3.3 Conducted experiments
3.3.4 Experiments at BTS-VR investigating SO 3 formation
dilution of flue gas by air ingress or by unreacted oxidant gas. The methodology for correction of gas concentrations is introduced in section 3.6.4. The reference oxygen corresponds always to the value used for calculation of the flue gas productionV˙FG,dryfor the respective experiment.
The corrections of acid gas concentrations in DSI experiments do not consider dilution of flue gases by sorbent injection gases in particular. Instead, the reference acid gas concentration levely¯i,re f that is used for calculation of the injection stoichiometriesαi and acid gas removal efficienciesηihas been determined during feeding of sorbent injection gas but without addition of sorbents. For all other DSI experiments, it is assumed that the sorbent injection gas stream, or more accurately, the ratio between the flue gas production and this stream was constant.
In the figures in section 4.2.2 of this thesis, showing results of DSI experiments, trendlines based on 2ndorder polynomial fits are plotted for the sake of clarity. Even though this fitting approach yields a good agreement to the experimental results, it has not been optimized and its utilization does not imply a general principle describing the capture of acid gases by DSI.
In all oxy-fuel tests, an overall O2 concentration in the oxidant gas of 28·10−2 mm33, wet, was maintained, with the remainder being CO2, H2O, NO, SO2, and Hg. This corresponds to simulated recirculation ratios of about 70 %.
To ease the identification of different air and oxy-fuel experiments studying SO3 formation, all tests are labeled with a code allowing for identification of the experimental settings. In addition to air fired experiments (labeled “A”), in total eight different oxy-fuel settings, seven with injection of H2O, NO, SO2, and Hg0, and one with combustion in pure CO2/O2(i.e. removal of 100 % of all impurities; experiments labeled “CO2”) were investigated. The fuels and recycle removal efficiencies for SO2 and Hg in simulated oxy-fuel experiments are included in the labels of the respective experiments. For example, the label C3-O2S5H indicates an oxy-fuel experiment with coal C3 and simulated removal of about 20 % of the SO2 (label: 2S) and of about 50 % of the Hg (label: 5H) from the recirculated gas. Coals C1, C2, and C3 were all tested under A, OC, and 2S5H conditions (approx. 20 % SO2and 50 % Hg removal) and coal C1 was Table 3.11: List of indices denominating experimental settings for different fuels, combus-tion atmospheres, simulated recycle and once-through impurity removal efficiencies from the recirculated flue gas for H2O, SO2and Hg, and simulated NOxconcentrations in the recirculated gas in experiments studying SO3formation at BTS-VR.
sim. removal efficiencies in % ηH2O ηSO2 ηHд
index coal air/oxy
rec. o.-t. rec. o.-t. rec. o.-t.
sim.yNO,dry in rec. flue gas
in10−6 mm33
C1-A
C1
air - - -
-C1-CO2 oxy 100 100 100 100 100 100 0
C1-5S5H oxy 25 9 52 25 44 19 890
C1-2S5H oxy 25 9 23 8 44 19 890
C1-2S0H oxy 25 9 23 8 7 2 890
C1-2S8H oxy 25 9 23 8 80 54 890
C1-0S5H oxy 25 9 3 1 44 19 890
C1-0S0H oxy 25 9 3 1 7 2 890
C2-A
C2
air - - -
-C2-CO2 oxy 100 100 100 100 100 100 0
C2-2S5H oxy 26 9 24 8 52 24 860
C3-A
C3
air - - -
-C3-CO2 oxy 100 100 100 100 100 100 0
C3-2S5H oxy 26 9 25 9 63 34 860
C4-A C4 air - - -
-C4-8S0H oxy 48 20 78 49 0 0 950
investigated with five additional oxy-fuel settings (permutations of approx. 0 %, 20 %, and 50 % SO2 and approx. 0 %, 50 %, and 80 % Hg removal). Coal C4 was tested in air and one oxy-fuel setting (C4-8S0H: approx. 80 % SO2 and 0 % Hg removal). Also moisture recirculation was simulated. For coals C1, C2, and C3 oxy-fuel experiments, approx. 25 % drying (recycle) of the recirculated gas and for coal C4, 50 % drying (recycle) was simulated. In addition, NO was added to the oxidant gases to simulate a NO content of the recirculated flue gasyNO of between 860 and 950·10−6 mm33, dry. Since H2O and NO levels were kept constant in the simulated oxy-fuel experiments for each fuel, they are not considered in the labels of the experiments. Table 3.11 lists all experiments including their labels, the simulated oxy-fuel recycle and once-through impurity removal efficiencies, and simulated NO concentrations in the recirculated flue gas.
