3rd Oxyfuel Combustion Conference
SO3/SOx Ratio in Oxy-Fuel Combustion – The Impact of
Operational Conditions
Daniel Fleig*, Klas Andersson, Filip Johnsson
Department of Energy and Environment, Division of Energy Technology,
Chalmers University of Technology, SE-412 96 Göteborg, Sweden
Keywords: oxyfuel; sulfur; SO 3; SO2; H2SO4; corrosion
1. Introduction
The formation of SO3 is in general undesired during boiler operation since it increases the risk for corrosion on
heat transfer surfaces. Literature shows that oxy-fuel firing leads to significantly higher SO2 concentrations
compared to air-firing due to recycling of flue gases. An increase in SO2 concentration will lead in general to a
higher SO3 concentration, however the SO3/SOx ratio decreases somewhat as the SO2 concentration increases, as
shown by our modeling work [1]. Measurements performed under oxy-coal conditions reveal a concentration of SO3
that is several times higher than that seen under air-fired conditions [2-8]. Comparing the available data, it is clear
that coal types with a calcium rich ash tend to have the lowest SO3/SOx ratio in the stack. The data also demonstrate
that higher concentrations of SO2 lead to higher concentrations of SO3. However, it is unclear as to which other
factors influence the SO3/SOx formation ratio in oxy-fuel combustion. Thus, there is a need to clarify the conditions
for SO3 formation in oxy-fuel combustion using a more general approach. Therefore, in this work the effect of oxyfuel operating conditions on the gas-phase SO3 formation is characterized with the aim to provide possibilities to
limit the SO3/SOx formation ratio.
2. Experiments
Experiments were performed in the Chalmers 100 kWth oxy-fuel test unit as shown in Figure 1. Propane was used
as fuel and SO2 was injected into the feed gas (oxidizer). The formed SO3 was measured in the flame, after the flame
as well as downstream of the furnace with a controlled condensation method. Tests were performed for both air and
oxy-fuel combustion applying wet and dry flue-gas recycling (FGR) conditions and varying the oxygen to fuel ratio
(λ), fuel load, FGR ratio, and SO2 concentration. Table 1 lists the main experimental test conditions during the
experiments, as well as the concentration of O2, SO2, H2O, and CO2 in the flue gas. The fuel input was equivalent to
*
Corresponding author. Tel.: +46 31-772-1453.
E-mail address: [email protected].
2
60 kWth in all cases, except for the one OF30 case with 75 kWth. The concentration of SO2 and the λ value was
varied in additional experiments, which are not listed in Table 1.
Figure 1. The Chalmers 100 kWth oxy-fuel test unit. M1 to M15 denote measurement positions: in-flame
measurements M2 – M5, outlet SO3 concentration M8, flue-gas composition M13, and feed-gas composition M15.
Table 1. Experimental conditions used in the test cases.
Test case
O2 in feed gas
[vol.%, dry]
Type
of FGR
Oxidizer volume
flow [m3(STP)/h]
λ
21
30
30
46
25
40
dry
dry
wet
dry
dry
77.7
50.1
62.6
56.5
63.0
35.6
1.38
1.25
1.25
1.28
1.31
1.18
Air
OF30
OF30 75 kWth
OF46w
OF25
OF40
XO2 [%]
5.39
5.39
5.39
5.39
5.39
5.39
Flue-gas composition (M13)
XSO2 [ppm]
XH2O [%]
XCO2 [%]
885
2438
1625
870 & 1379
2438
2438
12.0
18.7
18.7
54.0
15.6
24.7
8.6
~71
~71
37.9
~74
64.6
3. Results and Discussion
Figure 2 shows the influence of oxygen to fuel ratio on the SO 3/SOx ratio for the OF30 case and for the air-fired
case. As seen, the SO3/SOx ratio increases with increasing oxygen to fuel ratio. This is in line with experience shown
in literature [9, 10] from air combustion. With respect to industrial applications it should be mentioned that when
lowering the oxygen to fuel ratio, the SO2 concentration increases, assuming that the burnout of sulfur from the fuel
is not affected, which would counteract the benefit of reduced SO 3 formation at lowered oxygen to fuel ratio.
Figure 2. SO3/SOx ratio for different oxygen to fuel ratios for the OF30 and air case.
Daniel Fleig/ Energy Procedia 00 (2013) 000–000
3
Figure 3 shows the measured conversion ratio SO3/SOx for the OF30 75 kWth case and the OF30 case (60 kWth).
It is obvious that the thermal load has a significant impact on the SO 3/SOx ratio. This is mainly due to an increased
temperature level in the furnace and due to the changed temperature field. The SO3/SOx ratio was on average 40%
higher in the OF30 case with 75 kWth fuel load compared to the reference conditions at 60 kWth. A trend to higher
SO3/SOx ratios for an increase in boiler load was also reported by Crumley and Fletcher [11]. In oxy-fuel
combustion during wet FGR (OF46w case) the SO3/SOx ratios were found to be higher than during dry FGR (OF30
case) at similar temperature conditions, as also shown in Figure 3.
Figure 4 shows the measured SO3/SOx ratio for the OF25, OF30 and OF40 cases. The SO3 formation is
significantly enhanced in the OF30 case compared to the OF25 case. The measured SO 3/SOx ratio was highest for
the OF40 case. In general, the increase in SO3/SOx ratio with increasing OF number i.e. reduced FGR ratio can be
explained by an increase of 1) average furnace temperature, 2) residence time, 3) H2O concentration, and 4) O2
concentration in the feed gas. The consequence of this is that it is advantageous to operate oxy-fuel power plants at
low OF numbers (e.g. OF25) if it is of interest to keep the SO3/SOx ratio low.
Figure 3. Impact of fuel load and wet FGR on the
SO3/SOx ratio.
Figure 4. Influence of FGR ratio on the SO3/SOx ratio.
References
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