Energy & Fuels 1993,7, 1013-1020
1013
Relation between Functional Forms of Coal Nitrogen and Formation of NO, Precursors during Rapid Pyrolysis Shinji Kambara,'vt Takayuki Takarada, Yasuhiro Yamamoto, and Kunio Kat0 Department of Biological and Chemical Engineering, Gunma University, Kiryu, Gunma 376, Japan, and Idemitsu Kosan Co., Coal Research Laboratories, 3-1 Sodegaura, Chiba 299-02, Japan Received January 1 1 , 1993. Revised Manuscript Received August 2, 199P
The formation of NO, precursors during rapid pyrolysis was investigated for 20 coals covering wide ranks at temperatures of 853-1488 K to elucidate the influence of coal properties on NO, formation during coal combustion. It was found that the main compositions of volatile nitrogen are HCN, NH3, and N2 under these experimental conditions. Their yields are strongly dependent on coal types and pyrolysis temperature. By analyzing functional forms of coal nitrogen in parent coals and chars after pyrolysis using X-ray photoelectron spectroscopy, the relation between nitrogen functionality and formation of nitrogen-containing species has been further demonstrated. Three nitrogen functional forms of pyrrole type, pyridine type, and quaternary nitrogen are observed for all coals. A proportion of each functional form is dependent on coal type. The variation of the nitrogen functional forms in chars during pyrolysis is quantitatively compared with the yield of nitrogen-containing species. Quaternary nitrogen converts finally to NH3, and a part of the pyrrole and pyridine type nitrogen converts to HCN.
Introduction
It has been well recognized that the major source of nitrogen oxide from coal combustion is conversion of fuel nitrogen1 (Le., fuel NO,). Thevolatile nitrogen conversion to NO, is the primary contributor to fuel NO, emissions.2 The fate of coal nitrogen during coal combustion has been studied by several investigators with the result that there is no simple relation between coal nitrogen and nitric oxide formed by combustion.3~4The behavior of various nitrogen-containing species as NH3, amines, or pyridine in relation to nitric oxide formation was investigated in gas flames.s*s Also Miyamae et al.' indicated that volatile nitrogen such as HCN and NH3 affects NO, formation during coal combustion. In order to control NO, emissions, it is important to predict the composition of volatile nitrogen (NO, precursors) for various ranks of coal since NO, emissions vary significantly with coal properties. The fundamental research on the thermal decomposition of fuel nitrogen can be traced to the latter half of the 1970s. Early studies of coal nitrogen decomposition investigated the devolatilization rate of coal nitrogen, the amount of the yield, the nitrogen distribution during devolatilization, and the formation of nitrogen-containing species during pyro1ysis.g-10 As a result of these studies, + Idemitau Kosan Co.
Abstract published in Advance ACS Abstracts, October 1, 1993. (1) Pershing, D. W.; Wendt, J. 0. L. Symp. (Int.) Combust., [Proc.] 1976, 16, 389. (2) Pershing, D. W.; Wendt, J. 0. L.2nd. Eng. Chem. Process Des. Den 1979, la,#. (3) Chen, 5. L.;Heap, M. P.; Pershing, D. W.; Martin, G.B. Fuel 1982, 61, 1218. (4) Glass,J. W.; Wendt, J. 0. L. Symp. (Znt.)Combust., [Roc.] 1982, 19, 1243. (5) Fenimore, C. P. Combust. Flame 1976,26, 249. (6) DeSoete, G.G. Symp. (2nt.) Combuat., [Proc.] 1974, 15, 1093. (7) Miyamae, 5.;Kiga,N.;Ikebe, H.; Suzuki,K. Ishikawajimu-Harimu Giho. 1987,27,152. (8) Pohl,J. H.; Sarofim, A. F. Symp. (Int.) Combust., [Roc.] 1976, 16, 491.