Deviations from the desired SO2and Hg removal efficiencies, mentioned above, can be observed, but experiments with different coals and similar removal efficiencies are considered to represent comparable oxy-fuel setups. The oxy-fuel setting for experiments with coal C4 differs somewhat from the experiments with coals C1, C2, and C3, since these experiments have been conducted in a different research project with different focus. Nonetheless, they have been included here to complement the results obtained with coals C1, C2, and C3.
The recycle removal efficiencies mentioned above refer to calculated, theoretical concentration maxima for H2O, SO2, and Hg (yi,max,oxy,dry andyH2O,max,oxy,wet) in an oxy-fuel process with no impurity removal and no air ingress and an exit O2 concentration of 3·10−2 mm33, dry. Those maxima can be calculated, according to equations 3.13 (i: SO2, Hg; j: S or Hg) and 3.14, on basis of the fuel analyses.
yi,max,oxy,dry =
γj MM,j 1
1−yO2,exc,dry* . ,
γS
MM,S + γC
MM,C+ γN 2MM,N+
/
-(3.13)
yH2O,max,oxy,wet =
γH
2MM,H + γW MM,W γW
MM,W + γH
2MM,H + 1
1−yO2,exc,dry · * . ,
γS
MM,S + γC
MM,C+ γN 2MM,N+
/
-(3.14)
To simplify the determination of the operational conditions for the different experiments, Aspen Plus (Version 8.6) models for mass balancing of oxy-fuel recycle combustion systems with once-through impurity removal efficiencies, as listed in table 3.11, were prepared for the different fuels. In these models, a simplified fuel conversion, producing only CO2, H2O, SO2, N2, HCl, and Hg, is assumed. The recycle impurity removal efficiencies listed in table 3.11 can be calculated from the once-through ones and the recirculation ratio, according to equation 4.11. Figure 3.8 shows a schematic of the oxy-fuel model used for calculating the oxidant gas
dryer
SO2 & Hg removal filter O2 in recycle
part-ially dried
recycle & O2
flue gas recycle, wet out
H2O recycle out
SO2 & Hg out ash out combustor
coal in
ηH2O,loc
ηHg,loc ηSO2,loc
ξrec
Figure 3.8: Schematic process flow diagram used in Aspen Plus models for balancing oxy-fuel process configurations for simulated oxy-oxy-fuel experiments at BTS-VR with different extents of recycle gas cleaning (i.e.ηH2O,ηSO2, andηHд).
compositions. The Aspen Plus model has been used to calculate the oxidant gas compositions for the different experiments and the corresponding recirculation ratios that would be required in actual oxy-fuel recycle combustion to maintain the desired oxidant O2 concentration of 28·10−2 mm33, wet. In table 3.12 the information on oxidant compositions, simulated recirculation ratios, and coal feeds is summarized.
During the experiments studying SO3 formation with Australian coals (C1, C2, C3) at BTS-VR, the facility’s electrically heated furnace was operated with a constant temperature of all five heating zones (i.e.ϑBTS1toϑBTS5= 1350◦C). In experiments with coal C4, a temperature profile from 1300◦C to 1100◦C was applied (see table 3.8). The facility’s flue gas duct was trace heated, to reduce heat losses and accomplish a temperature and residence time profile comparable to actual power plant systems. Indicative temperature profiles averaged over the individual air and CO2/O2 experiments investigating SO3formation with coals C1, C2, and C3, are presented in figure 3.936. The temperatures and residence times in certain temperature ranges can impact the formation and capture of SO3. However, in all experiments with coals C1, C2, and C3, the temperature and residence time profiles were similar and hence, the results of the experiments studying SO3are assumed to be comparable. Due to the lower furnace outlet temperature, with coal C4 the temperature profile is somewhat altered, compared to the other SO3 formation experiments (see figure 3.4). However, along the flue gas path the temperature profiles for the different experiments converge more and more and reach very similar temperatures at the
36Lines between temperature measurements (indicated by black dots) are linearly interpolated, and the temperature between burner and first temperature measurement of the furnaceϑBT S1is assumed to be constant.
Table 3.12: Simulated recirculation ratios (ξrec), coal feeds (M˙coal), and oxidant composi-tions for studies on SO3 formation at BTS-VR with coals C1, C2, C3, and C4.
