Article pubs.acs.org/EF
Pressurized Oxy-fuel Combustion: A Study of Selected Parameters Janusz A. Lasek,* Krzysztof Głód, Marcin Janusz, Krzysztof Kazalski, and Jarosław Zuwała Institute for Chemical Processing of Coal, ul. Zamkowa 1, 41-803 Zabrze, Poland ABSTRACT: Oxy-fuel combustion is a potential low-emission technique for energy conversion. This paper presents the results of oxy-fuel experiments under high-pressure conditions. The influence of process parameters, such as reactor pressure and temperature, on the emission of NOx, N2O, and other compounds was tested. A new oxy-fuel experimental setup is presented. The experiments were conducted using a laboratory-scale (fuel input of up to 3 kg/h) pressurized fluidized-bed combustor. The feedstock used was “Ziemowit” coal, and the tests were carried out under oxy-fuel and air-fired conditions. The temperature inside the reactor was in the range of 750−900 °C. Generally, NOx emission decreases significantly under higher pressure; however, the details of this trend depend upon the experimental conditions. The effects of pressure and temperature on NOx and N2O emissions are discussed.
1. INTRODUCTION Developing new combustion techniques helps to improve energy efficiency and reduce emissions. Oxy-fuel combustion is considered a potential low-emission technique for energy conversion. Recently, it has also been intensively investigated.1,2 Among the many advantages of the oxy-fuel technique, one advantage is especially worth considering. The release of nitrogen oxides (NO and NO2, collectively referred to as NOx) is tightly restricted because of favorable conditions for the reduction of these pollutants. The impact of molecular nitrogen is limited, and thus, the thermal formation of NOx is very limited. Additionally, an increase in the temperature can cause a decrease in NOx emission during oxy-fuel combustion.1,3 Previously, the transformation of nitrogen oxides (NOx and N2O) under high pressure has been investigated mainly under air-fired conditions. The observations from these studies4−10 are consistent: increasing pressure decreases NO emission. Pressurized oxy-fuel combustion (POFC) is not a well-understood scientific topic. Only a few POFC installations are described in the literature. Examples include the following: University of Nevada (U.S.A.)/ThermoEnergy Power Systems (U.S.A.)/ CANMET (Canada) (15 MWe, coal/biomass),11−13 Enel Ingegneria e Innovazione (Italy) (5 MWt),14,15 and Institute for Chemical Processing of Coal, Zabrze (Poland) [laboratoryscale bubbling fluidized-bed combustor (BFBC), 21 kW]. POFC has also been studied by computational modeling. Seepana and Jayanti16 used numerical modeling to investigate the influence of pressure on the flame structure and NO creation. At optimal conditions, NO emission can be abated by increasing the pressure. Hong et al.17,18 presented modeling calculations for the power produced by an oxy-fuel plant. Atmospheric pressure or higher pressure in the combustion chamber was assumed. The results of the calculations showed that an increase in the pressure caused an increase in net power efficiency. The effect was greatest in the pressure range of 1−10 bar. A positive influence of the pressure on combustion efficiency was additionally observed by Heitmeir et al.19 This paper presents the results of air-fired and oxy-fueled combustion experiments conducted at atmospheric and higher © 2012 American Chemical Society
pressures. In particular, the influences of pressure and temperature on NO and N2O emissions were tested.
2. EXPERIMENTAL SECTION 2.1. Pressurized Fluidized-Bed Combustor (FBC). Experiments were carried out using a pressurized fluidized-bed (bubbling) combustor. The simplified scheme of the reactor is shown in Figure 1.
