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Changes in the forms of nitrogen and oxygen during rapid coal pyrolysis
Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1685
Changes in the Forms of Nitrogen and Oxygen during Rapid Coal Pyrolysis Mathew Watt, William Allen, and Thomas Fletcher Department of Chemical Engineering, Brigham Young University, Provo, UT 84602 USA 1. INTRODUCTION During coal combustion fuel nitrogen is the major source of nitrogen oxide pollution. This makes the study of nitrogen in coal and coal pyrolysis products an important area of study for the design and implementation of NOx control strategies. It has long been felt that nitrogen is located within the nitrogen macromolecule as pyridinic and pyrrolic structures with some amines forms [ 1-2]. Only recently with the use of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) has the pyridinic and pyrrolic forms of nitrogen been confirmed [3-4]. As shown in Fig. 1, these methods have shown approximate amounts of the different nitrogen functional groups. How these forms are released during pyrolysis, however, is poorly understood. 100 0
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80
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Pyrrolic Forms
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.~ Z
I Pyridinic Forms
2o 0
9
70
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75
"
I
80
"
I
85
"
I
90
"
95
% Carbon (daf) in Parent Coal Figure 1.
XPS and XANES data showing nitrogen functional forms in coal.
Very little has been done in the study of nitrogen forms in coal pyrolysis products. Kelemen et. al. [4] have studied a limited number of chars with the use of XPS and found that the levels of quaternary nitrogen went down while the forms of pyridinic nitrogen went up. Other studies have shown similar results [5]. The chars used in these studies were produced at low heating rates and temperatures and may not be applicable to the high heating rates and temperatures encountered in commercial pulverized coal furnaces. It has been found that at high heating rates and temperatures, nitrogen release during pyrolysis is a function of coal rank. Figure 2 shows nitrogen release data from single particle entrained flow experiments, where char samples were collected immediately after pyrolysis in a laminar flow flat flame burner [6]. It is apparent that large differences in the nitrogen volatiles occur over a range of coal rank and among coals of the same rank.
1686
rn ~D ~D ~D
0.7
Smith-Roland- Subbit.
0.60.5-
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Lower Wilcox - Lignite V
A
13
Dietz- Subbit.
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Illinois #6 - HV Bit.
0.2-
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Blue #1 - HV Bit.
0.1-
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Hiawatha- HV Bit.
Z
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Beulah Zap - Lignite
0.0 60
65
I
I
I
I
70
75
80
85
90
Pittsburgh #8 - HV Bit. Lower Kittaning - LV Bit.
%C (daf) in Parent Coal Pochohontas #3 - LV Bit. Figure 2.
Fractional nitrogen release as a function of coal rank [6].
Comparing the XPS and XANES data of Fig. 1 and the nitrogen volatile data of Fig. 2 does not give any clear picture of the reasons for the major differences in nitrogen release between different coals. This study explores how nitrogen functional groups react during rapid pyrolysis. Analysis of pyrolysis products, such as tars and chars are analyzed using Xray photoelectron spectroscopy (XPS). Insights into the pyrolysis reactions of nitrogen species are discussed. 2. EXPERIMENTAL Coals were pyrolyzed at atmospheric pressures in a drop tube reactor [7]. Pulverized coal pyrolysis experiments were performed in 100% Nitrogen at temperatures of approximately 1000 K and at heating rates of 104 K/s. Residence times were varied between 70 and 200 ms within the reactor to obtain chars and tars that corresponded to different amounts of pyrolyzed mass release. A water cooled, nitrogen quench, transpiring wall collection probe was used to obtain samples of chars and tar. Tar and char were separated aerodynamically using a virtual impactor and cyclone, in a manner similar to that used by Fletcher [8]. Additional pyrolysis experiments were performed by injecting entrained pulverized coal particles through a methane/hydrogen/air flat flame burner operating under fuel-rich conditions. This allowed for chgr and tar samples to be produced at temperatures of 1500 to 1700 K with heating rates of 10" K/see. The flat flame burner experiments used a collection system similar to that used in the drop tube experiments. The coals used in this study are shown in Table 1. The coals were chosen due to the wellcharacterized nature and the large literature information available. Other investigators have also used some of the chosen coals in their investigations of nitrogen evolution during pyrolysis [4-6, 8-9]. Chars and tars were analyzed for the different coal nitrogen and oxygen compositions at Exxon Research using established techniques [4].