it can be stated that the amount of volatile nitrogen increases with increasing temperatures, the gaseous compounds are Nz, NH3, and HCN, and the tars were an important intermediate in the volatile nitrogen evolution process. However, the influence of coal properties on the formation of nitrogen-containing species remained to be established, because only a few coal types were used in these experiments. For a comprehensive understanding of the effect of coal types on the formation of NO, precursors, it is necessary to study nitrogen structures in coal. It is presumed that the fuel nitrogen conversion to nitrogen-containingspecies depends on nitrogen structures in coal as CO and C02 evolution during pyrolysis has a relationship to coal structures.'lJ2 An X-ray photoelectron spectroscopy (XPS) technique is available for the direct measurement of functional forms of coal nitrogen. Bartle and Wallacel3 investigated nitrogen functionality in coal and tar after pyrolysis using XPS. It was found that the nitrogen structures in the parent coal and its derived products are predominantly pyrrole and pyridine types. Burchill and Welch14examined the variation of nitrogen functionality with eight UK coals covering the carbon range 80-95 w t 96 using XPS. It was found that functional forms of coal nitrogen consist of pyrrole and pyridine type structures for the eight UK coals and that the content of each form varies with the coal rank. Those studies seemed to provide a new knowledge about the distribution of nitrogen functionality in coals, but the relation between functional forms of coal (9) Blair, D. W.; Wendt, J. 0. L.; Bartok, W. Symp. (Znt.) Combust., [Roc.] 1976, 16, 476. (10) Solomon, P. R.; Colket, M. B. Fuel 1978,57, 749. (11) Xu, W. C.; Tomita, A. Fuel 1987, 66, 627. (12) Solomom, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R. Energy Fuels 1990,4,319. (13) Bartle, K. D.; Wallace S. Fuel Process Technol. 1987,15,361. (14) Burchill, P.; Welch, L. S. Fuel 1989,68, 100.
0887-0624/93/2507-1013$04.00/00 1993 American Chemical Society
1014 Energy & Fuels, Vol. 7, No. 6, 1993
Kambara et al. Table I. Fuel Analysis ultimate analysis [wt % ,dafJ
proximate analysis [wt % ,dbl Coal A
source
Canada Australia B Australia C Australia D Australia E F Australia Australia G Australia H China I Australia J Australia K Australia L Australia M Australia N Australia 0 P Canada Japan Q U.S.A. R U.S.A. S T Australia " 0 = 100- (C + H + N
ash
volatile matter
10.0 12.4 16.2 8.0 12.6 12.4 7.4 8.3 5.8 8.8 13.2 9.7 11.1 7.3 9.6 7.7 11.8 5.1 8.1 1.5
20.8 26.3 29.6 30.3 26.6 3 1.9 29.0 31.8 31.4 39.1 32.8 33.8 30.7 30.6 41.8 38.3 44.8 43.5 48.8 47.1
fixed carbon 69.2 61.4 54.2 61.7 60.8 55.7 63.7 60.0 62.9 52.1 54.0 56.5 58.1 62.1 48.6 54.0 43.4 51.4 43.0 51.4
C
H
N
0"
S
88.1 85.1 84.9 84.6 83.9 82.6 82.5 81.9 81.9 81.8 81.5 81.1 80.8 80.8 80.1 78.0 76.2 72.8 69.2 65.4
4.5 4.8 5.0 5.1 4.7 5.2 4.6 5.2 4.8 5.6 5.4 5.3 5.1 4.5 5.9 5.3 6.1 4.6 4.9 4.4
1.20 1.70 1.92 2.25 1.74 1.40 1.90 1.84 1.01 1.82 1.88 1.76 1.35 1.82 1.40 1.03 1.20 1.08 0.90 0.56
5.8 8.0 7.6 7.1 9.3 10.5 10.5 10.3 11.8 10.0 10.7 11.3 12.3 12.7 12.0 15.6 16.5 21.3 25.0 29.4
0.37 0.40 0.6 1 0.94 0.36 0.32 0.51 0.74 0.50 0.78 0.51 0.49 0.39 0.25 0.55 0.14 0.07 0.16 0.04 0.28
+ S).