Figure 1. Simplified scheme of the experimental setup: 1, pressurized fuel tank; 2, decompression, cooling, and cleaning module; 3, gas analyzers; and 4, decompression. The reactor was made of a stainless-steel tube, and its main parameters are as follows: a maximal pressure of 15 bar, temperature of 950 °C, reactor diameter of 0.075 m, height of 1 m, bed height of 0.25 m, maximal gas flow stream of 44 kg/h, fuel tank capacity of 0.01 m3, and fuel feed of 0.5−3 kg/h. The silica sand of the bed was Special Issue: International Conference on Carbon Reduction Technologies Received: October 27, 2011 Revised: February 2, 2012 Published: February 2, 2012 6492
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laid on a ceramic sieve (mesh of 1 mm). Streams of process gases (CO2, O2, N2, and air; technical purity, cylinders delivered by Linde Gas) were measured, heated, and fed into the bottom of the reactor. The temperature inside the reactor was measured using K-type thermocouples (with shells). According to the certificate provided by the thermocouple producer, the measurement uncertainty was below 5 °C. The thermocouples were placed above the bed (diameter of 2 mm) and top zone of the reactor (diameter of 2 mm). This kind of measurement equipment was applied because of the reactor construction and its scale. The total oxidizing gas flow was maintained in a range of 8−20 kg/h. The flow rate of each gas stream was controlled by valves. The pressure inside the reactor was automatically regulated by a spring controller. The flue gas was continuously analyzed for CO2 (0−100 vol %), CO (0−6000 ppmv and 0−50 vol %), H2O (0−30 vol %), NO (0−1000 ppmv), N2O (0−200 ppmv), NO2 (0−200 ppmv), SO2 (0−6000 ppmv), and SO3 (0−6000 ppmv) using a Fourier transform infrared (FTIR) analyzer (GASMET DX4000), and O2 (inlet and outlet) was measured by a paramagnetic analyzer (Oxymat 61) and a zirconium sensor analyzer (AMS Analysen). The measurement uncertainty of presented analyzers was below 1.5% of the total measuring range. A “Ziemowit” coal was introduced continuously into the combustion zone using a screw feeder. 2.2. Fuel and Bed Material. As mentioned above, “Ziemowit” coal (diameter of 1.4−1.7 mm) and silica sand (diameter of 1− 1.4 mm) were used as the fuel and bed material, respectively. The fuel analyses were conducted in an accredited laboratory (Accreditation Certificate AB 081) according to proper procedures. The investigations involved proximate and ultimate analyses of the chemical composition of ash after coal combustion. 2.3. Operating Conditions. Before every experiment, silica sand (diameter of 1−1.4 mm and volume of 500 mL) was introduced into the reactor and heated (in an air stream) to 800 °C by electrical heating elements until reaching steady-state conditions. After this operation, proper amounts of coal and oxidant were continuously introduced into the combustion zone and the heating elements were turned off. After the introduction of coal, temperature and pressure drop on the fluidized bed were maintained in ranges of 750−900 °C and 10−16 mbar, respectively. The inlet oxygen molar fraction for the oxy-fuel tests was 20 and 30 vol %. The total pressure inside the combustion chamber was maintained in the range of 1−4.5 bar. The exhaust gas from the combustion process was cooled, decompressed, and introduced into a chimney. A portion of the exhaust gas was sampled from the flue line and introduced into the gas analyzer. The process parameters (temperature at representative points in the facility, pressure, pressure drop, gas stream rates, and concentrations of the analyzed gases in the exhaust) were continuously measured and collected.
Table 1. Proximate and Ultimate Analyses of Ziemowit-Type Coal Proximate Analysis (wt %) moisture (wt %) ash volatile matter fixed carbon LHV (kJ/kg) Ultimate Analysis (wt %) C H N O S P Cl F