3. RESULTS & DISCUSSION As noted in Fig. 3, for the Wyodak chars, the forms of nitrogen found by XPS do not change much as a function of mass release. The major changes show that the pyridinic forms of nitrogen increase slightly while the quaternary forms decrease slightly. It is thought [4] that
1687 a portion of the quaternary nitrogen forms are protonated nitrogen functionalities. As pyrolysis takes, it is possible that the hydrogen is scavenged from the nitrogen to form other pyrolysis products. The functional form of nitrogen would then increase pyridinic nitrogen and decrease quaternary nitrogen, as analyzed by XPS. Table 1. Elemental analysis of coals examined Coal Rank Coal ID %C (da0 Wyodak Subbit. Argonne Prem. 85 Blind Canyon HV Bit. Utah Power 81 Pittsburgh #8 HV Bit. PSOC 1551D 84 Illinois #6 HV Bit. PSOC 1493D 77 Pocahontas #3 LV Bit. PSOC 1508D 91
%H (daf) 5.4 5.8 5.5 5.0 4.6
%N (daf) 1.6 1.6 1.7 1.5 1.6
%S+O (daf) 7.2 11.1 8.2 17.0 3.3
%Ash (mf) 19.8 4.7 4.1 15.1 15.9
8O
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Forms
8
o
r,j 9r'-J )
40
Pyridinic
Forms
9
20 9
Z
Quaternary
o
'
0
I
10
'
I
20
Forms '
I
30
'
I
40
'
I
50
'
60
% Mass Release
Figure 3.
XPS forms of nitrogen found in Wyodak chars as a function of mass release
It is interesting to note that quaternary forms of nitrogen are present in all of the chars. These forms must be relatively stable, since pyrolysis conditions for these samples were relatively harsh; the quaternary forms may not only be protonated pyridines. If such were the case, very little quaternary nitrogen groups would be left in the char with 60% mass release. Oxygen analysis was also performed on the Wyodak char samples using XPS, shown in Fig. 4, showing that oxygen is lost preferentially at the beginning of pyrolysis and then is released at the same rate as total mass is released. It is possible that the oxygen chemistry is somehow coupled to the nitrogen volatile chemistry in some form. Oxygen has a strong affinity for hydrogen, and pyrolysis products could include species such as H20 and phenols. Further investigation of the solid phase chemistry is necessary before any such speculated mechanisms can be verified. 4. SUMMARY AND CONCLUSIONS The use of XPS was used to determine how nitrogen functional forms in chars change during pyrolysis. It was found that only small changes occur in the overall functional forms of nitrogen. The quaternary nitrogen forms decrease slightly and pyridinic nitrogen forms increase almost the same amount. This seems to show, as has been indicated previously [4],
1688 that many of the quaternary XPS forms are protonated pyridines. Not all the quaternary nitrogen forms are lost during pyrolysis, showing that some quaternary forms are more stable than previously thought. It is thought that coal organic oxygen may play a role in fuel nitrogen chemistry. Additional analytical methods are needed, such as 15N NMR [10], to determine the forms of nitrogen in coal that affect coal-dependent nitrogen pyrolysis behavior.
25
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15
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9
0
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10
'
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% Mass Release Figure 4.
% Organic oxygen present in the Wyodak chars as a function of mass release.
REFERENCES 1. Pohl, J.H. and A.F. Sarofim, 16th Symposium (Int.) on Combustion, p. 491 (1977) 2. Solomon, P.R. and M.B. Colket, Fuel, 57, 749 (1978) 3. Mullins, O.C., Mitra-Kirtley, S., Elp, J. V., Cramer, S. P., Applied Spectroscopy, 47(8), 1268 (1993) 4. Kelemen, S. R., M. L. Gorbaty, and P. J. Kwiatek, Energy & Fuels,. 8, 896 (1994) 5. Wojtowicz, M. A., J. R. Pels, and J. A. Moulijn, Submitted to Fuel, (1993) 6. Mitchell, R.E., Hurt, R. H., Baxter, L. L., Hardesty, D. R., Compilation of Sandia Coal Char Combustion Data and Kinetic Analyses, Milestone Report, Sandia Report S AND92-8208, (1992). 7. Monson, C.R., Germane, G.J., Energy & Fuels, 7(6), 928, (1993) 8. Fletcher, T. H., Comb. & Flame, 78, 223 (1989). 9. Freihaut, J.D., W.M. Proscia, and J.C. Mackie, Comb. Sci. Tech., 93, 323 (1993) 10. Pugmire, R. J., "Project 1A. NMR Analysis of Coal and Char Structure," Advanced Combustion Engineering Research Center, 9th Year Progress Report, 2 (1995) ACKNOWLEDGMENTS This work was sponsored by the Advanced Combustion Engineering Research Center at Brigham Young University. Funds for this center are received from the National Science Foundation, the State of Utah, 40 industrial participants, and the U.S. Department of Energy. We would also like to thank Dr. Simon Kelemen at Exxon Research for performing the XPS analyses.