nitrogen and formation of NO, precursors was not considered. Nelson et al.15investigated the distribution of nitrogencontaining components in tarsafter pyrolysis in a fluidized bed reactor. They concluded that the formation of HCN and NH3 are a result of secondary reactions of nitrogencontaining tarsreleased early in the devolatilizationprocess and that HCN formation may be related to the greater fraction of pyrrole type nitrogen in the parent coal. Kelly et al.16 investigated the relation between functional forms of coal nitrogen and composition of nitrogen-containing species in tars after pyrolysis. These results produced many concepts relating to the thermal decomposition processof fuel nitrogen, but they were insufficient to draw a general conclusion. This is because the coals used in the experiment were only of two types, and the results are qualitative because they do not display a total nitrogen balance. On the other hand, several researchers3J7J8 have investigated the relation between NO, formation and coal properties in pilot-scale combustion. Some correlations to estimate NO, emissions levels in pulverized coal combustion were proposed. The correlations include coal nitrogen content, volatile matter content, volatile nitrogen yields, pyrolysis HCN yields, and maximum flame temperatures in a pulverized coal combustion furnace. Also the simulation of NO, formation has been carried out using a simplified model of fuel nitrogen devolatilization proc e s ~ e s These . ~ ~ correlations and the simulations based on bulk coal properties, however, have proved unsatisfactory in estimating NO, emissions levels for various ranks of coal. It is clear that a comprehensive study covering a wide variety of coal ranks, as mentioned above, is no doubt needed to reach a quantitative conclusion about the subject. Therefore, the objective of the present work is to investigate the relation between functional forms of (15) Nelson, P.F.;Kelly, M. D.; Womat, M . J. Fuel 1991, 70,403. (16) Kelly, D. K.; Buckley, A. N.; Nelson, P. F.hoc-lnt. Conf.Coal Sci. 1991, 356. (17) Wendt, J. 0. L. Prog. Energy Combust. Sci. 1980,6,201. (18) Okazaki, K.; Shishido,H.; Niahikawa, T.; Ohtake, K. Nihon Kikai Cakkai Ronbunshu B 1985,51,1549. (19) Smith, P. J.; Smoot, L. D.;Combust. Sci. Technol. 1980,23,17.
I
I
Coal sample
LLClj"
Temperature controller
Hra!tnq Chamber
4
D
I +
:'
Heated auto sampler
Exhaust
Camer gas (He 50mllmin) (He 50m'
""1
Gas Chromatography
4BS+ Heating line Heated block
Figure 1. Schematicdiagramof the pyrolysis reactor and analysis system employed.
coal nitrogen and fuel nitrogen conversion to NO, precursors by using various ranks of coal.
Experimental Section Fuels. A total of 20 coals covering wide ranks from semianthracite to blown cod was used in this experiment. The proximate and ultimate analyses are given in Table I. The carbon content ranged from 65.4 96 to 88.196 on a dry-ash-freebasis. The particle size was sieved to 64-74 pm. The Pyroprobe. The achematic diagram of the pyrolysis system is shown in Figure 1. A pyroprobe Type 120 (Chemical Data System Inc.) was used for rapid thermal decomposition. The 2-5-mg dried coal samples were loaded in the middle of a quartz tube of 1.3 mm inside diameter and 30 mm length. The coal samples were fixed with a small amount of glass wool. The quartz tube was inserted into the platinum coil of the pyroprobe, which was attached in the small chamber that was preheated to 573 K in a helium atmosphere (50 mL/min). The maximum temperature and the heating rate of the platinum coil are 1673 K and 75 X 103 K/s, respectively. The final temperature of the coal samples was measured in-situ a t the center of the samples with a thermocouple during pyrolysis. Figure 2 displays an example of the temperature history for coal E during pyrolysis a t a controlled temperature of 1373 K. The temperature of the coal particles was lower than the controlled temperature, and the time taken to reach maximum temperature was 5 s. The
Formation of NO, Precursors during Pyrolysis 10001, 945
I
I
,
I
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,
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Maximum temperature
.a 800 5
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-
-
-
-
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Holdtime
2001
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I I I
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Energy & Fuels, Vol. 7, No. 6, 1993 1015
6
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1
8
10
12
14
16
18
Time
J
20
[sec]
Figure 2. The temperature history of the pyroprobe for coal E (the Newlands coal from Australia) at a controlled temperature of 1100 OC.