5.1 22.4 28.77 43.73 19910 56.5 3.6 0.87 10.6 0.26 0.013 0.61 0.01
Table 2. Chemical Composition of Ash ash content (wt %) SiO2 Al2O3 Fe2O3 CaO MgO P2O5 SO3 Mn3O4 TiO2 SrO Na2O K2O
50.6 25.45 7.04 3.35 3.04 0.14 3.43 0.18 1.07 0.04 2.8 2.28
measured concentrations of additional compounds (NO, N2O, and SO2) and the fuel input. Although the concentration of NO2 and SO3 were continuously measured, the results were not included in the tables because the measured concentrations of these gases in all experiments were less than 3 ppmv. Figure 2 shows an example of measured parameters as functions of time for the following experimental conditions: oxy-fuel (25:75 O2/ CO2) and a total pressure of 4.5 bar. For this period of time, the molar fractions of gaseous compounds in the exhaust gas are presented in Figures 3−5. These three figures show the concentrations of the main compounds (CO2, H2O, CO, and O2), nitrogen oxides (NO and N2O), and SO2, respectively. Although POFC is a very attractive method for energy conversion, it was in fact a challenge to obtain stable operating conditions for high-pressure experiments. In every test, it was important to maintain a proper level of fluidization to achieve good mixing of the fuel and the material bed. From experience and earlier investigations (performed using a transparent reactor), we know that this condition can be satisfied when the pressure drop (Δp) of the bed (silica sand, 500 mL) is greater than 15 mbar. We observed the onset of fluidization at an apparent velocity of 0.38 m/s. However, good mixing was observed at velocities above 0.7 m/s. Additionally, Δp is a good parameter to adjust to control mixing. Proper mixing was observed for fluctuations of Δp higher than 1 mbar. Figure 2 shows that Δp was about 17 mbar and that the fluctuations of Δp were about 1.2 mbar. Many important process factors, such as gas streams, pressure, fuel input, and other factors, influenced the measured
3. RESULTS AND DISCUSSION 3.1. Primary Investigations. Tables 1 and 2 show the results of the fuel analyses. Table 1 includes proximate and ultimate analyses of the “Ziemowit” coal. Table 2 shows the chemical composition of ash from the combustion of the analyzed coal. Tables 3 and 4 show the results of the experiments. The tables include average, maximum, and minimum values of every measured parameter. Table 3 presents the main combustion parameters, such as bed temperature, temperature above the bed, total pressure, and pressure drop of the bed. Particularly, the experiments were carried out at airfired conditions and different temperatures (see experiment numbers 1−5 in Table 3), at oxy-fuel conditions (20 vol % O2 in CO2) and different temperatures (see experiment numbers 6−16 in Table 3), and at pressurized conditions at air-fired (experiment number 17) and oxy-fuel (experiment numbers 18−22) at different O2/CO2 ratios and temperatures. Additionally, concentrations of the main exhaust gas compounds (CO2, H2O, CO, and O2) are presented. Table 4 presents the 6493
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oxy 20:80
oxy 25:75
22
oxy 20:80
14
oxy 30:70
oxy 20:80
13
21
oxy 20:80
12
20
oxy 20:80
11
oxy 20:80
oxy 20:80
10
oxy 25:75
oxy 20:80
9
19
oxy 20:80
8
18
oxy 20:80
7
air
oxy 20:80
6
17
air
5
oxy 20:80
air
4
16
air
3
oxy 20:80
air
15
air
2
oxidant
1
number
6494
4.5 bar
4.2 bar
4.5 bar
4.4 bar
4.3 bar
4.4 bar
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
atm
pressure
17.6
20.5
23.9
17
21.6
15.2
19.1
19.5
19.7
17.6
17.5
19.5
18
18
19
11.5
11.6
18.4
9.8
10.7
10.8
11.5
average
16.5
19.5
21.6
16.8
20.5
14.7
18
18.5
18.7
17
16.5
19
17.5
17.5
18
11.3
11.3
17.8
9.5
10.3
10.3
11
minimum
Δp (mbar)
18.7
21.4
25.9
17.2
22.6
15.4
20.4
20.1
21.2
18
18.2
20.5
18.5
18.5
20
11.7
11.9
19.3
10.4
11.5
11.5
12
maximum
849