1685
Changes in the Forms of Nitrogen and Oxygen during Rapid Coal Pyrolysis Mathew Watt, William Allen, and Thomas Fletcher Department of Chemical Engineering, Brigham Young University, Provo, UT 84602 USA 1. INTRODUCTION During coal combustion fuel nitrogen is the major source of nitrogen oxide pollution. This makes the study of nitrogen in coal and coal pyrolysis products an important area of study for the design and implementation of NOx control strategies. It has long been felt that nitrogen is located within the nitrogen macromolecule as pyridinic and pyrrolic structures with some amines forms [ 1-2]. Only recently with the use of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) has the pyridinic and pyrrolic forms of nitrogen been confirmed [3-4]. As shown in Fig. 1, these methods have shown approximate amounts of the different nitrogen functional groups. How these forms are released during pyrolysis, however, is poorly understood. 100 0
r,j ,,_,
80
~
6O
"~
40
Pyrrolic Forms
0
.~ Z
I Pyridinic Forms
2o 0
9
70
I
75
"
I
80
"
I
85
"
I
90
"
95
% Carbon (daf) in Parent Coal Figure 1.
XPS and XANES data showing nitrogen functional forms in coal.
Very little has been done in the study of nitrogen forms in coal pyrolysis products. Kelemen et. al. [4] have studied a limited number of chars with the use of XPS and found that the levels of quaternary nitrogen went down while the forms of pyridinic nitrogen went up. Other studies have shown similar results [5]. The chars used in these studies were produced at low heating rates and temperatures and may not be applicable to the high heating rates and temperatures encountered in commercial pulverized coal furnaces. It has been found that at high heating rates and temperatures, nitrogen release during pyrolysis is a function of coal rank. Figure 2 shows nitrogen release data from single particle entrained flow experiments, where char samples were collected immediately after pyrolysis in a laminar flow flat flame burner [6]. It is apparent that large differences in the nitrogen volatiles occur over a range of coal rank and among coals of the same rank.
1686
rn ~D ~D ~D
0.7
Smith-Roland- Subbit.
0.60.5-
~D O
Lower Wilcox - Lignite V
A
13
Dietz- Subbit.
O
0.4-
O
tX
Illinois #6 - HV Bit.
0.2-
O
Blue #1 - HV Bit.
0.1-
O
Hiawatha- HV Bit.
Z
0.3-
O O .,-4
Beulah Zap - Lignite
0.0 60
65
I
I
I
I
70
75
80
85
90
Pittsburgh #8 - HV Bit. Lower Kittaning - LV Bit.
%C (daf) in Parent Coal Pochohontas #3 - LV Bit. Figure 2.
Fractional nitrogen release as a function of coal rank [6].
Comparing the XPS and XANES data of Fig. 1 and the nitrogen volatile data of Fig. 2 does not give any clear picture of the reasons for the major differences in nitrogen release between different coals. This study explores how nitrogen functional groups react during rapid pyrolysis. Analysis of pyrolysis products, such as tars and chars are analyzed using Xray photoelectron spectroscopy (XPS). Insights into the pyrolysis reactions of nitrogen species are discussed. 2. EXPERIMENTAL Coals were pyrolyzed at atmospheric pressures in a drop tube reactor [7]. Pulverized coal pyrolysis experiments were performed in 100% Nitrogen at temperatures of approximately 1000 K and at heating rates of 104 K/s. Residence times were varied between 70 and 200 ms within the reactor to obtain chars and tars that corresponded to different amounts of pyrolyzed mass release. A water cooled, nitrogen quench, transpiring wall collection probe was used to obtain samples of chars and tar. Tar and char were separated aerodynamically using a virtual impactor and cyclone, in a manner similar to that used by Fletcher [8]. Additional pyrolysis experiments were performed by injecting entrained pulverized coal particles through a methane/hydrogen/air flat flame burner operating under fuel-rich conditions. This allowed for chgr and tar samples to be produced at temperatures of 1500 to 1700 K with heating rates of 10" K/see. The flat flame burner experiments used a collection system similar to that used in the drop tube experiments. The coals used in this study are shown in Table 1. The coals were chosen due to the wellcharacterized nature and the large literature information available. Other investigators have also used some of the chosen coals in their investigations of nitrogen evolution during pyrolysis [4-6, 8-9]. Chars and tars were analyzed for the different coal nitrogen and oxygen compositions at Exxon Research using established techniques [4].