1.0
1.2
1.4
1.6
coal particle temp, K 853 1038 1218 1488
diff, K 120 135 155 185
maximum heating rate, which was calculated from the history, was approximately lo00 K/s. The temperature history and the maximum heating rate had no significant influenceon coal types. Table I1 lista the final particle temperatures for coal E (the Newlands coal from Australia) at each controlled temperature. It was found that the difference between the controlled temperature and the particle temperature was 120-185 K. In view of the time taken to reach the maximum temperature, the heating interval was adjusted for 5 a. The pyrolysis was carried out at particle temperatures ranging from 853 to 1488 K for all coals, as listed in Table 11. The weight loss of samples, the content of nitrogen retained in the char, and compositionof devolatilizedgases were measured in this experiment. The char after pyrolysis was taken out of a quartz tube, and then ita weight loss was determined: the weight loss includes the amount of tar evolved,which was deposited on the quartz tube and the glass wool. The amount of tars was also measured. The chars of more than 50 mg were collected after pyrolysis, and then the nitrogen content remaining in the chars was analyzed by the Kjeldahl method. The experiment was performed at least 10 times for each coal at each temperature, and the data were found to be reproducible to about f10% on repeated runs. All devolatilized gases during pyrolysis were quicklyintroduced into the column of a gas chromatograph through a sampling line heated to 573 K. HCN and other nitrogen-containing species present in the pyrolysis producta were analyzed using a flame thermionizing detector (FTD). Detection of NHa was accomplished by an absorption system in which the entire pyrolysis gas was passed through a solution (0.05N H&0,,50 mL) which was then subjected to standard specific ion electrode analysis. In order to obtain a sufficient concentration for analysis, gases were accumulated in a single solution at least 5 times for each coal under the same experimental condition. Hydrocarbons of C1C1, inorganic gas componenta, and molecular nitrogen were measured by the TCD-FID connected in series. XPS Measurements. X-ray photoelectron spectroscopy (ShimazuESCA-K1) with MgKa radiation was used to determine the functional forms of coal nitrogen. The spectrometer was run at a pass energy of 32 eV with an X-ray source power of 300 W. The test samples were mounted uniformly by pressing them onto a double-sided adhesive tape. A test area of 1-mmdiameter
2.0
2.2
il
Table 11. Final Particle Temperatures for Coal E at Each Controlled Temperature controlled temp, K 973 1173 1373 1673
1.8
Bulk N/C atom [%I Figure 3. A comparison of the surface nitrogen concentration determined by XPS with that from bulk analysisfor parent coals. (X-ray source, Mg; voltage, 10 kV; current, 30 mA, pass energy, 32 eV, step, 0.1 eV; repeat, 5 times).
800
1000 1200 1400 1600
Pyrolysis temperature [K] Figure 4. Weight loss as a function of pyrolysistemperature for typical coals. (Particle sizes, 69 wm; heating rate, lo00 K/s; hold time, 5 8.) on the surface of the samples was analyzed. As the ESCA-K1 can accommodate 10 samples on a sample table in a vacuum chamber,these sampleswere analyzedusing a routine. Analyzing positions which were decided exactly by using a microscopewere kept in the computer's memory, and the analyses were carried out automatically 3-5 times at different positions on the surface in order to obtain N 1s spectra of sufficient quality and representative data. Spectra were recorded at a pressure of less than 1X 1o-BPa. To compensate for sample charging,all binding energieswere referenced to carbon 1s at 285 eV. The spectra are plotted as electron intensity versus binding energy. The reproducibility of the data was reasonable. Comparison of surface nitrogen concentration by X P S with bulk analysis by the Kjeldahl method was made to check the sensitivity of the detector using the procedure of Perry and Grint," expressing concentrations as N/C atomic ratios. The correlation was confirmed for each routine, because electron intensity varies significantly with the pressure of the sample chamber. The correlation between surface nitrogen and bulk nitrogen shows a good linear relation for each measurement as shown in Figure 3.
Results Fuel Nitrogen Decomposition. Typical results of the variation of weight loss with pyrolysis temperature are shown in Figure 4. The weight loss initially increases with (20)Perry, D.L.;Grint, A. Fuel 1983,62, 1024.
1016 Energy & Fuels, Vol. 7,No. 6,1993
Kambara et al.
5
80
-8
70 60
I
2 0 -
50
c 40
a
-
.-
30 20 10 0 800 1000 1200 1400 1600
800 1000 1200 1400 1600
Pyrolysis temperature [K]
Pyrolysis temperature [K] Figure 5. Tar yields of typical coals at different temperatures.