861
864
870
875
904
875
796
820
764
834
805
887
885
833
846
868
809
837
830
830
810
831
820
831
830
827
898
871
786
809
756
810
800
885
882
830
842
866
803
830
825
825
800
minimum
857
904
925
910
896
920
877
802
829
770
865
810
890
890
838
849
870
815
850
835
835
820
maximum
T above bed (°C) average
Table 3. Experimental Results: Main Process Parameters
844
856
858
820
868
884
833
764
767
715
836
785
867
865
805
854
896
713
895
845
845
810
average
835
812
821
780
814
882
830
760
765
710
820
780
865
862
800
838
888
705
890
840
840
780
minimum
849
904
924
860
892
886
835
767
771
720
862
790
870
870
810
864
900
719
910
855
855
835
maximum
T top zone (°C)
13.7
9.4
20.1
10
14.1
12.7
9.1
10.5
9.9
10
8.3
11.8
7.5
7.5
11
13
11
10.6
8
10.5
12
12.5
average
11.9
13
13.9
7
7.8
11.7
6.3
9.1
6.4
8.1
7.3
11
7
7
10.5
10.5
6
8.9
4
7
10.5
9.5
minimum
15.5
4.2
23
14
19
13.7
11.4
12.4
12.4
12.4
8.6
12
8
8
12
18
16
12.5
10
13.5
15
18
maximum
O2 out (vol %)
88
91
80
80
86
8
92
91
91
91
92
85
90
89
87
91
93
10.4
13
9
8
7
average
87
88
76
77
81
7
90
90
89
89
91
0
0
87
86
89
91
9.3
11
8
6
4
minimum
CO2 (vol %)
89
94
85
82
91
8
93
92
93
92
94
0
0
90
89
95
95
11.4
15
12
10
10
maximum
520
690
590
970
380
490
1400
1300
1100
1600
1800
2400
3200
1600
1700
860
1080
370
1600
320
320
650
average
400
190
134
430
100
460
880
940
650
1200
900
0
2200
1300
1300
540
540
210
810
270
270
220
mini mum
CO (ppmv)
930
2200
1500
1300
1330
520
2600
1700
2600
2100
3300
0
3900
1900
1900
1080
1600
820
3250
540
430
1900
maximum
1.5
3.9
4.4
2.7
4.2
0.7
1.6
1.5
1.4
1.2
3.2
3.7
3.7
3.8
3.7
2.3
1.9
1
3.5
3.5
3.5
3.0
H2O (vol %)
Energy & Fuels Article
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Table 4. Experimental Results: Additional Process Parameters NO (ppm) number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
oxidant air air air air air oxy oxy oxy oxy oxy oxy oxy oxy oxy oxy oxy air oxy oxy oxy oxy oxy
20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 25:75 20:80 30:70 20:80 27:75
N2O (ppm)
SO2 (ppm)
pressure
average
minimum
maximum
average
minimum
maximum
average
minimum
maximum
fuel input (kg/h)
atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm 4.4 bar 4.4 bar 4.4 bar 4.5 bar 4.2 bar 4.5 bar
280 345 380 410 365 100 93 71 95 92 69 146 70 89 65 94 150 66 52 83 54 55
175 270 345 400 345 90 86 65 91 91 67 134 65 71 62 85 147 52 45 72 44 53
390 400 410 420 385 110 104 76 102 94 73 160 82 100 76 104 156 79 58 95 64 58
30 23 30 22 31 22 29 55 29 30 69 42 84 67 87 41 11 30 27 28 51 50
16 16 24 19 28 16 17 51 26 29 65 30 74 61 80 38 13 13 16 14 30 43
281 346 378 410 36 100 93 71 95 92 69 50 92 81 92 43 151 51 52 41 75 54
500 470 600 750 920 600 550 540 620 730 510 347 770 810 720 610 315 400 480 350 481 326
270 450 475 700 850 480 400 500 570 680 500 340 670 700 600 575 280 310 360 260 361 263
670 550 775 900 1050 740 600 560 640 760 520 362 830 920 760 650 360 650 710 680 665 371
0.52 0.52 0.52 0.52 0.29 0.40 0.40 0.60 0.55 0.60 0.60 0.47 0.27 0.40 0.40 0.60 0.60 1.34 1.34 1.41 1.6 1.22
Figure 4. Measured nitrogen oxides (NO, NO2, and N2O) emission during oxy-fuel combustion (75:25 vol % CO2/O2) as a function of time, with a fuel input of 1.22 kg/h and a total pressure 4.5 bar.
Figure 2. Main process parameters (total pressure, pressure drop of the bed, temperature above the bed, and temperature at the top zone of the bed) measured during oxy-fuel combustion (75:25 vol % CO2/ O2) as a function of time, with a fuel input of 1.22 kg/h.
Figure 5. Measured sulfur dioxide emission during oxy-fuel combustion (75:25 vol % CO2/O2) as a function of time, with a fuel input of 1.22 kg/h and a total pressure of 4.5 bar.
Figure 3. Main exhaust gas compounds (CO2, H2O, CO, and O2) measured during oxy-fuel combustion (75:25 vol % CO2/O2) as a function of time, with a fuel input of 1.22 kg/h and a total pressure of 4.5 bar.