3. RESULTS & DISCUSSION As noted in Fig. 3, for the Wyodak chars, the forms of nitrogen found by XPS do not change much as a function of mass release. The major changes show that the pyridinic forms of nitrogen increase slightly while the quaternary forms decrease slightly. It is thought [4] that
1687 a portion of the quaternary nitrogen forms are protonated nitrogen functionalities. As pyrolysis takes, it is possible that the hydrogen is scavenged from the nitrogen to form other pyrolysis products. The functional form of nitrogen would then increase pyridinic nitrogen and decrease quaternary nitrogen, as analyzed by XPS. Table 1. Elemental analysis of coals examined Coal Rank Coal ID %C (da0 Wyodak Subbit. Argonne Prem. 85 Blind Canyon HV Bit. Utah Power 81 Pittsburgh #8 HV Bit. PSOC 1551D 84 Illinois #6 HV Bit. PSOC 1493D 77 Pocahontas #3 LV Bit. PSOC 1508D 91
%H (daf) 5.4 5.8 5.5 5.0 4.6
%N (daf) 1.6 1.6 1.7 1.5 1.6
%S+O (daf) 7.2 11.1 8.2 17.0 3.3
%Ash (mf) 19.8 4.7 4.1 15.1 15.9
8O
rj Pyrrolic 60
Forms
8
o
r,j 9r'-J )
40
Pyridinic
Forms
9
20 9
Z
Quaternary
o
'
0
I
10
'
I
20
Forms '
I
30
'
I
40
'
I
50
'
60
% Mass Release
Figure 3.
XPS forms of nitrogen found in Wyodak chars as a function of mass release
It is interesting to note that quaternary forms of nitrogen are present in all of the chars. These forms must be relatively stable, since pyrolysis conditions for these samples were relatively harsh; the quaternary forms may not only be protonated pyridines. If such were the case, very little quaternary nitrogen groups would be left in the char with 60% mass release. Oxygen analysis was also performed on the Wyodak char samples using XPS, shown in Fig. 4, showing that oxygen is lost preferentially at the beginning of pyrolysis and then is released at the same rate as total mass is released. It is possible that the oxygen chemistry is somehow coupled to the nitrogen volatile chemistry in some form. Oxygen has a strong affinity for hydrogen, and pyrolysis products could include species such as H20 and phenols. Further investigation of the solid phase chemistry is necessary before any such speculated mechanisms can be verified. 4. SUMMARY AND CONCLUSIONS The use of XPS was used to determine how nitrogen functional forms in chars change during pyrolysis. It was found that only small changes occur in the overall functional forms of nitrogen. The quaternary nitrogen forms decrease slightly and pyridinic nitrogen forms increase almost the same amount. This seems to show, as has been indicated previously [4],
1688 that many of the quaternary XPS forms are protonated pyridines. Not all the quaternary nitrogen forms are lost during pyrolysis, showing that some quaternary forms are more stable than previously thought. It is thought that coal organic oxygen may play a role in fuel nitrogen chemistry. Additional analytical methods are needed, such as 15N NMR [10], to determine the forms of nitrogen in coal that affect coal-dependent nitrogen pyrolysis behavior.
25
2O ext)
0
15
.~-.i
~
10
5
9
0
I
10
'
I
20
'
I
30
'
I
40
'
I
50
"
60
% Mass Release Figure 4.
% Organic oxygen present in the Wyodak chars as a function of mass release.
REFERENCES 1. Pohl, J.H. and A.F. Sarofim, 16th Symposium (Int.) on Combustion, p. 491 (1977) 2. Solomon, P.R. and M.B. Colket, Fuel, 57, 749 (1978) 3. Mullins, O.C., Mitra-Kirtley, S., Elp, J. V., Cramer, S. P., Applied Spectroscopy, 47(8), 1268 (1993) 4. Kelemen, S. R., M. L. Gorbaty, and P. J. Kwiatek, Energy & Fuels,. 8, 896 (1994) 5. Wojtowicz, M. A., J. R. Pels, and J. A. Moulijn, Submitted to Fuel, (1993) 6. Mitchell, R.E., Hurt, R. H., Baxter, L. L., Hardesty, D. R., Compilation of Sandia Coal Char Combustion Data and Kinetic Analyses, Milestone Report, Sandia Report S AND92-8208, (1992). 7. Monson, C.R., Germane, G.J., Energy & Fuels, 7(6), 928, (1993) 8. Fletcher, T. H., Comb. & Flame, 78, 223 (1989). 9. Freihaut, J.D., W.M. Proscia, and J.C. Mackie, Comb. Sci. Tech., 93, 323 (1993) 10. Pugmire, R. J., "Project 1A. NMR Analysis of Coal and Char Structure," Advanced Combustion Engineering Research Center, 9th Year Progress Report, 2 (1995) ACKNOWLEDGMENTS This work was sponsored by the Advanced Combustion Engineering Research Center at Brigham Young University. Funds for this center are received from the National Science Foundation, the State of Utah, 40 industrial participants, and the U.S. Department of Energy. We would also like to thank Dr. Simon Kelemen at Exxon Research for performing the XPS analyses.