increasing pyrolysis temperature and finally approaches a constant value at a temperature of nearly 1400 K. It agrees with many previously published results. The asymptotic value of the weight loss is 1.1-1.5 times the amount of volatile matter found in proximate analyses using 20 coals. Tar yields are given in Figure 5 for typical coals. The yields were determined by the amount of tars deposited on the quartz tube and the glass wool. It was observed that the sampling line heated to 573 K from the reactor to the gas chromatograph was without deposition of tars. Tar yields in this experiment are low relative to yields from previous studies under rapid pyrolysis. It is likely that the tars convert to light gases by secondary reactions because of the high temperature and some residence time in the reactor. Low tar yields and the decrease in the yield above 1073 K have been shown to be a result of the secondary decomposition of tars. Nitrogen-containing heavy molecular species in tars are almost converted to light molecular species in this experiment. To establish a relation between functional forms of coal nitrogen and nitrogen evolution patterns, it is essential to identify N species in primary pyrolysis products. When determining the mechanisms of nitric oxide formation during coal combustion, however, it is important to estimate the yields of the final decomposition products of nitrogen-containing species. Hence, it is instructive to determine the yields of light molecular N species such as HCN and NH3 for various ranks of coals. Figure 6 presents nitrogen loss for typical coals as a function of pyrolysis temperature. The nitrogen loss was determined by measuring the content of nitrogen retained in char. It is obvious that the nitrogen loss is much more sensitive to pyrolysis temperature than the weight loss is. Similar results were discussed by Blair et al.9 Although they showed the amount of nitrogen lost increased with temperature with roughly the same slope for two bituminous coals and a subbituminous coal, in this experiment the temperature dependence of the nitrogen loss differs with coal type as shown in Figure 6. While coals E and B (the Oakdale coals from Australia) contained the same amount of fuel nitrogen and volatile matter of ASTM, coal E has much more volatile nitrogen than coal B. It is impossible to estimate the amount of volatile nitrogen from proximate and ultimate analyses. The yields of NHs, Nz, and HCN as a fraction of total nitrogen are summarized in Figures 7 and 8 as a function
Figure 6. Variation of nitrogen loss with pyrolysis temperature for typical coals. (Dot line: coal B and coal E have the same amount of volatile matter and nitrogen.)
= m 8 .-c z IO ,g v)
nal
.->r
2 5 b
I" z 0 800 1000 1200 1400 1600
Pyrolysis temperature [K] Figure 7. Variation of NH3 or Nz yields as a proportion of total nitrogenwith pyrolysistemperature for typical coals. (Solidlines, NH3; dot lines, N2.)
800 1000 1200 1400 1600
Pyrolysis temperature [K] Figure 8. Variation of HCN yields as a proportion of total nitrogen with pyrolysis temperature for typical coals.
of pyrolysis temperature for typical coals. The fuel nitrogen conversion to NH3 tends to level off a t about 1218 K for the coals used in this experiment. The increase of Nz yields is accelerated in the high-temperature region, but the yields were very low as compared with those of NHBand HCN. The yields of HCN are strongly dependent on pyrolysistemperature and reach 80 76 in the distribution of the volatile nitrogen species for all coals in the temperature range examined here. The yields of NH3, Nz,and HCN for all of the coals except those indicated in Figures 7 and 8 are between that of coal T and coal A (the Eaglemountine coals from Canada).
Formation of NO, Precursors during Pyrolysis
Energy & Fuels, Vol. 7,No. 6,1993 1017
Table 111. Yields of Nitrogen-ContainingSpecies and the Proportion of Nitrogen Retained in Chars at a Pyrolysis TemDerature of 1488 K volatile matter, yield, % retained detected ~
A B
wt%,db 25.3 28.9 34.5 37.6 39.6 36.2 32.4 35.7 36.0 43.7 38.2 37.9 34.4 35.5 48.9 44.1 49.8 47.6 53.8 56.9
60
1
A
~~~
Nz ch", % 0.1 78.0 0.3 72.9 C 0.2 61.4 D 0.3 53.3 E 0.5 52.4 F 0.8 54.3 G 0.1 59.6 H 0.7 58.7 I 0.3 55.4 J 0.4 57.2 K 0.8 58.0 L 0.7 60.4 M 0.8 57.0 N 0.4 60.8 0 1.3 49.2 P 0.8 50.2 Q 3.3 34.4 R 1.6 39.4 S 3.9 16.8 T 3.9 20.7 "Total N = NH3 + HCN + Nz + char N.