(I&C) part], it had some limitations. Because of these limitations, almost all of the measured parameters (see Figures 3−5) changed significantly. Nevertheless, specific differences between particular results can be observed. In particular, the influences
parameters significantly. Although the experimental setup was very well-equipped [especially the instrumentation and controls 6495
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of the temperature and pressure on nitrogen oxide transformation are discussed in the next section. Differences in carbon monoxide emission can be observed between air-fired and oxy-fuel combustion (see Table 3). Under oxy-fuel conditions, the emission of carbon monoxide is higher in comparison to air-fired combustion. This result can be explained by the following chemical reactions. Boudouard reaction:1,2,20 CO2 + C(s) ↔ 2CO
Table 5. Calculated ICL number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
(R1)
A reaction in the presence of hydrogen radicals:2 CO2 + H ↔ CO + OH
(R2)
Carbon dioxide dissociation, a strong endothermic reaction:1,21 CO2 ↔ CO + 0.5O2
(R3)
A higher concentration of carbon monoxide under oxy-fuel conditions was observed by Glarborg and Bentzen.21 Oxygen was in much greater excess and carbon monoxide was observed at a much higher level (>1000 ppmv) than in the air-fired test. In this study, the Boudouard reaction played a predominant role because of the low combustion temperature and the presence of char inside the reaction zone. Excluding the influence of the oxy-fuel atmosphere, the CO emission from the combustor seems to be still high, even at air-fired conditions. The high fluctuations of the measured concentration were especially observed under pressurized conditions during the change of the screw feeder rotation rate. Additionally, some of the coal particles (falling into the reactor) could be fragmented and going up to the top zone of the reactor. Another reasonable explanation of high CO emission is specific conditions of the combustion. Because of gas flow limitations (especially at pressurized conditions), the combustor has a relatively small size (diameter of 0.075 m and height of 1 m). Thus, the residence time of the gases in the combustor was relatively short. On the other hand, a proper gas flow rate must be maintained to fluidize the bed. The concentration of CO in exhaust gas from the large-scale industrial boilers is around 100 ppm (on the basis of 6% O2 as a reference content); however, the concentration profile over the combustor height can depict much higher CO concentrations. For example, CO concentration profiles over a horizontal central line across the furnace showed the CO gradients between 500 and 2500 ppm.22 Therefore, combustion efficiency of the rig is discussed, and the results are shown in Table 5. The loss of the incomplete combustion has been calculated. The main equation for the calculation of the incomplete combustion loss (ICL) is as follows: ICL = VedQCOzCO × 100/QF (%), where Ved is the volume of dry exhaust gas at normal conditions [standard temperature and pressure (STP)] after combustion of 1 kg of fuel )mn3/kg), QCO is the low heating value of carbon monoxide (12 644 kJ/mn3), zCO is the volumetric fraction of CO in exhaust gas, and QF is the low heating value of the fuel (kJ/kg). Table 4 also presents the level of SO2 in exhaust gas. Contradictory observations on the SO2 emission from oxy-fuel combustion are reported in the literature. Some researchers experimentally proved a decrease when comparing to combustion in air, whereas the others found no differences between oxy and air combustion. Additionally, the disagreements between experimental findings and the computations (on the basis of equilibrium modeling) have been described in the literature. These disagreements were expected as a consequence of the
oxidant air air air air air oxy oxy oxy oxy oxy oxy oxy oxy oxy oxy oxy air oxy oxy oxy oxy oxy
20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 20:80 25:75 20:80 30:70 20:80 25:75
pressure
ICL (%)
atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm atm 4.3 bar 4.3 bar 4.4 bar 4.3 bar 4.2 bar 4.5 bar
0.5 0.2 0.2 0.8 0.2 0.7 0.7 1.1 0.8 1.7 1.8 1.0 1.0 0.7 0.9 0.8 0.4 0.3 0.6 0.6 0.4 0.4
