BELO 2 - ResearchGate

The Differences in Sulfur Capture Behaviour by Fly Ash
between Air and Oxy-fuel Combustion
L. P. Belo1, R. Spörl2, K. V. Shah1, L. K. Elliott1, R. J. Stanger1,
J. Maier2, T. F. Wall1
1 Chemical
Engineering, The University of Newcastle, Callaghan NSW 2308, Australia
2 Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Germany
Experimental
Introduction
Oxy-fuel combustion is one of the developing
technologies deemed to solve the problem in CO2
emissions. In oxy-fuel, coal is burned in pure oxygen
and recirculated flue gas (RFG) to minimize the N2 in
the system and concentrate the CO2 prior to
compression,
transport
and
storage.
The
concentrations of impurities such as SO2 reaches up to
4 times due to the RFG.
Fly ash was sampled from the bag filter (Fig. 3)
maintained at 225 ± 30 °C (inlet) and 195 ± 15 °C
(outlet). The furnace maintained at a wall temperature
of 1350 °C and with a constant product rate of 11.5 m3
(STP)/h with an outlet O2 concentration of 3%. The
concentrations of SO2 entering and exiting the furnace
used in obtaining the fly ash is shown in Table 1.
Due to the increased concentrations of SO2 during
oxy-fuel combustion, the formation of SO3 is believed
to differ. With this, the extent of sulfation of the basic
oxides in the fly ash is expected to differ.
Fig. 3. Schematic of the 20 kWth experimental
rig at IFK, Stuttgart [1,2]
Fig. 2. Horizontal furnace used for the thermal evolved gas experiments.
Table 1. SO2 concentrations used in obtaining the fly ash samples [1]
Fig. 1. Sulfur routes during coal combustion
The objective of this study was to evaluate the
differences in sulfur capture behaviour by fly ash (FA)
obtained during air firing and oxy-fuel firing.
Coal
A
B
C
SO2 in oxidant SO2 in flue gas
(ppm, dry)
(ppm, dry)
air
oxy
air
oxy
0
824
199
1235
0
1569
367
2578
0
1723
444
2802
Thermal Ash Decomposition
Thermal decomposition experiments
were carried out using an electrically
heated horizontal furnace with a
heating rate of 5°C/min to a
temperature of 1400 ± 20 °C, and
held at temperature for 2 hrs with 1.5
L/min N2 used as the carrier gas.
Results and Discussion
(1) Sulfur release from heating fly ash shows that
evolution of sulfur species align on the plot.
(2) The amount of sulfur species [ppm SO2 in N2]
released during oxy-fuel is 2-3 times greater than in
air (shaded)
(3) 400°C to 800°C: first set of peaks (Fig 4 a) could be Fe2(SO4)3/Al2(SO4)3.
(4) 800°C to 1100°C: second set of peaks more prominent in all oxy-fuel FA, improved capture
mechanism during this temperature range during oxy-fuel combustion; may be MgSO4/K2SO4.
(5) >1300°C: third set of peaks. May be a combination of CaSO4 and Na2SO4.
The Eschka method (Australian Standards) was used to
obtain Total Sulfur from the ash. Eschka Sulfur was
plotted against calcium and alkali & alkaline-earth
metals (AAEMs) and was found (Fig. 5a and 5b) higher
CaO (and AAEM Oxides) captures higher sulfur and not
different between air and oxy. Eschka sulfur was also
plotted against Coal S/Ash and was found that
relationship was not linearly increasing (Fig. 6); other
coal specific factors might be involved in capture and
retention of sulfur in FA.
Fig. 5. Calcium and AAEMs content versus amount
of sulfur present in the FA (determined by Eschka)
‘Decomposable sulfur’ released upon heating and
Eschka sulfur which is total sulfur by digesting with acid.
Thermal decomposition shows that oxy-fuel FA is 2-3
greater in ‘decomposable sulfur’ whereas comparing
Eschka Sulfur shows only 17-23% more sulfur in oxy-fuel
FA than air FA.
Table 2. Sulfur balance and the comparison of Ash Decomposition and the
Eschka Methods [1].
Fly Ash
Sample
Fig. 6. Amount of Sulfur in FA (by Eschka) versus
the ratio of Sulfur/Ash in coal.
A-Air
A-Oxy
B-Air
B-Oxy
C-Air
C-Oxy
Sulfur Content
mg S/g FA
Decomposition
Eschka
0.85
1.67
0.48
1.09
1.65
4.05
1.71
2.00
1.29
1.59
4.21
4.95
Decomposition
Eschka
Eschka
Coal sulfur
% of ash S from
% fuel S
decomposition captured by Ash
50%
53%
83%
63%
38%
19%
68%
24%
39%
59%
82%
69%
Conclusions
Fig. 4. Thermal gas evolution from the decomposition of FA under N2
gas. Shaded (pink) region is the difference between oxy-fuel and airfired FA
Acknowledgments
The authors wish to acknowledge financial assistance provided through
Xstrata Coal Low Emissions R&D Corporation Pty Ltd; the Australian
National Low Emissions Coal Research and Development (ANLEC R&D).
ANLEC R&D is supported by Australian Coal Association Low Emissions
Technology Limited and the Australian Government through the Clean
Energy Initiative.
The Institute of Combustion and Power Plant
Technology (IFK), University of Stuttgart, Germany for the collaborative
research initiative.
• Thermal evolved gas analysis under N2 indicated greater capture by fly ash during oxy-fuel.
• Extend of sulfur species released upon heating, ‘decomposable sulfur’, is 2-3 times greater in
oxy-fuel than its air counterpart.
• Ash chemistry did not differ between oxy-fuel and air-fired fly ash.
References
[1] Belo, L. P.; Spörl, R.; Shah, K. V.; Elliott, L. K.; Stanger, R. J.; Maier, J.; Wall, T. F. Energy & Fuels 2014, 28, 5472-5479.
[2] Spörl, R.; Belo, L.; Shah, K. V.; Stanger, R.; Giniyatullin, R.; Maier, J.; Wall, T. F.; Scheffknecht, G. Energy & Fuels 2014, 28, 123.
[3] Stanger, R.; Wall, T. Progress in Energy and Combustion Science 2011, 37, 69.
[4] Belo, L. P.; Elliott, L. K.; Stanger, R. J.; Spörl, R.; Shah, K. V.; Maier, J.; Wall, T. F. Energy & Fuels. DOI:10.1021/ef5020346

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