coal
NH3 HCN 2.5 21.2 3.4 25.3 4.3 29.6 9.0 35.2 6.8 42.7 7.2 35.5 4.4 31.6 5.1 31.1 8.7 31.0 6.1 31.3 4.9 30.5 6.0 31.0 6.8 36.1 4.2 33.6 9.7 48.6 6.9 44.2 10.7 50.6 11.2 46.6 11.9 62.4 12.1 65.1
~
totalN,"% 101.7 101.9 95.5 97.8 102.3 97.8 95.6 95.6 95.5 95.0 94.2 98.0 100.8 99.0 108.7 102.0 98.9 98.7 95.0 101.7
HCN yields in this work are higher than those of Nelson et al.16 because of the secondary decomposition of tars. Coals H and T are the Bayswater and the Yalloun coals, respectively, from Australia, which are the same coals as they used. It is found that HCN yields in this experiment are 2-3 times than their results at 1173 K. Table I11 lists the yields of nitrogen-containing species and the proportion of nitrogen retained in the char at 1488 K for the 20 coals. Detected total nitrogen (NH3 + HCN + N2 + char N) is approximately95-100% of nitrogen in the raw coals. It is evident that decomposed fuel nitrogen is almost always converted to NH3, N2, and HCN. It is difficult to measure nitrogen content in tar since the tar yields are low as shown in Figure 5, although some nitrogen may be present in the tars. Small amounts of CHsCN, CHzCHCN, and CsH6CN are detected below the pyrolysis temperature of 1218 K. The yields of these species are found to be negligible as compared to the composition of volatile nitrogen, because their conversions are lower than 0.01-0.1% for each pyrolysis temperature. The yields of NH3, N2, and HCN at 1488 K are shown in Figure 9 as a function of carbon content. Although there is somewhat of a scatter, an increase in the yields with decreasing carbon content can still be found. It is clear that the fuel nitrogen conversion to NO, precursors is strongly dependent on the coal types. It was too difficult to use the bulk factors such as carbon content and O/C and N/C atomic ratios for prediction of their yields. XPS Studies. The XPS technique is an effectual method to determine functional forms of coal nitrogen. Some dramatic improvements concerning the sensitivity and the resolution of a detector have been successfully made in recent years. The progress has played a role in improvement of the quantity limit of nitrogen content: it can be applied to the analysis of the functional forms for the raw coal containing above 0.1% N. Figure 10 shows a typical N 1s spectrum for coal B. From earlier work,l3 it is known that the peaks at 398.7 f 0.2 and 400.3 f 0.2 eV can be ascribed to pyridine and pyrrole type nitrogen, respectively. The N 1s spectrum
L lo 1488K
0
1 65
0
0
0 0
0 g
o
1488K NH3 0
70 75 80 85 Carbon content [M%,d.a.f]
90
Figure 9. Relation between the yields of major volatile nitrogen species and the carbon content at a pyrolysis temperature of 1488 K. 1600
0 400
-
-
Coal 6
N=l.7%, d.a.f
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is fitted with two Gaussian-shaped peaks at 398.7 i 0.2 and 400.3 f 0.2 eV (referenced to the C 1s peak at 285 eV in each sample). It is observed that there is a small residual below 15% of total N 1s area at high binding energy for all coals in this study when the nitrogen peaks were resolved using symmetric components of Gaussian line shape at 398.7 f 0.2 and 400.3 f 0.2 eV. Kelly et a1.I6 have also pointed out the peak a t a binding energy near 401.4 eV for one coal or another. Although this component is quoted for quaternary nitrogen,13insufficient evidence has been obtained to make this assignment. For example, it is found that the peak at 401.4 f 0.2 eV is observed in 5-aminosalicylic acid having secondary amine groups as shown
Kambara et al.
1018 Energy &Fuels, Vol. 7,No. 6,1993
Table IV. Distribution of Functional Forms of Coal Nitrogen for Various Ranks of Coal
functional form, % fuel N, wt% daf 1.20 1.70 1.92 2.25 1.74 1.40 1.90 1.84 1.01 1.82 1.88 1.76 1.35
coal A
B C
D E F G H I J K L M N 0 P
2.8 5.8 3.8 11.7 6.3 8.5 4.9 4.1 8.9 10.5 5.2 8.2 6.7 7.2 9.2
1.82
1.40 1.03 1.20 1.08 0.90 0.56
Q R S T 4.01
Y
quarternary
11.2
12.9 11.6 14.2 14.2
A
1
3.01
pyrrole-N 80.2 66.8 69.3 55.4 67.9 64.5 72.4 73.6 70.8 62.9 69.0 69.7 65.0 68.9 63.6 61.4 54.0 59.2 52.8 51.3
1
ratio
pyridine-N
I
5-aminosalicyclicacid NHzCsH3(0H)COOH
pyrrole/quarternary
pyrrole/pyridine 4.7 2.4 2.6 1.7 2.6 2.4 3.2 3.3 2.4 2.4 2.7 3.2 2.3 2.9 2.3 1.6 1.6
17.0 27.4 26.9 32.9 25.8 27.0 22.7 22.3 29.2 26.6 25.8 22.1 28.3 23.9 27.2 38.6 33.1 29.2 33.1 34.5
28.6 11.5 18.2 4.7 10.8 7.6 14.8 18.0 8.0 6.0 13.3 8.5 9.7 9.6 6.9 5.5 4.2 5.1 3.7 3.6
2.0
1.6 1.5 I
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Pyrolysis char of coal B
VI
0.