computations assuming conditions immediately after combustion, whereas the measurements involved downstream conditions. Another plausible explanation of these differences is the kinetically controlled heterogeneous reaction of S fuel conversion. The sulfur capture in the reaction with ash compounds is also taken into account. Despite inconsistent investigation results, the one reasonable trend was very clearly observed. Among different process factors, such as temperature, oxygen excess, and type of fuel, the most important factor influencing SO2 emission is the sulfur content of the fuel.1 Insignificant dependence of SO2 emission as a function of the pressure is reported in the literature. However, some researchers noticed the decreased emission from pressurized air-fired combustors.22 In the described research, some differences between pressurized and atmospheric pressure experiments have also been observed. Assuming that a fuel concentration in the bed is almost proportional to the pressure and the SO2 emission does not significantly depend upon the temperature,22 it can be noticed (see result numbers 6, 19, and 21) that SO2 emission was almost 30% lower in the case of pressurized conditions (calculations based on 6% O2 as the reference content). 3.2. Impact of the Temperature. The temperature is an important factor that influences the conversion of combustion products, especially nitrogen oxides. Fluidized-bed combustion is considered a low-NOx process. This type of combustion is caused mainly by the low temperature inside the reaction zone. However, these conditions are preferable for the creation of N2O (which is considered a greenhouse gas). Figures 6−8 illustrate the impact of the temperature on NO and N2O transformation under air-fired and oxy-fuel conditions at atmospheric pressure. Figures 6 and 7 show that the measured concentration of NO (mg/mn3) in oxy-fuel combustion is less than one-third of that measured for the air-fired test (under comparable conditions). Lower emissions of NO from the oxy-fuel combustors are explained in the literature1,2 by the elimination of molecular nitrogen inside the reaction zone and as a consequence of the 6496
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excess and a high CO2 concentration, the reduced formation of NO is caused by the limitation of the O/H radical pool, particularly O. Additionally, NO can be intensely reduced on a char surface because of the reaction with CO, with the concentration being higher at oxy-fuel conditions. To support our results, we carried out experiments using the Flexi-Burn procedure.23 It involves the flexible change of a combustion regime from air-fired to oxy-fuel during real time of the process. The combustion was continuously switched from air-fired to oxy-fuel conditions. The concentrations of measured NO and N2O as a function of time are shown in Figure 9. It can be Figure 6. Influence of the temperature on NO and N2O conversion under air-fired conditions and atmospheric pressure. The concentration values are based on 6% O2 as a reference level.
Figure 9. Emission of NO and N2O at flexible change of the combustion conditions (under atmospheric pressure) from air-fired to oxyfuel. Conditions: air-fired [fuel feed, 0.29 kg/h; flow rate, 5.35 m3/h (STP); and O2(out), 10.6 vol %] and oxy-fuel [fuel feed, 0.27 kg/h; O2/CO2, 20:80; flow rate, 4.98 m3/h (STP); and O2(out), 10 vol %].
Figure 7. Influence of the temperature on NO and conversion under oxy-fuel (20:80 vol % O2/CO2) conditions and atmospheric pressure. The concentration values are based on 6% O2 as a reference level.
noticed from the figure that the NO concentration dropped from around 350 to around 90 ppm. Jia et al.24,25 studied NO creation under oxy-fuel conditions in a circulated FBC. Despite differences in the fluidization regime and other conditions between this study and that by Jia et al., the comparison of presented results was performed because of the similar scale of the experimental setups. The molar fraction of NO from an oxy-fuel circulated FBC (investigated by Jia et al.)24,25 was in the range of 123−394 ppmv at a temperature of approximately 850 °C. This NO concentration is higher than that found in the discussed research, which was about 90 ppm (e.g., see numbers 6−8 in Table 4). The discrepancy is caused by various factors, including differences in oxygen excess [Jia et al. reported an average of 4 vol % O2(out); in the present study, the average was 12 vol % O2(out)] and different amounts of nitrogen inside the coal (Jia et al. had an average N = 1.5 wt %, while the average N = 0.87 wt % was obtained during the research). Calculating this molar fraction into mg/mn3 (STP conditions based on 6% O2 as the reference content), the results of Jia et al. are 142−455 mg/mn3 compared to 208 mg/mn3. It should be mentioned that the effects of fuel composition on NO transformation are not well-understood.1 Very interesting results were obtained for oxy-fuel conditions. The NO and N2O results are shown separately for a clearer presentation. NO emission increases with the temperature to about 850 °C. A further increase of the temperature causes a decrease of NO emission. It is consistent with the literature data. The NO yield from conversion of char N increased with the temperature to a certain temperature, above which it decreases with the temperature. The maximum of the NO yield depended upon the coal rank and type of combustor. The fact that NO decreases with the temperature above 850 °C
Figure 8. Influence of the temperature on N2O conversion under oxyfuel (20:80 vol % O2/CO2) conditions and atmospheric pressure. The concentration values are based on 6% O2 as a reference level.