oxidant compositions
ξrec M˙coal yH2O yO2 yO2 yCO2 ySO2 yNOx cHд
m3 m3
kg
h 10−2 mm33 10−6 mm33 mµg3,ST P
index
- - wet wet dry dry dry dry dry
C1-A - 2.1 - 20.9 20.9 - - -
-C1-CO2 - 2.8 0 28.0 28.0 72.0 0 0 0
C1-5S5H 69.8 2.8 16.9 28.0 33.7 66.3 515 654 17.8
C1-2S5H 69.8 2.8 16.9 28.0 33.7 66.3 824 654 17.8
C1-2S0H 69.8 2.8 16.9 28.0 33.7 66.3 824 654 29.4
C1-2S8H 69.8 2.8 16.9 28.0 33.7 66.3 824 654 6.5
C1-0S5H 69.8 2.8 16.9 28.0 33.7 66.3 1030 654 17.8
C1-0S0H 69.8 2.8 16.9 28.0 33.7 66.3 1030 654 29.4
C2-A - 1.5 - 20.9 20.9 - - -
-C2-CO2 - 2.1 0 28.0 28.0 72.0 0 0 0
C2-2S5H 70.6 2.1 20.5 28.0 35.2 64.8 1569 631 8.9
C3-A - 1.4 - 20.9 20.9 - - -
-C3-CO2 - 1.9 0 28.0 28.0 72.0 0 0 0
C3-2S5H 70.5 1.9 17.5 28.0 34.0 66.0 1687 634 3.7
C4-A - 1.3 - 20.9 20.9 - - -
-C4-8S0H 73.1 1.7 12.1 28.0 31.9 68.1 1779 700 17.7
fabric filter inlet. Nonetheless, it should be considered that the different temperature profiles for coal C4 and the other coals (C1, C2, C3) may have impacted the results.
The temperature and residence time profile for coal C1, C2, and C3 experiments was determined on basis of the gas flow rates through the furnace, flue gas duct, and filter, the temperatures that were measured along the system, and its geometry. This calculation is subject to uncertainties in the assumptions (e.g. clean furnace and pipes) and differences in the actual temperatures that were reached in individual experiments. This approach was validated in an experiment using a tracer gas and a gas analyzer to determine the actual gas travel times along the furnace and flue gas duct in dedicated tests. A difference between measured residence times and calculated ones of +/- 15 % was observed, which gives an indication of the accuracy of the calculated temperature profiles. A general observation that was made in the air and simulated oxy-fuel experiments is that the temperatures in the flue gas duct in simulated oxy-fuel operation are somewhat higher than in air firing. This is related to the different thermal properties of the flue gases in both combustion modes. The difference is most pronounced at the inlet of the flue
0 2 4 6 8 10 12 0
200 400 600 800 1000 1200 1400
time ins ϑin◦ C
CO2/O2
air furnace
filter
Figure 3.9:Indicative temperature profile along the furnace, flue gas duct, and filter of BTS-VR during air and CO2/O2experiments studying SO3formation with coals C1, C2, and C3 (for coal C4, see figure 3.4).
gas duct where heat losses, due to the high temperature, are highest. The average temperature difference between both combustion modes reaches up to about 40◦C at the duct inlet and then reduces to approx. 20◦C at the filter outlet. For individual experiments, temperature variations along the flue gas duct were observed, with duct inlet temperatures in air firing ranging from about 630◦C to 765◦C and temperatures at the filter inlet between about 185◦C and 225◦C. In oxy-fuel experiments those temperature ranges were approx. 660 to 865◦C and 190 to 210◦C.
The average temperature drop between filter in- and outlet ranged between 25◦C and 30◦C in both combustion modes.
3.3.4.2 Conducted measurements and data processing
In the experiments studying SO3 formation at BTS-VR, O2, CO2, CO, SO2, and NOx were measured continuously before fabric filter. H2O concentrations were calculated based on coal composition and water injection. SO3 was measured discontinuously before and after the BTS-VR’s fabric filter. This filter was cleaned before each individual SO3 sampling. During the course of a measurement, the filter cleaning was discontinued. This implies an increase of the ash loading of the fabric filter during individual measurements as well as differing ash build-up rates for different coal feed rates and ash contents. Such conditions are similar to an actual power plant system, where ash build-up rates vary with the fuel. In the experiments investigating SO3formation at least two but in most cases three repetitive measurements were conducted that were used to calculate average SO3 concentrations that are reported in this thesis.
Based on the SO2and SO3 concentrations measured before the fabric filter of BTS-VRy¯SO3,b.f., SO2 to SO3 conversion ratiosκ23 can be calculated. The calculation ofκ23 was conducted, according to equation 3.15. This correlation is based on a well established standard to calculate
κ23at SCR catalysts for NOx reduction [205] and does not consider the concentration of SO3in the denominator. Due to the relatively low concentrations of SO3 in combustion flue gases, compared to the SO2concentrations, the difference between the calculation ofκ23according to equation 3.15 and according to a correlation that also considers the concentration of SO3 in the denominator is low and does not impact the interpretation of the results obtained. The SO3 capture efficiency at BTS-VR’s fabric filterηSO3,f il. is calculated, according to equation 3.16, on basis of the average SO3concentrations measured before (y¯SO3,b.f.) and after (y¯SO3,a.f.) the filter.
κ23 =
y¯SO3,b.f.
y¯SO2,b.f. (3.15)
ηSO3,f il. =y¯SO3,b.f.−y¯SO3,a.f.
y¯SO3,b.f. (3.16)