO2 concentration also
has influence on several
mineral components (Ca, Fe,
Mg).
60
MgSiO3 0ppmSO2
MgO 0ppmSO2
MgSiO4 0ppmSO2
MgCO3 0ppmSO2
40
MgSO4 2000ppmSO2
MgSiO3 2000ppmSO2
MgO
2000ppmSO2
MgSiO4 2000ppmSO2
20
0
1000
800
600
400
Temperature ℃
200
7
Typical component vaporization
Vaporization of silica in different atmospheres (HSK)
14
Air
CO2:O2=8:2
Vaporization rate wt.%
12
10
8
6
4
2
0
800
900
1000
1100
Temperature
1200
1300
1400
℃
 8% of SiO2 vaporization in different atmospheres; the vaporization
of SiO2 is easy in the high temperature at air atmosphere
compared with oxyfuel atmosphere.
Effect of CO2 concentration on vaporization
10
CO2:O2=9:1
CO2:O2=8:2
CO2:O2=7:3
vaporization rate wt.%
8
6
4
2
0
800
900
1000
1100
Temperature
1200
1300
1400
℃
Influence of O2 concentration on silica vaporization (HSK)
 High O2 results the decrease of silica vaporization, reduction
condition is helpful for Si vaporization.
9
Effect of occurrence on vaporization
12
Caol
LTA
SiO2
Vaporization rate wt.%
10
8
6
4
2
0
800
900
1000
1100
Temperature
1200
1300
1400
℃
Influence of silica occurrence on its vaporization (HSK)
 Compared with different silica occurrences, the silica in coal got
highest vaporization rate, because of the carbonaceous in coal.
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Quantitative of melting process
Chemical composition
Stickiness of deposit
Physical characteristics
Ash deposit
slagging
Ash fusion temperature
Melting process
Multivariate phase diagram
Mineral Quantitative &
Melting Thermal Analysis
LTA Mineral composition
Mineral transformation
Thermal analysis
Mineral Quantitative & Melting Thermal
Analysis (MQTA)
Mineral Quantitative
& Melting Thermal
Analysis(MQTA)
Mineralogy
composition
Rietveld Mineral
Quantitative
Mineral
transformation
Thermal analysis
method(STA)
Melting mass
change
MQTA
Mineral melting
thermal analysis
Melting heat
change
Theoretical basis
Melting
reaction
No mass
change
Melting reaction enthalpy change theory
Endothermic
Endothermic process without
weight change
The progress of melting reaction is
proportional to the endotherm during the
process, thus the reaction progress can
be reflected by DSC curve.
The reaction kinetic can be
established based on DSC
signal which reflects
melting energy in theory.
H
S′
Reactant
=
α=
conversion rate
Ho
S
Reactant
H 0 − H S − S ′ S ′′
=
=
concentration 1 − α = H
S
S
0
Theoretical basis
melting
Mineral
Ash deposit
O
H
Si
Mineral
transformation
Fe
S
Al
Mineral
evaporation
Melting
endotherm
Total
endotherm
Other reaction
endotherm
 Different components melt at different temperatures that will result in the overlap of endothermic
peaks. In addition, the formation process of ash not only contained ash melting but also
accompanied the evaporation of some components.
Melting endotherm
There is a weak endothermic
peak without weight loss from
point A (809℃) to point B (1013℃)
in the DSC curve. This is a typical
melting process.
There are obvious endothermic
peaks between BC and CD in DSC
curve, the BC section with an
obvious weight loss has mineral
composition evaporation, whereas
the CD section without mass
change belongs to another melting
process.
Mineral quantitative of LTA
煤样
Coal
LTA
高岭石
Kaolinite
伊利石
Illite
石英
Quartz
方解石
Calcite
烧石膏
Bassinite
黄铁矿
Pyrite
锐钛矿
Anantase
XLT
18.7
8.5
21.3
22.9
13.8
23.6
5.5
4.4
Weight loss during mineral transformation
高岭石： Al 2 SiO2 O5 (OH ) 4 450
→ 2 H 2 O + Al 2 O3 ⋅ 2 SiO2
Kaolinite
伊利石： 2 KAl 2 Si3 AlO10 (OH ) 2 850
→ 2 H 2 O + K 2 O ⋅ 3 Al 2 O3 ⋅ 6 SiO2
Illite
815
方解石： CaCO3 →
CaO + CO2
Calcite
→ CaSO4 + 0.5 H 2 O 1200
→
 CaO + SO2 + 0.5O2
烧石膏： CaSO4 ⋅ 0.5 H 2 O 
Bassinite
黄铁矿： 2 FeS 2 + 5.5O2 500
→ Fe2 O3 + 4 SO2
Pyrite
Weight loss of
evaporation
Total weight
loss
Weight loss of
mineral partition
Effect of evaporation of mineral
矿物熔融吸热
总吸热
Mineral melting endothermic
Endothermic
矿物蒸发吸热
Mineral evaporation endothermic
蒸发量
蒸发焓变
Evaporation
Evaporation enthalpy
n
H LTA = ∑ H iCi
1
10.57KJ/g
蒸发吸热
Evaporation endothermic
Melting of pyrite in oxyfuel combustion
TG-DTG-DSC of pyrite in oxyfuel
DSC curves of pyrite in different atmosphere
 Compared to the traditional coal combustion atmosphere, the lose weight of pyrite
decomposition increased in oxyfuel atmosphere, high CO2 concentration resulted the
shortening of the decomposition process and the slight extended of oxidation process.
And the melting endothermic of pyrite is higher up to 30.4 J/g in oxyfuel atmosphere
compared to 5.0 J/g in air combustion.
TG-DTG curves of calcite
TG curves of calcite
DTG curves of calcite
 The decomposition of calcite was delayed obviously in oxyfuel atmosphere, TG-DSC
curves indicated the decomposition temperature is up to 867.5 ℃ in oxyfuel
atmosphere, and 697.8 ℃ in air combustion; the loss weight peak occurred at 923.5
℃, higher than 826.3℃ in air atmosphere.
Melting of calcite in oxyfuel combustion
TG-DTG-DSC of calcite in oxyfuel
DSC curves of calcite in different atmosphere
 And the melting endothermic of calcite is higher up to 32.4 J/g in oxyfuel atmosphere,
compared to 15.3 J/g in air combustion. The melting temperature of minerals are
lower in oxyfuel atmosphere compare to those in air, which results mineral melting
easier and probably cause serious ash deposit in oxyfuel.
Comparison of melting curve and AFT
0.5
Melting fraction
0.4
XLT
IDT
HT
FT
0.3
0.2
0.1
0
600
800
1000
1200
1400
Temperature/℃
Compared with the ash fusion temperatures that determined by national standard GB/T219-1996,
the temperature of mineral began to melt is 300℃, lower than the initial deformation temperature
(IDT), which is consistent with the results of Hansen et al.
Summary
 A new method (MQTA) was proposed to
describe the melting process of mineral
components;
 Mineral vaporization in oxyfuel atmosphere is
lower than traditional air combustion;
 While the melting of minerals in oxyfuel is easy
than that in air atmosphere;
 The deposition is depend on the combination of
mineral vaporization and melting process.
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Thanks for your attention!
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