thermal NO limitation. The conversion of fuel N is a main source of the NO emission from combustion of bituminous coals. According to the explanation derived from Toftegaard et al.,1 up to 20% of the total NOx formed from pulverized coal combustion in air is due to thermal NOx and about 80−100% is derived from fuel N. However, the same authors underscored that only few studies have been reported on the impact of the high CO2 concentration on the nitrogen chemistry in the oxyfuel combustion. CO2 inhibits NO formation at both stoichiometric and fuel-lean conditions, but more particular investigations are needed for a full understanding of this phenomenon. It is not entirely clear what is a mechanism of this impact; however, it is explained by two potential effects. At a condition with oxygen 6497
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can be explained in terms of the competition between the char + O2 and the char + NO reactions. At high temperatures and/or larger particles, the char + O2 reaction is promoted because the char + O2 reaction is also determined by oxygen transport into the char surface. As a consequence, it leads to a decrease of NO emission.26 Additionally, the NO reduction under higher temperature can be catalytically increased by coal ash.27 FBCs can be sources of significant N2O emission. The concentration of N2O in air-fired FBC exhaust gases is in the range of 20−200 ppm, depending upon the temperature and amount of excess air in the reaction zone.28 A higher N2O emission was observed during oxy-fuel combustion in comparison to air-fired conditions (compare Figures 6 and 8). We have confirmed the well-known correlation between increasing temperature and decreasing N2O emission. It is explained by the two main effects. N2O conversion decreases with the temperature, partly because N2O formation is less favored and partly because of homo- and heterogeneous decomposition of N2O.26 However, in the case of oxy-fuel combustion (see Figure 8) this trend is clearer. In other words, under the tested combustion conditions, oxy-fuel combustion shows greater production of N2O and a stronger temperature dependence is observed. The effect of oxy-fuel combustion on N2O creation has been observed by other researchers. Yoshiie et al.29 noticed an increase in N2O emission during oxy-fuel combustion. The emission from a laboratory-scale drop-tube pulverized coal furnace was more than 2 times higher compared to that of an air-fired reactor. 3.3. Impact of the Pressure. Air-fired combustion at high pressure is a low-NOx process.4−10 The influence of pressure on NO and N2O production must be understood if POFCs are to be used in real power plants. Figures 10 and 11 show the influence of the pressure on NO and N2O conversion.
Figure 11. Influence of the pressure on N2O transformation during air-fired and oxy-fuel combustion. Conditions: air-fired [atm pressure/ 845 °C/12 vol % O2(out) and 4.5 bar/884 °C/12.7 vol % O2(out)] and oxy-fuel [O2/CO2: 20:80 vol %; atm pressure/854 °C/13 vol % O2(out) and 4.5 bar/820 °C/10 vol % O2(out)].
increased the residence time for the diffusion of NO throughout the char particles and, consequently, further reduced the amount of NO in the char particles. In fact, increasing pressure does not significantly influence NO creation but does create preferential conditions for NO reduction on the char surface via the following reaction:6,7 NO + C →
1 N2 + CO 2
(R4)
It can be assumed that reaction R4 also occurs under oxy-fuel conditions. However, the effect of the pressure on NO emission is stronger in the air-fired case (see Figure 10), probably because of the higher concentration of carbon monoxide at the boundary layer of coal particles under oxy-fuel conditions. The higher concentration of carbon monoxide (from the Boudouard reaction R1) can change the equilibrium state of reaction R4 and, as a consequence, inhibit NO reduction. If reaction R4 is promoted under higher pressure, increasing the O2 content in the oxidant (O2/CO2 mixture) should enhance the reaction of fuel N + O2 and lead to an increase of NO emission. This remark is confirmed in Figure 12. The NO concentration
Figure 10. Influence of the pressure on NO transformation during air-fired and oxy-fuel combustion. Conditions: air-fired [atm pressure/845 °C/ 12 vol % O2(out) and 4.5 bar/884 °C/12.7 vol % O2(out)] and oxy-fuel [O2/CO2, 20:80 vol %; atm pressure/854 °C/13 vol % O2(out) and 4.5 bar/820 °C/10 vol % O2(out)].