Char N=85.0%
0 L
1
I '
-
1200
E
600
VI
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1000
3
0
0
800
0
cn
3
0
400
200
- 408
n
404
400
396
Binding E n e r g y [eV]
Figure 11. C 1s and N 1s XPS spectra of 5-aminosalicylicacid.
in Figure 11, but the simple compound may be an inadequate coal model. Further qualitative determination of this peak is needed. A component at a binding energy of 401.4 f 0.2eV was usually required to achieve an acceptable fit along with the pyrrole and pyridine type nitrogen in this work. For all coals used in this experiment, functional forms of coal nitrogen were characterized as one of three types: pyrrole type, pyridine type, and quaternary nitrogen. The proportion of each functional form for coals and chars was determined by the peak synthesis method. In Table IV, the distribution of functional forms for parent coals are summarized. Pyrrole type is the most common in the distribution of nitrogen functional forms as indicated by previous studies. Jones et al.21 revealed that a pyrrole/pyridine ratio in coals was an approximately (21) Jones, R. B., McCourt, C. B.; Swift,P. Roc.-Int. Kohlenwiss. Tag., 1981.
I
1
404
402
I
400
I
I
I
398
396
394
Binding energy [eV]
Figure 12. Variation of nitrogen functional forms in chars during pyrolysis for coal B at different temperatures.
constant concentration ratio of 2:l. In this work the ratio of pyrrole type to pyridine type ranges from 1.51(coal T) to 4.7:l (coal A), and a pyrrole/quaternary ratio ranges from 3.7:l for coal S (the Usibelli coal from USA) to 28.6:l for coal A. It is found that the proportion of each functional form depends on the coal type. The variation of nitrogen functionality in char with pyrolysis temperature for coal B is shown in Figure 12. N 1s spectra of chars were resolved by the same procedure as that of the parent coals. At first the spectra are fitted with two Gaussian-shaped peaks at binding energies of pyrrole and pyridine types, and then the residual area at high binding energy is fitted with a Gaussian-shaped peak. The proportion of functional forms of coal nitrogen is determined as the fraction of area for each form to total N 1s area, and the content of each nitrogen functionality
Formation of NO, Precursors during Pyrolysis
Energy & Fuels, Vol. 7, No. 6,1993 1019
1
0.8 0.6 0.4 7 0.2
v1
a
I
K
O 1
u-
0.8 0.6 0.4
'0C
h: g v)
Coals
2,
O
M
+
T
* s O M
- 1Pyridine-N
* s
Y
0.8 0.6 0.4 0.2
?0 4 & 5 ! f l 800
Raw coal
.- 10 3
1000 1200 1400 1600
Pyrolysis temperature [K]
Figure 13. Decomposed fraction of each functional form as a function of pyrolysis temperature for various ranks of coal. is determined from the bulk nitrogen concentration for each char sample. The waveform of the nitrogen functionality for chars is clearly apparent as compared with those of the parent coals. The peak area of all nitrogen functional forms decreases with increasing temperature, and quaternary nitrogen disappears completely above 1218 K.
Discussion The relation between functional forms of coal nitrogen and the behavior of fuel nitrogen conversion to NH3 and HCN is discussed quantitatively as follows. Figure 13showsthe variation of the decomposedfraction of each functional form of coal nitrogen as a function of pyrolysis temperature for typical coals. It is found that quaternary nitrogen is evolved easily at low temperatures and usually decomposes completely at 1218K for all coals used in this study. This suggests that the peak at a binding energy of 401.4 eV is due to thermal instability. The decomposed fraction of pyridine type nitrogen increases in proportion to pyrolysis temperature, and 5080 7% of the pyridine type nitrogen is decomposed at 1488 K for semianthracite and brown coal. For pyrrole type nitrogen, the decomposed fraction has a low value in the low-temperature region and tends to increase above 1037 K although the fraction differs with different coal types. The decomposed fraction of pyrrole type nitrogen is lower than that of pyridine type for many coals. This suggests that pyrrole type nitrogen is more difficult to decompose than pyridine nitrogen, but it is doubtful in view of the thermal stability of pyrrole and pyridine. Moreover these results do not agree with previous studies carried out by Jones et aL21although nitrogen functional forms of chars at high temperatures were not investigated in their work. It is believed that fuel-bound nitrogen is bonded as complex structures rather than a simple pyrrole and pyridine structure. Therefore the results in Figure 13 cannot be explained by only the thermal stability of such simple compounds. The decomposition behavior of pyrrole type nitrogen depends on more coal types than that of quaternary and
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Calculated NH, yields [%N in coal] Figure 14. Comparison of calculated NHs yields based on quaternary nitrogen loss and observed NHs yields at each temperaturefor various ranks of coal. (CoalA was not detected.)