Figure 12. Impact of the O2/CO2 ratio on NO conversion under pressurized conditions (4.4 bar).
Figures 10 and 11 show that, in both cases (oxy and air), increasing the pressure led to NO and N2O inhibition. However, this tendency is more distinct for the air-fired conditions. The impact of the pressure on NO and N2O creation under oxy-fuel conditions has not been intensively investigated before. This scientific area needs more particular investigations. Some data can be found in the literature for air-fired or model atmospheres containing NO in the balance gas.4−10 For example, Lin et al.5 found that, with increasing pressure, NOx emission decreased dramatically. They explained that the pressure
(mg/mn3, STP) was calculated on the basis of 6% O2 as the reference content. It can be noticed from Figure 12 that increasing the O2/CO2 ratio results in increasing the NO concentrations. Figure 11 shows that an increase in the pressure caused a decrease in N2O emission for both air-fired and oxy-fuel combustion. As in the NO case (see Figure 10), the effect of the pressure is stronger for the air-fired conditions. Few studies 6498
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in the literature describe the effect of the pressure on N2O emission under oxy-fuel conditions. Moreover, the results for air-fired combustion are not consistent. Some researchers observed higher N2O emissions for pressurized reactors.8,9 Others5,10 concluded that the pressure has no effect on N2O formation. Some even found lower N2O emissions at higher pressures. This issue deserves more investigation, especially for the case of oxy-fuel conditions.
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4. CONCLUSION This paper presents the results of tests performed using a recently developed experimental setup. We hope that it will expand our understanding of the POFC process. POFC is a very attractive method of combustion. However, the important questions in this field have not yet been addressed. It can be concluded on the basis of the cited literature and the presented results that POFC technology will become an environmentally attractive option when the process can be carried out at a pressure up to 4.5 bar. During the oxy-fuel test, a higher concentration of carbon monoxide was observed in comparison to air-fired conditions. This result can be explained by the Boudouard reaction, which was favored by the combustion conditions (specifically, the high concentration of CO2). A relatively high CO concentration and its fluctuation was also caused by specific conditions of the combustion in the experimental rig (work of the screw feeder). It was also observed for SO2 that emission is mainly determined by fuel parameters. A decrease of SO2 emission has been observed under pressurized conditions; however, the issue needs more particular studies. A lower molar fraction of NO (65−146 ppmv) has been observed for oxy-fuel combustion compared to air-fired tests (280−410 ppmv). At oxy-fuel conditions, NO emission increases with the temperature to about 850 °C. This result can be explained by the enhancement of fuel N transformation into NO. A further increase of the temperature causes a decrease of NO emission. It can be explained by the creation of preferential conditions for the char + NO reaction. In contrast to NO, the measured N2O molar fractions were higher for the oxy-fuel conditions than for the air-fired combustion. Additionally, the relationship was more significant (as a function of the temperature) than that observed in the case of oxy-fuel conditions. A positive influence of the pressure on the abatement of NO and N2O emissions has been observed. The strongest decrease was observed under air-fired conditions. In this case, NO emission was reduced by more than half at a pressure of 4.5 bar (in comparison to atmospheric pressure conditions).
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +4832-271-00-41. Fax: +4832-271-08-09. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This scientific work was supported by the National Center for Research and Development, as a Strategic Project under Project PS/E/2/66420/10 “Advanced Technologies for Energy Generation: Oxy-combustion Technology for PC and FBC Boilers with CO2 Capture”. This support is gratefully acknowledged. 6499
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(28) Khan, A. A.; de Jong, W.; Jansens, P. J.; Spliethoff, H. Fuel Process. Technol. 2009, 90, 21−50. (29) Yoshiie, R.; Ueki, Y.; Naruse, I. Effects of flue gas recirculation on NO and N2O formations in coal combustion. Proceedings of the 2nd International Oxyfuel Combustion Conference; Yeppoon, Queensland, Australia, Sept 12−16, 2011.
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