pyridine types. This presumed that other nitrogen forms are present within the pyridine peak. Bartle and Wallacel3 observed a binding energy of 399.4 eV for primary amines, but it cannot be resolved due to overlap between pyridine and pyrrole nitrogen. The fuel nitrogen conversion to NH3 tends to level off at approximately 1218 K as shown in Figure 7. This behavior coincided with the results that quaternary nitrogen decomposes completely at about 1218 K (Figure 13). It seems that the lower the temperature for conversion to NH3 is, the lower the temperature for content of quaternary nitrogen is. Hence, it is considered that quaternary nitrogen might be converted finally to NH3. Figure 14 shows the comparison of observed NH3 yields and calculated NH3 yields in which it is assumed that the lost of quaternary nitrogen at each temperature is due to conversion to NH3 . For all coals studied here, a good agreement between them has been found. Although how intermediate speciesform from quaternary nitrogen during pyrolysis is unknown, fuel nitrogen conversion to NH3 in the high-temperature region can be estimated by determination of quaternary nitrogen using XPS. NH3 is the most important species as it is a NO, precursor during coal combustion. It significantly affects NO, formation at early stages during combustion in order to have a large oxidation and reduction rate.22 Hence the peak at a binding energy of 401.4 eV is required for evaluation of NO, formation. As quaternary nitrogen converts finally to NH3, it is assumed that residual nitrogen forms convert to HCN and a little N2. The relation between HCN yields during pyrolysis and nitrogen loss of pyridine and pyrrole type nitrogen is demonstrated by the comparison between the observed and calculated yields at each temperature as shown in Figure 15. Calculated HCN yields were expressed on the same basis as those of NH3. Both yields have a good agreement, however, HCN yield cannot be predicted by XPS measurements. For estimation of HCN yields, further studies are needed to characterize pyridine and pyrrole nitrogen that are devolatized easily during pyrolysis. Conclusions
The pyrolysis experiment and the measurelpent of the nitrogen functional forms using XPS have been performed (22) Kambara, S.; Takarada, T.; Nakagawa, N.; Kato, K. Kagaku Kogaku Ronbunshu. 1992, 18,920.
Kambara et al.
1020 Energy & Fuels, Vol. 7, No. 6, 1993
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Calculated HCN yields [%N in coal] Figure 15. Comparison of calculated HCN yields based on loss of pyridine and pyrrole type nitrogen and observed HCN yields at each temperature for various ranks of coal.
for coals covering a wide range of ranks under rapid heating conditions. The following conclusions are obtained. 1. In this experiment the main compositions of volatile nitrogen are HCN, NH3, and N2. Their yields depend on coal type and pyrolysis temperature. The fuel nitrogen conversion to NH3 tends to level off at a temperature of approximately 1218K. The yields of HCN and N2 increase with increasing the temperature, but N2 yields are very low as compared with the yields of NH3 and HCN.
2. The nitrogen functional forms of pyrrole type, pyridine type, and quaternary nitrogen are observed for all coals used in this study by XPS measurements. The ratio of pyrrole to pyridine type ranges from 1.51to 4.7:1, and the pyrrole/quaternary ratio ranges from 3.7:l to 28.6: 1. The proportion of each functional form depends on coal type. 3. The variation of the nitrogen functional forms in the coals and the char is quantitatively compared with the yield of nitrogen-containing species. The relation between observed NH3 yields and calculated NH3 yields based on evolution of quaternary nitrogen indicates a good agreement at each temperature. The yields of NH3 can be estimated by determination of quaternary nitrogen using XPS. 4. The relation between HCN yields during pyrolysis and nitrogen loss of pyridine and pyrrole type nitrogen is demonstrated by the comparison between the observed and calculated yields a t each temperature. It seems that all decomposed nitrogen for pyridine and pyrrole type nitrogen is converted finally to HCN. Acknowledgment. The authors gratefully acknowledge their colleagues, F. Ogata and M. Toyoshima for the assistance in the experiments and the analysis.
